980MPa-grade continuous annealing dual-phase steel with different yield strengths and preparation method of 980MPa-grade continuous annealing dual-phase steel

Abstract

The invention discloses 980MPa-grade continuous annealing dual-phase steel with different yield strengths and a preparation method of the 980MPa-grade continuous annealing dual-phase steel. The 980 MPa grade continuous annealing dual-phase steel with the different yield strengths comprises CR550 / 980DP steel and CR700 / 980DP steel. The preparation method comprises the following steps that (a) a smelting procedure is conducted, specifically, smelting is conducted according to chemical components of 980 MPa grade continuous annealing dual-phase steel with different yield strengths, a plate blank is cast, and the chemical components of the dual-phase steel comprise C, Si, Mn, Cr, a small amount of Nb, Ti, Cu, P, S, Als, N, B and the balance Fe and inevitable impurities; (b) a hot rolling process; (c) pickling and rolling; and (d) carrying out a continuous annealing process to obtain the 980MPa-grade continuous annealing dual-phase steel with different yield strengths.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled strip production, and specifically relates to a 980MPa grade continuous annealing dual-phase steel plate with different yield strengths produced by a continuous annealing unit and a preparation method thereof. Background Art

[0002] The development trend of the modern automobile industry is to reduce fuel consumption and emissions as much as possible while ensuring safety performance. Reducing the weight of the car body by thinning the high-strength steel used in automobiles is considered to be one of the most scientific and effective ways to promote the sustainable development of the automobile industry. Dual-phase steel has the advantages of good strength and plasticity, high work hardening rate, etc., and has become one of the most widely used advanced high-strength steels. According to the requirements of the national standard GB / T20564.2-2017, two grades of products, CR550 / 980DP and CR700 / 980DP, are classified according to different yield strengths to meet the use requirements of different parts.

[0003] In the prior art, duplex steel generally adds a higher Mo content (0.13~0.24%), which increases the alloy cost. In other prior art, water is used to rapidly cool the strip steel (120~150℃ / s) to 50-60℃ during continuous annealing, and then heated to 250℃ at a heating rate of 20~30℃ / s for overaging. Conventional continuous annealing units use gas cooling, which cannot achieve such a high cooling rate and does not have a reheating function. This technology is not easy to promote on conventional continuous annealing units; in addition, the prior art does not consider the production of products with different yield strengths with the same composition to meet the needs of different users.

[0004] Therefore, how to use a continuous annealing unit to produce 980MPa grade continuously annealed dual-phase steel plates with different yield strengths has become a difficult problem that urgently needs to be solved. Summary of the invention

[0005] Based on this, in order to solve the above-mentioned deficiencies in the prior art, a 980MPa grade continuous annealing dual-phase steel plate with different yield strengths produced by a continuous annealing unit and a preparation method thereof are provided.

[0006] In order to achieve the above purpose, the following technical solutions are adopted: The present invention provides a method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths, comprising the following steps: (a) Smelting process: smelting according to the chemical composition of 980 MPa grade continuously annealed dual-phase steel with different yield strengths and casting into slabs, the chemical composition of the dual-phase steel includes, by mass percentage, C: 0.065%-0.105%, Si: 0.35%-0.80%, Mn: 1.95%-2.45%, Cr: 0.50-0.80%, Nb: 0.010-0.035%, Ti: 0.025-0.060%, Cu: 0.020-0.10%, P≤0.014%, S≤0.008%, Als: 0.025%-0.070%, N≤0.0055%, B: 0.0010%-0.0035%, and the remaining elements are Fe and unavoidable impurities; (b) hot rolling process: the slab obtained in step (a) is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling to obtain a hot rolled coil; (c) Pickling process: the hot rolled coil obtained in step (b) is pickled and then cold rolled into thin strip steel; (d) Continuous annealing process: The thin strip steel obtained in step (c) is subjected to heating, soaking, slow cooling, rapid cooling, and overaging steps to obtain 980 MPa grade continuously annealed dual-phase steel with different yield strengths.

