50mn steel bar and rolling method thereof

Abstract

The present application relates to the technical field of metal material deformation heat treatment, and especially relates to a 50Mn steel bar and a rolling method thereof, the rolling method sequentially comprises the following processes: a heating process, a rough rolling process, a medium rolling process, a first water cooling process, a reducing-diameter unit finishing rolling process, a second water cooling process, a cooling bed cooling process; wherein, in the reducing-diameter unit finishing rolling process, the finish rolling temperature is 760-820 DEG C; in the second water cooling process, the cooling speed of the steel billet is not less than 50 DEG C / s, and the steel billet is cooled to not more than 720 DEG C. The rolling method of the 50Mn steel bar provided by the present application controls the finish rolling temperature to be 760-820 DEG C, and simultaneously uses the water tank fast cooling after rolling to be not more than 720 DEG C, so that the tensile strength of the 50Mn steel bar after hot rolling is 795-828 MPa, the yield strength is 474-506 MPa, the actual grain size is 7-8 levels, and the normalizing can be omitted for delivery.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material deformation heat treatment, in particular to a 50Mn steel bar and a rolling method thereof. Background Art

[0002] 50Mn steel is a commonly used, high-quality carbon structural steel with a low alloying element count, simple smelting, and low cost. Its main chemical composition (by mass percentage) is: C: 0.48-0.56%, Si: 0.17-0.37%, Mn: 0.7-1.0%, P: ≤0.035%, S: ≤0.035%, Cr: ≤0.25%, Ni: ≤0.3%, Cu ≤0.25%, with the remainder being matrix Fe and unavoidable impurities. It exhibits high hardenability and excellent overall mechanical properties after heat treatment. It is widely used in the manufacture of parts subject to high wear and stress, such as shafts, gear shafts, spindles, connecting rods, and worm gears.

[0003] When an automobile parts factory uses 50Mn steel bars to produce automobile steering shafts, it requires the material to have a tensile strength of ≥750MPa, a yield strength of ≥450MPa, and an actual grain size of 6-8. However, the tensile strength of 50Mn steel bars in the ordinary hot-rolled state is 650-700MPa, the yield strength is 390-430MPa, and the actual grain size is 5-6. Its grains are relatively coarse and the strength is relatively low. The mechanical properties and actual grain size are difficult to meet the design requirements, and it needs to be normalized before delivery. Normalizing treatment not only increases the process cost and weakens the low-cost advantage of 50Mn steel, but also increases carbon emissions, which is not conducive to the green and sustainable development of the steel industry. Summary of the Invention

[0004] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a 50Mn steel bar and a rolling method thereof. The rolling method of the 50Mn steel bar provided by the present invention controls the final rolling temperature to 760-820°C by using water tanks along the rolling line, and simultaneously uses a water tank after rolling to quickly cool the temperature to no more than 720°C, so that the tensile strength of the 50Mn steel bar after hot rolling is 795-828MPa, the yield strength is 474-506MPa, and the actual grain size is 7-8 levels, which meets the requirements of normalized mechanical properties and actual grain size, and can be delivered without normalizing.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a rolling method for a 50Mn steel bar, using a 50Mn steel billet as a raw material, the rolling method sequentially comprising the following steps: a heating step, a rough rolling step, an intermediate rolling step, a first water cooling step, a finishing rolling step in a reducing and sizing mill, a second water cooling step, and a cooling step on a cooling bed, to ultimately obtain a finished bar;

[0007] Wherein, in the finishing rolling process of the reducing and sizing mill, the final rolling temperature is 760-820°C;

[0008] In the second water-cooling step, the cooling rate of the steel billet is not less than 50°C / s, and the steel billet is cooled to a temperature not greater than 720°C.

[0009] The rolling method of the 50Mn steel bar provided by the present invention controls the final rolling temperature to 760-820°C by using water tanks along the rolling line, and simultaneously uses a post-rolling water tank for rapid cooling to no more than 720°C, so that the tensile strength of the 50Mn steel bar after hot rolling is 795-828MPa, the yield strength is 474-506MPa, and the actual grain size is 7-8 levels, which meets the requirements of normalized mechanical properties and actual grain size, and can be delivered without normalizing.

