Cold heading steel wire rod for non-standard fastener and preparation method thereof

Abstract

The present application belongs to the technical field of cold heading steel, and particularly relates to a cold heading steel wire rod for non-standard fasteners and a preparation method thereof. The chemical composition is as follows: C: 0.13-0.16%, Si: 0.35-0.45%, Mn: 0.50-0.70%, B: 0.0018-0.0030%, Cr: 0.20-0.30%, Ti: 0.020-0.030%, Al: ≤0.005%, P and S: ≤0.025%, Ni and Cu: ≤0.20%, N: 50-70ppm, Ti / N≥3, and the rest is iron and inevitable impurities. The present application effectively improves the heat treatment performance of the steel, effectively improves the plasticity and strength of the steel, realizes the production of high-plasticity 7.8-grade non-standard screws, and meets the special needs of the railway industry in the production of non-standard fasteners by optimizing the chemical composition of the steel and combining the process flow of converter+RH smelting, cogging rolling+wire rod rolling.

Description

Technical Field

[0001] This invention belongs to the field of cold heading steel technology, and relates to a cold heading steel wire rod for non-standard fasteners and its preparation method. Background Technology

[0002] In recent years, the fastener industry has been trending towards stability in line with the expected goals of national macro-control. With my country's proposed transformation and upgrading of the manufacturing industry and the intensification of market competition, Chinese fastener companies will only be able to obtain profits through quality improvement and quantitative growth in the future. At the same time, the fastener industry is currently facing the requirement of industrial upgrading; competition in low-end products is becoming increasingly fierce, resulting in correspondingly lower profits, making the need for transformation and upgrading increasingly strong. The development trend of the fastener industry can be summarized in three aspects: the direction of product structure adjustment is becoming increasingly clear; the domestic high-end fastener market is huge and growing rapidly; and the trend of green production and industrial clusters.

[0003] Fasteners are characterized by their wide variety of types and specifications, diverse performance and applications, and extremely high degree of standardization, serialization, and generalization; they are often referred to as standard parts. Classified by product form, fasteners can be divided into twelve major categories: bolts, studs, screws, nuts, self-tapping screws, wood screws, retaining rings, washers, pins, rivets, assemblies and connecting pairs, and weld studs. They are a widely used category of basic mechanical components, applied in industries such as automotive, railway, machinery, power, and electronics. Standard parts are available in nine performance grades: 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9.

[0004] Standard fasteners offer a relatively complete range of performance grades, meeting most manufacturing needs. However, they cannot meet the specific performance requirements of industries such as railways, aviation, and shipbuilding, necessitating the development of non-standard fasteners. Non-standard fasteners, as the name suggests, are non-standard parts, generally referring to fasteners designed and manufactured according to user requirements without national standards, lacking standardization, serialization, and universality. In recent years, with the rapid development of my country's railways and the high degree of autonomy and innovation in railway products, the demand for fasteners in the railway industry has continued to increase in both quantity and variety, with non-standard fasteners accounting for a growing proportion. Specifically, the domestic railway industry requires a 7.8 grade non-standard screw, demanding suitable strength and hardness, as well as high plasticity. Currently, there are no suitable materials or processing methods to meet the requirements of 7.8 grade screws. Specifically, if materials commonly used in manufacturing 6.8 grade standard parts, such as SWRCH22A, SWRCH25K, and S25C, are used and the heat treatment process is adjusted, the resulting screws have low yield strength and tensile strength, failing to meet the requirements. If materials commonly used in manufacturing 8.8 grade standard parts, such as 10B21, SWRCH35K, and 20CrMo, are used and the heat treatment process is adjusted, the resulting screws do not meet the requirements for elongation and reduction of area. Therefore, domestic fastener manufacturers, using existing cold-heading steel wire rods as raw materials, find it difficult to produce non-standard screws that meet the requirements by adjusting the heat treatment process. Practical experience has shown an urgent need to develop a new type of cold-heading steel wire rod with good heat treatment performance, suitable strength and hardness, and excellent plasticity to meet the railway industry's demand for 7.8 grade non-standard fasteners. Summary of the Invention

[0005] To address the aforementioned issues, this invention optimizes the chemical composition of steel and combines a converter + RH smelting process with a billet rolling + wire rod rolling process. This effectively improves the heat treatment performance of steel, enhances its plasticity and strength, and enables the production of high-plasticity 7.8 grade non-standard screws, meeting the special needs of the railway industry for producing non-standard fasteners.

