KR102020399B1 - High-strength ferritic stainless steel and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a high-strength ferritic stainless steel sheet used in automobiles, fasteners for general hoses, and the like, and a method of manufacturing the same.
More specifically, the present invention provides a ferritic stainless steel sheet having a high strength with a tensile strength of 520 MPa or more and no surface defects caused by inclusions, and having a high cleanness, and a method of manufacturing the same.

Description

High strength ferritic stainless steel sheet and manufacturing method thereof {HIGH-STRENGTH FERRITIC STAINLESS STEEL AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a high-strength ferritic stainless steel sheet used in automobiles, fasteners for general hoses, and the like, and a method of manufacturing the same.

Austenitic stainless steels, which are mainly used as stainless steels, are well known STS 304 series, which are commonly used in building interior and exterior materials such as elevators. However, STS 304 steel has a high content of high elemental nickel (Ni), which is disadvantageous in price competitiveness.

Thus, ferritic steel, that is, ferritic stainless steel, which has similar characteristics to that of STS 304 steel and has a competitive price, is increasingly used.

In addition, the ferritic stainless steel has a low thermal expansion rate, good surface gloss, moldability, and oxidation resistance, and thus is widely used in heat-resistant appliances, sink top plates, exterior materials, home appliances, and electronic products.

On the other hand, fasteners (Fasteners) used in automobiles or general hoses are a kind of parts for fastening plastic hoses or pipes, and need high strength and have no defects such as cracks when bending. Therefore, it is required that the material used for the fastener or the like be free of plastic discontinuous sites such as inclusions.

In recent years, the use of stainless steel as a material for fasteners and the like is increasing due to the demand for corrosion resistance as it is used not only indoors but also in outdoor environments such as automobiles.

Low chromium ferritic stainless steel is a steel containing about 11% by weight of chromium (Cr), and has a disadvantage in that corrosion resistance is poor because it reduces the content of nickel, chromium, etc., compared to conventional stainless steels.

Thus, attempts have been made to use a relatively high content of 430 (Cr) based 430 series as a fastener, but there is a limitation in application due to low tensile strength.

Therefore, there is a demand for the development of stainless steel that can satisfy high cleanliness while having high strength to be suitable for fastener applications such as automobiles or general hoses.

Japanese Patent Publication No. 4428550

An aspect of the present invention is to provide a ferritic stainless steel sheet having a high strength of 520 MPa or more and no surface defects due to inclusions and having a high cleanness, and a method of manufacturing the same.

One aspect of the present invention, in weight%, carbon (C): 0.065 to 0.090%, nitrogen (N): 0.035 to 0.060%, silicon (Si): 0.6 to 1.0%, manganese (Mn): 0.01 to 0.70% , Aluminum (Al): 0.06 ~ 0.15%, Chromium (Cr): 15.0 ~ 17.0%, provides a high-strength ferritic stainless steel sheet containing the remaining iron (Fe) and unavoidable impurities, satisfying the following relations 1 to 4.

[Relationship 1]

2 [Si] + 10 ([C] + [N]) ≥ 2.2

[Relationship 2]

[Si] / [Al] ≤ 15

[Relationship 3]

[Al] / [N] ≥ 2.5

[Relationship 4]

75 ≤ ([Al] / [N]) × γ ≤ 150

(γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]-12.0 [Mo]-52.0 [ Al], and each element of the formulas 1 to 4 means a weight content.)

Another aspect of the invention, the step of preparing a cold rolled steel sheet that satisfies the above-described alloy composition and relations 1 to 4; Annealing the cold rolled steel sheet at 830˜980 ° C .; And it provides a method for producing a high strength ferritic stainless steel sheet comprising the step of bright annealing the cold rolled steel sheet at 800 ~ 900 ℃ in a reducing atmosphere.

According to the present invention, it is possible to provide a high-quality ferritic stainless steel sheet having a high or high leasing grade.

The ferritic stainless steel sheet of the present invention can be advantageously used as a material for fasteners for automobiles or general hoses.

1 is a graph showing the relationship between the value of relational expression 1 and the tensile strength according to an embodiment of the present invention.
Figure 2 graphically illustrates the relationship between Equation 2 and inclusion stretch length in accordance with one embodiment of the present invention.
3 is a graph showing a relationship between relation 4 and a lowering characteristic according to an embodiment of the present invention.

The inventors of the present invention have studied in depth to provide a ferritic stainless steel having high strength to be suitable for use as a fastener for automobiles, general hoses, etc., and without surface defects caused by inclusions.

