JP2022548715A - Low Cr ferritic stainless steel with improved tube expandability and method for producing the same - Google Patents

Abstract

A low Cr ferritic stainless steel with improved pipe expandability and a method for producing the same are provided. In weight percent, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5 % or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (0 ), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and inevitable impurities, corresponding to a depth of 100 μm or less from the surface The ratio (Gs/Gc) of the average crystal grain size (Gs) of the region to the average crystal grain size (Gc) of the central region is 1.5 or less, and the following formula (1) is satisfied. . Formula (1): Cr+3Si+10Sn+2Cu≧17 (wherein Cr, Si, Sn, and Cu mean the content (% by weight) of each element) [selection] FIG.

Description

TECHNICAL FIELD The present invention relates to a low Cr ferritic stainless steel with improved tube expandability and a method for producing the same, and more particularly to a low Cr ferritic stainless steel for automobile exhaust systems with improved tube expandability and a method for producing the same. .

Stainless steels are generally classified according to their chemical composition and metallographic structure. Stainless steels can be classified into austenite, ferrite, martensite, and dual phase based metallographic structures.
Ferritic stainless steel has excellent corrosion resistance in spite of the addition of only a small amount of expensive alloying elements, and is highly price competitive compared to austenitic stainless steel. In particular, ferritic stainless steels such as STS409L, 439, and 436L are used as materials for automobile exhaust system members such as muffler cases, pipes, and plates, which are applied within a temperature range of 400°C.

For example, STS409L steel uses about 11% Cr and stabilizes carbon (C) and nitrogen (N) with titanium (Ti) to prevent weakening of the weld zone and improve workability. It has been most widely used because it is mainly used at temperatures below °C and has imperfect corrosion resistance to components of condensed water generated from automobile exhaust systems.
STS439 steel stabilizes carbon (C) and nitrogen (N) with titanium (Ti) and contains about 17% chromium (Cr). The STS436L steel is a steel obtained by adding about 1% molybdenum (Mo) to the STS439 steel, and has excellent corrosion resistance against condensed water and rust corrosion resistance.

On the other hand, in recent years, the penetration rate of automobiles has increased rapidly in various countries such as China, Central and South America, and India. ing. For example, South Korea and Japan regulate the sulfur (S) component in gasoline components to 10 ppm or less, but China's gasoline regulation value is 500 ppm or less, but actually it is higher than that. It is presumed that sulfur (S) is contained.
The sulfur (S) component in gasoline is concentrated into SO ₄ ^2- ions contained in the condensed water component of automobile exhaust gas, and changes to sulfuric acid (H ₂ SO ₄ ) with a pH of 2 or less, exhibiting high corrosiveness.
Thus, in areas where gasoline contains a large amount of sulfur (S), STS409L steel, which is used as an automobile muffler material, gradually contains 17% or more of chromium (Cr), such as STS439 and 436L steels. However, we are in a situation where we have no choice but to replace with high chromium stainless steel materials. For this reason, with the rise in resource prices, there is a demand for the development of stainless steel materials that do not add expensive elements such as molybdenum (Mo), or that have condensed water corrosion resistance equal to or greater than that of STS439 steel or 436L steel even with the addition of trace amounts. It is

On the other hand, in the actual automobile exhaust system environment, not only corrosion of the inner surface condensed water caused by condensed water but also external surface corrosion caused by snow removal salt and seawater occur at the same time. The fact is that the development and spread of stainless steel is inadequate, and it is impossible to replace it with the existing STS439 steel.
In recent years, the trend of automobile exhaust system parts is increasing the number of exhaust system parts in the lower part of the automobile. Therefore, there is a demand for improvement in tube expandability compared to existing products.
Therefore, in consideration of not only internal condensed water corrosion but also external corrosion, there is a demand for the development of a ferritic stainless steel with condensed water corrosion resistance equal to or greater than that of the existing 439 steel or 436L steel material, and with improved tube expandability. It is

The aim of the present invention is to optimize the Sn, Si and Cu contents to ensure resistance to external corrosion and internal condensate water corrosion comparable to high Cr ferritic stainless steels without increasing the Cr content. Another object of the present invention is to provide a ferritic stainless steel with improved pipe expandability and a method for producing the same.

