Non-quenched and tempered cold heading steel wire rod and manufacturing method thereof

Abstract

The invention discloses a non-quenched and tempered cold heading steel wire rod, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.25 to 0.35 percent of C, 0.20 to 0.40 percent of Si, 1.1 to 1.6 percent of Mn, 0.05 to 0.15 percent of V, 0.02 to 0.04 percent of Al and the balance of Fe. The mass percentage contents of C and V also meet the condition that 1g [V] [C] is less than 6.72-9500 / T, T represents the spinning temperature of the hot-rolled wire rod, and the unit is K. The invention further discloses a manufacturing method of the non-quenched and tempered cold heading steel wire rod. The manufacturing method comprises the following steps: smelting and casting; rolling the wire rod; and cooling: after rolling, cooling to 780-800 DEG C at the cooling speed of 3-4 DEG C / s, and then continuously cooling to room temperature at the cooling speed of lower than 2 DEG C / s. The invention further discloses a non-quenched and tempered fastener. The non-quenched and tempered fastener is manufactured after the non-quenched and tempered cold heading steel wire rod is subjected to drawing and cold heading.

Description

Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, and in particular to a wire rod and a manufacturing method thereof. Background Art

[0002] The production of high-strength bolts of grade 8.8 and above often requires an "annealing + quenching + tempering" process. Using non-quenched and tempered cold-headed steel to produce bolts eliminates the annealing and tempering (i.e., quenching + tempering) processes, shortening the bolt processing time. This is especially true when producing slender bolts, as it prevents quenching-induced bending. Furthermore, using non-quenched and tempered cold-headed steel to produce bolts is low-carbon, green, energy-efficient, and environmentally friendly, offering significant economic and social benefits.

[0003] Non-quenched and tempered steel can be categorized by processing technology: hot forging non-quenched and tempered steel, direct cutting non-quenched and tempered steel, and cold-work-strengthened non-quenched and tempered steel. Cold-work-strengthened non-quenched and tempered steel utilizes microalloying, controlled rolling, and controlled cooling to disperse and precipitate carbonitrides of the microalloying elements, resulting in precipitation strengthening and grain refinement. Cold deformation further enhances the steel's strength, ensuring it meets the required performance without quenching and tempering. Currently, this cold-work-strengthened non-quenched and tempered steel is primarily used in the manufacture of fasteners such as high-strength bolts.

[0004] The compositional characteristics of currently available cold-work-hardened, non-quenched and tempered steels are generally low in carbon, high in manganese, and supplemented with microalloying elements such as vanadium, niobium, and titanium. Some grades also contain high levels of silicon. This compositional design for cold-work-hardened, non-quenched and tempered steels produces fine, dispersed carbides of the microalloying elements during wire rolling and cooling, which act as a secondary phase strengthening, enhancing both strength and toughness.

[0005] Chinese patent document CN101619414A, published on January 6, 2010, and entitled "Method for rolling 10.9-grade niobium-containing non-quenched and tempered cold-heading steel and hot-rolled wire rod thereof," discloses a method for rolling 10.9-grade niobium-containing non-quenched and tempered cold-heading steel and hot-rolled wire rod thereof. The steel comprises the following components by weight: C 0.08-0.14%, Si 0.03-0.35%, Mn 1.80-2.30%, P ≤ 0.025%, S ≤ 0.015%, B 0.0005-0.003%, Ti 0.01-0.03%, Al 0.010-0.050%, Nb 0.02%-0.04%, with the remainder being iron and trace impurities. The hot-rolled wire rod rolling method is as follows: heating temperature 1050-1250°C, roughing and intermediate rolling temperatures 1000-1050°C, finishing rolling temperature 750-830°C, deformation amount 50-60%, deformation rate 20 / s, spinning temperature 730-850°C, and two-stage controlled cooling after spinning: rapid cooling at a cooling rate of ≥5°C / s above 550°C, and slow cooling at a cooling rate of 0.1-3°C / s below 550°C. The non-quenched and tempered cold heading steel material is obtained by adding alloying elements such as B, Ti, and Nb to C, Si, and Mn.

