Production method of free-cutting normalizing-free fine grain steel for diesel engine camshaft

Abstract

The invention relates to a production method of free-cutting normalizing-free fine grain steel for a diesel engine camshaft, which comprises the following steps of: sequentially performing converter smelting, LF (ladle furnace) refining, RH (Ruhrstahl Heraeus) vacuum degassing, soft blowing and micro-denaturation treatment, square billet continuous casting, heating, rolling and finishing on follow-up smelting raw materials through innovative design of chemical components, and combining innovation of smelting and rolling processes to realize normalizing-free, high-quality and high-quality free-cutting normalizing-free fine grain steel. Manufacturing cost is reduced, carbon emission in the process is reduced, and production efficiency is improved.

Description

Technical Field

[0001] This invention relates to a method for producing free-machining, non-normalizing, fine-grained steel for diesel engine camshafts, belonging to the field of special steel and its smelting technology. Background Technology

[0002] Currently, domestic diesel engines mainly use medium carbon steel to manufacture camshafts. The conventional camshaft steel production process includes: selecting medium carbon steel material suitable for camshaft manufacturing → selecting a suitable normalizing furnace → placing the camshaft in the normalizing furnace → maintaining the temperature for a certain period of time after the camshaft reaches the normalizing temperature → removing the camshaft from the normalizing furnace and cooling it after the temperature maintenance is completed.

[0003] In the existing technology, the production of steel for camshafts requires normalizing followed by machining, heat treatment, and surface nitriding. Offline normalizing is a common metal heat treatment process for homogenizing the microstructure, which creates conditions for subsequent machining and heat treatment.

[0004] However, the normalizing process typically requires transferring the workpiece from the production line to a dedicated normalizing furnace for heating and cooling. The heating process in the normalizing furnace consumes a significant amount of energy, including electricity and natural gas. This not only increases production costs but also places a substantial energy burden on the environment. Furthermore, the process involves multiple steps such as loading and unloading, transportation, heating, and cooling, which are time-consuming and reduce production efficiency. Simultaneously, during offline normalizing, factors such as the shape, size, and placement of the workpiece make it difficult to ensure temperature uniformity throughout the heating and cooling process. Uneven temperature can lead to inhomogeneous microstructure, affecting product quality and performance. Therefore, it is necessary to provide a production process for diesel engine camshaft steel that eliminates the need for normalizing, thereby reducing manufacturing costs, carbon emissions, and improving production efficiency while ensuring steel quality. Summary of the Invention

[0005] This invention provides a method for producing free-machining, non-normalizing fine-grained steel for diesel engine camshafts. Through innovative design of chemical composition and innovation of smelting and rolling processes, normalizing is eliminated, reducing manufacturing costs, carbon emissions, and improving production efficiency.