The low Cr ferritic stainless steel with improved pipe expandability of the present invention has C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), and Si: 1% by weight. 0-2.0, Mn: 0.5% or less (excluding 0), Cr: 9.0-15.0%, Ti: 0.1-0.5%, Sn: 0.05-0. 2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and inevitable impurities and the ratio (Gs/Gc) of the average crystal grain size (Gs) in the region corresponding to the depth of 100 μm or less from the surface and the average crystal grain size (Gc) in the central region is 1.5 or less, It is characterized by satisfying the following formula (1).
Formula (1): Cr+3Si+10Sn+2Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.

In addition, the low Cr ferritic stainless steel with improved pipe expandability of the present invention can satisfy the following formula (2).
Formula (2): Cr+2Si+15Sn+5Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
Furthermore, the low Cr ferritic stainless steel of the present invention preferably has a tube expansion ratio defined by the following formula (3) of 25% or more.
Formula (3): (Df-D0)/D0*100
Here, Df means the length of the hole in the machined portion after molding, and D0 means the length of the initial machined hole.

The low Cr ferritic stainless steel of the present invention preferably has an elongation of 30% or more in the direction perpendicular to the rolling direction.
In addition, in the low Cr ferritic stainless steel of the present invention, the average grain size in the region corresponding to the depth of 100 μm or less from the surface is preferably 50 μm or less.

The method for producing a low-Cr ferritic stainless steel with improved pipe expandability according to the present invention comprises, in weight percent, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.5%. 05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remaining Fe and The steps of hot-rolling, cold-rolling and cold-rolling annealing of a slab containing unavoidable impurities and satisfying the following formula (1), and cold-rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis are included. characterized by
Formula (1): Cr+3Si+10Sn+2Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.

In the method for producing low Cr ferritic stainless steel of the present invention, the slab can satisfy the following formula (2).
Formula (2): Cr+2Si+15Sn+5Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
Also, in the method for producing low Cr ferritic stainless steel of the present invention, the slab is preferably hot rolled at a temperature of 1,020 to 1,180.degree.
Furthermore, it is preferable to carry out cold rolling annealing in the temperature range of 900 to 1,100° C. in the method for producing the low Cr ferritic stainless steel of the present invention.

According to an embodiment of the present invention, the present invention provides a low-Cr ferritic stainless steel that can improve the workability of pipe expansion and ensure the resistance to external corrosion and internal condensed water corrosion of the STS439 level, and a method for producing the same. be able to.

4 is a graph showing the results of an external surface corrosion test caused by snow removal salt for each steel type in an automobile exhaust system environment. 2 is a graph showing the results of corrosion resistance evaluation by an external surface corrosion index defined by Cr+3Si+10Sn+2Cu in an automobile exhaust system environment. 2 is a graph showing the results of corrosion resistance evaluation by an internal corrosion index defined by Cr+2Si+15Sn+5Cu in an automobile exhaust system condensed water environment. FIG. 10 is a diagram showing a scale structure after cold rolling annealing in Example 2; FIG. 12 is a diagram showing a scale structure after cold rolling annealing in Comparative Example 12; 2 is a photograph showing the surface state of the cold-rolled steel sheet after cold-rolling pickling in Example 2 through neutral salt electrolysis and sulfuric acid electrolysis, and the surface state after corrosion resistance evaluation, and (a) omits the mixed acid immersion step. When cold-rolling pickling under the conditions of neutral salt electrolysis and sulfuric acid electrolysis is introduced, (b) is a diagram showing that rust generation is less and the timing of rust generation is delayed. 2 is a photograph showing the surface state after cold-rolling pickling of Example 2 through neutral salt electrolysis, sulfuric acid electrolysis, and mixed acid (nitric acid + hydrofluoric acid) immersion, and the surface state after corrosion resistance evaluation. When hydrofluoric acid is used, (b) is a diagram of a state in which a large number of rusts are generated due to the influence of pits formed on the surface. 4 is a photograph of observation of microstructures according to changes in cold-rolled annealing temperature in Example 2. FIG. 10 is a photograph of observation of the microstructure according to the change in cold rolling annealing temperature of Comparative Example 12. FIG.

