Component-normalized variable-strength dual-phase steel plate and flexible manufacturing method therefor

Abstract

Dual-phase steel and a flexible manufacturing method therefor. The dual-phase steel comprises the following chemical components in percentage by mass: 0.06%-0.08% of C, 0.4%-0.6% of Si, 2.4%-2.6% of Mn, 0.01%-0.05% of Al, 0.02%-0.04% of Ti, 0.0015%-0.0025% of B, and the balance of Fe and inevitable impurities. The microscopic structure of the variable-strength dual-phase steel comprises martensite having a volume fraction of 30% or above; the remaining structure is ferrite, retained austenite, and nano-precipitates; and the formability is good. In the present disclosure, a component normalization design and a flexible manufacturing process are used, steel of multiple strength grades required for a body-in-white is produced, the dual-phase steel can be flexible to reach a tensile strength of 450-1310 MPa, the formability is good, the product of strength and elongation is larger than 11000%, and the hole expansion ratio is larger than 35%.

Description

Technical field

[0001] The present disclosure relates to a manufacturing technology of dual-phase steel, in particular to a composition-normalized variable-strength dual-phase steel plate and a flexible manufacturing method therefor.Background Art

[0002] With the intensification of the global energy crisis and environmental issues, energy conservation and safety have become the main development directions of the automobile manufacturing industry. Reducing vehicle weight is one of the measures for energy conservation and emission reduction. Ultra-high strength steel, which has good mechanical and service properties, is suitable for the manufacture of automobile structural members and can be widely used to effectively reduce vehicle weight. In recent years, the demand for ultra-high strength steel for automobiles has been increased continuously.

[0003] There are a wide variety of ultra-high strength steels for automobiles. Dual-phase steels have good strength and plasticity, but have a hole expansion ratio (about 20%-35%) which is much lower than that of soft steels for traditional automobiles. Although bainitic steels and complex-phase steels have high hole expansion ratio, their elongation is too low. Therefore, on the basis of not reducing the elongation of dual-phase steels, the development of products with improved hole expansion should have a wide range of application scenarios.

[0004] A body-in-white requires the use of high-strength steels with a variety of mechanical performance, which requires different composition design and manufacturing processes traditionally.

[0005] Chinese patent application CN103215516A discloses a high-strength hot-rolled Q&P steel and manufacturing method therefor, which comprises the following chemical composition: C: 0.15%-0.40%, Si:1.0%-2.0%, Mn:1.5%-3.0%, P≤0.015%, S≤0.005%, Al: 0.3%-1.0%, N≤0.006%, Ti:0.005%-0.015%, with a balance of Fe; and which has a yield strength of ≥700MPa, a tensile strength of ≥1300MPa, and an elongation of >10%. It inhibits the precipitation of cementite by increasing the content of Si on the basis of the composition of common C-Mn steel, refines the austenite grains by micro-Ti treatment, and accelerates the austenite transformation kinetics during air cooling process by increasing the content of Al. Meanwhile, it adopts a hot continuous rolling process combined with a segmented cooling process to obtain a steel with a structure of proeutectoid ferrite + martensite + retained austenite. Additionally, the alloy cost is significantly reduced.

[0006] Chinese patent application CN104451436A discloses a bainitic-martensitic-austenitic complex-phase wear-resistant steel plate and its manufacturing method. The steel plate comprises the chemical composition in weight percentage: C: 0.20-0.40; Mn: 0.30-1.50; Si: 0.80-1.20; Cr: 0.60-1.00; Ni: 0.20-0.60; Mo: 0.20-0.40; Cu: 0.20-0.50; B: 0.0005-0.003; S≤0.010, P≤0.015, with a balance of Fe and unavoidable impurity elements. The rolled material can have a bainite-martensite-retained austenite complex-phase microstructure, with a volume fraction of retained austenite being 5-15%. The material has a yield strength of greater than 1000MPa, a tensile strength of greater than 1300MPa, an elongation of greater than 15%, and a hardness of HB420-500. Its machinability and weldability meet the requirements of equipment manufacturing, and its abrasive wear resistance is no less than 1.3 times that of Hardox450, and no less than 1.5 times that of Hardox450 under weakly acidic conditions.

