Production method of steel 38MnS6 for non-quenched and tempered crankshaft

Abstract

The invention relates to the technical field of steel production, and discloses a production method of non-quenched and tempered crankshaft steel 38MnS6, which comprises the following steps: S1, smelting treatment; s2, continuous casting treatment; s3, rolling treatment is conducted; s4, detecting the quality of a real product; the production method of the steel 38MnS6 for the non-quenched and tempered crankshaft comprises the following steps: adding ferro-aluminum and feeding an aluminum wire behind a furnace along a steel flow in a tapping process for precipitation and deoxidation; in the later tapping stage, slag infrared ray automatic detection and sliding plate slag stopping are adopted, and the thickness of steel ladle top slag is controlled to be smaller than or equal to 20 mm; whole-course bottom argon blowing is carried out behind the furnace so as to promote alloy melting and floating removal of deoxidized products; through common design of smelting, continuous casting and rolling, sulfide distribution in steel is controlled, sulfide is prevented from being distributed in a net shape along the grain boundary, and therefore the product quality is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of steel processing, and specifically provides a production method of 38MnS6 steel for non-quenched crankshafts. Background Art

[0002] The crankshaft of a passenger car engine is one of the most important components in an automobile engine and the main carrier of the power output of the automobile. It is subjected to bending and torsional loads during operation, and the crankshaft is required to have sufficient fatigue strength and impact toughness. The 38MnS6 steel product is currently widely used in the manufacture of crankshafts for passenger car engines;

[0003] 38MnS6 is a non-quenched steel for forging. For cost considerations, the composition design of this steel grade does not add fine-grained strengthening alloy elements with high costs, but instead adds relatively high Mn and S contents. Previous research results have shown that adding sulfur can prevent the growth of austenite grains. Under the same conditions, the austenite grain size of the steel containing 0.068% sulfur is 15-20 μm smaller than that of the steel containing 0.010% sulfur; compared with the steel containing 0.010% sulfur, the proeutectoid ferrite nucleation rate of the steel containing 0.068% sulfur is high, the growth rate is low, and the amount of proeutectoid ferrite is relatively large. Therefore, the steel containing 0.068% sulfur can obtain a fine and dispersed ferrite + pearlite structure. Since the solubility of S in steel is extremely small, it generally exists in the form of compounds such as iron sulfide and manganese sulfide in the steel, and the eutectics of many sulfides such as Fe-FeS and FeS-FeO have very low melting points. Therefore, when the processing temperature of the steel product exceeds the eutectic temperature of the sulfide in the steel, the brittleness of the steel product will increase sharply and the plasticity will decrease, resulting in cracking during forging and rolling of the steel product; however, the influence of sulfur on the processing performance of the steel product is closely related to the manganese content. Because the affinity of manganese for sulfur is stronger than that of iron and manganese sulfide is formed to replace iron sulfide, when the Mn / S ratio is greater than 3, the steel product has good processing performance and will not produce hot brittleness, and the melting point of manganese sulfide is relatively high and it is distributed inside the grains. Therefore, in order to avoid grain growth during the forging heating of 38MnS6, further improve the processing performance of the product, and at the same time ensure that the processed product has better fatigue strength and impact toughness, it is necessary to design appropriate internal control chemical compositions, and appropriate process measures should also be taken during the production process to control the distribution of sulfides in the steel and avoid the formation of a network of sulfides along the grain boundaries; in view of this, we propose a production method of 38MnS6 steel for non-quenched crankshafts. Summary of the Invention

[0004] The purpose of the present invention is to provide a production method of 38MnS6 steel for non-quenched crankshafts to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A production method of 38MnS6 steel for non-quenched crankshafts includes the following steps:

[0006] S1, Smelting treatment;

[0007] S2, Continuous casting treatment;

[0008] S3, Rolling treatment;

[0009] S4, Physical quality inspection of products.

[0010] Optionally, the S1 includes:

[0011] The end-point composition of the converter is controlled with P ≤ 0.012% and C ≥ 0.08%, and the tapping temperature ≥ 1620°C;

[0012] During the tapping process, the alloy addition amount is accurately calculated according to the ladle furnace arrival composition requirements for batching addition, and about 500 kg of lime and 300 kg of refining synthetic slag are added for slag washing;

[0013] During the tapping process, about 300 kg of ferrosilicon aluminum is added along the steel flow and 200 m of aluminum wire is fed behind the furnace for precipitation deoxidation;

[0014] In the later stage of tapping, infrared automatic detection of slag entrainment and slide gate slag blocking are adopted to control the thickness of the top slag in the ladle ≤ 20 mm; bottom blowing argon is carried out throughout the process behind the furnace to promote the melting of the alloy and the floating and removal of deoxidation products.

