A high-strength Q&P steel with low yield strength ratio and grade of 890MPa and its production method

Abstract

This invention provides a high-strength Q&P steel with a low yield strength ratio of 890MPa and its production method. The chemical composition of the thick plate, by mass percentage, is: C≤0.1%, Si 0.4~0.6%, Mn 1.4~1.8%, Cr 0.4~0.8%, Ni 0.6~1.0%, Mo 0.2~0.6%, Nb 0.02~0.08%, V 0.02~0.08%, Ti 0.01~0.03%, Al 0.01~0.05%, Cu 0.8~1.5%, with the balance being Fe and unavoidable impurities. The production process, based on the above composition, employs controlled rolling and cooling combined with offline salt bath Q&P heat treatment to prepare a 15mm thick high-strength steel with a low yield strength ratio of 890MPa. The controlled rolling and cooling process uses a two-stage rolling process: water cooling to 400~420℃ after rolling, followed by air cooling to room temperature. The high-strength steel plate of this invention, with a yield strength ≥ 890 MPa, a yield strength ratio ≤ 0.87, an elongation ≥ 15%, and an impact energy ≥ 100 J at -40℃, exhibits a good overall performance balance. The advantage of this invention lies in the use of a salt bath Q&P heat treatment process, which achieves a good combination of high strength, high and low temperature toughness, and a low yield strength ratio.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a high-strength Q&P steel with a low yield strength ratio of 890MPa and its production method. Background Technology

[0002] High-strength steel plates are widely used in machinery, mining, marine, and bridge construction. As a structural material, high-strength steel requires not only high strength but also, depending on the application, high and low temperature toughness and a low yield strength ratio. Currently, commonly used technologies for improving steel performance include high-purity steel metallurgy, low-cost high-performance microalloying, TMCP technology, and offline / online heat treatment strengthening technology for medium and heavy plates.

[0003] For medium-thick plates with higher strength levels, traditional quenching and tempering heat treatment processes (quenching + high-temperature tempering) are often used to produce structural steel plates with higher performance stability and uniformity. Although traditional quenching and tempering heat treatment processes have good strength and toughness, the yield strength ratio is relatively high (>0.95). In order to reduce the yield strength ratio, the quenching + two-phase region secondary quenching + tempering (QLT) process is currently often used. However, the production process is more complex and it is difficult to achieve a combination of high strength and toughness with a low yield strength ratio.

[0004] Chinese invention patent CN108914006A discloses an ultra-high strength quenched and tempered steel plate with excellent thickness-direction properties and its manufacturing method. By combining alloy composition design with controlled rolling and cooling and traditional quenching and tempering heat treatment processes, a yield strength ReH ≥ 890 MPa, tensile strength Rm ≥ 940 MPa at room temperature, elongation after fracture A ≥ 15%~20%, and impact energy KV2 ≥ 100 J at -40℃ can be obtained. However, the yield strength ratio of this high-strength steel is greater than 0.94, which is not conducive to engineering applications.

[0005] Chinese invention patent CN110331334A discloses a corrosion-resistant marine engineering steel with a yield strength ≥890MPa and its production method. Good corrosion resistance is achieved through the addition of elements such as RE, Sn, Mg, and Ca, and the yield strength is greater than 890MPa through traditional quenching and tempering heat treatment. However, the yield-to-tensile strength ratio using the quenching and tempering process is still relatively high.

[0006] Chinese invention patent CN105950997A discloses a high-toughness, high-strength thick plate and its production method. The rolled steel plate is heat-treated using a quenching + sub-temperature quenching + tempering process to achieve a yield strength ≥1200MPa and a yield strength ratio ≤0.85. However, the elongation after fracture is ≥12%, and the impact energy at -40℃ is ≥65J. However, its toughness and plasticity are not high, and the carbon content is greater than 0.2%, which makes it unsuitable for welding.

