A low yield ratio 1030MPa grade high-strength Q&P steel and its production method

Abstract

The present invention provides a low-yield ratio 1030 MPa grade high-strength Q&P steel and a production process thereof. The chemical composition of the thick plate is, by mass percentage, as follows: C 0.15-0.17%, Si 0.4-0.6%, Mn 1.4-2.6%, Cr 0.4-0.8%, Ni 1.2-1.6%, 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 0.8-1.5%, with the balance being Fe and unavoidable impurities. The production process utilizes the above composition, controlled rolling and controlled cooling, and an offline salt bath Q&P heat treatment process to produce a 15 mm thick low-yield ratio 1030 MPa grade high-strength steel. The controlled rolling and controlled cooling process adopts two-stage rolling, and after rolling, it is water-cooled to 400-420℃, and then air-cooled to room temperature. The heat treatment adopts the salt bath two-step Q&P process, with a complete austenitization temperature of 900-920℃, a salt bath quenching temperature of 200-250℃, a salt bath partitioning temperature of 350-400℃, and finally water-cooled to room temperature. The yield strength of the low yield ratio 1030MPa grade high-strength steel plate of the present invention is ≥1030MPa, the yield ratio is ≤0.85, the elongation is ≥14%, and the impact energy at -40℃ is ≥45J. The advantage of the present invention is that the salt bath Q&P heat treatment process is used to obtain a good performance match between high strength and low yield ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of steel materials, and in particular to a low-yield ratio 1030 MPa grade high-strength Q&P steel and a production method thereof. Background Art

[0002] High-strength steel plates are widely used in machinery, mining, ocean, bridges and other fields. With the increasing requirements for the strength level of high-strength steel, the requirements for low yield ratio are also getting higher and higher. The quenching + medium temperature / low temperature tempering heat treatment process is widely used to produce high-strength structural steel plates with higher performance stability and uniformity. Although the quenching + tempering heat treatment process has good strength and toughness, the yield ratio is relatively high (>0.9). At present, the quenching + two-phase zone secondary quenching + tempering (QLT) process is mostly used to reduce the yield ratio, but the two-phase zone secondary quenching makes it difficult to reach the yield strength of 1030MPa. The high-strength steel plates produced by the Q&P heat treatment process have a low yield ratio, but currently this process is only used to produce thin plates and it is difficult to reach a strength of 1030MPa.

[0003] Chinese invention patent publication number CN106544590 discloses a 1000MPa-grade high-toughness, high-performance, uniform, easily weldable, extra-thick steel plate and its manufacturing method. Through composition design, controlled rolling and controlled cooling, combined with a quenching process at 890-910°C and tempering at 200-240°C, the plate achieves a yield strength ≥1030MPa, a tensile strength ≥1100MPa, an elongation ≥10%, and a low-temperature Charpy impact energy of ≥50J at -40°C. This high-strength steel has low elongation and a high yield strength ratio (>0.9).

[0004] Chinese invention patent publication number CN108315671 discloses an ultra-high-strength steel with a yield strength of 1000 MPa and a low yield strength ratio, and its preparation method. Through a high-Si and high-Mn composition design, controlled rolling and controlled cooling combined with 850-1000°C quenching and 200-400°C tempering heat treatment, the steel achieves an elongation of 13.8-16.9%, an impact energy of ≥100 J for a 10 mm thick specimen at -20°C, and ≥36 J for a 2.5 mm thick specimen at -20°C. This high-strength steel exhibits good strength-ductility and a low yield strength ratio, but exhibits a high Si content and low low-temperature toughness.

[0005] Chinese invention patent publication number CN111748732 discloses a 1000MPa-grade high-toughness, high-magnetism hot-rolled yoke steel and its production method. Through a composition design with a carbon content of 0.2-0.5%, controlled rolling and controlled cooling combined with a quenching process at 850-950°C and tempering at 500-700°C, the steel achieves a yield strength of 1000MPa or higher, a tensile strength of 1050MPa or higher, an elongation of 15%, and a low-temperature Charpy impact energy of 20J or higher at -20°C. This high-strength steel has a high yield strength ratio (>0.93), low low-temperature toughness, and a high carbon content.