[0006] A cold heading steel wire rod for non-standard fasteners and its preparation method, including chemical composition design and preparation process flow. The preparation method of the wire rod of the present invention is described in detail below.

[0007] The mechanical properties of grade 7.8 screws required by the railway industry are as follows: tensile strength 730-780 MPa, yield strength ≥585 MPa, yield ratio ≥0.8, elongation A ≥30%, and reduction of area Z ≥75%.

[0008] Chemical composition design:

[0009] The main chemical composition range of the cold heading steel wire rod for non-standard fasteners described in this invention, by weight percentage, is: C: 0.13-0.16%, Si: 0.35-0.45%, Mn: 0.50-0.70%, B: 0.0018-0.0030%, Cr: 0.20-0.30%, Ti: 0.020-0.030%, Al: ≤0.005%, P, S: ≤0.025%, Ni, Cu ≤0.20%, N: 50-70ppm, Ti / N ≥3, with the remainder being iron and unavoidable impurities.

[0010] To improve the heat treatment performance of steel, the cold heading steel of this invention increases the Si content, decreases the Al content, and adds small amounts of B and Cr. The Ti and N content is designed to improve the plasticity of the steel. The C and Mn content is designed to ensure that the steel has appropriate strength and hardness to cooperate with heat treatment and meet the mechanical properties of non-standard fasteners. Other elements are controlled as residual or harmful elements.

[0011] Preparation process flow:

[0012] The preparation process is as follows: converter primary refining—RH vacuum refining—continuous casting of steel billets—bill rolling—wire rolling—wire controlled cooling.

[0013] Initial refining in the converter:

[0014] The primary smelting process in the converter uses molten iron and scrap steel as furnace charge, with scrap steel accounting for 10%–20% of the total mass. In the early stages of primary smelting, oxygen is blown for decarburization and desiliconization, followed by bottom blowing with argon gas, and the addition of lime and slagging agents for slag formation. In the later stages of primary smelting, deoxidation and alloying operations are performed. Specifically, silicon carbide is first added for deoxidation at tapping, and silicon manganese and ferrochrome are added for alloying when one-third of the steel has been tapped. The tapping temperature is ≥1630℃, and the carbon content is controlled at 0.05%–0.08%.

[0015] RH vacuum refining:

[0016] The RH vacuum refining process adopts a "high vacuum + large circulation" smelting process. The high vacuum degree is required to be <60Pa, the high vacuum time is ≥45min, and the number of molten steel circulations is ≥5 times. After the high vacuum circulation, the molten steel is sampled and the chemical composition is tested. After the RH vacuum is broken, carbon wire, ferrotitanium wire, and ferroboron wire are fed in through a wire feeder. Nitrogenating agent is added to adjust the chemical composition to the product requirements. Then, soft blowing is performed for ≥15min.

[0017] Continuous casting of steel billets:

[0018] The billet section of the continuous casting is 300mm*325mm. The secondary cooling water adopts a medium cooling intensity of 0.6-0.8L / kg and the cooling method is air mist cooling. The continuous casting adopts slow casting speed control, and the casting speed is required to be 0.8-1.0m / min. The superheat of the continuous casting furnace is controlled at 30-40℃, and the superheat of the continuous casting furnace is controlled at ≤20℃. The continuous casting billet is cooled in a sheltered pile. The electromagnetic stirring of the crystallizer is turned on and the end electromagnetic stirring is turned off. The billet is lightly pressed down 4 times, with a total reduction of 16mm.

