A method for controlling surface quality of composite microalloyed gear steel

Abstract

The present invention belongs to the technical field of steel metallurgy and steelmaking, and specifically relates to a surface quality control method for composite microalloyed gear steel. The present invention indirectly reduces the cooling intensity of the primary shell of the ingot through medium-to-high speed casting and ultra-weak cooling, thereby reducing the solidification volume shrinkage and phase transformation volume shrinkage during the formation of the primary shell; and adopts a large inverted taper crystallizer copper tube to reduce the air gap between the solidified shell of the ingot and the inner wall of the crystallizer copper tube; adopts high-viscosity protective slag on the crystallizer liquid surface to improve the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the crystallizer copper tube. Combining the above measures, the present invention starts from reducing the cooling intensity of the primary shell of the ingot, reducing the air gap between the solidified shell of the ingot and the inner wall of the crystallizer copper tube, and improving the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the crystallizer copper tube, so that the tissues of various parts of the solidified shell of the ingot are uniformly solidified, effectively avoiding the generation of crack sources due to stress in the solidified shell of the ingot, and improving the surface quality of the ingot.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy and steelmaking, and particularly relates to a surface quality control method for composite micro-alloyed gear steel. Background Art

[0002] With the rapid development of vacuum high-temperature carburizing furnaces and their processes in my country, the increase in carburizing heat treatment process temperature and the improvement in carburizing efficiency can easily lead to product quality problems such as coarse grains and mixed crystals in the gear carburizing heat treatment process, which puts higher requirements on the quality of steel.

[0003] To prevent grain coarsening during high-temperature carburizing, composite microalloying with Nb, Ti, V, N, and Al has become a trend toward grain refinement. However, due to the immaturity of the current production process for composite microalloyed gear steel, cracks are very likely to form on the surface of the ingot during the casting process, and the surface cracking of the rolled steel is a serious problem. This is especially true for gear steel, which is a peritectic reaction steel. The formation of the primary ingot shell is accompanied by significant phase transformation and volume shrinkage, which increases the tendency to crack. As a result, the overall product qualification rate is only around 50%, seriously affecting customer delivery and increasing pressure on process cost control. Summary of the Invention

[0004] The purpose of the present invention is to provide a surface quality control method for composite microalloyed gear steel, which indirectly reduces the cooling strength of the primary shell of the ingot by medium-high casting speed and ultra-weak cooling, reduces the solidification volume shrinkage and phase change volume shrinkage during the formation of the primary shell, and prevents the generation of thermal stress and tissue stress cracks; and adopts a large inverted taper crystallizer copper tube to reduce the air gap between the solidified shell of the ingot and the inner wall of the crystallizer copper tube, and prevents the formation of hot spots in the shell due to the existence of the air gap, resulting in thermal stress and tissue stress cracks; adopts a high-viscosity protective slag on the liquid surface of the crystallizer to improve the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the crystallizer copper tube, and prevents To prevent the slag film between the solidified shell of the ingot and the inner wall of the copper tube of the crystallizer from being too thin or too thick, resulting in poor lubrication and heat transfer of the slag film, uneven cooling of the ingot shell, and generation of thermal stress and structural stress cracks, the present invention combines the above measures, starting from reducing the cooling intensity of the primary shell of the ingot, reducing the air gap between the solidified shell of the ingot and the inner wall of the copper tube of the crystallizer, and improving the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the copper tube of the crystallizer, so that the tissues of various parts of the solidified shell of the ingot are uniformly solidified, effectively avoiding the source of cracks in the solidified shell of the ingot due to stress, and suppressing the generation of cracks in the subsequent rolling process, thereby effectively improving the surface quality of the ingot.

