JP2000109951A - High strength hot rolled steel sheet excellent in stretch-flanging property and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thin high strength hot rolled steel sheet of <=3.5 mm sheet thickness excellent in stretch-flanging properties and to provide a method for producing it. SOLUTION: As to this producing method, a steel slab contg., by weight, 0.05 to 0.30% C, <=1.0% Si, 1.5 to 3.5% Mn, <=0.02% P, <=0.005% S, <=0.150% Al and <=0.0200% N and moreover contg. one or two kinds of 0.003 to 0.20% Nb and 0.005 to 0.20% Ti is heated at <=1200 deg.C and is thereafter subjected to hot rolling in such a manner that the finish rolling starting temp. is controlled to 950 to 1050 deg.C, and the finish rolling finishing temp. is controlled to >=800 deg.C, immediately after the completion of the rolling, cooling is started, and it is continuously cooled at the average cooling rate of 20 to 150 deg.C/sec and is coiled at 300 to 550 deg.C to form a fine bainitic structure of <=3.0 μm average grain size contg. no coarse grains of >10 μm grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet suitable for use in automobile bumper parts, high-strength parts such as impact beams, and more particularly to a tensile strength TS of 780 MPa or more and a stretch flangeability. The present invention relates to an excellent high-strength hot-rolled steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art With the recent movement to reduce the weight of automobile bodies, attention has been paid to the expansion of the application area of high-strength steel sheets. Above all, high-strength steel sheets exceeding 1000 MPa have been put to practical use for bumper parts and impact beams used for suppressing deformation of the cabin during a collision. In fact, these steel sheets are manufactured through a cold rolling process except for those having a thickness exceeding 3.2 mm. The main reason is that in the case of cold-rolled steel sheets, out-of-plane deformation of the steel sheets, that is, disorder of the shape, can be relatively easily suppressed by the furnace roll during continuous annealing, and the product shape is good. It is. Meanwhile, plate thickness 3.
A major cause of the failure to apply hot-rolled steel sheets as thin high-strength steel sheets of 2 mm or less, particularly 3.0 mm or less, is that in the cooling process after hot rolling,
This is because effective tension cannot be applied to the steel sheet, and the disorder of the shape of the steel sheet cannot be suppressed unlike a cold-rolled steel sheet.

[0003] The reason why the hot-rolled steel sheet has not been put into practical use as a thin high-strength steel sheet in the above-mentioned thickness range is that it is disadvantageous in securing the material in addition to the above-mentioned steel sheet shape. That is, it is usually difficult to obtain a uniform and fine structure equivalent to that obtained by cold rolling and annealing in the state of hot rolling, and this difference has been reflected in the difference in workability between the two. However, this difference is remarkable in local ductility represented by stretch flange processing, which is also reflected in bending and hole expanding. By the way, several proposals have been made on the hole expandability of a high-strength hot-rolled steel sheet. For example, JP-A-61-19733 and JP-A-62-196336 disclose that a bainite phase is excellent as a metal structure in consideration of stretch flangeability. This means that when a simple C-Si-Mn-based component system is hot rolled and then accelerated cooling is performed to form a structure mainly composed of bainite, hole expandability (stretch flange workability) is improved. is there. On the other hand, as a means to achieve high tensile strength without performing accelerated cooling after hot rolling, hardenability improving elements such as Cu, Ni, Cr, and Mo, which are conventionally adopted in the field of thick steel plates, are used. A method of adding is known.

[0004]

However, the hole expandability of the steel sheet manufactured by the manufacturing method disclosed in the above-mentioned JP-A-61-19733 and JP-A-62-196336 is certainly ferrite-marten. Although it is superior to steel sheets with a site structure, it has not yet reached the level where it can sufficiently meet today's required levels. In addition, in this conventional technique, the change in the structure is relatively sensitive to the change in the hot rolling conditions, so that the change in the material tends to increase, which hinders the continuous and automatic press forming. It was also.

In addition, the method of adding a hardenability improving element such as Cu, Ni, Cr, and Mo requires a large amount of expensive alloying elements, which is disadvantageous not only in terms of cost but also in terms of recycling. Therefore, there is a problem that scrap management is complicated. Further, in this method, it is necessary to add the above-mentioned alloying element to such an extent that the alloying element completely becomes a martensite single phase.If the adding amount is insufficient, the obtained structure is a mixed structure of ferrite and martensite, Or it became a structure partially containing a small amount of pearlite and bainite, and it was not easy to obtain the desired good stretch flangeability.

