CN114585754B

CN114585754B - Low CR ferritic stainless steel with improved tube expansion workability and method for producing the same

Info

Publication number: CN114585754B
Application number: CN202080072912.3A
Authority: CN
Inventors: 柳汉振; 郑一鄼; 金会勋
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2019-09-17
Filing date: 2020-07-08
Publication date: 2024-09-20
Anticipated expiration: 2040-07-08
Also published as: JP2022548715A; KR20210032765A; KR102255119B1; JP7465955B2; CN114585754A; WO2021054587A1

Abstract

Disclosed is a low Cr ferritic stainless steel having improved tube expanding workability. The disclosed ferritic stainless steel contains up to 0.01 wt% of C (excluding 0), up to 0.01 wt% of N (excluding 0), 1.0 wt% to 2.0 wt% of Si, up to 0.5 wt% of Mn (excluding 0), 9.0 wt% to 15.0 wt% of Cr, 0.1 wt% to 0.5 wt% of Ti, 0.05 wt% to 0.2 wt% of Sn, up to 1.0 wt% of Cu (excluding 0), up to 0.035 wt% of P (excluding 0), and up to 0.01 wt% of S (excluding 0), the balance of Fe and unavoidable impurities, has a ratio (Gs/Gc) of an average grain size (Gs) at a region corresponding to a depth of up to 100 μm from a surface to an average grain size (Gc) at a center region of up to 1.5, and satisfies the following expression (1). Expression (1): cr+3Si+10Sn+2Cu is more than or equal to 17 (wherein Cr, si, sn and Cu represent the content (weight percent) of each element).

Description

Low CR ferritic stainless steel with improved tube expansion workability and method for producing the same

Technical Field

The present disclosure relates to ferritic stainless steel, and more particularly, to low Cr ferritic stainless steel having improved pipe expanding workability for an exhaust system of a vehicle and a method of producing the same.

Background

Generally, stainless steel is classified according to chemical compounds or metallic structures. Stainless steel may be classified into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and duplex stainless steel according to the metallic structure.

Ferritic stainless steel has excellent corrosion resistance by using a small amount of an expensive alloy element, thereby having higher price competitiveness than austenitic stainless steel. In particular, ferritic stainless steels such as STS 409L, STS 439 and STS 436L have been used for materials for automotive exhaust systems such as muffler boxes, pipes and plates, and are suitable for use at temperatures of 400 ℃ or below.

For example, STS 409L steel, which is a type of steel having improved workability by using about 11% of chromium (Cr) and by stabilizing carbon (C) and nitrogen (N) with titanium (Ti) to prevent sensitization of welded portions, is mainly used at a temperature lower than 700 ℃ and has been most widely used due to a degree of corrosion resistance to condensate components generated in an exhaust system of an automobile.

STS 439 steel, as a type of steel in which carbon (C) and nitrogen (N) are stabilized by titanium (Ti), contains about 17% chromium (Cr). In addition, STS 436L steel, as a type of steel having excellent corrosion resistance and rust resistance to condensate, was produced by adding about 1% molybdenum (Mo) to STS 439 steel.

Sulfur (S) in the gasoline component is concentrated in condensate of automobile exhaust gas to SO4 ^2- ions and converted to highly corrosive sulfuric acid (H ₂SO₄) having a pH of 2 or less.

As described above, STS 409L steel used as a material for mufflers of automobiles should be inevitably replaced with high Cr stainless steels containing 17% or more of chromium (Cr), such as STS 439 and STS 436L steel, in the field where gasoline contains a large amount of sulfur (S). Therefore, due to the increase in resource prices, it is necessary to develop stainless steel materials as follows: without the use of expensive elements such as molybdenum (Mo) or by using trace amounts of the same, has corrosion resistance to condensate equivalent to STS 439 or STS 436L steel material.

Meanwhile, in the actual environment of an automobile exhaust system, external corrosion is caused by deicing salt or sea salt in addition to internal condensate corrosion caused by condensate. However, considering that the development of ferritic stainless steel in such an external corrosive environment is insufficient, substitution with conventional STS 439 steel is impossible.

Further, as the number of exhaust system components located in the lower portion of the automobile increases, the shape of the exhaust system components tends to become more complicated to improve the space efficiency in the lower portion of the automobile, and there is a need for improving the pipe expanding workability as compared to the existing exhaust system.

Therefore, in view of the external corrosion and the internal condensate corrosion, it is necessary to develop a ferritic stainless steel: which has improved tube expansion workability and condensate corrosion resistance equivalent to or superior to that of conventional STS 439 or STS 436L steel.