[0007] Chinese invention patent CN109536850A discloses a high-strength, low-yield-strength-ratio thick steel plate and its production process. It employs a controlled rolling and cooling combined with a single quenching process, followed by a two-phase zone quenching and tempering heat treatment. This process yields a yield strength ≥800MPa, a yield-strength ratio ≤0.93, an impact energy ≥200J at -50℃, an elongation ≥18%, and a reduction of area ≥70%. However, its drawback is that the yield strength is lower than 890MPa.

[0008] Chinese invention patent CN111945076A discloses a 980MPa grade bainitic Q&P steel for automobiles and its production method. Through compositional design combined with Q&P heat treatment, the steel directly enters the over-aging stage after quenching, achieving a tensile strength of over 980MPa, a yield strength of 700-780MPa, and an elongation of 19-22%. However, the yield strength is relatively low, the thickness is thin, and there is no data on low-temperature toughness; therefore, it is only suitable for the automotive sheet metal industry.

[0009] The Q&P heat treatment process enables steel to achieve a microstructure composed of martensite and retained austenite at room temperature, resulting in high strength, plasticity, and toughness, while maintaining a low yield strength ratio, thus achieving excellent comprehensive mechanical properties. It has already seen widespread successful application in the automotive steel sheet industry, and its theoretical development is relatively mature. However, this theory is less applied to thick plates, and theoretical research in this area is currently lacking in China, necessitating further research. Summary of the Invention

[0010] The purpose of this invention is to provide a high-strength Q&P steel with a low yield strength ratio of 890MPa and its production method, overcoming the shortcomings of the prior art. It is suitable for 10~20mm thick plates. Through alloy composition design, controlled rolling and cooling and offline salt bath two-step Q&P heat treatment process, the phase composition is controlled to obtain high strength, high and low temperature toughness and low yield strength ratio.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] One technical solution: a high-strength Q&P steel with a low yield strength ratio of 890MPa, wherein the composition of the steel plate by mass percentage is: C ≤0.1%, Si 0.4~0.6%, Mn 1.4~1.8%, Cr 0.4~0.8%, Ni 0.6~1.0%, Mo 0.2~0.6%, Nb 0.02~0.08%, V 0.02~0.08%, Ti 0.01~0.03%, Al 0.01~0.05%, Cu 0.8~1.5%, with the balance being Fe and unavoidable impurities.

[0013] Technical Solution Two: A production method for high-strength Q&P steel with a low yield strength ratio of 890MPa, including smelting and casting, characterized by the following specific preparation steps: 1) Rolling the thick slab using a two-stage rolling process: initial rolling temperature ≥1150℃, after descaling, the first stage involves 6-7 passes of rolling, and after the intermediate slab is heated to 890-920℃, the second stage involves 6-7 passes of rolling, with a final rolling temperature ≥850℃; 2) Water cooling to 400-420℃ after rolling, followed by air cooling to room temperature; 3) Heat treatment of the hot-rolled plate using an offline salt bath two-step Q&P heat treatment process: complete austenitization temperature is 900-920℃, salt bath quenching temperature is 200-250℃, salt bath distribution temperature is 400-450℃, and finally water cooling to room temperature to obtain high-strength Q&P steel with a low yield strength ratio of 890MPa.

[0014] The microstructure of the finished low yield strength ratio 890MPa grade high-strength steel is tempered lath martensite, with a yield strength ≥890MPa, yield strength ratio ≤0.87, elongation ≥15%, and impact energy ≥100J at -40℃.

[0015] The alloy composition design of this invention is relatively complex, with a high alloy content and the addition of various microalloying elements. The following is a detailed analysis and explanation of the role and dosage selection of the main alloy components in the low yield strength ratio 890MPa grade high-strength steel of this invention:

[0016] C: C strengthens the steel plate through solid solution treatment. Too low a content results in low strength, while too high a content reduces weldability and low-temperature toughness. Therefore, the C content in this invention is less than 0.1%.

[0017] Si: Si strengthens steel plates through solid solution treatment, but excessive Si content can lead to large inclusions, thereby reducing weldability and low-temperature toughness. Therefore, the Si content in this invention is between 0.4% and 0.6%.

[0018] Mn: Mn can improve the hardenability of steel plates and enhance their strength through solid solution strengthening. However, excessive Mn content will reduce weldability and low-temperature toughness. Therefore, the Mn content in this invention is 1.4~1.8%.