[0005] The technical problem to be solved by the present invention is achieved by adopting the following technical solution: A method for controlling the surface quality of composite microalloyed gear steel comprises the following steps:

[0006] S1. Use medium-high drawing speed casting during the casting process, and control the drawing speed of the casting process within a reasonable drawing speed range to ensure the internal quality of the casting and casting safety, that is, to prevent steel leakage and meet the requirements of grain refinement. The medium-high drawing speed casting mode is used to reduce the cooling time of the primary shell of the casting in the crystallizer, indirectly reducing the cooling intensity of the primary shell of the casting, thereby reducing the solidification volume shrinkage and phase transformation volume shrinkage during the formation of the primary shell, and preventing the generation of thermal stress and structural stress cracks.

[0007] S2. The crystallizer adopts ultra-weak cooling control to control the water flow rate of the crystallizer at 130-135m 3 / h; Advantages: Within the reasonable cooling system range that ensures the internal quality of the ingot and the safety of casting, the ultra-weak cooling control of the crystallizer is adopted to indirectly reduce the cooling intensity of the primary shell of the ingot, reduce the solidification volume shrinkage and phase transformation volume shrinkage during the formation of the primary shell, and prevent the generation of thermal stress and structural stress cracks;

[0008] S3. Use a large back-tapered copper tube in the mold, and control the back-taper of the tube within 1.00-1.15% / m. Advantages: Within a reasonable back-taper range to ensure the internal quality of the ingot and casting safety, the use of a large back-tapered copper tube in the mold can effectively reduce the air gap between the solidified shell of the ingot and the inner wall of the mold copper tube, preventing the formation of hot spots in the shell due to the existence of air gaps, which can cause thermal stress and structural stress cracks.

[0009] S4. Use high-viscosity mold slag for lubrication and insulation of the mold surface, controlling the mold surface slag viscosity between 0.85 and 0.95 Pa·s. Advantages: Within a reasonable mold surface slag viscosity range that ensures the internal quality of the ingot and casting safety, using high-viscosity mold slag on the mold surface can effectively improve the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the mold copper tube. This prevents the slag film from becoming too thin or too thick, resulting in poor slag film lubrication and heat transfer, uneven cooling of the ingot shell, and the generation of thermal stress and structural stress cracks.

[0010] The technical solution of the present invention further includes the following steps: S5, the billets are placed in a straight-line pit for slow cooling: After the billets come off the production line, they are promptly stacked in a straight-line manner for slow cooling. Advantages: The straight-line pit slow cooling can effectively connect small gaps or even seamlessly between different billets, achieving slow and uniform cooling of the billets, and avoiding thermal stress and structural stress cracks caused by poor billet slow cooling.

[0011] The technical solution of the present invention is as follows: in step S5, a heat preservation cover is added above the slow cooling pit, the slow cooling time of the billet is more than 36 hours, and the temperature out of the pit is less than 150°C.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] The present invention adopts a medium-to-high speed casting mode to reduce the cooling time of the primary shell of the casting billet in the crystallizer, indirectly reducing the cooling intensity of the primary shell of the casting billet, thereby reducing the solidification volume shrinkage and phase change volume shrinkage during the formation of the primary shell, and preventing the generation of thermal stress and structural stress cracks.

[0014] The present invention adopts ultra-weak cooling control of the crystallizer to indirectly reduce the cooling intensity of the primary shell of the casting billet, reduce the solidification volume shrinkage and phase change volume shrinkage during the formation of the primary shell, and prevent the casting billet from generating thermal stress and structural stress cracks.

[0015] The present invention adopts high-viscosity protective slag on the liquid surface of the crystallizer, which can more effectively improve the uniformity of the slag film between the solidified shell of the casting blank and the inner wall of the crystallizer copper tube, realize uniform cooling of the casting blank shell, and prevent the casting blank from generating thermal stress and structural stress cracks.

[0016] The present invention adopts a straight-line pit for slow cooling, which can well realize small gaps or even seamless connection between different castings, and the castings can better realize slow and uniform cooling, avoiding poor slow cooling effect of the castings to generate thermal stress and structural stress cracks.