[0007] Chinese patent application CN102776438A discloses a niobium-lanthanum micro-alloyed Mn-B series ultra-high strength steel plate and its heat treatment process, which belongs to the field of heat treatment technology of steel materials. The steel plate comprises the following chemical composition in weight percentage: C: 0.14%-0.35%; Mn: 1.5%-2.0%; Si: 0.6% -1.0%; P ≤ 0.015%; S ≤ 0.002%; Nb: 0.01%-0.06%; B: 0.0005%- 0.0040%, La: 0.001% -0.5%, with a balance of Fe and unavoidable impurities. The heat treatment process comprises the following steps: austenitization is carried out at a temperature of 880-940°C with a holding time of 0.5-5 hours before water quenching; tempering is carried out at a temperature of 190-250°C with a holding time of 1-15 hours. The steel plates obtained by the heat treatment process have excellent mechanical performance, with a tensile strength of 1200-1400MPa, a yield strength of 1000-1300MPa, an elongation of 6-15%. It has the characteristic of low production cost and can be used in industrial production of steel sheets with a thickness of 5-25mm.

[0008] Chinese patent application CN102321841A discloses a steel for track plates with a tensile strength of 1300MPa.The steel comprises the chemical composition in weight percentage: C: 0.20-0.30%, Mn: 0.80-1.40%, Si: 0.15-0.35%, P: 0-0.015%, S: 0-0.016%, Cr: 0-0.30%, Ni: 0- 0.25%, Cu: 0-0.30%, Ti: 0.01-0.02%, Al: 0.02-0.06%, B: 0.0005-0.0035%, with a balance of Fe and unavoidable impurity elements. The steel for track plates has a tensile strength of 1340MPa or above, and an elongation after fracture of less than 12%. The section steel has a "U"-shaped notch impact absorbing energy of greater than 72J, high strength, few quenching and internal cracks, and a long service life.

[0009] It can be seen from the above patent applications that the existing high-strength steels are produced by using a single composition design scheme and the corresponding single manufacturing process design.

[0010] The technical solutions described in Chinese patent applications CN103215516A and CN104451436A are to obtain sufficient retained austenite by adding a high content of Si and Al, and to obtain high elongation through the TRIP effect of retained austenite, without considering hole expansion performance.

[0011] The ultra-high strength steels described in Chinese patent applications CN102776438A and CN102321841A respectively have good mechanical performance through the addition of micro-alloys such as niobium, lanthanum, nickel, cadmium, and copper, but they fail to meet the indicators of high elongation and high hole expansion performance.Summary

[0012] One of the objects of the present disclosure is to provide a composition-normalized variable-strength dual-phase steel plate and a flexible manufacturing method therefor. By using a single chemical composition, steel of multiple strength grades required for a body-in-white can be produced, which can be flexible to reach a tensile strength of 450-1310MPa, have good formability, a product of strength and elongation of greater than 11000, and a hole expansion ratio of greater than 35%. The microstructure of the dual-phase steel comprises ferrite + martensite + retained austenite + nanoprecipitates, wherein, the martensite has a volume fraction of 30% or more, and sufficient retained austenite (greater than 3%) + sufficient nanoprecipitates (the mass fraction of nanoprecipitates with a size of less than 50nm in the total precipitates is greater than 8%).

[0013] In order to achieve the above object, the technical solution of the present disclosure is: A dual-phase steel, which comprises the following chemical composition in mass percentage: C: 0.06-0.08%, Si: 0.4-0.6%, Mn:2.4-2.6%, Al:0.01-0.05%, Ti:0.02-0.04%, B:0.0015-0.0025%, P≤0.015%, S≤0.003%, N≤0.005%, with a balance of Fe and unavoidable impurities; The microstructure of the dual-phase steel according to the present disclosure comprises ferrite + martensite + retained austenite + nanoprecipitates, wherein, martensite has a volume fraction of 30% or more; The dual-phase steel according to the present disclosure has a tensile strength of 450-1310MPa, a product of strength and elongation of greater than 11000%, and a hole expansion ratio of greater than 35%.

[0014] Further, the balance is Fe and unavoidable impurities.

[0015] In some embodiments, the dual-phase steel of the present disclosure is a composition-normalized variable-strength dual-phase steel.

[0016] In some embodiments, the nanoprecipitates in the dual-phase steel of the present disclosure are ε carbides.

[0017] In some embodiments, the volume fraction of retained austenite in the dual-phase steel of the present disclosure is greater than 3%.

[0018] In some embodiments, in the dual-phase steel of the present disclosure, the mass fraction of nano-precipitates with a size of less than 50nm in the total precipitates is greater than 8%.

[0019] In some embodiments, the dual-phase steel of the present disclosure has a yield strength of 310-1200MPa.

