JP2002038221A - Production method for high purity chromium-containing thin steel plate excellent in press formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high purity Cr-containing thin steel plate having a high r-value and less Δr without generating a flaw on the surface of a steel plate during hot rolling. SOLUTION: This high purity Cr-containing thin steel plate is produced by casting, hot rolling, cold rolling and finish annealing a steel containing one or two elements among, by mass, 3-30% Cr, <=0.02% C, <=0.02% N, Ti and Nb in a range of (Ti+Nb)/(C+N)>8 and the balance Fe with inevitable impurities. In this case, the relation of the ferrite particle diameter R (mm) of a cast piece and the winding-up temperature T ( deg.C) in the hot rolling satisfies the formulas, T>=55×R+400 (when R<=6 mm) and T>=730 (when R>6 mm).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a high-purity C excellent in press formability, characterized by high r-value (Rankford value) and small Δr (in-plane anisotropy of r-value).
The present invention relates to a method for producing an r-containing thin steel sheet.

[0002]

2. Description of the Related Art Cr-containing thin steel sheets have excellent corrosion resistance and heat resistance, and are used in a wide range of applications such as home appliances, kitchen appliances, and automobile exhaust system parts. Particularly for processing applications requiring high deep drawability, high r-values are required for thin steel sheets.
Methods for increasing the r value have been studied in the past. For example, Japanese Patent Application Laid-Open No. 61-261460 discloses a method in which the steel component is made low in C and N, and Ti is added. A method for applying annealing is disclosed. Although a high r value can be obtained by this technique, Δr is large, and there remains a problem that the quality and yield of a molded product are reduced.

A technique for reducing Δr is disclosed in
Japanese Patent Application Laid-Open No. 9-59717 discloses a technique in which the coefficient of friction between a rolled material and a rolling roll is reduced to 0.3 or less in rough rolling in hot rolling and a large reduction of 40 to 75% is applied. In the technology, there is a problem that the possibility of occurrence of flaws on the steel sheet surface during hot rolling increases.

[0004]

An object of the present invention is to provide a method for producing a high-purity Cr-containing thin steel sheet having a high r value and a small Δr without causing any flaws on the steel sheet surface during hot rolling. It is in.

[0005]

Means for Solving the Problems The present inventors have developed a high-purity C
In producing an r-containing thin steel sheet, the effect of the relationship between the ferrite grain size of the slab and the coiling temperature in hot rolling on the r value and Δr of the thin steel sheet after cold rolling annealing was examined in detail. As a result, it has been found that a steel sheet having a high average r value and a small Δr can be obtained by setting the relationship between the ferrite grain size of the slab and the coiling temperature in hot rolling in the range shown in FIG. Thus, the present invention has been completed.

The gist of the present invention for solving the above problems is as follows. (1) In mass%, Cr: 3 to 30%, C ≦ 0.02%, N ≦ 0.
02%, one or two of Ti and Nb: (Ti + Nb) / (C
+ N) ≧ 8, and the balance of the steel consisting of Fe and unavoidable impurities is turned into a thin steel sheet in a manufacturing process consisting of casting, hot rolling, cold rolling and finish annealing. A method for producing a high-purity Cr-containing thin steel sheet excellent in press formability, wherein a relationship between R (mm) and a winding temperature T (° C.) in hot rolling is in a range represented by the following formula. T ≧ 55 × R + 400 (R ≦ 6 mm) T ≧ 730 (R> 6 mm) (2) Further, Mg: 0.0005 to 0.005% A method for producing a high-purity Cr-containing thin steel sheet having excellent press formability. (3) The method for producing a high-purity Cr-containing thin steel sheet having excellent press formability according to the above (1) or (2), further comprising: S ≧ 0.4 × C in mass%. (4) Further, by mass%, Cu ≦ 2.0%, Ni ≦ 2.0%, Mo ≦ 2.
The method for producing a high-purity Cr-containing thin steel sheet excellent in press formability according to any one of the above (1) to (3), which comprises 0% of one or more kinds. (5) Further, in mass%, V ≦ 0.5%, Zr ≦ 0.5%, W ≦ 0.
The press formability according to any one of the above (1) to (4), comprising one or more of 5%, Co ≦ 0.5%, and Sn ≦ 0.5%. Method for producing high-purity Cr-containing thin steel sheet with excellent performance. (6) The high-purity Cr-containing thin steel sheet having excellent press formability according to any one of the above (1) to (5), further containing B ≦ 0.005% by mass%. Production method.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
First, the present invention will be described in detail based on the following experimental results. High-purity Cr-containing steel was melted in a vacuum melting furnace, and the composition of the steel and the casting start temperature were changed to obtain 100 mm thick slabs having different ferrite grain sizes. Here, for example, by adding Mg to the steel component, the ferrite grain size can be reduced,
Also, the ferrite grain size can be reduced by lowering the casting start temperature. The obtained slab is 1150 ° C
, And the thickness was increased to 20 mm by 6-pass hot rough rolling and further to 3 mm by 6-pass hot finish rolling.

