A duplex stainless steel wire rod with high surface quality for cold heading and its manufacturing method

Abstract

A duplex stainless steel wire rod with high surface quality for cold heading and its manufacturing method. The component mass percentages of the duplex stainless steel wire rod are as follows: C ≤ 0.030%, Si 0.40 - 0.60%, Mn 4.05 - 4.40%, P ≤ 0.040%, S ≤ 0.0015%, Cr 20.60 - 21.10%, Ni 1.00 - 2.00%, Mo ≤ 0.60%, V ≤ 0.13%, Cu ≤ 0.50%, N 0.10 - 0.17%, Co ≤ 0.15%, Ti ≤ 0.030%, Nb ≤ 0.030%, Al ≤ 0.10%, and the balance is Fe and other inevitable impurities; and it satisfies: Cr / Ni equivalent ratio: 2.6 - 2.9. The tensile strength of the duplex stainless steel wire rod described in the present invention is ≥ 620 MPa, the elongation is ≥ 25%, the reduction of area is ≥ 50%, and the content ratio of the ferrite phase in the structure is 40 - 60%.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel production, and in particular relates to a duplex stainless steel wire rod with high surface quality for cold heading and a manufacturing method thereof. Background Art

[0002] In recent years, the use of duplex stainless steel has increased year by year. At present, the manufacturing process of duplex stainless steel wire rods produced by domestic manufacturers is generally: electric arc furnace - AOD furnace - LF furnace - continuous casting - hot rolling - offline solution. The duplex stainless steel wire rods produced by this manufacturing process have defects such as warping and cracks on the wire rod surface caused by rolling cracking due to the imbalance of the two-phase ratio and the uncoordinated deformation during the production process; in the use process of downstream customers, there are cold heading cracking problems caused by the large proportion of γ phase (austenite phase) and the difference in γ phase morphology.

[0003] In the production process of duplex stainless steel wire, due to the high γ phase content at the edge of the ingot, the ingot is subjected to uneven stress during rolling, resulting in defects such as warping and cracks in the finished product caused by rolling cracking. Therefore, regulating the structure of the ingot has become an indispensable link.

[0004] The downstream use of duplex stainless steel wire products is mainly cold heading fastener bolts. There are two common problems in the downstream use process: First, due to the large proportion of γ phase, the wire rod has high hardness, high yield strength, high forming difficulty, and high mold loss; second, due to the difference in γ phase morphology, the impact work of the wire rod decreases, and it is easy to crack after cold heading. Both problems of duplex stainless steel wire rod in the downstream cold heading process are related to the tissue content and morphology. Therefore, regulating the solid solution state wire rod structure of duplex stainless steel has become an indispensable link.

[0005] Chinese patent CN114932146A discloses "a method for rolling super duplex stainless steel wire", the round billet size of which is φ160mm. Compared with square billets, round billets do not have the problem of excessive temperature drop at the corners. The quality rate of the factory's duplex stainless steel wire has increased from 0% to 80%.

[0006] Chinese Patent CN105821346A discloses "An economic duplex stainless steel wire rod and its manufacturing method". In this patent, the chemical composition of the duplex stainless steel in mass percentage is: C: 0.01 - 0.05%, Si: 0.1 - 1.0%, Mn: 2.5 - 4.0%, Ni: 1.5 - 2.5%, Cr: 20.5 - 22.5%, Mo: 0.2 - 0.8%, N: 0.12 - 0.18%, W: 0.05 - 0.30%, P ≤ 0.035%, S ≤ 0.015%, and the balance is Fe and inevitable impurities; after the batching for the manufacturing method of the stainless steel is completed, smelting, continuous casting, rolling, solution treatment, and pickling are carried out in sequence. The stainless steel wire rod can be used for wire rod applications such as processing and manufacturing stainless steel springs, partially replacing the uses of 304 wire rod stainless steel.

[0007] Chinese Patent CN115725811A discloses "A production method of a second-generation duplex stainless steel wire rod". In this patent, the chemical composition of the duplex stainless steel in mass percentage is: C ≤ 0.03%, Mn: 1.30 - 1.60%, Si: 0.045 - 0.065%, P ≤ 0.030%, Cr: 22.30 - 22.60%, Ni: 4.55 - 4.75%, Mo: 3.02 - 3.10%, and the balance is Fe and inevitable impurities; the continuous casting billet is heated in a heating furnace and then sent to a rolling mill to be rolled into wire rods with a specification of φ5.5 - 23 mm.

