JP2014141703A - High strength hot rolled steel sheet excellent in appearance and balance between elongation and hole-expandability and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having a tensile strength of 590 MPa or more and excellent in appearance and a balance between elongation and hole-expandability, and to provide a method of producing the high strength hot rolled steel sheet.SOLUTION: There is provided the high strength hot rolled steel sheet which has a chemical constituent composition containing, by mass%, C:0.02% to 0.10%, Si:0.1% or less, Mn:0.5% to 2.0%, Al:0.2% to 1.0%, N:0.01% or less, Ti:0.01% to 0.11%, and which has a ferrite structure fraction of 90% or more, a martensite particle diameter of 10 μm or less, a martensite structure fraction of 1% to 10%, an X ray random ratio in a {211}<011> orientation parallel to a rolling face and a rolling direction of 3.0 or less. The method of producing the high strength hot rolled steel sheet is also provided in which a slab having the above chemical constituent composition is re-heated to 1,200°C or higher, final finish rolling is performed on the slab at a final finish rolling reduction of 20% or more and a finishing temperature of 880 to 1,000°C, cooling of the slab is stared within 0.01 sec. to 1.0 sec. after completion of the finish rolling, the slab is cooled to 600 to 750°C at a cooling rate of 30°C/sec. or more, is air cooled for 3 to 10 sec. and cooled to 200°C or lower at the cooling rate of 30°C/sec. or more, and the slab is rolled up.

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more and excellent in appearance and excellent in balance between elongation and hole expansibility, and a method for producing the same.

  In recent years, for the purpose of improving the fuel consumption and collision safety of automobiles, efforts have been made to reduce the weight of the vehicle body by applying high-strength steel sheets. It is important to ensure press formability when applying high-strength steel sheets. For example, in order to improve the surface design of automotive wheel discs, the Si scale pattern is eliminated as much as possible, stretch processing and burring processing are performed, so the steel sheet used as the material has an excellent appearance and requires high elongation and hole expansibility It becomes.

  Patent Document 1 proposes a hot-rolled steel sheet with an excellent balance between elongation and hole expansibility by increasing the martensite structure fraction to 3% or more and less than 10%, and strengthening ferrite with Ti and Nb as an alternative to its strength. Has been.

  Patent Document 2 proposes a composite steel of ferrite and martensite in which Al is added in order to prevent generation of Si scale, which causes deterioration of chemical conversion properties, and the ratio of ferrite becomes 40% or more.

JP 2011-184788 A Japanese Patent Laid-Open No. 2005-120438

  In the invention described in Patent Document 1, since Ti and Nb are added for the precipitation strengthening of ferrite, a texture develops during hot rolling, and the plastic anisotropy of ferrite is strong, so that the hole expandability is sufficient. There wasn't. Further, since 0.5% or more of Si is added, a streak pattern (hereinafter referred to as a scale pattern) is generated on the steel sheet due to the scale generated during hot rolling, and the appearance is deteriorated.

  The invention described in Patent Document 2 improves appearance and chemical conversion treatment by adding Al as an alternative to Si. However, by adding Al, the ferrite transformation start temperature is increased, coarse ferrite and martensite are formed, cracks are likely to occur at the interface between ferrite and martensite, and elongation and hole expansibility are not sufficient.

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more, which is excellent in appearance, and excellent in the balance between elongation and hole expansibility, and a method for producing the same.

  As a result of various investigations on means for solving the above problems, the present inventors have formed by improving the texture formed during hot rolling and adding Al by Ti or Nb added as a strength substitute for martensite. The present inventors have found that it is important to control the austenite structure immediately before transformation in order to solve the two problems of hole expandability deterioration due to coarse martensite. Specifically, the final stage finish reduction is set to 20% or more, and the finish rolling temperature is set to 880 ° C. or more and 1000 ° C. or less to promote austenite recrystallization and improve the texture. Furthermore, by starting cooling within 0.01 seconds or more and 1.0 seconds after the end of rolling, recrystallization in a short time becomes possible and fine recrystallized austenite is formed. The transformation from fine recrystallized austenite has many ferrite nucleation sites and the transformation proceeds quickly, so fine ferrite is formed during air cooling. As a result, it has been found that since austenite remaining during air cooling also remains fine, the martensite after transformation can be refined.

