CN118166190A - Hot-rolled wire rod for 12.9-grade low-chromium fire-reducing bolt and manufacturing method thereof - Google Patents
Hot-rolled wire rod for 12.9-grade low-chromium fire-reducing bolt and manufacturing method thereof Download PDFInfo
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 51
- 239000011651 chromium Substances 0.000 title claims abstract description 44
- 229910052804 chromium Inorganic materials 0.000 title claims abstract description 27
- 150000003839 salts Chemical class 0.000 claims abstract description 133
- 238000005496 tempering Methods 0.000 claims abstract description 83
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 54
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 48
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 43
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 33
- 238000009987 spinning Methods 0.000 claims abstract description 22
- 229910001567 cementite Inorganic materials 0.000 claims abstract description 13
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims abstract description 13
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- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 8
- 239000000126 substance Substances 0.000 claims description 8
- 239000012535 impurity Substances 0.000 claims description 7
- 229910052698 phosphorus Inorganic materials 0.000 claims description 7
- 229910052717 sulfur Inorganic materials 0.000 claims description 7
- 229910052710 silicon Inorganic materials 0.000 claims description 4
- 229910052748 manganese Inorganic materials 0.000 claims description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 239000013078 crystal Substances 0.000 abstract description 15
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
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- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 229910001182 Mo alloy Inorganic materials 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
The invention relates to a hot rolled wire rod for a 12.9-grade low-chromium fire-down bolt and a manufacturing method thereof, wherein the hot rolled wire rod is designed by adopting a C-Si-Mn-Cr-Mo component with high Mn and low Cr, the wire rod after spinning is quenched and tempered in an online molten salt bainite phase region, most of austenitic structure is firstly promoted to be transformed into quenched bainite, then the tempered bainite is converted into tempered bainite through isothermal tempering, a small amount of austenitic structure is transformed into martensite, pearlite and ferrite, the newly generated martensite is transformed into tempered martensite, a cementite sheet layer of the newly generated pearlite is transformed into spheroidized structure, finally, the newly generated pearlite sheet is slowly cooled through a roller way, and the hot rolled wire rod is manufactured into a microstructure which comprises the tempered bainite as a main part, a small amount of tempered pearlite, the rest tempered martensite and a mixed structure consisting of crystal/block ferrite, the tensile strength reaches 961-1010 MPa, the area shrinkage rate reaches 50% -54%, and the hot rolled wire rod can be used for the green and efficient manufacturing of a 12.9-grade fastener bolt.
Description
Technical Field
The invention belongs to the technical field of annealing-reducing hot rolled wire rods, and particularly relates to a hot rolled wire rod for a 12.9-grade low-chromium fire-reducing bolt and a manufacturing method thereof.
Background
With the continuous high-strength development situation of the bolts for the fasteners, the requirement of the ultra-high strength bolts for the 12.9-grade fasteners, which are suitable for the development of the automobile industry, is growing. The hot rolled wire rod for the ultra-high strength bolt of the 12.9-level fastener generally adopts a high Cr and microalloying thought to improve the strength grade, and a high-strength component system with high alloy content can increase the plastic loss of the wire rod, so that a bolt mill needs two spheroidizing annealing procedures, and the spheroidizing annealing is used for improving the deformability of the wire rod, thereby meeting the processing performance requirements of drawing and cold heading in bolt manufacturing and avoiding cold heading cracking. In order to solve the problems of energy consumption, time consumption, high emission and the like caused by multiple spheroidizing annealing procedures, a hot rolled wire rod for a 12.9 grade fire reducing bolt and a manufacturing method thereof are required to be developed, and the hot rolled wire rod is used for realizing efficient green manufacturing of an ultra-high strength fastener bolt so as to meet the development requirements of the automobile industry.
The hot rolled wire rod for the 12.9-grade ultrahigh-strength bolt in the prior art is mainly manufactured by adopting a stelmor air-cooled wire-controlled cooling process after spinning, and the performance of the hot rolled wire rod is difficult to meet the technical difficulty and cause of the application requirement of fire decline comprises the following steps:
(1) The high Cr and microalloyed 12.9 grade hot rolled wire rod steel grade has higher hardenability, aggravates the risk of separating out martensite and bainite hard brittle phases when the stelmor wire is cooled fast, increases the content of pearlite and ferrite mainly through low-temperature rolling and wire spitting, keeps warm and slowly cools, reduces the wire rod extremely poor and the risk of wire breakage drawn by reducing fire caused by the brittle phases, for example: according to the manufacturing process for the wire rod for simplifying the annealing 12.9-level fastener disclosed in the patent CN117802291A, the C-Si-Cr-Mo component design is adopted, the Cr content is 0.8% -1.2%, the low-temperature wire-laying and heat-preservation slow cooling process design is combined, the metallographic structure is pearlite and ferrite, the tensile strength is 800MPa, the reduction of area is more than 40%, on one hand, the heat-preservation slow cooling is in a higher temperature state for a longer time, the ferrite content in the structure can be increased, the plasticity of the wire rod is limited, but the strength is obviously reduced, the strength of the wire rod needs to be strengthened by a large amount before tempering, the plastic loss is larger, the cold heading cracking risk is aggravated, on the other hand, the low-temperature rolling and wire-laying are high in requirements on rolling mill equipment capacity and motor load, the equipment wear and the wire-laying difficulty risks are caused, and in the prior art, although the wire-laying temperature is reduced by passing water cooling after rolling, the wire-laying temperature is increased, the wire rod is subjected to large bubble interference due to passing water cooling after rolling, and the mechanical property fluctuation and surface defects are increased.