As described above, the tensile strength is 780 MPa or more,
In particular, high-strength hot-rolled steel sheet of 780-1300MPa,
mm or less as a normal hot-rolled steel sheet 3.
It has been extremely difficult to produce a hot-rolled steel sheet having high strength, good stretch flangeability, and a quality that can withstand practical use in a wide range of sheet thicknesses up to a thickness exceeding 0 mm.
For this reason, the development of a hot rolled steel sheet manufacturing technology that solves this problem is strongly desired, and the emergence of a technology for manufacturing with a low-alloy composition in which the amount of alloying elements is as small as possible, particularly in terms of cost reduction of the steel sheet. Was required. In addition, in order to increase the strength of the final product, in addition to increasing the strength of the steel sheet itself, strengthening using bake hardening, that is, increasing the strength in the baking process after forming-painting, increasing the strength in the final product It is conceivable to secure high strength. However, in conventional hot-rolled steel sheets, it is not easy to stably control the amount of bake hardening, and no attempt has been made to develop a hot-rolled steel sheet that effectively utilizes this phenomenon.

Accordingly, an object of the present invention is to solve such a problem in the prior art, and to provide a thin high-strength hot-rolled steel sheet having excellent stretch flangeability, high bake hardenability, and a thin material and a method for producing the same. Is to do. Another object of the present invention is to provide such a high-strength hot-rolled steel sheet with a thickness of 3.5 mm.
An object of the present invention is to provide an inexpensive manufacturing technique that can be manufactured even with a low alloy composition for the following hot-rolled steel sheets. further,
An object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and a bake hardening amount of 70 MPa or more as target values of specific properties of the steel sheet.

[0008]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive experiments from the viewpoint of steel composition, production conditions and the like, and have repeated studies. As a result, in terms of material, even if expensive steel elements are not necessarily used, if a steel is adjusted to an appropriate component composition range and manufactured under appropriate hot rolling and cooling conditions, a uniform fine pan can be obtained. It was found that a structure mainly composed of inite was formed, and good mechanical properties were stably obtained. Among these manufacturing conditions, it was also found that the control of the cooling pattern after hot rolling was particularly important. In other words, the cooling on the hot run table has focused on the winding temperature and the average cooling rate from the start of cooling to the winding temperature. The cooling rate at each position on the table) was not considered. However, in such a conventional method, the mechanical characteristics fluctuate greatly due to a change in the metal structure, and a material of a uniform material that can be used practically cannot be obtained. On the other hand, the inventors have found that cooling to a winding temperature after hot rolling should be performed continuously without interruption on a hot run table while maintaining a predetermined cooling rate (relatively slow cooling). Found to be extremely effective in solving the above problems. Then, it was concluded that the intended target could be achieved by combining this with an appropriate steel composition and hot rolling conditions (slab heating temperature, finish rolling temperature).

The present invention has been completed based on such findings, and the gist thereof is as follows. (1) C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, M
n: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 w
t% or less, Al: 0.150 wt% or less, N: 0.0200 wt% or less, and Nb: 0.003 to 0.20 wt%, Ti: 0.005 to 0.20
wt%, one or two wt%, the balance is composed of Fe and unavoidable impurities.
A high-strength hot-rolled steel sheet excellent in stretch flangeability, comprising a structure mainly composed of fine bainite of 3.0 μm or less.

(2) A high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized in that the steel sheet according to (1) further contains B: 0.0005 to 0.0040 wt% in addition to the above components.

(3) In the steel sheet according to the above (1) or (2), in addition to the above components, Cu: 0.02 to 1.0 wt%,
Ni: 0.02 to 1.0 wt%, Cr: 0.02 to 1.0 wt%, Mo: 0.02
1 to 1.0 wt% in total of 1.
A high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by being contained in a range of 0 wt% or less.

(4) The steel sheet according to any one of the above (1) to (3), wherein in addition to the above components, Ca: 0.0005 to
High strength hot rolled steel sheet with excellent stretch flangeability, characterized by containing 0.0050 wt%.

(5) The high-strength hot-rolled steel sheet according to the above (1) to (4), which has a composition in which the aspect ratio of bainite grains is 1.5 or less, and is excellent in stretch flangeability.

(6) C: 0.05 to 0.30 wt%, Si: 1.0 wt%
Hereinafter, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S:
0.005 wt% or less, Al: 0.150 wt% or less, N: 0.0200 wt%
% Nb: 0.003 to 0.20 wt%, Ti: 0.005%
After heating a steel slab containing up to 0.20 wt% of one or two components at a temperature of 1200 ° C or less, hot rolling at a finish rolling end temperature of 800 ° C or more, and within 2 seconds after the end of rolling Of high-strength hot-rolled steel sheets with excellent stretch flangeability, characterized by continuously cooling to a winding temperature at a cooling rate of 20 to 150 ° C / sec and winding at 300 to 550 ° C. Production method.

