WO2020085849A1 - High-strength and high-ductility nonmagnetic steel having excellent weldability, and manufacturing method therefor - Google Patents

Abstract

The present invention relates to nonmagnetic steel that can be suitably used for components in which an eddy current occurs, such as a distribution board and a transformer, and, more specifically, to nonmagnetic steep having excellent strength, ductility and weldability, and a manufacturing method therefor.

Description

High strength and high ductility non-magnetic steel with excellent weldability and manufacturing method thereof

The present invention relates to a non-magnetic steel material that can be suitably used in parts that generate eddy currents, such as a switchboard and a transformer, and more particularly, to a non-magnetic steel material having excellent strength and ductility as well as weldability and a method for manufacturing the same.

Materials such as switchboards and transformers generally require high strength and excellent non-magnetic properties. To meet these conditions, conventionally, stainless steel in which nickel (Ni) and chromium (Cr) are added in large amounts has been used. However, stainless steel has the disadvantages of low strength and high price.

Ferritic or martensitic stainless steel may be applied to increase the strength of the non-magnetic steel, but since the ferritic or martensitic stainless steel has high magnetism, not only does power loss due to eddy current occur, but also the price is very high. It has the disadvantage of being expensive.

Accordingly, in order to have low strength and high strength and non-magnetic properties, steel materials having an austenite phase have been developed by controlling the contents of manganese (Mn) and carbon (C) in steel.

These austenite steel grades have the advantage of stably maintaining the austenite phase even at room temperature and cryogenic temperature by controlling the content of the two elements, so that the non-magnetic properties can be well maintained.

On the other hand, it is necessary to prevent deterioration of physical properties due to welding in manufacturing a steel material having high strength and non-magnetic properties as an intended part, and thus, securing weldability of the non-magnetic steel material has emerged.

Therefore, there is a need to develop a non-magnetic steel material having high strength and non-magnetic properties as well as excellent weldability.

One aspect of the present invention is to provide a non-magnetic steel material having excellent high weldability and high strength and high ductility at a low manufacturing cost from optimizing alloy composition.

Another aspect of the present invention is to provide a method for manufacturing the non-magnetic steel described above.

The subject of this invention is not limited to the above-mentioned matter. Additional subject matter of the present invention is described in the overall contents of the specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding the additional subject matter of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention, by weight, carbon (C): 0.03 ~ 0.50%, silicon (Si): 0.3% or less, manganese (Mn): 15-30%, chromium (Cr): 2.0% or less (0 % Excluded), molybdenum (Mo): 0.5% or less (excluding 0%), titanium (Ti): 0.01 to 0.1%, vanadium (V): 0.01 to 0.5%, aluminum (Al): 0.2 to 1.0%, phosphorus ( P): 0.1% or less, sulfur (S): 0.01 wt% or less, nitrogen (N): 0.03% or less, the balance contains other unavoidable impurities and Fe, high strength and high ductility visa with excellent weldability with austenite single phase structure Provides sex steel.

Another aspect of the present invention, the step of reheating the steel slab having the above-described alloy composition to 1100 ~ 1250 ℃; Preparing a thick steel plate by finishing hot rolling the reheated steel slab at 800 to 1000 ° C; And cooling the thick steel sheet at a cooling rate of 10 ° C./s or higher. The present invention provides a method of manufacturing a high strength and high ductility nonmagnetic steel material having excellent weldability.

According to the present invention, it is possible to provide a steel material having excellent non-magnetic properties at a low cost. In addition, the steel material of the present invention is not only excellent in strength and ductility, but also has an effect of having excellent weldability.

1 is a graph showing the results of measuring the permeability of invention steel and comparative steel according to an embodiment of the present invention.

The inventors of the present invention have studied in depth to provide a non-magnetic steel material having excellent strength and high ductility as well as excellent non-magnetic properties and excellent weldability. As a result, it was found that it is possible to provide an optimum component system capable of significantly improving the phase stability of the non-magnetic steel.

In particular, in the present invention, in addition to alloying elements such as C and Mn, Al is added at a certain content to prevent carbon from forming carbides, and by further adding Cr and Mo, strength, ductility, and weldability can be further improved. There will be technical significance to it.

Hereinafter, the present invention will be described in detail.

