Production process of cr-ni-mo high carbon alloy steel bar

Abstract

This invention relates to the field of high alloy steel production technology, and in particular to a production process for Cr-Ni-Mo medium carbon high alloy steel bars. The production process includes the following steps: (1) Rolling: The temperature of the heating zone of the billet is 1100-1250℃, the initial rolling temperature is 1050-1200℃, the rolling process is uncontrolled, the final rolling temperature is 900-1000℃, and the rolled material is air-cooled; (2) Heat treatment: The rolled material is heat-treated at a temperature of (A1 point - 20)℃, held for 20-25 hours, and then cooled in the furnace; (3) Finishing and delivery. This patent optimizes the steel rolling process and increases the rolling temperature to achieve appropriate grain coarsening, thereby reducing the stability of austenite and thus reducing the residual austenite content in the hot-rolled steel, which is beneficial for subsequent heat treatment to reduce the hardness of the delivered product.

Description

Technical Field

[0001] This invention relates to the field of high alloy steel production technology, and in particular to a production process for Cr-Ni-Mo medium carbon high alloy steel bars. Background Technology

[0002] Cr-Ni-Mo alloy structural steel bars are widely used in engineering machinery, automotive parts, and other fields due to their high hardenability and excellent comprehensive mechanical properties. Examples include 30CrNiMo, 36CrNiMo, 40CrNiMo, and 30Cr2Ni4Mo. As the content of Cr, Ni, and Mo alloying elements increases, the bars gradually become harder and their machinability deteriorates. Therefore, for Cr-Ni-Mo high-alloy steel, customers often require steel mills to perform annealing heat treatment to reduce hardness before delivery.

[0003] Currently, there are many heat treatment processes for reducing hardness, such as spheroidizing annealing and high-temperature tempering. For alloy steels with high Cr, Ni, and Mo content, the austenite has high stability. Articles such as "The Influence of Heat Treatment Process on Mechanical Properties and Microstructure of 23CrNi3MoA Drill Bit Steel" and "Research on Annealing Process of 30CrNi4MoA Steel" both found that low-temperature annealing is effective in reducing hardness. However, as the annealing time of the bar stock increases, the impact performance after quenching and tempering shows a decreasing trend. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the problem that as the content of Cr, Ni and Mo alloying elements increases, the bar stock gradually becomes harder and the processing performance deteriorates, in the prior art, and to provide a production process for Cr-Ni-Mo medium carbon high alloy steel bars.

[0005] The technical solution adopted by this invention to solve its technical problem is: a production process for Cr-Ni-Mo medium-carbon high-alloy steel bars, wherein the chemical composition of the medium-carbon high-alloy steel by weight percentage is C: 0.20-0.40%, Si: 0.1-0.5%, Mn: 0.40-1.00%, Cr: 1.00-2.00%, Ni: 2.00-4.30%, Mo: 0.15-0.80%, V≤0.20%;

[0006] The production process includes the following steps: (1) Rolling: The temperature of the heating zone of the billet is 1100-1250℃, the initial rolling temperature is 1050-1200℃, the rolling process is uncontrolled, the final rolling temperature is 900-1000℃, and the billet is air-cooled after rolling.

[0007] (2) Heat treatment: The rolled material is heat treated at a temperature of (A1 point - 20)℃, and then cooled in the furnace after holding for 20-25 hours.

[0008] (3) Finishing and delivery.

[0009] Further, the bar specifications in step (1) are 14-50mm.

[0010] The beneficial effects of this invention are: the invention provides a production process for Cr-Ni-Mo medium-carbon high-alloy steel bars;

[0011] (1) The higher the content of Cr, Ni and Mo in steel, the higher the stability of austenite, resulting in hot-rolled round bars containing a certain amount of residual austenite. The residual austenite in steel has a great influence on the hardness after heat treatment. The heat treatment process of single-pass low-temperature annealing (annealing temperature below Ac1) is difficult to transform and decompose the residual austenite. Instead, the residual austenite is transformed into martensite structure when cooled after heat treatment, and the delivery hardness is difficult to meet the requirements. This patent optimizes the steel rolling process and increases the rolling temperature to achieve the purpose of appropriate grain coarsening, thereby reducing the stability of austenite and reducing the content of residual austenite in hot-rolled steel, which is beneficial to the subsequent heat treatment to reduce the delivery hardness of the product.

[0012] (2) As the alloy content in steel increases, the transformation rate of hot-rolled structure during high-temperature tempering is slow. To achieve the required delivery hardness, a relatively long heat treatment is required. However, if the heat treatment time is too long, the carbides precipitated in the steel will coarsen severely, which will affect the impact toughness of the product. Therefore, this patent combines the optimization of the rolling process with the design of the heat treatment time to meet the product's requirements for impact toughness and delivery hardness. Attached Figure Description

[0013] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0014] Figure 1 This is a metallographic diagram of the residual austenite in the hot-rolled material of Example 1;

[0015] Figure 2 This is a metallographic schematic diagram of the residual metal in the hot-rolled material of Comparative Example 1;

[0016] Figure 3 This is a metallographic schematic diagram showing the size of the carbide particles in Example 1;

[0017] Figure 4 This is a metallographic schematic diagram of the carbide particle size in Comparative Example 2. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0019] A production process for Cr-Ni-Mo medium-carbon high-alloy steel bars, wherein the chemical composition of the medium-carbon high-alloy steel by weight percentage is: C: 0.20-0.40%, Si: 0.1-0.5%, Mn: 0.40-1.00%, Cr: 1.00-2.00%, Ni: 2.00-4.30%, Mo: 0.15-0.80%, V≤0.20%;

[0020] The production process includes the following steps: (1) Rolling: The temperature of the heating zone of the billet is 1100-1250℃, and the initial rolling temperature is 1050-1200℃. If the initial rolling temperature is too high, decarburization will be aggravated; if the temperature is too low, the composition uniformity will be poor. The rolling process is not controlled. The rolling process is not water-cooled. Therefore, the rolling process temperature is relatively high, which can appropriately coarsen the grains, improve the stability of the austenite structure, reduce the transformation of the residual austenite in the subsequent cooling process, and lay the foundation for the next step of annealing heat treatment. The final rolling temperature is 900-1000℃. After rolling, air cooling is used. If water cooling is used after rolling, the thermal stress will be large and the bending of the rolled material will be more serious. Therefore, air cooling is selected after rolling. The bar specifications in step (1) are 14-50mm.

