CN114058801A - Method for refining boron steel grains, high-strength and high-toughness boron steel and application thereof - Google Patents
Method for refining boron steel grains, high-strength and high-toughness boron steel and application thereof Download PDFInfo
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- 229910000712 Boron steel Inorganic materials 0.000 title claims abstract description 141
- 238000000034 method Methods 0.000 title claims abstract description 32
- 238000007670 refining Methods 0.000 title claims abstract description 8
- 238000010438 heat treatment Methods 0.000 claims abstract description 32
- 238000005242 forging Methods 0.000 claims abstract description 17
- 238000005098 hot rolling Methods 0.000 claims abstract description 16
- 238000001816 cooling Methods 0.000 claims description 55
- 238000010791 quenching Methods 0.000 claims description 45
- 230000000171 quenching effect Effects 0.000 claims description 45
- 238000005496 tempering Methods 0.000 claims description 25
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 21
- 229910001562 pearlite Inorganic materials 0.000 claims description 17
- 238000004321 preservation Methods 0.000 claims description 17
- 229910000859 α-Fe Inorganic materials 0.000 claims description 17
- 229910001563 bainite Inorganic materials 0.000 claims description 14
- 239000013078 crystal Substances 0.000 claims description 11
- 239000000463 material Substances 0.000 abstract description 17
- 239000002184 metal Substances 0.000 abstract description 2
- 239000002689 soil Substances 0.000 abstract description 2
- 239000010959 steel Substances 0.000 description 42
- 229910000831 Steel Inorganic materials 0.000 description 41
- 230000000052 comparative effect Effects 0.000 description 26
- 238000005520 cutting process Methods 0.000 description 7
- 238000010606 normalization Methods 0.000 description 7
- 230000009466 transformation Effects 0.000 description 6
- 229910001566 austenite Inorganic materials 0.000 description 5
- 238000005096 rolling process Methods 0.000 description 5
- 238000007599 discharging Methods 0.000 description 4
- 238000005498 polishing Methods 0.000 description 4
- 230000009286 beneficial effect Effects 0.000 description 3
- 238000009826 distribution Methods 0.000 description 3
- 238000005034 decoration Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000009313 farming Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000004154 testing of material Methods 0.000 description 2
- 238000000691 measurement method Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 239000000126 substance Substances 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
- C21D1/28—Normalising
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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
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- 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
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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
- C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- 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
Abstract
The invention relates to the technical field of metal heat treatment, in particular to a method for refining boron steel grains, high-strength and high-toughness boron steel and application thereof. According to the invention, the boron steel after hot rolling or forging is subjected to normalizing treatment, so that a finer and uniformly distributed microstructure is obtained, a banded structure in the material is improved, and the tissue inheritance problems of coarse grains and banded structures in the boron steel after hot rolling or forging in the subsequent heat treatment in the prior art are effectively solved. The fine and evenly distributed microstructure can effectively improve the strength, hardness and toughness of the boron steel workpiece, thereby improving the wear resistance of the workpiece and prolonging the service life of the key parts for high-speed plough to dig into the soil.
Description
Technical Field
The invention relates to the technical field of metal heat treatment, in particular to a method for refining boron steel grains, high-strength and high-toughness boron steel and application thereof.
Background
In recent years, with the continuous development of modern agriculture, the improvement of agricultural mechanization and automation level greatly improves the productivity and competitiveness of agriculture, and further promotes the sustainable, rapid and healthy development of agricultural economy.
Among mechanized farming implements, the high-speed plough is considered to be the most efficient farming implement, and mainly bears the impact and friction of external objects such as soil, stones, tree roots and the like on the plough body during the service process, so the material for preparing the high-speed plough needs to have higher strength and wear resistance to ensure that the high-speed plough has longer service life.
At present, the material for preparing the high-speed plough is mainly boron steel. The conventional treatment process is to directly perform pressure quenching and low-temperature tempering treatment on the boron steel after hot rolling or forging. In the actual process, coarse grains and banded structures exist in the boron steel after hot rolling or forging, and the structural inheritance is very easy to occur in the subsequent heat treatment process, so that the mechanical property of a workpiece is seriously damaged, and the service life of the high-speed plough is shortened.
