A high-strength Q&P steel with low yield strength ratio and grade 1100MPa and its production method

Abstract

This invention provides a high-strength steel with a low yield strength ratio of 1100MPa and its production method. The steel plate has the following chemical composition by mass percentage: C 0.15~0.17%, Si 0.58~0.6%, Mn 2.53~2.6%, Cr 0.53~0.8%, Ni 1.44~1.6%, Mo 0.2~0.6%, Nb 0.052~0.08%, V 0.02~0.08%, Ti 0.019~0.03%, Al 0.01~0.05%, Cu 0.83~1.5%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength ≥1100MPa, a yield strength ratio ≤0.88, an elongation ≥14%, and an impact energy ≥30J at -40℃. The finished steel plate has a thickness of 15mm and a microstructure of tempered lath martensite. The high-strength steel plate of this invention, with a yield strength ≥1100MPa, yield strength ratio ≤0.88, elongation ≥14%, and impact energy ≥30J at -40℃, exhibits a low yield-to-tensile ratio. The advantage of this invention lies in the fact that the salt bath Q&P heat treatment process achieves a good balance between high strength and a low yield-to-tensile 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 1100MPa and its production method. Background Technology

[0002] High-strength steel plates are widely used in machinery, mining, marine, and bridge construction. With increasing demands for strength levels, the requirement for low yield strength ratios is also rising. Increasing carbon content or using quenching followed by low-temperature tempering heat treatment is widely used to produce 1100MPa grade structural steel plates with even higher strength levels. While increasing carbon content combined with traditional quenching and tempering processes provides good toughness and plasticity, the higher carbon content is detrimental to welding, and the yield strength ratio is relatively high (>0.9). Quenching followed by low-temperature tempering heat treatment improves strength through dislocation strengthening, resulting in relatively lower carbon content, but the high dislocation density leads to relatively poor plasticity. High-strength steel plates produced using the Q&P heat treatment process have a low yield strength ratio, but this process is currently only used for producing thin plates, and the presence of retained austenite makes it difficult to achieve a yield strength of 1100MPa.

[0003] Chinese invention patent CN104513936A discloses a tempered high-strength steel with a yield strength of 1100MPa and its production method. This method involves increasing the carbon content and combining controlled rolling and controlled cooling with aluminum content... c3 The high-strength steel is obtained by quenching at (30~80)℃ and then tempering at 150~350℃, resulting in a yield strength of 1100~1200MPa, a tensile strength ≥1250MPa, an elongation ≥8%, and a Charpy impact energy ≥40J at -40℃. This high-strength steel has a relatively high carbon content (0.17~0.21%) and a relatively low elongation (≤13%).

[0004] Chinese invention patent CN111996437A discloses a method for producing thick, high-toughness, 1100MPa-grade ultra-high-strength steel plates. By increasing the carbon content and using a heat treatment process combining controlled rolling and cooling with 900℃ quenching and 200℃ tempering, a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation ≥10%, and Charpy impact energy ≥30J at -60℃ are obtained. This high-strength steel has a relatively high carbon content (0.17~0.27%) and a relatively low elongation (≤13%).

[0005] Chinese invention patent CN114277307A discloses a 1100MPa grade high-strength steel for engineering machinery and its production method, which utilizes complex composition design and controlled rolling and cooling combined with A c3The high-strength steel is obtained by quenching at (30~80)℃ and tempering at 600~650℃, resulting in a yield strength ≥1100MPa, tensile strength ≥1150MPa, elongation ≥16%, and Charpy impact energy ≥100J at -60℃. This high-strength steel has a relatively high carbon content (0.2~0.24%) and a high yield strength ratio (>0.9).

[0006] Chinese invention patent CN113373370A discloses a 1100MPa grade bridge shell steel and its manufacturing method. Through a high-C, high-Si composition design combined with controlled rolling and segmented controlled cooling, a yield strength ≥1100MPa, tensile strength ≥1300MPa, elongation A ≥18%, and impact energy ≥80J at -20℃ are obtained. This high-strength steel exhibits good strength and plasticity, but its low-temperature toughness is only measured up to -20℃. Furthermore, its high C content (0.17~0.23%) and Si content (1.0~1.3%) are unfavorable for thick plate welding.