[0006] Chinese patent publication number CN101220439A, published on July 16, 2008, and titled "Non-quenched and tempered duplex cold-heading steel for high-strength fasteners and its manufacturing method," discloses a non-quenched and tempered duplex cold-heading steel for high-strength fasteners and its manufacturing method. The composition and weight percentages of the non-quenched and tempered duplex cold-heading steel for high-strength fasteners are as follows: C: 0.06-0.15%; Si: 0.60-0.90%; Mn: 1.40-2.0%; P: ≤0.025%; S: ≤0.025%; Al: ≤0.04%; N: ≤0.0060%; Fe: balance. Summary of the Invention

[0007] One of the objectives of the present invention is to provide a non-quenched and tempered cold-forged steel wire rod that has low strength and good cold deformation capability. Furthermore, this wire rod reduces wear on machining tools during fastener production, eliminates the need for spheroidizing annealing and post-cold-forging tempering, and significantly reduces the production cost of high-strength fasteners using this non-quenched and tempered cold-forged steel wire rod.

[0008] To achieve the above object, the present invention provides a non-quenched and tempered cold heading steel wire rod, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0009] C: 0.25~0.35%, Si: 0.20~0.40%, Mn: 1.1~1.6%, V: 0.05~0.15%, Al: 0.02~0.04%;

[0010] The mass percentages of C and V also satisfy lg[V][C]<6.72-9500 / T, where T represents the spinning temperature of the hot-rolled wire rod, in K.

[0011] Furthermore, in the non-quenched and tempered cold heading steel wire rod of the present invention, the mass percentage of each chemical element is:

[0012] C: 0.25-0.35%, Si: 0.20-0.40%, Mn: 1.1-1.6%, V: 0.05-0.15%, Al: 0.02-0.04%; the balance is Fe and other inevitable impurities.

[0013] Existing non-quenched and tempered cold-heading steel wire rods generally do not contain V. However, the inventors have discovered that the addition of V to non-quenched and tempered cold-heading steel can form vanadium carbide precipitation phases with C. At high temperatures, these vanadium carbide precipitation phases tend to grow to form large particles, which is not conducive to improving the strength of the steel. Therefore, to ensure the quantity and dispersion of vanadium carbide, the inventors have creatively proposed a relationship between the V and C content and the spinning temperature.

[0014] In the non-quenched and tempered cold heading steel wire rod of the present invention, the design principles of the chemical elements are specifically described as follows:

[0015] C: In the non-quenched and tempered cold-heading steel wire rod described herein, carbon is the most important element affecting the steel's cold plastic deformation. Higher carbon content increases steel strength, but reduces cold deformation performance. Therefore, to ensure a good balance of strength and toughness for the material described herein, the carbon content in the steel must be controlled within a reasonable range. Therefore, in the non-quenched and tempered cold-heading steel wire rod described herein, the mass percentage of carbon is controlled between 0.25% and 0.35%.

[0016] Si: In the non-quenched and tempered cold-heading steel wire rod described herein, Si can moderately increase the tensile strength of the steel. However, excessive Si content is not recommended, as it can negatively impact the steel's plasticity, particularly its cold plastic deformation. Therefore, in the non-quenched and tempered cold-heading steel wire rod described herein, the Si content is controlled within a range of 0.20% to 0.40% by weight.

[0017] Mn: In the non-quenched and tempered cold-heading steel wire rod described herein, the alloy strengthening effect of Mn causes much less plasticity loss than does C. Therefore, maintaining a certain Mn content is essential for almost all non-quenched and tempered steels. However, excessive Mn content can adversely affect the steel's plasticity and microstructure. Therefore, considering both the beneficial and adverse effects of Mn, the mass percentage of Mn in the non-quenched and tempered cold-heading steel wire rod described herein is controlled between 1.1% and 1.6%.

[0018] V: In the non-quenched and tempered cold-heading steel wire rod described in the present invention, V is a strong carbide-forming element. It bonds strongly with carbon and forms stable V. This element is a typical carbide with a high melting point, high hardness, and high dispersion, and is one of the main causes of precipitation strengthening. However, excessive V content not only increases production costs but also leads to excessively high strength, hindering cold deformation. Therefore, in the non-quenched and tempered cold-heading steel wire rod described in the present invention, the mass percentage of V is controlled between 0.05% and 0.15%.