(2) Compared with ferrite and pearlite structures, martensite and bainite structures have high-density dislocation and substructure, spheroidizing annealing is easier, pearlite transformation is avoided by quick cooling after the spinning temperature is lowered, and then heat preservation and slow cooling promote bainite and/or martensite transformation, so that spheroidizing annealing time is shortened, for example: the cold heading steel wire rod for the high-strength fastener disclosed in the patent CN116121512A and the production method thereof adopt the component design of C-Si-Mn-Cr-Al-Mo, the Cr content is 0.85% -0.95%, and the bainitic+martensitic content is more than or equal to 75% and the bainitic content is more than or equal to 60% by combining the low-temperature wire-laying, air-cooling rapid cooling and heat-preserving slow cooling process design; for example, a cold heading steel wire rod disclosed in patent CN116875916A, a preparation method and application thereof, wherein the area percentage of pearlite is 45% -60%, the area percentage of bainite is 25% -40%, and the area percentage of ferrite is 10% -25%, so that an annealing process can be simplified; on the one hand, the bainite and martensite in the structure have poor plasticity, are extremely easy to break in the coiling and uncoiling process of a bolt mill, the downstream transportation process of the coil from a steel mill, and even the coiling process of the steel mill, and have narrow production window; on the other hand, when the air cooling speed is improved, the temperature difference between the air receiving surface and the leeward surface of the wire rod can be increased, the performance of the wire rod is extremely poor, and the risks of wire drawing breakage and cold heading cracking are caused.
Disclosure of Invention
The invention aims to solve at least one of the technical problems to a certain extent, and provides a hot rolled wire rod for a 12.9-grade low-chromium fire-down bolt and a manufacturing method thereof.
The technical scheme adopted for solving the technical problems is as follows:
A manufacturing method of a hot rolled wire rod for a 12.9-grade low-chromium hypofire bolt comprises the following chemical components in percentage by mass: c:0.35 to 0.42 percent of Si:0.10 to 0.30 percent of Mn:0.85% -1.05%, cr:0.35 to 0.50 percent, less than or equal to 0.015 percent of P, less than or equal to 0.008 percent of S, and Mo:0.15 to 0.35 percent, and the balance of Fe and unavoidable impurities; the manufacturing method comprises the following steps:
The wire rod produced by wire spinning in the rolling control procedure is firstly subjected to online molten salt quenching, so that the wire rod is cooled to a bainitic phase region at a cooling speed of more than or equal to 30 ℃/s, most of austenitic structure is promoted to be transformed into quenched bainite, then is subjected to online molten salt heating tempering, isothermal tempering is carried out, so that the quenched bainite is transformed into tempered bainite, a small amount of austenitic structure is transformed into martensite, pearlite and ferrite, new martensite is transformed into tempered martensite, a cementite sheet layer of the new pearlite is transformed into spheroidized structure, and finally, the wire rod is slowly cooled by a roller way, so that the hot rolled wire rod with a microstructure comprising 55% -60% by volume of tempered bainite, a small amount of tempered pearlite, the balance tempered martensite and ferrite which are mixed structures consisting of edges and blocks is manufactured.
The design basis of the chemical components and the mass percentages of the hot rolled wire rod comprises:
(1) Carbon: c is the most basic strengthening element in steel, and along with the increase of carbon content, the tensile strength and hardenability of the steel can be improved, the transformation from austenite to quenched bainite is facilitated, but the excessive content can lead to the reduction of the ductility and cold heading performance of the steel, so that the hot rolled wire rod can reach the final strength grade through tempering under the fire reducing process in order to give consideration to the cold heading performance of the steel, and the mass percentage of C is controlled to be 0.35-0.42%.
(2) Silicon: si is a main deoxidizing element in steel, and can be dissolved in ferrite to block dislocation movement, so that the ferrite is reinforced; however, excessive silicon is easy to form inclusion, promotes grain boundary segregation of impurity elements, reduces low-temperature impact toughness of wire rod plasticity, and increases cold heading forming difficulty, so that the mass percentage of Si is controlled to be 0.10% -0.30%.
(3) Manganese: mn and Cr and Mo alloy elements are combined to increase the austenite stability, further improve the hardenability of the steel and promote the transformation of quenched bainite, but the excessive Mn content is easy to aggravate the segregation in the solidification process of the steel billet and reduce the uniformity of the steel, so that the Mn content is properly improved for ensuring the alloy strength and certain hardenability under the condition of Cr reduction, and the mass percentage of Mn is controlled to be 0.85-1.05%.
(4) Chromium: the Cr element is a ferrite forming element that can promote the formation of acicular ferrite to obtain a ferrite structure with good toughness; meanwhile, cr is a medium-strength carbide forming element and a hardenability improving element, has the function of refining a structure, but if the content is too high, the brittleness tendency of steel is increased, and the drawing and cold heading performances of the wire rod are deteriorated, so that the Cr content is properly reduced, and the mass percentage of Cr is controlled to be 0.35-0.50%.
(5) Molybdenum: mo is mainly in a solid solution form in steel, so that the hardenability can be obviously improved, the transformation of quenched bainite is promoted in a wider temperature range, the tempering stability of the wire rod is improved, and the coarsening of grains is effectively inhibited, but the production cost of the wire rod is improved due to the excessively high Mo content, so that the mass percentage of Mo is controlled to be 0.15-0.35%.
(6) Phosphorus, sulfur: the lower the P element and the S element are, the better, so that the P is less than or equal to 0.015 percent and the S is less than or equal to 0.008 percent.
On the basis of the design of the high Mn low Cr C-Si-Mn-Cr-Mo components, the invention adopts the direct salt bath after spinning to carry out online molten salt quenching and heating tempering, the wire rod can be cooled from the spinning temperature to the bainite temperature range at extremely high cooling speed through online molten salt quenching, so that most of high-temperature austenitic tissues are transformed into quenched bainite tissues, the matrix strength is provided, a small amount of austenitic tissues are reserved, and the preparation on the tissues is carried out for subsequent rising temperature tempering. On the other hand, compared with an air cooling rapid cooling process after low-temperature wire spinning, the method can avoid the loss of wire rod plasticity caused by the generation of a martensitic structure in a structure during quenching and the increase of the final tempering martensitic content, and simultaneously, compared with a relative air cooling or water cooling rapid cooling process, the method utilizes the high heat exchange capability of molten salt, and the wire rod can exchange heat more uniformly when passing through the molten salt, so that the temperature difference of the wire rod can be reduced, and further, the fluctuation of the mechanical property of the wire rod is reduced.