[0007] Chinese invention patent CN109136779A discloses a method for preparing martensitic matrix 1100MPa grade rare earth Q&P steel. Through composition design combined with Q&P heat treatment, a tensile strength ≥1100MPa and elongation ≥20% can be obtained. This high-strength steel has a low yield strength ratio, but its yield strength is below 1100MPa, and its thickness is relatively thin (1.8mm). Furthermore, the high C content (0.15~0.22%) and Si content (0.6~1.7%) lead 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 1100MPa 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 is a high-strength Q&P steel with a low yield strength ratio of 1100MPa. The composition of this 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 0.6-1.5%, 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 1100MPa, 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 reaches 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 350-450℃, and finally water cooling to room temperature to obtain high-strength Q&P steel with a low yield strength ratio of 1100MPa.

[0012] The microstructure of the finished high-strength steel with a low yield strength ratio of 1100MPa is tempered lath martensite, with a yield strength ≥1100MPa, a yield strength ratio ≤0.88, an elongation ≥14%, and an impact energy ≥30J at -40℃.

[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 low yield strength ratio 1100MPa grade high-strength steel of this invention:

[0014] C: C can significantly improve the strength of steel plates through solid solution strengthening, but excessive content will adversely affect the plasticity and low-temperature toughness of the steel plates. In order to ensure a yield strength of 1100 MPa, the C content in this invention is 0.15~0.17%.

[0015] Si: Si can improve strength through solid solution strengthening, but it can produce large-sized inclusions. Excessive Si content can lead to a decrease in low-temperature toughness. Therefore, the Si content in this invention is 0.4~0.6%.

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

[0017] Cr: Cr enhances strength through solid solution strengthening and improves the hardenability of steel plates. 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, and inhibit the hot brittleness caused by Cu. Therefore, the Ni content in this invention is 0.7~1.6%.

[0019] Mo: Mo can improve strength by refining grains, and can also improve the hardenability and corrosion resistance of steel plates. However, if the Mo content is too high, it will reduce the weldability. Therefore, the Mo content in this invention is 0.2~0.6%.

[0020] Cu: Cu can enhance strength through precipitation strengthening, improve hardenability of steel plates, and improve corrosion resistance of steel plates. However, excessive Cu can lead to hot brittleness and must be used in combination with Ni. Therefore, the Cu content in this invention is 0.6~1.5%.

[0021] Nb, V, Ti, and Al: Nb, V, Ti, and Al enhance strength through precipitation strengthening. Adding trace amounts to steel can form dispersed nanoscale precipitates; however, the effect is not significant below 0.01%. The nanoscale precipitates of Nb and Ti can improve strength and low-temperature toughness by refining the grain size. However, excessively high Ti content leads 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 the present invention are as follows:

[0025] (1) The composition design of this invention is to add appropriate amounts of Nb, V, Ti, Al and Cu to form a large number of dispersed fine nano phases in steel. The strength and toughness are improved by fine grain strengthening and precipitation strengthening. Combined with the C content of 0.15~0.17%, the Mn content of 1.4~2.6% and the Ni content of 0.7~1.6% and the soft and hard phase structure obtained by the offline salt bath two-step Q&P heat treatment process, the strength and toughness are improved. The yield strength of the steel plate is ≥1100MPa, the elongation is ≥14%, and the impact energy at -40℃ is ≥30J.

[0026] (2) An offline salt bath two-step Q&P heat treatment process is adopted. Through quenching and partitioning, a tempered lath bainitic structure with strength differences is formed to reduce the yield strength ratio while ensuring strength. By controlling the quenching temperature and partitioning temperature, the composition of the soft and hard phases of the steel is adjusted. Combined with composition design and the strengthening of microalloyed precipitates, high strength and a low yield strength ratio can be obtained. Under the premise that the carbon content of the steel plate is 0.15~0.17%, a yield strength ≥1100MPa and a yield strength ratio ≤0.88 can be achieved. Attached Figure Description

[0027] Figure 1 The microstructure of the 1100MPa grade high-strength steel with low yield strength ratio in Example 1;

[0028] Figure 2 The microstructure of the 1100MPa grade high-strength steel with low yield strength ratio in Example 2;

[0029] Figure 3 The microstructure of the 1100MPa grade high-strength steel with low yield strength ratio in Example 3;

[0030] Figure 4 The microstructure of the 1100MPa grade high-strength steel with low yield strength ratio in Example 4;

[0031] Figure 5 The microstructure of the 1100MPa grade high-strength steel with low yield strength ratio in Example 5. Detailed Implementation

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The present invention relates to a high-strength steel with a low yield strength ratio of 1100MPa, the chemical composition of which, 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 0.6~1.5%, with the balance being Fe and unavoidable impurities.