[0007] In some embodiments, the chemical composition of the dual-phase steel includes, by mass percentage, C: 0.075%~0.095%, Si: 0.40%~0.60%, Mn: 2.00%~2.25%, Cr: 0.55%~0.75%, Nb: 0.015%~0.030%, Ti: 0.035%~0.055%, Als: 0.027%~0.063%, Cu: 0.035%~0.085%, P≤0.012%, S≤0.005%, N≤0.0035%, B: 0.0015%~0.0035%, and the balance is Fe and unavoidable impurities.

[0008] In some embodiments, the 980 MPa grade continuously annealed dual phase steels with different yield strengths include CR550 / 980DP steel and CR700 / 980DP steel.

[0009] In some embodiments, one or more of the following are included: In step (c), the cold rolling reduction rate is 40%-70%; In step (d), for CR550 / 980DP steel: heating to an ambient temperature of 780°C to 810°C, slowly cooling to a slow cooling end temperature of 660°C to 680°C after holding for a certain period of time, and then rapidly cooling to a rapid cooling end temperature of 265°C to 300°C; the over-aging temperature is 250°C to 285°C, and the skin-pass elongation range is 0.15% to 0.30%; for CR700 / 980DP steel: heating to an ambient temperature of 835°C to 865°C, slowly cooling to a slow cooling end temperature of 715°C to 735°C after holding for a certain period of time, and then rapidly cooling to a rapid cooling end temperature of 300°C to 335°C; the over-aging temperature is 285°C to 320°C, and the skin-pass elongation range is 0.50% to 0.65%.

[0010] In some embodiments, in step (b), for CR550 / 980DP steel: the final rolling temperature is 860°C-910°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 575-625°C; for CR700 / 980DP steel: the final rolling temperature is 840°C-900°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 520-570°C.

[0011] In some embodiments, in step (c), when the thickness of the thin strip steel is the same, the cold rolling reduction ratio of the CR550 / 980DP steel is about 3% higher than that of the CR700 / 980DP steel.

[0012] In some embodiments, in step (d), for CR550 / 980DP steel: the unit speed is 105 m / min-145 m / min; for CR700 / 980DP steel: the unit speed is 90 m / min-130 m / min.

[0013] The present invention also provides a 980 MPa grade continuously annealed dual-phase steel with different yield strengths, including CR550 / 980DP steel and CR700 / 980DP steel, which are prepared using the method as described above.

[0014] In some embodiments, the microstructure of CR550 / 980DP steel is composed of 40%-45% ferrite, 35%-40% martensite and 15%-25% tempered martensite, and its yield strength is 615-690 MPa, tensile strength is 985-1030 MPa, and total elongation A is 80 The average elongation is 7.5%~12.0%, the yield strength ratio is 0.64-0.76, and the hole expansion rate is 30%~43%.

[0015] In some embodiments, the microstructure of CR700 / 980DP steel is composed of 35%-40% ferrite, 20%-25% martensite and 35%-45% tempered martensite, and its yield strength is 730-785 MPa, the tensile strength is 1005-1065 MPa, and the total elongation A is 80 It is 10.5%~13.5%, the uniform elongation is 6.8%~10.5%, the yield strength ratio is 0.69-0.78, and the hole expansion rate is 38%~50%.

[0016] The present invention has the following beneficial technical effects: The 980MPa grade continuously annealed dual-phase steel with different yield strengths disclosed in the present invention achieves the production of 980MPa grade continuously annealed dual-phase steel with different yield strength levels by the same chemical composition through process optimization, which not only reduces the loss caused by mixed casting of steel types, but also improves the production organization rhythm. The Nb-Ti-B composite microalloyed low C-Si-Mn-Cr system is adopted, the Nb element is used to refine the grains, and the Ti element is used to fix the N element to give full play to the hardenability of B. At the same time, through the adjustment of the hot rolling process and the continuous annealing process, the 980MPa grade continuously annealed dual-phase steel with different yield strengths is obtained.