Further, the wire rod is subjected to online molten salt heating tempering, so that the wire rod and the molten salt are subjected to isothermal tempering, on one hand, compared with martensite and bainite tissues obtained by an air cooling rapid cooling and heat preservation slow cooling process after spinning, the molten salt isothermal tempering can enable quenched bainite to be converted into tempered bainite, the strength of the quenched bainite is reserved, the plasticity of the quenched bainite is obviously improved, meanwhile, after a small amount of high-temperature austenite tissues in the quenched tissue are converted into martensite, newly generated fresh martensite is converted into tempered martensite tissues under the high-temperature tempering, the strength of the martensite is reserved, the plasticity of the martensite is obviously improved, further, the plasticity of the hot rolled wire rod is effectively improved, and breakage of the wire rod in the coiling and uncoiling process of a bolt factory, the downstream transportation process of the coil from a steel factory, and even the coiling process of the steel factory is avoided; on the other hand, compared with ferrite and pearlite obtained by stelmor heat preservation slow cooling after spinning at a low temperature, the ferrite and pearlite are mainly composed of a molten salt, the ferrite and pearlite are heated and tempered, on the basis that most tempered bainite in the structure provides matrix strength, a small amount of high-temperature austenitic structure in the quenched structure is converted into a small amount of tempered pearlite and a ferrite structure along crystal or block shape, as the temperature of the wire rod is consistent with the temperature of the molten salt and kept isothermal, the wire rod is slowly cooled to have a longer high-temperature state, the newly generated pearlite cementite sheet is converted into a spheroidized structure to obtain tempered pearlite, the matrix structure is softened at a high temperature for a long time, so that the strength and plasticity of the wire rod are matched, the high-temperature state of the wire rod after being heated and tempered by the molten salt can be continued in the further roller way slow cooling process, further toughening of the wire rod structure and further spheroidized cementite sheet are promoted, further final wire rod structure regulation and control are realized, and the subsequent spheroidized annealing difficulty is reduced, and the application fields of manufacturing 12.9-grade high strength fasteners under the annealing are used.
Preferably, the controlled rolling process adopts billets to sequentially perform initial rolling, intermediate rolling and finish rolling, the initial rolling temperature is controlled to 1050-1100 ℃, the initial rolling reduction is controlled to 35-45%, and compared with the low-temperature controlled rolling caused by low-temperature wire laying in the prior art, the controlled rolling process has the problems of mill wear and lower rolling speed, and the invention adopts the online molten salt quenching after wire laying, so that the cooling capacity is greatly improved, the initial rolling temperature can be improved, the rolling efficiency is improved, the initial rolling reduction is increased, columnar crystals are crushed as much as possible, preparation is made for refining grains, and the refined tissue grains remain in the final hot rolled wire rod tissue, thereby being beneficial to improving the tissue strength.
Preferably, the intermediate rolling temperature is controlled to be 1000-1050 ℃, the intermediate rolling reduction is controlled to be 25-35%, and the bloom subjected to initial rolling can be further compressed and extended to obtain rolled pieces with proper temperature and size.
Preferably, the final rolling temperature is controlled to be more than or equal to 950 ℃, the final rolling reduction is 15% -20%, the final rolling temperature can be properly increased, the number of grain boundaries is increased, then spheroidizing nucleation points are increased, and the spheroidizing difficulty is reduced.
Preferably, the spinning temperature is controlled to be more than or equal to 920 ℃, so that the wire rod tissue is austenitized and preparation is made for subsequent tissue transformation.
Preferably, the temperature of the online molten salt quenching is controlled to be 420-450 ℃, and the quenching time is controlled to be 5-25 s; the lower the temperature of online molten salt quenching is, the longer the quenching time is, the higher the content of quenched bainite is, but the lower the quenching temperature is, the too long quenching time can lead to the too high content of quenched bainite, the increase of the proportion of tempered bainite, the decrease of the proportion of tempered pearlite and even the generation of too much tempered martensite, and the decrease of the plasticity of wire rods is caused, otherwise, the too high temperature and the too short quenching time lead to the lower content of quenched bainite and the decrease of the strength of a matrix, so the temperature and the quenching time of online molten salt quenching can be controlled, the proportion of austenite structure conversion into quenched bainite is controlled, and the preparation on the structure is made for the subsequent online molten salt heating tempering.
Preferably, as the temperature difference of the wire rod from the wire-laying temperature to the quenching temperature is larger, in order to control the molten salt temperature rise of online molten salt quenching and further control the quenching temperature precision, the molten salt circulation amount of the online molten salt quenching is 460-670 t/h, and the molten salt temperature rise is controlled to be less than or equal to 10 ℃ by adopting the larger molten salt circulation amount.
Preferably, the tempering temperature of the online molten salt heating tempering is controlled to be 590-640 ℃, and the tempering treatment time is controlled to be 500-650 s; the higher the tempering temperature of online molten salt temperature rising tempering is, the higher the content of along-grain/block ferrite in the tissue is, the lower the strength of the wire rod is, the spheroidization of cementite sheets is not facilitated even if the tempering time is too short, even the plasticity of the wire rod is obviously reduced because the quenched bainite is not completely converted into tempered bainite, the higher the content of tempered martensite in the tissue is, the plasticity loss of the wire rod is caused, the longer the tempering treatment time is, the higher the content of tempered pearlite in the tissue is when the wire rod is softened for a long time at a high temperature, but the higher the energy consumption is when the tempering treatment time is too long, and the strength loss is larger, so the tempering temperature and the tempering treatment time of online molten salt temperature rising tempering can be controlled, the tissue conversion is further controlled, and the strength and plasticity matching of the wire rod is realized.
Preferably, the temperature difference from the quenching temperature to the tempering temperature of the wire rod is relatively small, so that the energy consumption is reduced, the temperature precision of online molten salt temperature rising tempering is controlled, the molten salt circulation amount of the online molten salt temperature rising tempering is 330-440 t/h, and the molten salt temperature rising can be controlled to be less than or equal to 10 ℃ by using the relatively small molten salt circulation amount.
