WO2024029679A1 - High corrosion resistance and high strength stainless steel and method for manufacturing same - Google Patents

Abstract

A high corrosion resistance and high strength stainless steel, according to one example of the present invention, comprises, in wt%, 0.01-0.1% of C, 0.01-0.1% of N, 0.01-1.0% of Si, 0.01-3.0% of Mn, 10.0-20.0% of Cr, 0.001-1.0% of Al, at most 0.05% of P, at most 0.01% of S, and the remainder in Fe and other unavoidable impurities, wherein the distribution of carbides with a diameter of at least 0.5 ㎛ is at most 7/100 ㎛² per unit area, and the microstructure is dual phase with a martensite phase and a ferrite phase, and the martensite phase is at least 20% in area ratio.

Description

High-corrosion-resistant, high-strength stainless steel and manufacturing method thereof

The present invention relates to a highly corrosion-resistant, high-strength stainless steel composed of dual phases of martensite and ferrite after cold rolling annealing heat treatment, and a method of manufacturing the same.

Ferritic stainless steel products, which are widely used in various kitchen utensils, home appliances, and automobile parts, are increasingly requiring high functionality such as high strength and weight reduction. In particular, there is a demand for extreme cost reduction as well as improved energy efficiency through lightweighting by reducing weight by reducing thickness through increased strength. However, in normal ferritic stainless steels, there is no phase transformation, so there is a limit to improving strength through grain refinement.

To solve this problem, the martensite phase is formed in the hot rolling annealing step to reduce roffing that may be generated in the cold rolling process, thereby securing a cold rolled steel sheet with an attractive surface. However, when the martensite phase is distributed in the hot rolling annealing step, the strength increases. Therefore, during cold rolling, problems such as cracks or tearing of the steel sheet may occur during rolling due to the rolling load. To solve this problem, the reduction rate per pass can be reduced during cold rolling, but this increases the number of passes and causes economic loss. Stainless steel that can secure strength without reducing corrosion resistance is required.

The purpose of the present invention to solve the above-described problems is to provide a high-corrosion-resistant, high-strength stainless steel and a method of manufacturing the same by controlling the distribution of carbides and forming a martensite phase in the cold rolling annealing step.

However, the problems to be solved by the embodiments of the disclosed invention are not limited to the above-described problems and can be expanded in various ways within the scope of the technical idea included in the present invention.

High-corrosion-resistant, high-strength stainless steel according to one embodiment is, in weight percent, C: 0.01 to 0.1%, N: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.01 to 3.0%, Cr: 10.0 to 20.0%. , Al: 0.001 ~ 1.0%, P: 0.05% or less, S: 0.01% or less, including the remaining Fe and other inevitable impurities, and the distribution of carbides with a diameter of 0.5 ㎛ or more is 7/100 ㎛ ² or less per unit area, It may be a high-corrosion-resistant, high-strength stainless steel with a microstructure of two phases, a martensite phase and a ferrite phase, and where the martensite phase has an area ratio of 20% or more.

The high-corrosion-resistant, high-strength stainless steel according to one embodiment may be a high-corrosion-resistant, high-strength stainless steel that satisfies Equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10. In formula (1), C, N, Ni, Mn, Cr, Si, and Al represent the weight percent of each element.

The high-corrosion-resistant, high-strength stainless steel according to one embodiment may be a high-corrosion-resistant, high-strength stainless steel that has a pitting potential of 70 mV or more and a yield strength of 350 MPa or more.

The highly corrosion-resistant, high-strength stainless steel according to one embodiment may be a highly corrosion-resistant, high-strength stainless steel with a tensile strength of 500 MPa or more.

The highly corrosion-resistant, high-strength stainless steel according to one embodiment may be a highly corrosion-resistant, high-strength stainless steel with a hardness of 200Hv or more.

The high-corrosion-resistant, high-strength stainless steel according to one embodiment may be a high-corrosion-resistant, high-strength stainless steel in which the martensite phase itself has a hardness of 400 Hv or more.

The highly corrosion-resistant, high-strength stainless steel according to one embodiment may be a highly corrosion-resistant, high-strength stainless steel in which the aspect ratio of ferrite grains is 2.0 or less.

A method of manufacturing high corrosion resistance and high strength stainless steel according to an embodiment includes, in weight %, C: 0.01 ~ 0.1%, N: 0.01 ~ 0.1%, Si: 0.01 ~ 1.0%, Mn: 0.01 ~ 3.0%, Cr: 10.0 ~ Reheating the slab containing 20.0%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.01% or less, the remaining Fe and other inevitable impurities to 1050 to 1250°C; Hot rolling and hot rolling annealing; It includes the steps of cold rolling and cold rolling annealing at 950 to 1100°C, satisfies the following equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10, and has a microstructure This may be a method of manufacturing high-corrosion-resistant, high-strength stainless steel, which is a two-phase martensite phase and a ferrite phase. In formula (1), C, N, Ni, Mn, Cr, Si, and Al represent the weight percent of each element.

