JP2007186744A - Continuously cast slab for steel sheet and method for producing the same, and steel sheet and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a slab for steel sheet in which press cracking caused by large-sized inclusions is hard to occur, to provide a method for producing the same, a steel sheet, and a method for producing the same. <P>SOLUTION: The composition of inclusions in a continuously cast slab is controlled to the following, and then rolling is performed at a draft of ≥150: 80≤Ti oxide(%)+REM oxide(%)+Al<SB>2</SB>O<SB>3</SB>(%); CaO(%)≤5; 10≤Ti oxide(%); 15≤Al<SB>2</SB>O<SB>3</SB>(%)≤70; and 5≤REM oxide(%)≤0.31×Al<SB>2</SB>O<SB>3</SB>(%)+15. By this method, the steel sheet in which press cracking caused by large-sized nonmetallic inclusions upon press working is hard to occur can be obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a slab for a thin steel plate and a method for producing the same, and a thin steel plate obtained thereby and a method for producing the same.

The molten steel refined in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this excess oxygen is generally deoxidized by Al, a strong deoxidizing element with a strong affinity for oxygen. Is. However, Al generates Al ₂ O ₃ inclusions by deoxidation, and these aggregate and coalesce into coarse alumina clusters. This alumina cluster has a problem in that it adheres to the inner wall of the tundish nozzle and immersion nozzle used for pouring from the tundish into the mold and causes nozzle clogging. In particular, low carbon molten steel, which is a material for thin steel sheets with a low carbon concentration and a high dissolved oxygen concentration after refining, has a very large amount of alumina clusters, and measures to reduce alumina inclusions are a major issue.

In order to solve such problems associated with Al deoxidation, Patent Document 1 proposes a method for forming a CaO—Al ₂ O ₃ composite oxide having a low melting point by adding Ca to molten Al deoxidized steel. Yes. However, the metal Ca has a low boiling point, is difficult to add to molten steel, and the yield is not stable. Furthermore, when the yield is low, the balance between the CaO and Al ₂ O ₃ concentrations in the inclusions becomes poor and clustering is easier than in the alumina inclusions, so that there is a problem that both nozzle clogging and surface properties are deteriorated. It was.

  On the other hand, Patent Document 2 has recently developed a method of deoxidizing with Ti without adding Al. Such an Al-less Ti deoxidation method does not produce a clustered oxide. Therefore, the surface defects resulting from inclusions becoming clustered are reduced. However, in the case of this Ti deoxidation, a new problem has arisen in that Ti oxide in a solid state adheres and grows on the inner surface of the tundish nozzle, and instead induces nozzle clogging.

As a thin steel sheet that solves this problem (prevention of nozzle clogging) and is less prone to surface defects and rusting due to cluster inclusions, Patent Document 3 discloses 0.0005 mass of Ca and metal REM in titanium killed molten steel. % In which the total amount of CaO and REM oxides is 5% by mass or more and 50% by mass or less, Ti oxide is 90% by mass or less, Al ₂ A titanium killed steel material characterized by mainly containing oxide inclusions with O ₃ of 70% by mass or less is disclosed.

JP 58-154447 A JP-A-8-239731 Japanese Patent Laid-Open No. 11-343516

  The nozzle clogging of the titanium killed steel material having the above characteristics tended to decrease as compared with the conventional Al deoxidized steel. However, press cracks may occur during processing of thin steel sheets, and as a result of detailed investigation of the site where the press cracks occurred, it was found that large non-metallic inclusions were the starting point.

  That is, the titanium killed steel material having the above characteristics has a problem that press cracks due to non-metallic inclusions may occur in some cases.

  The present invention is intended to solve the above-mentioned problems of the prior art, and after preventing nozzle clogging, a slab for thin steel sheet which is less prone to press cracking due to large inclusions, and a method for producing the same, And it is providing the thin steel plate obtained by it, and its manufacturing method.