[0017] The surface quality control method of the composite microalloyed gear steel of the present invention can effectively improve the surface quality of the composite microalloyed gear steel, overcome the problem of serious cracks on the gear steel surface caused by the sensitivity of the composite microalloying elements to the crack source, and achieve a qualified rate of more than 90% for the surface magnetic leakage detection of the composite microalloyed gear steel according to the GB / T32547 standard Level 3, effectively solving the crack problem of the gear steel caused by composite microalloying and peritectic reaction. DETAILED DESCRIPTION

[0018] Example 1

[0019] Prepare 260mm*300mm specification section, finished material A method for controlling the surface quality of a composite microalloyed gear steel of a certain specification according to an embodiment of the present invention comprises the following steps:

[0020] S1. The casting of the billet adopts medium-high pulling speed casting, and the pulling speed of the billet casting process is controlled at 0.52m / min.

[0021] S2. The crystallizer adopts ultra-weak cooling control to control the water flow of the crystallizer at 130m 3 / h.

[0022] S3. Use a large back-tapered copper tube for the crystallizer and control the back-taper of the copper tube at 1.05% / m.

[0023] S4. The crystallizer liquid surface is lubricated and insulated with high-viscosity protection slag, and the viscosity of the protection slag on the crystallizer liquid surface is controlled at 0.85 Pa·s.

[0024] S5. The ingots are put into the pit in a straight line for slow cooling: after the ingots come off the line, they should be put into the pit in a straight line for slow cooling in time. An insulation cover is added above the slow cooling pit. The slow cooling time of the ingots is 36 hours, and the temperature out of the pit is 120℃.

[0025] The composite micro-alloyed gear steel prepared by the method for controlling the surface quality of the composite micro-alloyed gear steel in Example 1 had a pass rate of 92% in surface magnetic flux leakage testing at level 3 according to GB / T32547 standard.

[0026] Example 2

[0027] Prepare 260mm*300mm specification section, finished material A method for controlling the surface quality of a composite microalloyed gear steel of a certain specification according to an embodiment of the present invention comprises the following steps:

[0028] S1. The casting of the billet adopts medium-high pulling speed casting, and the pulling speed of the billet casting process is controlled at 0.57m / min.

[0029] S2. The crystallizer adopts ultra-weak cooling control to control the water flow of the crystallizer at 135m 3 / h.

[0030] S3. Use a large back-tapered copper tube for the crystallizer and control the back-taper of the copper tube at 1.12% / m.

[0031] S4. The crystallizer liquid surface is lubricated and insulated with high-viscosity protection slag, and the viscosity of the protection slag on the crystallizer liquid surface is controlled at 0.80 Pa·s.

[0032] S5. The ingots are put into the pit in a straight line for slow cooling: After the ingots come off the line, they should be stacked in a straight line for slow cooling. An insulation cover is placed above the slow cooling pit. The slow cooling time of the ingots is 42 hours, and the temperature out of the pit is 105℃.

[0033] The composite micro-alloyed gear steel prepared by the method for controlling the surface quality of the composite micro-alloyed gear steel in Example 2 had a pass rate of 91.5% in surface magnetic flux leakage testing at level 3 according to GB / T32547 standard.

[0034] Comparative Example 1

[0035] Prepare 260mm*300mm specification section, finished material The composite micro-alloyed gear steel of the specification is prepared by using the existing composite micro-alloyed gear steel preparation process, which includes the following steps:

[0036] S1. The casting process of the billet is carried out at a low casting speed, and the casting speed is controlled at 0.45m / min.

[0037] S2. The crystallizer adopts weak cooling control and controls the water flow of the crystallizer at 145m 3 / h.

[0038] S3. Control the back taper of the copper tube of the crystallizer to 0.90% / m.

[0039] S4. The crystallizer liquid surface is lubricated and insulated with low-viscosity protection slag, and the viscosity of the protection slag on the crystallizer liquid surface is controlled at 0.48 Pa·s.

[0040] S5. The ingots are placed in a cross-shaped pit for slow cooling: After the ingots come off the line, they should be placed in a cross-shaped stacking pit for slow cooling in a timely manner. An insulation cover is placed above the slow cooling pit. The slow cooling time of the ingots is 24 hours, and the temperature out of the pit is 91°C.