[0008] Chinese Patent CN114904914A discloses "A rolling method of duplex stainless steel wire rods". The rolling method includes continuous casting billet preparation, heating, blooming, intermediate billet heating, high-speed wire rolling, and cooling, and further includes intermediate billet grinding. This invention optimizes the blooming heating temperature and the reduction in the blooming process, optimizes the intermediate billet heating temperature and the temperature in the high-speed wire rolling process, and is equipped with an induction heating device for online temperature increase during the rolling process, solving the problems of cracking during the rolling of duplex stainless steel, surface peeling and cracks on the finished product, and having a uniform structure without precipitation.

[0009] The above patents related to duplex stainless steel wire rods mainly describe the composition design, shape change of the casting billet, continuous casting process, hot rolling process, heat treatment process, uses of the wire rod, and quality improvement of duplex stainless steel wire rods, and do not involve a duplex stainless steel wire rod with high surface quality for cold heading uses and its manufacturing method. Summary of the Invention

[0010] The object of the present invention is to provide a duplex stainless steel wire rod with high surface quality for cold heading and its manufacturing method. The tensile strength of the duplex stainless steel wire rod is ≥620 MPa, the elongation is ≥25%, the reduction of area is ≥50%, and the ferrite phase content is in the proportion of 40% - 60%, avoiding cold heading cracking after downstream cold heading processing; there is no cracking during the rolling process of the duplex stainless steel wire rod, and there are no defects such as scale off and cracks on the surface of the wire rod, with good surface quality; and the production cost is relatively low, the forming performance of the customer during downstream cold heading processing is good, and the product meets the standard performance requirements of fasteners.

[0011] To achieve the above object, the technical solution of the present invention is:

[0012] A duplex stainless steel wire rod with high surface quality for cold heading, the mass percentage of its components is: C ≤ 0.030%, Si: 0.40% - 0.60%, Mn: 4.05 - 4.40%, P ≤ 0.040%, S ≤ 0.0015%, Cr: 20.60 - 21.10%, Ni: 1.00 - 2.00%, Mo ≤ 0.60%, V ≤ 0.13%, Cu ≤ 0.50%, N: 0.10 - 0.17%, Co ≤ 0.15%, Ti ≤ 0.030%, Nb ≤ 0.030%, Al ≤ 0.10%, and the balance is Fe and other inevitable impurities; and at the same time satisfy:

[0013] Cr / Ni equivalent ratio: 2.6 - 2.9;

[0014] Cr equivalent = Cr + 1.5Mo + 2Si + 1.5Ti + 5.5Al + 5V;

[0015] Ni equivalent = Ni + Co + 0.5Mn + 30C + 25N + 0.3Cu;

[0016] The microstructure of the duplex stainless steel wire rod is austenite + ferrite, and the ferrite phase content accounts for 40 - 60%. The austenite microstructure is round and close to island-shaped.

[0017] The tensile strength of the duplex stainless steel wire rod is ≥620 MPa, the elongation is ≥25%, and the reduction of area is ≥50%.

[0018] In the component design of the duplex stainless steel wire rod of the present invention:

[0019] C: C is an austenite-forming element. In duplex stainless steel, the presence of carbon will not only promote the formation of σ phase (harmful phase), but also reduce the corrosion resistance of stainless steel. During the downstream cold heading processing of fasteners, cold heading cracking will occur due to the presence of σ phase. In the present invention, the C content is controlled to be ≤0.030%.

[0020] Si: Si can dissolve in ferrite and austenite. The lower the silicon content, the rounder and closer to island-like the austenite structure morphology in the continuous casting billet. However, too low a silicon content will increase the difficulty and cost in the preparation process. In this invention, the Si content is controlled to be 0.40% - 0.60%.

[0021] Mn: Mn enhances the stability of the austenite phase structure and replaces nickel to a certain extent. However, manganese will deteriorate the corrosion resistance. In this invention, the Mn content is controlled to be 4.05 - 4.40%.