[0006] The technical solution adopted by this invention to solve its technical problem is: A free-machining, non-normalizing, fine-grained steel for diesel engine camshafts comprises the following chemical element composition by mass percentage: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, P≤0.015%, S: 0.040%-0.060%, Cr: 0.10%-0.15%, Al: 0.040%-0.050%, Cu≤0.20%, Ni≤0.05%, Mo≤0.05%, [O]≤15ppm, with the balance being Fe. The production method of free-machining, non-normalizing, fine-grained steel for diesel engine camshafts specifically includes the following steps: Step S1: Select sulfur-controlled and low-phosphorus molten iron, and after pretreatment, transport it to a top-and-bottom blown converter for smelting. During the smelting process, a secondary slag-forming method is used for dephosphorization. During the tapping process, pre-melted refining slag, lime and quartz sand are added for slag-forming. Aluminum cake is added for pre-deoxidation. At the same time, silicon-manganese alloy and high-carbon ferrochrome are added to initially adjust the composition of the molten iron until all elements in the molten iron reach the preset range when it is refined to the station. A sliding plate slag deflector is used to control slag discharge, with the phosphorus return rate of the slag after steelmaking set to ≤0.001%. Step S2: LF refining process, during which silicon carbide, calcium carbide, and aluminum particles are added for slag surface deoxidation, while controlling the Al content in the deoxidation process to be within the range of 0.060%-0.080%; The composition of the refining slag was adjusted, and its basicity R was set to 3.5-5.0. The sulfur content of the molten steel during the refining process was controlled within the range of 0.050%-0.060%. Lime and pre-melted refining slag are used in proportion according to the degree of steel peroxidation in order to adjust the composition of the molten steel to the preset value before the end of LF refining. Step S3: RH refining process, set the vacuum degree in the RH furnace to ≤50Pa, the high vacuum holding time to ≥25min, and the vacuum breaking [H] in the molten steel to ≤1.2ppm; Step S4: Soft blowing and micro-modification treatment. Before soft blowing, silicon calcium wire is fed in through a wire feeder for micro-calcium modification treatment. After feeding the wire, soft blowing is turned on to collect and remove the inclusions. Step S5: In the billet continuous casting process, an arc-shaped billet continuous casting machine is used to implement multi-flow constant speed casting, and the superheat is controlled at 25℃-35℃. The continuous casting process is carried out with full protection casting production. A two-stage electromagnetic stirring device combining M-EMS and F-EMS is used, and light pressure is applied at the end of the liquid core solidification at the straightening machine. Step S6: Rolling process. After the continuous casting billet produced in step S5 is cooled to room temperature, it is cold-loaded and then heated at high temperature in a regenerative walking beam furnace. After high-temperature heating, several continuous rolling mills are combined with KOCKS rolling mills for high-precision rolling. After rolling, it is cooled by water and the temperature range after water cooling is preset. Step S7: The steel rolled in step S6 is sequentially subjected to straightening, shot blasting, chamfering, ultrasonic testing, and surface testing for finishing. Furthermore, in step S1, the selected sulfur-controlled and low-phosphorus molten iron contains the following chemical elemental composition by mass percentage: P ≤ 0.012%, S: 0.050%-0.070%; Furthermore, in step S1, during smelting in a top-and-bottom blown converter, the molten iron temperature is controlled to be ≥1300℃, and a secondary slag-forming method is used until the converter endpoint P≤0.012%; During slag formation, add 200 kg of pre-melted refining slag, 400 kg of lime, and 50 kg of quartz sand. Upon refining, the molten iron contains the following chemical element composition by mass percentage: C: 0.35%-0.40%, Si: 0.05%-0.10%, Mn: 0.65%-0.72%, Cr: 0.05%-0.10%; Furthermore, in step S2, before the LF refining is completed, the chemical element composition of the molten steel is adjusted to include the following mass percentages: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, S: 0.040%-0.055%, Cr: 0.10%-0.15%, Al: 0.050%-0.060%; Furthermore, in step S4, before soft blowing, 50m of silicon-calcium wire is fed in through a wire feeder, and after feeding the wire, soft blowing is started and the soft blowing time is set to 25min-30min. Furthermore, in step S5, a 210mm×210mm arc-shaped square billet continuous casting machine is used, the multi-flow casting range is set to 4-5 flows, and the casting speed is constant at 1.2m / min; The implementation of full-process, fully protected casting production in continuous casting specifically includes setting a cooling water flow rate of 135m³. 3 / h, while using low-silicon intermediate ladle covering agent and high-sulfur steel special crystallizer protective slag; At the end of the liquid core solidification at the tension leveler, apply light pressure with a pressing height set to 5-7mm; Furthermore, in step S6, the diffusion is carried out in a regenerative walking beam furnace at a high temperature of 1200℃-1230℃ for 2.5-3 hours. For high-precision rolling, an 18-stand continuous rolling mill and 4 KOCKS rolling mills are used. The preset temperature range after water cooling is 880℃-920℃.

[0007] By employing the above technical solutions, the present invention has the following beneficial effects compared to the prior art: 1. The diesel engine camshaft free-machining non-normalizing fine-grained steel provided by the present invention refines the grain by optimizing the chemical composition and adding a certain amount of Cr and Al elements, while adjusting the content of S elements to combine with Mn elements in the steel to improve its machinability, thereby improving the surface quality of the camshaft. 2. The production method of free-machining, non-normalizing fine-grained steel for diesel engine camshafts provided by the present invention adopts a light-reduction continuous casting process to further improve low-magnification quality and reduce element and microstructure segregation; the rolling process adopts high-temperature heating and diffusion to reduce microstructure segregation; thus achieving a uniform microstructure. 3. The production method of free-cutting, non-normalizing fine-grained steel for diesel engine camshafts provided by the present invention involves water cooling after rolling, replacing the normalizing process. This achieves a uniform microstructure by controlling the phase transformation process, without affecting the matrix microstructure. Attached Figure Description

[0008] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0009] Figure 1 This is a flowchart of the production method of free-machining, non-normalizing fine-grained steel for diesel engine camshafts provided by the present invention. Detailed Implementation

[0010] The present invention will now be described in further detail with reference to the accompanying drawings. The specific dimensions used in this embodiment are merely illustrative and do not limit the scope of protection of the present invention.