[0019] Cr: Cr can improve the hardenability of steel plates and enhance their strength through solid solution strengthening. Therefore, the Cr content in this invention is 0.4% to 0.8%.

[0020] Ni: Ni can improve the hardenability of steel plates, as well as their strength and low-temperature toughness. Therefore, the Ni content in this invention is 0.6% to 1.0%.

[0021] Mo: Mo can improve the hardenability of steel plates and increase their strength by refining the grain size. Therefore, the Mo content in this invention is 0.2% to 0.6%.

[0022] Cu: Cu can improve the hardenability of steel plates, enhance strength through precipitation strengthening, and improve the corrosion resistance of steel plates. Therefore, the Cu content in this invention is 0.8~1.5%.

[0023] Nb, V, Ti, Al: Adding trace amounts of Nb, V, Ti, and Al to steel can form dispersed nanoscale precipitates, which enhance strength through precipitation strengthening. The nanoscale precipitates of Nb and Ti can improve strength and low-temperature toughness by refining the grain size. However, excessively high Ti content can lead to the formation of large-sized TiN particles, reducing the low-temperature toughness of the steel. Therefore, in this invention, the Nb and V contents are 0.02~0.08%, the Ti content is 0.01~0.03%, and the Al content is 0.01~0.05%.

[0024] In terms of rolling process, this invention employs a two-stage controlled rolling and cooling process. By controlling the reduction amount in both stages and the intermediate billet temperature, recrystallization is promoted, and the mixed crystal zone is avoided, thereby obtaining a fine and uniform microstructure. After rolling, water cooling + air cooling is used to cool to room temperature, controlling the grain size and improving the strength of the steel plate.

[0025] In terms of heat treatment, this invention employs an offline salt bath two-step Q&P heat treatment process. Through the two-step process of quenching and partitioning, a multiphase microstructure is formed to reduce the yield strength ratio while ensuring strength. By controlling the quenching temperature and partitioning temperature, the phase composition of the steel is adjusted, thereby obtaining high strength, high and low temperature toughness, and a low yield strength ratio.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1) The composition design of this invention involves adding appropriate amounts of Nb, V, Ti, Al, and Cu to form a large number of dispersed fine nanophases in steel. Through fine grain strengthening and precipitation strengthening, the strength and toughness are improved, resulting in a steel plate with an impact energy of ≥100J at -40℃.

[0028] 2) An offline salt bath two-step Q&P heat treatment process is adopted. Through quenching and partitioning, a tempered lath martensitic structure with strength differences is formed while ensuring strength, thereby reducing the yield strength ratio. By controlling the quenching temperature and partitioning temperature, the composition of soft and hard phases in the steel is adjusted. Combined with the strengthening effect of microalloyed precipitates, high strength, high and low temperature toughness, and a low yield strength ratio are obtained. Under the premise that the carbon content of the steel plate is less than 0.1%, a yield strength ≥890MPa and a yield strength ratio ≤0.87 can be achieved. Attached Figure Description

[0029] Figure 1 The microstructure of the 890MPa grade high-strength steel with low yield strength ratio in Example 1;

[0030] Figure 2The microstructure of the 890MPa grade high-strength steel with low yield strength ratio in Example 2;

[0031] Figure 3 The microstructure of the 890MPa grade high-strength steel with low yield strength ratio in Example 3;

[0032] Figure 4 The microstructure of the 890MPa grade high-strength steel with low yield strength ratio in Example 4;

[0033] Figure 5 The microstructure of the 890MPa grade high-strength steel with low yield strength ratio in Example 5; Detailed Implementation

[0034] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0035] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0036] The components of the embodiments of the invention described and shown in the accompanying drawings can typically be arranged and designed in numerous different configurations. Therefore, the following detailed description of the embodiments of the invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention.