[0019] Al: In the non-quenched and tempered cold heading steel wire rod of the present invention, Al is a deoxidizer added to the steel. Controlling the mass percentage of the Al element within the range of 0.02 to 0.04% is beneficial to grain refinement and improving the strength and toughness of the steel.

[0020] Furthermore, among the inevitable impurities in the non-quenched and tempered cold heading steel wire rod of the present invention, S≤0.010%, P≤0.015%, and N≤0.005%.

[0021] It should be noted that in the non-quenched and tempered cold heading steel wire rod of the present invention, the elements S, P and N are all impurity elements in the steel. If technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the steel should be reduced as much as possible.

[0022] S: In the non-quenched and tempered cold-heading steel wire rod described in the present invention, S is a harmful element in steel. It can cause hot brittleness, deteriorate the hot working properties of the steel, and negatively affect the material's corrosion resistance. Therefore, in the non-quenched and tempered cold-heading steel wire rod described in the present invention, the mass percentage of S is controlled to S ≤ 0.010%.

[0023] P: In the non-quenched and tempered cold-heading steel wire rod described herein, excessive P content can reduce the steel's toughness and plasticity, leading to cold brittleness. Furthermore, P has a strong work-hardening effect on the steel, leading to severe segregation within the steel, which can easily cause wire rod breakage during drawing and cold-heading cracking. Therefore, the steel grade designed in this invention utilizes an extremely low P content. In the non-quenched and tempered cold-heading steel wire rod described herein, the P content is controlled to ≤ 0.015% by weight.

[0024] N: In the non-quenched and tempered cold-heading steel wire rod described herein, N can form large precipitates with V, which is detrimental to improving the steel's strength. Therefore, to ensure the quantity and dispersion of V precipitation, the N content must be strictly controlled. In the non-quenched and tempered cold-heading steel wire rod described herein, the N content is controlled to ≤ 0.005% by weight.

[0025] Furthermore, the microstructure of the non-quenched and tempered cold heading steel wire rod described in the present invention is ferrite+pearlite.

[0026] Furthermore, in the non-quenched and tempered cold heading steel wire rod of the present invention, its tensile strength is 730-780 MPa and its elongation is 25-30%.

[0027] Another object of the present invention is to provide a method for manufacturing non-tempered cold-headed steel wire rod, which optimizes the manufacturing process. The non-tempered cold-headed steel wire rod produced by this manufacturing method has good cold deformation ability. After subsequent drawing and cold heading, non-tempered fasteners that meet the use requirements can be effectively prepared.

[0028] In order to achieve the above object, the present invention provides a method for manufacturing non-quenched and tempered cold heading steel wire rod, which comprises the steps of:

[0029] (1) Smelting and casting;

[0030] (2) Wire rolling;

[0031] (3) Cooling: After rolling, cool to 780-800°C at a cooling rate of 3-4°C / s, and then continue to cool to room temperature at a cooling rate lower than 2°C / s.

[0032] In the present invention, to obtain fine, dispersed vanadium carbide precipitates, the residence time of the wire rod in the high-temperature zone after rolling must be shortened. Therefore, the present invention controls the cooling process to cool the wire rod to 780-800°C at a cooling rate of 3-4°C / s after rolling, and then continue cooling to room temperature at a cooling rate of less than 2°C / s.

[0033] Furthermore, in step (2) of the manufacturing method of the present invention, the steel billet is heated to 1000-1080°C and subjected to high-speed wire rolling, the final rolling temperature is controlled to be 830-920°C, and the high-speed wire drawing temperature is 850-920°C.

[0034] Another object of the present invention is to provide a non-tempered fastener prepared from the above-mentioned non-tempered cold-headed steel wire rod, which has high strength and does not require spheroidizing annealing during processing and eliminates the tempering treatment after cold heading, which can significantly reduce the production cost of making high-strength fasteners.

[0035] In order to achieve the above-mentioned object, the present invention provides a non-quenched and tempered fastener, which is produced by drawing and cold heading the non-quenched and tempered cold heading steel wire rod described in the present invention.

[0036] Furthermore, in the non-quenched and tempered fastener of the present invention, its tensile strength is greater than 900 MPa and its elongation is greater than or equal to 12%.

