JP2008156734A - High-strength hot-dip galvanized steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet having excellent drawability and its manufacturing method. <P>SOLUTION: Fracture in a vertical wall portion at deep drawing is suppressed by improving the structure of a surface layer of a steel sheet. Specifically, a structure having the following characteristics is provided: ferrite phase of ≤10 μm average grain size and martensite phase of 30 to 90% volume fraction are contained; the ratio of hardness in a sheet-thickness surface layer to hardness in a sheet-thickness center ranges from 0.6 to 1; the maximum depth of cracks propagating from the interface between a plating layer and a steel sheet toward the inner part on the steel sheet side and of recessed portions is 0 to 20 μm; and the area ratio of smooth portions except the cracks and the recessed portions is 60 to 100%. In order to obtain the steel sheet having such a structure, e.g. hot-dip galvanizing treatment is performed as follows: an atmosphere in a heat treatment furnace in the course from a ≥600°C temperature-raising process, via an annealing temperature, to a cooling process down to 450°C is controlled so that it has 2 to 20% hydrogen concentration and -60 to -10°C dew point; after holding at 760 to 860°C annealing temperature for 10 to 500 sec, cooling is done at 1 to 30°C/sec average cooling rate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, which is suitable for use in automobile parts that are required to be pressed into a strict shape, and a method for producing the same.

It is important that high strength hot-dip galvanized steel sheets used for automobile parts and the like have excellent workability in addition to high strength due to the characteristics of their applications. Recently, high-strength steel sheets have been demanded for automobile bodies from the viewpoint of improving fuel efficiency by reducing the weight of the vehicle body and ensuring collision safety, and their application has been expanded. Conventionally, light processing has been mainly used, but application to complex shapes is also being studied. However, the workability tends to decrease as the strength of the steel plate increases, and cracking during press forming is a major problem in applying the high strength steel plate. Therefore, it is required to improve workability, that is, drawability, bendability, stretchability and stretch flangeability according to the part shape.
For example, Patent Documents 1 to 5 disclose methods for improving formability by limiting steel components and structures, optimizing hot rolling conditions, and annealing conditions. For example, Patent Documents 6 to 10 describe a plated layer and a steel plate ground iron interface structure.
Japanese Patent No. 2761095 Japanese Patent No. 2761096 Japanese Patent No. 3376882 JP 2001-303178 A JP 2002-206139 A JP 2001-355043 Japanese Patent Laid-Open No. 2003-171752 Japanese Patent Laid-Open No. 2004-18970 Japanese Unexamined Patent Publication No. 2004-18971 JP 2004-211157 A

In difficult-to-mold parts, the deformation of the material during pressing is often composite molding in which drawing molding or stretch molding is combined. In drawing, cracks often occur in the punch shoulder or vertical wall. Many punch shoulder breaks are observed in cylindrical drawing shapes, etc., and square wall drawing and hat-shaped drawing with open cross section may cause breakage at the vertical wall. It was difficult to achieve both strength and excellent drawability. It is widely known that drawability has a correlation with r value, but in cold-rolled annealed steel sheet or hot-dip galvanized steel sheet with transformation structure strengthened, r value is generally less than 1.0, and r value is improved. It is extremely difficult to impart drawability by means of. In this regard, Patent Documents 1 to 5 do not suggest anything regarding improvement of drawability.
Patent Document 6 discloses a steel having a structure of a high-strength hot-rolled steel sheet mainly composed of granular bainitic ferrite or quasi-polygonal ferrite, and crystal grains between individual crystals at the interface between the plating layer and the steel sheet. It describes a plated steel sheet characterized in that the boundary is etched by a width of 2.0 μm or less. It is disclosed that the effect is effective for securing plating adhesion, and that the lack of crystal grains due to damage during punching is prevented and the hole expansion rate is improved only by ensuring adhesion. However, there is no knowledge about the hardness ratio between the steel sheet surface layer and the center, which is important for drawability, the cracks progressing in the steel sheet at the interface between the plating layer and the steel sheet, and the smooth portion of the steel sheet surface. Patent Document 6 does not have any knowledge about cold-rolled steel sheets that are excellent in drawability and are composed of ferrite and martensite that have different structures and completely different thermal histories in Patent Document 6 and the present invention.
Patent Document 7 describes only the depth of the grain boundary oxidation phase existing in the steel sheet for the steel having ferrite and bainite as the main phase, and does not describe the properties of the interface and has no knowledge about the formability.
Patent Documents 8 to 10 include residual austenite, steel with ferrite and bainite as the main phase, the amount of oxide inside the steel within 10 μm from the interface in the cross-sectional observation, and the amount inside the steel sheet with an index of area ratio However, there is no knowledge about the interface state between the plating layer and the ground iron.

As described above, none of the conventional techniques has been studied on a high-strength hot-dip galvanized steel sheet having excellent drawability and its manufacturing method.
The present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet having excellent drawability and a method for producing the same.