[0020] In some embodiments, the dual-phase steel of the present disclosure has a hole expansion ratio of 36-55%.

[0021] In some embodiments, the dual-phase steel of the present disclosure has a tensile strength of 450-590MPa, preferably 470-590MPa, a yield strength of 310-370MPa, a product of strength and elongation of 12,000-14,500%, and a hole expansion ratio of 36-40%.

[0022] In some embodiments, the dual-phase steel of the present disclosure has a tensile strength of 591-980MPa, preferably 650-980MPa, a yield strength of 360-580MPa, a product of strength and elongation of 12500-14700%, and a hole expansion ratio of 45-55%.

[0023] In some embodiments, the dual-phase steel of the present disclosure has a tensile strength of 981-1310MPa, preferably 1000-1310MPa, a yield strength of 600-1200MPa, a product of strength and elongation of 11500-16000%, and a hole expansion ratio of 40-50%.

[0024] In some embodiments, in the microstructure of the dual-phase steel according to the present disclosure, martensite has a volume fraction of 32-91%, retained austenite has a volume fraction of 4-15%, and ferrite has a volume fraction of 2-65%.

[0025] In some embodiments, in the dual-phase steel of the present disclosure, the mass fraction of nano-precipitates with a size of less than 50nm in total precipitates is 9-30%. In some embodiments, in the dual-phase steel of the present disclosure, the mass fraction of nano-precipitates with a size of less than 50nm in total precipitates is 9-28%.

[0026] In the composition design of the variable-strength dual-phase steel according to the present disclosure: C: The addition of C element helps to improve the strength of the steel and ensure the occurrence of martensite phase transformation. The content of C is controlled at 0.06%-0.08 in the present disclosure, this is because, when the mass percentage of C is less than 0.06%, it is impossible to ensure sufficient martensite is generated during annealing, and the strength of the steel plate is affected; when the mass percentage of C is higher than 0.08%, a peritectic reaction will occur during the continuous casting process, which is very unsuitable for high drawing speed continuous casting production. Si: Si effectuates solid solution strengthening. Si may help to improve the tempering resistance of martensite and inhibit the precipitation and growth of Fe3C. Especially when the tempering temperature is relatively high, ε carbides may be formed to inhibit the precipitation of Fe3C. However, Si is not good for the surface of the steel plate. The content of Si is controlled at 0.4-0.6%. If the content of Si is lower than 0.4%, it is not conducive to inhibiting the precipitation and growth of Fe3C during tempering. If the content of Si is higher than 0.6%, it tends to affect the quality of surface. Mn: The addition of Mn is conducive to improve the hardenability of the steel and effectively improve the strength of the steel plate. The content of Mn is controlled at 2.4- 2.6% in the present disclosure. The reason is that,a large amount of carbides are generated during the hot coiling and insulation cover annealing process of the present disclosure, resulting in insufficient carbon equivalent of the matrix structure. When the mass percentage of Mn is less than 2.4%, the insufficient carbon equivalent leads to insufficient hardenability, and sufficient martensite cannot be generated during the annealing process, then the strength of the steel plate is insufficient; when the mass percentage of Mn is higher than 2.6%, the carbon equivalent is significantly increased, which has a negative impact on welding performance and delayed cracking resistance. Hence, the content of Mn is controlled at 2.4-2.6% in the present disclosure. Al: The addition of Al has an effect of deoxidation and grain refinement. Therefore, the content of Al is controlled at 0.01 - 0.05% in the present disclosure. Ti: 0.02-0.04% of Ti is added, because Ti is the main compound element of the precipitate, and Ti also shows a strong effect of inhibiting the growth of austenite grains to refine the grains at high temperature. However, too much carbonnitride-forming elements such as Nb and Ti in low-carbon steel may affect the subsequent phase transformation, so the upper limit of the alloying element content needs to be controlled. B: Boron is an element that can significantly improve the hardenability. The addition of boron may promote the formation of martensite and ensure the strength of martensitic steel. However, after the grain boundary defects are filled, if more boron is added, the plasticity will decrease due to the precipitation of "boron phase" at the grain boundary. 0.0015-0.0025% of B is added in the present disclosure. If the content of B is less than 0.0015%, the effect of B is insufficient; and if it is higher than 0.0025%, it is not conducive to the plasticity of steel.

