Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a 50Mn steel bar and a rolling method thereof, and the rolling method of the 50Mn steel bar provided by the invention controls the final rolling temperature to 760-820 ℃ through a rolling line along a water tank, and simultaneously utilizes the water tank to be quickly cooled to not more than 720 ℃ after rolling, 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, the requirements of normalizing mechanical property and actual grain size are met, and normalizing delivery can be avoided.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
In a first aspect, the invention provides a rolling method of a 50Mn steel bar, which uses a 50Mn steel billet as a raw material, and sequentially comprises the following steps: heating, rough rolling, middle rolling, first water cooling, reducing sizing mill finish rolling, second water cooling, and cooling by a cooling bed to obtain a finished bar;
Wherein, in the finish rolling process of the reducing sizing mill, the finish rolling temperature is 760-820 ℃;
in the second water cooling process, the cooling speed of the steel billet is not less than 50 ℃/s, and the steel billet is cooled to not more than 720 ℃.
The rolling method of the 50Mn steel bar provided by the invention controls the final rolling temperature to 760-820 ℃ through the water tank along the rolling line, and simultaneously utilizes the water tank after rolling to be quickly cooled to not more than 720 ℃, 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, the requirements of the mechanical property in a normalizing state and the actual grain size are met, and the normalizing delivery can be avoided.
Further, in the second water cooling process, the cooling speed of the steel billet is 50-150 ℃/s.
Further, in the second water cooling process, the steel billet is cooled to 660-720 ℃.
Further, in the heating procedure, the temperature of the heated soaking section is 1150-1230 ℃, and the heat preservation time of the soaking section is 30-90 min;
And/or in the heating process, the heating process is divided into four sections of heating, namely a preheating section, a first heating section, a second heating section and a soaking section, wherein the temperature of the preheating section is 600-750 ℃, the temperature of the heating section 1 is 1000-1160 ℃, the temperature of the heating section 2 is 1120-1200 ℃, and the total heating time in the heating process is 240-360 min;
and/or, in the rough rolling process, the initial rolling temperature is 1060-1140 ℃;
and/or, in the middle rolling process, the middle rolling temperature is 880-980 ℃;
And/or, the 50Mn steel billet comprises the following components in percentage by mass: c:0.48 to 0.56 percent of Si:0.17 to 0.37 percent of Mn:0.7 to 1.0 percent, P: less than or equal to 0.035 percent, S: less than or equal to 0.035 percent, cr: less than or equal to 0.25 percent, ni: less than or equal to 0.3 percent, less than or equal to 0.25 percent of Cu, and the balance of matrix Fe and unavoidable impurities.
Further, in the first water passing cooling process or the second water passing cooling process, water passing cooling is carried out by using a water tank, wherein the water quantity of the water tank is 200-350L/min, and the water pressure is 7-9 Mpa;
and/or in the first water passing cooling process, the cooling speed of the steel billet is 25-70 ℃/s, and the water passing speed of the steel billet is 1.2-4 m/s;
And/or in the second water passing cooling process, the water passing speed of the steel billet is 2-8 m/s.
Further, the 50Mn billet is subjected to surface treatment after heating and before rough rolling.
Further, the surface treatment includes high pressure water descaling.
Further, in the cooling process of the cooling bed, the upper cooling bed temperature is 660-720 ℃;
And/or, in the cooling procedure of the cooling bed, using a heat preservation cover for heat preservation and cooling, then taking out the heat preservation cover for cooling the cooling bed, wherein the speed of heat preservation and cooling in the heat preservation cover is 1-2 ℃/s, and the temperature of taking out the heat preservation cover is 500-600 ℃;
And/or, after the cooling process, the rolling method further comprises shearing, collecting and bundling.
In a second aspect, the present invention provides a 50Mn steel bar obtained by the rolling method of the first aspect.
Further, 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 area shrinkage is 45-49%, the actual grain size is 7-8 grade, the microstructure is F+P, and the ferrite volume ratio is 26.5-30.2%.