[0010] Furthermore, in the second water cooling step, the cooling rate of the steel billet is 50-150° C. / s.

[0011] Furthermore, in the second water cooling step, the steel billet is cooled to 660-720°C.

[0012] Furthermore, in the heating process, the temperature of the heated soaking section is 1150-1230° C., and the holding time of the soaking section is 30-90 minutes;

[0013] And / or, in the heating process, the heating is divided into four stages, namely a preheating stage, a heating stage, a heating stage 2, and a soaking stage, the temperature of the preheating stage is 600-750° C., the temperature of the heating stage 1 is 1000-1160° C., the temperature of the heating stage 2 is 1120-1200° C., and the total heating time in the heating process is 240-360 min;

[0014] and / or, in the rough rolling process, the starting rolling temperature is 1060-1140° C.;

[0015] and / or, in the intermediate rolling process, the intermediate rolling temperature is 880-980° C.;

[0016] And / or, in terms of mass percentage, the composition of the 50Mn steel billet includes: C: 0.48-0.56%, Si: 0.17-0.37%, Mn: 0.7-1.0%, P: ≤0.035%, S: ≤0.035%, Cr: ≤0.25%, Ni: ≤0.3%, Cu≤0.25%, and the rest is matrix Fe and unavoidable impurities.

[0017] Furthermore, in the first water-penetrating cooling step or the second water-penetrating cooling step, a water tank is used for water-penetrating cooling, the water volume of the water tank is 200-350 L / min, and the water pressure is 7-9 MPa;

[0018] and / or, in the first water-penetrating cooling step, the cooling rate of the steel billet is 25-70° C. / s, and the water-penetrating speed of the steel billet is 1.2-4 m / s;

[0019] And / or, in the second water-penetrating cooling step, the water-penetrating speed of the steel billet is 2 to 8 m / s.

[0020] Furthermore, the 50Mn steel billet is surface treated after heating and before rough rolling.

[0021] Furthermore, the surface treatment includes high-pressure water descaling.

[0022] Furthermore, in the cooling bed cooling process, the upper cooling bed temperature is 660-720°C;

[0023] and / or, in the cooling process of the cooling bed, using a heat preservation cover for heat preservation and cooling, and then exiting the heat preservation cover for air cooling of the cooling bed, the heat preservation and cooling speed in the heat preservation cover is 1-2°C / s, and the temperature exiting the heat preservation cover is 500-600°C;

[0024] And / or, after the cooling process on the cooling bed, the rolling method further includes shearing, collecting and bundling.

[0025] In a second aspect, the present invention provides a 50Mn steel bar, which is obtained by the rolling method described in the first aspect.

[0026] Furthermore, the specifications of the 50Mn steel bar are 20-60 mm, the tensile strength is 795-828 MPa, the yield strength is 474-506 MPa, the elongation after fracture is 19%-21%, the cross-sectional shrinkage rate is 45%-49%, the actual grain size is 7-8, the microstructure is F+P, and the ferrite volume ratio is 26.5%-30.2%.

[0027] The present invention utilizes the mechanisms of grain refinement and second-phase strengthening, combined with the characteristics of 50Mn steel, to design a rolling process that controls its microstructure and performance. Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] (1) The rolling method of the 50Mn steel bar provided by the present invention controls the final rolling temperature to 760-820°C by using water tanks along the rolling line, and simultaneously uses a water tank after rolling to quickly cool the temperature to 660-720°C, so that the 50Mn steel bar has a tensile strength of 795-828MPa, a yield strength of 474-506MPa, an elongation after fracture of 19%-21%, a cross-sectional shrinkage of 45%-49%, an actual grain size of 7-8, and a microstructure of F+P, wherein the volume ratio of ferrite is 26.5%-30.2%, which meets the requirements of mechanical properties in the normalized state and actual grain size microstructure performance, and can be delivered without normalizing.

