Quenched and tempered steel plate with yield strength of 1000 MPa and production method thereof

Abstract

The invention discloses a quenched and tempered steel plate with yield strength of 1000MPa and a production method thereof. The quenched and tempered steel plate comprises the following components in percentage by weight: 0.15-0.2% of C, 0.05-0.3% of Si, 0.80-1.60% of Mn, more than or equal to 0.6% of Cr, less than or equal to 0.2% of Mo, more than or equal to 0.06% of V, less than or equal to 0.005% of N, less than or equal to 0.030% of P, less than or equal to 0.005% of S, less than or equal to 0.004% of O and the balance of Fe and inevitable impurities. And the contents of the elements must simultaneously meet the following relation: 1% < = 3Mo + 0.6 Cr + 5V < = 1.5%. According to the component design, under the conditions that low Mo or even no Mo is adopted, and the contents of C and Mn are low, the tempering softening resistance and precipitation strengthening effects of high Cr and V are utilized, high strength, low cost and good comprehensive performance of the steel plate are achieved, the yield strength of the steel plate is larger than or equal to 1000 MPa, the tensile strength of the steel plate is larger than or equal to 1050 MPa, and the ductility gt is obtained; the impact energy at-40 DEG C is gt; the power is 120 J.

Description

Technical Field

[0001] The invention relates to the technical field of quenched and tempered steel plates and their manufacturing, and in particular to a quenched and tempered steel plate with a yield strength of 1000MPa and a production method thereof. Background Art

[0002] Construction machinery is an important part of the equipment industry. With the development of large-scale, high-end and lightweight construction machinery industry, the strength level of steel used in it has been continuously improved, generally rising from the original 500-600MPa to 1000MPa or even 1300MPa. Due to the harsh service environment of high-strength steel used in construction machinery, in addition to high strength, there are strict requirements on the elongation, bending performance, toughness and welding performance of the steel plate.

[0003] At present, quenched and tempered high-strength steel with a yield strength of 1000MPa has been widely used in the engineering machinery industry. In order to ensure good plasticity and high toughness, this type of steel has a high tempering temperature. In order to maintain an ultra-high strength of more than 1000MPa under high-temperature tempering conditions, a high content of Mo element needs to be added to resist softening during tempering.

[0004] Chinese invention patent CN113430458A discloses an ultra-high strength steel plate with a yield strength of 1040 MPa or above and a manufacturing method thereof, wherein the steel plate comprises: C: 0.07-0.12%, Si: 0.05-0.15%, Mn: 0.70-1.40%, P≤0.015%, S≤0.005%, Cu: 0.76-1.40%, Cr: 0.20-0.60%, Ni: 2.0-4.0%, Mo: 0.20-0.80%, Nb≤0.10%, V: 0.02-0.12%, Ti≤0.02%, Al≤0.04%, and the remainder comprises Fe and unavoidable impurity elements. In order to obtain higher strength, Mo: 0.20-0.80% is added to the steel.

[0005] Chinese invention patent CN114277306 A discloses a 1000MPa grade high-strength steel for engineering machinery and its production method, the chemical composition weight percentage of which is: C: 0.16-0.20%, Si: 0.30-0.50%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.40-0.70%, W: 0-0.70%, Ni: 0.10-0.50%, Cu: 0-0.40%, Nb: 0.010-0.030%, Ti: 0.010-0.030 %, V: 0.010~0.050%, B: 0.0005~0.0030%, Al: 0.02~0.06%, Ca: 0.001~0.004%, N: 0.002~0.005%, P≤0.020%, S≤0.0050%, O≤0.0040%, the rest is Fe and unavoidable impurities; and the above elements must also satisfy the following relationship 6≤(Mo+0.93W+0.52Cr+0.21Mn+0.55) / C≤10. In order to obtain high strength steel, Mo: 0.40~0.70% is added.

[0006] Chinese invention patent CN102560274A discloses a quenched and tempered ultra-high strength steel with a yield strength of 1000MPa, whose chemical composition is (by weight): C: 0.15-0.20%; Si: 0.10-0.40%; Mn: 1.00-1.50%; Cr: 0.30-0.50%; Mo: 0.30-0.50%; Ni: 0.20-0.50%; B: 0.0010-0.0030%; Nb: 0.010-0.030%; V: 0.030-0.050%; Ti: 0.010-0.020%; Alt: 0.030-0.050%; P≤0.020%; S≤0.010%, and the rest is Fe and unavoidable impurities, and Mo: 0.30-0.50% is also added to the steel.

