Gigapascal-class bainitic steel having an ultrahigh yield ratio and a method for producing the same

Abstract

The present invention discloses a gigapascal-grade bainite steel having an ultra-high yield ratio, which contains, in addition to Fe, the following chemical elements in mass percent: C: 0.12-0.24%, Si: 0.2-0.5%, Mn: 1.3-2.0%, B: 0.001-0.004%, and at least one of Cr, Nb, Ti, and Mo, with the proviso that Cr≦0.4%, Nb≦0.06%, Ti≦0.1%, and Mo≦0.4%. The present invention also discloses a manufacturing method and annealing process for use in the above-mentioned steel.

Description

Technical Field

[0001] The present invention relates to a steel type and a manufacturing method thereof, particularly to gigapascal-class bainite steel and a manufacturing method thereof.

Background Art

[0002] Under the concept of the new era of "green - safety", as the requirements for the strength of automotive structural members and safety members are increasing day by day, gigapascal-class high-strength steel is becoming one of the automotive structural materials that major automotive manufacturers are most concerned about.

[0003] In recent years, in order to ensure the normal use of automobiles and the safety of passengers, there are more and more automotive structural members (such as body and frame system parts) that require "zero deformation" within the service life. Therefore, high material performance is required, and the higher the yield strength or yield ratio, the better. Currently, materials with high yield strength or high yield ratio are attracting more and more attention from automotive manufacturers, and the market demand for gigapascal steel with high yield strength or high yield ratio is also increasing day by day.

[0004] The gigapascal-class high-strength steel described in conventional invention patents generally does not have a high yield ratio. The yield strength of dual-phase steel, which accounts for 90% of the market share of gigapascal-class automotive high-strength steel, is only 0.6 - 0.75. Although the yield ratio of some other products such as martensite steel, quenching and partitioning steel (Q&P steel), and multiphase steel has increased slightly, it is only about 0.75 - 0.85.

[0005] For example: Chinese Patent Document (Patent Publication CN103361577A, Publication Date October 23, 2013, Title "High Yield Ratio High Strength Steel Sheet with Excellent Workability") discloses a high yield ratio high strength steel sheet, whose microstructure mainly consists of ferrite, martensite, tempered martensite, and bainite. Although its tensile strength reaches 980 MPa or more, the yield ratio is only ≧0.68, still not meeting the latest requirements of the automotive parts market for high yield ratio gigapascal-class steel sheets.

[0006] Also, for example, Chinese Patent Document (Patent Publication No. CN106170574A, publication date November 30, 2016, title "High yield ratio high-strength cold-rolled steel sheet and manufacturing method thereof") discloses a high yield ratio high-strength cold-rolled steel sheet and its manufacturing method. The steel sheet structure mainly includes ferrite, retained austenite, martensite, a small amount of bainite, and tempered ferrite. Although its tensile strength reaches 980 MPa or more, the yield ratio is only ≧0.75 and at most does not exceed 0.8. Therefore, it still cannot meet the market demand for gigapascal-class high-strength steel with a yield ratio of 0.9 or more.

[0007] On the other hand, although some patent documents disclose high yield ratio steel sheets with a yield ratio of 0.9 or more and their manufacturing methods, the tensile strength of the steel sheets described in these patent documents all do not reach the 980 MPa level.

[0008] For example, Chinese Patent Document (Patent Publication No. CN102719736A, publication date October 10, 2012, title "Steel for ultra-fine grain slip path with a yield ratio of 0.9 or more and production method thereof") discloses a steel sheet with a yield ratio of 0.9 or more obtained by using an ultra-fine grain structure, but its tensile strength is only at the 700 MPa level.

[0009] Thus, at present, the tensile strength of the steel sheet reaching the gigapascal level and the yield ratio being 0.9 or more are two mutually contradictory technical indicators. The technical problem behind this contradiction is that the tissue control technology for realizing an ultra-high yield ratio of 0.9 or more is very difficult.

[0010] First, in order to achieve a high yield ratio, the matrix structure in the steel plate needs to be relatively uniform. For example, the matrix consists of single bainite or single martensite. It is difficult to obtain a high yield ratio for a steel plate with a multi-phase or complex-phase matrix structure, such as a steel plate that simultaneously contains ferrite, retained austenite, tempered martensite, and martensite in its matrix structure. In order for the strength of the steel plate to reach the gigapascal level, the mutual cooperation of the multi-phase structure is required, such as typical ferrite / martensite dual-phase steel and advanced high-strength steel containing retained austenite with the TRIP effect introduced. This is the first layer of technical contradiction.

[0011] Even for a single bainite or single martensite structure, due to the movement of transformation and work hardening caused by processing strain, it is difficult for the yield ratio of the steel plate to reach 0.9 or more. Usually, the yield ratio of a steel plate with a single martensite or bainite matrix is about 0.8 - 0.9.

[0012] Therefore, in order to further obtain a steel plate with an ultra-high yield ratio and increase the yield strength of the material, a complex component process design is required to prevent the movement of transformation. For example: Chinese Patent Document (Patent Publication CN101910436A, Publication Date December 8, 2010, Title "High-Strength Cold-Rolled Steel Plate with Excellent Weather Resistance and Its Manufacturing Method") discloses a method of increasing the yield strength of the material by introducing a large amount of expensive solid solution alloys such as Cr, Zr, Co, and W. However, considering the complex manufacturing process and relatively high alloy addition amount in conventional gigapascal-class ultra-high-strength steel, there are doubts about whether the above-mentioned complex process technology or the addition of expensive alloys for further increasing the yield ratio is suitable for introduction into conventional gigapascal-class ultra-high-strength steel with an already extremely complex structure. This is the second layer of technical contradiction.

