A method for improving the hardenability of gear steel bar and gear steel bar

Abstract

The present invention provides a method for improving the hardenability of a gear steel bar and the gear steel bar, belonging to the technical field of continuous casting processes. The method comprises: casting molten steel with qualified composition through a continuous casting machine; adopting a reduction process in the solidification stage; and rolling the hot ingot after solidification; during the continuous casting process, the tundish superheat is 40-45°C, and the electromagnetic stirring current of the crystallizer is 50-150A; the reduction amount is controlled by changing the solid phase ratio at the center of the ingot in the reduction process, the solid phase ratio at the center of the reduction area is 0.05-1.0, and the reduction amount in the continuous casting reduction area corresponding to the solid phase ratio being less than 0.3 is not less than 1 mm and not more than 3 mm; and the rolling section compression ratio is not less than 90%. By controlling the superheat, electromagnetic stirring of the crystallizer, the pressing process and the rolling process, the equiaxed crystal area of ​​the ingot and the bar is reduced, and the formation of semi-macro point segregation in the equiaxed crystal area is suppressed. The cast structure in the hardenability test area of ​​the bar is transformed from equiaxed crystals to columnar crystals, and the cross-sectional solute range and standard deviation as well as the uniformity of the solute distribution in the dendrite structure are further improved. The difference between the A and B surfaces of J9 and J15 are both ≤1.5HRC, and the hardenability index is qualified, which is conducive to reducing the uneven distortion of subsequent gear heat treatment and improving product homogeneity.

Description

Technical Field

[0001] The invention belongs to the technical field of continuous casting processes, and in particular relates to a method for improving the hardenability of a gear steel bar and the gear steel bar. Background Art

[0002] Gears, as key components in automotive transmission mechanisms, withstand impact, bending, and contact stresses during operation, requiring excellent toughness and wear resistance. Carburizing and quenching are commonly used methods to achieve a hard surface and tough core. Typically, when the hardenability of gear steel fluctuates significantly, uneven distortion can occur during the heat treatment process, directly deteriorating the gear's service performance and lifespan. Therefore, current high-end automotive users require not only a hardenability bandwidth of ≤4 HRC for the same batch of steel, but also a ≤2 HRC difference between the A and B surfaces of each bar (J9 and J15).

[0003] The hardenability of steel is closely related to the uniformity of matrix solute distribution. The more uniform the solute distribution, the smaller the hardenability fluctuation. Generally, during the solidification process of the ingot, solute redistribution and selective crystallization will occur. The as-cast matrix will inevitably have uneven composition. Considering the influence of solidification structure morphology and size on solute distribution, the matrix composition unevenness can reach a semi-macroscopic or macroscopic scale, which cannot be completely removed by rolling and heat treatment. It is inherited to the gear, resulting in increased hardenability fluctuations and causing anisotropy of heat treatment deformation. In addition, controlling the number of passes in the rolling process and the cross-sectional compression ratio of each pass is crucial to the hardenability of steel. If the interface compression ratio of each pass cannot be accurately controlled, it may lead to uneven grain refinement and uneven solute distribution, which in turn affects the hardenability of the gear steel. Summary of the Invention

[0004] To address the above-mentioned issues, the present invention provides a method for improving the hardenability of gear steel bars and a gear steel bar. By controlling superheat, mold electromagnetic stirring, reduction, and rolling processes, the equiaxed crystal region of the ingot and bar is reduced, while the formation of semi-macroscopic point segregation in the equiaxed crystal region is suppressed. The as-cast structure in the bar's hardenability test area transforms from equiaxed crystals to columnar crystals, further improving the cross-sectional solute range and standard deviation, as well as the uniformity of solute distribution in the dendrite structure. The A and B surface differences for J9 and J15 are both ≤1.5HRC, indicating qualified hardenability indicators. This helps reduce uneven distortion during subsequent gear heat treatment and improves product homogeneity.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] On the one hand, the present invention provides a method for improving the hardenability of gear steel bars, wherein the difference between the A and B surfaces of J9 and J15 of the gear steel is not greater than 1.5HRC, and the method is characterized in that it includes: casting molten steel with qualified composition through a continuous casting machine; adopting a reduction process in the solidification stage; after solidification, rolling the hot ingot; during the continuous casting process, the overheat degree of the ladle is 40-45°C, and the electromagnetic stirring current of the crystallizer is 50-150A; the reduction process controls the reduction amount by changing the solid phase ratio at the center of the ingot, the solid phase ratio at the center of the reduction area is 0.05-1.0, and in the continuous casting reduction area corresponding to the central solid phase ratio being less than 0.3, the reduction amount is not less than 1mm and not more than 3mm; the rolling section compression ratio is not less than 90%.

[0007] Furthermore, the casting process parameters are: constant temperature and constant pulling speed during the continuous casting process, the pulling speed is 0.95-1.15m / min, the secondary cooling water volume is 0.33-0.35L / kg, and the electromagnetic stirring current at the end of solidification is 100-200A.

[0008] Furthermore, the maximum single pressing amount satisfies the following formula:

[0009]

[0010] Among them, ε is the maximum critical strain, v is the working speed, h is the thickness of the billet, r is the diameter of the reduction roll, b is the roll spacing, and a is the maximum single reduction.