(7) C: 0.05 to 0.30 wt%, Si: 1.0 wt%
Hereinafter, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S:
0.005 wt% or less, Al: 0.150 wt% or less, N: 0.0200 wt%
% Nb: 0.003 to 0.20 wt%, Ti: 0.005%
After heating a steel slab containing up to 0.20 wt% of any one or two components at 1200 ° C or less, the finish rolling start temperature is 950 to 1050 ° C, and the finish rolling end temperature is 800 ° C or more. The rolling is started within two seconds after the end of the rolling, continuously cooled to a winding temperature at a cooling rate of 20 to 150 ° C./sec, and wound at a temperature of 300 to 550 ° C. A method for manufacturing high-strength hot-rolled steel sheets with excellent flangeability.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a description will be given of the reason why the above-mentioned configuration is limited. C: 0.05 to 0.30 wt% C is an element effective for strengthening the transformed structure, and its effect is exhibited by adding 0.05 wt% or more. However, when the content exceeds 0.30 wt%, the nugget portion of spot welding becomes significantly hard, and the weldability is reduced, making it difficult to apply as a steel sheet for automobiles. Therefore, the amount of C is limited to the range of 0.05 to 0.30 wt%. In addition, from the viewpoint of stabilizing the tensile properties of the steel sheet,
A range of 0.20% by weight or less is preferable.

Si: 1.0 wt% or less Si is a useful element used to increase temper softening resistance when utilizing transformation structure strengthening. On the other hand, Si
Has the effect of increasing the hot deformation resistance of steel, 1.0 wt%
If the addition exceeds the above range, the tendency becomes particularly remarkable, and it becomes difficult to perform hot rolling to a thin material aimed at by the present invention. Therefore, the addition amount is 1.0 wt% or less. In applications where scale defects on the surface are a problem, the content is desirably suppressed to 0.8 wt% or less.

Mn: 1.5 to 3.5 wt% Mn is an element effective for preventing hot cracking due to S, and is desirably added according to the amount of S. Mn is
Since it has the effect of making crystal grains finer, it is necessary from the viewpoint of improving the material. And, in particular, in the present invention, in the low-temperature transformation phase mainly composed of bainite, the steel is strengthened by the quenching property improving effect of Mn, and TS: 780 M
Secure Pa or more. To achieve these effects, at least 1.5 wt% must be added. As the amount of added Mn increases, more stable strength is obtained, and the uniformity of the material is improved. However, even if it is added in excess of 3.5 wt%, not only the effect is saturated, but also the hot deformation resistance increases, making it difficult to reduce the thickness by hot rolling.
Further, excessive addition deteriorates weldability and formability of a welded portion. For these reasons, the upper limit of the amount added is set to 3.5 wt%. For applications requiring better corrosion resistance and moldability, the range is preferably 3.2 wt% or less.

P: not more than 0.02 wt% In general, P may be added as a solid solution strengthening element for the ferrite phase in a high-strength steel sheet having a relatively low strength in a two-phase structure. Solid solution strengthening of P cannot be expected in steel plates of MPa or more. When P contains a large amount of C, Mn, or the like, P hardens the steel and lowers stretch flangeability. Further, P has a strong tendency to segregate at a specific position in the plate thickness direction, resulting in weld embrittlement due to segregation. For these reasons, the content of P is 0.02 wt% or less,
Preferably, it must be limited to 0.01 wt% or less.

S: 0.005 wt% or less S is a harmful element that exists as inclusions in steel, reduces ductility of the steel sheet, and further deteriorates corrosion resistance.
In particular, as in the present invention, the notch sensitivity increases in the case of high strength, so that the amount of MnS-based inclusions, which are sources of stress concentration, must be reduced as much as possible. Therefore, it is necessary to reduce S as much as possible, and the upper limit is 0.005 wt%. In applications where particularly good workability is required, 0.00
It is desirable to make it 2 wt% or less.

Al: 0.150 wt% or less Al is an element added as a deoxidizing element and is useful for improving the cleanliness of steel and refining the structure. In order to exert such effects, it depends on the deoxidation technology of molten steel, but it is necessary to add approximately 0.010 wt% or more. However, an excessive content deteriorates the surface properties and lowers the strength of the steel sheet.
Therefore, Al is added in a range of 0.150 wt% or less. In addition,
From the viewpoint of the stability of the material, it is desirable to add in the range of 0.010 to 0.080 wt%.

N: 0.0200 wt% or less When N is contained in excess of 0.0200 wt%, the heat ductility of the steel is reduced, and internal defects and surface defects of the steel sheet are easily generated, and slab cracking during continuous casting is performed. Increase the occurrence of. Therefore, the upper limit of N is set to 0.0200 wt%. In addition, the range of 0.0020 to 0.0150 wt% is preferable from the viewpoint of stability of the material and improvement of the yield in consideration of the entire manufacturing process. Since N has an effect of lowering the transformation point of steel, it is effective to add N within the above range when a thin steel sheet is manufactured and the transformation point is not greatly reduced and rolling is not desired.