A method of manufacturing a highly corrosion-resistant, high-strength stainless steel according to an embodiment may be a method of manufacturing a highly corrosion-resistant, high-strength stainless steel in which the distribution of carbides with a diameter of 0.5 ㎛ or more is 7/100 ㎛ ² or less per unit area.

The method of manufacturing highly corrosion-resistant, high-strength stainless steel according to an embodiment may be a method of manufacturing highly corrosion-resistant, high-strength stainless steel in which the hot rolling annealing step is performed at a temperature range of 750 to 900°C.

A method of manufacturing high corrosion resistance and high strength stainless steel according to an embodiment may be a method of manufacturing high corrosion resistance and high strength stainless steel in which the martensite phase may be 20% or more in area ratio after cold rolling annealing, and the hardness of the martensite phase may be 400 Hv or more.

A method of manufacturing highly corrosion-resistant, high-strength stainless steel according to an embodiment may be a method of manufacturing highly corrosion-resistant, high-strength stainless steel in which the aspect ratio of ferrite grains is 2.0 or less.

According to an embodiment of the present invention, it is possible to provide a stainless steel that simultaneously satisfies high corrosion resistance and high strength having dual phases of martensite and ferrite after cold rolling annealing heat treatment, and a method of manufacturing the same.

Figure 1 is a photograph showing the microstructure of a comparative example according to cold rolling annealing temperature.

Figure 2 is a photograph showing the microstructure of the invention example according to the cold rolling annealing temperature.

Figure 3 is a photograph showing the microstructure and carbide of a comparative example according to cold rolling annealing temperature.

Figure 4 is a photograph showing the microstructure and carbide of the invention example according to the cold rolling annealing temperature.

High-corrosion-resistant, high-strength stainless steel according to one embodiment is, in weight percent, C: 0.01 to 0.1%, N: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.01 to 3.0%, Cr: 10.0 to 20.0%. , Al: 0.001 ~ 1.0%, P: 0.05% or less, S: 0.01% or less, including the remaining Fe and other inevitable impurities, and the distribution of carbides with a diameter of 0.5 ㎛ or more is 7/100 ㎛ ² or less per unit area, It may be a high-corrosion-resistant, high-strength stainless steel with a microstructure of two phases, a martensite phase and a ferrite phase, and where the martensite phase has an area ratio of 20% or more.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression, unless the context clearly requires it to be singular. In addition, terms such as “comprise” or “comprise” used in the present application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to indicate other features. It should be noted that it is not used to preliminarily rule out the existence of any elements, steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be viewed as having the same meaning as generally understood by those skilled in the art to which the present invention pertains. Therefore, unless clearly defined in this specification, specific terms should not be interpreted in an overly idealistic or formal sense.

In addition, in this specification, "about", "substantially", etc. are used in the meaning of or close to that value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used accurately to aid understanding of the present invention. It is used to prevent unscrupulous infringers from unfairly exploiting disclosures where absolute figures are mentioned.

The present invention increases the stability of the austenite phase in the ferrite matrix, and performs cold rolling annealing heat treatment at a temperature at which the austenite phase is generated, so that the final product consists of a dual phase of ferrite and martensite through martensite phase transformation during cooling. We attempted to improve strength depending on the martensite fraction formed in the ferrite matrix. In addition, due to concerns about load, cracks, or rupture during the cold rolling stage, hot rolling annealing was performed directly below Ac1, which is the ferrite single phase region, to form a soft ferrite single phase, and after completion of cold rolling, heat treatment was performed at a temperature at which an austenite phase is generated in the cold rolling annealing stage. The aim was to improve strength by inducing martensite phase transformation during cooling.

In the cold rolling annealing step, heat treatment is performed at a temperature at which the austenite phase is formed, and the M ₂₃ C ₆ type carbides precipitated on the ferrite phase are dissolved into the matrix. According to the present invention, it is possible to provide a stainless steel and a manufacturing method thereof that can secure high strength without deteriorating corrosion resistance by controlling the distribution of these carbides.

In addition, by controlling the aspect ratio of ferrite grains, it is possible to provide stainless steel and a manufacturing method thereof that do not deteriorate the molding quality of the final product.

