JP2007247049A - High strength hot rolled steel sheet having excellent stretch-flanging property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having excellent ductility and stretch-flanging properties severely required as those of automobile parts and also having a tensile strength of ≥590 MPa. <P>SOLUTION: In the high tensile strength steel sheet with a ferrite single phase structure having a tensile strength of ≥590 MPa, the steel sheet has a composition comprising, by mass, 0.01 to 0.2% C, 0.01 to 1.5% Si and 0.25 to 3% Mn, and further comprising, as carbide forming element(s), one or more kinds selected from 0.03 to 0.2% Ti, 0.01 to 0.2% Nb, 0.01 to 0.2% V and 0.01 to 0.2% Mo, and the balance Fe with inevitable impurities, and in which the content of solid solution C in the C content is ≥0.002%, and the ferrite single phase structure is composed of hard ferrite crystal grains A where precipitated carbides with the maximum diameter of ≤8 nm are dispersed into the crystal grains at a number density of 1×10<SP>17</SP>to 5×10<SP>18</SP>pieces/cm<SP>3</SP>, soft ferrite crystal grains B where precipitated carbides with the maximum diameter of ≤8 nm are dispersed into the crystal grains at a number density of 1×10<SP>15</SP>to 5×10<SP>16</SP>pieces/cm<SP>3</SP>, and also, the volume fraction of the hard ferrite crystal grains A occupying in the whole volume, A/(A+B) is 0.1 to 0.5. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength hot-rolled steel sheet used for high-strength structural parts such as automobiles. Specifically, the steel sheet is excellent in burring workability and stretch flangeability, and is difficult to cause damage to the end face when the steel sheet is punched. The present invention relates to a high-strength hot-rolled steel sheet.

  In recent automobile parts, while weight reduction is emphasized from the viewpoint of energy saving, safety and durability tend to be emphasized, and the strength of steel material members is rapidly increasing compared to the past. Progressing. As a result, in contrast to the high formability required for conventional skin panels, structural members are also required to have workability with different indices such as hole expansibility and high strength with excellent workability. Hot-rolled steel sheets have been developed.

  When attention is focused on high-strength steel having a tensile strength of 590 MPa or more, it is not easy to achieve the required strength only by conventional solid solution strengthening methods such as carbon, silicon, and manganese. Originally, the ferrite structure alone is insufficient in strength, so the ductility of a composite structure steel sheet combined with hard phases such as bainite and martensite was studied.

  As a result, for example, a steel plate using a mixed structure of ferrite and bainite (see, for example, Patent Document 1) or a steel sheet in which a martensite structure is further mixed (see, for example, Patent Document 2) has been developed.

  However, these steel sheets have low stretch flangeability for steel sheets having a tensile strength of 590 MPa or more. In other words, these steel sheets are excellent in uniform elongation and the like, but have the disadvantage that the stretch flangeability represented by hole expansibility is rather deteriorated, and have the workability required for high-strength steel sheets in recent years. In order to achieve this, it has begun to be considered that there is a limit to the steel structure in which hard phases such as bainite and martensite are mixed.

  On the other hand, a method for producing a hot-rolled steel sheet with improved hole expansibility by a structure mainly composed of bainite has been proposed (for example, see Patent Document 3), but this steel sheet is applied because it has poor elongation characteristics. There were restrictions on the parts. Further, a steel sheet having a structure composed of phases called ferrite and bainitic ferrite and having excellent stretch flangeability is disclosed (for example, see Patent Document 4), but lacks ductility.

  Summarizing these technical courses, steel sheets with a conventional two-phase structure (Dual Phase: for example, ferrite + martensite, ferrite + bainite, etc.) have a soft ferrite phase and are therefore excellent in ductility. The high-strength steel sheet lacking stretch flangeability due to its large hardness difference and having a single-phase structure has a uniform structure and excellent stretch flangeability, but has a problem of lacking ductility.

  Thus, the improvement of ductility and stretch flangeability of high-strength steel sheets are contradictory indicators, and it has been difficult to improve both at the same time.

  In contrast to the study of utilizing a strengthened structure such as martensite and bainite, technical development to increase the strength while maintaining the excellent deformability of the original uniform structure of the ferrite phase has begun to be examined again in recent years. Attention has been focused on a strengthening method in which fine carbides are precipitated using easily carbide-forming elements such as Ti and Nb and the interaction between these carbides and dislocations is utilized.

  As such a steel sheet, composite precipitates are studied, fine carbides containing Ti and Mo are dispersed and precipitated in the ferrite structure, and the C solid solution amount in the steel is made 0.002% or less, whereby the tensile strength is increased. A steel sheet having a thickness of 780 MPa or more, a total elongation (uniform elongation + local elongation) of 20% or more, and a hole expansion ratio of 70% or more is disclosed (for example, see Patent Document 5).

  It is thought that the feature of this technology is to obtain a uniform structure by uniformly distributing fine carbides, but the idea is that while obtaining the maximum strength by the particle dispersion strengthening method, the precipitated carbides are further refined This is to ensure ductility.