As a result, by optimizing the alloy component relationship of ferritic stainless steel, it can be confirmed that it is possible to provide a high-purity ferritic stainless steel sheet having high gloss and high gloss and without defects such as surface slivers due to inclusions. The present invention has been completed.

On the other hand, the present invention reveals that the steel does not contain Ni, Mo and the like in the alloy composition. However, it may contain a very small amount of impurities.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In accordance with an aspect of the present invention, the high-strength ferritic stainless steel sheet is in weight percent, carbon (C): 0.065 to 0.090%, nitrogen (N): 0.035 to 0.060%, silicon (Si): 0.6 to 1.0%, manganese ( Mn): 0.01 to 0.70%, aluminum (Al): 0.06 to 0.15%, chromium (Cr): 15.0 to 17.0%, and the content relation of some components is proposed in the present invention (hereinafter, relations 1 to 4). It is preferable to satisfy).

Hereinafter, the reason for restricting the alloy composition and the component relationship of the high strength ferritic stainless steel sheet of the present invention as described above will be described in detail. At this time, the content of each component means weight% unless otherwise specified.

C: 0.065 ~ 0.090%

Carbon (C) is an element advantageous for improving the strength of steel, and precipitates as (Cr, Fe) ₂₃ C ₆ , (Cr, Fe) ₇ C ₃ carbide in the annealing process to improve strength. In order to sufficiently obtain such an effect, it is preferable to include C in an amount of 0.065% or more, but when the content is excessively more than 0.09%, there is a problem that the workability is inferior by lowering the elongation of steel.

Therefore, in the present invention, the C may be included as 0.065 to 0.090%.

N: 0.035-0.060%

Nitrogen (N) can be precipitated as a nitride of Cr ₂ N in the annealing process similar to the carbon can be improved in strength. To this end, it is preferable to include N in more than 0.035%, but if the content exceeds 0.060% excessively, there is a problem that not only inhibits the workability, but also causes a stretcher strain defect of the cold-rolled product.

Therefore, in the present invention, the N may include 0.035 to 0.060%.

Si: 0.6 ~ 1.0%

Silicon (Si) is added for the deoxidation effect in steelmaking and is a ferrite stabilizing element. In general, when a large amount of Si is contained in the stainless steel, hardening of the material may be caused to inhibit ductility, so that the amount is controlled to 0.4% by weight or less.

However, in the present invention, by increasing the content of Si to improve the tensile strength and yield strength by the solid solution strengthening effect as one means. To this end, according to one aspect of the present invention may include Si more than 0.6%. However, when the content exceeds 1.0%, there is a problem that the elongation of the steel is lowered.

Therefore, in the present invention, the Si may be included in 0.6 to 1.0%.

Mn: 0.01 ~ 0.70%

Manganese (Mn) is an austenite stabilizing element and serves to suppress roping and ridging. For this purpose, it is preferable to include more than 0.01%, but if the content exceeds 0.70%, the manganese fume (fume) is generated during welding, causing the MnS precipitation, there is a problem that the elongation is lowered.

Therefore, in the present invention, the Mn may be contained in an amount of 0.01 to 0.70%.

Al: 0.06 ~ 0.15%

Aluminum (Al) is an element added as a deoxidizer during steelmaking. In one aspect of the present invention, Al may be included in an amount of 0.06% or more to control the number and length of stretch inclusions generated during Si deoxidation. However, when the content exceeds 0.15%, there is a problem of lowering austenite fraction and inferior degumming.

Therefore, in the present invention, the Al may be included in an amount of 0.06 to 0.15%.

Cr: 15.0-17.0%

Chromium (Cr) is an element added to improve the corrosion resistance of steel. In the ferritic stainless steel, the critical content of Cr is 12% by weight, but in one aspect of the present invention, since the grain boundary corrosion may occur due to the inclusion of C and N, Cr may be included in an amount of 15.0% or more. However, if the content exceeds 17.0%, the steel manufacturing cost rises and becomes economically disadvantageous.

Therefore, in the present invention, the Cr may include 15.0 to 17.0%.

In the ferritic stainless steel sheet of the present invention that satisfies the above-described alloy composition, it is preferable that the component relations of some elements satisfy the following Equations 1 to 4.

[Relationship 1]

2 [Si] + 10 ([C] + [N]) ≥ 2.2

[Relationship 2]

[Si] / [Al] ≤ 15

[Relationship 3]

[Al] / [N] ≥ 2.5

[Relationship 4]

75 ≤ ([Al] / [N]) × γ ≤ 150

(γ is an austenite fraction, γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]- 12.0 [Mo] -52.0 [Al], and each of the elements of the relations 1 to 4 represents a weight content.)

