A heat treatment process for steel

Abstract

The invention relates to the field of heat treatment process, and discloses a heat treatment process for steel. It includes the following steps: rough machining of the steel ingot, then quenching and tempering, and using rapid water cooling to obtain the first processed part; annealing the first processed part at 730°C to obtain a balanced ferrite + pearlite structure, then annealing, and then recrystallizing and refining the fine grains at 910°C-930°C high temperature normalizing, and mist cooling after hanging out, cooling to 400-500°C, and then air cooling to obtain the second processed part; the second processed part is subjected to fluctuating isothermal tempering at two temperatures to completely transform the structure and obtain a molded part. The drill tool material is subjected to temperature and controlled forging, controlled cooling to refine the grains and homogenize the grains, and the advanced post-forging heat treatment process is applied to re-arrange the uneven grain structure after forging, and further refine the grains. The performance of the steel is maximized to meet the mechanical performance requirements of high strength and high toughness.

Description

Technical Field

[0001] The invention relates to the field of heat treatment technology, in particular to a heat treatment technology for steel. Background Art

[0002] The high strength and high toughness of 4330VMOD have been favored by the mining, quarrying and oil drilling industries in recent years. With the widespread implementation of modern mining technology, the service conditions of mining drill tools have become worse, and mining drill tools have become the most vulnerable parts in drilling tools. According to the development of the drilling industry in recent years, through the research on the 4330VMOD material stabilizer product, its high strength and high toughness characteristics have also provided the most powerful equipment for the drilling industry. HF3000 alloy is a strengthening method formed by surfacing tungsten carbide alloy blocks to the surface of the drill pipe stabilizer with wear-resistant surfacing dressings, and the welding bonding surface of the alloy reaches more than 97%. The contact area of ​​the wear-resistant material in the abrasive formation is 100%, and it has good abrasion resistance and impact resistance. This type of drilling tool has a long service life, good wear resistance, a wide range of applications, and a good market prospect.

[0003] However, when the above-mentioned material is subjected to existing heat treatment, the material has a strong grain inheritance phenomenon, and is prone to produce coarse grains and mixed crystals. In actual production, due to the high temperature of the material after forging, the grains are coarse and uneven, and the structure is generally residual austenite + martensite + bainite. The phenomenon of low grain size often occurs, and repeated normalizing cannot solve the problem. In addition, the material needs to withstand heavy loads during the drilling process, and is very prone to damage. Summary of the invention

[0004] In order to solve the above technical problems, the invention provides a heat treatment process for steel to solve the above background technology problems.

[0005] To achieve the above object, the technical solution of the invention is as follows:

[0006] A heat treatment process for steel comprises the following steps:

[0007] The steel ingot is subjected to rough machining, then subjected to quenching and tempering treatment, and rapid water cooling is used to obtain a first machined part;

[0008] The first workpiece is annealed at 730°C to obtain a balanced ferrite + pearlite structure, and then normalized at 910°C-930°C for recrystallization and grain refinement, and then mist-cooled after being hung out to 400-500°C, and then air-cooled to obtain a second workpiece;

[0009] The second processed part is subjected to isothermal annealing at two temperatures to completely transform the structure and obtain a formed part.

[0010] As a preferred solution of the invention, in the steel ingot, P≤0.015%, S≤0.015%, and P+S<0.025%.

[0011] As a preferred embodiment of the invention, during the tempering treatment, the temperature is first raised to 500-600°C and maintained at a uniform temperature, then raised to 910-930°C, and then kept for 5 hours for normalizing, followed by air cooling to 350-400°C, and step-heated to 880-900°C for quenching treatment.

[0012] As a preferred solution of the invention, before quenching, cold water needs to be injected and stirring needs to be strengthened.

[0013] As a preferred solution of the invention, two temperatures of 650°C + 730°C are adopted during isothermal tempering.

[0014] As a preferred embodiment of the invention, the components of the molded part are: C is 0.31%, Si is 0.19%, Mn is 0.58%, P is 0.01%, S is <0.005%, Cr is 0.96%, Ni is 2.54%, Mo is 0.49%, and V is 0.07%.

[0015] As a preferred embodiment of the invention, the properties of the molded part are as follows: tensile strength = 1172 MPa, yield strength = 1079 MPa, elongation = 17%, cross-sectional shrinkage = 60%, and impact energy = 96 / 126 / 114 J.

[0016] In summary, the present invention has the following beneficial effects: the steel ingot is subjected to quenching and tempering treatment after roughing, and a rapid water cooling heat treatment method is adopted, so that the austenite in the core of the workpiece can completely avoid the cooling rate of the pearlite and upper bainite transformation, so that the forging can obtain lower bainite or lower bainite plus martensite structure along the entire cross section.

[0017] By controlling the temperature and forging, controlling the cooling and refining the grains and homogenizing the grains of the drill tool materials, and applying advanced post-forging heat treatment technology, the uneven grain structure after forging is re-arranged and the grains are further refined, so that the performance of the steel can be maximized and meet the mechanical performance requirements of high strength and high toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 It is a schematic diagram of the process flow chart of the invention. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the invention to clearly and completely describe the technical solutions in the embodiments of the invention. Obviously, the described embodiments are only part of the embodiments of the invention, not all of the embodiments. Based on the embodiments in the invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the invention.

[0021] Example

[0022] Figure 1 As shown, the present invention is a heat treatment process for steel, comprising the following steps: rough machining of the steel ingot, followed by quenching and tempering, and rapid water cooling to obtain a first machined part.

