WO2023121133A1 - Steel plate for exhaust system steel pipe having improved corrosion resistance and formability, and method for producing same - Google Patents

Abstract

The present specification discloses: a steel plate for an exhaust system steel pipe, wherein the steel plate is economical and improved in terms of corrosion resistance and formability by controlling the alloy components and production method; and a method for producing the steel plate. The steel plate which is for a steel pipe and has improved corrosion resistance and formability according to an embodiment of the present invention may include 0.001-0.02 wt% of C, 0.4-1.0 wt% of Si, 0.1-0.5 wt% of Mn, 8.0-10.0 wt% of Cr, 0-0.3 wt% of Ni (exclusive of 0), 0-0.3 wt% of Cu (exclusive of 0), 0.02-0.2 wt% of Al, 0.001-0.02 wt% of N, and 0.05-0.4 wt% of Ti, with the remainder including Fe and inevitable impurities.

Description

Steel plate for exhaust system steel pipe with improved corrosion resistance and formability and manufacturing method thereof

The present invention relates to a steel sheet for exhaust system steel pipe with improved corrosion resistance and formability and a method for manufacturing the same, and more particularly, to a steel sheet for exhaust system steel pipe with improved corrosion resistance and formability while being economical by controlling alloy components and a manufacturing method, and a steel sheet for exhaust system steel pipe with improved corrosion resistance and formability, and the same It's about manufacturing methods.

Steel pipes for exhaust systems of internal combustion engines, such as automobiles, are subject to molding, such as bending and expansion, as needed, so they are required to have excellent formability. In addition, since the steel pipe for an exhaust system of an internal combustion engine is exposed to the outside, a certain level of corrosion resistance is also required.

The most common way to ensure corrosion resistance is to use stainless steel. Stainless steel for the exhaust system is ferritic stainless steel containing at least 10.5% by weight of Cr. However, since the stainless steel has a relatively high Cr content, price competitiveness is poor. Therefore, there is a need for stainless steel for exhaust system steel pipes with a reduced Cr content.

However, when the amount of Cr is lowered, there is a problem that corrosion resistance deteriorates. In addition, there is a problem that the toughness of the welded part is poor. Therefore, there is a demand for a method for securing a steel sheet for an exhaust system having excellent corrosion resistance and formability while reducing Cr.

An object of the present invention for solving the above problems is to provide a steel sheet for an exhaust system that is economical by reducing the Cr content and has improved corrosion resistance and formability, and a manufacturing method thereof.

Steel sheet for steel pipe with improved corrosion resistance and formability according to an embodiment of the present invention, in weight%, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more and 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu: more than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: 0.05% or more 0.4% or less, including the remaining Fe and unavoidable impurities, the texture index represented by the formula (1) below may satisfy 2 or more and 5.5 or less.

Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component.

In addition, the steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may have a coating index of 30% or more represented by the following formula (2).

Equation (2): Si _max + Al _max

In the above formula (2), Si _max and Al _max mean the maximum values of Si and Al content measured in a range of 20 nm from the steel sheet surface.

In addition, the steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may have an Ericsson value of 4.4 mm or more.

In addition, the steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may have a pitting potential of 70 mV or more.

In addition, in the method for manufacturing a steel sheet for a steel pipe with improved corrosion resistance and formability according to an embodiment of the present invention, in weight%, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu: more than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Preparing a slab containing Ti: 0.05% or more and 0.4% or less, the remainder Fe and unavoidable impurities, and having a texture index represented by Equation (1) below that satisfies 2 or more and 5.5 or less; Reheating the slab to 1150 to 1240 ° C; preparing a hot-rolled material by hot-rolling the reheated slab; Cold-rolling the hot-rolled material to produce a cold-rolled material; It may include the step of annealing heat treatment of the cold-rolled material at 900 to 1020 ℃.

Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component.

In addition, in the method for manufacturing a steel sheet for a steel pipe having improved corrosion resistance and formability according to an embodiment of the present invention, the hot-rolled material may have a thickness of 2.5 to 4.5 mm.

In addition, in the method for manufacturing a steel sheet for a steel pipe having improved corrosion resistance and formability according to an embodiment of the present invention, the cold-rolled material may have a thickness of 0.5 to 2.0 mm.

According to one embodiment of the present invention, it is possible to provide a steel sheet for an exhaust system that is economical by reducing the Cr content and has improved corrosion resistance and formability, and a manufacturing method thereof.