[0029] (2) The 50Mn steel rolling process of the present invention controls the final rolling temperature to 790±30℃ by using water tanks along the rolling line, so that the material is deformed at a temperature slightly higher than the Ar3 phase transformation point, accumulating a large number of dislocations to promote phase deformation nucleation. At the same time, the post-rolling water tank is used to quickly cool the temperature to 690±30℃, allowing these nucleation points to form more pearlite structure, achieving the effect of fine grain strengthening and second phase strengthening.

[0030] (3) Water tanks are arranged along the rolling line of the present invention. By controlling the water pressure and water volume of the water tanks, the final rolling temperature and the temperature after the final rolling water rapid cooling are controlled. This technology can accurately control the temperature conditions of each key point to obtain the required round bar, and has the characteristic of high controllability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 This is a metallographic photograph of the 50Mn steel bar produced in Example 1 of the present invention;

[0033] Figure 2 This is a metallographic photograph of the 50Mn steel bar produced in Example 2 of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0035] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0036] In a first aspect, the present invention provides a rolling method for a 50Mn steel bar, using a 50Mn steel billet as a raw material, the rolling method sequentially comprising the following steps: a heating step, a rough rolling step, an intermediate rolling step, a first water cooling step, a finishing rolling step in a reducing and sizing mill, a second water cooling step, and a cooling step on a cooling bed, to ultimately obtain a finished bar;

[0037] Wherein, in the finishing rolling process of the reducing and sizing mill, the final rolling temperature is 760-820°C (for example, 760°C, 780°C, 800°C or 820°C);

[0038] In the second water-cooling step, the cooling rate of the steel billet is not less than 50°C / s, and the steel billet is cooled to a temperature not greater than 720°C.

[0039] To reduce production costs, shorten processing steps, and reduce carbon emissions, it is proposed to optimize the rolling process of 50Mn steel bars (Φ20-Φ60) so that the material in the hot-rolled state meets the microstructure and performance requirements of a certain automotive parts factory for 50Mn steel bars described in the background technology, achieving normalizing-free delivery. The 50Mn steel bar rolling method provided by the present invention controls the final rolling temperature to 760-820°C through water tanks along the rolling line, and simultaneously uses a post-rolling water tank to rapidly cool the temperature to no more than 720°C. This ensures that the tensile strength of the 50Mn steel bar after hot rolling is 795-828MPa, the yield strength is 474-506MPa, and the actual grain size is 7-8, meeting the normalized mechanical properties and actual grain size requirements, and can be delivered without normalizing.

[0040] The Ar3 phase transformation point of 50Mn steel is 755℃, and the Ar1 phase transformation point is 725℃. In order to improve the deformation-induced nucleation ability and provide more nucleation particles for grain refinement, the final rolling temperature is selected to be around 790℃ (slightly higher than the Ar3 temperature). In order to improve the transformation ability of austenite to pearlite, prevent excessive precipitation of ferrite structure at the nucleation points, and refine the actual grains at the same time, the steel after final rolling is rapidly cooled to around 690℃ (slightly lower than the Ar1 temperature) through water, thereby obtaining more pearlite structure and finer grains, thereby achieving the purpose of strengthening the mechanical properties of the material.

[0041] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the second water cooling step, the cooling rate of the steel billet is 50-150°C / s (for example, it can be 50°C / s, 70°C / s, 90°C / s, 110°C / s, 130°C / s or 150°C / s).

[0042] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the second water cooling step, the steel billet is cooled to 660-720°C (for example, 660°C, 680°C, 700°C or 720°C).

[0043] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the heating process, the temperature of the heated soaking section is 1150-1230°C (for example, it can be 1150°C, 1170°C, 1190°C, 1210°C or 1230°C), and the holding time of the soaking section is 30 to 90 minutes (for example, it can be 30 minutes, 60 minutes or 90 minutes).