[0008] The obtained hot-rolled steel sheet was placed in a heat-retention furnace at a varied temperature in order to simulate the heat history at the time of winding.
After holding for 1 hour in a heat preservation furnace, air-cooled, and cold-rolled to 1.0 mm
The steel sheet was subjected to finish annealing at 850 ° C. to obtain a thin steel sheet. FIG. 1 shows the relationship between the average r value and Δr of the thin steel sheet, the ferrite grain size of the slab, and the coiling temperature during hot rolling (the heat retaining furnace temperature in the experiment). From FIG. 1, by setting the ferrite grain size R (mm) and the winding temperature T (° C.) during hot rolling in the range of the following formula, the average r value is increased,
In addition, Δr can be reduced. T ≧ 55 × R + 400 (R ≦ 6 mm) T ≧ 730 (R> 6 mm)

Next, the limited range of the components of the present invention will be described. The component content is% by mass. Cr: 3 to 30% or less. In order to obtain excellent corrosion resistance and heat resistance, the content must be 3% or more.
However, even if the content exceeds 30%, the required performance is already sufficiently satisfied, and this merely leads to an increase in alloy cost.

C: Must be 0.02% or less.
If the content exceeds 0.02%, the corrosion resistance of the weld heat affected zone is likely to be degraded due to precipitation of Cr carbide at the grain boundary. Further, since it is an interstitial solid solution, it is an element that significantly strengthens the steel and degrades the r value. If the content exceeds 0.02%, the workability deteriorates, and Ti, N for fixing C
b The required amount increases. For these reasons, the upper limit is made 0.02%.

N: It is necessary to be 0.02% or less. C
It is an element that remarkably strengthens the steel and degrades the r-value because it forms a solid solution in an interstitial manner as in the case of. If the content exceeds 0.02%, the workability is deteriorated, and T for fixing N is further reduced.
Since the required amount of i and Nb increases, the upper limit is set to 0.02.
%.

Ti, Nb: one or two of Ti, Nb
Nb needs to be at least 8 times C + N. Ti and Nb are easily bonded to C and N, and have the effect of substantially reducing the amount of C and N dissolved in steel, thereby improving workability. This effect can be obtained by 8 × as Ti + Nb.
A content of less than (C + N) is insufficient.

The above are the basic components of the present invention, but the following components can be added as needed. When Mg is contained in the range of 0.0005 to 0.005%, an effect of reducing the ferrite grain size of the continuous cast slab can be obtained. As a result, the average r
The value can be increased and Δr can be decreased. The effect of reducing the ferrite grain size of the continuous cast slab is 0.000
If the content is less than 5%, it is insufficient. If the content exceeds 0.005%, the effect is saturated and only the corrosion resistance is deteriorated.

S: If 0.4 or more times C, Ti and C
From TiC type to Ti ₄ C ₂
It can be S ₂ type. As a result, C is more sufficiently fixed, and there is an effect of increasing the r value. However, if S is excessively contained, the corrosion resistance is deteriorated.
It is desirable to be 11%.

