Process for improving corner cracks of 16MnCr5 steel hot rolled blank

Abstract

The application discloses a process for improving corner crack of 16MnCr5 steel hot-rolled blank, which comprises the steps of slow cooling before heating, heating, blooming rolling and slow cooling after rolling; the slow cooling before heating step: the blank is first slowly cooled for more than or equal to 72 hours; the heating step: the segmented heating control is adopted, the heating temperature of the first heating section is 850-950 DEG C, the heating temperature of the second heating section is 1110-1150 DEG C, the temperature of the soaking section is 1100-1180 DEG C, and the total heating time is controlled to be 4-5 hours; the blooming rolling step: nine passes of rolling are adopted, the reduction of the first four passes is 50-60 mm, the reduction of the middle four passes is 20-30 mm, and the reduction of the last pass is 10-20 mm; the hot-rolled blank is obtained; and the slow cooling after rolling step: the hot-rolled blank is slowly cooled for more than or equal to 72 hours. The application reduces or eliminates the generation of surface cracks of the wire rod, guarantees good hardenability of the product, and effectively improves the product quality.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, and in particular to a process for improving corner cracks in hot-rolled 16MnCr5 steel billets. Background Technology

[0002] 16MnCr5, a low-carbon alloy steel, is a commonly used alloy quenched and tempered steel. It boasts advantages such as stable composition, low levels of harmful elements, high steel purity, small decarburized layer, and few surface defects. It is easy to spheroidize anneal, exhibits low cold heading cracking rate, and provides stable heat treatment quality and uniform hardness, making it widely used in various industries. The 16MnCr5 material can be used to produce automotive wheel bolts, cylinder head bolts, and other 12.9 grade high-strength bolts. 16MnCr5 products also possess advantages such as good hardenability, moderate hardness, and long fatigue life. The production process of low-carbon alloy steel 16MnCr5, due to its high chromium content which readily forms carbides, shifts the cooling C-curve to the right, increasing its undercooling stability in the austenitic state and lowering the critical cooling rate. When low-carbon alloy steel billets are rolled at high temperatures to the required dimensions, they are then cooled in air. The increased cooling rate on the billet surface generates stress that leads to martensitic structure formation. This stress, after being heated and rolled into wire rod, can cause corner cracks on the wire rod surface.

[0003] Therefore, how to eliminate the internal stress in the billet generated after high-temperature rolling of carbon alloy cold heading steel billets, that is, to avoid the problem of wire cracking caused by billet stress, and to ensure the good hardenability of alloy cold heading steel, is an urgent problem to be solved to support the high-performance and low-cost production of high-end low-carbon alloy steel. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a process for improving the corner cracks of 16MnCr5 steel hot-rolled billets, so as to avoid the cracking of wire rod caused by billet stress.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the steps of slow cooling before heating, heating, billet rolling and slow cooling after rolling; The preheating slow cooling step: The billet is first slow cooled for ≥72 hours; The heating process employs segmented heating control, with the first heating stage at 850–950°C, the second heating stage at 1110–1150°C, and the soaking stage at 1100–1180°C; the total heating time is controlled to be 4–5 hours. The billet rolling process involves 9 passes, with the first four passes using a reduction of 50-60 mm, the middle four passes using a reduction of 20-30 mm, and the last pass using a reduction of 10-20 mm to obtain a hot-rolled billet. The post-rolling slow cooling step: The hot-rolled billet is slow-cooled for ≥72 hours.

[0006] Furthermore, in the heating step, the holding time at a temperature ≥1100℃ is controlled within 1.5 to 2 hours.

[0007] Furthermore, in the billet rolling step, the cast billet is rolled from a 280×325mm billet into a 160×160mm hot-rolled billet.

[0008] The principle of this invention is as follows: Improve the heating process of low-carbon alloy steel billets to reduce the stability of the steel after austenitization; ensure the hardenability of the product by adjusting the appropriate reduction amount; control the cooling rate of the billet during the subsequent cooling process after rolling to avoid the brittle zone of martensitic transformation, improve the stability of austenitization, ensure the stabilization of austenite grains, and enhance resistance to deformation cracking. Experiments show that the improved production process for low-carbon alloy steel 16MnCr5 can completely eliminate the internal stress of the billet and avoid corner cracks on the steel surface. Figure 6 The graph showing the relationship between elongation and temperature indicates that the billet exhibits better plasticity at temperatures of 850℃ and above; therefore, the heating temperature for a specific period is limited to 850℃. Figure 7 The relationship between the reduction of area and temperature shows that the plasticity of the billet reaches its peak at 950℃, and decreases if the temperature is too high. Therefore, the upper limit of the heating temperature is set at 950℃. By controlling the heating temperature, the plasticity of the billet is effectively guaranteed.

