KR20190075517A - High-manganese steel sheet having excellent welding strength and method for manufacturing thereof - Google Patents

Abstract

According to an embodiment of the present invention, in regard to providing high-manganese steel which is suitable as a vehicle material, provided are high-manganese steel having excellent strength, which has excellent ductility as well as gigapascal-glass ultra-high strength while suppressing a welding crack, and a manufacturing method thereof. The high-manganese steel includes carbon, manganese, phosphorus, sulfur, aluminum, silicon, vanadium, tungsten, titanium, boron, and nitrogen.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-manganese steel sheet having excellent welding strength and a method of manufacturing the same. BACKGROUND ART [0002]

More particularly, the present invention relates to a high manganese steel sheet having excellent welding strength and a method of manufacturing the same.

As the environmental pollution control is gradually increasing, the use of high-strength steels has been increasing to increase the fuel consumption of automobiles in response to this, and recently, research on the commercialization of high-strength steels having tensile strengths of 980 MPa or more has been increasing .

High-strength steels for automobiles are typically composed of transformed organic (TRIP), abnormal (DP) and high manganese.

Of these, dual phase steels usually have a reheated slab rolled in austenite section and then cooled to a temperature lower than the Ms (martensitic transformation start temperature) during the cooling process to transform the austenite into martensite do. As the ratio of martensite phase increases, the strength increases and the ductility increases as the ratio of ferrite phase increases. In this case, when the martensite phase is formed excessively to improve the strength, the ferrite fraction is relatively increased And the ductility is lowered.

The trassformation induced plastisity steel forms austenite in the process of hot rolling the reheated slab, and then controls the cooling rate and the cooling termination temperature during the cooling process to partially retain the ostenite at room temperature, It is a steel type that improved ductility at the same time. That is, the residual austenite is transformed into martensite by processing during plastic deformation, and the transformation is locally caused by the transformation phase formed by the firing organic transformation as well as the improvement of the ductility.

Such TRIP steels can improve both strength and ductility by maintaining a metastable olefin phase at room temperature at a certain rate or more (for example, 10 vol% or more). To this end, Si, Mn, etc. are added to inhibit the formation of carbides It is necessary to increase the amount of carbon in the ostenite, and there is a drawback in that the alloy component must be strictly controlled in order to secure an appropriate material.

On the other hand, studies on high manganese steels containing a large amount of Mn in steel have been actively carried out in order to secure moldability together with high strength. Such high manganese steels are formed by adding a large amount of manganese to form twin and dislocation (Moldability) by suppressing the slip deformation due to the impact strength (see Patent Document 1).

However, when high manganese steel is welded to be applied as a part of an automobile material, welding cracks occur in the welded quartz. This is more pronounced as the strength is higher (see FIG. 1).

Therefore, in providing a high strength steel suitable as a material for automobiles, it is required to develop a steel material capable of forming a good welded portion at the time of welding as well as strength and ductility.

International Patent Publication WO1993-013233

An aspect of the present invention is to provide a high manganese steel sheet excellent in welding strength in which generation of welding cracks is suppressed at the time of welding while having super high strength of Giga Pascal grade and excellent ductility in providing high manganese steel suitable for automobile materials, And a method for producing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

An aspect of the present invention provides a method of manufacturing a semiconductor device, which comprises 0.5 to 1.0% of carbon (C), 10 to 25% of manganese (Mn), not more than 0.8% of phosphorus (P) (Si): 0.3% or less (excluding 0%), vanadium (V): 0.5-1.0%, tungsten (W): 0.3-1.0%, titanium (Ti) The steel having excellent weld strength and having a Vickers hardness of less than 450 Hv and containing a residual Fe and other unavoidable impurities and having a Vickers hardness of the welded portion formed after welding is 0.1 to 1.0%, boron (B): 0.001 to 0.003%, nitrogen (N) to provide.

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising: reheating a steel slab satisfying the alloy composition described above at a temperature range of 1100 to 1250 占 폚; Subjecting the reheated steel slab to finish hot rolling in a temperature range of 800 to 950 캜 to produce a hot-rolled steel sheet; Winding the hot rolled steel sheet in a temperature range of 400 to 700 ° C after cooling; Picking up the wound hot-rolled steel sheet and cold-rolling the steel sheet at a reduction ratio of 30 to 60% to produce a cold-rolled steel sheet; And continuously annealing the cold-rolled steel sheet at a temperature in the range of 650 to 900 占 폚.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a high-manganese steel having excellent strength and ductility as well as being capable of suppressing the generation of cracks in the welded joint during welding and having excellent welding strength.