Disclosure of Invention

Technical problem

Provided are a ferritic stainless steel having improved pipe expanding workability by optimizing contents of Sn, si and Cu without increasing Cr contents and external corrosion resistance and internal condensate corrosion resistance equivalent to those of high Cr ferritic stainless steel, and a method for producing the same.

Technical proposal

According to one aspect of the present disclosure, a low Cr ferritic stainless steel having improved tube expansion workability contains, in weight percent (wt.%) up to 0.01% of C (excluding 0), up to 0.01% of N (excluding 0), 1.0% to 2.0% of Si, up to 0.5% of Mn (excluding 0), 9.0% to 15.0% of Cr, 0.1% to 0.5% of Ti, 0.05% to 0.2% of Sn, up to 1.0% of Cu (excluding 0), up to 0.035% of P (excluding 0), up to 0.01% of S (excluding 0), and the balance of Fe and unavoidable impurities, wherein a ratio (Gs/Gc) of an average grain size (Gs) of a region within a depth of 100 μm from a surface to an average grain size (Gc) of a central region is 1.5 or less, and satisfies the following expression (1),

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

(Wherein Cr, si, sn and Cu represent the contents (wt.%) of the respective elements).

In addition, the low Cr ferritic stainless steel according to one embodiment of the present disclosure may satisfy the following expression (2),

Expression (2): cr+2Si+15Sn+5Cu is greater than or equal to 17

Further, according to an embodiment of the present disclosure, the pipe expansion ratio defined by the following expression (3) may be 25% or more,

Expression (3): (D _f-D₀)/D₀. Times.100)

(Wherein D _f represents the diameter of the hole of the machined part after machining, and D ₀ represents the diameter of the hole that was originally machined).

Further, according to an embodiment of the present disclosure, the elongation in the direction perpendicular to the rolling direction may be 30% or more.

Further, according to one embodiment of the present disclosure, the average grain size of the region within a depth of 100 μm from the surface may be 50 μm or less.

According to another aspect of the present disclosure, a method for producing a low Cr ferritic stainless steel having improved tube expanding workability includes: hot rolling a steel slab comprising, in weight percent (wt%) up to 0.01% C (excluding 0), up to 0.01% N (excluding 0), 1.0% to 2.0% Si, up to 0.5% Mn (excluding 0), 9.0% to 15.0% Cr, 0.1% to 0.5% Ti, 0.05% to 0.2% Sn, up to 1.0% Cu (excluding 0), up to 0.035% P (excluding 0), up to 0.01% S (excluding 0), and the balance Fe and unavoidable impurities, and satisfying the following expression (1); cold rolling and annealing; cold rolling and pickling are carried out through neutral salt electrolysis and sulfuric acid electrolysis,

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

Furthermore, according to one embodiment of the present disclosure, the billet may satisfy the following expression (2),

Expression (2): cr+2Si+15Sn+5Cu is greater than or equal to 17

Furthermore, according to one embodiment of the present disclosure, the steel slab may be hot rolled at a temperature of 1,020 ℃ to 1,180 ℃.

Furthermore, according to one embodiment of the present disclosure, the cold rolling annealing may be performed at a temperature of 900 ℃ to 1,100 ℃.

Advantageous effects

According to embodiments of the present disclosure, low Cr ferritic stainless steel having improved pipe expanding workability and external corrosion resistance and internal condensate corrosion resistance equivalent to those of STS 439 steel, and a method of producing the same, are provided.

Drawings

Fig. 1 is a graph showing the results of an external corrosion test performed using deicing salts or the like for different steel types in the environment of an automobile exhaust system.

Fig. 2 is a graph showing the evaluation result of corrosion resistance in the environment of an automobile exhaust system based on an external corrosion index defined as cr+3si+10sn+2cu.

Fig. 3 is a graph showing the results of corrosion resistance evaluation in the condensate environment of an automobile exhaust system based on the internal corrosion index defined as cr+2si+15sn+5cu.

Fig. 4 is a view showing the oxide skin structure of the steel of example 2 after cold rolling annealing. Fig. 5 is a view showing the oxide skin structure of the steel of comparative example 12 after cold rolling annealing.

Fig. 6 shows photographs showing the surface state of the cold-rolled steel sheet of example 2 after cold-rolling pickling by neutral salt electrolysis and sulfuric acid electrolysis and the surface state after corrosion resistance evaluation.

Fig. 7 shows photographs showing the surface state of the cold-rolled steel sheet of example 2 after cold-rolling pickling by neutral salt electrolysis, sulfuric acid electrolysis and immersing in a mixed acid solution (nitric acid+hydrofluoric acid) and the surface state after corrosion resistance evaluation.

