A process for reducing the hardness and curvature of 42CrMo in hot-rolled state

Abstract

The invention discloses a process for reducing the hot-rolled hardness and curvature of 42CrMo, including the following steps: heating: heating the billet stepwise, the stepwise heating is sequentially composed of a preheating section, a heating section, a second heating section, a soaking section, and a second soaking section; the temperature of the preheating section is <800°C, the temperature of the heating section is ≤950°C, the temperature of the second heating section is 1050-1150°C, the temperature of the soaking section is 1100-1140°C, and the temperature of the soaking section is 1100-1140°C; rolling: rolling the billet, the start rolling temperature is 1000-1040°C; slow cooling of the cooling bed: controlling the temperature of the upper cooling bed entering the insulation cover to 690-710°C through online air cooling and water tank water cooling. Under the premise that the length of the heating furnace remains unchanged, the present invention cancels the non-heating section, relatively prolongs the second heating section, the soaking section, and the soaking section, so that the diffusion time of the high temperature section is more sufficient and the diffusion of the austenite component is more uniform.

Description

Technical Field

[0001] The invention relates to a process method for steel blanks, in particular to a process method for reducing the hardness and curvature of 42CrMo in a hot-rolled state. Background Art

[0002] The applicant's prior application CN109013715A discloses a rolling method for reducing the hot-rolled hardness and curvature of 42CrMo, the steps of which include (1) step-wise heating of the billet; (2) rolling the billet; and (3) cooling the rolled steel. The metallographic microstructure of 42CrMo rolled by this method is ferrite, pearlite and bainite, which avoids the generation of a large amount of block-shaped needle-shaped martensite structure with high strength and hardness and poor plasticity in the central area of ​​the steel. This effectively improves the hot-rolled hardness of the steel, making the hot-rolled hardness between 240 and 300 HB, and also solves the problem of curvature degradation caused by reduced hardness, making the proportion of hot-rolled curvature of the steel within 4‰ above 97.34%. It can be delivered directly in the hot-rolled state without the need for subsequent annealing to reduce hardness and straightening to improve curvature. It shortens the production cycle and reduces production consumption. The prior application CN109013715A can achieve a hardness range of 240-300HB, and a hot-rolled curvature within 4‰; however, when the hardness is ≥280HB, the surface cracking ratio increases during quenching, and the consumption of cutting and sawing auxiliary materials increases; and when the hardness is between 220 and 260HB, the surface cracking is almost zero, and the consumption of cutting and sawing auxiliary materials returns to normal levels.

[0003] The existing patent application CN 106216391 B belongs to the field of special steel production and processing in the metallurgical industry, and involves a rolling production method for reducing the hardness of 42CrMo. The 42CrMo ingot is used as the raw material, and the following processes are included in sequence: heating, descaling, rough rolling, cooling after rough rolling, continuous rolling, water cooling, finishing rolling of the bar sizing unit, cooling on the cooling bed, and finally the finished bar is obtained. The process method is simple to operate on site, and it is easy to realize online automatic control, which reduces the labor intensity of workers; through the online controlled rolling and controlled cooling process, the straightness of the 42CrMo steel grade is greatly improved; the subsequent heat treatment process (annealing process) is reduced, the production cost is saved, the production cycle is shortened, and the market competitiveness of steel is improved; the hardness value range of the obtained 42CrMo steel is 220~260HBW; the metallographic structure is ferrite and pearlite; the banded structure is ≤2 level; the grain size is ≥8 level; the mechanical properties can meet the national standards and user needs. However, customers currently have higher requirements for hardness and bending, and it is necessary to further optimize the production process and reduce production costs

[0004] Therefore, it is urgent to solve the above problems. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a process for reducing the hardness and curvature of 42CrMo in hot-rolled state with more uniform diffusion of austenite components.

[0006] Technical solution: To achieve the above purpose, the present invention discloses a process for reducing the hardness and curvature of 42CrMo in hot-rolled state, comprising the following steps:

[0007] (1) Heating: The billet is heated in a stepwise manner, and the stepwise heating is sequentially composed of a preheating section, a heating section, a second heating section, a soaking section, and a second soaking section; wherein the temperature of the preheating section is <800°C, the temperature of the heating section is ≤950°C, the temperature of the heating section is 1050-1150°C, the temperature of the soaking section is 1100-1140°C, and the temperature of the soaking section is 1100-1140°C;

[0008] (2) Steel rolling: The billet is rolled at a starting temperature of 1000-1040°C;

[0009] (3) Slow cooling of the cooling bed: Through online air cooling and water cooling in the water tank, the temperature of the upper cooling bed entering the insulation cover is controlled at 690~710℃.