[0015] Optionally, the S1 further includes:

[0016] During the LF furnace refining process, silicon carbide and aluminum pellets are used for diffusion deoxidation;

[0017] The holding time of the refining white slag is controlled at about 35 min, the total slag amount is controlled above 12 kg / ton of steel, nitrogen alloy wire is fed after the molten steel temperature reaches 1590°C to increase nitrogen, the composition is fine-tuned according to the target value after the refining slag becomes white slag, a small amount of silica sand is added in the later stage of refining to adjust the basicity of the refining slag, the basicity control target is 4.0, and 0.4 - 0.6 m / t of steel of calcium wire is fed after refining for inclusion modification treatment, and soft blowing is carried out for 6 - 8 min after the calcium wire is fed and then sulfur wire is fed to increase sulfur.

[0018] The vacuum treatment uses an RH furnace to ensure that the vacuum time is controlled above 12 min to ensure that the H content in the steel is controlled below 1.5×10-6. Since a certain amount of nitrogen content will be removed during the vacuum treatment process, nitrogen is added by circulating nitrogen blowing during the vacuum treatment process to ensure the relative stability of the nitrogen content in the steel. After breaking the vacuum, the soft blowing argon time is controlled ≥ 20 min.

[0019] Optionally, the S2 includes:

[0020] For the continuous casting of 38MnS6 test production, a relatively high specific water volume of 0.35 L / kg is adopted for secondary cooling and steel casting is carried out with a constant casting speed of 0.80 m / min;

[0021] The superheat of molten steel in the tundish is controlled at 18 - 28 °C, and the temperature drop in the tundish is controlled within 10 °C.

[0022] During the casting process, the liquid level in the mold and the stopper rod stroke are stably controlled, and the liquid level fluctuation is controlled within ±5 mm. The mold uses electromagnetic stirring parameters of 250 A and 2.5 Hz to promote the floating of inclusions, increase the equiaxed crystal ratio of the continuous casting billet, reduce central segregation and porosity, and improve the surface quality at the same time.

[0023] Optionally, the step S3 further includes:

[0024] The temperature in the preheating section of the rolling heating furnace is controlled below 680 °C, the soaking section temperature is controlled at 1180 - 1210 °C for high-temperature diffusion, the total residence time in the furnace is controlled at 200 - 300 min, the high-pressure water descaling pressure is controlled at 28 MPa, and there is no residual scale on the surface after descaling.

[0025] Optionally, the step S4 further includes:

[0026] S41, detection of composition and gas content; the composition control in the smelting production process is stable, and the composition detection of the tundish melting meets the internal control requirements. The detected gas N content in the rolled finished product is (121 - 125)×10 -6 , the H content is 1.1×10 -6 , the total oxygen content is (7 - 8)×10 -6 ;

[0027] S42, macrostructure detection; after pickling the cross-section of the rolled finished round steel, there shall be no visible pores, shrinkage cavities, internal cracks, inclusions and other defects to the naked eye;

[0028] S43, analysis and detection of non-metallic inclusions; the rolled round steel product is detected and analyzed for non-metallic inclusions. By scanning electron microscopy, the composition of type A inclusions is all MnS and a very small amount of FeS;

[0029] S44, grain size detection; the grain size is the size of austenite grain. The finer the grain, the higher the strength of the steel, and the better the plasticity and toughness;

[0030] S45, banded structure detection; the presence of banded structure defects in steel will make the mechanical properties of the metal anisotropic, and the direction parallel to the banded structure is significantly better than its perpendicular direction.