The high-corrosion-resistant, high-strength stainless steel according to an example of the present invention has, in weight percent, C: 0.01 ~ 0.1%, N: 0.01 ~ 0.1%, Si: 0.01 ~ 1.0%, Mn: 0.01 ~ 3.0%, Cr: 10.0 ~ 20.0%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.01% or less, and may include the remaining Fe and other unavoidable impurities. Below, the reason for limiting the alloy composition will be explained in detail.

The content of C may be 0.01% to 0.1%.

C is an austenite stabilizing element and has the effect of expanding the austenite phase area, forming light martensite upon cooling, thereby improving the strength of steel. To obtain these effects, a content of 0.01% or more is required. In order to sufficiently expand the austenite region to obtain the desired strength, preferably 0.02% or more is required. However, if the C content exceeds 0.1%, the steel sheet hardens and ductility significantly decreases, and if the amount of martensite produced is too large, formability cannot be obtained. In addition, when a large amount of Cr carbide is generated due to excessive C addition, corrosion resistance is decreased due to decrease in Cr. Therefore, the C content is in the range of 0.01 to 0.1%. Preferably it may be in the range of 0.02 to 0.1%.

The content of N may be 0.01 to 0.1%.

N, like C and Mn, is an austenite stabilizing element and has the effect of expanding the austenite region. To obtain this effect, the N content must be 0.01% or more. However, when the N content exceeds 0.1%, ductility rapidly decreases due to the solid solution strengthening effect of N, and precipitation of Cr nitride causes a decrease in Cr, resulting in a decrease in corrosion resistance. Therefore, the N content is in the range of 0.01 to 0.1%. Preferably it may be in the range of 0.02 to 0.1%, and more preferably in the range of 0.01 to 0.07%.

The Si content may be 0.01 to 1.0%.

Si is an element that acts as a deoxidizing agent when melting steel. To obtain this effect, a content of 0.01% or more is required. However, if the Si content exceeds 1.0%, the steel sheet becomes hard, the rolling load during hot rolling increases, and surface defects such as sticking are caused. In addition, Si is a ferrite stabilizing element, and excessive addition of Si reduces the stability of austenite. Therefore, the Si content is in the range of 0.01 to 1.0%. Preferably, it may be in the range of 0.20 to 0.50%.

The content of Mn may be 0.01 to 3.0%.

Mn, like C, is an austenite phase stabilizing element and has the effect of expanding the austenite phase area. To obtain this effect, a content of 0.01% or more is required. However, when the Mn content exceeds 3.0%, the amount of MnS produced increases and corrosion resistance deteriorates. Therefore, the Mn content is in the range of 0.01 to 3.0%. Preferably it may be 0.2 to 1.0%.

The content of Cr may be 10.0 to 20.0%.

Cr is an element that has the effect of improving corrosion resistance by forming a passive film on the surface of the steel sheet. This effect appears when the Cr content is 10.0% or more, and corrosion resistance improves as the Cr content increases. Additionally, Cr is a ferrite stabilizing element and has the effect of suppressing the formation of an austenite phase. If the Cr content is less than 10.0%, too much austenite phase is generated, and the desired formability cannot be obtained. Therefore, the Cr content is set to 10.0% or more. However, if the Cr content exceeds 20.0%, the austenite phase is not generated and the required martensite phase fraction cannot be secured. Therefore, the Cr content is in the range of 10.0 to 20.0%. Preferably it may be in the range of 12.0 to 18.0%.

The Al content may be 0.001 to 1.0%.

Al is an element that acts as a deoxidizing agent like Si. To obtain this effect, a content of 0.001% or more is required. However, when the Al content exceeds 1.0%, Al inclusions such as Al ₂ O ₃ increase, and surface properties tend to deteriorate. Therefore, the Al content is in the range of 0.001 to 1.0%. Preferably, it may be in the range of 0.001 to 0.1%.

The content of Ni may be 0.01% to 1.0%.

Ni is a representative austenite stabilizing element, but is an expensive element, increasing manufacturing costs. In addition, while it has the effect of improving corrosion resistance, when added in large amounts, impurities in the material increase and the elongation rate decreases. Therefore, in the present invention, Ni may optionally be further included in an amount of 0.01 to 1.0%. Preferably it may be 0.5% or less.

The content of P may be 0.05% or less.

Since P is an element that promotes grain boundary destruction due to grain boundary segregation, it is an unavoidable impurity that is preferably as low as possible. Therefore, the P content is set to 0.05 or less. More preferably, it may be 0.03% or less.

The S content may be 0.01% or less.

S is an element that exists as sulfide-based inclusions such as MnS and reduces ductility and corrosion resistance, and its adverse effects are particularly noticeable when the content exceeds 0.01%. Therefore, the S content is an unavoidable impurity for which it is desirable to keep the content as low as possible. Therefore, the S content is set to 0.01% or less. More preferably, it may be 0.005% or less.