Therefore, there is a need for an improved boron steel treatment process that further improves the strength and wear resistance of boron steel.
Disclosure of Invention
In view of the above, the invention provides a method for refining boron steel grains, high-strength wear-resistant boron steel and application thereof. The method provided by the invention can solve the problem of tissue inheritance of coarse grains and banded structures in the boron steel after hot rolling or forging in the subsequent heat treatment, and obviously improves the hardness and wear resistance of the boron steel.
In order to achieve the above object, the present invention provides the following technical solutions:
method for refining boron steel grainsThe method comprises the following steps: normalizing the boron steel after hot rolling or forging; the normalizing treatment comprises the following steps: heating the hot rolled or forged boron steel to Ac3Keeping the temperature for 10-45 min at 20-90 ℃, and then cooling by air.
Preferably, the heating rate is 250-300 ℃/min.
Preferably, the hot rolled or forged boron steel is 28MnB5, 27MnCrB5 or 33MnCrB 5.
The invention also provides high-toughness boron steel which is obtained by sequentially performing pressure quenching and tempering on the boron steel after normalizing treatment; the normalizing treatment is carried out according to the method of the technical scheme.
Preferably, the content of pearlite and ferrite in the boron steel after the normalizing treatment is not less than 95%, the content of lower bainite is not more than 5%, and the hardness of the boron steel after the normalizing treatment is 20-24 HRC.
Preferably, the pressure of the pressure quenching is 6-8 MPa; the quenching temperature of the pressure quenching is 880-920 ℃; the quenching heat preservation time of the pressure quenching is 0.25-0.75 min; the cooling mode of the pressure quenching is water cooling, the cooling rate of the water cooling is 10-30 ℃/s, and the cooling time of the water cooling is 30-120 s; the tempering temperature is 180-200 ℃; and the tempering heat preservation time is 1-3 h.
Preferably, the number percentage of crystal grains with the grain size of 5-10 mu m in the high-strength and high-toughness boron steel is not less than 89.5%.
The invention also provides the application of the high-strength and high-toughness boron steel in the technical scheme in a high-speed plough.
The invention also provides a method for refining the boron steel grains, which comprises the following steps: normalizing the boron steel after hot rolling or forging; the normalizing treatment comprises the following steps: heating the hot rolled or forged boron steel to Ac3Keeping the temperature for 10-45 min at 20-90 ℃, and then cooling by air. The invention carries out normalizing treatment on the boron steel after hot rolling or forging, converts the austenitized boron steel into austenite again and then cools the austenite, thereby obtaining finer and uniformly distributed microstructures, reducing banded structures in the material and effectively solving the problem of hot rolling in the prior artOr the problem of structural inheritance of coarse grains and banded structures in the boron steel after forging in the subsequent heat treatment. The fine and evenly distributed microstructure can effectively improve the strength, hardness and toughness of the workpiece, thereby improving the wear resistance of the workpiece and prolonging the service life of the high-speed plough.
The invention also provides the high-strength and high-toughness boron steel, which can be obtained by sequentially carrying out pressure quenching and low-temperature tempering on the boron steel after normalizing treatment. Experimental results of the embodiment show that the high-strength and high-toughness boron steel provided by the invention has the hardness of 52HRC, the tensile strength of 1690MPa, the yield strength of 1400MPa and the elongation after fracture of 9%, and has good wear resistance and strength and toughness.
The invention also provides application of the high-strength and high-toughness boron steel in the scheme in a high-speed plough. The high-strength and high-toughness boron steel provided by the invention has higher strength and wear resistance, and the high-speed plough prepared from the boron steel is beneficial to prolonging the service life of the high-speed plough.