[0013] Therefore, in order to obtain a gigapascal-class ultra-high-strength steel with a yield ratio of 0.9 or more, it is necessary to overcome numerous technical difficulties such as the above-mentioned first layer of technical contradiction and the second layer of technical contradiction, which has not yet been realized in conventional patent technology.

[0014] Based on this, in order to solve the above problems, gigapascal-class bainitic steel with an ultra-high yield ratio is expected. This gigapascal-class bainitic steel simultaneously has an ultra-high yield ratio, ultra-high strength, excellent hole expansion performance and bending performance, and can be used in the manufacture of automotive structural members to realize the new design concept of "green-safety" for automobiles.

Summary of the Invention

Problems to be Solved by the Invention

[0015] One object of the present invention is to provide gigapascal-class bainitic steel with an ultra-high yield ratio. The present invention can obtain gigapascal-class bainitic steel with an ultra-high yield ratio through reasonable chemical composition design. This gigapascal-class bainitic steel has a tensile strength ≥ 980 MPa, a yield strength ≥ 900 MPa, a yield ratio ≥ 0.9, and a hole expansion rate ≥ 55%. Since it simultaneously has an ultra-high yield ratio, ultra-high strength, excellent hole expansion performance and bending performance, it can be used in the production of automotive structural members and has good general prospects and application values.

Means for Solving the Problems

[0016] To achieve the above object, the present invention proposes gigapascal-class bainitic steel with an ultra-high yield ratio, which contains the following chemical elements in mass percentage content in addition to Fe and inevitable impurities: C: 0.12 - 0.24%; Si: 0.2 - 0.5%; Mn: 1.3 - 2.0%; B: 0.001 - 0.004%; Al: 0.01 - 0.05%; At least one of Cr, Nb, Ti, Mo, provided that Cr ≤ 0.4%, Nb ≤ 0.06%, Ti ≤ 0.1%, Mo ≤ 0.4%.

[0017] Furthermore, in the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention, the mass percentage content of each chemical element is: C: 0.12 to 0.24%; Si: 0.2 to 0.5%; Mn: 1.3 to 2.0%; B: 0.001 to 0.004%; Al: 0.01 to 0.05%; At least one of Cr, Nb, Ti, Mo, provided that Cr ≤ 0.4%, Nb ≤ 0.06%, Ti ≤ 0.1%, Mo ≤ 0.4%; The balance is Fe and other inevitable impurities.

[0018] In the technical solution according to the present invention, specifically, the design principle of each chemical element is as follows. C: In the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the C element is one of the important elements for controlling the phase transformation of the microstructure in carbon steel. At the same time, the C element also has a great influence on the strength of the steel plate. The C element can form alloy carbides together with other alloy elements to increase the strength of the steel plate. If the C element content in the steel is less than 0.12%, the strength of the steel does not meet the target requirements; also, if the C element content in the steel exceeds 0.24%, martensite structure and coarse cementite are likely to be generated, and the performance of the steel plate deteriorates. Based on this, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of C is between 0.12% and 0.24%.

[0019] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the C element may also be between 0.15% and 0.20%.

[0020] Si: In the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the Si element is an essential element for deoxidation in steelmaking. It not only has a certain solid solution strengthening effect but also has a certain influence on the formation of bainite (the higher the content of the B element in the steel, the easier it is to form carbon-free bainite). It should be noted that if the Si element content in the steel is less than 0.2%, it is difficult to obtain a sufficient deoxidation effect; also, if the Si element content in the steel exceeds 0.5%, it is easy to form color differences in the iron oxide scale or tiger stripe patterns, which is disadvantageous to the surface quality of the steel sheet for automobiles. Based on this, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of Si is between 0.2% and 0.5%.

[0021] Mn: In the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the Mn element is a main additive element and one of the important elements for controlling the phase transformation in the steel. It should be noted that since the Mn element has a low cost, it is not only an effective element for increasing the strength of the steel but also a relatively important solid solution strengthening element. However, it should be noted that the Mn element content in the steel should not be too high. If the Mn element content in the steel is too high, the corrosion resistance and welding performance will deteriorate, and at the same time, the grain size tends to coarsen, and the plasticity and toughness of the steel will decrease. Based on this, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of Mn is between 1.3% and 2.0%.

[0022] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Mn element may also be between 1.6% and 2.0%.

[0023] B: In the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the B element is not only beneficial to the formation of bainite in the steel, but also has a great influence on the strength and hardness of the steel plate. It should be noted that when the content of the B element in the steel is less than 0.001%, the strength of the steel does not meet the target requirements; also, when the content of the B element in the steel exceeds 0.004%, brittle borides are likely to be generated, affecting the hole expansion performance and bending performance of the steel plate. Based on this, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of B is set between 0.001% and 0.004%.