The remaining ingredient is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, all of them are not specifically mentioned in this specification.

Hereinafter, a high-corrosion-resistant, high-strength stainless steel according to an embodiment of the present invention having the above-described alloy composition will be described.

In the high-corrosion-resistant, high-strength stainless steel according to an example of the present invention, the distribution of carbides with a diameter of 0.5 ㎛ or more may be 7/100 ㎛ ² or less per unit area. If the distribution of carbides exceeds 7/ ^100㎛2 per unit area, corrosion resistance and high strength cannot be secured at the same time.

The high-corrosion-resistant, high-strength stainless steel according to an example of the present invention has a microstructure of two phases, a martensite phase and a ferrite phase, where the martensite phase may have an area ratio of 20% or more. If the martensite phase is less than 20% in terms of area, high strength cannot be secured.

High-corrosion-resistant, high-strength stainless steel according to an example of the present invention prevents a decrease in corrosion resistance due to carbide precipitation when carbides are dissolved in the matrix and the distribution of carbides with a diameter of 0.5 ㎛ or more is less than 7 per unit area / 100 ㎛ ² . It can be done, and at the same time, high strength can be secured by securing the martensite phase area ratio of 20% or more, so it can be a stainless steel that satisfies high corrosion resistance and high strength at the same time.

In the high-corrosion-resistant, high-strength stainless steel according to an example of the present invention, the value of equation (1) may be 10 or more.

Equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al)

In formula (1), C, N, Ni, Mn, Cr, Si, and Al represent the weight percent of each element.

If the value of equation (1) is less than 10, the austenite phase stability is low and the austenite phase does not actively occur, so the phase transformation to martensite may not occur. Therefore, it is controlled to be 10 or more, and preferably to be 30 or more. Through this, after forming the austenite phase at high temperature, the martensite phase transformation can be smoothed during cooling, and the martensite phase can be secured at an area ratio of 20% or more.

The high-corrosion-resistant, high-strength stainless steel according to an example of the present invention has a pitting potential of 70 mV or more and a yield strength of 350 MPa or more, thereby satisfying both corrosion resistance and strength.

In addition, the high-corrosion-resistant, high-strength stainless steel according to an example of the present invention may have a tensile strength of 500 MPa or more and a steel hardness of 200 Hv or more. Additionally, the hardness of the martensite phase may be 400 Hv or more.

In addition, the high-corrosion-resistant, high-strength stainless steel according to an example of the present invention may have an aspect ratio of ferrite grains of 2.0 or less. The aspect ratio of ferrite grains refers to the ratio of the length of the ferrite grains in the rolling direction divided by the length of the ferrite grains in the thickness direction. In the present invention, this is expressed as equation (2).

Equation (2): Ar = Dr / Dt

Here, Ar is the aspect ratio of the ferrite grain, Dr is the rolling direction length of the ferrite grain, and Dt is the thickness direction length of the ferrite grain.

Ferrite grains are about 30 to 50㎛ in size, and if unrecrystallized ferrite grains elongated in the rolling direction are distributed, there is a high possibility that the molding quality will be inferior, such as in ridging. Therefore, it is desirable that the ferrite phase in which phase transformation does not occur is distributed as equiaxed recrystallized grains as much as possible to prevent the molding quality of the final product from deteriorating. Accordingly, the present invention can improve molding quality by controlling the aspect ratio of ferrite grains to 2.0 or less.

Hereinafter, a method for manufacturing a high-corrosion-resistant, high-strength stainless steel according to an embodiment of the present invention having the above-described alloy composition will be described.

The high-corrosion-resistant, high-strength stainless steel of the present invention can be manufactured through the following processes: reheating, hot rolling, finish rolling, hot rolling annealing, cold rolling, and cold rolling annealing of a slab having the above-described alloy composition.

The method for manufacturing high corrosion resistance and high strength stainless steel of the present invention is, in weight percent, C: 0.01 to 0.1%, N: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.01 to 3.0%, Cr: 10.0 to 20.0%. , Al: 0.001 ~ 1.0%, P: 0.05% or less, S: 0.01% or less, reheating the slab containing the remaining Fe and other unavoidable impurities; Hot rolling and hot rolling annealing; It may include cold rolling and cold rolling annealing.

A slab having the above-described alloy composition may have a value of equation (1) of 10 or more.

Equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al)

In formula (1), C, N, Ni, Mn, Cr, Si, and Al represent the weight percent of each element.

If the value of equation (1) is less than 10, the austenite phase stability is low and the austenite phase does not actively occur, so the phase transformation to martensite may not occur. Therefore, it can be controlled to 10 or more, and preferably to 30 or more to form an austenite phase at high temperature and then to smoothly transform the martensite phase during cooling.