High-strength and highly ductile non-magnetic steel with excellent adhesion according to an aspect of the present invention is in weight percent, carbon (C): 0.03 to 0.50%, silicon (Si): 0.3% or less, manganese (Mn): 15 to 30% , Chrome (Cr): 2.0% or less (excluding 0%), molybdenum (Mo): 0.5% or less (excluding 0%), titanium (Ti): 0.01 to 0.1%, vanadium (V): 0.01 to 0.5%, aluminum (Al): 0.2 to 1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.01 wt% or less, nitrogen (N): 0.03% or less.

Hereinafter, the reason for controlling the alloy component of the non-magnetic steel material provided in the present invention will be described in detail. At this time, unless otherwise specified, the content of each component means weight%, and the proportion of tissue is based on the area.

Carbon (C): 0.03 ~ 0.50%

Carbon (C) is an important element for securing an austenite structure in steel, and by containing such C in a certain amount or more, the stability of austenite can be sufficiently secured. In the present invention, for the above-described effect, the C may be included in an amount of 0.03% or more, and on the other hand, when the content of the C exceeds 0.30%, carbides are precipitated when exposed to a high temperature such as a roll for a long time, so that the non-magnetic properties Although lowered, in the present invention, since the formation of carbide is reduced by adding a certain amount of aluminum (Al), C may be included up to 0.50%.

Therefore, in the present invention, C can be contained in an amount of 0.03 to 0.50%.

Silicon (Si): 0.3% or less

Silicon (Si) does not significantly affect the lamination defect energy of steel and is usually used as a deoxidizer. When the Si content exceeds 0.3%, manufacturing costs increase, and oxides are excessively formed, and thus the surface quality of the product may be deteriorated.

Therefore, the Si may be included in an amount of 0.3% or less, and 0% is excluded in consideration of a level inevitably added in the steel manufacturing process.

Manganese (Mn): 15-30%

Manganese (Mn) is an important element that plays a role in stabilizing the austenite structure, and it needs to be contained at 15% or more in order to obtain a low permeability of steel. In particular, when the content of C is low, when the Mn is added to less than 15%, an α'-martensitic phase is formed, thereby degrading the non-magnetic properties. On the other hand, if the content of the Mn exceeds 30%, the manufacturing cost increases significantly, and there is a problem in that the surface quality is deteriorated by forming internal oxidation or processing cracks during heating in the hot working step.

Therefore, in the present invention, Mn may be included in 15 to 30%.

Chromium (Cr): 2.0% or less (excluding 0%)

Chromium (Cr) is an effective element for improving the strength by suppressing high-temperature oxidation to reduce surface defects and strengthen solid solution. When a large amount of Cr is added, manufacturing cost increases, and coarse carbides are formed to reduce strength. Therefore, in consideration of this, the Cr may be included in an amount of 2.0% or less, and 0% is excluded.

Molybdenum (Mo): 0.5% or less (excluding 0%)

Molybdenum (Mo) is an effective element for increasing the precipitation strengthening effect by making the precipitation phase fine. When such a large amount of Mo is added, the alloy cost increases, and the precipitation phase becomes coarse, so that the above-described effect cannot be sufficiently obtained. Therefore, in consideration of this, the Mo may be included in 0.5% or less, and 0% is excluded.

Titanium (Ti): 0.01 ~ 0.1%

Titanium (Ti) is an element that reacts with nitrogen (N) inside the steel to precipitate nitrides and form twins, and can be added to secure the strength and formability of the steel. In addition, Ti forms a precipitation phase to improve the yield strength. Since such an effect can be obtained even with a small amount of addition, it can be added at 0.01% or more. However, if the content exceeds 0.1%, the precipitates are excessively formed, which may cause cracks during rolling or forging, and may deteriorate formability and weldability.

Therefore, in the present invention, Ti may be included in an amount of 0.01 to 0.1%.

Vanadium (V): 0.01 ~ 0.5%

Vanadium (V) is useful for improving strength by forming carbides, nitrides, etc. by reacting with carbon, nitrogen, and the like inside the steel. Particularly, in the case of austenitic high-manganese steel, high solubility at a high temperature of 900 ° C or higher, and low solubility at a temperature of 600 to 800 ° C, is an element having a large precipitation strengthening effect. In order to sufficiently obtain the precipitation strengthening effect, it is preferable to contain the above V in 0.01% or more. However, when the content exceeds 0.5%, precipitates are formed excessively, and hot workability, such as rolling or forging, decreases, and there is a risk of cracking.