Nb: 0.003 to 0.20 wt%, Ti: 0.005 to 0.20
wt% These elements are extremely important elements that contribute to the refinement and homogenization of the structure. In the present invention, by combining with a relatively low slab heating temperature, the target fine crystal structure of 3.0 μm or less can be obtained. It is possible to obtain. This effect is at least 0.003 wt% or more for Nb and
It can be obtained by adding 0.005 wt% or more. However, if any element is added in excess of 0.20 wt%, the effect is saturated and the risk of slab cracking during continuous casting increases. Therefore, Nb is 0.003-0.20wt, Ti is 0.005--0.
Add in the range of 20 wt%.

Next, the optional elements are described. B: 0.0005 to 0.0040 wt% B contributes not only to the refinement of the structure of the steel sheet but also to the suppression of ferrite transformation of the steel. Extremely effective. These effects are exhibited by adding 0.0005 wt% or more. On the other hand, even if added in excess of 0.0040 wt%, the effect is saturated. Therefore, B is 0.0005
It is added as needed in the range of 0.0040 wt%.

Cu: 0.02 to 1.0 wt%, Ni: 0.02 to 1.0 wt%
%, Cr: 0.02 to 1.0 wt%, Mo: 0.02 to 1.0 wt% in a total amount of 1.0 wt% or less These elements delay the transformation after the end of hot rolling, and can be effectively used for strengthening by the transformation structure. The strength of the steel sheet can be increased. The effect can be obtained by adding 0.02 wt% or more, but if added excessively, the deformation resistance during hot rolling increases, the chemical conversion property and the surface treatment property in a broader sense deteriorate, and further, the hardening of the welded part is reduced. This leads to a decrease in weldability. Therefore, the addition amount of these elements is 1.0 wt.
%, And the total amount is limited to 1.0 wt%. It should be noted that any of these elements shows the same behavior when used alone or in combination.

Ca: 0.0005 to 0.0050 wt% Ca is an element useful for detoxifying S in steel.
In the case of a structure mainly composed of fine bainite having a relatively large Mn content, the improvement of stretch flangeability by addition of Ca is particularly large. Such effects are exhibited when added at 0.0005 wt% or more. However, even when added over 0.0050 wt%, not only the effect is saturated but also the surface properties tend to be deteriorated, and the surface treatment characteristics are deteriorated. There is a risk of doing. Therefore, the addition amount of Ca is set in the range of 0.0005 to 0.0050 wt%. In addition, considering the balance of various qualities, 0.0010-0.0035wt
% Is preferred.

Fine Bainite Structure The metal structure in the present invention needs to be mainly composed of fine bainite (having an area ratio of at least 90% bainite). The distinction between bainite and martensite (not tempered) is determined from the difference in strength, but it is difficult to distinguish between bainite and tempered martensite. Therefore,
In the present invention, paying attention to the precipitation state of carbides, bainite means that carbides are mainly precipitated in the grains or at lath boundaries, and temper martensite means that carbides are also frequently precipitated at former austenite grain boundaries. And Examination of the relationship with the stretch flangeability based on such a structure determination criterion showed that, even at the same strength, the structure mainly composed of bainite showed much better stretch flangeability. The reason was presumed to be that carbides precipitated at the former austenite grain boundaries and those coarsely precipitated had an adverse effect on this property.

Average grain size and aspect ratio of bainite structure The finer the bainite structure, the better the stretch flangeability. In this sense, regulation of the crystal grain size is also an important requirement. The average grain size of the bainite structure here is
It is calculated according to the method of determining the average grain size of ferrite (JIS G0552), and is determined from the average of the values measured over the entire thickness of each sheet thickness cross section in the rolling direction and the direction perpendicular to the rolling direction. When the average particle size measured in this way is 3.0 μm or less, stretch flangeability is significantly improved. In addition, in the conventional precipitation-strengthened steel sheet, there is an example in which a bainite structure of 3.0 μm or less is partially obtained, but a partially coarse structure is included, and the average grain size is 3.0% in the entire plate thickness. μ
Nothing was below m. The bainite structure preferably has no mixed grains, that is, no large grains having a grain diameter of more than 10 μm. The average particle size of the bainite structure is desirably 2.5 μm or less when better stretch flangeability is required. The aspect ratio of the bainite grains is preferably 1.5 or less from the viewpoint of workability. Here, the aspect ratio refers to the ratio of the major axis to the minor axis of the bainite grains, with the major axis generally in the rolling direction and the minor axis in the thickness direction.

