Homogeneous sk85 pickled strip and method for manufacturing the same

Abstract

The present application relates to a kind of homogenization SK85 pickling strip steel and its manufacturing method, steel chemical composition is as follows by weight percentage:0.83%≤C≤0.90%, 0.1%≤Si≤0.3%, 0.3%≤Mn≤0.6%, Cr≤0.3%, Ce≤0.007%, Al≤0.02%, P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.002%, N≤0.006%, the balance is Fe and inevitable impurities;Production process includes smelting, continuous casting, billet heating, rolling and cooling, normalizing and pickling, without hood annealing and cold rolling, finished product structure is fine pearlite structure, finished product plate hardness is ≥250HV, surface hardness difference is within ±5HV, tensile strength 1200~1300MPa, yield strength 900~1000MPa, elongation after break ≥15%.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a homogeneous SK85 pickled strip steel and its efficient manufacturing method. Background Technology

[0002] SK85 is a typical carbon tool steel in the JIS standard. Due to its high carbon content and good wear resistance, it is widely used in the manufacturing of precision cutting tools, elastic components, textile needles, and cold forming dies. However, due to its high carbon content, conventional smelting and hot rolling processes for producing SK85 steel plates can easily lead to problems such as carbon segregation causing graphite and other precipitates, excessive non-metallic inclusions in the segregation bands, uneven carbide distribution, severe work hardening, and difficulty in controlling the plate shape. These issues make it difficult to meet the requirements for subsequent precision stamping, bending, and heat treatment. Currently, an annealing + cold rolling method is commonly used, involving multiple cold rolling and bell-type annealing to obtain a more uniform spheroidal pearlite structure. However, this method has a long production cycle and low efficiency.

[0003] With the increasing demand for hypereutectoid tool steel strips such as SK85 from the precision machinery, textile equipment, hardware tools, and automotive parts industries, traditional ordinary high-carbon steel products can no longer meet the requirements of high-end components. Developing SK85 steel with uniform microstructure, stable performance, and excellent surface quality is of great significance for improving the lifespan and reliability of subsequent precision parts and achieving the localization of key basic materials.

[0004] Chinese patent application No. 202510737533.6 discloses "A method for improving the surface quality of SK85 hot-rolled steel sheet produced by a short-process technology." The chemical composition of the steel is C: 0.78%–0.90%, Si: 0.15%–0.30%, Mn: 0.2%–0.5%, Cr: 0.20%–0.30%, P: 0.01%–0.03%, and Fe as the matrix element. The SK85 hot-rolled strip steel is produced using the MCCR short-process technology, with composition, continuous casting, heating, lubrication, and cooling processes adjusted according to thickness differences. Laminar flow cooling with edge shielding and rapid slow cooling after coiling are used to prevent edge cracking defects. The microstructure is fine pearlite, with good strength, plasticity, and toughness matching, meeting the requirements of fine blanking and subsequent processing. However, it does not mention specific inclusions, segregation, and performance indicators. The use of semi-endless rolling and endless rolling results in small intermediate billet thickness, low compression ratio, and serious problems such as coarse microstructure and segregation in the finished product, affecting its performance.

[0005] Chinese patent application No. 200510110462.X discloses a "high-purity steel strip for textile needles and its production method." The steel composition is C: 0.9%–1.0%, Mn: 0.35%–0.50%, Si: 0.15%–0.30%, P≤0.030%, Cr: 0.15%–0.30%, and S≤0.007%. The raw hot-rolled steel strip undergoes a process of one annealing-one cold rolling-two annealing-two cold rolling-three annealing to obtain a spherical pearlitic cold-rolled high-carbon strip steel. 95% of the carbides have a particle size of 1.5–2.5 μm, with no Widmanstätten structure or banded structure, and a hardness of Hv255±15. By precisely controlling the reduction rate and process parameters, the thickness tolerance of the finished 0.4 mm steel strip reaches ±0.003 mm. This method employs multiple bell-type annealing and multiple cold rolling processes, resulting in complex processes, low production efficiency, and high costs. Furthermore, its design with extremely low sulfur content significantly reduces hot brittleness and inclusions, but the cost of desulfurization during smelting is relatively high.