A slab having the above-described alloy composition can be reheated to 1050 to 1250°C and hot rolled. The finish rolling temperature can be 700 to 950°C.

In addition, hot rolling annealing can be performed at 750°C to 900°C, which is directly below the Ac1 temperature, which is the ferrite single phase region, and more preferably, hot rolling annealing heat treatment is performed at 800°C to 850°C to form a soft ferrite single phase. .

A hot-rolled sheet composed of a single phase of soft ferrite can be cold-rolled at room temperature. After completing cold rolling, cold rolling can be annealed. Cold rolling annealing can be performed at 950 to 1100°C. Strength can be improved by heat treating above 950°C, which is the temperature above 900°C where the austenite phase is formed, to induce martensite phase transformation during cooling. However, when annealed at a temperature exceeding 1100℃, formability may be reduced due to coarsening of ferrite grains, or orange peel phenomenon due to coarsening of grains may appear on the surface of the bent part in severely bent parts. It is preferable to heat treat at the following temperature.

In addition, the method for producing high-corrosion-resistant, high-strength stainless steel of the present invention can cause the M ₂₃ C ₆ type carbides precipitated on the ferrite phase to be dissolved in the matrix by heat treating the cold rolling annealing temperature at a temperature at which the austenite phase is generated. there is.

The method for producing high-corrosion-resistant, high-strength stainless steel of the present invention involves cold rolling annealing heat treatment at a temperature at which an austenite phase is formed, so that carbides are dissolved in the matrix, and the distribution of carbides with a diameter of 0.5 ㎛ or more is 7 per unit area / 100 ㎛ ² or less. You can.

In the method for manufacturing high corrosion resistance and high strength stainless steel of the present invention, the hardness of the martensite phase after cold rolling annealing may be 400 Hv or more. Carbon decomposed from carbides increases the stability of the austenite phase, forming an austenite phase, and when cooled, a martensite phase with high hardness can be formed with a BCT structure.

In the method for manufacturing high corrosion resistance and high strength stainless steel of the present invention, the martensite phase may be 20% or more in area ratio after cold rolling annealing. When cold rolling annealing heat treatment is performed at a temperature at which the austenite phase is generated, the area ratio of the martensite phase increases as the temperature increases, and the yield strength and tensile strength may increase accordingly.

When carbides are dissolved in the matrix and the distribution of carbides with a diameter of 0.5㎛ or more is less than 7 per unit area/ ^100㎛2 , a decrease in corrosion resistance due to carbide precipitation can be prevented, and at the same time, a martensite phase with high hardness is generated. , strength can be secured by increasing the area ratio of the martensite phase.

Stainless steel manufactured according to the method for manufacturing high corrosion resistance and high strength stainless steel according to an example of the present invention has a pitting potential of 70 mV or more and a yield strength of 350 MPa or more, which can satisfy both corrosion resistance and strength.

In addition, the stainless steel manufactured according to the method for manufacturing high corrosion resistance and high strength stainless steel according to an example of the present invention may have a tensile strength of 500 MPa or more and a hardness of the steel may be 200 Hv or more.

In addition, the stainless steel manufactured according to the method for manufacturing high-corrosion-resistant, high-strength stainless steel according to an example of the present invention satisfies the aspect ratio of ferrite grains expressed in equation (2) of 2.0 or less, so that the molding quality of the final product can also be excellent. there is. Equation (2) is Ar = Dr / Dt, where Ar is the aspect ratio of the ferrite grain, Dr is the length of the ferrite grain in the rolling direction, and Dt is the length of the ferrite grain in the thickness direction.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

{Example}

In weight percent, C: 0.035%, N: 0.038%, Si: 0.32%, Mn: 0.5%, Cr: 16.3%, Al: 0.003%, Ni: 0.09%, P: 0.02% or less, S: 0.004% or less A cold-rolled annealed material was manufactured through the process of reheating the slab containing Fe and other inevitable impurities in the temperature range of the present invention - hot rolling - finishing rolling - hot rolling annealing - cold rolling - cold rolling annealing. In particular, in order to determine the effect of the cold rolling annealing heat treatment temperature in the present invention, it was manufactured within the scope of the present invention, but the cold rolling annealing heat treatment temperature was varied as shown in Table 1 below.

Table 1 below shows the fraction of martensite phase in examples according to cold rolling annealing heat treatment temperature.