1 is a photograph taken of the Ericsson test results of Example 2.

Figure 2 is a photograph taken of the Ericsson test results of Comparative Example 2.

Steel sheet for steel pipe with improved corrosion resistance and formability according to an embodiment of the present invention, in weight%, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more and 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu: more than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: 0.05% or more 0.4% or less, including the remaining Fe and unavoidable impurities, the texture index represented by the formula (1) below may satisfy 2 or more and 5.5 or less.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention may be embodied in other forms without being limited to only the embodiments presented herein. In the drawings, in order to clarify the present invention, illustration of parts irrelevant to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.

Throughout the specification, when a certain component is said to "include", it means that it may further include other components without excluding other components unless otherwise stated.

Expressions in the singular number include plural expressions unless the context clearly dictates otherwise.

Hereinafter, the reason for limiting the numerical value of the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, units are % by weight.

Steel sheet for steel pipe with improved corrosion resistance and formability according to an embodiment of the present invention, in weight%, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more and 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu: more than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: 0.05% or more 0.4% or less, the remaining Fe and unavoidable impurities may be included.

The content of C (carbon) may be 0.001% or more and 0.02% or less.

C is an element added to ensure product strength. Considering this, C may be added in an amount of 0.001% or more. However, when the content of C is excessive, corrosion resistance of the steel sheet may be deteriorated by forming a compound with Cr at the welded portion. Considering this, the upper limit of the C content may be limited to 0.02%. Preferably, the content of C may be 0.005% or more and 0.01% or less.

The content of Si (silicon) may be 0.4% or more and 1.0% or less.

Si is a major element that can simultaneously improve corrosion resistance and formability of steel. Considering this, Si may be added in an amount of 0.4% or more. However, when the content of Si is excessive, strength may be excessively high and moldability may be deteriorated. Considering this, the upper limit of the Si content may be limited to 1.0%. Preferably, the content of Si may be 0.5% or more and 0.8% or less.

The content of Mn (manganese) may be 0.1% or more and 0.5% or less.

Mn is an element that can be added for improving hardenability and effective pickling. Considering this, Mn may be added in an amount of 0.1% or more. However, when the content of Mn is excessive, formability may be deteriorated by accelerating the formation of martensitic phase in the weld zone. Considering this, the upper limit of the Mn content may be limited to 0.5%. Preferably, the content of Mn may be 0.2% or more and 0.4% or less.

The content of Cr (chromium) may be 8.0% or more and 10.0% or less.

In the present invention, Cr may be added in an amount of 8.0% or more to secure minimum corrosion resistance and formability. However, if the content of Cr is excessive, it is out of the spirit of the present invention to pursue low cost. Considering this, the upper limit of the Cr content may be limited to 10.0%. Preferably, the Cr content may be 8.5% or more and 9.5% or less.

The content of Ni (nickel) may be more than 0% and 0.3% or less.

Ni is an element that can deteriorate formability by accelerating the formation of martensitic phase in the weld zone. Considering this, the upper limit of the Ni content may be limited to 0.3%.

The content of Cu (copper) may be greater than 0% and 0.3% or less.

Cu, like Ni, is an element that can deteriorate formability by accelerating martensite phase formation. Considering this, the upper limit of the Cu content may be limited to 0.3%. Preferably, the content of Cu may be greater than 0% and less than or equal to 0.28%.

The content of Al (aluminum) may be 0.02% or more and 0.2% or less.

Al is an element that can simultaneously improve corrosion resistance and formability of steel. Considering this, Al may be added in an amount of 0.02% or more. However, when the Al content is excessive, it may be difficult to control the alloy components in molten steel. Considering this, the upper limit of the Al content may be limited to 0.2%. Preferably, the content of Al may be 0.03% or more and 0.1% or less.

The content of N (nitrogen) may be 0.001% or more and 0.02% or less.

N, like C, is an element effective in improving the strength of a steel sheet. Considering this, N may be added in an amount of 0.001% or more. However, when the N content is excessive, corrosion resistance of the steel sheet may be deteriorated by forming a compound with Cr at the welded portion. Considering this, the upper limit of the N content may be limited to 0.02%. Preferably, the N content may be 0.005% or more and 0.01% or less.

