Magnetic crack control method of 42CrMo alloy steel for crankshaft

Abstract

The invention relates to a magnetic crack control method of 42CrMo alloy steel for a crankshaft, and belongs to the technical field of ferrous metallurgy. In order to solve the problem that the center of an existing 42CrMo alloy steel bloom for a crankshaft is too large in porosity, so that magnetic track cracks are easily formed on the surface of the crankshaft, the invention provides a magnetic track crack control method of 42CrMo alloy steel for the crankshaft. By optimizing the continuous casting pulling speed, the specific water flow and electromagnetic stirring parameters and introducing the casting blank solidification tail end soft reduction technology, the core quality of the 42CrMo alloy bloom is remarkably improved, the center porosity level is reduced to 0.5 level or below from the original 1.5-2.0 level, raw materials with higher quality are provided for the subsequent rolling and forging process, and the production cost is reduced. And the rejection rate caused by magnetic marks and cracks is reduced, the reject rate is reduced from 3-5% to 0, the production cost is indirectly reduced, and remarkable economic benefits are brought.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel metallurgy, and particularly relates to a magnetic mark crack control method for 42CrMo alloy steel for crankshafts. BACKGROUND

[0002] 42CrMo alloy steel is widely used in the manufacture of key components such as engine crankshafts due to its excellent comprehensive mechanical properties. Due to the harsh service environment, the surface quality of such components is extremely strict. However, when 42CrMo bars with a specification of 100 mm or more are used to forge crankshafts, magnetic mark cracks often occur. Magnetic mark cracks are a special type of surface defect, mainly manifested as linear marks during magnetic powder detection. Research shows that such defects are not caused by inclusions, but by the incomplete elimination of internal porosity in the casting blank during rolling, which is then extruded to the surface of the crankshaft during forging.

[0003] 42CrMo alloy has a high content, and its bloom is prone to form significant core porosity during solidification. Currently, when 250mm×280mm bloom is rolled into 42CrMo bars with a diameter of 100mm or more, the core porosity defect of the casting blank is difficult to fully weld due to insufficient compression ratio. During the subsequent processing of forging crankshafts, due to the asymmetry of deformation, the core porosity defect of the casting blank is easily extruded to the surface of the crankshaft, forming a magnetic mark crack defect, which ultimately leads to the rejection of the finished crankshaft. Patent application No. CN115323107A discloses a steel production method for eliminating large-specification crankshaft magnetic mark defects. This method mainly reduces the magnetic mark defects on the surface of the crankshaft by reducing the center shrinkage of the continuous casting blank and improving the equiaxed grain size. However, this method is difficult to effectively improve the level of core porosity defects in the casting blank in actual production.

[0004] There is an urgent need to develop a process technology that can stably control the core porosity of the bloom below 0.5 level, to effectively inhibit the expansion of the core porosity defect during the forging of the crankshaft, and to avoid the resulting surface magnetic mark crack defect. SUMMARY

[0005] To solve the problem of excessive core porosity of 42CrMo alloy steel bloom for crankshafts leading to the formation of magnetic mark cracks on the surface of the crankshaft, the present application provides a magnetic mark crack control method for 42CrMo alloy steel for crankshafts.

[0006] The technical solution of the present application is as follows:

[0007] A kind of magnetic trace crack control method of 42CrMo alloy steel for crankshaft, in the preparation process of 42CrMo alloy steel big square billet, the continuous casting speed is controlled to be 0.65 m / min, the overheat degree control of this speed is 20~40 ℃;Crystal water quantity is 2350 L / min;Specific water quantity control is 0.20 L / kg, distribution ratio is 36 / 39 / 25%;First end electromagnetic stirring parameter control is 150 A / 2.0 Hz, continuous stirring;End electromagnetic stirring parameter control is 100 A / 8.0 Hz, alternate stirring, alternate stirring time is 10 seconds-3 seconds-10 seconds;Light press-down control is 0 / 2 / 4 / 4 mm.

