JPH0681036A - Production of ferritic stainless steel sheet excellent in ridging characteristic and workability - Google Patents

Abstract

PURPOSE:To produce a ferritic stainless steel sheet excellent in ridging characteristic and workability by subjecting a slab of a ferritic stainless steel having a chemical composition specified by a specific equation to hot roughing and finish hot rolling under respectively specified conditions, coiling the resulting plate, and then applying cold rolling and annealing to the plate. CONSTITUTION:A slab of a ferritic stainless steel where chemical components are balanced so that FIX represented by an equation becomes 35-80 and which has a dual phase structure of ferrite and austenite at hot working temp. is produced. Hot roughing is started by heating the slab up to 1150-1250 deg.C, and finish hot rolling is completed while regulating the finish rolling outlet side velocity and the finish rolling outlet side temp. to >=7.0m/s and >=860 deg.C, respectively, followed by coiling at 650 deg.C. Then the resulting hot rolled steel strip is subjected to a combination of cold rolling and annealing treatments, by which a cold rolled steel sheet is produced. By this method, the ferritic stainless steel having superior surface characteristics and excellent in ridging characteristic and workability can be obtained without causing reduction in productivity and rise in costs.

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 which is excellent in ridging property and workability for press forming, bending, roll forming and the like. The steel of the present invention is put on the market in the form of a cold-rolled steel strip or a cold-rolled steel sheet, but in the present specification, these are collectively referred to as a steel sheet.

[0002]

2. Description of the Related Art Ferrite stainless steel represented by SUS430 has good corrosion resistance and workability, does not contain expensive Ni, and has economic advantages over austenitic stainless steel. Therefore, it is widely used mainly for durable consumer goods. However, there is a phenomenon in which ferritic stainless steel sheets cause peculiar wrinkled surface irregularities called ridging during press forming. Ridging not only impairs the aesthetics of the molded product and lowers its commercial value, but it also increases the polishing load to remove it.
This is a major problem in the processing of ferritic stainless steel.

There have been many reports on the origin of ridging, but after all, the unit region consisting of a group of crystal grains with a close orientation derived from the casting structure or the hot-rolled sheet structure in the cold-rolled annealed sheet Is also preserved, and it is considered that each unit region exhibits different deformation behavior during processing such as press forming, resulting in ridge-like surface undulations in the rolling direction of the steel sheet. Therefore, to improve the ridging property, it is effective to miniaturize and pulverize this unit area, and various improvement measures have been proposed from this viewpoint.

Conventionally, the production of ferritic stainless steel sheets has been carried out by hot rolling a continuously cast slab into a hot rolled steel strip and annealing the hot rolled sheet in a box furnace or a continuous annealing furnace.
The general method is to pickle the product, perform one cold rolling or multiple cold rolling processes including intermediate annealing, and then recrystallize and anneal the product. Although hot-rolled sheet annealing may be omitted, hot-rolled sheet annealing may be performed for thin plate press forming applications where ridging and workability are important, especially for deep drawing. It is often manufactured by performing cold rolling twice including intermediate annealing.

The hot rolling conditions that are standardly adopted in the production of such a conventional ferrite cold-rolled stainless steel sheet are as follows.

Regarding the rolling speed, which is important in terms of production efficiency, in the conventional ferrite stainless steel, the finishing rolling exit speed is about 4.0 to 6.0 m / s. This is about 8 m / s or more for austenitic stainless steel and about 1 for ordinary steel.
It is considerably slower than 0m / s or more.

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

[0008] More positively, so-called delay rolling, in which rolling passes under relatively high temperature and high pressure reduction are performed, or the material is temporarily held during rolling to increase the interpass time, is effective in improving ridging. Is also known. For example
According to 45-34016, at least 50% reduction of hot rolling is performed at a temperature of 871 ° C or less, and the rolling is temporarily stopped during the hot rolling, and the material is not higher than 760 to 871 ° C. A method for improving ridging has been proposed, in which a subsequent hot rolling is performed after cooling to a temperature.

[0009]

The conventional improvement of the ridging property by lowering the hot rolling temperature has a problem that the rolling speed is inevitably reduced in order to lower the temperature. For this reason, productivity cannot be avoided. In fact, as mentioned above, the finish rolling speed of ferritic stainless steel is much slower than that of austenitic stainless steel, and its productivity is poor. Delayed rolling also causes an increase in rolling time and a drop in productivity. Therefore, this is not an economical method either.

