A low-cost bolt steel, a production method and a low-cost bolt

Abstract

The invention discloses a low-cost steel for bolts, a production method and a low-cost bolt. The steel for bolts contains C, Si, Mn, Alt, Nb, Cr, Cu, N and Tb elements, wherein the Cl-corrosion resistance index WR value is ≥5.6, the A value is ≥1.25, and the J value is ≤0.020. The structure of the low-cost steel for bolts is ultrafine-grained ferrite+degenerate pearlite, and the inclusions in the steel are all plastic inclusions with a size of less than 7.5 μm. The hot-rolled wire rod of the low-cost steel for bolts according to the invention is subjected to one drawing or two drawing steps, and then is heat-insulated at 200 to 300° C. for 40 to 60 minutes to obtain the low-cost bolt. The method saves heat treatment cost, and the low-cost bolt has a long service life, greatly reduces the manufacturing cost and replacement cost of high-strength fasteners, meets the dual-carbon development needs, and can control the strength level of the low-cost bolt to be above level 8.8 by controlling the total area reduction rate during drawing. The low-cost bolt is suitable for manufacturing high-quality low-cost bolts of level 8.8 and above.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading steel, and in particular relates to a low-cost steel for bolts, a production method and a low-cost bolt. Background Art

[0002] Bolts are mainly produced by cold heading steel. Open-air equipment such as communication towers, bridges, and wind power plants are connected by bolts. However, due to the complex application environment, they are often exposed to H formed by water, oxygen, salt, and acid in the environment. + , O 2- , Cl- plasma penetration, ordinary bolts will be seriously corroded. At present, the main measure is to carry out surface treatment before the bolts leave the factory, but this cannot guarantee the long-term corrosion resistance requirements of the bolts. The bolts will rust in a few months or even a few years. Repeated painting or replacement of bolts is still required, which is costly. It is urgent to develop low-cost cold heading steel materials with corrosion resistance.

[0003] Chinese patent CN101397634B discloses atmospheric corrosion resistant 08CrNiCu low alloy high strength annealing-free cold heading steel and production process, characterized in that the chemical composition ratio is C: 0.05-0.90%, Si: 0.15-0.25%, Mn: 0.55-0.65%, P: 0.005-0.025%, S: 0.005-0.02%, Cr: 0.65-0.75%, Ni: 0.25-0.30%, Cu: 0.35-0.40%, and the rest is Fe and unavoidable impurities; it can be used to process 8.8 grade bolts and matching nuts. The disadvantage is that the steel of the invention adds precious alloy elements such as Ni and Cr, and the cost is relatively high.

[0004] Chinese patent CN108070796A discloses a delayed fracture 1040MPa grade weather-resistant bolt, the mass percentage of which chemical composition is: C: 0.21-0.32, Si: 0.10-0.50, Mn: 0.60-1.00, P: 0.008-0.020, S: ≤0.005, Cr: 0.82-1.20, Ni: 0.25-0.50, Cu: 0.25-0.50, Mo: 0.05-0.20, Nb: 0.015-0 .060, V: 0.015-0.090, Ti: 0.008-0.035, B: 0.0008-0.0035, Al: 0.015-0.040, Ca: 0.003-0.007, Zr: 0.015-0.045, Re: 0.010-0.045, the balance is Fe and unavoidable impurities; the preparation method of the above bolts mainly adopts conventional high-purity smelting-continuous casting-rolling technology, and the bolts produced are both resistant to delayed fracture and atmospheric corrosion. However, the addition of Ti and Zr elements in this method will form nitride inclusions, which is harmful to the toughness of the material, and the alloy addition in the steel is relatively large, and the strength level is also above 1000MPa.

[0005] At present, there are many studies on corrosion-resistant construction steels such as plates and steel sections in my country, and the products are relatively mature, but there are few studies on corrosion-resistant cold heading steels. With the complexity of the application environment of fasteners, it is urgent to develop low-cost cold heading steel materials with corrosion resistance. Summary of the invention

[0006] The object of the present invention is to provide a low-cost bolt steel and a production method thereof. The metallographic structure of the low-cost bolt steel is ultrafine-grained ferrite + degenerate pearlite, and the ferrite grain size is below 7.5 μm. Compared with traditional Cr-Ni-Cu corrosion-resistant steel, the alloy cost is low, and the corrosion resistance can reach more than twice that of the 8.8 grade bolt steel commonly used in the market.