[0007] Chinese invention patent CN116676541A discloses a low-cost 960MPa ultra-high strength steel plate, whose chemical composition mass percentage is: C: 0.18-0.25%, Mn: 1.6-1.8%, Cr: 0.5-0.6%, V: 0.04-0.06%, Ti: 0.01-0.02%, B: 0.001-0.002%, and the rest is Fe and unavoidable impurities. Although the steel does not add Mo element, it mainly uses higher C and Mn content to improve strength.

[0008] If the carbon equivalent is low and the Mo content is low, the strength after quenching and tempering is difficult to reach 1000MPa. For example, Chinese invention patent CN114318120A discloses a high-strength steel for engineering machinery, whose Mo content is 0.10-0.30%, and the yield strength can only be guaranteed to be ≥800MPa.

[0009] In recent years, the price of Mo alloys has skyrocketed, significantly increasing the alloy cost and steel price of such products, limiting the expansion of their market and wider application. Therefore, it is urgent to develop high-strength quenched and tempered high-strength steel with low Mo alloy content. Summary of the invention

[0010] The object of the present invention is to provide a quenched and tempered steel plate with a yield strength of 1000MPa and a production method thereof. The quenched and tempered steel plate has a yield strength of ≥1000MPa, a tensile strength of ≥1050MPa, an elongation of>18%, and an impact energy of>120J at -40°C. The steel plate has ultra-high strength and low cost, and good comprehensive performance.

[0011] To achieve the above object, the technical solution of the present invention is:

[0012] A quenched and tempered steel plate with a yield strength of 1000 MPa, wherein the composition by weight percentage is: C: 0.15-0.2%, Si: 0.05-0.3%, Mn: 0.80-1.60%, Cr≥0.6%, Mo≤0.2%, V≥0.06%, N≤0.005%, P≤0.030%, S≤0.005%, O≤0.004%, and the rest includes Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship:

[0013] 1%≤3Mo+0.6Cr+5V≤1.5%.

[0014] Furthermore, the steel plate further comprises one or more of Nb≤0.050%, Ti≤0.050%, Cu≤0.5%, Ni≤0.5%, B≤0.003%, Al≤0.06%, Ca≤0.004%, and RE≤0.005%.

[0015] Furthermore, the remainder is Fe and other inevitable inclusions.

[0016] The microstructure of the steel plate of the present invention is tempered martensite + cementite within 50nm + Cr7C3 and VC or (V, Mo)C carbide of 2-10nm.

[0017] The steel plate of the present invention has a yield strength of ≥1000MPa, a tensile strength of ≥1050MPa, an elongation of>18%, and an impact energy of>120J at -40°C.

[0018] In the steel plate composition design of the present invention:

[0019] C: C is a basic element in steel and an important element for strengthening martensitic steel. C above 0.15% can ensure the strength of the quenched steel plate; however, a higher C content will lead to an increase in the overall C equivalent, which is prone to cracking during welding. Therefore, the C content of the present invention is controlled at 0.15-0.2%.

[0020] Si: Si above 0.05% can play a good deoxidation role, and Si exceeding 0.3% is prone to produce red iron scale. The silicon content of the present invention is controlled within 0.05-0.3%.

[0021] Mn: Mn element can improve the hardenability of steel when it is above 0.8%. When the Mn content exceeds 1.6%, it is easy to produce segregation and inclusions such as MnS, which deteriorates the toughness of martensitic high-strength steel. In the present invention, the Mn content is controlled at 0.80-1.60%.

[0022] Cr: Cr can improve the hardenability of steel and is conducive to the formation of full martensite structure during quenching; in addition, it has the effect of resisting tempering softening. Studies have found that when the Cr content is ≥0.60%, Cr carbide Cr7C3 can also be formed during high-temperature tempering, which plays a role of precipitation strengthening or secondary hardening. Therefore, the present invention controls the Cr content to be ≥0.60%.

[0023] Mo: Mo element can improve the hardenability of steel, which is conducive to the formation of full martensite structure during quenching; Mo can inhibit the growth and coarsening of carbides during high temperature tempering, and has the effect of resisting high temperature tempering softening and softening of welded joints; however, too high Mo content will lead to a significant increase in cost. Therefore, the present invention controls the Mo content to ≤0.20%.

[0024] V: V element can refine the grains and improve the toughness of steel. V reacts with C during tempering to form carbides, which have a precipitation strengthening effect, resist tempering softening, and improve the strength of quenched and tempered steel. Therefore, the present invention controls the V content to be ≥ 0.06%.

