JPH09287021A - Production of high purity ferritic stainless hot rolled steel strip excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high purity ferritic stainless hot rolled steel strip excellent in workability with obviated processes without executing cold rolling and annealing. SOLUTION: At the time of subjecting a high purity ferritic stainless steel strip having a compsn. contg., by weight, <=0.01% C, <=0.8% Si, <=0.8% Mn, 10 to 13% Cr, 0.05 to 0.3% Al, 10(C+N) to 0.2% Ti and <=0.01% N, satisfying C+N <=0.015, and the balance Fe with inevitable impurities to hot rolling, the rolling is executed at a draft of >=80% in the temp. range of 1,000 to 1,100 deg.C, and its temp. is held to 950 to 1,050 deg.C for >=5min, and thereafter, finish rolling is executed. If required, the above steel strip is subjected to annealing at 800 to 1,000 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled / annealed high purity ferritic stainless hot rolled steel strip which is used as an automobile exhaust system material, an electric appliance, a building material and the like and is relatively inexpensive and excellent in workability. The present invention relates to a method of manufacturing in a reduced number of steps without performing.

[0002]

2. Description of the Related Art So-called high-purity ferritic stainless steel strips containing 10 to 13% of Cr and fixing carbonitrides with Ti are generally inexpensive as stainless steels, and are generally used as exhaust system materials for automobiles. Has recently been used for home appliances and metal fittings for construction. Ferrite-based stainless steel is generally manufactured through hot rolling, annealing, hot pickling, cold rolling, annealing, and pickling after hot rolling. Highly pure ferritic stainless steel containing 10 to 13% of Cr is strongly demanded by the market to be cheaper than ordinary ferritic stainless steel typified by SUS430, and thus is manufactured with high productivity. For this reason, various innovations have been made in the past.

Efforts have been made to omit hot-rolled sheet annealing for the purpose of simplifying the manufacturing process without impairing workability. As a method of omitting hot-rolled sheet annealing, a method of winding a steel strip after hot rolling at a high temperature has already been disclosed (Japanese Patent Laid-Open No. 52-95527). According to this conventional method, it is possible to omit hot rolled sheet annealing, but in order to obtain good workability, it is necessary to continuously perform cold rolling and annealing. Only by omitting the hot-rolled sheet annealing by the conventional method, the metallographic structure of the hot-rolled steel strip is not sufficiently recrystallized in the hot rolling step, so that the recrystallized structure obtained by cold rolling and annealing is In comparison, ductility and deep drawability are reduced and ridging is increased.
Therefore, subsequent to hot rolling, it is necessary to adjust the metal structure by cold rolling and annealing.

[0004]

DISCLOSURE OF THE INVENTION The object of the present invention is to provide hot workability without cold rolling / annealing, or components for obtaining good workability in hot rolling / annealing, hot rolling, hot rolling. It is to provide conditions for sheet annealing.

[0005]

Means for Solving the Problems In the hot rolling of a highly pure ferritic stainless steel strip, the present invention limits the amounts of C and N added to the raw material, and in the so-called rough rolling of hot rolling, the rolling temperature, the reduction ratio, By setting the heat retention temperature and time after the completion of rough rolling, the hot rolled sheet is annealed as it is, and if necessary, hot rolled sheet is annealed after hot rolling to obtain good workability.

That is, the gist of the present invention is
% By weight, C: 0.01% or less, Si: 0.8% or less, Mn: 0.8% or less, Cr: 10 to 13%, Al: 0.05 to 0.3%, Ti: 10 ( C + N) to 0.2%, N: 0.01% or less is included, C + N ≦ 0.015% is satisfied, and when hot rolling a highly pure ferritic stainless steel strip with the balance being Fe and inevitable impurities. 1100 ° C
A high-purity ferrite excellent in workability, characterized by rolling at a rolling reduction of 80% or more in a temperature range of 1000 ° C or more, heat-retaining at 950 ° C or more and 1050 ° C or less for 5 minutes or more, and finish rolling. A method for producing a hot-rolled stainless steel strip, or a method for producing a highly pure ferritic stainless hot-rolled strip having excellent workability, which is characterized by annealing the steel strip at 800 ° C. or higher and 1000 ° C. or lower. .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention observed the as-hot-rolled metallographic structure of a high-purity ferritic stainless steel before cold rolling, and found that coarse ferrite grains generated in a slab during the hot rolling process. It was observed that the ferrite single-phase structure became coarse drawn ferrite-based grains with almost no recrystallization. In such a coarse deformed ferrite grain structure, strength is high due to work hardening, elongation and r value are low, and ridging is large. In addition, ductility is improved even when hot-rolled sheet annealing is performed,
It was confirmed that ridging was large and good workability could not be obtained.

