A heat treatment method for improving anisotropy of high-strength cold-rolled steel plate and high-strength cold-rolled steel plate

Abstract

The present invention relates to a heat treatment method for improving the anisotropy of a high-strength cold-rolled steel plate and a high-strength cold-rolled steel plate, wherein the final rolling temperature is controlled to be not less than Ar3+50°C in hot rolling, the cold rolling reduction ratio is controlled to be not less than 40% in cold rolling, and a uniform annealing step and a two-stage austenite isothermal quenching step are provided after cold rolling. The high-strength cold-rolled steel plate is also provided, and the yield strength of the steel plate finally obtained is above 650Mpa, and the structure is a fine and uniform bainite structure with a small amount of tempered martensite particles distributed therein, and the bending anisotropy index Ibend is less than 0.25. The heat treatment method of the present invention is suitable for improving the bending anisotropy of a variety of high-strength cold-rolled steel plates with a yield strength of more than 650MPa, and the composition of the high-strength cold-rolled steel plate suitable for reducing the bending anisotropy by the process of the present invention is limited to obtain a preferred low bending anisotropy effect.

Description

Technical Field

[0001] The invention relates to the field of steel, and in particular to a heat treatment method for improving the anisotropy of a high-strength cold-rolled steel plate and the obtained high-strength cold-rolled steel plate with low anisotropy. Background Art

[0002] The automobile industry uses a large number of cold-rolled steel sheets when producing cars. For the purpose of weight reduction and safety, a large number of cold-rolled high-strength steel sheets are used for the manufacture of automotive structural safety parts. The main deformation mode of high-strength steel is bending. However, due to the obvious anisotropy of high-strength steel sheets, when the bending ridge line is parallel to the rolling direction, the bending performance is significantly lower than when the bending ridge line is perpendicular to the rolling direction, which is not conducive to the application of materials.

[0003] The prior art for improving the anisotropy of high-strength steel plates found through novelty search includes:

[0004] Japanese patent P2007-9272A discloses a method for manufacturing a steel plate with low anisotropy. The method comprises the following steps: using C: 0.008-0.02%, Si≤0.02%, Mn: 0.15-0.25%, P≤0.02%, S≤0.015%, N≤0.0035%, Al: 0.065-0.2, Ti≤(Ti-(48 / 14)N-(48 / 32)S / ) / (48 / 12)C≤2.0, with an average grain size of less than 20 microns, -0.2≤△r≤0.2; hot rolling heating at 1050-1300℃, hot rolling at Ar3 or above, cold rolling at 80-90% after pickling, annealing and leveling at a recrystallization temperature of -850℃. The purpose of the patent is to control the plane anisotropy △r, and the main means is to control the Al content.

[0005] Japanese patent P2009-132981A discloses a method for manufacturing a 440MPa (TS) cold-rolled steel sheet with low elongation anisotropy. C: 0.03-0.2%, Si≤1.5%, Mn: 1-2.5%, P: 0.005-0.1%, S≤0.01%, N≤0.01%, Al: 0.005-1.5%. The steel sheet contains 85-99% ferrite and 1-15% martensite. The process used: heating the slab at more than 1000℃, hot rolling at more than 850℃, and coiling at 600-750℃. The cold rolling reduction rate is between 60-90%; annealing between 800-950℃, slow cooling to 500℃, and then cooling at a cooling rate of ≥3℃ / s. The elongation is expressed by |△El|=|El0-2EL45+EL90| / 2, and can be controlled between 0-2.

[0006] Japanese patent P2010-156016A discloses a method for manufacturing hot-rolled steel plates with a small toughness anisotropy of more than 950MPa (TS). The steel is made of C: 0.08-0.15%, Si: 0.3-1.5%, Mn: 1.5-2.5%, P: ≤0.01%, S≤0.004%, N≤0.01%, Al: 0.01-0.05%, Ti: 0.03-0.15%, B: 0.0003-0.001%, O≤0.005%. The hot rolling is carried out at Ar3 or above, and the steel is coiled after cooling to below 100℃ at 30℃ / s. The impact toughness of the steel in L direction / impact toughness in C direction is between 0.9-1.3.

