KR20090070503A - High managese steel plate, hot rolled steel plate, cold rolled steel plate, galvanized steel plate having excellent deep drawability and manufacturing method thereof - Google Patents

Abstract

A high manganese steel sheet, hot rolled steel sheet, cold rolled steel sheet, galvanized steel sheet and manufacturing methods thereof are provided to produce a steel sheet for vehicle with high tensile strength over 800MPa, elongation ratio over 30%, and excellent deep drawability. A high manganese steel sheet comprises C 0.3~0.9 weight%, Mn 15~30 weight%, Al 0.01~4.0 weight%, Si 0.1~1.0 weight%, P less than 0.05 weight%, S less than 0.01 weight%, and N less than 0.04 weight%. The steel sheet also includes at least one selected from the group consisting of Nb 0.02~0.1 weight%, Ti 0.01~0.1 weight%, and V 0.025~0.5 weight%. The micro-structure of the high manganese steel is austenite single phase structure. A manufacturing method of the high manganese steel sheet comprises hot-rolling at 850~1000°C, winding at 700°C or below, and cold-rolling at the reduction rate of 30~80%.

Description

High-strength high manganese steel, hot rolled steel, cold rolled steel, plated steel and excellent manufacturing method of deep drawing property {High Managed Steel Plate, Hot Rolled Steel Plate, Cold Rolled Steel Plate, Galvanized Steel Plate Having Excellent Deep Drawability and Manufacturing Method Thereof}

The present invention relates to a high-strength high manganese steel excellent in workability, and more particularly, high-strength high manganese steel having excellent elongation, high work hardening index, and excellent deep drawing property, and hot rolled steel sheet manufactured using the same, cold rolling It relates to a steel sheet and a manufacturing method thereof.

With increasing interest in environmental pollution, fuel efficiency and safety in the automotive industry, automakers are expanding the application of lightweight and high-strength materials. This trend is active because the materials applied to various structural members in addition to automobile parts have active characteristics.

However, the material generally has the property of decreasing elongation with increasing strength. Therefore, it is difficult to satisfy both the strength directly related to safety and the elongation associated with proper workability. In order to overcome these problems, steel grades such as abnormal tissue steels and metamorphic organic-plastic steels, which are excellent in formability, have been developed in the past, but these steel grades also have high strength and low elongation, so that automotive parts such as automobile structural members and inner plate materials are used. There is a limit to manufacturing a part having a complicated shape because it does not satisfy the required workability.

Particularly, in case of using low carbon steel high strength steel as the automotive steel plate, when the tensile strength is 800MPa or more, the elongation must be secured up to 30% or more, so that it is easy to apply to complex shaped parts, but the high strength of conventional low carbon steel series In steel, there is a limit that it is difficult to obtain these properties.

Therefore, in order to solve these problems, automakers are using a complicated and bypassing process of simplifying the shape of parts or forming and re-welding them into several parts. However, when manufacturing parts by welding, the properties of the weld are inevitably weaker than those of the base metal, and therefore they are not only limited to the design of the vehicle body but also deteriorated in part characteristics due to the inferiority of the weld. There is still a disadvantage that the process cost increases significantly.

Therefore, automakers can form parts with complex shapes and continuously demand materials having high strength and excellent workability and deep drawing in order to increase design freedom in designing a vehicle body.

The present invention provides a high manganese type high strength steel sheet that can be manufactured stably and easily even in complex automobile products due to the high strength of the steel sheet itself and excellent processability and deep drawing property so as to meet the requirements of the above-described automobile manufacturers. I would like to.

The present invention is in weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.04 High strength, characterized in that it further comprises one or two or more components selected from the group consisting of less than or equal to Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1% and V: 0.025 to 0.5% Provides high manganese steel.

Furthermore, the present invention provides a hot rolled steel sheet that hot rolled the high manganese steel, and a cold rolled steel sheet that is cold rolled.

Furthermore, the present invention provides a high manganese plated steel sheet in which the cold rolled steel sheet is subjected to electroplating or alloyed hot dip plating.

Furthermore, in the present invention, by weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: For steel slabs containing 0.04% or less, additionally comprising one or two or more components selected from the group consisting of Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1% and V: 0.025 to 0.5% ,

Reheating step to homogenize by heating to 1050 ~ 1300 ℃;

Hot rolling step of producing a steel sheet by finishing hot rolling at 850 ~ 1000 ℃; And

A winding step of winding the steel sheet at a temperature of 700 ° C. or less;

It provides a method for producing a high manganese hot rolled steel sheet comprising a.

