JP2007270168A - Method for producing chromium-containing ferritic steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a Cr-containing ferritic steel sheet, wherein a ridging can be improved in the wide component range. <P>SOLUTION: In the method for producing the Cr-containing ferritic steel sheet, when the continuously cast slab containing, in mass, 0.003-0.020% C, 0.05-1.0% Mn, 0.05-1.5% Si, ≤0.04% P, ≤0.006% S, 9.0-30% Cr, 0.001-0.09% Al, 0.003-0.015% N and 0.05-2.5% at least one kind of element selected from Ti, Nb, Cu, Ni, Mo, V, W, and the balance Fe with inevitable impurities, is hot-rolled, the hot-rolling is performed at ≥65% of the total rolling-reduction ratio in the same direction as the casting direction, and the hot-rolling of ≥30% in the total rolling reduction ratio is performed in the temperature zone higher than the recrystallized temperature. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a Cr-containing ferritic steel sheet, particularly a Cr-containing ferritic heat-resistant steel sheet or stainless steel sheet used in automobiles, home appliances, kitchens, and the like.

  Cr-containing ferritic heat-resistant steel sheets and stainless steel sheets such as SUH409 steel and SUS430 steel have excellent heat resistance and corrosion resistance, and are cheaper than austenitic stainless steels containing a large amount of Ni. , Household appliances, automobile exhaust system members, etc.

  However, in such a Cr-containing ferritic steel sheet, due to the coarse columnar crystal structure formed at the time of slab casting, a group of crystal grains with a close orientation remains in a band shape even after cold rolling / annealing. Tensile surface undulations called ridging are likely to occur due to tensile processing. This ridging increases the load of the surface polishing process in applications that require aesthetics of the product, and causes processing defects in applications that require workability, so it is required to be reduced as much as possible.

As techniques for improving ridging, many methods have been disclosed, such as a method of suppressing the formation of a columnar crystal structure depending on the casting conditions of the slab and a method of breaking the columnar crystal structure of the slab by hot rolling or the like. For example, Patent Document 1 discloses a method of increasing the equiaxed crystal ratio of a slab by setting the Ti / Al content ratio Ti / Al in the steel to 8 or more, and Patent Document 2 includes Y and Mg. A method of increasing the equiaxed crystal ratio of the slab by adding inclusions and dispersing inclusions including these, and further, Patent Document 3 increases the stability of the austenite component, heating temperature during hot rolling, rolling temperature In addition, a method has been proposed in which the rolling rate is controlled to break the columnar crystal structure and make it random.
JP-A-9-49010 JP 2002-285292 A JP 2000-256750 A

  However, the methods described in Patent Documents 1 to 3 can improve the ridging after cold rolling / annealing by increasing the equiaxed crystal ratio of the slab or by breaking down the columnar crystal structure and making it finer. And may not be applicable to a wide range of Cr-containing ferritic steel sheets.

  An object of this invention is to provide the manufacturing method of the Cr containing ferritic steel plate which can improve the ridging after cold rolling and annealing in a wide component range.

The inventors of the present invention have examined the slab columnar crystal structure in order to improve ridging, and have found the following.
1) As schematically shown in FIG. 1, the columnar crystal structure of the continuously cast slab is inclined in the direction opposite to the casting direction from the surface layer of the slab toward the center.
2) When hot rolling a slab, ridging can be improved by hot rolling a predetermined temperature range in the same direction as the casting direction at a predetermined reduction rate.

  The present invention was made based on such findings, and in mass%, C: 0.003-0.020%, Mn: 0.05-1.0%, Si: 0.05-1.5%, P: 0.04% or less, S: 0.006% Hereinafter, Cr: 9.0-30%, Al: 0.001-0.09%, N: 0.003-0.015%, at least one element selected from Ti, Nb, Cu, Ni, Mo, V, W: 0.05- When hot-rolling continuously cast slab composed of 2.5%, balance Fe and unavoidable impurities, the total rolling reduction is 65% or more in the same direction as the casting direction, and among the hot rolling Provided is a method for producing a Cr-containing ferritic steel sheet characterized in that the total rolling reduction is 30% or more in a temperature range above the recrystallization temperature.

  According to the present invention, a Cr-containing ferritic steel sheet with improved ridging can be produced even with a continuously cast slab having a low equiaxed crystal ratio of less than 60% in a wide component range. The Cr-containing ferritic steel sheet produced by the method of the present invention is suitable for kitchen appliances, home appliances, automobile exhaust system members, and the like. Here, the equiaxed crystal ratio is the ratio of the cross-sectional area occupied by the equiaxed crystal structure to the total cross-sectional area in the cross section perpendicular to the casting direction of the slab, as schematically shown in FIG. The equiaxed crystal structure is composed of crystal grains in which the ratio a / b of the major axis a to the minor axis b is 1.5 or less, and is usually formed in the center of the slab.

  The present invention is described in detail below.

1) ingredients
C: C needs to be 0.003% or more in order to obtain a predetermined strength as a member. However, C forms 0.020% or less because Cr carbide is formed, causing intergranular corrosion and toughness reduction.

  Mn: Mn is effective as a deoxidizing element, but if added excessively, the workability is lowered, so 0.05 to 1.0%.

  Si: Si is effective as a deoxidizing element and is an element that improves corrosion resistance. However, if added excessively, pickling property and workability are lowered, so 0.05 to 1.5%.

  P: P is 0.04% or less because P significantly reduces ductility.

  S: S is not more than 0.006% in order to form sulfide with Mn and reduce corrosion resistance and workability.