First, the tensile strength of 520 MPa or more can be secured by controlling the relationship between the contents of C, N, and Si, which affects the strength improvement of the steel, by using the above relational expression 1.

If the value of the relation 1 is less than 2.2, the strength cannot be sufficiently secured.

Next, the length control of the elongated inclusions is possible by controlling the relationship between the content of Si and Al, which affects the formation of the inclusions, using the above expression (2).

If the value of the relation 2 exceeds 15, that is, if the content of Si is excessively high compared to Al, it is impossible to control the length of the stretched inclusions, which may result in inferior surface quality of the steel.

Next, for the purpose of ensuring excellent dropping property of the steel, the component relationship between Al and N is controlled by the above expression 3, and at the same time [γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]-12.0 [Mo]-52.0 [Al]] to control the relationship with the austenite phase fraction expressed by the above equation 4 It is preferable.

If the value of the relation 3 is less than 2.5 or the value of the relation 4 does not satisfy 75 to 150, the unloading grade of the steel sheet cannot be secured to the 1 or 2 grade, and there is a risk of the occurrence of the strainer strain. Increases.

The remaining component of the present invention is iron (Fe). However, in the usual steel manufacturing process, impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

The ferritic stainless steel sheet of the present invention satisfying the above-described alloy composition and relations 1 to 4 includes a ferrite phase as a main phase, and preferably includes some austenite phase.

The austenite phase may include an area fraction of 30 to 50%.

In addition, the ferritic stainless steel sheet of the present invention may include inclusions whose stretched length is controlled to 20 ~ 200㎛, in this way by controlling the stretched length of the inclusions can be obtained the effect of preventing defects by inclusions. .

More preferably, the stretching length of the stretching inclusion observed in the thickness direction center part of the final product satisfies 20 to 200 µm within an area of 100 µm x 10,000 µm.

In this case, the inclusions are Si inclusions, for example, SiO inclusions, but is not limited thereto.

As described above, the ferritic stainless steel sheet of the present invention may have a tensile strength of 520 MPa or more, and have a lowering grade of 1 to 2 grades.

Hereinafter, a method of manufacturing a high strength ferritic stainless steel sheet which is another aspect of the present invention will be described in detail.

The high strength ferritic stainless steel sheet of the present invention can be produced using a method for producing a conventional ferritic stainless steel. For example, the prepared steel slab may be hot rolled to produce a hot rolled product, and then subjected to annealing for the hot rolled product and then cold rolled to produce a cold rolled product. Thereafter, the produced cold rolled product may be subjected to a bright annealing process. On the other hand, before annealing and annealing with respect to a hot rolled product, bath annealing can be performed further, This also can be performed on normal conditions.

At this time, the annealing heat treatment is preferably carried out within a few minutes in the temperature range of 830 ~ 980 ℃.

If the temperature during the annealing heat treatment is less than 830 ℃ recrystallization is inhibited or the ferrite of the form pressed in the rolling direction is present as it may deteriorate characteristics such as ductility. Therefore, it is preferable to perform annealing heat treatment at 830 degreeC or more, and it is not necessary to specifically limit the upper limit temperature, It can limit to 980 degreeC in consideration of energy efficiency, manufacturing cost, etc. In addition, the holding time in the above temperature range can be limited to within 2 minutes, preferably 10 to 60 seconds.

After completing the annealing heat treatment according to the above, it is possible to remove the scale formed on the surface of the steel by pickling.

At this time, the electrolytic pickling may be used as the pickling process, preferably, the neutral salt electrolytic pickling passing through the neutral salt electrolyzer and the sulfuric acid electrolytic pickling passing through the sulfuric acid electrolytic bath are then immersed in a mixed acid bath containing hydrofluoric acid in some cases. A mixed acid deposition step can be performed.

The neutral salt electrolytic pickling, sulfuric acid electrolytic pickling and mixed acid deposition step can be carried out under generally performed conditions, and the conditions are not particularly limited.

After completion of the annealing heat treatment or after completion of the pickling step, a bright annealing step can be performed, which is preferably performed at a temperature range of 800 to 900 ° C. in a reducing atmosphere.

When the bright annealing temperature is less than 800 ° C., there is a problem that the elongation is inferior due to lack of recrystallization, whereas when the temperature exceeds 900 ° C., the elongation is inversely transformed into austenite and the elongation is inferior.

