S35c cover annealing hot-rolled steel sheet having concentrated particle size distribution characteristics and manufacturing method thereof

Abstract

The application discloses a S35C cover annealing hot-rolled steel plate with a concentrated particle size distribution characteristic and a manufacturing method. In the hot-rolling process, the opening rolling temperature is controlled to be 1010-1050 DEG C, the final rolling temperature is 860-890 DEG C, the laminar cooling speed is 25-35 DEG C / s, and the coiling temperature is 640-660 DEG C. In the cover annealing process, the heating speed is controlled to be 15-20 DEG C / h in the range of 300-710 DEG C, the temperature is raised to 710 DEG C, and then the temperature is kept for 8-10 h. Then, the cooling stage is entered, the cooling speed is 15-20 DEG C / h in the range of 710 DEG C to 500 DEG C, and the cooling speed is 30-40 DEG C / h below 500 DEG C. In the hot-rolling stage, the final annealing precursor structure is provided by matching the final rolling temperature, the cooling speed and the coiling temperature. In the annealing stage, the uniform spheroidization of the cementite and the accurate particle size control are realized by the cooperation of the heating speed, the holding temperature and time and the segmented cooling speed.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to an S35C bell-type annealed hot-rolled steel plate with concentrated particle size distribution characteristics and its manufacturing method. Background Technology

[0002] Medium carbon structural steel, due to its balanced combination of strength, toughness, and machinability, is widely used in automotive powertrain systems, engineering machinery transmission components, and other fields. S35C, as a typical medium carbon steel, achieves a balanced combination of strength, toughness, and machinability due to its carbon content falling within the critical range of 0.32% to 0.38%, supplemented by optimized composition of 0.15% to 0.35% silicon and 0.60% to 0.90% manganese. Its hot-rolled tensile strength is ≥530MPa and its yield strength is ≥315MPa. After heat treatment, its performance is further improved, making it widely used in automotive powertrain systems, lightweight aerospace structural components, engineering machinery transmission components, and high-end equipment mold bases. Currently, the rapid development of the new energy vehicle and aerospace industries has placed higher demands on the dimensional accuracy, surface quality, and mechanical property stability of medium carbon steel plates, driving the industry towards efficient, precise, and green manufacturing.

[0003] The industrial production of S35C hot-rolled steel plates typically follows a process of "continuous casting billet heating - hot rolling - laminar flow cooling - coiling". However, the microstructure of the hot-rolled state is ferrite-lamellar pearlite, with high hardness and uneven distribution, resulting in significant internal stress. This directly leads to difficulties in subsequent cold stamping and machining, increased die wear, and even problems such as part cracking and dimensional deviations. To address this issue, the industry generally adopts annealing processes for softening and microstructure homogenization.

[0004] Currently, S35C annealed hot-rolled steel sheets are mostly prepared using a bell-type annealing process. However, existing products with a thickness of 8mm or more have unique problems such as poor microstructure uniformity, wide cementite particle size distribution, severe surface decarburization, and large hardness fluctuations in the thickness direction. The cementite particle size distribution is wide, ranging from 0.1μm to 3μm, and some areas still retain lamellar pearlite. The spheroidization rate is usually between 30% and 80%. The surface decarburization is severe (the decarburization depth reaches 140~500μm), which leads to large hardness fluctuations in the thickness direction (the hardness HRB in the thickness direction fluctuates by 6~10), and the fluctuations are greater the thickness, which affects the subsequent cold working performance and the fatigue life of the parts. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an S35C bell-type annealed hot-rolled steel plate with concentrated cementite particle size, high spheroidization rate, and stable processing performance, as well as a manufacturing method thereof.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for manufacturing S35C bell-annealed hot-rolled steel plate with concentrated particle size distribution characteristics includes a hot rolling process and a bell-annealing process. In the hot rolling stage, the matching of finishing rolling temperature, cooling rate and coiling temperature provides a suitable precursor structure for subsequent annealing. In the annealing stage, the uniform spheroidization of cementite and precise control of particle size are achieved through the synergy of heating rate, holding temperature and time and segmented cooling rate.

[0007] In the hot rolling process, the rough rolling stage is rolled in the austenite recrystallization zone, with the descaling water at the rough rolling inlet and outlet fully open to ensure that the iron oxide scale is completely removed; the finishing rolling start temperature is controlled at 1010~1050℃, and the finishing rolling temperature is controlled at 860~890℃; after rolling, the strip is coiled after laminar flow cooling, with a laminar flow cooling rate of 25~35℃ / s and a coiling temperature of 640~660℃; The steel plate thickness specifications cover 8~12mm, and the control deviation of the finishing rolling temperature, finishing rolling temperature, cooling rate and coiling temperature under the same thickness specification shall not exceed ±10℃.