In order to solve the above-described problems, the present invention has the following configuration.
(1) By mass%, C: 0.005% or less, Si: 1% or less, Mn: 3% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.001% or more 0.015% or less, Ti: 0.005% or more and 0.3% or less, REM: 0.001% or more and 0.004% or less, Ca: 0.0004% or less, N: 0.004% or less, It is a continuous cast slab characterized in that the average composition of nonmetallic inclusions consisting of the remaining Fe and inevitable impurities and having a volume of 27,000 μm ³ or more contained inside is in the following range.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
10 ≦ Ti oxide (%)
15 ≦ Al ₂ O ₃ (%) ≦ 70
5 ≦ REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15
(2) The continuous casting according to (1), wherein the additional casting component further contains at least one of mass%, Nb: 0.1% or less, and B: 0.05% or less. It is a slab.
(3) Al deoxidation treatment is performed so that the dissolved oxygen concentration is 30 ppm or more and 200 ppm or less, then Ti-containing alloy is added to perform Ti deoxidation, and then REM-containing alloy is added to melt molten steel, The method for producing a continuous cast slab according to (1) or (2), wherein continuous casting is performed.
(4) A method for producing a thin steel sheet, wherein the continuous cast slab according to (1) or (2) is rolled at a reduction ratio of 150 or more and the final thickness is 1.2 mm or less. .
(5) By mass%, C: 0.005% or less, Si: 1% or less, Mn: 3% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.001% or more 0.015% or less, Ti: 0.005% or more and 0.3% or less, REM: 0.001% or more and 0.004% or less, Ca: 0.0004% or less, N: 0.004% or less, and the balance Fe and inevitable impurities, in the range average composition of the following volume 27,000Myuemu ³ or more non-metallic inclusions contained therein, and a volume 125,000Myuemu ³ or more non-metallic inclusions contained therein It is a thin steel plate characterized in that the number of steel sheets is 60 pieces / kg or less.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
10 ≦ Ti oxide (%)
15 ≦ Al ₂ O ₃ (%) ≦ 70
5 ≦ REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15
(6) The thin steel sheet according to (5), wherein the additional steel further contains at least 1% by mass in the range of Nb: 0.1% or less and B: 0.05% or less as an additional component. It is.

  It is possible to obtain a thin steel plate that is less prone to press cracking due to large non-metallic inclusions during pressing.

Large non-metallic inclusions remaining in the thin steel plate have a great influence on the occurrence of press cracks. FIG. 1 is a diagram showing the number of non-metallic inclusions with a volume of 125,000 μm ³ or more remaining in a thin steel sheet and the index of occurrence of press cracks. To rise.

  The inventors have made molten steel with various compositions of nonmetallic inclusions based on the titanium killed material described in Patent Document 3, conducted test casting, and investigated nonmetallic inclusions in the slab. did. Furthermore, rolling was performed from the slab while changing the rolling conditions in various ways, and the nonmetallic inclusions in the obtained thin steel sheet were investigated. Some of them were also subjected to a press test to confirm the occurrence of press cracks. As a result, it has been found that the inclusion composition in the slab and the rolling method of the slab have a great influence on the large non-metallic inclusions remaining in the thin steel sheet. That is, by controlling the inclusion composition in the slab within a specific range and performing sufficient rolling, large non-metallic inclusions in the slab are crushed and rendered harmless during rolling. On the contrary, it was found that large non-metallic inclusions remain in the thin steel sheet when the inclusion composition is out of the proper range, or when the inclusion composition is appropriate and rolling is not sufficient.

Table 1 is a table showing the relationship between the composition of non-metallic inclusions having a volume of 27,000 μm ³ or more, the inclusion residual rate in the thin steel sheet, and the number of inclusions. The rolling conditions are all the same. From a slab having a thickness of 240 mm, the steel sheet is rolled to a thickness of 3 mm by hot rolling (temperature: 1200 to 850 ° C.) and further to a thickness of 1 mm by cold rolling. It is. The inclusion residual rate is a value obtained by dividing the number in the thin steel plate by the number in the slab for nonmetallic inclusions having a volume of 125,000 μm ³ or more. If it is 0, it means that there is no inclusion of that size in the thin steel sheet, and if the number increases, a large number of inclusions remain in the thin steel sheet.