FIG. 1 is a view showing a conventional plate breaking type of a conventional high strength steel.
Fig. 2 shows a cross-shaped welding form for measuring the cross-tensile strength of the welded portion.
FIG. 3 shows the results of the hardness measurement and the welded portion of the invention steel 4 and the comparative steel 1 according to an embodiment of the present invention.

The inventors of the present invention have confirmed that there is a problem that cracks occur in the welded portion when welding the high manganese steel in providing the high manganese steel suitable for automobile material, and have devised a method for solving the problem.

As a result, it has been confirmed that cracking in the welded portion can be suppressed by improving the welding strength of the welded portion during welding by optimizing the alloy composition and the manufacturing conditions of the base metal to which the welding is performed, that is, the high manganese steel, and completed the present invention .

Hereinafter, the present invention will be described in detail.

A high manganese steel sheet having an excellent weld strength according to one aspect of the present invention is characterized by containing 0.5 to 1.0% of carbon (C), 10 to 25% of manganese (Mn), 0.8% or less of phosphorus (P) (Si): 0.3% or less (excluding 0%), vanadium (V): 0.5-1.0%, tungsten (W): 0.3-1.0% 0.02 to 0.1% of titanium (Ti), 0.001 to 0.003% of boron (B), and 0.015% or less of nitrogen (N).

Hereinafter, the reasons for limiting the alloy composition of the high manganese steel sheet provided in the present invention will be described in detail. Here, the content of each component means weight% unless otherwise specified.

C: 0.5 to 1.0%

Carbon (C) is an element added for securing strength of steel. In order to secure a desired level of strength in the present invention, C may be contained at 0.5% or more. However, if the content exceeds 1.0%, carbides are precipitated and the formability is deteriorated. There is a problem that the interval between the liquid phase line temperature and the solid phase line temperature is increased and the main composition is inferior.

Therefore, in the present invention, C may be contained in an amount of 0.5 to 1.0%.

Mn: 10 to 25%

Manganese (Mn) is an advantageous element for stabilizing the austenite structure. For this, Mn may be contained at 10% or more, and if it is less than 10%, there is a problem that martensite is excessively formed and ductility is decreased. On the other hand, if the content exceeds 25%, the manufacturing cost increases greatly, and the internal oxidation is severely generated during heating in the hot rolling step, resulting in poor surface quality. In addition, the stability of austenite is greatly increased, so that the formation of epsilon martensite is inhibited and the strength is decreased.

Therefore, in the present invention, Mn may be contained in an amount of 10 to 25%.

P: not more than 0.8%

Phosphorus (P) is an element easily segregated in the steel, which promotes cracking during casting. In order to prevent this, P can be limited to 0.8% or less, and 0% is excluded considering the level inevitably added in manufacturing.

S: not more than 0.05%

Sulfur (S) forms a coarse manganese sulfide (MnS) to generate defects. If the content is excessive, the risk of hot brittleness increases. Therefore, in order to prevent this, S can be limited to 0.05% or less, and 0% is excluded considering the level that is inevitably added in manufacturing.

Al: 0.01 to 0.5%

Aluminum (Al) is an effective element for deoxidation. In particular, in a steel containing a large amount of Mn, it is an element which increases the stacking defect energy and affects the formation of twin and epsilon martensite phases.

If the content of Al is excessive, an oxide is formed at grain boundaries to deteriorate high-temperature ductility, cracks, and the like, resulting in a problem that surface quality is poor. Particularly, when the content exceeds 0.5%, the concentration of the oxide concentrated in the surface layer increases, and the risk of occurrence of cracking during welding becomes large. On the other hand, in order to make the content of Al less than 0.01%, an excessive cost is required, and the above-mentioned effect can not be sufficiently obtained.

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

Si: 0.3% or less (excluding 0%)

Silicon (Si) is used as a deoxidizer such as aluminum (Al). However, if the content of Si is excessive, oxide is formed at grain boundaries to deteriorate high-temperature ductility, cracks, and the like, resulting in poor surface quality. Therefore, Si may be contained within a range that does not impair the surface quality of the steel. Since Si has higher oxidation resistance than Al, it can be limited to 0.3% or less. However, 0% is excluded.

V: 0.5 to 1.0%

Vanadium (V) is a precursor for forming precipitates, which can be added in order to secure strength and moldability in one aspect of the present invention. V is effective for forming a fine precipitate phase because austenitic steel has a high high temperature solubility but a low room temperature solubility.