[0027] In the technical solution described in the present disclosure, the impurity elements include P, N, and S. The lower the content of impurity is controlled, the better the implementation effect is. The content of P is controlled at P≤0.015%. MnS formed by S seriously affects the formability, so that the content of S is controlled at S≤0.003%. Since N is prone to causing cracks or bubbles on the slab surface, the conten of N is controlled to be ≤0.005% in the present disclosure.

[0028] The flexible manufacturing method of the dual-phase steel according to the present disclosure comprises the following steps: 1) Smelting and casting The above composition is smelt and cast into a slab; 2) Hot rolling When preparing a dual-phase steel with a tensile strength of 450-590MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860 - 900°C and a coiling temperature is controlled at 231-260°C; When preparing a dual-phase steel with a tensile strength of 591-980MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860 - 900°C and a coiling temperature is controlled at 201-230°C; When preparing a dual-phase steel with a tensile strength of 981-1310MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860 - 900°C and a coiling temperature is controlled at 160-200°C; 3) Hot coiling and insulation cover annealing When preparing the dual-phase steel with a tensile strength of 450-590MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 0.5-2 hours, and an internal heat of the steel coil is utilized by the insulation cover with a temperature dropping rate per hour of less than 6°C / s; When preparing the dual-phase steel with a tensile strength of 591-980MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 2.1-4 hours, and an internal heat of the steel coil is utilized by insulation cover with a temperature dropping rate per hour of less than 6°C / s; When preparing the dual-phase steel with a tensile strength of 981-1310MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 4.1-6 hours, and an internal heat of the steel coil is utilized by insulation cover with a temperature dropping rate per hour of less than 6°C / s; 4) Cold rolling, a cold rolling reduction ratio is controlled at 0 - 50%; 5) Continuous annealing When preparing the dual-phase steel with a tensile strength of 450-590MPa, an annealing temperature is 760-820°C and a holding time is 50-100s; then the steel is cooled at a rate of 3 - 10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 50-150°C / s; When preparing the dual-phase steel with a tensile strength of 591-980MPa, an annealing temperature is 760-820°C and a holding time is 50-100s; then the steel is cooled at a rate of 3- 10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 151-350°C / s; When preparing the dual-phase steel with a tensile strength of 981-1310MPa, an annealing temperature is 760-820°C, and a holding time is 50-100s; then the steel is cooled at a rate of 3 - 10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 351-600°C / s; 6) Tempering When preparing the dual-phase steel with a tensile strength of 450-590MPa, a tempering temperature is 160-260°C and a tempering time is 0.5-3 hours; When preparing the dual-phase steel with a tensile strength of 591-980MPa, a tempering temperature is 160-260°C and a tempering time is 2-6 hours; When preparing the dual-phase steel with a tensile strength of 981-1310MPa, a tempering temperature is 160-260°C and a tempering time is 2-4 hours; 7) Temper rolling, with a temper rolling rate of 0-0.3%.

[0029] Preferably, the casting in step 1) adopts a thin slab continuous casting process.

[0030] Herein, when a cold rolling reduction ratio is 0%, it means that the cold rolling step has not been carried out.

[0031] In some embodiments, when preparing the dual-phase steel with a tensile strength of 450- 590MPa, a holding time in the hot rolling step is 0.5-3.5 hours. When preparing the dual-phase steel with a tensile strength of 450-590MPa, a coiling temperature in the hot rolling step is 232- 260°C.

[0032] In some embodiments, when preparing the dual-phase steel with a tensile strength of 591- 980MPa, a holding time in the hot rolling step is 0.5-2.5 hours. When preparing dual-phase steel with tensile strength of 591-980MPa, a coiling temperature in the hot rolling step is 203-228°C.

[0033] In some embodiments, when preparing the dual-phase steel with a tensile strength of 981-1310MPa, a holding time in the hot rolling step is 1.2-3.2 hours.

[0034] In some embodiments, in the hot coiling insulation cover annealing step, when preparing the dual-phase steel with a tensile strength of 450-590MPa, an annealing time is 0.5-2 hours; when preparing the dual-phase steel with a tensile strength of 591-980MPa, an annealing time is 2.2 - 4 hours; when preparing the dual-phase steel with a tensile strength of 981-1310MPa, an annealing time is 4.3-6 hours.

[0035] In some embodiments, in the continuous annealing step, when preparing the dual-phase steel with a tensile strength of 450-590MPa, cooling to room temperature is carried out at a rate of 50-150°C / s; when preparing the dual-phase steel with a tensile strength of 591-980MPa, cooling to room temperature is carried out at a rate of 160-350°C / s; when preparing the dual-phase steel with a tensile strength of 981-1310MPa, cooling to room temperature is carried out at a rate of 360-600°C / s.