[0006] Chinese patent application No. 202110827891.8 discloses "an SK85 carbon tool steel". The chemical composition of the steel is C: 0.80%~0.90%, Si≤0.35%, Mn≤0.50%, P≤0.030%, S≤0.030%, Cr≤0.30%, Ni≤0.25%, Cu≤0.25%. The metallographic structure of the SK85 cold-rolled steel strip is uniform granular / spheroidal pearlite, without coarse grains or network carbides, and the hardness is HV170~250. The process involves stress-relief annealing at 650℃ in a bell-type annealing furnace for 10 to 12 hours, pickling, softening annealing at 730℃ in a bell-type annealing furnace for 14 hours, cold rolling, and repeated spheroidizing annealing at 680℃, 720℃, and 750℃ in a bell-type annealing furnace. It uses a bell-type annealing furnace for repeated cyclic annealing, which is a complex process with low production efficiency and high cost. In addition, the surface is prone to oxidation and decarburization, which affects wear resistance and fatigue life.

[0007] Chinese patent application No. 201911340909.0 discloses a "manufacturing process for high-carbon tool steel SK85 cold-rolled wide strip". The chemical composition of the steel is C: 0.82%~0.88%, Si: 0.12%~0.28%, Mn: 0.25%~0.45%, P≤0.025%, S≤0.010%, Cr≤0.05%, Ni≤0.05%, Cu≤0.05%, with the remainder being Fe and impurities. The production process is: LF furnace refining - continuous casting - hot rolling - pickling - first cold rolling + first spheroidizing annealing + second cold rolling + second spheroidizing annealing + third cold rolling. The cold-rolled steel strip has a uniform and fine spheroidized pearlite structure without network carbide defects. The hardness is stable at HV170~250. However, it requires multiple spheroidizing annealing and cold rolling processes, which are complex, have low production efficiency, and high costs.

[0008] Chinese patent application No. 202511531303.0 discloses an "efficient manufacturing process for SK85 cold-rolled wide steel strip of high-carbon tool steel." The production process mainly includes hot-rolling pretreatment, nano-composite lubrication, and online induction heat treatment. Multi-pass warm rolling reduces deformation resistance and edge crack risk. Combined with precise shape and residual stress control, SK85 cold-rolled wide steel strip can be produced without offline annealing. The resulting strip has a grain size below 5μm, uniform carbide precipitation without network precipitates, and a uniform and dense microstructure. The hardness uniformity standard deviation is <2HRC, the difference within the same plate is ≤8μm, the thickness accuracy is ±1μm, and the residual stress after aging is ≤30MPa. However, it does not mention the spheroidization rate, strength, toughness, fatigue, and other performance indicators after cold rolling. High-carbon steel without spheroidization treatment suffers from high pearlite lamellar brittleness and typical segregation problems, affecting its performance. Furthermore, this method is only suitable for thin steel plates below 2mm; it is not applicable to thicknesses above 2mm. Summary of the Invention

[0009] This invention addresses the problems of severe carbon segregation, unstable microstructure and properties after heat treatment, and low production efficiency of annealing and cold rolling in SK85 high-carbon hypereutectoid tool steel. It provides a homogeneous SK85 pickled strip steel and its manufacturing method. The hot-rolled pickled strip steel has a thickness of 2-10 mm, and the finished microstructure is fine lamellar pearlite with pearlite clusters not exceeding 15 μm. The hardness is above 250 HV, the surface hardness difference is within ±5 HV, the tensile strength is 1200-1300 MPa, the yield strength is 900-1000 MPa, and the elongation after fracture is ≥15%.

[0010] To achieve the above objectives, the present invention employs the following technical solution: A homogeneous SK85 pickled strip steel, the chemical composition of which, by weight percentage, is: 0.83%≤C≤0.90%, 0.1%≤Si≤0.3%, 0.3%≤Mn≤0.6%, Cr≤0.3%, Ce≤0.007%, Al≤0.02%, P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.002%, N≤0.006%, with the balance being Fe and unavoidable impurities; the production process includes smelting, continuous casting, billet heating, rolling and cooling, normalizing and pickling, the finished product has a fine pearlite structure, the pearlite lamellar size does not exceed 15μm, the interlamellar spacing does not exceed 0.015μm, and the carbon segregation degree ≤1.05.

[0011] The finished steel plate has a hardness ≥250HV, a surface hardness difference within ±5HV, a tensile strength of 1200~1300MPa, a yield strength of 900~1000MPa, and an elongation after fracture ≥15%.