[0024] Al: In the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the Al element is added to the steel only as a deoxidizing element, which can remove the O element in the steel and ensure the performance and quality of the steel. Therefore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of Al is set between 0.01% and 0.05%. In the prior art, in order to bring about solid solution strengthening, the Al element is added in a large amount (≥0.1%) to the steel as a ferrite-forming element and an element for suppressing carbide precipitation, or the phase transition temperature, bainite formation kinetics, and carbide precipitation kinetics are changed by adding Al to change the phase transition of the steel material, forming retained austenite or carbon-free bainite, and finally increasing the strength of the steel material. However, since the component control and process adjustment of the present invention can already obtain a gigapascal-class bainite steel with an ultra-high yield ratio, there is no need to add a large amount of the Al element, and an increase in cost and a significant increase in the difficulty of steelmaking can be avoided.

[0025] Ti, Cr, Nb, and Mo: In the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention, Ti, Cr, Nb, and Mo are added to the steel as optional alloying elements to form the precipitation of fine and dispersed carbide secondary phases, and further increase the strength and yield ratio of the steel plate. Also, it should be noted that Cr and Mo elements can extend the growth periods of pearlite and ferrite in the CCT curve and suppress the formation of pearlite ferrite. Therefore, a bainite structure is easily obtained during cooling, and the hole expansion rate of the steel is easily increased.

[0026] Thus, the above four alloying elements affect the microstructure control of the steel plate and the corresponding annealing process, and the influencing factors on carbide formation directly affect the formation ratio and morphology of carbides. Based on this, in the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention, the mass percentages of Cr, Nb, Ti, and Mo are Cr ≤ 0.4%, Nb ≤ 0.06%, Ti ≤ 0.1%, and Mo ≤ 0.4%, respectively.

[0027] Also, due to the addition of the above alloying elements, the cost of the material increases. Therefore, considering the performance and cost management comprehensively, the technical solution according to the present invention preferably adds at least one of Cr, Nb, Ti, and Mo to the steel. In some preferred embodiments, the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention contains at least 0.1 - 0.4% of Cr. In some preferred embodiments, the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention contains at least 0.1 - 0.4% of Mo. In some preferred embodiments, the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention contains at least one or both of Cr and Mo. In some preferred embodiments, the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention contains at least 0.1 - 0.4% of Cr and 0.1 - 0.4% of Mo.

[0028] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the mass percentage of each chemical element satisfies at least one of the following items: C: 0.15 to 0.20%, Mn: 1.6 to 2.0%.

[0029] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, among other inevitable impurities, P ≤ 0.015% and / or S ≤ 0.004%.

[0030] In the above technical solution, both P and S are impurity elements in the steel. As long as it is technically possible, in order to obtain quenched and tempered steel with better performance and superior quality, the content of impurity elements in the steel should be reduced as much as possible.

[0031] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, at least one of the following chemical elements is further contained: 0 < Cu ≤ 0.2%, 0 < Ni ≤ 0.2%, 0 < V ≤ 0.2%, 0 < Ce ≤ 0.2%.

[0032] In the technical solution according to the present invention, the above Cu, Ni, V, and Ce elements can all further improve the performance of the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention.

[0033] Furthermore, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention satisfies 0.18 ≤ M ≤ 0.27, where M = Cr / 2.5 + Ti + V / 5 + Nb / 1.7 + Mo / 1.7, and where Cr, V, Nb, Ti, and Mo represent the numerical values before the percentage symbols of the mass percentage contents of the respective chemical elements.

[0034] In the above technical solution, it should be noted that in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, in order to control the mass percentage of a single chemical element and obtain a gigapascal-class bainite steel with better performance and quality, preferably, M can be set to 0.18 ≤ M ≤ 0.27. However, M = Cr / 2.5 + Ti + V / 5 + Nb / 1.7 + Mo / 1.7, where Cr, V, Nb, Ti, and Mo represent the numerical values before the percentage symbols of the mass percentage content of each chemical element.

[0035] It should be noted that in the present invention, if M is too high, coarse carbides are likely to form, and the hole expansion rate and bending performance of the steel deteriorate; also, if M is too low, sufficient carbide precipitation phases cannot be formed, and the strength and yield ratio of the steel are insufficient. Therefore, in the present invention, in order to ensure that nano-order, sub-micro-order or micro-order granular carbides are dispersed and precipitated in the steel and the diameter size of the maximum granular carbide precipitation phase, M can be set to 0.18 ≤ M ≤ 0.27.

[0036] Furthermore, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention satisfies 0.20 ≤ C b ≤ 0.27, where the content C b of the equivalent bainite carbon element = C - (Mo + Nb) / 8 - (Ti + V) / 4 - Cr / 12 + Ni / 10 + Mn / 20 + B × 10, and each element in the formula represents the numerical value before the percentage symbol of the mass percentage content of this element.

[0037] In the above technical solution, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, since the alloying elements and the M value affect the precipitation of carbides, the content C b of the equivalent bainite carbon element in the steel is indirectly affected. It should be noted that in the present invention, if C b is too low, a sufficient single bainite matrix structure cannot be formed; also, if C bIf it is too high, the hardness of the bainite becomes too large, and the bending and hole expansion performance of the steel deteriorates. Therefore, in the present invention, in addition to controlling the mass percentage of a single chemical element, in order to effectively ensure that the phase proportion of acicular lower bainite in the steel is 90% or more, preferably, C b may be set to 0.20 ≤ C b ≤ 0.27.