[0036] In the manufacturing method according to the present disclosure: The present disclosure adopts a flexible manufacturing process and adjusts different process controls with respect to different composition design for dual-phase steels with different strength grades, so as to achieve the required mechanical performance.

[0037] The main changes for the flexible process with respect to different strength grades in the present disclosure are as follows: Adjustment of hot rolling and coiling temperature parameters: For a steel with relatively high strength, a lower hot rolling and coiling temperature and an annealing holding temperature of the insulation cover are required, which is to prevent precipitates from growing and to achieve a better precipitation strengthening effect.

[0038] Adjustment of insulation cover annealing parameters: For a steel with higher strength, a longer holding time is required to obtain more precipitates.

[0039] Adjustment of continuous annealing parameters: For a steel with higher strength, a faster cooling rate is required to achieve higher strength. The amount of martensite obtained may be controlled by the different cooling rates of flexible control, resulting in the final products with different strength grades.

[0040] Adjustment of tempering holding time parameters: During the tempering process, for a steel with higher strength, a longer holding time is required to obtain more retained austenite and nano-precipitates. However, if the holding time is too long, the strength of a continuous annealing product may be decreased. Therefore, for different strength grades of steel, it is necessary to provide a reasonable holding time process interval to ensure both strength and sufficient retained austenite and nano-precipitates.

[0041] The insulation cover annealing process parameters, annealing process parameters, and tempering process parameters are flexibly controlled. The amount and size of retained austenite and fine diffused precipitates obtained may be controlled to ensure the volume fraction of retained austenite is greater than 3%, the mass fraction of nano-precipitates with a size of less than 50nm in the total precipitates is greater than 8% in the final microstructure, so as to provide different strength grades and overall performance matching with the products. The dual-phase steel has a tensile strength of 450-1310MPa, a product of strength and elongation of greater than 11000, and a hole expansion ratio of greater than 35%.

[0042] Compared with the prior art, the present disclosure has the beneficial effects as follows: The present disclosure adopts a composition normalized design and a flexible manufacturing technology, and sets different processes of hot coiling insulation cover annealing, insulation cover annealing, continuous annealing and tempering to obtain a dual-phase steel with a tensile strength of 450-1310MPa.

[0043] The development and application of many high-performance steels with different composition and mechanical performance make a body-in-white safe and lightweight. However, those multi-steel scheme creates numerous challenges, including resistance spot welding of dissimilar steels, process optimization and recycling. A single chemical composition is used to produce multiple strength grades of steels required for a body-in-white to addresses this issue in the present disclosure.

[0044] After hot-coiling, an insulation cover annealing is quickly carried out. Combined with the reasonable composition and process design, fine diffusely distributed ε carbides are generated by heat preservation annealing for a long time at a lower temperature. Then, through reasonable process design, the fine diffused ε carbides are inherited into the final continuously annealed product. The fine diffused precipitates in the final annealed product improve the overall strength, reduce the strength difference in each phase, reduce the strength difference between grain boundaries and intragranules, and strengthen the grain boundaries during the deformation process, so as to improve the strength and the hole expansion ratio of the steel. Flexible hot coil coiling temperature and holding time may be controlled to obtain the amount and size of the fine diffused precipitates, ensure that the mass fraction of nano-precipitates with a size of less than 50nm in the total precipitates is greater than 8%, and different strength grades and overall performance matching with the products are obtained.

[0045] Another feature of the present disclosure is the adoption of discontinuous ultra-low temperature long-duration tempering, with the aim of regenerating fine diffusely distributed ε carbides, without reducing the strength of the martensitic structure due to excessive tempering, and allowing the untransformed austenite to be carbon-rich, so as to obtain an adequate amount of retained austenite (more than 3%) + an adequate amount of nanoprecipitates (ensuring that the mass fraction of nanoprecipitates with a size of less than 50nm in the total precipitates is greater than 8% in the final structure) after cooling. It contributes significantly to both strength and elongation. The tempering temperature and tempering time can be flexible adjusted to control the amount and size of fine diffused precipitates obtained, thereby obtaining different strength grades and overall performance matching with the product.

[0046] Compared with the prior art, the technical solutions in Chinese patent applications CN103215516A and CN104451436A obtain sufficient retained austenite by adding high content of Si and Al, and achieve high elongation through the TRIP effect of retained austenite, without considering the hole expansion performance, which is different from the concept of the present disclosure.