In addition, the measuring method of the magnitude | size of a nonmetallic inclusion is as follows. First, a non-metallic inclusion is obtained by electrolyzing 500 to 1000 g of a slab or thin steel plate and magnetically selecting the residue. The obtained non-metallic inclusions were classified with a mesh of 30 μm, and those remaining on the mesh were defined as non-metallic inclusions having a volume of 27,000 μm ³ or more. Furthermore, the nonmetallic inclusions were classified with a 50 μm mesh, and those remaining on the mesh were defined as nonmetallic inclusions having a volume of 125,000 μm ³ or more.

From this result, when the composition of nonmetallic inclusions with a volume of 27,000 μm ³ or more satisfies the following range, the residual rate is extremely low, and by controlling within this range, large nonmetallic inclusions in the thin steel sheet can be reduced, and the press No cracking occurs. In the present specification,% indicates mass%.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
15 ≦ Al ₂ O ₃ (%)
REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15

At this time, the reason is limited to non-metallic inclusions having a volume of 27,000 μm ³ or more because non-metallic inclusions of a size smaller than this may be mixed during solidification or in the solid phase. . It has also been confirmed that the composition of non-metallic inclusions in the slab and the composition of non-metallic inclusions in the thin steel plate are almost the same.

Table 1 also shows the state of nozzle clogging when test casting was performed. If Al ₂ O ₃ is too large _Al 2 _{O 3} nozzles obstruction caused occurs. When the amount of REM oxide is too small, nozzle clogging due to a composite oxide of Al ₂ O ₃ and Ti oxide occurs. When there is too little Ti oxide, nozzle clogging occurs due to a composite oxide of Al ₂ O ₃ and REM oxide. When nozzle blockage occurs in this way, stable operation cannot be performed, which is not preferable. Therefore, it is necessary to control the inclusion composition as follows from the viewpoint of suppressing nozzle clogging and performing stable operation.
Al ₂ O ₃ (%) ≦ 70
5 ≦ REM oxide (%)
10 ≦ Ti oxide (%)

Further, in order to completely prevent nozzle clogging and perform stable casting for a long time, it is more preferable that the following conditions are satisfied in addition to the inclusion composition range described above.
Al ₂ O ₃ (%) ≦ 50

FIG. 2 is a graph showing the relationship between the reduction ratio of the slab and the number of non-metallic inclusions having a volume of 125,000 μm ³ or more in the thin steel plate. At this time, the No. of Table 1 is used for rolling. A slab containing inclusions having the composition shown in 1 was used. When the reduction ratio is 150 or more, the number of non-metallic inclusions is 60 / kg or less, but when the reduction ratio is less than 150, the number of inclusions may increase. That is, in order to sufficiently pulverize and detoxify inclusions by rolling, rolling with a reduction ratio of 150 or more is necessary, and rolling is preferably performed to satisfy the conditions.

From these findings, it is necessary to control the internal nonmetallic inclusion composition in the following range for the slab according to the present invention.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
10 ≦ Ti oxide (%)
15 ≦ Al ₂ O ₃ (%) ≦ 70, preferably 15 ≦ Al ₂ O ₃ (%) ≦ 50
5 ≦ REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15

Here, the Ti oxide is the concentration of Ti oxide in the nonmetallic inclusion. Ti oxides have forms such as TiO ₂ , Ti ₂ O ₃ , and Ti ₃ O ₅ , which are values converted as TiO ₂ concentrations. The REM oxide is the sum of La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ and Nd ₂ O ₃ concentrations.

The reason why the concentration of each component is limited as described above is as follows.
When the sum of Ti oxide + REM oxide + Al ₂ O ₃ is less than 80%, other oxides such as CaO, MnO, and SiO ₂ are contained in inclusions, and the grinding behavior of inclusions during rolling changes. Not good for that.