[0019] Billet rolling:

[0020] The billet rolling process involves heating a 300mm*325mm cross-section continuous casting billet to 1100-1150℃ in a walking beam furnace. Before the billet enters the furnace, it is coated with an anti-decarburization coating. The heated billet is then rolled into a 160mm*160mm cross-section hot-rolled billet using a 7-stand alternating horizontal and vertical rolling mill. The hot-rolled billet is then cooled in a sheltered environment.

[0021] Wire rod rolling:

[0022] The wire rod rolling process involves heating a 160mm*160mm cross-section hot-rolled billet to 950-1000℃ and rolling it into hot-rolled wire rod through a high-speed wire rod continuous rolling mill. The finishing mill has a rolling temperature of 820-850℃ and a final rolling temperature of ≤860℃.

[0023] Wire rod cooling:

[0024] The wire rod is cooled at a controlled temperature of 790-820℃. After being spun, it enters the Steyrmo controlled cooling line for controlled cooling at a cooling rate of 0.5-0.6℃ / s to ≤500℃. Then it enters the circulation area and is naturally cooled to <100℃ before being bundled and packaged.

[0025] Using wire rod as raw material, screws are processed through conventional processing techniques including pickling, phosphating, drawing, cold heading, wire rolling, heat treatment (quenching and tempering at 870±10℃ and quenching oil, tempering at 500±10℃), and surface treatment.

[0026] This invention optimizes the chemical composition of steel. For example, Si+Al synergistically affects the hardenability of steel, B+Cr synergistically affects the heat treatment performance and strength of steel, Ti+N synergistically affects the plasticity of steel, and C+Mn synergistically affects the strength and hardness of steel. Through the combination design of elements, a new steel grade is obtained. Combined with the wire rod preparation process and manufacturing parameters, the heat treatment performance of steel is effectively improved, and the plasticity and strength of steel are enhanced. This enables the production of high-plasticity 7.8 grade non-standard screws, meeting the special needs of the railway industry for producing non-standard fasteners. Detailed Implementation

[0027] The specific chemical composition of TC15B cold-heading steel wire rod for non-standard fasteners is as follows: Wt, %

[0028] C Yes Mn B Cr You Al N P.S Ni、Cu 0.13-0.16 0.35-0.45 0.50-0.70 0.0018-0.0030 0.20-0.30 0.020-0.030 ≤0.005 0.0050-0.0070 ≤0.025 ≤0.20

[0029] Note: Ti / N≥3

[0030] The wire rod manufacturing process is as follows: converter primary refining—RH vacuum refining—continuous casting of steel billets—bill rolling—wire rod rolling—controlled cooling of wire rod.

[0031] The wire rod specification is Φ25.0mm.

[0032] Example 1

[0033] 1. Chemical composition

[0034] The chemical composition, by weight percentage, is: C: 0.14%, Si: 0.40%, Mn: 0.65%, B: 0.0022%, Cr: 0.28%, Ti: 0.023%, Al: 0.002%, P: 0.015%, S: 0.006%, Ni: 0.02%, Cu: 0.03%, N: 55ppm.

[0035] 2. Initial refining in a converter

[0036] The primary smelting process in the converter uses molten iron and scrap steel as the furnace charge, with a converter load of 140 tons and scrap steel accounting for 14%. In the early stages of primary smelting, oxygen is blown for decarburization and desiliconization as per general requirements, followed by bottom blowing with argon gas and the addition of lime and slagging agents for slag formation. In the later stages of primary smelting, deoxidation and alloying operations are performed. Silicon carbide is first added for deoxidation at tapping, and silicon manganese and ferrochrome are added for alloying when one-third of the steel has been tapped. The tapping temperature is 1639℃, and the carbon content is controlled at 0.06%.

[0037] 3. RH vacuum refining

[0038] RH vacuum refining is performed at a high vacuum level of 50Pa for 55 minutes, with molten steel circulating 7 times. After high vacuum circulation, molten steel is sampled and its chemical composition is tested. After RH vacuum breaking, carbon wire, ferrotitanium wire, and ferroboron wire are fed in through a wire feeder, and nitrogen-enhancing agent is added to adjust the chemical composition to the product requirements. Then, soft blowing is performed for 18 minutes.