[0008] Based on the above observations and test results, the present inventors thought that the workability of the hot rolled sheet could be improved by recrystallizing the ferrite present in the slab during the hot rolling process. The present inventors paid attention to the heat retention effect between the so-called rough rolling and finish rolling in the hot rolling process, and examined the components that cause recrystallization, the rough rolling condition and the heat retaining condition during this period. As a result, in wt%, 11Cr-0.5Si-0.4Mn-0.05A
When heat is kept at 1000 ° C based on l-0.1Ti,
As shown in FIG. 1, the reduction of C and N promotes the progress of recrystallization, and the total reduction rate at 1100 to 1000 ° C. is 8%.
It was found that a recrystallization rate of 80% or more can be obtained after heat retention if the content is 0% or more.

FIG. 1 shows that steels having various changes in C and N are melted in a laboratory to prepare an ingot having a thickness of 110 mm, which is heated to 1175 ° C. and then the finish thickness is changed at 1100 to 1000 ° C. By carrying out hot rolling while changing the reduction ratio, the recrystallization rate in the metallographic structure of the steel sheet immediately after water cooling at a temperature of 1000 ° C. for 10 minutes after completion of the rolling, the C + N amount, and the reduction rate were shown. is there.

In addition to the study shown in FIG. 1, the effects of heat retention temperature and time were examined. As a result, the C + N content was set to 0.015% or less, and the total rolling reduction of the rough rolling was set to 80% or more. It was found that 80% or more of the recrystallized structure can be obtained by heat-retaining at 950 ° C. or more and 1050 ° C. or less for 5 minutes or more. Further, when the recrystallized structure is 80% or more as described above, the inventors of the present invention can obtain a good workability even with hot rolling, with a large elongation and a large r value and a small ridging. After confirmation, the present invention was completed.

Next, the reasons for limiting the components of the present invention will be described. C
Reduces the r value, which is an index representing the deep drawability. Also, it reduces the corrosion resistance. In particular, Cr
Forms carbides, thereby increasing the susceptibility to intergranular corrosion and causing sensitization. Therefore, the lower the C, the better. further,
In the present invention, for the purpose of recrystallizing coarse ferrite grains after rough rolling of hot rolling, it is desirable that C and N be as low as possible, and it is necessary to reduce C + N ≦ 0.015% as described above. C is generally reduced by secondary refining such as VOD, but extremely lowering C unnecessarily increases refining cost, so the upper limit of C is set to 0.01% in the present invention.

Si is an element effective for enhancing the oxidation resistance, and it is effective to add Si in an amount of preferably about 0.5%. However, if added excessively, the toughness decreases, so the upper limit is set to 0.
8%. Mn is not only used as a deoxidizing element,
It is added to enhance the strength, but if added in a large amount, the workability is impaired, so the upper limit is made 0.8%.

Cr is an essential element for obtaining heat resistance, oxidation resistance and corrosion resistance. In the present invention, it is set to 10 to 13% in consideration of heat resistance and workability required as a relatively inexpensive automobile exhaust system material or oil combustion device material. Al is a deoxidation product remaining, and is added in an amount of 0.01% or more for the purpose of deoxidation. However, if added excessively, the cleanliness of steel is reduced and the workability is adversely affected, so the upper limit is 0. 1%.

Ti is added in an amount of 10 (C + N)% or more for the purpose of preventing the deterioration of the corrosion resistance of the heat affected zone. However, if added in a large amount, the solid solution Ti reduces the toughness, so the upper limit is made 0.2%. N is preferably as low as possible for the same reason as C, but the upper limit is made 0.01% in consideration of the influence on the recrystallization behavior after rough rolling and the refining cost.

Next, the reasons for limiting the rolling conditions in the present invention will be described. In the present invention, utilizing the strain of hot rolling rough rolling,
In order to recrystallize coarse ferrite during heat retention after rough rolling, it is necessary to add a strain sufficient to recrystallize even coarse ferrite grains. Rough rolling temperature is 1100 ℃
If it exceeds, the ferrite will only recover after rolling, and the strain necessary for recrystallization will not accumulate. On the other hand, if the rough rolling temperature is too low and falls below 1000 ° C, it takes time to raise the temperature to the heat retention temperature. Therefore, the rolling temperature is 1100 to 100
Set to 0 ° C. The rolling temperature mentioned here relates to the rolling that contributes to recrystallization in the rough rolling, and does not specify the entire rough rolling temperature. 11 to start rough rolling to obtain the shape
There is no problem even if the temperature is higher than 00 ° C.

At the rolling temperature of the present invention, in order to obtain coarse ferrite recrystallization in the steel composition having the components related to the present invention, the total rolling reduction is 80% as described above.
It is necessary to add the above distortion. The upper limit of the rolling reduction is not particularly defined because it may be determined by the slab thickness and the hot-rolled finished thickness.