That is, the gist of the present invention is as follows.
(1) By mass% C: 0.02% or more, 0.10% or less,
Si: 0.1% or less,
Mn: 0.5% or more, 2.0% or less,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.2% or more, 1.0% or less,
N: 0.01% or less,
Ti: 0.01% or more and 0.11% or less, with the balance being a chemical composition composed of Fe and inevitable impurities, the proportion of the structure being 90% or more of the ferrite structure fraction, and martensite The particle size of the particles is 10 μm or less, the structure fraction is 1% or more and 10% or less, the X-ray random ratio of the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is 3.0 or less. A high-strength hot-rolled steel sheet with an excellent appearance and an excellent balance between elongation and hole expansibility.
(2) Further, by mass% Nb: 0.01% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0030% or less Mo: 0.02% or more, 0.5% or less,
Cr: 0.02% or more, 1.0% or less,
A high-strength hot-rolled steel sheet having an excellent appearance according to (1) and having an excellent balance between elongation and hole expansibility.
(3) The steel having the chemical composition described in (1) or (2) above is made into a slab by continuous casting, reheated to a temperature range of 1200 ° C or higher, and after rough rolling, a plurality of rolling mills are arranged in series. In the case of continuous finish rolling in the finish rolling mill row, the final finish rolling is performed at a reduction rate of 20% or more, the final finish rolling finish temperature is 880 to 1000 ° C. .01 seconds or more, start cooling within 1.0 seconds, cool to 600 to 750 ° C. at a cooling rate of 30 ° C./second or more, air cool for 3 to 10 seconds, and then at a cooling rate of 30 ° C./second or more. A method for producing a high-strength hot-rolled steel sheet that is excellent in appearance as described in (1) or (2) and excellent in balance between elongation and hole expansibility, wherein the steel sheet is cooled to 200 ° C. or lower and wound.

  The present invention has a predetermined chemical component composition, the proportion of the structure is 90% or more of the ferrite structure fraction, the particle size of martensite is 10 μm or less, and the structure fraction is 1% or more and 10% or less. Yes, the X-ray random ratio of the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is 3.0 or less, so that the appearance is excellent in a high strength hot rolled steel sheet having a tensile strength of 590 MPa or more. It is possible to achieve a balance between excellent elongation and hole expansibility.

  When hot-rolling a slab having the above-mentioned predetermined chemical composition, the finish rolling temperature is set to 880 ° C. or higher and 1000 ° C. or lower to promote austenite recrystallization and improve the texture. Furthermore, the finishing reduction ratio is set to 20% or more, and cooling is started within 0.01 seconds or more and 1.0 seconds after the end of rolling, thereby enabling recrystallization in a short time and making fine recrystallized austenite. Come on. The transformation from fine recrystallized austenite has many ferrite nucleation sites and the transformation proceeds quickly, so fine ferrite is formed during air cooling. As a result, since austenite remaining during air cooling also remains finely, it becomes possible to refine the martensite after transformation, and it is possible to manufacture a steel sheet having the crystal structure and the X-ray random ratio of the present invention. .

  The present invention is directed to a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more. In order to achieve improvement in hole expansibility in such a high-strength steel sheet, it is effective to set the ferrite fraction to 90% or more and the martensite fraction to 10% or less. As a means for obtaining these, as shown in Patent Document 1, it is necessary to promote ferrite transformation by performing intermediate air cooling in a run-out table (hereinafter referred to as ROT) in a hot rolling process with a steel sheet added with Si. Become. On the other hand, since a scale pattern due to Si scale is generated in Si, a poor appearance when using a steel plate becomes a problem. When Si is not added, it is effective to lower the finish rolling temperature in order to promote ferrite transformation. However, when the temperature is lowered, {211} <parallel to the rolling surface of the steel sheet texture and parallel to the rolling direction. 110> develops, and the anisotropy of plastic deformation becomes strong and the hole expandability deteriorates. Therefore, the balance between elongation and hole expandability could not be improved without addition of Si.