[0007] Another object of the present invention is to provide a low-cost bolt, which is obtained by drawing the hot-rolled wire rod of the low-cost bolt steel in the present invention in one or two steps and then keeping it at 200-300°C for 40-60 minutes, thereby saving the heat treatment cost and having a long service life, greatly reducing the manufacturing cost and replacement cost of high-strength fasteners. By controlling the total area reduction rate during drawing, the strength level of the low-cost bolt can be controlled to be above level 8.8, which is suitable for making high-quality low-cost bolts of level 8.8 and above.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A low-cost bolt steel, comprising, by weight percentage: C 0.10%-0.20%, Si 0.35%-0.55%, Mn 1.00%-1.20%, Alt 0.020%-0.030%, Nb 0.010%-0.020%, Cr 1.10%-1.40%, Cu 0.10%-0.20%, N 0.0060%-0.0080%, Tb 0.0050%-0.0080%, Ni≤0.10%, P≤0.015%, S≤0.015%, O≤0.0015%, and the remainder is Fe and other inevitable impurities;

[0010] In order to ensure that the steel has sufficient corrosion resistance, the Cl-corrosion resistance index WR value of the low-cost bolt steel is ≥5.6.

[0011] WR=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 +54.2(%Tb);

[0012] The steel of the present invention eliminates the traditional annealing and tempering steps when processing bolts, and only requires aging treatment. In order to ensure that the steel has excellent comprehensive strength and plasticity, the ferrite grain size needs to be controlled below 7.5 μm, so the J value needs to be controlled to be ≤0.020, J=((%Nb) / 41) / ((%C) / 6+(%N) / 7);

[0013] In order to ensure that the steel of the present invention has a high purity, it is necessary to control the A value to be ≥ 1.25, A = (% Tb) / ((% S) + (% O)).

[0014] The metallographic structure of the low-cost hot-rolled steel wire rod for bolts is ultrafine-grained ferrite+degenerate pearlite, and the ferrite grain size is below 7.5 μm.

[0015] The low-cost hot-rolled steel wire rod for bolts m 680~730MPa, A≥43%, Z≥70%, one-fifth of the cold forging is qualified.

[0016] After the low-cost hot-rolled steel wire rod for bolts is drawn through one or two drawing passes, the strength level reaches level 8.8 at a surface reduction rate of 25% to 30%, with Rm≥800MPa, Rp0.2≥640MPa, A≥12%, Z≥50%, and yield strength ratio≥0.8.

[0017] After one or two drawing passes, the low-cost hot-rolled wire rod of bolt steel reaches a strength level of 9.8 when the area reduction rate is 30% to 37% but not including 30%, Rm≥900MPa, Rp0.2≥720MPa, A≥10%, Z≥48%, and the yield strength ratio is ≥0.8.

[0018] The production method of the low-cost bolt steel comprises the following steps: molten iron pretreatment → electric furnace or converter smelting → LF furnace refining → RH refining → billet continuous casting → billet heating → high-speed wire rolling → Stelmor cooling line cooling → Φ5.5-50mm wire rod finished product.

[0019] In the electric furnace or converter smelting step, the electric furnace or converter end point C≤0.04%, P≤0.006%; slag blocking is used for tapping, the amount of slag is strictly controlled, the slag layer thickness of the ladle is ≤100mm, and the tapping time is ≥5min.

[0020] In the LF furnace refining step, the LF refining time is ≥30 min, and the white slag holding time is ≥20 min, so as to ensure sufficient deoxidation, desulfurization and dephosphorization.

[0021] In the RH refining step, the RH vacuum control time is ≥20 minutes, and Tb blocks are added 10 minutes to 12 minutes before breaking the vacuum. If it is earlier than this time, Tb is easily over-oxidized, resulting in a low yield. If it is later than this time, the Tb element cannot fully react in the molten steel, and the inclusions are insufficiently plasticized.

[0022] In the billet heating step, the soaking temperature of the heating furnace is controlled at 1040° C. to 1100° C. Above this temperature, decarburization and coarsening of grains are likely to occur on the billet surface, and below this temperature, Nb cannot be fully dissolved in austenite.

