A method for controlling the microstructure of hot-rolled CrMo steel bars

Abstract

The present invention discloses a method for controlling the microstructure of hot-rolled CrMo steel bars. The method comprises the following steps: 1) heating a continuous cast billet in a heating furnace; 2) subjecting the heated billet to high-pressure water descaling, rough rolling, intermediate rolling, and finish rolling; 3) transporting the rolled bar to a cooling bed via a conveyor roller; and 4) cooling the bar on the cooling bed, with the bars closely packed and the cooling bed insulation shield positioned less than 1 meter from the bar surface, at a cooling rate of 0.14 to 0.25°C / s. This method can control the microstructure of bars of this specification series to be ferrite and pearlite, eliminate the formation of bainite, and reduce the hardness of the bars after hot rolling.

Description

Technical Field

[0001] The present invention relates to a structure control method in the hot rolling production process of bars, specifically a method for controlling the steel structure by applying the cooling rate of the hot rolling process, more specifically a structure control method for hot-rolled CrMo steel bars. Background Art

[0002] 42CrMo steel is a commonly used alloy structural steel in engineering. After quenching and tempering, it exhibits high fatigue strength and low-temperature impact toughness. It is widely used in the manufacture of high-strength, large-section, important mechanical parts such as gears, rear axles, and connecting rods. However, during rolling mill production, the cooling rate of Ø60-Ø75 steel grades was high, resulting in a large amount of bainite in the steel structure during inspection. This resulted in high hardness and made sawing difficult for downstream users.

[0003] Chinese patent application CN110157867A discloses a method for controlling white abnormal structure in large-scale CrMo steel components. The specific process of this control method is as follows: (1) microalloying is performed under the basic CrMo composition conditions; (2) high-temperature diffusion annealing is used to reduce or eliminate segregation; (3) pearlitization annealing is used to eliminate the structural inheritance caused by high-temperature diffusion annealing; (4) the size and number of carbides are controlled through a carbide control process, thereby further refining the grain size and structure; (5) the M / A island size is refined through a "quenching + pre-tempering + tempering" process to obtain the final structure.

[0004] Chinese patent application CN111636029A discloses a method for reducing the banding grade of fully annealed CrMo-based half-axle steel. The method comprises: sequentially performing molten iron pretreatment - converter steelmaking - refining - continuous casting - heating of the cast ingot - rolling to obtain a steel with a banding grade of ≤2.5; wherein the mass percentage of sulfur in the molten iron obtained after the molten iron pretreatment is ≤0.01%; the endpoint of the converter steelmaking operation is a carbon mass percentage of ≥0.08%, a phosphorus mass percentage of ≤0.01%, and a slag volume of 5-7 t; the mass percentage of sulfur in the molten steel during the refining process is ≤0.005%; the content of oxygen in the refined molten steel is ≤12 ppm, and the mass percentage of aluminum is 0.025-0.035%; and the endpoint temperature of the rolling is 830-980°C.

[0005] Chinese patent application CN106435133A discloses a method for reducing the Brinell hardness of hot-rolled CrMo steel bars. The production process for the hot-rolled CrMo steel bars includes a heating process and a rolling process. The heating process includes a heating section and a soaking section. To reduce the Brinell hardness of the hot-rolled CrMo steel bars, the heating section temperature is controlled between 1050°C and 1120°C, the soaking section temperature is controlled between 1050°C and 1110°C, and the final rolling temperature is controlled between 900°C and 950°C. This method can reduce the Brinell hardness of the hot-rolled CrMo steel bars from approximately 310 HBW to approximately 260 HBW. Compared to controlled rolling and controlled cooling processes, this method eliminates the need for production line modifications to the controlled cooling process; instead, three to five water-cooling units and control equipment can be added to the production line, depending on the process layout. This method can reduce energy consumption by 5-10%, lowering production costs and adapting to various CrMo steel specifications. Note: Bar Brinell hardness is the Brinell hardness value measured at the 1 / 2 radius of the bar cross section.

[0006] Chinese patent application CN111363975A discloses a controlled rolling and cooling method for hot-rolled medium-carbon CrMo steel wire rod that can be directly drawn. The steel billet is heated to a temperature no higher than 1100°C for 1.5 to 2 hours; the finishing mill and mini mill are rolled at ultra-low temperatures no higher than 750°C; the spinning temperature is kept at an ultra-low temperature no higher than 750°C; and the cooling rate of the wire rod on the Stelmor controlled cooling line is kept at no higher than 0.12°C / s. This process effectively promotes the precipitation of proeutectoid structure, ensures the complete transformation of supercooled austenite on the Stelmor controlled cooling line, effectively prevents the transformation of bainite and martensite, and simultaneously achieves a fine-grained structure with a ferrite grain size of no less than 12. This ensures that the medium-carbon CrMo steel wire rod has excellent drawability in the hot-rolled state and can be cold-drawn with a single reduction of no less than 30%, reducing the cost of a single spheroidizing annealing process for downstream users.

