Ultrafine-structure bainite steel and preparation method thereof

Abstract

The invention relates to superfine structure bainite steel and a preparation method thereof, and relates to the technical field of bainite steel, and the superfine structure bainite steel comprises the following chemical components in percentage by weight: 0.72 to 0.98 percent of C, 1.45 to 1.60 percent of Si, 1.85 to 2.00 percent of Mn, less than or equal to 0.005 percent of P, less than or equal to 0.005 percent of S, 0.95 to 1.34 percent of Cr, 1.30 to 1.60 percent of Ni, 0.20 to 0.30 percent of Mo and the balance of Fe and inevitable impurities. The preparation method mainly comprises the following steps: (1) alloy smelting; (2) homogenization treatment and cogging; (3) high-temperature rolling; (4) austenitizing and intense temperature deformation treatment; (5) isothermal bainite transformation heat treatment; and (6) partition treatment. Compared with the prior art, the yield strength of the alloy steel is not lower than 1000 MPa, the tensile strength of the alloy steel is not lower than 1900 MPa, and the uniform ductility of the alloy steel is not lower than 10.0%. The adopted alloy components are low in cost, and meanwhile, the adopted preparation process is simple and can be used for industrial preparation of high-performance steel.

Description

Technical Field The invention relates to the technical field of metal materials and processing, and in particular to an ultrafine structure bainite steel and a preparation method thereof. Technical Background

[0001] In 2003, Caballero, Bhadeshia and others used high-carbon and high-silicon alloy steel to keep warm at low temperature for a long time, and obtained nano-bainite steel with both ultra-high strength and good toughness. Its ultimate tensile strength exceeds 2.2GPa, and it has a certain elongation (5-30%). The research shows that this nano-bainite steel is mainly composed of ultra-fine nano-scale bainite ferrite and two kinds of residual austenite with different morphologies. One is the film-like residual austenite between bainite ferrite. Due to its high carbon characteristics, it is not easy to undergo phase transformation during the strain process, which can effectively improve the plasticity of the material; the other is the blocky residual austenite distributed between the bainite bundles. During the strain process, deformation-induced martensitic phase transformation will occur, which is harmful to the plasticity of the material.

[0002] Refining the grain size of the structure and preparing fine-grained and ultra-fine-grained materials is an effective way to improve the strength of the material while maintaining a certain plasticity, thereby overcoming the "inversion" problem of the material's strength-plasticity, that is, improving the strength of the material often requires sacrificing a certain plasticity. Fine-grained and ultra-fine-grained structures can be prepared in steel through various deformation processes, achieving excellent mechanical properties. Among various methods, large deformation warm rolling is the most suitable grain refinement method for actual industrial production. Through large deformation warm rolling, ultra-fine-grained structures with high dislocation density can be obtained, thereby achieving high strength and toughness of the material.

[0003] Since austenite is below the Ac1 temperature during large deformation warm rolling, it is a metastable state, and it is necessary to achieve the relative stability of austenite through reasonable alloy composition regulation. Cr, Ni, and Mn are typical austenite stabilizing elements. By adding a certain amount of Cr, Ni, and Mn, the material can be processed without phase change, effectively expanding the processing range of warm rolling, thereby achieving uniform stability of the structure and performance of the material after processing. By adding strong carbide-forming elements such as Cr and Mo, combined with the deformation heat treatment process, fine and dispersed nano-scale precipitation can be introduced into the material, thereby significantly improving the mechanical properties of the material.

[0004] "A method for preparing ultra-high strength medium carbon nano-bainite steel" (CN112981277A) adopts the method of room temperature rolling of the obtained bainite steel to cause deformation-induced phase transformation of part of the blocky retained austenite, thereby reducing the volume fraction of the residual austenite with poor stability, avoiding the residual austenite in the material in the early stage of strain from being transformed into hard and brittle martensite due to the deformation-induced phase transformation effect, and improving the mechanical properties of the material. However, this method involves room temperature rolling, and since medium and high carbon steels are hard and brittle, it is not suitable for this preparation method.