[0023] In the high-speed wire rolling step, ultra-low temperature rolling is adopted, the finishing rolling temperature is controlled at 760°C to 790°C, and the sizing temperature is controlled at 770°C to 800°C. In this optimal rolling temperature range, low-temperature large deformation plastic deformation and deformation-induced ferrite phase transformation are carried out to promote the precipitation of proeutectoid ferrite and increase the ferrite content. At the same time, the pinning effect of Nb carbonitrides precipitated at the grain boundaries is used to refine the ferrite grains; the spinning temperature is controlled at 760°C to 800°C, and the roller reference speed is controlled at 0.12m / s to 0.15m / s. At this spinning temperature and roller reference speed, neither Mapei hard phase structure is produced nor degenerate pearlite structure can be obtained. The final hot-rolled wire rod structure is ultrafine-grained ferrite + degenerate pearlite structure, which has the characteristics of high strength and good plasticity and toughness. It can be directly subjected to large deformation drawing and cold heading without annealing, and the strength level of 8.8 or above can be achieved through work hardening effect.

[0024] The present invention also provides a low-cost bolt, which is obtained by subjecting the hot-rolled wire rod of the low-cost bolt steel of the present invention to one or two drawing steps, and then keeping it at 200-300°C for 40-60 minutes, wherein the total area reduction rate during drawing is controlled at 25%-37%, and when the area reduction rate is 25%-30%, the strength level of the bolt can reach level 8.8, Rm≥800MPa, Rp0.2≥640MPa, A≥12%, Z≥50%, and the yield strength ratio is ≥0.8; when the area reduction rate is 30%-37% but not including 30%, the strength level of the bolt reaches level 9.8, Rm≥900MPa, Rp0.2≥720MPa, A≥10%, Z≥48%, and the yield strength ratio is ≥0.8. When the reduction rate is lower than 25%, the strength level of the bolt is less than 8.8. When the reduction rate is higher than 37%, it will lead to greater loss of the drawing die. After drawing into fine wire, it is directly cold headed into bolts. The finished bolts need to be stabilized and kept at 200℃~300℃ for 40~60min. If the temperature is lower than this range or the time is too short, the residual stress cannot be completely eliminated. If the temperature is higher than this range or the time is too long, the cost will increase and the strength will decrease.

[0025] In the low-cost bolt steel provided by the present invention, the functions and controls of various components are as follows:

[0026] C: C is the most basic and effective strengthening element in steel. In addition, it forms carbonitrides with Nb and other elements in the steel of the present invention, which plays a role in fine grain strengthening and dispersion strengthening. However, as its content increases, ductility decreases. The C content is controlled at 0.10% to 0.20%.

[0027] Si: Si can accelerate the precipitation of proeutectoid ferrite during steel rolling, which promotes grain refinement. In addition, Si is mainly enriched on the steel surface, which also improves the stability of the rust layer and improves corrosion resistance. However, the increase of Si element will increase the diffusion of carbon in steel and aggravate the decarburization of steel. The Si content is controlled at 0.35% to 0.55%.

[0028] Mn: Mn and Fe form a solid solution, which improves the hardness and strength of ferrite and austenite in steel. Manganese in steel reduces the critical transformation temperature and plays a role in refining pearlite. However, excessive Mn will reduce the plasticity of steel. Excessive Mn will cause the growth of corrosion product particles and increase the corrosion rate. The Mn content is controlled at 1.00% to 1.20%.

[0029] Al: Al is a strong deoxidizing element and improves the oxidation resistance of steel. However, as the Al content increases, the amount of coarse carbonitride inclusions increases. The Al content is controlled at 0.020% to 0.030%.