[0007] Japanese patent P2012-172159A discloses a method for manufacturing high-strength cold-rolled steel sheets with uniform deformation ability and strong local deformation ability. C: 0.01-0.4%, Si: 0.001-2.5%, Mn: 0.001-4%, P: 0.001-0.15%, S: 0.0005-0.03%, N: 0.0005-0.01%, Al: 0.001-2%, O: 0.0005-0.01%. Hot rolling is performed with Ar3 or above, and coiling is performed after cooling to below 100°C at 30°C / s. The steel is pickled after hot rolling, cold rolled by more than 30%, annealed at 750-900℃, cooled to between 580℃-720℃ at 12℃ / s, then secondary cooled to between 200-600℃ at 4-300℃ / s, and then over-aged at below 600℃ to finally obtain the required steel plate.

[0008] Japanese patent P2014-5514A discloses a method for manufacturing cold-rolled hot-dip galvanized steel sheets with good fatigue performance and small ductility anisotropy of 780MPa (TS) or above. C: 0.05-0.2%, Si: 0.8-2.0%, Mn: 2-3%, P: 0.001-0.05%, S: 0.0001-0.01%, N: 0.0005-0.01%, Al: 0.001-0.3%, Ti: 0.005-0.05%, B: 0.0003-0.005%, O≤0.005%. The steel is heated at 1050℃ or above, hot rolled at 800℃ or above, cooled to below 600℃ at a cooling rate of 50℃ / s, and coiled at 400-550℃. After pickling, cold rolling of more than 30% is carried out, hot-dip galvanizing is carried out by heating to 580-720℃ at 5℃ / s, heating to 750-900℃ at <2℃ / s and holding for 15-600s, cooling to 550-750℃ at >5℃ / s, and then cooling to 450-550℃ for galvanizing. The average grain diameter of the steel is less than 10 microns, |ARL-2ARD+ARC| / 2<0.2, |ELL-2ELD+ELC| / 2<1.0, |UELL-2UELD+UELC| / 2<0.6.

[0009] The inventions for improving the anisotropy of steel plates in the above-mentioned prior art involve strength, elongation, grain size, △r value, etc., and none of them involve the anisotropic improvement of the bending properties of high-strength steel. Summary of the invention

[0010] The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art and provide a heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheets and a low-anisotropy cold-rolled steel sheet obtained based on the method. The heat treatment method of the present invention and the obtained cold-rolled steel sheet eliminate or reduce the adverse effects of bending anisotropy during the manufacture of high-strength steel through reasonable process design, reduce the anisotropy of the steel sheet, and thus improve the overall forming performance of the steel sheet.

[0011] In order to achieve the above invention purpose, the technical solution provided by the present invention is as follows:

[0012] A heat treatment method for improving the anisotropy of a high-strength cold-rolled steel plate, the heat treatment method comprising hot rolling, pickling and cold rolling, wherein the final rolling temperature in the hot rolling is controlled to be not less than Ar3+50°C, the cold rolling reduction ratio in the cold rolling is controlled to be not less than 40%, and a uniform annealing step and a two-stage austenite isothermal quenching step are further provided after the cold rolling;

[0013] In the uniform annealing step, after two-stage medium-temperature long-term heating and heat preservation treatment, uniform and fine grain distribution and cementite distribution, as well as low aspect ratio of grains and cementite are obtained, the aspect ratio of grains is controlled between 1-1.2, and the aspect ratio of cementite is controlled between 1-1.3; the aspect ratio is the ratio of the maximum diameter to the minimum diameter of the nearly circular grains or carbides;

[0014] In the two-stage austenite isothermal quenching process, the steel plate is rapidly heated to the full austenite temperature range, kept warm for a short time, then rapidly cooled, then subjected to medium-temperature isothermal quenching, and finally cooled to room temperature;