[0025] Cr, Mo and V are strong carbide forming elements, and their affinity with C is greater than that of Fe with C, so they can inhibit C from combining with Fe to form cementite, and inhibit the coarsening of cementite. Inhibiting the precipitation or coarsening of cementite can increase the C content dissolved in martensite, improve the strength of matrix martensite, and on the other hand, inhibit the occurrence of temper brittleness. In addition, Mo, Cr, V can react with C to form nano-scale carbides during high-temperature tempering, which plays a role in precipitation strengthening. Therefore, Mo, Cr and V play a role in tempering softening resistance, and improve the strength of steel plates. However, when the content of Cr and V is too much, alloy carbides Cr7C3 and VC will coarsen during high-temperature tempering, especially alloy carbides VC of V, which are easy to coarsen, reduce the precipitation strengthening efficiency, and significantly deteriorate the toughness of the steel plate.

[0026] Therefore, it is necessary to control the content of Mo, Cr and V within a reasonable range, which can inhibit the coarsening of cementite on the one hand and avoid the growth of alloy carbides on the other hand. According to the strength of the binding force between Mo, Cr and V and C and the precipitation effect of alloy carbides, the precipitation control index PCI = 3Mo + 0.6Cr + 5V can be defined. Experiments show that when PCI ≥ 1%, the precipitation of cementite can be significantly inhibited, and the size of the precipitated cementite can be controlled within 50nm to ensure the strength and toughness of the matrix martensite. When PCI ≤ 1.5%, the size of Cr7C3 and VC or (V, Mo) C precipitation phases is 2 to 10nm, which can play a significant precipitation strengthening effect and ensure the strength of the steel plate after high temperature tempering. When PCI ≥ 1.5%, the VC or (V, Mo) C precipitation phase coarsening is more serious, which reduces the precipitation strengthening efficiency and significantly reduces the toughness; at the same time, too high Mo, Cr and V will also significantly increase the cost of steel. Therefore, the present invention sets the PCI range as: 1%≤PCI≤1.5%.

[0027] Ni: Ni element has the function of promoting plastic flow, improving the plasticity and toughness of steel; too high Ni content will lead to an increase in carbon equivalent, deteriorating welding performance, and Ni is a precious metal, which will increase the cost. Therefore, the present invention controls the Ni content to ≤ 0.50%.

[0028] Cu: Cu element can produce a certain precipitation strengthening effect during tempering. In addition, adding a certain amount of Cu element can improve the corrosion resistance of ultra-high strength steel for engineering machinery. Therefore, the present invention controls the Cu content to ≤0.50%.

[0029] Nb: Nb is a microalloying element, which forms nano-scale precipitates with elements such as C and N, inhibiting the growth of austenite grains during heating; Nb can increase the non-recrystallization critical temperature Tnr and expand the production window. Therefore, the present invention controls the niobium content to ≤ 0.050%.

[0030] Ti: Ti can react with N to consume N in the matrix and reduce the deterioration of performance caused by N. Ti also refines grains and improves the toughness of steel. Too much Ti will cause the coarsening of TiN inclusions and deteriorate the toughness of steel. Therefore, the present invention controls the Ti content to ≤0.05%.

[0031] B: A trace amount of B can improve the hardenability and strength of steel; however, B exceeding 0.0030% is prone to segregation and forms carbon-boron compounds, which seriously deteriorates the toughness and welding performance of steel. Therefore, the present invention controls the boron content to ≤ 0.0030%.

[0032] RE: Rare earth elements can improve the morphology of inclusions in steel, making the inclusions smaller and more dispersed. Especially for the high Ti content steel of the present invention, it can significantly improve the size and morphology of TiN inclusions and improve the toughness and fatigue performance of steel. Too much rare earth elements will increase the cost. Therefore, the present invention controls the RE content to ≤0.0050%.

[0033] Al: As a deoxidizer, it can refine the grains and improve the impact toughness. When the Al content exceeds 0.06%, Al oxide inclusion defects are likely to occur. Therefore, the Al content is controlled to be ≤ 0.06% in the present invention.

[0034] Ca: Ca can act as a purifier in the steel smelting process and improve the toughness of the steel; however, when the Ca content exceeds 0.004%, it is easy to form Ca compounds with larger sizes, which will deteriorate the toughness. Therefore, the present invention controls the Ca content to ≤ 0.004%.

[0035] N: The present invention requires strict control of the range of N element. N content exceeding 0.005% easily leads to the formation of coarse precipitate particles, which deteriorates toughness. Therefore, the present invention controls the N content to ≤ 0.005%.