[0031] Compared with the prior art, the present invention provides a production method of non-quenched and tempered steel 38MnS6 for crankshafts, having the following beneficial effects:

[0032] The production method of non - quenched and tempered steel 38MnS6 for crankshafts involves adding ferrotitanium along the steel flow during tapping and feeding aluminum wire after the furnace to conduct precipitation deoxidation; during the late stage of tapping, infrared automatic detection of slag falling and slide gate slag blocking are used to control the thickness of the ladle top slag ≤ 20 mm; argon bottom - blowing is carried out throughout the process after the furnace to promote the melting of alloys and the floating and removal of deoxidation products; through the joint design of smelting, continuous casting, and rolling, the distribution of sulfides in the steel is controlled to avoid the net - shaped distribution of sulfides along the grain boundaries, thus ensuring product quality. Brief Description of the Drawings

[0033] Figure 1 It is a schematic flow diagram of the present invention. Detailed Embodiments

[0034] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

[0035] As Figure 1 shown, the present invention provides a technical solution: a production method of non - quenched and tempered steel 38MnS6 for crankshafts, including the following steps:

[0036] S1. Smelting treatment;

[0037] In the converter, control the end - point composition P ≤ 0.012%, C ≥ 0.08%, and the tapping temperature ≥ 1620 °C; during tapping, accurately calculate the alloy addition amount according to the ladle furnace arrival - station composition requirements for batching and adding, and add about 500 kg of lime and 300 kg of refined synthetic slag for slag washing; during tapping, add about 300 kg of ferrotitanium along the steel flow and feed 200 m of aluminum wire after the furnace to conduct precipitation deoxidation; during the late stage of tapping, use infrared automatic detection of slag falling and slide gate slag blocking to control the thickness of the ladle top slag ≤ 20 mm; argon bottom - blowing is carried out throughout the process after the furnace to promote the melting of alloys and the floating and removal of deoxidation products.

[0038] During the refining process of the LF furnace, mainly use silicon carbide and a small amount of aluminum grains for diffusion deoxidation; control the holding time of the refined white slag at about 35 min, control the total slag amount above 12 kg / ton of steel, feed nitrogen alloy wire to increase nitrogen after the molten steel temperature reaches 1590 °C, fine - tune the composition according to the target value after the refined slag becomes white slag, add a small amount of silica sand during the late stage of refining to adjust the basicity of the refined slag, and control the target basicity at 4.0; after refining is completed, feed 0.4 - 0.6 m / t of steel with calcium wire for inclusion modification treatment, and perform soft blowing for 6 - 8 min after feeding the calcium wire and then feed sulfur wire to increase sulfur.

[0039] RH furnace is used for vacuum treatment to ensure that the vacuum time is controlled above 12 min, so as to ensure that the H content in the steel is controlled below 1.5×10-6. Since a certain amount of nitrogen content will be removed during the vacuum treatment process, nitrogen gas is blown in a circulating manner during the vacuum treatment process to increase nitrogen, so as to ensure the relative stability of the nitrogen content in the steel. After breaking the vacuum, the soft blowing argon time is controlled ≥20 min to ensure that inclusions have sufficient floating and removal time, and improve the cleanliness of the molten steel.

[0040] S2. Continuous casting treatment;

[0041] The quality of the continuous casting billet determines the quality of the final product. In addition to having a high purity, low harmful gas and impurity element content, the continuous casting billet also needs to have a good solidification structure and uniform composition distribution. In addition, since the sulfur content of this steel grade is controlled relatively high, it is extremely unfavorable for the surface quality control of the casting billet. Therefore, for the 38MnS6 experimental production, the specific water volume of the secondary cooling of the continuous casting is relatively high at 0.35 L / kg and the constant casting speed of 0.80 m / min is used for steel casting; the superheat of the tundish molten steel is controlled at 18 - 28 °C, and the temperature drop of the tundish is controlled within 10 °C; during the casting process, the liquid level of the mold and the stopper rod stroke are controlled stably, and the liquid level fluctuation is controlled within ±5 mm; the electromagnetic stirring parameters of 250 A and 2.5 Hz are used in the mold to promote the floating of inclusions, increase the equiaxed crystal ratio of the continuous casting billet, reduce the central segregation and porosity, and at the same time improve the surface quality; and the electromagnetic stirring parameters of 250 A and 8.0 Hz at the solidification end and the soft reduction technology are applied to reduce the central segregation and central porosity of the 38MnS6 casting billet. The macrostructure of the produced casting billet is good, and the macrostructure ratings of general porosity and central porosity are less than 1.5 levels. After pickling the surface of the casting billet, no obvious cracks, pores and other defects are found.

