CN113073179B - Heat treatment method of low-carbon structural steel for cold extrusion - Google Patents
Heat treatment method of low-carbon structural steel for cold extrusion Download PDFInfo
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- 229910000746 Structural steel Inorganic materials 0.000 title claims abstract description 57
- 229910052799 carbon Inorganic materials 0.000 title claims abstract description 57
- 238000000641 cold extrusion Methods 0.000 title claims abstract description 53
- 238000010438 heat treatment Methods 0.000 title claims abstract description 52
- 238000000034 method Methods 0.000 title claims abstract description 47
- 238000001816 cooling Methods 0.000 claims abstract description 76
- 238000004321 preservation Methods 0.000 claims abstract description 38
- 238000000137 annealing Methods 0.000 claims abstract description 12
- 238000007599 discharging Methods 0.000 claims abstract description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 23
- 239000000463 material Substances 0.000 claims description 15
- 238000004513 sizing Methods 0.000 claims description 13
- 230000001681 protective effect Effects 0.000 claims description 7
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical group [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 claims description 3
- 229910001562 pearlite Inorganic materials 0.000 abstract description 23
- 229910000859 α-Fe Inorganic materials 0.000 abstract description 21
- 229910000831 Steel Inorganic materials 0.000 description 46
- 239000010959 steel Substances 0.000 description 46
- 230000000052 comparative effect Effects 0.000 description 17
- 238000004519 manufacturing process Methods 0.000 description 13
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 8
- 239000002184 metal Substances 0.000 description 4
- 229910052751 metal Inorganic materials 0.000 description 4
- 229910052757 nitrogen Inorganic materials 0.000 description 4
- 239000002994 raw material Substances 0.000 description 4
- 239000007789 gas Substances 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical group C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 238000010583 slow cooling Methods 0.000 description 2
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 239000003345 natural gas Substances 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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Abstract
The invention provides a heat treatment method of low-carbon structural steel for cold extrusion, which comprises the following steps: step one, placing a low-carbon structural steel blank in a heating furnace, raising the temperature of the furnace to a first heat preservation temperature of 740-; then, cooling to a second heat preservation temperature, and preserving heat for 3-5 hours at the second heat preservation temperature; and step two, cooling the blank subjected to heat preservation treatment at the second heat preservation temperature to 550 ℃, discharging, and finally air-cooling to room temperature to obtain the low-carbon structural steel finished product for cold extrusion. The hardness of the low-carbon structural steel obtained by the multi-stage isothermal annealing process is less than or equal to 125HBW, the hardness uniformity is less than or equal to 10HBW, and the structure is a ferrite and spherical pearlite structure, so that the requirements of the hardness and the structure of large deformation (the deformation rate is more than or equal to 70%) of cold extrusion are met.
Description
Technical Field
The invention belongs to the technical field of heat treatment of low-carbon structural steel, and particularly relates to a heat treatment method of low-carbon structural steel for cold extrusion.
Background
Cold extrusion is widely used for manufacturing various small parts, and metal flows to generate plastic deformation after being pressed in a die cavity. If the plasticity of the metal is poor, the manufactured part is difficult to reach the required size and shape, and even has the risk of cracking, so the cold extrusion processing mode has high requirements on the plasticity, the structure and the hardness uniformity of the metal raw material.
The low-carbon structural steel (the carbon content is not more than 0.18%), the steel structure has high plasticity and low hardness, the general hot rolling hardness is less than or equal to 143HBW, the hardness uniformity is less than or equal to 20HBW, the structure is ferrite and lamellar pearlite, and the low-carbon structural steel is suitable for being used as a metal raw material for cold extrusion. But for some cold extrusion production parts with large deformation, the required performance is higher, the hardness of the material is required to be less than or equal to 125HBW, the hardness uniformity is less than or equal to 10HBW, and the structure is ferrite + spherical pearlite structure.
For such steel grades for cold extrusion having a very low carbon content, hardness reduction, hardness uniformity improvement, and difficulty in structure spheroidization are large, so that a heat treatment process for hardness reduction and structure spheroidization needs to be studied.
