JP3241114B2 - Method for producing ferritic stainless steel sheet excellent in ridging property and workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel sheet having excellent ridging properties and workability, which is used for press forming, bending, and roll forming. The steel of the present invention is marketed in the form of a cold-rolled steel strip or a cold-rolled steel sheet, and these are collectively referred to as a steel sheet in this specification.

[0002]

[Prior Art] Ferrite stainless steel represented by SUS430 has good corrosion resistance and workability, does not contain expensive Ni, and has economical advantages compared to austenitic stainless steel. It is widely used mainly for durable consumer goods. However, a ferrite stainless steel sheet has a phenomenon in which a unique wrinkle-like surface unevenness called ridging is generated during press forming. Ridging not only impairs the aesthetics of the molded product and lowers its commercial value, but also increases the polishing load to remove it.
This is a major problem in processing ferritic stainless steel.

[0003] There have been many reports on the origin of ridging, but in the end, a unit region consisting of a group of crystal grains with similar orientations derived from the structure of the cast or hot rolled sheet is formed in the cold rolled annealed sheet. It is thought that each unit area shows different deformation behavior during press forming and other processing, and undulates the surface in the rolling direction of the steel sheet. Therefore, it is effective to refine and pulverize this unit area to improve the ridging property, and various improvement measures have been proposed from this viewpoint.

Conventionally, in the production of ferritic stainless steel sheets, a continuous cast slab is formed into a hot-rolled steel strip by hot rolling, and after performing hot-rolled sheet annealing in a box furnace or a continuous annealing furnace,
It is a common method to pickle, perform cold rolling once or cold rolling several times including intermediate annealing, and recrystallize and annealing to produce a product. However, although hot-rolled sheet annealing may be omitted in some cases, hot-rolled sheet annealing is performed especially for press forming of thin sheets where ridging property and workability are important for deep drawing. It is often manufactured by performing cold rolling twice including intermediate annealing.

[0005] In the production of such conventional ferritic stainless steel cold-rolled steel sheets, the hot rolling conditions that are standardly employed are as follows.

[0006] The rolling speed, which is important in terms of production efficiency, is approximately 4.0 to 6.0 m / s in the finish rolling exit speed in the conventional ferrite stainless steel. This is about 8m / s or more for austenitic stainless steel and about 1m for ordinary steel.
It is much slower than 0m / s or more.

It is known that low-temperature rolling is effective as a ridging improvement measure in hot rolling. For this reason, ferritic stainless steel is usually hot-rolled at a lower temperature than austenitic stainless steel. ing. When ridging property is particularly important for processing applications, the finish hot rolling finish temperature is regulated to be lower than that of general ferritic stainless steel.

More positively, so-called delay rolling, in which rolling passes are performed under high pressure at a relatively low temperature and where the material is temporarily on standby during rolling to increase the time between passes, is effective in improving ridging. Is also known. For example,
No. 45-34016 discloses that at least 50% reduction of hot rolling is performed at a temperature of 871 ° C or less, and that rolling is temporarily stopped during hot rolling and the material is not higher than 760 to 871 ° C. A ridging improvement method has been proposed in which hot rolling is performed at a later stage after cooling to a temperature.

[0009]

The conventional improvement in ridging properties by lowering the hot rolling temperature involves a problem that the lowering of the hot rolling temperature necessarily involves a reduction in the rolling speed. Therefore, a decrease in productivity cannot be avoided. In fact, as mentioned above, the finish rolling speed of ferritic stainless steel is considerably slower than that of austenitic stainless steel, and the productivity is inferior. Delay rolling also causes an increase in the rolling time and a reduction in productivity. Therefore, this is not an economic method.

[0010] In addition, although low-temperature hot rolling is certainly effective in improving ridging properties, there is a problem that surface flaws are easily generated and the surface quality of the steel strip is deteriorated. this is,
When the rolling temperature is lowered, the deformation resistance of the material to be rolled increases, so that the rolling load increases and seizure occurs between the roll and the material to be rolled, and a part of the material to be rolled is transferred to the roll surface as an adhered substance. It is considered to be due to the subsequent transfer to the steel strip surface. Also, when the rolling temperature is low, the oxide film formed on the steel strip surface becomes thinner, which may further promote baking. For this reason, low-temperature hot rolling has the problem of increasing the process load, such as the need to polish the surface of the steel strip in subsequent steps.