[0012] A method for manufacturing homogeneous SK85 pickled steel strip includes the following steps: (1) Smelting: This includes converter smelting and LF electric arc furnace refining; the converter produces basic slag with phosphorus ≤0.014%, sulfur ≤0.010%, and residual oxygen ≤0.05% before tapping; the LF produces acidic slag with free oxygen ≤0.010% and residual oxygen ≤0.015% before leaving the station. (2) Continuous casting: The superheat of the tundish casting is ≤25℃. The continuous casting adopts light reduction and electromagnetic stirring. The reduction amount is 5~10mm, the reduction rate is 2~6mm / m, the electromagnetic stirring current is 300~400A, and the frequency is 4~10Hz. The continuous casting speed is 0.9~1.2m / min, and the billet thickness is 170~250mm. (3) Heating of the billet: The billet is hot-charged and fed into the heating furnace at a temperature ≥500℃. The heating furnace uses a reducing atmosphere with an excess air coefficient of 0.7–0.9. The preheating section temperature is 600–700℃, with a holding time of ≥40 minutes. The first heating section temperature is 1000–1100℃, the second heating section temperature is 1130–1200℃, and the soaking section temperature is 1150–1200℃. The residual oxygen content is 0.7%–1%, and the total time in the furnace is 180–200 minutes. (4) Rolling and cooling: The thickness of the intermediate billet is 40-50 mm, the first pass reduction of roughing is 40-50 mm, the roughing speed is 5-8 m / s, and the roughing start temperature is 1130-1200℃; (2) the finishing speed is 6-10 m / s, the finishing start temperature is 1050-1100℃, and the finishing finish temperature is 800-880℃; the steel plate exits the finishing mill and enters laminar flow cooling, and is cooled to 650-750℃ at a cooling rate of 20-40℃ / s before being coiled. (5) Normalizing and pickling: The steel coil is fed into the normalizing and pickling unit. The normalizing speed is 40-70 m / min, the normalizing temperature is 800-850℃, and the holding time is 3-5 min. The first cooling is at a cooling rate of 5-10℃ / s to 400-500℃, and the second cooling is at a cooling rate of 10-15℃ / s to 50-100℃. Then it is pickled in the pickling solution.

[0013] In step (1), lime and lightly calcined dolomite are added during the converter smelting process to create alkaline slag, with an alkalinity R of 2.0 to 3.0; silica, lime and fluorite are added during the LF electric furnace refining process to create acidic slag, with an acidic slag alkalinity R ≤ 0.9.

[0014] In step (4), high-pressure water descaling is used before roughing and finishing, and the descaling water pressure is not less than 20MPa; roughing is carried out by 3+1 rolling, and finishing is carried out by 7-pass continuous rolling.

[0015] In step (5), the pickling solution is an HCl pickling solution with a temperature of 40-70℃ and a concentration of 20%-35%, and the pickling speed is 70-100m / min.

[0016] Compared with the prior art, the beneficial effects of the present invention are: To address the problems of severe carbon segregation, unstable microstructure and properties after heat treatment, and low production efficiency of annealing and cold rolling in SK85 high-carbon hypereutectoid tool steel, homogeneous SK85 pickled strip steel with a thickness of 2–10 mm was produced through improvements in chemical composition and processes. By controlling the process parameters of smelting, continuous casting, billet heating, rolling and cooling, normalizing, and pickling, a fine and uniform pearlitic microstructure was obtained, with pearlite lamellars not exceeding 15 μm, interlamellar spacing not exceeding 0.015 μm, and carbon segregation not exceeding 1.05. The finished steel plate has a hardness of over 250 HV, a surface hardness difference within ±5 HV, a tensile strength of 1200–1300 MPa, a yield strength of 900–1000 MPa, and an elongation after fracture ≥15%. No RH refining is required, and all non-metallic inclusions do not exceed grade 1.0. Attached Figure Description

[0017] Figure 1 This is a typical metallographic photograph of a homogeneous SK85 pickled strip steel as described in this invention. Detailed Implementation

[0018] The present invention discloses a homogeneous SK85 pickled strip steel with the following chemical composition by weight percentage: 0.83%≤C≤0.90%, 0.1%≤Si≤0.3%, 0.3%≤Mn≤0.6%, Cr≤0.3%, Ce≤0.007%, Al≤0.02%, P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.002%, N≤0.006%, with the balance being Fe and unavoidable impurities. The production process includes smelting, continuous casting, billet heating, rolling and cooling, normalizing and pickling. The finished product has a fine pearlite structure with a pearlite lamellar size not exceeding 15μm, a lamellar spacing not exceeding 0.015μm, and a carbon segregation degree ≤1.05.

