Low-temperature-impact-resistant medium-carbon high-strength non-quenched and tempered steel and preparation method thereof

Abstract

The invention discloses low-temperature-impact-resistant medium-carbon high-strength non-quenched and tempered steel and a preparation method thereof, and belongs to the technical field of non-quenched and tempered steel manufacturing. The non-quenched and tempered steel comprises the following chemical components in percentage by mass: 0.30 to 0.35 percent of C, 0.45 to 0.60 percent of Si, 1.25 to 1.60 percent of Mn, 0.16 to 0.20 percent of V, 0.030 to 0.035 percent of Nb, 0.01 to 0.03 percent of Ti, 0.30 to 0.36 percent of Cr, 0.021 to 0.030 percent of N, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S and the balance of Fe and inevitable impurities. According to the high-strength and high-toughness non-quenched and tempered steel and the preparation method thereof, carbonitride precipitation is formed through microalloying elements V, Nb and Ti, recrystallization in the rolling process is inhibited to refine grains, transformation of pearlite is delayed by inhibiting formation of cementite in the cooling process, and finally the high-strength and high-toughness non-quenched and tempered steel with the structure of ferrite, bainite and self-tempered martensite is obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of non-quenched and tempered steel manufacturing, and relates to a medium-carbon high-strength non-quenched and tempered steel resistant to low-temperature impact and a preparation method thereof. Background Art

[0002] Quenched and tempered steel is mainly used in mining and other construction machinery service scenarios. In order to achieve a good combination of mechanical properties such as strength, plasticity and toughness, traditional quenched and tempered steel often uses heat treatment processes such as quenching and high-temperature tempering, which consumes a lot of energy and does not conform to the green and sustainable development concept of the steel industry. Non-quenched and tempered steel avoids quenching and tempering by adding micro-alloying elements such as V, Nb, Ti, etc., as well as controlled forging (rolling), controlled cooling and other methods, so that its comprehensive mechanical properties reach or even exceed those of quenched and tempered steel. Due to its superior environmental friendliness and excellent comprehensive mechanical properties, non-quenched and tempered steel has been widely used in important parts such as rods and shafts in the automotive industry, achieving good economic and social benefits.

[0003] At present, the carbon content of non-quenched and tempered steel is mainly concentrated in 0.3~0.5wt%. Micro-alloying elements such as V, Ti, and Nb are added to control rolling (forging) and controlled cooling to inhibit recrystallization, refine grains, and obtain dispersed precipitation of carbonitrides to play the role of precipitation strengthening and further refine grains, so as to obtain ferrite pearlite non-quenched and tempered steel, so that the comprehensive mechanical properties of non-quenched and tempered steel are close to or reach those of quenched and tempered steel. Generally speaking, the tensile strength of non-quenched and tempered steel is generally not higher than 1200MPa, the yield strength is not higher than 1000MPa, and it is difficult to ensure excellent low-temperature impact toughness. Summary of the invention

[0004] The invention provides a medium-carbon high-strength non-quenched and tempered steel resistant to low-temperature impact and a preparation method thereof. By reducing the carbon content, introducing microalloy elements such as V, Nb, Ti, Cr, and combining controlled rolling and controlled cooling processes, the low-temperature impact-resistant high-strength non-quenched and tempered steel is obtained, which has a structure of ferrite, bainite and self-tempered martensite, a tensile strength of 1300MPa, a yield strength of 1000MPa, and a U-notch impact energy of 16-20J at -20°C.

[0005] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: A medium-carbon high-strength non-quenched and tempered steel resistant to low-temperature impact, the chemical composition and mass percentage of which are: C: 0.30-0.35%, Si: 0.45-0.60%, Mn: 1.25-1.60%, V: 0.16-0.20%, Nb: 0.030-0.035%, Ti: 0.01-0.03%, Cr: 0.30-0.36%, N: 0.021-0.030%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

[0006] Furthermore, the microstructure of the non-quenched and tempered steel has, by area percentage, a ferrite content of 15-20%, a bainite content of 60-70%, and a self-tempered martensite content of 10-25%.

[0007] Furthermore, the non-quenched and tempered steel has a tensile strength of 1300-1350 MPa, a yield strength of 1000-1050 MPa, an elongation of 15-20%, a U-notch impact energy of 16-20 J at -20°C, and a hardness of 360-400 HBW.