  In the present invention, as an alternative to Si, ferrite transformation can be promoted with Al, and ferrite coarsened by addition of Al can be avoided by transformation from fine austenite. The finishing temperature is in the range of 880 ° C. to 1000 ° C., the reduction rate is 20% or more, cooling is started within 0.01 seconds and 1.0 seconds after finishing rolling, and 600 at a cooling rate of 30 ° C./second or more After cooling to 750 ° C. to 750 ° C., air cooling for 3 to 10 seconds, cooling to 200 ° C. or less at a cooling rate of 30 ° C./second or more, and winding up, the surface appearance is excellent, and the ferrite fraction is 90% As described above, the martensite grain size is 10 μm or less, the structure fraction is 1% or more and 10% or less, the steel sheet texture is parallel to the rolling surface, and X in {211} <011> orientation parallel to the rolling direction. By making the line random ratio 3.0 or less, it became possible to obtain an excellent balance between elongation and hole expansibility.

The individual constituent requirements of the present invention will be described in detail below.
First, the reasons for limiting the components of the present invention will be described.

  C is an important element that determines the strength of the present invention. In order to obtain the target strength, it is necessary to contain 0.02% or more. Preferably it is 0.04% or more. However, if the content exceeds 0.10%, the toughness deteriorates, so the upper limit is made 0.10%.

  Si is an element necessary for preliminary deoxidation, but it causes an appearance defect, so is made 0.1% or less. Preferably it is 0.07% or less.

  Mn is effective for increasing the strength as a hardenability and solid solution strengthening element. In order to obtain the target strength, 0.5% or more is necessary. If added excessively, MnS harmful to toughness isotropic properties is generated, so the upper limit is made 2.0% or less.

  P is preferably as low as possible, and if contained in excess of 0.1%, workability and weldability are adversely affected and fatigue characteristics are also reduced.

  S is desirably as low as possible, and if it is too large, inclusions such as MnS that are harmful to toughness isotropic properties are generated. When severe low temperature toughness is required, the content is preferably 0.006% or less.

  Al is an important element for obtaining the characteristics of the present invention. In order to promote ferrite transformation during ROT cooling after finish rolling, it is necessary to add 0.2% or more. However, when added excessively, alumina precipitated in a cluster form is generated and the toughness is deteriorated, so the upper limit is made 1.0%.

  N forms Ti and precipitates at a higher temperature than S, and not only reduces Ti effective for fixing S but also forms coarse Ti nitrides and deteriorates toughness. Therefore, it is made 0.01% or less.

  Ti is an element necessary for obtaining strength in the present invention. In order to enhance precipitation of ferrite and obtain an excellent balance between elongation and hole expansibility, it is necessary to add 0.01% or more. However, if over 0.11% is added, inclusions due to TiN are generated and the hole expansion property deteriorates. Therefore, the Ti content is set to 0.01% or more and 0.11% or less.

  Although not essential for satisfying the required characteristics, it is preferable to add the following elements in order to reduce manufacturing variability or to further improve the strength.

  Nb can increase the strength by reducing the crystal grain size of the hot-rolled steel sheet and by NbC. The effect is obtained when the Nb content is 0.01% or more. On the other hand, if it exceeds 0.10%, the effect is saturated, so the upper limit is made 0.10%.

  Ca is a suitable element for refining the structure by dispersing many fine oxides in the deoxidation of molten steel, and fixing the S in the steel as spherical CaS for desulfurization of the molten steel. It is an element that suppresses the generation of objects and improves hole expansibility. These effects are obtained when the addition amount is 0.0005%, but since saturation occurs at 0.0030%, the Ca content is set to 0.0005% or more and 0.0030% or less.