[0038] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, its microstructure is mainly acicular lower bainite, and the phase proportion of acicular lower bainite is 90% or more.

[0039] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, its microstructure further contains a precipitation phase of nano-order, sub-micro-order or micro-order granular carbides that are dispersed and precipitated, and the total of the phase proportion of the precipitation phase of granular carbides + acicular lower bainite is 99% or more.

[0040] Furthermore, in the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, the diameter of the largest precipitation phase of granular carbides is 2 μm or less.

[0041] Furthermore, the gigapascal-class bainite steel with an ultra-high yield ratio described in the present invention has a tensile strength of ≥ 980 MPa, preferably ≥ 1000 MPa, a yield strength of ≥ 900 MPa, preferably ≥ 950 MPa, a yield ratio of ≥ 0.9, preferably ≥ 0.95, and a hole expansion rate of ≥ 55%, preferably ≥ 60%. In a preferred embodiment, for the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention, its tensile strength ≥ 1000 MPa, yield strength ≥ 910 MPa, yield ratio ≥ 0.9, and hole expansion rate ≥ 55%.

[0042] Furthermore, the gigapascal-class bainite steel with an ultra-high yield ratio according to the present invention has a yield strength of ≥ 950 MPa and a yield ratio of ≥ 0.95; more preferably, its tensile strength is ≥ 1000 MPa and the hole expansion rate is ≥ 60%.

[0043] Furthermore, the gigapascal-class bainitic steel with an ultra-high yield ratio according to the present invention has an elongation rate of ≧9.0%. Another object of the present invention is to provide an annealing process for the gigapascal-class bainitic steel with the above-mentioned ultra-high yield ratio. This annealing process has an important effect on the performance of the steel, and by reasonable process design and control of related process parameters, the gigapascal-class bainitic steel with an ultra-high yield ratio can be obtained.

[0044] To achieve the above object, the present invention proposes an annealing process for the gigapascal-class bainitic steel with the above-mentioned ultra-high yield ratio, including the following steps: (a) In the heating stage, heat to the soaking temperature Ts at a heating rate of 50 °C / s or less. However, Ts is 840 - 900 °C; (b) In the soaking stage, hold at the temperature Ts for 5 minutes or less; (c) In the slow cooling stage, cool to (Ts - 80) - (Ts - 140) °C at a first cooling rate of 15 °C / s or less; (d) In the rapid cooling stage, cool to (Ts - 490) - (Ts - 440) °C at a second cooling rate of (130 - Q) °C / s or less; however, Q = C×180 + Si×10 + Mn×30 + Ni×50 + Cr×15 + Mo×15 + B×2000, and each element in the formula represents the numerical value before the percentage symbol of the mass percentage content of this element; (e) In the self-regulating temperature control cooling stage, cool for 10 - 40 s at a third cooling rate. However, [(Q - 80) / 12] ℃ / s ≦ third cooling rate ≦ [(Q - 80) / 8] ℃ / s ; (f) Finally, in the air cooling stage, air-cool the strip to room temperature.

[0045] In the technical solution according to the present invention, it should be noted that the above-mentioned annealing process includes a heating stage, a soaking stage, a slow cooling stage, a rapid cooling stage, a self-regulating temperature control cooling stage, and an air cooling stage, which plays an important role in the performance of the gigapascal-class bainitic steel according to the present invention.

[0046] In step (a), in the heating stage, it is necessary to heat to a soaking temperature Ts of 840 - 900°C at a heating rate of 50°C / s or less, preferably to a soaking temperature of 840 - 870°C. However, the heating rate in the heating stage should not be too high. Otherwise, the uniformity of the strip structure will decrease. Also, it should be noted that if the soaking temperature Ts is lower than the above-mentioned soaking design temperature range, more than 90% acicular lower bainite structure cannot be obtained in the strip; if the soaking temperature Ts exceeds the above-mentioned soaking design temperature range, the crystal grains of the strip will coarsen and the forming performance of the steel will deteriorate. In some embodiments, the heating rate in step (a) is 5 - 45°C / s.

[0047] In step (b), preferably, the holding time is 1 minute or more. For example, the holding time is 1 minute to 4.5 minutes.

[0048] In step (c), in the slow cooling stage, it is necessary to cool to (Ts - 80) - (Ts - 140)°C at a first cooling rate of 15°C / s or less. However, the first cooling rate in the slow cooling stage should not be too high. Otherwise, not only energy will be wasted, but the strip structure will become non-uniform. Also, it should be noted that if the slow cooling temperature is lower than the above-mentioned slow cooling design temperature range, more than 90% bainite structure cannot be obtained in the strip; if the slow cooling temperature is higher than the above-mentioned slow cooling design temperature range, in the subsequent rapid cooling stage, higher cooling capacity and higher temperature accuracy control ability are required, so the deterioration of the strip structure uniformity due to insufficient cooling capacity or temperature accuracy control ability is likely to occur, and the product performance will deteriorate. Preferably, the first cooling rate in step (c) is 5 - 15°C / s, preferably 5 - 12°C / s.