[0008] The functions of some alloying elements in the non-quenched and tempered steel of the present invention are as follows: C: A strong interstitial solid solution strengthening element in steel, which can significantly improve the strength and hardness of steel, but at the same time damages the toughness and welding performance of steel. C can stabilize austenite, ensuring that the residual austenite content during cooling is sufficient to achieve a good ratio of each phase, and at the same time can reduce the Ms point to obtain a lower low-temperature transformation zone. In order to meet the demand for low-temperature impact toughness, the present invention designs the C addition amount of non-quenched and tempered steel to be 0.30-0.35%.

[0009] Si: has a strong solid solution strengthening effect, can effectively strengthen ferrite, inhibit the formation of carbides (especially Fe3C), retain carbon in the residual austenite, improve its stability, and obtain lower bainite and self-tempered martensite. The Si addition amount of the non-quenched and tempered steel designed by the present invention is 0.45-0.60%.

[0010] Mn: It also has a strong solid solution strengthening effect. Its solid solution strengthening effect in ferrite is second only to P and Si. It is also a low-cost element for expanding the austenite phase region and has the effect of promoting the bainite transformation of steel. When the Mn content exceeds 2.0%, an overly stable austenite will be formed, which cannot be transformed into bainite and self-tempered martensite, which is not helpful for improving strength and plasticity, and will also have an adverse effect on toughness. The Mn addition amount of the non-quenched and tempered steel designed by the present invention is 1.25-1.60%.

[0011] Nb, Ti: For medium carbon non-quenched and tempered steel, Ti forms fine TiN during the liquid crystal nucleation process, which has the effect of refining the original austenite grains. Nb has the effect of refining ferrite grains and improving the morphology of pearlite lamellae by forming Nb(C,N); it can also promote the transformation of pearlite lamellae from neat parallel arrangement to spheroidization and disordered morphology, and significantly improve plastic toughness. The invention designs the addition amount of Nb and Ti to be Nb: 0.030-0.035%, Ti: 0.01-0.03%.

[0012] Cr: It is a medium-strong carbide-forming element, which inhibits the formation of high-temperature ferrite, dissolves in ferrite to play a solid solution strengthening role, can improve the strength and hardness of steel, can hinder the diffusion of C in austenite, improve the stability of austenite, shift the C curve to the right, and improve hardenability. The Cr addition amount of the non-quenched and tempered steel designed by the present invention is 0.30-0.36%.

[0013] On the other hand, the present invention also provides a method for preparing the above-mentioned low-temperature impact-resistant medium-carbon high-strength non-quenched and tempered steel, which comprises a smelting process, a continuous casting process, a heating process, a controlled rolling process and a cooling process; The controlled rolling and cooling process comprises the steps of rough rolling, intermediate rolling, finishing rolling, laminar cooling, cooling bed cooling, secondary water cooling and pile cooling for the ingot in sequence; The inlet temperature of laminar cooling is 850~890℃, and the outlet temperature is 700~750℃; the upper cooling bed return temperature of cooling bed is 680~720℃, and the lower cooling bed temperature is 600~650℃.

[0014] Furthermore, the smelting process is to carry out electric furnace smelting according to the formula, with the weight proportion of molten iron entering the furnace being ≥60%; the tapping temperature is 1560-1620°C, and the C in the molten steel at the end of the electric furnace is ≥0.10wt%, and P is ≤0.010wt%; the LF ladle refining process is ≥50min; the vacuum degree in the VD vacuum refining process is less than 67Pa and maintained for 8-10min, V, Nb, and Ti elements are added, and the air is broken after 15-20min, and the argon blowing time is 10-15min.

[0015] Furthermore, the heating process includes preheating, heating and soaking stages; the temperature of the preheating stage does not exceed 950°C, and the time is 50 to 60 minutes; the primary heating temperature is 1060 to 1150°C, and the secondary heating temperature is 1150 to 1220°C; the soaking stage temperature is 1220 to 1250°C, and the time is 20 to 30 minutes.

[0016] Furthermore, the controlled rolling and cooling process has a starting rolling temperature of 1050-1130°C, a final rolling temperature of 890-920°C, a deformation amount after rolling of 85-90%, a secondary water cooling inlet temperature of 600-640°C, and an outlet temperature of 150-200°C; a stack cooling and slow cooling cooling rate of 0.1-0.3°C / min, and after stack cooling to 70-80°C, air cooling to room temperature.