[0022] P: P is a harmful element that easily leads to an increase in the cold brittleness of steel. In this invention, the P content is controlled ≤ 0.040%.

[0023] S: S is a harmful element that forms sulfide inclusions, which not only reduces the toughness and plasticity of steel but also affects the corrosion resistance of duplex steel. When the S content in duplex steel ≤ 0.0015%, less iron sulfide is generated in the continuous casting billet, which can effectively reduce the harm of iron sulfide in the continuous casting billet to the wetting of grain boundaries and reduce the rolling cracking ratio. In this invention, the S content is controlled ≤ 0.0015%.

[0024] Cr: Cr is a ferrite-forming element. A higher Cr content can promote the formation of intermetallic phases. In this invention, the Cr content is controlled to be 20.60 - 21.10%.

[0025] Ni: Ni is an element that stabilizes austenite. Adding Ni delays the formation of harmful intermetallic phases in austenitic stainless steel. The cost of Ni metal is high. Considering the cost, in this invention, the Ni content is controlled to be 1.00% - 2.00%.

[0026] Mo: Mo is a ferrite-forming element that can improve the corrosion resistance but also increases the tendency of stainless steel to form σ phase. In this invention, the Mo content is controlled ≤ 0.60%.

[0027] Cu: The Cu element can enhance the corrosion resistance of stainless steel and improve the mechanical properties of duplex stainless steel. In this invention, the Cu content is controlled ≤ 0.50%.

[0028] V: V can play a role in refining grains, improving toughness, and improving the corrosion resistance of steel. In this invention, the V content is controlled ≤ 0.13%.

[0029] N: N is a strong austenite-forming element. It is the most effective solid-solution strengthening element and low-cost alloying element. Increasing the N content in the structure can improve the mechanical properties of the material and lower the precipitation temperature of harmful phases. In this invention, the N content is controlled to be 0.10 - 0.17%.

[0030] Co: Co can improve the heat resistance and wear resistance, but too much content will deteriorate the toughness of duplex steel. In this invention, the Co content is controlled ≤ 0.15%.

[0031] Ti: The Ti element has the function of stabilizing carbides, but excessive content will deteriorate the toughness of duplex steel. In the present invention, the Ti content is controlled to be ≤0.030%.

[0032] Nb: The Nb element can refine grains, improve corrosion resistance, strengthen precipitation, and optimize welding performance, but the cost is high. In the present invention, the Nb content is controlled to be ≤0.030%.

[0033] Al: Excessive Al content is likely to form brittle Al2O3 inclusions in the molten steel. In the present invention, the Al content is controlled to be ≤0.10%.

[0034] Cr equivalent: The Cr equivalent represents the ability of stainless steel to form ferritic structure;

[0035] Ni equivalent: The Ni equivalent represents the ability of stainless steel to form austenitic structure.

[0036] It is found in the research of the present invention

[0037] When the content of α-phase (ferritic phase) in the continuous casting billet is 40 - 60%, the proportion of the two phases is balanced, which can effectively avoid stress concentration and rolling cracking caused by the large difference in the proportion of the two phases in the blank.

[0038] The traditional process relies on experience to adjust a single element to control the ferritic content in the continuous casting billet. However, in the research of the present invention, by establishing a quantitative model, the Cr / Ni equivalent ratio is directly related to the ferritic content of the continuous casting billet for the first time, realizing precise control of the ferritic content in the duplex stainless steel continuous casting billet.

[0039] Through research experiments and regression analysis, the quantitative relationship between the Cr / Ni equivalent ratio and the ferritic content is established, and the following formula is obtained:

[0040] The ferritic content (%) in the continuous casting billet = 66×(Cr / Ni equivalent ratio) - 131.6:

[0041] Combined with the above research viewpoints and verified by production, when the Cr / Ni equivalent ratio in the duplex steel is 2.6 - 2.9, the content of α-phase (ferritic phase) in the obtained continuous casting billet structure is stably within the target range of 40 - 60%.