[0011] Therefore, a ferrite-based steel which improves the ridging property without deteriorating the productivity and surface quality of the hot-rolled steel strip, and securing the good deep drawability and ductility required for the cold-rolled steel sheet for processing. The economical industrial hot rolling technology of stainless steel has not yet been completed. The object of the present invention is in this respect.

[0012]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors conducted a detailed study on the effects of alloy composition, metal structure and hot rolling conditions on ridging properties and workability of ferritic stainless steel. I went.

As a result, the following several useful facts have been found. If the balance of steel composition is strictly controlled in the direction of increasing the amount of γ phase at high temperature, and high temperature and high speed hot rolling is performed in the finish hot rolling in the presence of a large amount of austenite, ferrite phase fragmentation and Strain accumulation can be achieved, recovery and recrystallization of the ferrite phase is promoted after winding or hot rolling annealing, and a ferritic stainless steel sheet with excellent ridging properties and workability after cold rolling and annealing can be obtained. I understand. In other words, even if high-temperature and high-speed hot rolling is used to ensure good surface quality and high productivity in hot-rolled steel strip, excellent ridging properties after cold rolling and annealing can be achieved by appropriate control of γmax described later. Workability can be imparted.

In addition, when an appropriate amount of B is added,
After the high-speed hot rolling, the γ phase becomes dispersed, and the ridging property and workability are further improved after cold rolling and annealing, and the workability is further improved by adding appropriate amounts of Ti, Nb, Zr and V. I found out.

The present invention has been completed on the basis of these findings, and the gist of the present invention is to provide a ferritic stainless steel exhibiting a two-phase structure of ferrite and austenite in a hot working temperature range by rough hot rolling. And hot-rolled steel strip through finish hot rolling, and when a cold-rolled steel sheet or steel strip is manufactured by combining cold rolling and annealing, the γmax expressed by the following formula (1) must be 35 or more and 80 or less. Manufacture ferrite stainless steel slabs in which the chemical components are balanced as follows (however, this steel does not mean that all the components in the formula (1) have been added, and the components that do not contain Γmax is calculated as 0%),
Rough hot rolling was started by heating to 150 to 1250 ° C, and the finish rolling exit speed was 7.0 m / s or more and the finish rolling exit temperature was 860 m / s.
This is a method for producing ferritic stainless steel sheets with excellent ridging properties and workability, characterized by finishing hot rolling at a temperature of ℃ or higher and winding at 650 ℃ or higher. .gamma.max = 420 (% C) +470 (% N) +23 (% Ni) +7 (% Mn) -11.5 (% Cr) -11.5 (% Si) -23 (% V) -49 (% Ti) -50 (% Nb) -50 (% Zr) +189 (1)

[0016]

[Action] γmax is an index corresponding to the maximum austenite phase amount at a high temperature such as a hot rolling temperature range, and the steel targeted by the present invention is basically a ferritic stainless steel having a γmax of 35 to 80. Any steel may be used. Preferred ferritic stainless steels that can be advantageously achieved by the present invention are, by mass%, C: 0.10% or less, Si: 0.75% or less, Mn: 2.0% or less, Ni: 0.50% or less, Cr: 10.00 to 20.00%, N: 0.04
% Or less, B: 0.0010 to 0.0300%, and Ti: 0.01 to 0.30%, Nb: 0.01 to 0.30%, Zr:
0.01 to 0.30% or V: steel containing one or more of 0.01 to 0.30%, the balance being Fe and unavoidable impurities, and γmax of 35 to 80.

The operation and effect of the present invention will be specifically described below with reference to typical experimental results.

Table 1 shows the chemical components of the test steels. Steel A
Γmax, which has a common chemical component as SUS430, is 17.0
7% Cr steel. Steel B has a higher γmax of 45.1 than steel A,
C steel has a γmax of 42.1 and contains B (boron). These steels were smelted in a vacuum high-frequency melting furnace and cast into molds lined with refractories to reduce the solidification rate and obtain a cast structure close to that of actual slabs, producing 100 kg ingots. From this steel ingot, a sample for hot rolling of 40 mm thickness × 100 mm width × 1 mm length was collected.

