Performance-adjustable ultrahigh-strength steel and manufacturing method thereof

Abstract

The invention relates to performance-adjustable ultrahigh-strength steel and a manufacturing method thereof.The performance-adjustable ultrahigh-strength steel comprises, by mass, 0.3%-0.4% of C, 0.4%-0.6% of Si, 1.0%-2.6% of Mn, 0.4%-1.1% of Cr, 0.1%-0.3% of Cu, 0.05%-0.25% of Mo, 0.2%-1.0% of Al, 0.05%-0.2% of Ti and the balance Fe and inevitable impurities, and the manufacturing method of the performance-adjustable ultrahigh-strength steel comprises the steps that after induction smelting, forging and hot rolling are conducted, in the steel with the same component, the steel with the same component is subjected to hot rolling, and then the steel with the same component is subjected to hot rolling; three heat treatment processes including subcritical annealing, quenching partitioning or quenching-tempering are developed, microweaving control over ferrite, martensite, retained austenite, carbide and the like is achieved, the performance-adjustable ultra-high-strength steel with the tensile strength being 758-2016 MPa and the elongation being 7.9-21.5% is obtained, and multiple purposes of one steel can be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy steels, and in particular relates to an ultra-high strength steel with adjustable properties and a manufacturing method thereof. Background Art

[0002] With the continuous in-depth research on steel materials, ultra-high strength steel has been widely used in the automotive field, military equipment and marine platforms, including rocket engine casings, armored vehicle steel plates and large transport aircraft landing gears. There are many types of steel corresponding to different mechanical properties.

[0003] Steel car bodies have a long history of development. Automobile bodies are composed of hundreds of parts, which are made of more than 10 kinds of steel materials with different compositions, and their different properties are applied to different parts of the car. The difficulty of manufacturing multiple steels: it increases the difficulty of optimizing process parameters in multiple processes such as chemical metallurgy and physical metallurgy in steel mills; the existence of multiple steel grades makes the most classic stamping, welding and painting links in automobile manufacturing extremely complicated, especially the welding problem of dissimilar materials; the sorting, recycling and reuse of scrap steel from scrapped car bodies has become a challenge. Advantages of using one steel instead of multiple steels: steel grades with a single composition can obtain different microstructures through existing mature production processes, thereby providing better mechanical properties than existing steel grades and realizing the lightweighting of steel white bodies; the complexity of various links such as steel white body design, procurement, production and manufacturing can be reduced by simplifying steel grades. Summary of the invention

[0004] The purpose of the present invention is to provide an ultra-high strength steel with adjustable performance and a method for manufacturing the same. By regulating the microstructures of ferrite, martensite, residual austenite and carbide, an ultra-high strength steel with adjustable performance having a tensile strength of 758-2016MPa and an elongation of 7.9-21.5% is obtained. It is expected that one steel can replace multiple steels, thereby solving the problem of multiple steels in automobile manufacturing.

[0005] To achieve the above purpose, the specific technical solution adopted by the present invention is:

[0006] A performance-adjustable ultra-high strength steel, whose chemical composition comprises C 0.3-0.4, Si 0.4-0.6, Mn 1.0-2.6, Cr 0.4-1.1, Cu 0.1-0.3, Mo 0.05-0.25, Al 0.2-1.0, Ti 0.05-0.2, and the balance is Fe and unavoidable impurities.

[0007] The above-mentioned performance-adjustable ultra-high strength steel and its manufacturing method include the following steps:

[0008] (1) Preparing materials according to the target ultra-high strength steel composition and performing vacuum induction suspension melting;

[0009] (2) Forging treatment;

[0010] (3) Hot rolling water cooling;

[0011] (4) subjecting the steel obtained in step (3) to subcritical annealing;

[0012] or

[0013] The steel obtained in step (3) is subjected to quenching and partitioning, austenitizing treatment, and air cooling to room temperature after salt bathing;

[0014] or

[0015] The steel obtained in step (3) is quenched and tempered.