The invention designs a rolling process for controlling the structure and the performance of the steel by combining the characteristics of the 50Mn steel and applying fine grain strengthening and second phase strengthening mechanisms, and compared with the prior art, the invention has the beneficial effects that the rolling process at least comprises the following steps:
(1) The rolling method of the 50Mn steel bar provided by the invention controls the final rolling temperature to 760-820 ℃ through a rolling line along a water tank, and simultaneously utilizes the water tank after rolling to be rapidly cooled to 660-720 ℃, 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 area shrinkage is 45-49%, the actual grain size is 7-8 levels, the microstructure is F+P, the ferrite volume ratio is 26.5-30.2%, the requirements of normalized mechanical property and actual grain size tissue property are met, and normalized delivery can be avoided.
(2) The rolling process of the 50Mn steel in the invention controls the finishing temperature 790+/-30 ℃ through the water tank along the rolling line, so that the material is deformed at the temperature slightly higher than the Ar3 phase transition point, a large amount of dislocation is accumulated to promote phase transition nucleation, and meanwhile, the water tank is rapidly cooled to 690+/-30 ℃ after rolling, so that more pearlite structures are formed at the nucleation points, and the effects of fine grain strengthening and second phase strengthening are achieved.
(3) According to the invention, the water tanks are distributed along the rolling line, the final rolling temperature and the temperature after the final rolling penetrating water is cooled quickly are controlled by controlling the water pressure and the water quantity of the water tanks, and the temperature conditions of all key points can be accurately controlled by the technology, so that the required round bar is obtained, and the technology has the characteristic of high controllable degree.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described in the following in conjunction with the embodiments of the present invention. It will be apparent to those skilled in the art that the examples are merely to aid in understanding the invention and are not to be construed as a specific limitation thereof. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention. The process parameters for the specific conditions not noted in the examples below are generally as usual.
The endpoints of the ranges and any values disclosed in the present invention are not limited to the precise range or value, and the range or value should be understood to include values close to the range or value. For numerical ranges, one or more new numerical ranges may be obtained in combination with each other between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point values, and are to be considered as specifically disclosed in the present invention.
In a first aspect, the invention provides a rolling method of a 50Mn steel bar, which uses a 50Mn steel billet as a raw material, and sequentially comprises the following steps: heating, rough rolling, middle rolling, first water cooling, reducing sizing mill finish rolling, second water cooling, and cooling by a cooling bed to obtain a finished bar;
wherein, in the reducing mill finishing step, the finishing temperature is 760-820 ℃ (for example, 760 ℃, 780 ℃, 800 ℃ or 820 ℃);
in the second water cooling process, the cooling speed of the steel billet is not less than 50 ℃/s, and the steel billet is cooled to not more than 720 ℃.
In order to reduce production cost, shorten processing procedures and reduce carbon emission, the rolling process of the 50Mn steel bar (phi 20-phi 60) is optimized, so that the material meets the structural performance requirement of a certain automobile part factory on the 50Mn steel bar in the background art in a hot rolling state, and the normalizing-free delivery is realized. The rolling method of the 50Mn steel bar provided by the invention controls the final rolling temperature to 760-820 ℃ through the water tank along the rolling line, and simultaneously utilizes the water tank after rolling to be quickly cooled to not more than 720 ℃, 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, the requirements of the mechanical property in a normalizing state and the actual grain size are met, and the normalizing delivery can be avoided.
The Ar3 phase transformation point of the 50Mn steel is 755 ℃, the Ar1 phase transformation point is 725 ℃, more nucleation points are provided for improving the deformation induction nucleation capability and refining grains, and the finishing rolling temperature is about 790 ℃ (slightly higher than the Ar3 temperature); in order to improve the transformation capability of austenite to pearlite, prevent excessive ferrite structures from being precipitated at nucleation points, refine actual grains, and rapidly cool the rolled steel to about 690 ℃ through water (slightly lower than Ar1 temperature), so as to obtain more pearlite structures and finer grains, and achieve the purpose of reinforcing the mechanical properties of the material.
In the above rolling method of 50Mn steel bar, as an alternative embodiment, in the second through-water cooling process, the cooling rate of the billet is 50 to 150 ℃/s (for example, may be 50 ℃/s, 70 ℃/s, 90 ℃/s, 110 ℃/s, 130 ℃/s or 150 ℃/s),
In the above-mentioned method for rolling a 50Mn steel bar, as an alternative embodiment, in the second through-water cooling step, the billet is cooled to 660-720 ℃ (for example, 660 ℃, 680 ℃, 700 ℃ or 720 ℃).
