A bainite high strength steel sheet having different yield ratio and tensile strength, and a manufacturing method and application thereof

Abstract

The application discloses a bainite high-strength steel plate with different yield ratio and tensile strength, and a manufacturing method and application thereof. The bainite high-strength steel plate has chemical components including C, Si, Mn, Cr, Mo, Ti, B, Als and Fe, and simultaneously satisfies 10 <= Ti / B <= 25. The application adopts a reduced alloy component design and is combined with a matched heat-treatment-free TMCP process, so that two kinds of bainite high-strength steel plates with a yield ratio <= 0.70 and a tensile strength >= 1000 MPa and a yield ratio > 0.85 and a tensile strength >= 1200 MPa can be produced under a chemical component system. The former has the advantages of high strength, high hardness and easy forming, and is suitable for manufacturing high-strength structural parts such as mixer truck tank bodies, which have relatively high requirements on cold bending performance and wear resistance. The latter has the advantages of high wear resistance, high plastic deformation resistance and high plate shape quality, and is suitable for manufacturing wear-resistant structural parts such as automobile compartment bodies, which have very high requirements on wear resistance and rigidity.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of hot-rolled steel plate production, and particularly relates to a bainite high-strength steel plate with different yield strength ratios and tensile strengths and a manufacturing method and application thereof. BACKGROUND

[0002] The bainite structure is widely used in the production of high-strength steel in recent years due to its good matching of strength, hardness, plasticity and toughness.

[0003] For the bainite steel plate, the relevant disclosed patents searched are as follows: the patents with publication numbers CN112575263A, CN112575264A, CN112553543A and the like disclose a granular bainite and acicular ferrite structure, and a high content of Ti (Ti content is as high as 0.5-0.8%) is added to the chemical composition, which causes that Ti cannot be completely solid-solved into the matrix during heating, and a large amount of micron-level TiC precipitates are formed in combination with the chemical composition with a medium C content design, which can improve the wear resistance of the material to a certain extent, but a large amount of micron-level TiC hard particles are not conducive to the plasticity and toughness of the material, and the highest tensile strength of the material is only 900 MPa, and the strength level is not high.

[0004] The patent with publication number CN109234612A discloses a bainite structure, and the highest tensile strength of the material is 900 MPa, the strength level of the material is low, and the yield strength ratio is 0.89-0.90, which is high. The patents with publication numbers CN114908291A and CN114892080A disclose a bainite structure, and the highest tensile strength of the material is 1000 MPa, but a high content of precious alloy elements Mo, Cu, Ni and rare earth elements is added to the alloy composition, and the alloy cost is high. The patents with publication numbers CN114058945A, CN107747056A, CN110747405A and CN106636899A disclose a bainite structure, and the highest tensile strength of the material is 1000 MPa, but the production process must adopt tempering or quenching + tempering or cold rolling + annealing heat treatment, the material strength is improved mainly through the tempering or quenching + tempering or annealing process, the process flow is long, and the production cost is high.

[0005] The patent with publication number CN113430467A discloses "a thin-gauge 1400MPa grade bainite steel and its manufacturing method", which is produced by thin strip continuous casting and rolling process. The difference from the traditional hot continuous rolling process is that (1) the fine-grained as-cast structure obtained under the rapid solidification conditions of double-roller thin strip continuous casting, combined with the rapid cooling of the gas mist cooling system, can form a bainite structure with very fine grain size, which is easy to achieve high strength, and the alloy addition amount is lower, and the rolling cooling process parameters are not suitable for the traditional hot continuous rolling process; (2) due to the as-cast structure with a depth of about 2mm at the edge of the cast strip, the edge quality of the thin strip cast and rolled strip is poor and must be trimmed.

[0006] In summary, among the disclosed patents retrieved about bainite steel plates, on the one hand, the highest tensile strength of the material is only 900MPa, the strength level is not high and the yield ratio is high; on the other hand, the tensile strength of the material reaches 1000MPa, but the alloy composition is high or the tempering or quenching + tempering or cold rolling + annealing heat treatment process must be added, the process flow is long and the production cost is high; on the other hand, the tensile strength of the material reaches 1400MPa, and the alloy composition and process parameters are not suitable for the traditional hot continuous rolling process. SUMMARY