  Further, among these oxides, when CaO is contained in inclusions, the pulverizability at the time of rolling becomes extremely poor, so that large non-metallic inclusions remain in the thin steel sheet, causing press cracks. Therefore, CaO needs to be 5% or less.

When the Ti oxide is less than 10%, Al ₂ O ₃ and REM oxide in the inclusions increase and nozzle clogging occurs, so it is necessary to make it 10% or more.

When Al ₂ O ₃ is less than 15%, the pulverizability during rolling deteriorates and large non-metallic inclusions tend to remain in the thin steel sheet. Further, if Al ₂ O ₃ is large, severe nozzle clogging occurs and stable casting cannot be performed, so 70% or less is necessary.

When the REM oxide exceeds (0.31 × Al ₂ O ₃ (%) + 15), the inclusion residual ratio increases as shown in Table 1. That is, there are many large non-metallic inclusions in the thin steel sheet, and the results of press cracking deteriorate. On the other hand, if the REM oxide is less than 5%, the risk of nozzle clogging increases, so the REM oxide is preferably 5% or more.

Also, if Al ₂ O ₃ is 70% or less, a small amount of stable casting is possible, but if the casting amount increases, the amount of deposits on the nozzle increases. In order to ensure productivity by mass casting, Al ₂ O ₃ is more preferably 50% or less.

  As for the reduction ratio of the slab, it is necessary to ensure a reduction ratio of 150 or more as shown in FIG. Here, the reduction ratio is the ratio of the thickness of the slab to the thickness of the final product. When the reduction ratio is less than 150, the inclusion residual rate increases, that is, a large number of large non-metallic inclusions exist in the thin steel sheet, and the results of press cracking deteriorate.

  If the reduction ratio of 150 or more is secured, the influence of other rolling conditions is small. That is, the temperature conditions of hot rolling and cold rolling, the respective reduction ratios, etc. can be set to values that satisfy the required characteristics of the steel material.

  The thickness of the thin steel sheet product at this time is 1.2 mm or less. This range is appropriate for the thickness of the thin steel plate to be pressed. When the thickness exceeds 1.2 mm, the processing becomes difficult, and press cracks due to large non-metallic inclusions in the thin steel plate are less likely to occur.

In producing the slab of the present invention, first, molten steel is deoxidized with an Al-containing alloy, and oxide inclusions mainly composed of Al ₂ O ₃ are generated in the steel. At this time, it is necessary to control the dissolved oxygen concentration after deoxidation to 30 ppm or more and 200 ppm or less. If it is less than 30 ppm, a large amount of Al ₂ O ₃ is produced, and the composition of non-metallic inclusions is mainly Al ₂ O _{3, which} is not good. If it exceeds 200 ppm, the amount of Ti oxide produced by the subsequent Ti deoxidation increases, and the inclusion composition falls outside the satisfactory range.

After the Al deoxidation, the Ti-containing alloy is subsequently deoxidized to produce non-metallic inclusions mainly composed of Al ₂ O ₃ and Ti oxide in the steel. If the Ti concentration at this time is too low, the ratio of the Ti oxide in the inclusion becomes small, which is not preferable. If the Ti concentration is too high, the amount of Ti oxide produced increases, and the thin steel plate hardens and the workability deteriorates.

  Furthermore, about the slab concerning this invention, after deoxidizing with Ti alloy, the alloy containing REM is added and a nonmetallic inclusion composition is controlled. As an alloy containing REM, in addition to metal Ce, misch metal, Fe alloy, and the like are also assumed. However, alloys containing Ca should be avoided because CaO in the inclusions increases and the risk of press cracking increases.

  At this time, if the REM concentration is too low, the REM oxide concentration in the non-metallic inclusions is small and it is difficult to obtain a predetermined effect. If the REM concentration is too high, the REM oxide concentration in the non-metallic inclusions increases, resulting in poor pulverization during rolling and cause press cracks.