[0005] "A method for preparing niobium-containing nano-bainite steel with good thermal stability" (CN112981215A) not only accelerates the bainite phase transformation by adding a small amount of niobium, but also significantly improves the mechanical stability of nano-bainite in the high temperature zone. However, this method uses niobium, which is expensive in industrial production, and increases production costs. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing ultrafine microstructure bainitic steel in order to overcome the defects existing in the above method. The present invention adopts high-carbon low-alloy steel, avoids the use of expensive metal elements such as Co and Nb in industrial production, has low production cost, and controllable process path. Nano-bainitic steel containing nano-precipitates is obtained through an adapted multi-pass warm rolling-partitioning process. The volume fraction of bainitic ferrite is greater than 60%, the thickness of acicular bainite is ≤200nm, the equivalent size of ultrafine residual austenite is ≤1.0μm, and nano-scale precipitates with an equivalent size of ≤100nm are distributed in the structure, achieving higher strength and excellent uniform elongation, and the yield strength is not less than 1000MPa, the tensile strength is not less than 1900MPa, and the uniform elongation is not less than 10.0%.

[0007] To achieve the above object, the technical solution of the present invention is:

[0008] An ultrafine structure bainite steel comprises the following elements in percentage by mass: C: 0.72-0.98%, Si: 1.40-1.67%, Mn: 1.75-2.10%, P≤0.005%, S≤0.005%, Cr: 0.90-1.34%, Ni: 1.30-1.60%, Mo: 0.20-0.30%, and the rest are Fe and unavoidable impurities.

[0009] Furthermore, an ultrafine structure bainitic steel comprises the following elements in percentage by mass: C: 0.78-0.80%, Si: 1.49-1.57%, Mn: 1.89-1.90%, P≤0.005%, S≤0.005%, Cr: 0.97-1.06%, Ni: 1.35-1.37%, Mo: 0.24-0.30%, and the rest are Fe and unavoidable impurities, wherein, in order to control production costs, the total percentage by mass of the alloy after optimization of the present invention is not higher than 7%, in order to introduce nano-precipitation as a strengthening phase, a certain amount of Cr and Mo are added to the optimized alloy of the present invention as strong carbide forming elements, and the percentage by mass of Cr+Mo is not less than 1.3%, in order to expand the processing area of ​​strong warm deformation and ensure that the bainite phase transformation time is not too long, the optimized alloy composition of the present invention controls the percentage by mass of austenite stabilizing elements Mn+Cr+Ni to be not less than 4.0% and not more than 4.4%.

[0010] The present invention provides a method for preparing ultrafine microstructure bainite steel, comprising the following steps: alloy smelting, homogenization treatment and blanking, high temperature rolling, austenitization and strong warm deformation treatment, low temperature long time isothermal quenching, and partitioning treatment. The specific steps are as follows:

[0011] Step (1) alloy smelting: preparing the ingredients according to the above ingredients, uniformly stirring and smelting in a vacuum induction furnace, and casting into an ingot;

[0012] Step (2) homogenization and blanking: the ingot obtained in step (1) is homogenized at 1150-1300°C for a holding time of not less than 4.0 hours, and then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of not less than 1200°C, and is drawn multiple times to finally obtain a slab.

[0013] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1000-1200°C in a vacuum or argon-protected hot furnace, kept warm for 2.0-5.0 hours, and then hot rolled. The final rolling temperature is not less than 900°C, the cumulative deformation is greater than 50%, and air-cooled to room temperature after rolling to finally obtain a plate with a thickness of 20-30 mm.

[0014] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 950-1150°C for 30-60 minutes, air-cooled to 600-700°C, and subjected to 3-4 passes of large deformation rolling, with the cumulative deformation amount being more than 70%.

[0015] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 200-350°C and kept at a constant temperature for 8.0-10.0 hours, and then air-cooled to room temperature.

[0016] Step (6) distribution treatment: the plate obtained in step (5) is subjected to carbon distribution treatment at 200-350°C for a distribution time of 4.0-16.0 hours, and then air-cooled to room temperature.

[0017] Furthermore, the alloy smelting in step (1) adopts a smelting method which is a combination of vacuum induction and electroslag remelting.

[0018] Furthermore, the rolling method for the intense warm deformation treatment in step (4) is 2 consecutive synchronous rolling passes plus 2 consecutive asynchronous rolling passes.

[0019] Furthermore, the intense warm deformation treatment in step (4) is performed by alternating synchronous rolling and asynchronous rolling, with the deformation amount of each pass being not less than 15% and the cumulative deformation amount being not less than 80%.

[0020] Furthermore, the cooling method used for quenching in the medium-temperature bainite transformation heat treatment in step (5) is ammonium nitrate salt quenching, and the cooling rate is ≥100°C / s.