Disclosure of Invention
The invention aims to provide a heat treatment method of low-carbon structural steel for cold extrusion, which at least solves the problems of high hardness and poor hardness uniformity of the low-carbon structural steel for cold extrusion with large deformation at present.
In order to achieve the above purpose, the invention provides the following technical scheme:
a heat treatment method of low carbon structural steel for cold extrusion comprises the following steps:
step one, placing a low-carbon structural steel blank in a heating furnace, raising the temperature of the furnace to a first heat preservation temperature of 740-; then cooling to a second heat preservation temperature, and preserving heat for 3-5h at the second heat preservation temperature;
and step two, cooling the blank subjected to heat preservation treatment at the second heat preservation temperature to 550 ℃, discharging, and finally air-cooling to room temperature to obtain the low-carbon structural steel finished product for cold extrusion.
In the heat treatment method of the low carbon structural steel for cold extrusion, the second heat preservation temperature is preferably 690-730 ℃.
In the above heat treatment method for the cold extrusion low carbon structural steel, preferably, the cooling treatment in the second step includes a two-stage cooling process, the cooling rates of the two stages are a first cooling rate and a second cooling rate in turn, and the first cooling rate is less than the second cooling rate;
the blank after the heat preservation treatment at the second heat preservation temperature is cooled to 600-630 ℃ at the first cooling speed and then is immediately cooled at the second cooling speed.
In the heat treatment method of the low carbon structural steel for cold extrusion as above, preferably, the first cooling rate is not more than 30 ℃/h.
In the heat treatment method of the low carbon structural steel for cold extrusion as above, preferably, the second cooling rate is not more than 50 ℃/h.
In the above method for heat-treating a low-carbon structural steel for cold extrusion, it is preferable that the temperature raising rate at which the furnace temperature is raised to the first holding temperature in the first step is 50 to 100 ℃/h.
In the above heat treatment method for the low carbon structural steel for cold extrusion, preferably, in the first step, the cooling rate when the temperature is reduced from the first heat preservation temperature to the second heat preservation temperature is not more than 30 ℃/min.
In the heat treatment method of the low carbon structural steel for cold extrusion, preferably, a plurality of low carbon structural steel blanks are placed in a heating furnace, the plurality of blanks are stacked layer by layer in the furnace, and each layer is separated by a sizing block;
preferably, the shim member is a ferrous material.
In the above heat treatment method of the low carbon structural steel for cold extrusion, it is preferable that the distance between the two adjacent low carbon structural steel blanks in the same layer is not less than 50 mm.
In the above method for heat-treating a low-carbon structural steel for cold extrusion, the heating furnace is preferably a car-type annealing furnace into which a protective atmosphere is introduced.
Has the beneficial effects that:
according to the heat treatment method of the low-carbon structural steel for cold extrusion, disclosed by the invention, the steel structural part after heat treatment can be used for cold extrusion production parts with larger requirements on deformation, the hardness of the obtained low-carbon structural steel is less than or equal to 125HBW, the hardness uniformity is less than or equal to 10HBW, and the structure is a ferrite and spherical pearlite structure; the method achieves the aims of reducing the hardness of the low-carbon structural steel, improving the uniformity of the hardness, and simultaneously changing the structure form into a ferrite and spherical pearlite structure, and meets the requirements of the hardness and the structure of large deformation (the deformation rate is more than or equal to 70%) of cold extrusion.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. Wherein:
FIG. 1 is a graph illustrating annealing curves of a heat treatment process of a low carbon structural steel according to an embodiment of the present invention;
FIG. 2 is a metallographic structure diagram showing a microstructure of a steel material ingot obtained by heat treatment in example 1 of the present invention.
Detailed Description
The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings. It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict.
In the description of the present invention, the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are for convenience of description of the present invention only and do not require that the present invention must be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention. The terms "connected" and "connected" used herein should be interpreted broadly, and may include, for example, a fixed connection or a detachable connection; they may be directly connected or indirectly connected through intermediate members, and specific meanings of the above terms will be understood by those skilled in the art as appropriate.
