JP4285869B2 - Method for producing Cr-containing thin steel sheet - Google Patents

Description

【００２９】
【表２】

【００３０】
【発明の効果】
以上に説明した通り、本発明により耐リジング性に優れた高純度Ｃｒ含有薄鋼板を提供することが可能となった。
【図面の簡単な説明】
【図１】Ｍｇ：０．００２８％を含む高純Ｃｒ含有鋼を過熱度を変化させて鋳造し、熱間圧延、冷間圧延さらに仕上げ焼鈍して薄鋼板とし、リジング高さを測定した結果である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a Cr-containing thin steel sheet, particularly a high-purity Cr-containing thin steel sheet having excellent ridging resistance.
[0002]
[Prior art]
Cr-containing steel is excellent in corrosion resistance and heat resistance, and is used in a wide range of applications such as building materials and automotive parts.
In particular, among so-called thin steel sheets with a thickness of 5 mm or less, high purity Cr-containing thin steel sheets that reduce C and N, and further fix these with added Ti to reduce the amount of dissolved C and N in the steel are: Since the press limit and deep drawing limit are high, they are used for applications requiring high workability such as automobile exhaust system members.
[0003]
When a Cr-containing thin steel plate typified by ferritic stainless steel is press-formed, striped irregularities along the rolling direction called ridging may occur. Ridging not only impairs the aesthetics of the molded product, but also causes a polishing load to remove the ridging, which is a problem when press-forming a Cr-containing thin steel sheet.
[0004]
In order to improve the ridging resistance, it is necessary to finely divide the coarse colony band resulting from the solidified structure at the time of casting. This method promotes recrystallization during hot rolling, and the solidified structure is made of equiaxed crystals. A method of making grains has been proposed.
For techniques for finely dividing the colony band by promoting recrystallization of ferritic stainless steel in the hot rolling process and the annealing process after hot rolling, for example, Japanese Patent Laid-Open Nos. 4-160117 and 5-179358 are disclosed. It is disclosed in the publication. These techniques can improve the ridging resistance of a general-purpose ferritic stainless steel sheet in which the amounts of C and N are not particularly reduced. However, in the high-purity Cr-containing steel that is the subject of the present invention, the slab structure before hot rolling is coarse compared to such a general-purpose steel. The effect of reducing ridging is hardly obtained.
[0005]
Also, as a method for suppressing the generation of ridging by finely dividing the colony band by equiaxial crystallization and fine graining of the slab structure, for example, Japanese Patent Application Laid-Open No. 52-47522 is effective for low temperature casting and electromagnetic stirring. Techniques for combining them are disclosed. However, even in this technique, the effect is small with respect to the high-purity Cr-containing steel that is the subject of the present invention, and columnar crystals grow coarsely particularly in the vicinity of ¼ part in the thickness direction of the slab.
[0006]
Japanese Patent Application Laid-Open No. 10-324956 discloses a technique for equiaxial crystallization and fine graining of a slab structure by utilizing an oxide containing Mg as an inoculation nucleus for solidification. Although this technique can greatly improve the ridging resistance of ferritic stainless steel, such steel cannot be obtained unless solidification of the molten steel is started within 180 seconds after the addition of Mg. In the actual machine production line, it takes time to transfer molten steel from the refining equipment to which the Mg is added to the casting equipment, and to wait for the timing at which casting is possible, so it is very difficult to start solidification within 180 seconds after the addition of Mg.
[0007]
[Problems to be solved by the invention]
As described above, it has been impossible to sufficiently improve the ridging resistance of the high purity Cr-containing thin steel sheet by the conventional manufacturing technique.
Then, the subject of this invention is providing the manufacturing method of the high purity Cr containing thin steel plate excellent in ridging resistance by dividing | segmenting a colony band finely.
[0008]
[Means for Solving the Problems]
The present inventors have studied in detail the composition, size, and distribution of inclusions effective for finely dividing the colony band of high-purity Cr-containing steel. Moreover, the manufacturing method effective in producing | generating such an inclusion was also investigated. As a result, it became clear that the ridging resistance of the thin steel sheet is greatly improved when Mg-based oxides having a maximum diameter in the range of 0.5 to 10 μm are present in the cross section of the product plate at 3 pieces / mm ² or more. . Furthermore, it discovered that such an oxide was produced | generated by adding 0.0005-0.005% of Mg to high purity Cr containing steel, and making molten steel superheat degree 50 degrees C or less, and completed this invention. . In addition, molten steel superheat degree is a molten steel temperature at the time of making liquid phase temperature into a standard.
[0009]
That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.015% or less, Cr: 3 to 30%,
Ti: 8 × (C + N) to 0.4%, N: 0.02% or less,
Mg: 0.0005 to 0.005%
Steel, the balance being Fe and inevitable impurities, cast at a molten steel superheat degree of 50 ° C. or less, hot-rolled , and scraping the steel plate after the hot rolling at 650 ° C. or higher, or After holding the steel strip after the hot rolling at a temperature range of 800 to 1100 ° C. for 10 minutes or less, cold rolling and finish annealing are performed, and after the finish annealing, Mg having a maximum diameter of 0.5 to 10 μm. A method for producing a Cr-containing thin steel sheet, comprising 3 or more oxides per mm ² .