  However, this steel sheet does not take into account the punched end face damage that occurs during the stretch flange processing of the steel sheet, and suppresses the end face damage that occurs during the punching of the steel sheet while maintaining the above total elongation and tensile strength. I could not.

  Further, although steel sheets containing Ti and Nb and improving the balance between ductility and stretch flangeability by devising production conditions have been disclosed (for example, see Patent Document 6), the structure containing carbide is detailed. It is not stipulated, and it has not reached the point of presenting steel design guidelines.

JP-A-8-269615 Japanese Unexamined Patent Publication No. 7-150291 JP-A-4-88125 JP-A-6-172924 JP 2002-322540 A Japanese Patent Laid-Open No. 2005-120435

  The present invention has been made in order to solve the above-described conventional problems. In a thin steel plate of 590 MPa class or more, the present invention achieves both excellent stretch flangeability and ductility, and further prevents damage to the punched end face. It intends to provide a high strength hot rolled steel sheet.

  This invention is made | formed based on the said new knowledge, The place made into the summary is as follows.

(1) In a high-strength hot-rolled steel sheet composed of a ferrite single-phase structure having a tensile strength of 590 MPa or more, (a) in mass%, C: 0.01 to 0.2%, Si: 0.01 to 1.5%, Mn: 0.25 to 3%, further, as carbide forming elements, Ti: 0.03 to 0.2%, Nb: 0.01 to 0.2%, V: 0.01 to 0.2 %, And Mo: 0.01 to 0.2% of one or more, and the balance consists of Fe and inevitable impurities, (b) out of the C amount, the amount of solid solution C Is 0.002% or more by mass%, and (c) the ferrite single-phase structure is (c-1) precipitated carbides having a maximum diameter of 8 nm or less in the crystal grains are number density 1 × 10 ^{17 to} 5 × 10 ¹⁸ particles / cm ³ dispersed in hard ferrite crystal grains A, and (c-2) precipitated carbides having a maximum diameter of 8 nm or less are contained in the crystal grains at a number density of 1 × 10 ^{15 to} 5 × The volume fraction A / (A + B) of the hard ferrite crystal grains A consisting of the soft ferrite crystal grains B dispersed at 10 ¹⁶ particles / cm ³ and (d) occupying the total volume is 0.1 to 0.5. A high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by being

  (2) When the grain boundary segregation C amount in the hard ferrite crystal grains A and the soft ferrite crystal grains B is averaged in the range of 2 nm on both sides of the grain boundary position, it is 0.5% to 3% in terms of number of atoms. A high-strength hot-rolled steel sheet excellent in stretch flangeability as described in (1) above.

  By applying the present invention, it becomes possible to supply a hot-rolled high-strength steel sheet having a tensile strength of 590 MPa class or more, an unprecedented stretch flangeability-ductility balance, and suppressing the occurrence of damage on the punched end face. Furthermore, the balance can be controlled.

  Details of the present invention will be described below.

  In order to achieve both ductility and stretch flangeability, the present inventors have intensively studied the behavior related to strengthening and ductility by the dispersion precipitation method of fine carbides in order to achieve both ductility and stretch flangeability.

  As a result, the present inventors, unlike the conventional ferrite single structure consisting only of the same ferrite crystal grains, have a ferrite single structure consisting of two types of ferrite grains having different carbide precipitation states, which has never been achieved. And a steel sheet that can achieve both tensile flange strength and tensile strength.

  That is, as a result of the study, the present inventors have obtained ferrite grains having excellent ductility as crystal grains of the steel sheet structure, but by making the grains into two types of ferrite grains having different precipitated carbide densities, the ductility is improved. And a steel sheet with improved stretch flangeability.

  In addition, regarding the damage to the punched end face, the present inventors believe that when maximum strength is obtained by the dispersion of precipitated carbides, solid solution carbon is reduced and segregation carbon at grain boundaries is reduced. The present inventors have found that the damage of the punched end face is suppressed by partially dispersing and precipitating the precipitated carbide in the ferrite crystal grains to leave segregated carbon at the grain boundaries.