FIG. 1 shows the result of examining the relationship between the stretch flangeability (hole expanding test) and the average grain size of the bainite structure. The test steel sheet is 0.08wt% C-0.21wt% Si-3.
0 wt% Mn-0.040 wt% Al-0.0030 wt% N-0.025 wt% Nb
-Steel slabs of -0.015 wt% Ti-0.0025wt% B-0.0020wt% Ca are widely varied with slab heating temperature of 950 ~ 1300 ℃, finish rolling temperature of 750 ~ 980 ℃, cooling rate of 10 ~ 200 ℃, By adjusting the winding temperature, bainite can be reduced to 90%.
% Is a hot-rolled steel sheet with a thickness of 2.0 mm (tensile strength TS is 960 to 1200 MPa). From FIG. 1, it can be seen that the hole expandability is significantly improved when the average grain size of the bainite structure is 3.0 μm or less. In addition, we confirmed that this hole expansion rate was not simply correlated with TS,
Even with the same TS, a finer structure can achieve an improvement in the hole expansion rate. The hole expansion test conforms to the Iron and Steel Federation Standards and punches a 10 mmφ hole (clearance 12.5
% Constant) and formed by a conical punch having a vertical angle of 60 °.

By setting the metal structure to be a bainite-based structure and the crystal grain size within the above range, it is possible to obtain a large bake hardenability aimed at by the present invention, specifically, a bake hardenability of 70 MPa or more. Becomes Although a certain degree of bake hardenability can be obtained by using bainite as the metal structure, a remarkable increase in the amount of bake hardening can be achieved by further refining the structure. The reason for obtaining such an effect is that bainite itself contains a large amount of solute C and N in supersaturation, and that a large amount of C and N exist relatively stably in a solid solution state in the crystal grain boundary itself. Presumed to be for that.

Next, the manufacturing conditions will be described. The slab is desirably manufactured by a continuous casting method in order to prevent macroscopic segregation of components, but it is also possible to manufacture the slab by an ingot making method or a thin slab casting method. After production, the slabs are cooled to room temperature and then heated again. Energy saving process steps such as direct rolling can be applied without any problem. However, from the viewpoint of uniformity and fineness of the initial structure, even when performing direct rolling, etc., once,
It is more desirable to reheat after completing the γ → α transformation.

Slab heating temperature (SRT): 1200 ° C. or less The slab heating temperature has a great influence on the γ grain size. Conventionally,
In the production of a high-strength steel sheet as in the present invention, Nb, in steel of a component to which a carbonitride forming element such as Ti is added,
In order to use precipitation strengthening effectively, these elements must be completely dissolved as an initial state, and the SRT is generally set to a high temperature of about 1250 ° C. or higher. On the other hand, the present inventors restricted the SRT to 1200 ° C. or less, and despite the fact that the steel was a high-tensile steel containing Nb and Ti, some of the added Nb and Ti remained in an undissolved state. It was found that the uniformity and the fineness of the hot-rolled structure were remarkably improved by the heat treatment. In this case, the deformation resistance tends to increase as compared with the conventional high SRT method, but the degree of increase in the deformation resistance is relatively small because dynamic recrystallization occurs in the rough rolling step. In the present invention, the precipitation strengthening effect may be reduced, but the reduced amount can be compensated for by making the structure into a uniform and fine bainite-based structure, and the remarkable structure is made finer and more uniform. The point is brought. In order to make the structure more uniform and finer, the SRT is desirably 1100 ° C. or lower, more preferably 1050 ° C. or lower.

Finish rolling start temperature (entrance temperature of finishing mill): 950 to 5050 ° C. In the present invention, dynamic recrystallization is caused by rough rolling and
By promoting at least one to four passes of finish rolling, an increase in deformation resistance in finish rolling can be suppressed.
In addition, dynamic recrystallization is effective not only for reducing deformation resistance during rolling, but also makes equiaxed grains due to recrystallization and can advantageously achieve an aspect ratio of bainite grains of 1.5 or less. In order to promote dynamic recrystallization in finish rolling, the finish rolling start temperature is important.
By setting the temperature to 950 to 1,050 ° C., dynamic recrystallization is promoted, and an increase in deformation resistance can be suppressed.

Finish rolling finish temperature (outlet temperature of finish rolling mill): 800 ° C. or more By setting the hot finish rolling finish temperature to 800 ° C. or more, a uniform and fine hot rolled sheet structure can be obtained. However, when the finish rolling end temperature is lower than 800 ° C., the structure of the steel sheet expands and becomes non-uniform, and a part of the processed structure remains, thereby increasing the risk of various problems during forming. Therefore, the finish rolling finish temperature should be 800 ° C or higher.
In order to further improve the mechanical properties, it is desired that the temperature be 820 ° C. or higher. Although the upper limit of the finish rolling end temperature does not need to be particularly defined, it is usually 950 ° C. or lower due to the relationship with the SRT.

