CN111876676B - A production method of cold-rolled steel for 1800MPa grade hot stamping forming - Google Patents
A production method of cold-rolled steel for 1800MPa grade hot stamping forming Download PDFInfo
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- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 28
- 238000004519 manufacturing process Methods 0.000 title claims description 21
- 238000000137 annealing Methods 0.000 claims abstract description 70
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 49
- 239000010959 steel Substances 0.000 claims abstract description 49
- 238000000034 method Methods 0.000 claims abstract description 35
- 238000003466 welding Methods 0.000 claims abstract description 16
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- 239000000126 substance Substances 0.000 claims abstract description 7
- 239000012535 impurity Substances 0.000 claims abstract description 6
- 238000005554 pickling Methods 0.000 claims abstract description 6
- 238000001816 cooling Methods 0.000 claims description 51
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- 229910052739 hydrogen Inorganic materials 0.000 claims description 29
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- 238000005096 rolling process Methods 0.000 claims description 19
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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Abstract
本发明公开了一种1800MPa级热冲压成形用冷轧钢，按重量百分比由以下化学成分组成：C:0.25～0.35%；Si:0.20～0.40%；Mn:1.10～1.45%；P:≤0.020%；S:≤0.012%；Als:0.030～0.060%；Cr:0.25～0.60%；Ti:0.030～0.060%；B:0.0025～0.0055%；其余为Fe和不可避免杂质。其工艺路线为：原料钢带热轧→一次罩式炉退火→连续闪光对焊→酸洗→冷连轧→二次罩式炉退火→平整→成品。其热冲压成形后屈服强度为1359～1369MPa，抗拉强度1821～1846MPa，断后伸长率6.5～6.0%的1800MPa级热冲压成形用冷轧钢带，满足1800MPa级热冲压成形用钢带的使用要求，具有低成本、高效率的特点。
The invention discloses a 1800MPa grade cold-rolled steel for hot stamping forming, which is composed of the following chemical components according to weight percentage: C: 0.25-0.35%; Si: 0.20-0.40%; Mn: 1.10-1.45%; P: ≤ 0.020 %; S: ≤0.012%; Als: 0.030～0.060%; Cr: 0.25～0.60%; Ti: 0.030～0.060%; B: 0.0025～0.0055%; the rest are Fe and inevitable impurities. The process route is: hot rolling of raw steel strip → annealing in a bell furnace → continuous flash butt welding → pickling → continuous cold rolling → annealing in a second bell furnace → flattening → finished product. The yield strength after hot stamping is 1359-1369MPa, the tensile strength is 1821-1846MPa, and the elongation after breaking is 6.5-6.0%. It has the characteristics of low cost and high efficiency.
Description
Technical Field
The invention belongs to the technical field of cold-pressure high-strength steel production, and particularly relates to a production method of cold-rolled steel for 1800 MPa-level hot stamping forming.
Background
With the continuous development of the automobile industry towards light weight, the research on stronger and lighter materials such as advanced high-strength steel and high-strength alloy for automobiles is also deepened continuously. When the tensile strength of the cold-rolled high-strength steel for the automobile exceeds 1000MPa, the plastic deformation range of the steel becomes small, and the formability is poor; and the required stamping force in the processing process is large, the requirement on stamping equipment is high, and the production cost is higher. When the part is pressed and deformed at room temperature, the part is easy to crack and wrinkle, the rebound of the formed part is serious, and the like, and especially when the strength reaches more than 1300MPa, the conventional cold forming process can hardly be used.
Compared with the continuous development of cold-forming high-strength steel products, the continuous maturity of the hot forming technology also provides a new choice for processing the steel with rigid strength. The existing common hot forming steel is 22MnB5 steel, the strength after quenching can reach 1500MPa, and the total elongation rate reaches 5-6%. However, the cold-rolled steel strip for 1800MPa hot stamping is still relatively lacking at present, mainly the strength of the hot-rolled steel strip for 1800MPa hot stamping can reach 700-900 MPa, the plasticity is low, and the cold rolling is difficult to carry out. 1800 MPa-grade hot rolled or pickled steel strips are produced by part of steel mills, and compared with cold rolled steel strips, the hot rolled steel strips have the characteristics of narrow producible thickness range, low dimensional tolerance precision of finished products and poor surface quality.