That is, the present invention provides a steel sheet satisfying all of ductility (20% or more), stretch flangeability (hole expandability: 80% or more), tensile strength (590 MPa or more), and punching end face damage resistance. To obtain technical means to solve this technical problem,
(A) Based on a ferrite single-phase structure having good ductility, and by precipitating and dispersing finely precipitated carbides having a size (8 nm or less) that does not reduce stretch flangeability (hole expandability) in the crystal grains of the structure, Improve the tensile strength,
(B) In order to ensure an appropriate amount of grain boundary segregation C that contributes to the improvement of punching end face damage resistance in the ferrite single phase structure, while maintaining a predetermined amount of C solid solution in the steel, (B -1) The number density of the finely precipitated carbides is high (1 × 10 ¹⁷ / cm ³ or more) and the number density of the intragranular precipitated carbides is low (5 × 10 ¹⁶ pieces / cm ³ or less) soft. Two types of ferrite crystal grains, ferrite crystal grains B, are formed to maintain good tensile strength (590 MPa or more) of the steel sheet. (B-2) Improved ductility compared to conventional precipitation strengthened ferrite high strength steel sheets (20% or more)
(C) The number density (upper limit) of the precipitated carbides of the hard ferrite crystal grains A and the number density (lower limit) of the precipitated carbides of the soft ferrite crystal grains B are appropriate so that the hardness difference between these crystal grains does not become excessively large. By setting it within the range (1 × 10 ^{15 to} 5 × 10 ¹⁸ pieces / cm ³ ), stretch flangeability (hole expansion rate: 80%) compared to conventional high-strength steel sheets of ferrite and bainite or martensite. )
(D) By setting the volume fraction A / (A + B) of the hard ferrite crystal grains A in the total volume of the ferrite single-phase structure to an appropriate range (0.1 to 0.5), the tensile strength (A / (A + B) lower limit) and ductility (A / (A + B) upper limit),
Is the technical idea.

Furthermore, the present invention provides a technical means for improving the punching end face damage resistance,
(E) Grain boundary segregation C of the hard ferrite crystal grains A and the soft ferrite crystal grains B is adjusted to 0.5 to 3% in atomic weight% by adjusting the precipitation amount of the intra-grain fine precipitation carbides. ,
Is also a technical idea.

  In the present invention, the ferrite single-phase structure is used, which means that the matrix other than the precipitate is substantially composed of the ferrite structure. In other words, as long as the volume fraction of ferrite can be ensured to be 97% or more as long as the steel plate characteristics targeted by the present invention are not impaired, it is allowed to contain precipitates of a structure phase or components other than those specified in the ferrite structure. .

  Further, the ferrite means ferrite crystal grains that are BCC as a crystal structure, may include dislocations, and includes polygonal ferrite, ashkel ferrite, bainetic ferrite, and the like.

  The size of the precipitated carbide is the maximum diameter of the precipitated carbide, that is, the diameter when the precipitated carbide is spherical, the diagonal length when it is plate-like, and the average value of the measured values is the size of the precipitated carbide.

  In the present invention, the above-mentioned precipitated carbide means not only carbides but also carbonitrides in which nitrogen is slightly mixed in the carbides.

  Next, the reason for limitation relating to the microstructure of the steel sheet, which is a feature of the present invention, will be described.

  In order to satisfy the ductility, stretch flangeability, and tensile strength of the steel sheet under the above technical idea, the present inventors have precipitated in two types of ferrite crystal grains constituting a ferrite single phase structure. The optimum conditions for the size and number density of carbides were studied experimentally.

  The steel materials having the component compositions shown in Table 1 were hot rolled, and various steel plates shown in Table 2 were produced under different heat treatment conditions. In addition, the unit of the analysis value of the chemical component in a table | surface is the mass%. About the tensile strength of each steel plate, the No. 5 test piece as described in JISZ 2201 was produced, and it evaluated by performing the tensile test according to the test method as described in JIS Z2241. Further, the stretch flangeability of the steel sheet was evaluated by conducting a hole expansion test according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.

  When observing small precipitated carbides with a transmission electron microscope, it is difficult to distinguish them from dislocations. Therefore, FIM observation and three-dimensional atom probe measurement methods are used to measure the size and density of precipitated carbides in ferrite grains. Then, it went as follows.

  First, a needle-like sample is prepared from a sample to be measured by cutting and electropolishing. At this time, a focused ion beam processing method may be used together with the electropolishing method. Next, the presence or absence of precipitated carbide is observed with a relatively wide field of view by FIM observation, the size of any 30 precipitated carbides is measured, and the average value is obtained.

  In the three-dimensional atom probe measurement, the accumulated data can be reconstructed to obtain an actual distribution image of atoms in real space. From the volume of the three-dimensional distribution image of the precipitated carbide and the number of the precipitated carbide, the number density of the precipitated carbide (precipitate density) can be obtained.

  In the present invention, the size of the precipitated carbide in the ferrite crystal grains is the maximum value of the precipitated carbide, that is, the average value of 30 values obtained by measuring the diameter when the precipitated carbide is spherical, and the diagonal length when it is plate-like. And

  FIG. 1 shows the relationship between the precipitated carbide size (nm) in the ferrite crystal grains of the steel sheet and the hole expansion ratio λ (%) of the steel sheet. According to FIG. 1, when the average size of the precipitated carbide in the ferrite crystal grains exceeds 8 nm, the hole expansion rate of the steel sheet is significantly reduced. For this reason, in this invention, the size (nm) of the precipitation carbide | carbonized_material in the ferrite crystal grain of a steel plate shall be 8 nm or less.

  Further, the average size (nm) of the precipitated carbide is preferably 0.4 to 5 nm from the viewpoint of stably maintaining the strength of the steel sheet without reducing the hole expansion ratio due to the precipitated carbide in the ferrite crystal grains. .