[0047] The ultra-high strength steels described in Chinese patent applications CN102776438A and CN102321841A respectively achieve good mechanical performance through the addition of micro-alloys such as niobium, lanthanum, nickel, cadmium, and copper, but they cannot achieve the indicators of high elongation and high hole expansion performance according to the present disclosure.Detailed Description

[0048] The present disclosure will be further explained and illustrated with reference to the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

[0049] Table 1 lists the compositions of the steels of examples in the present disclosure, with a balance of Fe and other unavoidable impurities except P, S and N. Table 2 lists the manufacturing process parameters for the steel plates of examples in the present disclosure. Table 3 lists the microstructure ratios of the steels of examples in the present disclosure. Table 4 lists the relevant performance parameters of the steel plates of examples in the present disclosure.

[0050] The flexible processes comprise the steps as follows: Flexible process 1: When the dual-phase steel has a tensile strength of 450-590MPa, the hot rolling coiling temperature is 231-260°C, the annealing time for the insulation cover annealing is 0.5-2 hours; the steel is cooled to room temperature at a rate of 50 to 150 ° C / s during continuous annealing; and the tempering time is 0.5 - 3 hours; Flexible process 2: When the dual-phase steel has a tensile strength of 591-980MPa, the hot rolling coiling temperature is 201-230°C; the annealing time for the insulation cover annealing is 2.1-4 hours; the steel is cooled to room temperature at a rate of 151-350 °C / s during continuous annealing; and the tempering time is 2-6 hours; Flexible process 3: When the dual-phase steel has a tensile strength of 981-1310MPa, the hot rolling coiling temperature is 160-200 °C; the annealing time for the insulation cover annealing is 4.1 -6 hours; the steel is cooled to room temperature at a rate of 351-600°C / s during continuous annealing; and the tempering time is 2-6 hours.

[0051] As it can be seen from Table 3, the microstructure of the dual-phase steel of the present disclosure comprises ferrite + martensite + retained austenite + nano-precipitates, wherein, the martensite has a volume fraction of 30% or more, retained austenite has a volume fraction of greater than 3%, and the mass fraction of nano-precipitates with a size of less than 50nm in the total precipitates is greater than 8%.

[0052] As it can be seen from Table 4, in the present disclosure a component normalization design and a flexible manufacturing method by adjusting parameters such as continuous annealing and tempering process are used to obtain a martensitic steel with a tensile strength of 450-1310MPa grade. The product of steel plate has good formability and delayed cracking resistance, a product of strength and elongation of greater than 11000, and a hole expansion ratio of greater than 35%. Table 1 (Unit: weight percentages)No.CSiMnAlPSNTiBExample 10.0650.412.430.0150.0130.00270.00450.0210.0024Example 20.0770.422.580.0250.0120.00230.00350.0360.0023Example 30.0710.582.430.0110.0050.00260.00290.0210.0025Example 40.0760.542.470.0410.0090.00170.00360.0260.0022Example 50.0620.532.490.0230.0150.00140.00340.0400.0016Example 60.0670.502.410.0430.0110.00120.00380.0370.0019Example 70.0630.462.530.0130.0120.00180.00240.0340.0015Example 80.0730.492.590.0480.0140.00220.00470.0380.0017Example 90.0780.512.520.0260.0110.00240.00320.0320.0024Example 100.0640.592.550.0360.0100.00240.00210.0370.0024Example 110.0610.482.480.0440.0140.00170.00470.0210.0018Example 120.0790.582.580.0470.0120.00280.00310.0390.0019 Table 2 No.Hot rollingCold rollingAnnealingTemperingHeating temperat ure, °CHoldin g time, hFinal rolling temperature, °CCoiling temperature / insulation cover annealing temperature, °CInsulation cover annealing time, hCold rolling reduction rate, %Annealin g tempera ture, °CHoldin g time, sCooli ng rate v1, C / sRapid cooling start temperatu re, °CRapid cooling speed v2, °C / sRapid cooling final temperatu re, °CTemperi ng tempera ture, °CTemper ing time, hEx.1 (Flexible Process 1)12500.586026022082010010660150Room temperatu re2600.6Ex. 2 (Flexible Process 1)12301.58802501.510810908670100Room temperatu re1601Ex. 3 (Flexible Process 1)11002.59002321.82076080366550Room temperatu re1802Ex. 4 (Flexible Process 1)11503.58902400.5-80050776080Room temperatu re2003Ex. 5 (Flexible Process 2)11301.58702032.2-790503750350Room temperatu re2002Ex. 6 (Flexible Process 2115018602233.4-7806010720300Room temperatu re2605Ex. 7 (Flexible Process 2)11800.58802282.8-770707660160Room temperatu re1606Ex.8 (Flexible Process 2)12302.59002104308101008700250Room temperatu re2204Ex. 9 (Flexible Process 3)11001.28901604.3208158010760600Room temperatu re1604Ex. 10 (Flexible Process 3)12302.2900200630805903700400Room temperatu re2603Ex. 11 (Flexible Process 3)12503.29001904.5507651003660360Room temperatu re2202Ex. 12 (Flexible Process 3)12001.58601805.5-780505680450Room temperatu re1904 Table 3 Volume fraction of Martensite, %Volume fraction of Retained austenite, %Volume fraction of Ferrite, %Mass fraction of 50 nm precipitate in total precipitates, %Example 1324649Example 24055513Example 33985319Example 438125015Example 554104619Example 667151827Example 775121328Example 870102025Example 97991218Example 108710317Example 11917226Example 128191025 Note: The volume fraction and mass fraction of the microstructure are tested in accordance with GB / T 13298. Table 4 Yield strength (MPa)Tensile strength (MPa)Elongation (JIS5# specimen) (%)A product of strength and elongation (%)Hole expansion ratio (%)Example 134747825.412141.237Example 245558621.212423.238Example 336656925.114281.936Example 431854626.114250.640Example 536465422.214518.850Example 654778617.914069.449Example 757697413.112759.455Example 857381616.413382.448Example 9767102313.513810.549Example 10103312289.812034.443Example 11118913108.911712.447Example 12602108714.515761.548 Note: The yield strength, tensile strength and elongation are tested in accordance with GB / T 228.1, and the hole expansion ratio is tested in accordance with GB / T 24524.