[0036] P, S and O: P, S and O as impurity elements affect the plasticity and toughness of steel. The control ranges of the present invention are P≤0.030%, S≤0.0050%, and O≤0.0040%, respectively.

[0037] The method for producing a quenched and tempered steel plate with a yield strength of 1000 MPa of the present invention comprises the following steps:

[0038] 1) Smelting and casting

[0039] Smelting and casting into slabs according to the above ingredients;

[0040] 2) Heating

[0041] The heating temperature is 1200-1250℃. When the core of the ingot reaches the temperature, the insulation time is

[0042] >1.5h;

[0043] 3) Rolling

[0044] The final rolling temperature is 820~920℃;

[0045] 4) Cooling and coiling

[0046] The rolled steel plate is cooled to 500-700°C at a cooling rate of ≥60°C / s, then coiled and air-cooled to room temperature;

[0047] 5) Heat treatment

[0048] Quenching treatment: heat the steel plate to 820-950℃, keep the temperature for 5-40min after the core of the steel plate reaches the temperature, and then quickly cool it to room temperature at a cooling rate of ≥20℃ / s;

[0049] Tempering treatment, the tempering temperature is 450-650℃, the core of the steel plate is kept warm after reaching the temperature, the holding time is ≥5min, and finally air-cooled to room temperature.

[0050] Preferably, the final rolling reduction ratio is 15 to 20%.

[0051] In the manufacturing method of the present invention:

[0052] During the quenching treatment, the quenching temperature is 820-950℃. The temperature above 820℃ is to ensure that the steel plate can be completely austenitized; the temperature below 950℃ is to prevent the coarsening of austenite grains and deterioration of the toughness of the steel; cooling to room temperature at ≥20℃ / s is to obtain a full martensitic structure and ensure the strength of the steel.

[0053] During the tempering treatment, since the tempering temperature of the steel of the component system described in the present invention exceeds 450°C and the core of the steel plate is kept warm for more than 5 minutes after reaching the furnace temperature, it is ensured that Mo, Cr and V in the alloy can react with C within the temperature range to form fine Cr7C3 and VC or (V, Mo)C precipitation phases, which brings precipitation strengthening and improves the yield strength of the steel; at the same time, the internal stress of the steel plate can be effectively removed, and the local dislocation in the martensite can be restored to improve the plasticity of the steel; when the tempering temperature exceeds 650°C, the alloy carbides will coarsen, which will deteriorate the toughness of the steel; the best match of strength and plasticity can be achieved by adjusting the tempering temperature and time.

[0054] Beneficial effects of the present invention:

[0055] In terms of composition design, the present invention adopts low Mo or even no Mo and low C and Mn contents, and utilizes high Cr and V element addition to control 1%≤3Mo+0.6Cr+5V≤1.5%, inhibits the precipitation of cementite, and controls the precipitated cementite within 50nm, and forms Cr7C3 and VC or (V,Mo)C precipitation phases, and utilizes its tempering softening resistance and precipitation strengthening effect to achieve high strength of the steel plate. Compared with conventional ultra-high strength steel that uses a higher content of Mo element to resist softening during tempering to make the yield strength greater than 1000MPa, the cost is reduced, and the adverse effect of adding higher C and Mn element contents on welding performance when no Mo element is added is avoided.

[0056] The present invention controls the temperature and time of quenching and tempering heat treatment on the basis of component design to prevent grain coarsening and control the size of cementite, Cr7C3 and VC or (V, Mo) C carbides. The size of cementite is controlled within 50 nm, and Cr7C3 and VC or (V, Mo) C precipitation phases with a size of 2 to 10 nm are formed, thereby obtaining the best precipitation strengthening effect and the best strength-plasticity matching, thereby achieving ultra-high strength at a lower alloy content. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a metallographic structure photo of the steel plate after heat treatment in Example 4 of the present invention;

[0058] Figure 2 This is a scanning electron microscopic photograph of the steel plate after heat treatment in Example 4 of the present invention. DETAILED DESCRIPTION

[0059] The present invention will be further described below in conjunction with the embodiments and drawings.

[0060] The composition of the steel example of the present invention is shown in Table 1, and the manufacturing process parameters of the steel example of the present invention are shown in Table 2.

[0061] Depend on Figure 1 As can be seen from the photo in the middle, the structure after heat treatment is tempered martensite structure. Figure 2 From the scanning electron microscopy photos, it can be seen that there are a large number of dispersed nano-scale precipitates in the organization, among which the slightly larger ones are cementite below 50nm, and the very fine ones are Cr7C3 and VC or (V,Mo)C of 2-10nm.