Drawings
FIG. 1 is a temperature-expansion curve of 28MnB5 in the temperature rise process measured by a Gleeble-3500 thermal simulator;
FIG. 2 is a temperature-expansion curve of 28MnB5 in a cooling process measured by a Gleeble-3500 thermal simulator;
FIG. 3 is a continuous cooling curve (CCT curve) of 28MnB5 steel;
FIG. 4 is a 28MnB5 steel isothermal transformation curve (TTT curve);
FIG. 5 is a normalizing process diagram of 28MnB5 steel;
FIG. 6 shows the metallographic structure of a hot-rolled plate of 28MnB5 steel;
FIG. 7 is a comparison of the metallographic structure of normalized boron steel of example 1 and non-normalized boron steel of comparative example 1;
FIG. 8 is a comparison of the metallographic structures of the high-toughness boron steel of example 1 and the high-toughness boron steel of comparative example 1;
FIG. 9 is a graph comparing the grain distributions of the high-toughness boron steel of example 1 and the high-toughness boron steel of comparative example 1;
FIG. 10 is a comparison of the metallographic structure of the normalized boron steel of example 2 and the non-normalized boron steel of comparative example 2;
FIG. 11 is a comparison of the metallographic structures of the high-toughness boron steel of example 2 and the high-toughness boron steel of comparative example 2;
FIG. 12 is a graph comparing the grain distribution of the high-toughness boron steel of example 2 and the high-toughness boron steel of comparative example 2.
Detailed Description
The invention provides a heat treatment method of boron steel, which comprises the following steps: normalizing the boron steel after hot rolling or forging; the normalizing treatment comprises the following steps: heating the hot rolled or forged boron steel to Ac3Keeping the temperature for 10-45 min at 20-90 ℃, and then cooling by air.
In the prior art, the conventional treatment process is to directly perform pressure quenching and tempering treatment on the boron steel after hot rolling or forging, and the invention heats the boron steel after hot rolling or forging until the internal structure is transformed into austenite again, then discharges the boron steel for air cooling, thereby obtaining fine and uniformly distributed pearlite and ferrite structures and reducing banded structures existing in the original material. The fine and evenly distributed microstructures can effectively improve the strength, hardness and toughness of the workpiece, thereby improving the wear resistance of the workpiece and prolonging the service life of the high-speed plough.
The invention heats the boron steel after hot rolling or forging to Ac3Keeping the temperature 20-90 ℃ above the temperature for 10-45 min. In the present invention, the hot-rolled or forged boron steel is preferably 28MnB5, 27MnCrB5 or 33MnCrB5, more preferably 28MnB5, and the chemical composition of the 28MnB5 preferably includes, in mass percent: 0.25-0.32 wt% of C; 0.15-0.40 wt% of Si; 1.00-1.50 wt% of Mn; p is less than or equal to 0.035 wt%; s is less than or equal to 0.035 wt%; cr is less than or equal to 0.3 wt%; al is less than or equal to 0.02 wt%; b0.0008 to 0.0050 wt%. The hot rolled or forged boron steel is preferably a hot rolled plate.
In the embodiment of the invention, the boron steel after hot rolling or forging is preferably cut, polished to be smooth and then subjected to normalizing treatment; the cutting is preferably cutting along the rolling or forging direction, the invention has no special requirement on the cutting size and can be determined according to the actual requirement; the grinding is preferably performed using a grinder.
In the present invention, it is preferable to heat the boron steel after hot rolling or forging to Ac 320 to 90 degrees above the temperatureMore preferably 30 to 80 ℃, further preferably 40 to 60 ℃, and most preferably 50 ℃; preferably, the temperature is kept for 15-45 min, more preferably for 25-35 min, and further preferably for 30 min; heating the hot rolled or forged boron steel to Ac3The heating rate of 20 to 80 ℃ above is preferably 150 to 350 ℃/min, more preferably 200 to 300 ℃/min, and still more preferably 250 ℃/min. In the present invention, a temperature rise rate of 250 deg.C/min is employed. The invention preferably adopts a Gleeble-3500 thermal simulator to measure the Ac of the steel3And (3) temperature. The invention controls the heat preservation temperature and the heat preservation time within the ranges, adopts the strategies of high heat preservation temperature and low heat preservation time, can ensure that the internal structure of the boron steel is transformed into austenite again in the heating process, has high austenitizing degree and cannot generate coarse grains.
After the heat preservation is finished, the invention cools the boron steel after the heat preservation by air. The invention has no special requirement on the air cooling mode, and the steel after heat preservation can be cooled to room temperature in the air. The invention cools the re-austenitized steel material in the air, and in the cooling process, the structure in the steel material is converted into pearlite and ferrite more, and smaller grains are obtained, and the banded structure in the steel material is reduced.
The invention also provides high-strength and high-toughness boron steel which is obtained by sequentially carrying out pressure quenching and tempering on the boron steel treated by the technical scheme.