In addition, although low temperature hot rolling is certainly effective in improving the ridging property, it has a problem that surface defects easily occur and the surface quality of the steel strip deteriorates. this is,
When the rolling temperature is low, the deformation resistance of the material to be rolled increases, so 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 that this is due to the transfer to the surface of the steel strip thereafter. In addition, when the rolling temperature is low, the oxide film formed on the surface of the steel strip becomes thinner, which may further promote the baking. For this reason, low temperature hot rolling has a problem in that the process load is increased, for example, the surface polishing of the steel strip is required in the subsequent process.

Therefore, a ferrite system which improves ridging property without deteriorating the productivity and surface quality of the hot rolled steel strip, while ensuring good deep drawability and ductility required for the cold rolled steel sheet for working. Economical industrial hot rolling technology for stainless steel has not yet been perfected. 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 have made a detailed study on the effects of alloy composition, metal structure and hot rolling conditions on the ridging property and workability of ferrite stainless steel. I went.

As a result, some useful facts were found as follows. When the composition balance of steel is strictly controlled in the direction of increasing the amount of γ phase at high temperature, and hot rolling at high temperature and high speed is carried out in the state where a large amount of austenite phase coexists in the finish hot rolling, the ferrite phase is fragmented and Strain accumulation can be promoted, recovery and recrystallization of the ferrite phase is promoted after winding or during hot rolling annealing, and a ferrite stainless steel sheet with excellent ridging property and workability after cold rolling / annealing can be obtained. I understood it. In other words, even if high-temperature / high-speed hot rolling that ensures good surface quality and high productivity in the hot-rolled steel strip is adopted, it is possible to obtain excellent ridging property after cold rolling / annealing by appropriate control of γmax described later. Workability can be imparted.

In addition, if an appropriate amount of B is added, high temperature
After high-speed hot rolling, the γ phase becomes a dispersed distribution form, further improving the ridging property and workability after cold rolling and annealing, and further improving the workability by adding an appropriate amount of Ti, Nb, Zr and V. I found out that

The present invention has been completed on the basis of such findings, and the gist of the invention is to perform rough hot rolling of a ferrite stainless steel having a dual phase structure of ferrite and austenite in the hot working temperature range. And when producing a cold rolled steel sheet or strip by combining cold rolling and annealing to form a hot rolled strip after finishing hot rolling, γmax expressed by the following formula (1) is 35 or more and 80 or less. A slab of ferritic stainless steel with a balanced chemical composition is manufactured (however, this steel does not mean that all the constituents in Eq. (1) are added, and the constituents that do not contain Γmax is calculated assuming 0%), and this slab is 1
Coarse hot rolling was started by heating to 150 to 1250 ° C, the finish rolling exit speed was 7.0 m / s or more, and the finish rolling exit temperature was 860
This is a method for producing ferritic stainless steel sheets with excellent ridging and workability, which is characterized by finishing hot rolling at ℃ or above and winding at 650 ℃ or above. γ 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]

[Function] γmax is an index corresponding to the maximum amount of austenite phase at high temperature such as hot rolling temperature range, and the steel targeted by the present invention is basically made of ferritic stainless steel having γmax of 35 or more and 80 or less. It just needs to be steel. The preferred ferritic stainless steels that can be advantageously achieved by the present invention are 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%, Ti: 0.01 to 0.30%, Nb: 0.01 to 0.30%, Zr:
0.01 to 0.30% or V: 0.01 to 0.30% of 1 type or 2 types or more, the balance of Fe and inevitable impurities, and γmax of 35 to 80.

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

Table 1 shows the chemical composition of the test steel. A steel
Γmax, which has general chemical composition as SUS430, is 17.0 of 17.0
7% Cr steel. Steel B has a higher γmax of 45.1, which is higher than that of Steel A.
C steel is a steel having a γmax of 42.1 and containing B (boron). These steels were melted in a vacuum high-frequency melting furnace and cast in a refractory-lined mold to slow the solidification rate and obtain a cast structure close to that of an actual slab, producing a 100 kg ingot. A sample for hot rolling having a thickness of 40 mm, a width of 100 mm, and a length of 1 mm was taken from this steel ingot.

[0019]

[Table 1]

These samples were hot-rolled and cold-rolled under the conditions shown in Table 2 to prepare hot-rolled steel sheets with a plate thickness of 3.6 mm. In each case, the hot rolling speed was not changed and was kept constant at 5 m / s. This hot-rolled sheet is annealed at 850 ° C x 6h, descaled, and cold-rolled to a thickness of 0.7mm at 830 ° C x 1
It was annealed for min.