[0007] In view of the problems of existing bainite steel plate production, such as strength level <1000MPa, high yield ratio, or strength level ≥1000MPa, high alloy cost, and long process flow, the present application provides a bainite high-strength steel plate with different yield ratios and tensile strengths, as well as a manufacturing method and application thereof. By adopting a reduced alloy composition design and combining a matching heat treatment-free TMCP process, a chemical composition system can be used to produce two bainite high-strength steel plates with different yield ratios, i.e. a yield ratio ≤0.70 and a tensile strength ≥1000MPa, and a yield ratio >0.85 and a tensile strength ≥1200MPa. The bainite high-strength steel plate with a yield ratio <0.70 and a tensile strength ≥1000MPa has high strength, high hardness, and easy forming, etc., and is suitable for manufacturing high-strength structural parts such as mixer truck tanks which require high cold bending performance and wear resistance. The bainite high-strength steel plate with a yield ratio >0.85 and a tensile strength ≥1200MPa has high wear resistance, high plastic deformation resistance, and high plate shape quality, etc., and is suitable for manufacturing wear-resistant structural parts such as automobile bodies which require high wear resistance and rigidity.

[0008] To achieve the above-mentioned purposes, the technical solutions adopted by the present application are as follows:

[0009] The bainite high-strength steel plate has the following chemical components and percentage by weight: C: 0.15-0.23%; Si: 0.10-0.50%; Mn: 0.80-1.50%; Cr: 0.80-1.50%; Mo: 0.08-0.30%; Ti: 0.020-0.070%; B: 0.0008-0.0030%; Als: 0.30-0.70%; P: ≤0.012%; S: ≤0.003%; N: ≤0.0030%; O: ≤0.0030%; and the rest is Fe and inevitable inclusions; and the following conditions are met simultaneously: 10≤Ti / B≤25.

[0010] The metallographic structure of the bainite high-strength steel plate is granular bainite + nanometer precipitates or lath bainite.

[0011] The bainite high-strength steel plate has the following properties: the yield strength / tensile strength ratio is ≤0.70, the tensile strength is ≥1000 MPa, the elongation A 50 ≥16%, the hardness is ≥300 HB, and the cold bending performance reaches 180° and D=1a; or the yield strength / tensile strength ratio is >0.85, the tensile strength is ≥1200 MPa, and the elongation A 50 ≥12%, the hardness is ≥360 HB, and the cold bending performance reaches 150° and D=4a.

[0012] A method for manufacturing bainite high-strength steel plates with different yield strength / tensile strength ratios and tensile strengths under the same composition, the cast blank with the composition of the bainite high-strength steel plate as described in the application is heated, rolled, cooled, coiled, leveled and opened; and the bainite high-strength steel plates with different yield strength / tensile strength ratios and tensile strengths are produced by controlling the rolling, cooling, coiling, leveling processes.

[0013] In the heating step, after the slab enters the heating furnace, it enters the preheating section, the first heating section, the second heating section and the soaking section in sequence, the temperature at the end of the second heating section is ≥1180℃, the soaking time is ≥30 min, and the slab discharge temperature is 1215-1265℃, the holding time of the slab temperature ≥1215℃ is ≥40 min.

[0014] In the rolling step, the slab is rolled by using a 2-stand rough rolling and a 7-stand finishing rolling hot continuous rolling mill unit, the cumulative reduction rate of rough rolling is ≥80%, the cumulative reduction rate of finishing rolling is ≥85%, and constant speed rolling is adopted, the finishing rolling speed v of the F7 stand is ≥6.0 m / s.

[0015] For the bainite high-strength steel plate with the yield strength / tensile strength ratio ≤0.70 and the tensile strength ≥1000 MPa:

[0016] In the rolling step, the rough rolling finishing temperature R2DT is 1080-1130℃, the finishing rolling finishing temperature FDT is 860-910℃, and the finishing rolling speed v of the F7 stand is ≥6.5 m / s.

[0017] In the cooling and coiling step, a segmented cooling mode is adopted, first rapidly cooling to 630-680℃ at a cooling rate of 90-120℃ / s, then air cooling for 6-12s, and then cooling to 520-580℃ at a cooling rate of ≥30℃ / s for coiling, and air cooling to room temperature after coiling;

[0018] In the skin pass step, the rolling force of the skin pass mill is 500-700t, the bending force is 60-80t, and the skin pass speed is 60-80m / min.