[0042] S3. Rolling treatment;

[0043] In this experimental production, 280-square billets are used to roll round steel with a diameter of φ90 mm, and the rolling reduction ratio reaches more than 12, meeting the customer's requirement of greater than 7. Since the sulfur content of this steel grade is relatively high, segregation and porosity defects are likely to occur during the solidification of the casting billet. The heating temperature should be appropriately increased during rolling heating, and the soaking and heat preservation time should be extended to ensure the homogenization of the composition and structure; at the same time, considering that the high sulfur steel has strong crack thermal sensitivity, the preheating temperature must be well controlled. In this experimental rolling, the temperature of the preheating section of the heating furnace is controlled below 680 °C, the temperature of the soaking section is controlled at 1180 - 1210 °C for high-temperature diffusion, the total time in the furnace is controlled at 200 - 300 min, the high-pressure water descaling pressure is controlled at about 28 MPa, and the descaling effect is good. There is no residual scale on the surface after descaling; during the rolling process, the reduction of the 1# stand is 55 mm, the reduction of the 2# stand is 64 mm, the inlet temperature of the 7-stand finishing mill is about 980 °C, and the finished product is produced at the 12th stand. There are no obvious crack defects on the online surface after pickling. After rolling, the round steel is slowly cooled with a cooling bed and a heat preservation cover.

[0044] S4. Physical quality inspection of the product;

[0045] S41. Detection of composition and gas content;

[0046] The composition control during the smelting production process is stable, and the composition detection of the tundish melting meets the internal control requirements. The detected gas N content of the rolled finished product is (121 - 125)×10 -6 , the H content is 1.1×10 -6 , and the total oxygen content is (7 - 8)×10 -6 , meeting the requirements of the customer's technical agreement, and the refining measures and continuous casting protective casting control are good.

[0047] S42. Macrostructure detection;

[0048] After pickling the cross-section of the rolled finished round steel, there shall be no defects such as visible pores, shrinkage cavities, internal cracks, inclusions, etc. by the naked eye. The measured results are good, meeting the requirements of the technical agreement. The macrostructure and grading data of the round steel are as follows:

[0049] General porosity Center porosity Center segregation Standard ≤ Grade 2 ≤ Grade 2 ≤ Grade 2 Measured Grade 1.0 Grade 1.0 Grade 0.5

[0050] S43. Analysis and detection of non-metallic inclusions;

[0051] The rolled round steel product is detected and analyzed for non-metallic inclusions. Since the sulfur content of this steel grade is controlled at a relatively high level, the inclusions are mainly A-type sulfides, and the sulfide particles are small and diffusely distributed. There are no B and C type inclusions, and there are a small amount of D-type spherical oxides. The rating of A-type inclusions is all between 2 - 2.5 levels, and the D-type inclusions are 0.5 - 1.0 levels. Under a 100-fold microscope, the inclusions all meet the requirements of the technical agreement well; through scanning electron microscopy analysis, the composition of A-type inclusions is all MnS and a very small amount of FeS, and the composition and distribution of sulfides have achieved ideal results.

[0052] S44. Grain size detection;

[0053] The grain size is the size of the austenite grain. The finer the grains, the higher the strength, plasticity, and toughness of the steel. According to GB / T6394, the austenite grain size of the product is detected by the direct etching method to be 8 - 8.5 levels. Under a 500-fold microscope, the grain size rating is 8 - 8.5 levels.

[0054] S45. Banded structure detection;

[0055] The presence of banded structure defects inside steel will make the mechanical properties of the metal anisotropic. The direction parallel to the banded structure is significantly better than the perpendicular direction. During the processing of steel products, cracking is likely to occur at the junction. For parts that require heat treatment after processing, banded structure defects will cause relatively large thermal deformation. In severe cases, stress concentration will be caused, leading to cracking. In order to evaluate the banded structure of the trial-produced steel products this time, samples of rolled round steel were taken and its banded structure was detected under a 100-fold microscope. The banded structure was rated at level 1.0, meeting the customer's requirement of not exceeding level 3.0.

[0056] The above text has generally described the present invention in detail. However, based on the present invention, some modifications or improvements can be made, which are obvious to those of ordinary skill in the technical field. Therefore, modifications or improvements made without departing from the spirit of the present invention are within the protection scope of the present invention.

Claims

1. A method for producing non-quenched and tempered crankshaft steel 38MnS6, characterized in that: The steps include: S1, smelting treatment; S2, continuous casting process; S3, rolling treatment; S4. Physical quality inspection of products.