Therefore, in the present invention, V may be included in an amount of 0.01 to 0.5%.

Aluminum (Al): 0.2 ~ 1.0%

Aluminum (Al) is added as a deoxidizer and is an effective element to prevent the formation of carbides in the steel. In addition, there is an effect of improving the moldability by adjusting the fraction of twins. It is preferable to contain 0.2% or more in order to sufficiently acquire the above-described effect. However, when the content exceeds 1.0%, the tendency to form oxide increases, and welding of the molten pool becomes poor during arc welding, resulting in poor welding, and the quality of the surface of the product is deteriorated due to the formation of oxide.

Therefore, in the present invention, Al may be included in 0.2 to 1.0%, and more advantageously, in 0.2 to 0.8%.

Phosphorus (P): 0.1% or less

Phosphorus (P) is an element that promotes segregation and causes cracking during casting, and is preferably contained as low as possible. When the content of P exceeds 0.1%, castability may deteriorate, so the P may include 0.1% or less.

Sulfur (S): 0.01% or less

Sulfur (S) is an element that inhibits the physical properties of steel by forming inclusions such as MnS. Therefore, it is preferable to contain as low as possible, and if the content exceeds 0.01%, there is a problem of hot brittleness. Therefore, the S may be included in 0.01% or less.

Nitrogen (N): 0.03% or less

Nitrogen (N) combines with titanium (Ti) to form Ti nitride, but when its content exceeds 0.03%, the free nitrogen remaining after binding with Ti causes aging hardening, which significantly inhibits the toughness of the base material. , Inducing cracks on the surface of slabs and steel plates, and thus inhibiting the surface quality. Therefore, the N may be included in 0.03% or less.

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

It is preferable that the nonmagnetic steel of the present invention having the above-described alloy composition has an austenite single phase structure as a microstructure. By having an austenite single-phase structure as described above, it is possible to maintain non-magnetic properties even when receiving external energy.

Particularly, the non-magnetic steel of the present invention has an austenite phase with high stability from optimization of alloy composition, and from this, may have a property of a relative magnetic permeability of 1.01 or less in a magnetic field of 50 kA / m.

The loss due to the eddy current of the material exposed to the electromagnetic field is closely related to the magnetism of the material. The larger the magnetism, the greater the eddy current generation and the higher the loss. In general, magnetism is proportional to the permeability (μ). That is, the magnetic permeability increases as the permeability increases. The permeability is defined as the ratio of the induced magnetic field (B) to the magnetic field (H) to be magnetized, that is, μ = B / H. In other words, reducing the magnetic permeability decreases the magnetic properties of the material, thus preventing eddy current loss on the surface when exposed to an electric field, thereby increasing energy efficiency. Therefore, it is advantageous to use a non-magnetic steel sheet having no magnetism to prevent energy loss.

In addition, the steel material of the present invention is a thick steel plate having a thickness of 10 to 40 mm, and has excellent strength and ductility, and can specifically secure a tensile strength of 450 MPa or more and an elongation of 55% or more.

Hereinafter, a method of manufacturing a high strength and high ductility nonmagnetic steel according to another aspect of the present invention will be described in detail.

First, after preparing a steel slab satisfying the above-described alloy composition, the steel slab is reheated at 1100 to 1250 ° C.

When the temperature of the steel slab is reheated to less than 1100 ° C, the rolling load may be excessively taken during subsequent hot rolling, whereas when the temperature exceeds 1250 ° C, internal oxidation occurs severely and surface quality may deteriorate.

Therefore, the re-heating of the steel slab can be carried out at 1100 ~ 1250 ℃.

The reheated steel slab can be hot rolled according to the above to produce a thick steel plate. At this time, it is preferable to finish hot rolling at 800 to 1000 ° C.

If the temperature during finishing hot rolling is less than 800 ° C, there is a problem that the load increases during rolling. On the other hand, the higher the temperature during the hot rolling of the finish, the lower the deformation resistance and the easier rolling, whereas the target strength cannot be secured due to the coarsening of the structure, so it is preferable to limit it to 1000 ° C or less.

Thereafter, the thick steel plate manufactured according to the above-described method can be cooled.

