High-quality free-cutting die steel cast billet and manufacturing method therefor

Abstract

A high-quality free-cutting die steel cast billet and a manufacturing method therefor, belonging to the field of steelmaking methods. The mass percentage of element S in the steel cast billet is 0.05%-0.15%. The manufacturing method comprises: blast furnace smelting, converter smelting, LF furnace refining, RH furnace refining, slab continuous casting, and insulation pit slow cooling processes. The present invention obtains high-sulfur-content and high-purity molten steel during the converter and refining processes, controls brittle inclusions in the molten steel, reduces the total content and the size of oxide inclusions, and uses optimization design of continuous casting process parameters, so as to achieve composition and structure homogenization as well as the surface and internal quality enhancement of cast billets, thereby producing high-cleanliness and high-quality free-cutting alloy die steel cast billets.

Description

A high-quality free-cutting die steel blank and its preparation method

[0001] Methodology

[0002] The invention relates to a high-quality free-cutting die steel casting blank and a preparation method thereof, belonging to the field of steelmaking methods.

[0003] Background Methods

[0004] Free-cutting alloy die steel is primarily used in various molds and mold bases. It facilitates machining and cutting, reduces cutting resistance, extends tool life, facilitates chip removal, and improves surface finish. Due to its extremely high demands on machining performance and product quality, it places very high demands on sulfide morphology, billet purity, ingot shape, and ingot surface and internal quality. Good sulfide morphology helps improve machining performance. Excessive small particle inclusions can lead to uncontrolled sulfide nucleation and growth, resulting in uncontrolled sulfide morphology, which in turn affects machining performance. Large particle inclusions or foreign hard inclusions hinder the control of machining performance and increase tool wear. At the same time, ingots with good shape, surface quality, and internal quality have positive implications for reducing billet loss and improving product qualification rates during rolling and subsequent processing.

[0005] Foreign free-cutting steel is primarily produced using electric arc furnace melting, secondary refining, and continuous casting, or converter smelting, secondary refining, and continuous casting. This mature process, with reliable product quality, has gradually replaced electric arc furnace melting and die casting. Typical foreign free-cutting steel production processes include: Japan's Daido Steel Plant uses a 70t EAF-LF-RH-C process followed by initial rolling and slab production; Japan's Aichi Steel Plant uses an 80t EAF-LF-RH-CC process followed by initial rolling and slab production or continuous rolling; and Japan's Sumitomo Steel Plant uses a KR-70BOF-70t VAD / RH-CC process. Domestic free-cutting steel production primarily utilizes an electric furnace and die casting process. Only a few steel mills have explored continuous casting for some free-cutting steels, and the resulting product quality is inconsistent, resulting in a significant reliance on imports for free-cutting steel.

[0006] Controlling sulfur in sulfide-based free-cutting steels is a challenging aspect of their production, as the yield of sulfur in the steel significantly impacts the overall sulfur content. Currently, the main methods used include direct addition of sulfur powder, pyrite ore powder, commercially pure ferrous sulfur, and sulfur or ferrous sulfide alloy core wire. Furthermore, traditional white slag processes during refining, such as the high-alkalinity CaO-SiO2-Al2O3 slag system, are highly susceptible to excessive desulfurization of the molten steel, hindering the stability of sulfur control.

[0007] The castability of free-cutting steel is another major challenge to its stable production. The high oxygen and sulfur contents of free-cutting steel significantly reduce the surface tension of the molten steel, making slag separation difficult. This results in slag entanglement, numerous surface and subsurface defects, and even steel breakout, hindering continuous casting. The high manganese and oxygen contents in free-cutting steel react chemically and physically with certain components of the refractory at high temperatures, corroding the refractory and causing overflow or tundish breakout during continuous casting. Free-cutting steel has high viscosity and poor fluidity, requiring elevated pouring temperatures to ensure castability. However, free-cutting steel is also crack-sensitive, necessitating a weak cooling system. These conflicting requirements make the transition from die casting to continuous casting extremely challenging. Therefore, continuous casting process control is crucial in the current free-cutting steel production process.

[0008] The inventors searched the literature in this field and found the following patents:

[0009] (1) Patent CN101580912B provides a production process for low-carbon, high-sulfur free-cutting steel, the main components of which include C 0.14%-0.20%, Si 0.17%-0.37%, Mn 1.30%-1.60%, P < 0.040%, and S 0.080%-0.130%. An electric furnace is used for oxygen smelting without power supply, and the ratio of hot iron to scrap steel in the electric furnace reaches 7:1-3. The carbon oxygen gun at the furnace door and the coal oxygen auxiliary gun at the furnace wall alternately blow oxygen into the furnace for carbon-enriched smelting; the steel and slag are left at the eccentric furnace bottom for tapping, and one or more steel core aluminum, Ba-Ca-Si or composite refining deoxidizers are added at the end of the furnace for pre-deoxidation and alloying, and then slag is formed; the refining furnace forms low-basicity slag, and the composition weight ratio of the refined slag is: CaO 40%-50%, SiO2 15%-30%, Al2O3 20%-35%, MgO 5%-15%; the refining furnace is tapped with Ca or Ca-Si wire, and the soft blowing time is 8-15 minutes before the pit is removed and the ladle is transferred to the continuous casting platform. Continuous casting uses a one-machine, four-strand arc continuous casting machine, with argon-sealed shroud protection and argon-blown protection in the tundish. A weak cooling process is used for the secondary cooling process, and the crystallizer uses electromagnetic stirring at a current of 600-700A and a frequency of 4-5Hz. This produces continuous casting billets with stable chemical composition and good quality.

[0010] (2) Patent CN104357761B provides a sulfur-containing easy-cut die steel smelting process, comprising the following steps: Step 1, selecting high-quality scrap steel, high-quality pig iron, and sponge iron as raw materials, controlling the content of harmful elements: the total content of As, Sn, Pb, Sb, and Bi is not more than 0.05%; Step 2, smelting in an alkaline electric arc furnace, melting the raw materials, blowing oxygen for decarburization, dephosphorization, heating, degassing, and removing inclusions, tapping in an eccentric bottom electric furnace, only tapping without slag, making the [P] in the molten steel ≤ 0.006%, the temperature ≥ 1640°C, and adding precipitated deoxidizer, lime, and partially baked alloy with the steel flow during tapping; Step 3, refining in a ladle furnace, vacuum degassing, controlling the sulfur content to reach the target composition, and tapping; Step 4, casting into ingots. By increasing the sulfur content in the steel, the cutting performance of the steel during processing is improved, and the polishing performance and surface roughness of the steel are effectively improved.

