JP3416858B2 - Stainless steel manufacturing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a stainless steel material suitable for reducing the incidence of surface flaws and obtaining a high-quality mirror-finished steel sheet. 2. Description of the Related Art High-grade stainless steel sheets, particularly mirror-finished stainless steel used for decoration and reflectors (for example, JIS)
SUS304 equivalent steel) plates are usually melted in steelmaking furnaces such as AOD furnaces, VOD furnaces and converters, cast into slabs by continuous casting, and the slab surface is cleaned, then hot-rolled, acidified. It is manufactured by a step of buffing after washing, cold rolling, and bright annealing. [0003] In this type of mirror-finished stainless steel sheet, extremely minor flaws appear on the surface after buffing in the final step, and these are easily detected as defects. high. [0004] One of the causes of this ground flaw is the effect of inclusions generated during the steelmaking stage. For example, if large inclusions of several tens of μm or more appear on the surface of the mirror-finished product, or even inclusions of a size smaller than that at the slab stage, they are extended in the rolling process to 0.5 to several mm. When it appears on the product surface as a group of linearly extending inclusions, it is detected as a large linear flaw by visual observation. There is a demand for a refining and casting method that does not generate large inclusions and can control the inclusions so that they do not extend linearly even when subjected to rolling in the manufacturing stage of the mirror finishing stainless steel. . [0006] Japanese Patent Application Laid-Open No. 58-130215 discloses that in the refining of stainless steel for thin plate spring material, the production of non-metallic inclusions is suppressed in order to improve the fatigue resistance.
A deacidification method to control its morphology is shown. However, in the present invention, it is not clear whether Al can be used as a deoxidizing agent and the limit value of the Al content in relation to surface flaws that affect the specularity. [0007] JP-A-3-267312 and JP-A-Hei-4
Japanese Patent No. 99215 discloses a method for producing stainless steel for mirror finishing, which maintains sol. Al in molten steel at a low value only for the refining or finishing process. However, it is difficult to achieve a higher level of specularity with steel obtained by controlling the sol.Al value only for a point in time when molten steel is in the furnace. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide:
It is an object of the present invention to provide a method for producing a raw steel capable of obtaining a high quality mirror surface with a small occurrence rate of large flaws or linear flaws in a mirror finished stainless steel product stage. The gist of the present invention resides in the following method for producing a mirror finishing stainless steel. [0010] In the manufacturing process of stainless steel for mirror finishing, from the decarburization and reduction process to the casting process, Al
It is required to control the sol.Al content in steel to 50 ppm or less with no added material and to control the slag composition from the addition of Si to the end of the reduction step in weight% under the following conditions. A method for producing stainless steel for mirror finishing. 1.0 ≦ (% CaO) / (% SiO ₂ ) ≦ 1.5 (% Al ₂ O ₃ ) ≦ 10% (% MgO) ≦ 10% The above “Al-containing substance” includes metal Al. "Stainless steel for mirror finishing" includes both ferritic and austenitic duplex stainless steels and austenitic alloys such as high Ni, but the above method is preferably applied to austenitic stainless steels. The present invention is intended to reduce the incidence of large flaws or linear flaws in mirror-finished stainless steel, and to determine the composition of the molten steel as a condition suitable for producing high quality products. By controlling the deoxidizing conditions, in particular, the deoxidizing conditions and the slag composition conditions in the refining and refining period, it is characterized in that the inclusions are changed into harmless inclusions in terms of quality. More specifically, the following is described. In order to suppress the formation of large Al ₂ O ₃ clusters that cause large flaws, the upper limit of the sol. Al value in steel from the refining process to the casting process was determined. Al-containing substances are not added not only in the furnace in each of the decarburization and reduction steps, but also in the ladle, the tundish, and the mold. Inevitably Si alloys