[0021] Furthermore, the holding temperature selected for the isothermal bainite transformation heat treatment in step (5) is 200-250°C.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The material prepared by the present invention has excellent comprehensive mechanical properties, with a yield strength exceeding 1000MPa, an ultimate tensile strength exceeding 1900MPa, and a uniform elongation of 10.0%. The material can replace some high-cost high-alloy steels and be widely used in wear-resistant materials and load-bearing structural parts.

[0024] 2. The processing and preparation process adopted in the present invention is relatively simple. Compared with special processing techniques such as low-temperature dynamic deformation, equal-diameter angular extrusion, asynchronous rolling, etc. used in other large deformation technologies, the medium-temperature rolling method adopted in the present invention is easier to implement in industrial production.

[0025] 3. The ultrafine structure bainite steel described in the present invention has a relatively stable austenite in the warm deformation temperature range, which is convenient for strong warm deformation processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The process route diagram adopted by the present invention;

[0027] Figure 2 The microstructure of the ultrafine structure bainite steel of Example 6;

[0028] Figure 3 This is the engineering stress-strain curve of the ultrafine structure bainitic steel of Example 6. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0030] Example 1

[0031] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.73%, Si: 1.50%, Mn: 1.85%, Cr: 0.97%, Ni: 1.40%, Mo: 0.26%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0032] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0033] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 1150°C for 4 hours, and then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1250°C and drawn multiple times to obtain a slab.

[0034] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1000°C in a vacuum or argon-protected hot furnace, kept warm for 2 hours and then hot rolled, with a final rolling temperature of 920°C and a cumulative deformation of 52%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 29 mm.

[0035] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 950°C for 30 minutes, air-cooled to 600°C, and then subjected to two synchronous rolling passes and one asynchronous rolling pass, with a cumulative deformation of 72%, to obtain a rolled plate with a final thickness of 8 mm.

[0036] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 340°C for constant temperature insulation for 8 hours, and then air-cooled to room temperature.

[0037] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 320° C. for 4 hours, and then air-cooled to room temperature.

[0038] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite. The yield strength of the plate is 1021 MPa, the tensile strength is 1916 MPa, and the uniform elongation is 16.6%.

[0040] Example 2

[0041] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.78%, Si: 1.43%, Mn: 1.82%, Cr: 1.00%, Ni: 1.45%, Mo: 0.25%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0042] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0043] Step (2) homogenization and blanking: the ingot obtained in step (1) is homogenized at 1170°C for 5 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1230°C, and drawn multiple times to obtain a slab.

[0044] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1050°C in a vacuum or argon-protected hot furnace, kept warm for 2 hours and then hot rolled, with a final rolling temperature of 930°C and a cumulative deformation of 55%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 27 mm.

[0045] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 970°C for 40 minutes, air-cooled to 600°C, and then subjected to two synchronous rolling passes and one asynchronous rolling pass, with a cumulative deformation of 77%, to obtain a rolled plate with a final thickness of 6.2 mm.

[0046] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 320°C for constant temperature insulation for 8 hours, and then air-cooled to room temperature.

[0047] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 330°C for 4 hours and then air-cooled to room temperature.

[0048] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite, and the yield strength of the plate is 1034 MPa, the tensile strength is 1986 MPa, and the uniform elongation is 16.0%.

[0049] Example 3

[0050] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.81%, Si: 1.52%, Mn: 1.87%, Cr: 1.07%, Ni: 1.45%, Mo: 0.27%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0051] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0052] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 12000°C for 5 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1200°C and drawn multiple times to obtain a slab.

[0053] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1050°C in a vacuum or argon-protected hot furnace, kept warm for 4 hours and then hot rolled, with a final rolling temperature of 920°C and a cumulative deformation of 58%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 25 mm.

[0054] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1000°C for 60 minutes, air-cooled to 630°C, and then subjected to two synchronous rolling passes and one asynchronous rolling pass, with a cumulative deformation of 73%, to obtain a rolled plate with a final thickness of 6.8 mm.

[0055] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 300°C for constant temperature insulation for 8 hours, and then air-cooled to room temperature.

[0056] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 300°C for 7 hours, and then air-cooled to room temperature.

[0057] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite. The yield strength of the plate is 1057 MPa, the tensile strength is 1976 MPa, and the uniform elongation is 15.8%.

[0058] Example 4

[0059] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.79%, Si: 1.52%, Mn: 1.85%, Cr: 1.03%, Ni: 1.36%, Mo: 0.28%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0060] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0061] Step (2) homogenization and blanking: the ingot obtained in step (1) is homogenized at 1270°C for 8 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1240°C and drawn multiple times to obtain a slab.