Claims

1. A dual-phase steel, which comprises the following chemical compositions in mass percentages: C: 0.06-0.08%, Si: 0.4-0.6%, Mn: 2.4-2.6%, Al: 0.01-0.05%, Ti: 0.02-0.04%, B: 0.0015-0.0025%, P≤0.015%, S≤0.003%, N≤0.005%, with a balance of Fe and other unavoidable impurities; the microstructure of the dual-phase steel comprises ferrite + martensite + retained austenite + nanoprecipitates, wherein, martensite has a volume fraction of 30% or more, retained austenite has a volume fraction of greater than 3%, and the mass fraction of nanoprecipitates with a size of less than 50 nm in total precipitates is greater than 8%; the dual-phase steel has a tensile strength of 450-1310MPa, a product of strength and elongation of greater than 11000%, and a hole expansion ratio of greater than 35%.

2. The dual-phase steel according to claim 1, wherein the balance is Fe and other unavoidable impurities.

3. The dual-phase steel according to claim 1 or 2, wherein the nanoprecipitates are ε carbides.

4. The dual-phase steel according to claim 1 or 2, wherein the martensite has a volume fraction of 32-91%, the retained austenite has a volume fraction of 4-15%, and the ferrite has a volume fraction of 2-65%; and / or the mass fraction of nanoprecipitates with a size of less than 50nm in the total precipitates is 9-30%, such as 9-28%.

5. The dual-phase steel according to any one of claims 1-4, wherein the dual-phase steel has a yield strength of 310-1200MPa.

6. The dual-phase steel according to any one of claims 1-5, wherein the dual-phase steel has a hole expansion ratio of 36-55%.

7. The dual-phase steel according to any one of claims 1-4, wherein the dual-phase steel has a tensile strength of 450-590MPa, preferably 470-590MPa, a yield strength of 310-370MPa, a product of strength and elongation of 12,000-14,500%, and a hole expansion ratio of 36-40%; or the dual-phase steel has a tensile strength of 591-980MPa, preferably 650-980MPa, a yield strength of 360-580MPa, a product of strength and elongation of 12,500-14,700%, and a hole expansion ratio of 45-55%; or the dual-phase steel has a tensile strength of 981-1310MPa, preferably 1000-1310MPa, a yield strength of 600- 1200MPa, a product of strength and elongation of 11500-16000%, and a hole expansion ratio of 40-50%.