[0019] The mechanism of action of each alloy component in the homogeneous SK85 pickled strip steel described in this invention is as follows: C: 0.83%~0.9%.

[0020] Carbon (C) is a major solid solution strengthening element in steel. In this invention, C also reacts with Mn, Si, and Ce to form fine carbides, improving strength, hardness, and wear resistance. If the C content is below 0.83%, it is difficult to guarantee the strength and hardness after heat treatment. On the other hand, if the C content is above 0.9%, the excessive strength will deteriorate the toughness and plasticity of the steel, and the high rolling force during cold rolling of thick gauges will easily lead to strip breakage. Therefore, this invention controls the C content between 0.83% and 0.90%.

[0021] Mn: 0.3%~0.6%.

[0022] Manganese (Mn) is relatively inexpensive and is an excellent deoxidizer and desulfurizer, as well as an essential element for ensuring the strength and toughness of steel. Manganese and iron can form a solid solution indefinitely, increasing hardness and strength. In this invention, Mn combines with Ce and S to form MnCeS. This avoids the formation of FeS at grain boundaries, which can cause hot cracks and affect stamping formability. Furthermore, the fine MnCeS precipitates also enhance strength and impact toughness. Too low an Mn content will not meet the requirements for high strength and hardness, while too high an Mn content will severely affect deep-drawing formability due to segregation and increase production costs. Therefore, considering both cost and performance requirements, this invention controls the Mn content to 0.3%–0.6%.

[0023] Si: 0.1%~0.3%.

[0024] Si is a common element in steel and is the main deoxidizer in this invention. As a non-carbide-forming element, Si, when dissolved in ferrite, can improve hardenability and tempering resistance, increase strength and hardness after heat treatment, and enhance wear resistance. This invention utilizes the fine precipitates formed by Si and Ce to hinder dislocation movement, thereby improving the elasticity and yield strength of tools (such as textile needles) and preventing deformation. Si is also a strong graphitizing element; high-carbon hypereutectoid steels are prone to wear resistance and fatigue performance issues due to graphite precipitation. Therefore, this invention controls the Si content to 0.1%–0.3%.

[0025] Cr≤0.3%.

[0026] Chromium is a strong carbide-forming element in steel. This invention adds Cr to form composite carbides with C, Mn, and Ce, inhibiting the segregation of C and Mn and improving the uniformity of the microstructure. Chromium can also increase the A3 and A1 temperatures, shift the GS line to the upper left, slow down austenite decomposition, improve hardenability, and enhance the oxidation resistance and corrosion resistance of steel.

[0027] Ce≤0.007%.

[0028] Ce, as a microalloying element, forms composite precipitates with C and Cr, which can promote carbide nucleation, pearlite spheroidization and refinement, and improve wear resistance and fatigue strength. In this invention, Ce can prevent carbon from accumulating at dendrite ends, reducing segregation and graphitization. On the other hand, this invention utilizes Ce to combine with O, S, and Mn to form fine inclusions, allowing them to float fully and avoiding the formation of elongated MnS inclusions by S and Mn, thus improving toughness and fatigue performance.

[0029] Harmful elements: P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.002%, Al≤0.02%, N≤0.006%.

[0030] P, S, H, O, Al, and N are all unavoidable harmful impurities in steel. Their presence will seriously deteriorate the toughness of steel. Therefore, this invention controls the content of P, S, H, O, Al, and N in steel to be as low as possible.

[0031] The present invention discloses a method for manufacturing homogeneous SK85 pickled strip steel, comprising the following steps: (1) Smelting: This includes converter smelting and LF electric arc furnace refining. In the converter, basic slag is produced, and slag-blocking operations are implemented during tapping to reduce the amount of slag carried over, requiring a slag thickness ≤80mm. Before tapping, phosphorus ≤0.014%, sulfur ≤0.010%, and residual oxygen ≤0.05%. In the LF furnace, acidic slag is produced, with free oxygen ≤0.010% and residual oxygen ≤0.015% before leaving the furnace. Ce alloy is then added, and inclusions are modified and fully floated to improve steel purity. The level of various non-metallic inclusions in the finished steel does not exceed grade 1.0. Ce can also form fine carbides with C, reducing carbon segregation at dendrite tips.

