JP5106153B2 - Stainless steel with excellent surface properties - Google Patents

Description

  The present invention relates to a Ti-containing stainless steel having excellent surface properties.

  Stainless steel containing Ti tends to generate Ti nitride (TiN) in the melting stage, and the clustered TiN becomes a surface defect of the slab and may remain as a surface defect after rolling. In addition, Ti-containing stainless steel also produces hard Al—Mg—Ti-based oxides and Ca—Ti-based oxides at the melting stage, and surface defects due to coarse inclusions are likely to occur. is there.

  As a countermeasure, as in Patent Document 1, by appropriately controlling the Ti, N, and Al concentrations in the steel, a method of preventing the accumulation of inclusions and the generation of large inclusions due to adhesion and aggregation on the inner wall of the nozzle Is disclosed.

  Further, as disclosed in Patent Document 2, a method for preventing nozzle clogging in which inclusions adhere and aggregate on the nozzle inner wall by appropriately controlling the Ti, Al, and Ca concentrations in the steel is disclosed.

Further, as in Patent Document 3, by properly controlling the Ti and N concentrations in the steel and the (SiO ₂ ) concentration in the slag of the refining furnace, a method of preventing nozzle clogging in which inclusions adhere and aggregate on the nozzle inner wall Is disclosed.

Further, as disclosed in Patent Document 4, a method for preventing deckle due to accumulation of TiN-based inclusions by appropriately controlling Ti, N, and Si concentrations in steel is disclosed.
JP 56-35755 A JP-A-8-39206 JP-A-3-28314 Japanese Patent No. 3925697

  The above-described conventional techniques are intended for deckles caused by TiN resulting from accumulation of inclusions in the mold and exfoliated substances that exfoliate inclusions that cling to the inner wall of the injection nozzle during casting and mix into the mold. . With these methods, complete casting is possible, and large slab defects can be reduced, but they cannot be completely eliminated. Therefore, it is necessary to remove defects by surface care of the slab and to perform rolling, resulting in a decrease in yield. . In addition, it is necessary to perform surface grinding after rolling, and the current process is complicated.

  The present invention provides a stainless steel having excellent surface properties, in which the surface quality of a slab is further improved in a stainless steel containing Ti, and surface flaws are not generated during hot rolling.

The present invention has been made to solve the above-mentioned problems, and unlike the above-described prior art, in order to control deoxidation and inclusion composition, a slight amount of Mg is controlled, and further improvement of surface quality is achieved. The gist is as follows.
(1) By mass%, C: 0.005 to 0.07%, Si: 0.4% or less, Mn: 0.5 to 4.0%, P: 0.05% or less, S: 0.01 %: Cr: 16-19%, Ni: 13% or less, Ca: 0.0005-0.0035%, Mg: 0.0001-0.0015%, Al: 0.01-0.07%, Ti : 0.1 to 0.5%, N: 0.02% or less, balance Fe and inevitable impurities, and satisfy both Ti × N: 0.0050 or less, Al / Ti: 0.10 or more Stainless steel with excellent surface properties.

(2) The above (1), further comprising one or more of Mo: 3.0% or less, Cu: 3.5% or less, and Nb: 0.5% or less in terms of mass%. Stainless steel with excellent surface properties as described in 1.

  The present invention is to obtain stainless steel with excellent surface properties by controlling the inclusions in steel by adjusting the Ti, Al, Ca, Mg, and N concentrations in stainless steel to render them harmless. .

  In the present invention, in order to prevent the formation of clustered TiN, in addition to controlling the Ti and N concentrations, the Si concentration is regulated.

  FIG. 1 shows the relationship between Ti × N and the occurrence of shaving after thick plate rolling. From the result of visual observation of the state of occurrence of whipping on the thick plate, the evaluation was divided into ○, Δ, and ×. ○ indicates that there is no problem as a product with no maintenance at all, Δ indicates that it can be remedied by grinding (entire maintenance), and × indicates that a failure that cannot be used at all has occurred. The Si concentration is in the range of 0.3 to 0.4%. When Ti × N is 0.030 or less, TiN is hardly generated at the molten steel stage, so that the rate of occurrence of shaving is low.