8. A manufacturing method of the dual-phase steel according to any one of claims 1 to 7, wherein it comprises the following steps: 1) Smelting and casting The composition according to claim 1 or 2 is smelted, and casted into slabs; 2) Hot rolling When preparing a dual-phase steel with a tensile strength of 450-590MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860-900°C and a coiling temperature is controlled at 231-260°C; When preparing a dual-phase steel with a tensile strength of 591-980MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860-900°C and a coiling temperature is controlled at 201-230°C; When preparing a dual-phase steel with a tensile strength of 981-1310MPa, the slab is heated to 1100-1250°C with a holding time of 0.5 hours or more; a final rolling temperature is controlled at 860-900°C and a coiling temperature is controlled at 160-200°C; 3) Hot coiling and insulation cover annealing When preparing the dual-phase steel with a tensile strength of 450-590MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 0.5-2 hours, and an internal heat of the steel coil is utilized by insulation cover with a temperature dropping rate per hour of less than 6°C / s; When preparing the dual-phase steel with a tensile strength of 591-980MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 2.1-4 hours, and an internal heat of the steel coil is utilized by insulation cover with a temperature dropping rate per hour of less than 6°C / s; When preparing dual-phase steel with a tensile strength of 981-1310MPa, after coiling, an insulation cover annealing is carried out with an annealing time of 4.1-6 hours, and an internal heat of the steel coil is utilized by insulation cover with a temperature dropping rate per hour of less than 6°C / s; 4) Cold rolling, a cold rolling reduction ratio is controlled at 0-50%; 5) Continuous annealing When preparing the dual-phase steel with a tensile strength of 450-590MPa, an annealing temperature is 760-820°C and a holding time is 50-100s; then the steel is cooled at a rate of 3-10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 50-150°C / s; When preparing the dual-phase steel with a tensile strength of 591-980MPa, an annealing temperature is 760-820°C and a holding time is 50-100s; then the steel is cooled at a rate of 3-10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 151-350°C / s; When preparing the dual-phase steel with a tensile strength of 981-1310MPa, an annealing temperature is 760-820°C, and a holding time is 50-100s; then the steel is cooled at a rate of 3-10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 351-600°C / s; 6) Tempering When preparing the dual-phase steel with a tensile strength of 450-590MPa, a tempering temperature is 160-260°C and a tempering time is 0.5-3 hours; When preparing the dual-phase steel with a tensile strength of 591-980MPa, a tempering temperature is 160-260°C and a tempering time is 2-6 hours; When preparing the dual-phase steel with a tensile strength of 981-1310MPa, a tempering temperature is 160-260°C and a tempering time is 2-6 hours; 7) Temper rolling, with a temper rolling rate of 0-0.3%.

9. The manufacturing method of the dual-phase steel according to claim 8, wherein, in step 1), the casting adopts a thin slab continuous casting process.

10. The manufacturing method of the dual-phase steel according to claim 8, wherein, when preparing the dual-phase steel with a tensile strength of 450-590MPa, a holding time in the hot rolling step is 0.5-3.5 hours; when preparing the dual-phase steel with a tensile strength of 450-590MPa, a coiling temperature in the hot rolling step is 232-260°C.

11. The manufacturing method of the dual-phase steel according to claim 8, wherein, when preparing the dual-phase steel with a tensile strength of 591-980MPa, a holding time in the hot rolling step is 0.5-2.5 hours; when preparing dual-phase steel with tensile strength of 591-980MPa, a coiling temperature in the hot rolling step is 203-228°C.

12. The manufacturing method of the dual-phase steel according to claim 8, wherein, when preparing the dual-phase steel with a tensile strength of 981-131 0MPa, a holding time in the hot rolling step is 1.2-3.2 hours.

13. The method for manufacturing dual-phase steel according to claim 8, wherein, in the hot coiling and insulation cover annealing step, when preparing the dual-phase steel with a tensile strength of 450-590MPa, an annealing time is 0.5-2 hours; when preparing the dual-phase steel with a tensile strength of 591-980MPa, an annealing time is 2.2-4 hours; when preparing the dual-phase steel with a tensile strength of 981-1310MPa, an annealing time is 4.3-6 hours.

14. The manufacturing method of the dual-phase steel according to claim 8, wherein, in the continuous annealing step, when preparing the dual-phase steel with a tensile strength of 450-590MPa, the steel is cooled to room temperature at a rate of 50-150°C / s; when preparing the dual-phase steel with a tensile strength of 591-980MPa, the steel is cooled to room temperature at a rate of 160-350°C / s; when preparing the dual-phase steel with a tensile strength of 981-1310MPa, the steel is cooled to room temperature at a rate of 360-600°C / s.

Images