[0010] Among them, the heating rate of the preheating section is ≤18.08℃ / min.

[0011] Preferably, the heating rate of the heating stage is ≤4.89°C / min.

[0012] Furthermore, the target temperature of the second heating stage is 1110°C, and the heating rate is ≤6.42°C / min.

[0013] Furthermore, the target temperature of the soaking stage is 1120°C, and the heating rate is ≤0.28°C / min.

[0014] Furthermore, the second stage of equalizing heat adopts differential heating of the head and tail of the billet. The target temperature of the head of the billet in the second stage of equalizing heat is 1110℃, and the target temperature of the tail of the billet is 1130℃.

[0015] Furthermore, the time from heating the second stage to taking out of the furnace is ≥100 min.

[0016] Preferably, in step (3), the upper cooling bed has a multiple length of 94 to 106 meters, the multiple upper cooling bed is located within the insulation cover, the multiple slow cooling time within the insulation cover is ≥20 minutes, and the cooling rate is ≤0.17°C / s.

[0017] Furthermore, in step (3), the length of the heat preservation cover covers the entire length of the cooling bed rack, and the width covers the width of the cooling bed, and observation and sampling space is reserved at the head and tail of the width.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0019] (1) On the premise that the length of the heating furnace remains unchanged, the non-heating section is reduced, and the original six-section type is optimized to a five-section type; by canceling the non-heating section, the second heating section, the first soaking section and the second soaking section are relatively extended, so that the diffusion time of the high-temperature section is more sufficient, the diffusion of the austenite component is more uniform, and the uniformity of the austenite structure is improved by optimizing the heating process; at the same time, differential heating is achieved in the second heating section to ensure the consistency of the subsequent rolling head and tail temperature, that is, near-isothermal rolling is achieved through differential heating.

[0020] (2) The billet is rolled, and the temperature difference between the head and the tail is controlled at about 20°C during heating, so as to achieve near-isothermal rolling at the head and the tail during rolling, thereby ensuring the consistency of the rolling temperature of the rod and avoiding uneven structure and performance differences caused by the temperature difference between the head and the tail during rolling; that is, near-isothermal rolling is achieved through differential heating; the starting rolling temperature is 1000-1040°C.

[0021] (3) In order to obtain the appropriate structure and achieve the required performance, the temperature of the upper cooling bed entering the insulation cover is controlled at 690-710°C through online air cooling and 7 water tanks, where the temperature at which austenite begins to transform into ferrite + pearlite has been taken into account. Through post-rolling controlled cooling, the temperature entering the insulation cover and the multiple-length insulation time are controlled to achieve a hot-rolled hardness of ≤240HB, which is further improved on the basis of the original indicators.

[0022] (4) The multiple length is in the heat preservation cover. When the cooling rate is ≤0.17℃ / s, the transformation temperature of austenite to ferrite + pearlite can be promoted. When the slow cooling time is ≥20min, the transformation of ferrite + pearlite can reach more than 85%, thereby reducing the hot-rolled hardness. At the same time, the V-shaped restriction and rotation straightening effect of the cooling bed rack on the bar can correct the physical deformation, thereby ensuring that the hot-rolled curvature is ≤3‰ and the bad bending ratio is within 1.20%, which is better than the existing technology in which the hot-rolled curvature is ≤4‰ and the bad bending ratio is about 2.66%; the hot-rolled curvature exceeds 3‰ and the bad bending ratio is within 1.20%, further improving the performance indicators on the basis of the existing performance indicators;

[0023] (5) The process of the present invention is simple and convenient to operate, and is easy to realize automation and mass production. The steel produced can be directly tempered or silver-brightened without straightening, thereby reducing the straightening process and saving costs of about RMB 130 / ton. DETAILED DESCRIPTION

[0024] The technical solution of the present invention is further described below.

[0025] The present invention discloses a process for reducing the hardness and curvature of 42CrMo in a hot-rolled state, comprising the following steps:

[0026] (1) Heating: The billet is heated in a step-by-step manner, which is composed of a preheating stage, a heating stage, a second heating stage, a soaking stage, and a second soaking stage in sequence. The purpose of this heating step is to heat the billet to a temperature range of austenite single-phase solid solution structure, so that there is enough time for homogenization and dissolution of carbides. In the final stage, the head and tail of the billet are heated differentially, so that the subsequent rolling is closer to isothermal rolling, thereby obtaining uniformity of structure and consistency of performance.