	Cold rolled annealing temperature (℃)	Martensite phase fraction (%)
Comparative Example 1	820	0
Comparative Example 2	890	0
Comparative Example 3	910	0
Comparative Example 4	930	0
Invention Example 1	950	26.5
Invention Example 2	1000	37.3
Invention Example 3	1030	40.1
Invention Example 4	1050	42.5

Figures 1 and 2 are photographs showing the microstructure of examples according to cold rolling annealing heat treatment temperature. Changes in microstructure were observed using an SEM electron microscope. It can be confirmed that Comparative Examples 1 to 4 were composed of a ferrite phase. In Invention Examples 1 to 4, in Figure 2, the white upper part corresponds to the ferrite phase, and the black upper part corresponds to the martensite phase. It can be seen from Table 1, Figures 1 and 2 that the fraction of martensite phase in the ferrite matrix increases as the cold rolling annealing heat treatment temperature increases.

Comparative Examples 1 to 4, in which the cold rolling annealing heat treatment temperature does not fall within the range of the present invention, consist only of a ferrite phase, and as the temperature rises, a martensite phase may exist, but only in such a trace amount that it cannot be measured. . On the other hand, it can be confirmed that Inventive Examples 1 to 4, which fall within the scope of the present invention, have a cold rolling annealing heat treatment temperature of 950°C or higher, making it possible to secure a fraction of martensite phase of 20% or more in the ferrite matrix.

Table 2 below shows the number of M ₂₃ C ₆ type carbides per unit area according to the cold rolling annealing heat treatment temperature.

	Cold rolled annealing temperature (℃)	Number of carbides (piece/ 100㎛2 )
Comparative Example 1	820	10.6
Comparative Example 3	910	8.2
Invention Example 1	950	6.5
Invention Example 2	1000	0.8
Invention Example 4	1050	0.3

Figures 3 and 4 are photographs showing the microstructure of the example according to the cold rolling annealing heat treatment temperature and the number of carbides of the M ₂₃ C ₆ type (piece/100㎛ ² ). Changes in microstructure and carbide were observed using an SEM electron microscope. In Figures 3 and 4, the martensite phase is expressed as M, and the ferrite phase is expressed as F. As in Comparative Example 1 and Comparative Example 3, it can be confirmed that a large amount of carbides are present when the hot rolling annealing temperature is less than 950°C.

Through Figure 4, it can be confirmed that the martensite phase is generated at a temperature of 950°C or higher, and it can be confirmed that the carbide is dissolved in solid solution and its distribution gradually decreases. It can be seen from Table 2, Figures 3 and 4 that as the cold rolling annealing heat treatment temperature increases, the fraction of martensite phase in the ferrite matrix increases and the number of carbides per unit area decreases.

In comparative examples where the cold rolling annealing heat treatment temperature does not fall within the range of the present invention, it can be confirmed that the number of carbides per unit area exceeds 7. On the other hand, it can be confirmed that in the invention examples falling within the scope of the present invention, the cold rolling annealing heat treatment temperature is 950°C or higher, and the number of carbides per unit area is reduced to 7 or less.

Table 3 below shows the formal dislocation, yield strength, tensile strength, elongation, and hardness of the examples.

	Official potential (mV/100㎂)	yield strength (MPa)	tensile strength (MPa)	hardness of steel (Hv)
Comparative Example 1	106.8	305.3	479.0	143.7
Comparative Example 2	61.7	327.9	478.5	145.5
Comparative Example 3	50.3	333.4	477.2	148.4
Comparative Example 4	58.2	338.5	484.0	148.1
Invention Example 1	78.1	367.5	609.7	204.7
Invention Example 2	123.0	391.7	700.4	210.7
Invention Example 3	141.4	410.7	723.6	219.8
Invention Example 4	93.3	451.8	736.0	255.1

Through Table 3, when the cold rolling annealing heat treatment temperature of the present invention is satisfied and the area fraction of the martensite phase is 20% or more and the number of carbides per unit area is 7 or less, the pitting potential satisfies 70 mV or more and the yield strength is 350 MPa. By satisfying the above requirements, it can be seen that it is possible to secure stainless steel that satisfies high strength while preventing a decrease in corrosion resistance. The pitting potential was measured at 0.333 mV/sec in a 3.5% NaCl solution at a temperature of 30°C.

In Comparative Examples 2 to 4, in which a very small amount of martensite begins to be generated as the cold rolling annealing temperature increases, the yield strength and tensile strength can be seen to increase compared to Comparative Example 1, and correspond to the cold rolling annealing temperature or higher within the range of the present invention. It can be seen that in Invention Examples 1 to 4, as the martensite fraction increases to 20% or more, excellent strength can be secured with a yield strength of 350 MPa or more and a tensile strength of 500 MPa or more.

