WO2018030690A1 - Lean duplex stainless steel having excellent bending processability - Google Patents

Abstract

A lean duplex stainless steel having excellent bending processability is disclosed. According to one embodiment of the present invention, the lean duplex stainless steel comprises, by wt%, 0.01-0.06% of C, 0.2-1.0% of Si, 3.5-6.5% of Mn, 18.5-22.5% of Cr, 0.05-0.25% of N, and the balance of iron (Fe) and other inevitable impurities, wherein Cr + Mn is 26.0-28.5% and Cr/Mn is 3.4-4.1.

Description

Lean duplex stainless steel with excellent bendability

The present invention relates to a lean duplex stainless steel, and relates to a lean duplex stainless steel excellent in bending workability.

In general, austenitic stainless steel having good processability and corrosion resistance contains iron (Fe) as a base metal, and contains chromium (Cr) and nickel (Ni) as main raw materials, and molybdenum (Mo) and copper (Cu), etc. It is developed in various steel grades to suit various purposes by adding other elements of.

300-based stainless steel, which has excellent corrosion resistance and workability, includes expensive raw materials such as Ni and Mo, and as an alternative, 400-based stainless steel has been discussed, but moldability does not reach 300-based stainless steel. This exists. In the case of thick plate with less molding than heat / cold rolled material, it can be applied to corrosion resistance of 400 series depending on the usage environment.However, it is used as a thick plate due to the thermal shock characteristics and deterioration of welding part of 400 series stainless steel. There are many restrictions.

On the other hand, the duplex stainless steel mixed with the austenitic phase and the ferrite phase has all the advantages of the austenitic and ferritic systems, and various types of duplex stainless steel have been developed to date.

Alloy elements Ni, Mo and the like have a disadvantage of expensive price, and the interest in cheap stainless steel is increasing steadily. As a result, attempts have been made to develop lean alloys with low content of expensive alloying elements. This trend is also seen in duplex stainless steels with microstructures consisting of ferrite and austenite phases.

Patent document 1 relates to an austenitic-ferritic stainless steel characterized by a low Ni content and a high N content. The lean duplex stainless steel has high elongation while maintaining high strength by controlling the stability of the austenite phase. Characterized in that. Lean duplex stainless steel has excellent corrosion resistance and has high strength characteristics due to the grain size reduction effect by the formation of two-phase structure, and its use is gradually increasing.

Stainless oral tubes or structural tubes have a variety of shapes, depending on the application, and also the required materials (corrosion resistance and formability). Therefore, 400 series, 200 series and 300 series stainless steels are used according to the shape and material requirements, and 400 series and 200 series stainless steels, which are inexpensive, are mainly used.

Lean Duplex stainless steel is expected to be applied not only to outdoor decorative pipes with excellent corrosion resistance but also to structural pipes due to its high strength characteristics, but due to its relatively high price and easy cracking when bending, compared to 400 and 200 stainless steels. Use has been limited. Therefore, in order to replace 400 series and 200 series stainless steel tubes, it is necessary to further reduce the use of expensive alloy elements, and in particular, it is necessary to secure bending workability to make a tube having a complicated cross-sectional shape such as a rectangular shape. Is required.

 (Patent Document 1) Korean Unexamined Patent Publication No. 10-2009-0005252 (published Jan. 12, 2009)

Embodiments of the present invention are austenitic and ferritic phase that can improve the bending workability of stainless steel by minimizing the content of expensive alloying elements such as Ni, Cu, Mo, etc. and optimizing the sum and ratio of Cr, Mn in the component system of the duplex stainless steel An object is to provide a lean duplex stainless steel with a two-phase structure.

Lean duplex stainless steel having excellent bending workability according to an embodiment of the present invention, in weight%, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 3.5 to 6.5%, Cr: 18.5 to 22.5%, N: 0.05 to 0.25%, balance iron (Fe) and other unavoidable impurities, Cr + Mn: 26.0 to 28.5%, Cr / Mn: 3.4 to 4.1.

In addition, according to an embodiment of the present invention, Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.5% or less.

