Heat treatment method for improving hardenability of medium-carbon structural steel

Abstract

The invention relates to a heat treatment method for improving the hardenability of medium-carbon structural steel, and belongs to the field of steel heat treatment. The temperature is increased to 690 + / -(0-5) DEG C along with the furnace according to the speed of 5 + / -(0-0.2) DEG C / min, and heat preservation is conducted according to the condition that the diameter of the round steel is larger than or equal to 2.5 min / mm *; quickly cooling to 600 + / -(0-5) DEG C according to the speed of 8 + / -(0-0.2) DEG C / min after heat preservation is completed, and preserving heat according to the condition that the diameter of the round steel is greater than or equal to 1.5 minutes per millimeter * greater than or equal to 1.5 minutes; then cooling to below 100 DEG C along with the furnace, discharging and air-cooling to room temperature; the steel is reheated to 860 + / -(0-5) DEG C, heat preservation is conducted according to the diameter larger than or equal to 2.5 min / mm * round steel, then water quenching is conducted, and the water temperature is smaller than or equal to 25 DEG C. Through a large number of experimental studies, on the premise that chemical elements such as Ni, Mo, V and B capable of improving the hardenability of the steel are not additionally added, the hardenability of the medium-carbon structural steel is remarkably improved through the heat treatment technology.

Description

Technical Field

[0001] The invention relates to a heat treatment method for medium carbon steel. Background Art

[0002] Through quenching heat treatment, the metallographic structure of medium carbon structural steel changes from ferrite + pearlite to martensite, and the hardness increases from 20~22HRC to more than 50HRC; such a transformation makes medium carbon structural steel, a low-cost steel, widely used in many fields of industrial manufacturing, such as automobile parts manufacturing, engineering machinery parts manufacturing, equipment manufacturing and other fields. However, since medium carbon structural steel does not contain precious alloy elements, such as nickel (Ni), molybdenum (Mo), vanadium (V), etc., the quenching hardness of round steel of medium carbon structural steel reaches a depth of more than 50HRC after quenching heat treatment, that is, the depth of the hardened layer is only 4~5mm. In many application scenarios, it is hoped that round steel can have a deeper hardened layer, that is, better hardenability, to improve the overall strength and hardness of parts; traditional quenching heat treatment technology cannot further improve the hardenability of medium carbon structural steel to obtain a deeper hardened layer, and can only improve the hardenability of steel by increasing the content of alloy elements that are beneficial to hardenability.

[0003] Chinese patent publication number CN 106319383A discloses a nitrogen-containing high-chromium wear-resistant steel and a preparation process thereof. The nitrogen-containing high-chromium wear-resistant steel is composed of the following mass fractions: carbon 1.95-2.08%, silicon 1.24-1.35%, chromium 23.5-24.8%, copper 0.4-0.45%, boron 0.08-0.13%, nitrogen 0.15-0.2%, nickel 1.2-1.4%, molybdenum 0.11-0.14%, scandium 0.005-0.009%, manganese 0.25-0.4%, phosphorus ≤0.02%, sulfur ≤0.02%, and the balance is iron. The invention can improve hardenability through high chromium.

[0004] The Chinese patent publication number CN 107012393A discloses a method for producing medium-carbon chromium steel with low cost and high hardenability. It adopts the design concept of fine-tuning Mo+V to improve hardenability, obtain good tempered troostite structure, meet the quality requirements of high-end crankshaft steel, and open up a new path to successfully solve the problem of insufficient hardenability of 40Cr and high production cost of 42CrMoA. The content of Mn, Cr, Mo, V and other elements by weight is controlled according to the target values ​​of 0.78%, 1.08%, 0.06% and 0.04%, thereby improving the hardenability of steel.