In the invention, the pressure of the pressure quenching is preferably 6-8 MPa, more preferably 7MPa, the quenching temperature of the pressure quenching is preferably 880-920 ℃, more preferably 890-900 ℃, more preferably 900 ℃, the quenching heat preservation time of the pressure quenching is preferably 0.25-0.75 h, more preferably 0.4-0.5 h, more preferably 0.5h, the cooling mode of the pressure quenching is preferably water cooling, the cooling rate of the water cooling is preferably 10-30 ℃/s, more preferably 20-30 ℃/s, more preferably 30 ℃/s, the cooling time of the water cooling is 30-120 s, more preferably 30-70 s, more preferably 32s, the tempering temperature is preferably 150-250 ℃, more preferably 180-200 ℃, more preferably 200 ℃, the tempering heat preservation time is preferably 1-3 h, more preferably 2-3 h, more preferably 3 h. Under the conditions, the high-strength ductile steel can form a curved surface structure more easily, and is convenient to apply and prepare the parts of the high-speed plough.
In the present invention, the pearlite and ferrite contents of the boron steel after the normalizing treatment are not less than 95%, more preferably not less than 96%; the lower bainite content is not higher than 5%, more preferably not higher than 4%; in the invention, after the normalizing treatment, austenite grains in the material are refined, the number of fine grains in the material is increased, the contents of pearlite, ferrite and lower bainite in the boron steel after the normalizing treatment are controlled within the range, the microstructure distribution in the boron steel is optimized, and after the subsequent pressure quenching and tempering treatment, the strength, hardness and toughness of the material can be effectively improved, thereby improving the comprehensive mechanical property of the material. The hardness of the boron steel after the normalizing treatment is preferably 20-24 HRC, more preferably 22-24 HRC, and further preferably 23-24 HRC.
In the invention, the number percentage of crystal grains with the grain size of 5-10 mu m in the high-strength and high-toughness boron steel is preferably not less than 89.5%; more preferably not less than 89%. According to the invention, the number percentage of the crystal grains with the grain size of 5-10 μm in the high-strength and high-toughness boron steel is preferably controlled within the range, so that the strength, hardness and toughness of a workpiece are improved, and the refined crystal grains are matched with pearlite, ferrite and lower bainite with proper content in the steel, so that the hardness and toughness of the steel are well matched, the wear resistance of the steel is improved, and the processability and service life of the steel are favorably improved.
The invention also provides the application of the high-strength and high-toughness boron steel in the high-speed plough, preferably the key soil-penetrating member of the high-speed plough. The application of the high-strength and high-toughness boron steel in the high-speed plough is not particularly limited, and the high-strength and high-toughness boron steel can be applied by adopting a method well known by the technical personnel in the field.
The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
The 28MnB5 steel used was a commercially available product, which was a hot-rolled plate. The components (wt%) are as follows: c: 0.25; si: 0.30; mn: 1.29; p: 0.0051; s: < 0.00020; cr: 0.30; mo: 0.04; ni: 0.1; al: 0.02; co: 0.01; ti; 0.04; b: 0.0027.
the continuous cooling transformation point of the 28MnB5 steel was measured using a Gleeble-3500 thermal simulator. FIG. 1 is a temperature-expansion curve of 28MnB5 measured by Gleeble-3500 for temperature rise, in which Ac was measured1＝712℃、Ac3830 ℃. FIG. 2 is a temperature-expansion curve of 28MnB5 in the cooling process, measured by Gleeble-3500, and measured as Ms 383 ℃ and Mf 279 ℃. FIGS. 3 and 4 show isothermal transformation curve and continuous cooling transformation curve of 28MnB5, and the transformation point is determined by combining the transformation point shown in FIGS. 1 and 2, the normalizing temperature is set to 850 deg.C, the holding time is 0.5h, and air cooling is directly carried out after the holding is finished. The specific normalizing process is shown in figure 5.