  Mo is an effective element for precipitation strengthening of ferrite. In order to obtain this effect, addition of 0.02% or more is desirable. However, the addition of a large amount increases the cracking susceptibility of the slab and makes it difficult to handle the slab, so the upper limit is made 0.5%.

  Cr is an element effective for improving the steel sheet strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, the addition of a large amount decreases the ductility, so the upper limit is made 1.0%.

  Next, the crystal structure of the steel sheet of the present invention will be described.

  A composite structure steel is a steel sheet in which a hard structure such as martensite is dispersed in a ferrite that is soft and excellent in elongation, and achieves high elongation while having high strength. However, high strain concentrates in the vicinity of the hard structure, and the crack propagation speed increases, so that there is a drawback that the hole expandability is lowered.

  In order to suppress deterioration of the hole expandability due to martensite, it is necessary to make the particle size of martensite 10 μm or less and the structure fraction to 10% or less as shown in Patent Document 1. In order to secure the balance between fatigue characteristics and elongation and strength, 1% or more is necessary. Further, as a means for substituting for the reduction of martensite that ensures the strength of the steel sheet while ensuring the elongation, the structural fraction of ferrite that is precipitation strengthened by Ti is required to be 90% or more. However, when Ti is added, since recrystallization of austenite during finish rolling is suppressed, a strong work texture is formed by finish rolling. Therefore, it has been found that the steel sheet texture after transformation shows a strong degree of accumulation and the hole expandability is inferior. Therefore, in addition to the optimization of the structure fraction, the X-ray random ratio of the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is set to 3.0 or less in the texture of the steel sheet. It is possible to achieve both high elongation and hole expandability.

  Next, a manufacturing method will be described.

  When hot rolling a continuous cast slab having the chemical composition of the present invention (hereinafter referred to as slab), the slab is first heated to 1200 ° C. or higher. When the slab is heated at less than 1200 ° C., TiC is not sufficiently dissolved in the slab, and Ti required for precipitation strengthening of ferrite is insufficient.

  The heated slab is subjected to rough rolling, and further subjected to continuous finish rolling in a finish rolling mill row in which a plurality of rolling mills are arranged in series. At this time, the final finish rolling reduction ratio (final rolling final stage reduction ratio) is 20% or more, and the final finish rolling finish temperature FT is 880 to 1000 ° C. In order to cause austenite recrystallization at a high temperature, a rolling reduction of 20% or more is required. If it is less than 20%, the driving force required for recrystallization is not sufficient, causing grain growth from the final finish rolling to the start of cooling, and martensite becomes coarse, resulting in poor hole expandability. When the finish rolling temperature is less than 880 ° C., the recrystallization of austenite does not proceed, the steel plate texture is parallel to the rolling surface, and the X-ray random ratio in the {211} <011> orientation parallel to the rolling direction is over 3.0. The hole expandability is inferior. If it exceeds 1000 ° C., the crystal grain size of austenite becomes coarse and the dislocation density rapidly decreases, so that the ferrite transformation is significantly delayed and a ferrite structure fraction of 90% or more cannot be obtained.

  Primary cooling after finish rolling starts within 0.01 seconds and within 1.0 seconds. In order to complete recrystallization of austenite after rolling, it is necessary to air-cool after finish rolling for 0.01 seconds or more. However, if the air cooling time is long, coarsening of recrystallized austenite crystal grains occurs, ferrite transformation is greatly delayed, and coarse martensite is formed. In order to suppress voids generated at the interface between ferrite and martensite and to obtain excellent hole expansibility, it is important to make the martensite particle size 10 μm or less. For this purpose, since it is necessary to suppress the coarsening of the austenite crystal grains, the primary cooling starts within 1.0 seconds.