The cooling is preferably performed at a cooling rate sufficient to suppress the formation of grain boundary carbides, and more preferably at a cooling rate of 10 ° C / s or more.

When the cooling rate is less than 10 ° C / s during cooling, it is difficult to avoid carbide formation, and carbides precipitate on grain boundaries during cooling, and there is a problem that the strength is deteriorated along with a decrease in ductility due to premature fracture of steel.

Since the present invention is advantageous as the cooling rate is faster, there is no need to specifically limit the upper limit as long as it is within the range of accelerated cooling. However, considering that the cooling rate is difficult to exceed 80 ° C / s during normal accelerated cooling, the upper limit may be limited to 80 ° C / s or less.

Meanwhile, cooling of the thick steel sheet may be stopped at 500 ° C or lower. Even if accelerated cooling is performed as described above, when cooling is stopped at an excessively high temperature, there is a fear that carbides are generated and grown. When a large amount of carbide is produced, there is a problem that the austenite stability decreases and the permeability characteristics decrease.

Although the cooling is performed at room temperature, there is no difficulty in securing the intended physical properties, and the lower limit of the cooling end temperature is not particularly limited.

The final steel (thick steel plate) of the present invention obtained by completing the process of hot rolling and cooling has a high-stability austenite phase as a microstructure, whereby it is excellent in high strength and high ductility as well as weldability and non-magnetic properties. Can have.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of rights of the present invention is determined by matters described in claims and reasonably inferred therefrom.

(Example)

After preparing each steel slab having the alloy composition shown in Table 1 below, the steel slab was reheated at 1200 ° C, and then hot rolled at 950 ° C to prepare each thick steel sheet. Thereafter, the produced thick steel sheets were cooled to 20 ° C / s to complete cooling at 400 ° C.

Then, the mechanical properties (yield strength (YS), tensile strength (TS), elongation (El)) and permeability of each of the thick steel plates prepared above were measured, and the results are shown in Table 2 below. At this time, the yield strength (YS) is represented by a 0.2% offset value.

The permeability is expressed as the relative permeability, which is the ratio of the permeability in vacuum and the permeability in a specific atmosphere. In the present invention, the relative permeability (μ), which is the ratio of permeability in vacuum and atmosphere, was measured using paramagnetic measuring equipment.

On the other hand, mechanical properties were processed into plate-shaped specimens in accordance with ASTM E8 / E8M, the standard test method for tensile testing, and then evaluated by a one-way tensile testing machine.

Then, after the flux cored arc welding (FCAW) with a heat input amount of 1.5 KJ / cm for each specimen, the welded surface was observed with the naked eye to evaluate weldability.

River Burn	Alloy composition (% by weight)												division
	C	Si	Mn	P	S	Al	Cr	Mo	Ti	Ni	V	N
One	0.05	0.3	0	0.02	0.003	0.2	15	1.1	2.1	25	0.3	0.005	Conventional steel
2	0.20	0.3	30	0.02	0.003	0.2	2	0.3	0.06	0	0.3	0.005	Invention Steel 1
3	0.10	0.3	30	0.02	0.003	0.2	2	0.3	0.06	0	0.3	0.005	Invention Steel 2
4	0.03	0.3	30	0.02	0.003	0.2	2	0.3	0.06	0	0.3	0.005	Invention Steel 3
5	0.10	0.3	27	0.02	0.003	0.2	2	0.3	0.06	0	0.3	0.005	Invention Steel 4
6	0.45	0.01	18.3	0.08	0.003	0.95	0	0	0.085	0	0.01	0.0087	Comparative steel 1
7	0.05	0.3	25	0.02	0.003	0.2	15	0.3	0.06	0	0.3	0.005	Comparative steel 2
8	0.51	0.01	18	0.09	0.004	0.01	0	0	0.046	0	0	0.010	Comparative steel 3
9	0.61	0.01	18.5	0.09	0.002	2.68	0	0	0.073	0	0.015	0.0065	Comparative River 4

division	Relative permeability	Mechanical properties			Weldability
		YS (MPa)	TS (MPa)	El (%)
Conventional steel	1.004	203	624	35	-
Invention Steel 1	1.002	160	591	67	Good
Invention Steel 2	1.001	159	532	78	Good
Invention Steel 3	1.000	159	492	64	Good
Invention Steel 4	1.001	159	650	58	Good
Comparative steel 1	1.010	538	960	59	Bad
Comparative steel 2	1.050	157	398	26	Good
Comparative steel 3	1.070	484	1106	60.4	Good
Comparative River 4	1.010	529	849	50	Bad

As shown in Tables 1 and 2, the invention steels 1 to 4 satisfying both the alloy composition and the manufacturing conditions of the present invention can be confirmed that the relative magnetic permeability is measured to be less than 1.01, and the strength and ductility as well as the weldability are good. can confirm.