[0006] Chinese invention patent publication number CN114277306 discloses a 1000 MPa-grade high-strength steel for engineering machinery and its production method. Through complex composition design, controlled rolling and controlled cooling combined with a heat treatment process of Ac3+(50-100)°C quenching followed by 600-650°C tempering, the steel achieves yield strength ≥1000 MPa, tensile strength ≥1050 MPa, elongation A ≥18%, and -60°C impact energy ≥100 J. This high-strength steel exhibits excellent strength, toughness, and ductility, but the quenching followed by high-temperature tempering results in a high yield strength ratio (>0.93).

[0007] Chinese invention patent publication number CN108193138 discloses a 980 MPa-grade cold-rolled high-strength Q&P steel for automotive use and its production method. Through compositional design combined with a Q&P heat treatment process, the steel achieves a yield strength of 550 MPa or higher, a tensile strength of 980 MPa or higher, and an elongation of 20% or higher. This high-strength steel has a low yield strength ratio, resulting in a lower strength grade and thinner thickness. High C and Si contents in the compositional design of thick plates can lead to poor low-temperature toughness and weldability. Summary of the Invention

[0008] The present invention aims to provide a low-yield ratio 1030 MPa grade high-strength Q&P steel and a production method thereof, which overcomes the shortcomings of the prior art and is suitable for 10-20 mm thick plates. By designing the alloy composition, controlling the phase composition, and conducting a two-step Q&P heat treatment process using controlled rolling and cooling, and an offline salt bath, the steel achieves high strength and a low yield ratio.

[0009] To achieve the above object, the present invention is implemented through the following technical solutions:

[0010] Technical solution 1: A low yield ratio 1030 MPa grade high-strength Q&P steel, the composition of the steel plate 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 1.2-1.6%, 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 0.8-1.5%, and the balance is Fe and unavoidable impurities.

[0011] Technical Solution 2: A method for producing low yield ratio 1030MPa grade high strength Q&P steel, including smelting, casting, characterized in that the specific preparation steps are: 1) using a two-stage rolling process to roll the thick slab: the starting rolling temperature

[0012] ≥1150℃, after descaling, the first stage is rolled in 6 to 7 passes, and after the intermediate billet is heated to 890 to 920℃, the second stage is rolled in 6 to 7 passes, and the final rolling temperature is ≥850℃; 2) after rolling, it is water-cooled to 400 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 austenitization temperature is 900 to 920℃, the salt bath quenching temperature is 200 to 250℃, the salt bath partitioning temperature is 350 to 400℃, and finally water-cooled to room temperature to obtain a low yield ratio 1030MPa grade high-strength Q&P steel.

[0013] The microstructure of the finished product low yield ratio 1030MPa grade high strength steel is tempered lath martensite, and the yield strength

[0014] ≥1030MPa, yield strength ratio ≤0.85, elongation ≥14%, -40℃ impact energy ≥45J.

[0015] The alloy composition design of the present invention is relatively complex, with a high alloy content and the addition of multiple microalloying elements. The following is a detailed analysis and explanation of the functions and dosages of the main alloying components contained in the low-yield ratio 1030 MPa grade high-strength steel of the present invention:

[0016] C: C can significantly improve the strength of steel plates through solid solution strengthening, but too high a content can adversely affect the plasticity and low-temperature toughness of the steel plates. To ensure a yield strength of 1030 MPa, the C content in the present invention is 0.15-0.17%.

[0017] Si: Si improves strength through solid solution strengthening, but it will produce large-sized inclusions. Too high a Si content will lead to a decrease in low-temperature toughness. In order to ensure toughness and plasticity, the Si content in the present invention is 0.4-0.6%.

[0018] Mn: Mn improves the strength through solid solution strengthening and can improve the hardenability of the steel plate. However, too high a Mn content will lead to a decrease in low-temperature toughness. Therefore, the Mn content in the present invention is 1.4-2.6%.