[0019] For the bainite high-strength steel plate with a yield ratio >0.85 and a tensile strength ≥1200MPa:

[0020] In the rolling step, the rough rolling final rolling temperature R2DT is 1060-1110℃, and the finish rolling final rolling temperature FDT is 800-850℃;

[0021] In the cooling and coiling step, the cooling rate V is V0+k*h 1 / 2 cooling to 420-480℃ for coiling, and air cooling to room temperature after coiling; wherein V0 represents the bainite phase transformation critical cooling rate, with units of ℃ / s, V0=10-20℃ / s; k represents the correction coefficient of the cooling rate, k=14.5℃ / s·mm 1 / 2 ; h represents the thickness of the steel plate, with units of mm;

[0022] In the skin pass step, the rolling force of the skin pass mill is 700-900t, the bending force is 80-100t, and the skin pass speed is 30-50m / min.

[0023] In the cooling and coiling step, preferably cooling to 420-480℃ at a cooling rate of ≥40℃ / s for coiling, and air cooling to room temperature after coiling.

[0024] The bainite high-strength steel plate with a yield ratio ≤0.70 and a tensile strength ≥1000MPa has a granular bainite and nano precipitate TiC microstructure, with a proportion of TiC precipitates ≤18nm in size ≥88%; an elongation A 50 ≥16%, a hardness ≥300HB, and a cold bending performance of 180°, D=1a is qualified.

[0025] The bainite high-strength steel plate with a yield ratio >0.85 and a tensile strength ≥1200MPa has a lath bainite microstructure, with a bainite lath width of 150-350nm; an elongation A 50 ≥12%, a hardness ≥360HB, and a cold bending performance of 150°, D=4a is qualified.

[0026] The chemical composition of the bainite high-strength steel plate provided by the application adopts a medium-C low-Si and Mn+Cr+Mo+Ti+B composite micro-alloying design idea, wherein,

[0027] C: 0.15%-0.23%, C as a basic element in steel plays a very important role in improving the strength and hardness of the steel. In order to obtain higher strength and hardness, the content of C must be ensured to be greater than 0.17%, but the content of C cannot be too high. When the content of C is >0.24%, the size of the martensite-austenite islands formed is larger and the proportion is higher, which is not conducive to improving the plasticity and toughness of the material.

[0028] Si: 0.10-0.50%, Si has a strong solid solution strengthening effect and can improve the strength of the material. Considering that the content of Cr in the application is relatively high (0.80~1.50%), Cr can improve the adhesion of the iron oxide scale of the steel plate to the matrix. If a high Si component design is adopted, the thickness of the oxide on the surface of the steel plate will be thick and difficult to remove, which is not conducive to improving the surface quality of the steel plate. Therefore, the application adopts a low Si alloy component design, and the content of Si is 0.10-0.50%.

[0029] Mn: 0.80~1.50%, Mn as a strengthening element of steel can expand the austenite phase region, significantly improve the hardenability of the steel, and improve the strength of the steel. In order to ensure that the tensile strength of the material is ≥1000MPa, the content of Mn should be controlled to be greater than 0.80%, but the content of Mn should not be too high. Too high content of Mn will reduce the elongation of the material and is not conducive to the welding performance and forming performance of the material.

[0030] Cr: 0.80~1.50%, Cr can increase the solid solution strengthening effect of the material, improve the hardenability and strength and hardness of the material, and thus be conducive to improving the wear resistance of the material. In order to ensure that the tensile strength of the material is ≥1000MPa, the content of Cr should be controlled to be greater than 0.80%, but when the content of Cr is too high, high-temperature tempering brittleness of the material is easily caused at a coiling temperature of 520-580℃, which leads to an increase in the ductile-brittle transition temperature of the material and is not conducive to the control of the low-temperature impact toughness of the material. Based on the requirement of the low-temperature toughness control of the material, the content of Cr is ≤1.50%.

[0031] Mo: 0.08%~0.30%, for the target organization is granular bainite high-strength steel, Mo and C have strong binding force and high-temperature thermal stability, and can form nanoscale (Ti, Mo) C precipitates during the cooling air cooling section and coiling process, which can refine the precipitate size and have better precipitation strengthening effect; for the target organization is lath bainite high-strength steel, because the C content of the application is 0.15~0.23%, the C content is relatively high, and the pearlite structure is easily formed during the cooling process, Mo as a stable austenite element can inhibit the decomposition of austenite, can significantly delay the pearlite phase change, avoid the formation of pearlite structure during the cooling process, and is beneficial to promote the bainite structure transformation to obtain the target lath bainite structure.