[0030] Nb, N: Nb and carbon and nitrogen in steel form carbonitrides, which have the effects of grain refinement and dispersion strengthening. The steel of the present invention utilizes the atomic ratio of Nb and N and C to ensure that the J value is less than or equal to 0.020, J=((%Nb) / 41) / ((%C) / 6+(%N) / 7), and ensures that a sufficient amount of Nb carbonitride is obtained. When the J value exceeds 0.020, the carbonitride is coarse and becomes a brittle inclusion to deteriorate the plasticity of the steel. Then, in combination with a suitable final rolling temperature, the steel utilizes a large reduction in the austenite region for severe thermal deformation, controls the final rolling temperature to be within a temperature range of 20 to 60° C. above the AC3 point to cause a pro-eutectoid ferrite phase transformation, combines deformation-induced ferrite phase transformation, and the pinning effect of Nb carbonitride on the grain boundary, obtains a microstructure dominated by ultrafine ferrite+degenerate pearlite microstructure, and a ferrite grain size of less than 7.5 μm, thereby obtaining sufficient comprehensive strength and plasticity, and the hot-rolled wire rod can directly obtain a strength of more than 800 MPa through large deformation drawing hardening without annealing. The Nb content is controlled within the range of 0.010% to 0.020%, and the N content is controlled within the range of 0.0060% to 0.0080%.

[0031] Cr: Cr element significantly improves toughness and strength in steel. Cr can form a dense oxide film on the surface of steel, improve the passivation ability of steel, and improve the weather resistance of steel. However, excessive Cr increases the brittleness tendency of steel. The Cr content is controlled at 1.10% to 1.40%.

[0032] Cu: The prominent role of Cu in steel is to improve the corrosion resistance of steel. The cathode contact between steel and the Cu secondary precipitation on the surface can promote the anodization of steel and form a rust layer with good protection. The copper element can also change the hygroscopicity of the rust layer, thereby increasing the critical humidity. However, Cu produces high crack sensitivity in steel. The Cu content is controlled at 0.10% to 0.20%.

[0033] Tb: Tb is also a key element of the steel of the present invention. Large-sized inclusions in the steel become defect points, which will reduce the toughness and service life of the steel. In corrosion-resistant steel, pitting corrosion is also likely to occur, accelerating the corrosion rate. The steel of the present invention adds Tb to modify the inclusions in the steel, while promoting the dispersion and distribution of fine spherical inclusions, thereby improving the purity of the molten steel. The steel of the present invention also controls the A value ≥ 1.25, A = (% Tb) / ((% S) + (% O)), ensuring that all inclusions are fully plasticized and the size is below 8.0 μm, thereby improving the comprehensive performance of the steel and the service life of the bolts. In addition, Tb is enriched in the rust layer of the steel, accelerating the cathode reduction of the rust layer, inhibiting the anodic dissolution, and significantly improving its corrosion resistance. The steel of the present invention reduces the content of precious metals Ni and Cu in traditional Cr-Ni-Cu corrosion-resistant steel by adding Tb element, effectively saving alloy cost. In order to make the corrosion resistance of the steel of the present invention reach more than twice that of ordinary 8.8 grade bolt steel, the chemical composition ratio needs to ensure that the corrosion resistance index WR value is ≥5.6, WR=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 +54.2 (% Tb), through the combined effect of various corrosion-resistant elements, the density and stability of the rust layer are improved, the self-corrosion potential and ion exchange resistance of the steel are increased, and the corrosion current is reduced. If it is lower than this value, the corrosion resistance of the steel of the present invention cannot be achieved.

[0034] Ni: The corrosion resistance of Ni is similar to that of Cr. However, Ni is a precious metal element, and considering the cost, the Ni content is controlled to be ≤0.10%.

[0035] S and P: Sulfur easily forms MnS inclusions with manganese in steel, which is harmful to the processing properties of steel; P is an effective corrosion-resistant element, but it is not suitable to be added to bolt steel because P is an element with a strong segregation tendency, which will cause cold brittleness and lead to cold heading cracking. Control P≤0.015%, S≤0.015%.

[0036] O: O forms oxide inclusions in steel, which damages the processing performance and service life of the steel. Control O≤0.0015%.

[0037] The low-cost bolt steel provided by the present invention is obtained by accurately controlling the content of each component, the Cl-corrosion resistance index WR value, the J value, and the A value to obtain a bolt steel with corrosion resistance, high strength and toughness, and a long service life. The inclusions in the bolt steel are all plastic inclusions with a size of less than 8.0 μm, one-fifth of which are qualified for cold heading forging, and have excellent drawing and cold heading properties. After the bolt is made into a finished product, the mechanical properties of the bolt can reach 8.8 to 9.8 levels.