In addition, according to one embodiment of the present invention, the volume fraction of the ferrite matrix in the microstructure may be 50 to 75%.

In addition, according to an embodiment of the present invention, the elongation of the stainless steel may be 30 to 40%.

Embodiments of the present invention can manage the alloy components, such as Ni, Cu, Mo, etc. in the component system of the duplex stainless steel to minimize or exclude these components to save resources and minimize the manufacturing cost of the duplex stainless steel.

In addition, the bendability of the stainless steel can be improved by optimizing the sum and ratio of Cr and Mn.

1 is a photograph of the processed surface after 180 ° bending by using a lean duplex stainless steel according to an embodiment of the present invention.

Figure 2 is a photograph of the processed surface after 180 ° bending using lean duplex stainless steel according to a comparative example of the present invention.

Lean duplex stainless steel having excellent bending workability according to an embodiment of the present invention, in weight%, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 3.5 to 6.5%, Cr: 18.5 to 22.5%, N: 0.05 to 0.25%, balance iron (Fe) and other unavoidable impurities, Cr + Mn: 26.0 to 28.5%, Cr / Mn: 3.4 to 4.1.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to fully convey the spirit of the present invention to those skilled in the art. The invention is not limited to the examples presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Lean duplex stainless steel having excellent bending workability according to an embodiment of the present invention, in weight%, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 3.5 to 6.5%, Cr: 18.5 to 22.5%, N: 0.05 to 0.25%, balance iron (Fe) and other unavoidable impurities.

The content of C is 0.01 to 0.06%.

C may be used in place of an expensive element such as Ni as an austenite phase forming element, and is an effective element for increasing material strength by solid solution strengthening.

When excessive addition of C forms segregation and coarse carbide in the center of the material during manufacturing, adversely affects the post-hot rolling-annealing-cold rolling-cold annealing process and Cr is effective for corrosion resistance at the ferrite-austenite phase boundary. Since it easily bonds with carbide forming elements such as to lower the Cr content around the grain boundary to reduce the corrosion resistance, it is preferable to add within the range of 0.06% or less in order to maximize the corrosion resistance. Therefore, it is preferable to limit the content of C in the range of 0.01 to 0.06%.

The content of Si is 0.2 to 1.0%.

Si is partially added for the deoxidation effect and is an element that is concentrated in ferrite during annealing heat treatment as a ferrite phase forming element.

Si should be added at least 0.2% to secure an appropriate ferrite phase fraction. However, excessive addition of more than 1.0% sharply increases the hardness of the ferrite phase to lower the elongation, decreases slag fluidity during steelmaking, forms inclusions in combination with oxygen, and lowers corrosion resistance. Therefore, it is preferable to limit the content of Si in the range of 0.2 to 1.0%.

The content of Mn is 3.5 to 6.5%.

Mn is an element which increases melt flow control, deoxidizer and nitrogen solubility, and is added in place of expensive Ni as an austenite forming element.

If Mn is less than 3.5%, it is difficult to secure a proper austenite phase fraction even when the content of N, which is another austenite forming element, is controlled when Ni and Cu are managed as impurities. When Mn is more than 6.5%, it becomes difficult to ensure corrosion resistance, and it becomes difficult to control the phase ratio due to the excessive austenite phase. Therefore, it is preferable to limit the content of Mn in the range of 3.5 to 6.5%.

The content of Cr is 18.5 to 22.5%.

Cr, together with Si, is a ferrite phase stabilizing element that plays an important role in securing the ferrite phase and is an essential element for securing corrosion resistance.

Increasing the Cr content increases the corrosion resistance, but the content of expensive Ni or other austenite forming elements must be increased to maintain the phase fraction. Therefore, it is preferable to limit the content of Cr in the range of 18.5 to 22.5%.

The content of N is 0.05 to 0.25%.

N together with C and Ni contributes to stabilization of the austenite phase. N is one of the elements in which thickening occurs in the austenite phase during annealing.