Fig. 8 shows a photograph of the microstructure of example 2 according to the temperature change in the cold rolling annealing.

Fig. 9 shows a photograph of the microstructure of comparative example 12 according to the temperature change in cold rolling annealing.

Detailed Description

A low Cr ferritic stainless steel with improved tube expanding workability according to an embodiment of the present disclosure contains up to 0.01% of C (excluding 0), up to 0.01% of N (excluding 0), 1.0% to 2.0% of Si, up to 0.5% of Mn (excluding 0), 9.0% to 15.0% of Cr, 0.1% to 0.5% of Ti, 0.05% to 0.2% of Sn, up to 1.0% of Cu (excluding 0), up to 0.035% of P (excluding 0), up to 0.01% of S (excluding 0), and the balance of Fe and unavoidable impurities in weight percentage (wt%), wherein a ratio (Gs/Gc) of an average grain size (Gs) of a region within a depth of 100 μm from a surface to an average grain size (Gc) of a central region is 1.5 or less, and is expressed as follows,

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the disclosure to those of ordinary skill in the art to which the disclosure pertains. The present disclosure is not limited to the embodiments shown herein, but may be presented in other forms. In the drawings, for clarity of description of the present disclosure, parts irrelevant to the description are omitted, and the size of elements may be exaggerated for clarity.

Throughout the specification, unless the context requires otherwise, the term "comprise/comprise" does not exclude other elements but may further comprise further elements.

As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise. Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

The present inventors have conducted various studies to enhance the resistance of low-cost low-Cr ferritic stainless steel to external corrosion occurring due to deicing salts or sea salts and improve pipe-expanding workability, and have found those described below.

Generally, the Cr content is increased to enhance corrosion resistance. However, since the use of expensive Cr increases manufacturing costs, increasing Cr content is not an ideal target for development.

In the present disclosure, si, sn, and Cu are selected as candidate alloying elements to enhance the resistance of ferritic stainless steel to external corrosion and internal condensate corrosion. On the other hand, although Sn is known as an element that deteriorates hot workability, the present inventors found that deterioration of hot workability can be effectively controlled by adjusting the Sn content to 0.2% or less.

Further, the present inventors found that by adding 0.5% or less of Cu and 1% to 2% of Si in combination with Sn, the resistance of an automobile exhaust system to external corrosion is rapidly enhanced while hot workability is obtained.

Meanwhile, cu is an element that enhances resistance to external corrosion and internal condensate corrosion. However, as the Cu content increases, the grain size of the ferritic stainless steel increases rapidly on the surface layer, resulting in a problem that tube expanding workability cannot be obtained once the tube is made.

Accordingly, the present inventors found that by adjusting the Si content to 1.0% or more in a state where the Cu content is 0.5% or less, the growth of crystal grains on the surface layer is suppressed, and the composition ratio is optimized in consideration of the external corrosion resistance and the pipe expanding workability.

Referring to fig. 1, in the case where the Cr content is 11% and the corrosion depth is about 0.6mm without adding other alloying elements thereto, in the case where the alloying elements Sn, cu, and Si are separately added and the Cr content is 11%, the corrosion depth is in the range of 0.4mm to 0.5mm, indicating a slight decrease in the corrosion depth as compared with the 11Cr STS steel.

On the other hand, in the case where the alloying elements Sn, cu and Si were added in combination and the Cr content was 11%, it was confirmed that the corrosion depth rapidly decreased to a level of 0.1mm, indicating that corrosion resistance equivalent to that of 18Cr STS steel could be obtained.

A low Cr ferritic stainless steel with improved pipe expanding workability according to one embodiment of the present disclosure contains up to 0.01% C (excluding 0), up to 0.01% N (excluding 0), 1.0% to 2.0% Si, up to 0.5% Mn (excluding 0), 9.0% to 15.0% Cr, 0.1% to 0.5% Ti, 0.05% to 0.2% Sn, up to 1.0% Cu (excluding 0), up to 0.035% P (excluding 0), up to 0.01% S (excluding 0), and the balance Fe and unavoidable impurities in weight percentage (wt%).

Hereinafter, the reason for numerical limitation of the content of the alloy element in the embodiment of the present disclosure will be described. Hereinafter, unless otherwise indicated, units are% by weight.

The content of C and N is at most 0.01% (excluding 0), respectively.

Carbon (C) and nitrogen (N) are interstitial elements that form Ti (C, N) carbonitrides. As the contents of C and N increase, dissolved C and N, which do not form Ti (C, N) carbonitrides, deteriorate the elongation and low temperature impact characteristics of the material when used for a long time at a temperature below 600 ℃ after welding, and combine with Cr to form Cr carbide such as Cr ₂₃C₆, thereby causing intergranular corrosion. Therefore, the upper limits of C and N are set to 0.01%.