[0038] In the production method of low-cost bolt steel provided by the present invention, a process route of molten iron pretreatment → electric furnace or converter smelting → LF furnace refining → RH refining → billet continuous casting → billet heating → high-speed wire rolling → Stelmor cooling line cooling → Φ5.5-50mm wire rod finished product is adopted to produce low-cost bolt steel with a metallographic structure of ultrafine ferrite + degenerate pearlite. After the low-cost bolt steel is made into a finished bolt product, the mechanical properties of the bolt can reach 8.8-9.8 levels. In addition, due to the ultrafine structure and small-sized plastic inclusions, the bolt has a high fatigue life, and the number of axial fatigue cycles under a load of 100KN is greater than 2 million times.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. In order to ensure that the steel has sufficient corrosion resistance, the present invention controls the Cl-corrosion resistance index WR value to be ≥5.6, WR=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu)2+54.2(%Tb). Through the composite effect of various corrosion-resistant elements, the density and stability of the rust layer are improved, the self-corrosion potential and ion exchange resistance of the steel are improved, and the corrosion current is reduced. The corrosion resistance is more than twice that of ordinary 8.8 grade bolt steel;

[0041] 2. The steel of the present invention modifies the inclusions in the steel by adding the Tb element, and at the same time promotes the dispersion distribution of fine spherical inclusions, thereby improving the purity of the molten steel. It also controls the A value ≥ 1.25, A = (%Tb) / ((%S)+(%O)), to ensure that all inclusions are fully plasticized and the size is below 8.0μm, thereby improving the comprehensive performance of the steel and the service life of the bolts. In addition, Tb is enriched in the rust layer of the steel, accelerating the cathode reduction of the rust layer, inhibiting the anodic dissolution, and significantly improving its corrosion resistance. The steel of the present invention reduces the content of precious metals Ni and Cu in traditional Cr-Ni-Cu corrosion-resistant steels by adding the Tb element, effectively saving alloy costs;

[0042] 3. The steel of the present invention utilizes the atomic ratio of Nb and N, C to ensure that the J value is ≤ 0.020, J = ((%Nb) / 41) / ((%C) / 6+(%N) / 7), to ensure that a sufficient amount of Nb carbonitride is obtained, and then combined with a suitable final rolling temperature, a large reduction in the austenite region is used for severe thermal deformation, and the final rolling temperature is controlled to be above the AC3 point in the temperature range of 20 to 60°C to cause a pro-eutectoid ferrite phase transformation, combined with deformation-induced ferrite phase transformation and the pinning effect of Nb carbonitride on the grain boundary, the obtained structure is mainly ultrafine ferrite + degenerate pearlite structure, and the ferrite grain size is below 7.5μm, thereby obtaining sufficient comprehensive strength and plasticity, and the hot-rolled wire rod can directly obtain a strength of more than 800MPa through large deformation drawing hardening without annealing;

[0043] 4. Compared with the traditional Cr-Ni-Cu corrosion-resistant steel, the steel of the present invention has low alloy cost and corrosion resistance that is more than twice that of the 8.8-grade bolt steel commonly used in the market; in addition, the steel of the present invention does not need to undergo annealing and tempering heat treatment when processed at the user's site, and can be directly drawn and cold headed to reach a level above 8.8, and has a high fatigue life. The steel of the present invention saves alloy cost and subsequent heat treatment cost at the same time, and has a long service life, greatly reducing the comprehensive cost of high-strength fasteners. It is suitable for the production of high-quality and low-cost bolts of 8.8 and above, and can also be used for the production of other fasteners.

[0044] 5. The metallographic structure of the low-cost bolt steel provided by the present invention is ultrafine ferrite + degenerate pearlite, the ferrite grain size is below 7.5μm, the inclusions in the steel are all plastic inclusions, the size is below 8.0μm, and the mechanical properties of the steel are: R m 680~730MPa, A≥43%, Z≥70%, one-fifth cold forging qualified, with excellent drawing and cold heading performance, compared with the same level of cold heading steel SWRCH35K, NaHSO3 weekly immersion test relative corrosion rate ≤50%. After the bolt is made into a finished product, the mechanical properties of the bolt can reach 8.8~9.8. In addition, due to the ultra-fine structure and small-sized plastic inclusions, the bolt has a high fatigue life, and the number of axial fatigue cycles under 100KN load is >2 million times. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is the metallographic structure diagram of the bolt steel in Example 1;

[0046] Figure 2 is the metallographic structure diagram of the bolt steel in Comparative Example 1;