[0023] Since P in steel will cause cold brittleness and S will cause hot brittleness, P and S will form phosphides and sulfides during the solidification of molten steel and precipitate at the grain boundaries, causing intergranular brittleness and easily forming sulfide inclusions, which seriously affect the plasticity, toughness and fatigue properties of the steel.

[0024] When purchasing steel ingots, the P content in the steel ingots should be strictly controlled to be ≤ 0.015%, S ≤ 0.015%, and P+S < 0.025%.

[0025] When the steel ingot is rough-machined as mentioned above, the following process steps need to be taken: sawing both ends of the forging, ultrasonic flaw detection of the forging, drawing cross lines on the end face, drilling the center hole, rough machining of the outer circle, ultrasonic flaw detection of the forging, inspection, rough machining of the inner hole, inspection, and finally transfer to the tempering process.

[0026] The quenching and tempering process adopts the treatment method of normalizing + quenching and tempering. Through the normalizing process of air cooling and then air cooling, the cooling effect is enhanced, the grains are further refined, and full organizational preparation is made for the quenching and tempering treatment.

[0027] Before quenching, cold water needs to be injected and stirring should be strengthened to greatly shorten the steam film stage (the air mold generated in the steam film stage will affect the cooling of the workpiece and reduce the cooling rate) in the initial stage of quenching and cooling of the steel workpiece, or even reduce the air mold stage to zero. In this way, the early quenching cooling rate can be greatly increased, the quenching effect can be improved, the workpiece cooling can avoid the critical cooling curve of pearlite and upper bainite, and more quenched martensite structure can be obtained inside, and finally the tempered troostite structure can be obtained, thereby improving the impact toughness of the forging.

[0028] The first workpiece is annealed at 730°C to obtain a balanced ferrite + pearlite structure, and the balanced structure is subjected to subsequent process to block the inheritance of the structure, and then normalized at 910°C-930°C for recrystallization and grain refinement to ensure that the structure is completely austenitized, and then mist-cooled after hanging out to 400-500°C. Such a large supercooling rate is an important condition to ensure grain refinement, and then air-cooled to avoid mixing into bainite structure during transformation to obtain the second workpiece;

[0029] The second workpiece is subjected to isothermal tempering at two temperatures to achieve complete structural transformation and obtain a molded part. The isothermal tempering is carried out at two temperatures: 650°C + 730°C.

[0030] The components of the molded part are: C is 0.31%, Si is 0.19%, Mn is 0.58%, P is 0.01%, S is less than 0.005%, Cr is 0.96%, Ni is 2.54%, Mo is 0.49%, and V is 0.07%.

[0031] The properties of the molded parts are as follows: tensile strength = 1172 MPa, yield strength = 1079 MPa, elongation = 17%, cross-sectional shrinkage = 60%, and impact energy = 96 / 126 / 114 J.

[0032] In the present invention, the steel ingot is subjected to quenching and tempering treatment after roughing, and a rapid water cooling heat treatment method is adopted, so that the austenite in the core of the workpiece can completely avoid the cooling rate of the pearlite and upper bainite transformation, so that the forging obtains lower bainite or lower bainite plus martensite structure along the entire cross section.

[0033] By controlling the temperature and forging, controlling the cooling and refining the grains and homogenizing the grains of the drill tool materials, and applying advanced post-forging heat treatment technology, the uneven grain structure after forging is re-arranged and the grains are further refined, so that the performance of the steel can be maximized and meet the mechanical performance requirements of high strength and high toughness.

[0034] The above description of the disclosed embodiments enables one skilled in the art to make or use the invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A heat treatment process for steel, characterized in that: The steps include: The steel ingot is subjected to rough machining, then subjected to quenching and tempering treatment, and rapid water cooling is used to obtain a first machined part; Among them, during the quenching and tempering treatment, the temperature is first raised to 500-600℃ and maintained at a uniform temperature, then raised to 910-930℃, and then kept at this temperature for 5 hours for normalizing, then air-cooled to 350-400℃, and step-heated to 880-900℃ for quenching treatment; The first workpiece is annealed at 730°C to obtain a balanced ferrite + pearlite structure, and then normalized at 910°C for recrystallization and grain refinement, and then mist-cooled after being hung out to 400-500°C, and then air-cooled to obtain a second workpiece; The second workpiece is subjected to isothermal annealing at two temperatures to completely transform the structure and obtain a molded part; Among them, two temperatures of 650°C+730°C are adopted during isothermal tempering.

2. The heat treatment process for steel according to claim 1, characterized in that: In the steel ingot, P≤0.015%, S≤0.015%, and P+S<0.025%.

3. The heat treatment process for steel according to claim 1, characterized in that: Before quenching, cold water must be injected and stirring must be increased.

4. The heat treatment process for steel according to claim 1, characterized in that: The components of the molded part are: C is 0.31%, Si is 0.19%, Mn is 0.58%, P is 0.01%, S is less than 0.005%, Cr is 0.96%, Ni is 2.54%, Mo is 0.49%, and V is 0.07%.

5. The heat treatment process for steel according to claim 1, characterized in that: The properties of the molded part are as follows: tensile strength = 1172 MPa, yield strength = 1079 MPa, elongation = 17%, cross-sectional shrinkage = 60%, and impact energy = 96 / 126 / 114 J.

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