(2) In steel, by mass%,
Mo: 2.0% or less, Ni: 2.0% or less, Cu: 2.0% or less, containing one or more of Cr-containing thin steel sheets as described in (1) above Method.
(3) In steel, further in mass%,
Nb: 0.5% or less, V: 0.5% or less, Zr: 0.5% or less, or one or two or more of Cr content as described in (1) or (2) above Manufacturing method of thin steel sheet.
(4) The method for producing a Cr-containing thin steel sheet according to any one of (1) to (3), wherein the steel further contains B: 0.005% or less by mass%. .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
First, the present invention will be described in detail based on the following experimental results.
By mass%, C: 0.0050%, Cr: 17.2%, Ti: 0.20%, Mo: 1.2%, N: 0.0068%, Mg: 0.0028% High purity Cr-containing steel was melted in the laboratory and cast with varying superheat. Further, FIG. 1 shows the result of measuring the ridging height of the obtained slab by hot rolling, cold rolling, and finish annealing to form a thin steel plate. The ridging height was obtained by applying a tensile strain of 16% to the thin steel plate and measuring the height of the unevenness in the direction perpendicular to the rolling direction.
As is apparent from FIG. 1, the ridging height of the finish annealed plate is lowered by lowering the degree of superheating of the molten steel, and has a very excellent ridging resistance when the degree of superheating of the molten steel is 50 ° C. or less.
[0011]
Investigation of these thin steel sheets revealed that the ridging height corresponds to the density of the Mg-based oxide. That is, as the density of the Mg-based oxide increases, the ridging height decreases, and when the density of the Mg-based oxide having a maximum diameter of 0.5 to 10 μm is 3 pieces / mm ² or more, the ridging height decreases greatly. Was.
[0012]
Next, the limited range of this invention is described.
C: It is necessary to set it as 0.015 mass% or less. If it exceeds 0.015%, corrosion resistance deterioration of the weld heat-affected zone due to grain boundary precipitation of Cr carbide tends to occur. In addition, it is an element that reinforces steel remarkably because it dissolves in an interstitial form. If it exceeds 0.015%, workability deteriorates, and further, the necessary amount of Ti for fixing C increases. For these reasons, the upper limit was made 0.015%.
[0013]
Cr: It is necessary to set it as 3 to 30 mass%. In order to obtain excellent corrosion resistance and heat resistance, it is necessary to contain 3% or more. However, even if the content exceeds 30%, the required performance has already been sufficiently satisfied, and it merely increases the alloy cost.
[0014]
It is necessary to be not less than 0.4% by mass at least 8 times Ti: C + N. Ti is easily bonded to C and N, and has the effect of substantially reducing the amount of C and N dissolved in the steel. As a result, workability can be improved. This effect is insufficient with a content of less than 8 × (C + N). Moreover, even if it contains exceeding 0.4%, the hot workability of steel will be reduced and it will cause the flaw generation | occurrence | production in hot rolling, Therefore The upper limit was made into 0.4%.
[0015]
Mg: It is necessary to be 0.0005% or more and 0.005% or less. Addition of 0.0005% by mass or more is necessary to obtain an Mg-based oxide that is an additive element forming the basis of the present invention and that finely divides a colony band. However, even if the content exceeds 0.005%, the effect is saturated and only the corrosion resistance deteriorates, so the upper limit was made 0.005% by mass.
[0016]
N: It is necessary to set it as 0.02 mass% or less. It is an element that reinforces steel remarkably because it dissolves in an interstitial form in the same way as C, and if it exceeds 0.02%, workability deteriorates, and further, the necessary amount of Ti for fixing N increases. Therefore, the upper limit was made 0.02%.
[0017]
Mo, Ni, Cu: For applications that require high corrosion resistance, it is effective to add one or more of these elements within a range of 2.0% by mass or less. Even if the content exceeds 2.0%, the effect is saturated and the alloy cost is increased, so the upper limit was made 2.0%.
[0018]
Nb, V, Zr: It is effective to add one or more of these elements within a range of 0.5% by mass or less, particularly for applications that require high corrosion resistance of the weld. These elements easily combine with C and N, and suppress the precipitation of Cr carbonitride in the weld heat affected zone, thereby improving the corrosion resistance. Even if the content exceeds 0.5%, the effect is saturated, resulting in an increase in alloy costs and a decrease in ductility, so the upper limit was made 0.5%.
[0019]
B: For applications that require particularly high secondary workability, it is effective to add in a range of 0.005% by mass or less. B has an effect of repairing defects in the steel generated when the product is processed and improving secondary workability. However, if the B content exceeds 0.005%, the hot workability of the steel is lowered, and cracks and wrinkles occur during hot rolling, so the upper limit was made 0.005%.
[0020]