[0047] Figure 3 is the metallographic structure diagram of the bolt steel in Comparative Example 6;

[0048] Figure 4Schematic diagram of plastic inclusions in the bolt steel in Example 2;

[0049] Figure 5 Schematic diagram of plastic inclusions in the bolt steel in Comparative Example 5. DETAILED DESCRIPTION

[0050] The invention provides a low-cost bolt steel, which contains, by weight percentage, C 0.10%-0.20%, Si 0.35%-0.55%, Mn 1.00%-1.20%, Alt 0.020%-0.030%, Nb 0.010%-0.020%, Cr 1.10%-1.40%, Cu 0.10%-0.20%, N 0.0060%-0.0080%, Tb 0.0050%-0.0080%, Ni≤0.10%, P≤0.015%, S≤0.015%, O≤0.0015%, and the rest is Fe and other inevitable impurities;

[0051] The Cl-corrosion resistance index WR value of the low-cost bolt steel is ≥5.6.

[0052] WR=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 +54.2(%Tb);

[0053] J value ≤ 0.020, J = ((%Nb) / 41) / ((%C) / 6+(%N) / 7);

[0054] A value ≥ 1.25, A=(%Tb) / ((%S)+(%O)).

[0055] The production method of the low-cost bolt steel comprises the following steps: molten iron pretreatment → electric furnace or converter smelting → LF furnace refining → RH refining → billet continuous casting → billet heating → high-speed wire rolling → Stelmor cooling line cooling → Φ5.5-50 mm wire rod finished product.

[0056] In the electric furnace or converter smelting step, the electric furnace or converter end point C≤0.04%, P≤0.006%; slag blocking is used for tapping, the amount of slag is strictly controlled, the slag layer thickness of the ladle is ≤100mm, and the tapping time is ≥5min.

[0057] In the LF furnace refining step, the LF refining time is ≥30 min, and the white slag holding time is ≥20 min, so as to ensure sufficient deoxidation, desulfurization and dephosphorization.

[0058] In the RH refining step, the RH vacuum control time is ≥20 minutes, and 10 minutes to 12 minutes before breaking the vacuum.

[0059] In the billet heating step, the soaking temperature of the heating furnace is controlled at 1040°C to 1100°C.

[0060] In the high-speed wire rolling step, ultra-low temperature rolling is adopted, the finishing temperature is controlled at 760°C to 790°C, the sizing temperature is controlled at 770°C to 800°C, the spinning temperature is controlled at 760°C to 800°C, and the roller reference speed is controlled at 0.12m / s to 0.15m / s.

[0061] The present invention is described in detail below with reference to the embodiments.

[0062] The chemical composition and weight percentage of the bolt steel in each embodiment and comparative example are shown in Table 1.

[0063] Table 1

[0064]

[0065]

[0066] The production process route of the hot-rolled bolt steel wire rod in each embodiment and comparative example is: molten iron pretreatment → electric furnace or converter smelting → LF furnace refining → RH refining → billet continuous casting → billet heating → high-speed wire rolling → Stelmor cooling line cooling → Φ5.5~50mm wire rod finished product.

[0067] The production process parameters of the hot-rolled bolt steel wire rods in each embodiment and comparative example are shown in Table 2.

[0068] Table 2

[0069]

[0070]

[0071] A sample with a length of 15 mm was taken from the bolt steel wire rod in the embodiment and the comparative example, the cross section was polished, etched with 4% nitric acid alcohol, and the structure was evaluated according to GB / T 13298 "Methods for Examination of Metal Microstructures".

[0072] Inclusion size measurement: The inclusion size on the longitudinal surface of steel was measured and statistically calculated using an ASPEX scanning electron microscope.

[0073] The metallographic structure, inclusion size, and cold heading condition of the bolt steel wire rod in each embodiment and comparative example are shown in Table 3.

[0074] Samples were taken from the bolt steel wire rods in each embodiment and comparative example to perform tensile properties, KV2 impact test, notch sensitivity, delayed fracture resistance, and fatigue test tests. The specific test process is as follows:

[0075] Tensile properties: Refer to GB / T 228.1 "Metallic materials tensile test Part 1: Room temperature test method", process standard tensile specimens, conduct tensile tests, and test R m , A, Z values.