Cu, Ni, Mo: For applications requiring particularly high corrosion resistance, it is effective to add one or more of these elements in the range of 2.0% or less.
If the content exceeds 2.0%, the effect is saturated and alloy cost is increased. Therefore, the upper limit is set to 2.0%.

V, Zr, W, Co, Sn: In applications where high corrosion resistance is required especially for welds, one or more of these elements may be added in a range of 0.5% or less. It is valid. These elements easily bond with C and N,
Prevents the precipitation of Cr carbonitride in the heat affected zone by welding and improves corrosion resistance. If the content exceeds 0.5%, the effect is saturated, alloy cost increases, and workability deteriorates, so the upper limit was made 0.5%.

B: For applications requiring particularly high secondary workability, it is effective to add it in the range of 0.005% or less. B has the effect of repairing defects in steel generated when processing the product and improving the secondary workability. However B
If the content exceeds 0.005%, the hot workability of the steel is reduced, and cracks and flaws occur during hot rolling, so the upper limit was made 0.005%.

Si, Mn: Although not particularly limited in the present invention, they may be unavoidably mixed for the purpose of deoxidation, or may be added for obtaining strength or the like. When used in a high-temperature oxidizing atmosphere such as an automobile exhaust system, the effect on the r value and Δr is small even if added as needed.

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. The thin steel sheet of the present invention is manufactured by a manufacturing process including casting, hot rolling, cold rolling, and finish annealing of steel having the above-described components. In the present invention, as is apparent from FIG. 1, the ferrite grain size R (mm) of the slab is
The relationship with the winding temperature T (° C.) in the hot rolling is defined as the range of the following expression. T ≧ 55 × R + 400 (R ≦ 6 mm) T ≧ 730 (R> 6 mm)

As a result, the average r value can be increased and Δr can be reduced. The reason is considered as follows. The average r value and Δr largely depend on the recrystallization state of the hot-rolled sheet. When the hot-rolled sheet is completely recrystallized, the average r-value increases, but the rolling direction is 45 °.
The r value in the inclined direction becomes extremely small, and Δr becomes large. Further, if a large amount of unrecrystallized portion remains in the hot-rolled sheet, the average r value decreases. When the hot-rolled sheet has a partially recrystallized structure and the proportion of the non-recrystallized portion is relatively small, the average r value is high and Δr is small.

The smaller the ferrite grain size of the slab, the more recrystallization during hot rolling is promoted. In addition, as the winding temperature is higher, recrystallization at the time of winding after hot rolling is promoted. If the relationship between the ferrite grain size of the slab and the coiling temperature after hot rolling is limited to the shaded area in FIG. 1, a partial recrystallized structure in which the proportion of unrecrystallized portions is relatively small is obtained. As a result, the average r value of the thin steel sheet increases, and Δr decreases.

Although there is no particular upper limit for the winding temperature, it does not usually exceed 900 ° C. from the general hot rolling temperature. Further, in the present manufacturing process in which annealing is not performed between hot rolling and cold rolling, the hot-rolled sheet does not have a completely recrystallized structure. Other manufacturing conditions may use a conventionally known method.

[0023]

EXAMPLE 12 types of high purity Cr-containing steels A to L having the chemical components shown in Table 1 were melted and continuously cast to a thickness of 250.
mm slab. Table 1 also shows the ferrite grain size of each slab. Here, the ferrite grain size of the slab is such that a cross section perpendicular to the casting direction of the slab shows a macrostructure due to corrosion by an aqueous solution of HCl + H ₂ O ₂ , in the vicinity of the center of the width, and 1/4 in thickness
The ferrite grain size near the part was measured by a line segment method. When the ferrite grains were equiaxed, the average diameter was used. When the ferrite grains were columnar, the average value of the width and length of the columnar crystals was used as the ferrite grain size.