[0049] In step (d), in the rapid cooling stage, it is necessary to cool to (Ts - 490)~(Ts - 440)°C at a second cooling rate of (130 - Q)°C / s or more; provided that Q = C×180 + Si×10 + Mn×30 + Ni×50 + Cr×15 + Mo×15 + B×2000. However, if the second cooling rate in the rapid cooling stage is insufficient or the cooling temperature exceeds (Ts - 440)°C, the phase transition of bainite will occur earlier than expected, and a high-temperature bainite structure (such as upper bainite or equiaxed bainite) will be generated. Therefore, not only can it not be ensured that the phase proportion of acicular lower bainite in the steel is 90% or more, but also the latent heat of phase transition will be significantly reduced, subsequent self-recuperation temperature-controlled cooling cannot be realized, the material structure will become abnormal, and the steel plate and steel strip cannot obtain an ultra-high yield ratio. Also, if the cooling temperature in the rapid cooling stage is less than (Ts - 490)°C, a martensite structure will be generated, and the hole expansion rate and bending performance of the steel will decrease.

[0050] In step (e), in the self-recuperation temperature-controlled cooling stage, when the strip steel is executed as per the design parameters in the rapid cooling stage, the self-recuperation phenomenon of temperature is realized due to the large release of the latent heat of phase transition of the strip steel. Due to the self-recuperation temperature, the temperature of the strip steel can rapidly, uniformly, and efficiently rise by 50~120°C, so the uniform and dispersed precipitation of carbides is promoted. In order to fully precipitate carbides and keep the precipitation size fine, it is necessary to control the strip steel temperature and cool for 10~40 s at the third cooling rate, provided that [(Q - 80) / 12] ℃ / s ≤ the third cooling rate ≤ [(Q - 80) / 8] ℃ / s .

[0051] It should be noted that if the third cooling rate in the self-recuperation temperature-controlled cooling stage is too low or the control cooling time is too long and does not meet the above design requirements of the present invention, the carbide precipitation is likely to coarsen, and the hole expansion rate and bending performance will deteriorate; also, if the third cooling rate is too high or the control cooling time is too short, the carbide precipitation is likely to be insufficient, and the steel cannot obtain an ultra-high yield ratio performance with a yield ratio of 0.9 or more.

[0052] Another object of the present invention is to provide a method for manufacturing a gigapascal-class bainite steel having the above-described ultra-high yield ratio. According to this manufacturing method, the gigapascal-class bainite steel having an ultra-high yield ratio according to the present invention can be effectively produced.

[0053] To achieve the above object, the present invention proposes a method for manufacturing a gigapascal-class bainite steel having an ultra-high yield ratio, including the following steps: (1) Smelting and casting; (2) Hot rolling; (3) Cooling and coiling after rolling; (4) Pickling and cold rolling; (5) The annealing process described above.

[0054] In the technical solution according to the present invention, in the above manufacturing method, the operation steps in steps (1) to (4) of the pre-annealing process are mainly for obtaining a steel plate or steel strip with uniform components and initial structure, and for achieving uniform and stable structure and performance during the subsequent annealing process. The annealing process in step (5) has an important effect on the performance of the steel plate.

[0055] Furthermore, in the manufacturing method according to the present invention, in step (2), the heating temperature is set to 1150 - 1260°C; the starting temperature of finish rolling is set to 1100 - 1220°C, and the finishing temperature of finish rolling is set to 900 - 950°C.

[0056] Furthermore, in the manufacturing method according to the present invention, in step (3), the cooling rate is set to 30 - 150°C / s, Volume and the coiling temperature is set to 450 - 580°C.

[0057] Furthermore, in the manufacturing method according to the present invention, in step (4), the cold rolling reduction ratio is set to 50% or less.

[0058] Furthermore, in the manufacturing method according to the present invention, the gigapascal-class bainite steel having the ultra-high yield ratio is the gigapascal-class bainite steel having an ultra-high yield ratio according to any one of the embodiments in the present text.

Advantages of the Invention

[0059] The gigapascal-class bainitic steel with an ultra-high yield ratio and its manufacturing method according to the present invention have the following advantages and beneficial effects as compared with the prior art: Based on the premise that the chemical element components and processes are relatively simple and controllable, through the optimal combination of alloying elements and the innovative adjustment of the annealing process, it is ensured that the matrix structure of the steel plate is a simple and single bainite structure. By introducing the release of the latent heat of phase transition to realize the self-recuperation temperature of the steel strip, not only the energy consumption is reduced, but also the rapid, uniform, and efficient control of the recuperation temperature of the strip steel is realized, inducing the fine second-phase dispersion precipitation, and a gigapascal-class bainitic steel with an ultra-high yield ratio and good formability can be obtained.

[0060] The present invention can obtain a gigapascal-class bainitic steel with an ultra-high yield ratio, with a tensile strength ≥ 980 MPa, a yield strength ≥ 900 MPa, a yield ratio ≥ 0.9, and a hole expansion ratio ≥ 55% through reasonable chemical composition design. This gigapascal-class bainitic steel simultaneously has an ultra-high yield ratio, ultra-high strength, excellent hole expansion performance, and bending performance, can be used for manufacturing automobile structural members, meets the new design concept of "green-safety" for automobiles, and has good general prospects and application values.

[0061] The annealing process according to the present invention has an important effect on the performance of the steel. This annealing process includes a heating stage, a soaking stage, a slow cooling stage, a rapid cooling stage, a self-recuperation temperature control cooling stage, and an air cooling stage, and through reasonable process design and control of related process parameters, a gigapascal-class bainitic steel with an ultra-high yield ratio can be obtained.