[0036] This invention relates to a high-strength Q&P steel with a low yield strength ratio of 1100MPa, 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 heated to 890-920℃, and then subjected to 6-7 passes of rolling in the second stage, 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) An offline salt bath two-step Q&P heat treatment process is used to heat-treat the hot-rolled plate: the complete austenitizing temperature is 900-920℃, the salt bath quenching temperature is 200-250℃, 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 1100MPa. Its thickness is 15mm. Its yield strength is ≥1100MPa, yield ratio ≤0.88, elongation ≥14%, and impact energy at -40℃ ≥30J. Its microstructure is tempered lath martensite, and the microstructure of the steel plate is as follows: Figures 1-5 As shown.

[0037] Example 1

[0038] The chemical composition of the steel plate by mass percentage is: C 0.16%, Si 0.51%, Mn 2.53%, Cr 0.53%, Ni 0.79%, Mo 0.47%, Nb 0.054%, V 0.047%, Ti 0.019%, Al 0.022%, Cu 0.83%, with the balance being Fe and unavoidable impurities.

[0039] 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 1160℃, after descaling, the first stage involves 7 passes of rolling, and after the intermediate slab is heated to 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 890MPa. The mechanical properties are shown in Table 1, and typical microstructure photographs of the steel plate are shown below. Figure 1As shown, the microstructure is tempered lath martensite.

[0040] Example 2

[0041] 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.

[0042] 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 1160℃, after descaling, the first stage involves 7 passes of rolling, and after the intermediate slab reaches 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 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.

[0043] Example 3

[0044] The chemical composition of the steel plate by mass percentage is: C 0.17%, Si 0.55%, Mn 1.6%, Cr 0.53%, Ni 1.57%, Mo 0.5%, Nb 0.052%, V 0.062%, Ti 0.018%, Al 0.04%, Cu 0.9%, with the balance being Fe and unavoidable impurities.

[0045] 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 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 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.

[0046] Example 4

[0047] The chemical composition of the steel plate by mass percentage is: 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.

[0048] 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 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.

[0049] Example 5

[0050] The chemical composition of the steel plate by mass percentage is: C 0.16%, Si 0.49%, Mn 1.41%, Cr 0.56%, Ni 1.44%, Mo 0.22%, Nb 0.051%, V 0.03%, Ti 0.019%, Al 0.044%, Cu 1.41%, with the balance being Fe and unavoidable impurities.

[0051] 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 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 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 5 As shown, the microstructure is tempered lath martensite.

[0052] Table 1 Mechanical properties of the low yield strength ratio 1100MPa grade high-strength steel involved in the embodiments of the present invention

[0053]

[0054] Note: According to the GB / T228.1-2010 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 / T229-2007 test standard, the Charpy impact specimen size is 10×10×55mm, and the sampling position is longitudinal.

[0055] 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 steel with a low yield strength ratio of 1100 MPa, characterized in that, The chemical composition of the steel plate, by mass percentage, is: C 0.15~0.17%, Si 0.58~0.6%, Mn 2.53~2.6%, Cr 0.53~0.8%, Ni 1.44~1.6%, Mo 0.2~0.6%, Nb 0.052~0.08%, V 0.02~0.08%, Ti 0.019~0.03%, Al 0.01~0.05%, Cu 0.83~1.5%, with the balance being Fe and unavoidable impurities; The steel plate has a yield strength ≥1100MPa, a yield strength ratio ≤0.88, an elongation ≥14%, and an impact energy ≥30J at -40℃. The finished thickness of the steel plate is 15mm, and the microstructure is tempered lath martensite.

2. The method for producing a high-strength steel with a low yield strength ratio of 1100 MPa 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 350~450℃, and finally water cool to room temperature to obtain a high-strength Q&P steel with a low yield strength ratio of 1100MPa.

3. The method for producing a high-strength steel with a low yield strength ratio of 1100 MPa according to claim 2, characterized in that: The tensile strength of hot-rolled steel plates is ≥1200MPa and the elongation is ≥14%.

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