[0015] The final steel plate has a yield strength of more than 650 MPa, and its structure is a fine and uniform bainite structure with a small amount of tempered martensite particles distributed therein, the volume fraction of the tempered martensite particles is less than or equal to 10%, and the bending anisotropy index Ibend is less than 0.25, wherein the bending anisotropy index Ibend = (the 180-degree inner bend angle diameter of the bending ridge line parallel to the rolling direction - the 180-degree inner bend angle diameter of the bending ridge line perpendicular to the rolling direction) / the 180-degree inner bend angle diameter of the bending ridge line perpendicular to the rolling direction.

[0016] In the above-mentioned heat treatment method for improving the anisotropy of high-strength cold-rolled steel plates, in the uniform annealing step, the cold-rolled unannealed steel coil is heated, and a protective atmosphere of nitrogen or hydrogen is used, and the temperature is raised to between 650-720°C and kept warm for 1-3 hours; then it is cooled to between 500-600°C and kept warm for 10-20 hours. After the insulation is completed, it is cooled with the furnace for more than 12 hours, and the heat treatment is terminated when the furnace temperature drops below 200°C.

[0017] In the above-mentioned heat treatment method for improving the anisotropy of high-strength cold-rolled steel plates, in the uniform annealing step, the uniform and fine ferrite grain distribution and cementite distribution obtained are, under ideal conditions, the ferrite grain diameter is 5-15 microns, and the cementite particle diameter is not greater than 1 micron.

[0018] In the above-mentioned heat treatment method for improving the anisotropy of high-strength cold-rolled steel plate, in the two-stage austenite isothermal quenching link, the steel plate is heated to the full austenite temperature range at a heating rate of ≥25°C / s, kept warm for a short time of 10-60s, and then quickly cooled at a cooling rate of ≥30°C / s, cooled to 360-460°C for isothermal quenching. γ is prone to bainite transformation in the temperature range of 360-460°C. The holding time in this temperature range is set to 500-1500s. After the isothermal quenching is completed, it is cooled to below 100°C, and then heated to the temperature range of 200-350°C, kept warm for more than 500s, and cooled to room temperature after the insulation is completed.

[0019] In the above heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheet, in the final steel sheet structure, the average grain diameter of the bainite is not greater than 10 microns.

[0020] In the above heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheet, the components and percentages of the steel sheet are as follows:

[0021] C: 0.08∽0.30wt%: Mn: 1.0∽3.0wt% Cr: 0-0.7%,

[0022] P: ≤0.02wt% S: ≤0.003wt% Al: 0.02-0.08% N: ≤0.005wt%, and also contains one or more of Nb, V, and Ti, the content of which does not exceed 0.1%,

[0023] The rest is composed of Fe and inevitable impurities.

[0024] In the above heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheet, 0.1-0.4% Mo or 0.7-1.8% Si, or both, may be added to the steel sheet, but the total amount does not exceed 1.8%.

[0025] A low anisotropy high-strength cold-rolled steel plate, the composition and percentage of the steel plate are as follows:

[0026] C: 0.08∽0.30wt%: Mn: 1.0∽3.0wt% Cr: 0-0.7%,

[0027] P: ≤0.02wt% S: ≤0.003wt% Al: 0.02-0.08% N: ≤0.005wt%, and also contains one or more of Nb, V, and Ti, the content of which does not exceed 0.1%,

[0028] The rest is composed of Fe and unavoidable impurities;

[0029] 0.1-0.4% Mo or 0.7-1.8% Si may be added to the steel plate, or both may be added at the same time, but the total amount shall not exceed 1.8%;

[0030] The yield strength of the steel plate after heat treatment reaches more than 650Mpa, and its structure is fine and evenly distributed bainite structure and a small amount of tempered martensite particles. The average grain diameter of the bainite is not more than 10 microns, the volume fraction of the tempered martensite particles is less than or equal to 10%, and the bending anisotropy index Ibend is less than 0.25.