In addition, the reducing atmosphere is preferably 100% hydrogen atmosphere.

As described above, the bright annealing heat treatment in the present invention is carried out in a reducing atmosphere, so that the stainless steel sheet is formed with a passivation film having a smooth surface state of several nm thickness, not in the form of a high temperature oxidation scale having a thickness of several μm formed in a normal oxidizing atmosphere. To be manufactured.

After the light annealing is completed under the above-described conditions, temper rolling can be further performed. In one aspect of the invention it can be carried out in 1-pass during the temper rolling.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. 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 )

After preparing a cold rolled steel sheet having the alloy composition and the component relationship of Table 1, each cold rolled steel sheet annealing heat treatment at 840 ℃. Thereafter, bright annealing was performed at 840 ° C. in a 100% hydrogen atmosphere.

Tensile strength, ridging grades and stretcher strain defects were evaluated for each stainless steel sheet manufactured according to the above, and the results are shown in Table 2 below.

At this time, the tensile strength of the JIS 13 B specimens, and then subjected to a tensile test at a crosshead speed (crosshead speed) 20mm / min.

The leasing grade is the leaching height grade (Wt basis) measured after 15% tension of each stainless steel sheet. Grade 1 is less than 10 μm, Grade 2 is more than 10 μm and less than 15 μm, and grade 3 is more than 15 μm and less than 20 μm. 4 The grade is shown in the case of 20㎛ or more, where Grade 1 and Grade 2 correspond to the target range in the present invention.

In addition, 1 / 2t point (where t means thickness (mm)) of each stainless steel plate was observed with the optical microscope, and the average extending | stretching length of the interference | inclusion stretched in 100 micrometer x 10,000 micrometer area was measured.

division Alloy composition (% by weight) Relationship C Si Mn Cr Al N Relation
One Relation
2 Relation
3 Relation
4 Inventive Steel 1 0.065 0.6 0.3 16.2 0.10 0.035 2.2 6.0 2.86 81 Inventive Steel 2 0.085 0.9 0.3 16.2 0.11 0.035 3.0 8.2 3.14 103 Inventive Steel 3 0.075 0.6 0.3 16.2 0.15 0.040 2.35 4.0 3.75 121 Inventive Steel 4 0.080 0.8 0.3 16.2 0.09 0.035 2.75 8.9 2.57 85 Inventive Steel 5 0.090 0.9 0.3 16.2 0.09 0.035 3.05 10.0 2.57 92 Comparative Steel 1 0.040 0.7 0.3 16.2 0.09 0.030 2.1 7.8 3.0 45 Comparative Steel 2 0.030 0.3 0.3 16.2 0.09 0.040 1.3 3.3 2.25 45 Comparative Steel 3 0.090 1.0 0.3 16.2 0.06 0.035 3.25 16.7 1.71 63 Comparative Steel 4 0.020 0.8 0.3 16.2 0.09 0.045 2.25 8.9 2.0 25 Comparative Steel 5 0.070 1.0 0.3 16.2 0.09 0.045 3.15 11.1 2.0 62 Comparative Steel 6 0.070 0.8 0.3 16.2 0.01 0.040 2.7 80.0 0.25 9 Comparative Steel 7 0.070 0.7 0.3 16.2 0.08 0.035 2.45 8.8 2.29 69

(In Table 1, relations 1 to 4 are represented by [(2 × [Si]) + 10 ([C] + [N])], [[Si] / [Al]], [[Al] / [N]], respectively). , [([A] l / [N]) × γ], where γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]-12.0 [Mo]-52.0 [Al].

division Tensile Strength (MPa) Leasing Class Inclusion Stretch Length
(Μm) Stretcher strain
flaw Inventive Steel 1 522 Grade 2 62 Not Occurred Inventive Steel 2 564 Grade 1 70 Not Occurred Inventive Steel 3 529 Grade 2 35 Not Occurred Inventive Steel 4 547 Grade 2 85 Not Occurred Inventive Steel 5 566 Grade 1 102 Not Occurred Comparative Steel 1 517 Grade 4 68 Not Occurred Comparative Steel 2 489 Grade 5 27 Occur Comparative Steel 3 575 Grade 3 214 Occur Comparative Steel 4 524 Grade 4 204 Occur Comparative Steel 5 571 Grade 3 312 Occur Comparative Steel 6 547 Grade 4 407 Occur Comparative Steel 7 532 Grade 3 325 Occur

As shown in Table 1 and 2, the invention steels 1 to 5 satisfying both the alloy composition and the component relationship proposed in the present invention satisfies all of the tensile strength of 520MPa or more, the stretching length of the inclusions stretched in the thickness center It can confirm that 20-200 micrometers are satisfied. In addition, it can be seen that it is possible to provide a high-quality ferritic stainless steel sheet because the leasing grade satisfies all grades 1 and 2 and has excellent unloading characteristics, and no strainer strain defects occur.