The low Cr ferritic stainless steel with improved tube expandability according to one embodiment of the present invention has C: 0.01% or less (excluding 0) and N: 0.01% or less (excluding 0) by weight. , Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0 .05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and inevitable impurities, and the ratio (Gs/Gc) of the average crystal grain size (Gs) in the region corresponding to the depth of 100 μm or less from the surface and the average crystal grain size (Gc) in the central region is 1. It is 5 or less and satisfies the following formula (1).
Formula (1): Cr+3Si+10Sn+2Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The following examples are presented to fully convey the spirit of the invention to those of ordinary skill in the art to which the invention pertains. This invention is not limited to the embodiments presented herein, but may be embodied in other forms. In the drawings, parts irrelevant to the description may be omitted to clarify the present invention, and the sizes of components may be exaggerated to facilitate understanding.
Throughout the specification, when a part "includes" a component, this does not exclude other components, but may further include other components, unless specifically stated to the contrary. means
Singular references include plural references unless the context clearly dictates otherwise. Exemplary embodiments according to the invention are described in detail below with reference to the accompanying drawings.

The inventors of the present invention conducted various studies to improve the resistance to external corrosion caused by snow removal salt, seawater, etc. of low-cost, low-Cr ferritic stainless steel and to improve the pipe expandability. As a result, the following findings were obtained. rice field.
In order to improve the corrosion resistance, the Cr content is generally increased, but Cr is also expensive as a raw material and causes an increase in manufacturing cost, which is not a desirable development direction.

In the present invention, Si, Sn, and Cu are selected as alloying elements for improving the external corrosion and internal condensed water corrosion resistance of ferritic stainless steel. On the other hand, Sn is known as an element that reduces hot workability. However, the present inventors found that the reduction in hot workability can be effectively controlled when the Sn content is controlled to 0.2% or less.
In addition, by adding 0.5% or less of Cu and 1 to 2% of Si in combination with Sn, hot workability is ensured, and the external corrosion resistance of automobile exhaust systems is rapidly improved. Found it.

On the other hand, Cu is an element that improves external corrosion resistance and internal condensed water corrosion resistance. There is a problem that workability cannot be ensured during processing.
Therefore, the inventors of the present invention found that the growth of surface layer crystal grains can be suppressed if the Si content is secured at 1.0% or more while the Cu content is 0.5% or less. The alloy composition was optimized in consideration of workability.

FIG. 1 is a graph showing the results of an external surface corrosion test caused by snow removal salt or the like for each steel type in an automobile exhaust system environment.
As shown in FIG. 1, when Cr is contained by 11% without adding other alloying elements, the average corrosion depth is about 0.6 mm. When added alone, the corrosion depth is 0.4 to 0.5 mm, so it can be confirmed that it is slightly less than the 11Cr STS steel.
On the other hand, when the alloying elements Sn, Cu, and Si are simultaneously added in a state containing 11% Cr, the corrosion depth sharply decreases at the level of 0.1 mm, and corrosion resistance at the level of 18Cr STS steel can be secured. confirmed.

The low Cr ferritic stainless steel with improved pipe expandability according to one aspect of the present invention has, in weight %, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.5%. 05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remaining Fe and Consists of unavoidable impurities.
The reasons for limiting the numerical values of the content of the metal-containing component in the examples of the present invention will be described below. Below, the unit is % by weight unless otherwise specified.

The content of C and N is 0.01% or less (excluding 0).
Carbon (C) and nitrogen (N) are interstitial elements that form Ti(C,N) carbonitrides.
When the content of C and N is high, dissolved C and N, which cannot form Ti(C,N) carbonitrides, deteriorate the elongation and low-temperature impact properties of the material, and are used for a long time at 600°C or less after welding. In this case, the upper limits of C and N are limited to 0.01% because they combine with Cr to form Cr carbides such as Cr ₂₃ C ₆ to cause intergranular corrosion.
In addition, when the C+N content is high, steelmaking inclusions are increased due to the increase in Ti content, which causes surface defects such as scabs. Furthermore, there is a problem that the nozzle clogging phenomenon occurs during continuous casting, and the elongation and impact properties are deteriorated.

