A method for improving high temperature grain size mixing and surface decarburization of gear steel for cold forging

Abstract

The invention relates to a method for improving high-temperature grain size mixed crystal and surface decarburization of gear steel for cold and warm forging, and belongs to the field of iron and steel metallurgy. The method specifically relates to hot rolling forming of gear steel, and comprises a pre-rolling heating process: heating temperature of each section: 1250-1280°C in a soaking section, 1200-1250°C in a heating section, and ≤900°C in a preheating section; heating speed of each section: heating time of the preheating section is 120-140 minutes, heating time of the heating section is 30-50 minutes, and heating time of the soaking section is 50-60 minutes; atmosphere in a heating furnace: residual oxygen content is controlled to be below 0.5%; high-pressure water is used to descale the billet after it is taken out of the furnace, the descaling water pressure is adjusted to more than 250Kg, the rolling temperature is started to be ≥1150°C, a secondary descaling machine in the rolling process and a control process of cooling water in the rolling process: the secondary descaling is closed during the rolling process, and the opening degree of cooling water in each rolling pass is reduced, and the water cooling pressure is adjusted to 3.5Kg to ensure that the post-rolling temperature is above 1000°C. The method is used to improve the high-temperature grain size mixing and surface decarburization of gear steel.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and in particular relates to a method for improving high-temperature grain size mixed crystal and surface decarburization of gear steel. Background Art

[0002] With the serious imbalance between supply and demand in the steel industry, production capacity seriously exceeds demand, and quality competition and cost competition are becoming increasingly severe. Various steel users are constantly creating new processing technologies to achieve their continuous cost reduction while improving product performance requirements. For example, gear steel for cold forging can reduce heating costs for users while reducing steel processing waste and surplus materials, thereby improving the utilization rate of steel. The gear steel for cold forging in this application meets the needs of such users due to its superior high-temperature grain size and decarburization performance, thereby increasing product competitiveness.

[0003] In order to achieve the two performance requirements of high temperature grain size and decarburization of gear steel for cold forging, there are mainly the following difficulties in combination with the current production process path:

[0004] 1. There are many uncertainties in the realization of key parameters of the heating system. The higher heating temperature is quite different from the heating process of other types of steel. The continuity of rolling production and the conversion of types and specifications are subject to more production restrictions, which may cause other types of steel on the production line to overheat or even overburn. At the same time, the strict control range of each heating time results in a small emergency scope for workers to deal with abnormal risks, which may easily cause the heating time to be too long and cause excessive decarburization of the steel billet.

[0005] 2. The degree of cooling water opening of each key equipment in the rolling process affects the cooling effect of the rolling equipment, which may easily cause equipment damage.

[0006] 3. The temperature after rolling is above 1000℃, and the collection after rolling is prone to large bending deformation caused by high temperature, which affects the subsequent straightening process and even causes accidents that damage the straightening equipment. Summary of the invention

[0007] In view of the above-mentioned prior art, the present invention provides a method for improving the high-temperature grain size mixing and decarburization of gear steel for cold forging, including redesigning and improving the steel billet heating and rolling processes, so as to stably produce products that meet the needs of users of gear steel for cold forging.

[0008] The technical scheme of the present invention is: a method for improving the high temperature grain size mixed crystal and surface decarburization of gear steel for cold forging, which specifically relates to hot rolling forming of gear steel billet, including pre-rolling heating process and rolling process, wherein

[0009] Heating process: ① Heating temperature parameters of each section: soaking section 1250~1280℃, heating section 1200~1250℃, preheating section ≤900℃, to ensure that the starting rolling temperature is above 1150℃ and the post-rolling temperature is above 1000℃, to ensure that the high-temperature grain size of this product reaches above level 5; ② Heating speed: the first heating time is 120~140 minutes, the second heating time is 30~50 minutes, and the soaking section is 50~60 minutes. According to the actual production situation, the step rhythm of the billet is adjusted within the required range to ensure that the heating time of each section is within the design range to achieve the prerequisite of the required high-temperature grain size; ③ Atmosphere in the heating furnace: Due to the process limitation of this higher heating temperature, the product's decarburization sensitivity is increased. Through the current intelligent winding system, the residual oxygen content is controlled below 0.5% to limit the sensitivity of decarburization.