Preferably, the roller way slow cooling adopts a roller way to convey the wire rods into a heat preservation cover, so that the wire rods are slowly cooled to below 300 ℃ at a cooling speed of 0.4-0.9 ℃/s for collecting and coiling, the temperature of the wire rods after being subjected to salt bath is reduced too fast and softening effect is reduced after the wire rods are subjected to salt bath at too high cooling speed, and the wire rods are excessively long in online time and influence the production efficiency and cost after being cooled at too low cooling speed, so that the cooling speed of the wire rods is further controlled, the production rhythm can be considered, the slow cooling promotes further toughening of wire rod tissues and further spheroidization of cementite sheets, and the softening effect of the wire rods is improved.
A hot rolled wire rod for 12.9-grade low-chromium hypo-fire bolts, which is obtained by the method for producing a hot rolled wire rod for 12.9-grade low-chromium hypo-fire bolts according to any one of the above.
Preferably, the volume percentage of tempered pearlite is 25% -30%, the volume percentage of tempered martensite is 2% -8%, in the mixed structure, the strength of tempered bainite is obviously better than that of crystal/columnar ferrite, the strength of tempered martensite is higher than that of tempered bainite, but the plasticity is lower than that of other structures, the plasticity of crystal/block ferrite is better than that of tempered pearlite, but the strength is obviously lower than that of tempered pearlite, so that the volume percentage of each structure is further controlled, and the mixed structure mainly comprising tempered bainite and tempered pearlite and a small amount of crystal/columnar ferrite and tempered martensite can be obtained, so that the strength and high plasticity of the wire rod are matched.
Preferably, the diameter of the hot rolled wire rod is 6.0-10.0 mm, the tensile strength Rm is 961-1010 MPa, the area reduction rate Z is 50% -54%, and the hot rolled wire rod has better tensile strength and obviously improved area reduction rate compared with the conventional air-cooled wire rod on the basis of the design of the reinforced components with high Mn and low Cr.
The use of the hot rolled wire rod for the 12.9 grade low chromium reduced fire bolt comprises the step of omitting a spheroidizing annealing process at least once when the hot rolled wire rod is used for preparing the 12.9 grade bolt.
Compared with the prior art, the invention has the beneficial effects that:
(1) Aiming at the current situation that the strength of the wire rod is lost due to the fact that ferrite and pearlite mainly structures are generated through heat preservation and slow cooling under high Cr and microalloying or the wire rod plasticity is lost due to the fact that martensite or bainite mainly structures are generated through quick cooling after wire laying, the production method of the hot rolled wire rod for the 12.9-grade low-chromium hypofire bolt adopts the low Cr chemical composition design and the controlled rolling combined with the on-line molten salt bainite phase zone quenching and tempering and roller way slow cooling technology design, the hot rolled wire rod with the tempered bainite mainly, a small amount of tempered pearlite, the rest of tempered martensite and the mixed structure consisting of along with the crystal/blocky ferrite is obtained, and the strength and plasticity of the wire rod are matched through matrix structure regulation and control and the toughening and spheroidization of the wire rod structure, so that the method has good industrial applicability, is favorable for reducing the subsequent annealing difficulty, and can be used for the 12.9-grade bolt annealing green high-efficiency manufacturing, and the drawing wire breakage and the cold heading cracking risk is reduced.
(2) Aiming at the current situation that the existing preparation method of the cold heading steel hot rolled wire rod for the 12.9-grade fastener needs low-temperature wire-laying, the rolling mill abrasion and the rolling speed are lower, the invention can adopt higher initial rolling, middle rolling and finishing rolling temperatures, improve the rolling efficiency, reduce the abrasion to the rolling mill, refine the structure grains by adopting large reduction, and reduce the spheroidizing difficulty.
(3) Aiming at the current situation that the hot rolled wire rod of cold heading steel for the existing 12.9-grade fastener improves the strength grade through a high Cr and microalloying thought, a bolt factory needs two spheroidizing annealing procedures to meet the requirements on the drawing performance and the cold heading performance of bolt manufacture, and meanwhile, the hot rolled wire rod for the 12.9-grade low-chromium reduced fire bolt is successfully developed due to the current situation that the wire breakage and the cold heading cracking risk are improved in the drawing process of the bolt factory caused by the addition of high Cr and microalloying elements.
Drawings
The foregoing and/or additional aspects and advantages of the invention will become apparent and may be better understood from the following description of embodiments taken in conjunction with the accompanying drawings in which:
FIG. 1 is a metallographic structure diagram of example 1 of the present invention;
FIG. 2 is a metallographic structure diagram of comparative example 2 of the present invention;
FIG. 3 is a metallographic structure diagram of example 2 of the present invention;
FIG. 4 is a metallographic structure of example 3 of the present invention.
Detailed Description
The embodiments described below are exemplary only and are not intended to limit the description of the features and characteristics of the invention, in order to set forth the best mode of carrying out the invention, intended to illustrate it and to enable those skilled in the art to practice it, without any limitation to its scope, which is defined solely by the claims appended hereto.
Example 1:
The manufacturing method of the hot rolled wire rod for the 12.9-grade low-chromium fire-reducing bolt comprises the following chemical components in percentage by mass: c:0.39%, si:0.12%, mn:0.85%, cr:0.45%, P:0.014%, S:0.007%, mo:0.24% of Fe and the balance of unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, quenching and tempering in an online molten salt bainitic phase zone, slow cooling by a roller way and coil collecting, and specifically comprises the following steps of:
The controlled rolling is used for heating a billet with the specification of 160mm multiplied by 160mm through a heating furnace to obtain a high-temperature billet with rolling plasticity, rolling the billet coming out of the heating furnace into a wire rod with the specification of 9mm through a rolling line sequentially through initial rolling, intermediate rolling and final rolling, crushing columnar crystals as much as possible by using high Wen Chuga high reduction, preparing for grain refinement, further compressing and extending the high-temperature billet by intermediate rolling, improving the number of grain boundaries by final rolling, reducing the spheroidization difficulty, and specifically: the initial rolling temperature is controlled to 1070 ℃, the initial rolling reduction is 40%, the intermediate rolling temperature is 1020 ℃, the intermediate rolling reduction is 28%, the finishing rolling temperature is 960 ℃, and the finishing rolling reduction is 16%; the wire rod of rolling line is used for making wire rod through the wire rod machine into wire rod, controlling the wire rod temperature to 932 ℃, austenitizing to prepare on the follow-up structure, and conveying the wire rod along the roller way after the wire rod is scattered on the roller way.