[0009] The beneficial effects of adopting the above technical solution are as follows: This invention improves the stability of austenitization during the rolling of red billets by controlling the heating temperature, heating time, and reduction amount in stages during heating and billet rolling; before and after billet rolling, large square billets and hot-rolled billets are slowly cooled to eliminate the internal stress of the billets generated after high-temperature rolling, avoiding wire rod cracking defects caused by internal stress in the billets during subsequent wire rod rolling; This invention reduces or eliminates the generation of surface cracks in wire rods and ensures that the product has good hardenability, effectively improving product quality. Attached Figure Description

[0010] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0011] Figure 1 These are metallographic photographs of the wire obtained from subsequent rolling in Embodiment 1 of the present invention; Figure 2 These are metallographic photographs of the wire obtained from subsequent rolling in Embodiment 2 of the present invention; Figure 3 This is a surface photograph of the steel billet obtained in Example 3 of the present invention; Figure 4 These are metallographic photographs of the wire obtained from subsequent rolling in Embodiment 3 of the present invention; Figure 5 This is a diagram of the corner cracks in the steel billet obtained in Comparative Example 1; Figure 6 This is a graph showing the relationship between the elongation of the 16MnCr5 steel and temperature. Figure 7 This is a graph showing the relationship between the reduction of area of ​​the 16MnCr5 steel and temperature. Detailed Implementation

[0012] Example 1: The specific process for improving the corner cracks of 16MnCr5 steel hot-rolled billets is as follows.

[0013] (1) Slow cooling before heating: The large square billet is subjected to slow cooling for 72 hours; (2) Heating: A regenerative heating furnace is used with segmented heating control. The heating temperature of the first heating stage is 850℃, the heating temperature of the second heating stage is 1110℃, and the heating temperature of the heat soaking stage is 1100℃. The total heating time is controlled within 5 hours, and the heat preservation time of the high temperature stage (≥1100℃) is controlled within 2 hours. (3) Rolling of billet: The billet is rolled from 280×325mm to 160×160mm in 9 passes. The first four passes use a reduction of 50mm, the middle four passes use a reduction of 20mm, and the last pass uses a reduction of 10mm to obtain the hot-rolled billet. (4) Slow cooling after rolling: The hot-rolled billet is slow-cooled for 72 hours.

[0014] The 16MnCr5 steel billet obtained in this embodiment, as well as the subsequently rolled wire rod, were subjected to low-magnification pickling according to national inspection standards. No corner cracks were found on the surfaces of both the billet and the wire rod. Metallographic photographs of the wire rod obtained from the subsequently rolled steel billet are shown below. Figure 1 ;Depend on Figure 1 It is evident that this method effectively eliminates stress cracking on the steel surface, ensures the hardenability of the steel, and guarantees the stabilization of austenite grains.

[0015] Example 2: The specific process for improving the corner cracks of 16MnCr5 steel hot-rolled billets is as follows.

[0016] (1) Slow cooling before heating: The large square billet is subjected to slow cooling for 72 hours; (2) Heating: A regenerative heating furnace is used with segmented heating control. The heating temperature of the first heating stage is 950℃, the heating temperature of the second heating stage is 1150℃, and the heating temperature of the heat soaking stage is 1180℃. The total heating time is controlled within 4 hours, and the heat preservation time of the high temperature stage (≥1100℃) is controlled within 1.5 hours. (3) Rolling of billet: The billet is rolled from 280×325mm to 160×160mm using 9 passes. The first four passes use a reduction of 55mm, the middle four passes use a reduction of 25mm, and the last pass uses a reduction of 15mm to obtain the hot-rolled billet. (4) Slow cooling after rolling: The hot-rolled billet is slow-cooled for 72 hours.