[0017] The 980MPa grade continuously annealed dual-phase steel with different yield strengths disclosed in the present invention: The microstructure of CR550 / 980DP steel is composed of 40%-45% ferrite, 35%-40% martensite and 15%-25% tempered martensite. Its yield strength is 615~690MPa, tensile strength is 985~1030MPa, and total elongation A 80 The average elongation is 7.5%~12.0%, the yield strength ratio is 0.64-0.76, and the hole expansion rate is 30%~43%.

[0018] The microstructure of CR700 / 980DP steel is composed of 35%-40% ferrite, 20%-25% martensite and 35%-45% tempered martensite. Its yield strength is 730~785MPa, tensile strength is 1005~1065MPa, and total elongation A 80 It is 10.5%~13.5%, the uniform elongation is 6.8%~10.5%, the yield strength ratio is 0.69-0.78, and the hole expansion rate is 38%~50%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 The scanning electron microscope image of CR550 / 980DP steel of an embodiment of the present invention; Figure 2 is a scanning electron microscope image of CR700 / 980DP steel according to an embodiment of the present invention; Figure 3 1 is a stress-strain curve of continuously annealed dual-phase steel with different yield strengths of 980 MPa according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0022] It should be understood that the embodiments of the present invention shown in the exemplary embodiments are only illustrative. Although only a few embodiments are described in detail in the present invention, it is easy for those skilled in the art to appreciate that multiple modifications are feasible without actually departing from the teaching of the subject matter of the present invention. Accordingly, all such modifications should be included within the scope of the present invention. Without departing from the gist of the present invention, other replacements, modifications, changes and deletions may be made to the design, operating conditions and parameters of the following exemplary embodiments.

[0023] In an embodiment of the present invention, the main difference between the two grades of products, CR550 / 980DP and CR700 / 980DP, is the difference in their minimum yield strength. CR550 / 980DP is a cold-rolled dual-phase steel, where "CR" stands for cold rolled, 550 means the prescribed minimum yield strength is 550 MPa, and 980 means the prescribed minimum tensile strength is 980 MPa. CR700 / 980DP is also a cold-rolled dual-phase steel, where 700 means the prescribed minimum yield strength is 700 MPa, and 980 means the prescribed minimum tensile strength is 980 MPa. In summary, the main difference between the two grades of products, CR550 / 980DP and CR700 / 980DP, is the difference in their minimum yield strength, and they are suitable for different automobile manufacturing needs. With the continuous development of the automobile industry and the improvement of consumers' requirements for automobile quality, the application prospects of these two high-strength steels in automobile manufacturing will be broader.

[0024] Based on the above purpose, a first aspect of an embodiment of the present invention provides a 980 MPa grade continuously annealed dual-phase steel with different yield strengths, wherein the chemical composition of the dual-phase steel includes, by mass percentage, C: 0.065%~0.105%, Si: 0.35~0.80%, Mn: 1.95%~2.45%, Cr: 0.50~0.80%, Nb: 0.010~0.035%, Ti: 0.025~0.060%, Cu: 0.020~0.10%, P≤0.014%, S≤0.008%, Als: 0.025%~0.070%, N≤0.0055%, B: 0.0010%~0.0035%, and the remaining elements are Fe and unavoidable impurities.

[0025] In a preferred embodiment, the 980 MPa grade continuously annealed dual-phase steel with different yield strengths has the following chemical compositions, by mass percentage: C: 0.075%-0.095%, Si: 0.40%-0.60%, Mn: 2.00%-2.25%, Cr: 0.55%-0.75%, Nb: 0.015%-0.030%, Ti: 0.035%-0.055%, Als: 0.027%-0.063%, Cu: 0.035%-0.085%, P≤0.012%, S≤0.005%, N≤0.0035%, B: 0.0015%-0.0035%, and the balance is Fe and unavoidable impurities.

