A high-strength Q&P steel with low yield strength ratio and grade of 960MPa and its production method

Abstract

This invention provides a high-strength Q&P steel with a low yield strength ratio of 960MPa and its production method. Its chemical composition, by mass percentage, is: C 0.15–0.17%, Si 0.4–0.6%, Mn 1.4–2.6%, Cr 0.4–0.8%, Ni 0.7–1.6%, 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 0.8–1.6%, 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 960MPa. The controlled rolling and cooling process employs a two-stage rolling process, and the heat treatment uses a two-step salt bath Q&P process. The complete austenitizing temperature is 900–920℃, the salt bath quenching temperature is 200–300℃, the salt bath partitioning temperature is 350–450℃, and finally, it is water-cooled to room temperature. The low yield strength ratio 960MPa grade high-strength steel plate of this invention has a yield strength ≥960MPa, a yield strength ratio ≤0.81, an elongation ≥15%, and an impact energy ≥35J at -40℃. The advantage of this invention lies in achieving a good performance match between high strength and a low yield strength ratio through the salt bath Q&P heat treatment process.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and in particular to a high-strength Q&P steel with a low yield strength ratio of 960MPa and its production method. Background Technology

[0002] High-strength steel plates are widely used in machinery, mining, marine, and bridge construction. As structural materials, high-strength steel requires increasingly lower yield strength ratios. 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). To reduce the yield strength ratio, the quenching + two-phase zone secondary quenching + tempering (QLT) process is currently widely used, but the production process is relatively complex, and it is difficult to achieve a yield strength of 960MPa. High-strength steel plates obtained by the Q&P heat treatment process all have the characteristic of low yield strength ratios, but currently this process can only produce thin plates below 10mm, and the strength level is relatively low.

[0003] Chinese invention patent CN110318008A discloses a thick high-strength steel plate with a yield strength of 960 MPa against lamellar tearing and its production method. Through controlled rolling and cooling combined with quenching at 900~940℃ and tempering at 560~620℃, a yield strength ≥960 MPa, tensile strength ≥1000 MPa, elongation ≥12%, Charpy impact energy at -40℃ ≥30 J, and Z-direction tensile reduction of area ≥35% can be obtained. However, the yield strength ratio of this high-strength steel is excessively high (>0.94).

[0004] Chinese invention patent CN110863143A discloses a 960MPa grade ultra-high strength steel with excellent low-temperature toughness and its manufacturing method. Through controlled rolling and cooling combined with quenching at 800~870℃ and tempering at 500~600℃, the resulting steel exhibits a yield strength ≥960MPa, tensile strength 990~1100MPa, elongation after fracture ≥15%, and Charpy impact energy ≥230J at -60℃. This high-strength steel possesses ultra-high strength and good toughness and plasticity, but its yield strength ratio is slightly high (>0.81).

[0005] Chinese invention patent CN112553526A discloses a 960MPa grade ultra-high strength structural steel, a steel pipe, and its manufacturing method. Through composition design combined with a quenching process at 890~930℃ followed by tempering at 590~630℃, a steel pipe with a yield strength ≥1027MPa, tensile strength ≥1085MPa, elongation ≥21%, and Charpy impact energy ≥45J at -40℃ is obtained. This process is suitable for high-strength steel pipes, especially those with excessively high yield-to-strength ratios (>0.93).

[0006] Chinese invention patent CN114000056A discloses a low yield strength ratio marine engineering steel plate with a yield strength of 960 MPa and its preparation method. Through composition design, controlled rolling and cooling combined with two-phase annealing + full austenitizing quenching + medium-low temperature tempering heat treatment, a yield strength ReH ≥ 960 MPa, tensile strength Rm ≥ 1100 MPa, yield strength ratio YR ≤ 0.95, elongation after fracture A ≥ 12%, and impact toughness at -40℃ ≥ 69 J is obtained. This high-strength steel has slightly lower plasticity and a slightly higher yield strength ratio (> 0.84).

[0007] Chinese invention patent CN110093552A proposes a high-strength, high-ductility Q&P steel plate with excellent weldability and its preparation method. Through composition design combined with Q&P heat treatment process, a yield strength ≥600MPa, tensile strength ≥1000MPa, and elongation ≥20% can be obtained. The high-strength steel obtained by Q&P process has a low yield strength ratio, but the strength level is insufficient and the thickness is relatively thin. The high C and Si content also leads to poor low-temperature toughness and weldability. Summary of the Invention

[0008] The purpose of this invention is to provide a high-strength Q&P steel with a low yield strength ratio of 960MPa 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 and low yield strength ratio.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] One technical solution: A high-strength Q&P steel with a low yield strength ratio of 960MPa, characterized in that the chemical composition of the steel plate by mass percentage is: C 0.15~0.17%, Si 0.4~0.6%, Mn 1.4~2.6%, Cr 0.4~0.8%, Ni 0.7~1.6%, 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 0.8~1.6%, with the balance being Fe and unavoidable impurities.