  The present inventors have intensively studied to solve the above problems. As a result, it is possible to suppress fracture at the vertical wall during deep drawing by improving the steel surface layer structure (hardness, smoothness), and to obtain a high-strength hot-dip galvanized steel sheet with excellent drawability I found.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In mass%, C: 0.03-0.12%, Si: 0.01-1.0%, Mn: 1.5-2.5%, P: 0.001-0.05%, S: 0.0001-0.005%, Al: 0.005-0.15%, N : 0.001-0.01%, Cr: 0.01-0.5%, Ti: 0.005-0.05%, Nb: 0.005-0.05%, V: 0.005-0.5%, B: 0.0003-0.0030%, the balance being Fe and inevitable impurities And having a structure including a ferrite phase with an average crystal grain size of 10 μm or less and a martensite phase with a volume fraction of 30 to 90%, the ratio of the sheet thickness surface hardness to the sheet thickness center hardness is 0.6 to 1, The maximum depth of cracks and recesses extending from the interface between the layer and the steel plate to the inside of the steel plate is 0 to 20 μm, and the area ratio of the smooth part other than the cracks and the recesses is 60% to 100% High strength hot-dip galvanized steel sheet.
[2] In the above [1], the composition further contains at least one of mass: Mo: 0.01 to 0.5%, Cu: 0.01 to 0.5%, and Ni: 0.01 to 0.5%. Strength hot dip galvanized steel sheet.
[3] When the steel composed of the component described in [1] or [2] is hot-rolled, pickled and cold-rolled, and subjected to hot-dip galvanizing treatment to produce a hot-dip galvanized steel sheet, the heat In the intermediate rolling, the slab heating temperature is 1150 to 1300 ° C, the finish rolling temperature is 850 to 950 ° C, the winding temperature is 400 to 600 ° C, and in the pickling, the bath temperature is less than 10 to 100 ° C and the hydrochloric acid concentration is 1 In the hot dip galvanizing treatment, the atmosphere in the heat treatment furnace from the temperature rising process of 600 ° C. or higher to the cooling process to 450 ° C. through the annealing temperature is 2 to 20% in hydrogen concentration and -60 to -10 dew point. A method for producing a high-strength hot-dip galvanized steel sheet, wherein the steel sheet is held at an annealing temperature of 760 to 860 ° C. for 10 to 500 seconds and then cooled at an average cooling rate of 1 to 30 ° C./second.
[4] The method for producing a high-strength hot-dip galvanized steel sheet according to [3], further comprising an alloying treatment after the hot-dip galvanizing treatment.

  In addition, in this specification,% which shows the component of steel is all mass%. In the present invention, the high-strength hot-dip galvanized steel sheet is a cold-rolled steel sheet having a tensile strength (hereinafter sometimes referred to as TS) of 780 MPa or more.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having excellent drawability can be obtained. The high-strength hot-dip galvanized steel sheet obtained by the present invention satisfies all the properties of strength and formability required for automobile parts, and is suitable as an automobile part that is press-formed into a strict shape.

The present invention was obtained as a result of diligent investigations regarding the improvement of drawability of high-strength hot-dip galvanized steel sheets, and includes a ferrite phase having an average crystal grain size of 10 μm or less and martensite having a volume fraction of 30 to 90%. The ratio of the surface thickness hardness to the center thickness of the plate thickness is 0.6 to 1, and the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel plate to the inside of the steel plate is 0 to 20 μm Furthermore, the area ratio of the smooth portion other than the cracks and the recesses is 60% to 100%. In this way, by improving the structure, especially the steel sheet surface layer structure, and making the hardness and smoothness as described above, it becomes possible to suppress breakage at the vertical wall during deep drawing, thereby reducing drawability. An excellent high-strength hot-dip galvanized steel sheet can be obtained.
Below, the reason for limitation of the chemical composition and structure | tissue of steel for obtaining the high intensity | strength hot-dip galvanized steel plate excellent in draw formability is demonstrated.

First, the reasons for limiting the chemical composition (composition) of steel in the present invention are as follows.
C: 0.03-0.12%
The strength of the martensite phase tends to be proportional to the C content, and C is indispensable for strengthening steel using the martensite phase. To obtain a TS of 780 MPa or more, 0.03% or more is necessary, and TS increases as the amount of C increases. On the other hand, if the content exceeds 0.12%, the spot weldability is remarkably deteriorated, the martensite phase is excessively hardened, and the formability such as stretch flangeability tends to be lowered. Therefore, C is set to 0.03% or more and 0.12% or less. From the viewpoint of stably securing TS of 780 MPa or more and obtaining better formability and spot weldability, the preferable range of C content is 0.06% or more and 0.10% or less.

Si: 0.01 to 1.0%
Si is an element that contributes to strength improvement by solid solution strengthening. However, if the content is less than 0.01%, there is no effect of addition. On the other hand, the content exceeding 1.0% not only saturates the effect but also reduces the ductility of the ferrite phase. Moreover, when it contains excessively, the scale of a hard-peeling property is produced | generated at the time of hot rolling, and the surface property of a steel plate is deteriorated, In addition, it segregates and concentrates on the steel plate surface, a crystal grain boundary, etc., and a material deteriorates. From the above, Si is 0.01% or more and 1.0% or less, preferably 0.02% or more and 0.5% or less.

Mn: 1.5-2.5%
Mn contributes to strength, and such an effect is recognized by containing 1.5% or more. On the other hand, if the content exceeds 2.5% excessively, it becomes a structure where the transformation point is partially different due to segregation of Mn, etc., and as a result, the ferrite phase and martensite phase exist in a band shape, resulting in a non-uniform structure. Processability decreases. In addition, segregates and concentrates on the steel plate surface, grain boundaries, etc., and deteriorates the material. Accordingly, Mn is set to 1.5% to 2.5%, preferably 1.8% to 2.4%.