  Moreover, in the ferrite crystal grain boundary in the steel sheet, the grain boundary becomes a preferential nucleation site, and a relatively coarse grain boundary precipitated carbide may precipitate at the grain boundary. In the present invention, the size of the precipitated carbides at the ferrite grain boundaries does not have to be particularly limited because no adverse effect is exerted.

  FIG. 2 shows the relationship between the number density of precipitated carbides of two types of ferrite crystal grains and the ductility and hole expansion ratio of the steel material.

  The vertical axis and the horizontal axis represent the number density of average precipitated carbides of the hard ferrite crystal grains A and the soft ferrite crystal grains B, respectively. Further, in the figure, a region satisfying both the target steel plate elongation (≧ 20%) and tensile strength (≧ 590 MPa) is indicated by (1), and the target steel plate hole expansion rate (≧ 80%) Regions satisfying both tensile strengths (≧ 590 MPa) are indicated by (2).

  Here, of the two types of ferrite grains, the grain with the higher number density of precipitated carbide is the hard ferrite crystal grain A, and the grain with the lower number density of precipitated carbide is the soft ferrite crystal grain B. Also for the ferrite crystal grains, the size of the precipitated carbide was 8 nm or less, which is in the size range in which the hole expansion rate is not lowered in FIG.

  In FIG. 2, in both the hard ferrite crystal grains A and the soft ferrite crystal grains B, as the number density of precipitated carbides present in the crystal grains increases, the strength of the steel sheet increases accordingly. When the number density of precipitated carbides in the grains becomes excessively high, the elongation and hole expansion rate of the steel sheet decrease (region (4) in the figure).

  Conversely, in both the hard ferrite crystal grains A and the soft ferrite crystal grains B, the lower the number density of precipitated carbides present in the crystal grains, the more advantageous in securing the elongation of the steel sheet. When the number density becomes excessively low, the tensile strength of the steel sheet decreases (region (3) in the figure).

  Further, in the case where the ferrite single-phase structure is composed of two types of ferrite crystal grains in which the number density of precipitated carbides in the crystal grains is different, both the hole expansion ratio and elongation of the steel sheet are maintained while maintaining the tensile strength of the steel sheet. In order to improve the workability, it is necessary to set the number density of the precipitated carbides in the two types of ferrite crystal grains within an optimum range.

That is, as can be seen from FIG. 2, in order to satisfy both the tensile strength of the steel sheet of 590 MPa or more and the high ductility of 20% or more, the number density of precipitated carbides of the hard ferrite crystal grains A is set to 1 × 10. ¹⁷ / cm ³ or more, and it is necessary to make the number density of carbide precipitates soft ferrite grain B 5 × 10 ¹⁶ atoms / cm ³ or less and (range in FIG. (1)). The reason is considered that soft ferrite grains B having a low density of precipitated carbides ensure ductility.

Furthermore, in order to satisfy both the high strength of the steel sheet with a tensile strength of 590 MPa or more and the high stretch flangeability with a hole expandability of 80% or more, the upper limit of the number density of precipitated carbides of the hard ferrite crystal grains A and the softness The lower limit of the number density of the precipitated carbides of the ferrite crystal grains B needs to be within the range of 1 × 10 ^{15 to} 5 × 10 ¹⁸ pieces / cm ³ (range (2) in the figure).

  This is because, empirically, it is known that the steel sheet having a uniform structure is better for the stretch flangeability of the steel sheet when the strength is the same. This is probably because stretch flangeability improves as the number density is closer to each other.

  That is, when the number density of precipitated carbides in two types of ferrite crystal grains is greatly different from each other, the difference in hardness between the two types of ferrite crystal grains is large, and the hard ferrite crystal grains A are stretched locally in the steel sheet. It is considered that it becomes the starting point of cracking when this happens.

  As described above, in the conventional high-strength steel sheet (DP steel sheet) of ferrite and bainite or martensite, the reason why the stretch flangeability of the steel sheet is bad is that the ferrite soft phase and the hard phase of bainite or martensite This is because the hardness difference is large. Therefore, in the present invention, when the number density of precipitated carbides in the two types of ferrite crystal grains is increased, the stretch flangeability of the steel sheet is deteriorated as in the case of the dual-phase high-strength steel sheet (DP steel sheet). Absent.

From the above, in the present invention, in order to satisfy all the properties of elongation of 20% or more and hole expansion ratio of 80% or more at a tensile strength of 590 MPa or more of the target steel plate, In addition to setting the carbide size to 8 nm or less that does not impair hole expansibility, the number density of precipitated carbides in the hard ferrite crystal grain A is set to 1 × for two types of ferrite crystal grains constituting the ferrite single structure. 10 ^{17 to} 5 × 10 ¹⁸ pieces / cm ^3, and the number density of precipitated carbides in the soft ferrite crystal grains B is set to 1 × 10 ^{15 to} 5 × 10 ¹⁶ pieces / cm ³ .