[0019]

[Table 1]

These samples were subjected to high-temperature hot rolling and low-temperature hot rolling under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 3.6 mm. In each case, the hot rolling speed was not changed and was kept constant at 5 m / s. The hot-rolled sheet is annealed at 850 ° C. for 6 hours, descaled, and cold-rolled to a thickness of 0.7 mm.
Min annealing was performed.

From the obtained cold-rolled annealed sheet, a tensile test specimen having a parallel part 35 mm wide × 120 mm long was taken in parallel with the rolling direction.
The ridging appearing on the surface with 0% tensile strain applied was evaluated. The ridging was evaluated by measuring the center line average roughness Ra in a direction perpendicular to the rolling direction using a surface roughness meter. The results are summarized in FIG.

[0022]

[Table 2]

FIG. 1 shows that γmax and hot rolling temperature have a very interesting effect on the ridging properties of ferritic stainless steel. In other words, in the case of steel A having a low γmax, the effect of the hot rolling temperature on Ra is large, and in high temperature hot rolling, Ra is significantly increased and the ridging property is inferior. The effect is relatively small and Ra is independent of any hot rolling temperature.
Is small and has excellent ridging properties even at high temperature hot rolling.

Further, from the results shown in FIG. 1, the center line average roughness equal to or less than that of the cold-rolled annealed sheet when the steel A corresponding to the conventional conditions is hot-rolled at a low temperature is γmax of about 35 in the hot-rolled hot-rolling. It can be seen that the above is obtained. In steel C containing B (boron), Ra becomes even smaller, and very good ridging properties are exhibited even in hot rolling at high temperatures.

The results in FIG. 1 show that even if the rolling speed is increased and the hot rolling temperature is increased in hot rolling, good ridging properties can be ensured if γmax is increased. It indicates that it promotes the effect.

The reason why good ridging characteristics can be obtained even at high temperature hot rolling when γmax is increased is considered as follows.

As described above, γmax corresponds to the maximum amount of austenite at a high temperature, and since the precipitation nose of austenite is in the range of 1050 to 1100 ° C., increasing γmax cannot be achieved in the temperature range in the latter half of rough hot rolling. Will increase the amount of austenite. However, steel containing a large amount of Cr, such as ferrite stainless steel, has a higher γ →
The α transformation is slow, and austenite formed in the rough hot rolling stage is brought almost as it is during the finish hot rolling.

Therefore, by increasing γmax, the austenite phase, which has a higher hot deformation resistance than the ferrite phase, is subjected to finish hot rolling in a state in which a large amount of the austenite phase coexists with the ferrite phase. It is thought that this contributes to the recovery and recrystallization of the ferrite phase after winding or during annealing of a hot-rolled sheet, and as a result, the ridging property is improved. Further, since high-speed hot rolling increases the strain rate, it provides an effect of accelerating the accumulation of strain in the ferrite phase, similarly to hot rolling at substantially low temperature.

On the other hand, the operation for B can be considered as follows. FIG. 2 is a photograph showing the metallographic structure of the hot rolled sheet of steel A in the test, and FIG. 3 is a photograph showing the metallographic structure of the hot rolled sheet of steel C (steel containing boron B). From the comparison of the two figures, it can be seen that in the steel C containing B (boron), the transformation phase, which was the austenite phase during hot rolling, is dispersed and distributed in the form of grains, and the steel A, which is a strip and has a high elongation, It can be seen that the metal structures differ greatly.

Therefore, B does not affect the amount of austenite, but rather changes the size and distribution of the austenite phase, and further enhances the effect of the austenite phase during hot rolling. it is conceivable that. The mechanism of the effect of B on the distribution of the transformed phase is not yet clear at this time, but probably there are many precipitates involving B in ferrite, which form It might be a site.

The reasons for limiting the numerical values of the requirements specified in the present invention will be described below.

As described above, γmax is an index of the maximum austenite amount in the hot rolling temperature range. This γma
The definition of x is the most important point of the present invention.When γmax is 35 or less, the amount of austenite at high temperatures is small, and sufficient ridging characteristics can be obtained when high-speed and high-temperature hot rolling specified in the present invention is performed. Absent. On the other hand, to increase γmax, C,
It is necessary to increase the amount of austenite forming elements such as N, Mn, and Ni. However, these elements are specified as γmax of 80 or less because they cause hardening of the material and increase in cost.