[0062] Moreover, the production process of the manufacturing method according to the present invention is unique. By adopting the above-mentioned annealing process, the performance of the obtained gigapascal-class bainitic steel is ensured. The obtained gigapascal-class bainitic steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion performance and bending performance.

Brief Description of the Drawings

[0063]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0064] Hereinafter, based on the drawings and specific examples, the gigapascal-class bainite steel having an ultra-high yield ratio according to the present invention and its manufacturing method will be further detailedly interpreted and described. However, the interpretation and description do not inappropriately limit the technical solution of the present invention.

[0065] Examples 1-14 and Comparative Examples 1-10 The gigapascal-class bainite steel having an ultra-high yield ratio of Examples 1-14 was produced by the following steps: (1) Smelting and casting were carried out with the chemical composition shown in Table 1.

[0066] (2) Hot rolling: The heating temperature was set to 1150 - 1260 °C; the finishing rolling start temperature was set to 1100 - 1220 °C, and the finishing rolling end temperature was set to 900 - 950 °C.

[0067] (3) Cooling after rolling and Volume Quenching: The cooling rate was set to 30 - 150 °C / s, Volume and the quenching temperature was set to 450 - 580 °C.

[0068] (4) Pickling and cold rolling: The cold rolling reduction rate was set to 50% or more. (5) Annealing.

[0069] It should be noted that in step (5), the annealing process includes the following steps: (a) In the heating stage, it was heated to the soaking temperature Ts at a heating rate of 50 °C / s or less. However, Ts was set to 840 - 900 °C.

[0070] (b) In the soaking stage, it was held at the temperature Ts for 5 minutes or less. (c) In the slow cooling stage, it was cooled to (Ts - 80) - (Ts - 140) °C at a first cooling rate of 15 °C / s or less.

[0071] (d) In the rapid cooling stage, it was cooled to (Ts - 490) - (Ts - 440) °C at a second cooling rate of (130 - Q) °C / s or more; however, Q = C×180 + Si×10 + Mn×30 + Ni×50 + Cr×15 + Mo×15 + B×2000.

[0072] (e) In the self - returning temperature control cooling stage, it was cooled for 10 - 40 s at a third cooling rate. However, [(Q - 80) / 12] ℃ / s ≤ third cooling rate ≤ [(Q - 80) / 8] ℃ / s 。

[0073] (f) Finally, in the air - cooling stage, the strip was air - cooled to room temperature. Also, it should be noted that the gigapascal - class bainitic steel with ultra - high yield ratio in Examples 1 - 14 according to the present invention was all produced in the above steps, and its chemical composition and related process parameters all meet the design specification control requirements of the present invention.

[0074] Also, the comparative steels of Comparative Examples 1 - 10 were similarly produced in the steps of smelting and casting, hot rolling, post - rolling cooling, Volume pickling, cold rolling, and annealing. However, the chemical compositions and related process parameters of Comparative Examples 1 - 6 do not all meet the parameter requirements designed in the present invention; although the chemical compositions of Comparative Examples 7 - 14 meet the design requirements of the present invention, there are process parameters that do not all meet the design requirements of the present invention.

[0075] However, in the examples and comparative examples of the present invention, the chemical element components of Comparative Example 7 and Example 1 are the same, the chemical element components of Comparative Example 8 and Example 2 are the same, the chemical element components of Comparative Example 9 and Example 6 are the same, and the chemical element components of Comparative Example 10 and Example 11 are the same.

[0076] Table 1 shows the mass percentage blending ratios (%) of each chemical element of the gigapascal-class bainitic steel with an ultra-high yield ratio in Examples 1-14 and the comparative steels in Comparative Examples 1-10.

[0077] [Table 1]

[0078] Note: In the above table, Cb = C - (Mo + Nb) / 8 - (Ti + V) / 4 - Cr / 12 + Ni / 10 + Mn / 20 + B×10, and each element in the formula represents the numerical value before the percentage symbol of the mass percentage content of this element; M = Cr / 2.5 + Ti + V / 5 + Nb / 1.7 + Mo / 1.7, where Cr, V, Nb, Ti, and Mo represent the numerical values before the percentage symbol of the mass percentage content of each chemical element.

[0079] Tables 2-1 and 2-2 show the specific process parameters of the gigapascal-class bainitic steel with an ultra-high yield ratio in Examples 1-14 and the comparative steels in Comparative Examples 1-10.

[0080] [Table 2-1]

[0081] [Table 2-2]

[0082] (Continued from Table 2-2) TIFF0007701973000004.tif35170

[0083] Note: In the above table, Q = C × 180 + Si × 10 + Mn × 30 + Ni × 50 + Cr × 15 + Mo × 15 + B × 2000, and each element in the formula represents the numerical value before the percentage symbol of the mass percentage content of this element.

[0084] For the gigapascal-class bainitic steel with ultra-high yield ratio of Examples 1-14 and the comparative steels of Comparative Examples 1-10, relevant mechanical property measurements were carried out, and the mechanical property measurement results of each obtained example and comparative example are listed in Table 3. The relevant performance measurement means are as follows.

[0085] Samples were taken respectively for the gigapascal-class bainitic steel with ultra-high yield ratio of Examples 1-14 and the comparative steels of Comparative Examples 1-10, and the yield strength and tensile strength of the steel were measured with JIS 5# tensile samples along the transverse direction, and the hole expansion rate and bending performance of the steel were measured in the middle region of the plate.