In the present invention, it is considered that the Mg-based oxide acts as a solidification nucleus of ferrite during casting, and the slab structure is equiaxed and refined to finely divide the colony band. By acting as pinning particles during heating, it is considered that there is an effect of suppressing the grain growth of ferrite and promoting recrystallization during hot rolling. In order to produce an Mg-based oxide having such an effect, after adding Mg to the steel in the range of 0.0005% or more and 0.005% or less as described above, the superheat degree of the molten steel at the time of casting is set. It is necessary to make it 50 degrees C or less. When the superheated degree of molten steel exceeds 50 ° C, the density of Mg-based oxides during solidification decreases and the slab structure becomes coarse columnar crystals, and ferrite grains grow during reheating of hot rolling, and hot rolling Since the recrystallization at the time is delayed, the colony band is not finely divided.
[0021]
Examples of Mg-based oxides include Mg and Ti composite oxides in addition to Mg alone, and inevitable impurities such as Al and Ca and Mg composite oxides. As long as Mg is contained in the product, it has a function of finely dividing the colony band. Furthermore, even if these oxides are covered with TiN, there is an effect of finely dividing the colony band.
[0022]
Regarding the size of the Mg-based oxide, those having a maximum diameter in the range of 0.5 to 10 μm have a function of finely dividing the colony band. This is because if the maximum diameter is less than 0.5 μm, it does not act as a solidification nucleus of ferrite during casting. Moreover, when the maximum diameter exceeds 10 μm, such a function does not appear.
[0023]
Regarding the number of Mg-based oxides, the maximum diameter of 0.5 μm to 10 μm is 3 / mm ^{2 in the} thin steel plate. It must exist. Mg oxide with a maximum diameter of 0.5 μm to 10 μm is 3 / mm ^{2 in the} thin steel plate. If the amount is less than 1, the function of finely dividing the colony band is insufficient, and improvement in ridging resistance cannot be expected.
The size and number of Mg-based oxides can be determined by selecting the type of Mg raw material (metal Mg or Mg alloy, etc., and its size), adjusting the amount of dissolved oxygen and the type of oxide before adding Mg, or molten steel or mold. The timing can be adjusted by appropriately selecting the timing and method of adding Mg.
[0024]
For applications that require higher ridging resistance, the steel sheet after hot rolling is scraped at 650 ° C. or higher, or the steel strip after hot rolling is at a temperature range of 800 to 1100 ° C. for 10 minutes or less. It is good to hold. By promoting recrystallization by these treatments, the colony band is further finely divided, and a high purity Cr-containing thin steel sheet having very excellent ridging resistance is obtained. Here, even if the steel sheet after hot rolling is scraped off at a temperature of less than 650 ° C., the effect of promoting recrystallization is small.
Even when the steel strip after hot rolling is held at a temperature of less than 800 ° C., the colony band is not sufficiently divided because the effect of promoting recrystallization is small. Moreover, when it hold | maintains at the temperature exceeding 1100 degreeC, or when it hold | maintains exceeding 10 minutes, a grain will grow and the recrystallization at the time of the finish annealing after cold rolling will be delayed, and workability will deteriorate.
[0025]
【Example】
Thirteen types of high-purity ferritic stainless steels A to M having the chemical components shown in Table 1 were melted and cast into 250 mm thick slabs by continuous casting under the molten steel superheat degree shown in Table 2. These cast slabs were hot-rolled to a thickness of 3 mm, and then the temperature was changed to scrape them to obtain hot-rolled steel strips. Some hot-rolled steel strips were annealed. These steel strips were cold-rolled to a thickness of 0.6 mm and then subjected to finish annealing to obtain thin steel plates.
A surface parallel to the rolling surface of the obtained thin steel plate was mirror-polished and electrolytically etched to extract an extracted replica. This replica was observed with a transmission electron microscope capable of component analysis, and the number of Mg-based oxides having a maximum diameter of 0.5 to 10 μm was counted. The ridging height of this thin steel plate was also measured. These measurement results are also shown in Table 2.
[0026]
For steels A to J to which Mg is added in an amount of 0.0005% or more, when the molten steel superheat degree during casting is 50 ° C. or less, the maximum diameter of the finish annealed plate is 0.5 to 10 μm. The density is 3 pieces / mm ² or more, and excellent ridging resistance is obtained. Further, when the hot-rolled steel sheet is scraped off at a high temperature of 650 ° C. or higher or the hot-rolled steel strip is annealed at a temperature range of 800 to 1100 ° C. for 10 minutes or less, the ridging resistance is further improved.
[0027]
For steels A to J to which Mg is added in an amount of 0.0005% or more, when the superheated degree of molten steel at the time of casting exceeds 50 ° C., the maximum diameter of the finish annealed plate is 0.5 to 10 μm. The density is less than 3 pieces / mm ² and the ridging resistance is not improved.
On the other hand, for steels K to M with an Mg addition amount of less than 0.0005%, the ridging resistance is hardly improved even when the molten steel superheat degree during casting is lowered to 50 ° C. or less. This is because the density of the Mg-based oxide having a maximum diameter of 0.5 to 10 μm in the finish annealed sheet is less than 3 pieces / mm ² .
[0028]
[Table 1]