In the above-mentioned rolling method of 50Mn steel bar, as an alternative embodiment, in the heating step, the temperature of the heated soaking stage is 1150-1230 ℃ (for example, 1150 ℃, 1170 ℃, 1190 ℃, 1210 ℃ or 1230 ℃), and the holding time of the soaking stage is 30-90 min (for example, 30min, 60min or 90 min).
In the above rolling method of 50Mn steel bar, as an alternative embodiment, the heating step is divided into four heating stages, which are a preheating stage, a heating stage and a soaking stage in this order, wherein the temperature of the preheating stage is 600-750 ℃ (for example, 600 ℃, 650 ℃, 700 ℃ or 750 ℃), the temperature of the heating stage 1 is 1000-1160 ℃ (for example, 1000 ℃, 1050 ℃, 1100 ℃ or 1160 ℃), the temperature of the heating stage2 is 1120-1200 ℃ (for example, 1120 ℃, 1140 ℃, 1160 ℃, 1180 ℃ or 1200 ℃), and the total heating time in the heating step is 240-360 min (for example, 240min, 270min, 300min, 330min or 360 min).
In the above-mentioned rolling method of 50Mn steel bar, as an alternative embodiment, in the rough rolling step, the initial rolling temperature is 1060 to 1140 ℃ (for example, 1060 ℃, 1080 ℃, 1100 ℃, 1120 ℃ or 1140 ℃).
In the above-mentioned rolling method of 50Mn steel bar, as an alternative embodiment, in the intermediate rolling step, the intermediate rolling temperature is 880-980 ℃, and may be 880 ℃, 900 ℃, 910 ℃, 930 ℃, or 980 ℃, for example.
In the rolling method of the 50Mn steel bar, as an optional embodiment, the components of the 50Mn steel billet include, in mass percent: c:0.48 to 0.56 percent of Si:0.17 to 0.37 percent of Mn:0.7 to 1.0 percent, P: less than or equal to 0.035 percent, S: less than or equal to 0.035 percent, cr: less than or equal to 0.25 percent, ni: less than or equal to 0.3 percent, less than or equal to 0.25 percent of Cu, and the balance of matrix Fe and unavoidable impurities.
In the rolling method of 50Mn steel bar, as an optional implementation manner, in the first water passing cooling process or the second water passing cooling process, water tank is used in the water passing cooling process after finish rolling of the middle rolling mill and the reducing mill, the final rolling temperature and the upper cooling bed temperature are controlled by controlling the water amount and the water pressure of the water tank, the water amount of the water tank is 200-350L/min (for example, 200L/min, 250L/min, 300L/min or 350L/min, and the water pressure is 7-9 Mpa).
In the above rolling method for 50Mn steel bar, as an alternative embodiment, in the first water passing-through step, the cooling rate of the billet is 25 to 70 ℃/s (for example, 25 ℃/s, 30 ℃/s, 35 ℃/s, 40 ℃/s, 45 ℃/s, 50 ℃/s, 60 ℃/s or 70 ℃/s), and the water passing rate of the billet is 1.2 to 4m/s, for example, 1.2m/s, 2m/s, 3m/s or 4m/s. In the rolling method of 50Mn steel bar, as an alternative embodiment, in the second water passing-through cooling step, the water passing speed of the billet may be 2 to 8m/s, for example, 2m/s, 4m/s, 6m/s, or 8m/s.
In the present invention, in the first water passing cooling process or the second water passing cooling process, the cooling rate of the billet can be controlled by controlling the water amount of the water tank, the water pressure, and the speed of the billet passing through the water tank.
In the rolling method of 50Mn steel bar, as an alternative embodiment, the 50Mn billet is further subjected to surface treatment after heating and before rough rolling.
In the rolling method of 50Mn steel bar, as an alternative embodiment, the surface treatment includes high-pressure water descaling.
In the above-mentioned rolling method of 50Mn steel bar, as an alternative embodiment, in the cooling process of the cooling bed, the upper cooling bed temperature (i.e., the temperature after the finish rolling through water cooling) is 660 to 720 ℃ (for example, 660 ℃, 680 ℃, 700 ℃ or 720 ℃).