[0008] Further, the size of the 42CrMo alloy steel big square billet is 250×280 mm.

[0009] Further, the chemical composition of the 42CrMo alloy steel big square billet includes, by weight percentage: C: 0.38~0.45%, Si: 0.15~0.35%, Mn: 0.50~0.80%, Cr: 0.90~1.20%, Mo: 0.15~0.25%, and the rest is Fe and unavoidable impurities; the 42CrMo alloy steel big square billet preparation process includes BOF converter smelting→LF refining→-VD vacuum refining→CC big square billet continuous casting→heat treatment→rolling.

[0010] Further, the BOF converter smelting adopts high-pull blow operation, the tapping endpoint control endpoint C: 0.10~0.20%, endpoint P≤0.015%, and endpoint temperature is not lower than 1610 ℃;Tapping deoxidation adopts 60~120 kg / 100 t aluminum ingot precipitation deoxidation, and the in-place oxygen content after refining is controlled to be ≤300 ppm.

[0011] Further, the first time power supply argon flow control of the LF refining is 300~500 NL / min, the first time composition full analysis sample is taken, at this time, the refining slag has formed white slag, the slag composition is CaO: 56±2wt%, SiO2≤6wt%, Al2O3: 27~29wt%, MgO≤5wt%, FeO≤0.5wt%, R: 7~9;According to the first time composition full analysis result, the alloy is fine-tuned, the argon flow control after the first time power supply is 100~250 NL / min, diffusion deoxidation is carried out, and white slag is maintained.

[0012] Further, the final slag composition of the LF refining is CaO: 55±2wt%, SiO2≤7wt%, Al2O3: 28~30wt%, MgO≤5wt%, FeO≤0.5wt%, R: 7~9;The amount of SiC deoxidizer used in LF process is 1.5 kg / t, and the white slag maintaining time during refining is not less than 30 min.

[0013] Further, the VD vacuum refining is connected with argon, the argon is adjusted to 100-150 NL / min, the vacuum treatment is started, when the vacuum degree is less than or equal to 67 Pa, the vacuum treatment is continued for 15-25 min, the H after vacuum is less than or equal to 1.2 ppm, the ladle covering agent is added after vacuum, the argon flow is adjusted to 50-120 NL / min, the soft argon blowing is started, and the soft argon blowing time is not less than 15 min.

[0014] Further, the continuous casting blank obtained by the continuous casting is hot sent or pit slow cooling, the slow cooling time is not less than 48 hours, and the blank temperature after the slow cooling is less than or equal to 200 DEG C.

[0015] Further, the heat treatment includes heating 1 section, heating 2 section and soaking section, the temperature of the heating 1 section is not more than 1120 DEG C, the heating temperature of the heating 2 section is 1200-1240 DEG C, the heating temperature of the soaking section is 1190-1230 DEG C, the furnace-out temperature after the heat treatment is completed is 1190-1230 DEG C, and the heating time of the heat treatment is not less than 150 min.

[0016] Further, the rolling open-rolling temperature is not less than 1000 DEG C, the material is produced by 10 passes of continuous rolling, and the processing ratio is not less than 7.5.

[0017] The beneficial effects of the present application are as follows:

[0018] The present application significantly improves the center quality of the 42CrMo alloy square billet by optimizing the continuous casting speed, specific water quantity, electromagnetic stirring parameters and introducing the light pressing-down technology at the end of the billet solidification, reduces the center loose level from the original 1.5-2.0 level to below 0.5 level, provides higher quality raw materials for the subsequent rolling and forging process, successfully solves the magnetic mark crack defect problem in the forging crankshaft process of the 42CrMo steel, significantly improves the surface quality of the crankshaft, reduces the waste rate caused by the magnetic mark crack, reduces the proportion of such waste products from 3-5% to 0, indirectly reduces the production cost, and brings significant economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The macrostructure photo of the 42CrMo alloy steel square billet prepared in Example 1 is shown in the figure;

[0020] Figure 2 The macrostructure photo of the 42CrMo alloy steel square billet prepared in Comparative Example 1 is shown in the figure;