[0031] The above-mentioned bending anisotropy index Ibend is defined as (180-degree inner bend angle diameter with the bending ridge line parallel to the rolling direction - 180-degree inner bend angle diameter with the bending ridge line perpendicular to the rolling direction) / 180-degree inner bend angle diameter with the bending ridge line perpendicular to the rolling direction.

[0032] Based on the above technical solution, the heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheets and the low-anisotropy high-strength cold-rolled steel sheets of the present invention have the following technical advantages compared with the prior art:

[0033] 1. The present invention can effectively reduce the bending anisotropy of high-strength steel plates through two-stage homogenization annealing and two-stage austenite isothermal quenching + tempering process design, thereby improving the overall forming performance of the steel plates. In the homogenization annealing process, the insulation temperature of stage I is relatively high and the time is relatively short, and the insulation time in the middle temperature zone of stage II is relatively long, which is conducive to obtaining fine ferrite grains and spheroidal carbides that are superior to ordinary homogenization annealing. In the isothermal quenching stage, a two-stage process of isothermal quenching plus tempering is selected to ensure as sufficient bainite transformation as possible and avoid the presence of high-hardness martensite. After the above treatment, the steel plate has good uniformity of organization, and the organization is mainly composed of low-carbon bainite with high toughness and a small amount of tempered martensite, showing excellent performance and low anisotropy.

[0034] 2. The process of the present invention is applicable to high-strength steel with a wide range of composition designs, but its effect is related to the composition design to a certain extent. The composition suitable for reducing bending anisotropy in the process of the present invention is limited to obtain the preferred low bending anisotropy effect. The composition of the steel plate must ensure that the strength reaches more than 650MPa after heat treatment in the process of the present invention. Therefore, the types and contents of C, Mn, Cr, Si, and Mo in the alloying elements must be sufficient to ensure the necessary strength and organizational type after heat treatment. The C content cannot be too high, otherwise low-carbon bainite and low-carbon martensite cannot be formed.

[0035] 3. The cold-rolled steel sheet or steel strip of the present invention can be austempered for a long time to make the microstructure grow slowly after nucleation, small and uniform, and there is no obvious banded structure in the steel sheet. The final process requires low-temperature tempering of the austempered steel coil to improve the toughness of a small amount of martensite that may be formed during the final cooling process after austempering. The final steel sheet exhibits lower bending anisotropy, and the difference in its transverse and longitudinal bending performance is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of two-stage heat preservation in a heat treatment method for improving the anisotropy of a high-strength cold-rolled steel plate according to the present invention.

[0037] Figure 2 It is a schematic diagram of austenite isothermal quenching in a heat treatment method for improving the anisotropy of a high-strength cold-rolled steel plate of the present invention. DETAILED DESCRIPTION

[0038] Hereinafter, we will further elaborate on a heat treatment method for improving the anisotropy of a high-strength cold-rolled steel sheet and a high-strength cold-rolled steel sheet with low anisotropy obtained according to the present invention in combination with the accompanying drawings and specific embodiments, so as to more clearly understand the operation process and component composition of the present invention. However, this shall not limit the scope of protection of the present invention.

[0039] Under existing process conditions, the following process is usually adopted to manufacture cold-rolled steel sheets or steel strips:

[0040] After hot rolling, the continuous casting slab is stretched to form a banded structure, which is retained after cold rolling; the banded structure is retained after conventional annealing, and the steel plate has a large anisotropy, which is manifested as the bending transverse direction is significantly lower than the longitudinal direction. Taking the steel plate with yield strength ≥650MPa and tensile strength ≥980MPa as an example, when the bending ridge is parallel to the rolling direction, the minimum bending diameter (inner) of 180-degree bending is 3.5 times the plate thickness, and when the bending ridge is perpendicular to the rolling direction, the minimum bending diameter (inner) of 180-degree bending is 2.5 times the plate thickness.