[0026] Among them, the microstructure of CR550 / 980DP steel is composed of 40%-45% ferrite, 35%-40% martensite and 15%-25% tempered martensite, with a yield strength of 615~690MPa, a tensile strength of 985~1030MPa, and a total elongation of A 80 The microstructure of CR700 / 980DP steel is composed of 35%-40% ferrite, 20%-25% martensite and 35%-45% tempered martensite. Its yield strength is 730~785MPa, tensile strength is 1005~1065MPa, and total elongation A is 11.0%~14.5%, uniform elongation is 7.5%~12.0%, yield strength ratio is 0.64-0.76, and hole expansion ratio is 30%~43%. 80 It is 10.5%~13.5%, the uniform elongation is 6.8%~10.5%, the yield strength ratio is 0.69-0.78, and the hole expansion rate is 38%~50%.

[0027] Carbon (C) is an important element for stabilizing austenite and improving the strength of dual-phase steel. Carbon mainly affects the volume fraction of austenite formed during annealing. The diffusion process of carbon in austenite or ferrite actually plays a role in controlling the growth of austenite grains. As the carbon content increases, the volume fraction of austenite increases, and then the martensitic structure formed after cooling increases, and the hardenability of martensite also increases, resulting in an increase in the strength of the material. At the same time, the plasticity of the material decreases, resulting in an increase in the hardness difference between the two phases. For steel parts with higher flanging and hole expansion requirements, the carbon content should not be too high. If the carbon content is too low, it will lead to insufficient hardenability, resulting in bainite transformation or insufficient strength, and other alloys need to be added to improve the hardenability, resulting in an increase in alloy cost. Therefore, the C content of the present invention is 0.065%~0.105%, preferably 0.075%~0.095%.

[0028] Silicon (Si) plays a significant role in solid solution strengthening in steel, and effectively inhibits the precipitation of carbides and delays pearlite transformation during phase transformation. When the critical zone is kept warm, Si accelerates the diffusion of carbon into austenite, has a significant purification effect on ferrite, improves the purity of ferrite in dual-phase steel, promotes ferrite formation and expands the process window. On the other hand, too high a silicon content will increase the brittleness of martensite, resulting in poor toughness, and form high-melting-point oxides on the surface of the steel plate, affecting the surface quality of the steel plate. It is necessary to control the silicon content at a certain level, which can not only strengthen the ferrite but also not significantly reduce the brittleness and surface quality. Therefore, the Si content of the present invention is: 0.35~0.80%, preferably 0.40%~0.60%.

[0029] Manganese (Mn) is a good deoxidizer and desulfurizer, and is also a commonly used solid solution strengthening element in steel. Mn can combine with C to form a variety of carbides to play a role in precipitation strengthening, and can also be dissolved in the matrix to enhance the solid solution strengthening effect. Mn is easy to combine with S to form a high melting point compound MnS, thereby eliminating or weakening the hot brittleness caused by FeS and improving the hot working properties of steel. Mn can improve the stability of austenite, shift the C curve to the right, and thus significantly reduce the critical cooling rate of martensite. However, when the Mn content is too high, it is easy to enrich the surface to form manganese oxides during the annealing process, resulting in a decrease in surface quality. More importantly, the Mn content is too high and it is easy to cause segregation to form a banded structure, which increases the anisotropy of the material. Therefore, in the present invention, the Mn content is 1.95% to 2.45%, preferably 2.00% to 2.25%.

[0030] Chromium (Cr) is a medium-strong carbide-forming element. Like manganese, it can improve the hardenability of steel. When added to steel in combination with other alloying elements, it can greatly improve the hardenability of steel, thereby delaying pearlite transformation and bainite transformation, and expanding the coiling window. Cr is also a solid solution strengthening element, which plays a role in strengthening the matrix. On the other hand, too high Cr content will greatly improve the hardenability of steel, thereby greatly increasing the strength. Since Cr has obvious cost advantages over Mo, it is added to duplex steel in large quantities. However, Cr will also cause a decrease in plasticity and deteriorate the forming and welding properties of steel. Therefore, the Cr content in the present invention is 0.50~0.80%, preferably 0.55%~0.75%.