[0011] (3) Patent CN103600049B provides a process for controlling and improving the internal quality of die steel continuous casting thick slabs by controlling the water volume in the vertical section, bending section and straightening section of the continuous casting machine. The water flow in the vertical section is controlled at 12-19m 3 / h; in the curved section, the arc water flow rate is controlled at 18-26m 3 / h, the outer arc water flow rate is controlled at 20-28m 3 / h; the arc water volume in the straightening section is controlled at 4-7m 3 / h, the outer arc water volume is controlled at 6-10m 3 / h. By controlling the inner and outer arc water flow in the vertical and curved sections, the temperature reversal during the cooling process of the slab is reduced, reducing or eliminating cracks caused by thermal stress. By controlling the inner and outer arc water flow in the straightening section, the slab temperature in the straightening zone is controlled to 900-1100°C, reducing or eliminating cracks caused by straightening force. Near the solidification endpoint, a sector-shaped section is implemented to transfer the reduction force to the center of the slab, controlling and improving center segregation, center porosity, and shrinkage holes in thick slabs.

[0012] (4) Patent CN103556065B provides a method for producing free-cutting plastic mold steel plates, the process steps of which are molten iron pretreatment → 120-ton converter steelmaking → LF refining → RH vacuum treatment → continuous casting → heating → rolling → normalizing → tempering → finishing → performance testing → ultrasonic testing. The chemical composition percentage of the steel is: C = 0.35-0.45%, Si = 0.25-0.35%, Mn = 1.50-1.60%, P ≤ 0.008%, S = 0.05-0.10%, Als = 0.010-0.045%, Cr = 1.80-2.00%, Mo = 0.15-0.20%. The free-cutting plastic mold steel plates produced with a thickness of less than 120 mm have a hardness of 30-36 HRC with a difference of less than ±0.5 HRC and pass the first-level ultrasonic testing, meeting the requirements for free-cutting plastic mold steel plates.

[0013] (5) Patent CN104988434B provides a production process for sulfur-containing plastic mold steel thick plates, the thickness of which can reach 200 mm, overcoming the problems of serious sulfide segregation and looseness inside the steel plates in existing sulfur-containing mold steel thick plates. In addition, a production process for the sulfur-containing plastic mold steel thick plates is provided, which is formed from 450 mm thick continuous casting billets. Through a reasonable heating system, rolling process, and heat treatment system, the obtained sulfur-containing plastic mold steel thick plates have good internal quality, no pinholes, and can meet the Φ2.0 mm defect equivalent detection requirements, which are more stringent than the JB / T4730.3-2005 Grade I flaw detection standard requirements. The steel plates have uniform structure and the hardness difference between the same plates is within 2.0 HRC.

[0014] The following conclusions were found after research:

[0015] (1) Patent CN101580912B provides a production process for low-carbon, high-sulfur free-cutting steel. However, high-sulfur molten iron and high-sulfur scrap steel are not used in the electric furnace, resulting in a large amount of subsequent sulfur addition and high cost. At the same time, the steel composition described in this invention patent does not contain Al, but steel core aluminum, Ba-Ca-Si or composite refining deoxidizers are added during the electric furnace tapping and LF refining process for deoxidation and alloying. Such composite deoxidizers can significantly reduce the oxygen content in molten steel and slag, while causing the presence of high-melting-point inclusions of alumina in the molten steel, which is detrimental to the processing performance of the product. In addition, Ca-Si wire is fed during refining and tapping, and the Ca element can directly react with S to desulfurize, which is not conducive to the stable control of the S content. High-alkalinity protective slag is used, but this type of protective slag has a high melting point and poor fluidity, and has unsatisfactory lubrication and thermal conductivity effects on the billet, which is not conducive to the control of surface quality. At the same time, there is no reduction process, which is not conducive to the control of center porosity and segregation.

[0016] (2) Although patent CN104357761B provides a smelting process for sulfur-containing easy-cutting die steel, lime and fluorite are added to slag during the refining process to adjust the sulfur content, which seriously affects the desulfurization of the molten steel. Therefore, after vacuum deoxidation and degassing, the slag must be transported back to the LF refining furnace for reheating, and the slag basicity must be adjusted to 2-3. The basicity of 2.5-3.0 is too high. After desulfurization, the slag must be fed back to the sulfur line for adjustment. This complex process flow is detrimental to the quality of the molten steel and the control of production costs.

[0017] (3) Although patent CN103600049B provides a process for controlling and improving the internal quality of thick slabs cast in continuous casting of mold steel, when controlling the cooling of the slab, although the water flow rate in the vertical and curved sections is controlled to reduce the reheating of the slab during cooling, reduce or eliminate cracks caused by thermal stress, and increase the cooling intensity to reduce reheating, the cooling of the outer side or corners of the slab is not considered. For high-sulfur and high-alloy steels, excessive cooling of the edges and corners is prone to cracking. In addition, it is difficult to stably improve segregation and core quality by relying solely on end reduction. End electromagnetic stirring and reduction should be used in combination, because excessive single reduction can easily cause cracks formed by reduction, while too little reduction will not significantly improve center segregation and looseness.

[0018] (4) Although patent CN103556065B provides a method for producing free-cutting plastic mold steel plates, it uses lime and synthetic slag to form slag during the refining process, and adds metallic aluminum and calcium carbide for deoxidation. The modified slag method has a very serious desulfurization problem. At the same time, a large amount of aluminum wire is fed into the refining and smelting process to control the aluminum content at 0.04%-0.06%. The comprehensive desulfurization amount in the LF refining process is very large. In the converter smelting process, measures are proposed to control the sulfur content, resulting in the feeding of a large amount of sulfur wire, up to 900-1000m, in the later stage of refining. The continuous casting process adopts low superheat casting and weak cooling water distribution, which easily causes excessive cooling at the corners or surface, and easily causes cracks during straightening.

[0019] (5) Although patent CN104988434B provides a production process for thick plates of sulfur-containing plastic mold steel, it does not perform a sulfur control process in the converter. Instead, it adjusts the sulfur content of the molten steel after the RH vacuum degassing is completed, and then performs calcium treatment and feeds the Ti-Fe wire. This will cause a large amount of desulfurization of the molten steel, which is detrimental to the cleanliness of the molten steel and the control of sulfide morphology. The continuous casting process uses low superheat pouring, and the tundish induction heating controls the superheat stability. The overall temperature of the molten steel is low, and the process control of secondary cooling water distribution and atomized water cooling is not detailed and clear, which cannot guarantee the cooling effect of the billet, which is detrimental to the surface and internal quality control of the billet.

[0020] Conventional processes for producing sulfur-containing free-cutting alloy mold steels employ no desulfurization during the KR process or use some high-sulfur scrap. However, high-basicity slag systems are used in the converter for dephosphorization, resulting in some sulfur removal. Furthermore, alloying and carbon powder are added to the converter to fully deoxidize the steel, and even aluminum or other strong deoxidizing alloying elements are added to reduce the oxygen content of the molten steel, creating medium-to-high basicity slag systems. Slag diffusion deoxidation and calcium treatment during the LF refining process also significantly desulfurize the molten steel, necessitating the addition of large amounts of sulfur wire or ferropyrite during the LF refining process. This results in poor sulfur control stability. The continuous casting process utilizes low superheat combined with medium-to-low intensity cooling, resulting in low molten steel temperature control, which hinders the melting of mold powder and its separation from the ingot. Furthermore, the secondary cooling water control and reduction ratio are not precisely designed based on the cooling characteristics of the ingot in each section. This results in low overall production efficiency for sulfur-containing free-cutting alloy mold steels, poor control of sulfur and inclusions, and high production costs.