[0042] The manufacturing method of the high surface quality duplex stainless steel wire rod for cold heading described in the present invention includes the following steps:

[0043] 1) Smelting and casting

[0044] Smelt and cast into a continuous casting billet with a cross-sectional size of 160×160 - 220×220 mm according to the above composition;

[0045] 2) Heating

[0046] Control the temperature in the preheating section of the heating furnace ≤ 1000°C, the temperature in the heating section is 1020 - 1100°C, the temperature in the soaking section is 1120 - 1170°C, and the total time in the furnace is 2.0 - 4.0 h;

[0047] 3) Rolling

[0048] Control the rough rolling starting temperature at 1050 - 1120°C, the rough rolling speed at 0.8 - 1.2 m / s, the rough rolling reduction at 75 - 80%, and the finish rolling reduction at 25 - 85%;

[0049] 4) Cooling

[0050] Control the wire laying temperature of the wire laying machine at 1000 - 1100°C, open the heat preservation cover and turn on the cooling fan to make the cooling rate of the wire rod on the Stelmor roller path ≥ 60°C / min. The temperature of the wire rod entering the water cooling unit is 950 - 1020°C, and the temperature of the wire rod leaving the water cooling unit ≤ 200°C;

[0051] 5) Solution treatment

[0052] The solution temperature is 930 - 1080°C, and the time in the furnace is 1.5 h - 3.0 h. Among them, the temperature in the low - temperature section is 930 - 1020°C, and the time in the low - temperature section ≤ 1.0 h. The temperature in the high - temperature section is 1040 - 1080°C, and the time in the high - temperature section ≥ 1.0 h; The temperature of the wire rod before entering the water ≥ 1000°C, the cooling time of the wire rod ≤ 15 min, and the temperature of the wire rod leaving the water ≤ 60°C.

[0053] In the manufacturing method of the duplex stainless steel wire rod described in the present invention:

[0054] In step 2), through the high - temperature Glibble tensile test curve and muffle furnace simulation, as Figure 1 shown, control the temperature in the soaking section of the heating furnace at 1120 - 1170°C, and the total time in the furnace at 2.0 - 4.0 h. Preferably, control the total time in the furnace at 2.0 - 2.5 h, which can avoid the coarsening of grains caused by a long time in the furnace and obtain good high - temperature plasticity.

[0055] In step 3), control the rough rolling speed at 0.8 - 1.2 m / s. Since the ferrite structure is easy to deform at both high and low temperatures, while the austenite structure has difficulty in recrystallization after the temperature decreases and the deformation difficulty increases, a slow rough rolling speed is selected during rough rolling to promote the recrystallization of austenite, with a small difference in the deformation rate between the ferrite structure and the austenite structure, improving the plasticity of the slab.

[0056] For the duplex stainless steel slab described in the present invention, after rough rolling, and then through medium rolling, pre - finish rolling, the wire rod size is fixed after finish rolling, where the finish rolling reduction is 25% - 85%, and the finish rolling speed is 50 - 90 m / s.

[0057] In step 4), control the wire laying temperature of the wire laying machine to be 1000~1100°C, open the heat preservation cover and turn on the cooling fan. Control the cooling rate of the wire rod on the Stelmor roller path to be ≥60°C / min. The temperature of the wire rod entering the water cooling unit is 950~1020°C, and the temperature of the wire rod leaving the water cooling unit is ≤200°C.

[0058] In step 5), control the temperature of the low temperature section to be 930~1020°C, and the temperature of the high temperature section to be 1040~1080°C. The solution treatment is designed with a low temperature section and a high temperature section. The low temperature section is designed to make the mechanical properties of the coil less different; the high temperature section is designed to improve the toughness of the duplex stainless steel to reduce the cracking ratio of cold heading bolts for downstream customers.

[0059] The design of the solution low temperature section helps the heat conduction of the duplex stainless steel coil, avoiding uneven heating at different positions of the inner and outer circles of the coil due to too fast heating speed, large differences in mechanical properties of the wire rods in the same coil, and uneven hardness problems when downstream customers cold heading the coil.