Further, in the present invention, after the rough rolling, heat is kept to perform recrystallization, but at least 950 ° C., 5 for the progress of recrystallization.
It is necessary to keep heat for more than a minute. However, if the heat is kept above 1050 ° C., the amount of scale formation increases, which causes surface defects in finish rolling, so the upper limit of the heat retention temperature is set to 1050 ° C. The upper limit of the heat retention time is not particularly defined because it may be determined in consideration of the slab thickness, the hot rolled finished thickness, and the like.

Since the steel sheet recrystallized after rough rolling is subjected to work again by finish rolling, in the present invention, after hot rolling, annealing is carried out if necessary in order to eliminate the influence of the work and obtain necessary workability. The lower limit of the annealing temperature is 800 ° C., which is the lower limit at which a nearly complete recrystallized structure is obtained. Further, if the annealing temperature is too high, the crystal grains become coarse, which may cause roughening of the surface during product processing. Therefore, the upper limit temperature of annealing is set to 1000 ° C. The annealing method may be so-called box annealing, continuous annealing,
Therefore, any method of heating, holding and cooling may be used.

[0019]

EXAMPLE Steels having the components shown in Table 1 were melted in a laboratory, and 50
kg ingots were produced. After heating to 1175 ° C., the temperature was 1100 to 10
After rolling at 00 ° C and heat retention, finish rolling was performed. The rolling finishing temperature was 860 to 840 ° C., and the finished plate thickness was 2 mm.
Immediately after rolling, insert into an electric furnace to reproduce the winding,
After holding at 750 ° C. for 1 hour, the furnace was cooled. Some of the steel sheets were annealed under the conditions shown in Table 2. From the obtained steel sheet, a JIS 13B tensile test side and a JIS 5 tensile test piece were produced parallel to the rolling direction. A tensile test was carried out using a JIS 13B tensile test piece to measure the yield stress and elongation. Also, using JIS No. 5 tensile test pieces, 15%, 2
Tension to 0% was performed, and the r value and the ridging height were measured, respectively.

When the conditions of the present invention are satisfied, the elongation and r value are excellent, the ridging height is low, and good workability is achieved without cold rolling and annealing. However, in Comparative Examples 1, 2, and 3, the reduction ratio of 1100 to 1000 ° C. and 110
Since the heat retention temperature and heat retention time after rolling at 0 to 1000 ° C deviate from the scope of the present invention, the elongation, the r value and the ridging height are inferior as compared with the case of the present invention. In Comparative Example 4, the hot-rolled sheet was annealed to obtain a good elongation, but the C + N amount deviated from the range of the present invention, so the r value was poor and the ridging height was large. The rolling and heat retention conditions of Comparative Example 5 are in accordance with the present invention,
Since the temperature of hot-rolled sheet annealing deviates from the range of the present invention, the elongation, r-value and ridging height are poor.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

EFFECTS OF THE INVENTION According to the present invention, a cold rolled and annealed high purity ferritic stainless hot rolled steel strip used for automobile exhaust system materials, electric appliances, building materials, etc., which is relatively inexpensive and has excellent workability. Since it can be manufactured without any process, the industrial effect is very large.

[Brief description of drawings]

FIG. 1 11Cr-0.5Si-0.4Mn-0.05
After heating Al-0.1Ti steel to 1175 ° C., 1100-110
Hot rolling is performed at different rolling reductions by changing the finishing thickness at 1000 ° C.
After holding for a minute, immediately after water cooling, the relationship between the recrystallization rate in the metal structure of the steel sheet, the C + N amount, and the rolling reduction is shown.
It is a figure which shows that a recrystallization structure of 80% or more can be obtained if the total reduction rate of 1100 to 1000 ° C. is 80% or more in addition to 15% or less.

Claims (2)

[Claims] 1. By weight%, C: 0.01% or less, Si: 0.8% or less, Mn: 0.8% or less, Cr: 10 to 13%, Al: 0.01 to 0.1%. , Ti: 10 (C + N) to 0.2%, N: 0.01% or less is included, C + N ≦ 0.015% is satisfied, and the balance is Fe and unavoidable impurities. When rolling, rolling with a rolling reduction of 80% or more in a temperature range of 1100 ° C. or lower and 1000 ° C. or higher, and 950 ° C. or higher,
A method for producing a high-purity ferritic stainless hot-rolled steel strip excellent in workability, which comprises heat-retaining at 1050 ° C. or lower for 5 minutes or more and finish rolling. 2. The steel strip according to claim 1 having a temperature of 800.degree.
A method for producing a high-purity ferritic stainless hot-rolled steel strip excellent in workability, characterized by annealing at 0 ° C or less.