The content of Si is 1.0-2.0%.
Silicon (Si) is an element that acts as a deoxidizing agent during the steelmaking process and stabilizes the ferrite phase. As the Si content increases, a concentration phenomenon occurs around grain boundaries, and the concentrated Si has the effect of suppressing the growth of grains. In the present invention, 1.0% or more of Si is added in order to increase corrosion resistance and suppress the growth of surface layer crystal grains in a condensed water atmosphere. However, if the content is excessive, there is a problem that the ductility and formability are lowered, so in the present invention, the upper limit is limited to 2.0%.

The content of Mn is 0.5% or less (excluding 0).
Manganese (Mn) is an element that stabilizes austenite, and when the Mn content increases, it forms precipitates such as MnS and reduces pitting corrosion resistance. increases, the upper limit is limited to 0.5%.

The Cr content is 9.0-15.0%.
Chromium (Cr) is an element that forms a passive film that suppresses oxidation and stabilizes ferrite. In the present invention, 9.0% or more of Si is added to ensure corrosion resistance in a condensed water atmosphere. However, if the content is excessive, the production cost increases and the workability and impact properties are deteriorated, so the upper limit is limited to 15.0%.

The content of Ti is 0.1-0.5%.
Titanium (Ti) is an element that forms Ti(C,N) carbonitrides to prevent intergranular corrosion. Ti preferentially combines with interstitial elements such as carbon (C) and nitrogen (N) to form precipitates Ti(C,N) carbonitrides, thereby causing solid solution C and solid solution N in the steel. is an element effective for securing the corrosion resistance of steel by reducing the amount of Cr and suppressing the formation of a Cr-depleted region. However, if the content is excessive, Ti-based inclusions are formed to cause a large amount of surface defects such as scabs, which causes nozzle clogging during continuous casting. Limited to 5%.

The Sn content is 0.05-0.2%.
Tin (Sn) is an essential element for ensuring corrosion resistance in a condensed water atmosphere, which is the target of the present invention. good. However, if the content is excessive, there is a problem that the hot workability is lowered and the efficiency of the manufacturing process is lowered, so the upper limit is limited to 0.2%.

The content of Cu is 1.0% or less (excluding 0).
Copper (Cu) is an essential element for ensuring corrosion resistance in a condensed water atmosphere, which is the target of the present invention, and is added to ensure corrosion resistance equal to or higher than that of 18Cr STS439 steel. However, if the content is excessive, there is a problem that not only the material cost increases but also the hot workability deteriorates, so the upper limit is limited to 1.0%.

The content of P is 0.035% or less (excluding 0).
Phosphorus (P) is an unavoidable impurity contained in steel, and is an element that forms grain boundary segregation and MnS precipitates and is the main cause of deterioration of hot workability. Control is preferred. In the present invention, the P content is controlled to 0.035% or less.

The content of S is 0.01% or less (excluding 0).
Sulfur (S) is an unavoidable impurity contained in steel, and is an element that forms grain boundary segregation and MnS precipitates and is the main cause of deterioration of hot workability. Control is preferred. In the present invention, the S content is controlled to 0.01% or less.

The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, so this cannot be completely eliminated. Since these impurities are known to any person skilled in the art of normal manufacturing processes, their full content is not specifically mentioned herein.

On the other hand, the low Cr ferritic stainless steel with improved pipe expandability according to an embodiment of the present invention can satisfy the following formula (1).
Formula (1): Cr+3Si+10Sn+2Cu)≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
In the present invention, as a result of evaluating the corrosion resistance in a solution simulating the external surface corrosion environment of ferritic stainless steel, the external surface corrosion index represented by the formula (1) was derived.

FIG. 2 is a graph showing the results of corrosion resistance evaluation by the external surface corrosion index defined by Cr+3Si+10Sn+2Cu in an automobile exhaust system environment. In FIG. 2, the corrosion depth of the existing STS439 steel was measured to be 1 mm, and the external corrosion index was limited to 17 or more in order to ensure external corrosion resistance equal to or greater than that of the STS439 steel.
As shown in FIG. 2, when the external corrosion index is less than 17, the corrosion depth exceeds 1 mm, and resistance to external corrosion caused by snow removal salt or seawater of the STS439 steel level cannot be ensured.