P: 0.001 to 0.05%
P is an element that deteriorates weldability, and when the amount of P exceeds 0.05%, the effect appears remarkably. On the other hand, excessive reduction of the amount of P is accompanied by an increase in production cost in the steelmaking process. Accordingly, P is 0.001% to 0.05%, preferably 0.001% to 0.015%, and more preferably 0.001% to 0.010%.

S: 0.0001 to 0.005%
If the amount of S increases, it may cause hot red hot brittleness and may cause problems in the manufacturing process. In addition, inclusion MnS is formed and present as a plate-like inclusion after cold rolling, thereby particularly reducing the ultimate deformability of the material and lowering the formability. If the amount of S is up to 0.005%, the above is not a problem. On the other hand, excessive reduction is accompanied by an increase in desulfurization cost in the steelmaking process. Accordingly, S is set to be 0.0001% or more and 0.005% or less, preferably 0.0001% or more and 0.0015% or less.

Al: 0.005% to 0.15%
Al is effective as a deoxidizer in the steelmaking process, and is also effective in separating non-metallic inclusions that reduce local ductility into the slag. In addition, Al has the effect of suppressing the formation of Mn and Si-based oxides on the surface layer that hinders plating properties during annealing and improving the plating surface appearance. Furthermore, by raising the Ac ₃ transformation point and expanding the ferrite + austenite two-phase region, there is also an effect of expanding the appropriate annealing temperature range. Addition of 0.005% or more is necessary to obtain such an action. On the other hand, if added over 0.15%, not only the steel component cost increases, but also the weldability decreases. Accordingly, Al is 0.005% to 0.15% or less, preferably 0.01% or more and 0.06% or less.

N: 0.001 to 0.01%
The influence of N on the material properties in the structure strengthened steel is not so great, and if it is 0.01% or less, the effect of the present invention is not impaired. On the other hand, from the viewpoint of improving workability by cleaning ferrite, the N content is preferably small, but the cost for steelmaking also increases, so the lower limit is made 0.001%. Accordingly, N is set to be 0.001% to 0.01%, preferably 0.001% to 0.0060%.

Cr: 0.01-0.5%
Cr is an effective element for hardening hardening of steel. In order to obtain this effect, addition of 0.01% or more is required. On the other hand, when Cr exceeds 0.5%, this effect is saturated, and the surface quality is significantly reduced. Therefore, Cr is 0.01% to 0.5%, preferably 0.04% to 0.2%.

Ti: 0.005-0.05%
Ti forms fine carbides and fine nitrides with C or N in steel, and thus effectively acts to refine the hot-rolled sheet structure and the steel sheet structure after annealing and to impart precipitation strengthening. In order to obtain this effect, it is necessary to add 0.005% or more of Ti. On the other hand, if it exceeds 0.05%, the effect is saturated. Therefore, Ti is 0.005% to 0.05%, preferably 0.01% to 0.04%.

Nb: 0.005-0.05%
Nb is an element that contributes to strength improvement by solid solution strengthening or precipitation strengthening. In addition, by strengthening ferrite, stretch flangeability is improved through the effect of reducing the hardness difference from the martensite phase. Such an effect is obtained when the Nb content is 0.005% or more. On the other hand, when it contains excessively, a hot-rolled sheet will harden | cure and the rolling load in hot rolling and cold rolling will increase. In addition, the ductility of the ferrite deteriorates and the workability decreases. For this reason, the Nb content is 0.05% or less. From the viewpoint of strength and workability, Nb is preferably 0.01% or more and 0.04% or less.
V: 0.005-0.5%
V precipitates carbides, suppresses the coarsening of the ferrite phase in the heating stage during continuous annealing, makes the steel structure fine and uniform, and significantly improves stretch flangeability. V is an element effective for strengthening steel. In order to obtain these effects, V needs to be added in an amount of 0.005% or more. On the other hand, when the added amount of V exceeds 0.5%, these effects are saturated. Therefore, V is 0.005% to 0.5%, preferably 0.01% to 0.2%.

B: 0.0003 to 0.0030%
B enhances hardenability, suppresses the formation of ferrite that occurs during the annealing and cooling process, and contributes to obtaining a desired amount of martensite. In order to obtain this effect, the amount of B needs to be 0.0003% or more. On the other hand, when the amount of B exceeds 0.0030%, the above effect is saturated. From the above, B is in the range of 0.0003% to 0.0030%. Preferably, B is 0.0003% or more and 0.0020% or less.

The steel sheet of the present invention can achieve the desired characteristics with the above-mentioned essential additive elements, but can contain the following elements depending on the desired characteristics.
One or more of Mo: 0.01-0.5%, Cu: 0.01-0.5%, Ni: 0.01-0.5%
Mo is an element effective for strengthening the quenching of steel. To obtain this effect, addition of 0.01% or more is required. However, this effect is saturated when the Mo content exceeds 0.5%. Therefore, when Mo is contained, the content is in the range of 0.01% to 0.5%. More preferably, it is 0.05% or more and 0.35% or less.
Cu and Ni are elements that contribute to strength. When added for strengthening, 0.01% or more of each is required. On the other hand, even if added excessively, the effect is saturated, so the upper limit of Cu and Ni is 0.5%. Therefore, when it contains, Cu and Ni shall be 0.01% or more and 0.5% or less. More preferably, Cu and Ni are each 0.01% or more and 0.3% or less.
In addition, it has the effect of controlling the form of sulfide inclusions without significantly changing the plating property, which is the essence of the present application, and this effectively contributes to improving the formability of REM or crystals on the steel sheet surface layer. Sb or the like having the effect of regulating the size may be contained in the range of 0.0001% to 0.1%. The remainder other than the above is made of Fe and inevitable impurities.