[0011] Furthermore, when the solid fraction at the center of the reduction zone is between 0.05 and 1.0, the total reduction is calculated as follows:

[0012]

[0013] in, is the reduction coefficient, which ranges from 0.8 to 1.1, A is the total amount of reduction, V R is the pressing rate, t is the time, h is the thickness of the billet, W is the width of the billet, T L is the liquidus temperature of the steel grade, T S is the solidus temperature of the steel grade.

[0014] Furthermore, the relationship between the reduction and the central solid fraction is:

[0015] When 0.05≤f s ≤0.08, the pressing amount is 1mm; when 0.18≤f s ≤0.20, the pressing amount is 2mm; when 0.55≤f s ≤0.58, the pressing amount is 2mm; when 0.98≤f s ≤1, the pressing amount is 2mm.

[0016] Furthermore, the steel used for rolling the bar is 20CrMnTiH steel.

[0017] Furthermore, the rolling process is as follows: heating the hot ingot to a temperature of 1160-1200°C and keeping it warm for 150 minutes, starting rolling at a temperature of 1060-1100°C, finishing rolling at a temperature of 900-1000°C, rolling it into a φ70mm bar, and then cooling it to obtain the bar.

[0018] Furthermore, 4 to 6 rolling passes are used, with the cross-sectional compression ratio of the first rolling pass being no less than 50%, the cross-sectional compression ratio of the last rolling pass being 10% to 25%, and the cross-sectional compression ratio of each of the remaining rolling passes being 30% to 45%.

[0019] Furthermore, the continuous casting machine is a full-arc continuous casting machine with six machines and six streams, and a casting section of 200mm×240mm.

[0020] A gear steel bar is prepared by the above method.

[0021] The beneficial effects brought about by the technical solution provided by the embodiment of the present invention include:

[0022] (1) The present invention adjusts the continuous casting process parameters, that is, increases the superheat, reduces the electromagnetic stirring of the crystallizer, and simultaneously implements light pressure at the end of solidification, and combines it with the subsequent rolling process. The equiaxed crystal zone of the prepared continuous casting billet is significantly reduced, and the solute distribution of the cross section of the billet is more uniform. The cross section solute C range and standard deviation of the continuous casting billet prepared by the continuous casting process parameter method of the present invention are reduced from 0.02wt.% and 0.007wt.% to 0.01wt.% and 0.003wt.%, respectively; the cross section Cr range and standard deviation are reduced from 0.09wt.% and 0.022wt.% to 0.05wt.% and 0.016wt.%, respectively, and the cross section Mn range and standard deviation are reduced from 0.08wt.% and 0.020wt.% to 0.03wt.% and 0.010wt.%, respectively, and the solute fluctuation is significantly reduced. The range and standard deviation of C in the cross section of the prepared rod were reduced from 0.02wt.% and 0.005wt.% to 0.01wt.% and 0.002wt.%, respectively; the range and standard deviation of Cr in the cross section were reduced from 0.04wt.% and 0.013wt.% to 0.03wt.% and 0.009wt.%, respectively; the range and standard deviation of Mn in the cross section were reduced from 0.04wt.% and 0.012wt.% to 0.01wt.% and 0.005wt.%, respectively; the solute distribution of the rod was more uniform; the as-cast structure in the hardenability test area of ​​the rod was transformed from equiaxed crystal to columnar crystal zone, and the average contents of solute C, Cr and Mn in the dendrite structure were reduced from 0.42wt.%, 1.95wt.% and 1.31wt.% to 0.41wt.% and 1.92wt.%, respectively. t.%, 1.27wt.%; the maximum segregation ratios of C, Cr and Mn solutes are reduced from 2.75, 2.24 and 3.73 to 2.41, 2.16 and 3.30 respectively; the differences between the three hardenability test results J9-A and J9-B of the prepared bars are reduced from 2.3HRC, 3.0HRC and 2.7HRC to 0.6HRC, 0.7HRC and 0.2HRC respectively; the differences between J15-A and J15-B are reduced from 4.1HRC, 2.5HRC and 3.2HRC to 0.3HRC, 1.5HRC and 1.3HRC respectively; the fluctuations in the hardenability of the A and B surfaces are significantly reduced, both of which are less than the 2HRC required by the index, and the bars prepared by the technical solution of the present application have excellent performance and obvious fluctuation reduction effect, which does not exceed 1.5HRC.

[0023] (2) In the continuous casting process of the present invention, the superheat is increased and the current of the electromagnetic stirring of the crystallizer is reduced, so that the equiaxed crystal rate of the ingot is reduced from 26.42% to 6.69%, and the equiaxed crystal rate of the rod is reduced from 24.89% to 4.09%; on this basis, the pressing roller is pressed down by 7mm in the range of the central solid phase fraction of 0.05-1.00, the convection heat transfer is enhanced, the local cooling rate increases sharply, the existing crystal nuclei solidify rapidly, and the dendrites cannot bridge, thereby reducing or even eliminating the aggregation of concentrated steel liquid in the center under the action of solidification negative pressure suction, and the final organization is fine equiaxed crystals, and the point segregation size is small and the number is small, so that the cast organization in the rod hardenability detection area is transformed from equiaxed crystals to columnar crystals, and the solute is evenly distributed, which is beneficial to improve the consistency of the A and B surfaces in the end quenching experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 Schematic diagram of sampling of the ingot prepared in an embodiment of the present invention;