On the other hand, the low Cr ferritic stainless steel with improved pipe expandability according to an embodiment of the present invention satisfies the following formula (2).
Formula (2): Cr+2Si+15Sn+5Cu≧17
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
In the present invention, as a result of evaluating the corrosion resistance in a solution simulating condensed water as well as the external corrosion environment of ferritic stainless steel, the internal corrosion index represented by the formula (2) was derived.

FIG. 3 is a graph showing the results of corrosion resistance evaluation by the internal corrosion index defined by Cr+2Si+15Sn+5Cu in an automobile exhaust system condensed water environment. In FIG. 3, the corrosion depth of the existing STS439 steel was measured to be 2.5 mm, and the internal corrosion index was limited to 17 or more in order to ensure external corrosion resistance equal to or greater than that of the STS439 steel.
As shown in FIG. 3, when the internal corrosion index is less than 17, the corrosion depth exceeds 2.5 mm, and the corrosion resistance in the condensed water environment of the STS439 steel level cannot be ensured.

As described above, when Cu and Si are added in combination with Sn, as the Cu content increases, the crystal grain size in the surface layer of the ferritic stainless steel increases rapidly, and the workability during pipe expansion after pipe making is reduced. cannot be guaranteed. In the present invention, the growth of surface layer crystal grains could be suppressed by controlling the Si content to 1.0 to 2.0% while the Cu content is 0.5% or less.

The low Cr ferritic stainless steel with improved pipe expandability according to one embodiment of the present invention has an average grain size (Gs) in the region corresponding to a depth of 100 μm or less from the surface and an average grain size (Gs) in the central region. The size (Gc) ratio (Gs/Gc) is 1.5 or less.
That is, in ferritic stainless steel, it is possible to control the growth of surface crystal grains distributed in a region of 100 μm or less from the surface compared to the internal crystal grains, thereby ensuring pipe expandability during pipe making. For example, the average crystal grain size (Gs) of the surface region is preferably 50 μm or less in consideration of the tube-making elongation.
Therefore, the ferritic stainless steel according to the disclosed examples has an expansion ratio of 25% or more defined by the following formula (3).
Formula (3): (D _f −D ₀ )/D ₀ *100
(Here, _Df means the length of the machined hole after molding, and _D0 means the length of the initial machined hole.)

The expansion rate is a material property that indicates how much a hole machined in a steel plate through various processing methods can be expanded without defects such as cracks and necking. )-(length of initial machined hole)*100/(length of initial machined hole).
Next, a method for producing a low Cr ferritic stainless steel with improved pipe expandability according to another aspect of the present invention will be described.
For example, a cold-rolled annealed steel sheet may be produced by hot-rolling a slab containing the above-described alloy composition, subjecting the hot-rolled hot-rolled steel sheet to annealing heat treatment, cold-rolling and cold-rolling annealing.

In the case of hot rolling conditions, the higher the slab heating temperature is, the more advantageous it is for recrystallization during the hot rolling operation. , 180°C.
The lower the finish rolling temperature during hot rolling, the higher the accumulated deformation energy during hot rolling, which is useful for recrystallization during annealing, which is advantageous for improving the elongation. On the other hand, if the finish rolling temperature is too low, sticking defects, that is, sticking of the material to the rolling rolls, are likely to occur.

On the other hand, if the cold rolling reduction of the material is too low, it is difficult to remove surface defects and ensure surface properties. It is preferable to limit the cold rolling reduction to 70 to 80%.
Next, the cold-rolled annealed steel sheet may be cold-rolled and pickled through neutral salt electrolysis and sulfuric acid electrolysis after undergoing a step of cold-rolling annealing in a normal temperature range of 900 to 1,100°C.

In the austenitic stainless steel of the present invention, Sn, Cu, and Si are added in combination at the same time, and scales are not annularly formed on the surface of the cold-rolled and annealed steel sheet, but are uniformly formed in a thin layer.
That is, the formation of a SiO ₂ scale layer after cold rolling annealing may be suppressed by including a certain amount of Sn. Therefore, conventionally, since the SiO ₂ scale layer is thickly formed in an annular shape, a mixed acid dipping step to which hydrofluoric acid and nitric acid are added has been performed in the cold rolling pickling step in order to remove such scales. Even if only neutral salt electrolysis and sulfuric acid electrolysis are performed without adding such hydrofluoric acid and nitric acid, a sufficient effect of cold rolling pickling can be obtained, and the process cost can be reduced.