[0039] 4. Continuous casting of steel billets

[0040] The billet section of the continuous casting is 300mm*325mm. The secondary cooling water adopts a medium cooling intensity of 0.6L / kg and the cooling method is air mist cooling. The continuous casting adopts slow casting speed control, and the casting speed requirement is 0.9m / min. The superheat of the continuous casting furnace is 19℃, and the continuous casting billet is cooled in a sheltered pile. The electromagnetic stirring of the crystallizer is turned on and the end electromagnetic stirring is turned off. The billet is lightly pressed down 4 times, with a total reduction of 16mm.

[0041] 5. Billet rolling

[0042] The billet rolling process involves heating a 300mm*325mm cross-section continuous casting billet to 1140℃ in a walking beam furnace. Before the billet enters the furnace, it is coated with an anti-decarburization coating. The heated billet is then rolled into a 160mm*160mm cross-section hot-rolled billet using a 7-stand alternating horizontal and vertical rolling mill. The hot-rolled billet is then cooled in a sheltered environment.

[0043] 6. Wire rod rolling

[0044] The wire rod rolling process involves heating a 160mm*160mm cross-section hot-rolled billet to 975℃ and rolling it into hot-rolled wire rod through a high-speed wire rod continuous rolling mill. The finishing mill has a rolling temperature of 825℃ and a final rolling temperature of 840℃.

[0045] 7. Wire rod cooling

[0046] The wire rod is cooled at 810℃ and then enters the Steyrmo controlled cooling line for controlled cooling at a rate of 0.55℃ / s to 465℃. After that, it enters the circulation area and is naturally cooled to 80℃ before being bundled and packaged.

[0047] Example 2

[0048] 1. Chemical composition

[0049] The chemical composition, by weight percentage, is: C: 0.16%, Si: 0.39%, Mn: 0.58%, B: 0.0027%, Cr: 0.25%, Ti: 0.026%, Al: 0.003%, P: 0.011%, S: 0.008%, Ni: 0.01%, Cu: 0.02%, N: 65ppm.

[0050] 2. Initial refining in a converter

[0051] The primary smelting process in the converter uses molten iron and scrap steel as the furnace charge, with a converter load of 141 tons and scrap steel accounting for 15%. In the early stages of primary smelting, oxygen is blown for decarburization and desiliconization as per general requirements, followed by bottom blowing with argon gas and the addition of lime and slagging agents for slag formation. In the later stages of primary smelting, deoxidation and alloying operations are performed. Silicon carbide is first added for deoxidation at tapping, and silicon manganese and ferrochrome are added for alloying when one-third of the steel has been tapped. The tapping temperature is 1645℃, and the carbon content is controlled at 0.07%.

[0052] 3. RH vacuum refining

[0053] The RH vacuum refining process involves a high vacuum of 55 Pa for 53 minutes, with 7 cycles of molten steel circulation. After the high vacuum circulation, molten steel samples are taken and their chemical composition is tested. After the RH vacuum is broken, carbon wire, ferrotitanium wire, and ferroboron wire are fed in through a wire feeder. A nitrogen-enhancing agent is added to adjust the chemical composition to meet product requirements. Then, soft blowing is performed for 20 minutes.

[0054] 4. Continuous casting of steel billets

[0055] The billet section of the continuous casting is 300mm*325mm. The secondary cooling water adopts a medium cooling intensity of 0.8L / kg and the cooling method is air mist cooling. The continuous casting adopts slow casting speed control, and the casting speed requirement is 1.0m / min. The superheat of the continuous casting furnace is 16℃, and the continuous casting billet is cooled in a sheltered pile. The electromagnetic stirring of the crystallizer is turned on and the end electromagnetic stirring is turned off. The billet is lightly pressed down 4 times, with a total reduction of 16mm.