[0026] Figure 2 Schematic diagram of sampling of the rod prepared in the embodiment of the present invention, a is the solute content detection position; b is the sampling position of the sample dendrite morphology, c is the sampling position of the rod hardenability detection, and d is the J9 and J15 positions;

[0027] Figure 3 This is a low-magnification metallographic image of a cross-section of a slab prepared in Example 1 of the present invention;

[0028] Figure 4 The solute content distribution at different positions of the ingot prepared in Example 1 of the present invention;

[0029] Figure 5 The solute content distribution at different positions of the rod prepared in Example 1 of the present invention;

[0030] Figure 6 This is an EPMA scanning result diagram of the hardenability test area of ​​the bar prepared in Example 1 of the present invention;

[0031] Figure 7 These are the A and B surface test results of the hardenability J9 and J15 of the bars prepared in Example 1 of the present invention.

[0032] Figure 8 Low-magnification metallographic image of the cross section of the ingot prepared in Comparative Example 1 of the present invention

[0033] Figure 9 The solute content distribution at different positions of the ingot prepared in Comparative Example 1 of the present invention;

[0034] Figure 10 The solute content distribution at different positions of the rod prepared in Comparative Example 1 of the present invention;

[0035] Figure 11 This is the EPMA scanning result of the hardenability test area of ​​the bar prepared in Comparative Example 1 of the present invention;

[0036] Figure 12 These are the A and B surface test results of the hardenability J9 and J15 of the bars prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] An embodiment of the present invention provides a method for improving the hardenability of gear steel bars, wherein the difference between the A and B surfaces of J9 and J15 of the gear steel is no more than 1.5HRC, comprising: casting molten steel with qualified composition through a continuous casting machine; adopting a reduction process in the solidification stage; after solidification, rolling the hot ingot; during the continuous casting process, the overheat degree of the tundish is 40-45°C, and the electromagnetic stirring current of the crystallizer is 50-150A; the reduction process controls the reduction amount by changing the solid phase ratio at the center of the ingot, the solid phase ratio at the center of the reduction area is 0.05-1.0, and in the continuous casting reduction area corresponding to the central solid phase ratio being less than 0.3, the reduction amount is not less than 1mm and not more than 3mm; the rolled section compression ratio is not less than 90%.

[0039] The present invention adjusts the continuous casting process parameters, i.e., increases the superheat, reduces the electromagnetic stirring of the crystallizer, and simultaneously implements soft reduction at the end of solidification, and combines the subsequent rolling process. The equiaxed crystal region of the prepared continuous casting billet is significantly reduced, and the solute distribution of the cross section of the billet is more uniform. The cross section solute C range and standard deviation of the continuous casting billet prepared by the continuous casting process parameter method of the present invention are reduced from 0.02wt.% and 0.007wt.% to 0.01wt.% and 0.003wt.%, respectively; the cross section Cr range and standard deviation are reduced from 0.09wt.% and 0.022wt.% to 0.05wt.% and 0.016wt.%, respectively; the cross section Mn range and standard deviation are reduced from 0.08wt.% and 0.020wt.% to 0.03wt.% and 0.010wt.%, respectively, and the solute fluctuation is significantly reduced. The range and standard deviation of C in the cross section of the prepared rod were reduced from 0.02wt.% and 0.005wt.% to 0.01wt.% and 0.002wt.%, respectively; the range and standard deviation of Cr in the cross section were reduced from 0.04wt.% and 0.013wt.% to 0.03wt.% and 0.009wt.%, respectively; the range and standard deviation of Mn in the cross section were reduced from 0.04wt.% and 0.012wt.% to 0.01wt.% and 0.005wt.%, respectively; the solute distribution of the rod was more uniform; the as-cast structure in the hardenability test area of ​​the rod was transformed from equiaxed crystal to columnar crystal zone, and the average contents of solute C, Cr and Mn in the dendrite structure were reduced from 0.42wt.%, 1.95wt.% and 1.31wt.% to 0.41wt.% and 1.92wt.%, respectively. t.%, 1.27wt.%; the maximum segregation ratios of C, Cr and Mn solutes are reduced from 2.75, 2.24 and 3.73 to 2.41, 2.16 and 3.30 respectively; the differences between the three hardenability test results J9-A and J9-B of the prepared bars are reduced from 2.3HRC, 3.0HRC and 2.7HRC to 0.6HRC, 0.7HRC and 0.2HRC respectively; the differences between J15-A and J15-B are reduced from 4.1HRC, 2.5HRC and 3.2HRC to 0.3HRC, 1.5HRC and 1.3HRC respectively; the fluctuations in the hardenability of the A and B surfaces are significantly reduced, both of which are less than the 2HRC required by the index, and the bars prepared by the technical solution of the present application have excellent performance and obvious fluctuation reduction effect, which does not exceed 1.5HRC.

