A component design method for reducing the proportion of bainite and martensite of hot-rolled 42CrMo steel

Abstract

A component design method for reducing the proportion of bainite and martensite of hot-rolled 42CrMo steel, the present application proposes an index proportional to the volume fraction of bainite and martensite hard phase in 42CrMo steel under different components and different cooling speed conditions I hardphase and a calculation method thereof, I hardphase The present application can reduce the tendency of generating bainite and martensite hard phase structure of 42CrMo steel during rolling and cooling process, thereby reducing the hardness of the rolled material, eliminating the original annealing process, improving the production efficiency and reducing the production cost.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel composition design and production, and particularly relates to a composition design method for reducing the proportion of bainite and martensite in hot-rolled 42CrMo steel. Background Art

[0002] 42CrMo alloy steel is often used to manufacture important parts with large cross-sections, such as shafts, gears, connecting rods, transmission gears, and supercharger gears, and it can also be used to replace quenched and tempered steels with higher nickel content. Due to its high carbon and chromium content, 42CrMo alloy steel exhibits high fatigue strength and good resistance to multiple impacts after quenching and tempering, as well as good low-temperature impact toughness and no significant temper brittleness. It is widely used in my country's machinery manufacturing industry.

[0003] To improve processing efficiency, most users are switching from band saw cutting to shearing. This places higher demands on the hot-rolled hardness of 42CrMo steel, requiring it to be less than 269HB. However, in actual production, 42CrMo alloy steel is prone to the formation of hard phases such as bainite and martensite. This high hot-rolled hardness not only requires steelmakers to add annealing processes, impacting production efficiency, but also makes it difficult for downstream users to cut the steel, hindering their use.

[0004] From the above, it can be seen that in order to meet customer requirements for the hardness of 42CrMo hot-rolled steel without adding additional heat treatment steps, it is urgently necessary to develop a composition design method to reduce the proportion of bainite and martensite in hot-rolled 42CrMo steel. Because different alloying elements in steel have different effects on various types of phase transformations during the cooling process, it is possible to reduce the proportion of bainite and martensite hard phases in 42CrMo hot-rolled steel from the perspective of composition optimization design within the national standard range and while meeting key indicators such as hardenability, thereby reducing the hardness of 42CrMo hot-rolled steel. However, because different alloying elements in steel have different effects on the transformation start temperature and transformation kinetics of bainite and martensite phase transformations, and this influence is also affected by the previous phase transformation process and cooling rate, it is difficult to establish a composition design method based on phase transformation laws. Therefore, a quantitative calculation method for the volume fraction of bainite and martensite hard phases in hot-rolled 42CrMo steel based on different alloying elements has not yet been established. Summary of the Invention

[0005] The present invention aims to provide a composition design method for reducing the proportion of bainite and martensite in hot-rolled 42CrMo steel, so that the hardness of the 42CrMo hot-rolled material can meet the requirements without the need for additional heat treatment processes such as annealing.

[0006] To solve the above problems, the technical solution adopted by the present invention is as follows: a composition design method for reducing the proportion of bainite and martensite in hot-rolled 42CrMo steel, the method proposes an index proportional to the volume fraction of bainite and martensite hard phase in 42CrMo steel under different compositions and different cooling rates. I hardphase and its calculation method.

[0007] The index of the present invention I hardphase Calculated by the following formula:

[0008]

[0009] Wherein, AusTemp is the austenitizing temperature, in °C; C, Cr, Mn, Mo, Ni, and Si are the mass percentages of the corresponding alloying elements in 42CrMo steel; and CoolingRate is the cooling rate of 42CrMo bar after rolling, in °C / min.

[0010] The hot-rolled 42CrMo steel of the present invention has a diameter of φ70-90 mm.

[0011] The hot-rolled 42CrMo steel obtained by the method of the present invention has a hardness of ≤269HB.

[0012] The present invention provides a composition design method for reducing the proportion of bainite and martensite in hot-rolled 42CrMo steel. In the composition design process of actual production, under the condition of meeting the hardenability of the user, the composition design calculation is performed within the scope specified by the national standard to make the index I hardphase Smaller, can reduce the tendency of 42CrMo steel to generate bainite and martensite hard phase structure during rolling cooling process, thereby reducing the hardness of the rolled material, eliminating the original annealing process, improving production efficiency and reducing production costs.