[0027] The temperature of the preheating section is less than 800°C, and the heating rate is ≤18.08°C / min. The step-by-step heating of the present invention eliminates the non-heating section of the conventional step-by-step heating, and reduces the non-heating section under the premise that the length of the heating furnace remains unchanged, adjusting the existing six-section type to a five-section type; the second heating section, the first soaking section and the second soaking section are relatively extended, so that the diffusion time of the high-temperature section is more sufficient and the diffusion of the austenite component is more uniform;

[0028] The temperature of the heating stage is ≤950°C, and the heating rate is ≤4.89°C / min. In the present invention, the temperature of the heating stage is lowered, and the plasticity of the steel is relatively poor at low temperatures. Slow heating can avoid excessive thermal stress, which may cause problems such as core tissue defects.

[0029] The temperature of the second heating stage is 1050-1150°C, the target temperature is 1110°C, and the heating rate is ≤6.42°C / min. The present invention limits the temperature of the second heating stage to 1050-1150°C, controls the upper and lower temperature difference within 100°C, and selects the middle value of 1110°C as the target temperature, which can create conditions for subsequent constant temperature diffusion and avoid excessive temperature increase and temperature decrease, resulting in uneven temperature.

[0030] The temperature of the equalizing section is 1100-1140°C, the target temperature is 1120°C, and the heating rate is ≤0.28°C / min. The present invention limits the temperature of the equalizing section to 1100-1140°C, controls the upper and lower temperature difference within 40°C, selects the middle value of 1120°C as the target temperature, and diffuses at a constant temperature to avoid excessive temperature increase or decrease, which causes uneven temperature.

[0031] The temperature of the second soaking stage is 1100-1140°C, the target temperature of the billet head is 1110°C, and the target temperature of the tail is 1130°C. The tail of the billet in the second soaking stage is far away from the furnace door, and the head of the billet is close to the furnace door. The target temperature difference between the head and the tail of the billet is about 20°C, and the general error is ±3°C; that is, differential heating of the head and the tail; the temperature of the second soaking stage of the present invention is limited to 1100-1140°C, and the upper and lower temperature differences are controlled within 40°C. The middle value is selected as the target temperature, and constant temperature diffusion is performed to avoid excessive temperature increase and temperature decrease, resulting in uneven temperature; at the same time, differential heating of the billet head and tail is adopted in the final stage, so that the subsequent rolling is closer to isothermal rolling, thereby obtaining uniformity of organization and consistency of performance;

[0032] The time from the second heating stage to the furnace exit is ≥100min to ensure the high temperature diffusion time;

[0033] (2) Steel rolling: The billet is rolled, and the temperature difference between the head and the tail is controlled at about 20°C during heating to achieve near-isothermal rolling at the head and tail during rolling, thereby ensuring the consistency of the rolling temperature of the rod and avoiding uneven structure and performance differences caused by the temperature difference between the head and the tail during rolling. That is, near-isothermal rolling is achieved through differential heating, and the starting rolling temperature is 1000-1040°C;

[0034] (3) Slow cooling of the cooling bed: In order to obtain the appropriate microstructure and achieve the required performance, the temperature of the upper cooling bed entering the insulation cover is controlled to be 690-710℃ through online air cooling and water cooling of 7 water tanks. The temperature at which austenite begins to transform to ferrite + pearlite has been taken into account. The length of the insulation cover covers the entire length of the cooling bed rack, and the width covers the width of the cooling bed. Space for observation and sampling is reserved at the head and tail of the width. For example: the number of cooling bed teeth is 80, the tooth root spacing is 0.1m, and the rack length is 80 teeth * 0.1m = 8m. The length of the insulation cover can be set to 8m; the width of the cooling bed is 120m, and 6m of observation and sampling space is reserved at the head and tail of the cooling bed. The width of the insulation cover can be set to 108m, and so on. That is, the width of the insulation cover is 108m, the length of the insulation cover is 8m, the width of the cooling bed is 120m, the length of the cooling bed is 12.5m, the number of cooling bed teeth in the insulation cover is 80, and the tooth root spacing is 100mm; taking the insulation cover with a width of 108 meters as an example, the length of the upper cooling bed is 94 to 106 meters, and the position of the upper cooling bed is inside the insulation cover; the slow cooling time in the insulation is ≥20min, and the cooling rate is about 0.17℃ / s. In the present invention, the multiple length is in the heat preservation cover, and when the cooling rate is about ≤0.17℃ / s, the transformation temperature of austenite to ferrite+pearlite can be promoted. When the slow cooling time is ≥20min, the transformation of ferrite+pearlite can be achieved to more than 85%, thereby reducing the hot-rolled hardness. At the same time, the V-shaped restriction and rotation straightening effect of the cooling bed rack on the bar can correct the physical deformation, thereby ensuring that the hot-rolled curvature is ≤3‰ and the bad bending ratio is within 1.20%, which is significantly better than the prior art in which the hot-rolled curvature is ≤4‰ and the bad bending ratio is about 2.66%, and the hot-rolled curvature exceeds 3‰ and the bad bending ratio is within 1.20%, which is further improved on the basis of the original performance indicators.