The content of Ti (titanium) may be 0.05% or more and 0.4% or less.

Ti is an element effective in securing corrosion resistance by preventing the formation of a compound of Cr, C, and N. Considering this, Ti may be added by 0.05% or more. However, when the content of Ti is excessive, formability may be deteriorated because the toughness of the steel is lowered. Considering this, the upper limit of the Ti content may be limited to 0.4%. Preferably, the content of Ti may be 0.1% or more and 0.3% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may satisfy a texture index of 2 or more and 5.5 or less represented by Equation (1) below.

Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component.

When the Cr content of the steel sheet for steel pipes is reduced, an austenite phase is formed while passing through a high temperature during welding of the steel pipe, and a phase transformation into a martensite phase may occur while cooling to room temperature. Therefore, when the Cr content of the steel sheet for steel pipes is reduced, the toughness of the welded portion is deteriorated, and cracks may easily occur when the steel pipe is bent or expanded.

In order to prevent the formation of martensite phase in the weld zone, it is necessary to control the alloy composition. In particular, impurities such as Ni, Cu, and Mn are easily added to molten steel used for manufacturing steel sheets. Since the impurities are caused by scraps used in molten steel, it is necessary to control the content of the impurities.

In the present invention, the formula (1) is disclosed as a texture index capable of controlling Ni, Cu, and Mn acting as impurities in molten steel along with Si and Al.

When the value of Equation (1) is less than 2, it is difficult to effectively suppress the generation of martensitic phase in the weld zone, and thus the formability of the steel sheet may be deteriorated. However, when the value of formula (1) exceeds 5.5, the formability of the steel sheet may deteriorate due to excessively high strength of the steel.

The steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may have a coating index of 30% or more represented by Equation (2) below.

Equation (2): Si _max + Al _max

In the above formula (2), Si _max and Al _max mean the maximum values of Si and Al content measured in a range of 20 nm from the steel sheet surface.

In order to secure corrosion resistance while reducing the Cr content, it is effective to form a film of Si and Al on the surface of the steel sheet. In order to form a film of Si and Al on the surface of the steel sheet, it is advantageous that the content of Si and Al on the surface of the steel sheet is high.

In the present invention, sufficient corrosion resistance can be secured by controlling the sum of the maximum values of Si and Al contents measured in the range of 20 nm from the surface of the steel sheet to 20% or more.

The steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention may have an Ericsson value of 4.4 mm or more and a pitting potential of 70 mV or more by controlling the alloy composition and manufacturing method.

Next, a method for manufacturing a steel sheet for a steel pipe having improved corrosion resistance and formability according to another aspect of the present invention will be described.

In the method for manufacturing a steel sheet for steel pipes with improved corrosion resistance and formability according to an embodiment of the present invention, in weight%, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more 0.5 % or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu: more than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: Preparing a slab containing 0.05% or more and 0.4% or less, the remaining Fe and unavoidable impurities, and having a texture index represented by Equation (1) below that satisfies 2 or more and 5.5 or less; Reheating the slab to 1150 to 1240 ° C; preparing a hot-rolled material by hot-rolling the reheated slab; Cold-rolling the hot-rolled material to produce a cold-rolled material; It may include the step of annealing heat treatment of the cold-rolled material at 900 to 1020 ℃.

Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component.

The component range of each alloy composition and the reason for limiting the numerical value of Formula (1) are as described above, and each manufacturing step will be described in detail below.

After manufacturing a slab satisfying the alloy composition and formula (1), a series of reheating, hot rolling, cold rolling, and annealing heat treatment may be performed.

First, the slab may be reheated to 1150 to 1240 ° C.

When the reheating temperature is low, it may be difficult to re-decompose the coarse precipitates generated during slab manufacturing. Considering this, the reheating temperature may be 1150°C or higher. However, when the reheating temperature is too high, Si on the surface of the slab is excessively oxidized and may be removed in the pickling process. Therefore, when the reheating temperature is too high, the Si content on the surface of the steel sheet is lowered, and thus corrosion resistance may be deteriorated. Considering this, the upper limit of the reheating temperature may be limited to 1240°C. Preferably, the reheating temperature may be 1200 ° C or more and 1240 ° C or less.

A hot-rolled material may be produced by hot-rolling the reheated slab, and a cold-rolled material may be manufactured by cold-rolling the hot-rolled material.