[0007] Chinese patent application CN106566998A discloses a method for manufacturing CrMo-based gear round steel. The composition, by weight, is as follows: C: 0.22%-0.27%, Si: 0.03%-0.12%, Mn: 1.20-1.45%, S≤0.020%, P≤0.020%, Cr: 0.30%-0.60%, Mo: ≤0.20%, Al: 0.015-0.040%, Ni≤0.10%, O≤15×10-6%, with the remainder being Fe and unavoidable impurities. The resulting round steel has Class A and Class D inclusions ≤1.5, and Class B and Class C inclusions ≤1.0. The round steel has a hardenability of 42-50 HRC for a 5mm diameter steel, 38-46 HRC for a 9mm diameter steel, and 32-40 HRC for a 15mm diameter steel. It changes the traditional technical idea of ​​increasing the hardenability of steel by adding large amounts of alloying elements such as Cr, Ni, and Mo. By appropriately increasing the C and Mn contents, significantly reducing the Mo and Cr contents, and replacing Ni alloys, it significantly reduces the production cost of the product and improves the competitiveness of the product while ensuring that the hardenability of the gear steel is not reduced.

[0008] Chinese patent application CN 102643965 A discloses a process for reducing the surface hardness of round steel. The process involves heating the round steel to 930-950°C before entering the cooling bed; transferring the round steel to the cooling bed within 7 minutes, with the temperature entering the cooling bed exceeding 800°C; feeding the round steel to a chain conveyor within 10 minutes; heating the unloaded round steel to 750-800°C, with the temperature entering the slow cooling pit exceeding 650°C; and collecting and bundling the round steel into the slow cooling pit, slowly cooling it to below 150°C before exiting. This process reduces the surface hardness of the round steel, but requires costly modification of the cooling bed in the existing production process and cannot be adjusted for the production of round steel of different specifications.

[0009] The methods used in the above patent applications are unable to obtain satisfactory hardness values ​​to meet the needs of downstream users. Therefore, there is an urgent need to develop a simple and fast rod structure control method that does not require major changes to the existing production process. Summary of the Invention

[0010] The object of the present invention is to provide a method for controlling the microstructure of hot-rolled CrMo steel bars, in particular a method for controlling the microstructure of hot-rolled CrMo steel bars with a specification of Ф60-Ф75. The method optimizes the heating system, controls the final rolling temperature and reduces the cooling rate of the bars on the transport rollers and the cooling bed, so that the microstructure is transformed into ferrite and pearlite, and the appearance of bainite is avoided.

[0011] In order to achieve the above object, the technical solution provided by the present invention is:

[0012] A method for controlling the microstructure of a CrMo steel hot-rolled bar, the method comprising the following steps:

[0013] 1) The continuous casting billet is heated in a heating furnace;

[0014] 2) The heated continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling, and finish rolling;

[0015] 3) The rolled bars are transported to the cooling bed via a conveyor roller;

[0016] 4) The bars are cooled on a cooling bed, the bars are densely packed, the insulation cover of the cooling bed is less than 1 meter away from the surface of the bars, and the cooling rate is 0.14-0.25℃ / S.

[0017] In the present invention, the chemical composition of the CrMo steel is as follows by mass percentage: C=0.39-0.42%, Si=0.17-0.37%, Mn=0.70-0.80%, Cr=1.0-1.10%, Mo=0.18-0.25%, P≤0.020%, S≤0.025%, and Ni≤0.30%.

[0018] Preferably, in step 1), the temperature of the preheating section is 500-850°C, the temperature of the heating section I is 900-1150°C, the temperature of the heating section II is 1150-1240°C, the temperature of the soaking section is 1170-1210°C, the holding time of the soaking section is ≥35 minutes, the total heating time is ≥190 minutes, and the temperature difference of the continuous casting billet cross section is <30°C.

[0019] More preferably, the temperature of the preheating section is 500-850°C, such as 500°C, 600°C, 700°C, 800°C, 850°C, etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0020] The temperature of heating stage I is 900-1150°C, such as 900°C, 950°C, 1000°C, 1100°C, 1150°C, etc. Other unlisted values ​​within this numerical range can also achieve the purpose of the present invention.