[0041] The anisotropy index formula is defined as follows:

[0042] Anisotropy index Ibend = (180-degree inner bend diameter with the bending ridge line parallel to the rolling direction - 180-degree inner bend diameter with the bending ridge line perpendicular to the rolling direction) / 180-degree inner bend diameter with the bending ridge line perpendicular to the rolling direction

[0043] According to the above calculation formula, the bending anisotropy index of the high-strength steel plate is calculated to be Ibend = (3.5-2.5) / 2.5 = 0.4. In the case of obvious bending anisotropy of the steel plate, its formability is affected by the short plate with poor performance, which is not conducive to the overall formability of the steel plate.

[0044] The present invention firstly relates to a heat treatment method for improving the anisotropy of high-strength cold-rolled steel plates, attempting to eliminate or reduce the adverse effects of bending anisotropy during the manufacture of high-strength steel through reasonable process design, reduce the anisotropy of the steel plates, and thus improve the overall forming performance of the steel plates.

[0045] The heat treatment method of the present invention comprises hot rolling, pickling and cold rolling, wherein the final rolling temperature is controlled to be not less than Ar3+50°C in the hot rolling, the cold rolling reduction ratio is controlled to be not less than 40% in the cold rolling, and a uniform annealing step and a two-stage austenite isothermal quenching step are further provided after the cold rolling:

[0046] In the uniform annealing step, after two-stage medium-temperature long-term heating and heat preservation treatment, a uniform and fine grain distribution and cementite distribution, as well as a lower aspect ratio of the grains and cementite are obtained. In the uniform and fine ferrite grain distribution and cementite distribution obtained here, under ideal conditions, the ferrite grain diameter is 5-15 microns, and the cementite particle diameter is not greater than 1 micron. The lower aspect ratio is defined as: the grain aspect ratio is controlled between 1-1.2, and the cementite aspect ratio is controlled between 1-1.3. In the uniform annealing step, when heating the cold-rolled unannealed steel coil, a protective atmosphere of nitrogen or hydrogen is used, the temperature is raised to between 650-720°C, and the temperature is kept for 1-3 hours; then it is cooled to between 500-600°C and heat-insulated for 10-20 hours. After the insulation is completed, it is cooled with the furnace for more than 12 hours, and the heat treatment of this step is terminated when the furnace temperature drops below 200°C.

[0047] The main purpose of the above homogenization heat treatment process is to fully diffuse the alloy elements in the steel by keeping the temperature in the recrystallization temperature range, obtain a uniform and fine distribution of recrystallized grains and cementite, and obtain a lower aspect ratio of the grains and cementite. Figure 1As shown, the high holding temperature in stage I is to obtain a higher recrystallization nucleation rate, and the short holding time is to avoid the long-term excessive growth of grains and the occurrence of obvious ripening effect. The ripening effect is that large grains eat small grains, resulting in excessive grains, large size differences and increased aspect ratio. In this holding stage, if the holding temperature is low, it is not conducive to improving the nucleation rate. If the holding temperature is too high, it will exceed the A1 temperature, resulting in phase change. If the holding time is short, the internal temperature of the steel coil cannot meet the requirements. If the holding time is too long, it will cause excessive growth of grains. In the second stage, the medium temperature zone is selected to ensure that the grains grow fully at a lower growth rate and eventually form fine grains. In this stage, if the holding temperature is too low, the equiaxed crystals are not well developed, the rolled hard structure is not completely eliminated, the grain aspect ratio is too high, and it is not conducive to the full homogenization and spheroidization of cementite. If the holding temperature is high, it is easy to cause the grain size to be too large, which is not conducive to the aspect ratio. If the holding time is short, it is not conducive to the full homogenization and spheroidization of the cementite. If the holding time is too long, it will waste energy.