In addition, in the case of Invention Examples 1 to 4 where the cold rolling annealing temperature is 950°C or higher, it corresponds to the temperature at which carbides are dissolved in solid solution, so the number of carbides per unit area in the ferrite matrix is 7 or less, and Cr due to precipitation of carbides It can be confirmed that the decline in the formal potential value due to consumption can be prevented. Accordingly, it can be confirmed that excellent corrosion resistance can be secured with a pitting potential of 70 mV or more.

In Comparative Examples 2 to 4, the pitting potential value is less than 70 mV, which means that the cold rolling annealing temperature corresponds to a temperature at which carbides are precipitated without being dissolved in solid solution, so the pitting potential is lowered by consuming Cr due to carbide precipitation.

However, in the case of Comparative Example 1, the pitting potential value corresponds to 70 mV or more, but it can be confirmed that the yield strength is low and the strength is not secured even though corrosion resistance is secured.

In addition, the hardness of the steel was expressed as the average value measured 10 times with a load of 1 kg using a micro Vickers hardness tester. In the case of Comparative Examples 1 to 4 in which the martensite phase does not exist, the hardness value was measured at around 140 Hv, confirming that the hardness was inferior. On the other hand, in the case of Invention Examples 1 to 4 in which the martensite phase is present at 20% or more, it can be confirmed that high hardness is obtained by measuring 200 Hv or more.

In the present invention, the transformation to the martensite phase is performed during the cold rolling annealing process after completing cold rolling, while the temperature at which carbide is dissolved in solid solution, and the cold rolling annealing heat treatment is performed in a temperature range corresponding to the temperature at which the austenite phase is generated, thereby forming the martensite phase during cooling. By inducing the transformation, the number of carbides per unit area is controlled to 7 or less and the martensite phase fraction is secured to 20% or more, thereby providing stainless steel that simultaneously satisfies corrosion resistance and strength.

Table 4 below shows the hardness values of the martensite phase and the ferrite phase itself.

	Hardness of martensite phase (Hv)	Hardness of ferrite phase (Hv)
Comparative Example 1	x	154.0
Comparative Example 3	x	155.0
Invention Example 1	428.7	157.6
Invention Example 2	535.7	164.4
Invention Example 4	546.7	159.9

The hardness of the martensite phase and the ferrite phase itself corresponds to the value measured with a micro hardness meter (load: 5g). Through this, it can be confirmed that the hardness of the ferrite phase itself corresponds to about 150 to 160 Hv in the invention example like the comparative example, and that the hardness of the martensite phase itself has a high hardness value of more than 400 Hv. That is, the hardness of the steel itself of the invention examples is determined by cold rolling annealing heat treatment in the austenite region, so that the M ₂₃ C ₆ type carbides precipitated on the ferrite phase are dissolved in the matrix, and at this time, the carbon decomposed from the carbides increases the stability of the austenite phase. An austenite phase is formed, and upon cooling, a martensite phase with a high hardness of the BCT structure is formed, thereby achieving high hardness.

Table 5 below shows the aspect ratio of ferrite grains.

	area	Average aspect ratio of ferrite grains in each region	Average aspect ratio of ferrite grains over the entire area
Comparative Example 1	1 (1t/5)	1.562	2.066
	2 (2t/5)	1.892
	3 (3t/5)	2.136
	4 (4t/5)	2.108
	5 (5t/5)	1.632
Invention Example 2	1 (1t/5)	1.485	1.490
	2 (2t/5)	1.480
	3 (3t/5)	1.493
	4 (4t/5)	1.508
	5 (5t/5)	1.483
Invention Example 3	1 (1t/5)	1.523	1.556
	2 (2t/5)	1.576
	3 (3t/5)	1.562
	4 (4t/5)	1.582
	5 (5t/5)	1.537
Invention Example 4	1 (1t/5)	1.564	1.559
	2 (2t/5)	1.543
	3 (3t/5)	1.536
	4 (4t/5)	1.532
	5 (5t/5)	1.619

The aspect ratio of ferrite grains according to an example of the present invention may be 2.0 or less. The aspect ratio of ferrite grains refers to the ratio of the length of the ferrite grains in the rolling direction divided by the length of the ferrite grains in the thickness direction. In the present invention, this is expressed as equation (2) and Ar = Dr / Dt. Here, Ar is the aspect ratio of the ferrite grain, Dr is the length of the ferrite grain in the rolling direction, and Dt is the length of the ferrite grain in the thickness direction. The present invention refers to the upper surface of the cross section in the rolling direction of the steel sheet in the thickness direction to determine the aspect ratio of the ferrite grain. After dividing into a total of 5 areas from the surface layer to the surface layer on the opposite side, the aspect ratio of 1000 ferrite grains in each of the 5 areas was measured, the average value was calculated in each area, and the average value was calculated to calculate the aspect ratio of the entire area. . This is the aspect ratio of the ferrite grains, and it can be controlled to 2.0 or less. It can be confirmed that in one embodiment of the present invention, deterioration in molding quality can be prevented by controlling the aspect ratio of ferrite grains to 2.0 or less.