The method comprises the following specific steps: cutting a 12mm thick plate into samples with the specification of 15 multiplied by 10 multiplied by 12mm along a rolling direction, polishing the samples smoothly by a grinding machine, heating the samples to 850 ℃ in a heating furnace, preserving heat for 0.5h, quickly taking out the completely austenitized samples from the heating furnace, and placing the samples in air to naturally cool to room temperature. The hardness of the sample before and after the normalization treatment was measured by a rockwell hardness tester, wherein the hardness of the sample before the normalization treatment was 19HRC and the hardness of the sample after the normalization treatment was 22 HRC.
FIG. 6 shows the metallographic structure of a 28MnB5 hot-rolled sheet, wherein the scale on the left is 200 μm and the scale on the right is 50 μm. As can be seen from fig. 6, before the normalizing treatment, a band-like structure was clearly present in the steel material, and the grains were varied in size, and the metallographic structure was ferrite and pearlite.
And (3) putting the normalized sample into the heating furnace again, heating to 920 ℃, keeping the temperature for 0.5h, taking the sample out of the furnace, performing pressure quenching, and performing water cooling, wherein the pressure of the pressure quenching is 7MPa, the quenching temperature is 920 ℃, the quenching time is 0.5h, the cooling speed of the water cooling is 30 ℃/s, and the cooling time is 32 s. And transferring the boron steel to a tempering furnace after water cooling, keeping the tempering temperature at 200 ℃, keeping the temperature for 3h, discharging the boron steel from the furnace after heat preservation, and air-cooling the boron steel to room temperature to obtain the high-strength and high-toughness boron steel.
Example 2
The other conditions were identical to those of example 1, except that the temperature of the normalizing treatment was selected to be 880 ℃.
The method comprises the following specific steps: cutting a 12mm thick plate into samples with the specification of 15 multiplied by 10 multiplied by 12mm along the rolling direction, polishing the samples smoothly by a grinding machine, heating the samples to 880 ℃ in a heating furnace, preserving heat for 0.5h, quickly taking out the completely austenitized samples from the heating furnace, and placing the samples in air to naturally cool to room temperature. The hardness of the sample before and after the normalization treatment was measured by a rockwell hardness tester, wherein the hardness of the sample before the normalization treatment was 19HRC and the hardness of the sample after the normalization treatment was 23 HRC.
And (3) putting the normalized sample into the heating furnace again, heating to 920 ℃, keeping the temperature for 0.5h, taking the sample out of the furnace, performing pressure quenching, and performing water cooling, wherein the pressure of the pressure quenching is 7MPa, the quenching temperature is 920 ℃, the quenching time is 0.5h, the cooling speed of the water cooling is 30 ℃/s, and the cooling time is 32 s. And transferring the boron steel to a tempering furnace after water cooling, keeping the tempering temperature at 200 ℃, keeping the temperature for 3h, discharging the boron steel from the furnace after heat preservation, and air-cooling the boron steel to room temperature to obtain the high-strength and high-toughness boron steel.
Comparative example 1
Other conditions were the same as in example 1, and only the normalizing treatment step was omitted, specifically: cutting a 12mm boron steel thick plate material with the same material as that in the embodiment 1 into a sample with the specification of 15 multiplied by 10 multiplied by 12mm along the rolling direction, and then polishing the sample smoothly by a grinder; heating the sample in a heating furnace to 920 ℃, preserving heat for 0.5h, taking the sample out of the heating furnace, performing pressure quenching, and cooling with water, wherein the pressure of the pressure quenching is 7MPa, the quenching temperature is 920 ℃, the quenching time is 0.5h, the cooling speed of the water cooling is 30 ℃/s, and the cooling time is 32 s. And transferring the molten steel to a tempering furnace after water cooling, keeping the tempering temperature at 200 ℃, keeping the temperature for 3h, discharging the molten steel from the furnace after heat preservation, and air-cooling the molten steel to room temperature to obtain the tough boron steel.
Comparative example 2
Other conditions were the same as in example 2, and only the normalizing treatment step was omitted, specifically: cutting a 12mm boron steel thick plate material with the same material as that in the embodiment 2 into a sample with the specification of 15 multiplied by 10 multiplied by 12mm along the rolling direction, and then polishing the sample smoothly by a grinder; heating the sample in a heating furnace to 920 ℃, preserving heat for 0.5h, taking the sample out of the heating furnace, performing pressure quenching, and cooling with water, wherein the pressure of the pressure quenching is 7MPa, the quenching temperature is 920 ℃, the quenching time is 0.5h, the cooling speed of the water cooling is 30 ℃/s, and the cooling time is 32 s. And transferring the molten steel to a tempering furnace after water cooling, keeping the tempering temperature at 200 ℃, keeping the temperature for 3h, discharging the molten steel from the furnace after heat preservation, and air-cooling the molten steel to room temperature to obtain the tough boron steel.