The online molten salt bainitic phase zone quenching and tempering adopts two sections of salt baths with molten salt arranged therein, wire rods after spinning are conveyed through the first section of salt baths by a roller way, the temperature of the wire rods is quickly reduced to the molten salt temperature for online molten salt quenching, so that the wire rods are cooled down to a bainitic phase zone at a cooling speed of 34 ℃/s, most of austenitic tissues are promoted to be converted into quenched bainitic, the wire rods are conveyed through the second section of salt baths by the roller way, the temperature of the wire rods is quickly increased to the molten salt temperature for online molten salt heating tempering, isothermal tempering enables the quenched bainitic to be converted into tempered bainitic, a small amount of austenitic tissues are simultaneously converted into martensite, pearlite and ferrite, newly generated pearlite sheets are converted into spheroidized tissues, and the following concrete is that: the temperature of online molten salt quenching is controlled at 440 ℃, the quenching time is controlled at 15s, the circulating amount of molten salt is 460t/h, and the temperature rise of molten salt is less than or equal to 10 ℃; the tempering temperature of the online molten salt temperature-rising tempering is controlled at 590 ℃, the tempering treatment time is controlled at 650s, the molten salt circulation amount is 330t/h, and the molten salt temperature-rising is less than or equal to 10 ℃.
The wire rod that the process of roll table slow cooling will come out from second section salt bath carries into the heat preservation cover with the roll table and carries out slow cooling treatment, and the slow cooling promotes the further toughening of wire rod tissue and the further spheroidization of cementite lamellar, improves wire rod softening effect, and is specific: slowly cooling the wire rod to 280 ℃ at a cooling speed of 0.4 ℃/s until the wire rod is collected; the coil collecting procedure is used for collecting the coil rod into coils through a coil collecting drum, and obtaining a hot rolled coil rod finished product after packaging and warehousing, and the metallographic structure diagram of the hot rolled coil rod finished product is shown in figure 1.
Comparative example 1:
A method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 1 in that: the manufacturing method comprises the following steps of low temperature hot rolling, spinning, stelmor heat preservation slow cooling and coil collection, and specifically comprises the following steps of: the initial rolling temperature of the low-temperature hot rolling process is controlled to be 915 ℃, the intermediate rolling temperature is controlled to be 860 ℃, and the final rolling temperature is controlled to be 810 ℃; the spinning process controls the spinning temperature to 790 ℃; the Steyr heat preservation slow cooling adopts the steps that the wire rods after spinning pass through a Steyr air cooling line, all fans and a heat preservation cover are closed, the wire rods are subjected to phase change in the cover at a cooling speed of 1.5 ℃/s, and the hot rolled wire rods are obtained after coil collection and coil discharging.
Comparative example 2:
a method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 1 in that: the manufacturing method comprises the following steps of rolling control, wire laying, online molten salt quenching and roller way slow cooling, and specifically comprises the following steps of: and a salt bath tank with molten salt is adopted in the online molten salt quenching, wire rods after spinning are conveyed through the salt bath tank by a roller way, the quenching temperature is controlled at 150 ℃, the quenching time is controlled at 20 seconds, and the hot rolled wire rods are obtained, and a metallographic structure diagram of the hot rolled wire rods is shown in figure 2.
Example 2:
The manufacturing method of the hot rolled wire rod for the 12.9-grade low-chromium fire-reducing bolt comprises the following chemical components in percentage by mass: c:0.38%, si:0.17%, mn:0.99%, cr:0.39%, P:0.015%, S:0.008%, mo:0.15% of Fe and the balance of unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, quenching and tempering in an online molten salt bainitic phase zone, slow cooling by a roller way and coil collecting, and specifically comprises the following steps of:
The controlled rolling is used for heating a billet with the specification of 160mm multiplied by 160mm through a heating furnace to obtain a high-temperature billet with rolling plasticity, rolling the billet coming out of the heating furnace into a wire rod with the specification of 8mm through a rolling line sequentially through initial rolling, intermediate rolling and final rolling, crushing columnar crystals as much as possible by using high Wen Chuga high reduction, preparing for grain refinement, further compressing and extending the high-temperature billet by intermediate rolling, improving the number of grain boundaries by final rolling, reducing the spheroidization difficulty, and specifically: the initial rolling temperature is controlled to 1090 ℃, the initial rolling reduction is 43%, the intermediate rolling temperature is 1030 ℃, the intermediate rolling reduction is 32%, the finishing rolling temperature is 970 ℃, and the finishing rolling reduction is 17%; the wire rod of rolling line is used for being wire rod through the wire rod machine of throwing, and the wire rod temperature is 941 ℃ in the control wire rod throwing process, austenitizing is the preparation on the follow-up structure, and the wire rod is scattered on the roll table and is transported along the roll table.
The online molten salt bainitic phase zone quenching and tempering adopts two sections of salt baths with molten salt arranged therein, wire rods after spinning are conveyed through the first section of salt baths by a roller way, the temperature of the wire rods is quickly reduced to the molten salt temperature for online molten salt quenching, so that the wire rods are cooled down to a bainitic phase zone at a cooling speed of 38 ℃/s, most of austenitic tissues are promoted to be transformed into quenched bainitic, the wire rods are conveyed through the second section of salt baths by the roller way, the temperature of the wire rods is quickly increased to the molten salt temperature for online molten salt heating tempering, isothermal tempering enables the quenched bainitic to be transformed into tempered bainitic, a small amount of austenitic tissues are simultaneously transformed into martensite, pearlite and ferrite, and newly generated pearlite sheets are transformed into spheroidized tissues, specifically: the temperature of the online molten salt quenching is controlled at 430 ℃, the quenching time is controlled at 20s, the circulating amount of the molten salt is 670t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the tempering temperature of the online molten salt temperature-rising tempering is controlled at 640 ℃, the tempering treatment time is controlled at 600s, the molten salt circulation amount is 440t/h, and the molten salt temperature-rising is less than or equal to 10 ℃.