[0021] The temperature of heating section II is 1150-1240°C, such as 1150°C, 1180°C, 1200°C, 1220°C, 1240°C, etc. Other unlisted values ​​within this numerical range can also achieve the purpose of the present invention.

[0022] The temperature of the soaking section is 1170-1210°C, such as 1170°C, 1180°C, 1200°C, 1210°C, etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0023] The holding time of the soaking section is ≥ 35 minutes, and can be 35 minutes, 40 minutes, 50 minutes, etc., but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0024] The total heating time is ≥ 190 minutes, and can be 190 minutes, 200 minutes, 250 minutes, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0025] The temperature difference of the cross section of the continuous casting slab is less than 30°C. It can be 25°C, 20°C, 10°C, 5°C, etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0026] Preferably, in step 2), the temperature during rough rolling is 1030-1100°C, and the temperature after finishing rolling is 900-950°C.

[0027] More preferably, the rough rolling temperature is 1030-1100°C, such as 1030°C, 1050°C, 1080°C, 1100°C, 1240°C, etc. Other unlisted values ​​within this numerical range can also achieve the purpose of the present invention.

[0028] The temperature after finishing rolling is 900-950° C., such as 900° C., 910° C., 920° C., 930° C., 940° C., 950° C., etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0029] Preferably, in step 3), a roller heat insulation cover is provided on the transport roller, and the roller heat insulation cover is completely closed.

[0030] Preferably, in step 3), the temperature of the rod exiting the heat preservation cover is controlled at 950-1000° C., such as 950° C., 960° C., 970° C., 980° C., 990° C., 1000° C., etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0031] Preferably, the rod structure of the rod obtained in step 4) is ferrite and pearlite.

[0032] Preferably, the specification of the rod obtained in step 4) is Ø60-Ø75, such as Ø60, Ø65, Ø70, Ø75, etc. Other values ​​not listed within this numerical range can also achieve the purpose of the present invention.

[0033] Preferably, the hardness of the rod obtained in step 4) is: ≤216HB in the edge area, ≤223HB in the radius area, and ≤234HB in the core area.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention can control the microstructure of the bars of this specification series to be ferrite and pearlite, eliminate the generation of bainite microstructure, and reduce the hardness value of the bars after hot rolling. The present invention is simple and fast and does not require major changes to the existing production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 200X schematic diagram of the bar edge structure.

[0037] Figure 2 Schematic diagram of the rod radius area 200X.

[0038] Figure 3 Schematic diagram of the core area of ​​the bar 200X. DETAILED DESCRIPTION

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The invention discloses a microstructure control method for hot-rolled CrMo steel bars with a specification of Ø60-Ø75. The chemical composition of the steel is as follows: C=0.39-0.42%, Si=0.17-0.37%, Mn=0.70-0.80%, Cr=1.0-1.10%, Mo=0.18-0.25%, P≤0.020%, S≤0.025%, and Ni≤0.30%.

[0041] The process is as follows:

[0042] (1) The continuous casting billet is heated in a heating furnace, with the preheating section temperature of 500-850°C, the heating section temperature of 900-1150°C, the heating section temperature of 1150-1240°C, the soaking section temperature of 1170-1210°C, the soaking section holding time of ≥35 minutes, the total heating time of ≥190 minutes, and the temperature difference of the cross section of the continuous casting billet of less than 30°C;

[0043] (2) The continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling, and finishing rolling. The rough rolling temperature is 1030-1100°C, and the finishing rolling temperature is 900-950°C.

[0044] (3) The rolled bars are transported to the cooling bed via a conveyor roller. The conveyor roller is equipped with a roller insulation cover. The insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 950-1000°C.

[0045] (4) The bars are cooled on a cooling bed, the bars are densely packed, the insulation cover of the cooling bed is less than 1 meter away from the surface of the bars, and the cooling rate is 0.14-0.25℃ / S.

[0046] Example 1:

[0047] A method for controlling the microstructure of a Ф60 CrMo steel hot-rolled bar. The chemical composition of the steel is shown in Table 1. The microstructure after rolling is ferrite and pearlite. Figure 1 、 Figure 2 、 Figure 3 , steel hardness test is shown in Table 2.

[0048] The process is as follows:

[0049] (1) The continuous casting billet is heated in a heating furnace with a preheating section temperature of 500°C, a heating section temperature of 900°C, a heating section temperature of 1150°C, a soaking section temperature of 1170°C, a soaking section holding time of 35 minutes, a total heating time of 190 minutes, and a cross-sectional temperature difference of 30°C.