The left side of FIG. 7 shows the metallographic structure of the boron steel after normalization treatment in example 1, with a scale of 50 μm. It can be seen that the metallographic structure thereof was pearlite and ferrite, contained a small amount of lower bainite, and had a small amount of band structure and coarse grains. The content of pearlite and ferrite in the normalized boron steel is 95% and the content of lower bainite is 5% by quantitative metallographic determination. The right side of fig. 7 is a metallographic structure of the non-normalized boron steel of comparative example 1, on a scale of 50 μm, in which a band-like structure is clearly present. The content of pearlite and ferrite measured by a metallographic structure observation method is 92 percent and is lower than that of the normalized boron steel, and the content of lower bainite is 8 percent and is higher than that of the lower bainite in the normalized boron steel.
In FIG. 8, the left side shows the metallographic structure of the high-toughness boron steel of example 1, and the right side shows the metallographic structure of the high-toughness boron steel of comparative example 1, wherein the scales of the left and right pictures are both 50 μm. It can be seen that the microstructure of the high strength ductile steel obtained in example 1 is finer and more uniform and denser than that of the high strength ductile steel in comparative example 1. The quantity percentages of crystal grains with different grain diameters in the metallographic structure chart of the high-toughness boron steel of the example 1 and the high-toughness boron steel of the comparative example 1 are measured by ImageJ software, and the measurement results are shown in a chart of 9. As can be seen from FIG. 9, the percentage content of the crystal grains with the diameter of 5-10 μm in the high-toughness boron steel in example 1 is 89.9%, which is higher than the percentage content of the crystal grains with the diameter of 5-10 μm in the high-toughness boron steel in comparative example 1.
As can be seen from the graphs in FIGS. 7 to 9, the content of pearlite and ferrite in the boron steel after normalizing treatment is higher than that of the boron steel without normalizing treatment, the content of lower bainite is reduced, and after pressure quenching and tempering, the percentage content of crystal grains with the diameter of 5-10 mu m in the steel is increased, which is beneficial to improving the comprehensive mechanical property of the steel. Hardness of the high-strength and tough steel of example 1 and the high-strength and tough steel of comparative example 1 were measured by an HRS 150D rockwell hardness tester, and the hardness of the high-strength and tough boron steel of example 1 was 52HRC and the hardness of the high-strength and tough boron steel of comparative example 1 was 48 HRC. The post-fracture elongation, tensile strength and yield strength of the high-strength and high-toughness steel of example 1 and the high-strength and high-toughness steel of comparative example 1 are measured by adopting an Instron universal material testing machine model 5567, the post-fracture elongation of the high-strength and high-toughness boron steel of example 1 is 8.0%, the tensile strength is 1690MPa, the yield strength is 1410MPa, the post-fracture elongation of the high-strength and high-toughness boron steel of comparative example 1 is 7.0%, the tensile strength is 1580MPa, and the yield strength is 1300MPa, the experimental results show that the microstructure of the boron steel after normalizing treatment is optimized, and the comprehensive mechanical property of the steel is improved after pressure quenching and low-temperature tempering treatment, which is consistent with the results reflected by figures 7-9.
The left side of FIG. 10 shows the metallographic structure of the normalized boron steel of example 2, on a scale of 50 μm, and it can be seen that the metallographic structure thereof is pearlite and ferrite, and may contain a small amount of lower bainite, and the band structure and coarse grains are small. The content of pearlite and ferrite in the normalized boron steel is 96 percent and the content of lower bainite is 4 percent by adopting a quantitative metallographic measurement method. The right-hand side of FIG. 10 shows the metallographic structure of the unnormalized boron steel of comparative example 2, on a scale of 50 μm, in which a band-shaped structure is clearly present. The metallographic structure observation method is adopted to measure that the content of pearlite and ferrite in the boron steel which is not normalized is 92 percent, the content of the pearlite and the ferrite is lower than that in the boron steel which is normalized, and the content of lower bainite is 8 percent and is higher than that in the high-strength and high-toughness boron steel in the example 2.