Chinese invention patent CN110499460A discloses a cold-rolled steel strip for hot stamping forming and a production method thereof, wherein the chemical components of the cold-rolled steel strip comprise, by weight, 0.32-0.36 Wt% of C; 0.10-0.30 Wt% of Si; 1.30-1.50 Wt% of Mn; p is less than or equal to 0.020 Wt%; s is less than or equal to 0.005 Wt%; 0.020-0.060 Wt% of Als; 0.020-0.040 Wt% of Nb; 0.020-0.040 Wt% of Ti; b, 0.0015-0.0025 Wt%; 0.20-0.40 Wt% of Cr; n is less than or equal to 0.060 Wt%. The cold-rolled steel strip contains 0.020-0.040 Wt% of Nb, has high alloy cost, and has extremely high strength after hot rolling and high cold-rolling difficulty. However, the patent does not disclose the key processes and equipment of cold rolling welding, pickling and rolling.
Disclosure of Invention
According to the invention, through researches on acid pickling cold continuous rolling and two annealing processes of the cold-rolled steel strip for 1800 MPa-level hot stamping forming of a 27MnCrB5 component system, the cold-rolled steel strip for 1800 MPa-level hot stamping forming and the acid pickling cold continuous rolling and annealing process method thereof are provided, and the industrial production of the cold-rolled steel strip for 1800 MPa-level hot stamping forming by adopting a CSP process and a bell-type furnace process is realized.
Therefore, the invention adopts the following technical scheme:
the 1800 MPa-grade cold rolled steel for hot stamping forming comprises the following chemical components in percentage by weight: 0.25-0.35% of C; 0.20 to 0.40 percent of Si; 1.10 to 1.45 percent of Mn; p is less than or equal to 0.020%; s is less than or equal to 0.012 percent; 0.030-0.060% of Als; 0.25 to 0.60 percent of Cr; 0.030 to 0.060% of Ti; 0.0025-0.0055% of B; the balance of Fe and inevitable impurities.
Further, preferably, the cold-rolled steel consists of the following chemical components in percentage by weight: 0.27 to 0.32 percent of C; 0.20 to 0.35 percent of Si; 1.15 to 1.35 percent of Mn; p is less than or equal to 0.020%; s is less than or equal to 0.012 percent; 0.020-0.050% of Als; 0.30 to 0.55 percent of Cr; 0.035 to 0.050 percent of Ti; 0.0030-0.0050% of B; the balance of Fe and inevitable impurities.
Further, the yield strength of the cold-rolled steel ranges from 529 MPa to 541MPa, the tensile strength ranges from 656 MPa to 667MPa, and the elongation A after fracture50mm22.0 to 23.0%.
Further, the thickness of the cold-rolled steel is 1.20-1.50 mm.
The production method of the cold-rolled steel for 1800 MPa-level hot stamping forming comprises the following process steps:
s1, hot rolling the raw material steel strip, namely preheating the raw material steel strip and hot rolling the raw material steel strip;
s2, primary bell-type furnace annealing is carried out, the annealing temperature and the hot spot are 726-728 ℃ and 731-734 ℃ respectively, the heat preservation time is 17-19 h, and a hydrogen atmosphere is adopted in the annealing process;
s3 continuous flash butt welding, wherein the upsetting length is 3.2-3.6 mm, the upsetting force is 600-700 KN, the upsetting time is 0.2-0.3S, the annealing heating time is 10-15S, and the annealing cooling time is 12-18S;
s4 acid washing for removing the oxide layer;
s5, cold continuous rolling, wherein the total rolling reduction rate of the cold continuous rolling is 45-60%;
s6, secondary bell-type furnace annealing is carried out, the annealing temperature cold and hot points are 672-676 ℃ and 696-699 ℃ respectively, the heat preservation time is 17-19 h, and a hydrogen atmosphere is adopted in the annealing process;
s7, flattening to make the thickness of the finished product be 1.20-1.60 mm.
Further, in the primary bell-type furnace annealing, the heating rate of the temperature rising section from room temperature to the hot spot temperature of 650 ℃ is 79.7-80.0 ℃/h, the heating rate of 650-700 ℃ is 33.6-33.9 ℃/h, and the heating rate of 700-740 ℃ is 24.8-25.0 ℃/h; and when the temperature is lower than 580 ℃, air cooling full speed cooling is adopted, when the temperature is lower than 380 ℃, water cooling full speed cooling is adopted, and the tapping temperature is 95-98 ℃.