On the other hand, in the case of the conventional steel, that is, stainless steel containing a large amount of Cr and Ni, the relative permeability is low, but it is difficult to secure ductility, and the production cost is greatly increased by adding a large amount of expensive elements.

On the other hand, the comparative steel 2 containing a large amount of Cr and the comparative steel 3 containing a large amount of C and lacking the Al content, it can be seen that the permeability is very inferior to 1.01 or more.

In addition, it can be confirmed that the comparative steel 4 containing no large amounts of Cr and Mo, relatively high Al content containing comparative steels 1 and C, and excessive Al content has a magnetic permeability of 1.01 and poor weldability. It is judged that the arc stability is deteriorated due to the strong deoxidation effect by Al in the steel, and welding defects are caused due to surface bead defects. In addition, it has been confirmed that the recovery rate of other elements such as Ti is improved, resulting in deterioration of impact toughness, materials, etc., as precipitation phases of Al ₂ O ₃ and Ti (Al) (CN) are formed.

1 is a comparison of the results obtained by measuring the permeability values of the invention steel 2 and the comparative steel 3, and the invention steel 2 has an overall low magnetic permeability, whereas the comparative steel 3 has a high magnetic permeability.

As described above, when the alloy composition proposed in the present invention is satisfied as compared with a conventional steel containing a large amount of Cr and Ni, a non-magnetic steel material can be obtained at a lower cost. In addition, since the non-magnetic steel of the present invention is excellent in strength and ductility as well as weldability, it is described that the application uses will be expanded.

Claims (6)

1. In weight percent, carbon (C): 0.03 to 0.50%, silicon (Si): 0.3% or less, manganese (Mn): 15 to 30%, chromium (Cr): 2.0% or less (excluding 0%), molybdenum (Mo ): 0.5% or less (excluding 0%), titanium (Ti): 0.01 to 0.1%, vanadium (V): 0.01 to 0.5%, aluminum (Al): 0.2 to 1.0%, phosphorus (P): 0.1% or less, Sulfur (S): 0.01% by weight or less, Nitrogen (N): 0.03% or less, the balance including other inevitable impurities and Fe,

   High strength and highly ductile non-magnetic steel with excellent weldability with austenite single-phase structure.
2. According to claim 1,

   The steel material is a high strength and high ductility non-magnetic steel material having excellent weldability with a relative magnetic permeability of 1.01 or less at a magnetic field of 50 kA / m.
3. According to claim 1,

   The steel material is a high strength and high ductility non-magnetic steel with excellent weldability of tensile strength of 450 MPa or more and elongation of 55% or more.
4. In weight percent, carbon (C): 0.03 to 0.50%, silicon (Si): 0.3% or less, manganese (Mn): 15 to 30%, chromium (Cr): 2.0% or less (excluding 0%), molybdenum (Mo ): 0.5% or less (excluding 0%), titanium (Ti): 0.01 to 0.1%, vanadium (V): 0.01 to 0.5%, aluminum (Al): 0.2 to 1.0%, phosphorus (P): 0.1% or less, Sulfur (S): 0.01% by weight or less, nitrogen (N): 0.03% or less, the remainder of the steel slab containing other inevitable impurities and Fe reheating to 1100 ~ 1250 ℃;

   Preparing a thick steel plate by finishing hot rolling the reheated steel slab at 800 to 1000 ° C; And

   Cooling the thick steel sheet at a cooling rate of 10 ° C./s or higher;

   Method of manufacturing a high strength and high ductility non-magnetic steel excellent in weldability, including.
5. The method of claim 4,

   The cooling is performed at a cooling rate of 10 ~ 80 ℃ / s method of manufacturing a high strength and high ductility non-magnetic steel excellent in weldability.
6. The method of claim 4,

   The cooling is a method of manufacturing a high strength and high ductility non-magnetic steel having excellent weldability, which is terminated at 500 ° C or lower.

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