Furthermore, the present invention provides a high manganese hot rolled steel sheet by continuously annealing and plating the high manganese cold rolled steel sheet and the cold rolled steel sheet manufactured by cold rolling the high manganese hot rolled steel sheet at a reduction ratio of 30 to 80%. It provides a method of manufacturing.

According to the high manganese steel of the present invention and the steel sheet using the same, a tensile strength of 800 MPa or more and an elongation of 30% or more can be produced for automotive steel sheet having excellent deep drawing.

The present inventors have studied to provide a steel sheet having high strength and at the same time excellent in workability and deep drawing, and as a result, by appropriately adding Al to the austenitic single-phase high-manganese steel formed by adding a large amount of Mn in the steel, twins ( By controlling the formation rate of twinning, it has been found that the work hardening index can be increased to improve the tensile properties such as elongation and the deep drawing property, and the present invention has been achieved based on these facts.

Furthermore, by controlling the amount of C appropriately to the high manganese steel, the workability is improved, and addition of Si, Ti, Nb, etc., has ensured excellent yield strength.

Hereinafter, the component system constituting the steel of the present invention will be described in detail.

C: 0.3 ~ 0.9%

C is an element that can contribute to stabilization of the austenite phase. If the amount of C added is less than 0.3%, the α'-marthecite phase is formed during deformation, so that cracks occur during steelworking and ductility is lowered. On the other hand, if the amount of C is more than 0.9%, the austenite phase is stable. Too much increase may result in a lower workability due to the transition of the deformation behavior due to slip deformation. Therefore, the amount of C added is limited to 0.3 ~ 0.9%.

Mn: 15 ~ 30%

Mn is also an essential element for stabilizing the austenite phase, but at less than 15%, an α'-marthecite phase can be formed which impairs the formability, thereby increasing the strength but decreasing the ductility rapidly. On the other hand, when the amount of Mn added exceeds 30%, the occurrence of twins is suppressed to increase strength but decrease ductility. Furthermore, as the amount of Mn added increases, hot rolling cracking occurs well, and the production cost of products increases, so the upper limit of the amount of Mn added is limited to 30%.

Al: 0.01 ~ 4.0%

Al is usually added for deoxidation of steel, but in the present invention it is added to increase the work hardening index. In high manganese steels such as the present invention, Al improves ductility by suppressing formation of ε-marte cite phase by increasing the stacking fault energy in the slip surface of the steel as well as the stabilizing role of the ferrite phase. Furthermore, Al can suppress the formation of the epsilon-marte cite phase even in the case of low Mn addition amount, thus making a great contribution to minimizing Mn addition amount and improving workability.

In addition, the segregation of components is reduced when Al is added, so that the material becomes uniform and formability is increased. Therefore, it is necessary to add more than 0.01% because of this property, the ε-martensite is produced in the addition amount of less than that may increase the strength, but the deep drawing property may be drastically reduced. However, if the amount exceeds 4.0%, there are problems such as reducing the ductility by reducing the occurrence of twinning, deterioration of castability during continuous casting, and deterioration of the surface quality of the product due to surface oxidation during hot rolling. The upper limit is limited to 4.0% as it may.

Nb: 0.02-0.1%

Nb functions as a strong carbide forming element that forms carbide, such as Ti, in combination with C. The carbide formed at this time is an element that is effective for miniaturization of grain size by suppressing the growth of crystal grains, and usually forms a precipitated phase at a lower temperature than Ti. Add. However, at less than 0.02%, the effect is insignificant. On the other hand, if it is added more than 0.1%, excessive Nb segregates at grain boundaries, causing grain embrittlement or excessively coarsening of precipitated phases. Since it falls, the amount of Nb added is limited to 0.02 to 0.1%.

Ti: 0.01 ~ 0.1%

Ti is a strong carbide forming element that bonds with carbon to form carbides. The carbides formed at this time are effective for miniaturizing grain size by preventing the growth of grains. However, if the addition amount is less than 0.01%, the effect is insignificant, whereas if it exceeds 0.1%, the excess Ti segregates at the grain boundary, causing grain embrittlement, or excessively coarsening of the precipitate phase, thereby reducing the effect of grain growth. Therefore, the amount of Ti added was limited to 0.01 to 0.1%.

V: 0.025-0.5%

V functions as a strong carbide forming element that bonds with C to form carbides, such as Ti and Nb. In addition, since V forms a fine precipitated phase at low temperature, V is also an element having a large precipitation strengthening effect. However, when added in a small amount of less than 0.02%, the effect is insignificant, whereas if it exceeds 0.5%, excess Nb segregates at the grain boundaries, causing grain boundary odors, or excessively coarsening of the precipitate phases, thereby reducing the grain growth effect. Therefore, the amount of V added was limited to 0.025 to 0.5%.

