JPH0790383A - Production of ferritic stainless steel sheet excellent in ridging resistance - Google Patents

Abstract

PURPOSE:To provide a method for producing a ferritic stainless steel sheet in which the defects of the conventional techniques are solved and excellent in ridging resistance. CONSTITUTION:A slab or a steel ingot having a steel compsn. contg., by weight, <=0.08% C, <=2.0% Si, <=1.00% Mn, 13.0 to 18.0% Cr, <=0.6% Ni, 0.03 to 0.15% Al, 0.003 to 0.05% N, <=0.010% S, and the balance Fe with inevitable impurities is subjected to hot rolling, and the hot rolled sheet finishing temp. is regulated to the Ac1 point +20 to 50 deg.C. After the completion of finish rolling, it is cooled and is successively coiled around a coil at 400 to 700 deg.C. After the coiling, water cooling is immediately executed. Next, annealing is executed at a temp. and for a time in which the obtd. martensite is perfectly disappeared, and furthermore, the regulation of the grains in the hot rolled sheet is executed to attain the randomization of the crystal orientation. Moreover, a cold rolling-annealing stage for one time or >=two times including process annealing is executed. The steel compsn. may contain 0.03 to 0.5% Nb as well.

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 resistance.

[0002]

2. Description of the Related Art Conventionally, during hot rolling of ferritic stainless steel, finish rolling was carried out so that the final temperature of hot-rolled sheet was in the ferrite single phase region. Was the purpose. However, this tends to cause coarsening of the texture and crystal grains, and there is a problem in ridging resistance.

The "hot rolled sheet end temperature" is the hot rolling finish temperature measured by the surface temperature of the hot rolled sheet.

Regarding improvement of ridging resistance, for example, in Japanese Unexamined Patent Publication (Kokai) No. 52-131920, 50% or more of the casting structure is made equiaxed, and hot rolling for refining it and annealing of hot rolled sheet are performed. Stipulates the conditions.

Japanese Patent Publication No. 51-30008 discloses that a hot-rolled sheet is heated to a temperature of a two-phase region to cause an austenite phase to appear, and a martensite transformation is performed at the time of cooling to cool a ferrite and a martensite phase in a mixed structure state. It is said that ridging can be eliminated by carrying out hot rolling to divide the striped structure.

Further, Japanese Patent Application Laid-Open No. 1-111816 discloses 850
Hot rolling at ℃ or more, immediately cooling at 10 ℃ / s or more, 550
A method has been proposed for improving the ridging property by winding at a temperature of not higher than 0 ° C. to form a two-phase structure of ferrite and martensite, and then performing cold rolling at a cumulative reduction of 50% or more.

[0007] However, in this case, since the cold-rolled sheet has high strength and low ductility because it contains martensite during cold rolling, edge cracking of the coil occurs during cold rolling, and further coil rupture occurs. Therefore, cold rolling becomes impossible industrially.

On the other hand, after hot rolling, the obtained coil is allowed to cool for several tens of hours and then subjected to a long annealing process to soften the coil and spheroidize the carbides, even if the hot rolling is performed. During subsequent cooling, the crystal is recovered and recrystallized due to the heat contained, and the crystal grain size in the coil winding central portion becomes coarse.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for manufacturing a ferritic stainless steel sheet having excellent ridging resistance, which overcomes the above-mentioned drawbacks of the prior art.

[0010]

Therefore, the inventors of the present invention left the martensite layer and eliminated the martensite phase by the annealing treatment in the next step, and at the same time, randomized the texture and refined the crystal structure. By doing so, we focused on preventing the occurrence of ridging.

That is, the crystal structure is refined by temperature control of the hot rolling finishing step and the winding step, and water cooling.
In addition, the randomization of the texture makes it difficult to cause ridging.

From these viewpoints, the present inventors have conducted various studies, and as a result, in order to carry out hot rolling at a relatively low temperature such that transformation of the structure does not occur, the amount of Al and Nb added during hot rolling By increasing the final temperature of the hot-rolled sheet and changing the structure, it is possible to suppress coarsening of crystal grains during annealing and randomize the texture to prevent ridging. The present invention has been completed.

Here, the gist of the present invention is that, in% by weight, C: 0.08% or less, Si: 2.0% or less, M
n: 1.00% or less, Cr: 13.0 to 18.0%, Ni: 0.6% or less, Al: 0.03 to 0.15%, N: 0.003 to 0.05%, S:
Slabs or steel ingots with a steel composition of 0.010% or less, balance Fe and unavoidable impurities are hot-rolled, the hot rolled sheet final temperature is set to Ac ₁ point + 20 to 50 ° C, and after finishing rolling, cooling To produce 1 part martensite,
The coil is wound at 400 to 700 ° C, the coil is cooled with water immediately after winding, and then annealed at a temperature and for a time such that the obtained martensite completely disappears. It is a method for producing a ferritic stainless steel sheet having excellent ridging resistance, which comprises randomizing the crystal orientation by performing, and further performing cold rolling once or twice with intermediate annealing. .