[0032] Ti: 0.020%~0.070%, on the one hand, to refine the original austenite grain size during the heating process; on the other hand, the Ti solid-solved in the matrix during the heating process can precipitate a large amount of nanoscale TiC during the cooling air cooling section and coiling after heat preservation, which has the effect of precipitation strengthening, and improves the strength and hardness of the material. But the Ti content cannot be too high, considering that the C content of the application is 0.15~0.23%, according to the solid solubility product formula, when the Ti content is greater than 0.070%, the TiC full solid solution temperature is >1284℃, and the Ti that cannot be completely solid-solved in the matrix during the heating process forms a large amount of hard micron-sized TiC after rolling and cooling, which becomes the crack initiation source, which is not conducive to the plasticity and toughness of the material, and the cold bending forming performance. Furthermore, Ti is a strong carbonitride forming element, and the compound formed by Ti, C, N and O can refine the columnar grains of the weld, and compared with the addition of B alone, the addition of Ti and B has a more significant effect on improving the low-temperature toughness of the weld metal. But Ti and B need to satisfy: 10≤Ti / B≤25, when Ti content is too low or B is too high, which leads to Ti / B<10, on the one hand, the nanoscale precipitates TiC are too few to weaken the precipitation strengthening effect and B seriously inhibits the proeutectoid ferrite transformation to increase the phase transformation strengthening effect, which leads to the increase of the material strength; on the other hand, the remaining B that is not fixed by Ti will form FeB at the grain boundary, which leads to the deterioration of the plasticity and toughness of the material, the cold bending performance; when the Ti content is too high or the B content is too low, which leads to Ti / B>25, the increase of Ti content will increase the phase transformation temperature of austenite to ferrite, and the decrease of B content will weaken the inhibiting effect of proeutectoid ferrite, and the combined action of the two factors will lead to the increase of the proportion of proeutectoid ferrite, which in turn will reduce the strength of the material.

[0033] B: 0.0008%~0.0030%, B is an important element in the present application, which can improve the stability of austenite, and for the target structure of granular bainite high-strength steel, can avoid the formation of pearlite structure in the air cooling stage, and delay the transformation of austenite to ferrite. By adding a certain amount of B and appropriately prolonging the air cooling time, the proportion of ferrite can be controlled, the number and proportion of TiC precipitates can be increased, and the precipitation strengthening effect can be improved. For the target structure of lath bainite high-strength steel, B can improve the hardenability of the steel, avoid the formation of ferrite and pearlite structure during cooling, and is beneficial to the acquisition of the target lath bainite structure. At the same time, the addition of B can reduce the interfacial energy of the austenite grain boundary, which can improve the microstructure of the weld, reduce the formation of proeutectoid ferrite, and increase the proportion of acicular ferrite. The grain boundary of acicular ferrite is a large-angle grain boundary and the grain size is small, which can significantly improve the low-temperature toughness of the weld metal.

[0034] Als: 0.30%-0.70%, for the target structure of granular bainite high-strength steel, Al can significantly improve the austenite to ferrite transformation starting temperature, expand the ferrite phase transition region, inhibit the formation of cementite, promote the formation of ferrite structure during the cooling process, and ultimately be beneficial to the acquisition of granular bainite structure. For the target structure of lath bainite high-strength steel, Al as an element to expand the austenite region can reduce the unrecrystallization temperature of the material, create conditions for the stability of low-temperature rolling process of the material, and be beneficial to the acquisition of small austenite grain size, and then be beneficial to the acquisition of small bainite structure after rolling and cooling, and improve the strength and toughness of the material.

[0035] P, S as impurity elements, will have adverse effects on the plasticity, forming, welding and other properties of the steel, the lower the better, considering the production cost factor, in actual production, control P: ≤0.012%, S: ≤0.003%.

[0036] O, N as harmful gas elements, due to the high activity of Ti, will preferentially react with O, N to precipitate, affecting the yield of Ti, the amount and precipitation strengthening effect of TiC. Based on the yield of Ti and the improvement of TiC precipitation strengthening effect, N: ≤0.0030%; O: ≤0.0030%.