  As described above, when the composition of non-metallic inclusions is limited to the range of the present invention, Al, Ti, REM, and Ca that are closely related to non-metallic inclusions among the steel components are defined in the following ranges.

When Al exceeds 0.015%, Al ₂ O ₃ in the non-metallic inclusions exceeds 70%, causing nozzle clogging and deterioration of surface properties, so 0.015% or less. Further, if it is 0.01% or more, Al ₂ O ₃ in the non-metallic inclusions exceeds 50%, and if the casting amount is increased, nozzle clogging is caused. Further, if it is less than 0.001%, Al ₂ O ₃ in the inclusion is less than 15% and does not satisfy the predetermined range, so the content is made 0.001% or more.

  If Ti is less than 0.005%, the Ti concentration is too low, so the ratio of Ti oxide in the inclusions becomes small, which is not preferable. If the Ti concentration exceeds 0.3%, the amount of Ti oxide produced is too much, and the thin steel sheet is hardened and the workability deteriorates. For this reason, it is made into 0.005% or more and 0.3% or less.

  If the REM is less than 0.001%, the REM concentration is too low, so that the ratio of the REM oxide in the inclusion becomes small and the predetermined performance cannot be obtained. When the REM concentration exceeds 0.004%, the amount of REM oxide produced is too large, the REM oxide concentration in the non-metallic inclusions increases, the pulverizability during rolling deteriorates, and press cracks are caused. For this reason, it is made into 0.001% or more and 0.004% or less.

  If Ca exceeds 0.0004%, CaO in the non-metallic inclusions exceeds 5% and press cracks occur, so the content is made 0.0004% or less.

In addition, the component composition other than the above is limited within the following range.
C: 0.005% or less for application to high workability thin steel sheet.
Si: Si has an effect of strengthening steel, and an appropriate amount is included depending on the required strength. However, if it exceeds 1%, the deep drawability deteriorates, so the content is made 1% or less.
Mn: Mn has an effect of strengthening steel, and an appropriate amount is included depending on the required strength. However, if it exceeds 3%, the deep drawability deteriorates, so 3% or less.
P: P has an effect of strengthening steel, and an appropriate amount is included depending on the required strength. However, if it exceeds 0.15%, the deep drawability deteriorates, so the content is made 0.15% or less.
S: If it exceeds 0.05%, it causes wrinkles during rolling, so 0.05% or less.
N: N decreases the workability, so is 0.004% or less.

  If necessary, Nb may be added within a range of 0.1% by mass or less, and B may be added within a range of 0.05% by mass or less.

  Nb is an element effective for refining crystal grains of a steel sheet, and is effective in improving the deep drawability of a thin steel sheet. However, if added over 0.1% by mass, there is a possibility of causing a problem of significantly increasing the deformation resistance of the steel.

  By adding B, secondary work embrittlement when deep drawing or the like is performed can be prevented. However, if added over 0.05% by mass, there is a possibility of causing a problem that the deformation resistance of the steel is remarkably increased.

When a slab containing inclusions of the above composition is rolled under the above rolling conditions, the number of large non-metallic inclusions in the obtained thin steel sheet can be extremely reduced. As shown in FIG. 1 above, when the number of non-metallic inclusions with a volume of 125,000 μm ³ or more remaining in the thin steel plate exceeds 60 / kg, the press cracking index rapidly increases, so 60 / kg. The following is preferable.

  The 300 ton molten steel after the converter steel was decarburized with an RH vacuum degasser, and metal Al was added to the molten steel to lower the dissolved oxygen concentration in the molten steel to the required concentration. Then, Ti was deoxidized by adding metal Ti or Fe-Ti alloy to the molten steel. After refluxing for 5 minutes, the required amount of metal Ce, Fe-REM alloy, and misch metal alloy (containing Ce 48%, La 37%, and Nd 10%) was added to modify the inclusions. This process was performed five times for each example, and a total of 1500 ton of molten steel was cast with a two-strand slab continuous casting apparatus to produce a slab having a thickness of 240 mm. After the end of continuous casting, the tundish nozzle and the immersion nozzle were observed. In addition, a survey of non-metallic inclusions contained in the slab was conducted.