According to the invention, by carrying out a certain annealing treatment process on the low-carbon structural steel with higher plasticity, the annealed steel can release stress and increase the ductility and toughness of the material, the heat-treated steel is applied to a cold extrusion production raw material with large deformation, the deformation of the cold extrusion production raw material with large deformation generally meets the requirement that the deformation rate is more than or equal to 70%, the heat-treated steel has low hardness and good hardness uniformity, and the structure is ferrite + spherical pearlite structure. According to the mass percentage, the steel components of the low-carbon structural steel blank meet the following element content requirements: c is less than or equal to 0.18%, Si: 0.17% -0.37%, Mn: 0.35 to 0.65 percent of Cr and less than or equal to 0.25 percent of Cr. The strength and hardness of the spherical pearlite are low, the plasticity and the toughness are good, and the cold extrusion processing is convenient.
The embodiment of the invention provides a heat treatment method of low-carbon structural steel for cold extrusion, as shown in figure 1, the isothermal annealing curve in the invention is shown, and the heat treatment method in the invention comprises the following steps:
step one, placing the low-carbon structural steel blank in a heating furnace, raising the temperature of the furnace to a first heat preservation temperature of 740-; then, the temperature is reduced to a second heat preservation temperature, and heat preservation is carried out for 3-5h (such as 3.2h, 3.5h, 4h, 4.2h, 4.5h and the like) at the second heat preservation temperature.
In one embodiment of the present invention, the furnace is a car-type annealing furnace that is exposed to a protective atmosphere, wherein the protective atmosphere is an inert gas, such as nitrogen. The fuel used in the heating furnace is natural gas, and the uniformity of the furnace temperature is good. The heating furnace is internally provided with a material seat for placing steel blanks, in order to improve the working efficiency, a plurality of low-carbon structural steel blanks are placed in the heating furnace, the low-carbon structural steel blanks are stacked in the furnace layer by layer, and each layer is separated by a sizing block piece. The sizing block piece is a strip-shaped iron material, the sizing block piece is partially contacted with the low-carbon structural steel blank, the mutual contact surface area of the steel blanks is reduced, and the low-carbon structural steel blanks between every two layers are separated by the sizing block piece, so that the phenomenon of uneven heating caused by mutual contact of the blanks can be avoided. In other embodiments, the sizing elements may also be sheets or other shapes of ferrous material, and the sizing elements may be used to separate the mild structural steel blanks between the layers to reduce the contact area between the mild structural steel blanks. In the same layer, the distance between two adjacent low carbon structure steel blanks is not less than 50mm, and the sizing block pieces are used for separating between each layer and also ensuring the distance between two adjacent steel blanks in the same layer, so that each steel blank can be fully thermally treated, and the steel tissue is more uniform. Specifically, the low carbon steel blank according to the present invention is a round bar steel blank, but may be a steel having other shapes such as a square shape, and the present invention is not limited thereto.
In one embodiment of the present invention, the second temperature is 690-.
In the first step, the temperature rise speed is 50-100 ℃/h (such as 50 ℃/h, 55 ℃/h, 60 ℃/h, 65 ℃/h, 70 ℃/h, 75 ℃/h, 80 ℃/h, 85 ℃/h, 90 ℃/h, 95 ℃/h and 100 ℃/h) when the furnace temperature is raised to the first holding temperature, the production efficiency is influenced by too slow temperature rise, and the steel is heated unevenly due to too fast temperature rise. The cooling speed is not more than 30 ℃/min when the temperature is reduced from the first heat preservation temperature to the second heat preservation temperature. The hardness of steel can be influenced by too fast cooling, the tissue form of the steel is changed, and the production efficiency can be influenced by too slow cooling.
And step two, cooling the blank subjected to heat preservation treatment at the second heat preservation temperature to 550 ℃, discharging, and finally air-cooling to room temperature to obtain the low-carbon structural steel finished product for cold extrusion.
In an embodiment of the present invention, the cooling process in the second step includes two stages of cooling processes, which are a first cooling rate and a second cooling rate in sequence, and the first cooling rate is lower than the second cooling rate. I.e. the cooling is performed at a relatively slow rate at the beginning and then at a fast rate. The second cooling speed is higher than the first cooling speed, so that the annealing time can be shortened and the production efficiency can be improved on the premise of ensuring the structure and the hardness.