[0044] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the heating process, the heating is divided into four stages, namely, a preheating stage, a heating stage, a heating stage 2 and a soaking stage. The temperature of the preheating stage is 600-750°C (for example, it can be 600°C, 650°C, 700°C or 750°C), the temperature of the heating stage 1 is 1000-1160°C (for example, it can be 1000°C, 1050°C, 1100°C or 1160°C), and the temperature of the heating stage 2 is 1120-1200°C (for example, it can be 1120°C, 1140°C, 1160°C, 1180°C or 1200°C). The total heating time in the heating process is 240-360 min (for example, it can be 240 min, 270 min, 300 min, 330 min or 360 min).

[0045] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the rough rolling process, the starting rolling temperature is 1060-1140°C (for example, it can be 1060°C, 1080°C, 1100°C, 1120°C or 1140°C).

[0046] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the intermediate rolling process, the intermediate rolling temperature is 880-980°C, for example, it can be 880°C, 900°C, 910°C, 930°C or 980°C.

[0047] In the above-mentioned rolling method of the 50Mn steel bar, as an optional embodiment, the composition of the 50Mn steel billet includes, by mass percentage, C: 0.48-0.56%, Si: 0.17-0.37%, Mn: 0.7-1.0%, P: ≤0.035%, S: ≤0.035%, Cr: ≤0.25%, Ni: ≤0.3%, Cu ≤0.25%, and the rest is matrix Fe and unavoidable impurities.

[0048] A water tank is used in the water cooling process after the intermediate rolling and the finishing rolling of the reducing and sizing mill. The finishing temperature and the upper cooling bed temperature are controlled by controlling the water volume and water pressure in the water tank. In the above-mentioned rolling method of the 50Mn steel bar, as an optional embodiment, a water tank is used for water cooling in the first water cooling process or the second water cooling process. The water volume of the water tank is 200-350L / min (for example, it can be 200L / min, 250L / min, 300L / min or 350L / min), and the water pressure is 7-9Mpa.

[0049] In the above-mentioned 50Mn steel bar rolling method, as an optional embodiment, in the first water-cooling step, the cooling rate of the steel billet is 25-70°C / s (for example, 25°C / s, 30°C / s, 35°C / s, 40°C / s, 45°C / s, 50°C / s, 60°C / s, or 70°C / s), and the water-cooling speed of the steel billet is 1.2-4 m / s, for example, 1.2 m / s, 2 m / s, 3 m / s, or 4 m / s. In the above-mentioned 50Mn steel bar rolling method, as an optional embodiment, in the second water-cooling step, the water-cooling speed of the steel billet is 2-8 m / s, for example, 2 m / s, 4 m / s, 6 m / s, or 8 m / s.

[0050] In the present invention, in the first water-penetrating cooling step or the second water-penetrating cooling step, the cooling speed of the steel billet can be controlled by controlling the water volume and water pressure in the water tank and the speed at which the steel billet passes through the water tank.

[0051] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, the 50Mn steel billet is further subjected to surface treatment after heating and before rough rolling.

[0052] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, the surface treatment includes high-pressure water descaling.

[0053] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the cooling bed cooling process, the upper cooling bed temperature (i.e., the temperature after final rolling and water cooling) is 660-720°C (for example, it can be 660°C, 680°C, 700°C or 720°C).

[0054] In the above-mentioned rolling method of 50Mn steel bar, as an optional embodiment, in the cooling process of the cooling bed, an insulation cover is used for insulation cooling, and then the cooling bed is air-cooled after leaving the insulation cover. The insulation cooling rate in the insulation cover is 1-2°C / s, and the temperature outside the insulation cover is 500-600°C (for example, it can be 500°C, 520°C, 540°C, 560°C, 580°C or 600°C).

[0055] In the above-mentioned rolling method of 50Mn steel bars, as an optional embodiment, after the cooling process on the cooling bed, the rolling method further includes shearing, collecting and bundling.

[0056] In a second aspect, the present invention provides a 50Mn steel bar, which is obtained by the rolling method described in the first aspect.