[0031] The main function of niobium (Nb) in steel is to refine grains. Nb atoms are relatively large in size and have a significant atomic dragging effect during hot rolling heating to inhibit the growth of austenite grains. During the finishing rolling process, Nb-containing second phases (NbC, NbN) are precipitated to pin grain boundaries to inhibit the coarsening of austenite, and Nb has a significant effect of inhibiting recrystallization. Nb is the most effective and easy-to-operate element for grain refinement. It only requires "low temperature and high reduction rate" to achieve grain refinement and is one of the most widely used microalloying elements. Nb mainly exists in the form of NbC in dual-phase steel, which has significant grain refinement and dispersion precipitation strengthening effects. During continuous annealing and heating, undissolved NbC particles can pin ferrite grain boundaries, thereby refining grains. When the Nb content is too low, the grain refinement effect is not significant, but too high a Nb content will lead to a significant increase in alloy cost. Therefore, the Nb content in the present invention is 0.010~0.035%, preferably 0.015%~0.03%.

[0032] Titanium (Ti) is mainly precipitation strengthened in steel, combining with C and N in steel to form TiC and TiN, which plays a strong precipitation strengthening role and significantly refines the grains. The most important use of Ti in B-containing steel is to fix the N element, so that the B element is in a solid solution state, giving full play to its effect of significantly improving hardenability. In addition, Ti has the effect of significantly inhibiting recrystallization and further refining the grains, but too high a Ti content will significantly increase the deformation resistance during the rolling process and increase the load of the cold rolling unit. On the other hand, high Ti is prone to form liquid TiN, and coarse liquid TiN will cause the plastic toughness of the material to deteriorate dramatically. Therefore, the Ti content in the present invention is 0.025~0.060%, preferably 0.035%~0.055%.

[0033] Copper (Cu) has a good aging strengthening ability. Ferrite can be effectively strengthened through Cu nano-precipitates. After the strip is annealed and cooled to a certain temperature, it needs to be kept warm to effectively produce an aging strengthening effect. In addition, since the dispersed precipitation of Cu can inhibit the formation of coarse TiC and TiN, it plays a role in composite strengthening. However, too high Cu will increase the brittleness of steel, and make the strength of hot-rolled strip too high, and the edge cracks will be serious during cold rolling. Therefore, the content of Cu is set to 0.020~0.10%, preferably 0.035%~0.085%.

[0034] Aluminum (Al) is a common deoxidizer in steel. It can also form AlN to pin the grain boundary, thereby refining the grains. In addition, Al has a similar effect to Si and can inhibit the precipitation of carbides, thereby making the austenite fully carbon-rich. When the Al content is low, it is difficult to remove the O element in the molten steel; when the Al content is too high, a large amount of Al2O3 inclusions are easily generated, affecting the cleanliness of the steel. Therefore, the Als content in the present invention is 0.025% to 0.070%, preferably 0.027% to 0.063%.

[0035] Phosphorus (P) is an impurity element in steel, which is easy to segregate at the grain boundary and weaken the intercrystalline bonding force. During the rapid solidification process, a high P content is easy to cause cracking of the ingot. Therefore, in the present invention, the P content is ≤0.014%, preferably ≤0.012%.

[0036] Sulfur (S) is an impurity element in steel, which is easy to be segregated at the grain boundary and form FeS with low melting point with Fe in steel, which reduces the toughness of steel and easily causes fine microcracks on the surface of the ingot during rapid solidification. Therefore, the S content in the present invention is ≤0.008%, preferably ≤0.005%.

[0037] Boron (B) has the specific effect of improving hardenability and inhibiting recrystallization. B atoms are small and easily segregated at grain boundaries, which greatly inhibits the progress of recrystallization. At the same time, it significantly increases the incubation period of pearlite and bainite, thereby improving hardenability. However, B easily combines with N to form BN, which cannot inhibit recrystallization and improve hardenability. Therefore, the B content in the present invention is 0.0010% to 0.0035%, preferably 0.0015% to 0.0035%.