[0017] Explanation of the process mechanism of the present invention: The present invention selects appropriate laminar cooling outlet temperature, upper cooling bed return temperature and lower cooling bed temperature, so that the cooling curve passes through the ferrite and lower bainite transformation area, so that the medium carbon non-quenched and tempered steel has polygonal ferrite, lower bainite and self-tempered martensite structure formed after subsequent water cooling. Studies have shown that the large hardness difference between ferrite and martensite helps to enhance elongation. Plastic constraint (soft thin layer can be constrained by the surrounding hard matrix) can improve strength during deformation. In the present invention, the lower bainite acts as a soft thin layer and is strongly plastically constrained by the surrounding martensite during deformation, thereby improving the strength of the entire lower bainite-martensite structure. Benefiting from the presence of a large amount of lower bainite and self-tempered martensite, they induce additional uneven deformation between polygonal ferrite and lower bainite-self-tempered martensite, thereby enhancing the strain hardening ability of the non-quenched and tempered steel structure as a whole, thereby obtaining greater tensile strength. In addition, the first nucleation and growth of lower bainite divides the previous austenite grains, thereby reducing the size of the martensite package formed subsequently, obtaining a finer hard phase structure, and playing a role in grain refinement and toughening. During the low-temperature impact process, the ductile ferrite near the hard phase is considered to have a stress relief effect, which increases the impact toughness. When cracks pass through large-angle grain boundaries (such as martensite package boundaries and ferrite-martensite boundaries), their propagation direction will be deflected. The lower bainite will not only form large-angle grain boundaries with the surrounding martensite, but also induce more martensite package boundaries through the division of the previous austenite, thereby increasing the difficulty of crack propagation and improving impact toughness.

[0018] The beneficial effect of adopting the above technical solution is that: the present invention, by adding V, Nb, and Ti microalloying elements, gives full play to the strengthening effect of Nb and Ti on carbonitrides and nitrides and the effect of refining grains, and Cr stabilizes austenite, improves hardenability, obtains relatively fine ferrite and bainite main structures with good mechanical properties, and an appropriate amount of self-tempered martensite structure that plays an incompatible deformation strain strengthening role, which can also increase high-angle grain boundaries to prevent crack propagation and thus improve toughness.

[0019] The present invention obtains non-quenched and tempered steel with high strength and good low-temperature impact toughness by controlling the non-quenched and tempered steel smelting process, rolling process and cooling process, the tensile strength reaches 1300-1350MPa, the yield strength reaches 1000-1050MPa, the elongation reaches 15-20%, the U-notch impact energy at -20°C reaches 16-20J, and the hardness reaches 360-400HBW. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 3 is a microstructure diagram of the non-quenched and tempered steel obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with specific embodiments.

[0022] Example 1

[0023] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 69%; the tapping temperature is 1560°C, and the C=0.11wt% and P=0.008wt% in the molten steel at the end of the electric furnace; the LF ladle refining process is 52min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 8min, V, Nb, and Ti elements are added, and the air is broken after 20min, and the argon blowing time is 10min. After component detection, the mass percentage of each element in the non-quenched and tempered steel in this embodiment is: C: 0.33%, Si: 0.45%, Mn: 1.40%, V: 0.17%, Nb: 0.030%, Ti: 0.03%, N: 0.030%, Cr: 0.35%, P: 0.005%, S: 0.015%, and the balance is Fe and unavoidable impurities.

[0024] The temperature of the preheating section of the casting billet is 945℃, and the time is 60min; the primary heating temperature is 1070℃, and the secondary heating temperature is 1200℃; the temperature of the soaking section of the casting billet is 1220℃, and the time is 25min.

[0025] The starting rolling temperature is 1050℃, the final rolling temperature is 895℃; the deformation after rolling is 90%; the laminar cooling inlet temperature is 890℃, and the outlet temperature is 750℃; the upper cooling bed return temperature of the cooling bed is 700℃, and the lower cooling bed temperature is 645℃; the secondary water cooling inlet temperature is 630℃, and the outlet temperature is 150℃; the stack cooling slow cooling rate is 0.3℃ / min, and after stack cooling to 75℃, it is air cooled to room temperature.

[0026] The non-quenched and tempered steel sample of this embodiment was cut into a small test piece of 5 mm × 5 mm × 5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope. The non-quenched and tempered steel had a ferrite content of 18%, a bainite content of 67%, a self-tempered martensite content of 14%, and a small amount of retained austenite. Figure 1 shown.