  Subsequently, the slab was heated to 1200 ° C. and hot-rolled at a processing temperature of 1200 to 850 ° C. to produce a hot-rolled sheet having a thickness of 3 mm. Further, the hot-rolled sheet was cold-rolled to produce a cold-rolled sheet having a thickness of 1 mm. Investigation of non-metallic inclusions contained in the obtained cold-rolled sheet was conducted. Furthermore, the cold-rolled sheet was pressed and evaluated for press cracking.

  Table 2 shows the composition of the steel sheet obtained at this time, the composition of non-metallic inclusions in the slab, the residual ratio of inclusions in the product plate, the number, the results of nozzle clogging, and the occurrence of press cracks in the product plate. Show. In Table 2, no. 1 to 12 are examples of the present invention. 13 to 32 are comparative examples. No. shown in the following description of the present invention and comparative examples. No. in Table 2. It corresponds to.

(Invention Example 1) (No. 1)
The C concentration was adjusted to 0.0015% by mass after decarburization with an RH vacuum degasser. After adding metal Al, the dissolved oxygen concentration in the molten steel was reduced to 150 ppm, and then Ti deoxidation was performed using metal Ti. A Misch metal alloy was added as the REM alloy so that the REM amount was 0.1 kg / ton-steel.

  No deposits were observed on the tundish nozzle and the immersion nozzle after completion of the casting. Moreover, when the cold-rolled sheet was pressed, no press cracks were observed.

(Invention Example 2) (No. 2 to 11)
In the molten steel after decarburization treatment with an RH vacuum degassing apparatus, metal Al was added to reduce the dissolved oxygen concentration in the molten steel to 50 to 200 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. Inclusion reforming was performed by adding a misch metal alloy or an Fe-Si-REM alloy as the REM alloy so that the amount of REM was 0.05 to 0.1 kg / ton-steel.

  No deposits were observed on the tundish nozzle and the immersion nozzle after completion of the casting. Moreover, when the cold-rolled sheet was pressed, no generation of press cracks was observed.

(Invention Example 3) (No. 12)
Metal molten Al was added to the molten steel after the decarburization treatment with the RH vacuum degassing apparatus to reduce the dissolved oxygen concentration in the molten steel to 40 ppm, and then Ti deoxidation was performed using Fe-Ti. The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. Inclusion reforming was performed by adding a misch metal alloy as the REM alloy so that the amount of REM was 0.1 kg / ton-steel.

  This process was performed five times, and a total of 1500 tons of molten steel was cast with a two-strand slab continuous casting machine to produce slabs. After about 1200 tons of casting, nozzle clogging occurred, and the sliding plate for flow rate adjustment opened. It became a little bit and the hot water level was changed. Deposits were observed on the tundish nozzle and the immersion nozzle after completion of casting. On the other hand, when the above-mentioned cold-rolled sheet was pressed, almost no press cracks were observed.

(Comparative example 1) (No. 13-16)
In the molten steel after decarburization treatment with an RH vacuum degassing apparatus, metal Al was added to reduce the dissolved oxygen concentration in the molten steel to 50 to 200 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. A misch metal alloy was added as a REM alloy so that the amount of REM was 0.02 kg / ton-steel or less, and inclusion modification was performed.

This process was performed five times, and a total of 1500 tons of molten steel was cast with a two-strand slab continuous casting machine to produce slabs. From about 900 tons of casting, nozzle clogging occurred, and a sliding plate for flow rate adjustment opened. After a 1200 ton casting, the flow rate could not be adjusted and the casting was stopped. When the tundish nozzle and the immersion nozzle were observed after the continuous casting, non-metallic inclusions mainly composed of Ti oxide and Al ₂ O ₃ were observed. In this case, since the amount of REM alloy added was too small, the inclusion composition was out of the proper range.