[0021] Free-cutting alloy die steel is widely used in various molds and mold frames, facilitating machining and extending tool life. Due to the extremely high processing performance and product quality requirements, stringent requirements are placed on sulfide morphology control, billet purity, billet shape, and billet surface and internal quality. Therefore, achieving high-quality free-cutting alloy die steel requires carefully controlling the stability of the sulfur content, the morphology of sulfide inclusions, the billet oxygen content, purity, and billet quality. Based on this, the present invention provides a high-quality sulfur-containing free-cutting steel billet and a method for preparing the same. Summary of the Invention

[0022] In order to solve the above problems, the present invention discloses a high-quality free-cutting die steel ingot and a preparation method thereof. The specific method scheme is as follows:

[0023] A high-quality free-cutting die steel ingot has a chemical composition, in addition to iron, comprising the following by mass percentage: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, S: 0.05%-0.15%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, TO≤0.0015%, N≤0.0025%, H≤0.0002%, Alt≤0.0015%, Ti≤0.0010% and other inevitable impurity element components.

[0024] Furthermore, the central segregation C of the steel ingot is ≤1.0, and 9 points are taken at the center of the width direction of the steel ingot, the 1 / 4 position, and 1 cm from the inner arc to the outer arc edge. The ratios of the maximum and minimum values ​​of the Mn element to the average value of all points are within the range of 0.96-1.04, and the ratios of the maximum and minimum values ​​of the Cr element to the average value are also within the range of 0.96-1.04; the height or depth of the defects of surface vibration marks, pits, bumps, folds and pores is ≤2mm.

[0025] A method for manufacturing high-quality free-cutting die steel castings comprises the following steps:

[0026] Step 1: Blast furnace smelting: High-sulfur iron ore or charge is selected to smelt iron in a blast furnace to form molten iron, which is then tapped into a ladle. The mass percentages of C, Si, P, and S in the molten iron are as follows: C: 4.1%-4.5%, Si: 0.2%-0.6%, P≤0.12%, and S: 0.04%-0.08%. The temperature is 1350-1430°C. After tapping, the molten iron is not skimmed and is directly transported to a converter for smelting.

[0027] Step 2: Converter smelting: The converter is charged with 200±5t, the scrap ratio is 15%-20%, high sulfur scrap steel or ordinary scrap steel is used for smelting, the converter adopts double slag method, the converter adopts large bottom blowing and stirring in the early stage of blowing, the top gun is blown with low oxygen flow, pellets and lime are added to make slag, the slag basicity is 1.5-2.0, the T.Fe content in the slag is 20%-40%, the molten steel temperature is controlled to be ≤1450℃, and then 50%-70% of the slag is poured; in the second stage of blowing, the bottom blowing and top gun oxygen flow are increased, and the slag is divided into 2-4 steps. Batch adding of lime, light burning and pelletizing to form slag, stabilizing the slag basicity to 2.5-3.5, the T.Fe content in the slag to 15%-30%, blowing to a C mass content of 0.06%-0.15%, the molten steel temperature to 1600-1640°C, the oxygen mass content to 0.025%-0.045%, and the S mass content to 0.03%-0.09%, then stopping blowing and starting slag pouring, pouring out 60%-85% of the slag, then tapping, using a slide plate to block the slag, leaving the slag after tapping, and proceeding to the next furnace of smelting;

[0028] Step 3: Converter tapping: At the beginning of tapping, manganese alloy, silicon alloy, ferrochrome alloy, ferromolybdenum alloy and carbon powder are first added to the ladle for deoxidation and alloying. The bottom blowing flow rate is 100-300NL / min. When 85% of the steel is tapped, sulfur-containing slag, calcium silicate synthetic slag and lime are added to make slag. The slag basicity is controlled at 1.0-1.5. At the end of tapping, the bottom blowing flow rate is 400-800NL / min. After stirring for 3-5 minutes, the slag is transported to the LF furnace for treatment.

[0029] Step 4: LF furnace refining: The bottom blowing of the refining ladle is turned on throughout the entire process. The maximum argon flow rate of the air brick is 400-500NL / min when adding lime, high basicity synthetic slag and alloying. The maximum argon flow rate of the air brick is 200-400NL / min during the heating period, and the maximum argon flow rate of the air brick is 100-150NL / min at other times. During the refining process, lime or high basicity synthetic slag is added, the basicity of the ladle slag is controlled at 1.5-2.0, 2.0%≤T.Fe+MnO≤4.5%, manganese alloy, ferrochrome alloy and ferromolybdenum alloy are added to adjust the Mn, Cr and Mo components in the molten steel to the mass range of: Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, the tapping temperature is controlled at 1600-1620℃, and the steel is tapped into the RH furnace;

[0030] Step 5: RH furnace refining: RH enters the station for rapid vacuum treatment. When the vacuum degree drops below 50mbar, carbon powder is added to adjust the carbon content to the mass range C: 0.25%-0.55%. Then, ferrosilicon alloy, ferromanganese alloy and ferrosulfur alloy are added for alloying. Then, the working pressure of the vacuum chamber is quickly reduced to ≤1mbar. At the same time, calcium carbide and silicon carbide are added to the slag surface of the ladle to diffuse and deoxidize the slag. The basicity of the slag is controlled between 2.0-3.0, T.Fe+MnO≤2.0%, and the gas flow rate is increased to 150-200Nm 3 / min, processing time ≥15min, then turn off the second or third stage vacuum pump, increase the vacuum chamber pressure to above 20mbar, and increase the gas flow rate to 100-150Nm 3 / min, then RH net circulation, breaking air to tap steel, soft stirring and calming treatment after tapping, and then transported to continuous casting;

[0031] Step 6: Slab continuous casting: The slab continuous casting machine is used for casting, and the continuous casting is protected during the whole process. A low-alkalinity, low-alumina content tundish covering agent is used to control the high superheat of the molten steel in the tundish. A low-melting-point mold protection slag is used, and electromagnetic stirring is performed at the end of the mold.

[0032] Step 7: Cool slowly in the insulation pit.

[0033] Furthermore, in step 1, the amount of slag in the iron ladle is ≤3 kg / t.

[0034] Furthermore, the mass percentage of the components in the high-sulfur scrap steel used in the converter in step 2 is: P≤0.015%, 0.05%≤S≤0.25%, and the rest are conventional C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements;

[0035] The mass percentage of the ordinary scrap steel components used in the converter in step 2 is: P≤0.015%, S≤0.03%, and the rest are conventional C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements;

[0036] The scrap steel added into the converter is all high-sulfur scrap steel, all ordinary scrap steel, or a combination of high-sulfur scrap steel and ordinary scrap steel in any proportion.