Si: 0.1 ~ 1.0%

Si is a solid solution strengthening element that decreases the grain size by the solid solution effect and increases the yield strength. In addition, when an appropriate amount of Si is added to the steel to which Mn is added in a large amount as in the present invention, since a thin silicon oxide layer is formed on the surface to suppress oxidation of Mn, a thick Mn oxide layer formed after rolling in a cold rolled steel sheet is It can prevent formation. In addition, since the Si prevents the corrosion of the cold rolled steel sheet after annealing to improve the surface quality, and maintains the excellent surface quality as the base plate of the electroplating material is added 0.1% or more. However, if the addition amount is excessively excessive, Si oxide may be formed on the surface of the steel sheet during hot rolling, and the pickling effect may be reduced, thereby lowering the surface quality of the hot rolled steel sheet. In addition, excessive Si is concentrated on the surface of the steel sheet during high temperature annealing in the continuous annealing process and the continuous hot dip plating process, thereby reducing the wettability of the molten zinc on the surface of the steel sheet during hot dip plating. The upper limit thereof is 1.0%, and preferably 0.5% because it can be made.

P: 0.05% or less

P is an element that is inevitably contained in the manufacturing process, and the quality thereof, such as the formation of performance cracks, is reduced, so the range of addition is limited to less than 0.05%.

S: 0.01% or less

S is an element that is inevitably contained during manufacture, and may form coarse manganese sulfide (MnS), which may cause defects such as flange cracks, and it is preferable to suppress the addition amount as much as possible because it reduces the hole expandability of the steel sheet. Therefore, the addition amount is limited to 0.01% or less.

N: 0.04% or less

N is an element that acts with aluminum during the solidification process in the austenite grains to precipitate fine nitrides to promote twin formation. Therefore, the strength and ductility of the steel sheet is improved. However, when the addition amount is excessively more than 0.04%, the nitride may be excessively precipitated and the hot workability and elongation may be lowered, so the amount of nitrogen is limited to 0.04% or less.

Hereinafter, a method of manufacturing the high manganese steel of the present invention will be described in detail.

In general, the production of hot rolled steel sheet of high manganese steel uses the continuous casting method as in the manufacturing process of the general steel. The steel slab composed of the above-described component system is first subjected to a homogenization treatment in the range of 1050 to 1300 ° C.

In the present invention, the steel slab heating temperature during hot rolling is limited to 1050 to 1300 ° C., when the heating temperature exceeds 1300 ° C., since the grain size increases, surface oxidation may occur, the strength may decrease, or the surface may be inferior. to be. In addition, when heated to a temperature exceeding 1300 ℃ can form a liquid film in the columnar grain boundary of the steel slab may cause cracks during hot rolling. However, if the heating temperature is too low, it is difficult to secure the temperature during the finish rolling, so the rolling load increases due to the temperature decrease, so that the rolling cannot be sufficiently rolled to a predetermined thickness, and the lower limit thereof is limited to 1050 ° C.

Finishing hot rolling is performed in the range of 850 to 1000 ° C. for the steel slab in which the homogenization treatment is performed by heating. When the finish rolling temperature is lowered below 850 ° C., the rolling load may be increased, which may not only impair the rolling mill but may also deteriorate the quality of the steel sheet. On the other hand, if the rolling finish temperature is excessively high, surface oxidation may occur during rolling, so the rolling finish temperature is limited to 850 to 1000 ° C.

After finishing hot rolling, the steel sheet is hot rolled at a temperature of 700 ° C or lower. When the temperature of the hot rolled coil exceeds 700 ℃, since the thick oxide film and internal oxidation may occur on the surface of the hot rolled steel sheet it is not easy to remove the oxide layer during the pickling process. Therefore, the coiling temperature of the hot rolled steel sheet is preferably lowered to 700 ℃ or less.

After hot rolling, cold rolling is performed to match the shape and thickness of the steel sheet. Cold rolling is performed at a rolling reduction of 30 to 80%. The cold rolled steel sheet is subjected to a continuous annealing treatment. The annealing temperature needs to be performed at 700 ° C. or higher, because when the annealing temperature is too low, it is difficult to secure sufficient processability and transformation into austenite does not occur sufficiently to maintain an austenite phase at low temperatures. Since the high manganese steel of the present invention is an austenite steel which does not cause phase transformation, it is produced by annealing under ordinary annealing conditions of 700 ° C. or higher because heating can be ensured sufficiently when it is heated above the recrystallization temperature.