[0060] Control the temperature of the high temperature section to be 1040~1080°C, and the time of the high temperature section is ≥1.0 h. The higher the solution temperature, the higher the impact work performance, as Figure 2 shown. At the same time, the higher the solution temperature, the higher the temperature of the coil before entering the water. Enter the water above the σ-phase precipitation temperature, and control the temperature of the coil before entering the water to be ≥1000°C to avoid the precipitation of harmful σ-phase.

[0061] For duplex stainless steel wire rods, the content and morphology of the γ-phase (austenite phase) affect the mechanical properties of the wire rods. The higher the solution temperature, the lower the content of the γ-phase, the tensile strength and yield strength of the wire rods decrease, and the elongation increases; the higher the solution temperature, the more rounded and island-like the γ-phase microstructure, and the better the impact toughness of the material.

[0062] Wire rods with low tensile strength, low yield strength and high impact work are beneficial for cold heading forming. Therefore, by studying the change of the two-phase ratio of duplex stainless steel at different solution temperatures and deeply studying its microstructure, a microstructure control technology for duplex stainless steel wire rods is proposed, which solves the problems of poor cold heading forming performance and cold heading cracking of customers.

[0063] When the composition requirement of the duplex stainless steel wire rod described in the present invention is in the Cr / Ni equivalent ratio range of 2.6~2.9, through the solution treatment process described in the present invention, the ferrite phase ratio of the duplex steel wire rod is effectively controlled within 40%~60%.

[0064] Compared with the prior art, the difference of the present invention lies in:

[0065] 1. Compared with Chinese Patent CN114932146A, the process route for improving the surface quality of the present invention is different.

[0066] Chinese Patent CN114932146A discloses "A Rolling Method for Super Duplex Stainless Steel Wire Rod". This patent uses round billets instead of square billets. After heat treatment, the round billets are reheated and rolled, and the qualified product rate is increased from 0% to 80%.

[0067] In the composition design of the present invention, the size of the continuous casting billet is a common square billet with dimensions of 160*160~220*220; to ensure that the α-phase (ferrite phase) content in the continuous casting billet is 40~60%, the Cr / Ni equivalent ratio is controlled: 2.6~2.9.

[0068] In the present invention, to reduce the difference in the deformation rate between the ferrite structure and the austenite structure during rough rolling, the rough rolling speed is controlled at 0.8~1.2 m / s.

[0069] In the solution treatment process of the present invention, to improve the impact energy and reduce the precipitation of σ-phase, the temperature in the high-temperature section of the solution furnace is controlled at 1040~1080 °C, the time in the high-temperature section is ≥1.0 h, and the temperature before entering the water is ≥1000 °C.

[0070] 2. Compared with Chinese Patent CN105821346A, the product use, composition design requirements, rolling process requirements, and solution treatment process of the present invention are significantly different.

[0071] Existing Chinese Patent CN105821346A discloses "An Economical Duplex Stainless Steel Wire Rod and Its Manufacturing Method". After the batching is completed in the manufacturing method of the stainless steel, smelting, continuous casting, rolling, solution treatment, and pickling are carried out in sequence. The mass percentage of the chemical composition of the duplex stainless steel in the patent is: C: 0.01~0.05%, Si: 0.1~1.0%, Mn: 2.5~4.0%, Ni: 1.5~2.5%, Cr: 20.5~22.5%, Mo: 0.2~0.8%, N: 0.12~0.18%, W: 0.05~0.30%, P≤0.035%, S≤0.015%, and the balance is Fe and unavoidable impurities. In the rolling process of this patent, the continuous casting square billet is put into a heating furnace for heating, the heating temperature is 1100~1200 °C, and the rolling temperature is 950~1150 °C; in its solution treatment, the temperature of the off-line solution treatment is 1000~1200 °C.

[0072] In the composition design of the present invention, to ensure that the α-phase content in the continuous casting billet is 40~60%, the Cr / Ni equivalent ratio is controlled: 2.6~2.9. In the present invention, to reduce the difference in the deformation rate between the ferrite structure and the austenite structure during rough rolling, the rough rolling speed is controlled at 0.8~1.2 m / s. In the solution treatment process of the present invention, to improve the impact energy and reduce the precipitation of σ-phase, the temperature in the high-temperature section of the solution furnace is controlled at 1040~1080 °C, the time in the high-temperature section is ≥1.0 h, and the temperature before entering the water is ≥1000 °C.