Therefore, in the cold-rolled annealed steel sheet, the ratio (Gs/Gc) of the average grain size (Gs) in the region corresponding to the depth of 100 μm or less from the surface and the average grain size (Gc) in the central region is 1 0.5 or less.
That is, by controlling the growth of surface crystal grains, it is possible to ensure the pipe expansion workability during pipe making, so that when pipes made of ferritic stainless steel according to the disclosed embodiments are expanded, A rate of 25% or more can be secured.

Hereinafter, the present invention will be described in more detail through examples.
For the various alloy composition ranges shown in Table 1 below, a 120 mm thick ingot was cast by ingot melting, hot rolled at a temperature of 1,150 ° C., and a 3.0 mm thick hot rolled steel sheet. manufactured. After that, a cold-rolled steel sheet having a thickness of 1.2 mm was manufactured by cold rolling, and then cold-rolled annealing was performed at a temperature of 1,100° C. for 1 minute.
Thereafter, the cold-rolled annealed steel sheet was deposited at a molten salt temperature of 400° C. for 5 seconds, deposited in a nitric acid solution at 60° C. for about 10 seconds, and cold-rolled and pickled to produce a final cold-rolled pickled steel sheet. At this time, the concentration of the nitric acid solution was maintained at 110 g/L.

Table 1 below shows the alloy composition (% by weight), the value of formula (1), and the value of formula (2) for each experimental steel type.

The depth of corrosion was measured by simulating external corrosion caused by snow removal salt, seawater, etc. and internal corrosion caused by condensed water.
The outer surface corrosion test was performed by cutting test pieces of each example and comparative example into 150*70mm sizes, removing oil from the surface with caustic soda, and then heat-treating them in a heat-treating furnace maintained at 400°C for about 24 hours. did

A combined cycle corrosion test was then performed. Specifically, each test piece was sprayed with a 5% NaCl solution at 30 ° C. for 2 hours, dried in an atmosphere with a relative humidity of 25% and a temperature of 60 ° C. for about 4 hours, and then dried in an atmosphere with a relative humidity of 90% and a temperature of 50 ° C. The corrosion test was conducted by repeating 100 cycles, one cycle being maintained at 2 hours. Thereafter, each test piece was immersed in a 60% nitric acid solution to remove oxide scale, and the depth of corrosion was measured. The depth of corrosion was calculated by averaging 10 deepest points from each test piece.
The internal corrosion test was carried out by cutting test pieces of each example and comparative example into 40*70 mm sizes and pre-treating them in an electric furnace maintained at 400° C. for about 24 hours.

A solution of HCl, H ₂ SO ₄ in a condensed water mimicking environment was then prepared with a Cl ⁻ concentration of 50 ppm, an SO ₄ ²⁻ concentration of 100 ppm, and a pH maintained at 8.0. At this time, pH was adjusted to 8.0 with _NH3 solution. Thereafter, 10 mL of the test solution was injected into each test piece every 6 hours, and a corrosion test was performed by repeating 100 cycles.
On the other hand, the grain size of the region corresponding to the depth of 100 μm or less from the surface and the grain size of the central region corresponding to half the thickness were etched and measured using an optical microscope, and the average grain size and center of the surface region were measured. The average grain size ratio (Gs/Gc) in the partial region and the average grain size in the surface region are shown in Table 2 below.

In Tables 1 and 2, Comparative Examples 1 and 2 correspond to 11Cr STS409 steel with 11% Cr and 18Cr STS439 steel with 18% Cr, which are generally used as materials for automobile exhaust systems, respectively.

FIG. 2 is a graph showing the results of corrosion resistance evaluation by the external surface corrosion index defined by Cr+3Si+10Sn+2Cu in an automobile exhaust system environment.
As shown in FIG. 2, it can be confirmed that the external corrosion depth decreases linearly as the external corrosion index increases. In the cases of Examples 1 to 7, the corrosion depth was 1.0 mm or less, and external corrosion resistance equal to or higher than that of STS439 steel could be secured.