[0076] Cold forging: The wire rod is cold forged according to the following requirements: X = h1 / h = 1 / 5; (where: h is the height of the sample before cold forging (twice the wire rod diameter); h1 is the height of the sample after cold forging.) After the cold forging test, the surface of the sample shall not have visible cracks, cracks, cracks and hairline defects. A group of 10 samples, all qualified are considered qualified, otherwise the qualified rate is calculated.

[0077] Samples were taken from the square billets during the production of bolt steel in each embodiment and comparative example, and the samples were processed according to TB / T 2375-1993 "Cyclic Immersion Corrosion Test Method for Corrosion Resistant Steel for Railways", and the 100-hour immersion test in NaHSO3 solution was completed, and the corrosion weight loss rate was calculated. Each number was 10 groups, and the average value was calculated. The corrosion weight loss rate (W) was calculated as follows:

[0078]

[0079] Where: W——weight loss rate, g / (m 2 ·h); G0——original weight of the sample, g; G1——weight of the sample after the test, g; a——length of the sample, mm; b——width of the sample, mm; c——thickness of the sample, mm; t——test time, h.

[0080] The above test results are shown in Table 3.

[0081] Table 3

[0082]

[0083]

[0084] The bolt steel wire rods in the above-mentioned embodiments and comparative examples are processed into bolts, and the specific process is as follows: the hot-rolled wire rods are drawn through one or two drawing steps, and then kept at 200-300° C. for 40-60 min, and the total area reduction rate during drawing is controlled at 25%-37%. The specific process parameters for processing the bolts in the specific embodiments and comparative examples are shown in Table 4, and the properties of the bolts are shown in Table 5.

[0085] Table 4

[0086]

[0087] The bolt performance testing method in Table 4 above is as follows:

[0088] Mechanical properties: Finished bolts are tested for mechanical properties in accordance with GB-T 3098.1 "Mechanical properties of fasteners - bolts, screws and studs" to determine R m , Rp0.2, A, Z values ​​and calculate the yield strength ratio.

[0089] Fatigue test: Axial fatigue test is carried out on MTS fatigue testing machine, using axial stress control, strain cycle characteristic of R=-1, controlled load of 100KN, frequency of 10Hz, 20 bolts as a group, and the test is considered successful when the number of cycles of 80% of the samples exceeds 2 million times.

[0090] Table 5

[0091]

[0092] From the above content, it can be seen that the chemical composition of the steel, the production method and the bolt processing method of Examples 1 to 9 are properly controlled, the hot-rolled wire rods obtain ultrafine grain structure, high comprehensive strength and plasticity, high purity, excellent cold heading performance and drawing performance, and the bolt strength directly produced without annealing and quenching and tempering treatment reaches 8.8 to 9.8, and the fatigue life is high; in addition, the steel has high corrosion resistance, and the corrosion resistance is more than twice that of the 8.8-grade cold heading steel SWRCH35K (i.e., comparative example 6) commonly used in the market. The embodiment optimizes the design on the basis of traditional Cr-Ni-Cu corrosion-resistant steel, reduces the alloy cost of corrosion-resistant steel, and realizes ultrafine grains through the combination of component design and controlled rolling and controlled cooling process, thereby eliminating the special processing steps in the usual fastener processing, and further improves the fatigue life of the fastener through inclusion plasticization treatment, thereby achieving low cost and high life of the fastener.

[0093] Although the chemical composition of Comparative Example 1 is within the required range, the production method is not properly controlled. First, due to the late addition of Tb element, the plasticization of inclusions is not sufficient, and the maximum inclusion size in the steel reaches more than 10μm. In addition, the rolling process is not properly controlled, resulting in a large amount of bainite in the hot-rolled wire rod, which seriously deteriorates the plasticity of the steel, and the final elongation and surface reduction of the finished bolt are not in compliance; the bolt processing technology of Comparative Example 2 is improper, the drawing surface reduction is low, and the strength level of the finished bolt is lower than 8.8; the corrosion resistance WR value of Comparative Example 3 is lower than 5.6, and the corrosion resistance is poor; the fineness of Comparative Example 4 is too high .... The crystal index J value does not meet the requirements, and there are not enough fine dispersed niobium carbonitrides precipitated, the grain size is lower than that of the embodiment, which deteriorates the toughness and plasticity of the steel, and finally the plasticity and fatigue life of the bolt are poor; Comparative Example 5 is an example in which the purity index A value does not meet the requirements, resulting in a large inclusion size in the steel and a low fatigue life of the bolt; Comparative Example 6 is a common 8.8-grade cold heading steel SWRCH35K on the market, whose average grain size and maximum inclusion size are both large. Although the mechanical properties after drawing can barely reach 8.8 grade, the performance margin is small, and it has no corrosion resistance and a low fatigue life.

