Heat treatment process for spheroidized annealing high hardenability high carbon chromium steel

Abstract

The present invention relates to a heat treatment process for a spheroidized annealing state high hardenability high-carbon chromium steel. The microstructure of the high-carbon chromium steel before heat treatment is a spheroidized annealing structure, with granular carbides distributed on a ferrite matrix. Step 1: Heat the aforementioned high-carbon chromium steel product to 30°C to 50°C above Ac cm and hold at this temperature range. The holding time is 1.5 min / mm to 3.0 min / mm * d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm. Step 2: Quench the product after holding in an alkali bath at 190°C to 210°C until the bath temperature is reached. Step 3: Hold the product treated in Step 2 in the alkali bath for 25 to 35 min / mm * d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm, and then air cool after leaving the bath. This heat treatment process realizes a significant improvement in the strength, toughness, and hardness of the spheroidized annealing state high-carbon chromium steel.

Description

Technical Field

[0001] The present invention relates to the heat treatment of metal materials, and particularly to a heat treatment process for spheroidized annealed steel. Background Art

[0002] High-carbon chromium steel is the most commonly used steel type for manufacturing wear-resistant parts. The high-carbon chromium steels included in the Chinese standards are GCr4, GCr15, GCr15SiMn, etc. Whether the selected material for manufacturing mechanical parts is appropriate will have a great impact on its service performance and lifespan. The service performance of the material is jointly determined by the raw material and the heat treatment process of the raw material. For high-precision wear-resistant parts, it is required that they not only have high strength, high wear resistance, high elastic limit, appropriate hardness, but also have a certain impact toughness and good dimensional stability.

[0003] For parts with relatively small specifications (generally referring to diameter), GCr15 or GCr15SiMn steel with relatively low hardenability is often selected for production, and heat treatment processes that can give full play to their strength and toughness have been developed at home and abroad. For parts with a larger diameter, it is often difficult to meet the hardenability requirements using the aforementioned steel types, which will result in insufficient strength and toughness of the parts after heat treatment, large differences in mechanical properties between the surface and the core, and it is difficult to meet the comprehensive performance requirements of high-precision wear-resistant parts. Therefore, it is very necessary to re-design the chemical composition of traditional high-carbon chromium steel and develop materials with higher hardenability. For the development of new products, corresponding heat treatment processes are required to match. Generally speaking, wear-resistant steel needs to achieve the regulation of tissue performance through spheroidizing annealing and quenching and tempering processes. Through spheroidizing annealing, the softening of the material can be achieved and the lamellar pearlite can be transformed into granular carbide tissue, realizing the regulation of the carbide particle size and spacing. The structure and properties of the final product of the part need to be regulated through subsequent quenching processes. Therefore, studying the matching of strengthening phases such as martensite, bainite, carbide, and ductile phase retained austenite in the microstructure of spheroidized annealed wear-resistant steel after quenching heat treatment and realizing duplex structure design can achieve the comprehensive improvement of the strength and toughness of the material. Summary of the Invention

[0004] The purpose of the present invention is to provide a heat treatment process for spheroidized annealed high-carbon chromium steel aiming at the above-mentioned existing technology, so as to obtain a specific duplex structure and achieve a significant improvement in the mechanical properties of the steel, especially strength, toughness, and hardness.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: A heat treatment process for spheroidized annealed high-hardening high-carbon chromium steel, the microstructure of the high-carbon chromium steel before heat treatment is spheroidized annealing structure, with granular carbides distributed on the ferrite matrix. Step 1: Heat the aforementioned high-carbon chromium steel product to A cmAbove 30°C to 50°C (870 - 890°C), and hold at this temperature range. The holding time is 1.5 min / mm to 3.0 min / mm*d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm; Step 2: Quench the product after holding into an alkali bath at 190 - 210°C until the bath temperature; Step 3: Hold the product processed in Step 2 in the alkali bath for 25 - 35 min / mm*d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm, and then air cool after leaving the bath.

[0006] Preferably, before heat treatment, the average diameter of the granular carbides in the spheroidized annealing structure is 0.40 μm to 0.7 μm, and the distance between the granular carbides is 0.10 - 0.20 μm.