[0027] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0028] Example 2

[0029] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 65%; the tapping temperature is 1580°C, and C=0.10wt% and P=0.008wt% in the molten steel at the end of the electric furnace; the LF ladle refining process is 51min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 10min, V, Nb, and Ti elements are added, and the air is broken after 18min, and the argon blowing time is 13min. After component detection, the mass percentage of each element of the non-quenched and tempered steel in this embodiment is: C: 0.30%, Si: 0.50%, Mn: 1.25%, V: 0.16%, Nb: 0.035%, Ti: 0.01%, N: 0.025%, Cr: 0.36%, P: 0.025%, S: 0.005%, and the balance is Fe and unavoidable impurities.

[0030] The temperature of the preheating section of the casting billet is 940℃, and the time is 50min; the primary heating temperature is 1060℃, and the secondary heating temperature is 1150℃; the temperature of the soaking section of the casting billet is 1250℃, and the time is 20min.

[0031] The starting rolling temperature is 1120℃, the final rolling temperature is 890℃; the deformation after rolling is 85%; the laminar cooling inlet temperature is 870℃, and the outlet temperature is 730℃; the upper cooling bed temperature is 720℃, and the lower cooling bed temperature is 630℃; the secondary water cooling inlet temperature is 620℃, and the outlet temperature is 160℃; the stack cooling slow cooling rate is 0.1℃ / min, and after stack cooling to 70℃, it is air cooled to room temperature.

[0032] The non-quenched and tempered steel sample of this embodiment was cut into small test blocks of 5 mm×5 mm×5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope, and the non-quenched and tempered steel had a ferrite content of 15%, a bainite content of 60%, and a self-tempered martensite content of 25%.

[0033] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0034] Example 3

[0035] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 65%; the tapping temperature is 1620°C, and the C=0.11wt% and P=0.007wt% in the molten steel at the end of the electric furnace; the LF ladle refining process is 50min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 9min, V, Nb, and Ti elements are added, and the air is broken after 15min, and the argon blowing time is 14min. After component detection, the mass percentage of each element of the non-quenched and tempered steel in this embodiment is: C: 0.35%, Si: 0.56%, Mn: 1.60%, V: 0.20%, Nb: 0.033%, Ti: 0.02%, N: 0.026%, Cr: 0.30%, P: 0.015%, S: 0.005%, and the balance is Fe and unavoidable impurities.

[0036] The temperature of the preheating section of the casting billet is 949℃, and the time is 57min; the primary heating temperature is 1150℃, and the secondary heating temperature is 1220℃; the temperature of the soaking section of the casting billet is 1240℃, and the time is 30min.

[0037] The starting rolling temperature is 1130℃, the final rolling temperature is 920℃; the deformation after rolling is 86%; the laminar cooling inlet temperature is 885℃, and the outlet temperature is 700℃; the upper cooling bed temperature is 680℃, and the lower cooling bed temperature is 600℃; the secondary water cooling inlet temperature is 600℃, and the outlet temperature is 200℃; the stack cooling slow cooling rate is 0.2℃ / min, and after stack cooling to 80℃, it is air cooled to room temperature.

[0038] The non-quenched and tempered steel sample of this embodiment was cut into small test blocks of 5 mm×5 mm×5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope, and the non-quenched and tempered steel had a ferrite content of 20%, a bainite content of 70%, and a self-tempered martensite content of 10%.

[0039] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0040] Example 4

[0041] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 62%; the tapping temperature is 1598°C, and the C=0.11wt% and P=0.003wt% in the molten steel at the end of the electric furnace; the LF ladle refining process is 60min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 8min, V, Nb, and Ti elements are added, and the air is broken after 16min, and the argon blowing time is 15min. After component detection, the mass percentage of each element in the non-quenched and tempered steel in this embodiment is: C: 0.32%, Si: 0.60%, Mn: 1.50%, V: 0.19%, Nb: 0.031%, Ti: 0.028%, N: 0.021%, Cr: 0.32%, P: 0.008%, S: 0.009%, and the balance is Fe and unavoidable impurities.

[0042] The temperature of the preheating section of the casting billet is 947℃, and the time is 59 minutes; the primary heating temperature is 1130℃, and the secondary heating temperature is 1205℃; the temperature of the equalizing section of the casting billet is 1225℃, and the time is 25 minutes.

[0043] The starting rolling temperature is 1110℃, the final rolling temperature is 904℃; the deformation after rolling is 87%; the laminar cooling inlet temperature is 850℃, and the outlet temperature is 710℃; the upper cooling bed temperature is 690℃, and the lower cooling bed temperature is 650℃; the secondary water cooling inlet temperature is 640℃, and the outlet temperature is 185℃; the stack cooling slow cooling rate is 0.3℃ / min, and after stack cooling to 73℃, air cooling to room temperature.