[0040] The technical solution proposed in the present invention is used to prepare medium-carbon low-alloy steel ingots that meet the hardenability requirements of J9 and J15, with the difference between the A and B surfaces being less than 2HRC. In the prior art, the difference between the A and B surfaces of J9 and J15 is usually much greater than 2HRC. For example, in the prior art, it is believed that the flatness of the solidification interface of equiaxed crystals is worse than that of columnar crystals, which is more conducive to the formation of large-area point segregation, resulting in extremely large fluctuations in the solute distribution in the cross-section of the ingot, and the solute uniformity distribution is significantly correlated with the fluctuations in hardenability. The present invention is based on a continuous casting process regulated by solidification structure (high superheat, weak crystallizer electromagnetic stirring, implementation of light reduction at the end of solidification and combined with subsequent rolling process) to reduce the equiaxed crystal area of ​​the ingot and then improve the point segregation, thereby improving the uniformity of the solute distribution in the cross-section of the ingot and bar.

[0041] Molten steel with qualified composition after BOF+LF+VD refining is cast through a continuous casting machine. The casting process is maintained at a constant temperature and casting speed of 0.95-1.15 m / min, with a secondary cooling water ratio of 0.33-0.35 L / kg, and electromagnetic stirring at the end of solidification of 100-200 A. Maintaining low superheat during casting and strong electromagnetic stirring in the mold can increase the equiaxed grain ratio of the ingot, thereby reducing problems such as central shrinkage and central segregation caused by overdeveloped columnar grains. However, for the regulation of semi-macro segregation, inhibiting the development of equiaxed crystals is more effective in reducing the size of point segregation and solute concentration in the ingot. The flatness of the solidification interface of equiaxed crystals is worse than that of columnar crystals, which is more conducive to the formation of large-area point segregation. The casting process in the continuous casting process proposed by the present invention, through the combination of the above-mentioned process parameters with the pressure control method and the rolling process, finally achieves the technical indicators proposed by the present invention, so that the prepared continuous casting ingot can be further processed after being rolled into bars to prepare gear steel bars that meet the index requirements.

[0042] In order to ensure that cracks do not appear in the ingot during the pressing process, the molten steel flow rate is fast, the convective heat transfer is enhanced, the local cooling rate is increased sharply, the existing crystal nuclei are quickly solidified, and the dendrites cannot bridge each other, the maximum single pressing amount is limited. Specifically, the maximum single pressing amount satisfies the following formula:

[0043]

[0044] Wherein, ε is the maximum critical strain, and the embodiment of the present invention adopts 20CrMnTiH steel, and the value is 0.71%, v is the working speed, h is the thickness of the casting, r is the diameter of the reduction roll, b is the roll spacing, and a is the maximum single reduction.

[0045] In the embodiment of the present invention, v = 0.95-1.15 m / min; h = 200 mm; r = 350 mm; b = 855 mm. Through calculation, the maximum single pressing amount a is no more than 2 mm.

[0046] When the solid fraction in the center of the reduction zone is between 0.05 and 1.0, the total reduction is calculated as follows:

[0047]

[0048] in, is the reduction coefficient, which ranges from 0.8 to 1.1, A is the total amount of reduction, V R is the pressing rate, t is the time, h is the thickness of the billet, W is the width of the billet, T L is the liquidus temperature of the steel grade, T S is the solidus temperature of the steel grade.

[0049] The maximum single reduction and total reduction of the present invention comprehensively consider the geometric dimensions of the rollers and the composition content of the 20CrMnTiH steel proposed in the present invention, and thus prepare a continuous casting billet that meets the corresponding technical indicators.

[0050] In order to suppress both macro- and semi-macro-segregation, when 0.05≤f s ≤0.08, the pressing amount is 1mm; when 0.18≤f s ≤0.20, the pressing amount is 2mm; when 0.55≤f s ≤0.58, the pressing amount is 2mm; when 0.98≤f s ≤1, with a reduction of 2mm. The reduction rollers were pressed down by 7mm within the range of 0.05-1.00 in the center solid phase fraction, which enhanced convective heat transfer, dramatically increased the local cooling rate, and rapidly solidified the existing nuclei. This prevented bridges between dendrites, thereby reducing or even eliminating the accumulation of concentrated molten steel in the center under the action of negative pressure suction during solidification. The final microstructure was fine equiaxed crystals, with small and few point segregations. This transformed the as-cast microstructure in the hardenability test area of ​​the bar from equiaxed crystals to columnar crystals, resulting in a uniform solute distribution and facilitating improved consistency between the A and B surfaces in the end quenching test.

[0051] The gear steel used in the embodiment of the present invention is 20CrMnTiH steel, and the specific composition is shown in Table 1.

[0052] Table 1 Chemical composition of 20CrMnTiH steel (wt%)

[0053] serial number C Mn Si P S Cr Ti Standard range 0.17-0.23 0.80-1.20 0.17-0.37 ≤0.030 ≤0.030 1.00-1.20 0.04-0.10

[0054] The embodiment of the present invention is produced using a full-arc continuous casting machine with six machines and six streams, and a casting section of 200mm×240mm.

[0055] The rolling process is as follows: the hot ingot is heated to a temperature of 1160-1200°C and kept warm for 150 minutes, the starting rolling temperature is 1060-1100°C, the final rolling temperature is 900-1000°C, and the ingot is rolled into a φ70mm bar, which is then cooled to obtain the bar. Specifically, the rolling process is as follows: the ingot is hot-sent to the rolling mill after it comes off the production line, the heating section temperature is 1160°C, the soaking section temperature is 1200°C, the heating and holding time is 150 minutes, the starting rolling temperature is 1060-1100°C, the final rolling temperature is 900-1000°C, and the rolling specifications are as follows: After rolling, it is sent to a slow cooling pit for slow cooling, with a cooling rate of less than 0.63℃ / s.