[0007] Preferably, the elemental weight percentage content of the high-carbon chromium steel satisfies C: 0.95 - 1.05%, Si: 0.20 - 0.30%, Mn: 0.60 - 0.80%, Cr: 1.65 - 1.75%, Mo: 0.40 - 0.50%, and the balance is Fe and unavoidable impurities. Different from traditional high-carbon chromium steels, the design of this elemental composition meets the requirements of high hardenability, and the hardenability depth increases by 8 mm. After spheroidizing annealing, the microstructure of this component material is a fine and uniform spheroidized annealing structure, that is, granular carbides distributed on a ferrite matrix. The size of the granular carbides is smaller, with an average diameter of about 0.40 μm, the distance between the granular carbides is 0.10 - 0.20 μm, and the hardness is 221 HBW.

[0008] Preferably, in Step 1, the heating temperature of the product is Ac cm Above 35°C, (880°C).

[0009] Preferably, in Step 2, the temperature of the alkali bath is 200°C.

[0010] Preferably, in Step 2, the isothermal medium of the alkali bath is NaOH + KOH. Further, the isothermal medium of the alkali bath is 37 wt% NaOH + 63 wt% KOH to meet the required quenching cooling capacity.

[0011] Preferably, after heat treatment, the obtained structure of the product is a structure with hardening phase martensite existing on the basis of lower bainite, as well as retained austenite + granular carbides. The whole structure is mainly lower bainite, and the hardening phase martensite structure exists in a small amount as necessary.

[0012] In the above technical solution, the heating temperature in Step 1 is 870 - 890°C. The austenitizing heating temperature is mainly determined according to the phase transformation critical point of the steel. Usually, it is heated to Ac cm Above 30°C to 50°C. According to the above chemical composition, the Ac of this high-carbon chromium steel cmIt is about 845℃. If the heating temperature is too low, austenitization is not sufficient, and the granular carbides in the original spheroidizing annealing structure (larger than the granular carbides after heat treatment) are less dissolved. The granular carbides are easy to become the initiation points of cracks during service, reducing the toughness of the material after heat treatment; and if the heating temperature is too high, more carbides will be dissolved in the matrix, which will lead to an increase in the carbon content of the matrix. The high carbon content in the matrix is ​​also not good for the toughness after quenching heat treatment (this has always been a disadvantage of high-carbon materials). In addition, too high a heating temperature will also cause the austenite grains to coarsen and further deteriorate the toughness. Therefore, in the heat treatment process, the control of the heating temperature is extremely critical, and the optimal temperature is Ac. cm Above 35℃, that is 880℃.

[0013] In the above technical solution, the holding time of step 1 is 1.5 min / mm to 3.0 min / mm. If the holding time is too short, austenitization is insufficient, while if it is too long, excessive dissolution of particulate carbides will occur, excessive carbon content will enter the matrix, matrix toughness will decrease, austenite grains will coarsen, and energy will be wasted, which is also not conducive to obtaining the target structure and performance.

[0014] In the above technical solution, the isothermal temperature of step 2 is 190-210°C. In order to obtain good strength and toughness, the structure needs to be adjusted to a structure in which a small amount of hardened phase martensite exists on the basis of lower bainite. If the isothermal temperature is too high, pearlite or upper bainite structure is easily formed, which is not conducive to the formation of lower bainite and martensite; if the isothermal temperature is too low, more martensite structure is easily formed, which is not conducive to the formation of lower bainite and makes the material brittle.

[0015] In the above technical scheme, the holding time of step three is also particularly critical and has a significant impact on the final organization. In order to ensure that good martensite, lower bainite, residual austenite and granular carbide multiphase organization are obtained after isothermal quenching, the holding time needs to be regulated. The bainite transformation is between the diffusion transformation in the high temperature zone and the shear transformation in the low temperature zone. The formation of bainite requires a certain incubation period. At the beginning of isothermal treatment, there are more bainite nucleation points. The content of bainite increases with the extension of isothermal time. If the holding time is too short, the content of lower bainite is relatively small. If the holding time is too long, the content of martensite and residual austenite will be relatively small. The granular carbides after heat treatment are mainly nanometer-sized and distributed in the lattice of the organization where martensite is located. Compared with the larger granular carbides (spherical pearlite) before heat treatment, the granular carbides after heat treatment are finer in size and distributed in the lattice.