And cooling the blank subjected to the heat preservation treatment at the second heat preservation temperature at a second cooling speed immediately after the blank is cooled to 630 ℃ at a first cooling speed. Specifically, the first cooling rate is not more than 30 ℃/h (such as 10 ℃/h, 12 ℃/h, 14 ℃/h, 16 ℃/h, 18 ℃/h, 20 ℃/h, 22 ℃/h, 24 ℃/h, 26 ℃/h, 28 ℃/h and the like). The second cooling rate is not more than 50 ℃/h (such as 25 ℃/h, 30 ℃/h, 35 ℃/h, 40 ℃/h, 45 ℃/h, etc.). In the invention, the mixture is cooled to 600 ℃ and 630 ℃ (such as 600 ℃, 605 ℃, 610 ℃, 615 ℃, 620 ℃, 625 ℃ and 630 ℃) by adopting a slow cooling mode, and then is cooled to 550 ℃ by adopting a rapid cooling mode to be discharged, and two different cooling speeds are adopted in the cooling process so as to obtain the ferrite + spherical pearlite structure finally. The heat treatment mode of the invention is adopted to treat the steel blank with the low carbon structure, the hardness of the finally obtained steel is less than or equal to 125HBW, the uniformity of the hardness is less than or equal to 10HBW, and the microstructure of the steel is ferrite plus spherical pearlite microstructure.
Example 1
The embodiment provides a heat treatment process of low-carbon structural steel for cold extrusion, which comprises the following steps of:
placing 10 steel blanks in a trolley type annealing furnace, stacking a plurality of blanks on a material seat in the furnace layer by layer, separating each layer by using a sizing block piece, leading the distance between any two adjacent blanks in the same layer to be 50mm, introducing protective gas nitrogen into the furnace, heating the furnace to 740 ℃, keeping the temperature at the temperature for 4 hours at the heating speed of 70 ℃/h; then, the temperature is reduced to 690 ℃ at the speed of 30 ℃/h, and the temperature is kept for 4h at the temperature; cooling the blank after heat preservation treatment to 630 ℃ at the speed of 30 ℃/h, continuously cooling to 550 ℃ at the speed of 50 ℃/h, discharging, and air cooling to room temperature. Thus obtaining the steel material which has the hardness of 111-120HBW, the hardness uniformity of less than or equal to 10HBW and the metallographic structure of ferrite + spherical pearlite structure shown in figure 2 and can be processed by a large deformation cold extrusion machine.
Five samples of the round bar type steel material subjected to the heat treatment in the examples of the present invention were measured for surface hardness, center hardness and 1/2 radius hardness as shown in Table 1 below.
TABLE 1 hardness data of different steels after the same heat treatment
| Sample number | Surface hardness | Center hardness | 1/2 radius hardness |
| 1# | 114 | 115 | 116 |
| 2# | 111 | 112 | 115 |
| 3# | 118 | 115 | 117 |
| 4# | 118 | 116 | 120 |
| 5# | 120 | 117 | 118 |
Example 2
The embodiment provides a heat treatment process of low-carbon structural steel for cold extrusion, which comprises the following steps of:
placing 10 steel (namely No. 10 steel) blanks in a trolley type annealing furnace, stacking a plurality of blanks on a material seat in the furnace layer by layer, separating each layer by using a sizing block piece, leading the distance between any two adjacent blanks in the same layer to be 50mm, introducing protective gas nitrogen into the furnace, wherein the blanks are round bar steel, heating the furnace to 780 ℃, heating the temperature at a speed of 70 ℃/h, and preserving the heat at the temperature for 4 h; then, cooling to 730 ℃ at the speed of 30 ℃/h, and preserving heat for 4h at the temperature; cooling the blank after heat preservation treatment to 630 ℃ at the speed of 30 ℃/h, continuously cooling to 550 ℃ at the speed of 50 ℃/h, discharging, and air cooling to room temperature. The steel material with the hardness of 115-125HBW, the hardness uniformity of less than or equal to 10HBW, the metallographic structure of ferrite + spherical pearlite structure and the large deformation amount can be processed by a cold extruding machine can be obtained.