[0011] Technical Solution Two: A production method for high-strength Q&P steel with a low yield strength ratio of 960MPa, including smelting and casting, characterized by the following specific rolling and heat treatment steps: 1) Rolling the thick slab using a two-stage rolling process: initial rolling temperature ≥1150℃, first stage rolling of 6~7 passes after descaling, intermediate slabs are heated to 890~920℃ and then rolled in the second stage of 6~7 passes, 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 of 900~920℃, salt bath quenching temperature of 200~300℃, salt bath distribution temperature of 350~450℃, and finally water cooling to room temperature to obtain high-strength Q&P steel with a low yield strength ratio of 960MPa.

[0012] The microstructure of the finished low yield strength ratio 960MPa grade high-strength steel is tempered lath martensite, with a yield strength ≥960MPa, yield strength ratio ≤0.81, elongation ≥15%, and impact energy at -40℃ ≥35J.

[0013] 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 high-strength, low-alloy thick steel plate of this invention:

[0014] C: C, as a solid solution element, can significantly improve the strength of steel plates, but excessive content will adversely affect the toughness, plasticity, and weldability of the steel plates. Therefore, the C content in this invention is 0.15~0.17%.

[0015] Si: Si strengthens steel plates through solid solution treatment, but it can also produce large inclusions. Excessive Si content can lead to reduced weldability and low-temperature toughness. Therefore, the Si content in this invention is between 0.4% and 0.6%.

[0016] Mn: Mn strengthens steel plates through solid solution strengthening and can improve the hardenability of steel plates. However, excessive Mn content will lead to a decrease in weldability and low-temperature toughness. Therefore, the Mn content in this invention is 1.4~2.6%.

[0017] Cr: Cr strengthens the steel plate through solid solution strengthening and can improve the hardenability of the steel plate. Therefore, the Cr content in this invention is 0.4~0.8%.

[0018] Ni: Ni can improve the strength and low-temperature toughness of steel plates, as well as the hardenability and corrosion resistance of steel plates. Therefore, the Ni content in this invention is 0.7~1.6%.

[0019] Mo: Mo improves the strength of steel plates by refining grains, and can also improve the hardenability and corrosion resistance of steel plates. Therefore, the Mo content in this invention is 0.2~0.5%.

[0020] Cu: Cu strengthens the steel plate by precipitation, improves its hardenability and corrosion resistance. Therefore, the Cu content in this invention is 0.8-1.6%.

[0021] Nb, V, Ti, and Al: Nb, V, Ti, and Al enhance strength through precipitation strengthening. Their addition in trace amounts to steel can form dispersed nanoscale precipitates. 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%.

[0022] 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.

[0023] In terms of heat treatment, this invention employs an offline salt bath two-step Q&P heat treatment process. Through quenching and partitioning, a multiphase microstructure is formed to reduce the yield strength ratio while maintaining strength. By controlling the quenching and partitioning temperatures, the phase composition of the steel is adjusted, thereby achieving high strength and a low yield strength ratio.

[0024] Compared with the prior art, the advantages of this invention are:

[0025] 1) This invention utilizes alloy composition design, controlled rolling and cooling, and an offline salt bath two-step Q&P heat treatment process to control phase composition and obtain a 960MPa grade high-strength steel with high strength, high and low temperature toughness, and a low yield strength ratio. The composition design involves adding appropriate amounts of Nb, V, Ti, Al, and Cu to form a large number of dispersed fine nanophases in the steel. The strength and toughness are improved through fine grain strengthening and precipitation strengthening. Combined with a C content of 0.15~0.17% to improve strength and the soft and hard phase structure obtained by the offline salt bath two-step Q&P heat treatment process, the steel plate has a yield strength ≥960MPa and an impact energy ≥35J at -40℃.

[0026] 2) An offline salt bath two-step Q&P heat treatment process is employed. Through quenching and partitioning, a tempered lath bainitic structure with strength differences is formed while maintaining strength, thus reducing the yield strength ratio. By controlling the quenching and partitioning temperatures, the composition of the steel's soft and hard phases is adjusted. Combined with compositional design and the strengthening effect of microalloyed precipitates, high strength and a low yield strength ratio are achieved. With a carbon content of 0.15~0.17%, a yield strength ≥960MPa and a yield strength ratio ≤0.81 can be achieved.

[0027] 1. Elongation 15%, impact energy at -40℃ 35J. Attached Figure Description

[0028] Figure 1 The microstructure of the 960MPa grade high-strength steel with low yield strength ratio in Example 1;

[0029] Figure 2 The microstructure of the 960MPa grade high-strength steel with low yield strength ratio in Example 2;

[0030] Figure 3 The microstructure of the 960MPa grade high-strength steel with low yield strength ratio in Example 3;

[0031] Figure 4 The microstructure of the 960MPa grade high-strength steel with low yield strength ratio in Example 4;

[0032] Figure 5 The microstructure of the 960MPa grade high-strength steel with low yield strength ratio in Example 5. Detailed Implementation

[0033] 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.

[0034] 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.

[0035] 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.