The cold-rolled material may be annealed and heat treated at 900 to 1020 ° C.

When the annealing heat treatment temperature is low, recrystallization in the cold-rolled structure is insufficient, and workability may be deteriorated. In consideration of this, the annealing heat treatment temperature may be 900 °C or more. However, when the annealing heat treatment temperature is too high, as in the reheating process, the Si content of the steel surface is reduced, and thus corrosion resistance may be deteriorated. Considering this, the upper limit of the annealing heat treatment temperature may be limited to 1020 °C. Preferably, the annealing heat treatment temperature may be 920 ° C or more and 980 ° C or less.

In addition, in the method for manufacturing a steel sheet for steel pipes having improved corrosion resistance and formability according to an embodiment of the present invention, the hot-rolled material may have a thickness of 2.5 to 4.5 mm, and the cold-rolled material may have a thickness of 0.5 to 2.0 mm. can be However, the thickness of the hot-rolled material and the cold-rolled material is not limited thereto, and may be manufactured in various thicknesses depending on the purpose.

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

{Example}

For the various alloy composition ranges shown in Table 1 below, slabs were prepared, reheated to 1240 ° C, and then hot rolled to produce a 3 mm thick hot rolled material. Next, the hot-rolled material was cold-rolled to prepare a cold-rolled product having a thickness of 1 mm, followed by annealing heat treatment at 980° C. and pickling.

division	alloy component
	C	Si	Mn	Ni	Cu	Cr	Al	Ti	N
Example 1	0.006	0.85	0.30	0.03	0.02	9.5	0.11	0.19	0.007
Example 2	0.006	0.48	0.30	0.03	0.01	9.5	0.08	0.20	0.006
Example 3	0.007	0.78	0.28	0.02	0.15	8.8	0.04	0.17	0.006
Example 4	0.006	0.92	0.30	0.20	0.03	9.1	0.03	0.19	0.006
Example 5	0.006	0.80	0.30	0.25	0.10	9.1	0.15	0.15	0.006
Comparative Example 1	0.005	0.50	0.25	0.01	0.03	11.2	0.03	0.20	0.006
Comparative Example 2	0.006	0.48	0.45	0.03	0.35	0.0	0.01	0.21	0.009
Comparative Example 3	0.006	0.22	0.30	0.03	0.25	9.5	0.10	0.19	0.008
Comparative Example 4	0.008	0.21	0.31	0.03	0.03	8.8	0.10	0.20	0.003
Comparative Example 5	0.008	0.52	0.27	0.06	0.15	9.1	0.01	0.22	0.004
Comparative Example 6	0.006	0.26	0.29	0.02	0.03	8.9	0.01	0.17	0.006
Comparative Example 7	0.006	0.50	0.30	0.23	0.28	9.2	0.10	0.19	0.006

Equation (1) values, Equation (2) values, Ericsson values, and pit potentials are shown in Table 2 below. Equation (1) values are expressed by calculating Equation (1) below.

Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component.

The value of Equation (2) was expressed by calculating Equation (2) below.

Equation (2): Si _max + Al _max

In Equation (2), Si _max and Al _max mean the maximum values of Si and Al content measured in a range of 20 nm from the surface of the steel sheet.

On the other hand, the Si and Al contents measured in the range of 20 nm from the surface of the steel sheet were measured using glow discharge mass spectrometry, and may be measured using other equipment commonly used in industry or academia.

Ericsson value was measured as follows. After inserting the specimen between the upper and lower dies, the outer periphery of the specimen was fixed with a force of 20 kN. Thereafter, deformation was applied to the specimen at a speed of 5 to 20 mm/min using a spherical punch having a diameter of 20 mm. Thereafter, a punch was inserted until the specimen was fractured, and then the strain height of the specimen was measured at fracture. The higher the Ericsson value, the better the moldability.