[0073] 3. Compared with Chinese Patent CN115725811A, the composition and the heating furnace process of the present invention are different. Additionally, the present invention has a solution treatment process.

[0074] Chinese Patent CN115725811A discloses "A production method of second-generation duplex stainless steel wire rods". The mass percentages of the chemical components of the duplex stainless steel in this patent are as follows: C≤0.03%, Mn: 1.30 - 1.60%, Si: 0.045 - 0.065%, P≤0.030%, Cr: 22.30 - 22.60%, Ni: 4.55 - 4.75%, Mo: 3.02 - 3.10%, with the balance being Fe and inevitable impurities. In the rolling process of the said patent, the continuous casting billet is heated in a heating furnace. The temperature in the preheating section is raised to above 1000°C, the heating section is heated to 1130 - 1170°C with a holding time of ≤60 min, and the soaking section is heated to 1180 - 1210°C with a holding time of ≤60 min, and then it is sent to a rolling mill to roll into wire rods with a specification of φ5.5 - 23 mm.

[0075] In the composition design of the present invention, to ensure that the α-phase content in the casting billet is 40 - 60%, the Cr / Ni equivalent ratio is controlled at 2.6 - 2.9. In the present invention, to reduce the difference in the deformation rate between the ferrite structure and the austenite structure during rough rolling, the rough rolling speed is controlled at 0.8 - 1.2 m / s. The solution treatment process of the present invention is to increase the impact toughness and reduce the precipitation of σ-phase, and there is a solution treatment process.

[0076] 4. Compared with Chinese Patent CN114904914A, the present invention clarifies the requirements for the composition system, omits the blooming process, has a different heating furnace process, and has a solution treatment process.

[0077] The existing Chinese Patent CN114904914A discloses "A rolling method of duplex stainless steel wire rods". The rolling method includes continuous casting billet preparation, heating, blooming, intermediate billet heating, high-speed wire rolling, and cooling. Further, it also includes intermediate billet grinding. After intermediate billet grinding, the process temperature of the heating furnace is as follows: preheating section temperature: 800±20°C, time 60±20 min; heating section: temperature 1000±20°C, time 40±5 min; soaking section: 1230±10°C, time 30±5 min.

[0078] In the composition design of the present invention, to ensure that the α-phase content in the casting billet is 40 - 60%, the Cr / Ni equivalent ratio is controlled at 2.6 - 2.9. The continuous casting billet of the present invention does not go through the blooming process and directly enters the heating furnace to roll wire rods. In the present invention, to reduce the difference in the deformation rate between the ferrite structure and the austenite structure during rough rolling, the rough rolling speed is controlled at 0.8 - 1.2 m / s. The present invention is to increase the impact toughness and reduce the precipitation of σ-phase, and there is a solution treatment process.

[0079] The beneficial effects of the present invention:

[0080] (1)By restricting the Cr / Ni equivalent relationship of the alloy composition, the content of the α-phase (ferrite phase) in the continuous casting billet structure is 40 - 60%, and the two-phase ratio is balanced, which can effectively reduce defects such as scale and cracking caused by rolling cracking, and improve the surface quality of the wire rod;

[0081] (2)The billet of the present invention is heated at a lower temperature in the heating furnace, saving energy consumption;

[0082] (3)By controlling the rough rolling speed in the rolling process, the present invention reduces the scale and cracking defects formed by rolling cracking, and improves the surface quality of the wire rod;

[0083] (4)By controlling the residence time and temperature in the solution furnace, for the duplex stainless steel wire rod of the present invention, the ferrite phase ratio is 40 - 60%, the austenite microstructure is round and closer to the island shape, the impact energy is increased, the forming performance is good during the downstream cold heading processing by customers, and the product meets the standard performance requirements of fasteners.