[0056] 5. Billet rolling

[0057] The billet rolling process involves heating a 300mm*325mm cross-section continuous casting billet to 1130℃ in a walking beam furnace. Before the billet enters the furnace, it is coated with an anti-decarburization coating. The heated billet is then rolled into a 160mm*160mm cross-section hot-rolled billet using a 7-stand alternating horizontal and vertical rolling mill. The hot-rolled billet is then cooled in a sheltered environment.

[0058] 6. Wire rod rolling

[0059] The wire rod rolling process involves heating a 160mm*160mm cross-section hot-rolled billet to 990℃ and rolling it into hot-rolled wire rod through a high-speed wire rod continuous rolling mill. The finishing mill has a rolling temperature of 836℃ and a final rolling temperature of 855℃.

[0060] 7. Wire rod cooling

[0061] The wire rod is cooled at 805℃ and then enters the Steyrmo controlled cooling line for controlled cooling at a rate of 0.6℃ / s to 431℃. After that, it enters the circulation area and is naturally cooled to 70℃ before being bundled and packaged.

[0062] Comparative Example 1

[0063] Replace the Si content in the chemical composition of step 1 of Example 1 with 0.03%, and keep other conditions the same as in Example 1.

[0064] Comparative Example 2

[0065] Replace the Al content in step 1 of Example 1 with 0.040%, and keep other conditions the same as in Example 1.

[0066] Comparative Example 3

[0067] Replace the Cr content in the chemical composition of step 1 of Example 1 with 0.02%, and keep other conditions the same as in Example 1.

[0068] Comparative Example 4

[0069] Replace the content of B in the chemical composition of step 1 of Example 1 with 0.0003%, and keep other conditions the same as in Example 1.

[0070] Comparative Example 5

[0071] In Example 1, the Ti content in step 1 was replaced with 0.005% and the N content was replaced with 20 ppm, while other conditions remained the same as in Example 1.

[0072] Comparative Example 6

[0073] Replace the Ti content in step 1 of Example 1 with 0.005%, and keep other conditions the same as in Example 1.

[0074] Comparative Example 7

[0075] In Example 1, the continuous casting section of the billet in step 4 is 160mm*160mm, and step 5, billet rolling, is omitted. Other conditions are the same as in Example 1.

[0076] Comparative Example 8

[0077] In Example 1, the heating temperature of the hot-rolled billet in step 6 of wire rod rolling is replaced with 1150℃, the rolling temperature of the finishing mill is replaced with 900℃, and the final rolling temperature is replaced with 950℃. Other conditions are the same as in Example 1.

[0078] Comparative Example 9

[0079] Replace the spinning temperature in step 7 of Example 1 with 900℃ and the cooling rate with 0.2℃ / s, while keeping other conditions the same as in Example 1.

[0080] The non-standard fastener cold-heading steel wire rod produced in this embodiment and comparative example has a specification of Φ25.0mm. Using the wire rod as raw material, it undergoes a conventional processing procedure of "pickling—phosphating—drawing—cold heading—thread rolling—quenching and tempering heat treatment (heat treatment method is quenching + tempering, quenching temperature 870±10℃, quenching medium is quenching oil, tempering temperature 500±10℃)—surface treatment" to produce M24 non-standard screws. The mechanical properties and cold heading cracking rate of non-standard screws processed from wire rod using different methods are compared in Table 1 below:

[0081] Table 1

[0082]

[0083] Notes: 1. Mechanical performance requirements for high-plasticity grade 7.8 non-standard screws: tensile strength 730~780MPa, yield strength

[0084] ≥585Mpa, yield strength ratio ≥0.8, elongation A ≥30%, reduction of area Z ≥75%.

[0085] 2. In the fastener industry, the cracking rate of cold heading exceeds 10%, making normal production and processing impossible.