[0037] The present invention relates to a high-strength Q&P steel with a low yield strength ratio of 890MPa, the chemical composition of which, by mass percentage, is: C ≤0.1%, Si 0.4~0.6%, Mn 1.4~1.8%, Cr 0.4~0.8%, Ni 0.6~1.0%, Mo 0.2~0.6%, Nb 0.02~0.08%, V 0.02~0.08%, Ti 0.01~0.03%, Al 0.01~0.05%, Cu 0.8~1.5%, with the balance being Fe and unavoidable impurities.

[0038] This invention relates to a high-strength Q&P steel with a low yield strength ratio of 890MPa, produced using the following method: Specific rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is ≥1150℃, followed by 6-7 passes of rolling in the first stage after descaling; the intermediate slab is then heated to 890-920℃ before undergoing a second stage of 6-7 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 400-420℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 900-920℃, the salt bath quenching temperature is 200-250℃, the salt bath quenching temperature is 400-450℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. Its thickness is 15mm. Its yield strength is ≥890MPa, yield ratio ≤0.87, elongation ≥15%, and impact energy at -40℃ ≥100J. Its microstructure is tempered lath martensite. The microstructure of the steel plate is as follows: Figures 1-5 As shown.

[0039] Example 1

[0040] The chemical composition of the steel plate by mass percentage is: C 0.093%, Si 0.51%, Mn 1.55%, Cr 0.54%, Ni 0.81%, Mo 0.22%, Nb 0.054%, V 0.053%, Ti 0.02%, Al 0.03%, Cu 1.43%, with the balance being Fe and unavoidable impurities.

[0041] Example 1: The rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is 1150℃, after descaling, the first stage involves 6 passes of rolling, and after the intermediate slab is heated to 900℃, the second stage involves 7 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 400℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 900℃, the salt bath quenching temperature is 250℃, the salt bath partitioning temperature is 400℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 1 As shown, the microstructure is tempered lath martensite.

[0042] Example 2

[0043] The chemical composition of the steel plate by mass percentage is: C 0.091%, Si 0.5%, Mn 1.58%, Cr 0.52%, Ni 0.85%, Mo 0.24%, Nb 0.05%, V 0.051%, Ti 0.018%, Al 0.032%, Cu 1.4%, with the balance being Fe and unavoidable impurities.

[0044] Example 2: Rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is 1150℃, after descaling, the first stage involves 6 passes of rolling, and after the intermediate slab is heated to 890℃, the second stage involves 6 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 400℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 900℃, the salt bath quenching temperature is 200℃, the salt bath partitioning temperature is 400℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 2 As shown, the microstructure is tempered lath martensite.

[0045] Example 3

[0046] The chemical composition of the steel plate by mass percentage is: C 0.093%, Si 0.48%, Mn 1.5%, Cr 0.57%, Ni 0.91%, Mo 0.25%, Nb 0.052%, V 0.05%, Ti 0.022%, Al 0.025%, Cu 1.35%, with the balance being Fe and unavoidable impurities.

[0047] Example 3: Rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is 1150℃, after descaling, the first stage involves 7 passes of rolling, and after the intermediate slab is heated to 900℃, the second stage involves 6 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 400℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 900℃, the salt bath quenching temperature is 250℃, the salt bath partitioning temperature is 450℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 3 As shown, the microstructure is tempered lath martensite.

[0048] Example 4

[0049] The chemical composition of the steel plate by mass percentage is: C 0.09%, Si 0.48%, Mn 1.49%, Cr 0.5%, Ni 0.81%, Mo 0.44%, Nb 0.055%, V 0.053%, Ti 0.017%, Al 0.03%, Cu 0.8%, with the balance being Fe and unavoidable impurities.

[0050] Example 4: Rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is 1160℃, after descaling, the first stage involves 7 passes of rolling, and after the intermediate slab is heated to 910℃, the second stage involves 6 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 400℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 920℃, the salt bath quenching temperature is 250℃, the salt bath partitioning temperature is 450℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 4 As shown, the microstructure is tempered lath martensite.

[0051] Example 5

[0052] The chemical composition of the steel plate by mass percentage is: C 0.088%, Si 0.45%, Mn 1.68%, Cr 0.42%, Ni 0.75%, Mo 0.53%, Nb 0.035%, V 0.061%, Ti 0.013%, Al 0.042%, Cu 0.89%, with the balance being Fe and unavoidable impurities.