From the obtained cold rolled annealed plate, a tensile test piece having a parallel portion of 35 mm width × 120 mm length was taken in parallel with the rolling direction, and 2
The ridging that appeared on the surface was evaluated by applying 0% tensile strain. The ridging was evaluated by measuring the center line average roughness Ra in the direction perpendicular to the rolling direction using a surface roughness meter. The results are summarized in Fig. 1.

[0022]

[Table 2]

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

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

The results shown in FIG. 1 show that even if the rolling speed is increased and the hot rolling temperature is increased in hot rolling, good ridging property can be secured by increasing γmax, and B is further improved. It is shown to promote the effect.

The reason why good ridging characteristics can be obtained even with high temperature hot rolling by increasing γmax is considered as follows.

Γmax corresponds to the maximum amount of austenite at high temperature as described above, and the precipitation nose of austenite is in the range of 1050-1100 ° C. Therefore, increasing γmax in the temperature range in the latter half of rough hot rolling. Will increase the amount of austenite. However, for steels containing a large amount of Cr such as ferritic stainless steel, γ →
The α-transformation is slow, and the austenite produced in the rough hot rolling stage is brought almost as it is during the finish hot rolling.

[0028] Therefore, by increasing γmax, the austenite phase, which has a larger deformation resistance during hot rolling than the ferrite phase, will be finished hot-rolled in a state in which a larger amount of the austenite phase coexists with the ferrite phase. It is thought that this contributes to the formation and strain accumulation, which acts to promote the recovery and recrystallization of the ferrite phase after winding or during annealing of the hot rolled sheet, and as a result, the ridging property is improved. In addition, high-speed hot rolling increases the strain rate, so that it provides an action of promoting strain accumulation in the ferrite phase, as in the case of hot rolling at a substantially low temperature.

On the other hand, the action 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 above 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 both figures, in the steel C containing B (boron), the transformation phase, which was the austenite phase, was dispersed and distributed in the form of particles during hot rolling. It can be seen that the metal structures differ greatly.

Therefore, B serves not only to influence the amount of austenite, but rather to change the size and distribution of the austenite phase and to further enhance the effect of the austenite phase during hot rolling. it is conceivable that. The mechanism of the effect of B on the distribution state of this transformation phase is not always clear at present, but there are probably many precipitates in which B is involved in the ferrite, which results in the formation of austenite during coarse hot rolling. It seems that it will be a site.

The reasons for limiting the numerical values of the requirements defined by 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 temperature is small, and sufficient ridging characteristics can be obtained when high-speed / high-temperature hot rolling specified by the present invention is applied. Absent. On the other hand, in order to increase γmax, C,
It is necessary to increase the amount of austenite forming elements such as N, Mn, Ni, etc., but these cause hardening of the material and increase in cost, so γmax is specified to be 80 or less.

The heating temperature of the slab before rough hot rolling is 1150 ° C.
If the amount is less than the above, even if high-speed rolling is performed, the problems of productivity drop and surface defects, which are problems in conventional hot rolling, cannot be completely solved. On the other hand, adopting a slab heating temperature above 1250 ° C is disadvantageous because it raises the energy cost. Therefore, the slab heating temperature is 1150 to 1250 ℃.
The range is.

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

In order to lower the coiling temperature, it is necessary to take measures such as positive water cooling from the exit side of the finish rolling mill to the coiler, which leads to defective shape of the steel strip. On the contrary, in order to carry out high temperature winding, it is necessary to carry out hot rolling at an unnecessarily high temperature. Therefore, the slab heating temperature and finish rolling end temperature specified in the present invention are adopted, and the coiling temperature is set to 650 ° C. or higher as the coiling temperature that does not require special water cooling.

The steel targeted by the present invention may basically be a ferrite stainless steel having a γmax of 35 or more and 80 or less, but as a ferrite stainless steel that can advantageously achieve the object of the present invention. , C in mass%: 0.10%
Below, Si: 0.75% or less, Mn: 2.0% or less, Ni: 0.50%
Below, Cr: 10.00 to 20.00%, N: 0.04% or less, B: 0.00
Contains 10-0.0300%, Ti: 0.01 if necessary
~ 0.30%, Nb: 0.01 ~ 0.30%, Zr: 0.01 ~ 0.30% or V: 0.01 ~ 0.30%, containing 1 or 2 or more kinds, the balance consisting of Fe and unavoidable impurities, and γmax of 35 or more
Steel of 80 or less. The reasons for limiting the content of each component of this steel are as follows.

C is an element that increases γmax. Therefore, austenite in the hot rolling temperature range is increased, which is advantageous for the refinement of the structure and is preferable for improving the ridging property. However, C is also an element that increases the strength after cold rolling annealing, and if it is too high, the ductility decreases, so the upper limit is 0.10%.