[0086] However, the hole expansion rate of the steel was measured in the hole expansion test. A sample with a hole in the center of the male die was pressed into the female die, and the central hole of the sample was expanded until necking or through cracking appeared at the edge of the plate hole. Since the method of creating the initial hole in the center of the sample and the quality of the corresponding initial hole edge have a great influence on the measurement results of the hole expansion rate, the test and measurement method was carried out according to the hole expansion rate measurement method defined in the ISO / DIS16630 standard, and the central initial hole adopted the punch hole form (corresponding to the processing method with the worst quality of the initial hole edge). The 180° bending test was carried out according to the measurement method of bending performance defined in the GB / T232-2010 standard (bending diameter d = 1a).

[0087] Table 3 shows the measurement results of the mechanical properties of the gigapascal-class bainitic steel with ultra-high yield ratio of Examples 1-14 and the comparative steels of Comparative Examples 1-10.

[0088]

Table 3

[0089] As can be seen from Table 3, compared with the comparative steels of Comparative Examples 1-10, the gigapascal-class bainite steel with an ultra-high yield ratio of Examples 1-14 of the present invention has significantly superior mechanical properties.

[0090] The gigapascal-class bainite steel with an ultra-high yield ratio of Examples 1-14 of the present invention simultaneously has an ultra-high yield ratio, ultra-high strength, excellent hole expansion performance and bending performance, and its tensile strength is all ≧980 MPa, the yield strength is all ≧900 MPa, the yield ratio is all ≧0.9, and the hole expansion rate is all ≧55%.

[0091] In individual preferred embodiments, for example, in Example 1, the gigapascal-class bainite steel with an ultra-high yield ratio of Example 1 has a yield strength ≧950 MPa and a yield ratio ≧0.95, and has an ultra-high yield ratio and an ultra-high yield strength.

[0092] Figure 1 is a microstructural photograph of the gigapascal-class bainite steel of Example 1 magnified 3000 times.

[0093] As shown in Figure 1, the gigapascal-class bainite steel of Example 1 was cooled to the lower bainite phase region (the rapid cooling temperature meets the invention requirements) at a sufficiently fast cooling rate (the second cooling rate meets the invention requirements) in the rapid cooling stage. Therefore, the matrix of its microstructure is acicular lower bainite, and because the cooling rate in the self-recuperating temperature-controlled cooling stage is appropriate (the third cooling rate meets the invention requirements), the structure contains fine and dispersed precipitation of nano-order, sub-micro-order or micro-order granular carbide precipitation phases. However, the phase proportion of acicular lower bainite is 90% or more, the total phase proportion of granular carbide precipitation phase + acicular lower bainite is 99% or more, and the diameter of the largest granular carbide precipitation phase is 2 μm or less.

[0094] Figure 2 is a microstructural photograph of the comparative steel of Comparative Example 7 magnified 3000 times. As shown in Fig. 2, since the cooling rate during the rapid cooling stage cooling of the comparative steel of Comparative Example 7 is insufficient (the second cooling rate does not meet the invention requirements), when the comparative steel has not cooled down to the lower bainite phase region, that is, when bainite phase transformation occurs at a relatively high temperature and finally cools to an appropriate lower bainite phase region temperature, bulk equiaxed bainite mainly exists in the microstructure, acicular lower bainite is hardly contained, and carbide precipitation is not sufficiently fine and uniform.

[0095] Fig. 3 is a microstructural photograph of the comparative steel of Comparative Example 8 magnified 1000 times. As shown in Fig. 3, although the cooling rate during the rapid cooling stage cooling of the comparative steel of Comparative Example 8 is appropriate (the second cooling rate meets the invention requirements), the rapid cooling temperature is too high (the cooling temperature during the rapid cooling stage does not meet the invention requirements), so the microstructure is almost entirely a bulk equiaxed bainite structure, acicular lower bainite is hardly contained, and carbide precipitation is not sufficiently fine and uniform.

[0096] As can be seen from the above content, the present invention can obtain a gigapascal-class bainite steel with an ultra-high yield ratio of tensile strength ≥ 980 MPa, yield strength ≥ 900 MPa, yield ratio ≥ 0.9, and hole expansion ratio ≥ 55% through reasonable chemical composition design. This gigapascal-class bainite steel simultaneously has an ultra-high yield ratio, ultra-high strength, excellent hole expansion performance and bending performance, can be used for manufacturing automobile structural members, meets the new design concept of "green-safety" of automobiles, and has good general prospects and application values.

[0097] The annealing process according to the present invention has an important effect on the performance of the steel. This annealing process includes a heating stage, a soaking stage, a slow cooling stage, a rapid cooling stage, a self-return temperature control cooling stage and an air cooling stage, and through reasonable process design and control of related process parameters, a gigapascal-class bainite steel with an ultra-high yield ratio can be obtained.

[0098] Moreover, the production process of the manufacturing method according to the present invention is unique. By adopting the annealing process described above, the performance of the obtained gigapascal-class bainitic steel is ensured. The obtained gigapascal-class bainitic steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion performance and bending performance.

[0099] In addition, the combination method of each technical feature in the present application is not limited to the combination method described in the claims of the present application or the combination method described in the specific embodiments. All the technical features described in the present application can be freely combined or joined in any way as long as they do not conflict with each other.