The wire rod that the process of roll table slow cooling will come out from second section salt bath carries into the heat preservation cover with the roll table and carries out slow cooling treatment, and the slow cooling promotes the further toughening of wire rod tissue and the further spheroidization of cementite lamellar, improves wire rod softening effect, and is specific: slowly cooling the wire rod to 285 ℃ at a cooling speed of 0.9 ℃/s until the wire rod is collected; the coil collecting procedure is used for collecting the coil rod into coils through a coil collecting drum, and obtaining a hot rolled coil rod finished product after packaging and warehousing, and the metallographic structure diagram of the hot rolled coil rod finished product is shown in figure 3.
Comparative example 3:
A method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 2 in that: the temperature of the online molten salt quenching is controlled at 380 ℃, the quenching time is controlled at 30s, and the hot rolled wire rod is obtained after coil collection and coil discharging.
Comparative example 4:
a method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 2 in that: the temperature of the online molten salt quenching is controlled at 500 ℃, the quenching time is controlled at 5s, and the hot rolled wire rod is obtained after coil collecting and coil discharging.
Example 3:
The manufacturing method of the hot rolled wire rod for the 12.9-grade low-chromium fire-reducing bolt comprises the following chemical components in percentage by mass: c:0.42%, si:0.1%, mn:1.05%, cr:0.35%, P:0.014%, S:0.008%, mo:0.3% of Fe and the balance of unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, quenching and tempering in an online molten salt bainitic phase zone, slow cooling by a roller way and coil collecting, and specifically comprises the following steps of:
The controlled rolling is used for heating a steel billet with the specification of 220mm multiplied by 220mm through a heating furnace to obtain a high-temperature steel billet with rolling plasticity, rolling the steel billet coming out of the heating furnace into a wire rod with the specification of 10mm through a rolling line sequentially through initial rolling, intermediate rolling and final rolling, crushing columnar crystals as much as possible by using high Wen Chuga high reduction, preparing for refining crystal grains, further compressing and extending the high-temperature steel billet by intermediate rolling, improving the number of crystal boundaries by final rolling, reducing the spheroidization difficulty, and specifically: the initial rolling temperature is controlled to 1050 ℃, the initial rolling reduction is 35%, the intermediate rolling temperature is 1000 ℃, the intermediate rolling reduction is 25%, the finishing rolling temperature is 950 ℃, and the finishing rolling reduction is 15%; the wire rod of rolling line is used for being wire rod through the wire rod machine of throwing, and the wire rod temperature is controlled to 920 ℃, austenitizing becomes the preparation on the follow-up structure, and the wire rod is scattered on the roll table and is transported along the roll table.
The online molten salt bainitic phase zone quenching and tempering adopts two sections of salt baths with molten salt arranged therein, wire rods after spinning are conveyed through the first section of salt baths by a roller way, the temperature of the wire rods is quickly reduced to the molten salt temperature for online molten salt quenching, the wire rods are cooled to the bainitic phase zone at a cooling speed of 36 ℃/s, most of austenitic tissues are promoted to be converted into quenched bainitic, the wire rods are conveyed through the second section of salt baths by the roller way, the temperature of the wire rods is quickly increased to the molten salt temperature for online molten salt heating tempering, isothermal tempering enables the quenched bainitic to be converted into tempered bainitic, a small amount of austenitic tissues are simultaneously converted into martensite, pearlite and ferrite, new generated pearlite cementite sheets are converted into spheroidized tissues, and the following concrete is that: the temperature of online molten salt quenching is controlled at 420 ℃, the quenching time is controlled at 25s, the circulating amount of molten salt is 570t/h, and the temperature rise of molten salt is less than or equal to 10 ℃; the tempering temperature of the online molten salt temperature-rising tempering is controlled at 610 ℃, the tempering treatment time is controlled at 500s, the molten salt circulation amount is 400t/h, and the molten salt temperature-rising is less than or equal to 10 ℃.
The wire rod that the process of roll table slow cooling will come out from second section salt bath carries into the heat preservation cover with the roll table and carries out slow cooling treatment, and the slow cooling promotes the further toughening of wire rod tissue and the further spheroidization of cementite lamellar, improves wire rod softening effect, and is specific: slowly cooling the wire rod to 290 ℃ at a cooling speed of 0.5 ℃/s until the wire rod is collected; the coil collecting procedure is used for collecting the coil rod into coils through a coil collecting drum, and obtaining a hot rolled coil rod finished product after packaging and warehousing, and the metallographic structure diagram of the hot rolled coil rod finished product is shown in figure 4.
Comparative example 5:
a method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 3 in that: the tempering temperature of the online molten salt heating tempering is controlled at 660 ℃, the tempering treatment time is controlled at 680s, and the hot rolled wire rod is obtained after coil collection and coil discharging.
Comparative example 6:
A method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 3 in that: the tempering temperature of the online molten salt heating tempering is controlled at 550 ℃, the tempering treatment time is controlled at 360s, and the hot rolled wire rod is obtained after coil collection and coil discharging.