[0008] To ensure better and more complete spheroidization of pearlite in the thickness direction, slow heating is used in the annealing process within the range of 300~710℃. The heating rate is controlled at 15~20℃ / h, and the temperature is held at 710℃ for 8~10h. For hot rolls with a thickness of 8~12mm, the heat conduction is slow, and slow heating can ensure uniform temperature in the thickness direction and avoid uneven internal structure. Then, the cooling stage is entered. The cooling rate is 15~20℃ / h in the range of 710℃ to 500℃, and the cooling rate is 30~40℃ / h below 500℃, until the temperature is cooled to room temperature.

[0009] The cooling rate below 500℃ is adjusted according to the thickness of the steel plate: 30℃ / h when the steel plate thickness is 8mm, 35℃ / h when the steel plate thickness is 9mm, and 40℃ / h when the steel plate thickness is 10~12mm.

[0010] Furthermore, the chemical composition of S35C steel plate, by mass percentage, is as follows: C 0.32%~0.38%, Si 0.15%~0.35%, Mn 0.60%~0.90%, P≤0.030%, S≤0.035%, Cu≤0.30%, Ni≤0.20%, Cr≤0.20%, with the balance being Fe and unavoidable impurities, and Ni+Cr≤0.35%.

[0011] Furthermore, during the heat preservation stage of the bell-type annealing process, the furnace temperature uniformity is controlled within ±5℃, and during the cooling stage, the protective atmosphere dew point does not exceed -40℃.

[0012] The method of this invention controls the coiling temperature at 640~660℃ to avoid excessively coarse or fine pearlite lamellae in the hot-rolled state, providing a homogeneous matrix for the uniform dissolution and precipitation of cementite during annealing. Combined with differentiated cooling rates in stages after annealing, this prevents excessively slow cooling of thin steel plates, which could lead to abnormal cementite growth, while also preventing excessively rapid cooling of thick steel plates, which could inhibit the spheroidization process. Ultimately, this achieves a stable standard deviation of cementite particle size within 0.20μm across all thicknesses. Simultaneously, the combined effect of annealing holding temperature and heating rate addresses the balance between decarburization and spheroidization: slow heating at 20℃ / h reduces the oxidation rate on the steel plate surface, while low-temperature holding at 710℃ reduces carbon diffusion while ensuring spheroidization, controlling the surface decarburized layer depth to below 35μm.

[0013] Also provided is an S35C bell-type annealed hot-rolled steel plate prepared by the manufacturing method described above. In the microstructure at 1 / 4 of the thickness of the steel plate, spherical cementite is distributed in a ferrite matrix with a spheroidization rate of 80-90%. The cementite particle size distribution range is 0.12-1.50 μm, the average particle size is 0.47-0.57 μm, the standard deviation of particle size is ≤0.20 μm, the surface decarburization layer depth is ≤35 μm, and the Rockwell hardness HRB measured on the upper surface, 1 / 4, 1 / 2, 3 / 4 and lower surface in the thickness direction is 67-69, with the difference between the maximum and minimum values ​​≤2.0. This improves the tool life of subsequent cutting operations and the dimensional accuracy of stamped parts.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Significantly improved uniformity of structure: cementite particles are concentrated in the range of 0.1~1.5μm, with a standard deviation of particle size ≤0.2μm, exhibiting excellent particle size concentration distribution characteristics.

[0015] (2) Better spheroidization effect and shallower decarburization: After the cover is removed, the spheroidization rate of the steel plate is ≥80% and the surface decarburization layer depth is ≤35μm. The spheroidization is more complete and the decarburization control is better than that of conventional processes.

[0016] (3) Significantly improved hardness stability: Rockwell hardness HRB is stably controlled within the range of 67~69, and the thickness direction hardness HRB difference is ≤2, which reduces hardness fluctuation by more than 50% compared with existing technologies. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments and comparative examples. The chemical composition of the steel plates in the embodiments meets the requirements of the S35C standard: C 0.32%~0.38%, Si 0.15%~0.35%, Mn 0.60%~0.90%, P≤0.030%, S≤0.035%, Cu≤0.30%, Ni≤0.20%, Cr≤0.20%, with the balance being Fe and unavoidable impurities, and Ni+Cr≤0.35%.

[0018] The main hot rolling process parameters of each embodiment and comparative example of the present invention are shown in Table 1, and are executed according to the conventional austenite recrystallization zone rough rolling process, with the inlet and outlet descaling water fully open.

[0019] Table 1 Hot rolling process parameters of various embodiments and comparative examples of the present invention The main cover removal process parameters of the various embodiments and comparative examples of the present invention are shown in Table 2.

[0020] Table 2. Cover removal process parameters for various embodiments and comparative examples of the present invention. Table 3 shows the statistical results of cementite particle size and spheroidization rate in various embodiments and comparative examples of this invention. The microstructure at 1 / 4 of the steel plate thickness was observed using a ZeissSigma 300 field emission scanning electron microscope (SEM) at a magnification of 2000x, revealing spherical cementite uniformly distributed within the ferrite matrix. Image-Pro Plus image analysis software was used to randomly select five fields of view, with at least 100 particles counted in each field, and the cementite particle diameter was statistically analyzed. The proportion of spherical cementite area to the total area of ​​all cementite was calculated using the area method, and the spheroidization rate was then calculated.