Next, the reason for limiting the steel structure, which is one of the important requirements for the present invention, will be described.
The average crystal grain size of ferrite phase is 10 μm or less Crystal refinement contributes to the improvement of stretch flangeability of steel sheets. Therefore, in the present invention, the average crystal grain size of the ferrite phase in the composite structure is limited to 10 μm or less. If the average crystal grain size of the ferrite phase becomes excessively large, the surface of the steel sheet may be roughened after press forming. In addition, if the soft region and the hard region are present roughly, the deformation becomes non-uniform and the moldability deteriorates. Since the ferrite phase and the martensite phase are present uniformly and finely, the deformation of the steel sheet becomes uniform during forming. Therefore, it is desirable that the average crystal grain size of the ferrite phase is small. From the above, the average crystal grain size of the ferrite phase is 10 μm or less, and from the viewpoint of suppressing the deterioration of formability, the average grain size of the ferrite phase is preferably 1 μm or more and 5 μm or less.

The volume fraction of martensite phase is 30-90%
In order to obtain TS780 MPa or more, the steel sheet of the present invention needs to contain a martensite phase of 30% or more by volume fraction. The martensite phase is a hard phase and has an action of increasing the strength of the steel sheet by strengthening the transformation structure. In addition, since the generation of movable dislocation is accompanied at the time of transformation generation, it also has the effect of reducing the yield ratio of the steel sheet. On the other hand, if the martensite phase exceeds 90%, the strength becomes excessively high, the soft ferrite phase decreases, and it becomes difficult to ensure the moldability. From the above, the volume fraction of the martensite phase is 30% to 90%, preferably 40% to 70%.

Ratio of sheet thickness surface layer hardness to sheet thickness center hardness is 0.6-1
In order to obtain good drawability, in the steel sheet of the present invention, the ratio of sheet thickness surface layer hardness to sheet thickness center hardness (hereinafter also referred to as hardness ratio) is set to 0.6-1. When the hardness ratio is 0.6 to 1, the vertical wall portion at the time of drawing is a uniform structure in the plate thickness direction and is uniformly deformed. However, when the hardness ratio is less than 0.6, the surface layer is excessively softened from the thickness center, and when the hardness ratio is greater than 1, the surface layer is hardened. In this way, if the difference in hardness between the surface layer and the center of the plate thickness is large, that is, the structure is non-uniform, damage is caused at the plate thickness surface layer and the center of the plate thickness of the vertical wall that the steel plate receives due to the strain introduced at the time of drawing. Different. As a result, when the structure in the plate thickness direction is non-uniform in this way, the deformation behavior of the surface layer portion and the plate thickness center portion is different as the molding progresses, faster than when the structure has a uniform structure in the plate thickness direction. It will lead to breakage. From the above, the ratio of the sheet thickness surface layer hardness to the sheet thickness center hardness is set to 0.6 to 1. A preferred range is from 0.75 to 1.
In addition, the hardness of the surface layer and the center of the plate thickness is measured as a simple average value by measuring the hardness at a predetermined position on the cross section of the plate thickness parallel to the rolling direction at a pressing load of 100 g using a Vickers hardness tester. be able to. Further, in the present invention, the surface layer is defined as 0 μm at the interface between the plated layer and the steel plate, and the center of the thickness is 25 μm from the interface, and the center of the thickness is 1/2 of the actual measured thickness. For example, in the case of a measured plate thickness of 1.40 mm including the plating thickness, the surface layer is approximately 25 μm from the surface of the plated and ground iron interface, and the center of the plate thickness is approximately 0.70 mm from the plated surface.
In order to control the hardness ratio to 0.6 to 1, it is effective to perform low-temperature winding at, for example, 600 ° C. or less.