In the above-mentioned rolling method of 50Mn steel bar, as an alternative embodiment, in the cooling step of the cold bed, a heat-retaining cover is used for heat-retaining cooling, and then the cold bed air cooling is performed by taking out the heat-retaining cover, wherein the heat-retaining cooling speed is 1-2 ℃/s, the temperature of the heat-retaining cover is 500-600 ℃ (for example, 500 ℃, 520 ℃, 540 ℃, 560 ℃, 580 ℃ or 600 ℃).
In the rolling method of the 50Mn steel bar, as an alternative embodiment, the rolling method further includes shearing, collecting and bundling after the cooling process of the cooling bed.
In a second aspect, the present invention provides a 50Mn steel bar obtained by the rolling method of the first aspect.
In the 50Mn steel bar, as an alternative implementation mode, 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 area shrinkage is 45-49%, the actual grain size is 7-8 grade, the microstructure is F+P, and the ferrite volume ratio is 26.5-30.2%.
The present invention will be described in further detail with reference to specific examples and comparative examples.
The mass percentages of the respective elemental components in the 50Mn billets used in the following examples and comparative examples are shown in table 1.
TABLE 1 mass percent of each element component in 50Mn billet (balance Fe and unavoidable impurities)
| Element(s) | 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 |
Examples 1 to 5
The embodiment of the invention provides a rolling method of a 50Mn steel bar, which takes a 50Mn steel billet as a raw material, and sequentially comprises the following steps: heating, descaling with high-pressure water, rough rolling, middle rolling, first water cooling, finish rolling with a reducing sizing mill, second water cooling, cooling with a cooling bed, shearing, collecting and bundling to finally obtain a finished bar;
In the heating procedure, the heating is performed in a walking type furnace and is divided into four sections of heating, namely a preheating section, a heating section and a soaking section, wherein the temperature of the preheating section is 600-750 ℃, the temperature of the heating section 1 is 1000-1160 ℃, the temperature of the heating section 2 is 1120-1200 ℃, the temperature of the soaking section is 1150-1230 ℃, the heat preservation time of the soaking section is 30-90 min, and the total heating time in the heating procedure is 240-360 min.
In the rough rolling process, two rolling mills are used as equipment, and the initial rolling temperature is 1060-1140 ℃;
in the middle rolling process, the equipment is a two-roller mill, and the middle rolling temperature is 880-980 ℃;
In the first water passing cooling process, a water tank is used, the cooling speed of the steel billet is controlled by controlling the water quantity and the water pressure of the water tank and the water passing speed of the steel billet, so that the finish rolling temperature is controlled, the water quantity of the water tank is 200-350L/min, the water pressure is 7-9 Mpa, the water passing speed of the steel billet is 1.2-4m/s, and the cooling speed of the steel billet is 25-70 ℃/s;
In the finish rolling process of the reducing sizing mill, a three-roller reducing sizing mill is used as equipment, and the finishing temperature is 760-820 ℃;
In the second water passing cooling process, a water tank is used, the cooling speed of the steel billet is controlled by controlling the water quantity and the water pressure of the water tank and the water passing speed of the steel billet, so that the temperature of an upper cooling bed (namely the temperature after the final rolling water passing cooling) is controlled, the water quantity of the water tank is 200-350L/min, the water pressure is 7-9 Mpa, the water passing speed of the steel billet is 2-8m/s, the cooling speed of the steel billet is 50-150 ℃/s, and the steel billet is cooled to 660-720 ℃;
in the cooling process of the cooling bed, the upper cooling bed temperature (the temperature after final rolling through water cooling, namely the temperature of the billet after cooling by the second through water cooling process) is 660-720 ℃, the billet is subjected to heat preservation cooling by using a heat preservation cover, then the billet is subjected to cooling bed air cooling by taking out of the heat preservation cover, the heat preservation cooling speed in the heat preservation cover is 1-2 ℃/s, and the temperature of the billet taken out of the heat preservation cover is 500-600 ℃.