Further, when the content of c+n increases, the formation of steel-making inclusions increases with an increase in the Ti content, and thus surface defects such as scars (scab) are caused. Further, a nozzle (nozzle) may be clogged during the casting process, and problems such as elongation and impact property deterioration may occur, so the total content of c+n may be controlled to 0.02% or less.

The Si content is 1.0% to 2.0%.

Silicon (Si) is an element that serves as a deoxidizer and stabilizes the ferrite phase during the steelmaking process. As the Si content increases, si is enriched near the grain boundary, and the growth of crystal grains may be inhibited due to the enriched Si. In the present disclosure, si is preferably added in an amount of 1.0% or more to improve corrosion resistance under condensate environment and to suppress grain growth on the surface layer. However, excessive Si may deteriorate ductility and formability, and thus the upper limit of the Si content may be set to 2.0%.

The Mn content is at most 0.5% (excluding 0).

Manganese (Mn) is an austenite stabilizing element. As the Mn content increases, precipitates such as MnS are formed, thereby deteriorating pitting corrosion resistance. However, when the Mn content is too low, the cost for purification increases, and thus the upper limit of the Mn content may be set to 0.5%.

The Cr content is 9.0% to 15.0%.

Chromium (Cr) is an element that forms a passivation layer that suppresses oxidation and stabilizes ferrite. In the present disclosure, cr may be added in an amount of 9.0% or more to obtain corrosion resistance in a condensate environment. However, excessive Cr may increase manufacturing costs and deteriorate workability and impact characteristics. Therefore, the upper limit of the Cr content may be set to 15.0%.

The Ti content is 0.1% to 0.5%.

Titanium (Ti) is an element that forms Ti (C, N) carbonitrides to suppress grain boundary corrosion. Ti preferentially combines with interstitial elements such as carbon (C) and nitrogen (N) to form precipitates (Ti (C, N) carbonitrides), thereby reducing the amount of dissolved C and N in the steel and inhibiting the formation of Cr-depleted zones. Therefore, ti, as an element effective in obtaining corrosion resistance of steel, may be added in an amount of 0.1% or more in the present disclosure. However, excessive Ti may lead to the formation of Ti-based inclusions, leading to a large number of surface defects such as scars and clogging of the nozzle during the casting process. Therefore, the upper limit of the Ti content can be set to 0.5%.

The Sn content is 0.05% to 0.2%.

Tin (Sn), an essential element to obtain a desired level of corrosion resistance in the condensate environment in the present disclosure, may be added in an amount of 0.05% or more to obtain corrosion resistance equivalent to or better than that of STS 439 steel containing 18 Cr. However, excessive Sn may deteriorate hot workability and reduce efficiency of the manufacturing process. Therefore, the upper limit of the Sn content may be set to 0.2%.

The content of Cu is at most 1.0% or less (excluding 0).

Copper (Cu) is added as an essential element to obtain a desired corrosion resistance in a condensate environment according to the present disclosure to obtain corrosion resistance equivalent to or better than that of STS 439 steel containing 18Cr in the condensate environment. However, excessive Cu may not only increase the cost of raw materials but also deteriorate hot workability, and thus the upper limit of the Cu content may be set to 1.0%.

The content of P is at most 0.035% (excluding 0).

Phosphorus (P), an impurity inevitably contained in steel, is a main causative element of grain boundary segregation and MnS precipitate formation, and deteriorates hot workability, and therefore the P content is preferably controlled to be as low as possible. In the present disclosure, the P content is controlled to 0.035% or less.

The S content is at most 0.01% (excluding 0).

Sulfur (S), an impurity inevitably contained in steel, is a main causative element of grain boundary segregation and MnS precipitate formation, and deteriorates hot workability, and therefore, the S content is preferably controlled to be as low as possible. In the present disclosure, the S content is controlled to 0.01% or less.

The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may contain unintended impurities that are inevitably incorporated from the raw materials or the surrounding environment. In the present disclosure, the addition of other alloy components than the above-described alloy components is not excluded. Impurities are not specifically mentioned in this disclosure as they are known to any person skilled in the art of manufacture.

Meanwhile, the low Cr ferritic stainless steel having improved pipe expanding workability according to one embodiment of the present disclosure may satisfy the following expression (1).

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

Here, cr, si, sn, and Cu represent the content (wt%) of each element.