[0013] The beneficial effects of the above scheme are as follows: the composition design method for reducing the ratio of bainite and martensite in hot-rolled 42CrMo steel of the present invention is refined from a large amount of production data, and the index representing the ratio of bainite and martensite hard phase in 42CrMo steel under different cooling rates and different composition conditions is obtained. I hardphase , can quantitatively compare the ratio of bainite and martensite hard phase in 42CrMo steel with different compositions. In the actual production composition design process, under the premise of meeting the hardenability requirements of the user, within the composition range required by the national standard, the optimization algorithm is used to calculate the I hardphaseThe smaller the index, the more reasonable the ratio of the alloy elements in 42CrMo steel can be obtained, thereby reducing the proportion of bainite and martensite hard phases in the structure of 42CrMo steel hot-rolled material after cooling, thereby achieving the purpose of reducing the hardness of the rolled material. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The microstructure of the 42CrMo alloy steel hot-rolled material in Example 1;

[0015] Figure 2 The microstructure of the 42CrMo alloy steel hot-rolled material in Example 2;

[0016] Figure 3 The microstructure of the 42CrMo alloy steel hot-rolled material in Comparative Example 1;

[0017] Figure 4 This is the microstructure of the 42CrMo alloy steel hot-rolled material in Comparative Example 2. DETAILED DESCRIPTION

[0018] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0019] Example 1

[0020] In the embodiment of the present invention, the customer requires that the diameter of the 70mm 42CrMo bar represents the hardenability D I The value reaches 90mm or more. According to the national standard "GB / T 3077-2015", the main chemical element composition ranges of 42CrMo alloy structural steel are as follows: C: 0.38~0.45%, Cr: 0.9~1.2%, Mn: 0.5~0.8%, Si: 0.17~0.37%, Mo: 0.15~0.25%.

[0021] According to the present invention, the index of the volume fraction of the bainite and martensite hard phases in 42CrMo steel under different compositions and different cooling rates is proportional to the volume fraction of the bainite and martensite hard phases in 42CrMo steel. I hardphase Its calculation method can be used to calculate the composition of different alloy elements within the national standard composition range. I hardphase Index and D I Calculate the value and select the one that satisfies the D I Under the required conditions (>90mm) I hardphase The composition with the smaller index is designed as the final smelting composition.

[0022]

[0023] D I The value represents the ideal critical diameter of the steel, and the value is generally used to represent the hardenability of the steel. D I The value can be calculated according to the national standard GB / T 5216-2014 "Structural Steel with Guaranteed Hardenability". In the calculation process of the present application, in addition to meeting the component range specified in the national standard, the content range of each element is also adjusted according to the customer's requirement for hardenability. The method proposed in the present application is a method for reducing the influence of bainite and martensite to the maximum extent on the basis of meeting the component range requirements of the national standard and the customer's requirement for hardenability through component design.

[0024] The cooling curve of the 70mm diameter 42CrMo bar actually measured on the production site has an average cooling rate of 1.02℃ / s before phase change, so the CoolingRate is set to 61.2℃ / min and the AusTemp is set to 900℃ when calculating I hardphase

[0025] In the component range of the national standard 42CrMo alloy, the value calculated by different alloy element components is D I The minimum value is 67.83mm and the maximum value is 165.92mm. Under the condition of meeting the user's requirement D I The value calculated by different alloy element components is I hardphase The maximum value of the index is 61.5492 and the minimum value is 53.4873, wherein I hardphase The calculation results of the components with smaller indexes are shown in Table 1.

[0026] In the I hardphase In the component design with smaller indexes, the following components are finally selected as the target components for smelting the 42CrMo of this order considering the alloy cost: C: 0.38%, Cr: 0.9%, Mn: 0.65%, Si: 0.37%, Mo: 0.15%.

[0027] The microstructure of the 42CrMo hot-rolled material smelted by using the target components is shown in Figure 1 , and the hardness thereof is 265HB, which meets the user's requirement.

[0028] Table 1 Calculation results of chemical components of 70mm diameter 42CrMo alloy steel in Example 1

[0029]

[0030] ​Note: Table 1 does not list the entire composition calculation data, but only selects the results of some calculations. The actual calculation process will be within the composition range of the national standard, and each element will change by 0.01. D I Value and I hardphase The value is calculated, and the composition of different alloy elements is calculated. D I The minimum value is 67.83mm and the maximum is 165.92mm. The different alloy element compositions are calculated. I hardphase The maximum value of the index is 61.5492, and the minimum value is 53.4873. I hardphase The calculation results of the components with smaller indexes are listed. The rest of the examples are the same.

