1000mpa grade zr-containing multiphase steel and method for manufacturing the same

Abstract

The present application belongs to the technical field of hot continuous rolling steel production, and particularly relates to a 1000MPa grade Zr-containing complex phase steel and a preparation method thereof. The present application solves the technical problem of providing a 1000MPa grade Zr-containing complex phase steel and a preparation method thereof. The chemical components of the steel include, by weight percentage, C: 0.08-0.16%, Mn: 1.5-2.3%, Si: 0.05-0.20%, Cr: 0.2-0.4%, Nb: 0.03-0.05%, Ti: 0.06-0.10%, Zr: 0.02-0.05%, P≤0.015%, S≤0.005%, N≤0.0050%, O≤0.0015%, and the balance of Fe and inevitable impurities. The steel has a low yield strength ratio, good strength and ductility, a relatively low alloy cost, and a relatively simple controlled rolling and controlled cooling process, and is conducive to realizing stable industrial control.

Description

Technical Field

[0001] This invention belongs to the field of hot strip steel production technology, specifically relating to a 1000MPa grade Zr-containing multiphase steel and its preparation method. Background Technology

[0002] Since the 1970s, duplex steel has become one of the main steel types used in automotive lightweighting due to its low yield strength ratio, high work hardening rate, and good formability. Among the duplex steels extensively studied in the automotive field are martensitic duplex steel (FMDP) and bainitic duplex steel (FBDP). FMDP has a lower yield strength and a higher initial work hardening rate, but its elongation flange performance is poor. Therefore, researchers have developed FBDP with good elongation flange performance. In recent years, with the continuous development of high-strength and lightweight automotive industries, a large amount of research has been conducted on duplex steel and its microalloying methods. By reducing the hardness difference between the phases in the microstructure and minimizing the difference in deformation capacity, combined with the precipitation of the second phase in the ferrite phase, high-strength duplex steel with excellent strength and plasticity can be obtained. This can be widely applied to the manufacture of automotive parts with complex forming requirements and flanging / expansion requirements.

[0003] For example, patent CN101270436B discloses a hot-rolled multiphase steel plate and its manufacturing method. The steel composition is: C: 0.08%~0.25%, Si: 0.5%~2.0%, Mn: 0.5%~2.0%, Al: 0.010%~0.060%, N: ≤0.010%, P: ≤0.020%, S: ≤0.005%, Ti: ≤0.03%, Nb: ≤0.03%, with the remainder being iron and unavoidable impurities. It also provides a two-stage cooling process for controlling the multiphase microstructure, resulting in a steel plate with a thickness of 2.5~6.0 mm, tensile strength >1000 MPa, yield strength ≥500 MPa, yield ratio 0.51~0.80, elongation A80: 11%~20%, and no cracking when cold-bent d=4t. The microstructure is bainite and martensite (>90%) with a small amount of ferrite. This patent uses Si to increase the phase transformation temperature to promote the transformation of bainite and martensite, but it easily causes the problem of difficulty in controlling the hot-rolled iron oxide scale.

[0004] For example, patent CN109023036B discloses an ultra-high strength hot-rolled multiphase steel plate and its production method. The steel composition is as follows: C: 0.05%–0.15%, Si: 0.1%–0.8%, Mn: 1.2%–2.3%, P≤0.012%, S≤0.005%, Cr: 0.20%–0.70%, Mo: 0.10%–0.50%, Nb: 0.02%–0.06%, Ti: 0.05%–0.13%, Als: 0.015%–0.060%, with the remainder being Fe and unavoidable impurities. It comprehensively utilizes the effects of grain refinement strengthening, phase transformation strengthening, and precipitation strengthening to improve the steel's strength. Simultaneously, precipitation strengthening reduces the hardness difference between the two phases, improving porosity. However, this patent adds a high content of the precious metal Mo, resulting in high production costs.

[0005] Similarly, patent CN111270161B discloses a high elongation hot-rolled microstructure-controlled steel with a tensile strength ≥1000MPa and its production method. The steel composition is: C: 0.12~0.14%, Mn: 1.60~1.80%, Si: 0.70~0.80%, P≤0.008%, S≤0.002%, Als: 0.02%~0.04%; the cooling method of the steel is... The cooling process involves several stages: the first stage cools to 660–680°C at a rate of 100–200°C / s; the second stage cools to 620–640°C at a rate of 5–15°C / s; the third stage cools to 460–500°C at a rate of 50–100°C / s; the fourth stage cools to 440–458°C at a rate of 5–15°C / s; and the fifth stage cools to the winding temperature of 320–350°C at a rate of 100–200°C / s. This method has a complex cooling process, a narrow process window, and is difficult to scale up for mass production in industrial applications.

