JP6894515B2 - Ferritic stainless cold-rolled steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to ferrites stainless cold-rolled steel sheet and a manufacturing method thereof, and more particularly, precipitates and strength and acid resistance superior ferritic stainless capable of improving the strength and acid resistance by controlling a grain Regarding cold-rolled steel sheets and their manufacturing methods.

Of the stainless steels, ferritic stainless steels are widely used in building materials, food containers, home appliances, exhaust system parts of automobiles, and the like.

Ferritic stainless steel has recently been partially applied for automobile battery cells, and automobile manufacturers require higher strength and corrosion resistance than existing ferritic stainless steel in order to guarantee long-term battery performance. They are also demanding lower priced materials to lower the price of batteries.

There are methods such as work hardening, solid solution strengthening, and precipitation hardening as methods for increasing the strength of ferritic stainless steel in order to satisfy the demands of automobile manufacturers, but due to the characteristics of ferritic stainless steel without phase transformation. There is a problem that workability drops sharply during work hardening, and it is difficult to utilize Mo, Nb, etc., which have excellent solid solution strengthening effects, because they are expensive elements.

Conventionally, C is an element that impairs the workability of ferritic stainless steel, and is controlled to be almost 0.02 wt% or less unless it is used for special purposes. However, on the contrary, when a large amount of C is added, the strength of the ferritic stainless steel can be improved by the precipitation of carbides, and recently, if a certain degree of softness can be secured due to the development of processing technology, the strength and workability can be improved. It can be secured at the same time.

However, even if a large amount of C is added, if it is hot-rolled at a high temperature or the reduction rate is low and the winding temperature is high, the carbides cannot be finely deposited in the deformed structure and become coarse, so that it is difficult to refine the crystal grains. In addition, there is a problem that it is difficult to secure the desired strength.

Japanese Unexamined Patent Publication No. 2006-183081

When the present invention is intended controls the alloy components of ferritic stainless steel, excellent ferritic stainless cold that can improve the strength and acid resistance through the control of precipitates and the crystal grains of ferritic stainless steel To provide ferritic stainless steel.
In addition, a method for producing a ferritic stainless cold-rolled steel sheet that can improve strength and acid resistance through control of precipitates and crystal grains by controlling the slab reheating temperature, reduction rate, and winding temperature during hot rolling. To provide.

Ferrites stainless cold-rolled steel sheet of the present invention, in mass%, carbon (C): 0.1 to 0.2%, nitrogen (N): 0.005 to 0.05%, manganese (Mn): 0 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to 1.5%, The residue is iron (Fe) and other unavoidable impurities, and the number of carbides having a diameter of 100 nm or more per unit area is 50 to 200 ea / 100 μm ² .

The average crystal grain size of the ferrite-based stainless cold-rolled steel sheet is 10 μm or less, the tensile strength is 520 MPa or more, and the elongation rate is 20% or more.

According to the examples of the ferritic stainless cold-rolled steel sheet , the critical current density (Icrit) is 10 mA or less in a 5% sulfuric acid atmosphere.

In the method of manufacturing ferrites stainless cold-rolled steel sheet of the present invention, in mass%, carbon (C): 0.1 to 0.2%, nitrogen (N): 0.005 to 0.05%, manganese (Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to The formula (1) for hot rolling includes a step of hot rolling and a step of cold rolling of a ferritic stainless steel slab containing 1.5%, the rest of which is composed of iron (Fe) and other unavoidable impurities. It is characterized in that the value satisfies 1,000 or less.
15 * RHT / R4 + CT -------- Equation (1)
Here, RHT (° C.) means the slab reheating temperature, R4 (%) means the reduction rate of the R4 stand for rough rolling, and CT (° C.) means the take-up temperature.

The value of the formula (1) of the ferritic stainless cold-rolled steel sheet is characterized by satisfying 800 to 1,000.

The RHT of the ferritic stainless cold-rolled steel sheet is less than 1,250 ° C, the R4 is 40% or more, and the CT is less than 650 ° C.

The ferritic stainless cold-rolled steel sheet is characterized in that the number of carbides having a diameter of 100 nm or more per unit area is 50 to 200 ea / 100 μm ² , and the average crystal grain size is 10 μm or less.

According to the present invention, the alloy composition and hot rolling conditions of ferritic stainless steel can be controlled to improve the strength and acid resistance of the ferritic stainless cold-rolled steel sheet through the control of precipitates and crystal grains.

It is a graph for demonstrating the correlation between the hot rolling condition of a ferritic stainless steel, and the number of carbides of a cold-rolled steel sheet. It is a photograph which took the precipitation distribution state of the ferritic stainless cold-rolled steel sheet which concerns on one Example of this invention through a transmission electron microscope (TEM). It is a photograph of the precipitate distribution state of the ferritic stainless cold-rolled steel sheet according to the comparative example of the present invention taken through a transmission electron microscope (TEM). It is a graph for demonstrating the correlation between the number of carbides and tensile strength of a cold-rolled steel sheet of ferritic stainless steel.