[0011] As noted in the background section, domestic diesel engines currently primarily use medium carbon steel to manufacture camshafts, which require normalizing followed by machining, heat treatment, and surface nitriding. This results in high energy consumption, high costs, and long production cycles, while also causing quality control issues due to difficulties in ensuring temperature uniformity.

[0012] To address the aforementioned issues, this application provides a free-machining, non-normalizing fine-grained steel for diesel engine camshafts and its production method. First, it innovatively designs the chemical composition; second, it improves the smelting and rolling processes to achieve non-normalizing, reduce manufacturing costs, decrease carbon emissions, and improve production efficiency.

[0013] Regarding the design of the chemical composition, the free-machining, non-normalizing fine-grained steel for diesel engine camshafts provided in this application comprises the following chemical element composition by mass percentage: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, P≤0.015%, S: 0.040%-0.060%, Cr: 0.10%-0.15%, Al: 0.040%-0.050%, Cu≤0.20%, Ni≤0.05%, Mo≤0.05%, [O]≤15ppm, with the balance being Fe.

[0014] Among the aforementioned chemical elements, the addition of Cr and Al differs significantly from existing technologies. This difference is a highlight of the composition design. Cr can significantly improve the strength, hardness, wear resistance, and hardenability of steel. However, excessive Cr content will increase the brittle transition temperature and temper brittleness of steel. In this application, to improve the strength and hardenability of steel, the Cr content is controlled between 0.10% and 0.15%. Al is often used as a deoxidizer in steel. When the Al content is greater than 0.025%, it also plays a role in refining the grain size as the Al content increases. In this application, to fully refine the grain size of steel, water quenching during rolling is used instead of normalizing, and the Al content is limited to 0.040%-0.050%.

[0015] Another innovation in the chemical composition design is the limitation of the sulfur (S) content. S is a segregating element that reduces the plasticity and toughness of steel and also has an adverse effect on corrosion resistance. However, the steel grade in this application requires turning, which has very high requirements for cutting performance and surface finish. The added S element is needed to improve the cutting performance of the steel. Therefore, the S content in this application is strictly controlled, limited to 0.040%-0.060%. As for manganese (Mn), it can improve the strength, hardness, and hardenability of steel, and also has a solid solution strengthening effect. It can expand the austenite region, lower the transformation temperature of austenite to ferrite, and thus refine the ferrite grains and improve the strength and toughness of steel. However, too high a Mn content can easily cause segregation and increase the temper brittleness of steel. Therefore, the Mn content is controlled at 0.70%-0.75%. S and Mn form MnS inclusions in the steel. Through modification technology, the MnS inclusions are finely dispersed and spindle-shaped, which is beneficial for cutting. Therefore, S element can improve the cutting performance.

[0016] The aforementioned chemical composition also includes carbon (C), the most effective element for improving steel strength and hardenability. However, increasing the C content significantly reduces the steel's plasticity and impact toughness; C also increases the steel's cold brittleness and aging sensitivity. Considering the strength of the camshaft, this application limits the C content to between 0.44% and 0.46% based on existing 45 steel. Si acts as a reducing agent and deoxidizer in steel; killed steel generally contains more than 0.17% Si. Si can dissolve in ferrite and austenite to improve the steel's strength and hardness. This application adds 0.10%-0.15% Si primarily for deoxidation, while avoiding the impact of high Si on the nitriding layer depth on the camshaft surface. P (P) causes cold brittleness in steel, reduces its toughness and plasticity, and is a harmful element; therefore, the P content is strictly controlled to ≤0.015%. Cu is generally considered a harmful element in steel. When the Cu content exceeds 0.20% and the steel temperature exceeds 1100℃, oxidation and decarburization occur on the steel surface, which easily accumulates at the grain boundaries of the decarburized layer, leading to copper embrittlement and surface cracking, severely affecting the quality of the steel. Therefore, this application controls the Cu content in the smelting process to ≤0.20%. Ni can improve the strength of steel while maintaining good plasticity and toughness. It can also lower the brittle transition temperature of steel and improve the low-temperature impact toughness of steel. However, since nickel is a scarce resource and expensive, this steel does not add Ni, controlling Ni to ≤0.05%. Mo is a highly hardenable element that has a solid solution strengthening effect on ferrite, significantly improving the hardenability of steel. However, Mo is a precious metal, and this application does not specifically add Mo, controlling the residual Mo to ≤0.05%. Finally, [O] is controlled. Since O reduces the elongation and reduction of area of ​​steel, at lower temperatures and extremely low O content, the strength and plasticity of the material decrease sharply with the increase of O content. As the O content increases, the maximum impact value gradually decreases, while the brittle transition temperature increases rapidly, and the range of the brittle transition temperature also widens. At the same time, as the O content increases, the probability of oxide inclusions in the material increases significantly, thereby reducing the fatigue life of the material. Therefore, this application requires that the O content be stably controlled within 0.0015%.