[0019] Cr: Cr improves the strength through solid solution strengthening and can improve the hardenability of the steel plate. Therefore, the Cr content in the present invention is 0.4-0.8%.

[0020] Ni: Ni can improve the strength and low-temperature toughness of the steel plate, improve the hardenability of the steel plate, improve the corrosion resistance of the steel plate, and inhibit the hot brittleness caused by Cu. Therefore, the Ni content in the present invention is 1.2-1.6%.

[0021] Mo: Mo improves strength by refining grains, and can improve the hardenability and corrosion resistance of the steel plate. However, too high a Mo content will reduce welding performance. Therefore, the Mo content in the present invention is 0.2-0.6%.

[0022] Cu: Cu improves strength through precipitation strengthening, and can improve the hardenability and corrosion resistance of the steel plate. However, too much Cu will lead to hot brittleness and needs to be used in combination with Ni. Therefore, the Cu content in the present invention is 0.8-1.5%.

[0023] Nb, V, Ti, and Al: Nb, V, Ti, and Al increase strength through precipitation strengthening. Adding trace amounts to steel forms dispersed nanoscale precipitates, but additions below 0.01% have little effect. Nanoscale precipitates of Nb and Ti can improve strength and low-temperature toughness by refining grain size. However, excessive Ti content can lead to the formation of large TiN particles, reducing the steel's low-temperature toughness. Therefore, in the present invention, the Nb and V contents are set at 0.02-0.08%, the Ti content at 0.01-0.03%, and the Al content at 0.01-0.05%.

[0024] The present invention utilizes a two-stage controlled rolling and controlled cooling process. By controlling the reduction and intermediate billet holding temperature in both stages, recrystallization is promoted, avoiding the mixed crystal zone, thereby achieving a fine and uniform structure. After rolling, the steel is cooled to room temperature using a water-cooling and air-cooling method to control grain size and improve steel plate strength.

[0025] The present invention utilizes an offline, two-step salt-bath Q&P heat treatment process. This two-step process, quenching and partitioning, creates a multiphase structure to reduce the yield strength ratio while maintaining strength. By controlling the quenching and partitioning temperatures, the steel's phase composition is adjusted, resulting in high strength and a low yield strength ratio.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) The composition design of the present invention forms a large number of dispersed fine nanophases in the steel by adding appropriate amounts of Nb, V, Ti, Al, and Cu, and improves strength and toughness and plasticity through fine grain strengthening and precipitation strengthening. The soft and hard phase structures obtained by combining the C content of 0.15-0.17% and the Ni content of 1.3-1.5% with the offline salt bath two-step Q&P heat treatment process improve strength and toughness, resulting in a steel plate with a yield strength of ≥1030 MPa, an elongation of ≥14%, and an impact energy of ≥45 J at -40°C.

[0028] (2) An offline salt bath two-step Q&P heat treatment process is used. Through the two-step quenching and partitioning process, a tempered lath bainite structure with strength differences is formed on the basis of ensuring strength to reduce the yield strength ratio. By controlling the quenching temperature and partitioning temperature, adjusting the soft and hard phase composition of the steel, combining composition design with the strengthening of microalloy precipitation phases, high strength and a low yield strength ratio are achieved. Under the premise of a carbon content of 0.15-0.17% in the steel plate, a yield strength of ≥1030MPa and a yield strength ratio of ≤0.85 can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The microstructure of the low yield ratio 1030 MPa grade high strength steel in Example 1;

[0030] Figure 2 The microstructure of the low yield ratio 1030 MPa grade high strength steel in Example 2;

[0031] Figure 3 The microstructure of the low yield ratio 1030 MPa grade high strength steel in Example 3;

[0032] Figure 4 The microstructure of the low yield ratio 1030 MPa grade high strength steel in Example 4;

[0033] Figure 5 This is the microstructure of the low yield ratio 1030 MPa grade high-strength steel in Example 5. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0035] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] The components of the embodiments of the present invention generally described and shown in the drawings herein may be arranged and designed in countless 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 is merely representative of selected embodiments of the invention.