[0050] (2) The continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling and finishing rolling. The rough rolling temperature is 1030℃ and the finishing rolling temperature is 900-950℃.

[0051] (3) The rolled bars are transported to the cooling bed via a transport roller. The transport roller is equipped with a roller insulation cover. The insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 950-1000℃.

[0052] (4) The bars are cooled on a cooling bed, the bars are densely packed, the insulation cover of the cooling bed is less than 1 meter away from the surface of the bars, and the cooling rate is 0.14-0.25℃ / S.

[0053] Table 1 Chemical composition of steel by mass percentage (%)

[0054] C Si Mn Cr Mo P S Cu Ni 0.41 0.23 0.75 1.05 0.19 ≤0.020 ≤0.025 ≤0.20 ≤0.30

[0055] Table 2 Steel hardness test (HB)

[0056] Edge area Radius area Heart area Bar cross section 216 223 234

[0057] Example 2:

[0058] A method for controlling the microstructure of a Ф75 CrMo steel hot-rolled bar. The chemical composition of the steel is shown in Table 3. The microstructure after rolling is ferrite and pearlite. The hardness test of the steel is shown in Table 4.

[0059] The process is as follows:

[0060] (1) The continuous casting billet is heated in a heating furnace with a preheating section temperature of 850°C, a heating section temperature of 1150°C, a heating section temperature of 1240°C, a soaking section temperature of 1210°C, a soaking section holding time of ≥35 minutes, a total heating time of ≥190 minutes, and a cross-sectional temperature difference of 25°C.

[0061] (2) The continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling and finishing rolling. The rough rolling temperature is 1100℃ and the finishing rolling temperature is 950℃.

[0062] (3) The rolled bars are transported to the cooling bed via a transport roller. The transport roller is equipped with a roller insulation cover. The insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 1000℃.

[0063] (4) The bars are cooled on a cooling bed. The bars are densely packed, the insulation cover of the cooling bed is 0.5 meters away from the surface of the bars, and the cooling rate is 0.25℃ / s.

[0064] Table 3 Chemical composition of steel by mass percentage (%)

[0065] C Si Mn Cr Mo P S Cu Ni 0.40 0.28 0.70 1.05 0.18 ≤0.020 ≤0.025 ≤0.20 ≤0.30

[0066] Table 4 Steel hardness test (HB)

[0067] Edge area Radius area Heart area Bar cross section 215 220 231

[0068] Example 3:

[0069] A method for controlling the microstructure of a Ф65 CrMo steel hot-rolled bar. The chemical composition of the steel is shown in Table 5. The microstructure after rolling is ferrite and pearlite. The hardness test of the steel is shown in Table 6.

[0070] The process is as follows:

[0071] (1) The continuous casting billet is heated in a heating furnace with a preheating section temperature of 600°C, a heating section temperature of 1000°C, a heating section temperature of 1200°C, a soaking section temperature of 1180°C, a soaking section holding time of 50 minutes, a total heating time of 200 minutes, and a cross-sectional temperature difference of 10°C.

[0072] (2) The continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling and finishing rolling. The rough rolling temperature is 1050℃ and the finishing rolling temperature is 930℃.

[0073] (3) The rolled bars are transported to the cooling bed via a transport roller. The transport roller is equipped with a roller insulation cover. The insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 970°C.

[0074] (4) The bars are cooled on a cooling bed. The bars are densely packed, the insulation cover of the cooling bed is 0.8 meters away from the surface of the bars, and the cooling rate is 0.20℃ / s.

[0075] Table 5 Chemical composition of steel by mass percentage (%)

[0076] C Si Mn Cr Mo P S Cu Ni 0.41 0.23 0.75 1.05 0.19 ≤0.020 ≤0.025 ≤0.20 ≤0.30

[0077] Table 6 Steel hardness test (HB)

[0078] Edge area Radius area Heart area Bar cross section 210 218 229

[0079] Example 4:

[0080] A method for controlling the microstructure of a Ф70 CrMo steel hot-rolled bar. The chemical composition of the steel is shown in Table 7. The microstructure after rolling is ferrite and pearlite. The hardness test of the steel is shown in Table 8.