[0021] Figure 3 The macrostructure photo of the 42CrMo alloy steel forging crankshaft prepared in Example 2 is shown in the figure;

[0022] Figure 4 The macrostructure photo of the 42CrMo alloy steel forging crankshaft prepared in Comparative Example 2 is shown in the figure. DETAILED DESCRIPTION

[0023] The technical solutions of the present application are further described below in connection with the examples, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present application without departing from the spirit and scope of the present application shall be covered in the protection scope of the present application. The process equipment or device not specifically mentioned in the following examples is the conventional equipment or device in the art. If not specifically mentioned, the raw materials used in the examples of the present application are commercially available. If not specifically mentioned, the technical means used in the examples of the present application are conventional means known to those skilled in the art.

[0024] Example 1

[0025] The present embodiment provides a method for controlling magnetic mark cracks of 42CrMo alloy steel for crankshaft.

[0026] The specification of the 42CrMo alloy steel bloom in the present embodiment is 250x280mm. The chemical composition of the 42CrMo alloy steel bloom includes, by weight percentage: C: 0.38-0.45%, Si: 0.15-0.35%, Mn: 0.50-0.80%, Cr: 0.90-1.20%, Mo: 0.15-0.25%, and the rest is Fe and inevitable impurities.

[0027] The method for controlling magnetic mark cracks of 42CrMo alloy steel for crankshaft in the present embodiment is as follows: during the preparation of the 42CrMo alloy steel bloom, the continuous casting speed is controlled to be 0.65m / min, the superheating degree control at this speed is 20-40℃; the crystallizer water quantity is 2350L / min; the specific water quantity control is 0.20L / kg, and the distribution ratio is 36 / 39 / 25%; the first end electromagnetic stirring parameter control is 150A / 2.0Hz, continuous stirring; the last end electromagnetic stirring parameter control is 100A / 8.0Hz, alternating stirring, and the alternating stirring time is 10s-3s-10s; the light pressing down control is 0 / 2 / 4 / 4mm.

[0028] Example 2

[0029] The present embodiment provides a preparation process for the 42CrMo alloy steel bloom for crankshaft in Example 1, which includes BOF converter smelting→LF refining→VD vacuum refining→CC bloom continuous casting→heat treatment→rolling.

[0030] BOF converter smelting: about 90% of high-quality hot metal, about 10% of high-quality purchased and internal return scrap, 3500 kg of lime per furnace, high-ladle-slagging and top-blown operation, tapping endpoint control endpoint C: 0.20%, endpoint P≤0.015%, and endpoint temperature not lower than 1610℃; tapping deoxidation adopts 60~120 kg / 100t aluminum ingot precipitation deoxidation, and the in-place oxygen content after refining is controlled to be≤300ppm.

[0031] LF refining: connect argon at the station, add synthetic slag and lime 0~200 kg according to the fluidity of the slag after slagging, and then adopt 85% grade SiC for diffusion deoxidation after the slag is melted, the argon flow is controlled to be 500 NL / min during the first power transmission of LF refining, the first composition full analysis sample is taken, at this time, the refining slag has formed white slag, the slag composition is CaO: 56±2wt%, SiO2≤6wt%, Al2O3: 27~29wt%, MgO≤5wt%, FeO≤0.5wt%, and R: 7~9; at this time, the slag has strong abilities of arc burying, air insulation, desulfurization and inclusion adsorption.

[0032] According to the first composition full analysis result, the alloy composition and the molten steel temperature are adjusted to the process requirements, the argon flow is controlled to be 250 NL / min after the first power transmission of LF, diffusion deoxidation is carried out, and 85% grade SiC is adopted for diffusion deoxidation, at this time, the main task is white slag maintenance, the final LF slag composition is CaO: 55±2wt%, SiO2≤7wt%, Al2O3: 28~30wt%, MgO≤5wt%, FeO≤0.5wt%, and R: 7~9. The SiC deoxidizer consumption during the LF process is about 1.5 kg / t, the refining white slag maintenance time is≥30 minutes, no further composition adjustment and temperature adjustment are carried out after the end of the LF, so as to avoid secondary pollution of the molten steel. After the temperature and composition meet the requirements of the station, the station is left to enter the VD vacuum refining.