After heating these slabs at 1150 ° C., they were subjected to rough rolling in 6 or 7 passes and finish rolling in 6 passes to obtain hot-rolled steel sheets, and further subjected to cold rolling and finish annealing to obtain thin steel sheets. Table 2 also shows the winding temperature during hot rolling, the average r value, Δr, and ridging height of the obtained thin steel sheet. here,
The average r value and Δr of the thin steel sheet were measured based on “Plastic strain ratio test method for thin metal material” specified in JIS Z 2254. The test piece was a JIS No. 5 tensile test piece, and the applied strain was 15%. The ridging height was evaluated based on the height of unevenness generated when a JIS No. 5 tensile test piece was cut out in a direction parallel to the rolling direction and 16% strain was applied.

From Tables 1 and 2, it can be seen that the relationship between the ferrite grain size of the slab and the coiling temperature during hot rolling falls within the range of the hatched area in FIG. 3 to 7, 9, 12, 14, 15,
18 to 20, 23, and 24 indicate a high average r value and a small Δr
And it has excellent ridging characteristics. On the other hand, Comparative Example No. 1 in which the relationship between the ferrite grain size of the slab and the coiling temperature during hot rolling was out of the hatched range in FIG. 1,
2,8,10,11,13,16,17,21,22
Increases Δr.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As described above, according to the present invention, a high-purity Cr-containing thin steel sheet excellent in press formability having a high r value and a small Δr without causing any flaw on the steel sheet surface during hot rolling. Manufacturing has become possible. INDUSTRIAL APPLICABILITY The present invention can obtain tremendous benefits not only for the manufacturer but also for those who use the present thin steel sheet, and the industrial value is extremely high.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the average r value and the absolute value of Δr of a high-purity Cr-containing thin steel sheet, the relationship between the ferrite grain size of a slab and the winding temperature in hot rolling.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiko Takahashi 1-1, Tobata-cho, Tobata-ku, Kitakyushu Nippon Steel Corporation Yawata Works (72) Inventor, Takashi Moroboshi 1-1, Tobita-cho, Tobata-ku, Kitakyushu New Nippon Steel Corporation Yawata Works (72) Inventor Riki Ito 1-1 Hibata-cho, Tobata-ku, Kitakyushu-shi Nippon Steel Corporation Yawata Works (72) Inventor Ken Kimura 20-1 Shintomi, Futtsu-shi New Japan F-term in the Technology Development Division of Steel Corporation (reference) 4K037 EA02 EA04 EA05 EA10 EA11 EA12 EA13 EA14 EA17 EA18 EA19 EA20 EA25 EA31 EA32 EA33 EA35 EB03 FB00 FG00 FJ00 JA07

Claims (6)

[Claims] 1. Mass%, Cr: 3 to 30%, C ≦ 0.02%, N ≦ 0.02%, One or two of Ti and Nb: (Ti + Nb) / (C
+ N) ≧ 8, and the balance of the steel consisting of Fe and unavoidable impurities is turned into a thin steel sheet in a manufacturing process consisting of casting, hot rolling, cold rolling and finish annealing. A method for producing a high-purity Cr-containing thin steel sheet excellent in press formability, wherein a relationship between R (mm) and a winding temperature T (° C.) in hot rolling is in a range represented by the following formula. T ≧ 55 × R + 400 (R ≦ 6 mm) T ≧ 730 (R> 6 mm) 2. The method for producing a high-purity Cr-containing thin steel sheet having excellent press formability according to claim 1, further comprising Mg: 0.0005 to 0.005% by mass. 3. The method for producing a high-purity Cr-containing thin steel sheet excellent in press formability according to claim 1, further comprising S ≧ 0.4 × C in mass%. 4. The composition according to claim 1, further comprising one or more of Cu ≦ 2.0%, Ni ≦ 2.0%, and Mo ≦ 2.0% by mass%. The method for producing a high-purity Cr-containing thin steel sheet having excellent press formability according to any one of the above. 5. One or two of V ≦ 0.5%, Zr ≦ 0.5%, W ≦ 0.5%, Co ≦ 0.5%, Sn ≦ 0.5% by mass%. The method for producing a high-purity Cr-containing thin steel sheet having excellent press formability according to any one of claims 1 to 4, characterized by containing the above. 6. The method according to claim 1, further comprising: B ≦ 0.005% by mass%.
4. A method for producing a high-purity Cr-containing thin steel sheet having excellent press formability according to item 4.