[0037] The application provides a method for manufacturing bainite high-strength steel plates with different yield strength ratios and tensile strengths under the same ingredients, in which a high-temperature heating process is adopted by comprehensively considering small austenite grain size and high Ti solid solution amount, and the high-temperature heating process emphasizes prolonging the holding time of the high-temperature section instead of the total furnace time, and the casting blank discharge temperature is 1215-1265 DEG C, the purpose of which is to increase the proportion of Ti atoms solid-solved in the matrix, to create conditions for the precipitation of nanoscale TiC in the cooling air cooling section and coiling process of the bainite high-strength steel plate with a yield strength ratio of less than or equal to 0.70 and a tensile strength of greater than or equal to 1000 MPa; the holding time of the casting blank temperature of greater than or equal to 1215 DEG C is greater than or equal to 40 min, which is to allow the Ti atoms to have sufficient time to be fully solid-solved in the matrix, and to avoid the austenite grains being too coarse to improve the strength and the furnace-born iron oxide scale being too thick to control the surface quality.

[0038] In the rolling process, the high-temperature large deformation characteristics of the rough rolling stage are fully utilized to make the austenite fully recrystallize, the cumulative deformation amount of the non-recrystallization zone in the finish rolling stage is increased, and the lower finish rolling temperature is combined, which is beneficial to generate larger cumulative strain energy, increase the nucleation position, refine the austenite grain size and the grain size transformed from the austenite to the bainite, and improve the fine-grain strengthening effect. The constant speed rolling control strategy is mainly based on improving the rolling stability and the MT temperature, air cooling time and coiling temperature stability of the bainite high-strength steel plate with a yield strength ratio of less than or equal to 0.70 and a tensile strength of greater than or equal to 1000 MPa, which is beneficial to the mechanical property stability of the product. Compared with the bainite high-strength steel plate with a yield strength ratio of greater than 0.85 and a tensile strength of greater than or equal to 1200 MPa, the main purpose of increasing the finish rolling speed of the bainite high-strength steel plate with a yield strength ratio of less than or equal to 0.70 and a tensile strength of greater than or equal to 1000 MPa is to reduce the precipitation of micron-scale TiC in the austenite as much as possible, increase the precipitation amount of nanoscale TiC in the cooling air cooling section and coiling process, and improve the precipitation strengthening effect.

[0039] For the high-strength steel with target structure of granular bainite, yield ratio ≤0.70, and tensile strength ≥1000 MPa, a segmented cooling control strategy is adopted in the cooling process, which gives full play to the synergistic effect of three strengthening methods of fine-grain strengthening, precipitation strengthening and phase transformation strengthening, and avoids the problem that the one-stage continuous cooling process after rolling cannot simultaneously consider precipitation strengthening (requiring high coiling temperature) and phase transformation strengthening (requiring low coiling temperature). The first stage adopts concentrated cooling and increases the cooling rate mainly to increase the phase transformation undercooling degree, increase the nucleation position, refine the grain size of ferrite transformed from austenite and the grain size of bainite transformed from residual austenite, which is beneficial to improve the material strength and cold bending forming performance. The main purpose of rapid cooling to 630-680℃ is that this temperature interval is the best temperature interval for ferrite phase transformation, and is near the nose temperature of PTT curve, which can quickly form a certain proportion of soft ferrite organization and precipitate a large proportion of nanoscale TiC in ferrite grains, fully play the effect of precipitation strengthening and the effect of hard TiC on improving the wear resistance of the material. After rapid cooling to 630-680℃ and air cooling for 6-12s, if the air cooling time is too short, the amount of TiC precipitation is less; if the air cooling time is too long, the proportion of ferrite gradually increases and the grain size increases, the proportion of TiC precipitates is large, but the increase of material strength by precipitation strengthening is less than the decrease of phase transformation strengthening by the proportion of ferrite, which is not conducive to the improvement of material strength, hardness and wear resistance, and the length of laminar flow cooling line is limited and does not have the condition of long time air cooling. The third stage is rapidly cooled to 530-580℃ and coiled at medium temperature, on the one hand, fine bainite structure can be obtained, which fully plays the effect of phase transformation strengthening on the improvement of material strength; on the other hand, the TiC not precipitated in the cooling and air cooling stage will continue to precipitate, further increasing the effect of precipitation strengthening.