[0031] Example 2

[0032] In the embodiment of the present invention, the customer requires that the diameter of the 90mm 42CrMo bar represents the hardenability D I The value reaches more than 100mm. According to the national standard "GB / T 3077-2015", the main chemical element composition range of 42CrMo alloy structural steel has the following requirements: C: 0.38~0.45%, Cr: 0.9~1.2%, Mn: 0.5~0.8%, Si: 0.17~0.37%, Mo: 0.15~0.25%. According to the index of the volume fraction of bainite and martensite hard phase in 42CrMo steel under different composition and different cooling rate conditions proposed in the present invention, I hardphase Its calculation method can be used to calculate the composition of different alloy elements within the national standard composition range. I hardphase Index and D I Calculate the value and select the one that satisfies the D I Under the required value conditions (>100mm) I hardphase The composition with the smaller index is designed as the final smelting composition.

[0033] The cooling curve of the 90mm diameter 42CrMo bar measured at the production site has an average cooling rate of 1.01℃ / s before phase transformation. Therefore, when calculating I hardphase The CoolingRate is set to 60.6℃ / min, and the austenitizing temperature AusTemp is uniformly set to 900℃.

[0034] Within the range of the GB 42CrMo alloy composition requirements, the D I value calculated from different alloy element compositions is the minimum of 67.83 mm and the maximum of 165.92 mm. Under the condition of meeting the user requirement of D I value > 100 mm, the I hardphase index calculated from different alloy element compositions is the maximum of 61.5224 and the minimum of 54.0919, wherein the I hardphase index of the composition with smaller value is shown in Table 2.

[0035] In I hardphase the composition design with smaller I

[0036] The microstructure of the 42CrMo hot-rolled material smelted by using the target composition is shown in Figure 2 , and the hardness thereof is 260 HB, which meets the user requirement.

[0037] Table 2: Chemical composition calculation results of 90 mm diameter 42CrMo alloy steel in Example 1

[0038] Comparative Example 1

[0039] In the comparative example, the customer requirement of the 70 mm diameter 42CrMo bar represents the quenching property D I value reaches 90 mm or more, a general composition design route is adopted, wherein: C: 0.39%, Cr: 1.00%, Mn: 0.74%, Si: 0.24%, Mo: 0.19%. It is calculated that the D D I value of the 42CrMo smelted by using the composition is 108.02 mm, and the I I hardphase index thereof is 56.1484.

[0040] The microstructure of the 42CrMo hot-rolled material smelted by using the target composition is shown in Figure 3 , and the hardness thereof is 308 HB, which exceeds the user requirement and needs to be annealed.

[0041] Comparative Example 2

[0042] In the comparative example, the customer requirement of the 90 mm diameter 42CrMo bar represents the quenching property D IWhen the value reaches 100mm or more, the usual composition design route is adopted, among which: C: 0.40%, Cr: 1.10%, Mn: 0.76%, Si: 0.24%, Mo: 0.20%. D I The value is 121.0574mm, I hardphase The index is 57.5349.

[0043] The microstructure of 42CrMo hot rolled steel obtained by smelting the target composition is as follows: Figure 4 As shown, its hardness is 310HB, which exceeds the user's requirements and needs to be annealed.

Claims

1. A composition design method for reducing the ratio of bainite and martensite in hot-rolled 42CrMo steel, characterized in that: The method proposes an index proportional to the volume fraction of bainite and martensite hard phases in 42CrMo steel under different compositions and cooling rates. I hardphase and its calculation method; the index I hardphase Calculated by the following formula: ； Wherein, AusTemp is the austenitizing temperature, in °C; C, Cr, Mn, Mo, Ni, and Si are the mass percentages of the corresponding alloying elements in 42CrMo steel; CoolingRate is the cooling rate of 42CrMo bar after rolling, in °C / min; In the actual production composition design process, under the premise of meeting the user's hardenability requirements, within the composition range required by the national standard, the optimization algorithm is used to calculate I hardphase The index is small, and a reasonable ratio of each alloy element in 42CrMo steel is obtained, which reduces the proportion of bainite and martensite hard phases in the structure of 42CrMo steel hot-rolled material after cooling, thereby reducing the hardness of the rolled material.

2. A composition design method for reducing the ratio of bainite and martensite in hot-rolled 42CrMo steel according to claim 1, characterized in that: The hot-rolled 42CrMo steel has a diameter of φ70-90 mm.

3. A composition design method for reducing the ratio of bainite and martensite in hot-rolled 42CrMo steel according to claim 1, characterized in that: The hot-rolled 42CrMo steel obtained by the method has a hardness of ≤269HB.

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