The ferritic stainless steel having excellent strength and acid resistance of the present invention is, in mass%, carbon (C): 0.1 to 0.2%, nitrogen (N): 0.005 to 0.05%, manganese ( Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to 1. The number of carbides having a diameter of 100 nm or more per unit area is 50 to 200 ea / 100 μm ² , containing 5.5%, the rest consisting of iron (Fe) and other unavoidable impurities.

Hereinafter, the present invention will be described in detail with reference to the drawings. The following examples are presented in order to fully convey the idea of the present invention to a person who has ordinary knowledge in the technical field to which the present invention belongs. The present invention is not limited to the examples presented here, and may be embodied in other forms. The drawings may omit the illustration of parts unrelated to the description in order to clarify the present invention, and the size of the components may be exaggerated to help understanding.

The ferritic stainless steel having excellent strength and acid resistance according to an embodiment of the present invention has carbon (C): 0.1 to 0.2% and nitrogen (N): 0.005 to 0 in mass%. 05%, manganese (Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.50%, copper (Cu): It contains 0.3-1.5%, the rest consisting of iron (Fe) and other unavoidable impurities.

Carbon (C): 0.1 to 0.2%
The amount of carbon (C) is 0.1 to 0.2%. When the amount of carbon (C) is less than 0.1%, the amount of austenite generated during hot spreading decreases and the ferrite band structure remains without being destroyed, resulting in a crystal grain size. There is a problem that the strength becomes large, and there is a problem that the tensile strength of the final cold-rolled product decreases to less than 500 MPa. Further, when the amount of carbon (C) exceeds 0.2%, there is a problem that the carbides of the material are excessively increased and the elongation rate of the final product is lowered, and the surface quality and the corrosion resistance are lowered due to the removal of the carbides.

Nitrogen (N): 0.005-0.05%
The amount of nitrogen (N) is 0.005 to 0.05%. If the amount of nitrogen (N) is less than 0.005%, the manufacturing cost will increase due to the increase in refining time and the life of the refractory material, and the isobaric crystal of the slab due to the low degree of supercooling during casting. the rate is low, the amount of nitrogen (N) exceeds 0.05%, likely pinholes by nitrogen during slab casting occurs, per unit area of the Cr 2 _N precipitates in the final cold rolled product numerous pits (pit) surface quality by forming a is lowered to the surface of the final cold rolled product by the number is increased by Cr ₂ N precipitate around the formed chromium depleted regions (Cr depleted zone) of ..

Manganese (Mn): 0.01-0.5%
The amount of manganese (Mn) is 0.01-0.5%. If the amount of manganese (Mn) is less than 0.01%, there is a problem that the refining price becomes high, and if the amount of manganese (Mn) exceeds 0.5%, there is a problem that the elongation rate and the corrosion resistance are lowered. ..
Chromium (Cr): 12.0 to 19.0%
The amount of chromium (Cr) is 12.0 to 19.0%. If the amount of chromium (Cr) is less than 12.0%, there is a problem that the corrosion resistance is deteriorated, and if the amount of chromium (Cr) exceeds 19.0%, the elongation rate is lowered and the hot-rolled ticking (sticking) is performed. ) There is a problem that defects occur.
Nickel (Ni): 0.01-0.50%
The amount of nickel (Ni) is 0.01 to 0.50%. If the amount of nickel (Ni) is less than 0.01%, there is a problem that the refining price becomes high, and if the amount of nickel (Ni) exceeds 0.5%, impurities in the material increase and the elongation rate increases. There is a problem of decline.

Copper (Cu): 0.3-1.5%
The amount of copper (Cu) is 0.3-1.5%. If the amount of copper (Cu) is less than 0.3%, the critical current density ( _Icrit ) exceeds 10 mA in a 5% sulfuric acid atmosphere, and sufficient acid resistance cannot be ensured, so that copper (Cu) If the amount of copper exceeds 1.5%, not only the raw material price becomes excessively high, but also the hot workability is lowered and the growth rate of the final product is lowered.

In order to obtain the desired tensile strength in the final cold-rolled product of ferritic stainless steel, it is necessary to secure a large number of fine carbides, and the refinement of crystal grains is required.
The ferritic stainless steel having excellent strength and acid resistance of the present invention has a carbide having a diameter of 100 nm or more and having a number of carbides of 50 ea / 100 μm ² or more per unit area.
For example, the carbide is an M23C6 type carbide-based metal precipitate.
In order to increase the number of carbides per unit area, a sufficient deformed structure must be formed on the hot-rolled material during the hot-rolling process. When the deformed structure is not sufficiently formed, it is difficult to increase the amount of carbide because the carbide precipitation site (site) is not sufficient.