[0032] As a preferred method, lime and lightly calcined dolomite are added during the converter smelting process to create alkaline slag, with an alkalinity R of 2.0 to 3.0; silica, lime, and fluorite are added during the LF electric furnace refining process to create acidic slag, with an acidic slag alkalinity R ≤ 0.9.

[0033] (2) Continuous casting: The superheat of the tundish casting is ≤25℃. Continuous casting employs light reduction and electromagnetic stirring, with a reduction of 5–10 mm, a reduction rate of 2–6 mm / m, an electromagnetic stirring current of 300–400 A, and a frequency of 4–10 Hz. The continuous casting speed is 0.9–1.2 m / min, and the billet thickness is 170–250 mm. Uniform billet cooling helps reduce segregation, controls carbon segregation to no more than 1.05, and avoids the formation of segregated phases such as graphite.

[0034] (3) Heating: The billet is hot-charged and fed into the heating furnace at a temperature ≥500℃ to reduce the temperature difference on the billet surface and mitigate microsegregation. The heating furnace uses a reducing atmosphere with an excess air coefficient of 0.7–0.9; the preheating zone temperature is 600–700℃ with a holding time of at least 40 minutes; the first heating zone temperature is 1000–1100℃; the second heating zone temperature is 1130–1200℃; and the soaking zone temperature is 1150–1200℃. The residual oxygen content is 0.7%–1%, and the total time in the furnace is 180–200 minutes to prevent oxidation and decarburization of the billet while ensuring the rolling temperature to form various Ce carbides, thereby improving strength and toughness.

[0035] (4) Rolling and cooling: The intermediate slab thickness is 40–50 mm, the initial reduction in the roughing mill is 40–50 mm, the roughing mill rolling speed is 5–8 m / s, and the initial roughing mill temperature is 1130–1200℃. Low-temperature rolling is used in the roughing mill to control oxidation and decarburization, while simultaneously promoting Cr carbide precipitation and refining the microstructure. The finishing mill threading speed is 6–10 m / s, the finishing mill initial rolling temperature is 1050–1100℃, and the finishing mill final rolling temperature is 800–880℃. Low-temperature slow rolling is used in the finishing mill to help refine the austenite grains and prepare the microstructure for normalizing treatment. The steel plate exits the finishing mill and enters laminar flow cooling, cooling at a rate of 20–40℃ / s to 650–750℃ before coiling. Through high-temperature slow cooling, a lamellar pearlite microstructure is obtained, which easily dissolves carburization during normalizing treatment heating, accelerating the austenitization rate.

[0036] As a preferred method, high-pressure water descaling is performed before both roughing and finishing rolling, with a descaling water pressure of not less than 20 MPa to ensure the surface quality of the steel plate. Roughing is carried out using a 3+1 rolling process, and finishing rolling is carried out using a 7-pass continuous rolling process.

[0037] (5) Normalizing and pickling: The steel coil is fed into the normalizing and pickling unit. The normalizing speed is 40–70 m / min, the normalizing temperature is 800–850℃, and the holding time is 3–5 min to ensure full austenitization. The first cooling stage is performed at a rate of 5–10℃ / s to 400–500℃ to complete the normalizing treatment. This process transforms the pearlite phase into a fine pearlite structure, with pearlite lamellars not exceeding 15 μm and lamellar spacing not exceeding 0.015 μm (e.g., ...). Figure 1 (As shown). A slow initial cooling rate will result in coarse pearlite lamellae, while a fast initial cooling rate will produce sorbite or troostite, leading to excessively high hardness and brittleness in the steel plate. The second cooling is performed at a rate of 10–15°C / s to rapidly cool to 50–100°C, improving production efficiency. Then, pickling is carried out in a pickling solution to ensure excellent surface quality.

[0038] Preferably, the pickling solution is an HCl pickling solution with a temperature of 40-70℃ and a concentration of 20%-35%, and the pickling speed is 70-100m / min.

[0039] The finished steel plate has a hardness ≥250HV, a surface hardness difference within ±5HV, a tensile strength of 1200~1300MPa, a yield strength of 900~1000MPa, and an elongation after fracture ≥15%.

[0040] Unless otherwise specified, all percentages in this invention are percentages by mass.

