WO2019039768A1 - Low-ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance - Google Patents

Abstract

Disclosed is low-Ni austenitic stainless steel with improved hot workability and hydrogen embrittlement resistance which may occur as content of Mn and Ni decreases. The austenitic stainless steel according to the present invention comprises, by weight %: 0.05 to 0.15% of C; 0.2 to 0.7% of Si; 2.0 to 5.0% of Mn; 2.0 to 5.0% of Ni; 17.0 to 19.0% of Cr; less than 0.1% of P; less than 0.01% of S; 1.0 to 3.0% of Cu; 0.15 to 0.30% of N; and the balance being Fe and inevitable impurities, wherein a crack resistance index (CRN) value is 0 or more, and a Md30 value satisfies a range of -30 to 0°C.

Description

Low NI austenitic stainless steels excellent in hot workability and hydrogen embrittlement resistance

The present invention relates to a low-Ni austenitic stainless steel having a reduced Mn content, and more particularly, to an austenitic stainless steel improved in hot workability and hydrogen embrittlement which can be caused by reduction of Mn and Ni contents.

The work hardening type metastable austenitic stainless steels represented by STS304 and STS301 have excellent workability and corrosion resistance and are widely used in various applications. However, these steels have a disadvantage of high raw material costs due to high Ni content. Especially in recent years, raw material supply and demand are unstable due to extreme fluctuations in Ni raw material values, and material prices fluctuate accordingly, making it impossible to secure supply stability. Therefore, development of Ni-austenitic stainless steels with reduced Ni content from various materials suppliers is required.

Conventional Ni-austenitic stainless steels generally contain at least 5% by weight of Mn in order to reduce the cost of the material by reducing the amount of Ni to a certain amount or less and at the same time to ensure the stability of the austenite phase with decreasing Ni. However, when a large amount of Mn is added, a large amount of Mn fume is required during the steelmaking process to improve the environment. In addition, there is a problem of deteriorating mechanical properties such as elongation as well as deterioration of productivity in a manufacturing process and deterioration of surface corrosion resistance of a final cold-rolled material due to the formation of a rigid inclusion (MnS) due to the content of high Mn.

In order to solve this problem, it is preferable to reduce the Mn content. However, when the Mn content is reduced by a certain amount or more in the Ni-reduced austenitic stainless steel, the stability of the austenite phase is lowered and a large amount of delta ferrite is formed during casting , Which can lead to quality problems such as slab edge cracks and surface flaws during hot rolling.

In addition, in the case of a product requiring a highly corrosion-resistant surface, it is required to maintain the surface property formed in the final cold rolling up to the final product. When a brilliant annealing process in a hydrogen atmosphere is applied in such a manner as to maintain the final cold-rolling quality and surface properties and ensure good annealing property by proper annealing, the hydrogen- There is a problem that can be done.

Embodiments of the present invention solve the above problems and provide a low-Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance even when Mn is reduced.

Further, it is intended to provide a low-Ni austenitic stainless steel capable of securing corrosion resistance at a level of STS304 or STS301.

The low-Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance according to one embodiment of the present invention contains 0.05 to 0.15% of C, 0.2 to 0.7% of Si, 2.0 to 5.0% of Mn, Wherein the alloy contains 2.0 to 5.0% of Ni, 17.0 to 19.0% of Cr, less than 0.1% of S, less than 0.01% of S, 1.0 to 3.0% of Cu, 0.15 to 0.30% of N and Fe and unavoidable impurities. The value of the CRN value of the crack resistance index (CRN) represented by the formula (1) is 0 or more, and the Md30 value represented by the following formula (2) satisfies the range of -30 to 0 占 폚.

(1) CRN = 615 + 777C-26.3Si-1.8Mn + 46.2Ni-56Cr + 33.3Cu + 767N

(2) Md30 = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu)

Further, according to an embodiment of the present invention, the PREN value of formula (3) may satisfy 18 or more.

(3) PREN = Cr + 16N - 0.5Mn

Also, according to one embodiment of the present invention, it may further include at least one of 0.001 to 0.005% of B, and 0.001 to 0.003% of Ca in terms of% by weight.

Also, according to an embodiment of the present invention, the stainless steel may have an elongation of 50% or more.

The low-Ni austenitic stainless steels having excellent hot workability and hydrogen embrittlement resistance according to the embodiment of the present invention are capable of securing excellent hot workability by suppressing the formation of delta ferrite during reheating of the slab, and as a result, It is possible to solve the occurrence of minor cracks and quality problems.