[0057] In the above-mentioned 50Mn steel bar, as an optional embodiment, the specification of the 50Mn steel bar is 20-60 mm, the tensile strength is 795-828 MPa, the yield strength is 474-506 MPa, the elongation after fracture is 19%-21%, the cross-sectional reduction rate is 45%-49%, the actual grain size is 7-8, the microstructure is F+P, and the ferrite volume ratio is 26.5%-30.2%.

[0058] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0059] The mass percentages of the various elements in the 50Mn steel billets used in the following examples and comparative examples are shown in Table 1.

[0060] Table 1 Mass percentage of each element in 50Mn steel billet (balance is Fe and unavoidable impurities)

[0061] element C Si Mn P S Cr Ni Cu Content (wt%) 0.52 0.25 0.96 0.02 0.01 0.16 0.05 0.08

[0062] Examples 1-5

[0063] The embodiment of the present invention provides a rolling method for 50Mn steel bars, using 50Mn steel billets as raw materials. The rolling method sequentially includes the following steps: a heating step, high-pressure water descaling, a rough rolling step, an intermediate rolling step, a first water-penetrating cooling step, a finishing rolling step in a reducing and sizing mill, a second water-penetrating cooling step, a cooling step on a cooling bed, shearing, collecting, and bundling, to ultimately obtain finished bars.

[0064] In which, in the heating process, heating is carried out in a walking furnace and is divided into four heating sections, namely, a preheating section, a heating section, a heating section and a soaking section. The temperature of the preheating section is 600-750°C, the temperature of the heating section 1 is 1000-1160°C, the temperature of the heating section 2 is 1120-1200°C, the temperature of the soaking section is 1150-1230°C, the holding time of the soaking section is 30-90 minutes, and the total heating time in the heating process is 240-360 minutes.

[0065] In the rough rolling process, the equipment used is a two-roll rolling mill, and the starting rolling temperature is 1060-1140°C;

[0066] In the intermediate rolling process, the equipment used is a two-roll rolling mill, and the intermediate rolling temperature is 880-980°C;

[0067] In the first water-penetrating cooling step, a water tank is used. The cooling rate of the steel billet is controlled by controlling the water volume, water pressure, and water penetration speed of the steel billet, thereby controlling the final rolling temperature. The water volume of the water tank is 200-350 L / min, the water pressure is 7-9 MPa, the water penetration speed of the steel billet is 1.2-4 m / s, and the cooling rate of the steel billet is 25-70°C / s.

[0068] In the finishing rolling process of the reducing and sizing mill, the equipment used is a three-roll reducing and sizing mill, and the final rolling temperature is 760-820°C;

[0069] In the second water-penetrating cooling step, a water tank is used. The cooling rate of the steel billet is controlled by controlling the water volume, water pressure and water penetration speed of the steel billet, thereby controlling the temperature of the upper cooling bed (i.e., the temperature after the final water-penetrating cooling). The water volume of the water tank is 200-350 L / min, the water pressure is 7-9 MPa, the water penetration speed of the steel billet is 2-8 m / s, the cooling rate of the steel billet is 50-150°C / s, and the steel billet is cooled to 660-720°C.

[0070] In the cooling process of the cooling bed, the temperature of the upper cooling bed (the temperature after the final rolling water cooling, that is, the temperature of the billet after the second water cooling process) is 660-720 ° C, and an insulation cover is used for insulation cooling, and then the cooling bed is air-cooled after leaving the insulation cover. The insulation cooling rate in the insulation cover is 1-2 ° C / s, and the temperature outside the insulation cover is 500-600 ° C.

[0071] Five batches of 50Mn steel bars with a specification of 20 to 60 mm were produced according to the rolling method provided in Examples 1 to 5 of the present invention. The metallographic structure test was carried out in accordance with GB / T 13298-2015, the grain size test was carried out in accordance with ASTM E112-2013, and the tensile property test was carried out in accordance with GB / T 228.1-2021. The specific parameters of each embodiment are shown in Tables 2 and 3, and the test results are shown in Tables 4 and 5. Figure 1 and Figure 2 shown.