[0010] Rolling process: ① Setting of high-pressure water descaling parameters after the billet is out of the furnace. When producing this type of product, it is necessary to ensure that the descaling equipment operates normally, and at the same time adjust its pressure to more than 250Kg to ensure that the surface oxide scale is cleaned and the decarburization in the residual oxide scale area is avoided; ② Control process of the secondary descaling machine and cooling water in the rolling process. Since the data shows that the surface grain size exceeds the standard in a large proportion, which is related to the substantial cooling of the surface, the secondary descaling is closed during the rolling process, and the opening degree of each cooling water is reduced by 50%. It is preferred to adjust the water cooling pressure to 3.5Kg and execute it according to the high alloy steel production process to ensure that the post-rolling temperature is above 1000℃ to ensure the qualified grain size.

[0011] After rolling, the gear steel is air-cooled on a cooling bed to below 400°C.

[0012] Preferably, when the gear steel billet is heated together with other types of steel billets by step heating, the space steps between the types are greater than 15 steps.

[0013] Preferably, in the rolling process, the opening degree of the cooling water is increased during the rolling gap to avoid equipment damage caused by low cooling opening degree of the rolling equipment.

[0014] Preferably, a straightening grid is provided on the cooling bed to support the high-temperature gear steel after rolling.

[0015] Preferably, the gear steel is a low-carbon Mn-Cr gear steel, wherein the mass percentage of C is less than 0.2%.

[0016] Compared with the prior art, the advantages of the present invention are: the implementation of the above method helps to eliminate the phenomenon of high-temperature grain size mixed crystals in gear steel, and also reduces the decarburization of the gear steel surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1This is a decarburization detection diagram of a rolled product before the present invention is designed, the decarburization layer thickness is greater than 0.5% D, where D is the diameter of the round steel;

[0018] Figure 2 This is the grain size test diagram of the rolled product before the design of the present invention, full cross-section mixed crystal: 1-7 level;

[0019] Figure 3 This is a decarburization detection diagram of a product rolled after the method of the present invention is implemented, the thickness of the decarburization layer is ≤0.3%D, and D is the diameter of the round steel;

[0020] Figure 4 This is a decarburization detection diagram of the product rolled after the implementation of the method of the present invention, and the grain size of the entire cross section is level 7. DETAILED DESCRIPTION

[0021] The present invention is further described in detail below in conjunction with the accompanying drawings. The embodiments are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0022] This embodiment is aimed at low-carbon MnCr series gear steel products with requirements on grain size, and the main production process is: heating - high-pressure water descaling - rolling - cooling - sawing - finishing inspection - packaging and warehousing.

[0023] Through the previous production history test data and user feedback on product quality, the products mainly have problems with grain size mixing and decarburization exceeding the standard. Grain size mixing is difficult to save through large-scale heat treatment after testing. Decarburization exceeding the standard increases the surface peeling cost. If it occurs at the user's site, it will cause the product to be scrapped or the user to claim huge compensation. We will compare and analyze the previous production history test data and user feedback on product quality in combination with the corresponding rolling production process and formulate measures. Through continuous process exploration and process test practice summary, we have determined A new and stable production method was developed, mainly for changing the heating system and rolling process of the gear steel used for cold forging to: ① The heating temperature parameters of each section are: soaking section 1250~1280℃, heating section 1200~1250℃, preheating section ≤900℃, ensuring that the starting rolling temperature is above 1150℃ and the post-rolling temperature is above 1000℃, ensuring that the high-temperature grain size of this product reaches above level 5; ② Heating speed: 120-140 minutes for the first heating section, 30-50 minutes for the second heating section, 50-60 minutes for the soaking section 3. The atmosphere in the heating furnace: Due to the process limitation of this higher heating temperature, the product decarburization sensitivity is increased. The current intelligent winding system is used to control the residual oxygen content below 0.5% to limit the sensitivity of decarburization. 4. The high-pressure water descaling parameter setting after the billet is out of the furnace. When producing this type of product, it is necessary to ensure that the descaling equipment operates normally and adjust its pressure to more than 250Kg to ensure that the decarburization equipment is properly operated. Clean the surface oxide scale to avoid excessive decarburization in the residual oxide scale area; ⑤ The control process of the secondary descaling machine and the cooling water in the rolling process. Since the data shows that the surface grain size exceeds the standard in a large proportion, which is related to the substantial cooling of the surface, the secondary descaling is closed during the rolling process. At the same time, the opening degree of each cooling water is reduced by 50%, and the water cooling pressure is set to 3.5Kg. According to the high-alloy steel production process, the post-rolling temperature is guaranteed to be above 1000℃, and air cooling is carried out on the cooling bed to below 400℃ to ensure the grain size is qualified.