[0037] The low-yield ratio 1030 MPa grade high-strength steel of the present invention has the following components in percentage by mass: C 0.15-0.17%, Si 0.4-0.6%, Mn 1.4-2.6%, Cr 0.4-0.8%, Ni 1.2-1.6%, 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 0.8-1.5%, and the balance is Fe and unavoidable impurities.

[0038] The present invention discloses a low-yield ratio 1030 MPa grade high-strength Q&P steel. The production method comprises the following specific rolling and heat treatment steps: 1) rolling a thick slab using a two-stage rolling process: the starting rolling temperature is ≥1150°C, and after descaling, the first stage rolling is performed in 6-7 passes. After the intermediate slab is heated to 890-920°C, the second stage rolling is performed in 6-7 passes, and the final rolling temperature is ≥850°C; 2) after rolling, the slab is water-cooled to 400-420°C 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 austenitization temperature is 900-920°C, the salt bath quenching temperature is 200-250°C, the salt bath partitioning temperature is 350-400°C, and finally, the hot-rolled plate is water-cooled to room temperature to obtain a low-yield ratio 1030 MPa grade high-strength Q&P steel. The steel has a thickness of 15 mm. Its yield strength is ≥1030MPa, yield strength ratio is ≤0.85, elongation is ≥14%, and impact energy at -40℃ is ≥45J. Its microstructure is tempered lath martensite. The microstructure of the steel plate is as follows: Figures 1 to 5 shown.

[0039] Example 1

[0040] The chemical composition of the steel plate is as follows by mass percentage: 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%, and the balance is 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 starting rolling temperature is 1150°C, and after descaling, the first stage rolling is 6 passes. After the intermediate slab is heated to 890°C, the second stage rolling is 6 passes, and the final rolling temperature is ≥850°C; 2) after rolling, the slab is water-cooled to 400°C and then air-cooled to room temperature; 3) the hot-rolled plate is heat-treated by an offline salt bath two-step Q&P heat treatment process: the complete austenitization temperature is 900°C, the salt bath quenching temperature is 200°C, the salt bath partitioning temperature is 400°C, and finally water-cooled to room temperature to obtain a low yield ratio 890MPa grade high-strength Q&P steel. The mechanical properties are shown in Table 1. The typical microstructure photos of the steel plate are shown in Table 1. Figure 1 As shown, the microstructure is tempered lath martensite.

[0042] Example 2

[0043] The chemical composition of the steel plate is as follows by mass percentage: C 0.153%, Si 0.51%, Mn 2.42%, Cr 0.56%, Ni 1.5%, Mo 0.22%, Nb 0.052%, V 0.051%, Ti 0.02%, Al 0.04%, Cu 0.87%, and the balance is Fe and unavoidable impurities.

[0044] Example 2 The rolling and heat treatment steps are as follows: 1) a two-stage rolling process is used to roll the thick slab: the starting rolling temperature is 1160°C, and after descaling, the first stage rolling is 7 passes. After the intermediate slab is heated to 910°C, the second stage rolling is 6 passes, and the final rolling temperature is ≥850°C; 2) after rolling, the slab is water-cooled to 400°C and then air-cooled to room temperature; 3) the hot-rolled plate is heat-treated by an offline salt bath two-step Q&P heat treatment process: the complete austenitization temperature is 920°C, the salt bath quenching temperature is 200°C, the salt bath partitioning temperature is 350°C, and finally water-cooled to room temperature to obtain a low yield ratio 890MPa grade high-strength Q&P steel. The mechanical properties are shown in Table 1. The typical microstructure photos of the steel plate are shown in Table 1. Figure 2 As shown, the microstructure is tempered lath martensite.

[0045] Example 3

[0046] The chemical composition of the steel plate is as follows by mass percentage: 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%, and the balance is Fe and unavoidable impurities.

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

[0048] Example 4

[0049] The chemical composition of the steel plate is as follows by mass percentage: 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%, and the balance is Fe and unavoidable impurities.