(Comparative Example 2) (No. 17, 18)
In the molten steel after decarburization treatment by the RH vacuum degassing apparatus, metal Al was added to reduce the dissolved oxygen concentration in the molten steel to 210 to 250 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. Inclusion reforming was performed by adding a misch metal alloy as the REM alloy so that the amount of REM was 0.1 kg / ton-steel.

This treatment was performed five times, and a total of 1500 ton of molten steel was cast with a 2-strand slab continuous casting apparatus. When the tundish nozzle and the immersion nozzle were observed after the continuous casting, almost no deposits were observed. However, many large non-metallic inclusions remained in the cold-rolled sheet, and press cracks occurred during press working. In this case, the concentration of Al ₂ O _{3 in} the inclusions was low, and the inclusion composition was outside the appropriate range, and the residual rate of inclusions increased.

(Comparative example 3) (No. 19-22)
Metal Al was added to the molten steel after decarburization with an RH vacuum degassing device to reduce the dissolved oxygen concentration in the molten steel to 5 to 25 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. Inclusion reforming was performed by adding a misch metal alloy as the REM alloy so that the amount of REM was 0.1 kg / ton-steel.

  This treatment was performed five times, and a total of 1500 ton of molten steel was cast with a two-strand slab continuous casting machine to produce a slab. In each case, nozzle clogging occurred. About 19 and 20, casting was stopped on the way. When the tundish nozzle and the immersion nozzle were observed after completion, adhesion of nonmetallic inclusions mainly composed of Al2O3 was observed. In this case, since the Al deoxidation was not properly performed, the inclusion composition was out of the proper range.

(Comparative example 4) (No. 23-29)
In the molten steel after decarburization treatment with an RH vacuum degassing apparatus, metal Al was added to reduce the dissolved oxygen concentration in the molten steel to 50 to 200 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. The misch metal alloy was added as the REM alloy so that the REM amount was 0.12 to 0.2 kg / ton-steel, and the inclusion modification was performed.

  This treatment was performed five times, and a total of 1500 ton of molten steel was cast with a 2-strand slab continuous casting apparatus. When the tundish nozzle and the immersion nozzle were observed after the continuous casting, almost no deposits were observed. However, many large non-metallic inclusions remained in the cold-rolled sheet, and press cracks occurred during press working. In this case, the REM oxide concentration in the inclusions was high, and the inclusion composition was out of the proper range, and the inclusion residual rate increased.

(Comparative example 5) (No. 30-32)
In the molten steel after decarburization treatment with an RH vacuum degassing apparatus, metal Al was added to reduce the dissolved oxygen concentration in the molten steel to 50 to 200 ppm, and then Ti deoxidation was performed using Fe-Ti. . The amount of Fe—Ti alloy added for Ti deoxidation was changed in accordance with the target component value. Inclusion reforming was performed by adding a misch metal alloy as the REM alloy so that the amount of REM was 0.1 kg / ton-steel. Furthermore, Ca-Si alloy wire was added into the molten steel, and Ca treatment was performed.

  This treatment was performed five times, and a total of 1500 ton of molten steel was cast with a 2-strand slab continuous casting apparatus. When the tundish nozzle and the immersion nozzle were observed after the continuous casting, almost no deposits were observed. However, many large non-metallic inclusions remained in the cold-rolled sheet, and press cracks occurred during press working. In this case, since Ca was added, CaO was mixed in the inclusions, and the inclusion composition was out of the proper range, so that the inclusion residual rate increased.

  Next, the molten steel treated by the method shown in Example 1 of the present invention was cast with a continuous casting apparatus having a different mold size, and a slab having a thickness of 90 to 400 mm was produced. These slabs were hot-rolled and cold-rolled under several conditions to produce cold-rolled plates having a thickness of 0.5 to 1.2 mm. Investigation of non-metallic inclusions contained in the obtained cold-rolled sheet was conducted. Furthermore, the cold-rolled sheet was pressed and evaluated for press cracking.