[0062] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1100°C in a vacuum or argon-protected hot furnace, kept warm for 4 hours and then hot rolled, with a final rolling temperature of 970°C and a cumulative deformation of 60%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 24 mm.

[0063] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1130°C for 60 minutes, air-cooled to 600°C, and then subjected to 2 passes of synchronous rolling and 2 passes of asynchronous rolling, with a cumulative deformation of 90%, to obtain a rolled plate with a final thickness of 2.4 mm.

[0064] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 220°C for constant temperature insulation for 8 hours, and then air-cooled to room temperature.

[0065] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 330° C. for 16 hours, and then air-cooled to room temperature.

[0066] The microstructure of the prepared material is mainly composed of bainitic ferrite and residual austenite, and the yield strength of the plate is 1300 MPa, the tensile strength is 2200 MPa, and the uniform elongation is 10%.

[0067] Example 5

[0068] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.93%, Si: 1.57%, Mn: 2.03%, Cr: 1.32%, Ni: 1.54%, Mo: 0.30%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0069] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0070] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 1300°C for 8 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1300°C and drawn multiple times to obtain a slab.

[0071] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1200°C in a vacuum or argon-protected hot furnace, kept warm for 5 hours and then hot rolled, with a final rolling temperature of 1000°C and a cumulative deformation of 60%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 24 mm.

[0072] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1150°C for 60 minutes, air-cooled to 700°C, and then subjected to two synchronous rolling passes and one asynchronous rolling pass, with a cumulative deformation of 76%, to obtain a rolled plate with a final thickness of 5.8 mm.

[0073] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 350°C for constant temperature insulation for 10 hours, and then air-cooled to room temperature.

[0074] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 350°C for 16 hours and then air-cooled to room temperature.

[0075] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite, and the yield strength of the plate is 1140 MPa, the tensile strength is 2010 MPa, and the uniform elongation is 11.2%.

[0076] Example 6

[0077] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.80%, Si: 1.51%, Mn: 1.90%, Cr: 1.02%, Ni: 1.36%, Mo: 0.26%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0078] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0079] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 1200°C for 10 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1170°C and drawn multiple times to obtain a slab.

[0080] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1150°C in a vacuum or argon-protected hot furnace, kept hot-rolled for 4 hours, with a final rolling temperature of 1000°C and a cumulative deformation of 56%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 26 mm.

[0081] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1130°C for 60 minutes, air-cooled to 600°C, and then subjected to 2 passes of synchronous rolling and 2 passes of asynchronous rolling, with a cumulative deformation of 84%, to obtain a rolled plate with a final thickness of 4.2 mm.

[0082] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 200°C and kept at a constant temperature for 10 hours, and then air-cooled to room temperature.

[0083] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 220° C. for 16 hours, and then air-cooled to room temperature.

[0084] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite, and the yield strength of the plate is 1300 MPa, the tensile strength is 1984 MPa, and the uniform elongation is 14.9%.

[0085] Example 7

[0086] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.78%, Si: 1.49%, Mn: 1.83%, Cr: 0.94%, Ni: 1.32%, Mo: 0.21%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0087] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0088] Step (2) homogenization and blanking: the ingot obtained in step (1) is homogenized at 1170°C for 6 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1210°C and drawn multiple times to obtain a slab.

[0089] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1040°C in a vacuum or argon-protected hot furnace, kept warm for 2 hours and then hot rolled, with a final rolling temperature of 930°C and a cumulative deformation of 62%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 23 mm.

[0090] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1130°C for 60 minutes, air-cooled to 600°C, and then subjected to 2 passes of synchronous rolling and 2 passes of asynchronous rolling, with a cumulative deformation of 85%, to obtain a rolled plate with a final thickness of 3.4 mm.

[0091] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 250°C for constant temperature insulation for 9 hours, and then air-cooled to room temperature.

[0092] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 30°C for 8 hours, and then air-cooled to room temperature.

[0093] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite, and the yield strength of the plate is 1000 MPa, the tensile strength is 2016 MPa, and the uniform elongation is 12.8%.

[0094] Example 8

[0095] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.80%, Si: 1.55%, Mn: 1.94%, Cr: 1.27%, Ni: 1.33%, Mo: 0.21%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0096] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0097] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 1210°C for 8 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1200°C and drawn multiple times to obtain a slab.

[0098] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1050°C in a vacuum or argon-protected hot furnace, kept warm for 3 hours and then hot rolled, with a final rolling temperature of 920°C and a cumulative deformation of 57%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 26 mm.