If a substance containing Al must be added, the Al content is controlled to a certain value or less. In order to suppress the formation of ductile inclusions that cause linear ground defects (low melting point of inclusions), the slag basicity during the refining refining period is set low, and among the slag components, Al ₂ O ₃ And the upper limit of MgO concentration. [0017] Among the stainless steels, non-metallic inclusions, which are used for high-grade applications and are used for high-grade applications, are likely to appear as non-metallic inclusions on the product surface as ground flaws. Introduction of cleaning technology is required. In particular, when large inclusions (levels of several tens of μm or more) are formed in the steelmaking stage, they become flaws in the product stage. When ductile inclusions that are elongated and are not separated in the cold rolling step are formed, they appear as linear ground flaws having a length of 0.5 to several mm and are visually inferior, resulting in products with poor mirror finish. The following two manufacturing conditions are used to prevent the above-mentioned linear ground flaw from being generated in the mirror-finished stainless steel sheet. From the decarburization step to the reduction step and further to the casting step, an Al-containing substance such as metallic Al or an Al alloy,
It is necessary to control sol.Al in steel to 50 ppm or less without adding a flux or powder for continuous casting containing Al. Equipment that can be used in the decarburization and reduction steps
An AOD furnace, a VOD furnace, and the like, respectively. In the reduction step, slag conditions from the addition of Si to the end of the reduction step are set to 1.0 ≦ (% CaO) / (% SiO ₂ ) ≦ 1.5, and (% Al ₂ O ₃ ) ≦ 10% (% MgO) ≦ 10% needs to be controlled. First, the condition Al will be described. In the in-furnace smelting process from decarburization to reduction, for example, when metal Al is added, even if the sol.Al at the reduction end point is 50 ppm or less, the size of the steel is several tens μm. Large Al ₂ O ₃ clusters exist. The reason for this is that if Al is added, the sol.Al concentration is extremely high near the place where Al is added until Al in the steel is uniformly dispersed throughout the molten steel at the time of addition, high concentration sol.Al is to produce a large Al ₂ O ₃ clusters, therefore, large Al ₂ O ₃ clusters generated by adding phase of Al, a problem in the product stage for the case where it is held in the molten steel This is because it causes a large linear ground flaw. It is desirable that Al contained in Fe Si (ferrosilicon) or metallic Si, which is a Si source added in the reduction step as a deoxidizing agent, is as low as possible. This is because the above-mentioned large Al ₂ O ₃ cluster is also generated by the impurity Al in these Si sources. Al in metal Si is usually 0.5 Wt% or less, but there is no problem. For FeSi, a low Al product having an Al content of 1 to 0.5 Wt% is selected and used. Further, when an Al-containing substance is added in the ladle, this Al also causes a large Al ₂ O ₃ cluster to be formed, so that the Al-containing substance is not added to the ladle. Also at the time of casting, the heat for continuous casting used may contain Al for heat generation, and it is necessary to avoid the use of this kind of heat generating powder. This is because large Al ₂ O ₃ clusters are similarly formed by Al in the powder. In addition, even when Al is not added, there is a reaction in the smelting process where Al ₂ O ₃ in the slag or refractory is dissociated and eluted as sol. (For example, when a strong deoxidizing atmosphere occurs immediately after adding other deoxidizing elements such as Si and Mn to undeoxidized molten steel in an AOD furnace, VOD furnace, etc.) ₂ O ₃ clusters are formed, resulting in undesirable large flaws at the product stage. For this reason, it is not only necessary to avoid the addition of Al-containing substances in the steps from decarburization to casting, and even if they are not added, the sol. Must be kept below 50 ppm. FIG. 1 shows the maximum value of sol.Al and the coil caused by Al ₂ O ₃ inclusions in the process of manufacturing SUS304 for mirror finish material (electric furnace-AOD furnace-ladle-mold, molten steel amount 80 tons). The relationship with the ground flaw occurrence rate is shown. The coil thickness is 1.5 mm, and the slag composition is within the scope of the present invention described later. In addition, the ground flaw occurrence rate was defined by the following equation (1). Ground flaw occurrence rate (%) = [χ / (coil length (m) / 2)] × 100 (1) Here, χ represents a case where the coil is divided into coil molecules every 2 m. And the number of coil molecules having one or more ground flaws caused by inclusions. For example, if a coil of 200 m (100 coil molecules in 2 m units) has 10 coil molecules having one or more ground flaws, the ground flaw occurrence rate