In order to form a sufficiently deformed structure in the hot-rolled material, it is necessary to control the slab reheating temperature, the rough rolling reduction rate, and the hot-rolled coil winding temperature during the hot-rolling process, which will be described in detail later.
That is, the number of carbides having a diameter of 100 nm or more per unit area ^{can be achieved to be 50 ea / 100 μm 2} or more through the control of the hot spreading process conditions, and therefore, a tension of 520 MPa or more can be achieved by securing a large number of fine carbides. Strength can be ensured. If the process conditions are not met, the carbides become coarse and a sufficient amount of carbides cannot be obtained.
For example, when the number of carbides having a diameter of 100 nm or more per unit area is ^{less than 50 ea / 100 μm 2} , the amount of carbides is small, so that the crystal grains become coarse and the tensile strength decreases.

For example, the ferritic stainless steel has an average crystal grain size of 10 μm or less.
The tensile strength is 520 MPa or more,
The growth rate is 20% or more,
The critical current density (I _crit ) is 10 mA or less in a 5% sulfuric acid atmosphere.

According to the method for producing a ferritic stainless steel of the present invention, in terms of mass%, carbon (C): 0.1 to 0.2%, nitrogen (N): 0.005 to 0.05%, manganese (Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to 1.5% Includes a step of hot rolling and a step of cold rolling of a ferritic stainless steel slab consisting of iron (Fe) and other unavoidable impurities, and the value of the formula (1) during hot rolling is 1,000. Satisfy the following:
15 * RHT / R4 + CT -------- Equation (1)
Here, RHT (° C.) means the slab reheating temperature, R4 (%) means the reduction rate of the R4 stand for rough rolling, and CT (° C.) means the take-up temperature.

Ferritic stainless steel slabs are manufactured by continuously casting molten steel containing the above components. Then, the slab is hot-rolled to produce a hot-rolled coil having a thickness of 2 to 10 mm.
For example, the slab reheating temperature (RHT) is less than 1,250 ° C., the reduction rate (R4) of the rough-rolled R4 stand is 40% or more, and the take-up temperature (CT) is less than 650 ° C. At this time, the hot rolling conditions are carried out so that the value of the formula (1) satisfies 1,000 or less.

FIG. 1 is a graph for explaining the correlation between the hot rolling conditions of ferritic stainless steel and the number of carbides in cold-rolled steel sheets.
As shown in FIG. 1, when the value according to the formula (1) is 1,000 or less, it can be seen that ^{the number of carbides having a diameter of 100 nm or more per unit area is 50 ea / 100 μm 2 or more.}
Even if the carbon content is sufficient, if the hot rolling conditions of the formula (1) are not formed, a sufficient deformed structure is not formed in the hot-rolled material, so that carbide precipitation sites are not sufficiently formed.
In particular, when the winding temperature is as high as 650 ° C. or higher, coarsening of the precipitate occurs and the desired number of carbides cannot be obtained, and as a result, the crystal grains become coarse and the desired tensile strength in the final product is obtained. Can no longer be obtained.
For example, the value of the formula (1) can satisfy 800 to 1,000.
When the value of the formula (1) is less than 800, the temperature during hot rolling is excessively low, which causes a defect in the plate shape.

The hot-rolled plate material undergoes an annealing step, and in the annealing step, carbides are sufficiently precipitated through an annealing heat treatment at a temperature of 700 to 900 ° C. For example, the annealing heat treatment is carried out in a BAF annealing step. After the annealing heat treatment, a cold-rolled plate material having a thickness of less than 2 mm can be produced through cold rolling, and the final heat treatment can be carried out through heat treatment at a temperature of 800 to 900 ° C.
For example, in the cold-rolled plate material, the number of carbides having a diameter of 100 nm or more per unit area is 50 ea / 100 μm ² or more, and the average crystal grain size is 10 μm or less.

Hereinafter, the present invention will be described in more detail through examples.
Examples: Slabs of invention steels 1 to 4 and comparative steels 1 to 9 satisfying the components of Table 1 are produced through continuous casting, reheated under the hot rolling conditions of Table 2, and then hot-rolled to 5 mmt through hot rolling. Manufactured the coil. Then, an annealing heat treatment at 900 ° C. was carried out in the BAF annealing step. Then, a 1 mmt cold-rolled plate was produced through cold rolling, heat-treated at 900 ° C., and pickled with a surface shot ball treatment and a pickling solution containing sulfuric acid and hydrogen peroxide to produce a final product.