[0050] Example 4 The rolling and heat treatment steps are as follows: 1) a two-stage rolling process is used to roll the thick slab: the starting rolling temperature is 1150°C, and after descaling, the first stage rolling is 7 passes. After the intermediate slab is heated to 900°C, the second stage rolling is 6 passes, and the final rolling temperature is ≥850°C; 2) after rolling, the slab is water-cooled to 400°C 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 austenitization temperature is 900°C, the salt bath quenching temperature is 250°C, the salt bath partitioning temperature is 450°C, and finally water-cooled to room temperature to obtain a low yield ratio 890MPa grade high-strength Q&P steel. The mechanical properties are shown in Table 1. The typical microstructure photos of the steel plate are shown in Table 1. Figure 4 As shown, the microstructure is tempered lath martensite.

[0051] Example 5

[0052] The chemical composition of the steel plate is as follows by mass percentage: 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%, and the balance is Fe and unavoidable impurities.

[0053] The rolling and heat treatment steps of Example 5 are as follows: 1) a two-stage rolling process is used to roll the thick slab: the starting rolling temperature is 1160°C, and after descaling, the first stage rolling is 7 passes. After the intermediate slab is heated to 890°C, the second stage rolling is 7 passes, and the final rolling temperature is ≥850°C; 2) after rolling, water cooling is performed to 420°C, and then air cooling is performed 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 austenitization temperature is 920°C, the salt bath quenching temperature is 200°C, the salt bath partitioning temperature is 400°C, and finally water cooling is performed to room temperature to obtain a low yield ratio 890MPa grade high-strength Q&P steel. The mechanical properties are shown in Table 1, and a typical microstructure photograph of the steel plate is shown in Table 1. Figure 5 As shown, the microstructure is tempered lath martensite.

[0054] The mechanical properties of the low yield ratio 1030 MPa grade high strength Q&P steel obtained in the above embodiments of the present invention are compared in Table 1.

[0055] Table 1 Mechanical properties of low yield ratio 1030 MPa grade high strength steel in the embodiments of the present invention

[0056]

[0057] Note: According to the GB / T 228.1-2010 test standard, the tensile specimen adopts a gauge length of For the rod-shaped specimen, the sampling position is longitudinal sampling; according to the test standard GB / T 229-2007, the size of the Charpy impact specimen is 10×10×55mm, and the sampling position is longitudinal sampling.

[0058] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A low yield ratio 1030MPa grade high strength Q&P steel, characterized in that: The chemical composition of the steel plate 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 1.2~1.6%, Mo 0.2~0.6%, Nb0.02~0.08%, V 0.02~0.08%, Ti 0.01~0.03%, Al 0.01~0.05%, Cu 0.8~1.5%, and the balance is Fe and unavoidable impurities; The steel plate has a yield strength of ≥1030 MPa, a yield strength ratio of ≤0.85, an elongation of ≥14%, and an impact energy of ≥45 J at -40°C; The finished steel plate has a thickness of 15 mm and a microstructure of tempered lath martensite; The production method of the low yield ratio 1030MPa grade high-strength Q&P steel comprises the following specific rolling and heat treatment steps: 1) rolling the thick slab using a two-stage rolling process: the starting rolling temperature is ≥1150°C, and after descaling, the first stage rolling is 6-7 passes. After the intermediate slab is heated to 890-920°C, the second stage rolling is 6-7 passes, and the final rolling temperature is ≥850°C; 2) water cooling to 400-420°C after rolling, and then air cooling to room temperature; 3) heat treating the hot-rolled plate using an offline salt bath two-step Q&P heat treatment process: the complete austenitization temperature is 900-920°C, the salt bath quenching temperature is 200-250°C, the salt bath partitioning temperature is 350-400°C, and finally water cooling to room temperature to obtain the low yield ratio 1030MPa grade high-strength Q&P steel.

2. The low yield ratio 1030 MPa grade high strength Q&P steel according to claim 1, characterized in that: The salt bath partitioning temperature is 400-450°C.

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