Further, in order to obtain the above-mentioned characteristics of the steel sheet more stably, preferably, the density of precipitated carbide of the hard ferrite crystal grains A is set to 3 × 10 ¹⁷ to 2 × 10 ¹⁸ pieces / cm ³ , and the soft ferrite crystal grains B The density of precipitated carbide is set to 3 × 10 ¹⁵ to 2 × 10 ¹⁶ pieces / cm ³ .

1 and 2, the tensile strength, elongation, and hole expansion ratio of the steel sheet are satisfied. With respect to ferrite crystal grains A, the precipitation carbide has a size of 2 nm, the number density of the precipitation carbide is 6 × 10 ¹⁷ , and the ferrite crystal grains As for B, the volume of hard ferrite crystal grains A and soft ferrite crystal grains B is measured by FIM observation and three-dimensional atom probe method on a steel plate having a precipitated carbide size of 3 nm and a number density of precipitated carbide of 7 × 10 ^15. The volume fraction of the hard ferrite crystal grains A in the total volume of the steel sheet structure is estimated as “volume% of hard ferrite crystal grains A / (volume% of hard ferrite crystal grains A + volume% of soft ferrite crystal grains B)”. It was.

  The volume fraction was adjusted by changing the chemical composition of the steel sheet and the heat treatment conditions, and changing the volume percentage of the hard ferrite crystal grains A.

  FIG. 3 shows the relationship between the volume fraction of hard ferrite crystal grains A (A / A + B) in the total volume of the steel sheet structure, and the strength and ductility of the steel sheet. From FIG. 3, by setting the volume fraction of hard ferrite crystal grains A in the total volume of the steel sheet structure to be in the range of 0.1 to 0.5, both the tensile strength and the elongation of the steel sheet are satisfied, and the strength ductility balance is It was found that a good steel plate can be obtained. When the volume fraction of the hard ferrite crystal grains A in the steel sheet structure is smaller than 0.1, the strength is lowered, and it becomes difficult to secure a strength of 590 MPa or more.

  On the other hand, in order to ensure the ductility of the steel sheet, it is essential to include the soft ferrite crystal grains B in the ferrite single phase structure. When the volume fraction of the hard ferrite crystal grains A in the ferrite single phase structure is 0.5 or more, the strength increases, but as the ratio of the soft ferrite crystal grains B decreases, the ductility and stretch flangeability of the steel sheet decrease. Therefore, it becomes difficult to obtain an elongation of 20% or more of the steel sheet and a hole expansion ratio of 80% or more.

  Therefore, in this invention, the volume fraction of the hard ferrite crystal grain A which occupies for the whole volume of a steel plate shall be 0.1-0.5. More preferably, it is 0.15-0.3.

From the above results, the macro structure of the steel sheet of the present invention is a ferrite single phase structure composed of hard ferrite crystal grains A and soft ferrite crystal grains B, and the number density of precipitated carbides of the hard ferrite crystal grains A is 1 × 10 ^{17 to} 5 × 10 ¹⁸ pieces / cm ³ , the number density of precipitated carbides of soft ferrite crystal grains B is 1 × 10 ^{15 to} 5 × 10 ¹⁶ pieces / cm ³ , and the hard ferrite phase occupies the total volume in the ferrite single-phase structure The volume fraction of A (A / A + B) shall be 0.1-0.5.

  As a result, a high-tensile steel sheet having a tensile strength of 590 MPa or more, a stretch flangeability with a hole expansion ratio of 80% or more, and a ductility with an elongation of 20% or more and excellent in strength and workability is realized. It becomes possible.

  Further, in the present invention, as described above, by defining the precipitated carbide size and the precipitated carbide number density in the two types of ferrite crystal grains, an appropriate amount of precipitated carbide is dispersed and precipitated in each ferrite crystal grain. In addition to the C of the steel sheet components described later, solid solution C is secured by 0.002% or more, and by leaving an appropriate amount of segregated carbon segregating at the grain boundaries of each ferrite crystal, Damageability can also be maintained well. The amount of solute C is more preferably 0.005% or more.

  In the steel sheet of the present invention, in the cooling process after hot rolling of the steel sheet, the cooling is finished at a slightly earlier stage than the peak of the precipitation amount of the precipitated carbide, and partially in the ferrite crystal grains, the appropriate size and the appropriate number density By maintaining the solid solution C in the ferrite crystal grains while dispersing and precipitating carbides, it is possible to maintain the damage resistance of the end face when the steel sheet is punched together with the strength, ductility, and stretch flangeability. it can.

  Furthermore, in order to improve the damage resistance of the end face at the time of punching the steel sheet, it is preferable to appropriately set the grain boundary segregation C amount of the ferrite crystal grain boundary as follows.

  Table 2 shows the measurement result of the grain boundary segregation C amount and the result of visually observing the presence or absence of the end face defect by punching a 10 mm diameter hole as in the hole expansion test shown in the examples. FIG. 4 shows the relationship between the segregation C amount of ferrite grain boundaries in the steel sheet structure and the damage occurrence rate of the punched end face of the steel sheet.