Increasing the content of N may additionally increase the corrosion resistance and high strength. However, when the N content is excessive, it is difficult to stably manufacture the steel due to the surface defects caused by the generation of nitrogen pores during casting due to excess nitrogen solubility. Therefore, the content of N is preferably limited to the range of 0.05 to 0.25%.

For example, lean duplex stainless steel according to an embodiment of the present invention may include Ni: 0.5 or less, Cu: 0.5 or less, Mo: 0.5 or less.

Ni, together with Mn, Cu, and N, is an austenite stabilizing element and plays a major role in increasing the stability of the austenite phase.

However, in order to reduce the cost, instead of reducing the expensive Ni content as much as possible, it is possible to increase the ratio of other austenite phase forming elements Mn and N to sufficiently maintain the balance of the phase fraction by reducing Ni. In order to prevent the manufacturing cost of the product due to expensive Ni from rising, it is preferable not to add it. Therefore, the content of Ni is limited to 0.5% or less (including 0) in consideration of the content as impurities.

Cu is an element which suppresses work hardening resulting from formation of a work organic martensite phase, and contributes to soft nitriding of an austenitic stainless steel.

However, for cost reduction, it is preferable not to add it in order to prevent the manufacturing cost of the product due to the expensive Cu from rising. Therefore, the content of Cu is limited to 0.5% or less (including 0) in consideration of the content as impurities.

Mo is a very effective element for improving the corrosion resistance while stabilizing ferrite together with Cr.

However, in order to reduce the cost, it is preferable not to add it in order to prevent the manufacturing cost of the product due to the very expensive Mo from rising. Therefore, the content of Mo is limited to 0.5% or less (including 0) in consideration of the content as impurities.

That is, by managing alloy components such as Ni, Cu, Mo, etc. in the component system of the duplex stainless steel according to an embodiment of the present invention as impurities, it is possible to save resources by minimizing or excluding these components and to minimize the manufacturing cost of the duplex stainless steel Can be. Therefore, when the content of Ni, Cu, Mo exceeds 0.5%, respectively, there is a problem in that the production cost increases by increasing the content of Ni, Cu, and Mo, which are expensive metals.

In addition, the duplex stainless steel according to an embodiment of the present invention is Cr + Mn: 26.0 to 28.5%, Cr / Mn: 3.4 to 4.1.

The present invention relates to the composition of inexpensive lean duplex stainless steel for replacing 400 series and 200 series stainless steel decorative tubes. The inventors of the present invention study the formability of inexpensive lean duplex stainless steel containing Ni, Cu, and Mo, which are expensive alloying elements, as impurities other than target components, and have a relatively low elongation characteristic measured by a tensile test. As compared with the stainless steel which has the characteristic of this elongation high, the unusual phenomenon which was excellent in bending workability was discovered.

From this unusual phenomenon, excellent bendability only in the component steels defined by the specific range of the sum of Cr and Mn in weight percent of 26.0 to 28.5% and the ratio of Cr and Mn (Cr / Mn) of 3.4 to 4.1 It can be seen that it is implemented.

That is, the present invention controls the ratio of Cr and Mn to 26.0 to 28.5% by weight, and controls the ratio of Cr and Mn (Cr / Mn) to 3.4 to 4.1 to provide inexpensive stainless steel having excellent molding characteristics during bending. Can provide.

For example, the lean duplex stainless steel according to an embodiment of the present invention may have a volume fraction of ferrite matrix in the microstructure of 50 to 75%. If the volume fraction of the ferrite matrix is less than 50%, sufficient corrosion resistance may not be obtained, and if the volume fraction of the ferrite matrix is greater than 75%, the fraction of the austenite tissue may be reduced, and thus sufficient processability may not be obtained.

For example, the elongation of lean duplex stainless steel according to one embodiment of the present invention may be 30 to 40%.

If the elongation is less than 30%, there is a problem that the workability is lowered, if more than 40% there is a problem that cracking occurs during bending. In general, when the elongation is increased, the workability is considered to be increased, and the bending workability is also expected to increase. However, when the elongation is more than 40% when the total content and proportion of the component system and Cr and Mn of the present invention are satisfied, There is a problem that cracks occur during bending.