[0017] The 16MnCr5 steel billet obtained in this embodiment, as well as the subsequently rolled wire rod, were subjected to low-magnification pickling according to national inspection standards. No corner cracks were found on the surfaces of both the billet and the wire rod. Metallographic photographs of the wire rod obtained from the subsequently rolled steel billet are shown below. Figure 2 ;Depend on Figure 2 It is evident that this method effectively eliminates stress cracking on the steel surface, ensures the hardenability of the steel, and guarantees the stabilization of austenite grains.

[0018] Example 3: The specific process for improving the corner cracks of 16MnCr5 steel hot-rolled billets is as follows.

[0019] (1) Slow cooling before heating: The large square billet is subjected to slow cooling for 80 hours; (2) Heating: A regenerative heating furnace is used with segmented heating control. The heating temperature of the first heating stage is 900℃, the heating temperature of the second heating stage is 1130℃, and the heating temperature of the heat soaking stage is 1140℃. The total heating time is controlled at 4.5h, and the heat preservation time of the high temperature stage (≥1100℃) is controlled at 2h. (3) Rolling of billet: Nine passes are used to roll the billet from 280×325mm to 160×160mm. The first four passes use a reduction of 60mm, the middle four passes use a reduction of 30mm, and the last pass uses a reduction of 20mm. (4) Slow cooling after rolling: The hot-rolled billet is slow-cooled for 78 hours.

[0020] The 16MnCr5 steel billet obtained in this embodiment, as well as the subsequently rolled wire rod, were subjected to low-magnification pickling according to national inspection standards. No corner cracks were found on the surface of either the billet or the wire rod. A surface photograph of the obtained steel billet is shown below. Figure 3 Metallographic photographs of the wire rod obtained from the subsequent rolling of the steel billet are shown below. Figure 4 ;Depend on Figure 3-4 It is evident that this method effectively eliminates stress cracking on the steel surface, ensures the hardenability of the steel, and guarantees the stabilization of austenite grains.

[0021] Comparative Example 1: (1) Slow cooling before heating: Large square billet castings are not slow cooled and are directly hot-charged and then opened; (2) Heating: A regenerative heating furnace is used with segmented heating control. The heating temperature of the first heating stage is 800℃, the heating temperature of the second heating stage is 1060℃, and the heating temperature of the heat soaking stage is 1080℃. The total heating time is controlled within 4 hours.

[0022] (3) Rolling of billet: Nine passes are used to roll the billet from 280×325mm to 160×160mm. The first four passes use a reduction of 65mm, the middle four passes use a reduction of 35mm, and the last pass uses a reduction of 25mm. (4) Slow cooling after rolling: The hot-rolled billet is not slow cooled, but directly air-cooled.

[0023] The 16MnCr5 steel billet obtained in this comparative example, as well as the subsequently rolled wire rod, were subjected to low-magnification pickling according to national inspection standards. Corner cracks were found on the surface of the hot-rolled billet; photographs of the surface cracks of the obtained steel billet are shown below. Figure 5 .

Claims

1. A process for improving corner cracks in hot-rolled billets of 16MnCr5 steel, characterized in that: This includes steps such as preheating slow cooling, heating, billet rolling, and post-rolling slow cooling; The preheating slow cooling step: The billet is first slow cooled for ≥72 hours; The heating process employs segmented heating control, with the first heating stage at 850–950°C, the second heating stage at 1110–1150°C, and the soaking stage at 1100–1180°C; the total heating time is controlled to be 4–5 hours. The billet rolling process involves 9 passes, with the first four passes using a reduction of 50-60 mm, the middle four passes using a reduction of 20-30 mm, and the last pass using a reduction of 10-20 mm to obtain a hot-rolled billet. The post-rolling slow cooling step: The hot-rolled billet is slow-cooled for ≥72 hours.

2. The process for improving corner cracks in 16MnCr5 steel hot-rolled billets according to claim 1, characterized in that: In the heating step, the holding time for a temperature ≥1100℃ is controlled within 1.5 to 2 hours.

3. The process for improving corner cracks in 16MnCr5 steel hot-rolled billets according to claim 1 or 2, characterized in that: The billet rolling step involves rolling the cast billet from 280×325mm to 160×160mm hot-rolled billet.

Images