[0040] For the high-strength steel with target structure of lath bainite, the one-stage continuous cooling mode is adopted to cool to the low-temperature region B f + (20-50)℃ of bainite phase transformation temperature, mainly to obtain fine lath bainite structure, and the coiling temperature is lower than 400℃ to enter the martensite phase transformation temperature interval, the strength and hardness of the product can be guaranteed, but the plasticity and toughness of the material are difficult to guarantee, and the flatness is poor and the plate shape qualification rate is low, which is not conducive to the control of plate shape.

[0041] For the leveling process, mainly based on improving the strip shape and width direction performance uniformity, in the case of ensuring the strip shape, the control strategy of small leveling force and large bending roller force is preferred, the main purpose is to avoid the degree of work hardening of the strip to intensify, which is not conducive to the performance uniformity of the strip width direction. At the same time, in order to control the stability of the strip shape during the leveling process, the strip leveling speed should not be too fast. According to the different strength grades of the strip, the bainite high-strength steel with yield ratio > 0.85 and tensile strength ≥ 1200 MPa is compared with the bainite high-strength steel with yield ratio ≤ 0.70 and tensile strength ≥ 1000 MPa. The rolling force and bending roller force of the strip leveling machine are increased by 28-40% and 25-33% respectively.

[0042] Compared with the prior art, the present application has the following beneficial effects:

[0043] 1) The present application adopts an economic chemical design of medium-carbon low-silicon and manganese, chromium, molybdenum, titanium and boron compound micro-alloying, combined with a matching heat treatment-free TMCP process, and can produce two different yield ratios and different strength grades of bainite high-strength steel with one chemical composition system, which can meet the personalized special needs of users in multiple ways.

[0044] 2) Compared with the existing <1000MPa bainite steel plate, the corresponding bainite high-strength steel plate with tensile strength ≥ 1000MPa in the present application has a yield ratio < 0.70, has the advantages of high strength, high hardness and easy forming, and is suitable for manufacturing high-strength structural parts such as mixer truck tanks which have high requirements for cold bending performance and wear resistance.

[0045] 3) Compared with the existing bainite steel plate with strength grade ≥ 1000MPa, the corresponding bainite high-strength steel plate in the present application has a tensile strength ≥ 1200MPa and a yield ratio > 0.85, has the advantages of high wear resistance, high plastic deformation resistance and high flatness quality, and is suitable for manufacturing wear-resistant structural parts such as automobile bodies which have high requirements for wear resistance and stiffness.

[0046] 4) The two different yield ratios and different strength grades of bainite high-strength steel plates produced by the present application have a flatness control ≤ 5mm / m after being cut open and flattened, and a flatness qualification rate ≥ 95%, which better meets the use requirements of downstream users. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 It is a schematic diagram of the heating, rolling and cooling process of the method described in the present application;

[0048] Figure 2 It is a typical metallographic diagram corresponding to the steel plate of process 1 of example 1;

[0049] Figure 3 It is a typical metallographic diagram corresponding to the steel plate of process 2 of example 1;

[0050] Figure 4 This is a typical metallographic image of the steel plate corresponding to Process 1 in Example 2;

[0051] Figure 5 Here is a typical metallographic image of the steel plate corresponding to process 2 in Example 2;

[0052] Figure 6 Here is a typical metallographic image of the steel plate corresponding to Process 1 in Comparative Example 1;

[0053] Figure 7 Typical metallographic images of the steel plate corresponding to process 2 in Comparative Example 1;

[0054] Figure 8 Here is a typical metallographic image of the steel plate corresponding to Process 1 in Comparative Example 4;

[0055] Figure 9 The image shows a typical metallographic diagram of the steel plate corresponding to process 2 in Comparative Example 4. Detailed Implementation

[0056] This invention provides a bainitic high-strength steel plate, wherein the chemical composition and weight percentage content of the bainitic high-strength steel plate are as follows: C: 0.15~0.23%; Si: 0.10~0.50%; Mn: 0.80~1.50%; Cr: 0.80~1.50%; Mo: 0.08~0.30%; Ti: 0.020~0.070%; B: 0.0008~0.0030%; Als: 0.30~0.70%; P: ≤0.012%; S: ≤0.003%; N: ≤0.0030%; O: ≤0.0030%; the remainder being Fe and unavoidable inclusions; and must simultaneously satisfy: 10≤Ti / B≤25.

[0057] A method for manufacturing bainitic high-strength steel plates with different yield strength ratios and tensile strengths under the same composition includes the following steps: heating, rolling, cooling, coiling, leveling, and slitting a billet containing the composition of the bainitic high-strength steel plate described in this invention; and producing bainitic high-strength steel plates with different yield strength ratios and tensile strengths by controlling different rolling, cooling, coiling, and leveling processes.