Maximum depth of cracks and recesses extending from the interface between the plating layer and steel sheet to the inside of the steel sheet 0 to 20 μm, smooth area ratio other than cracks and recesses 60% to 100%
In order to obtain good drawability, in the steel sheet of the present invention, the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel sheet to the inside of the steel sheet is 0 to 20 μm. The area ratio of the smooth part is 60 to 100%, that is, the area ratio of cracks and recesses is 0 to 40%. In the case of drawing, the steel plate passes from the flange portion through the die shoulder portion and flows into the vertical wall portion. For example, in the case of cylindrical molding, the molding style is often such that it flows into the die after being subjected to drawing, bending, and bending back deformation, or in the case of hat molding, the molding is such that it flows into the die after being subjected to bending and bending back deformation. Therefore, strain is introduced into the steel sheet surface layer during bending back deformation. Here, if there is a crack or a recess at the interface between the plating layer and the steel sheet, and if it exists at a certain ratio or more, it becomes the starting point of crack generation at the time of drawing, leading to breakage, making it difficult to satisfy drawability. . If the depths of the cracks and recesses are 20 μm or less, they will not become apparent. In addition, if the existence ratio of cracks and recesses exceeds 40%, the drawability deteriorates. It is preferable that there are no cracks, but this requires additional steps such as a mechanical removal process such as cutting and a chemical removal process such as acid and alkali treatment after the cold rolling and before the heat treatment. The cost increases and it becomes difficult industrially. Therefore, the above upper limit is set as a range that does not become apparent.
From the above, the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel plate to the inside of the steel plate is 0 to 20 μm, and the area ratio of the smooth part other than the cracks and recesses is 60% to 100%. Preferably, the maximum depth of the crack and the recess is 0 to 5 μm, and the area ratio of the smooth portion other than the crack and the recess is 75% to 100%.
In the present invention, the crack progressing from the interface between the plating layer and the steel sheet to the inside of the steel sheet side is smooth, but the surface of the sheet is smooth, but linearly observed in an arbitrary cross-sectional structure observation, for example, along the grain boundary The concave portion is a crater shape in the plate surface observation, for example, a part of the steel plate surface is peeled off, and is not linear in the arbitrary cross-sectional structure observation, for example, one piece In the place where the crystal grains originally existed, it is a portion where the crystal is peeled off and observed in a hollow shape.
Also, the maximum depth is based on the position of the interface between the plating layer and the steel sheet steel (0 μm) .In the case of a crack, the depth in the thickness direction from the plating metal interface to the crack tip, and the depth of the recess is the opening. To the deepest bottom of the recess can be measured in the thickness direction, and will be described later in detail in the examples. Similarly, the smooth portion area ratio will be described later in detail in Examples.
Also, the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel sheet to the inside of the steel sheet is 0 to 20 μm, and the area ratio of the smooth part other than the cracks and the recesses is controlled to 60% to 100%. To do so, for example, the coiling temperature after hot rolling is controlled in the range of 400 to 600 ° C. to suppress surface scale generation and oxide generation inside the steel sheet, and the hydrogen concentration in the heat treatment furnace is 2 to 20%. In addition, it is effective to control the dew point in the range of -60 to -10 ° C.

Next, the manufacturing method of the high-strength hot-dip galvanized steel sheet of this invention is demonstrated.
First, slab is melted by continuous casting or ingot-making from molten steel adjusted to the above chemical component range. Subsequently, the obtained slab is cooled and then reheated or hot rolled as it is. At this time, the slab heating temperature is set to 1150 to 1300 ° C., and the finish rolling temperature is set to 850 to 950 ° C. in order to make the hot rolled sheet uniform and improve formability such as stretch flangeability. The coiling temperature is set to 400 ° C. or higher in order to improve surface properties and cold rolling properties. In addition, it is possible to reduce the sheet thickness variation in the width direction after cold rolling, to improve the surface properties by smoothing the surface by suppressing scale formation, and to reduce the band-like structure consisting of two phases of ferrite phase and pearlite phase. Sampling temperature shall be 600 ℃ or less. Next, pickling after hot rolling is performed at a bath temperature of less than 10-100 ° C. and a hydrochloric acid concentration of 1-20%. Next, after cold rolling, a desired plate thickness is obtained. The cold rolling rate is preferably 30% or more in order to improve ductility by promoting recrystallization of the ferrite phase. In hot dip galvanizing, the atmosphere in the heat treatment furnace from the temperature rising process of 600 ° C or higher to the cooling process to 450 ° C through the annealing temperature is maintained in a hydrogen concentration of 2-20% until just before the zinc bath process. In addition, the dew point is set to -10 to -60 ° C, and the annealing temperature of 760 to 860 ° C is maintained for 10 to 500 seconds, followed by cooling at an average cooling rate of 1 to 30 ° C / second. After cooling, the steel sheet is subsequently immersed in a molten zinc bath and galvanized. Or, further, after the galvanization, an alloying treatment of the plating can be performed. The adhesion amount of hot dip galvanizing can be controlled by gas wiping or the like, and the alloying treatment is sufficient to hold for 10 to 120 seconds in a temperature range of 450 to 600 ° C. As described above, a high-strength hot-dip galvanized steel sheet can be obtained by cooling to room temperature after plating or alloying.
Hereinafter, these manufacturing conditions will be described in detail.

Slab heating temperature: 1150 ~ 1300 ℃
It is necessary to redissolve the Ti, Nb, and V precipitates precipitated during casting, and the precipitates that exist in the heating stage are present as coarse precipitates in the steel sheet that is finally obtained, so they do not contribute to strength. . Heating above 1150 ° C contributes to strength, but heating above 1300 ° C causes decarburization from the surface layer and coarsening of austenite grains, increasing costs. It is also preferable to heat to 1150 ° C. or higher from the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface. From the above, the slab heating temperature is set in the range of 1150 ° C to 1300 ° C.