[0016] Compared with the prior art, the advantages of the present invention are as follows: The heat treatment of this application can significantly change the composition of the duplex structure after heat treatment of spheroidized annealing high-carbon chromium steel, ensuring that the final product has high strength, toughness and hardness. This heat treatment must be designed based on the spheroidized annealing structure and chemical compositions with high hardenability performance. After being treated by this process method, the microstructure of spheroidized annealing high hardenability high-carbon chromium bearing steel is martensite + lower bainite + retained austenite + granular carbide, and the tensile strength R m is 2230 - 2260 MPa, the yield strength R p0.2 is 1360 - 1450 MPa, the notch-free impact energy A K is 130 - 140 J, and the hardness is 60 - 62 HRC. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the microstructural diagram before heat treatment, which is the material structure before heat treatment described in all examples and comparative examples;

[0018] Figure 2 is Figure 1 the micrograph of granular carbide in the shown microstructure;

[0019] Figure 3 is the process schematic diagram of the isothermal quenching heat treatment of this application;

[0020] Figure 4 is the microstructure of Example 1;

[0021] Figure 5 is the microstructure of Example 2;

[0022] Figure 6 is the microstructure of Comparative Example 1;

[0023] Figure 7 is the microstructure of Comparative Example 2;

[0024] Figure 8 is the microstructure of Comparative Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention. The specifications of the products in the embodiments are round steel with a diameter of 50 mm, and the sampling position is the edge of the bar.

[0026] In this embodiment, the chemical composition of the high-carbon chromium steel by mass percentage is: 1.0% C, 0.25% Si, 0.70% Mn, 1.70% Cr, 0.40% Mo, Ni ≤ 0.10%, Ti ≤ 0.002%, P ≤ 0.005%, S ≤ 0.005%, O ≤ 0.001%, H ≤ 0.0002%, and the balance is Fe and unavoidable impurities. The round steel product is obtained through steelmaking, casting, and hot rolling. The product is subjected to spheroidizing annealing treatment to obtain a microstructure of fine and uniform spheroidized annealing structure. The average diameter of the granular carbides is 0.40 μm, the spacing of the granular carbides is 0.10 - 0.20 μm, the hardness is 221 HBW, and the microstructure is shown in Figure 1 , Figure 2 .

[0027] The heat treatment process parameters of the product in the embodiment are shown in Table 1. The microstructural photos after heat treatment are shown in Figure 4 , 5 . It can be seen that after heat treatment, the microstructures of Examples 1 and 2 are basically composed of lower bainite + martensite + granular carbides + retained austenite. After heat treatment, the tensile strength of Example 1 is 2236 MPa, the yield strength is 1363 MPa, the notch-free impact energy is 134 J, and the hardness is 61.2 HRC; after heat treatment, the tensile strength of Example 2 is 2245 MPa, the yield strength is 1415 MPa, the notch-free impact energy is 137 J, and the hardness is 61.6 HRC. It can be seen that after this heat treatment process, the microstructure of the spheroidized annealing state high hardenability high-carbon chromium steel is lower bainite + martensite + granular carbides + retained austenite. Since modern detection means cannot distinguish the relative content of lower bainite and martensite, generally only the content difference can be reflected through performance. The tensile strength R m is 2230 - 2260 MPa, the yield strength R p0.2 is 1360 - 1450 MPa, the notch-free impact energy A K is 130 - 140 J, and the hardness is 60 - 62 HRC. That is, the strength, toughness, and hardness are significantly improved after heat treatment.

[0028] The heat treatment process parameters adopted in Comparative Example 1 are shown in Table 1. The microstructural photos after heat treatment are shown in Figure 6As shown, the mechanical properties are shown in Table 2. It can be seen that after heat treatment, the microstructure of Comparative Example 1 is basically composed of lower bainite + granular carbide + retained austenite. Due to the relatively high isothermal quenching temperature, the formation of bainite is promoted, which is not conducive to the formation of martensite during subsequent air cooling. After heat treatment, the tensile strength of Comparative Example 1 is 2325 MPa, the yield strength is 1897 MPa, the notch-free impact energy is 161 J, and the hardness is 57.0 HRC. It can be seen that after this heat treatment process, there is no martensite structure in the microstructure of this spheroidized annealed high hardenability high-carbon chromium steel, and the hardness is also relatively low. The formation of martensite significantly increases the hardness.