For the above-mentioned effect, V may be contained at 0.5% or more. However, if the content exceeds 1.0%, a precipitate phase is excessively formed, and a fine crack may be formed during cold rolling, and the formability and weldability may be deteriorated.

Therefore, in the present invention, V may be included in the range of 0.5 to 1.0%.

W: 0.3 to 1.0%

Tungsten (W) is an element forming a carbide, and may be added in order to improve strength in one aspect of the present invention. Since W has a low solubility to V carbide when added together with V, it has an effect of inhibiting the growth of V carbide and is effective in promoting precipitation strengthening effect by V. In addition, since it has a high degree of solubility at a high temperature and a low solubility at room temperature, it is also effective in forming a fine precipitate phase.

In order to sufficiently obtain the above-mentioned effect, W may be contained at 0.3% or more. However, if the content exceeds 1.0%, a precipitate phase is excessively formed, and a fine crack may be formed during cold rolling, and the formability and weldability may be deteriorated.

Accordingly, in the present invention, W may be contained in an amount of 0.3 to 1.0%.

Particularly, in the present invention, the V is not much precipitated at the time of hot rolling but forms a precipitation phase in the continuous annealing process, and the precipitation strengthening effect can be maximized as it is formed finer than the quartz at such a high temperature. In addition, by additionally adding W together with V, an effect of further finely dividing the V precipitated phase can be obtained, thereby further improving the strength from this.

Ti: 0.02 to 0.1%

Titanium (Ti) is an element that reacts with nitrogen (N) in the steel to precipitate nitrides and form precipitation phases to improve strength. For this purpose, Ti may be added in an amount of 0.02% or more. However, if the content exceeds 0.1%, a precipitate phase is excessively formed, and a fine crack may be formed during cold rolling, and the formability and weldability may be deteriorated.

Therefore, in the present invention, Ti may be contained in an amount of 0.02 to 0.1%.

B: 0.001 to 0.003%

Boron (B) reacts with nitrogen (N) in the steel to precipitate nitrides and form precipitation phases to increase strength. In addition, there is an effect of enhancing the high temperature ductility by strengthening the grain boundaries to stabilize the grain boundaries at the time of high temperature deformation. For this purpose, B may be added in an amount of 0.001% or more. However, when the content exceeds 0.003%, there is a problem that a high temperature ductility is lost by forming a precipitate phase in the grain boundary.

Therefore, in the present invention, B may be contained in an amount of 0.001 to 0.003%.

N: 0.015% or less

Nitrogen (N) is an element which reacts with titanium (Ti), boron (B) or the like to form a nitride, The formed nitride has an effect of finely reducing the grain size. However, the nitrogen in the steel tends to exist as free nitrogen, and if it is high, the nitride is excessively formed and the formability is reduced.

Taking this into consideration, in the present invention, N may be included at 0.015% or less, and 0% is excluded.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The high manganese steel sheet of the present invention satisfying the above alloy composition may have a microstructure and a composite structure of austenite and martensite.

However, when the martensite fraction is excessively high, the brittleness is increased and the moldability is deteriorated. Therefore, it is preferable to limit the area to 15% or less (excluding 0%).

Thus, by having a composite structure, the strength and ductility of the high manganese steel sheet can be secured at a target level.

The high manganese steel sheet of the present invention having the above alloy composition and microstructure can secure strength and ductility, and can have a tensile strength of 1300 MPa or more and an elongation of 20 to 50%.

Hereinafter, a method for manufacturing a high manganese steel sheet having excellent welding strength, which is another aspect of the present invention, will be described in detail.

In one aspect of the present invention, the high manganese steel sheet provided in the present invention is a steel slab having a composition satisfying the alloy composition proposed in the present invention (reheating - hot rolling - cooling and winding - pickling and cold rolling - continuous annealing) ≪ / RTI >

Hereinafter, each step will be described in detail.

[Reheating steel slabs]

First, after a steel slab satisfying the alloy composition proposed in the present invention is prepared, it can be reheated. For example, in the reheating step, the steel slab may be charged into a heating furnace to uniformly heat the entire slab.

The reheating process can be carried out under normal conditions, and can be performed in a temperature range of, for example, 1100 to 1250 ° C. If the temperature at the time of reheating is less than 1100 ° C, the rolling load during the subsequent hot rolling may become excessively excessive, which may cause uneven rolling. On the other hand, the higher the heating temperature is, the easier the hot rolling is. However, if the temperature is higher than 1250 DEG C, the internal oxidation is severely caused and the surface quality is poor.