[0058] In the heating process, after the slab enters the heating furnace, it sequentially enters the preheating section, the first heating section, the second heating section, and the homogenization section. The temperature at the end of the second heating section is ≥1180℃, the homogenization time is ≥30min, the slab exiting the furnace is 1215~1265℃, and the holding time to control the slab temperature at ≥1215℃ is ≥40min.

[0059] In the rolling process, a 2-stand roughing mill and a 7-stand finishing mill hot continuous rolling mill are used for rolling. The cumulative reduction rate of the roughing mill is ≥80%, and the cumulative reduction rate of the finishing mill is ≥85%. Constant speed rolling is used.

[0060] For the bainite high-strength steel plate with a yield ratio ≤0.70 and a tensile strength ≥1000 MPa:

[0061] In the rolling step, the rough rolling final rolling temperature R2DT is 1080-1130℃, the finish rolling final rolling temperature FDT is 860-910℃, and the F7 rack final rolling speed v ≥6.5 m / s;

[0062] In the cooling and coiling step, a segmented cooling mode is adopted, first fast cooling to 630-680℃ at a cooling rate of 90-120℃ / s, then air cooling for 6-12s, and then cooling to 520-580℃ at a cooling rate ≥30℃ / s for coiling, and air cooling to room temperature after coiling;

[0063] In the flattening step, the flattening mill rolling force is 500-700t, the bending force is 60-80t, and the flattening speed is 60-80m / min.

[0064] For the bainite high-strength steel plate with a yield ratio >0.85 and a tensile strength ≥1200 MPa:

[0065] In the rolling step, the rough rolling final rolling temperature R2DT is 1060-1110℃, the finish rolling final rolling temperature FDT is 800-850℃, and the F7 rack final rolling speed v ≥6.0 m / s;

[0066] In the cooling and coiling step, the cooling rate V=V0+k*h 1 / 2 cooling to 420-480℃ for coiling, and air cooling to room temperature after coiling; wherein V0 represents the bainite phase transformation critical cooling rate, with the unit of ℃ / s, V0=10-20℃ / s; k represents the correction coefficient of the cooling rate, k=14.5℃ / s·mm 1 / 2 ; h represents the thickness of the steel plate, with the unit of mm;

[0067] In the flattening step, the flattening mill rolling force is 700-900t, the bending force is 80-100t, and the flattening speed is 30-50m / min.

[0068] The application will be described in detail below in combination with examples.

[0069] The chemical components of the examples and comparative examples are shown in Table 1, the rolling process parameters are shown in Table 2, the cooling process parameters are shown in Table 3, the flattening process parameters and the unevenness of the flattened plate are shown in Table 4, and the organizational type and mechanical properties are shown in Table 5.

[0070] Table 1 Chemical components of examples and comparative examples (mass percentage, wt%)

[0071]

[0072] Table 2 Rolling process parameters of examples and comparative examples

[0073]

[0074] Table 3 Cooling process parameters of examples and comparative examples

[0075]

[0076] Table 4 Leveling process parameters, flatness qualified rate of examples and comparative examples

[0077]

[0078] Note: The flatness of the leveled plate ≤8mm / m is qualified product, and the flatness qualified rate = the weight of the qualified product / the weight of the hot coil.

[0079] Table 5 Microstructure type, mechanical properties of examples and comparative examples

[0080]

[0081] The above detailed description of the reference examples of the bainite high-strength steel plate with different yield ratio and tensile strength, the manufacturing method and the application is illustrative rather than limiting, and several examples can be listed according to the defined range, and thus the changes and modifications without departing from the overall concept of the present application shall fall within the protection scope of the present application.