Example 3
Placing 10 steel blanks in a trolley type annealing furnace, stacking a plurality of blanks on a material seat in the furnace layer by layer, separating each layer by using a sizing block piece, setting the distance between any two adjacent blanks in the same layer as 80mm, introducing protective gas nitrogen into the furnace, wherein the blanks are round bar steel, heating the furnace to 760 ℃, setting the heating speed to 70 ℃/h, and keeping the temperature for 3 h; then, cooling to 710 ℃ at the speed of 20 ℃/h, and preserving the heat for 5h at the temperature; cooling the blank after heat preservation treatment to 630 ℃ at the speed of 30 ℃/h, continuously cooling to 550 ℃ at the speed of 50 ℃/h, discharging, and air cooling to room temperature. The steel material with the hardness of 113-123HBW, the hardness uniformity of less than or equal to 10HBW, and the metallographic structure of ferrite + spherical pearlite structure can be obtained and can be processed by a large-deformation cold extrusion machine.
Example 4
The difference between the embodiment 4 and the embodiment 1 is that the first cooling rate is 20 ℃/h, the second cooling rate is 45 ℃/h, and other steps and methods are the same as those of the embodiment 1, and are not repeated.
The steel prepared in the embodiment has the hardness of 112-120HBW, the hardness uniformity of less than or equal to 10HBW, and the metallographic structure of ferrite and spherical pearlite, and can be processed by a large-deformation cold extrusion machine.
Example 5
The difference between the example 5 and the example 2 lies in that the temperature rising speed of the heating furnace is changed to be 75 ℃/h, and other steps and methods are the same as those in the example 2, and are not repeated.
The steel prepared in the embodiment has the hardness of 114-123HBW, the hardness uniformity of less than or equal to 10HBW, and the metallographic structure of ferrite and spherical pearlite, and can be processed by a large-deformation cold extruder.
Example 6
The difference between embodiment 6 and embodiment 3 is that the distance between any two adjacent blanks in the same layer is changed, the distance between two adjacent blanks is 100mm, and other steps and methods are the same as those in embodiment 3, and are not repeated herein.
The steel prepared in the embodiment has the hardness of 112-122HBW, the hardness uniformity of less than or equal to 10HBW, and the metallographic structure of ferrite and spherical pearlite, and can be processed by a large-deformation cold extrusion machine.
Comparative example 1
The difference between this comparative example and example 1 is that the blank after heat preservation at 740 ℃ for 4 hours is subjected to primary cooling treatment at a cooling rate of 50 ℃/h, cooled to 550 ℃, taken out of the furnace, and then air-cooled to room temperature, and other steps and methods are the same as those in example 1, and are not repeated.
The hardness of the steel prepared in the comparative example is 110-135HBW, the uniformity of the hardness is less than or equal to 25HBW, the metallographic structure is a ferrite + lamellar pearlite structure, the cooling speed is high in the comparative example, the uniformity of the hardness of the steel after heat treatment is poor, and the use requirement of large-deformation cold extrusion production cannot be met.
Comparative example 2
The difference between this comparative example and example 1 is that the blank after the heat preservation treatment is cooled to 630 ℃ at a rate of 50 ℃/h, then cooled to 550 ℃ at a rate of 30 ℃/h, taken out of the furnace, and then cooled to room temperature by air after taken out of the furnace, and the other steps and methods are the same as those in example 1, and are not repeated here.
The hardness of the steel prepared in the comparative example is 115-135HBW, the uniformity of the hardness is less than or equal to 20HBW, the metallographic structure is a ferrite + spherical pearlite structure, the first cooling speed of the steel in the comparative example is higher than the second cooling speed, the first cooling speed is higher, the uniformity of the hardness of the steel is poor, and the use requirement of large-deformation cold extrusion production cannot be met.
Comparative example 3
The difference between this comparative example and example 1 is that the furnace temperature is increased to 900 ℃, the temperature is maintained for 4h, then the temperature is decreased to 800 ℃, the temperature is maintained for 4h, and other steps are the same as those in example 1, and are not described again.
The hardness of the steel prepared in the comparative example is 125-135HBW, the uniformity of the hardness is less than or equal to 10HBW, and the metallographic structure is ferrite and lamellar pearlite, so that the use requirement of large deformation cold extrusion production cannot be met.