[0014]

[Function] Conventionally, ferritic stainless steel (Example: SUS430)
In order to improve the mechanical properties, Al or Nb was added and the austenite transformation point was raised to enable high temperature hot rolling, but this method had a drawback that the ridging property deteriorates.

Therefore, in the present invention, the amount of aluminum added is reduced in order to perform grain size regulation of the structure and randomization of the texture, which is a technique for preventing ridging, and hot rolling is completed at the austenite transformation point. Then, it is tried to generate martensite by quenching.

[0016] Thus, according to the present invention, the process is omitted because it does not depend on the casting structure, and by rapidly cooling after hot rolling, the texture that causes ridging is randomized, and good ridging resistance is obtained. can get.

That is, in the conventional anti-ridging measures, BAF is used.
The grain size could not be controlled because the recrystallization was completed by the time the annealing was reached. Therefore, in the present invention, it is intended to improve the ridging resistance by preventing the coarsening of grain size and randomizing the crystal orientation by generating martensite which has not been generated conventionally in the rolling process.

Next, the reason why the steel composition, hot working and heat treatment conditions are defined as described above in the present invention will be described in detail together with its action. First, the reason for limiting the steel composition targeted in the present invention will be described. In addition, "%" that defines the steel composition is "% by weight".

C (carbon): C is an effective component for ensuring the strength of the steel sheet, but if it is added in excess of 0.08%, the formability is adversely affected, so the upper limit was made 0.08%. Si (Si): Si has the effect of improving the oxidation resistance of steel, but addition of more than 2.0% causes embrittlement of the steel, so the upper limit was made 2.0%.

Mn (manganese): Mn is an element necessary for improving the deoxidizing component and high temperature strength during steel making, but 1.00
% Is added, it causes embrittlement of the steel, so its upper limit is 1.00.
Defined as%. Cr (Chromium): Cr is an important element that improves the oxidation resistance, but if its content is less than 13.0%, it is insufficient, and 18.0%
If added over the range, heat fatigue resistance and moldability will deteriorate,
The Cr content is 13.0 to 18.0 because it is disadvantageous in terms of cost.
Defined as%.

Ni (Nickel): In the present invention, Ni is present as an impurity, and the permissible upper limit is 0.6%. Al (aluminum): Al is an element that improves hot workability and grain size, but since it is an element that makes it difficult for martensite transformation to occur, a component that causes martensite transformation and reduces the grain size. As the range, Al content
It was set at 0.03 to 0.15%.

S (sulfur): S is an element mixed as an impurity, but if its content exceeds 0.010%, it deteriorates the hot workability and adversely affects the workability. Was set to 0.010% or less.

N (nitrogen): N has a function of refining crystal grains, but if the content thereof is less than 0.003%, the desired effect due to the above function cannot be obtained, while if it exceeds 0.05%. Since the content of N tends to promote embrittlement, the content of N is set to 0.003 to 0.05%.

Nb (niobium): Nb also has a function of refining crystal grains, and may be added if desired. However, if its content is less than 0.03%, the desired effect due to the above action cannot be obtained, while if it exceeds 0.5%, the workability tends to deteriorate, so if it is added, the content of Nb is Was set to 0.03 to 0.5%, preferably 0.1 to 0.15%.

The ferritic stainless steel having such a steel composition has a ridging resistance after hot rolling, water cooling after winding, annealing, two or more cold rolling steps including intermediate annealing, and an annealing step. Excellent ferritic stainless steel is produced.

The hot rolling may be carried out under the conventional conditions except for coil winding. For example, in a ferritic stainless steel containing 13 to 18% of chromium, the austenite region necessary for producing a martensite phase is obtained. Since the temperature condition for heating is at least 850 ° C. or higher, it may be heated to, for example, 900 ° C. or higher prior to hot rolling. Preferably 90
It is 0 to 1000 ° C.

In the present invention, the hot-rolled sheet end temperature is defined as a temperature 20 to 50 ° C. higher than the Ac ₁ transformation point temperature. This means that in the ferritic stainless steel containing 13 to 18% of chromium, the martensitic transformation is performed. The temperature condition required to cause the above is heating above the Ac ₁ transformation point. Considering that the transformation amount necessary for this condition is obtained, the temperature above the Ac ₁ transformation point temperature
It is specified as 20 to 50 ℃.

After the hot rolling is finished, cooling is carried out. Since the purpose of this cooling is to precipitate an appropriate amount (20 to 40% by volume) of the martensite phase by rapid cooling, it is carried out, for example, by water cooling after the hot rolling. .

After cooling, the coil is wound at 400 to 700 ° C. If cooled to less than 400 ° C, problems such as deterioration of the surface shape will occur when wound on a coil.On the other hand, if it exceeds 700 ° C, recrystallization will start due to heat inclusion and the crystal grains may become coarse. Therefore, the coil winding temperature range is 4
It was set to 00-700 ℃.