[0048] In the two-stage austenite isothermal quenching process, the steel plate is rapidly heated to the full austenite temperature range, kept warm for a short time, then quickly cooled, and then medium-temperature isothermal quenching is performed, and finally cooled to room temperature. In the two-stage austenite isothermal quenching process, when further implemented, the steel plate is heated to the full austenite temperature range at a heating rate of ≥25℃ / s, kept warm for a short time of 10-60s, and then quickly cooled at a cooling rate of ≥30℃ / s, cooled to 360-460℃ for isothermal quenching, and kept warm in the temperature range of 360-460℃ where γ to bainite transformation is prone to occur. The holding time is set to 500-1500s. After the isothermal quenching is completed, it is cooled to below 100℃, and then heated to the temperature range of 200-350℃, kept warm for more than 500s, and cooled to room temperature after the holding is completed. The final steel plate structure is a fine and uniform bainite structure. Fine means that the average grain diameter of the bainite is not more than 10 microns. A small amount of tempered martensite particles are also distributed in the final steel plate structure. A small amount of tempered martensite particles means that the volume fraction of the tempered martensite particles in the structure is less than or equal to 10%. The performance is manifested as low bending anisotropy, and its transverse and longitudinal bending properties are basically the same, and the bending anisotropy index Ibend is less than 0.25.

[0049] In the above two-stage austenite isothermal quenching process, the steel plate is heated to the full austenite temperature range at a heating rate of ≥20℃ / s, and is kept warm for a short time of 10-60s. A low heating rate is not conducive to improving the austenite nucleation rate. If the holding time is too short, the grain growth will be uneven, and if the holding time is too long, it will easily lead to grain growth; rapid cooling at a cooling rate of ≥30℃ / s is to ensure that the austenite does not decompose into ferrite and pearlite. For isothermal quenching, 360-460℃ and 500-1500s are selected. The temperature range is selected in the temperature range where γ to bainite transformation is easy to occur. Too high or too low will reduce the transformation speed; the holding time is 500-1500s to ensure that as much bainite as possible is formed. If the time is short, the phase transformation is not sufficient. If the time is long, the phase transformation enters the S curve platform area, and the effect is not obvious. The final tempering treatment uses a temperature range of 200-350℃, and the holding time is controlled to be more than 500s, which can fully temper the newly formed martensite during the cooling process, thereby reducing the hardness. If the temperature is too low, the martensite toughness is poor, and if the temperature is too high, the strength of the steel plate will drop too significantly.

[0050] Based on the above process, the steel plate finally obtained by the present invention has a yield strength of more than 650Mpa, and its structure is a fine and uniform bainite structure. Fine means that the average grain diameter of bainite is not more than 10 microns. A small amount of tempered martensite particles are also distributed in the steel plate structure. A small amount means that the volume fraction of the tempered martensite particles in the structure is less than or equal to 10%. The bending anisotropy index Ibend is less than 0.25, and the bending anisotropy index Ibend = (180-degree inner bend angle diameter of the bending ridge parallel to the rolling direction - 180-degree inner bend angle diameter of the bending ridge perpendicular to the rolling direction) / 180-degree inner bend angle diameter of the bending ridge perpendicular to the rolling direction. The steel plate finally obtained exhibits lower bending anisotropy, and the difference in its transverse and longitudinal bending performance is greatly reduced.

[0051] In the above heat treatment method for improving the anisotropy of high-strength cold-rolled steel sheet,

[0052] The process of the present invention is applicable to high-strength steels with a wide range of composition designs, but its effect is related to the composition design. Therefore, the present invention limits the composition suitable for reducing bending anisotropy by the process of the present invention to obtain a preferred low bending anisotropy effect.

[0053] The present invention also provides a low anisotropy high-strength cold-rolled steel plate, which is prepared by the above-mentioned heat treatment method. The composition and percentage of the high-strength cold-rolled steel plate are as follows:

[0054] C: 0.08∽0.30wt%: Mn: 1.0∽3.0wt% Cr: 0-0.7%,

[0055] P: ≤0.02wt% S: ≤0.003wt% Al: 0.02-0.08% N: ≤0.005wt%, and also contains one or more of Nb, V, and Ti, with the content not exceeding 0.1%, and the rest is Fe and unavoidable impurities.