[0035] Example

[0036] The process method of the present invention is used to roll steel, and the parameters of each group of embodiments are shown in Table 1:

[0037] Table 1 Process parameters

[0038]

[0039] The Brinell hardness of the hot-rolled bar is measured by the nine-point method, in which the hardness value 1 is the hardness of the core, the hardness values ​​2, 3, 4 and 5 are at half the radius, and the hardness values ​​6, 7, 8 and 9 are near the surface. The average value of the φ28-80mm hot-rolled state is ≤240HB, as shown in Table 2.

[0040] Table 2 shows the Brinell hardness of the hot rolled bars

[0041]

[0042] In order to verify the improvement of curvature performance by the process method of the present invention, the curvature of the bar was measured, and the proportion of poor curvature with hot-rolled curvature exceeding 3‰ from January to June 2022 was 0.97%, as shown in Table 3. When the curvature is ≥ 3‰, and the hot-rolled curvature is within 3‰, it can meet the requirements of direct quenching and tempering or silver bright treatment without straightening, reducing the straightening process and saving costs of about 130 yuan / ton; the bar produced by the process method of the present invention can meet the above performance requirements.

[0043] Table 3 Cumulative hot-rolled curvature from January to June 2022

[0044]

[0045]

Claims

1. A process for reducing the hardness and curvature of 42CrMo in hot-rolled state, characterized in that: The steps include: (1) Heating: The billet is heated in a step-by-step manner, and the step-by-step heating is sequentially composed of a preheating section, a heating section, a second heating section, a soaking section, and a second soaking section; wherein the temperature of the preheating section is <800°C, the temperature of the heating section is ≤950°C, the temperature of the heating section is 1050-1150°C, the temperature of the soaking section is 1100-1140°C, and the temperature of the soaking section is 1100-1140°C; the soaking section adopts differential heating of the billet head and tail, and the target temperature of the billet head in the soaking section is 1110°C, and the target temperature of the billet tail is 1130°C; the time from the heating section to the furnace discharge is ≥100min; (2) Steel rolling: The billet is rolled at a starting temperature of 1000-1040°C; (3) Slow cooling of the cooling bed: Through online air cooling and water tank water cooling, the temperature of the upper cooling bed entering the insulation cover is controlled at 690-710°C; the length of the upper cooling bed is 94-106 meters, the position of the upper cooling bed is inside the insulation cover, the slow cooling time of the upper cooling bed in the insulation cover is ≥20 minutes, and the cooling rate is ≤0.17°C / s.

2. A process for reducing the hot-rolled hardness and curvature of 42CrMo according to claim 1, characterized in that: The heating rate of the preheating section is ≤18.08°C / min.

3. A process for reducing the hot-rolled hardness and curvature of 42CrMo according to claim 1, characterized in that: The heating rate of the heating stage is ≤4.89°C / min.

4. The process for reducing the hot-rolled hardness and curvature of 42CrMo according to claim 1, characterized in that: The target temperature of the second heating stage is 1110° C., and the heating rate is ≤6.42° C. / min.

5. The process for reducing the hardness and curvature of 42CrMo in hot-rolled state according to claim 1, characterized in that: The target temperature of the soaking stage is 1120° C., and the heating rate is ≤0.28° C. / min.

6. The process for reducing the hot-rolled hardness and curvature of 42CrMo according to claim 1, characterized in that: In step (3), the length of the heat preservation cover covers the entire length of the cooling bed rack, and the width covers the width of the cooling bed.

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