In addition, excellent hydrogen embrittlement resistance and workability can be ensured even if the production of the processed organic martensite due to the austenite phase stability is suppressed and the brass annealing process in the hydrogen atmosphere is carried out.

In addition, excellent corrosion resistance at the level of STS304 or STS301 can be secured.

FIG. 1 is a graph showing changes in martensite peak intensity and occurrence of hydrogen embrittlement according to Md30.

2 is a graph showing elongation change according to Md30.

The low-Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance according to one embodiment of the present invention contains 0.05 to 0.15% of C, 0.2 to 0.7% of Si, 2.0 to 5.0% of Mn, Wherein the alloy contains 2.0 to 5.0% of Ni, 17.0 to 19.0% of Cr, less than 0.1% of S, less than 0.01% of S, 1.0 to 3.0% of Cu, 0.15 to 0.30% of N and Fe and unavoidable impurities. The value of the CRN value of the crack resistance index (CRN) represented by the formula (1) is 0 or more, and the Md30 value represented by the following formula (2) satisfies the range of -30 to 0 占 폚.

(1) CRN = 615 + 777C-26.3Si-1.8Mn + 46.2Ni-56Cr + 33.3Cu + 767N

(2) Md30 = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu)

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

In order to solve the above-mentioned problems, the inventors of the present invention have made a correlation analysis between experimental hot workability evaluation results and predicted delta (δ) ferrites for various alloy components by using a crack resistance index (CRN) In addition, the stability of the austenite phase for each alloy component was examined to predict the hydrogen embrittlement resistance of the brass annealed material. In addition, by predicting the corrosion resistance using the internal resistance index (PREN), an alloy component having not only excellent hot workability but also excellent workability and corrosion resistance was derived.

The low-Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance according to one embodiment of the present invention contains 0.05 to 0.15% of C, 0.2 to 0.7% of Si, 2.0 to 5.0% of Mn, 2.0 to 5.0% of Ni, 17.0 to 19.0% of Cr, less than 0.1% of P, less than 0.01% of S, 1.0 to 3.0% of Cu, 0.15 to 0.30% of N and Fe and unavoidable impurities.

Hereinafter, the reason for limiting the numerical value of the content of the component component in the embodiment of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.05 to 0.15%.

C not only deteriorates the cold workability due to the effect of solid solution strengthening during the addition of an element effective for stabilizing the austenite phase, but also induces precipitation of Cr carbide due to latent heat after hot-coil coiling, Corrosion resistance and the like can be adversely affected. Therefore, the upper limit is set to 0.15%. In addition, as described above, it is preferable to add at least 0.05% to stabilize the austenite phase.

The content of Si is 0.2 to 0.7%.

Si plays a role of deoxidizing agent in steelmaking process and is effective to improve corrosion resistance. However, Si is an effective element for stabilizing the ferrite phase, and when added in excess, promotes the formation of delta (δ) ferrite in the cast slab, thereby lowering the hot workability and lowering the ductility and toughness of the steel due to the solid solution strengthening effect. Therefore, the upper limit is set to 0.7%.

The content of Mn is 2.0 to 5.0%.

Mn is an austenite phase stabilizing element generally added to Ni, which is effective for suppressing the formation of machined organic martensite to improve the cold rolling property, and its characteristics are effective when added at 2.0% or more. However, the amount of S inclusions (MnS) is increased in the case of excessive addition, resulting in a decrease in softness, toughness and corrosion resistance of the steel, and the upper limit is set to 5.0%.

The content of Ni is 2.0 to 5.0%.

Ni is an essential element for securing good hot workability and cold workability as an austenite phase stabilizing element. In particular, even if Mn is added in a certain amount or more, addition of 2.0% or more is essential. However, Ni is a costly raw material, which causes an increase in raw material costs when added in large amounts. The upper limit is set to 5.0%.

The content of Cr is 17.0 to 19.0%.

Cr is not only an element necessary for securing the corrosion resistance required for stainless steel but also effective for inhibiting the formation of martensite phase, and its characteristics are effective when added over 17.0%. On the other hand, the addition of a large amount promotes the formation of delta (delta) ferrite in the slab, resulting in a reduction in hot workability, so that the upper limit is set to 19.0%.

The content of P is less than 0.1%.

As the corrosion resistance and hot workability are deteriorated, the upper limit of P is set to 0.1%.

The content of S is less than 0.01%.

The upper limit of S is set to 0.01% as the corrosion resistance and hot workability are lowered.