  Primary cooling after finish rolling is performed at 30 ° C./second or more to 600 to 750 ° C., and intermediate air cooling is performed for 3 to 10 seconds. Since fine austenite has a high crystal grain growth rate, if it is less than 30 ° C./second, it grows during cooling and becomes a coarse structure. If the intermediate air cooling temperature is less than 600 ° C., the ferrite transformation is delayed, a high ferrite fraction cannot be obtained, and the elongation deteriorates. If it exceeds 750 ° C., TiC coarsely precipitates in the ferrite, so that sufficient ferrite precipitation strengthening cannot be obtained, and a tensile strength of 590 MPa cannot be obtained. Intermediate air cooling requires 3 seconds or more because of ferrite transformation. However, when air cooling exceeds 10 seconds, precipitation of bainite proceeds, resulting in inferior elongation and hole expansibility.

  After the intermediate air cooling, the secondary cooling is performed at 30 ° C./second or more to 200 ° C. or less and wound up. If it is less than 30 ° C./second, bainite transformation proceeds and martensite cannot be obtained, so that the tensile strength is lowered and the elongation is inferior. If the cooling temperature exceeds 200 ° C., the self-tempering effect of martensite occurs, so the tensile strength decreases and the elongation becomes inferior.

  Steel containing the components shown in Table 1 was melted in a converter and formed into a slab having a thickness of 230 mm by continuous casting. Thereafter, the slab was heated to a temperature of 1200 ° C. to 1250 ° C., subjected to rough rolling and finish rolling with a continuous hot rolling device, wound up after ROT cooling, and manufactured a hot-rolled steel sheet. Table 2 shows the steel type symbols used, the hot rolling conditions, and the steel plate thickness. In Table 2, “FT6” is the final finish rolling end temperature, “cooling start time” is the time from finish rolling to primary cooling, “primary cooling” is the average cooling rate from finishing finish rolling to the intermediate air cooling temperature, “Intermediate temperature” is the intermediate air cooling temperature after the primary cooling, “Intermediate time” is the intermediate air cooling holding time after the primary cooling, “Secondary cooling” is the average cooling rate from the intermediate air cooling to the winding, “Taking temperature” Is the temperature after the end of secondary cooling.

  The steel plate thus obtained was analyzed for the structural fraction and texture of ferrite, bainite and martensite using an optical microscope. The particle size of martensite was also investigated.

  As for the structural fractions of ferrite and bainite of the steel sheet, the area ratio was determined by image analysis in the field of view of 500 × 500 μm using an optical microscope after nital corrosion. The martensite structure fraction and particle size were determined by image analysis in a 500 × 500 μm field of view using an optical microscope after repeller corrosion.

  The texture analysis evaluated the X-ray random intensity ratio of {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction at ¼ part of the plate thickness. Using the EBSD (Electron Back Scattering Diffraction Pattern) method, the pixel measurement interval is 1/5 or less of the average grain size, and the measurement is performed in an area where 5000 or more crystal grains can be measured. From the distribution of ODF (Orientation Distribution Function) A random strength ratio of 3.0 or less was regarded as acceptable.

  For the steel sheet tensile test, a JIS No. 5 test piece was taken in the rolling width direction (C direction) of the steel sheet, and yield strength: YP (MPa), tensile strength: TS (MPa), and elongation: EL (%) were evaluated. .

  The hole expansion ratio: λ (%) was evaluated by the method specified in ISO 16630.

  The steel sheet appearance was evaluated by cutting the steel sheet 500 mm in the longitudinal direction at a position of 10 m on the outer periphery of the hot rolled coil, and measuring the area ratio of the scale pattern. The case where the area ratio of the scale pattern was 10% or less was designated as “◯”. On the other hand, the case where the area ratio of the scale pattern was more than 10% was designated as “x”.

  Table 3 shows the evaluation results of the structure fraction, martensite particle size, texture and material, and appearance.

  As shown in Table 3, the present invention example has a tensile strength of 590 MPa or more, a ferrite structure fraction of 90% or more, a martensite particle size of 10 μm or less, and a structure fraction of 1% or more and 10% or less. The X-ray random ratio of {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is 3.0 or less, and is excellent in the balance of appearance, elongation and hole expansibility.