[0056] Specifically, in order to improve the refinement of the bar grains and the uniformity of the solute distribution during the rolling process, thereby improving the hardenability of the steel, the present invention implements 4 to 6 rolling passes in the rolling process, and divides the rolling process into three stages: rough rolling, intermediate rolling, and finishing rolling according to the cross-sectional compression ratio of each pass. In the rough rolling stage, that is, the first rolling pass, the cross-sectional compression ratio is ensured to be no less than 50%. Through large-scale deformation, the casting defects are effectively eliminated, laying the foundation for the uniformity of the structure, and promoting the initial refinement of the grains, while promoting the initial uniform distribution of the solute elements. In the finishing rolling stage, that is, the last rolling pass, the cross-sectional compression ratio is controlled at 10% to 25%. Through precise small deformation, the extreme refinement of the grains is achieved, the solute distribution is made more uniform, and the surface quality of the bar is improved. The intermediate rolling stage covers the remaining rolling passes, with the cross-sectional compression ratio of each pass ranging from 30% to 45%. This stage further refines the grains, reduces segregation, provides conditions for the uniform distribution of solute elements, improves the internal structure, enhances the strength and toughness of the material, and ultimately ensures that the bar has excellent mechanical properties and hardenability.

[0057] An embodiment of the present invention further provides a gear steel bar, which is prepared using the above method.

[0058] Characterization of the prepared products:

[0059] 1) In order to measure the solute content fluctuation of the cross section of the continuous casting billet and bar, the sampling method for the continuous casting billet is as follows Figure 1 As shown, the inner arc side is the inner side of the ingot produced by the full-arc continuous casting machine, and the outer arc side is the outer side of the ingot produced by the full-arc continuous casting machine. Sampling and testing are carried out on the inner and outer arc sides and horizontal sides of the ingot cross section respectively; the sampling method for bars is as follows Figure 2 As shown in a, three surfaces are selected for measurement. In order to make the detection result more accurate, the distance between two adjacent surfaces is 20-30mm.

[0060] 2) In order to explore the dendritic morphology of the bar hardenability area, the sampling position of the dendritic morphology of the bar transverse sample is as follows: Figure 2 As shown in b, a sample C with a cross section of 10mm×10mm×10mm is taken at a distance of 15mm from the surface in the radial direction. The dendritic structure corrosion method of this study is: the sample is heated to 930℃ at 10℃ / min and kept warm for 0.5h, then cooled to 680℃ at 5℃ / min and kept warm for 1h, and then the furnace is cooled to room temperature; the sample is processed to the mirror level using a metallographic grinding machine and etched with 4% nitric acid alcohol for 20-30s, and the dendritic morphology can be observed. The C, Cr, and Mn solute distribution of the dendritic structure of sample C was scanned by an electron probe (EPMA1720, Shimadzu, Japan), and the scanning area was 3.00mm×2.25mm, as shown in FIG. Figure 2 As shown in the green rectangular area in b, the uppermost side of the scanning position of sample C is 15 mm away from the center; the scanning step size is 10 μm, and the EPMA surface scan includes the hardenability detection area.

[0061] 3) In order to explore the effect of different solidification structures on the hardenability of bars, a hardenability test experiment was carried out on 20CrMnTiH bars. The hardenability test standard for gear steel 20CrMnTiH mainly complies with GB / T 225-2006. Before preparing the hardenability sample, the φ70mm round steel needs to be pre-heat treated. The heat treatment system is 930℃ for 1h and then air-cooled to room temperature. The hardenability test standard complies with GB / T 225-2006. The experimental sampling location is as follows: Figure 2 As shown in the red area c, 3 samples were taken from each of the embodiment and comparative example bars. The included angle between the axis of each sample was 120°, and the size of the obtained sample was φ25mm×100mm. The samples were then subjected to end quenching experiments. After the quenched samples A and B surfaces were ground 0.5mm each, the hardness was measured at 9mm and 15mm from the end quenching surface. Figure 2 As shown in Figure d, the obtained data are the hardness values ​​of the A and B surfaces of J9 and J15.

[0062] In order to better illustrate the embodiments of the present invention, the present invention is further described in detail below through specific examples.

[0063] Example 1

[0064] This embodiment provides a method for improving the hardenability of gear steel bars, and a method for preparing gear steel bars using the method. The steel used for the bars is 20CrMnTiH. Qualified molten steel refined by BOF+LF+VD is cast using a six-machine, six-strand, full-arc continuous casting machine. The casting section is 200 mm × 240 mm, the tundish superheat is 40°C, the crystallizer electromagnetic stirring current is 100 A, the casting speed is 1.05 m / min, the secondary cooling water ratio is 0.33 L / kg, and the end electromagnetic stirring current is 150 A.

[0065] The solid fraction at the center fs =0.05, press down 1mm, at the center solid fraction f s =0.20, press down 2mm, at the center solid fraction f s =0.57, press down 2mm, at the center solid fraction f s =1.00, and press down 2mm. Take samples of the cooled ingot for analysis.