After being wound on the coil, water cooling of the coil such as dobu-zuke water cooling is performed. The coil is dipped in a water tank or the like in order to prevent the change of the structure due to latent heat after the coil is wound up, for example, at a cooling rate of 20 ° C./sec or more.

Here, the water-cooled dobu-zuke as an example of the coil water cooling is a process of cooling by immersing it in a water tank while it is wound around the coil. After cooling with water, the annealing treatment is performed under conditions of temperature and time such that martensite completely disappears.

That is, when martensite is produced after the annealing, the mechanical properties are significantly deteriorated, so that the annealing is carried out in a temperature / time range where the martensite completely disappears. It is sufficient to heat-treat at 900 ° C for 1 to 5 minutes.

Grain sizing, which is carried out subsequent to the annealing treatment for eliminating martensite, can generally be carried out under the condition of 850 ° C. to achieve the required randomization of crystal orientation. By this annealing treatment, the crystal orientation can be randomized as desired. Cold working performed next is not particularly limited,
For example, the annealing process is performed after performing cold working once or twice or more with intermediate annealing.

[0034]

EXAMPLES The operation and effects of the present invention will be described more specifically below based on examples. Steel ingots having the steel compositions shown in Table 1 were melted, hot-rolled under the conditions shown in Table 2 and used as test materials.

Castro-Tric to calculate the Ac ₁ transformation point
The following equation by ot was used. Ac ₁ = 35Cr-249.9C-280N + 73.15Si-114.8Ni-17.85Cu + 60.2
Mo + 170.1Nb + 290.15V + 61.95Ti + 749Al + 310
The relationship between the austenite transformation point temperatures using the above formula is also shown in Table 1, and the hot rolled sheet end temperature was determined based on this temperature.

These test materials were annealed at 830 ° C. for 12 hours and then subjected to ordinary cold rolling and annealing once to prepare test pieces for ridging, which were tested according to JIS 13B. A ridging property test and a Vickers hardness test were performed, and the martensite formation rate was visually measured.

The ridging resistance test was carried out by visually inspecting the ridging of the steel sheet surface after the 20% tensile test, and the results are shown in the following A to
The evaluation was performed by D. The ridging grade at this time is shown as wrinkle height as follows.

A: ≦ 10 μm, A ′: 11 to 15 μm, B: 1
6 to 25 μm, B ′: 26 to 30 μm, C: 30 to 50 μm, C ′:
51-60 μm, D: 60-80 μm, D ′: ≧ 80 μm, and grades of C or higher were adopted as appropriate values.

The results of the ridging resistance test are shown in Table 2. It can be seen that the final temperature of the hot-rolled sheet is 950 ° C or lower, the coiling temperature is not within the predetermined temperature range of 400 to 700 ° C, and the grade of C or higher cannot be obtained unless the coil is cooled with water.

In FIG. 1, it can be seen that under the condition that the conventional hot-rolled sheet is not water-cooled, the martensite production rate is not produced in the required amount, which is not suitable for preventing the coarsening of crystal grains. Figure 2
In the conventional cooling method of coil or coil, the middle part recovers and softens due to recrystallization, but the coil water-cooled material according to the present invention does not soften, and strain and martensite are retained. It turns out to be effective.

In FIG. 3, in the absence of conventional coil water cooling, recrystallization starts and a texture occurs, which is unsuitable for improving ridging resistance. Therefore, the maximum effect can be obtained by satisfying all of the addition amounts of Al and Nb, the hot rolled plate final temperature, the hot rolled plate cooling condition, the coil winding temperature, and the coil water cooling condition defined by the present invention.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

By carrying out the manufacturing according to the present invention,
It has become possible to produce ferritic stainless steels such as 430 Al steel with excellent ridging resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the final temperature of hot-rolled sheet and the martensite formation rate in sample No. 2 of the example.

FIG. 2 is a graph showing the relationship between hardness and cooling conditions in sample Nos. 5 and 6 of the examples.

FIG. 3 is a graph showing the relationship between coil winding temperature and Vickers hardness in sample No. 2 of the example.

Claims (2)

[Claims] 1. By weight%, C: 0.08% or less, Si: 2.0% or less, Mn: 1.00%
Below, Cr: 13.0 to 18.0%, Ni: 0.6% or less, Al: 0.03 to
0.15%, N: 0.003 to 0.05%, S: 0.010% or less, hot rolled a slab or steel ingot having a steel composition consisting of balance Fe and unavoidable impurities, the hot rolled sheet final temperature was Ac ₁ point + 20 to 50 ℃, after finishing rolling, cool down to 1
Part martensite is generated, the coil is continuously wound at 400 to 700 ° C., the coil is cooled with water immediately after winding, and then the obtained martensite is annealed at such a temperature and time that the martensite completely disappears. By further sizing the hot-rolled sheet, the crystal orientation is randomized.
A method for producing a ferritic stainless steel sheet having excellent ridging resistance, which comprises performing cold rolling once or twice or more with intermediate annealing. 2. The method for producing a ferritic stainless steel according to claim 1, wherein the steel composition further contains Nb: 0.03 to 0.5%.