Finishing rolling temperature: 850 ~ 950 ℃
The press formability (TS × λ ≧ 45000 MPa ·% and TS × El ≧ 18000 MPa ·%) can be remarkably improved by setting the finish rolling temperature to 850 ° C. or higher. When the finish rolling temperature is less than 850 ° C., it is remarkable after the hot rolling, especially in the edge portion in the width direction, etc., but the unrecrystallized austenite and recrystallized with the crystal grain size partially stretched by hot rolling Austenite may be mixed. A sized structure is formed from sized recrystallized austenite, and in the region where originally expanded grains are present, a ferrite phase and a low-temperature transformation phase exist in the vicinity of the former austenite grain boundary, forming a band structure. It will be. For example, if Mn, an austenite stabilizing element, is segregated due to casting segregation at the steelmaking stage, the Ar3 transformation point in that region is lowered, and the austenite region is obtained up to a low temperature. Moreover, it is thought that a non-recrystallized austenite exists during hot rolling as a result that the non-recrystallized temperature region and the rolling end temperature become the same temperature region as the temperature decreases. Thus, when it becomes a non-uniform | heterogenous structure | tissue, the uniform deformation | transformation of the material at the time of shaping | molding will be inhibited, and it will become difficult to have remarkably outstanding moldability.
In general, a hot-rolled scale is likely to grow in a short time in a high temperature region, and the scale thickness increases exponentially with time. If the temperature exceeds 950 ° C, the amount of oxides generated will increase rapidly, the iron-oxide interface will become rough, and the surface quality after pickling and cold rolling will deteriorate. Thus, when the surface quality after cold rolling deteriorates, in the hot dip galvanized steel sheet, which is the final product, the presence of cracks and recesses progressing into the steel plate side at the plating-steel interface becomes noticeable. It becomes easy to break. Further, the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during molding. Therefore, the finish rolling temperature is 850 to 950 ° C. In addition, the preferable conditions of finish rolling temperature are 900 degreeC-950 degreeC.

Winding temperature: 400 ~ 600 ℃
When the coiling temperature exceeds 600 ° C., decarburization from the surface layer and scale growth are remarkable and surface quality deteriorates. Also inside the steel sheet, Si and Mn-based oxides are formed in the grain boundaries and in the grains, microcracks after pickling, iron powder is peeled off after cold rolling, and the surface of the cold rolled material becomes a recess. Roughness and formability deteriorate. The coiling temperature is set to 400 ° C. or higher in order to suppress an increase in cold rolling load due to an increase in hot rolled sheet strength.

After hot rolling pickling bath temperature: less than 10-100 ° C, hydrochloric acid concentration: 1-20%
When the pickling bath temperature is less than 10 ° C and the hydrochloric acid concentration is less than 1%, removal of the scale generated during hot rolling and winding is incomplete, and the surface properties of the steel sheet deteriorate and the plating appearance is poor. On the other hand, when the pickling bath temperature is 100 ° C or higher, or when the hydrochloric acid concentration exceeds 20%, the scale quality is removed, and then the pickling is over pickled, the base iron-oxide interface becomes rough, the surface quality after pickling and cold rolling. In the hot-dip galvanized steel sheet, which is the final product, press formability is reduced due to cracks and recesses that have progressed to the inside of the steel sheet. From the above, after hot rolling, pickling bath temperature: less than 10-100 ° C, hydrochloric acid concentration: 1-20%. Note that the type of acid is not limited to hydrochloric acid as long as similar effects can be obtained. Further, in order to further improve and smooth the surface properties, an alkali treatment step may be added subsequent to the pickling.

Hot dip galvanizing treatment, hydrogen concentration in heat treatment furnace from temperature rising process above 600 ° C to cooling process through annealing temperature to 450 ° C: 2-20%, dew point: -60-10 ° C
In a plated steel sheet, the state of the steel sheet surface immediately before entering the hot dip zinc pot is important for the plateability and formability. Si and Mn-based oxides are likely to be generated in the high temperature range of 600 ° C or higher. Once heated to 600 ° C or higher, the hydrogen concentration in the furnace is less than 2% or the dew point is -10 before entering the zinc pot. When the steel sheet is exposed to a temperature exceeding ℃, Si and Mn are concentrated in the steel sheet surface layer and grain boundaries, and oxidation proceeds. The grain boundary strength decreases due to the generation of Si and Mn oxides, and when they come into contact with the in-furnace roll, the crystal grains adhere to the roll using the oxide as a medium and peel from the surface layer of the steel sheet, generating cracks and recesses. In this way, the steel plate is immersed in the zinc bath while the surface iron-oxide interface is roughened and the surface quality before entering the pot is deteriorated (with cracks and recesses extending into the steel plate side inside). Thus, a plating layer is formed on the steel plate. Due to such cracks and recesses formed in the vicinity of the galvanized steel sheet interface, the drawability is remarkably lowered.
The hydrogen concentration may be high and the dew point may be low, but from the viewpoint of steel sheet manufacturing cost, the hydrogen concentration is 20% or less and the dew point is -60 ° C or more. Moreover, it is preferable that the furnace gas other than hydrogen is nitrogen.