Example 4:
The manufacturing method of the hot rolled wire rod for the 12.9-grade low-chromium fire-reducing bolt comprises the following chemical components in percentage by mass: c:0.35%, si:0.3%, mn:0.97%, cr:0.5%, P:0.014%, S:0.006%, mo:0.35% of Fe and the balance of unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, quenching and tempering in an online molten salt bainitic phase zone, slow cooling by a roller way and coil collecting, and specifically comprises the following steps of:
The controlled rolling is used for heating a billet with the specification of 160mm multiplied by 160mm through a heating furnace to obtain a high-temperature billet with rolling plasticity, rolling the billet coming out of the heating furnace into a wire rod with the specification of 6mm through a rolling line sequentially through initial rolling, intermediate rolling and final rolling, crushing columnar crystals as much as possible by using high Wen Chuga high reduction, preparing for grain refinement, further compressing and extending the high-temperature billet by intermediate rolling, improving the number of grain boundaries by final rolling, reducing the spheroidization difficulty, and specifically: the initial rolling temperature is controlled to 1100 ℃, the initial rolling reduction is 45%, the intermediate rolling temperature is 1050 ℃, the intermediate rolling reduction is 35%, the finishing rolling temperature is 975 ℃, and the finishing rolling reduction is 20%; the wire rod of rolling line is used for being wire rod through the wire rod machine of throwing, and the wire rod is controlled to be wire rod temperature is 945 ℃, austenitizing becomes the preparation on the follow-up structure, and the wire rod is scattered on the roll table and is conveyed along the roll table.
The online molten salt bainitic phase zone quenching and tempering adopts two sections of salt baths with molten salt arranged therein, wire rods after spinning are conveyed through the first section of salt baths by a roller way, the temperature of the wire rods is quickly reduced to the molten salt temperature for online molten salt quenching, so that the wire rods are cooled down to the bainitic phase zone at a cooling speed of 35 ℃/s, most of austenitic tissues are promoted to be converted into quenched bainitic, the wire rods are conveyed through the second section of salt baths by the roller way, the temperature of the wire rods is quickly increased to the molten salt temperature for online molten salt heating tempering, isothermal tempering enables the quenched bainitic to be converted into tempered bainitic, a small amount of austenitic tissues are simultaneously converted into martensite, pearlite and ferrite, newly generated pearlite sheets are converted into spheroidized tissues, and the following concrete is that: the temperature of the online molten salt quenching is controlled at 450 ℃, the quenching time is controlled at 5s, the circulating amount of the molten salt is 500t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the tempering temperature of the online molten salt temperature-rising tempering is controlled at 625 ℃, the tempering treatment time is controlled at 560s, the molten salt circulation amount is 360t/h, and the molten salt temperature-rising is less than or equal to 10 ℃.
The wire rod that the process of roll table slow cooling will come out from second section salt bath carries into the heat preservation cover with the roll table and carries out slow cooling treatment, and the slow cooling promotes the further toughening of wire rod tissue and the further spheroidization of cementite lamellar, improves wire rod softening effect, and is specific: slowly cooling the wire rod to 285 ℃ at a cooling speed of 0.7 ℃/s until the wire rod is collected; the coil collecting procedure is used for collecting the coil rod into coils through the coil collecting drum, and obtaining a hot rolled coil rod finished product after packaging and warehousing.
Comparative example 7:
A method for manufacturing a hot rolled wire rod for a 12.9-stage bolt, which is different from example 4 in that: the manufacturing method comprises the steps of rolling control, wire laying, quenching in an online molten salt bainitic phase zone, tempering, air cooling and coil collecting, wherein the air cooling is realized by opening a heat insulation cover, conveying a coil from a second stage salt bath by using a roller way, and controlling the coil to be slowly cooled to 285 ℃ at a cooling speed of 3.2 ℃/s until the coil collecting is finished, and obtaining the hot rolled coil after the coil collecting is off line.
The hot rolled wire rods obtained in the above examples and comparative examples were subjected to structure and property detection: tensile testing was performed using the metal material tensile test section 1 of GB-T228.1-2021: room temperature test method, to obtain tensile strength and reduction of area, and to perform tissue detection according to the metal microstructure detection method of GB/T13298 standard, the comparison results obtained are shown in table 1 below:
TABLE 1 comparison of the composition of the hot rolled wire rods with the properties of the wire rod structure of the manufacturing process
Compared with the low Wen Tusi and stelmor heat-preservation slow cooling processes, the manufacturing method disclosed by the invention is based on the C-Si-Mn-Cr-Mo component design of high Mn and low Cr, and the control rolling is combined with the on-line molten salt bainite phase zone quenching and tempering and roller way slow cooling technology design, so that the transformation temperature interval of ferrite and pearlite can be skipped at extremely fast cooling speed, the temperature is cooled to the bainite temperature interval from the wire-laying temperature, most of high-temperature austenitic structure is transformed into quenched bainitic structure, the matrix strength is provided, the quenched bainitic is transformed into tempered bainite by molten salt isothermal tempering, a small amount of austenitic structure is transformed into martensite, pearlite and ferrite, the new martensite is transformed into tempered martensite by slow cooling at a high temperature for a longer time, and finally, the hot rolled wire rod of a mixed structure consisting of tempered bainite as a main part, a small amount of tempered pearlite and the rest tempered martensite and along the crystal/pearlite is obtained by roller way slow cooling, the hot rolled wire rod with the tensile strength of 9610150-50% and the shrinkage rate of the mixed structure can reach the market-grade of at least one-stage high-grade, and the high-quality fastener has good market-grade annealing prospect, and the application prospect is achieved at least in the market-grade of at least is high, and the market-grade is good.
As can be seen from the comparison results of examples 1-4 and comparative example 2, compared with quenching the austenitic structure into needle-shaped martensite in the martensite phase region after spinning, the manufacturing method of the invention promotes the transformation of most austenitic structure into quenched bainite, and then the tempering at the same temperature converts the newly-formed martensite into tempered martensite, so that the brittleness of the martensite can be obviously improved on the basis of better strength of the wire rod, the plastic loss of the wire rod caused by the martensite or bainite structure generated by the final structure is avoided, and the breakage of the wire rod in the coiling and uncoiling process of a bolt mill, the downstream transportation process of the coil from a steel mill, and even the coiling process of the steel mill is effectively avoided.