Cooling after hot finish rolling In the present invention, cooling after hot finish rolling is performed at a cooling rate of 20%.
It shall be cooled continuously to the winding temperature at ~ 150 ° C / sec. The purpose of controlling the cooling after rolling in this way is to finally obtain a uniform and fine bainite structure stably. In the present invention, in this step, without interrupting the cooling in the middle as in the prior art, from the finish rolling mill exit side on a so-called hot run table, forcibly cooled with cooling water, continuously until the winding temperature is reached. Achieved by cooling. The cooling rate when performing the above cooling is 20 to 150 ° C over the entire temperature range until the winding temperature is reached.
/ Sec range. At a speed lower than this range, sufficient strength cannot be obtained, while at a speed higher than this range, the variation in strength in the width direction and the longitudinal direction of the steel sheet increases.

As for cooling after hot rolling, water cooling is started immediately after rolling, and so-called gentle cooling having a lower heat transfer coefficient than usual is applied to achieve both uniformity of material and uniformity of shape. It is effective on the above. If the cooling is not started within 2 seconds after the end of the rolling, the processing strain added by the rolling is eliminated, and the effective microstructure is not achieved.
An uneven tissue mixed with a coarse tissue is obtained. For this reason,
It is necessary to start cooling within 2 seconds after the end of rolling. Further, when cooling a hot-rolled steel sheet having a thickness of 3.5 mm or less, which is a target of the present invention, on a hot run table, if the heat transfer coefficient at the time of cooling is a large value, 20 to 20 mm over the entire length and width directions of the steel sheet. It becomes difficult to maintain a cooling rate of 150 ° C./sec, and the uniformity of the material deteriorates. If the cooling rate is not uniform, the shape of the steel sheet will be disturbed, and the cooling rate will be more uneven, and the uniformity of the material will be further deteriorated. Considering these, the heat transfer coefficient during cooling is
It is preferable to be 1000 W / m ² · K or less. Also, when cooling on the hot run table, uniform cooling can be achieved by masking the cooling water at both ends in the width direction so as to prevent the cooling water from directly hitting the edges of the steel plate in order to prevent overcooling of the edges. The effect is more effectively exerted.

Winding temperature: 300 to 550 ° C. By lowering the winding temperature after hot rolling, the strength can be increased. Then, if the film is wound at a temperature of 550 ° C. or less, the target tensile strength of 780 MPa can be satisfied. However, if it is wound at a temperature lower than 300 ° C., the shape of the steel sheet deteriorates, and it becomes difficult to correct the shape thereafter, which causes problems in actual use. In addition, the uniformity of the material tends to deteriorate. Therefore, the winding temperature after hot rolling should be 300 to 550 ° C. When higher material uniformity can be required, a temperature range of 350 ° C or higher is desirable.
In addition, the shape of the steel sheet is preferably 25 mm or less in terms of wave height in consideration of preventing stumbling troubles and generation of flaws in a processing line after press forming or the like. In addition, the wave height is based on the Japan Iron and Steel Federation standard,
The wave height shall be measured on the surface plate.

The steel sheet of the present invention can be manufactured by a process incorporating the above conditions. However, it is necessary to employ the following steps singly or in combination to assist the steel sheet in terms of cross-sectional shape, dimensional accuracy, and material quality. This is desirable because further improvement in uniformity and the like can be achieved. First, the preceding material and the following material are joined and rolled continuously at the entrance of the finishing mill. By performing continuous rolling in this way, the so-called rolling unsteady portions at the front and rear ends of the material to be rolled are eliminated, and stable hot rolling conditions can be achieved over the entire length and width of the steel sheet. And such a rolling contributes greatly to improvement of the cross-sectional shape of a steel plate. Then, it becomes possible to stably improve the shape of the steel sheet over the entire length on the hot run table, and it becomes easy to obtain uniform cooling conditions in the longitudinal direction and the width direction of the steel sheet, which is advantageous in obtaining a uniform microstructure. Become. The joining method on the entry side of the finishing mill is not particularly limited, but examples include an induction heating welding method, a pressure welding method, a laser welding method, and an electron beam welding method. If the preceding material and the succeeding material are continuously rolled in this way, even when the rolled steel sheet is cooled on a hot run table, the steel sheet can always be given tension so that the shape of the steel sheet is kept good. This also makes it possible to prevent non-uniform cooling due to the bad shape of the steel plate. In addition, according to this rolling method, since the leading end of the material to be rolled can be stably threaded, low friction, which is difficult to apply from the viewpoint of threading properties and biting properties in ordinary single batch rolling, is used. Hot rolling with a coefficient, that is, hot rolling using a large amount of lubricant, can be performed, and the rolling load can be reduced. At the same time, the surface pressure of the roll can be reduced, so that the life of the roll can be extended. Further, the reduction of the coefficient of friction at the time of rolling is extremely effective also for making the structure uniform in the thickness direction. As described above, in a thin hot-rolled steel sheet, it is extremely effective to join the preceding material and the succeeding material and continuously roll them.