[0041] The composite micro-alloyed gear steel prepared by the existing composite micro-alloyed gear steel preparation process in Comparative Example 1 has a pass rate of 48.5% in the level 3 surface magnetic flux leakage detection according to GB / T32547 standard.

[0042] Comparative Example 2

[0043] Prepare 260mm*300mm specification section, finished material The composite micro-alloyed gear steel of the specification is prepared by using the existing composite micro-alloyed gear steel preparation process, which includes the following steps:

[0044] S1. The casting process of the billet is carried out at a low casting speed, and the casting speed is controlled at 0.48m / min.

[0045] S2. The crystallizer adopts weak cooling control and controls the water flow of the crystallizer at 140m 3 / h.

[0046] S3. Control the back taper of the copper tube of the crystallizer to 0.85% / m.

[0047] S4. The crystallizer liquid surface is lubricated and insulated with low-viscosity protection slag, and the viscosity of the protection slag on the crystallizer liquid surface is controlled at 0.67 Pa·s.

[0048] S5. The ingots are placed in a cross-shaped pit for slow cooling: After the ingots come off the line, they should be placed in a cross-shaped stacking pit for slow cooling in a timely manner. An insulation cover is placed above the slow cooling pit. The slow cooling time of the ingots is 27 hours, and the temperature out of the pit is 65°C.

[0049] The composite micro-alloyed gear steel prepared by the existing composite micro-alloyed gear steel preparation process in Comparative Example 2 has a pass rate of 52.9% in the surface magnetic flux leakage detection of level 3 according to GB / T32547 standard.

[0050] Any content not described in detail in the present invention can be based on conventional technical knowledge in the art.

[0051] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and are intended to be encompassed by the claims of the present invention.

Claims

1. A method for controlling the surface quality of composite microalloyed gear steel, characterized in that: The following steps are involved: S1. Use medium-high drawing speed casting during the casting process, and control the drawing speed of the casting process to 0.50-0.60m / min, so as to reduce the cooling time of the primary shell of the casting in the crystallizer, indirectly reduce the cooling intensity of the primary shell of the casting, and thus reduce the solidification volume shrinkage and phase transformation volume shrinkage during the formation of the primary shell, thereby preventing the generation of thermal stress and structural stress cracks; S2. The crystallizer adopts ultra-weak cooling control to control the water flow rate of the crystallizer at 130-135m 3 / h, indirectly reducing the cooling strength of the primary shell of the casting, reducing the solidification volume shrinkage and phase transformation volume shrinkage during the formation of the primary shell, and preventing the generation of thermal stress and structural stress cracks; S3. Use a large back-tapered copper tube for the crystallizer, and control the back-taper of the copper tube to 1.00-1.15% / m to reduce the air gap between the solidified shell of the ingot and the inner wall of the copper tube, and prevent the formation of hot spots in the shell due to the existence of the air gap, which will cause thermal stress and structural stress cracks. S4. Use high-viscosity protective slag to lubricate and insulate the liquid surface of the crystallizer. Control the viscosity of the protective slag on the liquid surface of the crystallizer at 0.85-0.95 Pa·s to improve the uniformity of the slag film between the solidified shell of the ingot and the inner wall of the copper tube of the crystallizer. Prevent the slag film between the solidified shell of the ingot and the inner wall of the copper tube of the crystallizer from being too thin or too thick, which will cause poor lubrication and heat transfer of the slag film, uneven cooling of the ingot shell, and thermal stress and structural stress cracks.

2. The surface quality control method of composite microalloyed gear steel according to claim 1, characterized in that: The following steps are also included: S5. The ingots should be placed in a pit in a straight line for slow cooling: after the ingots come off the line, they should be stacked in a straight line for slow cooling.

3. The surface quality control method of composite microalloyed gear steel according to claim 2, characterized in that: In step S5, a heat-insulating cover is added above the slow cooling pit, the slow cooling time of the ingot is more than 36 hours, and the temperature out of the pit is less than 150°C.