The content of Cu is 1.0 to 3.0%.

Cu is an austenite phase stabilizing element and is effective for softening the material. In order to exhibit such a softening effect, addition of 1.0% or more is essential. However, the addition of a large amount of Cu increases the cost of the material as well as the hot brittleness, thereby setting the upper limit to 3.0%.

The content of N is 0.15 to 0.30%.

N is an element effective for stabilizing the austenite phase and improving the corrosion resistance. When 0.15% or more is added, its characteristics are effective. On the other hand, when the excess amount of N is added, the cold workability is lowered due to the solid solution strengthening effect, and the upper limit is set to 0.30%.

Also, according to an embodiment of the present invention, at least one of 0.001 to 0.005% of B and 0.001 to 0.003% of Ca may be further included.

B is an effective element for suppressing the occurrence of cracks during casting and securing a good surface quality, and its characteristics are effective when 0.001% is added. On the other hand, when B is excessively added, nitride (BN) is formed on the surface of the product during the annealing / pickling process, resulting in a problem of deteriorating the surface quality. The upper limit is set to 0.005%.

Ca improves the cleanliness of the product by inhibiting the formation of MnS hard inclusions produced in the grain boundaries in the presence of high Mn, and its characteristics are effective when 0.001% is added. On the other hand, the excessive amount of Ca causes a decrease in hot workability due to the formation of Ca-based inclusions and a decrease in product surface quality, so that the upper limit is set to 0.003%.

It is known that the hot workability of such a high Mn low-Ni-containing austenitic stainless steel is correlated with the delta (?) Ferrite fraction distributed in the slab. This is a crack that occurs when the austenite and ferrite in a high temperature region exist in a complex state and the deformation resistance of each phase differs in the deformation amount applied in the rolling process. In order to secure hot workability, generation of delta (delta) ferrite is suppressed It is necessary to design alloy components and to derive hot working conditions. However, referring to the above-mentioned component system characteristics of the present invention, a large amount of solid solution strengthening elements such as C and N are likely to cause a large amount of cracks on the surface of the material due to a high hot rolling load at the time of hot working at a low temperature. Therefore, it is preferable to operate at a hot rolling temperature that does not cause occurrence of operation abnormality at the time of operation.

Specifically, the surface and edge quality of the hot rolled sheet was checked to determine whether cracks were formed or not, and the phase fraction of delta (δ) ferrite was predicted by phase analysis using computational simulation of alloy components. Respectively. The range of crack resistance index (CRN) expressed by Equation (1) is derived from the correlation between the experimental hot workability evaluation result and the predicted delta (δ) ferrite.

The low Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance according to one embodiment of the present invention has a value of crack resistance index (CRN) of 0 or more expressed by the following formula (1).

(1) CRN = 615 + 777C-26.3Si-1.8Mn + 46.2Ni-56Cr + 33.3Cu + 767N

When the crack resistance index (CRN) is 0 or more, cracks may not occur on the surface and the edge portion during hot working.

On the other hand, as described above, in the case of a product requiring a beautiful surface, the cold rolled steel sheet is generally subjected to bright annealing. Such a brass annealing is performed by heat treatment in a reducing atmosphere (Dew point -40 to -60 ° C.) using nitrogen (N ₂ ), hydrogen (H ₂ ) or the like, in the stainless steel cold rolled steel sheet, By preventing reoxidation, it is a heat treatment technology that keeps the surface bright and beautiful without changing the color and properties of the surface. As the atmospheric gas used for such brilliant annealing, brass annealing using hydrogen is the most common, because it is most widely used not only for high heat capacity but also for suppressing discoloration of the surface.

There is a point to be considered in application of the brass annealing of a hydrogen atmosphere to a stainless steel having a relatively reduced content of Ni and Mn as compared with a general austenitic stainless steel. This is due to the fact that the ultimate workpiece is likely to suffer from workability damping due to hydrogen embrittlement defects due to the penetration of hydrogen during the light annealing. In the case of stainless steels in which the austenite phase stabilizing elements such as Ni and Mn are reduced, the stressed organic or modified organic martensite is formed around the surface layer during the cold rolling before the final brass annealing, and the martensite phase formed in the surface layer is heat- Before being transformed into the austenite phase by the hydrogen atom, this hydrogen atom penetrates into the inside of some martensite phase. As the stress-induced or modified organic martensite is transformed into an austenite phase by brilliant annealing, the hydrogen atoms penetrated inside are not released to the outside but trapped in the atomic state in the surface layer. The hydrogen atoms penetrating into the surface layer are bare-out after a lapse of a certain time at room temperature on the ferrite or martensite phase, which is a general BCC and BCT structure, so that the hydrogen atoms do not greatly affect the physical properties. On the other hand, when the surface martensite phase is transformed into an austenite phase by the optical annealing, that is, when hydrogen atoms are present in the FCC lattice structure, the natural baking out of the hydrogen atoms is not smooth, do.