[0037] Furthermore, in step 3, when 20%-30% of the steel has been tapped from the converter, 30%-50% of manganese alloy, 40%-60% of silicon alloy, 85%-95% of ferrochrome alloy, 85%-95% of ferromolybdenum alloy, and 45%-65% of carbon powder are added.

[0038] Furthermore, the components of the manganese alloy include, by mass percentage: Mn ≥ 98%, P ≤ 0.010%, Al ≤ 0.0035%, Ti ≤ 0.0025%, and the remainder is Fe and other inevitable impurity components;

[0039] The silicon alloy comprises, by mass percentage, Si: 75%-80%, P≤0.015%, Al≤0.0050%, Ti≤0.0035%, and the remainder is Fe and other inevitable impurity components;

[0040] The components of the ferrochrome alloy include, by mass percentage, Cr: 55%-65%, P≤0.015%, Al≤0.003%, Ti≤0.003%, and the remainder is Fe and other inevitable impurity components;

[0041] The components of the molybdenum-iron alloy include, by mass percentage, Mo: 45%-55%, P≤0.018%, Al≤0.008%, Ti≤0.006%, and the rest are Fe and other inevitable impurity components.

[0042] Furthermore, in step 3, the amount of sulfur-containing slag added is 1.5-2.5 kg / t, wherein the sulfur mass percentage is 2.0%-4.0%;

[0043] The calcium silicate synthetic slag is added in an amount of 7-9 kg / t, and its components, calculated by mass percentage, include: CaO: 25%-35%, SiO2 45%-55%, Al2O3 ≤ 2%, MgO 3%-6%, and other inevitable components.

[0044] Furthermore, in step 4, the refining ladle is a double-permeable brick, the bottom blowing flow of the small-flow brick is 30%-60% of the flow of the large-flow permeable brick, and the CaO mass content in the lime component added during the refining process is: CaO ≥ 95%, and the rest are unavoidable impurity components.

[0045] Furthermore, the high basicity synthetic slag in step 4 comprises, by mass percentage, the following components: CaO: 65%-75%, SiO2: 0-20%, Al2O3≤5%-10%, MgO 3%-5%, and other inevitable components.

[0046] In the step 5, after the RH net circulation for 5 minutes, the air is broken for soft stirring and calming treatment, the soft stirring time is 5-10 minutes, and after the soft stirring is completed, the molten steel is allowed to stand for ≥10 minutes, and then transported to the continuous casting.

[0047] Furthermore, in step 6, the cross-sectional specifications of the slab continuous casting machine are 220mm×1800mm×2600mm or 320mm×1800mm×2600mm, the argon flow rate of the long water inlet of the large ladle is 150-200NL / min, the argon flow rate of the upper water inlet of the medium ladle is 5-10NL / min, and the argon flow rate of the stopper mechanism is 10-15NL / min.

[0048] Furthermore, the low basicity, low alumina content middling covering agent used in step 6 includes, by mass percentage, the following components: CaO: 30%-35%, SiO2: 50%-55%, MnO: 2%-5%, Al2O3≤2%, MgO: 3%-6%, and other inevitable components.

[0049] Furthermore, in step 6, the thickness of the crystallizer protective slag layer is 12-22 mm, and the consumption is 0.15-0.20 kg / t; CaO / SiO2 in the crystallizer protective slag is 0.70-0.85, and the components of the crystallizer protective slag include, by mass percentage: Na2O: 8-12%, Li2O: 2.5-3.5%, F: 10-15%, MgO: 1-3%, elemental C: 2-4%, and other inevitable impurity components.

[0050] Furthermore, in step 6, the superheat of the molten steel in the continuous casting tundish is 25-45°C, and the pouring tonnage is ≥25t; the tundish tonnage during normal pouring is ≥28t; the tundish tonnage when continuously casting and changing to a large ladle is ≥22t; the insertion depth of the submerged nozzle is 120-180mm, the crystallizer taper is 1.03%-1.07%, the continuous casting speed is controlled at 0.6-1.0m / min, the crystallizer electromagnetic stirring current is 450-650A, and the frequency is 6-8Hz.

[0051] Furthermore, in the step 7, the temperature entering the insulation pit for slow cooling is ≥500°C, and the temperature exiting the pit is ≤150°C.

[0052] Furthermore, the refractory materials of the refining ladle, continuous casting tundish, continuous casting ladle long nozzle, submerged nozzle and stopper rod in contact with the molten steel are all made of low-alumina material with Al2O3 mass content ≤5%.

[0053] The present invention is applied to protect the steel ingot produced by the above method.

[0054] The present invention applies to protect the mold made from the above steel billet.

[0055] The principle of the smelting process of the present invention is as follows:

[0056] Sulfur-based free-cutting steels achieve free-machining properties primarily through the addition of sulfur to the steel, combined with controlled sulfide inclusion morphology. Sulfur in sulfide-based free-cutting steels contributes to hot brittleness, but the addition of manganese, forming MnS, mitigates this detrimental effect while also improving machinability. Sulfur content in sulfide-based free-cutting steels typically ranges from 0.05% to 0.33%. Sulfides in the steel primarily exist as (Fe, Mn)S solid solutions.

[0057] Free-cutting alloy die steel is primarily used for various molds and mold bases. It offers advantages such as ease of machining, extended tool life, reduced cutting force, improved surface finish, and easier chip removal. Due to its demanding requirements for extremely high machining performance and product quality, it places very high demands on sulfide morphology control, billet purity, billet shape, and surface and internal quality of the ingot.

[0058] In order to obtain high-quality ingots, the quality of sulfur-containing free-cutting alloy mold steel ingots is comprehensively controlled from multiple dimensions, including the stability of the S element in the molten steel, control of brittle inclusions, control of ingot segregation and cracks, as well as production efficiency and cost control.

[0059] First of all, the S element is the key to ensuring the cutting performance of alloy die steel. When designing the smelting process, it is necessary to ensure the stable control of the S element while taking into account the control of production efficiency and cost:

[0060] (1) The molten iron raw materials are not desulfurized, and high-sulfur scrap steel is used in the converter, which further increases the sulfur content of the converter steel, increases the sulfur content of the raw materials, reduces the subsequent sulfur replenishment line or sulfur iron operation, reduces costs, and improves smelting efficiency.

[0061] (2) In the early stage of the converter, a low-alkalinity slag system is used. Through high gun position, low oxygen flow rate and the addition of pellets, the oxidation of the slag is improved, dephosphorization is enhanced, and the desulfurization effect of the slag is reduced. In the decarburization period of the converter, a medium-alkalinity slag system is used. At the same time, a small amount of pellets is added in multiple batches to stabilize the oxidation of the slag, avoid excessive alkalinity, reduced oxidation and desulfurization, and also inhibit phosphorus reversion, which comprehensively ensures low-phosphorus and high-sulfur steelmaking in the converter. In the process of converter tapping, some ferrosilicon, ferromanganese and carbon powder are added to reduce the degree of deoxidation, which can greatly reduce the reaction desulfurization of the molten steel slag during the tapping process to alleviate the problem of nitrogen absorption by the molten steel. At the same time, the molten iron desulfurization slag is mixed with acidic calcium silicate slag to reduce the alkalinity of the slag system, which can inhibit slag desulfurization and promote the return of S in the molten iron slag to the molten steel to increase sulfur.