[0029] The heat treatment process parameters used in Comparative Example 2 are shown in Table 1. The microstructure photograph after heat treatment is as Figure 7 shown, and the mechanical properties are shown in Table 2. It can be seen that after heat treatment, the microstructure of Comparative Example 2 is basically composed of lower bainite + granular carbide + retained austenite. Due to the relatively low heating temperature, less carbide dissolves into the matrix, and the content of granular carbide increases, which inhibits the formation of martensite. After heat treatment, the tensile strength of Comparative Example 2 is 2164 MPa, the yield strength is 1483 MPa, the notch-free impact energy is 98 J, and the hardness is 59.7 HRC. It can be seen that after this heat treatment process, there is no martensite structure in the microstructure of this spheroidized annealed high hardenability high-carbon chromium steel, and both the notch-free impact energy and the hardness are relatively low.

[0030] The heat treatment process parameters used in Comparative Example 3 are shown in Table 1. The microstructure photograph after heat treatment is as Figure 8 shown, and the mechanical properties are shown in Table 2. It can be seen that after heat treatment, the microstructure of Comparative Example 1 is basically composed of lower bainite + martensite + granular carbide + retained austenite. However, due to the low austenitizing heating temperature, less granular carbide dissolves ( Figure 8 there are more bright spots in the microstructure shown, mainly because less large granular carbide dissolves), and large granular carbide is likely to become the initiation point of cracks during service, reducing the toughness of the material after heat treatment. After heat treatment, the tensile strength of Comparative Example 3 is 2196 MPa, the yield strength is 1079 MPa, the notch-free impact energy is 83 J, and the hardness is 62.0 HRC. It can be seen that after this heat treatment process, both the yield strength and the notch-free impact energy of this spheroidized annealed high hardenability high-carbon chromium steel are relatively low.

[0031] Table 1 Heat treatment process parameters used in the examples and comparative examples

[0032] Number Heating temperature (°C) Heating time (min / mm) Isothermal temperature (°C) Insulation time (min / mm) Example 1 880 2.5 200 30 Example 2 880 3.0 200 30 Comparative Example 1 860 2.5 240 20 Comparative Example 2 840 2.5 200 20 Comparative Example 3 860 2.5 200 30

[0033] Table 2 Microstructure composition and mechanical properties after heat treatment of the examples and comparative examples

[0034]

[0035] In addition to the above embodiments, the present invention also includes other embodiments. Any technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. Heat treatment process for spheroidized annealing state high hardenability high carbon chromium steel, Characterized in that: The element weight percentage content of the high carbon chromium steel satisfies C: 0.95 - 1.05%, Si: 0.20 - 0.30%, Mn: 0.60 - 0.80%, Cr: 1.65 - 1.75%, Mo: 0.40 - 0.50%; The microstructure before heat treatment of the high carbon chromium steel is spheroidized annealing structure, with granular carbides distributed on the ferrite matrix. Before heat treatment, the average diameter of the granular carbides in the spheroidized annealing structure is 0.40 μm - 0.7 μm, and the spacing of the granular carbides is 0.10 - 0.20 μm; Step 1: Heat the aforementioned high-carbon chromium steel product to 30°C to 50°C above Ac cm 3, and hold it at this temperature range. The holding time is 1.5 min / mm to 3.0 min / mm * d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm. Step 2: Quench the product after holding in an alkali bath at 190 - 210°C until the product reaches the bath temperature. Step 3: Hold the product processed in Step 2 in the alkali bath for 25 - 35 min / mm * d, where d is the diameter of the round steel or the longest straight-line distance on the cross-section of the product, with the unit of mm, and then air cool after leaving the bath. After heat treatment, the product obtains a structure with lower bainite as the main structure, with hardening phase martensite and retained austenite + granular carbides existing. The entire structure is mainly lower bainite, and the hardening phase martensite structure exists in a small amount as a necessary component.

2. The heat treatment process for spheroidized annealing state high hardenability high carbon chromium steel according to claim 1, Characterized in that: In Step 1, the heating temperature of the product is Ac cm above 35°C.

3. The heat treatment process for spheroidized annealing state high hardenability high carbon chromium steel according to claim 1, Characterized in that: In step two, the alkali bath temperature is 200 °C.

4. The heat treatment process for spheroidized annealing state high hardenability high carbon chromium steel according to claim 1, Characterized in that: In step two, the isothermal medium of the alkali bath is NaOH + KOH.

5. The heat treatment process for spheroidized annealing state high hardenability high carbon chromium steel according to claim 4, Characterized in that: In step two, the isothermal medium of the alkali bath is 37wt% NaOH + 63wt% KOH.

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