Si is an element effective for deoxidation, but it has a large solid solution strengthening ability, and if its content is too high, the material hardens and the ductility decreases, so it is made 0.75% or less.

Mn is an austenite forming element and γmax
It can be effectively used for the control of, and has a small solid solution strengthening ability, and has little adverse effect on the material. However, addition of more than 2.0% causes deterioration of corrosion resistance and cost increase, so 2.0% is made the upper limit.

Like Mn, Ni is an austenite forming element and is an element effective for controlling γmax. But 0.50%
Addition of more than 0.5 causes hardening and cost increase, so 0.50% is made the upper limit.

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

N is an element which increases γmax like C,
Suitable for improving ridging property. However, addition of a large amount causes a decrease in ductility due to hardening and the occurrence of surface defects, so 0.04%
Is the upper limit.

As described above, B has the effect of uniformly dispersing the transformation phase distribution of the hot-rolled sheet and 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 increase and weldability deteriorates, so the appropriate content is specified to be 0.0010% to 0.0300%.

Each of Ti, Nb, Zr and 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 reduces γmax. Therefore, the appropriate content range for each is 0.01-
Regulate to 0.30%.

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

[0046]

[Examples] Steels having chemical compositions shown in Table 3 were melted, and each steel was hot-rolled according to the conditions of slab heating temperature, finish rolling speed and finish rolling temperature shown in Table 4. mm hot rolled steel strip. For each hot-rolled steel strip, after confirming the occurrence of surface defects due to seizure with the rolling rolls, annealing was performed at 850 ° C x 6h, and after descaling, the sheet thickness was 0.7
It was cold rolled to mm and annealed at 830 ° C for 1 min.

The cold-rolled steel strips thus obtained were examined for ridging property, tensile property and r-value. The ridging property was measured by pulling a tensile test piece with a parallel portion of 35 mm width × 120 mm length parallel to the rolling direction, applying a 20% tensile strain, and then using a surface roughness meter to measure the centerline average roughness in the direction perpendicular to the rolling direction. Ra was measured and evaluated according to the following 5 grades.

[0048]

Tensile properties and r-values were measured using JIS 13B test pieces in the rolling direction, rolling direction and 45 ^° direction, and rolling direction and 90 ^° direction.
The three directions were measured and the average was obtained. The obtained results are also shown in Table 4.

[0050]

[Table 3]

[0051]

[Table 4]

As is apparent from Table 4, Example No. a of the present invention
Each of the steels according to ~ g has no surface flaw in the hot-rolled steel strip and has excellent ridging property and workability.

On the other hand, in Comparative Example No. h, the used steel N
Since γmax of 0.8 is 22.8 (smaller than the range specified in the present invention), the ridging property is poor. In addition, the elongation and r value are low and the workability is poor.

Comparative Examples No. i to l subjected to low temperature / low speed hot rolling
Has relatively good ridging and workability,
Surface defects due to seizure with stainless steel occurred frequently on the hot-rolled steel strip. For this reason, surface polishing was required in a later process for commercialization.

In this example, hot-rolled sheet annealing was performed for a long time,
An example of the single cold rolling method without intermediate annealing was shown, but the same effect can be 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 and excellent ridging property and workability without lowering productivity and increasing cost. .

[Brief description of drawings]

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

FIG. 2 is a photograph showing a metallographic structure of a hot-rolled sheet of steel A in an example.

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

Claims (3)

[Claims] 1. A ferrite rolled stainless steel exhibiting a dual 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 cold rolling and annealing are performed. When manufacturing cold-rolled steel sheets or strips in combination, γmax shown in the following equation (1) is 35
A slab of ferrite stainless steel with a balanced chemical composition so as to be 80 or less is manufactured, and the slab is heated to 1150 to 1250 ° C to start rough hot rolling, and the finish rolling exit speed is 7.0. m / s or more and the finish rolling outlet temperature is
A method of manufacturing ferritic stainless steel sheets with excellent ridging and workability, characterized by finishing hot rolling at 860 ° C or higher and winding at 650 ° C or higher. γ 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. Ferrite-based stainless steel is mass%
Then, 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%, the balance consisting of Fe and unavoidable impurities, and .gamma.max of 35 or more and 80 or less. 3. Ferrite-based stainless steel is mass%
Then, 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: 0.01 to 0.30% of 1 type or 2 types or more, the balance consisting of Fe and inevitable impurities, and γm
The production method according to claim 1, wherein ax is 35 or more and 80 or less.