[0029]
[Table 2]

[0030]
【The invention's effect】
As explained above, according to the present invention, it has become possible to provide a high purity Cr-containing thin steel sheet having excellent ridging resistance.
[Brief description of the drawings]
FIG. 1 shows a result of measuring ridging height by casting a high purity Cr-containing steel containing Mg: 0.0028% while changing the degree of superheating, hot rolling, cold rolling and finish annealing to form a thin steel plate. It is.

Claims (4)

% By mass
C: 0.015% or less,
Cr: 3-30%
Ti: 8 × (C + N) to 0.4%,
N: 0.02% or less,
Mg: 0.0005 to 0.005%
Steel, the balance being Fe and inevitable impurities, cast at a molten steel superheat degree of 50 ° C. or less, hot-rolled , and scraping the steel plate after the hot rolling at 650 ° C. or higher, or After holding the steel strip after the hot rolling at a temperature range of 800 to 1100 ° C. for 10 minutes or less, cold rolling and finish annealing are performed, and after the finish annealing, Mg having a maximum diameter of 0.5 to 10 μm. 1mm ^{2 of} system oxide The manufacturing method of the Cr containing thin steel plate characterized by containing 3 or more per. In steel, further mass%,
Mo: 2.0% or less,
Ni: 2.0% or less,
The method for producing a Cr-containing thin steel sheet according to claim 1, comprising Cu: 2.0% or less of one kind or two or more kinds. In steel, further mass%,
Nb: 0.5% or less,
V: 0.5% or less,
The method for producing a Cr-containing thin steel sheet according to claim 1 or 2, comprising one or more of Zr: 0.5% or less. In steel, further mass%,
B: 0.005% or less is contained, The manufacturing method of the Cr containing thin steel plate of any one of Claims 1-3 characterized by the above-mentioned.

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