[Hot Rolling]

The reheated steel slab may be hot-rolled to produce a hot-rolled steel sheet. At this time, finishing hot rolling can be performed in a temperature range of 800 to 950 캜.

The higher the temperature at the finish hot rolling, the lower the deformation resistance and the easier the rolling. However, if the temperature is higher than 950 deg. C, the surface quality deteriorates. On the other hand, if the temperature is less than 800 ° C, there is a problem that the load increases during rolling.

[Cooling and winding]

The hot rolled steel sheet produced according to the above can be cooled and then wound in the form of a coil.

At this time, cooling can be carried out under normal cooling conditions, which can be carried out with water, and the conditions are not particularly limited. However, the water-cooling can be performed at a temperature in the range of 400 to 700 ° C, and then the film can be wound at that temperature.

If the temperature at the time of winding is less than 400 캜, a large amount of cooling water is required for cooling, and there is a problem that a load acts largely during winding. On the other hand, if the temperature exceeds 700 ° C., the reaction between the oxide film on the surface of the steel sheet and the steel sheet matrix proceeds during the cooling process after winding, which deteriorates the acidity.

[Pickling and Cold Rolling]

The rolled hot-rolled steel sheet may be picked and then cold-rolled to produce a cold-rolled steel sheet.

If the reduction rate in the cold rolling is too low, the strength of the product is lowered. Therefore, the reduction can be performed at 30% or more. On the other hand, the higher the reduction rate, the better the strength is secured, but the load on the rolling mill is excessively increased.

[Continuous Annealing]

The cold-rolled steel sheet produced according to the above can be continuously annealed.

The continuous annealing is a step for recrystallization of steel, and continuous annealing can be performed at 650 占 폚 or more for sufficient recrystallization to occur. On the other hand, when the temperature exceeds 900 DEG C, oxides are formed on the surface of the steel and the workability with the connected products before and after continuous operation is poor.

On the other hand, if necessary, the cold-rolled steel sheet after completion of the continuous annealing can be galvanized to produce a coated steel sheet. The plating conditions are not particularly limited, Plating can be performed.

The high manganese steel sheet of the present invention produced by the above-described series of steps may have a tensile strength of 1300 MPa or more and an elongation of 20 to 50% as described above.

Furthermore, since the hardness of the welded portion formed at the time of welding can be ensured to be less than the Vickers hardness of 450 Hv in the high manganese steel sheet of the present invention, the weld strength can be secured to 6 kN or more.

As described above, in the present invention, it is possible to secure the welding strength of 6 kN or more at the time of welding of the high-manganese steel sheet due to the suppression of formation of the martensite phase having brittleness at the welded portion during welding.

The strength of welds is generally called Cross Tensile Strength (CTS) and is expressed in cross tensile strength. The cross tensile strength is measured by measuring the force required to separate the two products by a butt welding in a cross shape as shown in Fig.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Each steel slab having the alloy composition shown in the following Table 1 was prepared, reheated at 1100 to 1250 ° C, and then subjected to a series of processes according to the conditions shown in Table 2 to prepare a cold rolled steel sheet.

The yield strength (YS), the tensile strength (TS) and the elongation (T-El) of the respective cold-rolled steel sheets were measured and the results are shown in Table 3 below. At this time, the specimens for the tensile test were prepared and then subjected to the tensile test standard of JIS standard according to the universal tensile tester at room temperature.

On the other hand, in order to evaluate the weldability of each cold rolled steel sheet, heterogeneous welding was applied. At this time, 980 MPa or more (DP) steel which is widely used as automobile material was used as a relative material. Welding conditions and tests were carried out by SEP 1220 standard method.