These hydrogen atoms are known to be factors that cause hydrogen embrittlement. Hydrogen atoms trapped in the material due to some processing or deformation are changed into hydrogen molecules. When a certain pressure is reached, cracks Acting as a starting point, causing the elongation to decrease.

Therefore, in the case of austenitic stainless steels having a relatively low Ni and Mn, it is necessary to control the amount of martensite phase formed on the surface due to work hardening in addition to the alloy component, thereby ensuring a good surface quality through the brightness annealing, It is possible.

Therefore, the low-Ni austenitic stainless steel excellent in hot workability and hydrogen embrittlement according to one embodiment of the present invention satisfies the Md30 value represented by the following formula (2) in the range of -30 to 0 占 폚.

(2) Md30 = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu)

In metastable austenitic stainless steels, martensitic transformation occurs by sintering at temperatures higher than the martensitic transformation starting temperature (Ms). The upper limit temperature causing the phase transformation by this processing is represented by the Md value, and the temperature (° C) at which 50% of the phase transformation to the martensite occurs when 30% deformation is given is referred to as Md30. When the Md30 value is high, it is easy to produce the processed organic martensite phase. On the other hand, if the Md30 value is low, it can be judged that it is relatively difficult to produce the processed organic martensite phase. The Md30 value is used as an index for determining the austenite stabilization degree of a conventional metastable austenitic stainless steel.

The correlation between Md30 indicating the degree of austenite stabilization and the amount of martensite phase introduced during cold rolling and the occurrence of hydrogen embrittlement due to the amount of martensite phase introduced into the surface layer during the light annealing The results of the experimental evaluation are shown in Fig.

Fig. 1 is a graph showing changes in martensite peak intensity and occurrence of hydrogen embrittlement according to Md30. The symbol? Indicates that hydrogen embrittlement has not occurred, and the symbol x indicates that hydrogen embrittlement has occurred. As the Md30 value increases, the peak intensity on the surface martensite due to a decrease in the phase stability of austenite increases, and when the peak intensity increases above a certain value, it is confirmed that hydrogen embrittlement occurs in the light annealing in a hydrogen atmosphere . Based on these results, it is confirmed that it is preferable to keep the Md30 value at 0 DEG C or less to suppress the hydrogen embrittlement.

Also, as in the present invention, in the case where a large amount of C and N is inevitably added to improve the phase stability of austenite as a result of reduction in the contents of Mn and Ni relative to conventional STS304 and STS301, the Md30 value can be reduced, The hardenability is increased and it is difficult to secure a desired elongation. Especially, it is necessary to control the lower limit of Md30 value in consideration of the fact that it is necessary to secure elongation of about 50% or more in general 300-series stainless steel applications.

FIG. 2 is a graph showing the elongation change according to Md30. In FIG. 2, the elongation is 50% or more and the elongation is less than 50%.

Referring to FIG. 2, it is confirmed that it is desirable to control the Md30 value to -30 ° C or more in order to secure an elongation of 50% or more in the alloy component range.

Also, as it is required to have similar corrosion resistance as existing STS304 or STS301, it is necessary to maintain the internal formula resistance index (PREN) value for the alloy component above a certain level.

Thus, according to an embodiment of the present invention, the PREN value of formula (3) may satisfy 18 or more.

(3) PREN = Cr + 16N - 0.5Mn

Hereinafter, preferred embodiments of the present invention will be described in detail.

Example

For the various alloy composition ranges shown in Table 1 below, a 200-ton slab was prepared and reheated at 1,230 ° C for 2 hours, followed by hot rolling to a thickness of 3t. The surface and edge quality of these hot rolled materials were checked to determine the index of hot workability through the occurrence of cracks. In addition, phase fraction of delta (δ) ferrite was predicted by phase analysis of alloy components. The crack resistance index (CRN) of the formula (1) derived from the correlation between the experimental hot workability evaluation result and the predicted delta (δ) ferrite was derived and shown in Table 2. The elongation was measured using a No. 5 test piece specified in JIS Z 2201 according to the tensile test method of a metal material specified in Japanese Industrial Standard JIS Z 2241.