According to the rolling method provided by the embodiment 1-5 of the invention, 5 batches of 50Mn steel bars with the specification of 20-60 mm are produced, the metallographic structure detection is carried out according to GB/T13298-2015, the grain size detection is carried out according to ASTM E112-2013, the tensile property detection is carried out according to GB/T228.1-2021, the specific parameters of each embodiment are shown in tables 2 and 3, and the detection results are shown in tables 4, 1 and 2.
TABLE 2
TABLE 3 Table 3
TABLE 4 Table 4
In table 4, F represents ferrite (phase), and P represents pearlite (structure).
Fig. 1 is a photograph of the metallographic structure of the 50Mn steel bar produced in example 1, fig. 2 is a photograph of the metallographic structure of the 50Mn steel bar produced in example 2, and it can be seen from fig. 1 and 2 that the metallographic structure of the 50Mn steel bar is f+p. The metallographic photographs of the 50Mn steel bars produced in the other examples were similar to those of examples 1 and 2.
As can be seen from Table 4, the invention combines the characteristics of 50Mn steel by using fine grain strengthening and second phase strengthening mechanisms, so that the tensile strength of 50Mn steel bars after hot rolling is 795-828 MPa, the yield strength is 474-506 MPa, the elongation after fracture is 19-21%, the area reduction is 45-49%, the actual grain size is 7-8 grade, the microstructure is F+P, the ferrite volume ratio is 26.5-30.2%, the performance requirement of normalizing structure is met, and the normalizing-free delivery is realized.
Comparative example 1
This comparative example provides a rolling process for 50Mn steel bar differing from example 1 only in the finishing temperature of 750 ℃. The rest of the settings are the same as in example 1.
Comparative example 2
This comparative example provides a rolling process for 50Mn steel bar differing from example 1 only in that the finishing temperature is 830 ℃. The rest of the settings are the same as in example 1.
Comparative example 3
This comparative example provides a rolling method for 50Mn steel bar differing from example 1 only in that the upper cooling bed temperature is 650 ℃. The rest of the settings are the same as in example 1.
Comparative example 4
This comparative example provides a rolling method for 50Mn steel bar differing from example 1 only in the upper cooling bed temperature of 730 ℃. The rest of the settings are the same as in example 1.
Comparative example 5
The present comparative example provides a rolling method of a 50Mn steel bar differing from example 1 only in that in the second through-water cooling process, the cooling rate of the billet is 45 ℃/s.
Comparative example 6
This comparative example provides a rolling method of a50 Mn steel bar differing from example 1 only in that in the second through-water cooling process, the cooling rate of the billet is 160 ℃/s.
The rolling method provided in comparative examples 1 to 4 was used to produce 4 batches of 50Mn steel bars of 20mm in size, the metallographic structure test was performed in accordance with GB/T13298-2015, the grain size test was performed in accordance with ASTM E112-2013, the tensile property test was performed in accordance with GB/T228.1-2021, and the test results of the 50Mn steel bars produced in the respective comparative examples are shown in Table 5.
TABLE 5
At least as can be seen from table 5:
(1) As is clear from example 1 and comparative example 1, when the finish rolling temperature is too low, the tensile strength and yield strength satisfy the protocol requirements, but mixed crystals appear due to the too low finish rolling temperature, the grain size is 4-8 grade, and the elongation after break and the reduction in area are reduced more.
(2) As is clear from examples 1 and 2, when the finishing temperature is too high, both the tensile strength and the yield strength are not satisfactory.
(3) As is clear from examples 1 and 3, the tensile strength and yield strength satisfy the protocol requirements, but the lower upper cooling bed temperature results in less ferrite structure and more decrease in elongation after break and end shrinkage.
(4) As is clear from examples 1 and 4, the upper cooling bed temperature was too high, the austenite structure was not sufficiently transformed, and a large amount of residual austenite was formed into a ferrite structure, and the tensile strength and the yield strength were not satisfactory.
(5) As is clear from examples 1 and 5, in the second through-water cooling step, when the cooling rate of the billet is too low, both the tensile strength and the yield strength are not satisfactory.
(6) As is clear from examples 1 and 6, in the second through-water cooling step, when the cooling rate of the billet is too high, the tensile strength and the yield strength satisfy the protocol requirements, but the cooling rate after rolling is too high, and the bainitic structure appears, so that the elongation after breaking and the end face shrinkage rate are greatly reduced.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.