[0006] Therefore, in order to improve the shortcomings of the existing high-strength multiphase steel production process, there is an urgent need to provide a hot-rolled multiphase steel with lower alloy cost and stronger adaptability to the production process, as well as its production method. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a 1000MPa grade Zr-containing multiphase steel and its preparation method. This steel exhibits a yield strength ≥600MPa, tensile strength ≥1000MPa, elongation ≥18%, passes 180° cold bending with d=2a, has a hole expansion rate ≥30%, and an impact energy at 0℃ ≥100J. Its microstructure has an average grain size ≤8μm, a ferrite proportion ≤30%, with the remainder being bainite and martensite. The levels of various inclusions, including sulfides, oxides, silicates, and precipitated TiN, are all ≤1.0.

[0008] This invention first provides a 1000MPa grade Zr-containing multiphase steel, the chemical composition of which by weight percentage includes: C: 0.08-0.16%, Mn: 1.5-2.3%, Si: 0.05-0.20%, Cr: 0.2-0.4%, Nb: 0.03-0.05%, Ti: 0.06-0.10%, Zr: 0.02-0.05%, P≤0.015%, S≤0.005%, N≤0.0050%, O≤0.0015%, with the balance being Fe and unavoidable impurities.

[0009] The microstructure of the above-mentioned 1000MPa grade Zr-containing multiphase steel has an average grain size of ≤8μm, a ferrite ratio of ≤30%, and the remainder is bainite and martensite.

[0010] This invention also provides a method for preparing the above-mentioned 1000MPa grade Zr-containing multiphase steel, comprising the following steps:

[0011] According to the set chemical composition of steel, it is smelted in a converter or electric furnace, and then refined, vacuum degassed and cast to obtain billets or ingots; the billets or ingots are then heated, hot continuous rolling, laminar flow cooling and coiled.

[0012] The heating temperature of the above-mentioned billet or ingot is 1240~1280℃, and the furnace time is ≥160min.

[0013] The steel plates are manufactured using a two-stage rolling process. The recrystallization zone is rolled at a temperature of 1100–1200℃, with 5–7 passes and a cumulative compression ratio of 4–8. The non-recrystallization zone is rolled at a temperature of 880–1080℃, with a cumulative compression ratio of 5–15.

[0014] The steel plates are cooled in stages: the first stage is cooled to 670-730℃ at 50-100℃ / s; the second stage is maintained at 3-6℃ / s for 4-6s, with the temperature controlled at 660-700℃; and the third stage is cooled to 380-480℃ at 50-100℃ / s.

[0015] The coiling temperature of the aforementioned steel plate is 380–480℃.

[0016] The finished steel plates obtained have a thickness of 2.0 to 10.0 mm.

[0017] Beneficial effects:

[0018] (1) The 1000MPa grade Zr-containing multiphase steel provided by the present invention has a low yield strength ratio, good strength and plasticity, good low temperature fracture toughness and hole expansion forming performance, and is suitable for processing into complex forming parts.

[0019] (2) The 1000MPa grade Zr-containing multiphase steel alloy provided by the present invention has a low cost. It does not select to add the precious element Mo, but adds elements with lower alloy cost such as Cr, Ti, and Zr. The controlled rolling and cooling process is relatively simple, which is conducive to achieving industrial-grade stable control. Attached Figure Description

[0020] Figure 1 This is a microstructure diagram of the multiphase steel of Example 1 of the present invention;

[0021] Figure 2 This is a morphology diagram of inclusions in the multiphase steel of Embodiment 1 of the present invention;

[0022] Figure 3 This is a microstructure diagram of the multiphase steel of Comparative Example 1 of the present invention;

[0023] Figure 4 This is a morphology diagram of TiN inclusions in the multiphase steel of Comparative Example 1 of this invention;

[0024] Figure 5 This is a morphology diagram of type A inclusions in the multiphase steel of Comparative Example 3 of the present invention;

