A preparation method for improving the sorbitizing rate of hot-rolled narrow strip steel and hot-rolled narrow strip steel

Abstract

The application relates to the technical field of metallurgical rolling, and provides a preparation method for improving the sorbitizing rate of hot-rolled narrow strip steel and the hot-rolled narrow strip steel, which comprises the following steps: providing a hot-rolled narrow strip steel casting blank with specific components; heating and rolling the casting blank to obtain a hot-rolled steel strip; performing post-rolling cooling, including preheating a bearing surface of a cooling conveying device and controlling the temperature of the bearing surface to be 650-700 DEG C; coiling in the range of 640-660 DEG C, and placing the hot-rolled steel strip in a heat preservation environment to perform self tempering by using residual heat. Through the synergistic effect of the bearing surface preheating, the strong water penetration temperature control, the phase change induction and the coiling residual heat self tempering, the temperature difference gradient on the upper and lower surfaces of the strip steel is eliminated, the microstructure sorbitizing rate is significantly improved, the pearlite interlamellar spacing is refined, the material performance uniformity is improved, and no noble alloy element needs to be added or equipment needs to be transformed.

Description

Technical Field

[0001] This application relates to the field of metallurgical steel rolling technology, specifically to a method for improving the sorbitization rate of hot-rolled narrow strip steel and the hot-rolled narrow strip steel itself. Background Technology

[0002] In the production of hot-rolled narrow strip steel for carbon tools, the microstructure evolution during the post-rolling cooling stage directly determines the mechanical properties and processing applicability of the final product. In existing technologies, during the post-rolling cooling of high-carbon narrow strip steel, the difference in heat dissipation conditions between the upper and lower surfaces leads to inconsistent undercooling, resulting in uneven distribution of the pearlite-to-sorbite transformation along the thickness direction. This microstructure inhomogeneity not only increases residual stress within the strip but can also lead to excessively high or low local hardness, severely affecting the processing stability of downstream cold-rolling users and the performance consistency of finished tools, springs, and other products. Furthermore, traditional post-rolling cooling processes often focus on strength enhancement, lacking synergistic control over hardness and microstructure uniformity after phase transformation. This makes it difficult to simultaneously meet the dual requirements of high sorbitization and suitable processing hardness without adding expensive alloying elements. Therefore, there is an urgent need for a preparation method that can effectively improve microstructure uniformity and precisely control the sorbitization rate through process optimization. Summary of the Invention

[0003] To address the problems of uneven microstructure and difficulty in hardness control in the post-rolling cooling of high-carbon hot-rolled narrow strip steel in existing technologies, this application proposes a method for improving the sorbitization rate of hot-rolled narrow strip steel and the hot-rolled narrow strip steel itself. This method significantly improves the sorbitization rate of the hot-rolled microstructure and enhances the uniformity of material properties without the need to add expensive alloying elements.

[0004] To achieve the above objectives, this application adopts the following technical solution: A method for improving the sorbitization rate of hot-rolled narrow strip steel includes: providing a hot-rolled narrow strip steel billet, wherein the chemical composition of the hot-rolled narrow strip steel billet, by weight percentage, comprises: C: 0.80%–0.95%, Si: 0.20%–0.30%, Mn: 0.35%–0.55%, P≤0.015%, S≤0.010%, Cr: The hot-rolled narrow strip steel billet is heated and rolled to obtain a hot-rolled steel strip. The heating temperature is 1180~1200℃, and the total heating time is 240~260min. The rolling process includes roughing and finishing, with a total reduction rate of 86%~88% in the roughing process and 91%~93% in the finishing process. The final rolling temperature is 900~920℃. Post-rolling cooling is performed, which includes preheating the bearing surface of the cooling conveyor and controlling the temperature of the hot-rolled steel strip entering the bearing surface of the cooling conveyor to be 650~700℃. The hot-rolled steel strip is coiled at 640~660℃ and placed in an insulated environment to self-temper using residual heat.

[0005] The composition range has been optimized for typical carbon tool steels such as SK5. While ensuring hardenability and wear resistance, the content of impurity elements is strictly controlled. Combined with the controlled rolling and cooling process of this application, it is easier to obtain a high-purity, uniformly structured sorbitic matrix, which is particularly suitable for high-end saw blades, precision springs, and other fields with stringent requirements for base material quality. The above scheme ensures the homogenization of the billet composition and the refinement of austenite grains through specific heating and rolling regimes, laying the microstructure foundation for subsequent phase transformation. By preheating and precisely controlling the temperature of the cooling conveyor bearing surface, the temperature gradient between the upper and lower surfaces of the strip is eliminated, placing the strip within a suitable sorbitic phase transformation temperature range and promoting the uniform transformation of the fine lamellar structure. Combined with coiling within a specific temperature range and self-tempering under residual heat in a holding environment, the latent heat and sensible heat of phase transformation are used to further homogenize the microstructure and adjust the hardness, thereby obtaining hot-rolled narrow strip steel with high sorbitivity and stable performance without increasing alloy costs.