FIG. 3 is a graph showing the results of corrosion resistance evaluation by the internal corrosion index defined by Cr+2Si+15Sn+5Cu in an automobile exhaust system condensed water environment.
As shown in FIG. 3, it can be confirmed that the internal corrosion depth linearly decreases as the internal corrosion index increases. In the case of ~7, the corrosion depth was 2.5 mm or less, and the internal corrosion resistance equal to or higher than that of STS439 steel could be secured.

FIG. 4 is a diagram showing the scale structure after cold rolling annealing in Example 2. FIG. FIG. 5 is a diagram showing the scale structure after cold rolling annealing of Comparative Example 12. FIG.
As shown in FIGS. 4 and 5, in the case of Comparative Example 12, which does not contain Sn, after cold-rolling annealing, SiO ₂ annealing scales are formed in an annular shape on the entire surface. In contrast, in the case of Example 2, which contains a Sn content of 0.05% or more, for example, 0.15%, the SiO ₂ annealing scale is not ring-shaped on the surface, and is very formed uniformly in a thin layer. Therefore, a sufficient cold-rolling pickling effect can be obtained without adding hydrofluoric acid during cold-rolling annealing and pickling.

FIG. 6 is a photograph showing the surface state of the cold-rolled steel sheet after cold-rolling pickling in Example 2 through neutral salt electrolysis and sulfuric acid electrolysis, and the surface state after corrosion resistance evaluation. When cold-rolling pickling under the neutral salt electrolysis-sulfuric acid electrolysis condition, omitting the process, is introduced, (b) is a diagram showing that rust generation is less and the time of rust generation is delayed. FIG. 7 is a photograph showing the surface state after cold-rolling pickling of Example 2 through neutral salt electrolysis, sulfuric acid electrolysis, and mixed acid (nitric acid + hydrofluoric acid) immersion, and the surface state after corrosion resistance evaluation. ) shows a state in which hydrofluoric acid is used, and (b) shows a state in which many rusts occur due to the influence of pits formed on the surface.
Corrosion resistance was evaluated using a combined cycle corrosion tester. The combined cyclic corrosion test conditions were salt spray (5% NaCl solution sprayed at 30°C for 2 hours), dry (relative humidity 25%, temperature 60°C for 4 hours), wet (relative humidity 90%, temperature 50°C). 1 cycle was repeated for 2 hours in a wet state, and under these conditions, corrosion resistance was evaluated by observing photographs of the surface of the test piece after repeating 5 cycles.

As shown in FIG. 7(a), when cold-rolling pickling under nitric acid/hydrofluoric acid mixed acid immersion conditions is introduced, the use of hydrofluoric acid creates pits where the base material is dissolved on the surface. It can be confirmed that many occurrences occur. Further, as shown in FIG. 7B, it can be confirmed that many rusts occur due to the influence of pits formed on the surface.
On the other hand, as shown in (a) of FIG. 6, when cold-rolling pickling under the conditions of neutral salt electrolysis and sulfuric acid electrolysis omitting the mixed acid immersion step was introduced, no pits were observed, and uniform stainless steel was obtained. surface was obtained. Moreover, as shown in FIG. 6(b), it can be confirmed that the occurrence of rust is less and the time of occurrence of rust is delayed.
That is, the ferritic stainless steel cold-rolled annealed steel sheet according to one embodiment of the present invention can completely remove the cold-rolled annealing scale through neutral salt electrolysis and sulfuric acid electrolysis, and is less likely to rust. Therefore, the time of rust generation is later than that of the comparative example, and a sufficient cold-rolled pickling effect can be obtained without the mixed acid process during pickling. be able to.

On the other hand, by changing the cold rolling annealing temperature of Example 2 and Comparative Example 12 from 900 to 1,030 ° C., the average grain size of the surface region and the average grain size of the central region in the thickness direction in the rolling direction TD plane Table 3 below shows the grain size ratio (Gs/Gc), the elongation ratio, and the presence or absence of cracks during tube expansion of 25% or more.
The elongation was measured in accordance with the JIS 2241 standard by processing the elongation in the direction perpendicular to the rolling direction into JIS 13B size. An expansion rate of 25% was given during pipe making, and the presence or absence of crack generation was checked.