[0033] VD vacuum refining: connect argon, adjust the argon to 150 NL / min, start vacuum treatment, continue for 25 min when the vacuum degree is≤67 Pa, the H after vacuum is≤1.2ppm, add ladle covering agent after vacuum, adjust the argon flow to 120 NL / min, and start soft argon blowing, the soft argon blowing time is not less than 15 min.

[0034] CC bloom continuous casting: the continuous casting parameters are controlled according to the control method of embodiment 1, the obtained continuous casting billet is taken for hot sending or pit slow cooling, the slow cooling time is not less than 48 hours, and the billet temperature after slow cooling is≤200℃.

[0035] Heat treatment: including heating 1 section, heating 2 section and soaking section, the temperature of heating 1 section is not more than 1120℃, the heating temperature of heating 2 section is 1240℃, the heating temperature of soaking section is 1230℃, the out furnace temperature of heat treatment is 1230℃, the heating time of heat treatment is 150min.

[0036] After the casting blank is discharged from the heating furnace, high-pressure water phosphorus removal treatment is taken, the opening rolling temperature is not less than 1000℃, the 10-pass continuous rolling is taken in the continuous rolling unit to obtain the 42CrMo rod with a diameter of 100mm, and the processing ratio is 8.

[0037] Comparative example 1

[0038] The comparative example provides a continuous casting control method of 42CrMo alloy steel for crankshaft.

[0039] The specification of the 42CrMo alloy steel bloom in the comparative example is 250*280mm, and the preparation process method of the 42CrMo alloy steel bloom is the same as that in example 2.

[0040] The continuous casting control method of the 42CrMo alloy steel for crankshaft in the comparative example is as follows:

[0041] In the preparation process of the 42CrMo alloy steel bloom, the continuous casting speed is controlled to be 0.70m / min, the superheating degree control for this speed is 20-30℃; the crystallizer water quantity is 2350L / min; the specific water quantity control is 0.30L / kg, the distribution ratio is 36 / 39 / 25%; the first end electromagnetic stirring parameter control is 150A / 2.0Hz, continuous stirring; the last end electromagnetic stirring parameter control is 200A / 6.0Hz, continuous stirring, and the light pressing down is not set in the comparative example.

[0042] Figure 1 The macrostructure photo of the 42CrMo alloy steel bloom prepared in example 1 is shown in the figure; Figure 2 The macrostructure photo of the 42CrMo alloy steel bloom prepared in comparative example 1 is shown in the figure; from Figure 1 and Figure 2 It can be seen from the comparison of the figures that the macrostructure center porosity level of the 42CrMo alloy steel bloom prepared in example 1 is 0.5 level, while the macrostructure center porosity level of the bloom prepared in comparative example 1 is 1.5 level.

[0043] Figure 3 The macrostructure photo of the forged crankshaft of the 42CrMo alloy steel prepared in example 2 is shown in the figure; Figure 4 The macrostructure photo of the forged crankshaft of the 42CrMo alloy steel prepared in comparative example 2 is shown in the figure; from Figure 3 and Figure 4 It can be seen from the comparison of the figures that, Figure 3 No obvious magnetic mark is found on the surface of the sample, indicating that the material has no crack defect; whileFigure 4 Linear magnetic mark aggregation (as shown by the red arrow) exists on the surface of the sample, which is consistent with the characteristics of crack defects. The results prove that the continuous casting control method provided by the application can effectively inhibit the formation of magnetic mark cracks on the surface of the crankshaft.