As a result, the present invention controls the reheating temperature, finish rolling temperature, hot rolling annealing, and cold rolling annealing temperatures for stainless steel in which the austenite phase is formed, and in particular, cold rolling annealing heat treatment above 950°C where the austenite phase is formed to prevent marten during cooling. Stainless steel and its products that can secure corrosion resistance and strength at the same time by inducing site phase transformation and solid solution of carbides, securing the martensite phase fraction of more than 20%, and controlling the number of carbides per unit area to 7 or less. It can be confirmed that the manufacturing method can be provided.

The pitting potential is more than 70mV, which confirms that it is possible to provide a stainless steel and a manufacturing method thereof that simultaneously satisfy high corrosion resistance and high strength due to excellent yield strength, tensile strength, and hardness, while not being inferior in corrosion resistance.

In addition, it can be confirmed that the effect of preventing deterioration of molding quality can be obtained by controlling the aspect ratio of ferrite grains to 2.0 or less.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art will recognize the present invention within the scope and spirit of the following claims. You will understand that various changes and modifications are possible.

According to one embodiment of the present invention, it is possible to provide a stainless steel that simultaneously satisfies high corrosion resistance and high strength with dual phases of martensite and ferrite after cold rolling annealing heat treatment, and a manufacturing method thereof, and its industrial applicability is recognized. do.

Claims (13)

1. By weight %, C: 0.01 to 0.1%, N: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.01 to 3.0%, Cr: 10.0 to 20.0%, Al: 0.001 to 1.0%, P: 0.05% Hereinafter, S: 0.01% or less, including the remaining Fe and other inevitable impurities,

   The distribution of carbides with a diameter of 0.5㎛ or more is 7/100㎛2 ^or less per unit area,

   The microstructure is in two phases: martensite phase and ferrite phase.

   A high-corrosion-resistant, high-strength stainless steel in which the martensite phase has an area ratio of 20% or more.
2. In claim 1,

   Highly corrosion-resistant, high-strength stainless steel that satisfies the following formula (1).

   Equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10

   (In equation (1), C, N, Ni, Mn, Cr, Si and Al refer to the weight percent of each element)
3. In claim 1,

   High-corrosion-resistant, high-strength stainless steel with a pitting potential of 70 mV or more and a yield strength of 350 MPa or more.
4. In claim 1,

   High-corrosion-resistant, high-strength stainless steel with a tensile strength of 500 MPa or more.
5. In claim 1,

   High-corrosion-resistant, high-strength stainless steel with a hardness of 200Hv or more.
6. In claim 1,

   A high-corrosion-resistant, high-strength stainless steel having a hardness of 400 Hv or more in the martensite phase.
7. In claim 1,

   High-corrosion-resistant, high-strength stainless steel with an aspect ratio of ferrite grains of 2.0 or less.
8. By weight %, C: 0.01 to 0.1%, N: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.01 to 3.0%, Cr: 10.0 to 20.0%, Al: 0.001 to 1.0%, P: 0.05% Hereinafter, reheating the slab containing S: 0.01% or less, the remaining Fe and other unavoidable impurities to 1050 to 1250°C;

   Hot rolling and hot rolling annealing; and

   It includes cold rolling and cold rolling annealing at 950 to 1100°C,

   A method of manufacturing a highly corrosion-resistant, high-strength stainless steel that satisfies the following equation (1) and has a microstructure of two phases: a martensite phase and a ferrite phase.

   Equation (1): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10

   (In equation (1), C, N, Ni, Mn, Cr, Si and Al refer to the weight percent of each element)
9. In claim 8,

   A method of manufacturing highly corrosion-resistant, high-strength stainless steel in which the distribution of carbides with a diameter of 0.5 ㎛ or more is 7/100 ㎛ ² or less per unit area.
10. In claim 8,

    A method of manufacturing high corrosion resistance and high strength stainless steel, wherein the hot rolling annealing step is performed at 750 to 900°C.
11. In claim 8,

    A method of manufacturing high corrosion resistance and high strength stainless steel, wherein the martensite phase is 20% or more in area ratio after the cold rolling annealing.
12. In claim 8,

    A method of manufacturing high corrosion resistance and high strength stainless steel, wherein the martensite phase has a hardness of 400 Hv or more after the cold rolling annealing.
13. In claim 8,

    A method of manufacturing highly corrosion-resistant, high-strength stainless steel with an aspect ratio of ferrite grains of 2.0 or less.

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