[0005] Chinese patent publication number CN 14134430A discloses a high hardenability 35SiMnCrMoB steel for wear-resistant parts of engineering machinery and a manufacturing method thereof, wherein the chemical composition weight percentage is C: 0.32-0.38%, Si: 1.20-1.50%, Mn: 1.20-1.40%, Cr: 1.00-1.30%, Ti: 0.020-0.050%, B: 0.0010-0.0040%, Mo: 0.20-0.30%, S≤0.010%, P≤0.025%, Cu≤0.20%, Al≤0.020%, N≤0.0060%; B is a surface active element, and the boron dissolved in austenite is adsorbed on the grain boundary to reduce the grain boundary energy, inhibit the nucleation rate of proeutectoid ferrite and pearlite, slow down the decomposition of supercooled austenite, delay the transformation of ferrite and improve the hardenability of the steel. Summary of the invention

[0006] The present invention proposes a heat treatment technology for improving the hardenability of medium carbon carbon structural steel, which no longer relies on increasing the content of alloy elements in the steel to improve the hardenability. Through a large number of experimental studies, the hardenability of medium carbon carbon structural steel is significantly improved through a special heat treatment technology without adding additional chemical elements such as Ni, Mo, V, B, etc. that can improve the hardenability of steel.

[0007] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention relates to: (1) The chemical composition of the medium carbon structural steel includes, by mass percentage, C: 0.42-0.48%, Si: 0.10-0.40%, Mn: 0.50-0.90%, P: ≤0.010%, S: ≤0.010%, Cr: ≤0.30%, Ni: ≤0.03%, Cu: ≤0.03%, Mo: ≤0.02%, Al: ≤0.050%, and the balance is Fe and unavoidable impurity elements, and no other chemical elements that can improve the hardenability of the steel are added to the steel.

[0008] In order to achieve the purpose of the present invention, a large number of experimental studies have been carried out, which show that a special heat treatment process is applied to medium carbon structural steel, so that C and Mn elements can be diffused in a larger range in the steel, the component segregation of the steel is greatly reduced, and the stability of the residual austenite is reduced; in the subsequent quenching process of such medium carbon structural steel, the primary austenite is more easily thermally transformed into martensite, thereby achieving the purpose of improving the hardenability of the steel.

[0009] The medium carbon structural steel is heated to 690±(0~5)℃ at a rate of 5±(0~0.2)℃ / min in the furnace, and kept warm according to the rule of ≥2.5min / mm; after the insulation is completed, the temperature is quickly lowered to 600±(0~5)℃ at a rate of 8±(0~0.2)℃ / min, and kept warm according to the rule of ≥1.5min / mm, and then cooled to below 100℃ in the furnace and air-cooled to room temperature.

[0010] The medium carbon structural steel that has undergone the above special process heat treatment is subjected to quenching heat treatment. The process is: heat the steel to 860± (0~5)℃, keep it warm according to the rule of ≥2.5 minutes / mm, and then water quench it with the water temperature ≤25℃.

[0011] After quenching, the hardness of the steel cross section is tested. For round steel with a diameter of less than 25 mm, the hardness is measured from the surface, and the quenching hardness of the entire cross section reaches above 50 HRC; for round steel with a diameter of ≥25 mm, the hardness is measured from the surface, and the depth of quenching hardness reaching above 50 HRC exceeds 12 mm.

[0012] The cross-sectional metallographic structure of the above quenched medium carbon structural steel is measured in accordance with GB / T38720: for round steel with a diameter of less than 25 mm, the quenching structure of the entire cross section reaches above level 6; for round steel with a diameter of ≥25 mm, the quenching structure reaches level 6 at a depth of more than 12 mm.

[0013] Compared with the prior art, the present invention has the following obvious advantages: 1. The present invention does not need to add precious alloy elements, such as Ni, Mo, V, etc., to medium carbon structural steel, which effectively reduces resource waste and production costs.

[0014] Second, the special heat treatment of the present invention is implemented by using a continuous annealing furnace with mature and stable process technology, which improves the utilization rate of the original equipment and expands the application scenarios and scope of the equipment.

[0015] 3. The hardenability of the steel treated by the special heat treatment process of the present invention is greatly improved under the general quenching heat treatment process conditions, and the depth of hardness>50HRC reaches more than 12mm, which is far beyond the hardness and hardened layer depth of the medium carbon structural steel in the general hot-rolled state after quenching.