From the comparison of examples 1 to 4, the comparison result of example 2 and comparative example 3 shows that the lower the temperature of online molten salt quenching is, the longer the quenching time is, the higher the content of quenched bainite is, but the lower the quenching temperature is, the longer the quenching time is, the higher the content of quenched bainite is, the ratio of tempered bainite is increased, the ratio of tempered pearlite is decreased, and even excessive tempered martensite is generated, resulting in the decrease of the plasticity of the wire rod; from the comparison of examples 1 to 4, the comparison results of example 2 and comparative example 4 show that the quenching bainite content is lower when the temperature and time of online molten salt quenching are too high and too short, and the matrix strength is reduced.
From the comparison of examples 1 to 4, the comparison results of example 3 and comparative example 5 show that the higher the tempering temperature of the online molten salt temperature rising tempering is, the longer the tempering treatment time is, the higher the content of the ferrite along the crystal/block in the structure is, the wire rod is excessively softened at high temperature for a long time, and the strength loss is larger; from the comparison of examples 1 to 4, the comparison result of example 3 and comparative example 6 shows that the lower the tempering temperature of the online molten salt temperature-rising tempering is, the too short tempering treatment time is, the quenched bainite is not completely converted into tempered bainite, and the plasticity of the wire rod is obviously reduced, so that the tempering temperature and tempering treatment time of the online molten salt temperature-rising tempering can be controlled, and the strength and plasticity matching of the wire rod can be further controlled.
As can be seen from the comparison results of examples 1-4 and comparative example 7, the roller way slow cooling can continue the high temperature state of the wire rod after the wire rod is heated and tempered by molten salt, further promote the further toughening of the wire rod structure and the further spheroidization of cementite sheets, further realize the regulation and control of the final wire rod structure, reduce the subsequent spheroidizing annealing difficulty, and have lower energy consumption and more economy compared with the direct prolonged tempering treatment time.
The above list of detailed descriptions is only specific to practical embodiments of the present invention, and they are not intended to limit the scope of the present invention, and all equivalent embodiments or modifications that do not depart from the spirit of the present invention should be included in the scope of the present invention.
Claims (10)
1. The manufacturing method of the hot rolled wire rod for the 12.9-grade low-chromium fire-reducing bolt is characterized by comprising the following chemical components in percentage by mass: c:0.35 to 0.42 percent of Si:0.10 to 0.30 percent of Mn:0.85% -1.05%, cr:0.35 to 0.50 percent, less than or equal to 0.015 percent of P, less than or equal to 0.008 percent of S, and Mo:0.15 to 0.35 percent, and the balance of Fe and unavoidable impurities; the manufacturing method comprises the following steps:
The wire rod produced by wire spinning in the rolling control procedure is firstly subjected to online molten salt quenching, so that the wire rod is cooled to a bainitic phase region at a cooling speed of more than or equal to 30 ℃/s, most of austenitic structure is promoted to be transformed into quenched bainite, then is subjected to online molten salt heating tempering, isothermal tempering is carried out, so that the quenched bainite is transformed into tempered bainite, a small amount of austenitic structure is transformed into martensite, pearlite and ferrite, new martensite is transformed into tempered martensite, a cementite sheet layer of the new pearlite is transformed into spheroidized structure, and finally, the wire rod is slowly cooled by a roller way, so that the hot rolled wire rod with a microstructure comprising 55% -60% by volume of tempered bainite, a small amount of tempered pearlite, the balance tempered martensite and ferrite which are mixed structures consisting of edges and blocks is manufactured.
2. The method for manufacturing a hot rolled wire rod for a 12.9-grade low-chromium reduced fire bolt according to claim 1, wherein the rolling control procedure is characterized in that billets are subjected to initial rolling, intermediate rolling and finish rolling in sequence, the initial rolling temperature is controlled to 1050-1100 ℃, the initial rolling reduction is controlled to 35% -45%, the intermediate rolling temperature is 1000-1050 ℃, the intermediate rolling reduction is controlled to 25% -35%, the finish rolling temperature is more than or equal to 950 ℃, and the finish rolling reduction is controlled to 15% -20%; the spinning temperature is controlled to be more than or equal to 920 ℃.
3. The method for manufacturing a hot rolled wire rod for a 12.9-grade low-chromium reduced fire bolt according to claim 1, wherein the temperature of the online molten salt quenching is controlled to be 420-450 ℃, and the quenching time is controlled to be 5-25 seconds.
4. The method for manufacturing a hot rolled wire rod for a 12.9-grade low chromium reduced fire bolt according to claim 3, wherein the tempering temperature of the online molten salt heating tempering is controlled to be 590-640 ℃, and the tempering treatment time is controlled to be 500-650 s.
5. The method for manufacturing a hot rolled wire rod for a 12.9-grade low-chromium hypoid fire bolt, which is characterized in that the circulation amount of molten salt for online molten salt quenching is 460-670 t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the circulation amount of the molten salt in the online molten salt temperature rising tempering is 330-440 t/h, and the molten salt temperature rising is less than or equal to 10 ℃.
6. The method for manufacturing a hot rolled wire rod for a 12.9-grade low-chromium reduced fire bolt according to claim 4, wherein the roller way slow cooling is characterized in that the roller way is used for conveying the wire rod into a heat preservation cover, so that the wire rod is slowly cooled to below 300 ℃ at a cooling speed of 0.4-0.9 ℃/s for collecting the wire rod.
7. A hot rolled wire rod for 12.9 grade low chromium reduced fire bolt, characterized in that it is obtained by the manufacturing method of the hot rolled wire rod for 12.9 grade low chromium reduced fire bolt according to any one of claims 1 to 6.
8. The hot rolled wire rod for 12.9 grade low chromium reduced fire bolt according to claim 7, wherein the tempered pearlite is 25% -30% by volume and the tempered martensite is 2% -8% by volume.
9. The hot rolled wire rod for 12.9 grade low chromium reduced fire bolt according to claim 7, wherein the diameter of the hot rolled wire rod is 6.0-10.0 mm, the tensile strength is 961-1010 mpa, and the area reduction is 50-54%.
10. Use of a hot rolled wire rod for a grade 12.9 low chromium reduced fire bolt according to claim 7, comprising the step of omitting at least one spheroidizing annealing step for the preparation of a grade 12.9 bolt.
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