As a result of evaluating the corrosion resistance of ferritic stainless steel in a solution simulating an external corrosion environment, an external corrosion index represented by expression (1) was obtained in the present disclosure.

Fig. 2 is a graph showing the evaluation result of corrosion resistance in the environment of an automobile exhaust system based on an external corrosion index defined as cr+3si+10sn+2cu. In fig. 2, since the corrosion depth of the conventional STS 439 steel is measured to be 1mm, the external corrosion index is limited to 17 or more to obtain external corrosion resistance equivalent to or superior to that of the STS 439 steel.

Referring to fig. 2, when the external corrosion index of the steel is less than 17, the corrosion depth exceeds 1mm, indicating that the resistance to external corrosion caused by deicing salt or sea salt equivalent to STS 439 steel cannot be obtained.

Meanwhile, the low Cr ferritic stainless steel having improved pipe expanding workability according to one embodiment of the present disclosure may satisfy the following expression (2).

Expression (2): cr+2Si+15Sn+5Cu is greater than or equal to 17

Here, cr, si, sn, and Cu represent the content (wt%) of each element.

In the present disclosure, as a result of evaluating corrosion resistance of ferritic stainless steel in a solution simulating condensate and in an external corrosive environment, an internal corrosion index represented by expression (2) was obtained.

Fig. 3 is a graph showing the results of corrosion resistance evaluation in the condensate environment of an automobile exhaust system based on the internal corrosion index defined as cr+2si+15sn+5cu. In fig. 3, since the corrosion depth of the conventional STS 439 steel was measured to be 2.5mm, the internal corrosion index was limited to 17 or more to obtain internal corrosion resistance equivalent to or superior to that of the STS 439 steel.

Referring to fig. 3, when the internal corrosion index of the steel is less than 17, the corrosion depth exceeds 2.5mm, indicating that corrosion resistance equivalent to that of STS 439 steel cannot be obtained in the condensate environment.

As described above, in the case of adding Cu and Si in combination with Sn, as the Cu content increases, the grain size of the surface layer of the ferritic stainless steel increases rapidly, resulting in a problem that workability cannot be obtained during pipe expansion after manufacturing the pipe. In the present disclosure, an attempt is made to suppress the growth of crystal grains on the surface layer by adjusting the Si content in the range of 1.0% to 2.0% in a state where the Cu content is 0.5% or less.

In the low Cr ferritic stainless steel with improved tube expanding workability according to one embodiment of the present disclosure, a ratio (Gs/Gc) of an average grain size (Gs) of a region within a depth of 100 μm from a surface to an average grain size (Gc) of a central region is 1.5 or less.

That is, by suppressing the growth of surface grains distributed in a region within a depth of 100 μm from the surface, compared with grains inside the ferritic stainless steel, it is possible to obtain tube expansion workability during the manufacturing process. For example, the average grain size (Gs) of the surface region may be 50 μm or less in view of the elongation for tube manufacturing.

Therefore, the pipe expansion ratio of the ferritic stainless steel according to one embodiment defined by the following expression (3) is 25% or more.

Expression (3): (D _f-D₀)/D₀. Times.100)

The expansion ratio indicates the ability of a material to enable expansion of a hole machined in a steel plate using various machining methods without causing defects such as cracks or necking, and is defined by the expression (diameter of a machined portion of the hole after machining) - (diameter of an originally machined hole) ×100/(diameter of an originally machined hole).

Hereinafter, a method for producing a low Cr ferritic stainless steel having improved tube expanding workability according to another embodiment of the present disclosure will be described.

For example, a steel slab having a chemical composition including the above alloy elements is hot-rolled, the hot-rolled steel sheet is subjected to an annealing heat treatment, and the annealed steel sheet is cold-rolled and annealed to produce a cold-rolled annealed steel sheet.

Regarding the conditions of hot rolling, as the heating temperature of the slab increases, recrystallization occurs more effectively during the hot rolling process. However, when the heating temperature is too high, a large number of surface defects are formed, and thus the upper limit of the hot rolling temperature may be set to 1,180 ℃.

During hot rolling, as the finishing temperature decreases, the deformation stored in the mild steel can increase to promote recrystallization during the annealing process, thereby increasing elongation. However, too low a finish rolling temperature easily causes adhesion defects of the material to the rolling mill (sticking), so the lower limit of the hot rolling temperature can be set to 1,020 ℃.

Meanwhile, when the cold rolling reduction is too low, it is difficult to remove surface defects and obtain surface characteristics. When the cold rolling reduction is too high, the r-bar value is increased to improve formability. Accordingly, the cold rolling reduction may be maintained in the range of 70% to 80%.