[0016] Furthermore, in step (2), the process parameters of the forging treatment are: pre-forging heating temperature is 1000-1250°C, and the holding time is 90-120min; hot forging temperature is 900-1150°C, and the forging ratio is 8-12.

[0017] Furthermore, in step (3), the hot rolling process parameters are: two passes are adopted, the hot rolling temperature of the first pass is 1000-1250°C, and the deformation is 50%-80%; the hot rolling temperature of the second pass is 850-1000°C, and the deformation is 40%-90%.

[0018] Furthermore, in step (4), the subcritical annealing temperature is 630-680° C. and the holding time is 30-90 min.

[0019] Furthermore, in step (4), the austenitizing treatment temperature is 820-920° C., and the holding time is 10-30 min; the salt bath temperature is 320-400° C., and the holding time is 10-30 min, and then air-cooled to room temperature.

[0020] Furthermore, in step (4), the quenching temperature is 900-980°C, and the holding time is 5-90 min; the tempering temperature is 200-300°C, and the holding time is 20-60 min.

[0021] Compared with the prior art, the present invention manufactures an ultra-high strength steel with adjustable performance. After three heat treatment processes of subcritical annealing, quenching and partitioning or quenching-tempering, by regulating the microstructures of ferrite, martensite, residual austenite and carbide, an ultra-high strength steel with adjustable performance of tensile strength of 758-2016MPa and elongation of 7.9-21.5% is obtained, thus realizing "one steel for multiple uses". BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 These are microstructure diagrams of the steel obtained by the subcritical annealing process in Example 1 at different magnifications.

[0024] Figure 2 These are microstructure diagrams of the steel obtained by the quenching and partitioning process in Example 2 at different magnifications.

[0025] Figure 3 The microstructure diagram of different areas of the steel obtained by the quenching-tempering process in Example 3.

[0026] Figure 4 These are tensile specimens with different heat treatment processes. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below according to specific embodiments. The protection scope of the present invention is not limited to the following embodiments, which are listed for illustrative purposes only and are not intended to limit the present invention in any way.

[0028] The design basis of the chemical composition of the present invention is as follows:

[0029] C is the most commonly used strengthening element. There is a direct relationship between the tensile strength of low alloy ultra-high strength steel and its carbon content. As the carbon content increases, the strength of the steel increases, but the plasticity and toughness of the steel decrease, and the processing properties such as machinability and weldability also decrease. Therefore, the C content designed is between 0.3% and 0.4%.

[0030] Si can dissolve in ferrite and austenite to improve the hardness and strength of steel. Si hinders the diffusion of C in austenite into the surrounding matrix, thus improving the thermal stability of austenite. However, too high a content of Si reduces toughness and plasticity, and seriously impairs cold working formability. Therefore, the designed Si content is between 0.4% and 0.6%.

[0031] Mn is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or weaken the hot brittleness of steel caused by sulfur, thereby improving the hot working properties of steel. The solid solution formed by manganese and iron improves the hardness and strength of ferrite and austenite in steel. The Mn content designed accordingly is between 1.0 and 2.6%.

[0032] Ti is a microalloying element. Adding a small amount of Ti can significantly change one or several properties of steel. Ti is one of the strong carbide-forming elements. It forms fine TiC particles with carbon and has the effect of refining grains. The Ti content designed accordingly is 0.05-0.2%.

[0033] Al is added to steel as a deoxidizer to form AlN precipitation phase with N element, which has the effect of inhibiting grain growth. Too much Al will reduce the hot working performance and welding performance of steel. Therefore, the Al content is designed to be between 0.2 and 1.0%.

[0034] Cu has a solid solution strengthening effect, and the degree of solid solution strengthening is similar to that of Mn. The addition of Cu can also refine the grains, improve the impact toughness and fatigue strength, and improve the weather resistance of steel. Too low Cu (x < 0.5%) will not have a precipitation strengthening effect, while too high Cu will easily cause hot brittleness, which is not conducive to welding and hot processing. The Cu content designed accordingly is between 0.1 and 0.3%.