The pitting potential was measured using a potentiostat equipment. At this time, when the specimen was immersed in the NaCl solution and a voltage of 20 mV/min was applied, the measured value of the pitting potential at which the current reached 100 ㎂ was shown. Here, the temperature of the NaCl solution was set to 30 °C and the concentration was set to 0.5%. On the other hand, the higher the pitting potential value, the better the corrosion resistance.

division	Equation (1)	Formula (2) (%)	Ericsson value (mm)	Formal potential (mV)
Example 1	5.4	48.9	4.5	105
Example 2	3.4	32.3	6.2	70
Example 3	2.8	39.3	5.9	85
Example 4	2.6	44.8	5.3	97
Example 5	2.2	50.3	4.4	108
Comparative Example 1	0.8	22.9	0.5	49
Comparative Example 2	1.0	19.2	0.5	41
Comparative Example 3	1.9	18.7	0.9	40
Comparative Example 4	1.6	24.7	1.5	53
Comparative Example 5	1.4	12.8	1.1	28
Comparative Example 6	1.1	32.0	0.8	69

Referring to Table 2, Examples 1 to 5 satisfied the alloy composition, component range, and Equation (1) and Equation (2) values presented in the present invention. Accordingly, Examples 1 to 5 satisfied the Ericsson value of 4.4 mm or more and the pitting potential of 70 mV or more. That is, Examples 1 to 5 were excellent in both corrosion resistance and formability. However, Comparative Examples 1 to 6 did not satisfy the value of 2 or more in the equation (1), and thus did not satisfy the Ericsson value of 4.4 mm or more. couldn't That is, Comparative Examples 1 to 6 were inferior in formability.

In Comparative Examples 1 to 5, the value of Equation (2) did not satisfy 30% or more, so the pitting potential value did not satisfy 70 mV or more. That is, Comparative Examples 1 to 5 were inferior in corrosion resistance.

1 is a photograph of the Ericsson test results of Example 2, and FIG. 2 is a photograph of the Ericsson test results of Comparative Example 2.

Referring to FIGS. 1 and 2 , it can be seen that the formability of the steel sheet for an exhaust system steel pipe according to an embodiment of the present invention is very excellent.

According to one embodiment of the present invention, it is possible to provide a steel sheet for an exhaust system that is economical by reducing the Cr content and has improved corrosion resistance and formability, and a method for manufacturing the same, and industrial applicability is recognized.

Claims (7)

1. In % by weight, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more and 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu : More than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: 0.05% or more and 0.4% or less, the remainder including Fe and unavoidable impurities,

   Steel sheet for steel pipes with improved corrosion resistance and formability that satisfies 2 or more and 5.5 or less in the structure index represented by the formula (1) below:

   Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

   (In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component).
2. The method of claim 1,

   Steel sheet for steel pipes with improved corrosion resistance and formability, having a film index of 30% or more, represented by the formula (2) below:

   Equation (2): Si _max + Al _max

   (In the above formula (2), Si _max and Al _max mean the maximum values of Si and Al content measured in the range of 20 nm from the steel sheet surface).
3. The method of claim 1,

   Steel sheet for steel pipes with an Ericsson value of 4.4 mm or more and improved corrosion resistance and formability.
4. The method of claim 1,

   Steel sheet for steel pipes with improved corrosion resistance and formability with a pitting potential of 70mV or more.
5. In % by weight, C: 0.001% or more and 0.02% or less, Si: 0.4% or more and 1.0% or less, Mn: 0.1% or more and 0.5% or less, Cr: 8.0% or more and 10.0% or less, Ni: more than 0% and 0.3% or less, Cu : More than 0% and 0.3% or less, Al: 0.02% or more and 0.2% or less, N: 0.001% or more and 0.02% or less, Ti: 0.05% or more and 0.4% or less, including the remainder Fe and unavoidable impurities, expressed by the following formula (1) Manufacturing a slab that satisfies a displayed tissue index of 2 or more and 5.5 or less;

   Reheating the slab to 1150 to 1240 ° C;

   preparing a hot-rolled material by hot-rolling the reheated slab;

   Cold-rolling the hot-rolled material to produce a cold-rolled material;

   Method for producing a steel sheet for steel pipes with improved corrosion resistance and formability, comprising the step of annealing and heat-treating the cold-rolled material at 900 to 1020 ° C.:

   Formula (1): (Si+2Al)/(Ni+Cu+Mn/2)

   (In the above formula (1), Si, Al, Ni, Cu and Mn mean the content (wt%) of each component).
6. The method of claim 5,

   The hot-rolled material has a thickness of 2.5 to 4.5 mm, a method for producing a steel sheet for a steel pipe with improved corrosion resistance and formability.
7. The method of claim 5,

   The cold-rolled material has a thickness of 0.5 to 2.0 mm, a method for producing a steel sheet for steel pipes with improved corrosion resistance and formability.

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