[0006] As one implementation, the step of preheating the bearing surface of the cooling conveyor includes: preheating the hot-rolled steel strip by running it on the bearing surface of the cooling conveyor before it contacts the hot-rolled steel strip. This implementation, by introducing a preheating medium before formal production, increases the enthalpy value of the bearing surface in advance, avoiding a sharp temperature drop on the lower surface of the first or initial strip due to contact with cold equipment. This reduces the difference in cooling rates between the upper and lower surfaces of the strip from the source, ensuring the consistency of microstructure transformation along the entire length.

[0007] As one implementation method, the post-rolling cooling further includes: subjecting the hot-rolled steel strip to forced water cooling to control the temperature of the bearing surface of the hot-rolled steel strip in the cooling conveyor device. Forced water cooling provides sufficient cooling capacity to suppress the precipitation of proeutectoid ferrite in the high-temperature region, forcing the supercooled austenite to undergo pearlite / sorbite transformation at a lower temperature. Simultaneously, by adjusting the forced water flow rate, the heat absorption of the bearing surface and the heat dissipation from the environment can be dynamically balanced, stably maintaining the temperature of the hot-rolled steel strip entering the bearing surface within the target window, ensuring the controllability of phase transformation kinetics.

[0008] As one implementation method, the step of coiling the hot-rolled steel strip at 640–660°C includes coiling the hot-rolled steel strip at 650°C. 650°C is near the lower limit of the optimal temperature range for sorbite transformation. Coiling at this temperature ensures sufficient phase transformation to obtain fine lamellar spacing while retaining enough residual heat for the subsequent self-tempering process. It is the optimal process point for balancing the degree of microstructure refinement and the tempering softening effect.

[0009] As one implementation method, the step of placing the hot-rolled steel strip in an insulated environment to utilize residual heat for self-tempering includes: placing the hot-rolled steel strip in a slow cooling pit for slow cooling, wherein the temperature inside the slow cooling pit is ≥300℃. The slow cooling pit provides a relatively enclosed insulated environment with a certain base temperature, effectively slowing down the heat dissipation rate of the outer layer of the steel coil, making the temperature difference between the inside and outside of the steel coil tend to be uniform; the pit temperature of ≥300℃ ensures the continuous progress of the self-tempering process, promotes the spheroidization or aggregation and growth of carbides, reduces the material hardness, improves machinability, and further eliminates structural stress.

[0010] In one embodiment, the hot-rolled steel strip has a central sorbitization rate of ≥65% and an average pearlite lamellar spacing of ≤280nm. This microstructure characteristic is a direct result of the aforementioned preparation process. The high sorbitization rate and fine lamellar spacing endow the steel with an excellent balance of strength and toughness, enabling it to maintain high strength while possessing good forming potential, thus meeting the stringent requirements of high-end tool manufacturing for substrate performance.

[0011] In addition, this application also provides a hot-rolled narrow strip steel with high sorbitization rate, which is prepared by the preparation method described above. The hot-rolled narrow strip steel has a central sorbitization rate of ≥65% and an average pearlite lamellar spacing of ≤280nm.

[0012] The above-mentioned products have achieved an ideal microstructure through a specific preparation process. The high sorbitization rate in the central region indicates that the microstructure transformation has penetrated deep into the core of the steel strip, overcoming the defect of "inconsistency between the surface and the interior" in traditional processes. The fine pearlite lamellar spacing means a higher interface strengthening effect and better comprehensive mechanical properties, providing a high-quality raw material guarantee for downstream deep processing.

[0013] Beneficial Effects: The technical solution provided in this application solves the technical problems of uneven microstructure and difficult hardness control in high-carbon hot-rolled narrow strip steel by constructing a synergistic process system of "preheating the bearing surface to achieve a uniform temperature difference + strong water quenching and temperature-controlled phase transformation + self-tempering with residual heat from coiling". Specifically, the preheating treatment of the bearing surface of the cooling conveyor eliminates the asymmetric cooling boundary conditions on the upper and lower surfaces of the strip steel from a physical perspective, allowing the supercooled austenite to undergo a consistent thermal history in the thickness direction, thereby obtaining a uniformly distributed sorbite microstructure. The bearing surface temperature is precisely controlled in the range of 650-700℃, taking advantage of the high nucleation rate and moderate growth rate of sorbite in this temperature range, which significantly refines the pearlite lamellar spacing to below 280nm, greatly improving the strength and toughness of the material. The self-tempering at 640-660℃ and placing it in a ≥300℃ holding environment cleverly utilizes the residual heat of rolling to complete the low-temperature heat treatment process, achieving microstructure stabilization and moderate hardness reduction without additional energy consumption, thus balancing high strength and good processing performance. The entire process is fully compatible with existing hot-rolled narrow strip steel production lines, requiring no addition of precious alloying elements or specialized heat treatment equipment, resulting in significant economic benefits and promotional value. Attached Figure Description