  Here, the amount C of grain boundary segregation is the average concentration of atoms (number of atoms%) contained within 2 nm on both sides of the grain interface of the ferrite crystal. This is the average C concentration obtained by dividing the range of 2 nm on both sides of the field position and calculating from the number of C atoms therein.

  From the results shown in Table 2, when solid solution C is present in the ferrite crystal grains in the steel sheet structure in an amount of 0.002% or more by mass%, segregation C remains at 0.5 atomic% or more at the grain boundaries. I understand that it will be possible. That is, from Table 2 and FIG. 4, it is possible to suppress the occurrence of damage on the punched end surface of the steel sheet.

  When the segregation C amount in the ferrite crystal grain boundary is 0.5 atomic% or more, the reason why the end face damage resistance of the steel sheet is improved is that the grain boundary is strengthened by the segregation C, and the grain boundary is This is thought to be due to the suppression of crack growth in the steel.

  Moreover, from Table 2 and FIG. 4, when the amount of segregation C at the ferrite grain boundary exceeds 3 atomic%, C concentrates at the grain boundary and suppresses the precipitation of cementite which reduces the stretch flangeability of the steel sheet. It becomes impossible, it becomes easy to generate | occur | produce the damage of the punching end surface of a steel plate, and it turns out that it is not preferable.

  As shown in FIG. 4, in the present invention, in order to stably improve the end face damage resistance during stretch flange processing of a steel sheet, the segregation C amount in ferrite crystal grains of the steel sheet is 0.5-3 atomic%. And more preferably 0.8 to 2%.

  The present invention realizes a ferrite single-phase structure composed of two types of ferrite crystal grains having different precipitation dispersion states of the intragranular precipitated carbide as a steel sheet structure, and has ductility (20% or more) and stretch flangeability (hole expansion) of the steel sheet. Property: 80% or higher), tensile strength (590 MPa or higher), and punching end face damage resistance, in order to realize a steel plate, it is necessary to define the component composition of the steel plate as follows.

  In the present invention, the target steel plate characteristics can be sufficiently obtained by the basic component composition described below, but it is a matter of course that other components may be added as long as the steel plate characteristics are not impaired. The Further, “%” shown below means “% by mass” unless otherwise specified.

  C: C is set to 0.01 to 0.2%. The reason why the C content is set to 0.01% or more is that when the C content is less than 0.01%, the strength decreases. On the other hand, the C content is set to 0.2% or less because the increase in carbon concentration exceeding 0.2% promotes the formation of cementite and the formation of transformation structures such as pearlite and martensite. This is because the manufacturing conditions for obtaining the ferrite phase to be handled become severe.

  Si: Si is effective for increasing the strength as a solid solution strengthening element. However, if the Si content is less than 0.01%, the effect of increasing the strength cannot be obtained. On the other hand, if the Si content exceeds 1.5% , Workability deteriorates. Therefore, the Si content is preferably in the range of 0.01 to 1.5%.

  Mn: Mn is necessary for deoxidation and desulfurization, and is also effective as a solid solution strengthening element. However, if the Mn content is less than 0.25%, the above effect cannot be obtained. When the content exceeds 3.0%, segregation is likely to occur, and stretch flangeability deteriorates. Therefore, the Mn content is preferably 0.25 to 3.0%.

  One or more of Ti, Nb, V, and Mo: In the present invention, one or more of Ti, Nb, V, and Mo are used as carbide precipitation elements in the ferrite crystal grains of the steel sheet. And contained in the steel sheet.

  Ti precipitates carbides in the ferrite crystal grains and contributes to an increase in the strength of the steel sheet by precipitation strengthening, and by this precipitation, it acts to fix C contributing to the formation of cementite, but less than 0.03%, On the other hand, when the content exceeds 0.2%, coarsening of the carbide is unavoidable, which causes a reduction in the number density in the crystal grains. Therefore, when Ti is contained, the Ti content is preferably 0.03 to 0.2%.

  V, Nb, and Mo are also added as precipitation strengthening elements that precipitate carbides in ferrite crystal grains and supplement Ti, and elements that act to fix C contributing to the formation of cementite, similarly to Ti. . However, when the content of these elements is less than 0.01%, the above effect is insufficient. On the other hand, when the content exceeds 0.2%, coarse precipitated carbides are generated, and the number density in the crystal grains is reduced. Causes to reduce. Therefore, when adding V, Nb, and Mo, it is preferable that the content is 0.01 to 0.2%.

  The above is the basic component composition of the steel sheet of the present invention. Further, the upper limit of the contents of N, P, S, and Al handled as inevitable impurity components in the steel sheet of the present invention is limited as follows. preferable.

  N: N forms TiN and lowers the workability of the steel sheet. In particular, if the N content exceeds 0.009%, coarse TiN is generated and the workability of the steel sheet deteriorates, so the N content is preferably limited to 0.009% or less.

  P: P acts as a solid solution strengthening element and increases the strength of the steel sheet. However, when the content is increased, the workability and weldability of the steel sheet are deteriorated, which is not preferable. In particular, when the P content exceeds 0.1%, the workability and weldability of the steel sheet are significantly reduced. Therefore, the P content is preferably limited to 0.1% or less.