Claims

1. A bainite high-strength steel sheet, characterized by, The chemical composition and weight percentage content of the bainite high-strength steel plate are: C: 0.15-0.23%; Si: 0.10-0.50%; Mn: 0.80-1.50%; Cr: 0.80-1.50%; Mo: 0.08-0.30%; Ti: 0.020-0.070%; Als: 0.30-0.70%; S: ≤0.003%; N: ≤0.0030%; O: ≤0.0030%; and the rest is Fe and inevitable inclusions; and 10≤Ti / B≤25 is simultaneously satisfied. The metallographic structure of the bainite high-strength steel plate is granular bainite and nano precipitates. B：0.0008~0.0030%； The chemical composition and weight percentage content of the bainite high-strength steel plate are: C: 0.15-0.23%; Si: 0.10-0.50%; Mn: 0.80-1.50%; Cr: 0.80-1.50%; Mo: 0.08-0.30%; Ti: 0.020-0.070%; Als: 0.30-0.70%; S: ≤0.003%; N: ≤0.0030%; O: ≤0.0030%; and the rest is Fe and inevitable inclusions; and 10≤Ti / B≤25 is simultaneously satisfied. P ：≤0.012%； The metallographic structure of the bainite high-strength steel plate is granular bainite and nano precipitates. The bainite high-strength steel plate component cast blank is heated, rolled, cooled, coiled, leveled, and opened. In the rolling step, the rough rolling final rolling temperature R2DT is 1080-1130℃, and the finish rolling final rolling temperature FDT is 860-910℃.

2. The bainitic high strength steel sheet according to claim 1, characterized in that, The bainitic high-strength steel plate has a yield strength ratio ≤ 0.70, tensile strength ≥ 1000 MPa, and elongation A. 50 ≥16%, hardness ≥300HB, cold bending performance reaches 180°, D=1a qualified.

3. A bainitic high strength steel sheet, characterized by, In the cooling and coiling step, a segmented cooling mode is adopted, first rapidly cooled to 630-680℃ at a cooling rate of 90-120℃ / s, then air-cooled for 6-12s, and then cooled to 520-580℃ at a cooling rate of ≥30℃ / s for coiling, and air-cooled to room temperature after coiling. In the leveling step, the rolling force of the leveler is 500-700t, the bending force is 60-80t, and the leveling speed is 60-80m / min. B：0.0008~0.0030%； The bainite high-strength steel plate component cast blank is heated, rolled, cooled, coiled, leveled, and opened. P ：≤0.012%； In the rolling step, the rough rolling final rolling temperature R2DT is 1060-1110℃, and the finish rolling final rolling temperature FDT is 800-850℃. In the leveling step, the rolling force of the leveler is 700-900t, the bending force is 80-100t, and the leveling speed is 30-50m / min. In the heating step, after the slab enters the heating furnace, it enters the preheating section, the first adding section, the second adding section, and the soaking section in sequence, the temperature at the end of the second adding section is ≥1180℃, the soaking time is ≥30min, the slab discharge temperature is 1215-1265℃, and the holding time of the slab temperature ≥1215℃ is ≥40min.

4. The bainitic high strength steel sheet according to claim 3, characterized in that, The bainitic high-strength steel plate has a yield strength ratio >0.85, tensile strength ≥1200MPa, and elongation A. 50 ≥12%, hardness ≥360HB, cold bending performance reaches 150°, D=4a qualified.

5. A method of manufacturing a bainitic high-strength steel sheet having a yield ratio of ≤ 0.70 and a tensile strength of ≥ 1000 MPa, characterized by, In the rolling step, 2-stand rough rolling and 7-stand finish rolling hot continuous rolling mill train are adopted for rolling, the cumulative reduction rate of rough rolling is ≥80%, and the cumulative reduction rate of finish rolling is ≥85%; constant speed rolling and F7-stand final rolling speed v≥6.0m / s are adopted. ​ ​ ​ 6. A method of manufacturing a bainitic high-strength steel sheet having a yield ratio of > 0.85 and a tensile strength of > 1200 MPa, characterized by, ​ ​ In the cooling and coiling step, the cooling rate V = V0+ k*h 1 / 2 Cooling to 420~480℃ for coiling, and air cooling to room temperature after coiling; wherein V0 represents a bainite phase transformation critical cooling rate, in °C / s, k represents a correction coefficient of the cooling rate, k = 14.5 °C / s·mm 1 / 2 ; h represents the thickness of the steel sheet, in mm; ​ 7. The production method according to claim 5 or 6, characterized by, ​ 8. The production method according to claim 5 or 6, characterized by, ​ 9. The production method according to claim 6, wherein In the cooling and coiling step, cooling is performed at a cooling rate of ≥ 40 °C / s to 420 to 480 °C and coiling is performed, and after coiling, air cooling is performed to room temperature.

10. Use of the bainite high-strength steel sheet according to any one of claims 1 to 4 as a steel for a mixer tank body or a steel for a car body.

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