Annealing temperature: When the holding annealing temperature is lower than 760 ° C for 10 to 500 seconds in the temperature range of 760 to 860 ° C, it becomes a band-like uneven structure due to the structure in which the crystal grains are expanded by cold rolling. If two phases are continuously present in the rolling direction, deformation of the steel sheet is hindered, so that cracks are likely to propagate in the thickness direction, and workability such as elongation, hole expansion rate, and bendability deteriorates. In addition, sufficient austenite phase does not exist during continuous annealing, and a martensite phase cannot be obtained in the final product, resulting in insufficient strength. On the other hand, when the annealing temperature is higher than 860 ° C., the crystal grain size becomes excessively coarse, the hole expansion rate decreases, the amount of ferrite phase formed decreases, and the elongation decreases. From the above, annealing is performed at a temperature of 760 ° C. or higher and 860 ° C. or lower.
If the residence time is less than 10 seconds, there is a high possibility that undissolved carbides will be present during annealing, and the austenite phase may be present at a lower temperature during annealing or at the cooling start temperature. It becomes. On the other hand, crystal grains tend to grow and become coarse due to long-term annealing, and when the time exceeds 500 seconds, the grain size of the austenite phase during heat annealing becomes coarse, and finally the structure of the steel sheet obtained after heat treatment becomes coarse, Hole expansion rate decreases. In addition, it is caused by coarse particles, which may cause rough skin after press molding, which is not preferable. In addition, since the amount of ferrite phase produced during the cooling process to the cooling stop temperature is also reduced, the elongation is also lowered. Therefore, in order to achieve both a finer structure and obtaining a uniform and fine structure by reducing the influence of the structure before annealing, the annealing time is 10 seconds or more and 500 seconds or less, preferably 20 More than 2 seconds and less than 200 seconds.

Average cooling rate: 1-30 ° C / sec The average cooling rate controls the abundance ratio of ferrite phase and martensite phase, and plays an important role in securing TS780MPa class or higher. That is, when the average cooling rate exceeds 30 ° C./sec, a martensite phase is excessively generated, so that it is easy to secure the TS780 MPa class, but the moldability deteriorates. On the other hand, if the average cooling rate is slower than 1 ° C./second, the ferrite phase becomes too much and TS is lowered. A preferred average cooling rate is 5 to 20 ° C./second.
Note that the average cooling rate in the present invention is the average cooling rate when cooling from the annealing temperature to 500 ° C., and the cooling after 500 ° C. is not particularly limited.
The cooling in this case is preferably gas cooling, but can be performed in combination using cooling, mist cooling, roll cooling, water cooling, or the like.
After cooling, it passes through a general hot dip galvanizing process, or in addition, it is reheated by an induction heating device or the like, and in some cases, passes through an alloying process to obtain a hot dip galvanized steel sheet.
After continuous annealing, the hot-dip galvanized steel sheet finally obtained may be subjected to temper rolling for the purposes of shape correction and surface roughness adjustment. Since it is stretched to form a rolled structure and the ductility is lowered, the rolling reduction in skin pass rolling is preferably 0.1 to 1.5%.

Using a slab having the components shown in Table 1, hot rolling, pickling, cold rolling, continuous annealing, and plating treatment were performed under the conditions shown in Table 2, and the plate thickness was 1.4 mm and the zinc coating amount was 45 g / m ² . An alloyed hot dip galvanized steel sheet was produced. The zinc plating bath temperature was 460 ° C., and the alloying of the plating was performed at 550 ° C. About the hot dip galvanized steel sheet obtained in this way, the material test shown below was done and the material characteristic was investigated. The results obtained are shown in Table 3.

(1) The thickness cross section (depth position is 1/4 position of the plate thickness) parallel to the direction of rolling of the steel plate was examined by observing with an optical microscope or a scanning electron microscope. For the ferrite phase crystal grain size, the crystal grain size was measured according to the method specified in JISZ0552 and converted to an average crystal grain size. The martensite phase volume fraction is obtained by obtaining the area occupied by the martensite phase in a 100 mm x 100 mm square area arbitrarily set by image analysis using a cross-sectional structure photograph with a magnification of 1000 times. Rate.
FIG. 1 shows a method of measuring the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel plate to the inside of the steel plate and the area ratio of the smooth portion other than the cracks and recesses. As shown in Fig. 1, the maximum depths of the recesses and cracks at the interface between the plating layer and the ground iron interface are as follows. The deepest value was adopted in the distance from the open end to the bottom of the recess that progressed toward the inside of the steel plate or the tip of the crack. In addition, as shown in FIG. 1, the area ratio of the smooth part is arbitrary using cross-sectional SEM photographs (1000 magnifications) of the surface layer of any 10 steel plates in the 100 μm × 100 μm region in the thickness cross section parallel to the rolling direction. The ratio of the total line length occupied by the smooth portion (smooth portion excluding the concave portion and the opening portion of the crack portion) was obtained with respect to the cross-sectional line length of 100 μm. In addition, the maximum depth and the area ratio were observed at 10 arbitrary cross-sections, and the average value was obtained for each.
(2) Tensile properties: Using JISZ2201 No. 5 test piece with the direction perpendicular to the rolling direction as the longitudinal direction (tensile direction), a tensile test based on JISZ2241 was conducted and evaluated. The evaluation standard for tensile properties was TS × El ≧ 14000 MPa ·% or higher.
(3) Hole expansion rate: Implemented based on the Japan Iron and Steel Federation Standard JFST1001. When a hole with an initial diameter of d ₀ = 10 mm was punched and the 60 ° conical punch was raised to widen the hole, the punch was stopped when the crack penetrated the plate thickness, and the punched hole diameter d after crack penetration was measured. The hole expansion rate (%) = ((d−d ₀ ) / d ₀ ) × 100. Tested at N = 3 and determined by simple average. The evaluation standard for the hole expansion rate was TS × λ ≧ 30000 MPa ·% or more.
(4) Drawability: Evaluated by a cylindrical deep drawing test with a punch diameter of 50 mmφ, a punch shoulder radius of 5 mmR, a die inner diameter of 55 mmφ, and a die shoulder radius of 5 mmR. For lubrication, a polyethylene sheet was used. When multiple samples with different diameters (thin plates on a disk) are used to perform cylindrical deep drawing and the drawing ratio (diameter of the sample relative to the punch diameter) is 1.5, the drawability is improved. .
(5) Hardness: The hardness of the plate thickness section parallel to the rolling direction was measured using a Vickers hardness tester under the condition of a pressing load of 100 g. The plating layer-base iron interface was the outermost layer, and the hardness (average of arbitrary 5 points) at a depth of 25 μm from the outermost layer was measured as the hardness of the plate thickness surface layer. Further, the hardness (average of arbitrary 5 points) at a depth of 0.70 mm from the outermost layer was determined as the hardness at the center of the plate thickness.