[0062] (3) During the LF refining process, lime and other substances are added to fine-tune the slag basicity after adjusting the composition of the molten steel. However, the basicity is controlled below 2.0. The slag basicity is adjusted in the later stage of refining, and small and medium bottom blowing is controlled to avoid violent slag-metal reaction and large-scale desulfurization. At the same time, the slag is controlled to have a certain degree of oxidizability to further reduce the desulfurization effect.

[0063] (4) The RH vacuum treatment process utilizes a deep vacuum carbon deoxidation method to further reduce the oxygen content of the molten steel, and adds all the silicon iron and manganese iron alloys for composite deoxidation to completely remove the oxygen content of the molten steel. In addition, the slag is modified by adding a slag surface deoxidizer to reduce the slag oxidation and reduce the slag's oxidizing effect on the molten steel. In addition, the RH treatment process does not open the ladle bottom blowing, and the slag's oxidizing effect on the molten steel is extremely weak after the slag's oxidation is reduced. After the molten steel and slag components meet the standards, the vacuum chamber pressure is increased and the lifting gas flow rate is reduced to reduce the RH molten steel circulation flux, reduce the erosion of the refractory material, control the foreign brittle inclusions such as alumina and magnesia spinel, and facilitate the floating removal of the inclusions. This achieves stable and efficient control of the molten steel's S content, while reducing the TO content and improving the cleanliness.

[0064] Secondly, the converter tapping and refining processes utilize an incompletely deoxidized, low-alkalinity, and weakly oxidizing slag smelting method. Combined with controlled Al and Ti content in the alloy and auxiliary materials, this method keeps brittle inclusions in the molten steel at extremely low levels. Furthermore, measures such as variable RH circulation flow and molten steel calming are employed to effectively reduce foreign inclusions and efficiently remove existing inclusions. High superheat pouring during the continuous casting process further promotes inclusion buoyancy. Controlling the amount and method of deoxidation with Si and Mn elements produces low-melting-point SiO2-MnO inclusions, while also effectively controlling brittle inclusions such as alumina, magnesia spinel, and titanium oxide, resulting in high-quality molten steel.

[0065] Finally, the continuous casting process uses a high superheat and weak cooling pouring process. Combined with the use of low-alkalinity, low-aluminum tundish covering agents, the desulfurization problem caused by the slag-metal reaction can be reduced, while preventing the introduction of alumina components into the molten steel. The use of low-melting-point mold powder, combined with high superheat and weak cooling processes, can improve the melting effect and fluidity of the mold powder, and promote the separation of slag and steel during the continuous casting process. In addition, the temperature of the ingot exiting the mold and the temperature of the secondary cooling stage during the continuous casting process need to be precisely controlled. This is because, during the solidification process of the continuous casting ingot after exiting the mold, in the third brittle zone below 900°C, the proeutectoid ferrite formed at the austenite grain boundaries weakens the grain boundary strength. Under the combined effects of thermal stress, structural stress, and mechanical straightening stress, cracking occurs along the grain boundaries. The addition of sulfur expands the precipitation range of the ferrite structure, exacerbating the occurrence of cracks.

[0066] In order to stabilize the quality of the casting, measures include:

[0067] (1) By controlling the water flow rate in the vertical section and the inner and outer arcs of the curved section, the temperature recovery during the cooling process of the billet is reduced, and cracks caused by thermal stress are reduced or eliminated.

[0068] (2) By controlling the water flow rate in the inner and outer arcs of the straightening section and accurately controlling the water volume in the middle and edge of the wide side, the temperature of the billet in the straightening section is controlled to be outside the brittle zone II, thereby reducing or eliminating cracks caused by straightening.

[0069] (3) Near the end of solidification, one sector is pressed down with a larger ratio, while the other two sectors are pressed down with a smaller ratio. This can transfer the pressing force to the center of the billet, control and improve the center segregation, center porosity and shrinkage of the thick slab, and at the same time avoid cracking caused by excessive pressing at a single position.

[0070] (4) The applied reduction is related to the thickness of the slab, which can meet the requirements of slab control and internal quality improvement of slabs of different thickness specifications and steel grades.

[0071] (5) The obtained ingot is immediately placed in a slow cooling pit for heat preservation, and the cooling rate and the temperature out of the pit are controlled to further control the microcracks generated during the cooling process.

[0072] By combining the above comprehensive steelmaking and continuous casting control methods, high-quality sulfur-containing free-cutting alloy die steel ingots can be obtained.

[0073] The beneficial effects of the present invention are:

[0074] (1) In order to ensure cutting performance, high-carbon, high-alloy, sulfur-containing free-cutting die steel has strict requirements on inclusion type, composition, uniformity of structure, surface quality, etc. Therefore, controlling inclusion type, composition and uniformity of structure, ingot segregation and cracks are the key to obtaining high-quality free-cutting alloy die steel. Analysis shows that inclusions are mainly controlled from two aspects: sulfide inclusion performance and reduction of brittle inclusions. In terms of composition control, the main consideration is how to control the content of S element economically and stably, while ensuring the uniform distribution of C element and alloying elements in the ingot. Since high-sulfur alloy die steel is very sensitive to cracks, it has extremely high requirements on continuous casting overheating, cooling process, cooling process, etc. to avoid the generation of surface or internal cracks in order to obtain ingots with low defect rate.

[0075] This invention systematically and comprehensively designs a steelmaking and continuous casting process. By obtaining high-sulfur, high-purity molten steel in the converter and refining processes, it controls brittle inclusions in the molten steel while reducing the total amount and size of oxide inclusions. Combined with the optimized design of continuous casting process parameters, this achieves uniform composition and microstructure of the ingot, while enhancing both surface and internal quality. This invention provides a high-quality, sulfur-containing, free-cutting alloy die steel and its preparation method, which can be used to produce high-purity, high-quality, free-cutting alloy die steel ingots.

[0076] (2) The present invention systematically and comprehensively designs an efficient and low-cost sulfur control process method for the entire process, from molten iron and scrap steel raw materials, converter smelting, converter steelmaking deoxidation and alloying and slag making process, refining and slag adjustment process, RH alloying process, etc., so as to achieve stable control of the S element in molten steel.

[0077] (3) The present invention innovatively designs a process of combining incomplete deoxidation and alloying of converter steelmaking with a low-basicity, high-sulfur slag system, and combines it with a LF refining process of a low-basicity, weakly oxidizing slag system to achieve stable control of sulfur elements.