Furthermore, the cold continuous rolling adopts five frames, the first frame reduction rate is 13.5-13.8%, the second frame reduction rate is 15.2-16.2%, the third frame reduction rate is 9.9-10.4%, the fourth frame reduction rate is 8.4-8.8%, and the fifth frame reduction rate is 1.8-1.9%.
Further, in the secondary bell-type furnace annealing, full-speed heating is carried out in a temperature rising section, the heat preservation time is 17-19 h, and a cooling stage is carried out after the heat preservation is finished; when the temperature is lower than 580 ℃, air cooling is adopted for full speed cooling; and (3) cooling at the temperature of less than 380 ℃ by adopting water cooling at full speed, wherein the tapping temperature is 47-49 ℃.
The 1800 MPa-grade hot stamping forming steel strip has the advantages that the strength can reach 700-900 MPa after hot rolling, the plasticity is low, and the carbon equivalent Ceq generally reaches 0.49-0.73%, so that the cold rolling deformation resistance is large, the welding performance is poor, and the stable cold rolling industrial production is difficult to realize. Therefore, the first softening annealing is needed, the strength of the strip steel is reduced, the cold rolling deformation resistance is reduced, cementite is uniformly distributed, the welding performance is improved, and the stable operation of the cold rolling industrial production is ensured. The 1800 MPa-level hot stamping forming steel strip after cold rolling is subjected to work hardening, the steel strip is in a cold hardening state, has higher strength, poorer plasticity and no machinability, and is fibrous in structure, free of obvious crystal grain appearance, and highly densely distributed with a large amount of dislocation and defects. Therefore, a second recrystallization annealing is required to perform recrystallization processes such as re-nucleation and growth of crystal grains, so as to obtain equiaxed crystal grains with uniform structures. Therefore, the steel strip has good plasticity and a certain strength, internal stress is eliminated, and work hardening caused by cold rolling is eliminated. The metallographic structure photograph of the hot rolled state, the primary annealed state and the secondary annealed state is shown in FIG. 1.
The invention has the beneficial effects that:
1. the invention adopts twice bell-type furnace annealing to solve the problems of high strength and large cold rolling difficulty of the hot-rolled steel strip after 1800MPa level hot forming of the 27MnCrB5 component system, successfully rolls the cold-rolled steel strip with the thickness of 1.2-1.6 mm, realizes the production of the cold-rolled hot-formed steel strip for the automobile with 1800MPa level high surface quality and thin ultra-high strength, and has the characteristic of low alloy cost;
2. according to the invention, a hot-rolled steel strip of 27MnCrB5 ingredient system 1800MPa grade hot-formed steel is used as a raw material, the first annealing is carried out by adopting a high-temperature large-flow hydrogen atmosphere, so that the strength of the hot-rolled steel strip is reduced, the gradient difference of the strength of a welding seam in continuous flash butt welding is also reduced, the stable production of continuous welding and rolling is facilitated, the iron oxide skin layer on the surface of the hot-rolled steel strip is reduced, the pickling speed is improved, the cold rolling time is shortened, the production cost is reduced, the surface quality after cold rolling is improved, and compared with a reversible cold rolling mill, the hot-rolled steel strip has the characteristics of low production cost and high efficiency;
3. the invention adopts the cover annealing furnace for annealing, and carries out annealing twice, so that the steel strip crystal grains are fully coarsened, the strength of the hot formed steel is reduced, the plasticity is improved, and the energy consumption of a production line in the production process is reduced; meanwhile, the leveling process is provided with higher conductivity, so that quality defects such as yellow spots and the like generated on the surface of the steel strip are effectively avoided, and the cold-rolled hot-forming steel strip with high plasticity and high surface quality is produced.
Drawings
FIG. 1 is a metallographic structure diagram in a hot rolled state;
FIG. 2 is a metallographic structure diagram in a primary annealed state;
FIG. 3 is a metallographic structure diagram in a secondary annealed state.
Detailed Description
In the following examples, a hot-rolled raw steel strip for hot stamping forming having a thickness of 2.4 to 3.2mm, a yield strength of 603 to 650MPa, a tensile strength of 740 to 765MPa, and an elongation A after fracture was used50mm23.0 to 25.0 percent; the composite material comprises the following chemical components in percentage by weight: 0.27 to 0.32 percent of C; 0.20 to 0.35 percent of Si; 1.15 to 1.35 percent of Mn; p is less than or equal to 0.020%; s is less than or equal to 0.012 percent; 0.020-0.050% of Als; 0.30 to 0.55 percent of Cr;0.035 to 0.050 percent of Ti; 0.0030-0.0050% of B; the balance of Fe and inevitable impurities.