On the other hand, Comparative steel 1 did not satisfy the relation 1 as the content of C and N out of the present invention showed a tensile strength of less than 520MPa, leasing grade was also inferior to the grade 4 grade.

Comparative steel 2 did not satisfy the relation 1 as the content of C and Si out of the present invention was very inferior in tensile strength and dripping. In addition, stretcher strain defects also occurred.

Comparative steels 3 to 7 did not satisfy at least one of the relations 2 to 4, and the inferior lagging property, the length of the stretched inclusions in the thickness center exceeded 200㎛, and the strainer strain defects occurred.

1 is a graph showing the relationship between the value of the relation 1 and the tensile strength, it can be seen that the tensile strength can be secured to 520MPa or more when the value of the relation 1 satisfies 2.2 or more.

FIG. 2 is a graph showing the relationship between the relationship 2 and the length of inclusion inclusions. When the value of the relationship 2 satisfies 15 or less, the length of the stretched inclusions may be controlled in the range of 20 to 200 μm.

3 is a graph showing the relationship between the relational expression 4 and the lowering characteristic, and when the value of the relational expression 4 satisfies 75 to 150, it is possible to secure the lagging property of the 1-2 grades.

Claims (7)

By weight%, carbon (C): 0.065 to 0.090%, nitrogen (N): 0.035 to 0.060%, silicon (Si): 0.6 to 1.0%, manganese (Mn): 0.01 to 0.70%, aluminum (Al): 0.06 0.15% chromium (Cr): 15.0-17.0%, containing the balance iron (Fe) and unavoidable impurities,
Satisfy the following relations 1 to 4,
A high strength ferritic stainless steel sheet comprising inclusions having an elongated length of 20 to 200 μm.

[Relationship 1]
2 [Si] + 10 ([C] + [N]) ≥ 2.2
[Relationship 2]
[Si] / [Al] ≤ 15
[Relationship 3]
[Al] / [N] ≥ 2.5
[Relationship 4]
75 ≤ ([Al] / [N]) × γ ≤ 150
(γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]-12.0 [Mo]-52.0 [ Al], and each element of the formulas 1 to 4 means a weight content.)
delete The method of claim 1,
The inclusion is a high strength ferritic stainless steel sheet is Si-based inclusions.
The method of claim 1,
The steel sheet is a high strength ferritic stainless steel sheet having a tensile strength of 520MPa or more, the leaching height is less than 15㎛.
By weight%, carbon (C): 0.065 to 0.090%, nitrogen (N): 0.035 to 0.060%, silicon (Si): 0.6 to 1.0%, manganese (Mn): 0.01 to 0.70%, aluminum (Al): 0.06 ˜0.15%, chromium (Cr): 15.0 to 17.0%, balance iron (Fe) and an unavoidable impurity, preparing a cold rolled steel sheet that satisfies the following relations 1 to 4;
Annealing the cold rolled steel sheet at 830˜980 ° C .; And
Bright annealing the cold rolled steel sheet at 800 ~ 900 ℃ in a reducing atmosphere
Method for producing a high strength ferritic stainless steel sheet comprising a.

[Relationship 1]
2 [Si] + 10 ([C] + [N]) ≥ 2.2
[Relationship 2]
[Si] / [Al] ≤ 15
[Relationship 3]
[Al] / [N] ≥ 2.5
[Relationship 4]
75 ≤ ([Al] / [N]) × γ ≤ 150
(γ (%) = 420 [C] + 470 [N] + 23 [Ni] + 9 [Cu] + 10 [Mn] + 180-11.5 [Cr]-11.5 [Si]-12.0 [Mo]-52.0 [ Al], and each element of the formulas 1 to 4 means a weight content.)
The method of claim 5,
The reducing atmosphere is 100% hydrogen atmosphere manufacturing method of high strength ferritic stainless steel sheet.
The method of claim 5,
After the annealing heat treatment, before the bright annealing, the method of manufacturing a high strength ferritic stainless steel sheet further comprising the step of pickling the cold rolled steel sheet.

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