[0078] (4) The full-process system design of the present invention takes into account the control of various impurity elements and inclusions in steel. Through the design of incomplete deoxidation and alloying during converter steelmaking and the refining of low-basicity, weakly oxidizing slag system, combined with RH deep vacuum carbon deoxidation and silicon and manganese composite deoxidation and variable circulation flow method, the impurity element content, TO content and gas content in steel are reduced. Combined with the control of refractory materials, alloys and auxiliary materials, the size and number of endogenous and exogenous inclusions are greatly reduced, and the cleanliness of molten steel is significantly improved.

[0079] (5) The continuous casting of the present invention adopts a high superheat weak cooling process mode, through precise segmented control of secondary cooling water distribution and pressing process, optimal crystallizer protective slag type, and reasonable control of crystallizer electromagnetic stirring and billet insulation and slow cooling process, etc., to obtain continuous casting billets with high surface quality and internal quality.

[0080] (6) The thickness deviation of the steel ingot of the present invention is within ±1.5 mm, the width deviation is within ±2.5 mm, the bulging or concavity of the wide side is within ±2.5 mm, and the bulging or concavity of the narrow side is within ±1.5 mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] FIG1 is a low-magnification image of a high-quality free-cutting die steel ingot according to the present invention. DETAILED DESCRIPTION

[0082] The present invention will be further described below in conjunction with specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0083] A method for casting high-quality free-cutting die steel blanks mainly comprises the following steps:

[0084] (1) Blast furnace smelting: High-sulfur iron ore or charge is selected to smelt iron in a blast furnace to form molten iron, which is then tapped into a ladle. The slag content, molten iron composition, and temperature of the ladle are shown in Table 1. After tapping, the molten iron is not slag-stripped and is directly transported to a converter for smelting.

[0085] Table 1 Hot metal related information

[0086] (2) Converter smelting: The converter charge and scrap ratio are shown in Table 2. High-sulfur scrap steel or ordinary scrap steel is used for smelting. The P and S components and usage ratios of the high-sulfur scrap steel and ordinary scrap steel are also shown in Table 2. The rest are conventional C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements. The converter adopts the double slag method for smelting. In the early stage of converter blowing, large bottom blowing and stirring are used, and the top gun is blown with low oxygen flow. Pellets and lime are added to make slag, and the temperature of the molten steel is controlled before slag is poured. In the second stage of blowing, the bottom blowing and top gun oxygen flow are increased, and lime, light burning, and pellets are added in multiple batches to make slag. The slag basicity and slag oxidation are stabilized. The blowing is carried out to the appropriate end point composition, and the blowing is stopped and the slag is poured. The relevant parameters are shown in Table 3. Then the steel is tapped. The C, O, and S contents and the molten steel temperature at the end of blowing are shown in Table 4. Each furnace uses a slide plate to block the slag. The slag is left after tapping and the next furnace is smelted.

[0087] Table 2 Information about hot metal and scrap steel

[0088] Table 3 Process control parameters

[0089] Table 4 Converter endpoint data

[0090] (3) BOF Tapping: At the beginning of BOF tapping, some manganese alloy, some silicon alloy, all ferrochrome, ferromolybdenum, and some carbon powder are added for deoxidation and alloying. The timing and proportion of addition, as well as the bottom blowing flow rate, are shown in Table 5. The main elements and some impurities of manganese alloy, silicon alloy, ferrochrome, and ferromolybdenum are shown in Table 6. The remainder is Fe and other unavoidable impurities. When 85% of the steel is tapped, sulfur-containing slag, calcium silicate synthetic slag, and lime are added to form slag. The slag basicity and bottom blowing flow rate are controlled. After stirring for a period of time, the slag is transported to the LF furnace for processing, as shown in Table 7. The main components of calcium silicate synthetic slag are shown in Table 8, as well as other unavoidable components.

[0091] Table 5 Information about hot metal and scrap steel

[0092] Table 6 Main elements and some impurity components of alloys

[0093] Table 7 Slag material and bottom blowing

[0094] Table 8 Main components of calcium silicate synthetic slag

[0095] (4) LF furnace refining: The bottom blowing of argon is opened in the refining ladle throughout the whole process. The refining ladle is a double-permeable brick. The flow rate of the bottom blowing of the small-flow brick is the ratio of the flow rate of the large-flow permeable brick. The maximum permeable brick argon flow rate during the feeding and alloying, heating and other time periods is as shown in Table 9; lime or high-alkalinity synthetic slag is added during the refining process. The main components of lime and high-alkalinity synthetic slag and the alkalinity oxidizing properties of the ladle slag are shown in Table 10. Manganese alloy, ferrochrome alloy and ferromolybdenum alloy are added to control the steel composition and the tapping temperature as shown in Table 11, and the steel is tapped into the RH furnace.

[0096] Table 9 Bottom blowing flow control

[0097] Table 10 Basicity and oxidizability of slag-making materials and LF refining end slag

[0098] Table 11 LF refining end point composition and temperature

[0099] (5) RH furnace refining: RH enters the station and is quickly vacuumed. When the vacuum drops below 50 mbar, carbon powder is added, and then ferrosilicon alloy, ferromanganese alloy, and ferrosulfur alloy are added for alloying, and then the working pressure of the vacuum chamber is quickly reduced; at the same time, calcium carbide and silicon carbide are added to the slag surface of the ladle to diffuse and deoxidize the slag, so that the basicity and oxidizability of the slag are controlled as shown in Table 12, the gas flow rate is increased to 150-200 Nm3 / h, the processing time is ≥15 min, and then the two to three-stage vacuum pumps are turned off, the vacuum chamber pressure is increased to above 20 mbar, and the gas flow rate is increased to 100-150 Nm 3 / h, after RH net circulation ≥5min, break the air for soft stirring and calming treatment. The specific flow pressure and time are shown in Table 13, and then transported to continuous casting.

[0100] Table 12 RH vacuum chamber pressure and RH slag basicity and oxidizability

[0101] Table 13 RH increase gas flow rate and soft stirring static treatment time