[0017] Preferably, the free-machining, non-normalizing fine-grained steel for diesel engine camshafts comprises the following chemical element composition by mass percentage: C: 0.45%, Si: 0.12%, Mn: 0.72%, P: 0.012%, S: 0.050%, Cr: 0.12%, Al: 0.045%, Cu: 0.20%, Ni: 0.01%, Mo: 0.01%, [O]: 10ppm, with the balance being Fe.

[0018] Based on the chemical element composition of the designed camshaft steel, this application provides a method for producing free-machining, non-normalizing, fine-grained steel for diesel engine camshafts. Figure 1As shown, the smelting raw materials undergo the following processes in sequence: converter smelting, LF refining, RH vacuum degassing, soft blowing and micro-modification treatment, billet continuous casting (light reduction), heating, rolling, and finishing.

[0019] Specifically, the following steps are included: Step S1: Select molten iron with controlled sulfur and low phosphorus content (P≤0.012%, S: 0.050%-0.070%), and after pretreatment, transport it to a top-and-bottom blown converter with a capacity of 90 tons or more for smelting, controlling the molten iron temperature to ≥1300℃; during the smelting process, a secondary slag-forming method is used for dephosphorization treatment to ensure that P≤0.012% at the converter endpoint; during the tapping process, 200kg of pre-melted refining slag, 400kg of lime, and 50kg of quartz sand are added for slag formation, aluminum cake is added for pre-deoxidation, and silicon-manganese alloy and high-carbon ferrochrome are added to initially adjust the composition of the molten steel. When the steel arrives at the refining station, the molten iron contains the following chemical element composition by mass percentage: C: 0.35%-0.40%, Si: 0.05%-0.10%, Mn: 0.65%-0.72%, Cr: 0.05%-0.10%; The slag discharge is controlled by a sliding plate slag deflector, and the phosphorus return rate of the slag after steelmaking is set to ≤0.001%.

[0020] Step S2: LF refining process. During the refining process, silicon carbide, calcium carbide, and aluminum particles are added to deoxidize the slag surface. These deoxidizers combine with the oxygen in the slag, thereby reducing the oxygen content in the molten steel. At the same time, the Al content is controlled within the range of 0.060%-0.080% during the deoxidation process to ensure the deoxidation effect of the molten steel. The composition of the refining slag is adjusted, and its basicity R is set to 3.5-5.0. High basicity refining slag is conducive to the desulfurization reaction and reduces the sulfur content by adsorbing sulfur in the steel. The sulfur content of the molten steel is controlled within the range of 0.050%-0.060% during the refining process, and the sulfur content in the molten steel is controlled within a certain range. Lime and pre-melted refining slag are used in proportion according to the degree of over-oxidation of the molten steel. In addition to their roles in slag formation and deoxidation, lime and pre-melted refining slag may also participate in the alloying process to adjust the composition of the molten steel. Before the end of LF refining, the chemical element composition of the molten steel is adjusted to include the following mass percentages: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, S: 0.040%-0.055%, Cr: 0.10%-0.15%, Al: 0.050%-0.060%.

[0021] Step S3: RH refining process, vacuum degassing is carried out in an RH furnace of 100 tons or more. There must be no cold steel in the vacuum chamber. The vacuum degree in the RH furnace is set to ≤50Pa, and the high vacuum holding time is ≥25min to ensure that the molten steel void [H] is ≤1.2ppm.

[0022] Step S4: Soft blowing and micro-modification treatment. Before soft blowing, 50m of silicon-calcium wire is fed in through a wire feeder for micro-calcium modification treatment. After feeding the wire, soft blowing is started and the soft blowing time is set to 25min-30min. Soft blowing requires the slag surface to move slightly and the molten steel to be not exposed, so as to ensure that the inclusions are quickly gathered and floated to the surface for removal.