From Table 3, the present invention example satisfies the drawing ratio of 1.5, and additionally satisfies all of TS × El ≧ 14000MPa ·% and TS × λ ≧ 30000MPa ·% at the same time. It can be seen that an excellent high-strength hot-dip galvanized steel sheet is obtained.
On the other hand, No. 7 whose hot-rolled slab heating condition is outside the scope of the present invention has a small smooth part area (a large area ratio of cracks and recesses) and inferior drawability.
No8, 9, 10, 11, 12 where any of hot rolling coiling temperature, pickling condition, hydrogen concentration, dew point condition is outside the scope of the present invention has a long maximum depth of cracks and recesses, and a smooth part area Since it is small, the drawability is inferior.
No13 with a high annealing temperature has a high austenite fraction during annealing, the amount of martensite finally obtained exceeds a predetermined amount, the TS × El value is low, and the formability is poor. No14 has an excessively long holding time at the annealing temperature, and the crystal grain size becomes too coarse, so that it stretches and is inferior in stretch flangeability.
No15, which has a fast cooling rate, has a small amount of ferrite, a TS that is too high, a low El, and a poor ductility.
No18, whose C component is outside the scope of the present invention, has an excessively hard martensite phase, and has poor stretch flangeability and drawability.
No19, whose Si component is outside the scope of the present invention, has a long crack depth of cracks and recesses and a small smooth part area, and therefore has poor stretch flangeability and drawability. No20, whose Mn component is outside the range of the present invention, has a large martensite phase volume fraction and is outside the range, resulting in high TS, low El, low TS × El value, and poor moldability.

The high-strength cold-rolled steel sheet of the present invention is suitably used not only for automobile parts but also for fields that require strict dimensional accuracy and workability, such as construction and home appliance fields.

It is a figure which shows the measuring method of the smooth part area ratio other than the crack and the maximum depth of a crack and the recessed part which have propagated from the interface of a plating layer and a steel plate side inside. (Example 1)

Claims (4)

In mass%, C: 0.03-0.12%, Si: 0.01-1.0%, Mn: 1.5-2.5%, P: 0.001-0.05%, S: 0.0001-0.005%, Al: 0.005-0.15%, N: 0.001- Contains 0.01%, Cr: 0.01-0.5%, Ti: 0.005-0.05%, Nb: 0.005-0.05%, V: 0.005-0.5%, B: 0.0003-0.0030%, the balance consists of Fe and inevitable impurities,
It has a structure containing a ferrite phase with an average crystal grain size of 10 μm or less and a martensite phase with a volume fraction of 30 to 90%,
The ratio of the sheet thickness surface layer hardness to the sheet thickness center hardness is 0.6 to 1, the maximum depth of cracks and recesses extending from the interface between the plating layer and the steel sheet to the inside of the steel sheet is 0 to 20 μm, and A high-strength hot-dip galvanized steel sheet characterized by having an area ratio of smooth parts other than cracks and recesses of 60% to 100%.   The high-strength molten zinc according to claim 1, further comprising at least one of Mo: 0.01 to 0.5%, Cu: 0.01 to 0.5%, and Ni: 0.01 to 0.5% in mass%. Plated steel sheet. To the steel comprising the component according to claim 1 or 2, hot rolling, pickling and cold rolling, and when producing a hot dip galvanized steel sheet through hot dip galvanizing treatment,
In the hot rolling, the slab heating temperature is 1150-1300 ° C, the finish rolling temperature is 850-950 ° C, the winding temperature is 400-600 ° C,
In the pickling, the bath temperature is less than 10-100 ° C., the hydrochloric acid concentration is 1-20%,
In the hot dip galvanizing treatment, the atmosphere in the heat treatment furnace from the temperature rising process of 600 ° C. or higher to the cooling process through the annealing temperature to 450 ° C. is set to a hydrogen concentration of 2 to 20% and a dew point of −60 to −10 ° C., 760 to A method for producing a high-strength hot-dip galvanized steel sheet characterized by holding at an annealing temperature of 860 ° C for 10 to 500 seconds and then cooling at an average cooling rate of 1 to 30 ° C / second.   The method for producing a high-strength hot-dip galvanized steel sheet according to claim 3, further comprising an alloying treatment after the hot-dip galvanizing treatment.

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