[0035] Mo can hinder the diffusion of atoms in steel during tempering. Compared with carbon steel at the same temperature, Mo generally delays the decomposition of martensite, that is, improves the steel's resistance to tempering softening. It refines austenite grains and increases the strengthening effect of grain boundaries. When the Mo content is high, Mo2C carbides are easily formed. The Mo content designed accordingly is between 0.05 and 0.25%.

[0036] The addition of Cr is used to improve the hardenability, hardness, strength and toughness of steel and improve its corrosion resistance. The addition of Cr can inhibit the growth of carbides during heat treatment, increase the tempering stability of steel, reduce the activity of carbon, and slow down the decarburization tendency of steel. At the same time, the addition of Cr can promote the formation of bainite and martensite, and the Cr content is designed to be between 0.4 and 1.1%.

[0037] The technical solution of the present invention is further described below according to specific embodiments. The protection scope of the present invention is not limited to the following embodiments which are only for illustrative purposes and do not limit the present invention in any way.

[0038] According to the composition, induction melting, forging treatment and hot rolling and water cooling are carried out. The chemical composition (wt%) of the smelted ingot is C 0.38, Si 0.56, Mn 2.0, Cr 0.4, Cu 0.25, Mo 0.2, Al 0.3, Ti 0.1, and the balance is Fe and inevitable impurities.

[0039] Example 1

[0040] Subcritical annealing was performed at 650°C for 60 min and then air-cooled to room temperature.

[0041] Example 2

[0042] Austenitizing was performed at 820°C and kept at this temperature for 10 min, then the alloy was cooled in a salt bath to 320°C and kept at this temperature for 10 min, and then air-cooled to room temperature.

[0043] Example 3

[0044] Austenitizing was performed at 920°C and kept at this temperature for 5 min, followed by water quenching to room temperature, followed by tempering at 200°C and kept at this temperature for 20 min, and air cooling to room temperature.

[0045] Example 4

[0046] Austenitizing was performed at 950°C and kept at this temperature for 60 min, followed by water quenching to room temperature, followed by tempering at 250°C and kept at this temperature for 30 min, and air cooling to room temperature.

[0047] Example 5

[0048] Austenitizing was performed at 980°C and kept at this temperature for 90 min, followed by water quenching to room temperature, followed by tempering at 300°C and kept at this temperature for 60 min, and air cooling to room temperature.

[0049] The steel materials of Example 1, Example 2 and Example 3 were characterized by SEM, and the obtained microstructures were as follows Figure 1-3 As shown, Figure 1 The microstructure is ferrite, pearlite and carbides dispersed in the ferrite matrix. Figure 2 The microstructure is ferrite, retained austenite and lath martensite. Figure 3 The microstructure is martensite, part of retained austenite and carbide.

[0050] Example 1 is kept at 650℃ for 60min, and then air-cooled to room temperature. The hardness of the two components can be adjusted by subcritical annealing. The purpose of keeping at 650℃ for 60min is to soften the matrix and eliminate internal stress. The structure obtained by the subcritical annealing heat treatment process is ferrite, pearlite, and carbides dispersed in the ferrite matrix. Ferrite has good plasticity and toughness, but low strength.

[0051] Example 2: Austenitizing was performed at 820°C and kept at this temperature for 10 minutes, then the salt bath was cooled to 320°C and kept at this temperature for 10 minutes, and then air-cooled to room temperature. The purpose of the salt bath was to control the cooling rate, and the purpose of selecting the salt bath temperature of 320°C was that the melting point of the nitrate used in the salt bath was near this temperature. The microstructure obtained after the austenitizing and salt bath heat treatment process was ferrite, residual austenite and lath martensite.

[0052] Example 3 is kept at 920°C for 5 minutes to ensure that the steel is completely austenitized, water quenched to room temperature, tempered at 200°C and kept for 20 minutes, and air cooled to room temperature. Examples 3, 4 and 5 all use traditional quenching-tempering heat treatment processes to reduce or eliminate the internal stress generated during quenching; prevent deformation or cracking of the workpiece; reduce the brittleness of the steel; stabilize the structure to stabilize the shape and size; and obtain the desired structure and performance. The structure obtained after quenching and tempering heat treatment processes is martensite, part of residual austenite and carbide. Martensite has high strength and hardness, but poor plasticity.