  FIG. 2 shows the relationship between the Si concentration and the occurrence of shave after thick plate rolling. Ti × N is in the range of 0.035 to 0.050. When the Si concentration is 0.4 or less, TiN is hardly generated at the molten steel stage, so that the rate of occurrence of hege is low, and Si has a large effect of suppressing TiN generation.

  Furthermore, in the present invention, generation of surface defects is prevented by suppressing generation of hard and harmful Al—Mg—Ti oxides and Ca—Ti oxides. FIG. 3 shows the relationship between Al / Ti and the rate of occurrence of lashes after thick plate rolling. When Al / Ti is less than 0.10, a large number of hard Ca—Ti-based oxides are generated, and thus surface defects frequently occur during rolling. Further, nozzle clogging tends to occur in the continuous casting process.

Ca is an effective element that can control the inclusion composition by adding a small amount. Moreover, Ca addition is effective for nozzle clogging in the continuous casting process. In order to produce a harmless Ca—Al—Ti oxide with a low melting point, 0.0005% or more must be added. However, if a large amount is added, a Ca—Ti-based hard oxide is generated, which causes surface flaws during rolling, so 0.0035% or less is desirable. In order to lower the melting point of inclusions, the concentration of (TiO ₂ ) in the inclusions is preferably less than 15%. FIG. 4 shows the relationship between the (TiO ₂ ) concentration in inclusions and the occurrence of lashes after thick plate rolling. Note that a large amount of TiN is excluded. When the (TiO ₂ ) concentration in the inclusion is 15% or less, the rate of occurrence of heges becomes low.

  Mg can be added as a deoxidizer as a Ni-Mg alloy or the like in a ladle or tundish. Addition of a small amount can deoxidize and lower melting point of inclusions. However, when Mg concentration in molten steel exceeds 0.0015%, harmful Al-Mg-Ti hard oxide is generated. Should be avoided. The slag and refractory used in the refining process contain Mg oxide, which may be generated by the reaction of the slag and refractory with molten steel. The Mg concentration is preferably 0.0005% or more.

  The reason for limiting the component composition (mass%) according to the present invention will be described together with the action of each element.

  C is a strong austenitizing element and is strengthened by solid solution, so 0.005% or more is added. However, if the content is increased, carbide is generated and the corrosion resistance is deteriorated, so the content is made 0.07% or less.

  Si is an element that acts as a deoxidizer during the melting of stainless steel, but in the present invention, it is necessary to control it to 0.4% or less from the viewpoint of preventing the formation of Ti inclusions.

  Mn is a deoxidizer and has an effect of improving hot workability, and is an element effective for fixing S as MnS and preventing the occurrence of red heat embrittlement due to the formation of FeS. However, if it is contained in a large amount, the refractory melting loss during melting will increase and the corrosion resistance will deteriorate, so it is 4.0% or less.

  P is an impurity in the steelmaking process, but if contained in a large amount, hot workability is impaired, so the upper limit is made 0.05% or less.

  S lowers the hot workability, makes it easier to generate cracking defects during hot rolling, and deteriorates the corrosion resistance, so the content is made 0.01% or less.

  Cr is a basic element of stainless steel and contributes to improvement of corrosion resistance and oxidation resistance, but has strong interaction with O and N, and its influence changes depending on the concentration level. Therefore, in order to achieve the target inclusion control in the present invention, the Cr concentration is set to 16 to 19%.

  Ni is an element having an effect of improving the corrosion resistance and toughness of steel, but in addition to being expensive, it has a strong interaction with Al, and its influence level changes at a high concentration.

  Al acts as a powerful deoxidizer when added in an amount of 0.01% or more. However, since a harmful Al—Mg—Ti hard oxide is produced when a large amount of Al is contained, the upper limit of Al is set to 0.06%.

  Ti is an element effective for improving corrosion resistance, and is added in an amount of 0.1% or more. However, if it exceeds 0.5%, the hot workability deteriorates rapidly, so it is necessary to control it to 0.5% or less.

  N is an element that contributes to the stabilization of austenite, etc., but at the same time, it is an element that is effective for improving the strength. There is a need to.