  Table 3 shows the slab thickness, product plate thickness, rolling reduction ratio, residual ratio of inclusions in the product plate, number, and the occurrence of press cracks as results. In Table 3, no. Nos. 33 to 45 are examples of the present invention. 46 to 49 are comparative examples. No. shown in the description of the present invention examples and comparative examples below. No. in Table 3. It corresponds to.

(Invention Example 4) (No. 33 to 45)
A slab having a thickness of 90 to 400 mm was hot-rolled and cold-rolled so that the reduction ratio was 150 or more to produce a cold-rolled sheet having a thickness of 0.5 to 1.2 mm. When the investigation of the nonmetallic inclusions contained in the obtained cold-rolled sheet was carried out, all of them were satisfied at 60 pieces / kg or less. Furthermore, when the cold-rolled sheet was pressed, no press cracks were observed.

(Comparative Example 6) (No. 46-49)
A slab having a thickness of 90 to 400 mm was hot-rolled and cold-rolled so as to be less than a reduction ratio, and a cold-rolled sheet having a thickness of 0.8 to 1.2 mm was produced. The investigation of non-metallic inclusions contained in the obtained cold-rolled sheet exceeded 60 pieces / kg. Furthermore, when the cold-rolled sheet was pressed, press cracks occurred.

  The present invention can be applied to the manufacture of a thin steel plate that is less prone to press cracking.

6 is a graph showing the relationship between the number of inclusions in a product plate and the frequency of occurrence of press cracks. It is the graph which showed the relationship between the reduction ratio of slab, and the number of inclusions in a product board.

Claims (6)

In mass%, C: 0.005% or less, Si: 1% or less, Mn: 3% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.001% or more and 0.015 %: Ti: 0.005% or more, 0.3% or less, REM: 0.001% or more, 0.004% or less, Ca: 0.0004% or less, N: 0.004% or less, and the balance Fe and A continuous cast slab comprising an inevitable impurity and having an average composition of nonmetallic inclusions having a volume of 27,000 μm ³ or more contained in the interior in the following range.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
10 ≦ Ti oxide (%)
15 ≦ Al ₂ O ₃ (%) ≦ 70
5 ≦ REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15 The continuous cast slab according to claim 1, further comprising at least one additional component in the range of mass%, Nb: 0.1% or less, and B: 0.05% or less as an additional component. Al deoxidation treatment is performed so that the dissolved oxygen concentration is 30 ppm or more and 200 ppm or less, then Ti-containing alloy is added to perform Ti deoxidation, and then REM-containing alloy is added to melt molten steel, and continuous casting is performed. The manufacturing method of the continuous cast slab of Claim 1 or 2 characterized by implementing. A method for producing a thin steel sheet, comprising rolling the continuous cast slab according to claim 1 or 2 at a reduction ratio of 150 or more to a final thickness of 1.2 mm or less. In mass%, C: 0.005% or less, Si: 1% or less, Mn: 3% or less, P: 0.15% or less, S: 0.05% or less, Al: 0.001% or more and 0.015 %: Ti: 0.005% or more, 0.3% or less, REM: 0.001% or more, 0.004% or less, Ca: 0.0004% or less, N: 0.004% or less, and the balance Fe and The average composition of nonmetallic inclusions that are unavoidable impurities and have a volume of 27,000 μm ³ or more contained in the interior is in the following range, and the number of nonmetallic inclusions that have a volume of 125,000 μm ³ or more contained therein is A thin steel sheet characterized by being 60 pieces / kg or less.
80 ≦ Ti oxide (%) + REM oxide (%) + Al ₂ O ₃ (%)
CaO (%) ≦ 5
10 ≦ Ti oxide (%)
15 ≦ Al ₂ O ₃ (%) ≦ 70
5 ≦ REM oxide (%) ≦ 0.31 × Al ₂ O ₃ (%) + 15 The thin steel sheet according to claim 5, further comprising, as an additional component, at least 1% by mass in a range of Nb: 0.1% or less and B: 0.05% or less.

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