[0066] After solidification, the hot ingot was heated to 1160℃ and kept warm for 150min. The starting rolling temperature was 1060℃ and the final rolling temperature was 950℃. The bar is then cooled to obtain the bar. Specifically, four rolling passes are used, with a cross-sectional compression ratio of 64% in one rolling pass, a cross-sectional compression ratio of 25% in the last rolling pass, and a cross-sectional compression ratio of 45% in each of the remaining rolling passes. The bar is sampled and analyzed.

[0067] like Figure 3 As shown, the equiaxed crystal ratio of the ingot prepared in this embodiment is 6.69%, and the center quality is generally good.

[0068] like Figure 4 As shown, it can be seen that the range and standard deviation of solute C in the cross section of the ingot prepared in this embodiment are 0.01wt.% and 0.003wt.%, respectively; the range and standard deviation of Cr in the cross section are 0.05wt.% and 0.016wt.%, respectively; and the range and standard deviation of Mn in the cross section are 0.03wt.% and 0.010wt.%, respectively.

[0069] like Figure 5 As shown, it can be seen that the range and standard deviation of solute C in the cross section of the rod prepared in this embodiment are 0.01wt.% and 0.002wt.%, the range and standard deviation of Cr in the cross section are 0.03wt.% and 0.009wt.%, and the range and standard deviation of Mn in the cross section are 0.01wt.% and 0.005wt.%.

[0070] like Figure 6 As shown, the hardenability test area of ​​the prepared rod is columnar crystal, and the average contents of solute C, Cr and Mn are 0.41wt.%, 1.92wt.% and 1.27wt.%, respectively; the maximum segregation ratios of C, Cr and Mn solutes are 2.41, 2.16 and 3.30, respectively.

[0071] Figure 7The test results of the A and B surfaces of the three hardenability test samples J9 and J15 are shown. It can be seen that the differences between the A and B surfaces of the three hardenability indicators of J9 are 0.6HRC, 0.7HRC and 0.2HRC respectively, and the differences between the A and B surfaces of J15 are 0.3HRC, 1.5HRC and 1.3HRC respectively. The fluctuation of the hardenability of the A and B surfaces is significantly reduced, and both are less than the required 2HRC.

[0072] Example 2

[0073] This embodiment provides a method for improving the hardenability of gear steel bars, and a method for preparing gear steel bars using the method. The steel used for the bars is 20CrMnTiH. Qualified molten steel refined by BOF+LF+VD is cast using a six-machine, six-strand, full-arc continuous casting machine. The casting section is 200 mm × 240 mm, the tundish superheat is 40°C, the crystallizer electromagnetic stirring current is 50 A, the casting speed is 0.95 m / min, the secondary cooling water volume is 0.33 L / kg, and the end electromagnetic stirring current is 100 A.

[0074] The solid fraction at the center f s =0.05, press down 1mm, at the center solid fraction f s =0.18, press down 2mm, at the center solid fraction f s =0.55, press down 2mm, at the center solid fraction f s =0.98, and press down 2mm. Take samples of the cooled ingot for analysis.

[0075] After solidification, the hot ingot is heated to 1160℃ and kept warm for 150min. The starting rolling temperature is 1060℃ and the final rolling temperature is 900℃. The bar is then cooled to obtain the bar, specifically, five rolling passes are used, with one rolling pass having a cross-sectional compression ratio of 50%, the last rolling pass having a cross-sectional compression ratio of 10%, and the remaining rolling passes having a cross-sectional compression ratio of 30-45%. The bar is sampled and analyzed.

[0076] The ingot prepared in this embodiment has an equiaxed crystal ratio of 7.0%, no central shrinkage cavity in the cross section, and the range and standard deviation of solute C in the cross section are 0.02wt.% and 0.004wt.%, respectively; the range and standard deviation of Cr in the cross section are 0.06wt.% and 0.018wt.%, respectively; and the range and standard deviation of Mn in the cross section are 0.05wt.% and 0.014wt.%, respectively.

[0077] The range and standard deviation of solute C in the cross section of the rod prepared in this example are 0.02wt.% and 0.002wt.%, respectively; the range and standard deviation of Cr in the cross section are 0.03wt.% and 0.010wt.%, respectively; and the range and standard deviation of Mn in the cross section are 0.02wt.% and 0.006wt.%, respectively.

[0078] The average contents of solute C, Cr and Mn in the dendrite structure of the rod prepared in this embodiment in the hardenability test area are 0.41wt.%, 1.93wt.% and 1.28wt.%, respectively; the maximum segregation ratios of solute C, Cr and Mn are 2.46, 2.18 and 3.38, respectively.

[0079] The A and B surface test results of the rolled material samples J9 and J15 prepared in this embodiment are as follows: the differences between the A and B surfaces of J9 are 0.9HRC, 1.2HRC and 0.9HRC respectively, and the differences between the A and B surfaces of J15 are 0.8HRC, 1.3HRC and 1.1HRC respectively.

[0080] Example 3

[0081] This embodiment provides a method for improving the hardenability of gear steel bars, and a method for preparing gear steel bars using the method. The steel used for the bars is 20CrMnTiH. Qualified molten steel refined by BOF+LF+VD is cast using a six-machine, six-strand, full-arc continuous casting machine. The casting section is 200 mm × 240 mm, the tundish superheat is 45°C, the crystallizer electromagnetic stirring current is 150 A, the casting speed is 1.15 m / min, the secondary cooling water volume is 0.35 L / kg, and the end electromagnetic stirring current is 200 A.