The process route for producing the 1800 MPa-grade cold-rolled hot-stamped formed steel strip is as follows: the specific process steps and process parameters of the hot rolling of the raw material steel strip → the annealing of a primary bell type furnace → the continuous flash butt welding → the acid washing → the cold continuous rolling → the annealing of a secondary bell type furnace → the flattening → the finished product are specifically explained by referring to the following examples, wherein the test method of the mechanical property of the product in the examples adopts the standard GB-T2975-1998 sampling regulation of the mechanical property and the process property test of the steel.
Example 1
Hot rolling a raw steel strip: the thickness is 3.2mm, the yield strength is 603MPa, the tensile strength is 740MPa, and the elongation after fracture is A50mmThe content was 25.0%.
the annealing cold and hot spot is 726 ℃/733 ℃, the heat preservation time is 18h, and the annealing process adopts hydrogen atmosphere. The heating rate is 79.9 ℃/h from room temperature to 650 ℃, and the average flow of hydrogen is 39.8m3H; the heating rate of 650-700 ℃ is 33.9 ℃/h, the heating rate of 700-740 ℃ is 24.8 ℃/h, and when the hot spot of the heating section is more than or equal to 650 ℃, the average flow of hydrogen is 25.0m3H is used as the reference value. The average flow of hydrogen is 17.8m at 6h before heat preservation3H is used as the reference value. When the temperature is cooled to 580 ℃, the air cooling mode is switched to full speed cooling, the air cooling is carried out to 380 ℃, the water cooling mode is switched to full speed cooling, and the tapping temperature is 98 ℃.
the welding mode is oil dripping and annealing mode, the flash voltage is 15.6V, the flash time is 15s, the upsetting length is 3.4mm, the upsetting force is 650KN, the upsetting time is 0.2s, the annealing heating time is 12s, and the annealing cooling time is 15 s.
the annealing cold and hot spot temperature is 673 ℃/698 ℃, and the annealing atmosphere is hydrogen. Heating the temperature rising section at full speed, keeping the temperature for 18h, entering a cooling stage after the heat preservation is finished, switching to an air cooling mode and cooling at full speed when the temperature is lowered to 580 ℃, switching to a water cooling mode and cooling at full speed when the temperature is lowered to 380 ℃, and discharging at 49 ℃. Average flow of hydrogen flow heating section is 39.9m3H, before the heat preservation time, the average flow of 1/3 sections of hydrogen is 24.8m3H, cooling to 500-350 ℃ and then ensuring the average flow of hydrogen to be 4.9m3/h。
the flat practical elongation is 1.49%, and the set value of the conductivity is 16.2 mS/cm. The specification of the final product is 1.60 multiplied by 1250mm, and the performance of the steel strip before hot stamping forming is as follows:
the hot stamping and forming properties of the steel strip are as follows:
example 2
Hot rolling a raw steel strip: the thickness is 3.0mm, the yield strength is 611MPa, the tensile strength is 752MPa, and the elongation after fracture is A50mmThe content was 24.5%.
the annealing cold and hot spot is 727 ℃/731 ℃, the heat preservation time is 18h, and the annealing process adopts a hydrogen atmosphere. The heating rate is 80.0 ℃/h from room temperature to 650 ℃, and the average flow of hydrogen is 39.9m3H; the heating rate of 650-700 ℃ is 33.6 ℃/h, the heating rate of 700-740 ℃ is 24.9 ℃/h, and when the hot spot of the heating section is more than or equal to 650 ℃, the average flow of hydrogen is 24.8m3H is used as the reference value. The average flow of hydrogen is 17.9m at 6h before heat preservation3H is used as the reference value. When the temperature is cooled to 580 ℃, the air cooling mode is switched to full speed cooling, the air cooling is carried out to 380 ℃, the water cooling mode is switched to full speed cooling, and the tapping temperature is 95 ℃.
the welding mode is oil dripping and annealing mode, the flash voltage is 15.6V, the flash time is 15s, the upsetting length is 3.5mm, the upsetting force is 650KN, the upsetting time is 0.3s, the annealing heating time is 12s, and the annealing cooling time is 15 s.