FIG. 8 is a photograph of observation of the microstructure according to the cold-rolling annealing temperature change in Example 2, and FIG. 9 is a photograph of observation of the microstructure according to the cold-rolling annealing temperature change of Comparative Example 12.
As shown in FIGS. 8 and 9, in the case of Comparative Example 12, it can be confirmed that the crystal grain size of the surface layer sharply increases from 930° C. or higher. On the other hand, in the case of Example 2, a uniform grain size distribution is exhibited in the surface layer and the center without abrupt changes in the grain size of the surface layer up to 1,030°C.

As shown in Table 3, the elongation value of Example 2 was 32-33%, which was lower than that of Comparative Example 12 by 1-2%. In the case of Example 2, the Si content is as high as 1% or more, which is considered to be due to the occurrence of the work hardening phenomenon.
Generally, the better the draw rate, the higher the expansion rate.
However, when the cold-rolled annealed steel sheet is made into a pipe and expanded by 25% or more, in the case of Comparative Example 12, the crystal grain sizes in the surface layer and the central portion are unevenly distributed, and cracks occur during pipe expansion. can be confirmed to occur.

In contrast, in the case of Example 2, 1.0% or more of Si was added, and the ratio of the average crystal grain size of the surface region and the average crystal grain size of the central region was controlled to 1.5 or less. By doing so, the occurrence of cracks was suppressed.
As described above, according to the disclosed embodiment, by controlling the alloy components and the component relational expressions, not only the condensed water corrosion but also the outer surface corrosion resistance is ensured, and the ferritic stainless steel with improved pipe expandability. Steel can be manufactured.

While illustrative embodiments of the invention have been described above, the invention is not so limited and those of ordinary skill in the art will appreciate the concept and scope of the claims set forth below. It is understood that various modifications and variations may be made without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY The ferritic stainless steel according to the present invention can be applied to automobile exhaust system materials because it can improve tube expandability and ensure resistance to external surface corrosion and internal surface condensed water corrosion at the STS439 level.

Claims (9)

% by weight, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (0 excluding), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remaining Fe and inevitable impurities,
The ratio (Gs/Gc) of the average crystal grain size (Gs) in the region corresponding to the depth of 100 μm or less from the surface and the average crystal grain size (Gc) in the central region is 1.5 or less,
A low Cr ferritic stainless steel with improved pipe expandability, which satisfies the following formula (1).
Formula (1): Cr+3Si+10Sn+2Cu≧17
(Here, Cr, Si, Sn, Cu mean the content (% by weight) of each element) 2. The low Cr ferritic stainless steel with improved pipe expandability according to claim 1, wherein the following formula (2) is satisfied.
Formula (2): Cr+2Si+15Sn+5Cu≧17
(Here, Cr, Si, Sn, Cu mean the content (% by weight) of each element) 2. The low Cr ferritic stainless steel with improved tube expandability according to claim 1, wherein the tube expansion rate defined by the following formula (3) is 25% or more.
Formula (3): (Df-D0)/D0*100
(Here, Df means the length of the machined hole after molding, and D0 means the length of the initial machined hole.) 2. The low Cr ferritic stainless steel with improved pipe expandability according to claim 1, wherein the elongation in the direction perpendicular to the rolling direction is 30% or more. 2. The low Cr ferritic stainless steel with improved pipe expandability according to claim 1, wherein the average grain size of the region corresponding to the depth of 100 μm or less from the surface is 50 μm or less. % by weight, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (0 excluding), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and inevitable impurities, and hot-rolling a slab that satisfies the following formula (1) When,
cold rolling and cold rolling annealing;
and cold rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis.
Formula (1): Cr+3Si+10Sn+2Cu≧17
(Here, Cr, Si, Sn, Cu mean the content (% by weight) of each element) 7. The method for producing low Cr ferritic stainless steel with improved pipe expandability according to claim 6, wherein the slab satisfies the following formula (2).
Formula (2): Cr+2Si+15Sn+5Cu≧17
(Here, Cr, Si, Sn, Cu mean the content (% by weight) of each element) 7. The method for producing low Cr ferritic stainless steel with improved pipe expandability according to claim 6, wherein the slab is hot rolled at a temperature of 1,020 to 1,180.degree. 7. The method for producing a low Cr ferritic stainless steel with improved tube expandability according to claim 6, wherein the cold rolling annealing is performed in a temperature range of 900 to 1,100°C.

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