[0014] Figure 1 This is a continuous cooling transformation curve of SK5 steel according to an embodiment of this application; Figure 2 This is a metallographic morphology diagram of the edge of the hot-rolled narrow strip steel according to an embodiment of this application; Figure 3 This is a metallographic image of the center of the hot-rolled narrow strip steel according to an embodiment of this application; Figure 4 This is a bar chart comparing the sorbitization rate under different processes in the embodiments of this application. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0016] This embodiment provides a method for improving the sorbitization rate of hot-rolled narrow strip steel, using SK5 carbon tool steel as an example. The SK5 steel strip in this embodiment is produced by billet refining using raw materials with the following chemical composition by weight percentage: C: 0.85%, Si: 0.23%, Mn: 0.41%, P: 0.010%, S: 0.001%, Cr: 0.22%, Ni: 0.05%, Cu: 0.07%. The phase transformation initiation point A1 of pearlite was simulated and determined to be 731℃ using JMatPro material property simulation software (see...). Figure 1 This provides a theoretical basis for formulating and implementing the plan.

[0017] The raw materials are sequentially smelted in an electric furnace, refined in an LF furnace, vacuumed in an RH furnace, and continuously cast to obtain steel billets to be processed. The billet size is 250×300×9800mm. The billet is then sequentially heated, descaled, rolled in a controlled manner, cooled in a controlled manner, and coiled at high temperature to obtain finished steel strip. The pre-obtained finished steel strip has a rolling specification of 2.50×360mm.

[0018] While maintaining the same steelmaking composition and rolling process, three implementation schemes (samples 1-3) and one comparative example (sample 4) were developed by adjusting the process parameters of the upper platen chain temperature for SK5 hot-rolled narrow strip steel. For all four tests, the SK5 hot-rolled steel strip samples were taken from the tail end of the strip. Samples with upper platen chain temperatures of 650℃, 680℃, 700℃, and 720℃ were numbered 1#, 2#, 3#, and 4#, respectively. Samples 1# to 3# had upper platen chain temperatures within the 650–700℃ range specified in this application, while sample 4# was processed using the original process.

[0019] The rolling test process parameters are shown in Table 1.

[0020] Table 1 Rolling Test Process

[0021] The heating temperature was set at 1190℃, and the total heating time was controlled at 250 minutes to ensure the homogeneity of the billet composition. Rolling was carried out under mill load conditions: roughing involved 10 passes with an initial rolling temperature of 1110℃ and a total roughing reduction of 87%; finishing involved 12 passes with a final rolling temperature of 910℃ and a total finishing reduction of 92%.

[0022] Post-rolling controlled cooling employs a forced water quenching method, adjusting the cooling rate to control the temperature of the upper flat chain. Before production, three ordinary Q195 steel strips are used to preheat the flat chain to reduce the temperature drop rate of the lower surface of the steel strip, minimize the temperature difference between the upper and lower surfaces, and ensure uniform temperature across the entire surface of the steel strip.

[0023] After coiling, the steel strip is placed in a slow cooling pit for slow cooling. The temperature inside the pit is ≥300℃. It relies on the residual heat for low-temperature self-tempering, which softens the material and reduces its hardness, thereby improving the uniformity of the final product's structure.

[0024] (1) Metallographic analysis Depend on Figure 2 and Figure 3As shown, the edge and center microstructures of the hot-rolled narrow strip steel SK5 samples at different top plate chain temperatures are pearlite and ferrite. As the top plate chain temperature after rolling continuously decreases (from 720℃ to 650℃), the average lamellar spacing of pearlite continuously decreases, and the lamellae become finer, decreasing from 443nm to 276nm (see Table 2 below), with a significant increase in sorbitization.

[0025] Table 2. Average lamellar spacing of pearlite in each sample at different upper plate chain temperatures.

[0026] (2) Soxhletification rate analysis Depend on Figure 4 It can be seen that the edge of the samples under all four schemes has a higher sorbitization rate than the central region. This is because the cooling rate is faster at the edge of the steel strip than at the center. The lower the temperature, the greater the undercooling, which accelerates the nucleation of ferrite or cementite and inhibits the growth of pearlite lamellar spacing, thus making it easier to obtain a sorbitic structure with finer lamellar spacing. At the same time, the edge sorbitization rate of sample #1 under scheme one (upper plate chain temperature 650℃) is 80% and the central sorbitization rate is 65%, which is significantly improved compared to the edge sorbitization rate of 50% and the central sorbitization rate of 40% under scheme four (original process, upper plate chain temperature 720℃).