[0023] During steelmaking, center segregation leads to uneven chemical composition in the central part of the steel, easily forming stress concentration points, which can then cause defects such as cracks during subsequent processing and use. Improving center segregation can reduce this risk and enhance the strength and toughness of the material. Therefore, in step S5, the billet continuous casting process, a 210mm×210mm arc-shaped billet continuous casting machine is used to implement multi-flow constant-speed casting. Preferably, the multi-flow casting range is set to 4-5 flows, and the casting speed is kept constant at 1.2m / min; the superheat is controlled at 25℃-35℃; and the cooling water flow rate is set to 135m³. 3 / h, while using low-silicon tundish covering agent and special crystallizer protective slag for high-sulfur steel, continuous casting is carried out with full protection casting production; a two-stage electromagnetic stirring device combining M-EMS and F-EMS is used to fully homogenize the structure and composition and improve the internal quality; and light pressure is used at the end of the liquid core solidification at the straightening machine, with the pressure height set at 5-7mm, to further improve the low magnification quality and reduce element and structure segregation.

[0024] Step S6: Rolling process. After cooling the continuously cast square billet produced in step S5 to room temperature, it is cold-loaded and heated in a regenerative walking beam furnace at a high temperature of 1200℃-1230℃ for 2.5-3 hours to further reduce microstructure segregation. After high-temperature heating, it is rolled with high precision using several continuous rolling mills in combination with KOCKS mills. Preferably, 18 continuous rolling mills and 4 KOCKS mills are used. After rolling, it is cooled by water to replace normalizing and homogenize the microstructure. The temperature range after water cooling is 880℃-920℃.

[0025] This is the most significant innovation of this application. Normalizing involves heating the steel to a certain temperature and then cooling it in air, while water cooling, by controlling the water flow rate and cooling time, can achieve a specific cooling rate. After water cooling, the temperature is maintained between 880℃ and 920℃. This temperature range and cooling rate allow for a transformation of the steel's internal structure, promoting microstructure homogenization. During water cooling, a phase transformation occurs as the steel rapidly cools from a high temperature. This phase transformation helps eliminate the inhomogeneous structure generated during rolling, allowing grains to recrystallize and grow, thus achieving a homogeneous structure. Simultaneously, the cooled microstructure is more stable, similar to the microstructure after normalizing, thereby achieving the goal of replacing normalizing to achieve a homogeneous structure.

[0026] Step S7: The steel rolled in step S6 is sequentially straightened, shot blasted, chamfered, ultrasonically tested, and surface tested to finish the steel and ensure its internal and external quality.

[0027] The present application provides Examples 1, 2, and 3, as well as a comparative example. The comparative example uses existing 45 steel. The chemical composition of the camshaft steel used in Examples 1, 2, and 3 is shown in Table 1. Table 1. Composition of steel for camshafts (wt%)

[0028] Table 2 shows a comparison of the gas content of Examples 1, 2, and 3, as well as the comparative examples. Table 2. Comparison of gas content in Examples 1, 2, and 3 with existing technologies.

[0029] Table 3 shows a comparison of the mechanical properties and surface hardening hardness of Examples 1, 2, 3, and the comparative examples. Table 3. Comparison of mechanical properties and surface hardening hardness of Examples 1, 2, and 3 with existing technologies.

[0030] Table 4 shows a comparison of the low-magnification tissues of Examples 1, 2, and 3, as well as the comparative examples. Table 4. Comparison of low-magnification tissue samples from Examples 1, 2, and 3 with existing technologies.

[0031] Table 5 Comparison of non-metallic inclusions in Examples 1, 2, and 3 with existing technologies

[0032] As shown in Tables 1, 2, 3, 4, and 5, the production method provided in this application for Examples 1, 2, and 3 shows that the D-type inclusions are stably controlled at ≤1.0 level, the B-type inclusions are stably controlled at 0 level, and the C-type and Ds-type inclusions are not detected. The process control level of this application has reached the international advanced level.

[0033] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0034] The meaning of "and / or" as used in this application includes situations where each exists alone or both exist simultaneously.

[0035] The term "connection" as used in this application can mean a direct connection between components or an indirect connection between components through other components.

[0036] 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 free-cutting normalizing-free fine-grain steel for a diesel engine camshaft, characterized in that: The chemical element composition includes the following mass percentages: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, P≤0.015%, S: 0.040%-0.060%, Cr: 0.10%-0.15%, Al: 0.040%-0.050%, Cu≤0.20%, Ni≤0.05%, Mo≤0.05%, [O]≤15ppm, and the balance is Fe.

2. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 1, characterized in that: The specific steps include: Step S1: Select sulfur-controlled and low-phosphorus molten iron, and after pretreatment, transport it to a top-bottom double-blown converter for smelting. During the smelting process, a secondary slag making method is used for dephosphorization. During the steelmaking process, pre-melted refined slag, lime and quartz sand are added for slag making, and aluminum cakes are added for pre-deoxidation. At the same time, silicon-manganese alloy and high-carbon ferrochrome are added to initially adjust the composition of the molten steel until the elements in the molten iron reach the preset range when the steel is refined to the station; The sliding plate slag stopper is used to control the slag discharge, and the phosphorus recovery amount of the slag discharge is set to ≤0.001%; Step S2: LF refining process, during which silicon carbide, calcium carbide and aluminum particles are added to deoxidize the slag surface, and the Al content is controlled to be in the range of 0.060%-0.080% during the deoxidation process; Adjust the composition of the refined slag, set its basicity R to 3.5-5.0, and control the S content of molten steel in the refining process to 0.050%-0.060%; Lime and pre-melted refining slag are used in proportion according to the degree of peroxidation of the molten steel to adjust the composition of the molten steel to the preset value before the end of LF refining; Step S3: RH refining process, setting the vacuum degree in the RH furnace ≤ 50Pa, the high vacuum holding time ≥ 25min, and the molten steel air breakage [H] ≤ 1.2ppm; Step S4: soft blowing and micro-modification treatment. Before soft blowing, calcium silicon wire is fed into the wire feeder for micro-calcium modification treatment. After feeding the wire, soft blowing is started to gather and float the inclusions for removal. Step S5: billet continuous casting process, using an arc-shaped billet continuous casting machine to implement multi-stream constant-speed casting, controlling the superheat to 25°C-35°C, and implementing full-process full-protection casting production; using M-EMS combined with F-EMS two-stage electromagnetic stirring device, and using light pressure at the end of liquid core solidification at the straightening machine; Step S6: rolling process, cooling the continuous casting billet produced in step S5 to room temperature and then cold loading, and then heating it at high temperature in a regenerative walking beam heating furnace; after high temperature heating, a plurality of continuous rolling units are used in combination with a KOCKS rolling mill for high-precision rolling, and after rolling, water cooling is performed, and the temperature range after water cooling is preset; Step S7: The steel material after rolling in step S6 is finished by straightening, shot blasting, chamfering, ultrasonic flaw detection and surface flaw detection.

3. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S1, the selected sulfur-controlled and low-phosphorus molten iron includes the following chemical element components in mass percentage: P≤0.012%, S: 0.050%-0.070%.

4. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S1, when smelting in a top and bottom double-blown converter, the molten iron temperature is controlled to be ≥1300°C, and a secondary slag making method is used until the converter end point P is ≤0.012%; When making slag, add 200kg pre-melted refined slag, 400kg lime and 50kg quartz sand; When refined to the station, the molten iron includes the following chemical element compositions in mass percentage: C: 0.35%-0.40%, Si: 0.05%-0.10%, Mn: 0.65%-0.72%, Cr: 0.05%-0.10%.

5. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S2, before the end of LF refining, the chemical element composition in the molten steel is adjusted to include the following mass percentages: C: 0.44%-0.46%, Si: 0.10%-0.15%, Mn: 0.70%-0.75%, S: 0.040%-0.055%, Cr: 0.10%-0.15%, Al: 0.050%-0.060%.

6. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S4, 50 m of calcium silicon wire is fed through a wire feeder before soft blowing, and soft blowing is started after wire feeding, and the soft blowing time is set to 25 min-30 min.

7. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S5, a 210 mm × 210 mm arc-shaped billet continuous casting machine is used, the multi-stream casting range is set to 4-5 streams, and the casting speed is constant at 1.2 m / min; The continuous casting process is implemented with full protection casting production, specifically including setting a cooling water volume of 135m 3 / h, and adopt low-silicon tundish covering agent and high-sulfur steel special mold protection slag; At the end of the liquid core solidification at the tension leveler, light pressure is used, and the pressure height is set to 5-7mm.

8. The method for producing free-cutting normalizing-free fine-grained steel for diesel engine camshaft according to claim 2, characterized in that: In step S6, the mixture is heated and diffused at 1200° C. to 1230° C. in a regenerative step-beam heating furnace for 2.5 to 3 hours. For high-precision rolling, 18 continuous rolling mills and 4 KOCKS rolling mills are used; The preset temperature range after water cooling is 880℃-920℃.

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