[0021] (2) Heat treatment: The rolled material is heat treated at a temperature of (A1 point - 20)℃. After holding at the temperature for 20-25h, it is cooled in the furnace. This part is a low-temperature annealing treatment, which transforms the martensitic structure of the rolled material into a high-temperature tempered structure, reduces the hardness of the material, and improves the subsequent processing performance. A1 point is the critical point, that is, the temperature at which austenite, ferrite and cementite coexist in equilibrium.

[0022] (3) Finishing and delivery.

[0023] Example 1:

[0024] This example illustrates the actual control effect of the invention on delivery hardness and product impact performance using the production process of a high Cr, Ni, Mo steel for engineering machinery, with a rolled bar specification of φ30mm.

[0025] (1) Steel smelting composition: C 0.32%, Si 0.20%, Mn 0.60%, Cr 1.50%, Ni 3.40%, Mo 0.40%, V 0.08%.

[0026] (2) The cross section of the continuous casting billet is 220mm*260mm. The cold billet of the continuous casting billet is heated in the heating furnace. The temperature of the high temperature section is 1180℃, and the initial rolling temperature is 1100℃. The rolling process is uncontrolled, and the final rolling temperature is 950℃. After rolling, the billet is air-cooled and then slowly cooled in the pit.

[0027] (3) After the rolled material is removed from the pit, it is heat-treated at a temperature of 670℃ and kept at that temperature for 25 hours before being cooled in the furnace.

[0028] The delivered hardness and quenched-temper impact properties are shown in Table 1. The delivered hardness is between 260-275 HBW, and the quenched-temper impact toughness AKU2 is between 140-170 J.

[0029] Comparison Case 1

[0030] Using steel grades with similar chemical composition and specifications as a comparative example, the production process is as follows:

[0031] (1) The cross section of the continuous casting billet is 220mm*260mm. The cold billet of the continuous casting billet is heated in the heating furnace. The temperature of the high temperature section is 1180℃, and the initial rolling temperature is 1100℃. The rolling process is water-controlled rolling, and the final rolling temperature is 830℃. After rolling, the billet is air-cooled and then slowly cooled in the pit.

[0032] (2) After the rolled material is removed from the pit, it is heat-treated at a temperature of 670℃ and kept at that temperature for 25 hours before being cooled in the furnace.

[0033] After lowering the rolling temperature, the metallographic structure of the hot-rolled material is as follows: Figure 1 As shown, compared with the example, the amount of residual metal in the tissue is significantly increased, which makes subsequent annealing to reduce hardness more difficult. The delivery hardness and quenched-temper impact properties are shown in Table 1. The delivery hardness is 290-300 HBW, and the quenched-temper impact toughness is AKU2 140-160J.

[0034] Comparison Case 2

[0035] Using steel of the same composition and specifications as a comparative example, the production process is as follows:

[0036] (1) The cross section of the continuous casting billet is 220mm*260mm. The cold billet of the continuous casting billet is heated in the heating furnace. The temperature of the high temperature section is 1180℃, and the initial rolling temperature is 1100℃. The rolling process is controlled without water piercing, and the final rolling temperature is 950℃. After rolling, it is air cooled and then put into the pit for slow cooling.

[0037] (2) After the rolled material is removed from the pit, it is heat-treated at a temperature of 670℃ and kept at that temperature for 45 hours before being cooled in the furnace.

[0038] After extending the annealing heat treatment time, the metallographic structure is as follows: Figure 2 As shown, compared with the example, the carbides in the tissue are significantly larger, which is detrimental to improving impact toughness. The delivery hardness and quenched-temper impact properties are shown in Table 1. The delivery hardness is 240-250 HBW, and the quenched-temper impact toughness is AKU2 70-120 J. Impact toughness shows a decrease and significant fluctuations.

[0039] Table 1 Test Results

[0040]

[0041] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A production process of a Cr-Ni-Mo system high-carbon high-alloy steel bar, characterized by, The chemical composition of the medium-carbon high-alloy steel is as follows in percentage by weight: C: 0.20-0.40%, Si: 0.1-0.5%, Mn: 0.40-1.00%, Cr: 1.00-2.00%, Ni: 2.00-4.30%, Mo: 0.15-0.80%, V≤0.20%; The production process comprises the following steps: (1) rolling: the temperature of the heating and soaking section of the casting blank is 1100-1250℃, the temperature of the starting rolling is 1050-1200℃, the rolling process is not controlled rolling, the temperature of the final rolling is 900-1000℃, and the rolled product is air-cooled; (2) heat treatment: the rolled product is subjected to heat treatment, the heat treatment temperature is selected as (A1 point-20)℃, and the heat treatment is maintained for 20-25h and then the heat treatment is cooled in the furnace; (3) finishing and delivery.

2. The production process of Cr-Ni-Mo system high-carbon alloy steel bar according to claim 1, characterized in that: The specification of the bar in the step (1) is Φ14-50mm.

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