In fig. 11, the left side shows the metallographic structure of the high-toughness boron steel of example 2, the right side shows the metallographic structure of the high-toughness boron steel of comparative example 2, and the scales of the two pictures are both 50 μm, so that it can be seen that the high-toughness steel of example 2 has a finer and more uniform microstructure and is more compact than the high-toughness steel of comparative example 2. The quantity percentage of the crystal grains with different grain diameters in the metallographic structure diagram of the normalized boron steel in example 2 and the non-normalized boron steel in comparative example 2 is measured by ImageJ, and the measurement result is shown in figure 12. As can be seen from FIG. 12, the percentage content of the grains with the diameter of 5-10 μm in the high-toughness boron steel in example 1 is 89.7%, which is slightly higher than the percentage content of the grains with the diameter of 5-10 μm in the high-toughness boron steel in comparative example 1.
As can be seen from FIGS. 10 to 12, the content of pearlite and ferrite in the normalized boron steel is higher than that of the non-normalized boron steel, the content of lower bainite is reduced, and after pressure quenching and tempering, the percentage content of crystal grains with the diameter of 5 to 10 mu m in the steel is increased, which is beneficial to improving the comprehensive mechanical property of the steel. Hardness of the high-strength and tough steel of example 2 and the high-strength and tough steel of comparative example 2 were measured by a rockwell hardness tester, and the hardness of the high-strength and tough boron steel of example 2 was 50HRC and the hardness of the high-strength and tough boron steel of comparative example 2 was 47 HRC. The elongation after fracture, tensile strength and yield strength of the high-strength tough steel of example 2 and the high-strength tough steel of comparative example 2 were measured by using an instron universal material testing machine model 5567, the elongation after fracture of the high-strength tough boron steel of example 2 was 9.0%, the tensile strength was 1685MPa, the yield strength was 1400MPa, and the elongation after fracture of the high-strength tough steel of comparative example 2 was 7.0%, the tensile strength was 1550MPa, and the yield strength was 1280 MPa. The above experimental results show that the microstructure of boron steel after normalizing treatment is optimized, and the comprehensive mechanical properties of steel are improved after pressure quenching and low-temperature tempering treatment, which is consistent with the results reflected in fig. 10-12.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (8)
1. A method for refining boron steel grains is characterized by comprising the following steps:
normalizing the boron steel after hot rolling or forging; the normalizing treatment comprises the following steps: heating the hot rolled or forged boron steel to Ac3Keeping the temperature for 10-45 min at 20-90 ℃, and then cooling by air.
2. The method according to claim 1, wherein the heating is performed at a temperature increase rate of 250 to 300 ℃/min.
3. The method of claim 1, wherein the hot rolled or forged boron steel is 28MnB5, 27MnCrB5, or 33MnCrB 5.
4. The high-strength and high-toughness boron steel is characterized by being obtained by sequentially carrying out pressure quenching and tempering on normalized boron steel; the normalizing treatment is carried out according to the method of any one of claims 1 to 3.
5. The high-strength and high-toughness boron steel according to claim 4, wherein the content of pearlite and ferrite in the boron steel after the normalizing treatment is not less than 95%, the content of lower bainite in the boron steel is not more than 5%, and the hardness of the boron steel after the normalizing treatment is 20-24 HRC.
6. The high-strength boron steel according to claims 4 to 5, wherein the pressure of the pressure quenching is 6 to 8 MPa; the quenching temperature of the pressure quenching is 880-920 ℃; the quenching heat preservation time of the pressure quenching is 0.25-0.75 min;
the cooling mode of the pressure quenching is water cooling, the cooling rate of the water cooling is 10-30 ℃/s, and the cooling time of the water cooling is 30-120 s;
the tempering temperature is 180-200 ℃; and the tempering heat preservation time is 1-3 h.
7. The high-toughness boron steel according to claim 4, wherein the number percentage of crystal grains with the grain size of 5-10 μm in the high-toughness boron steel is not less than 89.5%.
8. The use of the high-strength boron steel of any one of claims 4 to 7 in a high-speed plough.
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