  On the other hand, in Comparative Example 2, the intermediate air cooling temperature is high, Ti is coarsely precipitated in the ferrite, and sufficient precipitation strengthening cannot be obtained, so that the tensile strength is less than 590 MPa.

  Since Comparative Example 5 has a finishing temperature of less than 880 ° C., the anisotropy of the steel sheet texture is strong and the hole expandability is inferior.

  In Comparative Example 8, since the cooling start time after finish rolling exceeds 1.0 seconds, the coarsening of the austenite structure progresses and the ferrite transformation is greatly delayed, so that the elongation and hole expansibility are inferior.

  In Comparative Example 12, since the intermediate air cooling time was less than 3 seconds and the ferrite transformation did not proceed sufficiently, the elongation and hole expansibility were inferior.

  In Comparative Example 16, the intermediate air cooling time exceeds 10 seconds, the bainite transformation proceeds, and the martensite structure fraction cannot be obtained, so that the elongation is inferior.

  In Comparative Example 17, since the intermediate air cooling temperature is less than 650 ° C., the ferrite structural fraction cannot be obtained, and the elongation and hole expansibility are inferior.

  Since the finishing temperature of Comparative Example 20 exceeds 1000 ° C., the ferrite transformation is delayed due to coarsening of the austenite structure, and the elongation and hole expansibility are inferior.

  In Comparative Example 22, the coiling temperature was higher than 200 ° C., and bainite was generated. Therefore, the tensile strength was less than 590 MPa, and the elongation and hole expansibility were inferior.

  In Comparative Example 24, the final step reduction was less than 20%, so that the martensite was coarsened and exceeded 10 μm, so that the hole expandability was inferior and the recrystallization of austenite was not sufficient. The anisotropy is strong and the hole expandability is inferior.

  In Comparative Example 29, the component composition of the steel sheet is less than 0.2% by mass of Al, so that the ferrite transformation does not proceed and the elongation and hole expansibility are inferior.

  In Comparative Example 30, the composition of the steel sheet was such that the mass% of Si was more than 0.1%, so that a large number of scale patterns were seen on the appearance, exceeding 10% of the total.

Claims (3)

% By mass C: 0.02% or more, 0.10% or less,
Si: 0.1% or less,
Mn: 0.5% or more, 2.0% or less,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.2% or more, 1.0% or less,
N: 0.01% or less,
Ti: 0.01% or more, 0.11% or less It has a chemical composition composed of the balance Fe and unavoidable impurities, the proportion of the structure is 90% or more of ferrite, and the particle size of martensite is 10 μm. In the following, the structural fraction is 1% or more and 10% or less, the X-ray random ratio of the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is 3.0 or less. High-strength hot-rolled steel sheet with excellent appearance and excellent balance between elongation and hole expansibility. Furthermore, by mass% Nb: 0.01% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0030% or less Mo: 0.02% or more, 0.5% or less,
Cr: 0.02% or more, 1.0% or less,
A high-strength hot-rolled steel sheet having an excellent appearance according to claim 1 and having an excellent balance between elongation and hole expansibility.   A finishing mill in which the steel having the chemical composition according to claim 1 or 2 is made into a slab by continuous casting, reheated to a temperature range of 1200 ° C or higher, and after rough rolling, a plurality of rolling mills are arranged in series. When performing continuous finish rolling in a row, the final finish rolling reduction is 20% or more, the final finish rolling finish temperature is 880 to 1000 ° C., the finish rolling is performed for 0.01 seconds or more after finishing the finish rolling. Start cooling within 0.0 seconds, cool to 600-750 ° C. at a cooling rate of 30 ° C./second or more, air cool for 3-10 seconds, and then cool to 200 ° C. or less at a cooling rate of 30 ° C./second or more. The method for producing a high-strength hot-rolled steel sheet having excellent appearance and excellent balance between elongation and hole expansibility according to claim 1 or 2, characterized by being wound up.