[0041] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0042]

Example

[0043] Table 1 - Chemical composition of steel (%)

[0044] Table 2 - Smelting Process Parameters

[0045] Table 3 - Continuous casting and billet heating process parameters

[0046] Table 4 - Rolling and Cooling Process Parameters

[0047] Table 5 - Normalizing and Pickling Process Parameters

[0048] Table 6 - Mechanical Properties of Finished Products

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A homogeneous SK85 pickled steel strip, characterized in that, The chemical composition of the steel, by weight percentage, is: 0.83%≤C≤0.90%, 0.1%≤Si≤0.3%, 0.3%≤Mn≤0.6%, Cr≤0.3%, Ce≤0.007%, Al≤0.02%, P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.002%, N≤0.006%, with the balance being Fe and unavoidable impurities. The production process includes smelting, continuous casting, billet heating, rolling and cooling, normalizing and pickling. The finished product has a fine pearlite structure, with pearlite lamellar size not exceeding 15μm, lamellar spacing not exceeding 0.015μm, and carbon segregation ≤1.

05.

2. The homogeneous SK85 pickled strip steel according to claim 1, characterized in that, The finished steel plate has a hardness ≥250HV, a surface hardness difference within ±5HV, a tensile strength of 1200~1300MPa, a yield strength of 900~1000MPa, and an elongation after fracture ≥15%.

3. A method for manufacturing homogeneous SK85 pickled strip steel as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Smelting: This includes converter smelting and LF electric arc furnace refining; the converter produces basic slag with phosphorus ≤0.014%, sulfur ≤0.010%, and residual oxygen ≤0.05% before tapping; the LF produces acidic slag with free oxygen ≤0.010% and residual oxygen ≤0.015% before leaving the station. (2) Continuous casting: The superheat of the tundish casting is ≤25℃. The continuous casting adopts light reduction and electromagnetic stirring. The reduction amount is 5~10mm, the reduction rate is 2~6mm / m, the electromagnetic stirring current is 300~400A, and the frequency is 4~10Hz. The continuous casting speed is 0.9~1.2m / min, and the billet thickness is 170~250mm. (3) Heating of the billet: The billet is hot-charged and fed into the heating furnace at a temperature ≥500℃. The heating furnace uses a reducing atmosphere with an excess air coefficient of 0.7–0.

9. The preheating section temperature is 600–700℃, with a holding time of ≥40 minutes. The first heating section temperature is 1000–1100℃, the second heating section temperature is 1130–1200℃, and the soaking section temperature is 1150–1200℃. The residual oxygen content is 0.7%–1%, and the total time in the furnace is 180–200 minutes. (4) Rolling and cooling: The thickness of the intermediate billet is 40-50 mm, the first pass reduction of roughing is 40-50 mm, the roughing speed is 5-8 m / s, and the roughing start temperature is 1130-1200℃; (2) the finishing speed is 6-10 m / s, the finishing start temperature is 1050-1100℃, and the finishing finish temperature is 800-880℃; the steel plate exits the finishing mill and enters laminar flow cooling, and is cooled to 650-750℃ at a cooling rate of 20-40℃ / s before being coiled. (5) Normalizing and pickling: The steel coil is fed into the normalizing and pickling unit. The normalizing speed is 40-70 m / min, the normalizing temperature is 800-850℃, and the holding time is 3-5 min. The first cooling is at a cooling rate of 5-10℃ / s to 400-500℃, and the second cooling is at a cooling rate of 10-15℃ / s to 50-100℃. Then it is pickled in the pickling solution.

4. The method for manufacturing homogeneous SK85 pickled strip steel according to claim 3, characterized in that, In step (1), lime and lightly calcined dolomite are added during the converter smelting process to produce alkaline slag with an alkalinity R=2.0~3.0; silica, lime and fluorite are added during the LF electric furnace refining process to produce acidic slag with an alkalinity R≤0.

9.

5. The method for manufacturing homogeneous SK85 pickled strip steel according to claim 3, characterized in that, In step (4), high-pressure water descaling is used before roughing and finishing, and the descaling water pressure is not less than 20MPa; roughing is carried out by 3+1 rolling, and finishing is carried out by 7-pass continuous rolling.

6. The method for manufacturing homogeneous SK85 pickled strip steel according to claim 3, characterized in that, In step (5), the pickling solution is an HCl pickling solution with a temperature of 40-70℃ and a concentration of 20%-35%, and the pickling speed is 70-100m / min.

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