[0082] The solid fraction at the center f s =0.08, press down 1mm, at the center solid fraction f s =0.20, press down 2mm, at the center solid fraction f s =0.58, press down 2mm, at the center solid fraction f s =1, press down 2mm, and take samples of the cooled ingot for analysis.

[0083] After solidification, the hot ingot is heated to 1200℃ and kept at this temperature for 150min. The starting rolling temperature is 1100℃ and the final rolling temperature is 1000℃. The bar is then cooled to obtain the bar, specifically, six rolling passes are used, with one rolling pass having a cross-sectional compression ratio of 50%, the last rolling pass having a cross-sectional compression ratio of 10%, and the remaining rolling passes having a cross-sectional compression ratio of 30-45%. The bar is sampled and analyzed.

[0084] The ingot prepared in this embodiment has an equiaxed crystal ratio of 7.2%, no central shrinkage cavity in the cross section, and the range and standard deviation of solute C in the cross section are 0.02wt.% and 0.005wt.%, respectively; the range and standard deviation of Cr in the cross section are 0.07wt.% and 0.0170wt.%, respectively; and the range and standard deviation of Mn in the cross section are 0.06wt.% and 0.014wt.%, respectively.

[0085] The range and standard deviation of solute C in the cross section of the rod prepared in this example are 0.02wt.% and 0.004wt.%, respectively; the range and standard deviation of Cr in the cross section are 0.03wt.% and 0.011wt.%, respectively; and the range and standard deviation of Mn in the cross section are 0.02wt.% and 0.008wt.%, respectively.

[0086] The average contents of solute C, Cr and Mn in the dendrite structure of the rod prepared in this embodiment in the hardenability test area are 0.41wt.%, 1.93wt.% and 1.29wt.%, respectively; the maximum segregation ratios of solute C, Cr and Mn are 2.50, 2.19 and 3.42, respectively.

[0087] The A and B surface test results of the rolled material samples J9 and J15 prepared in this embodiment are as follows: the differences between the A and B surfaces of J9 are 1.0HRC, 1.3HRC and 1.1HRC respectively, and the differences between the A and B surfaces of J15 are 0.9HRC, 1.4HRC and 1.2HRC respectively.

[0088] Comparative Example 1

[0089] Different from Example 1, in Comparative Example 1, the superheat is 35° C., the electromagnetic stirring of the crystallizer is 300 A, and pressure reduction control is not adopted, that is, pressure reduction operation is not adopted in the range of the central solid phase fraction of 0-1.

[0090] like Figure 8 As shown, the equiaxed crystal ratio of the ingot prepared in this comparative example is 26.42%. Compared with the columnar crystal area, the interdendritic segregation in the equiaxed crystal area is more obvious, and the dark spots formed by solute segregation are large in size and large in number, which is a typical point segregation.

[0091] like Figure 9 As shown, the cross-sectional solute C range and standard deviation of the ingot prepared in this comparative example were 0.02 wt.% and 0.007 wt.%, respectively; the cross-sectional Cr range and standard deviation were 0.09 wt.% and 0.022 wt.%, respectively; and the cross-sectional Mn range and standard deviation were 0.08 wt.% and 0.020 wt.%, respectively. Compared with Example 1, the cross-sectional solute distribution of the ingot fluctuated significantly.

[0092] like Figure 10As shown, the range and standard deviation of cross-sectional solute C in the rod prepared in this comparative example were 0.02 wt.% and 0.005 wt.%, respectively; the range and standard deviation of cross-sectional Cr were 0.04 wt.% and 0.013 wt.%, respectively; and the range and standard deviation of cross-sectional Mn were 0.04 wt.% and 0.012 wt.%, respectively. Compared with the examples, the solute C, Cr, and Mn contents in the comparative example fluctuated significantly.

[0093] like Figure 11 As shown, the hardenability test area of ​​the rod prepared in this comparative example is a typical equiaxed crystal region. The average contents of C, Cr, and Mn in the dendrite structure are 0.42 wt.%, 1.95 wt.%, and 1.31 wt.%, respectively. The maximum segregation ratios of C, Cr, and Mn are 2.75, 2.24, and 3.73, respectively. Compared with the examples, the dendrite structure of the comparative example is significantly enriched in C, Cr, and Mn.

[0094] like Figure 12 As shown in Figure 3, the A and B surface test results of the bar samples J9 and J15 prepared in this comparative example show that the differences between the A and B surfaces of J9 are 2.3HRC, 3.0HRC, and 2.7HRC, respectively, and the differences between the A and B surfaces of J15 are 4.1HRC, 2.5HRC, and 3.2HRC, respectively. The differences between the A and B surfaces of the comparative example bars J9 and J15 are much greater than 2HRC.

[0095] Comparative Example 2

[0096] Different from Example 1, the superheat degree in this comparative example is 35°C.

[0097] The A and B surface test results of the three hardenability tests of the bar samples J9 and J15 prepared in this comparative example showed that the differences between the A and B surfaces of J9 were 2.1HRC, 2.5HRC and 2.4HRC respectively, and the differences between the A and B surfaces of J15 were 2.0HRC, 2.6HRC and 2.3HRC respectively.