  Mo is not a scale that is an effective element for improving the corrosion resistance, but has an effect of strengthening solid solution. If necessary, 0.05% or more is added. However, if it exceeds 3.0%, the hot workability deteriorates rapidly, so it is necessary to control it to 3.0% or less.

  Cu is an austenite stabilizing element and is an element having an action of improving the corrosion resistance. Therefore, it is desirable to add 0.2% or more. However, if it is contained in a large amount, the hot workability is impaired, so it is necessary to be 3.5% or less.

  Nb is an element effective for improving corrosion resistance and has an effect of strengthening solid solution, so 0.05% or more is added as necessary. However, if it exceeds 0.5%, the hot workability deteriorates rapidly, so it is necessary to control it to 0.5% or less.

  Stainless steel Nos. 1 to 15 containing the chemical composition shown in Table 1 were melted in an electric furnace and AOD process to produce a 143 mm thick continuous cast slab. These slabs were heated to 1200 ° C. and hot-rolled to a thickness of 10 mm on a thick plate rolling line.

Table 2 shows the state of occurrence of scabs on a thick plate. The state of occurrence of the lashes was evaluated by dividing into the above-mentioned circles, triangles, and circles. All the evaluations of the lashes of the steels of the present invention were ○. Table 2 also shows the form of inclusions and the amount of (TiO ₂ ) in the inclusions.

  The inclusions were subjected to composition analysis for any 10 inclusions having a size of 5 μm or more using an electron microscope and EDS. The inclusions of the steel of the present invention are harmless Ca—Al—Ti oxides, and the intended results were obtained.

Compared with this, since the comparative steel 16 was too high in Ti × N, galling occurred. Since the comparative steel 17 was too low in Al / Ti, scabs were generated. Moreover, the clogging in the immersion nozzle occurred at the end of continuous casting, and the casting was stopped. The comparative steel 18 had a low Ca, and galling occurred. Moreover, the clogging in the immersion nozzle occurred at the end of continuous casting, and the casting was stopped. Since the comparative steel 19 had an excessively high Mg, galling occurred. Since the comparative steel 20 was too high in Si, galling occurred. Since the comparative steel 21 was too high in Cr, galling occurred. Since the comparative steel 22 was too high in S, it caused baldness. Since the comparative steel 23 had too high P, scabs were generated. Since the comparative steel 24 was too high in Ni, galling occurred. Since the comparative steel 25 was too high in Al, galling occurred. Since the comparative steel 26 was too high in Mg, galling occurred. Since the comparative steel 27 was too high in Ca, galling occurred. Since the comparative steel 28 was too high in Ti, scabs were generated. Since the comparative steel 29 was too high in N, scabs were generated. Since the comparative steel 30 had Mo too high, the baldness was generated. Since the comparative steel 31 was too high in Cu, scabs were generated. Since the comparative steel 33 was too high in Nb, scabs were generated.

It is a figure which shows the result of having investigated the relationship between Ti * N and the occurrence of balding. It is a figure which shows the result of having investigated the relationship between Si density | concentration and the occurrence of shaving. It is a figure which shows the result of having investigated the relationship between Al / Ti and the occurrence of scab. It is a figure which shows the relationship between the (TiO2) density | concentration in inclusions, and scab generation.

Claims (2)

  In mass%, C: 0.005 to 0.07%, Si: 0.4% or less, Mn: 0.5 to 4.0%, P: 0.05% or less, S: 0.01% or less, Cr: 16 to 19%, Ni: 13% or less, Ca: 0.0005 to 0.0035%, Mg: 0.0001 to 0.0015%, Al: 0.01 to 0.06%, Ti: 0.00. 1 to 0.5%, N: 0.02% or less, remaining Fe and inevitable impurities, and satisfying both Ti × N: 0.0050 or less and Al / Ti: 0.10 or more Stainless steel with excellent surface properties.   The surface property according to claim 1, further comprising one or more of Mo: 3.0% or less, Cu: 3.5% or less, and Nb: 0.5% or less in terms of mass%. Of excellent stainless steel.

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