is 10%. As described above, by suppressing the maximum value of sol. Al in the manufacturing process to 50 ppm, the probability of forming large Al ₂ O ₃ cluster-like inclusions is reduced, and the generation of Al ₂ O ₃ inclusions is reduced. It is possible to suppress the occurrence rate of ground flaws. In the actual production process, fluctuations in various smelting conditions (for example, fluctuations in the molten steel temperature for each charge, component segregation in the molten steel, etc.) cause large fluctuations and segregations in the sol.Al concentration locally. Often. Therefore, if the sol.Al concentration is controlled more accurately and on the safe side, it is desirable that the sol.Al in the steel is always 15 ppm or less. Next, the slag composition under the conditions will be described. The inclusions in the molten steel before the deoxidation of Si during melting are Cr ₂ O ₃ .MnO, but after the addition of Si, trace amounts of Ca, Mg, Al, etc. from the slag and refractory are removed from the Si source. Al elutes, and the inclusion morphology changes over time as shown in (2) below. Cr ₂ O ₃ .MnO → Cr-Mn- (Si)-(Ca)-(Mg)-(Al) -O [Cr ₂ O ₃ .MnO-based non-ductile inclusion] → Si-Ca-Mg -Al- (Cr)-(Mn) -O (2) (Ductile inclusions) In the notation of inclusion composition in (2) above, () The element in the symbol indicates that the composition ratio of the element in the inclusion is relatively low. For example, Cr-Mn- (Si) - (Ca) - (Mg) - (Al) inclusions represented by -O mainly Cr ₂ O ₃ and containing MnO, Other small amount of SiO ₂ , CaO, MgO, Al ₂ O ₃ (eg, wt%
In each case 20% or less). Cr-Mn- (Si)-(Ca)-(Mg)-(Al) -O-based inclusions (Cr ₂ O ₃ .MnO-based non-ductile inclusions) appear on the surface of the material steel slab. However, since this is a non-ductile inclusion having a high melting point, it is not elongated even in the rolling step after the casting step. For this reason, although it is observed as a point-like flaw, it is not noticeable and does not pose a problem. On the other hand, the morphological change of inclusions has progressed further, and Si-C
If it appears as a-Mg-Al- (Cr)-(Mn) -O-based inclusions on the surface of the base steel slab, it is a ductile inclusion with a relatively low melting point. Is done. Therefore, since it is observed as a linear ground flaw, there is a problem in quality. For example, as shown in FIG. _{2 (a), Cr 2 O} 3 ·
When MnO-based non-ductile inclusions are observed on the product surface, the inclusions are not elongated, and in many cases, the flaws are short in length in the rolling direction and have no problem. . On the other hand, when the Si-Ca-Mg-Al- (Cr)-(Mn) -O-based ductile inclusions are observed on the product surface, as shown in FIG. It becomes a state ground flaw. Control is performed so that the form of inclusions after the addition of Si is limited to Cr ₂ O ₃ .MnO-based non-ductile inclusions, in other words,
In order to prevent this non-ductile inclusion from changing to the form of Si-Ca-Mg-Al- (Cr)-(Mn) -O-based ductile inclusion, it is necessary to add Si after the addition of Si in the reduction step. Slag conditions until the end of the reduction step are controlled to 1.0 ≦ (% CaO) / (% SiO ₂ ) ≦ 1.5, and (% Al ₂ O ₃ ) ≦ 10%, (% MgO) ≦ 10% There is a need. The change of the inclusion form shown in the above (2) is as follows.
The stronger the deoxidizing power of the molten steel, the faster it progresses. Also, [S
i] When the level is constant, the larger the slag basicity expressed by (% CaO) / (% SiO ₂ ), the stronger the deoxidizing power of Si acts, and the change reaction proceeds more. Therefore, as a method for controlling the form of inclusions, attention was first focused on the control of slag basicity. Since the activity of SiO _{2 in} the slag can be controlled by controlling the slag basicity, the Si deoxidizing power of the molten steel can be changed. Therefore, the reaction rate of the inclusion morphology change shown in the above (2) can be controlled. FIG. 3 shows an example of a change in inclusion morphology when the slag basicity is changed, as an example of the slag basicity and the Cr in the inclusions present in the product flaws during the reduction step in an 80 ton AOD furnace. The relationship with the ratio of ₂ O ₃ · MnO-based non-ductile inclusions is shown. FIG. 4 shows the relationship between the basicity of slag and the rate of occurrence of linear ground defects in products. As shown in FIGS. 3 and 4, when the slag basicity is 1.5 or less, most of the inclusions present at the flaws are Cr ₂ O ₃ .MnO-based non-ductile inclusions. Since the shape is not linear but point-like, there is little problem. On the other hand, when the slag basicity exceeds 1.5, Ca, Mg, Si, and Al are contained in the Cr ₂ O ₃ .MnO-based non-ductile inclusions.