[0016] 4. The steel treated by the special heat treatment process of the present invention has a significantly improved quenching structure level under the general quenching heat treatment process conditions, and the quenching structure reaches a depth of more than 12mm above level 6, which is also far higher than the quenching structure level of the general hot-rolled medium carbon structural steel after quenching. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention. Example 1

[0018] Medium carbon structural steel is used, and its chemical composition is as follows by mass percentage: C: 0.45%, Si: 0.25%, Mn: 0.76%, P: 0.008%, S: 0.003%, Cr: 0.16%, Ni: 0.01%, Cu: 0.02%, Mo: 0.01%, Al: 0.022%, and the remainder is Fe and unavoidable impurity elements, and no other chemical elements that can improve the hardenability of steel are added to the steel.

[0019] Medium carbon structural steel is used, and the diameter of the round steel bar is Φ24mm.

[0020] The above-mentioned steel was heated to 694°C in a furnace at a rate of 5.1°C / min and kept at this temperature for 1 hour. After the insulation was completed, the temperature was quickly cooled to 600°C at a rate of 8.0°C / min and kept at this temperature for 1.2 hours. Subsequently, the steel was cooled to 87°C in the furnace and air-cooled to room temperature.

[0021] The medium carbon structural steel that has undergone the above special process heat treatment is subjected to quenching heat treatment. The process is: heating the steel to 865°C, keeping it warm for 1 hour, and then water quenching.

[0022] After quenching, the hardness near the surface of the steel cross section is 53HRC, the hardness at the 1 / 2 radius position is 51.5HRC, and the center hardness is 51HRC.

[0023] The metallographic structure level of the cross section of the above steel materials is evaluated in accordance with GB / T38720: level 7 near the surface, level 6 at the 1 / 2 radius, and level 6 at the center. Example 2

[0024] Medium carbon structural steel is used, and its chemical composition is as follows by mass percentage: C: 0.45%, Si: 0.26%, Mn: 0.75%, P: 0.006%, S: 0.002%, Cr: 0.17%, Ni: 0.01%, Cu: 0.02%, Mo: 0.01%, Al: 0.019%, and the balance is Fe and unavoidable impurity elements, and no other chemical elements that can improve the hardenability of steel are added to the steel.

[0025] Medium carbon structural steel is used, and the diameter of the round steel bar is Φ45mm.

[0026] The above-mentioned steel was heated to 691°C in a furnace at a rate of 4.9°C / min and kept at this temperature for 2 hours. After the insulation was completed, the temperature was quickly cooled to 606°C at a rate of 8.1°C / min and kept at this temperature for 0.7 hours. The steel was then cooled to 90°C in the furnace and air-cooled to room temperature.

[0027] The medium carbon structural steel that has undergone the above special process heat treatment is subjected to quenching heat treatment. The process is: heating the steel to 862°C, keeping it warm for 2 hours, and then water quenching.

[0028] After quenching, the hardness of the steel cross section near the surface is 53HRC, and the hardness at 12mm from the surface is 51HRC.

[0029] The metallographic structure level of the cross section of the above steel materials was evaluated in accordance with GB / T38720, with level 7 near the surface and level 6 at a position 12 mm from the surface.

Claims

1. A heat treatment method for improving the hardenability of medium carbon structural steel, characterized in that: Raise the temperature to 690±(0~5)℃ with the furnace at a rate of 5±(0~0.2)℃ / min, and keep it warm for ≥2.5 minutes / mm*round steel diameter; after the insulation is completed, quickly cool down to 600±(0~5)℃ at a rate of 8±(0~0.2)℃ / min, and keep it warm for ≥1.5 minutes / mm*round steel diameter; then cool with the furnace to below 100℃, take it out of the furnace and air cool it to room temperature; reheat the steel to 860±(0~5)℃, keep it warm for ≥2.5 minutes / mm*round steel diameter, and then water quench, the water temperature is ≤25℃.

2. The method according to claim 1, characterized in that: The elemental composition of structural steel includes C: 0.42~0.48%, Si: 0.10~0.40%, Mn: 0.50~0.90%, P: ≤0.010%, S: ≤0.010%, Cr: ≤0.30%, Ni: ≤0.03%, Cu: ≤0.03%, Mo: ≤0.02%, Al: ≤0.050%, and the balance is Fe and unavoidable impurity elements.