The slab heating temperature before rough hot rolling is 1150 ° C.
If it is less than 1, even if high-speed rolling is performed, the problems of productivity reduction and surface flaw generation, which are problems in conventional hot rolling, cannot be completely solved. On the other hand, adopting a slab heating temperature exceeding 1250 ° C is disadvantageous because it raises energy costs. Therefore, the slab heating temperature is 1150-1250 ° C
Range.

The regulation of the finish rolling speed is an important point of the present invention, and is regulated to 7.0 m / s or more in order to increase the productivity, secure the rolling temperature, and obtain a hot-rolled steel strip having good surface quality. Regarding the finish rolling end temperature, if the above slab heating temperature of 1150 ° C or higher is used, the finishing temperature should be set to less than 860 ° C by controlling the rolling speed or strengthening the cooling of the strip during hot rolling. In addition, the risk of surface flaws caused by seizure with the roll increases. For this reason, the finish rolling finish temperature should be 860 ° C or higher.

In order to lower the winding temperature, it is necessary to take measures such as aggressive water cooling from the finish rolling mill outlet to the winding machine, resulting in a defective shape of the steel strip. Conversely, in order to perform high temperature winding, it is necessary to perform hot rolling more than necessary. Therefore, the slab heating temperature and the finish rolling end temperature specified in the present invention are adopted, and the winding temperature that does not require special water cooling or the like is specified as 650 ° C. or higher.

The steel targeted in the present invention may be basically a ferritic stainless steel in which the above-mentioned γmax is 35 or more and 80 or less, but as the ferrite stainless steel in which the object of the present invention can be advantageously achieved. ,% By mass C: 0.10%
Or less, Si: 0.75% or less, Mn: 2.0% or less, Ni: 0.50%
Below, Cr: 10.00-20.00%, N: 0.04% or less, B: 0.00
Contains 10 to 0.0300%, and additional Ti: 0.01 if necessary
~ 0.30%, Nb: 0.01 ~ 0.30%, Zr: 0.01 ~ 0.30% or V: 0.01 ~ 0.30%, the balance consists of Fe and unavoidable impurities, γmax is 35 or more
80 or less steel. The reasons for limiting the content of each component of the steel are as follows.

C is an element that increases γmax. For this reason, austenite in the hot rolling temperature range is increased, which works advantageously for microstructural refinement, and is preferable for improving ridging properties. However, C is also an element that increases the strength after cold-rolling annealing. If it is too high, ductility is reduced, so the upper limit is 0.10%.

Although Si is an effective element for deoxidation, it has a large solid solution strengthening ability, and if its content is too high, the material is hardened and ductility is reduced.

Mn is an austenite forming element and γmax
Effective in controlling the temperature and the solid solution strengthening ability is small, and the adverse effect on the material is small. However, addition exceeding 2.0% causes deterioration of corrosion resistance and cost increase, so the upper limit is 2.0%.

Ni, like Mn, is an austenite forming element and is an element effective for controlling γmax. But 0.50%
Addition exceeding 0.5% leads to hardening and cost increase, so the upper limit is 0.50%.

The lower limit of 10.00% of Cr is the minimum necessary for maintaining the corrosion resistance of stainless steel. On the other hand, a large amount causes deterioration in workability, so the upper limit is 20.00%.

N is an element that increases γmax similarly to C,
It is preferable for improving ridging properties. However, a large amount of addition causes a decrease in ductility and surface flaws due to hardening.
Is the upper limit.

As described above, B has the effect of uniformly dispersing the transformed phase distribution of the hot-rolled sheet, miniaturizing and dividing the unit region that causes ridging. The effect is 0.0010%
Less than is not enough. On the other hand, if it exceeds 0.0300%, slab casting defects will increase and weldability will decrease, so the appropriate content is specified as 0.0010% to 0.0300%.

Ti, Nb, Zr or V is an element effective for increasing the r value and improving the deep drawability. On the other hand, it is a ferrite-forming element and lowers γmax. Therefore, each of the appropriate content ranges is 0.01 to
Restrict to 0.30%.

In addition to the above components, Mo, Cu, Al
It is permissible to appropriately add other elements such as for the purpose of improving various properties such as corrosion resistance and oxidation resistance.

[0046]

EXAMPLE Steels having the chemical components shown in Table 3 were melted, and each steel was hot-rolled in accordance with the conditions of the slab heating temperature, the finish rolling speed and the finish rolling temperature shown in Table 4, and each had a sheet thickness of 3.6. mm hot-rolled steel strip. Each hot-rolled steel strip was checked for surface flaws caused by seizure with the rolling rolls, and then annealed at 850 ° C for 6 hours.
mm, and annealed at 830 ° C. × 1 min.