[0102] (6) Slab continuous casting: Slab continuous casting is performed using a slab continuous casting machine with a cross-section of 220 mm or 320 mm × 1800 mm × 2600 mm. The argon flow rate for the long water inlet of the main ladle, the upper water inlet of the middle ladle, and the stopper mechanism is shown in Table 14. Protective casting is performed throughout the continuous casting process, using a low-alkalinity, low-alumina tundish covering agent. The main components of the low-alkalinity, low-alumina tundish covering agent are shown in Table 15, as well as other unavoidable components. Control the high superheat of the molten steel in the tundish and use a low-melting-point mold slag. The thickness, consumption, and main components of the mold slag layer are shown in Table 16, as well as other unavoidable impurity components. Electromagnetic stirring is performed at the end of the mold; the current of the mold electromagnetic stirring is 450-650 A, and the frequency is 6-8 Hz. The superheat of the molten steel in the continuous casting tundish is 25-45℃, and the tonnage of the opening casting is ≥25t; the tonnage of the tundish during normal casting is ≥28t; the tonnage of the tundish when continuously casting and changing to a large ladle is ≥22t; the insertion depth of the submerged nozzle is 120-180mm, the taper of the crystallizer is 1.03%-1.07%, and the continuous casting speed is controlled at 0.6-1.0m / min, as shown in Table 17. The water volume control of the crystallizer during slab continuous casting is shown in Table 18; the secondary cooling section adopts the ultra-weak cooling mode, the secondary cooling water volume adopts the weak cooling mode, and the water volume of the fan-shaped sections from zone 1 to zone 11 are as follows: the water volume of the full roller section is 55-75NL / min, the inner and outer arcs of section 1 are 180-200NL / min, the inner and outer arcs of section 2 are 310-340NL / min, the inner and outer arcs of section 3 are 320-350NL / min, the inner arc of section 4 is 260-290NL / min, the inner and outer arcs of section 5 are 90-120NL / min, the inner arc of section 6 is 40-60NL / min, and the outer arc of section 6 is 60-80NL / min in, inner arc of section 7 20-40NL / min, outer arc of section 7 40-60NL / min, inner arc of section 8 35-55NL / min, outer arc of section 8 80-100NL / min, inner arc of section 9 25-45NL / min, outer arc of section 9 40-60NL / min, section 10 is consistent with section 9, inner arc of sections 11-14 15-25NL / min, outer arc of sections 11-14 25-45NL / min, specific control is shown in Table 19; total reduction of continuous casting billet and reduction ratio of sector sections 8-11 are controlled in sections as shown in Table 20.

[0103] Table 14 Protective atmosphere control

[0104] Table 15 Main components of low basicity and low alumina tundish covering agent

[0105] Table 16 Mold slag consumption and main components

[0106] Table 17 Protective atmosphere control

[0107] Table 18 Crystallizer water control

[0108] Table 19 Secondary cooling section water control

[0109] Table 20 Continuous casting reduction control

[0110] (7) The inlet temperature of the holding pit for slow cooling is ≥500℃, and the outlet temperature is ≤150℃, as shown in Table 21.

[0111] Table 21 RH vacuum chamber pressure and RH slag basicity and oxidizability

[0112] The refractory materials in contact with molten steel in the refining ladle, continuous casting tundish, long nozzle, submerged nozzle and stopper rod are all made of low-alumina material with Al2O3 mass content ≤5%.

[0113] The chemical composition of the obtained steel ingot, in addition to iron, also includes, by mass percentage: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, S: 0.05%-0.15%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, TO≤0.0015%, N≤0.0025%, H≤0.0002%, Alt≤0.0015%, Ti≤0.0010% and other inevitable impurity element components, as shown in Table 22. The steel ingot had a central segregation C of ≤1.0. At nine points uniformly sampled from the inner arc to the outer arc, at the center, the quarter-point, and the 1 cm edge of the ingot width, the ratio of the maximum, minimum, and average values ​​of the Mn and Cr elements ranged from 0.96 to 1.04. Surface defects such as vibration marks, pits, bumps, folds, and pores had a height or depth of ≤2 mm, as shown in Table 23. A photograph of the resulting steel ingot is shown in Figure 1.

[0114] Table 22 Main chemical composition of continuous casting slab

[0115] Table 23 Continuous casting billet quality

[0116] The method means disclosed in the scheme of the present invention is not limited to the method means disclosed in the above method means, but also includes a method scheme composed of any combination of the above method features.

[0117] With the above-mentioned ideal embodiment of the present invention as inspiration, and through the above description, relevant personnel can make various changes and modifications without departing from the scope of the method concept of this invention.

Claims

1. A method for manufacturing high-quality free-cutting die steel ingots, characterized in that: The chemical composition of the high-quality free-cutting die steel ingot, in addition to iron, also includes, by mass percentage, the following: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, S: 0.05%-0.15%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, TO≤0.0015%, N≤0.0025%, H≤0.0002%, Alt≤0.0015%, Ti≤0.0010% and other inevitable impurity element components. The method comprises the following steps: Step 1: Blast furnace smelting: High-sulfur iron ore or charge is selected to smelt iron in a blast furnace to form molten iron, which is then tapped into a ladle. The mass percentages of C, Si, P, and S in the molten iron are as follows: C: 4.1%-4.5%, Si: 0.2%-0.6%, P≤0.12%, and S: 0.04%-0.08%. The temperature is 1350-1430°C. After tapping, the molten iron is not skimmed and is directly transported to a converter for smelting. Step 2: Converter smelting: The converter is charged with 200±5t, the scrap ratio is 15%-20%, high sulfur scrap steel or ordinary scrap steel is used for smelting, the converter adopts double slag method, the converter adopts large bottom blowing and stirring in the early stage of blowing, the top gun is blown with low oxygen flow, pellets and lime are added to make slag, the slag basicity is 1.5-2.0, the T.Fe content in the slag is 20%-40%, the molten steel temperature is controlled to be ≤1450℃, and then 50%-70% of the slag is poured; in the second stage of blowing, the bottom blowing and top gun oxygen flow are increased, and the slag is divided into 2-4 steps. Batch adding of lime, light burning and pelletizing to form slag, stabilizing the slag basicity to 2.5-3.5, the T.Fe content in the slag to 15%-30%, blowing to a C mass content of 0.06%-0.15%, the molten steel temperature to 1600-1640°C, the oxygen mass content to 0.025%-0.045%, and the S mass content to 0.03%-0.09%, then stopping blowing and starting slag pouring, pouring out 60%-85% of the slag, then tapping, using a slide plate to block the slag, leaving the slag after tapping, and proceeding to the next furnace of smelting; Step 3: Converter tapping: At the beginning of tapping, manganese alloy, silicon alloy, ferrochrome alloy, ferromolybdenum alloy and carbon powder are first added to the ladle for deoxidation and alloying. The bottom blowing flow rate is 100-300NL / min. When 85% of the steel is tapped, sulfur-containing slag, calcium silicate synthetic slag and lime are added to make slag. The slag basicity is controlled at 1.0-1.

5. At the end of tapping, the bottom blowing flow rate is 400-800NL / min. After stirring for 3-5 minutes, the slag is transported to the LF furnace for treatment. Step 4: LF furnace refining: The bottom blowing of the refining ladle is turned on throughout the entire process. The maximum argon flow rate of the air brick is 400-500NL / min when adding lime, high basicity synthetic slag and alloying. The maximum argon flow rate of the air brick is 200-400NL / min during the heating period, and the maximum argon flow rate of the air brick is 100-150NL / min at other times. During the refining process, lime or high basicity synthetic slag is added, the basicity of the ladle slag is controlled at 1.5-2.0, 2.0%≤T.Fe+MnO≤4.5%, manganese alloy, ferrochrome alloy and ferromolybdenum alloy are added to adjust the Mn, Cr and Mo components in the molten steel to the mass range of: Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, the tapping temperature is controlled at 1600-1620℃, and the steel is tapped into the RH furnace; Step 5: RH furnace refining: RH enters the station for rapid vacuum treatment. When the vacuum degree drops below 50mbar, carbon powder is added to adjust the carbon content to the mass range C: 0.25%-0.55%. Then, ferrosilicon alloy, ferromanganese alloy and ferrosulfur alloy are added for alloying. Then, the working pressure of the vacuum chamber is quickly reduced to ≤1mbar. At the same time, calcium carbide and silicon carbide are added to the slag surface of the ladle to diffuse and deoxidize the slag. The basicity of the slag is controlled between 2.0-3.0, T.Fe+MnO≤2.0%, and the gas flow rate is increased to 150-200Nm 3 / min, processing time ≥15min, then turn off the second or third stage vacuum pump, increase the vacuum chamber pressure to above 20mbar, and increase the gas flow rate to 100-150Nm 3 / min, then RH net circulation, breaking air to tap steel, soft stirring and calming treatment after tapping, and then transported to continuous casting; Step 6: Slab continuous casting: The slab continuous casting machine is used for casting, and the continuous casting is protected during the whole process. A low-alkalinity, low-alumina content tundish covering agent is used to control the high superheat of the molten steel in the tundish. A low-melting-point mold protection slag is used, and electromagnetic stirring is performed at the end of the mold. Step 7: Cool slowly in the insulation pit.