[0084] For the stainless steel wire rod of the present invention, the tensile strength ≥ 620 MPa, the elongation ≥ 25%, the reduction of area ≥ 50%, and the ferrite phase content is in the proportion of 40% - 60%, ensuring that the proportion of cold heading cracking after downstream cold heading processing is below 0.1%, and there is no cracking during the rolling process of the duplex stainless steel; there are no defects such as scale and cracks on the surface of the wire rod, and the surface quality is good; the production cost is low and the performance is good, the forming performance is good during the downstream cold heading processing by customers, and the product meets the standard performance requirements of fasteners. BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Figure 1 It is a schematic diagram of the high-temperature area reduction and high-temperature strength of the bloom in Example 1 of the present invention;

[0086] Figure 2 It is a schematic diagram of the impact energy of the wire rod at different solution temperatures in Example 1 of the present invention;

[0087] Figure 3 It is a cross-sectional microstructure photo of the continuous casting billet in Example 1 of the present invention (α-phase content 45.3%);

[0088] Figure 4 It is a cross-sectional microstructure photo of the continuous casting billet in Comparative Example 1 (α-phase content 39.6%);

[0089] Figure 5 It is a solution microstructure photo of the wire rod in Example 5 of the present invention (α-phase content 51.1%);

[0090] Figure 6 It is a solution microstructure photo of the wire rod in Comparative Example 5 (α-phase content 39.8%). DETAILED DESCRIPTION OF THE INVENTION

[0091] The present invention will be further described below in conjunction with the examples and the drawings.

[0092] The component indexes of the embodiments and comparative examples of the present invention are shown in Table 1, and the balance contains Fe and other inevitable impurities; the manufacturing processes of the embodiments of the present invention are shown in Tables 2 and 3; the mechanical properties, the α-phase content of the continuous casting billet, and the α-phase content of the wire rod of the embodiments and comparative examples are shown in Table 4.

[0093] The mechanical property test of the material was carried out according to GB / T 228.1 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature".

[0094] The microstructure detection of the material was carried out according to GB / T 13305-2008 "Metallographic determination method for α-phase area content in stainless steel".

[0095] Compared with Comparative Examples 1 and 2, the Cr / Ni equivalent ratios of Examples 1 and 2 are 2.68 and 2.88 respectively, and the Cr / Ni equivalent ratios of Comparative Examples 1 and 2 are 2.49 and 2.48 respectively. The ferrite contents in the continuous casting billets of Examples 1 and 2 are 45.3% and 58.5% respectively, and the ferrite contents in the continuous casting billets of Comparative Examples 1 and 2 are 39.6% and 38.1% respectively. The ferrite content in the continuous casting billets of Examples 1 and 2 is higher than that in the continuous casting billets of Comparative Examples 1 and 2. The defective rates of peeling and cracking of Examples 1 and 2 are 1.0% and 1.5% respectively, and the defective rates of peeling and cracking of Comparative Examples 1 and 2 are 12.3% and 14.9% respectively. The α-phase content in the continuous casting billet of Example 1 and the α-phase content in the continuous casting billet of Comparative Example 1 are shown in Figure 3 、 Figure 4 。

[0096] Compared with Comparative Examples 3 and 4, the rough rolling speeds of Examples 3 and 4 are 0.91 m / s and 0.99 m / s respectively, and the rough rolling speeds of Comparative Examples 3 and 4 are 1.30 m / s and 1.29 m / s respectively. The defective rates of peeling and cracking of Examples 3 and 4 are 1.8% and 1.3% respectively, and the defective rates of peeling and cracking of Comparative Examples 3 and 4 are 3.6% and 3.9% respectively.

[0097] Compared with Comparative Examples 5 and 6, the high-temperature periods of Examples 5 and 6 are 85 min and 86 min respectively, and the high-temperature temperatures of Examples 5 and 6 are 1075 °C and 1079 °C respectively; the high-temperature periods of Comparative Examples 5 and 6 are 42 min and 45 min respectively, and the high-temperature temperatures of Comparative Examples 5 and 6 are 1021 °C and 1025 °C respectively. The α-phase contents of the wire rods of Examples 5 and 6 are 51.1% and 50.9% respectively, and the α-phase contents of the wire rods of Comparative Examples 5 and 6 are 39.8% and 45.8% respectively. The proportion of the α-phase content of the wire rod of Example 5 satisfying 40-60% is satisfied, and the austenite microstructure is more rounded and close to island-shaped, see Figure 5 、 Figure 6 。

[0098] In summary, by controlling the Cr / Ni equivalent and the actual ferrite content in the continuous casting billet, and reducing the rough rolling speed in the rolling process, the present invention reduces the defective rates of scale and cracking, and obtains duplex stainless steel wire rods with high surface quality. Moreover, by controlling the solution treatment time and temperature in the furnace, the present invention adjusts the α-phase ratio to be close to the optimal ratio of 40-60%, makes the austenite morphology and austenite structure round and close to island-shaped, reduces the tensile strength and yield strength of the duplex steel, and improves the impact energy of the material. The product has good forming performance in downstream cold heading processing for customers, and meets the standard performance requirements of fasteners.