[0056] 0.1-0.4% Mo or 0.7-1.8% Si, or both, can be added to the above steel plate, but the total amount shall not exceed 1.8%. If the Mo content is lower than 0.1%, the strength of the steel will decrease rapidly during tempering, and the effect will not be obvious; if the Mo content is higher than 0.25%, the hardenability of the steel will be too high, the yield strength will be too high, and the elongation will be low. If the Si content is lower than 0.7%, the strength of the steel will decrease rapidly during tempering; if the Si content is higher than 1.8%, the surface oxide scale of the steel will be too heavy and difficult to handle. The same is true for adding Mo and Si at the same time.

[0057] After the high-strength cold-rolled steel sheet containing the above-mentioned components undergoes the above-mentioned heat treatment process, the steel sheet finally obtained has a yield strength of more than 650Mpa, and its structure is a fine and uniform bainite structure with a small amount of tempered martensite particles distributed therein, the above-mentioned fine means that the average diameter of the bainite grains is not greater than 10 microns, and the small amount means that the volume fraction of the tempered martensite particles in the structure is less than or equal to 10%. The bending anisotropy index Ibend of the steel sheet finally obtained is less than 0.25, and the bending anisotropy index Ibend=(180-degree inner bend angle diameter of the bending ridge line parallel to the rolling direction-180-degree inner bend angle diameter of the bending ridge line perpendicular to the rolling direction) / 180-degree inner bend angle diameter of the bending ridge line perpendicular to the rolling direction.

[0058] It can be seen that the steel plate with the above-mentioned recommended composition can obtain a yield strength of more than 650Mpa after being processed by the above-mentioned specific process. At the same time, good bending anisotropy is also obtained.

[0059] Example

[0060] Table 1 shows the chemical composition of the embodiment suitable for using the present method to improve bending anisotropy. Comparative Example 1 also meets the composition range recommended by the present invention, but uses conventional processing technology, neither homogenization annealing nor two-stage isothermal quenching, but conventional hot rolling, cold rolling and quenching + tempering process. Table 1 Chemical composition of the embodiment, wt%

[0061]

[0062] The process examples of the present invention are shown in Table 2 and Table 3, where Table 2 is an example of homogenization annealing; Table 3 is an example of austenite isothermal quenching. The experimental steel was smelted, hot rolled, pickled and cold rolled, and then processed by the processes shown in Table 2 and Table 3 to finally obtain the mechanical properties shown in Table 5. It can be seen that after being processed by the process of the present invention, the bending anisotropy index Ibend of the embodiments is reduced to less than 0.3, while the Ibend value of the comparative example is 0.4.

[0063] Table 2 shows the key process parameters of homogenization annealing in the heat treatment process of the present invention.

[0064]

[0065] Table 3 is the key process parameters of two-stage austenite isothermal quenching in the heat treatment process of the present invention

[0066]

[0067] Table 4 shows the key process parameters of quenching and tempering of Comparative Example 1

[0068]

[0069] Table 5 Performance of experimental steel after treatment by the process of the present invention

[0070]

[0071]