[0081] The process is as follows:

[0082] (1) The continuous casting billet is heated in a heating furnace with a preheating section temperature of 800°C, a heating section temperature of 1100°C, a heating section temperature of 1220°C, a soaking section temperature of 1200°C, a soaking section holding time of 40 minutes, a total heating time of ≥200 minutes, and a temperature difference of the cross section of the continuous casting billet of less than 30°C.

[0083] (2) The continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling and finishing rolling. The rough rolling temperature is 1050℃ and the finishing rolling temperature is 940℃.

[0084] (3) The rolled bars are transported to the cooling bed via a transport roller. The transport roller is equipped with a roller insulation cover. The insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 960°C.

[0085] (4) The bars are cooled on a cooling bed. The bars are densely packed, the insulation cover of the cooling bed is 0.8 meters away from the surface of the bars, and the cooling rate is 0.14°C / s.

[0086] Table 7 Chemical composition of steel by mass percentage (%)

[0087]

[0088]

[0089] Table 8 Steel hardness test (HB)

[0090] Edge area Radius area Heart area Bar cross section 211 220 230

[0091] Comparative Example 1

[0092] Compared to the control method of the present invention, except for the different heating furnace temperatures, the temperature of the transport rollers, and the cooling rate, all other aspects are the same. The resulting bar structure is partially ferrite and pearlite, and partially bainite. The hardness values ​​of the bars after hot rolling are shown in Table 9.

[0093] Table 9 Steel hardness test (HB)

[0094] Edge area Radius area Heart area Bar cross section 281 295 330

[0095] Comparative Example 2

[0096] Compared to the control method of the present invention, except for the different heating furnace temperatures, the temperature of the exit conveyor rollers, and the cooling rate, all other aspects are the same. The resulting bar structure is partially ferrite and pearlite, and partially bainite. The hardness values ​​of the bars after hot rolling are shown in Table 10.

[0097] Table 10 Steel hardness test (HB)

[0098] Edge area Radius area Heart area Bar cross section 265 283 311

[0099] As can be seen from the above examples, the control method of the present invention can achieve a bar structure consisting entirely of ferrite and pearlite, while also reducing the hardness by approximately 70 HB, achieving significant technical benefits. The present invention can control the structure of bars in this specification series to be ferrite and pearlite, eliminating the formation of bainite and reducing the hardness of the bars after hot rolling.

[0100] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed here one by one.

[0101] Any content not described in detail in the present invention can be based on conventional technical knowledge in the art.

[0102] 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 microstructure of a hot-rolled CrMo steel bar, the method comprising the following steps: 1) The continuous casting billet is heated in a heating furnace; the preheating section temperature is 500-850°C, the heating section temperature is 900-1150°C, the heating section temperature is 1150-1240°C, the soaking section temperature is 1170-1210°C, the soaking section holding time is ≥35 minutes, the total heating time is ≥190 minutes, and the temperature difference of the continuous casting billet cross section is <30°C; 2) After heating, the continuous casting billet undergoes high-pressure water descaling, rough rolling, intermediate rolling, and finishing rolling; the rough rolling temperature is 1030-1100℃, and the finishing rolling temperature is 900-950℃; 3) The rolled bars are transported to the cooling bed via a transport roller. The transport roller is equipped with a roller insulation cover. During the roller transport process, the roller insulation cover is completely closed and the temperature of the bars leaving the insulation cover is controlled at 950-1000℃. 4) The bars are cooled on a cooling bed with the bars densely packed and the cooling bed insulation cover less than 1 meter from the bar surface. The cooling rate is 0.14-0.25°C / s. The hardness of the obtained bars is: ≤216HB in the edge area, ≤223HB in the radius area, and ≤234HB in the core area. The chemical composition of the CrMo steel is as follows: C=0.39-0.42%, Si=0.17-0.37%, Mn=0.70-0.80%, Cr=1.0-1.10%, Mo=0.18-0.25%, P≤0.020%, S≤0.025%, and Ni≤0.30%.

2. The method for controlling the microstructure of a CrMo steel hot-rolled bar according to claim 1, characterized in that: The chemical composition percentages of the CrMo steel are as follows: C=0.41%, Si=0.23%, Mn=0.75%, Cr=1.05%, Mo=0.19%, P≤0.020%, S≤0.025%, and Ni≤0.30%.

3. The method for controlling the microstructure of a CrMo steel hot-rolled bar according to claim 1, wherein: Step 4) The bar structure of the obtained bar is ferrite and pearlite.

4. The method for controlling the microstructure of a CrMo steel hot-rolled bar according to claim 1, wherein: Step 4) The specifications of the obtained rods are Ф60-Ф75.

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