[0053] Example 5: Rolling and heat treatment steps are as follows: 1) A two-stage rolling process is used to roll the thick slab: the initial rolling temperature is 1160℃, after descaling, the first stage involves 7 passes of rolling, and after the intermediate slab reaches 890℃, the second stage involves 7 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, the slab is water-cooled to 420℃, and then air-cooled to room temperature; 3) The hot-rolled plate is heat-treated using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 920℃, the salt bath quenching temperature is 200℃, the salt bath partitioning temperature is 450℃, and finally, the slab is water-cooled to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 5 As shown, the microstructure is tempered lath martensite.

[0054] Table 1 Mechanical properties of high-strength, low-toughness, low-yield-strength thick steel plates prepared in Examples 1-5 of this invention

[0055]

[0056] Note: According to the GB / T 228.1-2010 test standard, the tensile specimen is a bar-shaped specimen with a gauge length of φ5, and the sampling position is longitudinal; according to the GB / T 229-2007 test standard, the Charpy impact specimen size is 10×10×55mm, and the sampling position is longitudinal.

[0057] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high-strength Q&P steel with a low yield strength ratio of 890MPa, characterized in that, The chemical composition of the low yield strength ratio 890MPa grade high-strength Q&P steel, by mass percentage, is as follows: C ≤0.1%, Si 0.4~0.6%, Mn 1.4~1.8%, Cr 0.52~0.8%, Ni 0.6~1.0%, Mo 0.44~0.53%, Nb 0.05~0.08%, V 0.02~0.08%, Ti 0.01~0.03%, Al 0.025~0.05%, Cu 0.8~1.5%, with the balance being Fe and unavoidable impurities; The low yield strength ratio 890MPa grade high-strength Q&P steel has a yield strength ≥890MPa, a yield strength ratio ≤0.87, an elongation ≥15%, and an impact energy ≥100J at -40℃. The microstructure of the low yield strength ratio 890MPa grade high-strength Q&P steel is tempered lath martensite. The specific rolling and heat treatment steps are as follows: 1) Roll the thick slab using a two-stage rolling process: the initial rolling temperature is ≥1150℃, after descaling, the first stage involves 6~7 passes of rolling, and after the intermediate slab is heated to 890~920℃, the second stage involves 6~7 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, water cool to 400~420℃, and then air cool to room temperature; 3) Heat treat the hot-rolled plate using an offline salt bath two-step Q&P heat treatment process: the complete austenitizing temperature is 900~920℃, the salt bath quenching temperature is 200~250℃, the salt bath distribution temperature is 400~450℃, and finally water cool to room temperature to obtain a high-strength Q&P steel plate with a low yield strength ratio of 890MPa.

2. The low yield strength ratio 890MPa grade high-strength Q&P steel according to claim 1, characterized in that, The finished thickness of the low yield strength ratio 890MPa grade high-strength Q&P steel plate is 15mm.

3. The method for producing a high-strength Q&P steel with a low yield strength ratio of 890MPa according to claim 1, characterized in that: The specific rolling and heat treatment steps are as follows: 1) Roll the thick slab using a two-stage rolling process: the initial rolling temperature is ≥1150℃, after descaling, the first stage involves 6~7 passes of rolling, and after the intermediate slab is heated to 890~920℃, the second stage involves 6~7 passes of rolling, with a final rolling temperature ≥850℃; 2) After rolling, water cool to 400~420℃, and then air cool to room temperature; 3) Heat treat the hot-rolled plate using an offline salt bath two-step Q&P heat treatment process: the complete austenitization temperature is 900~920℃, the salt bath quenching temperature is 200~250℃, the salt bath distribution temperature is 400~450℃, and finally water cool to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 890MPa.

4. The method for producing a high-strength Q&P steel with a low yield strength ratio of 890MPa according to claim 2, characterized in that: The tensile strength of hot-rolled steel plates is ≥1050MPa, and the elongation is ≥16%.

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