With respect to each of the obtained cold rolled steel strips, ridging properties, tensile properties, and r values were examined. The ridging property was determined by taking a tensile test specimen 35 mm wide x 120 mm length parallel to the rolling direction, applying a 20% tensile strain, and using a surface roughness meter to measure the center line average roughness in the direction perpendicular to the rolling direction. The Ra was measured and evaluated according to the following five grades.

[0048]

The tensile properties and r-values were measured using a JIS No. 13B test piece in the rolling direction, the rolling direction in the 45 ^° direction, and the rolling direction in the 90 ^° direction.
The measurement was performed in three directions, and the average was obtained. The results obtained are shown in Table 4.

[0050]

[Table 3]

[0051]

[Table 4]

As is clear from Table 4, the present invention example No. a
All of the steels according to (1) to (4) have excellent surface ridging and workability without any surface flaws in the hot-rolled steel strip.

On the other hand, Comparative Example No. h
Since γmax of o.8 was 22.8 (smaller than the range specified in the present invention), the ridging property was inferior. Also, the elongation and the r value are low and the workability is inferior.

Comparative Examples No. i-l Performed at Low Temperature and Low Speed Hot Rolling
Has relatively good ridging and processability,
Surface flaws caused by seizure with stainless steel occurred frequently in the hot-rolled steel strip. For this reason, product polishing required surface polishing in a later step.

In this embodiment, a long-time hot-rolled sheet annealing is performed.
Although the example of the single cold rolling method without intermediate annealing is shown, the same effect is obtained regardless of the presence or absence of hot rolling annealing, the method of hot rolled sheet annealing, and the presence or absence of intermediate annealing.

[0056]

As described above, according to the present invention, it is possible to obtain a ferritic stainless steel sheet having good surface properties, excellent ridging property and workability without causing a decrease in productivity and a rise in cost. .

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of γmax and hot rolling temperature on center line average roughness Ra for ridging evaluation.

FIG. 2 is a photograph showing a metal structure of a hot-rolled steel A sheet in Examples.

FIG. 3 is a photograph showing a metal structure of a hot rolled sheet of steel C in an example.

Claims (3)

(57) [Claims] 1. A ferritic stainless steel exhibiting a two-phase structure of ferrite and austenite in a hot working temperature range is formed into a hot-rolled steel strip through rough hot rolling and finish hot rolling, and is subjected to cold rolling and annealing. When a cold-rolled steel sheet or steel strip is manufactured in combination, the γmax expressed by the following equation (1) is 35.
A slab of ferritic stainless steel having a chemical composition balanced so as to be 80 or less was manufactured, and this slab was heated to 1150 to 1250 ° C to start rough hot rolling, and the finish-rolling exit speed was 7.0. m / s or more and the finish-rolling exit temperature is
A method for producing a ferritic stainless steel sheet with excellent ridging properties and workability, characterized by finishing hot rolling at 860 ° C or higher and winding at 650 ° C or higher. .gamma.max = 420 (% C) +470 (% N) +23 (% Ni) +7 (% Mn) -11.5 (% Cr) -11.5 (% Si) -23 (% V) -49 (% Ti) -50 (% Nb) -50 (% Zr) +189 (1) 2. The ferrite stainless steel has a mass% of
, C: 0.10% or less, Si: 0.75% or less, Mn: 2.0% or less, Ni: 0.50% or less, Cr: 10.00 to 20.00%, N: 0.04
%, B: 0.0010 to 0.0300%, the balance consists of Fe and unavoidable impurities, and γmax is 35 or more and 80 or less. 3. The ferrite stainless steel has a mass% of
, C: 0.10% or less, Si: 0.75% or less, Mn: 2.0% or less, Ni: 0.50% or less, Cr: 10.00 to 20.00%, N: 0.04
%, B: 0.0010 to 0.0300%, and Ti:
0.01 to 0.30%, Nb: 0.01 to 0.30%, Zr: 0.01 to 0.30%
Or V: contains 0.01 to 0.30% of one or more kinds, the balance being Fe and unavoidable impurities, and γm
The method according to claim 1, wherein ax is 35 or more and 80 or less.