[0025] Figure 6 This is a morphology diagram of TiN inclusions in the multiphase steel of Comparative Example 3 of the present invention. Detailed Implementation

[0026] This invention first provides a 1000MPa grade Zr-containing multiphase steel, the chemical composition of which by weight percentage includes: C: 0.08-0.16%, Mn: 1.5-2.3%, Si: 0.05-0.20%, Cr: 0.2-0.4%, Nb: 0.03-0.05%, Ti: 0.06-0.10%, Zr: 0.02-0.05%, P≤0.015%, S≤0.005%, N≤0.0050%, O≤0.0015%, with the balance being Fe and unavoidable impurities.

[0027] The reasons for the restrictions on the main alloying elements in the steel described in this invention are explained below.

[0028] Carbon (C) is an important component of the bainite and martensite phases in the microstructure of steel. Its content directly affects the hardness, strength, and other properties of the bainite and martensite phases. Therefore, the C content should not be too low. However, if the C content is too high, it may cause the steel described in this invention to form relatively coarse alloy cementite or precipitate a second phase during the second cooling process. This will not only reduce the precipitation strengthening effect but also reduce the phase transformation strengthening effect. Therefore, this invention controls the C content to be 0.08-0.16%.

[0029] In steel, manganese (Mn) not only plays a role in solid solution strengthening and improving toughness, but also improves the hardenability of austenite, promotes bainite and martensite transformation, and enhances the material's ability to suppress crack propagation by forming a high dislocation density microstructure, thus improving the material's toughness and plasticity. However, excessively high Mn content can easily cause segregation in the cast billet, affecting the uniformity of the microstructure. Therefore, the Mn content is controlled at 1.5%–2.3%.

[0030] Cr can improve the hardenability of steel and promote the transformation of bainite and martensite, while Nb can inhibit the pearlite phase transformation and promote the transformation of bainite and martensite. Therefore, the present invention adds Cr and Nb elements, with the contents controlled at 0.2-0.4% and 0.03-0.05%, respectively.

[0031] Ti and Zr have similar chemical properties and can both form nanoscale Ti / Zr(CN) second phases in ferrite. This precipitation strengthening improves strength while reducing the hardness difference between ferrite and bainite / martensite phases, thus reducing stress concentration during material forming. Furthermore, the addition of Zr reduces the formation of liquid TiN from Ti during the smelting process. Therefore, this invention adds Ti and Zr elements, with the contents controlled as follows: Ti: 0.06–0.10%, Zr: 0.02–0.05%.

[0032] High-strength steels microalloyed with Ti generally require control of the content of gaseous elements such as N and O. This is because during the smelting process, Ti reacts with N and O to form inclusions such as liquid-precipitated TiN and TiO2, which not only reduce the purity of the steel but also decrease the effective Ti content, thus reducing the precipitation strengthening effect of Ti. Therefore, this invention requires that the N and O contents be controlled at N ≤ 0.0050% and O ≤ 0.0015%, respectively.

[0033] This invention also provides a method for preparing the above-mentioned 1000MPa grade Zr-containing multiphase steel, comprising the following steps:

[0034] According to the set chemical composition, the smelting is carried out in a converter or electric furnace, followed by refining, vacuum degassing, and casting to obtain billets or ingots; the billets or ingots are then heated, hot continuous rolling, laminar flow cooling, and coiled.

[0035] The reasons for the limitations of the production process are explained below in conjunction with the control requirements for hot-rolled iron oxide scale of steel described in this invention.

[0036] To ensure that microalloying elements such as Ti, Zr, Cr, and Nb are fully dissolved in the furnace to exert phase transformation strengthening and precipitation strengthening effects, the heating temperature is set at a relatively high 1240–1280℃, and the furnace time is controlled at a relatively long ≥160 min.

[0037] Controlled rolling employs segmented rolling. In the roughing stage, within the austenite recrystallization region, a higher rolling temperature (1100–1200℃) is used to reduce rolling deformation resistance and promote core deformation of the steel plate. Simultaneously, a larger roughing compression ratio (4–8) is used to further promote austenite recrystallization, thereby improving the refinement and uniformity of the microstructure along the thickness direction of the steel plate. In the finishing stage, within the non-recrystallization region of austenite, a lower rolling temperature (880–1080℃) and a larger finishing compression ratio (5–15) are used to promote austenite flattening, increase dislocation density, and provide sufficient nuclei for subsequent phase transformations.