Claims

1. A method for controlling magnetic crack of 42CrMo alloy steel for crankshaft, characterized in that, In the preparation process of the 42CrMo alloy steel bloom, the continuous casting speed is controlled to be 0.65 m / min, the overheat degree of the speed is controlled to be 20-40 DEG C; the water quantity of the crystallizer is 2350 L / min; the specific water quantity is controlled to be 0.20 L / kg, the distribution ratio is 36 / 39 / 25%; the first end electromagnetic stirring parameter is controlled to be 150 A / 2.0 Hz, the continuous stirring is executed; the last end electromagnetic stirring parameter is controlled to be 100 A / 8.0 Hz, the alternate stirring is executed, the alternate stirring time is 10 seconds-3 seconds-10 seconds; the light pressing down control is 0 / 2 / 4 / 4 mm.

2. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 1, characterized in that, The specification of the 42CrMo alloy steel bloom is 250*280 mm.

3. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 1 or 2, characterized in that, The chemical composition of the 42CrMo alloy steel bloom includes, by weight percentage, C: 0.38-0.45%, Si: 0.15-0.35%, Mn: 0.50-0.80%, Cr: 0.90-1.20%, Mo: 0.15-0.25%, and the rest is Fe and inevitable impurities; the preparation process of the 42CrMo alloy steel bloom includes BOF converter smelting-LF refining-VD vacuum refining-CC bloom continuous casting-heat treatment-rolling.

4. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 3, characterized in that, The BOF converter smelting adopts high-pull blow operation, the tapping end point control end point C: 0.10-0.20%, end point P≤0.015%, end point temperature is not lower than 1610 DEG C; the tapping deoxidation adopts 60-120 kg / 100 t aluminum ingot precipitation deoxidation, and the in-place oxygen content after refining is controlled to be ≤300 ppm.

5. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 4, characterized in that, The first time power supply argon flow of the LF refining is controlled to be 300-500 NL / min, the first time composition full analysis sample is taken, at this time, the refining slag has formed white slag, the slag composition is CaO: 56±2wt%, SiO2≤6wt%, Al2O3: 27-29wt%, MgO≤5wt%, FeO≤0.5wt%, R: 7-9; the alloy is fine-tuned according to the first time composition full analysis result, the argon flow is controlled to be 100-250 NL / min after the first time power supply, the diffusion deoxidation is carried out, and the white slag is kept.

6. The method of claim 5, wherein the 42CrMo alloy steel for a crankshaft is characterized by: The final slag composition of the LF refining is CaO: 55±2wt%, SiO2≤7wt%, Al2O3: 28-30wt%, MgO≤5wt%, FeO≤0.5wt%, R: 7-9; the SiC deoxidizer dosage in the LF process is 1.5 kg / t, and the white slag keeping time is not less than 30 min.

7. The method of claim 6, wherein the 42CrMo alloy steel for a crankshaft is characterized by: The VD vacuum refining is connected with argon, the argon is adjusted to be 100-150 NL / min, the vacuum treatment is started, when the vacuum degree is ≤67 Pa, it is continuously kept for 15-25 min, the H after vacuum is ≤1.2 ppm, the ladle covering agent is added after vacuum, the argon flow is adjusted to be 50-120 NL / min, the soft argon blowing is started, and the soft argon blowing time is not less than 15 min.

8. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 7, characterized in that, The continuous casting blank obtained by the continuous casting is adopted for hot sending or pit slow cooling, the slow cooling time is not less than 48 hours, and the casting blank temperature after slow cooling is ≤200 DEG C.

9. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 8, characterized in that, The heat treatment comprises heating stage 1, heating stage 2 and soaking stage, the temperature of the heating stage 1 is not more than 1120 DEG C, the heating temperature of the heating stage 2 is 1200-1240 DEG C, the heating temperature of the soaking stage is 1190-1230 DEG C, the out furnace temperature of the heat treatment is 1190-1230 DEG C, and the heating time of the heat treatment is not less than 150 min.

10. The method for controlling magnetic indication cracks in 42CrMo alloy steel for crankshafts according to claim 9, characterized in that, The rolling opening temperature is not less than 1000 DEG C, 10-pass continuous rolling produces a material, and the processing ratio is not less than 7.5.

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