[0044] The non-quenched and tempered steel sample of this embodiment was cut into small test blocks of 5 mm×5 mm×5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope, and the non-quenched and tempered steel had a ferrite content of 18%, a bainite content of 68%, and a self-tempered martensite content of 14%.

[0045] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0046] Example 5

[0047] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 66%; the tapping temperature is 1615°C, and the C=0.12wt% and P=0.008wt% in the molten steel at the end of the electric furnace are 55min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 9min, V, Nb, and Ti elements are added, and the air is broken after 17min, and the argon blowing time is 11min. After component detection, the mass percentage of each element in the non-quenched and tempered steel in this embodiment is: C: 0.34%, Si: 0.49%, Mn: 1.45%, V: 0.18%, Nb: 0.031%, Ti: 0.015%, N: 0.022%, Cr: 0.35%, P: 0.008%, S: 0.009%, and the balance is Fe and unavoidable impurities.

[0048] The temperature of the preheating section of the casting billet is 950℃, and the time is 52min; the primary heating temperature is 1110℃, and the secondary heating temperature is 1200℃; the temperature of the equalizing section of the casting billet is 1230℃, and the time is 26min.

[0049] The starting rolling temperature is 1115℃, the final rolling temperature is 895℃; the deformation after rolling is 88%; the laminar cooling inlet temperature is 865℃, and the outlet temperature is 740℃; the upper cooling bed temperature is 695℃, and the lower cooling bed temperature is 640℃; the secondary water cooling inlet temperature is 625℃, and the outlet temperature is 165℃; the stack cooling slow cooling rate is 0.3℃ / min, and after stack cooling to 78℃, it is air cooled to room temperature.

[0050] The non-quenched and tempered steel sample of this embodiment was cut into small test blocks of 5 mm×5 mm×5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope, and the non-quenched and tempered steel had a ferrite content of 16%, a bainite content of 62%, and a self-tempered martensite content of 22%.

[0051] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0052] Example 6

[0053] The preparation method of the medium carbon high strength non-quenched and tempered steel resistant to low temperature impact in this embodiment is as follows: electric furnace smelting is carried out according to the formula, and the weight ratio of the molten iron in the furnace is 62%; the tapping temperature is 1600°C, and the C=0.11wt% and P=0.004wt% in the molten steel at the end of the electric furnace; the LF ladle refining process is 50min; the vacuum degree of the VD vacuum refining process is less than 67Pa and maintained for 9min, V, Nb, and Ti elements are added, and the air is broken after 19min, and the argon blowing time is 11min. After component detection, the mass percentage of each element in the non-quenched and tempered steel in this embodiment is: C: 0.31%, Si: 0.57%, Mn: 1.35%, V: 0.19%, Nb: 0.034%, Ti: 0.016%, N: 0.03%, Cr: 0.35%, P: 0.01%, S: 0.010%, and the balance is Fe and unavoidable impurities.

[0054] The temperature of the preheating section of the casting billet is 950℃, and the time is 58min; the primary heating temperature is 1090℃, and the secondary heating temperature is 1190℃; the temperature of the soaking section of the casting billet is 1230℃, and the time is 23min.

[0055] The starting rolling temperature is 1125℃, the final rolling temperature is 900℃; the deformation after rolling is 89%; the laminar cooling inlet temperature is 860℃, and the outlet temperature is 720℃; the upper cooling bed return temperature of the cooling bed is 689℃, and the lower cooling bed temperature is 650℃; the secondary water cooling inlet temperature is 630℃, and the outlet temperature is 195℃; the stack cooling slow cooling rate is 0.2℃ / min, and after stack cooling to 77℃, it is air cooled to room temperature.

[0056] The non-quenched and tempered steel sample of this embodiment was cut into small test blocks of 5 mm×5 mm×5 mm, and after grinding, polishing and etching, its microstructure was observed using an optical microscope, and the non-quenched and tempered steel had a ferrite content of 18%, a bainite content of 68%, and a self-tempered martensite content of 14%.

[0057] The mechanical properties of the non-quenched and tempered steel in this example are shown in Table 1.