[0038] The controlled cooling employs segmented cooling. The first segment uses rapid cooling at a rate of 50–100 °C / s to cool to 670–730 °C to improve austenite stability, thus retaining a large amount of retained austenite while the ferrite phase transformation occurs. The second segment uses slow cooling at a rate of 3–6 °C / s for 4–6 s, with the temperature controlled at 660–700 °C to promote the precipitation of carbonitrides such as Ti and Zr in the ferrite. The third segment uses rapid cooling at a rate of 50–100 °C / s to cool to 380–480 °C to promote the transformation of the retained austenite into bainite and martensite. The final microstructure obtained is precipitation-strengthened ferrite, as well as bainite and martensite, which reduces the hardness difference between the two phases, improves toughness and plasticity, and also increases the strength of the steel.

[0039] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0040] Example

[0041] Table 1 shows the composition of the embodiments and comparative examples of the present invention, Table 2 shows the production process parameters of the embodiments and comparative examples of the present invention, and Table 3 shows the mechanical properties and microstructure test results of the steels of the embodiments and comparative examples of the present invention.

[0042] The 1000MPa grade Zr-containing hot-rolled multiphase steel described in Examples 1-3 of this invention is produced by hot continuous rolling, laminar flow cooling, and coiling. During the hot continuous rolling process, parameters such as heating regime, rolling process, and laminar flow cooling process are strictly controlled. The resulting finished steel plate has a yield strength ≥600MPa, tensile strength ≥1000MPa, elongation ≥18%, 180° cold bending d=2a is qualified, hole expansion rate ≥30%, 0℃ impact energy ≥100J, and its microstructure has an average grain size ≤8μm, ferrite ratio ≤30%, with the remainder being bainite and martensite. The levels of various inclusions such as sulfides, oxides, silicates, and liquid-precipitated TiN are all ≤1.0. Figures 1-2 The image shows the microstructure and inclusion morphology of the test steel corresponding to Example 1.

[0043] Comparative Example 1 steel did not contain Zr and had a high N content, resulting in a higher level of TiN precipitation in the finished steel, which in turn led to a lower porosity (26%, lower than the required value of 30%). Simultaneously, the cooling rate in the first stage of laminar cooling for Comparative Example 1 steel was lower (44℃ / s, lower than the required value of 50℃ / s) and the temperature was higher (736℃, higher than the required value of 730℃), resulting in a higher average grain size (10μm, higher than the required value of 8μm). Furthermore, the cooling time in the second stage of laminar cooling for Comparative Example 1 steel was longer (8s, higher than the required value of 6s), resulting in a higher ferrite content (35%, higher than the required value of 30%). Under the combined influence of grain coarsening and a higher ferrite content, the tensile strength of Comparative Example 1 steel was lower (934MPa, lower than the required value of 1000MPa). Figures 3-4 The microstructure and liquid-precipitated TiN morphology of steel in Comparative Example 1 are shown.

[0044] The Cr, Nb, and Ti contents in the chemical composition of Comparative Example 2 steel are relatively low. The controlled rolling and controlled cooling process parameters meet the requirements of this invention. However, due to the low content of microalloying elements and the insufficient precipitation strengthening effect, the tensile strength of Comparative Example 2 steel is relatively low (963 MPa, which is lower than the required value of 1000 MPa).

[0045] The chemical composition of Comparative Example 3 steel contains high levels of impurity elements such as N, O, and S. The controlled rolling and cooling process parameters meet the requirements of this invention. The high levels of impurity elements lead to higher levels of Class A inclusions (sulfide inclusions), Class D inclusions (alumina inclusions), and liquid-precipitated TiN inclusions in Comparative Example 3 steel. Consequently, the elongation of Comparative Example 3 steel is low and the 180° cold bending d=2a is not up to standard. Figures 5-6 The morphology of type A inclusions and liquid-precipitated TiN in steel of Comparative Example 3 is shown.