Claims

1. A cold-heading steel wire rod for non-standard fasteners, characterized in that: The chemical composition of cold heading steel wire rod for non-standard fasteners, by weight percentage, is as follows: C: 0.13~0.16%, Si: 0.35~0.45%, Mn: 0.50~0.70%, B: 0.0018~0.0030%, Cr: 0.20~0.30%, Ti: 0.020~0.030%, Al: ≤0.005%, P≤0.025%, S: ≤0.025%, Ni≤0.20%, Cu≤0.20%, N: 50~70ppm, Ti / N≥3, with the remainder being iron and unavoidable impurities; The preparation of the cold heading steel wire rod for non-standard fasteners involves the following rolling process: hot-rolled billets are heated to 950–1000°C and rolled into hot-rolled wire rods using a high-speed wire rod continuous rolling mill; the finishing mill in the process of rolling the hot-rolled wire rods into hot-rolled wire rods operates at an initial rolling temperature of 820–850°C and a final rolling temperature of ≤860°C; controlled cooling is achieved by controlling the wire drawing temperature at 790–820°C, and after wire drawing, the wire enters the Steyrmore controlled cooling line for controlled cooling at a cooling rate of 0.5–0.6°C / s. The cold-heading steel wire rods used for non-standard fasteners are used to produce grade 7.8 non-standard screws, which meet the mechanical performance requirements of grade 7.8 non-standard screws: tensile strength 730~780MPa, yield strength ≥585MPa, yield ratio ≥0.8, elongation A ≥30%, and reduction of area Z ≥75%.

2. The method for preparing cold-heading steel wire rod for non-standard fasteners according to claim 1, characterized in that: The preparation process of the cold heading steel wire rod for non-standard fasteners is as follows: converter primary refining—RH vacuum refining—converter continuous casting—bill rolling—wire rod rolling—wire rod controlled cooling; RH vacuum refining: adopts high vacuum + large circulation process for smelting; high vacuum + large circulation process means that the high vacuum degree requirement is <60Pa, the high vacuum time is ≥45min, the number of steel circulations is ≥5 times, and the steel is sampled and tested for chemical composition after high vacuum circulation. Continuous casting of steel billets: The cross-section of the continuously cast steel billets is 300mm*325mm. The secondary cooling water adopts a cooling intensity of 0.6~0.8L / kg and the cooling method is air mist cooling. The continuous casting adopts slow casting speed control, and the casting speed is required to be 0.8~1.0m / min. The superheat of the continuous casting furnace is controlled at 30~40℃, and the superheat of the continuous casting furnace is controlled at ≤20℃. The continuously cast billets are cooled in a sheltered manner. The electromagnetic stirring of the crystallizer is turned on, the end electromagnetic stirring is turned off, and the steel billets are lightly pressed down. Billet rolling: The 300mm*325mm cross-section continuous casting billet is heated to 1100~1150℃ in a walking beam furnace. Before the billet enters the furnace, it is sprayed with anti-decarburization coating. The heated billet is rolled into a 160mm*160mm cross-section hot-rolled billet by a rolling mill. The hot-rolled billet is cooled in a sheltered place. Wire rod rolling: The 160mm*160mm cross-section hot-rolled billet is heated to 950~1000℃ and rolled into hot-rolled wire rod by a high-speed wire rod continuous rolling mill; the initial rolling temperature of the finishing mill in the process of rolling into hot-rolled wire rod by the high-speed wire rod continuous rolling mill is 820~850℃, and the final rolling temperature is ≤860℃. Controlled cooling of wire rod: The spinning temperature is controlled at 790-820℃. After spinning, the wire rod enters the Stellmore controlled cooling line for controlled cooling at a cooling rate of 0.5-0.6℃ / s to ≤500℃. After that, it enters the circulation area and is naturally cooled to <100℃ before being bundled and packaged.

3. The method for preparing cold-heading steel wire rod for non-standard fasteners according to claim 2, characterized in that; After RH ventilator is broken, the chemical composition is adjusted to meet product requirements, followed by soft blowing, with a soft blowing time of ≥15 minutes.

4. The method for preparing cold-heading steel wire rod for non-standard fasteners according to claim 2, characterized in that, In the continuous casting of steel billets, the billet is lightly reduced in 4 stands, with a total reduction of 16mm.

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