Along with this, the number of carbides having a diameter of 100 nm or more in the final produced cold-rolled steel sheet per unit area, average crystal grain size, tensile strength, elongation, and critical current density were measured in a 5% sulfuric acid atmosphere. Shown in 3.
A TEM Replica was prepared for the finally produced cold-rolled plate material, and the number of carbide precipitates ^{per unit area (100 μm 2) was measured.}

FIG. 2 is a photograph of the precipitate distribution state of the ferritic stainless cold-rolled steel sheet of the present invention taken through a transmission electron microscope (TEM). FIG. 3 is a photograph of the precipitation distribution state of the ferritic stainless cold-rolled steel sheet according to the comparative example of the present invention taken through a transmission electron microscope (TEM).
FIG. 2 is a photograph of the cold-rolled steel sheet according to the second embodiment, and FIG. 3 is a photograph of the cold-rolled steel sheet according to the second comparative example.
As shown in FIGS. 2 and 3, as in Comparative Examples 1 to 4, the value of 15 * RHT / R4 + CT according to the relational expressions relating to the slab reheating temperature during hot rolling, the R4 reduction rate, and the winding temperature is 1, Even if the carbon content exceeds 000 and is sufficient, the carbide precipitation site is not sufficient because a sufficient deformed structure is not formed on the hot-rolled material.
Not only that, as in Comparative Example 2, it can be seen that when the winding temperature is high, the precipitates become coarse and the desired number of carbides cannot be obtained.

When the copper content is too high as in Comparative Example 5, it is found that the elongation rate of the final product is 18.8% and the elongation rate decreases, and when the copper content is low as in Comparative Example 6. _{The critical current density (I crit} ) is 14.5 mA in a 5% sulfuric acid atmosphere, and sufficient acid resistance cannot be ensured.
It can be confirmed that when the carbon content is too large as in Comparative Examples 7 and 8, the number of carbides increases and the elongation rate decreases. It can be confirmed that when the carbon content is small as in Comparative Examples 9 to 13, the size of the crystal grains becomes large and the tensile strength decreases to less than 500 MPa.

FIG. 4 is a graph for explaining the correlation between the number of carbides and the tensile strength of a cold-rolled steel sheet of ferritic stainless steel.
As shown in FIG. 4, the number of carbides and the tensile strength of the cold-rolled steel sheet according to the above Examples and Comparative Examples are shown in a graph, and it can be seen that the tensile strength tends to increase as the number of carbides increases. ..
Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to this, and any person who has ordinary knowledge in the relevant technical field will describe the concept and scope of the claims described below. It should be understood that various changes and modifications are possible within the range that does not deviate from.
The ferritic stainless steel having excellent strength and acid resistance according to the embodiment of the present invention and its manufacturing method can be applied to building materials, food containers, home appliances, automobile exhaust system parts, automobile batteries and the like.

Claims (7)

By mass%, carbon (C): 0.1 to 0.2%, nitrogen (N): 0.005 to 0.05%, manganese (Mn): 0.01 to 0.5%, chromium (Cr) 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to 1.5%, the rest is iron (Fe) and other unavoidable Consists of impurities
A ferritic stainless cold-rolled steel sheet characterized in that the number of carbides having a diameter of 100 nm or more per unit area is 50 to 200 ea / 100 μm ² and the average crystal grain size is 10 μm or less. The ferrite-based stainless cold-rolled steel sheet according to claim 1, wherein the tensile strength is 520 MPa or more. The ferrite-based stainless cold-rolled steel sheet according to claim 1, wherein the elongation rate is 20% or more. The ferrite-based stainless cold-rolled steel sheet according to claim 1, wherein the critical current density (Icrit) is 10 mA or less in a 5% sulfuric acid atmosphere. By mass%, carbon (C): 0.1-0.2%, nitrogen (N): 0.005-0.05%, manganese (Mn): 0.01-0.5%, chromium (Cr) 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.3 to 1.5%, the rest is iron (Fe) and other unavoidable Including the steps of hot rolling and cold rolling of ferritic stainless steel slabs composed of impurities.
At the time of the hot rolling, the value of the formula (1) was satisfied with 1,000 or less.
The cold-rolled cold-rolled sheet is the number that 50~200ea / 100μm ² per unit area of the carbides having a diameter greater than 100 nm, ferritic stainless cold, wherein the average crystal grain size is 10μm or less Manufacturing method of ferritic stainless steel.
15 * RHT / R4 + CT -------- Equation (1)
Here, RHT (° C.) means the slab reheating temperature, R4 (%) means the reduction rate of the R4 stand for rough rolling, and CT (° C.) means the take-up temperature. The method for producing a ferritic stainless cold-rolled steel sheet according to claim 5, wherein the value of the formula (1) satisfies 800 to 1,000. The method for producing a ferritic stainless cold-rolled steel sheet according to claim 5, wherein RHT is less than 1,250 ° C, R4 is 40% or more, and CT is less than 650 ° C.

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