The Vickers hardness and tensile strength of the welds after welding were measured, and the average values were obtained after measuring three points. The results are shown in Table 3 below. The tensile strength was measured within the welding current range. The Vickers hardness was measured at the boundary between the two types of steel welded by spot welding, that is, at the portion where the martensite was well formed by diluting the steel pieces of the two types of hot dip galvanizing (Fig. 3 (B) part).

division Alloy composition (% by weight) C Mn P S Al Si Ti B W V N Comparative River 1 0.15 17.0 0.015 0.003 0 0 0.04 0.0015 0.20 0.30 0.007 Inventive Steel 1 0.55 19.1 0.014 0.002 0.24 0.21 0.05 0.0021 0.41 0.52 0.006 Invention river 2 0.58 17.3 0.016 0.003 0.34 0.12 0.06 0.0022 0.45 0.71 0.007 Invention steel 3 0.72 18.2 0.014 0.002 0.32 0.17 0.04 0.0019 0.64 0.62 0.008 Inventive Steel 4 0.84 16.1 0.015 0.003 0.48 0.22 0.03 0.0025 0.67 0.65 0.006 Comparative River 2 0.86 16.2 0.016 0.003 0.64 0.12 0.08 0.0010 0.70 0.67 0.007

division Finishing hot rolling
Temperature (℃) Coiling temperature
(° C) Cold rolling reduction ratio
(%) Annealing temperature
(° C) Comparative River 1 975 450 52 740 Inventive Steel 1 925 450 42 760 Invention river 2 921 450 52 770 Invention steel 3 915 450 55 780 Inventive Steel 4 900 450 56 770 Comparative River 2 900 450 50 760

division Mechanical properties Welding evaluation YS (MPa) TS (MPa) T-El (%) Hardness (Hv) Tensile strength (kN) Comparative River 1 650 950 44.0 474 4.8 Inventive Steel 1 816 1304 27.1 295 6.2 Invention river 2 864 1324 25.3 265 6.8 Invention steel 3 912 1398 26.2 250 10.1 Inventive Steel 4 1005 1501 18.0 267 11.5 Comparative River 2 1050 1588 10.4 276 11.6

As shown in Tables 1 to 3, inventive steels 1 to 4, in which the alloy composition and the manufacturing conditions satisfy all of the requirements proposed in the present invention, are not only excellent in strength and ductility, but also have a weld hardness of less than 450 Hv, Exceeded 6.0 kN.

On the other hand, comparative steel 1, in which the content of C, V, and W was insufficient, was not able to secure sufficient strength.

In addition, in the comparative steel 2 in which the content of Al was excessive, sufficient elements for influencing the precipitation strengthening effect were sufficiently added, and ultrahigh strength could be secured.

3 is a photograph of a weld formed after the welding of the invention steel 4 and the comparative steel 1, and shows the hardness value measured at a specific point.

As shown in Fig. 3, the hardness value of most of the inventive steel 4 (A) is less than 450 Hv in the region of the welded portion, whereas the comparative steel 1 (B) has the hardness value of most of 450 Hv in the welded region.

Claims (7)

(P): not more than 0.8%, sulfur (S): not more than 0.05%, aluminum (Al): 0.01 to 0.5% (Si): 0.3% or less (excluding 0%), vanadium (V): 0.5-1.0%, tungsten (W): 0.3-1.0%, titanium (Ti) : 0.001 to 0.003%, nitrogen (N): 0.015% or less, the balance Fe and other unavoidable impurities,
A high manganese steel sheet having excellent weld strength with a Vickers hardness of less than 450 Hv in a weld formed after welding.
The method according to claim 1,
Wherein the steel sheet comprises a microstructure and a composite structure of austenite and martensite.
The method according to claim 1,
Wherein the steel sheet has a tensile strength of 1300 MPa or more and an elongation of 20 to 50%.
The method according to claim 1,
And a tensile strength of the welded portion is 6.0 kN or more.
(P): not more than 0.8%, sulfur (S): not more than 0.05%, aluminum (Al): 0.01 to 0.5% (Si): 0.3% or less (excluding 0%), vanadium (V): 0.5-1.0%, tungsten (W): 0.3-1.0%, titanium (Ti) : 0.001 to 0.003%, nitrogen (N): 0.015% or less, the balance Fe and other unavoidable impurities in a temperature range of 1100 to 1250 캜;
Subjecting the reheated steel slab to finish hot rolling in a temperature range of 800 to 950 캜 to produce a hot-rolled steel sheet;
Winding the hot rolled steel sheet in a temperature range of 400 to 700 ° C after cooling;
Picking up the wound hot-rolled steel sheet and cold-rolling the steel sheet at a reduction ratio of 30 to 60% to produce a cold-rolled steel sheet; And
Continuously annealing the cold-rolled steel sheet in a temperature range of 650 to 900 ° C
Wherein the steel sheet has an excellent weld strength.
6. The method of claim 5,
Wherein the vanadium (V) carbide is formed during the continuous annealing process.
6. The method of claim 5,
Further comprising the step of zinc plating after the continuous annealing treatment.

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