Claims

1. A heat treatment method for improving the anisotropy of a high-strength cold-rolled steel sheet, the heat treatment method comprising hot rolling, pickling and cold rolling, characterized in that: In the hot rolling, the final rolling temperature is controlled to be not less than Ar3+50°C, and in the cold rolling, the cold rolling reduction ratio is controlled to be not less than 40%. After the cold rolling, a uniform annealing step and a two-stage austenite isothermal quenching step are provided; In the uniform annealing step, the cold-rolled unannealed steel coil is heated through a two-stage medium-temperature long-term heating and heat preservation treatment, and the temperature is raised to 650-720°C in a nitrogen or hydrogen protective atmosphere, and the temperature is kept for 1-3 hours; then the temperature is cooled to 500-600°C for 10-20 hours of heat preservation treatment, and after the heat preservation is completed, the furnace is cooled for more than 12 hours, and the heat treatment is terminated when the furnace temperature drops below 200°C, so as to obtain a uniform and fine grain distribution and cementite distribution, as well as a lower aspect ratio of the grains and cementite: the aspect ratio of the grains is controlled between 1.0-1.2, and the aspect ratio of the cementite is controlled between 1.0-1.3; In the two-stage austenite isothermal quenching process, the steel plate is heated to the full austenite temperature range at a heating rate of ≥25°C / s, kept warm for 10-60s, and then quickly cooled at a cooling rate of ≥30°C / s to 360-460°C for isothermal quenching. In the temperature range of 360-460°C where austenite to bainite transformation is prone to occur, the temperature is kept warm for 500-1500s. After the isothermal quenching is completed, the steel plate is cooled to below 100°C, and then heated to the temperature range of 200-350°C, kept warm for more than 500s, and cooled to room temperature after the insulation is completed. The yield strength of the steel plate finally obtained reaches more than 650MPa, and its structure is a fine and evenly distributed bainite structure and a small amount of tempered martensite particles, the volume fraction of the tempered martensite particles is less than or equal to 10%, and the bending anisotropy index Ibend is less than 0.25; The bending anisotropy index Ibend=(180-degree inner bend angle diameter when the bending ridge line is parallel to the rolling direction-180-degree inner bend angle diameter when the bending ridge line is perpendicular to the rolling direction) / 180-degree inner bend angle diameter when the bending ridge line is perpendicular to the rolling direction.

2. A heat treatment method for improving anisotropy of high-strength cold-rolled steel sheet according to claim 1, characterized in that: In the uniform annealing step, uniform and fine ferrite grain distribution and cementite distribution are obtained, the ferrite grain diameter is 5-15 microns, and the cementite particle diameter is not greater than 1 micron.

3. A heat treatment method for improving anisotropy of high-strength cold-rolled steel sheet according to any one of claims 1 or 2, characterized in that: The composition and percentage of the steel plate are as follows: C: 0.08~0.30wt%, Mn: 1.0~3.0wt%, Cr: 0~0.7wt%, P: ≤0.02wt%, S: ≤0.003wt%, Al: 0.02-0.08wt%, N: ≤0.005wt%, and also contains one or more of Nb, V, and Ti, with the content not exceeding 0.1wt%, The rest is composed of Fe and inevitable impurities.

4. A heat treatment method for improving anisotropy of high-strength cold-rolled steel sheet according to claim 3, characterized in that: 0.1-0.4wt% Mo or 0.7-1.8wt% Si is also added to the steel plate, or both are added at the same time, but the total amount does not exceed 1.8wt%.

5. A heat treatment method for improving anisotropy of high-strength cold-rolled steel sheet according to claim 1, characterized in that: In the finally obtained steel plate structure, the average grain diameter of the bainite is not greater than 10 μm.

6. A low anisotropy high strength cold rolled steel sheet manufactured by the heat treatment method according to any one of claims 1 or 2, characterized in that: The composition and percentage of the steel plate are as follows: C: 0.08~0.30wt%, Mn: 1.0~3.0wt%, Cr: 0~0.7wt%, P: ≤0.02wt%, S: ≤0.003wt%, Al: 0.02-0.08wt%, N: ≤0.005wt%, and also contains one or more of Nb, V, and Ti, with the content not exceeding 0.1wt%, The rest is composed of Fe and unavoidable impurities; The steel plate further contains 0.1-0.4wt% Mo or 0.7-1.8wt% Si, or both, but the total amount does not exceed 1.8wt%; The yield strength of the steel plate after heat treatment reaches more than 650MPa, and its structure is fine and evenly distributed bainite structure and a small amount of tempered martensite particles. The grain diameter of the bainite is not greater than 10 microns, the volume fraction of the tempered martensite particles is less than or equal to 10%, and the bending anisotropy index Ibend is less than 0.25.

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