Comparative example 4
The difference between this comparative example and example 1 is that a plurality of blanks are stacked layer by layer in a heating furnace, no sizing block is used for separation during stacking, the distance between two adjacent blanks in the same layer is 30mm, and other steps and methods are the same as those in example 1, and are not repeated again.
The hardness of the steel prepared in the comparative example is 112-127HBW, the uniformity of the hardness is less than or equal to 15HBW, and the metallographic structure is a ferrite and spherical pearlite structure.
Comparative example 5
The difference between this comparative example and example 1 is that when the blank after being kept at 690 ℃ for 4 hours is cooled to 640 ℃ at a speed of 30 ℃/h, then cooled to 550 ℃ at a speed of 50 ℃/h, and finally cooled to room temperature, and other steps and methods are the same as those in example 1, and are not repeated.
The hardness of the steel prepared in the comparative example is 115-128HBW, the uniformity of the hardness is less than or equal to 13HBW, the metallographic structure is ferrite + spherical + lamellar pearlite, the spheroidization effect of the steel can be influenced by the excessively high cooling speed at the temperature of above 630 ℃, the uniform spherical pearlite structure cannot be obtained, the uniformity of the hardness is poor, and the use requirement of large-deformation cold extrusion production cannot be met.
In conclusion, according to the heat treatment method of the low-carbon structural steel for cold extrusion, the steel structural part after heat treatment can be used for cold extrusion production parts with larger deformation requirements, and through the multi-stage isothermal annealing process, the hardness of the obtained low-carbon structural steel is less than or equal to 125HBW, the hardness uniformity is less than or equal to 10HBW, and the structure is a ferrite and spherical pearlite structure; the method achieves the aims of reducing the hardness of the low-carbon structural steel, improving the uniformity of the hardness, and simultaneously changing the structure form into a ferrite and spherical pearlite structure, and meets the requirements of the hardness and the structure of large deformation of cold extrusion.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. A heat treatment method of low-carbon structural steel for cold extrusion is characterized by comprising the following steps:
step one, placing a low-carbon structural steel blank in a heating furnace, raising the temperature of the furnace to a first heat preservation temperature of 740-; then, cooling to a second heat preservation temperature, and preserving heat for 3-5 hours at the second heat preservation temperature; the second heat preservation temperature is 690-730 ℃;
step two, cooling the blank subjected to heat preservation treatment at a second heat preservation temperature to 550 ℃, discharging, and finally air-cooling to room temperature to obtain a low-carbon structural steel finished product for cold extrusion;
the cooling treatment comprises a two-stage cooling process, wherein the cooling speeds of the two stages are a first cooling speed and a second cooling speed in sequence, and the first cooling speed is less than the second cooling speed; and cooling the blank subjected to the heat preservation treatment at the second heat preservation temperature to 600-630 ℃ at the first cooling speed, and then immediately cooling at the second cooling speed, wherein the first cooling speed is not more than 30 ℃/h, and the second cooling speed is not more than 50 ℃/h.
2. The method for heat-treating a low carbon structural steel for cold extrusion as set forth in claim 1, wherein the rate of temperature rise at the time of raising the temperature in the furnace to the first holding temperature in the first step is 50 to 100 ℃/h.
3. The heat treatment method of a low carbon structural steel for cold extrusion as set forth in claim 1, wherein in the first step, a cooling rate from the first holding temperature to the second holding temperature is not more than 30 ℃/min.
4. The method for heat-treating a low carbon structural steel for cold extrusion as set forth in claim 1, wherein a plurality of said low carbon structural steel blanks are placed in said heating furnace, and a plurality of said blanks are stacked one on another in the furnace, each of which is separated by a sizing member.
5. The heat treatment method of a low carbon structural steel for cold extrusion as set forth in claim 4, wherein the shim member is a ferrous material.
6. The method for heat-treating a low carbon structural steel for cold extrusion as set forth in claim 4 or 5, wherein a distance between two adjacent low carbon structural steel blanks in the same layer is not less than 50 mm.
7. The method for heat-treating a low carbon structural steel for cold extrusion as set forth in claim 1, wherein the heating furnace is a car type annealing furnace into which a protective atmosphere is introduced.
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