Lean duplex stainless steel according to an embodiment of the present invention, in weight percent, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 3.5 to 6.5%, Cr: 18.5 to 22.5%, N: 0.05 to Duplex stainless steel containing 0.25%, Ni: 0.5 or less, Cu: 0.5 or less, Mo: 0.5 or less, balance iron (Fe) and other unavoidable impurities, Cr + Mn: 26.0 to 28.5%, and Cr / Mn: 3.4 to 4.1 Hot-rolled steel slab, hot-rolled annealing heat treatment of the hot rolled steel sheet at a temperature of 1,050 to 1,150 ℃, and then cold-rolled, cold-rolled steel annealing heat treatment at a temperature of 1,050 to 1,150 ℃, pickling treatment to produce a duplex stainless steel do.

The lean duplex stainless steel slab of the above composition may be thick rolled by a conventional method, and the hot rolled steel sheet may have a thickness of 4 to 20 mm. For example, the hot rolled steel sheet may be annealed for 30 seconds to 60 minutes at a temperature of 1,050 to 1,150 ℃.

Thereafter, the hot rolled steel sheet may be cold rolled by a conventional method, and the cold rolled steel sheet may have a thickness of 0.1 to 5 mm. For example, the cold rolled steel sheet may be annealed for 10 seconds to 60 minutes at a temperature of 1,050 to 1,150 ℃.

Through the following examples will be described in more detail the present invention.

Invention steel and comparative steel

In order to include the component system according to each of the inventive steels and comparative steels of Table 1, each cast in the form of a 50 kg ingot of 140 mm thickness in a vacuum induction melting furnace. The cast ingot was subjected to a three-hour aging process in a heating furnace at 1,250 ° C., and then hot rolled to a plate width of 200 mm and a thickness of 4 mm, followed by air cooling. Hot-rolled annealing was performed on the air-cooled hot rolled plate at a temperature of 1,100 ° C. for 1 minute, and then cold rolled to a thickness of 1.5 mm after pickling. The cold rolled sheet was cold rolled and annealed at a temperature of 1,100 ° C. for 30 seconds, and pickled to prepare a duplex stainless steel cold rolled steel sheet specimen.

	Cr	Mn	Si	C	N	Cr + Mn	Cr / Mn
Inventive Steel 1	20.9	6.10	0.42	0.022	0.178	27.0	3.43
Inventive Steel 2	22.1	6.02	0.41	0.015	0.192	28.1	3.67
Invention Steel 3	21.2	6.00	0.39	0.050	0.195	27.2	3.53
Inventive Steel 4	22.1	5.99	0.40	0.050	0.183	28.1	3.69
Inventive Steel 5	21.1	5.91	0.40	0.058	0.215	27.0	3.57
Comparative Steel 1	19.0	6.00	0.41	0.019	0.184	25.0	3.17
Comparative Steel 2	19.7	5.85	0.42	0.018	0.182	25.6	3.37
Comparative Steel 3	19.0	6.07	0.37	0.049	0.188	25.1	3.13
Comparative Steel 4	19.9	6.10	0.39	0.050	0.189	26.0	3.26
Comparative Steel 5	20.9	5.01	0.71	0.100	0.105	25.9	4.17
Comparative Steel 6	21.2	3.92	0.38	0.098	0.103	25.1	5.41

The ferrite fraction of the material was measured using a ferrite scope on a 4 mm thick hot-annealed material. Ferrite scope is a device for measuring the fraction of the ferrite phase by using the magnetism of the material, it is shown in Table 2 by measuring the ferrite fraction using Fisher's "Ferritescope MP30".

A specimen of 180 mm in length and 20 mm in width was processed from a 1.5 mm thick cold rolled annealing plate in the direction perpendicular to rolling and used as a bending test piece. In the bending test, a 90 ° bend was first performed with a force of 10 ton size using a punch having a corner radius of 1.5 mm, followed by additional bending up to 180 degrees. The 180 ° bending test result was observed whether the sample was broken, and the results are shown in Table 2 below.