[0027] (3) Hardness analysis On average, the hardness of the edges of the same sample was about 11 HV higher than that of the center. For different samples (1# and 4#), the edge hardness at the same location was up to 32 HV higher. This is consistent with the results of metallographic observation of the sorbitization rate, that is, the lower the temperature of the upper plate chain, the higher the sorbitization rate, and the greater the hardness value of the sample. The results are shown in Table 3.

[0028] Table 3. Hardness test results of various samples at different upper plate chain temperatures.

[0029] (4) Mechanical property analysis As shown in Table 4, the tensile strength of sample 1#, which uses an upper plate chain temperature of 650℃, is 90MPa higher than that of sample 4#, which uses an upper plate chain temperature of 720℃ in the original process, and the mechanical properties are significantly improved.

[0030] Table 4. Tensile strength test results of various samples at different upper plate chain temperatures.

[0031] The above test results show that when the upper platen chain temperature is within the range of 650–700℃ as specified in this application, the sorbitization rate, lamellar spacing, hardness, and mechanical properties of hot-rolled narrow strip steel exhibit a regular changing trend. Specifically, the central sorbitization rate of sample #1 (650℃) reached 65%, the average pearlite lamellar spacing was 276 nm, the tensile strength reached 970 MPa, and the average hardness was 302 HV, all showing significant improvement. As the upper platen chain temperature increases, the undercooling gradually decreases, atomic diffusion ability increases, and the growth rate of cementite lamellae relatively increases, leading to a gradual coarsening of the lamellar spacing and a corresponding decrease in the sorbitization rate. However, as long as the upper platen chain temperature is strictly maintained within the range of 650–720℃, the steel strip can still obtain a matrix structure dominated by fine pearlite or sorbite, avoiding abnormally coarse structures caused by temperature runaway. The performance indicators of sample #4 (720℃, existing process) were significantly worse than those of samples #1 to #3, further proving that the range of process parameters defined in this application is not an arbitrarily selected numerical range, but an effective process window determined based on the thermodynamic and kinetic characteristics of material phase transformation.

[0032] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing hot-rolled narrow strip steel with improved sorbititization rate, characterized in that, include: A hot-rolled narrow strip steel billet is provided, wherein the chemical composition of the hot-rolled narrow strip steel billet, by weight percentage, comprises: C: 0.80%–0.95%, Si: 0.20%–0.30%, Mn: 0.35%–0.55%, P≤0.015%, S≤0.010%, Cr: 0.20%–0.25%, Ni: ≤0.20%, Cu: ≤0.20%, with the balance being Fe and unavoidable impurities; The hot-rolled narrow strip steel billet is heated and rolled to obtain hot-rolled steel strip; wherein the heating temperature is 1180~1200℃, the total heating time is 240~260min; the rolling includes roughing and finishing rolling, the total reduction rate of the roughing rolling is 86%~88%, the total reduction rate of the finishing rolling is 91%~93%, and the final rolling temperature is 900~920℃; Post-rolling cooling includes: preheating the bearing surface of the cooling conveyor and controlling the temperature of the hot-rolled steel strip entering the bearing surface of the cooling conveyor to be 650-700°C. The hot-rolled steel strip is coiled at 640–660°C and then placed in an insulated environment to undergo self-tempering using residual heat.

2. The preparation method according to claim 1, characterized in that, The step of preheating the bearing surface of the cooling conveyor includes: preheating the bearing surface of the cooling conveyor by running a preheating steel strip on the bearing surface of the cooling conveyor before the hot-rolled steel strip contacts the bearing surface of the cooling conveyor.

3. The preparation method according to claim 1, characterized in that, The post-rolling cooling also includes: subjecting the hot-rolled steel strip to forced water cooling to control the temperature of the hot-rolled steel strip as it enters the bearing surface of the cooling conveyor device.

4. The preparation method according to claim 1, characterized in that, The step of winding the hot-rolled steel strip at 640-660°C includes winding the hot-rolled steel strip at 650°C.

5. The preparation method according to claim 4, characterized in that, The step of placing the hot-rolled steel strip in an insulated environment to utilize residual heat for self-tempering includes: placing the hot-rolled steel strip in a slow cooling pit for slow cooling, wherein the temperature in the slow cooling pit is ≥300℃.

6. The preparation method according to any one of claims 1 to 6, characterized in that, The hot-rolled steel strip has a central sorbitization rate of ≥65% and an average pearlite lamellar spacing of ≤280nm.

7. A hot-rolled narrow strip steel with high sorbitivity, characterized in that, The hot-rolled narrow strip steel is prepared by any one of the preparation methods described in claims 1 to 7, wherein the central sorbitization rate of the microstructure is ≥65% and the average pearlite lamellar spacing is ≤280nm.

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