Subsequently, after cold-rolling annealing at a usual heat treatment temperature of 900 ℃ to 1,100 ℃, the cold-rolled annealed steel sheet may be pickled by neutral salt electrolysis and sulfuric acid electrolysis.

Since the ferritic stainless steel of the present disclosure is formed by simultaneously adding Sn, cu, and Si in combination, an oxide scale is formed not in a ring shape but in a uniform thin layer form on the surface of the cold-rolled annealed steel sheet.

That is, by adding a certain amount of Sn, the formation of SiO ₂ oxide scale after cold rolling annealing can be suppressed. Therefore, during the cold-rolling pickling process of cold-rolled steel, a process of immersing in a mixed acid solution containing hydrofluoric acid and nitric acid is conventionally performed to remove the SiO ₂ oxide skin layer formed in a thick ring shape. However, according to one embodiment, the cold rolling pickling effect can be sufficiently obtained only by neutral salt electrolysis and sulfuric acid electrolysis without adding hydrofluoric acid and nitric acid, so that the manufacturing cost can be reduced.

In the thus obtained cold-rolled annealed steel sheet, a ratio (Gs/Gc) of an average grain size (Gs) of a region within a depth of 100 μm from a surface to an average grain size (Gc) of a central region may be 1.5 or less.

That is, tube expansion workability may be obtained during the tube manufacturing process by suppressing growth of crystal grains on the surface, and a tube made of ferritic stainless steel according to one embodiment may have an expansion ratio of 25% or more during the tube manufacturing process.

Hereinafter, the present disclosure will be described in more detail with reference to the following examples.

The alloy elements having various chemical compositions shown in table 1 below were melted to prepare steel ingots having a thickness of 120mm, and the steel ingots were hot-rolled at a temperature of 1,150 ℃ to prepare hot-rolled steel sheets having a thickness of 3.0 mm. Subsequently, the hot rolled steel sheet was cold rolled to prepare a cold rolled steel sheet having a thickness of 1.2mm and cold rolled annealed at a temperature of 1,100 ℃ for 1 minute.

Then, the cold-rolled annealed steel sheet was immersed in a molten salt at 400 ℃ for 5 seconds, then immersed in a nitric acid solution at 60 ℃ for about 10 seconds, and then cold-rolled pickled to obtain a final cold-rolled pickled steel sheet. In this regard, the concentration of the nitric acid solution was maintained at 110g/L.

The chemical composition (wt%) of the alloy element of each steel type and the values of the expressions (1) and (2) are shown in table 1 below.

TABLE 1

The corrosion depths obtained therein were measured by simulating the environment in which external corrosion was caused by deicing salt, sea salt, etc., and the environment in which internal corrosion was caused by condensate, respectively.

The external corrosion test was performed as follows. Each sample according to examples and comparative examples was cut into a size of 150mm×70mm, the surface was cleaned with sodium hydroxide by removing oil, etc., and the sample was heat-treated in a furnace maintained at 400 ℃ for about 24 hours.

Subsequently, a composite cycle corrosion test was performed. Specifically, 100 corrosion test cycles were repeated and each cycle was performed as follows. Each sample was sprayed with a 5% NaCl solution at 30 ℃ for 2 hours, dried in an atmosphere of 25% relative humidity and a temperature of 60 ℃ for about 4 hours, and maintained in an atmosphere of 90% relative humidity and a temperature of 50 ℃ for 2 hours. After the corrosion test, each sample was immersed in a 60% nitric acid solution to remove scale, and then the corrosion depth was measured. The average of 10 deepest portions of each sample estimated and selected by visual observation was calculated as the corrosion depth.

The internal corrosion test was performed as follows. Each sample according to examples and comparative examples was cut to a size of 40mm x 70mm and pre-treated in an electric furnace maintained at 400 ℃ for about 24 hours.

Subsequently, HCl, H ₂SO₄ solutions were prepared in which Cl ^- and SO ₄ ^2- concentrations were maintained at 50ppm and 100ppm, respectively, and the pH was maintained at 8.0 to simulate a condensate environment. In this regard, the pH was adjusted to 8.0 using NH ₃ solution. Subsequently, 100 corrosion test cycles were repeated with 10mL of test solution applied every 6 hours.

Meanwhile, the grain sizes of the region within a depth of 100 μm from the surface and the central region located at a depth equal to half the thickness were measured by etching using an optical microscope, and the ratio of the average grain size of the surface to the average grain size of the central region (Gs/Gc) and the average grain size of the surface region are shown in table 2 below.

TABLE 2

In tables 1 and 2, comparative examples 1 and 2 show Cr11% STS 409 steel and Cr18% STS 439 steel, respectively, which are commonly used as materials for automobile exhaust systems.