[0100] Furthermore, it should be noted that the above-mentioned embodiments are only specific embodiments of the present invention. The present invention is not limited to the above embodiments. Similar changes and deformations thereof can be directly obtained by those skilled in the art from the disclosure content of the present invention or can be easily recalled. Therefore, it goes without saying that they belong to the protection scope of the present invention.

Claims

1. A gigapascal-class bainite steel sheet having an ultra-high yield ratio, containing the following chemical elements in mass percentage content: C: 0.12 - 0.24%; Si: 0.2 - 0.5%; Mn: 1.3 - 2.0%; B: 0.001 - 0.004%; Al: 0.01 - 0.05%; At least one of Cr, Nb, Ti, Mo, provided that Cr ≤ 0.4%, Nb ≤ 0.06%, Ti ≤ 0.1%, Mo ≤ 0.4%; The balance being Fe and other inevitable impurities, 0.18 ≤ M ≤ 0.27 (where M = Cr / 2.5 + Ti + V / 5 + Nb / 1.7 + Mo / 1.7, and Cr, V, Nb, Ti and Mo represent the numerical values before the percentage symbols of the mass percentage content of each chemical element) and, 0.20 ≤ C ｂ ≤ 0.27 (However, the content C of the equivalent bainite carbon element ｂ = C - (Mo + Nb) / 8 - (Ti + V) / 4 - Cr / 12 + Ni / 10 + Mn / 20 + B×10, where each element in the formula represents the numerical value before the percent symbol of the mass percent content of this element) satisfying, The microstructure of the gigapascal-class bainite steel sheet is mainly acicular lower bainite, and the phase proportion of acicular lower bainite is 90% or more. The gigapascal-class bainite steel sheet has a tensile strength ≥ 980 MPa, a yield strength ≥ 900 MPa, a yield ratio ≥ 0.9 and a hole expansion ratio ≥ 55%. A gigapascal-class bainite steel sheet having an ultra-high yield ratio, characterized by the above.

2. The mass percentage content of each chemical element satisfies at least one of the following items: C: 0.15 - 0.20%, Mn: 1.6 - 2.0% The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to Claim 1, characterized by the above.

3. In other inevitable impurities, P ≤ 0.015% and / or S ≤ 0.004%. The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to Claim 1, characterized by the above.

4. The following chemical elements: 0 < Cu ≤ 0.2%, 0 < Ni ≤ 0.2%, 0 < V ≤ 0.2%, 0 < Ce ≤ 0.2% The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to Claim 1, characterized by containing at least one of the above.

5. The microstructure further contains nano-order, sub-micro-order or micro-order granular carbide precipitation phases that are dispersed and precipitated, and the total phase proportion of granular carbide precipitation phase + acicular lower bainite is 99% or more. The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to Claim 1, characterized by the above.

6. The diameter of the largest granular carbide precipitation phase is 2 μm or less. The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to Claim 5, characterized by the above.

7. The gigapascal-class bainite steel sheet having an ultra-high yield ratio according to claim 1, characterized in that the yield strength is ≧ 950 MPa and the yield ratio is ≧ 0.

95.

8. An annealing process for use in the gigapascal-class bainite steel sheet having an ultra-high yield ratio according to any one of claims 1 to 7, characterized by including the following steps. In the heating stage, it is heated to the soaking temperature T at a heating rate of 50 °C / s or less, provided that T ｓ is up to; provided that T ｓ is 840 to 900 °C; (b) During the soaking stage, keep it at a temperature of T ｓ for 5 minutes or less; (c) In the slow cooling stage, cool at a first cooling rate of 15 °C / s or less to (T ｓ - 80) to (T ｓ - 140) °C; (d) In the rapid cooling stage, it is cooled at a second cooling rate of (130 - Q) °C / s or more to (T ｓ - 490) to (T ｓ - 440) °C; (e) In the self-return temperature control cooling stage, cooling is performed at a third cooling rate for 10 to 40 s, provided that [(Q - 80) / 12] °C / s ≦ third cooling rate ≦ [(Q - 80) / 8] °C / s; (f) Finally, in the air cooling stage, the strip is air cooled to room temperature; provided that Q = C × 180 + Si × 10 + Mn × 30 + Ni × 50 + Cr × 15 + Mo × 15 + B × 2000.

9. The following steps: (1) Smelting and casting; (2) Hot rolling; (3) Post-rolling cooling and coiling; (4) Pickling and cold rolling; and (5) The annealing process according to claim 8 A manufacturing method of a gigapascal-class bainite steel sheet having an ultra-high yield ratio, including: The gigapascal-class bainite steel sheet having an ultra-high yield ratio is as described in any one of claims 1 to 7 A manufacturing method of a gigapascal-class bainite steel sheet having an ultra-high yield ratio, characterized by this.

10. In step (2), the heating temperature is 1150 to 1260 °C; the finish rolling start temperature is 1100 to 1220 °C, and the finish rolling end temperature is 900 to 950 °C. The manufacturing method according to claim 9, characterized by this.

11. In step (3), the cooling rate is 30 to 150 °C / s, and the coiling temperature is 450 to 580 °C. The manufacturing method according to claim 9, characterized by this.

12. In step (4), the cold rolling reduction ratio is 50% or more. The manufacturing method according to claim 9, characterized by this.

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