[0046] Table 1 Chemical composition of the steels in the embodiments and comparative examples of the present invention

[0047] C Si Mn P S Cr Nb Ti Zr N O Example 1 0.13 0.15 1.89 0.009 0.003 0.31 0.037 0.091 0.034 0.0035 0.0012 Example 2 0.11 0.13 2.05 0.011 0.002 0.32 0.043 0.085 0.042 0.0029 0.0010 Example 3 0.09 0.08 1.71 0.008 0.003 0.29 0.048 0.079 0.029 0.0041 0.0011 Comparative Example 1 0.14 0.09 1.68 0.009 0.004 0.35 0.033 0.099 / <![CDATA[ 0.0051 ]]> 0.0013 Comparative Example 2 0.12 0.14 1.88 0.012 0.003 <![CDATA[ 0.18 ]]> <![CDATA[ 0.029 ]]> <![CDATA[ 0.055 ]]> 0.049 0.0033 0.0010 Comparative Example 3 0.10 0.07 2.09 0.007 <![CDATA[ 0.011 ]]> 0.38 0.047 0.093 0.027 <![CDATA[ 0.0055 ]]> <![CDATA[ 0.0022 ]]>

[0048] Table 2 Production process parameters for the examples and comparative examples

[0049]

[0050]

[0051] Table 2 (continued) Production process parameters of the examples and comparative examples

[0052]

[0053] Table 3. Mechanical properties and microstructure test results of the steels in the examples and comparative examples.

[0054]

[0055] Table 3 (continued) Mechanical properties and microstructure test results of the steels in the examples and comparative examples

[0056]

[0057]

Claims

1. A 1000MPa grade Zr-containing multiphase steel, characterized in that: Its chemical composition by weight percentage includes: C: 0.08-0.16%, Mn: 1.5-2.3%, Si: 0.05-0.20%, Cr: 0.2-0.4%, Nb: 0.03-0.05%, Ti: 0.06-0.10%, Zr: 0.02-0.05%, P≤0.015%, S≤0.005%, N≤0.0050%, O≤0.0015%, with the balance being Fe and unavoidable impurities; the average grain size of the steel's microstructure is ≤8μm, the ferrite proportion is ≤30%, and the remainder is bainite and martensite; The steel is prepared by the following steps: According to the set chemical composition of steel, it is smelted in a converter or electric furnace, then refined, vacuum degassed, and cast to obtain billets or ingots; the billets or ingots are then heated, hot continuous rolling, laminar flow cooling, and coiled. The heating temperature of the billet or ingot is 1240-1280℃, and the furnace time is ≥160min; The steel plate is rolled in two stages. The rolling temperature in the recrystallization zone is 1100-1200℃, and it is rolled in 5-7 passes with a cumulative compression ratio of 4-8. The rolling temperature in the non-recrystallization zone is 880-1080℃, and the cumulative compression ratio is 5-15. The steel plate is cooled in stages: the first stage is cooled to 670-730℃ at 50-100℃ / s; the second stage is maintained at 3-6℃ / s for 4-6s, with the temperature controlled at 660-700℃; and the third stage is cooled to 380-480℃ at 50-100℃ / s.

2. The method for preparing the 1000MPa grade Zr-containing multiphase steel according to claim 1, characterized in that: Includes the following steps: According to the set chemical composition of steel, it is smelted in a converter or electric furnace, then refined, vacuum degassed, and cast to obtain billets or ingots; the billets or ingots are then heated, hot continuous rolling, laminar flow cooling, and coiled. The heating temperature of the billet or ingot is 1240-1280℃, and the furnace time is ≥160min; The steel plate is rolled in two stages. The rolling temperature in the recrystallization zone is 1100-1200℃, and it is rolled in 5-7 passes with a cumulative compression ratio of 4-8. The rolling temperature in the non-recrystallization zone is 880-1080℃, and the cumulative compression ratio is 5-15. The steel plate is cooled in stages: the first stage is cooled to 670-730℃ at 50-100℃ / s; the second stage is maintained at 3-6℃ / s for 4-6s, with the temperature controlled at 660-700℃; and the third stage is cooled to 380-480℃ at 50-100℃ / s.

3. The method for preparing 1000MPa grade Zr-containing multiphase steel according to claim 2, characterized in that: The coiling temperature of the steel plate is 380–480℃.

4. The method for preparing 1000MPa grade Zr-containing multiphase steel according to claim 2, characterized in that: The obtained finished steel plate has a thickness of 2.0 to 10.0 mm.

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