The use of an edge heater for heating the widthwise end of the material to be rolled (sheet bar) on the entry side of the finishing mill is advantageous in making the temperature of the material to be rolled uniform in the width direction. is there. In the present invention, the uniformity of the temperature of the steel sheet during rolling and cooling on the hot run table is important. It is preferable to make the temperature distribution uniform in the width direction. In addition, since the temperature also tends to decrease at the longitudinal end of the material to be rolled, a heating device (hereinafter, referred to as a sheet bar heater) capable of heating the material to be rolled (sheet bar) over the entire width at the entrance side of the finishing mill. ), It is preferable to heat the temperature-reduced portion at the longitudinal end to make the longitudinal temperature distribution of the material to be rolled uniform. Further, when rolling after performing the joining, the sheet bar may be wound into a coil shape on the entry side of the joining apparatus.In this case, particularly, regarding the outermost winding and the innermost winding of the coil, It is particularly preferable to use the above-mentioned sheet bar heater because the temperature tends to decrease. When heating the material to be rolled using an edge heater or a sheet bar heater, it is recommended that the heating amount be such that the temperature difference in the final finish rolling is 20 ° C or less. It changes slightly depending on the composition and the like.

According to the method described above, TS is 780 MPa
As described above, the baking hardening amount has a characteristic of 70 MPa or more, and it is possible to uniformly impart good stretch flangeability in the longitudinal direction and the width direction of the steel sheet. In addition, since slow cooling is performed in the hot run, a hot-rolled steel sheet having an excellent shape can be manufactured. In addition, the uniformity of the material can be further improved by using a rolling method in which the leading material and the succeeding material are joined and finish rolling is performed, or by using a sheet bar heating by an edge heater and a sheet bar heater. .

The bake hardening amount is defined as an increase in yield stress when aged at 170 ° C. for 20 minutes after applying a tensile strain of 2%. This value is 70 MPa or more, preferably 10
At 0 MPa, when used as an actual part, it appears as an effect of increasing the strength of the part, and it is possible to achieve a reduction in the thickness of the steel sheet. In addition, the steel sheet after hot rolling is used after removing the oxidized layer on the surface by pickling, and furthermore,
It is used after skin pass rolling for surface roughness control or leveler processing for shape correction. Moreover, it is also possible to use it as it is without performing pickling. Furthermore, it is also possible to perform various electroplating and hot-dip plating on the surface layer.

[0042]

EXAMPLES Example 1 A steel having a composition shown in Table 1 and a balance of substantially Fe was prepared. The steel slab was hot-rolled under the conditions shown in Table 2 to a final finished sheet thickness of 1.6 mm, and pickled to obtain a test steel sheet. Observation of a microstructure, a tensile test, a bending test, and a hole expansion test were performed on each of the obtained hot-rolled steel sheets. For the tensile properties, JIS No. 5 test pieces were used. The bake hardening amount was defined as an increase in yield strength when a 2% prestrain was applied and aged at 170 ° C. for 20 minutes under standard conditions. In addition, the hole expandability test was performed by punching a hole of 10 mmφ (clearance 12.5% constant) according to the Iron and Steel Federation standard, and forming the hole with a conical bunch having a vertical angle of 60 °. Table 3 shows the test results. The tensile properties of the same hot-rolled steel sheet without pickling were also examined, but no difference in properties depending on the presence or absence of pickling was found. In addition, three points in the longitudinal direction of the steel plate (15 m from the tip, central position in the longitudinal direction, 15 m from the tail end), five points in the width direction (central position in the width direction, 25 mm from both edges, 100 mm from both edges) (Position), a total of 15 samples were collected, and the degree of variation in tensile strength was measured to evaluate the uniformity of the material.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

As shown in Tables 1 to 3, each of the steel sheets of the invention examples had a structure in which bainite occupied 90% or more in area ratio and had an average grain size of 3.0 μm or less. Also,
TS has characteristics of 780 MPa or more and bake hardening amount of 70 MPa or more, which indicates that the desired characteristics are satisfied.
Further, the bending workability and the hole expansion ratio were sufficient. In addition, the bainite referred to here is one in which carbides are precipitated mainly in the grains or on the lath boundaries, and the precipitation at the austenite grain boundaries is small.