[0037] The non-quenched and tempered cold heading steel wire rod and the manufacturing method thereof of the present invention have the following advantages and beneficial effects compared with the prior art:

[0038] The non-quenched and tempered cold heading steel wire rod described in the present invention can obtain 9.8 grade non-quenched and tempered cold heading steel wire rod with low strength and good cold deformation performance through reasonable component matching and process design. Its tensile strength is 730-780MPa and elongation is 25-30%.

[0039] The non-quenched and tempered cold heading steel wire rod of the present invention can be processed into non-quenched and tempered fasteners with excellent performance to meet use requirements after pickling, phosphorus saponification, drawing and cold heading processes.

[0040] The non-tempered cold-forging steel wire rod described in the present invention can reduce the loss of processing molds during the process of manufacturing fasteners, and does not require spheroidizing annealing during processing and avoids tempering treatment after cold heading, which can significantly reduce the production cost of manufacturing high-strength fasteners. DETAILED DESCRIPTION

[0041] The non-quenched and tempered cold-headed steel wire rod, non-quenched and tempered fastener, and manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0042] Examples 1-5 and Comparative Example 1

[0043] The non-quenched and tempered cold heading steel wire rods of Examples 1-5 of the present invention and the comparative steel plate of Comparative Example 1 were prepared by the following steps:

[0044] (1) smelting and refining, and casting into continuous casting billets or ingots under non-oxidizing conditions. Table 1 lists the chemical composition ratios of various embodiments and comparative examples;

[0045] (2) Wire rolling: heat the billet to 1000-1080°C and perform high-speed wire rolling. Control the final rolling temperature to 830-920°C and the high-speed wire drawing temperature to 850-920°C (i.e., 1123K-1193K).

[0046] (3) Cooling: After rolling, cool to 780-800°C at a cooling rate of 3-4°C / s, and then continue to cool to room temperature at a cooling rate lower than 2°C / s.

[0047] It should be noted that the chemical composition design and related manufacturing processes used in the non-quenched and tempered cold-headed steel wire rods of Examples 1-5 meet the specification requirements of the present invention. Accordingly, the chemical composition design and related manufacturing processes used in the comparative wire rod of Comparative Example 1 contain process parameters that do not meet the design requirements of the present invention.

[0048] Table 1 lists the mass percentages of the chemical elements in the non-quenched and tempered cold heading steel wire rods of Examples 1-5 and the comparative wire rod of Comparative Example 1.

[0049] Table 1. (wt%, the balance is Fe and other inevitable impurities except S, P, and N)

[0050]

[0051] Accordingly, Table 2 lists the specific process parameters of the non-quenched and tempered cold heading steel wire rods of Examples 1-5 and the comparative wire rod of Comparative Example 1 in the above process steps.

[0052] Table 2.

[0053]

[0054]

[0055] Samples of the non-quenched and tempered cold-headed steel wire rods obtained in Examples 1-5 and the comparative wire rod and comparative steel in Comparative Example 1 were taken, and their microstructures were observed using a metallographic microscope. Through microstructural observation, the inventors found that the microstructures of the non-quenched and tempered cold-headed steel wire rods in Examples 1-5 and the comparative wire rod in Comparative Example 1, produced using the manufacturing method described herein, were all ferrite + pearlite.

[0056] In addition, the non-quenched and tempered cold-headed steel wire rods of Examples 1-5 and the comparative wire rod of Comparative Example 1 were sampled again, and the wire rod samples of each example were subjected to relevant mechanical property tests. The obtained mechanical property test results are listed in Table 3. The relevant mechanical property test methods are as follows:

[0057] Tensile test: At room temperature, according to GBT228-2002 Metal Material Room Temperature Tensile Test Method, the tensile strength and elongation of the non-quenched and tempered cold heading steel wire rods obtained in Examples 1-5 and the comparative wire rod of Comparative Example 1 were tested.

[0058] Table 3 lists the mechanical property test results of the non-quenched and tempered cold heading steel wire rods of Examples 1-5 and the comparative wire rod of Comparative Example 1 obtained after hot rolling.

[0059] Table 3.