2. The method according to claim 1, characterized in that In the step 1, the amount of slag in the iron ladle is ≤3kg / t.

3. The method according to claim 1, characterized in that The mass percentage of the components in the high-sulfur scrap steel used in the converter in step 2 is: P≤0.015%, 0.05%≤S≤0.25%, and the rest are conventional C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements; The mass percentage of the ordinary scrap steel components used in the converter in step 2 is: P≤0.015%, S≤0.03%, and the rest are conventional C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements; The scrap steel added into the converter is all high-sulfur scrap steel, all ordinary scrap steel, or a combination of high-sulfur scrap steel and ordinary scrap steel in any proportion.

4. The method according to claim 1, wherein In step 3, when 20% to 30% of the converter steel is tapped, 30% to 50% of the manganese alloy, 40% to 60% of the silicon alloy, 85% to 95% of the ferrochrome alloy, 85% to 95% of the ferromolybdenum alloy, and 45% to 65% of the carbon powder are added.

5. The method according to claim 4, characterized in that The components of the manganese alloy include, by mass percentage, Mn ≥ 98%, P ≤ 0.010%, Al ≤ 0.0035%, Ti ≤ 0.0025%, and the remainder is Fe and other inevitable impurity components; The silicon alloy comprises, by mass percentage, Si: 75%-80%, P≤0.015%, Al≤0.0050%, Ti≤0.0035%, and the remainder is Fe and other inevitable impurity components; The components of the ferrochrome alloy include, by mass percentage, Cr: 55%-65%, P≤0.015%, Al≤0.003%, Ti≤0.003%, and the remainder is Fe and other inevitable impurity components; The components of the molybdenum-iron alloy include, by mass percentage, Mo: 45%-55%, P≤0.018%, Al≤0.008%, Ti≤0.006%, and the rest are Fe and other inevitable impurity components.

6. The method according to claim 1, characterized in that In step 3, the amount of sulfur-containing slag added is 1.5-2.5 kg / t, wherein the sulfur content by mass is 2.0%-4.0%; The calcium silicate synthetic slag is added in an amount of 7-9 kg / t, and its components, calculated by mass percentage, include: CaO: 25%-35%, SiO2 45%-55%, Al2O3 ≤ 2%, MgO 3%-6%, and other inevitable components.

7. The method according to claim 1, characterized in that In step 4, the refining ladle is a double-permeable brick, and the bottom blowing flow of the small-flow brick is 30%-60% of the flow of the large-flow permeable brick. The CaO mass content of the lime component added during the refining process is: CaO ≥ 95%, and the rest are unavoidable impurity components.

8. The method according to claim 1, characterized in that The high basicity synthetic slag in step 4 comprises, by mass percentage, the following components: CaO: 65%-75%, SiO2: 0-20%, Al2O3≤5%-10%, MgO 3%-5%, and other inevitable components.

9. The method according to claim 1, characterized in that In the step 5, after the RH net circulation for 5 minutes, the air is broken for soft stirring and calming treatment, the soft stirring time is 5-10 minutes, and after the soft stirring is completed, the molten steel is allowed to stand for ≥10 minutes, and then transported to the continuous casting.

10. The method according to claim 1, characterized in that In step 6, the cross-sectional specifications of the slab continuous casting machine are 220mm×1800mm×2600mm or 320mm×1800mm×2600mm, the argon flow rate of the long water inlet of the large ladle is 150-200NL / min, the argon flow rate of the upper water inlet of the medium ladle is 5-10NL / min, and the argon flow rate of the stopper mechanism is 10-15NL / min.

11. The method according to claim 1, wherein The low basicity, low alumina content middling covering agent used in step 6 includes, by mass percentage, the following components: CaO: 30%-35%, SiO2: 50%-55%, MnO: 2%-5%, Al2O3≤2%, MgO: 3%-6%, and other inevitable components.

12. The method according to claim 1, characterized in that The thickness of the crystallizer protective slag layer in step 6 is 12-22 mm, and the consumption is 0.15-0.20 kg / t; the CaO / SiO2 in the crystallizer protective slag is 0.70-0.85, and the components of the crystallizer protective slag include, by mass percentage: Na2O: 8-12%, Li2O: 2.5-3.5%, F: 10-15%, MgO: 1-3%, elemental C: 2-4%, and other inevitable impurity components.

13. The method according to claim 1, wherein In step 6, the superheat of the molten steel in the continuous casting tundish is 25-45° C., and the pouring tonnage is ≥25t; the tundish tonnage during normal pouring is ≥28t; the tundish tonnage during continuous casting and changing to a large ladle is ≥22t; the immersion nozzle insertion depth is 120-180mm, the crystallizer taper is 1.03%-1.07%, the continuous casting speed is controlled at 0.6-1.0m / min, the crystallizer electromagnetic stirring current is 450-650A, and the frequency is 6-8Hz.

14. The method according to claim 1, wherein In step 7, the temperature entering the insulation pit for slow cooling is ≥500°C, and the temperature exiting the pit is ≤150°C.

15. The method according to claim 1, wherein The refractory materials of the refining ladle, continuous casting tundish, continuous casting ladle long nozzle, submerged nozzle and stopper rod in contact with the molten steel are all made of low-alumina material with Al2O3 mass content ≤5%.

16. Steel ingot produced by the method according to claims 1-15.

17. The steel casting billet according to claim 16, characterized in that: The central segregation C of the steel ingot is ≤1.0 level. Nine points are taken at the center of the width direction of the steel ingot, the 1 / 4 position, and 1 cm from the inner arc to the outer arc edge. The ratios of the maximum and minimum values ​​of the Mn element to the average value of all points are within the range of 0.96-1.04, and the ratios of the maximum and minimum values ​​of the Cr element to the average value are also within the range of 0.96-1.04; the height or depth of surface vibration marks, pits, bumps, folds and pores are ≤2 mm.

18. A mold made from the steel billet according to claim 16.

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