[0099] The above content is the embodiment and related analysis description of the present invention, rather than limiting the patent scope of the present invention. Any equivalent exchange made using the content of the specification of the present invention, directly or indirectly applied in the relevant technical fields, is similarly included in the patent protection scope of the present invention.

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Claims

1. A duplex stainless steel wire rod with high surface quality for cold heading, characterized in that, The mass percentages of its components are as follows: C ≤ 0.030%, Si: 0.40 - 0.60%, Mn: 4.05 - 4.40%, P ≤ 0.040%, S ≤ 0.0015%, Cr: 20.60 - 21.10%, Ni: 1.00 - 2.00%, Mo ≤ 0.60%, V ≤ 0.13%, Cu ≤ 0.50%, N: 0.10 - 0.17%, Co ≤ 0.15%, Ti ≤ 0.030%, Nb ≤ 0.030%, Al ≤ 0.10%, and the balance is Fe and other inevitable impurities; and at the same time, the following are satisfied: Cr / Ni equivalent ratio: 2.6 - 2.9; Cr equivalent = Cr + 1.5Mo + 2Si + 1.5Ti + 5.5Al + 5V; Ni equivalent = Ni + Co + 0.5Mn + 30C + 25N + 0.3Cu; The microstructure of the duplex stainless steel wire rod is austenite + ferrite, and the ferrite phase content is 40% - 60%. The austenite tissue morphology is round and close to island-shaped; The tensile strength of the duplex stainless steel wire rod is ≥ 620 MPa, the elongation is ≥ 25%, and the reduction of area is ≥ 50%.

2. The manufacturing method of the duplex stainless steel wire rod with high surface quality for cold heading according to claim 1, characterized in that, It includes the following steps: 1) Smelting and casting Smelt and cast into a billet with a cross-sectional size of 160×160 - 220×220 mm according to the components of the duplex stainless steel wire rod described in Claim 1; 2) Heating Control the temperature in the preheating section of the heating furnace ≤ 1000°C, the temperature in the heating section is 1020 - 1100°C, the temperature in the soaking section is 1120 - 1170°C, and the total time in the furnace is 2.0 - 4.0 h; 3) Rolling Control the rough rolling starting temperature at 1050 - 1120°C, the rough rolling speed at 0.8 - 1.2 m / s, the rough rolling deformation at 75 - 80%, and the finish rolling deformation at 25 - 85%; 4) Cooling Control the wire laying temperature of the wire laying machine at 1000 - 1100°C, open the heat preservation cover and turn on the cooling fan to make the wire rod control the cooling rate ≥ 60°C / min on the Stelmor roller table. The temperature of the wire rod entering the water cooling unit is 950 - 1020°C, and the temperature of the wire rod leaving the water cooling unit is ≤ 200°C; 5) Solution treatment The solution temperature is 930 - 1080°C, and the time in the furnace is 1.5 - 3.0 h. Among them, the temperature in the low-temperature section is 930 - 1020°C, and the time in the low-temperature section is ≤ 1.0 h. The temperature in the high-temperature section is 1040 - 1080°C, and the time in the high-temperature section is ≥ 1.0 h; the temperature of the wire rod before entering the water is ≥ 1000°C, the cooling time of the wire rod is ≤ 15 min, and the temperature of the wire rod leaving the water is ≤ 60°C.

3. The manufacturing method according to claim 2, characterized in that, In step 2), the total time in the furnace is 2.0 - 2.5 h.

4. The manufacturing method according to claim 2, characterized in that, In step 3), the finish rolling speed is 50 - 90 m / s.

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