[0060]

[0061] As can be seen from Table 3, the non-quenched and tempered cold heading steel wire rods of Examples 1-5 of the present invention have excellent strength and good cold deformation ability (elongation is greater than 25%), their tensile strength is between 733-761 MPa, and their elongation is between 26.5-30%.

[0062] Since the comparative wire rod of Comparative Example 1 does not meet the chemical element composition design and related process design requirements of the present invention, the vanadium carbide precipitation phase in the wire rod is coarsened, the precipitation strengthening effect is weak, and its tensile strength is less than 730 MPa.

[0063] In addition, in order to verify that the non-tempered cold heading steel wire rod prepared by the present invention can be further used to prepare non-tempered fasteners with excellent performance, the non-tempered cold heading steel wire rods of Examples 1-5 prepared by the present invention and the comparative wire rod of Comparative Example 1 can be further subjected to pickling, phosphating, drawing, and cold heading processes to obtain non-tempered fasteners corresponding to Examples 1-5 and Comparative Example 1 and processed into excellent performance.

[0064] Accordingly, mechanical properties tests were conducted on the non-quenched and tempered fasteners corresponding to Examples 1-5 and Comparative Example 1, and the obtained mechanical properties test results are listed in Table 4. The relevant mechanical properties test methods are as follows:

[0065] Tensile test: At room temperature, according to GBT228-2002 Metal Material Room Temperature Tensile Test Method, the tensile strength and elongation of the non-quenched and tempered fasteners obtained in Examples 1-5 and Comparative Example 1 were tested.

[0066] Table 4.

[0067]

[0068] It can be seen from Table 4 above that the non-quenched and tempered fasteners corresponding to Examples 1-5 have tensile strengths greater than 900 MPa and elongations greater than 12%, respectively, and have good mechanical properties.

[0069] The non-quenched and tempered fastener corresponding to Comparative Example 1 does not meet the chemical element composition design and related process design requirements of the present invention, and its tensile strength is only 887 MPa, which does not meet the design requirements of the non-quenched and tempered fastener.

[0070] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0071] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A non-quenched and tempered cold heading steel wire rod containing Fe and unavoidable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.25~0.35%, Si: 0.20~0.40%, Mn: 1.1~1.6%, V: 0.05~0.15%, Al: 0.02~0.04%; The mass percentages of C and V also satisfy lg[V][C]<6.72-9500 / T, where T represents the spinning temperature of the hot-rolled wire rod, in K.

2. The non-quenched and tempered cold heading steel wire rod according to claim 1, characterized in that: The mass percentage of each chemical element is: C: 0.25-0.35%, Si: 0.20-0.40%, Mn: 1.1-1.6%, V: 0.05-0.15%, Al: 0.02-0.04%; the balance is Fe and other inevitable impurities.

3. The non-quenched and tempered cold heading steel wire rod according to claim 1 or 2, characterized in that: Among the inevitable impurities, S≤0.010%, P≤0.015%, and N≤0.005%.

4. The non-quenched and tempered cold heading steel wire rod according to claim 1 or 2, characterized in that: Its microstructure is ferrite + pearlite.

5. The non-quenched and tempered cold heading steel wire rod according to claim 1 or 2, characterized in that: Its tensile strength is 730-780 MPa and its elongation is 25-30%.

6. The method for manufacturing non-quenched and tempered cold heading steel wire rod according to any one of claims 1 to 5, characterized in that: It includes the steps of: (1) Smelting and casting; (2) Wire rolling; (3) Cooling: After rolling, cool to 780-800°C at a cooling rate of 3-4°C / s, and then continue to cool to room temperature at a cooling rate lower than 2°C / s.

7. The manufacturing method according to claim 6, wherein: In step (2), the steel billet is heated to 1000-1080°C and subjected to high-speed wire rolling, the final rolling temperature is controlled to be 830-920°C, and the high-speed wire drawing temperature is controlled to be 850-920°C.

8. A non-quenched and tempered fastener, characterized in that: The steel wire rod is made by drawing and cold heading the non-quenched and tempered cold heading steel wire rod as claimed in any one of claims 1 to 5.

9. The non-quenched and tempered fastener according to claim 8, wherein: Its tensile strength is greater than 900MPa and its elongation is greater than or equal to 12%.

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