[0098] Comparative Example 3

[0099] Different from Example 1, the electromagnetic stirring of the crystallizer in this comparative example is 300A.

[0100] The A and B surface test results of the three hardenability tests of the bar samples J9 and J15 prepared in this comparative example showed that the differences between the A and B surfaces of J9 were 1.9HRC, 2.4HRC and 2.1HRC respectively, and the differences between the A and B surfaces of J15 were 2.3HRC, 2.8HRC and 2.2HRC respectively.

[0101] Comparative Example 4

[0102] Different from Example 1, this comparative example does not adopt pressure reduction control, that is, no pressure reduction operation is adopted in the range of the central solid phase fraction of 0-1.

[0103] The A and B surface test results of the three hardenability tests of the bar samples J9 and J15 prepared in this comparative example showed that the differences between the A and B surfaces of J9 were 2.3HRC, 2.9HRC and 2.4HRC respectively, and the differences between the A and B surfaces of J15 were 3.8HRC, 2.8HRC and 3.3HRC respectively.

[0104] Comparative Example 5

[0105] Different from Example 1, the rolling process in this comparative example is: 5 rolling passes are adopted, the cross-sectional compression ratio of the first rolling pass is 45%, the cross-sectional compression ratio of the last rolling pass is 30%, and the cross-sectional compression ratio of each of the remaining rolling passes is 30% to 45%.

[0106] The A and B surface test results of the three hardenability tests of the bar samples J9 and J15 prepared in this comparative example showed that the differences between the A and B surfaces of J9 were 1.8HRC, 1.7HRC and 2.1HRC respectively, and the differences between the A and B surfaces of J15 were 1.9HRC, 2.2HRC and 1.8HRC respectively.

[0107] It can be seen from Example 1 and Comparative Examples 1-5 that the solute C, Cr and Mn in the cross-section of the ingot prepared by the technical solution provided by the present invention are evenly distributed, the equiaxed crystal area of ​​the ingot is reduced, the number of point segregations is small and the size is small; the dendrite structure in the hardenability test area of ​​the bar is columnar crystal, and the solute is evenly distributed; it meets the technical agreement indicators of downstream customers.

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving the hardenability of gear steel bars, wherein the difference between the A and B surfaces of J9 and J15 of the gear steel is not greater than 1.5HRC, characterized in that: include: The molten steel with qualified composition is cast through a continuous casting machine; During the solidification stage, a reduction process is used; After solidification, the hot ingot is rolled; During the casting process, the tundish superheat is 40-45°C, and the crystallizer electromagnetic stirring current is 50-150A. The reduction process controls the reduction by changing the solid phase ratio at the center of the ingot. The solid phase ratio at the center of the reduction area is 0.05-1.

0. When the solid phase ratio is less than 0.3, the reduction in the continuous casting reduction area is not less than 1mm and not more than 3mm. The rolling section compression ratio is not less than 90%. The casting process parameters are: constant temperature and constant pulling speed during continuous casting, pulling speed is 0.95-1.15m / min, secondary cooling water volume is 0.33-0.35L / kg, and electromagnetic stirring current at the end of solidification is 100-200A; The maximum single pressing amount satisfies the following formula: ； Wherein, ε is the maximum critical strain, v is the working speed, in m / min, h is the thickness of the ingot, in mm, r is the diameter of the reduction roll, in mm, b is the roller spacing, in mm, and a is the maximum single reduction, in mm; When the solid fraction in the center of the reduction zone is between 0.05 and 1.0, the total reduction is calculated as follows: ； ； in, is the reduction coefficient, which ranges from 0.8 to 1.1, A is the total amount of reduction, V R is the pressing rate, t is the time, h is the thickness of the billet, W is the width of the billet, T L is the liquidus temperature of the steel grade, T S is the solidus temperature of the steel grade; The relationship between the reduction and the central solid fraction is: when , the pressing amount is 1mm; when , the pressing amount is 2mm; when , the pressing amount is 2mm; when , the pressing amount is 2mm.

2. The method for improving the hardenability of gear steel bars according to claim 1, characterized in that: The steel used for the bar is 20CrMnTiH steel.

3. The method for improving the hardenability of gear steel bars according to claim 1, characterized in that: The rolling process is as follows: heating the hot ingot to a temperature of 1160-1200°C and keeping it warm for 150 minutes, starting rolling at a temperature of 1060-1100°C, finishing rolling at a temperature of 900-1000°C, rolling it into a φ70mm bar, and then cooling it to obtain the bar.

4. The method for improving the hardenability of gear steel bars according to claim 3, characterized in that: The rolling is performed in 4 to 6 passes, with the cross-sectional compression ratio of the first rolling pass being not less than 50%, the cross-sectional compression ratio of the last rolling pass being 10% to 25%, and the cross-sectional compression ratio of each of the remaining rolling passes being 30% to 45%.

5. The method for improving the hardenability of gear steel bars according to claim 1, characterized in that: The continuous casting machine is a full-arc continuous casting machine with six machines and six streams, and a casting section of 200mm×240mm.

6. A gear steel bar, characterized in that: The gear steel bar is prepared by the method according to any one of claims 1 to 5.

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