Referring to fig. 2, it was determined that the external corrosion depth decreased linearly as the external corrosion index increased. In the cases of examples 1 to 7 in which the external corrosion resistance index represented by expression (1) was 17 or more, the corrosion depth was 1.0mm or less, indicating that external corrosion resistance equivalent to or better than that of STS 439 steel was obtained.

Referring to fig. 3, it was determined that the internal corrosion depth linearly decreased as the internal corrosion index increased. In the cases of examples 1 to 7 in which the internal corrosion resistance index represented by expression (2) was 17 or more, the corrosion depth was 2.5mm or less, indicating that the internal corrosion resistance equivalent to or better than that of the STS 439 steel was obtained.

Referring to fig. 4 and 5, in the case of comparative example 12 containing no Sn, a ring-shaped SiO ₂ annealed oxide skin was formed on the entire surface after cold rolling annealing. However, in the case of example 2 containing Sn in an amount of 0.05% or more (e.g., 0.15%), the SiO ₂ annealed oxide scale is uniformly formed as a very thin layer on the surface, instead of being formed in a ring shape. Thus, during the cold rolling, annealing, pickling processes, a sufficient cold rolling pickling effect can be obtained without adding hydrofluoric acid thereto.

Fig. 6 shows photographs showing the surface state of the cold-rolled steel sheet of example 2 after cold-rolling pickling by neutral salt electrolysis and sulfuric acid electrolysis and the surface state after corrosion resistance evaluation. Fig. 7 shows photographs showing the surface state of the cold-rolled steel sheet of example 2 after cold-rolling pickling by neutral salt electrolysis, sulfuric acid electrolysis and immersing in a mixed acid solution (nitric acid+hydrofluoric acid) and the surface state after corrosion resistance evaluation.

The evaluation of corrosion resistance was performed by evaluating corrosion resistance using a composite cycle corrosion tester. The conditions for the composite cycle corrosion test are as follows. Each sample was sprayed with brine (2 hours at 30 ℃ with 5% nacl solution), dried (4 hours at 25% relative humidity at 60 ℃) and kept in a moist state (2 hours at 50 ℃ in a moist state with 90% relative humidity). Under the conditions, corrosion resistance was evaluated by observing photographs of the surfaces of the respective samples after repeating 5 cycles.

Referring to fig. 7 (a), in the case of introducing cold rolling pickling under the condition of immersing in a mixed acid solution of nitric acid and hydrofluoric acid, it was determined that a large number of pits (pids) in which a base material is dissolved are formed on the surface due to the use of hydrofluoric acid. Further, referring to fig. 7 (b), it was determined that a large amount of rust was formed due to pits formed on the surface.

In contrast, referring to fig. 6 (a), in the case of introducing cold rolling pickling under the condition of neutral salt electrolysis-sulfuric acid electrolysis, from which the process of immersing in the mixed acid solution is omitted, a uniform stainless steel surface is obtained without pits. Further, referring to fig. 6 (b), it is determined that rust formation is reduced and delayed.

That is, in the cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present disclosure, the cold-rolled annealed scale may be completely removed by neutral salt electrolysis and sulfuric acid electrolysis, and rust formation is reduced and delayed as compared to the comparative example. Therefore, although the process of immersing in the mixed acid solution is not performed during pickling, the effect of cold rolling pickling can be sufficiently obtained and the surface characteristics can be obtained, thereby reducing the process cost.

Meanwhile, when the cold rolling annealing temperatures of example 2 and comparative example 12 were changed from 900 ℃ to 1,030 ℃, the ratio (Gs/Gc) of the average grain size of the surface region to the average grain size of the central region in the thickness direction in the cross section (TD) in the rolling direction, the elongation, and the occurrence of cracks in the case of tube expansion of 25% or more were shown in the following table 3.

The elongation is measured by processing the elongation value in the direction perpendicular to the rolling direction into JIS13B size according to JIS2241 standard. The appearance of cracks was checked by applying a 25% pipe expansion rate during the pipe manufacturing process.

TABLE 3 Table 3

Fig. 8 shows a photograph of the microstructure of example 2 according to the temperature change of the cold rolling annealing. Fig. 9 is a photograph showing a change in temperature of the microstructure according to cold rolling annealing of comparative example 12.

Referring to fig. 8 and 9, in the case of comparative example 12, the grain size of the surface layer rapidly increased after the temperature exceeded 930 ℃. In contrast, in the case of example 2, a uniform grain size distribution was observed in the surface layer and the central region, without rapid changes in grain size up to 1,030 ℃.