[0099] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1130°C for 60 minutes, air-cooled to 600°C, and then subjected to 2 passes of synchronous rolling and 2 passes of asynchronous rolling, with a cumulative deformation of 82%, to obtain a rolled plate with a final thickness of 4.6 mm.

[0100] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 300°C for constant temperature insulation for 10 hours, and then air-cooled to room temperature.

[0101] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 300°C for 12 hours and then air-cooled to room temperature.

[0102] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite, and the yield strength of the plate is 1020 MPa, the tensile strength is 2130 MPa, and the uniform elongation is 13.4%.

[0103] Example 9

[0104] The chemical composition of an ultrafine structure bainite steel is as follows: C: 0.74%, Si: 1.41%, Mn: 1.78%, Cr: 0.93%, Ni: 1.32%, Mo: 0.24%, and the rest is Fe and unavoidable impurities; the preparation method thereof is as follows:

[0105] Step (1) Alloy smelting: The ingredients are prepared according to the above ingredients, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and then cast into an ingot.

[0106] Step (2) homogenization and blanking: The ingot obtained in step (1) is homogenized at 1160°C for 4 hours, then cooled to room temperature to obtain a blank with uniform structure and properties. The blank is forged at a forging temperature of 1200°C and drawn multiple times to obtain a slab.

[0107] Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1020°C in a vacuum or argon-protected hot furnace, kept warm for 2 hours and then hot rolled, with a final rolling temperature of 920°C and a cumulative deformation of 65%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 21 mm.

[0108] Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 1000°C for 45 minutes, air-cooled to 620°C, and then subjected to 2 passes of synchronous rolling and 2 passes of asynchronous rolling, with a cumulative deformation of 85%, to obtain a rolled plate with a final thickness of 3.2 mm.

[0109] Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 220°C for constant temperature insulation for 8 hours, and then air-cooled to room temperature.

[0110] Step (6) Distribution treatment: The plate obtained in step (5) is subjected to carbon distribution treatment at 220° C. for 14 h, and then air-cooled to room temperature.

[0111] The microstructure of the prepared material is mainly composed of bainitic ferrite and retained austenite. The yield strength of the plate is 1003 MPa, the tensile strength is 1932 MPa, and the uniform elongation is 14.3%.

[0112] It can be seen from the above embodiments that:

[0113] In the composition design of the ultrafine structure bainite steel described in the present invention:

[0114] The carbon mass fraction is between 0.72% and 0.98%, which reduces the martensitic transformation temperature and enables bainitic transformation to occur at low temperatures; the silicon mass fraction is between 1.45% and 1.60%, which can effectively hinder the formation of hard phase cementite and improve the plasticity of the material. The precipitation of nano-scale metal carbides is controlled by adding a certain amount of Cr and Mo elements. Mn, Cr, and Ni are all austenite stabilizing elements. By controlling the total mass fraction of Mn, Cr, and Ni in the range of 4.0 to 4.4%, on the one hand, austenite can achieve a higher stability in the warm deformation temperature range, thereby expanding the processing range, and on the other hand, it can also avoid the bainite phase transformation rate being too slow.

[0115] In the method for preparing ultrafine structure bainite steel of the present invention:

[0116] The large deformation warm rolling method is used to significantly refine the austenite structure, and the dual-phase bainite and austenite structure is obtained through the low-temperature bainite phase transformation and partitioning process, thereby achieving a good strength-plasticity match of the material.

[0117] On the other hand, the isothermal bainite transformation described in the present invention uses ammonium nitrate salt as a quenching medium, and the cooling rate is not less than 100°C / s, thereby retaining a large number of high-density dislocations introduced by warm deformation, avoiding other phase changes caused by slow cooling, and achieving a high degree of uniformity of organization and stability of mechanical properties. Then, by using a long-term partitioning treatment, the untransformed and relatively unstable austenite remaining in the isothermal bainite transformation stage is further transformed into bainitic ferrite. During this process, the supersaturated carbon in the bainitic ferrite diffuses into the surrounding residual austenite, thereby improving the stability of the residual austenite and preventing it from being prematurely transformed into a hard and brittle phase martensite that is not conducive to the plasticity of the material when strain occurs, causing the material to break and fail. Therefore, the ultrafine structure bainitic steel described in the present invention has excellent comprehensive mechanical properties.

[0118] The examples of the present invention are not limited to the contents mentioned above, and modifications made by those skilled in the art based on the basic ideas and content framework of the present invention should be within the protection scope of the present invention.