[0036] This invention relates to a low yield strength ratio 960MPa grade high-strength Q&P steel. The chemical composition of the steel plate, by mass percentage, is: C 0.15~0.17%, Si 0.4~0.6%, Mn 1.4~2.6%, Cr 0.4~0.8%, Ni 0.7~1.6%, 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 0.8~1.6%, with the balance being Fe and unavoidable impurities.

[0037] This invention relates to a high-strength Q&P steel with a low yield strength ratio of 960MPa, 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 200-300℃, the salt bath quenching temperature is 350-450℃, 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 960MPa. Its thickness is 15mm. Its yield strength is ≥960MPa, yield ratio ≤0.81, elongation ≥15%, and impact energy at -40℃ ≥35J. Its microstructure is tempered lath martensite. The microstructure of the steel plate is as follows: Figures 1-5 As shown.

[0038] Example 1

[0039] The chemical composition of the steel plate by mass percentage is as follows: C 0.16%, Si 0.52%, Mn 1.5%, Cr 0.54%, Ni 0.8%, Mo 0.48%, Nb 0.052%, V 0.05%, Ti 0.018%, Al 0.037%, Cu 1.43%, with the balance being Fe and unavoidable impurities.

[0040] 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 960MPa. 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.

[0041] Example 2

[0042] The chemical composition of the steel plate by mass percentage is as follows: C 0.15%, Si 0.56%, Mn 2.4%, Cr 0.57%, Ni 1.47%, Mo 0.23%, Nb 0.058%, V 0.05%, Ti 0.019%, Al 0.038%, Cu 0.84%, with the balance being Fe and unavoidable impurities.

[0043] 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 7 passes of rolling, the intermediate slab is heated to 900℃, and then 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 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 960MPa. 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.

[0044] Example 3

[0045] The chemical composition of the steel plate by mass percentage is as follows: C 0.16%, Si 0.51%, Mn 2.49%, Cr 0.54%, Ni 0.83%, Mo 0.22%, Nb 0.057%, V 0.048%, Ti 0.019%, Al 0.043%, Cu 1.43%, with the balance being Fe and unavoidable impurities.

[0046] 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 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 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 960MPa. 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.

[0047] Example 4

[0048] The chemical composition of the steel plate by mass percentage is as follows: C 0.16%, Si 0.46%, Mn 2.27%, Cr 0.57%, Ni 1.46%, Mo 0.48%, Nb 0.045%, V 0.044%, Ti 0.016%, Al 0.033%, Cu 1.36%, with the balance being Fe and unavoidable impurities.

[0049] 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 1150℃, after descaling, the first stage involves 6 passes of rolling, and after the intermediate slab reaches 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 350℃, 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 960MPa. 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.

[0050] Example 5

[0051] The chemical composition of the steel plate by mass percentage is: C 0.17%, Si 0.58%, Mn 1.55%, Cr 0.58%, Ni 1.52%, Mo 0.52%, Nb 0.055%, V 0.072%, Ti 0.016%, Al 0.049%, Cu 0.89%, with the balance being Fe and unavoidable impurities.

[0052] 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 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 300℃, 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 960MPa. 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.

[0053] Table 1 Mechanical properties of 960MPa grade high-strength steel with low yield strength ratio involved in the embodiments of the present invention

[0054]

[0055] Note: According to the GB / T 228.1-2010 test standard, the tensile specimen is a φ5 rod-shaped specimen with a gauge length of 25mm, 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.

[0056] 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 960MPa, characterized in that, The chemical composition of the 960MPa grade high-strength Q&P steel with low yield strength ratio is as follows (by mass percentage): C 0.15~0.17%, Si 0.4~0.6%, Mn 1.4~2.6%, Cr 0.4~0.8%, Ni 0.7~1.6%, 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 0.8~1.6%, with the balance being Fe and unavoidable impurities. The low yield strength ratio 960MPa grade high-strength Q&P steel has a yield strength ≥960MPa, a yield strength ratio ≤0.81, an elongation ≥15%, and an impact energy ≥35J at -40℃. The microstructure of the low yield strength ratio 960MPa grade high-strength Q&P steel is tempered lath martensite. The specific rolling and heat treatment steps of the 960MPa grade high-strength Q&P steel with low yield strength ratio 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 is rolled in 6~7 passes, the intermediate slab is heated to 890~920℃, and then the second stage is rolled in 6~7 passes, 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 200~300℃, the salt bath distribution temperature is 350~450℃, and finally water cool to room temperature to obtain the 960MPa grade high-strength Q&P steel plate with low yield strength ratio.

2. The low yield strength ratio 960MPa grade high-strength Q&P steel according to claim 1, characterized in that: The product thickness of the low yield strength ratio 960MPa 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 960MPa 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 200~300℃, the salt bath distribution temperature is 350~450℃, and finally water cool to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 960MPa.

4. The method for producing a high-strength Q&P steel with a low yield strength ratio of 960MPa according to claim 3, characterized in that: The tensile strength of hot-rolled steel plates is ≥1200MPa and the elongation is ≥14%.

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