Tensile test results for Comparative Examples and Inventive Steels are shown in Table 2 below. In the tensile test, specimens having a gage length of 50 mm and a width of 12.5 mm were taken from a cold rolled annealing plate having a thickness of 1.5 mm and perpendicular to the rolling direction. The material properties after five tensile tests each sample are shown in Table 2 below.

	Ferrite fraction (%)	YS (MPa)	TS (MPa)	EL (%)	Bending crack
Inventive Steel 1	63	423	670	33.7	Not Occurred
Inventive Steel 2	71	398	643	31.4	Not Occurred
Invention Steel 3	56	456	703	34.8	Not Occurred
Inventive Steel 4	64	423	670	32.5	Not Occurred
Inventive Steel 5	60	406	672	33.3	Not Occurred
Comparative Steel 1	45	473	839	33.4	Occur
Comparative Steel 2	55	434	689	45.1	Occur
Comparative Steel 3	36	476	883	33.8	Occur
Comparative Steel 4	45	476	754	43.7	Occur
Comparative Steel 5	59	415	678	32.7	Occur
Comparative Steel 6	62	401	668	31.7	Occur

Referring to Tables 1 and 2, the inventive steels of the present invention did not have bending cracks, but it can be seen that all of the comparative steels had bending cracks.

Figure 1 is a photograph of the processed surface after 180 ° bending using lean duplex stainless steel according to an embodiment of the present invention. Figure 2 is a photograph of the processed surface after 180 ° bending using lean duplex stainless steel according to a comparative example of the present invention.

Specifically, FIG. 1 is a photograph of the processed surface after the 180 ° bending for the inventive steel 1, and FIG. 2 is a photograph of the processed surface after the 180 ° bending for the comparative steel 6.

That is, referring to FIG. 1, cracks do not occur even after bending, and thus, bending characteristics may be excellent. Referring to Figure 2, it can be seen that the surface cracks are badly generated by the bending process.

Referring to Table 2, even if the elongation of the stainless steel according to the tensile test is high, the bending workability is lowered when the parameters of Cr + Mn and Cr / Mn in the present invention are not satisfied.

More specifically, in the case of Comparative Steel 2 and Comparative Steel 4, the elongation is 45.1% and 43.7%, respectively. The elongation is the highest compared to other steels, but it can be seen that cracks occur after 180 ° bending. Therefore, it can be seen that the control of the parameters relating to Cr and Mn, that is, Cr + Mn and Cr / Mn, as desired in the present invention, is the most important for securing bending workability.

Therefore, in order to secure excellent bendability of duplex stainless steel, the sum of chromium and manganese (Cr + Mn), which is a component element, is 26.0 to 28.5% by weight, and the ratio of chromium and manganese (Cr / Mn) by weight is 3.4. It can be seen that it should be controlled to 4.1.

As described above, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person of ordinary skill in the art may be within the scope and spirit of the following claims. It will be understood that various changes and modifications are possible.

Lean duplex stainless steel having excellent bending workability according to embodiments of the present invention is excellent in bending workability and can be variously applied such as an outdoor decorative pipe or an indoor structural pipe.

Claims (4)

1. By weight, C: 0.01-0.06%, Si: 0.2-1.0%, Mn: 3.5-6.5%, Cr: 18.5-22.5%, N: 0.05-0.25%, balance iron (Fe) and other unavoidable impurities ,

   Lean duplex stainless steel having excellent bending workability with Cr + Mn: 26.0 to 28.5% and Cr / Mn: 3.4 to 4.1.
2. The method of claim 1,

   Lean duplex stainless steel having excellent bending workability including Ni: 0.5% or less, Cu: 0.5% or less, and Mo: 0.5% or less.
3. The method of claim 1,

   Lean duplex stainless steel with excellent bendability with a volume fraction of ferrite matrix in the microstructure of 50 to 75%.
4. The method of claim 1,

   Lean duplex stainless steel having excellent bending workability, the elongation of the stainless steel is 30 to 40%.

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