Oxide components such as these increase, resulting in ductile inclusions having a low melting point, which become linear ground defects. If the slag basicity is less than 1.0, the refractory (magcro, magdro) tends to be severely melted, which is not desirable in operation. From the above results, the appropriate range of the slag basicity was 1.0 ≦ (% CaO) / (% SiO ₂ ) ≦ 1.5. Further, among the slag components, an important control item other than the slag basicity promotes a morphological change to Si-Ca-Mg-Al- (Cr)-(Mn) -O-based ductile inclusions. This is the concentration of Al ₂ O ₃ and MgO in the slag. If the concentration in these slags is too high, the amount of eluted Al and Mg tends to increase, and it is necessary to limit the upper limits of these concentrations. FIG. 5 shows the effect of Al ₂ O ₃ and Mg in slag on the rate of occurrence of linear flaws when the slag basicity is 1.5.
Indicates the effect of O. In both cases, when the concentration in the slag exceeds 10%, the rate of occurrence of linear ground defects starts to increase. Therefore, in addition to the slag basicity, the conditions of the slag composition are set as Al ₂ O ₃ ≦ 10%, MgO ≦
10%. The lower the concentration of Al ₂ O ₃ and MgO in the slag, the better. The production method of the present invention can be applied to both ferritic and austenitic duplex stainless steels and austenitic alloys such as high Ni, but is preferably applied to austenitic alloys. EXAMPLES (Example of the Present Invention) In melting SUS304 stainless steel,
1.5 to 2% to 0.04% or [C] in 80 ton AOD furnace
After decarburization to 0.05%, molten steel shown in Table 1 (after decarburization)
Got. Then, at the same time as adding low Al Si, reduction smelting is performed by contacting slag (slag amount per tens of kg of molten steel is several tens of kg) under various composition conditions shown in the table, and molten steel shown in the table (after reduction) I got [Table 1] A slab having a width of 1200 mm and a thickness of 206 mm was cast at a casting speed of 0.7 to 0.8 m / min using the above-described reduced molten steel by a vertical continuous casting machine. During this production, low Al
No Al-containing materials other than Si were added. After slab surface treatment, the slab was heated to 1200 ° C. and rolled to a thickness of 3.6 mm by a continuous hot rolling mill (rolling ratio: about 60). This hot-rolled sheet is pickled, and the thickness is reduced by a continuous cold rolling mill.
The steel sheet was rolled to 1.5 mm (rolling ratio: about 2.4), bright annealed, and secondarily cold rolled by a temper mill to produce a thin steel sheet having a thickness of 1.5 mm. After the steel sheet thus obtained was buffed, the occurrence rate of flaws was investigated using the above equation (1). The results are shown in Table 1. The ground flaw occurrence rate was 2% or less in each case, and extremely good results were obtained. (Comparative Example) As shown in Table 2, a slag condition or an Al addition condition was out of the range defined by the present invention, and a thin steel sheet was manufactured under the same conditions as those of the present invention except for these conditions. The rates were investigated. Table 2 shows these conditions and results. [Table 2] In Examples No. 4 to No. 7, the occurrence rate of ground flaws was as high as 2% or more. In the execution No. 8, the flaw generation rate was as low as 2% or less, but the slag basicity was too low.
Magro bricks, which are refractories of OD furnaces, suffered a lot of erosion, which was a problem in operation. According to the method of the present invention, it is possible to stably produce a stainless steel material suitable for obtaining a high quality mirror-finished product with few ground flaws.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1: Maximum value of sol.Al in molten steel and Al ₂ O in refining process
FIG. 3 is a diagram showing a relationship with a ground flaw occurrence rate caused by _three inclusions. FIG. 2 is a diagram showing an example of inclusions observed at a flaw. (a) is non-ductile and fine, and (b) is ductile and undesirably large. FIG. 3 is a graph showing the relationship between slag basicity and the ratio of Cr ₂ O ₃ .MnO-based non-ductile inclusions at flaws. FIG. 4 is a diagram showing a relationship between slag basicity and a linear ground defect generation rate. FIG. 5 is a diagram showing the relationship between (MgO) and (Al ₂ O ₃ ) concentrations in slag and the rate of occurrence of linear flaws.

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Claims (1)

(57) [Claims] [Claim 1] In the manufacturing process of stainless steel for mirror finishing, from the decarburization and reduction process to the casting process, Al
It is required to control the sol.Al content in steel to 50 ppm or less with no added material and to control the slag composition from the addition of Si to the end of the reduction step in weight% under the following conditions. A method for producing stainless steel for mirror finishing. 1.0 ≦ (% CaO) / (% SiO ₂ ) ≦ 1.5 (% Al ₂ O ₃ ) ≦ 10% (% MgO) ≦ 10%