  Cr: Cr needs to be 9.0% or more in order to improve corrosion resistance and heat resistance. However, if it exceeds 30%, the workability is remarkably lowered, so 9.0 to 30%.

  Al: Al is effective as a deoxidizing element, and is an element that fixes N and improves workability. However, if excessively added, the weldability is reduced, so 0.001 to 0.09%.

  N: N also needs to be 0.003% or more in order to obtain a predetermined strength as a member in the same manner as C, but if it is excessively contained, Cr nitride is formed, causing intergranular corrosion and toughness reduction. % Or less.

  Ti, Nb, Cu, Ni, Mo, V, W: Ti, Nb, Cu, Ni, Mo, V, W improve workability, corrosion resistance, and heat resistance, but if added excessively, strength increases and toughness This causes a decrease in workability, so that it is necessary to contain 0.05 to 2.5% of at least one element.

  The balance is Fe and inevitable impurities.

2) Hot rolling As described above, the columnar crystal structure of the continuously cast slab is inclined in the direction opposite to the casting direction from the surface layer of the slab toward the center. This is because in continuous casting, the molten steel is solidified by being pulled in the casting direction, so cooling and solidification proceed from the front and side surfaces in the casting direction, and the columnar crystals are directed in the direction opposite to the casting direction and toward the center of the slab. Is to grow.

  FIG. 3 schematically shows the structure of the slab when hot rolling is performed in the same direction as the slab casting direction and when hot rolling is performed in the direction opposite to the casting direction of the slab. When hot rolling is performed at a predetermined reduction rate in the same direction as the casting direction, the effect of breaking and dividing the columnar crystal structure of the slab is high. At this time, this temperature is higher than the recrystallization temperature at the predetermined reduction rate. When hot rolling is performed, recrystallization progresses in a form that erodes the harmful texture that forms the basis of ridging, so that randomization of the structure of the hot-rolled steel sheet is promoted, and ridging is improved. On the other hand, when hot rolling in the direction opposite to the casting direction of the slab is performed, a stretched structure is formed while maintaining the form of the columnar crystals, so that a texture harmful to ridging tends to remain in a band shape, Ridging becomes noticeable.

  In FIG. 4, the 15Cr-0.5Nb-1.5Mo steel in the component range of the present invention was obtained by changing the hot rolling direction, the total rolling reduction, and the total rolling reduction in the temperature range above the recrystallization temperature. The relationship between the equiaxed crystal ratio of a slab and the ridging after cold rolling and annealing is shown. Even in slabs where there is almost no equiaxed crystals by hot rolling in the same direction as the slab casting direction with a total rolling reduction of 65% or more, of which 30% or more in the temperature range above the recrystallization temperature. For slabs with a ridging of 30 μm or less and an equiaxed crystal ratio of 40%, the ridging can be 10 μm or less. In the ridging, 5 No. 5 tensile test pieces specified in JIS Z 2201 were sampled, the parallel part was mirror-finished, 15% tensile strain was applied, and the average swell height of the surface was measured. evaluated.

  The steel sheet after hot rolling is subjected to cold rolling and recrystallization annealing by an ordinary method after annealing as necessary.

  Steels a, b, and c having the component compositions shown in Table 1 were used, and slabs having a thickness of 110 to 220 mm having different equiaxed crystal ratios were produced by continuous casting while changing the degree of superheated molten steel and the strength of electromagnetic stirring. Then, the slab was made into a 3-5 mm hot-rolled steel sheet under the hot rolling conditions shown in Table 2, and annealed at 850-120 ° C. Subsequently, the annealed hot-rolled steel sheet was pickled and then made into a cold-rolled steel sheet having a rolling reduction of 50 to 60% and a thickness of 1.0 to 2.0 mm, and recrystallization annealing was performed at 850 to 1050 ° C. And the ridging of the cold rolled steel sheet after recrystallization annealing was measured by the above-mentioned method. The mean value of the undulation height was classified as “◎”, “10” below 20 μm, “以下”, “20” above 30 μm below “Δ”, and “30 μm” above “×”.

  The results are shown in Table 2. When the equiaxed crystal ratio of the slab is the same, it can be seen that the ridging is improved in the inventive example produced under the hot rolling conditions of the invention compared to the comparative example. Especially in the present invention example, when using a slab having an equiaxed crystal ratio of 38 to 42%, ridging is improved to 20 μm or less, and even when using a slab having an equiaxed crystal ratio of less than 5%, ridging is improved to 30 μm or less Has been.

It is a figure which shows typically the structure | tissue of the slab in the cross section parallel to a casting direction. It is a figure which shows typically the structure | tissue of the slab in a cross section perpendicular | vertical to a casting direction. It is a figure which shows typically the relationship and structure | tissue of a casting direction and a hot rolling direction. It is a figure which shows the relationship between the equiaxed crystal ratio of a slab, and ridging.

Claims (1)

  In mass%, C: 0.003-0.020%, Mn: 0.05-1.0%, Si: 0.05-1.5%, P: 0.04% or less, S: 0.006% or less, Cr: 9.0-30%, Al: 0.001-0.09% , N: 0.003 to 0.015%, at least one element selected from Ti, Nb, Cu, Ni, Mo, V, W: 0.05 to 2.5%, the remaining Fe and inevitable impurities were continuously cast When the slab is hot-rolled, a total rolling reduction of 65% or more is hot-rolled in the same direction as the casting direction, and a total rolling reduction of 30% or more of the hot rolling is a temperature range above the recrystallization temperature. A method for producing a Cr-containing ferritic steel sheet, characterized in that

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