  S: If the S content is too high, inclusions such as MnS are formed, the stretch flangeability is deteriorated, and further, cracking occurs during hot rolling, so it is preferable to reduce as much as possible. In particular, in order to prevent cracking during hot rolling and improve workability, the S content is preferably limited to 0.005% or less.

  Al: Al needs to be added in an amount of 0.002% or more for molten steel deoxidization, but it is preferably reduced in order to improve the workability of the steel sheet. When the Al content exceeds 0.5%, Al forms coarse precipitates such as nitrides and degrades the elongation of the steel sheet. In order to improve the elongation of the steel sheet, the Al content is preferably limited to 0.5% or less.

  In the present invention, in addition to the above basic components, for the purpose of improving the strength of the steel sheet, addition of Cr and W as a solid solution strengthening element is also effective. May be.

  Here, the manufacturing method which manufactures the hot rolled sheet steel of this invention is demonstrated. If the production method can suppress the precipitation carbide size to 8 nm or less, and the precipitation carbide density is within the range specified in the present invention, and the grain boundary segregation C concentration can be within the range of 0.5 to 3 atomic%, Any manufacturing conditions may be adopted, but one of the preferable manufacturing conditions is described below.

  First, a steel piece having the above component composition is reheated to a temperature of 1200 ° C. or higher. The slab may be a slab immediately after being manufactured in a continuous casting facility, or may be manufactured in an electric furnace. Moreover, the reheating to the temperature of 1200 degreeC or more is also a heating condition with respect to the slab manufactured directly from molten steel.

  The reason why the temperature is defined as 1200 ° C. or higher is to sufficiently decompose and dissolve the carbide forming element and carbon in the steel material. In some cases, the temperature may be raised to 1250 ° C. Then, hot rolling is performed after heating and holding for 1 hour or more, but it is preferable to end the hot rolling in the austenite region in order to suppress variations in the characteristics of the steel sheet.

  Next, after the hot rolling is completed, it is desirable to cool to 700 ° C. or less at a fast rate of 30 ° C./sec or more in order to suppress ferrite transformation and carbide formation as much as possible. Thereafter, in order to partially realize ferrite transformation and carbide precipitation, a holding treatment is performed for a short time between 2 seconds and 10 seconds at a temperature (T1) between 700 ° C. and 550 ° C. desirable. If the holding time is shorter than 2 seconds, ferrite transformation and carbide precipitation do not occur, and if it is longer than 10 seconds, the ferrite transformation proceeds, making it difficult to obtain the two types of ferrite grains of the present invention. Therefore, the holding time is preferably 2 to 10 seconds.

  Next, after holding at the T1 temperature for a short time, in order to suppress the coarsening of the partially precipitated carbide, it is between 600 ° C. and 480 ° C. at a cooling rate of 10 ° C./sec or more. Cool to temperature (T2). Thereafter, in order to cause ferrite transformation and precipitation in the untransformed portion, it is desirable to perform a short-time holding treatment again for 2 seconds to 10 seconds at the T2 temperature.

  Next, after performing a short-term holding treatment at the T2 temperature, it is cooled again to the winding temperature at a cooling rate of 10 ° C./sec or more. If the coiling temperature is less than 350 ° C., hard martensite is generated and stretch flangeability may be deteriorated. Therefore, the coiling temperature is preferably 350 ° C. or more. On the other hand, when the coiling temperature exceeds 600 ° C., the precipitation of carbide progresses in the crystal grains, the amount of segregation C at the grain boundaries decreases, and not only the end face damage occurs but also harmful to stretch flangeability. Winding at 600 ° C. or lower is preferable because there is a possibility that pearlite cementite is generated.

  When such a multi-stage cooling process is adopted, various steel structures are often formed depending on the starting components and the holding conditions in the middle, but by examining the various conditions related to cooling, two types of ferrites with different density of precipitated carbides When a structure composed of crystal grains is obtained, the present invention is completed, and a high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability can be produced.

  Examples of the present invention will be described together with comparative examples.

  Various materials having the component composition shown in Table 1 were dissolved. The component value of a table | surface is a chemical analysis value, and a unit is the mass%. Ti was mainly selected as the carbide forming element, and Nb, V, and Mo were selected in addition.

  Next, for each hot-rolled sheet, No. 5 test piece described in JIS Z 2201 was prepared, tested according to the test method described in JIS Z 2241, and evaluated for tensile strength. In the present invention, a steel sheet having a tensile strength of 590 MPa or more is targeted.

  The stretch flangeability was evaluated by conducting a hole expansion test according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. The presence or absence of damage on the punched end face was confirmed by punching a 10 mm diameter hole in the steel plate and visually observing the end face shape in the same manner as the hole expansion test.