Referring to table 3, in the case of example 2, the elongation was in the range of 32% to 33%, which was relatively lower by 1% to 2% than that of comparative example 12. This is considered to be because work hardening occurs due to a high Si content of 1% or more in the case of example 2.

Generally, as the elongation increases, the expansion increases accordingly.

However, when a tube was manufactured using a cold-rolled annealed steel sheet and a tube expanding process of 25% or more was performed, an uneven grain size distribution was observed in the surface layer and the central region of comparative example 12, and thus it was determined that cracks occurred during the tube expanding process.

In contrast, in the case of example 2, by adding Si in an amount of 1.0% or more, the occurrence of cracks was suppressed by adjusting the ratio of the average grain size of the surface region to the average grain size of the central region to 1.5 or less.

As described above, according to the disclosed embodiments, by adjusting the alloying elements and their chemical compositions, it is possible to produce ferritic stainless steel having improved pipe-expanding workability and resistance to not only condensate corrosion but also external corrosion.

While the present disclosure has been particularly described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure.

[ Industrial applicability ]

Ferritic stainless steel having improved pipe expanding workability and resistance to external corrosion and internal condensate corrosion equivalent to STS 439 according to the present disclosure may be applied to materials for automobile exhaust systems.

Claims

1. A low Cr ferritic stainless steel having improved tube expansion workability contains, in weight percent (wt%), up to 0.01% but not including 0 of C, up to 0.01% but not including 0 of N, 1.0% to 2.0% of Si, up to 0.5% but not including 0 of Mn, 9.0% to 15.0% of Cr, 0.1% to 0.5% of Ti, 0.05% to 0.2% of Sn, up to 0.5% but not including 0 of Cu, up to 0.035% but not including 0 of P, up to 0.01% but not including 0 of S, and the balance of Fe and unavoidable impurities,

Wherein a ratio (Gs/Gc) of an average grain size (Gs) of the region within a depth of 100 μm from the surface to an average grain size (Gc) of the central region is 1.5 or less,

Wherein the region within a depth of 100 μm from the surface has an average grain size (Gs) of 29 μm to 45 μm, and

The following expression (1) is satisfied,

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

Wherein Cr, si, sn and Cu represent the content (wt%) of each element.

2. The low Cr ferritic stainless steel with improved pipe expanding workability according to claim 1, wherein the following expression (2) is satisfied,

Expression (2): cr+2Si+15Sn+5Cu is greater than or equal to 17

Wherein Cr, si, sn and Cu represent the content (wt%) of each element.

3. The low Cr ferritic stainless steel with improved tube expansion workability according to claim 1, wherein the tube expansion ratio defined by the following expression (3) is 25% or more,

Expression (3): (D _f-D₀)/D₀. Times.100)

Where D _f represents the diameter of the hole of the machined part after machining, and D ₀ represents the diameter of the hole that was originally machined.

4. The low Cr ferritic stainless steel with improved tube expansion workability according to claim 1, wherein the elongation in the direction perpendicular to the rolling direction is 30% or more.

5. A method for producing a low Cr ferritic stainless steel with improved tube expansion workability, the method comprising:

Hot rolling a steel slab comprising, in weight percent (wt%) up to 0.01% but not including 0C, up to 0.01% but not including 0N, 1.0% to 2.0% Si, up to 0.5% but not including 0 Mn, 9.0% to 15.0% Cr, 0.1% to 0.5% Ti, 0.05% to 0.2% Sn, up to 1.0% but not including 0 Cu, up to 0.035% but not including 0P, up to 0.01% but not including 0S, and the balance Fe and unavoidable impurities, and satisfying the following expression (1);

Cold rolling and annealing; and

Cold rolling and pickling are carried out by neutral salt electrolysis and sulfuric acid electrolysis,

Expression (1): cr+3si+10Sn+2Cu is more than or equal to 17

Wherein Cr, si, sn and Cu represent the content (wt%) of each element.

6. The method for producing a low Cr ferritic stainless steel having improved pipe-expanding workability according to claim 5, wherein said steel slab satisfies the following expression (2),

Expression (2): cr+2Si+15Sn+5Cu is greater than or equal to 17

Wherein Cr, si, sn and Cu represent the content (wt%) of each element.

7. The method for producing a low Cr ferritic stainless steel with improved tube expansion workability according to claim 5, wherein the steel blank is hot rolled at a temperature of 1,020 ℃ to 1,180 ℃.

8. The method for producing a low Cr ferritic stainless steel with improved tube expansion workability according to claim 5, wherein the annealing is performed at a temperature of 900 ℃ to 1,100 ℃.