[0021] Table 3. Cementite particle size statistics and spheroidization rate results of various embodiments and comparative examples of the present invention. The surface decarburization and thickness hardness fluctuation results of the various embodiments and comparative examples of the present invention are shown in Table 4. The depth of the decarburized layer on the steel plate surface was observed using a metallographic microscope. Rockwell hardness (HRB) tests were performed at three points on the upper surface, one-quarter, one-half, three-quarters, and lower surface of the sample, according to GB / T 230.1 standard.

[0022] Table 4. Surface decarburization and hardness fluctuation results of various embodiments and comparative examples of the present invention. As shown in Tables 3 and 4, Examples 1-5 all achieved concentrated distribution of cementite particle size and low decarburization control: Comparative Example 1 had a coiling temperature as high as 700℃, resulting in severe segregation of the hot-rolled microstructure. Although the spheroidization rate was acceptable after annealing, the surface decarburized layer reached 146μm, and the hardness fluctuated by more than 4HRB; Comparative Example 2 had a coiling temperature of only 500℃, resulting in excessively fine pearlite lamellae in the hot-rolled state. After annealing, the spheroidization rate was less than 50%, and the standard deviation of the particle size reached 0.34μm; Comparative Example 3 increased the heating rate to 35℃ / h, and the cooling rate in the high-temperature section increased simultaneously, so the cementite did not have enough time to spheroidize, and the spheroidization rate was only 60%~70%; Comparative Example 4 reduced the holding temperature to 650℃, resulting in insufficient dissolution of cementite, with the lowest spheroidization rate of only 20%, and the average particle size was too large and the distribution was scattered; Comparative Example 5 extended the holding time to 15h, leading to excessive diffusion of surface carbon elements, and the decarburized layer depth surged to 553μm, completely losing its performance.

[0023] As can be seen, this invention achieves precise control of cementite particle size in the full thickness range of 8~12mm by coupling multiple parameters of hot rolling and annealing processes, taking into account spheroidization rate, decarburization control and hardness uniformity. It solves the pain point of existing technologies where single parameter adjustment cannot take into account multiple properties, and is suitable for the stable mass production of high-end structural parts.

[0024] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for manufacturing S35C bell-type annealed hot-rolled steel plate with concentrated particle size distribution characteristics, characterized in that, This includes hot rolling and annealing processes; In the hot rolling process, the finishing rolling start temperature is controlled at 1010~1050℃, the finishing rolling temperature is controlled at 860~890℃, the laminar cooling rate is controlled at 25~35℃ / s, and the coiling temperature is controlled at 640~660℃. In the bell-type annealing process, the heating rate is controlled at 15-20℃ / h in the range of 300-710℃ to raise the temperature to 710℃ and then hold it for 8-10h; then the cooling stage begins, with a cooling rate of 15-20℃ / h in the range of 710℃ to 500℃; and a cooling rate of 30-40℃ / h below 500℃.

2. The manufacturing method according to claim 1, characterized in that: The cooling rate below 500℃ is adjusted according to the thickness of the steel plate: 30℃ / h when the steel plate thickness is 8mm, 35℃ / h when the steel plate thickness is 9mm, and 40℃ / h when the steel plate thickness is 10~12mm.

3. The manufacturing method according to claim 1, characterized in that, The chemical composition of the S35C steel plate, by mass percentage, is as follows: C 0.32%~0.38%, Si 0.15%~0.35%, Mn 0.60%~0.90%, P≤0.030%, S≤0.035%, Cu≤0.30%, Ni≤0.20%, Cr≤0.20%, with the balance being Fe and unavoidable impurities, and Ni+Cr≤0.35%.

4. The manufacturing method according to claim 1, characterized in that, In the hot rolling process, the steel plate thickness ranges from 8 to 12 mm, and the control deviation of the finishing rolling start temperature, finishing rolling temperature, cooling rate and coiling temperature under the same thickness specification does not exceed ±10℃.

5. The manufacturing method according to claim 1, characterized in that, During the heat preservation stage of the bell-type annealing process, the furnace temperature uniformity is controlled within ±5℃, and during the cooling stage, the dew point of the protective atmosphere does not exceed -40℃.

6. An S35C bell-type annealed hot-rolled steel sheet prepared by the manufacturing method according to any one of claims 1 to 5, characterized in that, In the microstructure at 1 / 4 of the steel plate thickness, spherical cementite is distributed in a ferrite matrix, with a spheroidization rate of 80-90%. The cementite particle size distribution ranges from 0.12 to 1.50 μm, with an average particle size of 0.47-0.57 μm and a standard deviation of particle size ≤0.20 μm. The surface decarburization layer depth is ≤35 μm. The Rockwell hardness HRB measured on the upper surface, 1 / 4, 1 / 2, 3 / 4, and lower surface in the thickness direction is 67-69, with the difference between the maximum and minimum values ​​≤2.0.

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