  In addition, a columnar sample of 0.3 mm × 0.3 mm × 10 mm is cut out from the steel plate, the tip portion thereof is made into a sharp needle shape by electrolytic polishing, and the size and density of precipitated carbides and grain boundaries are measured by a three-dimensional atom probe measurement method. Nearby segregation C concentration was measured. Furthermore, the presence or absence of large precipitated carbides exceeding 8 nm was confirmed by TEM observation.

  A series of results are shown in Table 2 together with various production conditions. In Table 2, the steel types of the samples used for each test are described as A to D. About the steel plate produced on various manufacturing conditions, tensile strength (TS), elongation, and the hole expansion rate were measured. The size of the precipitated carbide as the material parameter is a size measured by FIM observation and a three-dimensional atom probe measurement method, and the density is the number of precipitated carbide per volume.

  Next, the outline of each data shown in Table 2 will be described.

  In Tests 1 to 4 using Sample A, in Test 1, since the volume ratio of hard ferrite crystal grains A having high strength was insufficient, a strength of 590 MPa or more could not be realized. The density of precipitated carbides increased, the ductility deteriorated, the grain boundary C concentration decreased, and the punched end face was damaged.

  In Tests 5 to 7 using Sample B, in Test 6, only one type of ferrite crystal grain is present and stretch flangeability is good, but sufficient ductility cannot be ensured. In Test 7, the size of precipitated carbide is large. Things are included and stretch flangeability is not enough.

  In Tests 8 to 11 using Sample C, in Test 8, the precipitated carbide density of the soft ferrite crystal grains B was very low and sufficient ductility was exhibited, but the stretch flangeability was not sufficient, and Test 10 Then, the density of precipitated carbides of the hard ferrite crystal grains A is high, and the stretch flangeability is deteriorated. Moreover, in the test 11, the precipitation carbide density of the soft ferrite crystal grain B is high, and the ductility is deteriorated.

  In the tests 12 to 14 using the sample D, in the test 12, the precipitated carbide density of the hard ferrite crystal grains A is low and the ductility is not sufficient, and in the test 13, the precipitated carbide density of the soft ferrite crystal grains B is low. The stretch flangeability is getting worse.

  In Tests 15 to 17 using Sample E, in Test 16, the precipitation carbide density of the hard ferrite crystal grains A is low, the ductility is deteriorated, and the precipitation carbide size is large, so that the stretch flangeability is sufficient. In Test 17, the precipitation carbides coarsened in ferrite crystal grains AB, the stretch flangeability and ductility did not reach sufficient values, the grain boundary C concentration decreased, and the punched end face was damaged. is doing.

  As described above, by applying the present invention, a hot-rolled high-strength steel sheet having a tensile strength of 590 MPa class or more, having an unprecedented stretch flangeability-ductility balance, and suppressing the occurrence of damage on the punched end face is supplied. And the balance can be controlled. Therefore, the present invention is extremely useful industrially.

It is a figure which shows the relationship between distribution (nm) of precipitation carbide size, and a hole expansion rate (%). It is a figure which shows the relationship between the precipitation carbide density (piece / cm < ³ >) of two types of ferrite crystal grains, elongation (%), and a hole expansion rate (%). It is a figure which shows the range of the volume fraction of the hard ferrite crystal grain A when a steel plate achieves elongation 20% or more and tensile strength 590 MPa or more. It is a figure which shows the relationship between the segregation C amount of the ferrite crystal grain boundary in a steel plate structure, and the damage incidence rate of the punching end surface of a steel plate.

Claims (2)

In a high-strength hot-rolled steel sheet consisting of a ferrite single-phase structure with a tensile strength of 590 MPa or more,
(a) By mass%, C: 0.01 to 0.2%, Si: 0.01 to 1.5%, Mn: 0.25 to 3.0%, and further, as a carbide forming element, One of Ti: 0.03-0.2%, Nb: 0.01-0.2%, V: 0.01-0.2%, and Mo: 0.01-0.2% Or contains two or more, the balance consists of Fe and inevitable impurities,
(b) Among the above C amounts, the solid solution C amount is 0.002% or more in mass%,
(c) The ferrite single-phase structure is (c-1) hard ferrite crystal grains in which precipitated carbides having a maximum diameter of 8 nm or less are dispersed at a number density of 1 × 10 ^{17 to} 5 × 10 ¹⁸ particles / cm ³ in the crystal grains. A and (c-2) soft ferrite crystal grains B in which precipitated carbides having a maximum diameter of 8 nm or less are dispersed in the crystal grains at a number density of 1 × 10 ^{15 to} 5 × 10 ¹⁶ particles / cm ³ , and
(d) A high-strength hot-rolled steel sheet excellent in stretch flangeability, wherein the volume fraction A / (A + B) of the hard ferrite crystal grains A occupying the entire volume is 0.1 to 0.5.   The amount of grain boundary segregation C in the hard ferrite crystal grains A and soft ferrite crystal grains B is 0.5% to 3% in terms of atomic% when averaged in the range of 2 nm on both sides of the grain boundary position. The high-strength hot-rolled steel sheet excellent in stretch flangeability according to claim 1.

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