[0038] Nitrogen (N) is an impurity element in steel. Due to its small atomic size, it is easy to enter the gaps between iron elements, significantly improve lattice distortion and greatly increase strength, but it will significantly deteriorate the plasticity and toughness of steel. At the same time, N is easy to combine with Al, B, and Ti in steel to form second phases such as AlN, BN and TiN, which strengthen and deteriorate the plasticity and toughness of steel. Especially when the N content is too high, it combines with Ti to form liquid TiN, which can reach micron level. Not only does it not play a strengthening role, but it also causes stress concentration and forms crack sources. The content of N element needs to be strictly controlled. Therefore, the N content in the present invention is ≤0.0055%, preferably 0.0035%.

[0039] A second aspect of the embodiments of the present invention provides a method for preparing 980 MPa grade continuously annealed dual-phase steel with different yield strengths, comprising the following steps: (a) Smelting process: Smelting the 980 MPa graded continuously annealed dual-phase steels with different yield strengths according to their chemical composition and casting them into slabs; (b) hot rolling process: the slab obtained in step (a) is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling to obtain a hot rolled coil; (c) Pickling process: the hot rolled coil obtained in step (b) is pickled and then cold rolled into thin strip steel; the cold rolling reduction rate is 40%-70%; (d) Continuous annealing process: The thin strip steel obtained in step (c) is subjected to heating, soaking, slow cooling, rapid cooling, and overaging steps to obtain 980 MPa grade continuously annealed dual-phase steel with different yield strengths.

[0040] In some embodiments, the 980 MPa grade continuously annealed dual phase steels with different yield strengths include CR550 / 980DP steel and CR700 / 980DP steel.

[0041] In some embodiments, in step (d), for CR550 / 980DP steel: heating to an absorptive temperature of 780°C to 810°C, slowly cooling to a slow cooling endpoint temperature of 660°C to 680°C after holding for a certain period of time, and then rapidly cooling to a rapid cooling endpoint temperature of 265°C to 300°C; the over-aging temperature is 250°C to 285°C, and the skin-pass elongation range is 0.15% to 0.30%. As the thickness of the strip increases, the absorptive temperature, the slow cooling endpoint temperature, and the over-aging temperature increase. The degree and skin-pass elongation gradually increase, and the unit speed gradually decreases; for CR700 / 980DP steel: heat to an absorptive temperature of 835℃~865℃, keep warm for a certain period of time and slowly cool to a slow cooling end temperature of 715℃~735℃, then quickly cool to a fast cooling end temperature of 300℃~335℃; the over-aging temperature is 285℃~320℃, the skin-pass elongation range is 0.50~0.65%, and the continuous annealing process is also flexibly adjusted with the thickness of the strip.

[0042] In some embodiments, in step (b), for CR550 / 980DP steel: the final rolling temperature is 860°C-910°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 575-625°C; for CR700 / 980DP steel: the final rolling temperature is 840°C-900°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 520-570°C.

[0043] In some embodiments, in step (c), when the thickness of the thin strip steel is the same, the cold rolling reduction ratio of the CR550 / 980DP steel is about 3% higher than that of the CR700 / 980DP steel.

[0044] In some embodiments, in step (d), for CR550 / 980DP steel: the unit speed is 105 m / min-145 m / min; for CR700 / 980DP steel: the unit speed is 90 m / min-130 m / min.

[0045] Example This embodiment provides four groups of 980MPa grade continuously annealed dual-phase steels with different yield strengths, namely, Examples 1-4, and Comparative Examples 1-2, whose chemical compositions are shown in Table 1; Table 1 Chemical composition of 980 MPa graded continuous annealing duplex steel with different yield strength (wt.%)

[0046] The specific process of the preparation method of the above-mentioned different yield strength 980MPa grade continuous annealing dual-phase steel plate is as follows: A. Smelting process: After the smelting process, a dual-phase steel slab with the chemical composition shown in Table 1 is prepared; B. Hot rolling process: The slab is heated, descaled, hot rolled and laminar cooled to obtain a hot rolled coil. The specific hot rolling process parameters are shown in Table 2. Table 2 Main process parameters of hot rolling of 980 MPa graded dual-phase steel with different yield strengths

[0047] C. Pickling process: After the hot-rolled coil is pickled, it is cold-rolled into thin strip steel.