division	C	Si	Mn	Ni	Cr	Cu	N
Comparative Example 1	0.095	0.46	2.91	3.48	17.8	1.58	0.16
Comparative Example 2	0.076	0.45	3.01	2.9	18.1	1.61	0.22
Comparative Example 3	0.102	0.43	4.37	3.54	18.1	1.44	0.25
Comparative Example 4	0.098	0.47	3.95	3.68	18.0	1.54	0.23
Example 1	0.097	0.46	3.42	3.48	18.1	1.43	0.19
Example 2	0.096	0.47	3.83	3.45	18.2	1.47	0.23
Example 3	0.096	0.44	3.94	3.31	18.2	1.5	0.21
Example 4	0.087	0.44	2.95	3.44	17.9	1.48	0.244

division	Crack occurrence	CRN	Md30 (占 폚)	Whether hydrogen embrittlement occurs	Elongation (%)	PREN
Comparative Example 1	×	10.8	14.8	○	38.0	18.9
Comparative Example 2	○	-0.47	7.0	○	46.0	20.1
Comparative Example 3	×	64.7	-43.4	×	46.8	19.9
Comparative Example 4	×	61.4	-34.8	×	49.7	19.7
Example 1	×	12.6	-3.9	×	54.3	19.4
Example 2	×	35.9	-27.0	×	50.2	20.0
Example 3	×	15.7	-15.2	×	52.5	19.6
Example 4	×	58.7	-17.8	×	50.5	20.3

In Comparative Example 2, cracks were generated in the surface and edge portions in the hot rolling and the CRN (Crack Resistance Index) was -0.47. In Comparative Examples 1, 3 and 4, cracks were not generated on the surface and the edge portion in hot rolling, and when the crack resistance index (CRN) derived according to the delta (delta) ferrite phase fraction was 0 or more, It is confirmed that this is an index.

Referring to FIG. 1 together with Tables 1 and 2, in the case of Comparative Examples 1 and 2, the peak intensity on the surface martensite of the cold rolled steel sheet was increased due to the Md30 value calculated from the component system exceeding 0 ° C, , Which is indicated by X in Fig. 1. On the other hand, in the case of Comparative Examples 3 and 4, the value of Md30 satisfies 0 deg. C or less and hydrogen embrittlement does not occur, but the Md30 value is less than -30 DEG C and the elongation is measured to be less than 50%. From this, it was found that the range of Md30 value should satisfy the range of -30 to 0 占 폚 so that the workability condition according to the hydrogen embrittlement resistance and the elongation of 50% or more can be obtained.

On the other hand, in the range of the composition range according to the present invention, the PRES value according to the formula (3) satisfies the value of 18 or more, so that excellent corrosion resistance at the STS304 level can be secured.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

The low-Ni austenitic stainless steels according to the embodiments of the present invention can secure excellent corrosion resistance and processability even when Mn is reduced, and can be applied to various applications such as household appliances.

Claims (4)

1. 0.05 to 0.15% of C, 0.2 to 0.7% of Si, 2.0 to 5.0% of Mn, 2.0 to 5.0% of Ni, 17.0 to 19.0% of Cr, less than 0.1% of S, less than 0.01% of S 1.0 to 3.0% of Cu, 0.15 to 0.30% of N, the balance of Fe and unavoidable impurities,

   Wherein a value of a crack resistance index (CRN) represented by the following formula (1) is 0 or more,

   A low Ni austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance having an Md30 value represented by the following formula (2) satisfying the range of -30 to 0 占 폚:

   (1) CRN = 615 + 777C-26.3Si-1.8Mn + 46.2Ni-56Cr + 33.3Cu + 767N

   (2) Md30 = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu)

   Here, C, Si, Mn, Ni, Cr, Cu and N mean the content (weight%) of each element.
2. The method according to claim 1,

   A low-Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance satisfying an internal resistance index (PREN) value of 18 or more represented by the following formula (3)

   (3) PREN = Cr + 16N - 0.5Mn

   Here, Cr, N, and Mn mean the content (weight%) of each element.
3. The method according to claim 1,

   A low Ni austenitic stainless steel which further contains at least one of 0.001 to 0.005% of B and 0.001 to 0.003% of Ca, in terms of% by weight, and is excellent in hot workability and hydrogen embrittlement resistance.
4. The method according to claim 1,

   The stainless steel is a low-Ni austenitic stainless steel excellent in hot workability and hydrogen embrittlement with an elongation of 50% or more.

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