[0024] The above heating process is applicable to billet specifications: 300-320 square meters. It is mainly applicable to low-carbon MnCr series gear steel products with requirements on grain size.

[0025] In order to eliminate the main difficulties or problems in the path of specific implementation method 1, it is mainly completed in the following ways: ① In order to eliminate the impact on the production of other varieties, separate furnace production is implemented according to the batch situation or the advantages of the step heating furnace are utilized. The empty steps between varieties are greater than 15 steps to avoid mutual influence. At the same time, time for changing specifications can be reserved to improve the heating quality; ② In order to avoid equipment damage caused by low cooling opening of rolling equipment, manual operation is performed to open the water volume to the maximum during the rolling gap to ensure sufficient cooling, and automatic control can be optimized through program optimization later; ③ Due to high-temperature cooling after rolling, bending caused by uneven cooling is easy to occur. By increasing the number of straightening grids on the cooling bed, it is ensured that it is fully and evenly cooled before shearing and collection to improve the straightness of the product.

[0026] In summary, by improving and innovating the heating and rolling process of rolled gear steel products and applying them to the production process of the entire product process to ensure the various indicators of the rolled products, stable and efficient production is achieved while meeting the needs of users.

Claims

1. A method for improving high temperature grain size mixed crystal and surface decarburization of gear steel for cold forging, characterized by: The method specifically relates to hot rolling forming of gear steel billets, including a pre-rolling heating process and a rolling process, wherein: Pre-rolling heating process: multi-stage heating is adopted, and the heating temperature of each section is: 1250-1280℃ in the soaking section, 1200-1250℃ in the heating section, and ≤900℃ in the preheating section; the heating speed of each section is: 120-140 minutes in the preheating section, 30-50 minutes in the heating section, and 50-60 minutes in the soaking section. The billet is heated in a step-by-step manner, and the pace speed of the billet on the roller is controlled to meet the heating time of each section; the atmosphere in the heating furnace: the residual oxygen content is controlled below 0.5%; Rolling process: After the billet is out of the furnace, it is descaled with high-pressure water. The descaling water pressure is adjusted to more than 250Kg. The billet is directly hot-rolled into gear steel of target specifications. The starting rolling temperature is ≥1150℃. The control process of the secondary descaling machine and the cooling water during the rolling process: During the rolling process, the secondary descaling is turned off, and the opening degree of the cooling water for each rolling pass is reduced. The water cooling pressure is adjusted to 3.5Kg to ensure that the temperature after rolling is above 1000℃.

2. The method for improving high temperature grain size mixed crystal and surface decarburization of cold forging gear steel according to claim 1, characterized in that: After rolling, the gear steel is air-cooled on a cooling bed to below 400°C.

3. The method for improving high temperature grain size mixed crystal and surface decarburization of cold forging gear steel according to claim 1, characterized in that: When the gear steel billet is heated step by step with other types of steel billets, the space steps between the types are greater than 15 steps.

4. The method for improving high temperature grain size mixed crystal and surface decarburization of cold forging gear steel according to claim 1, characterized in that: In the rolling process, the opening degree of the cooling water is increased in the rolling gap.

5. The method for improving high temperature grain size mixed crystal and surface decarburization of cold forging gear steel according to claim 2, characterized in that: The cooling bed is provided with a straightening grid for supporting the high-temperature gear steel after rolling.

6. The method for improving high temperature grain size mixed crystal and surface decarburization of cold forging gear steel according to claim 1, characterized in that: The gear steel is low carbon Mn-Cr series low carbon gear steel.

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