Claims

1. An ultrafine structure bainite steel, characterized in that: The weight percentage of its chemical composition is: C: 0.72-0.98%, Si: 1.40-1.67%, Mn: 1.75-2.10%, P≤0.005%, S≤0.005%, Cr: 0.90-1.34%, Ni: 1.30-1.60%, Mo: 0.20-0.30%, and the rest is Fe and unavoidable impurities.

2. The ultrafine structure bainite steel according to claim 1, characterized in that: The weight percentage of its chemical composition is: C: 0.78-0.80%, Si: 1.49-1.57%, Mn: 1.82-1.90%, P≤0.005%, S≤0.005%, Cr: 0.97-1.06%, Ni: 1.35-1.37%, Mo: 0.24-0.30%, and the rest is Fe and unavoidable impurities. Among them, the total mass percentage of the alloy is not higher than 7%, the mass percentage of Cr+Mo is not less than 1.3%, and the mass percentage of Mn+Cr+Ni is not less than 4.0% and not more than 4.4%.

3. The ultrafine structure bainite steel according to claim 2, characterized in that: The weight percentage of its chemical composition is: C: 0.80%, Si: 1.51%, Mn: 1.90%, P≤0.005%, S≤0.005%, Cr: 1.02%, Ni: 1.36%, Mo: 0.26%, and the rest are Fe and unavoidable impurities. The total mass percentage of the alloy is 6.85%, the mass percentage of Cr+Mo is 1.28%, and the mass percentage of Mn+Cr+Ni is 4.28%.

4. A method for preparing ultrafine structure bainite steel, characterized in that: The steps include: Step (1) alloy smelting: preparing the ingredients according to any one of claims 1 to 3, uniformly stirring and smelting in a vacuum induction furnace, and casting into an ingot; Step (2) homogenization and blanking: the ingot obtained in step (1) is homogenized at 1150-1300°C for a holding time of not less than 4.0 hours, and then cooled to room temperature to obtain a blank with uniform structure and properties. The obtained blank is forged at a forging temperature of not less than 1200°C, and is drawn multiple times to finally obtain a slab; Step (3) high temperature rolling: the slab obtained in step (2) is heated to 1000-1200° C. in a vacuum or argon protection hot furnace, kept warm for 2.0-5.0 hours, and then hot rolled, with the final rolling temperature not lower than 900° C., the cumulative deformation amount being greater than 50%, and air-cooled to room temperature after rolling, to finally obtain a plate with a thickness of 20-30 mm; Step (4) austenitization and strong warm deformation treatment: the plate obtained in step (3) is kept at 950-1150° C. for 30-60 min, air-cooled to 600-700° C., and subjected to 3-4 passes of large deformation rolling, with the cumulative deformation amount being more than 70%; Step (5) isothermal bainite transformation heat treatment: the plate obtained in step (4) is cooled to 200-350° C. and kept at a constant temperature for 8.0-10.0 h, and then air-cooled to room temperature; Step (6) distribution treatment: the plate obtained in step (5) is subjected to carbon distribution treatment at 200-350°C for a distribution time of 4.0-16.0 hours, and then air-cooled to room temperature.

5. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The alloy smelting method used in step (1) is a combination of vacuum induction and electroslag remelting.

6. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The rolling method for the intense warm deformation treatment described in step (4) is 2 passes of continuous synchronous rolling plus 2 passes of continuous asynchronous rolling.

7. The method for preparing ultrafine structure bainite steel according to claim 6, characterized in that: Synchronous rolling and asynchronous rolling are carried out alternately, with the deformation of each pass not less than 15% and the cumulative deformation not less than 80%.

8. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The cooling method used for quenching in the isothermal bainite transformation heat treatment in step (5) is ammonium nitrate salt quenching, and the cooling rate is ≥100°C / s.

9. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The holding temperature selected for the isothermal bainite transformation heat treatment in step (5) is 200-250°C.

10. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The microstructure of the prepared bainite steel consists of acicular bainite and ultrafine retained austenite, the volume fraction of bainite ferrite is ≥60%, the thickness of acicular bainite is ≤200nm, and the equivalent size of ultrafine retained austenite is ≤1μm.

11. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: Nanoscale precipitation exists in the prepared bainitic steel, and the equivalent size is ≤100nm.

12. The method for preparing ultrafine structure bainite steel according to claim 4, characterized in that: The yield strength of the prepared ultrafine structure bainite steel is not less than 1000MPa, the tensile strength is not less than 1900MPa, and the uniform elongation is not less than 10.0%.

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