[0058] The performance test of the product prepared in the embodiment of the present invention is carried out according to the following test standards: the tensile strength and yield strength are tested according to GB / T 228.1 "Metallic Material Tensile Test Part 1: Room Temperature Test Method" standard, and are carried out on a CMT 4102 universal electronic tensile testing machine; the hardness is tested according to GB / T 231.1 "Metallic Material Vickers Hardness Test Part 1: Experimental Method" standard, and is carried out on a HV-10002 microhardness tester; the impact energy is tested according to GB / T 229 "Metallic Material Charpy Pendulum Impact Test Method" standard, and the Charpy impact test at -20°C is carried out on a JB-30B impact testing machine; the test results are shown in Table 1.

[0059] Table 1 Test results of non-quenched and tempered steel properties in various examples

[0060] In summary, the present invention forms (V, Ti, Nb) (C, N) carbonitrides through V, Nb and Ti elements, thereby forming grain refinement and precipitation strengthening effects, delays ferrite transformation through an appropriate amount of Cr element, stabilizes austenite, improves hardenability, obtains ferrite, bainite and self-tempered martensite structures, and regulates the cooling method and cooling rate after rolling, thereby effectively achieving the purpose of controlling the microstructure of non-quenched and tempered steel, so that the tensile strength of the final ferrite martensite non-quenched and tempered steel is not less than 1300MPa, the yield strength is not less than 1000MPa, the impact energy at -20°C reaches more than 16J, and the cost is effectively controlled.

Claims

1. A medium carbon high strength non-quenched and tempered steel resistant to low temperature impact, characterized in that: Its chemical composition and mass percentage are: C: 0.30-0.35%, Si: 0.45-0.60%, Mn: 1.25-1.60%, V: 0.16-0.20%, Nb: 0.030-0.035%, Ti: 0.01-0.03%, Cr: 0.30-0.36%, N: 0.021-0.030%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

2. The low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 1, characterized in that: The microstructure of the non-quenched and tempered steel has, by area percentage, a ferrite content of 15-20%, a bainite content of 60-70%, and a self-tempered martensite content of 10-25%.

3. The low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 1 or 2, characterized in that: The non-quenched and tempered steel has a tensile strength of 1300-1350 MPa, a yield strength of 1000-1050 MPa, an elongation of 15-20%, a U-notch impact energy of 16-20 J at -20°C, and a hardness of 360-400 HBW.

4. A method for preparing a low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to any one of claims 1 to 3, characterized in that: It includes smelting process, continuous casting process, heating process, controlled rolling and cooling process; The controlled rolling and cooling process comprises the steps of rough rolling, intermediate rolling, finishing rolling, laminar cooling, cooling bed cooling, secondary water cooling and pile cooling for the ingot in sequence; The inlet temperature of laminar cooling is 850~890℃, and the outlet temperature is 700~750℃; the upper cooling bed return temperature of cooling bed is 680~720℃, and the lower cooling bed temperature is 600~650℃.

5. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 4, characterized in that: The smelting process is to carry out electric furnace smelting according to the formula, the weight proportion of molten iron entering the furnace is ≥60%; the steel tapping temperature is 1560-1620°C, and C≥0.10wt% and P≤0.010wt% in the molten steel at the end of the electric furnace.

6. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 5, characterized in that: In the smelting process, the LF ladle refining process is ≥50 minutes; in the VD vacuum refining process, the vacuum degree is less than 67Pa and maintained for 8 to 10 minutes, V, Nb and Ti elements are added, the air is broken after 15 to 20 minutes, and the argon blowing time is 10 to 15 minutes.

7. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 6, characterized in that: The heating process includes preheating, heating and soaking stages; the temperature of the preheating stage does not exceed 950°C and the time is 50 to 60 minutes; the primary heating temperature is 1060 to 1150°C and the secondary heating temperature is 1150 to 1220°C; the soaking stage temperature is 1220 to 1250°C and the time is 20 to 30 minutes.

8. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 7, characterized in that: The controlled rolling and cooling process has a starting rolling temperature of 1050-1130° C., a final rolling temperature of 890-920° C., and a deformation amount after rolling of 85-90%.

9. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to any one of claims 4 to 8, characterized in that: In the controlled rolling and cooling process, the secondary water cooling inlet temperature is 600-640°C and the outlet temperature is 150-200°C.

10. The method for preparing low temperature impact resistant medium carbon high strength non-quenched and tempered steel according to claim 9, characterized in that: The controlled rolling and cooling process comprises a slow cooling process with a cooling rate of 0.1-0.3°C / min, wherein the temperature is cooled to 70-80°C and then air-cooled to room temperature.

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