Example 2 0.15 wt% C-0.20 wt% Si-1.8 wt% Mn-0.009 wt% P-
0.001 wt% S-0.039 wt% Al-0.0025 wt% N-0.025 wt
A hot rolled steel sheet (pickling material) having a thickness of 2.6 to 1.2 mm was manufactured under the respective manufacturing conditions shown in Table 4 using a steel slab of% Nb-0.0015 wt% B-0.0020 wt% Ca as a raw material. When continuous hot rolling is applied, the tail end of the preceding material and the leading end of the succeeding material of the sheet bar having a thickness of 25 mm obtained by rough rolling are joined by heating and melt-welding at the finishing mill entry side. And finish rolling was performed continuously. Similar tests were performed on these test steel sheets, and the results shown in Table 5 were obtained. Tables 4 and 5
As shown in the table, bainite occupies 90% or more in area ratio (other structures are pearlite, martensite, or ferrite) in all the steel sheets of the invention, and the average grain size is 3.0 μm or less. Organization.
In addition, TS had characteristics of 780 MPa or more and bake hardening amount of 70 MPa or more, and showed good bending workability and hole expansion ratio. Note that the steel sheet crown (in the width direction center and edge)
The difference in plate thickness from the position at 25 mm) was 40 μm or less, which was good. In addition, a small-diameter ERW pipe was manufactured for the steel sheet of the invention example and the 1.4 mm cold-rolled sheet (continuously annealed material). As a result, the steel sheet of the invention example could be manufactured without any problems in terms of forming and product characteristics, similarly to the cooling plate, although adjustment of the optimum conditions for welding was necessary.

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

As described above, according to the present invention,
A thin, high-strength hot-rolled steel sheet having excellent stretch flangeability can be provided. Further, according to the present invention, by optimizing the chemical composition and hot rolling conditions, the shape is uniform,
It is possible to provide a high-strength hot-rolled steel sheet having high material uniformity. Therefore, the high-strength hot-rolled steel sheet according to the present invention can be used in place of the conventional high-strength cold-rolled steel sheet in terms of quality,
It greatly contributes to energy saving in the manufacturing process, reduction of the cost of products such as automobile strength members and impact beams (beam pipes).

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the crystal grain size of bainite and the hole expansion rate.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corporation (72) Inventor Nobuo Yamada 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Chiba Works (72) Inventor Takao Uchiyama 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Chiba Works

Claims (7)

[Claims] 1. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N : Contains not more than 0.0200 wt%, Nb: 0.003 to 0.20 wt%, and Ti: 0.005 to 0.20 wt%, and the balance is composed of Fe and unavoidable impurities. A high-strength hot-rolled steel sheet having excellent stretch flangeability, characterized by having a structure mainly composed of fine bainite having an average grain size of 3.0 μm or less. 2. The high-strength hot-rolled steel sheet according to claim 1, further comprising: B: 0.0005 to 0.0040 wt% in addition to the above components. 3. The steel sheet according to claim 1, further comprising Cu: 0.02 to 1.0 wt%, Ni: 0.02 to 1.0 wt%, Cr: 0.02 to 1.0 wt%, Mo: 0.02. A high-strength hot-rolled steel sheet having excellent stretch flangeability, characterized in that it contains at least one of 1.0 to 1.0 wt% in a total amount of 1.0 wt% or less. 4. The steel sheet according to claim 1, further comprising Ca: 0.0005 to 0.0050 wt% in addition to the above components. High strength hot rolled steel sheet. 5. The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength hot-rolled steel sheet has a composition in which the aspect ratio of bainite grains is 1.5 or less. 6. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N : A slab containing 0.0200 wt% or less, and containing one or two of Nb: 0.003 to 0.20 wt% and Ti: 0.005 to 0.20 wt%, is heated at 1200 ° C or less, and then finish rolling is completed. Hot-roll at 800 ° C or higher, start cooling within 2 seconds after the end of rolling, continuously cool to the winding temperature at a cooling rate of 20 to 150 ° C / sec, and wind at 300 to 550 ° C A method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability. 7. C: 0.05 to 0.30 wt%, Si: 1.0 wt% or less, Mn: 1.5 to 3.5 wt%, P: 0.02 wt% or less, S: 0.005 wt% or less, Al: 0.150 wt% or less, N : A slab containing 0.0200 wt% or less, and containing one or two of Nb: 0.003 to 0.20 wt% and Ti: 0.005 to 0.20 wt%, heated at 1200 ° C or less, and then finished rolling start temperature 950 ~ 1050 ℃, Finish rolling end temperature 800 ℃
As described above, hot rolling is performed, cooling is started within 2 seconds after the completion of rolling, the cooling is continuously performed at a cooling rate of 20 to 150 ° C./sec to a winding temperature, and the film is wound at 300 to 550 ° C. And
Manufacturing method of high strength hot rolled steel sheet with excellent stretch flangeability.