[0072] Table 2

[0073]

[0074] Table 3

[0075]

[0076]

[0077] Table 4

[0078]

[0079] In Table 4, F represents ferrite (phase) and P represents pearlite (structure).

[0080] Figure 1 This is a metallographic photograph of the 50Mn steel bar produced in Example 1. Figure 2 The metallographic structure photo of the 50Mn steel bar produced in Example 2 is shown in FIG. Figure 1 and Figure 2 It can be seen that the metallographic structure of the 50Mn steel bar is F+P. The metallographic structure photos of the 50Mn steel bars produced in the other embodiments are similar to those of Examples 1 and 2.

[0081] As can be seen from Table 4, the present invention utilizes the fine grain strengthening and second phase strengthening mechanisms, combined with the characteristics of 50Mn steel, so that the tensile strength of the 50Mn steel bar after hot rolling is 795-828 MPa, the yield strength is 474-506 MPa, the elongation after fracture is 19%-21%, the cross-sectional reduction rate is 45%-49%, the actual grain size is 7-8, the microstructure is F+P, and the volume ratio of ferrite is 26.5%-30.2%, which meets the performance requirements of the normalized state structure and realizes normalizing-free delivery.

[0082] Comparative Example 1

[0083] This comparative example provides a rolling method for a 50Mn steel bar, which differs from Example 1 only in that the final rolling temperature is 750° C. The remaining settings are the same as those in Example 1.

[0084] Comparative Example 2

[0085] This comparative example provides a rolling method for 50Mn steel bars, which differs from Example 1 only in that the final rolling temperature is 830° C. The remaining settings are the same as those in Example 1.

[0086] Comparative Example 3

[0087] This comparative example provides a rolling method for 50Mn steel bars, which differs from Example 1 only in that the temperature of the upper cooling bed is 650° C. The remaining settings are the same as those in Example 1.

[0088] Comparative Example 4

[0089] This comparative example provides a rolling method for 50Mn steel bars, which differs from Example 1 only in that the temperature of the upper cooling bed is 730° C. The remaining settings are the same as those in Example 1.

[0090] Comparative Example 5

[0091] This comparative example provides a rolling method for 50Mn steel bars, which differs from Example 1 only in that, in the second water cooling step, the cooling rate of the steel billet is 45°C / s.

[0092] Comparative Example 6

[0093] This comparative example provides a rolling method for a 50Mn steel bar, which differs from Example 1 only in that, in the second water cooling step, the cooling rate of the steel billet is 160°C / s.

[0094] Four batches of 50Mn steel bars with a specification of 20 mm were produced according to the rolling methods provided in Comparative Examples 1-4. The metallographic structure test was performed in accordance with GB / T 13298-2015, the grain size test was performed in accordance with ASTM E112-2013, and the tensile property test was performed in accordance with GB / T 228.1-2021. The test results of the 50Mn steel bars produced in each comparative example are shown in Table 5.

[0095] Table 5

[0096]

[0097]

[0098] From Table 5 we can at least see that:

[0099] (1) It can be seen from Example 1 and Comparative Example 1 that when the final rolling temperature is too low, although the tensile strength and yield strength can meet the requirements of the agreement, due to the low final rolling temperature, mixed crystals appear, the grain size is 4-8, and the elongation after fracture and the cross-sectional shrinkage rate are greatly reduced.

[0100] (2) It can be seen from Example 1 and Comparative Example 2 that when the final rolling temperature is too high, both the tensile strength and the yield strength are unqualified.

[0101] (3) It can be seen from Example 1 and Comparative Example 3 that the tensile strength and yield strength can meet the requirements of the agreement, but because the temperature of the upper cooling bed is low and the ferrite structure is less, the elongation after fracture and the end face shrinkage rate decrease significantly.

[0102] (4) It can be seen from Example 1 and Comparative Example 4 that due to the high temperature of the upper cooling bed, the austenite structure is not fully transformed, and the remaining austenite is mostly generated as ferrite structure, resulting in unqualified tensile strength and yield strength.