2. The method for producing non-quenched and tempered crankshaft steel 38MnS6 according to claim 1, characterized in that: The S1 includes: The converter control end point composition P≤0.012%, C≥0.08%, and the tapping temperature≥1620℃; During the steel-making process, the amount of alloy added is accurately calculated according to the composition requirements of the ladle furnace at the station, and about 500kg of lime and 300kg of refined synthetic slag are added for slag washing; During the steel-out process, about 300kg of aluminum iron is added along the steel flow and 200m of aluminum wire is fed behind the furnace for precipitation deoxidation; In the later stage of steel tapping, automatic infrared detection of slag and sliding plate slag blocking are used to control the slag thickness on the top of the ladle to ≤20mm; argon is blown from the bottom of the furnace throughout the process to promote alloy melting and the floating and removal of deoxidation products.

3. The method for producing non-quenched and tempered crankshaft steel 38MnS6 according to claim 1, characterized in that: The S1 further comprises: The LF furnace refining process uses silicon carbide and aluminum particles for diffusion deoxidation; The holding time of refined white slag is controlled at about 35 minutes, and the total slag amount is controlled at more than 12kg / ton of steel. After the molten steel temperature reaches 1590℃, the nitrogen alloy wire is fed to increase nitrogen. After the refined slag becomes white slag, the composition is fine-tuned according to the target value. In the later stage of refining, a small amount of silica sand is added to adjust the basicity of the refined slag. The basicity control target is 4.

0. After refining, 0.4-0.6m / t steel is fed into the calcium wire for inclusion denaturation treatment. After the calcium wire is fed, soft blowing is performed for 6-8 minutes to feed the sulfur line to increase sulfur. The vacuum treatment uses an RH furnace to ensure that the vacuum time is controlled at more than 12 minutes to ensure that the H content in the steel is controlled below 1.5×10-6. Since a certain amount of nitrogen will be removed during the vacuum treatment process, circulating nitrogen blowing is used to increase nitrogen during the vacuum treatment process to ensure that the nitrogen content in the steel is relatively stable. After breaking the air, the soft argon blowing time is controlled to be ≥20 minutes.

4. The method for producing non-quenched and tempered crankshaft steel 38MnS6 according to claim 1, characterized in that: The S2 includes: The 38MnS6 trial production continuous casting secondary cooling adopts a relatively high specific water volume of 0.35L / kg and a constant drawing speed of 0.80m / min for steel casting; The superheat of molten steel in the tundish is controlled at 18-28℃, and the temperature drop of the tundish is controlled within 10℃; During the pouring process, the liquid level and stopper rod stroke of the crystallizer are stably controlled, and the liquid level fluctuation is controlled within ±5mm; the crystallizer adopts 250A and 2.5Hz electromagnetic stirring parameters to promote the floating of inclusions, increase the equiaxed crystal rate of the continuous casting billet, reduce center segregation and looseness, and improve the surface quality.

5. The method for producing non-quenched and tempered crankshaft steel 38MnS6 according to claim 1, characterized in that: The S3 further comprises: The temperature of the preheating section of the rolling heating furnace is controlled below 680°C, the temperature of the soaking section is controlled at 1180-1210°C for high-temperature diffusion, the total time in the furnace is controlled at 200-300min, the high-pressure water descaling pressure is controlled at 28MPa, and no iron oxide scale remains on the surface after descaling.

6. The method for producing non-quenched and tempered crankshaft steel 38MnS6 according to claim 1, characterized in that: The S4 further comprises: S41, composition and gas content detection; the composition control of the smelting production process is stable, and the composition detection of the tundish smelting meets the internal control requirements. The N content of the gas detected in the rolled product is (121~125)×10 -6 , H content is 1.1×10 -6 , total oxygen content is (7~8)×10 -6 ; S42, macrostructure inspection: the cross-section of the rolled round steel product shall not have defects such as pores, shrinkage holes, internal cracks, inclusions, etc. visible to the naked eye after pickling; S43. Non-metallic inclusion analysis and testing: Non-metallic inclusions are tested and analyzed on the rolled round steel products. Scanning electron microscopy analysis shows that the components of Class A inclusions are all MnS and a very small amount of FeS. S44, grain size detection; grain size refers to the size of austenite grains. The finer the grains, the higher the strength of the steel, and the better the plasticity and toughness; S45. Banded structure detection: The presence of banded structure defects inside steel will make the mechanical properties of the metal anisotropic, and the direction parallel to the banded structure is significantly better than the perpendicular direction.

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