[0053] After the material is deformed, not only its shape and size change, but also its internal organization and related properties change, making it in a state with higher enthalpy. This situation is unstable. The steel obtained by smelting, rolling and three different heat treatment methods of the low alloy steel of the present invention is stretched, and the mechanical property test results are shown in Table 1. After subcritical annealing, the tensile strength of the steel is 758MPa, the yield strength is 550MPa and the elongation is 21.5%, and the corresponding organization is pearlite, ferrite and carbide precipitated on the ferrite matrix, and the obtained engineering stress-strain curve is a typical engineering stress-strain curve, and the aluminum des band is observed during the stretching process. After quenching and partitioning, austenitization and salt bath treatment, the tensile strength of the steel is 926MPa, the yield strength is 565MPa and the elongation is 17.1%, and the corresponding organization is ferrite, residual austenite and lath martensite. After quenching and tempering, the steel has a tensile strength of 2016MPa, a yield strength of 1707MPa and an elongation of 7.9%. The corresponding structures are mainly martensite, part of retained austenite and carbide. The tensile strength exceeds 2000MPa, which is an ultra-high strength martensitic steel with a wide range of applications. The three specific heat treatment process parameters are shown in Table 1. The boundary conditions of ultra-high strength steel with adjustable performance. Through subcritical annealing, quenching and partitioning, quenching-tempering and other heat treatment processes, combined with ferrite, martensite, retained austenite and carbide structure control, ultra-high strength steel with adjustable performance of 758-2016MPa and elongation of 7.9-21.5% can be obtained. Figure 4 It is the tensile specimen of Example 1-3 after being broken in the room temperature tensile test.

[0054] Table 1 Mechanical properties of the examples

[0055]

[0056] After three different heat treatment processes, different mechanical properties are obtained. Different mechanical properties can be obtained by regulating the structures of ferrite, martensite, residual austenite and carbide, thus realizing "one steel for multiple uses".

Claims

1. An ultra-high strength steel, characterized in that: Its chemical composition is C 0.3~0.4, Si 0.4~0.6, Mn 1.0~2.6, Cr 0.4~1.1, Cu 0.1~0.3, Mo 0.05~0.25, Al 0.2~1.0, Ti 0.05~0.2, and the balance is Fe and inevitable impurities.

2. A method for manufacturing ultra-high strength steel according to claim 1, characterized in that: The following steps are involved: (1) Preparing materials according to the target ultra-high strength steel composition and performing vacuum induction suspension melting; (2) Forging treatment; (3) Hot rolling water cooling; (4) subjecting the steel obtained in step (3) to subcritical annealing; or The steel obtained in step (3) is subjected to quenching and partitioning, austenitization treatment, and air cooling to room temperature after salt bath; or The steel obtained in step (3) is quenched and tempered.

3. The method according to claim 2, characterized in that In step (2), the process parameters of the forging treatment are: pre-forging heating temperature is 1000-1250°C, and the holding time is 90-120min; hot forging temperature is 900-1150°C, and the forging ratio is 8-12.

4. The method according to claim 2, characterized in that In step (3), the hot rolling process parameters are: two passes are used, the hot rolling temperature of the first pass is 1000-1250°C, and the deformation is 50%-80%; the hot rolling temperature of the second pass is 850-1000°C, and the deformation is 40%-90%.

5. The method according to claim 2, characterized in that In step (4), the subcritical annealing temperature is 630-680° C. and the holding time is 30-90 min.

6. The method according to claim 2, characterized in that In step (4), the austenitizing treatment temperature is 820-920°C, and the holding time is 10-30 min; the salt bath temperature is 320-400°C, and the holding time is 10-30 min, and then air-cooled to room temperature.

7. The method according to claim 2, characterized in that In step (4), the quenching temperature is 900-980°C, and the holding time is 5-90 min; the tempering temperature is 200-300°C, and the holding time is 20-60 min.

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