[0048] D. Continuous annealing process: After continuous annealing of cold-rolled thin strip steel, continuous annealing duplex steel with different yield strength of 980MPa is obtained. Among them, the absorptive temperature of CR550 / 980DP is 780℃~810℃, the slow cooling end temperature is 660℃~680℃, the fast cooling end temperature is 265℃~300℃, the over-aging temperature is 250℃~285℃, the unit speed is 105m / min~145m / min, and the skin-pass elongation is 0.15%~0.30%. As the thickness of the strip increases, the absorptive temperature, slow cooling end temperature, over-aging temperature and skin-pass elongation increase. The rate gradually increases, and the unit speed gradually decreases; the soaking temperature of CR700 / 980DP is 835℃~865℃, the slow cooling end temperature is 715℃~735℃, the fast cooling end temperature is 300℃~335℃, the over-aging temperature is 285℃~320℃, the unit speed is 90m / min~130m / min, and the skin-clearing elongation is 0.50~0.65%. The continuous annealing process is also flexibly adjusted with the thickness of the strip. The specific continuous annealing process parameters are shown in Table 3: Table 3 Main annealing process parameters of 980 MPa grade dual-phase steel with different yield strengths

[0049] The microstructure of the 980MPa graded continuous annealing dual-phase steel with different yield strength prepared by the above process is as follows: Figure 1 to Figure 2 The stress-strain curve is shown in Figure 3 The above continuous annealing dual-phase steel properties are tested according to GB / T228-2010 "Room Temperature Tensile Test Method for Metallic Materials", as shown in Table 4: Table 4 Properties of 980MPa grade continuously annealed dual-phase steel with different yield strengths

[0050] It can be seen that Examples 1-4 achieve the production of 980MPa grade continuously annealed dual-phase steel with different yield strength levels using the same chemical composition, which not only reduces the loss caused by mixed casting of steel grades, but also improves the production organization rhythm. The low C-Si-Mn-Cr system is made of Nb-Ti-B composite microalloying, and the Nb element is used to refine the grains and the Ti element is used to fix the N element to give full play to the hardenability of B. At the same time, through the adjustment of the hot rolling process and the continuous annealing process, 980MPa grade continuously annealed dual-phase steel with different yield strengths is obtained.

[0051] Comparative Case 1 adds less C and alloy elements, and its strength is far lower than the 980MPa grade product (780MPa strength grade product). Comparative Case 2 adds more C, Si and Al elements, which significantly deteriorates the welding performance (higher carbon equivalent Ceq and crack sensitivity index Pcm) and hole expansion performance (only 22%). At the same time, the alloy cost is greatly increased, and the manufacturing difficulty is also significantly increased.

[0052] The above are exemplary embodiments disclosed in the present invention, but it should be noted that various changes and modifications may be made without departing from the scope of the embodiments disclosed in the claims. Although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless explicitly limited to the singular.

[0053] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the above embodiments of the present invention, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths, characterized in that: The following steps are involved: (a) Smelting process: Smelting and casting into slabs according to the chemical composition of 980 MPa grade continuously annealed dual-phase steel with different yield strengths, wherein the chemical composition of the dual-phase steel includes, by mass percentage: C: 0.065%~0.105%, Si: 0.35~0.80%, Mn: 1.95%~2.45%, Cr: 0.50~0.80%, Nb: 0.010~0.035%, Ti: 0.025~0.060%, Cu: 0.020~0.10%, P≤0.014%, S≤0.008%, Als: 0.025%~0.070%, N≤0.0055%, B: 0.0010%~0.0035%, the remaining elements are Fe and unavoidable impurities; (b) hot rolling process: the slab obtained in step (a) is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling to obtain a hot rolled coil; (c) Pickling process: the hot rolled coil obtained in step (b) is pickled and then cold rolled into thin strip steel; (d) Continuous annealing process: The thin strip steel obtained in step (c) is subjected to heating, soaking, slow cooling, rapid cooling, and overaging steps to obtain 980 MPa grade continuously annealed dual-phase steel with different yield strengths.

2. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 1, characterized in that: The chemical composition of the dual-phase steel includes, by mass percentage, C: 0.075%-0.095%, Si: 0.40%-0.60%, Mn: 2.00%-2.25%, Cr: 0.55%-0.75%, Nb: 0.015%-0.030%, Ti: 0.035%-0.055%, Als: 0.027%-0.063%, Cu: 0.035%-0.085%, P≤0.012%, S≤0.005%, N≤0.0035%, B: 0.0015%-0.0035%, and the balance is Fe and unavoidable impurities.

3. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to any one of claims 1 to 2, characterized in that: 980MPa grade continuously annealed dual-phase steels with different yield strengths include CR550 / 980DP steel and CR700 / 980DP steel.

4. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 3, characterized in that: Include one or more of the following: In step (c), the cold rolling reduction rate is 40%-70%; In step (d), for CR550 / 980DP steel: heating to an ambient temperature of 780°C to 810°C, slowly cooling to a slow cooling endpoint temperature of 660°C to 680°C after holding, and then rapidly cooling to a rapid cooling endpoint temperature of 265°C to 300°C; the overaging temperature is 250°C to 285°C, and the skin-pass elongation range is 0.15% to 0.30%; for CR700 / 980DP steel: heating to an ambient temperature of 835°C to 865°C, slowly cooling to a slow cooling endpoint temperature of 715°C to 735°C after holding, and then rapidly cooling to a rapid cooling endpoint temperature of 300°C to 335°C; the overaging temperature is 285°C to 320°C, and the skin-pass elongation range is 0.50% to 0.65%.

5. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 3, characterized in that: In step (b), for CR550 / 980DP steel: the final rolling temperature is 860°C-910°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 575-625°C; for CR700 / 980DP steel: the final rolling temperature is 840°C-900°C, the laminar cooling adopts the front-stage cooling method, the upper and lower header cooling rates are 40%-60% and 65%-85%, and the coiling temperature is 520-570°C.

6. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 4, characterized in that: In step (c), the cold rolling reduction ratio of CR550 / 980DP steel is about 3% higher than that of CR700 / 980DP steel when the thickness of the thin strip steel is the same.

7. The method for preparing 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 4, characterized in that: In step (d), for CR550 / 980DP steel: the unit speed is 105m / min~145m / min; for CR700 / 980DP steel: the unit speed is 90m / min~130m / min.

8. A 980MPa grade continuously annealed dual phase steel with different yield strengths, characterized in that: The steel comprises CR550 / 980DP steel and CR700 / 980DP steel, which are prepared by the method according to any one of claims 1 to 7.

9. The 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 8, characterized in that: The microstructure of CR550 / 980DP steel is composed of 40%-45% ferrite, 35%-40% martensite and 15%-25% tempered martensite. Its yield strength is 615~690MPa, tensile strength is 985~1030MPa, and total elongation A 80 The average elongation is 7.5%~12.0%, the yield strength ratio is 0.64-0.76, and the hole expansion rate is 30%~43%.

10. The 980MPa grade continuously annealed dual-phase steel with different yield strengths according to claim 8, characterized in that: The microstructure of CR700 / 980DP steel is composed of 35%-40% ferrite, 20%-25% martensite and 35%-45% tempered martensite. Its yield strength is 730~785MPa, tensile strength is 1005~1065MPa, and total elongation A 80 It is 10.5%~13.5%, the uniform elongation is 6.8%~10.5%, the yield strength ratio is 0.69-0.78, and the hole expansion rate is 38%~50%.

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