[0062] The mechanical properties of the embodiments were tested according to the national standard GB / T228.1-2010 "Standard for Tensile Test of Metallic Materials". The steel plates after quenching and tempering in each embodiment were subjected to transverse tensile and longitudinal impact tests. The corresponding properties of the samples in each embodiment are shown in Table 3.

[0063] From the perspective of mechanical properties, the steel of the present invention has ultra-high strength, yield strength ≥1000MPa, tensile strength ≥1050MPa, elongation above 18%, and -40°C impact energy>120J. This is due to the specific element composition of the embodiments of the present invention, especially the specific combination of Cr, Mo, and V elements, which ensures that the steel plate still maintains high strength under high-temperature tempering conditions; at the same time, the high tempering temperature ensures the high elongation and toughness of the steel plate, achieving a good combination of strength and plasticity.

[0064] The overall content of 3Mo+0.6Cr+5V in Comparative Example 1 is too low, and the strength does not meet the requirements of the present invention. The overall content of 3Mo+0.6Cr+5V in Comparative Example 2 is too low, and although the strength can meet the requirements of the present invention, the impact energy at -40°C is low, at about 70J, which cannot meet the toughness requirements.

[0065] Although the components of Comparative Example 3 meet the requirements of the present invention, the tempering temperature is too high during the heat treatment process, resulting in low strength and toughness of the steel. Although the components of Comparative Example 4 meet the requirements of the present invention, the tempering temperature is low, and although the strength can meet the requirements of the present invention, the elongation and impact energy are low, and the plasticity is poor.

[0066] The quenched and tempered steel with a yield strength of 1000 MPa described in the present invention can be used in various fields requiring the use of high-strength steel, especially in the use of steel plates for engineering machinery requiring both high strength and high toughness.

[0067]

[0068]

[0069]

Claims

1. A quenched and tempered steel plate with a yield strength of 1000 MPa, wherein the composition by weight is: C: 0.15-0.2%, Si: 0.05-0.3%, Mn: 0.80-1.60%, Cr≥0.6%, Mo≤0.2%, V≥0.06%, N≤0.005%, P≤0.030%, S≤0.005%, O≤0.004%, and the rest includes Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: 1%≤3Mo+0.6Cr+5V≤1.5%.

2. The quenched and tempered steel plate with a yield strength of 1000 MPa according to claim 1, characterized in that: The steel plate further comprises one or more of Nb≤0.050%, Ti≤0.050%, Cu≤0.5%, Ni≤0.5%, B≤0.003%, Al≤0.06%, Ca≤0.004%, and RE≤0.005%.

3. The quenched and tempered steel plate with a yield strength of 1000 MPa according to claim 1 or 2, characterized in that: The rest is Fe and other inevitable inclusions.

4. The quenched and tempered steel plate with a yield strength of 1000 MPa as claimed in claim 1, 2 or 3, characterized in that: The microstructure of the steel plate is tempered martensite + cementite within 50nm + Cr7C3 and VC or (V, Mo)C carbides with a size of 2 to 10nm.

5. The quenched and tempered steel plate with a yield strength of 1000 MPa as claimed in claim 1, 2, 3 or 4, characterized in that: The steel plate has a yield strength of ≥1000 MPa, a tensile strength of ≥1050 MPa, an elongation of >18%, and an impact energy of >120 J at -40°C.

6. The method for producing a quenched and tempered steel plate with a yield strength of 1000 MPa as claimed in any one of claims 1 to 5, characterized in that: The steps include: 1) Smelting and casting Smelting and casting into slabs according to the composition of claim 1, 2 or 3; 2) Heating The heating temperature is 1200-1250℃. When the core of the ingot reaches the temperature, the heat preservation begins. The heat preservation time is >1.5h. 3) Rolling The final rolling temperature is 820~920℃; 4) Cooling and coiling The rolled steel plate is cooled to 500-700°C at a cooling rate of ≥60°C / s, then coiled and air-cooled to room temperature; 5) Heat treatment Quenching treatment: heat the steel plate to 820-950℃, keep the temperature for 5-40min after the core of the steel plate reaches the temperature, and then quickly cool it to room temperature at a cooling rate of ≥20℃ / s; Tempering treatment, the tempering temperature is 450-650℃, the core of the steel plate is kept warm after reaching the temperature, the holding time is ≥5min, and finally air-cooled to room temperature.

7. The production method according to claim 6, characterized in that: In step 3), the final rolling reduction rate is 15-20%.

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