[0094] The detailed description of a low-cost bolt steel, a production method and a low-cost bolt with reference to the above embodiments is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A low-cost bolt steel, characterized in that: Contains by weight: C 0.10%~0.20%, Si0.35%~0.55%, Mn 1.00%~1.20%, Alt 0.020%~0.030%, Nb 0.010%~0.020%, Cr 1.10%~1.40%, Cu 0.10%~0.20%, N 0.0060%~0.0080%, Tb 0.0050%~0.0080%, Ni≤0.10%, P≤0.015%, S≤0.015%, O≤0.0015%, the rest is Fe and other inevitable impurities; The Cl resistance of the low-cost bolt steel - Corrosion index WR value ≥ 5.6, WR = 26.01 (%Cu) + 3.88 (%Ni) + 1.20 (%Cr) + 1.49 (%Si) + 17.28 (%P) - 7.29 (%Cu) (%Ni) - 9.10 (%Ni) (%P) - 33.39 (%Cu) 2 +54.2(%Tb); J value ≤ 0.020, J = ((%Nb) / 41) / ((%C) / 6+(%N) / 7); A value ≥ 1.25, A=(%Tb) / ((%S)+(%O)); The metallographic structure of the low-cost hot-rolled steel wire rod for bolts is ultrafine-grained ferrite+degenerate pearlite, and the ferrite grain size is below 7.5 μm.

2. The low-cost bolt steel according to claim 1, characterized in that: The low-cost hot-rolled steel wire rod for bolts m 680~730MPa, A≥43%, Z≥70%, one-fifth of the cold forging is qualified.

3. The low-cost bolt steel according to claim 1, characterized in that: After the low-cost hot-rolled wire rod of bolt steel is drawn in one or two passes, the strength level reaches 8.8 at a surface reduction rate of 25% to 30%, with Rm≥800MPa, Rp0.2≥640MPa, A≥12%, Z≥50%, and yield strength ratio≥0.

8.

4. The low-cost bolt steel according to claim 2 or 3, characterized in that: After the low-cost hot-rolled wire rod of bolt steel is drawn in one or two passes, the strength level reaches 9.8 when the area reduction rate is 30% to 37% but not including 30%, Rm≥900MPa, Rp0.2≥720MPa, A≥10%, Z≥48%, and the yield strength ratio is ≥0.

8.

5. The method for producing low-cost bolt steel according to any one of claims 1 to 4, characterized in that: The production method comprises the following steps: molten iron pretreatment→electric furnace or converter smelting→LF furnace refining→RH refining→square billet continuous casting→square billet heating→high-speed wire rolling→Stelmor cooling line cooling→Φ5.5~50mm wire rod finished product.

6. The method for producing low-cost bolt steel according to claim 5, characterized in that: In the RH refining step, the RH vacuum control time is ≥20 min, and Tb blocks are added 10 min to 12 min before breaking the vacuum.

7. The method for producing low-cost bolt steel according to claim 5, characterized in that: In the billet heating step, the soaking temperature of the heating furnace is controlled at 1040°C to 1100°C.

8. The method for producing low-cost bolt steel according to claim 5, characterized in that: In the high-speed wire rolling step, ultra-low temperature rolling is adopted, the finishing rolling temperature is controlled at 760°C-790°C, the sizing temperature is controlled at 770°C-800°C, the spinning temperature is controlled at 760°C-800°C, and the roller reference speed is controlled at 0.12m / s-0.15m / s.

9. A low-cost bolt, characterized in that: The hot-rolled wire rod of the low-cost bolt steel according to any one of claims 1 to 4 is obtained by one drawing or two drawing steps and then heat-insulated at 200-300° C. for 40-60 minutes, and the total area reduction rate during drawing is controlled at 25%-37%.

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