the temperature of the annealing cold and hot spot is 672 ℃/696 ℃, and the annealing atmosphere is hydrogen. Heating the temperature rising section at full speed for 18h, cooling the temperature to 580 deg.C, switching to air cooling mode, cooling at full speed to 380 deg.CWhen the temperature is higher than the preset temperature, the water cooling mode is switched to the full-speed cooling mode, and the tapping temperature is 47 ℃. Average flow of heating section of hydrogen flow is 39.7m3H, before the heat preservation time, the average flow of 1/3 sections of hydrogen is 24.9m3H, cooling to 500-350 ℃ and then ensuring the average flow of hydrogen to be 4.8m3/h。
the flat practical elongation is 1.50%, and the set value of the conductivity is 16.5 mS/cm. The specification of the final product is 1.50 multiplied by 1250mm, and the performance of the steel strip before hot stamping forming is as follows:
the hot stamping and forming properties of the steel strip are as follows:
example 3
Hot rolling a raw steel strip: thickness of 2.4mm, yield strength of 650MPa, tensile strength of 765MPa, and elongation after fracture A50mmThe content was found to be 23.0%.
the annealing cold and hot spot is 728 ℃/734 ℃, the heat preservation time is 18h, and the annealing process adopts a hydrogen atmosphere. The heating rate is 79.7 ℃/h from room temperature to 650 ℃, and the average flow of hydrogen is 39.8m3H; the heating rate of 650-700 ℃ is 33.8 ℃/h, the heating rate of 700-740 ℃ is 25.0 ℃/h, and when the hot spot of the heating section is more than or equal to 650 ℃, the average flow of hydrogen is 24.9m3H is used as the reference value. The average flow of hydrogen is 18.0m at 6h before heat preservation3H is used as the reference value. When the temperature is cooled to 580 ℃, the air cooling mode is switched to full speed cooling, the air cooling is carried out to 380 ℃, the water cooling mode is switched to full speed cooling, and the tapping temperature is 97 ℃.
the welding mode is oil dripping and annealing mode, the flash voltage is 15.6V, the flash time is 15s, the upsetting length is 3.5mm, the upsetting force is 650KN, the upsetting time is 0.3s, the annealing heating time is 12s, and the annealing cooling time is 15 s.
the annealing cold and hot spot temperature is 676 ℃/699 ℃, and the annealing atmosphere is hydrogen. Heating the temperature rising section at full speed, keeping the temperature for 18h, entering a cooling stage after the heat preservation is finished, switching to an air cooling mode and cooling at full speed when the temperature is lowered to 580 ℃, switching to a water cooling mode and cooling at full speed when the temperature is lowered to 380 ℃, and discharging at 47 ℃. Average flow of hydrogen flow heating section is 39.6m3H, before the heat preservation time, the average flow of 1/3 sections of hydrogen is 24.8m3H, cooling to 500-350 ℃ and the average hydrogen flow rate is 5.0m3/h。
the flat practical elongation is 1.40%, and the set value of the conductivity is 16.4 mS/cm. The specification of the final product is 1.20 multiplied by 1250mm, and the performance of the steel strip before hot stamping forming is as follows:
the hot stamping and forming properties of the steel strip are as follows:
according to the actual processing and measured data, the material processed by the process has the advantages that the yield strength is increased by 155% on average, the tensile strength is increased by 178% on average, the elongation after fracture is reduced by 73% on average, and the material performance is increased remarkably.
In the two annealing processes, a hood-type annealing furnace is adopted to carry out annealing treatment on the steel, the first annealing temperature is 726-734 ℃, the second annealing temperature is 672-699 ℃, and the two-time heat preservation time is 18 hours. The annealing process is a process which is important for ensuring the performance of the final product, whether the yield strength can reach 1800MPa or not is realized, and the annealing process mainly depends on the annealing time and the annealing temperature of the process. Because the bell-type annealing furnace is adopted, high-temperature annealing cannot be realized, and the final high strength can be realized only by a fine grain strengthening and precipitation strengthening mode. Ensuring the precipitation strengthening effect of precipitated phase particles such as TIC and the like; meanwhile, the recrystallized grains grow partially and do not grow excessively, the sizes of the grains are ensured to be below 6 mu m, partial dislocation substructures exist at the same time, and the fine grain strengthening effect is obvious.
It should be noted that the above are only some embodiments of the present invention, and it should be noted that, for those skilled in the art, many modifications and substitutions can be made without departing from the technical principle of the present invention, and these modifications and substitutions should also be regarded as the protection scope of the present invention.
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