[0103] (5) It can be seen from Example 1 and Comparative Example 5 that when the cooling rate of the steel billet is too low in the second water cooling step, both the tensile strength and the yield strength are unqualified.

[0104] (6) It can be seen from Example 1 and Comparative Example 6 that when the cooling rate of the steel billet is too high in the second water-penetrating cooling process, the tensile strength and yield strength can meet the requirements of the agreement. However, due to the excessively fast cooling rate after rolling, bainite structure appears, resulting in a significant decrease in the elongation after fracture and the end face shrinkage.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for rolling 50Mn steel bars, characterized in that: Using 50Mn steel billet as raw material, the rolling method includes the following steps in sequence: heating step, rough rolling step, intermediate rolling step, first water cooling step, finishing rolling step of reducing and sizing mill, second water cooling step, cooling step on cooling bed, and finally obtaining finished bar; Wherein, in the finishing rolling process of the reducing and sizing mill, the final rolling temperature is 760-820°C; In the second water cooling step, the cooling rate of the steel billet is 50-150°C / s, and the steel billet is cooled to 660-720°C; In the rough rolling process, the starting rolling temperature is 1060-1140°C; In the intermediate rolling process, the intermediate rolling temperature is 880-980°C; In the cooling process of the cooling bed, a heat preservation cover is used for heat preservation and cooling, and then the cooling bed is air-cooled after exiting the heat preservation cover. The heat preservation and cooling speed in the heat preservation cover is 1-2°C / s, and the temperature outside the heat preservation cover is 500-600°C.

2. The method for rolling a 50Mn steel bar according to claim 1, wherein: In the heating process, the temperature of the soaking section is 1150-1230°C, and the holding time of the soaking section is 30-90 minutes; And / or, in the heating process, the heating is divided into four stages, namely a preheating stage, a first heating stage, a second heating stage and a soaking stage, the temperature of the preheating stage is 600-750° C., the temperature of the first heating stage is 1000-1160° C., the temperature of the second heating stage is 1120-1200° C., and the total heating time in the heating process is 240-360 min; And / or, in terms of mass percentage, the composition of the 50Mn steel billet includes: C: 0.48-0.56%, Si: 0.17-0.37%, Mn: 0.7-1.0%, P: ≤0.035%, S: ≤0.035%, Cr: ≤0.25%, Ni: ≤0.3%, Cu≤0.25%, and the rest is matrix Fe and unavoidable impurities.

3. The rolling method of 50Mn steel bar according to claim 1, characterized in that: In the first water-penetrating cooling step or the second water-penetrating cooling step, a water tank is used for water-penetrating cooling, the water volume of the water tank is 200-350 L / min, and the water pressure is 7-9 MPa; and / or, in the first water-penetrating cooling step, the cooling rate of the steel billet is 25-70° C. / s, and the water-penetrating speed of the steel billet is 1.2-4 m / s; And / or, in the second water-penetrating cooling step, the water-penetrating speed of the steel billet is 2-8 m / s.

4. The method for rolling a 50Mn steel bar according to any one of claims 1 to 3, characterized in that: The 50Mn steel billet is further subjected to surface treatment after heating and before rough rolling.

5. The method for rolling a 50Mn steel bar according to claim 4, wherein: The surface treatment includes high-pressure water descaling.

6. The method for rolling a 50Mn steel bar according to claim 1, wherein: In the cooling process of the cooling bed, the temperature of the upper cooling bed is 660-720°C; And / or, after the cooling process on the cooling bed, the rolling method further includes shearing, collecting and bundling.

7. A 50Mn steel bar, characterized in that: The 50Mn steel bar is obtained by the rolling method according to any one of claims 1 to 6.

8. The 50Mn steel bar according to claim 7, characterized in that The 50Mn steel bar has a specification of 20-60 mm, a tensile strength of 795-828 MPa, a yield strength of 474-506 MPa, an elongation after fracture of 19%-21%, a cross-sectional shrinkage of 45%-49%, an actual grain size of 7-8, and a microstructure of F+P, wherein the volume ratio of ferrite is 26.5%-30.2%.

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