The annealing steel sheet may be made of a plated steel sheet, in the present invention, it is possible to treat the electroplated steel sheet by electroplating or the alloyed hot-dip steel sheet by alloyed hot-dip plating. In order to manufacture the electroplated steel sheet, it is possible to perform electroplating under conventional methods and conditions used in the related art. In addition, the alloying hot-dip plating treatment is performed by performing a conventional alloying hot-dip plating treatment on the cold rolled steel sheet is continuous annealing, it is possible to produce an alloyed hot-dip steel sheet by this treatment.

When performing the electroplating process or alloying hot-dip galvanizing treatment for a conventional metamorphic steel, in most cases, appropriate heat treatment conditions are required, but the high manganese steel of the present invention is composed of austenite single-phase structure and does not occur because transformation does not occur Since there is no significant difference in mechanical properties even without heat treatment conditions, the steel sheet may be manufactured by plating under normal conditions.

The present invention will be described in more detail with reference to the following examples.

Table 1 is a table showing the composition of the inventive steel and comparative steels.

C Si Mn Al P S Nb V N (ppm) Molding result Annealing Temperature 800 Annealing Temperature 700 Inventive Steel 1 0.7 0.3 18 1.5 0.01 0.007 0 0 60 O O Comparative Steel 1 0.6 0.3 18 0 0.01 0.004 0 0 60 X X Inventive Steel 2 0.6 0.3 18 1.0 0.01 0.004 0 0 60 O O Inventive Steel 3 0.6 0.3 18 1.5 0.01 0.007 0 0 60 O O Inventive Steel 4 0.6 0.3 18 2.0 0.01 0.004 0 0 60 O O Comparative Steel 2 0 0.3 30 0 0.01 0.004 0 0 60 X X Comparative Steel 3 0.6 0.3 24 0 0.01 0.002 0 0 60 O X Comparative Steel 4 0.6 0.3 22 0.004 0.01 0.007 0 0 60 X X Inventive Steel 5 0.5 0.3 25 1.5 0.01 0.008 0 0 60 O O Inventive Steel 6 0.6 0.3 18 1.5 0.01 0.007 0.03 0 60 O O Inventive Steel 7 0.6 0.3 18 1.5 0.01 0.007 0.02 0.1 60 O O Inventive Steel 8 0.6 0.3 18 1.5 0.01 0.007 0.03 0.1 60 O O

Table 1 shows the compositions of the inventive steel and the comparative steel, and the steel ingots of each composition were maintained in a heating furnace at 1200 ° C. for one hour, and then hot rolled. Subsequently, pickling was performed on the hot rolled steel sheets and cold rolled at a cold rolling rate of 50%.

Each cold-rolled specimen was annealed at 800 ° C., and the overaging temperature was continuously annealed at 400 ° C., followed by hot dip galvanizing to prepare a plated steel sheet. Drawing experiments were carried out using the plated steel sheets. In this case, the inventive steels showed excellent deep drawing properties as a result of forming (O). (X).

In addition, the microstructure changes of the inventive and comparative steels were observed and shown in FIG. 1. Referring to FIG. 1, the invention steel to which Al was appropriately added showed a uniform structure of equiaxed crystals, but the comparison steel without Al was found to have a high degree of segregation in the rolling direction.

In particular, comparative steels with little or no Al added showed severe segregation, and in the cup forming experiments (see FIG. 2), wrinkles or breakage occurred in the cup forming process.

1 is a photograph showing the microstructure of the inventive steel and comparative steel according to the embodiment of the present invention according to the addition amount of Al

Figure 2 is an experimental photograph when the cup is formed of the invention steel and comparative steel of the embodiment of the present invention

Claims (8)

By weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.04% or less And high strength manganese steel further comprising one or two or more components selected from the group consisting of Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, and V: 0.025 to 0.5%. The high-strength high manganese steel according to claim 1, wherein the high manganese steel is an austenite single phase structure. A high manganese hot rolled steel sheet, wherein the high manganese steel of claim 1 or 2 is hot rolled at a temperature of 850 to 1000 ° C. and wound at a temperature of 700 ° C. or less. The high manganese cold rolled steel sheet according to claim 1 or 2, wherein the high manganese steel is hot rolled at a range of 850 to 1000 ° C. and cold rolled at a reduction ratio of 30 to 80%. By weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.04% or less For the steel slab which further comprises one or two or more components selected from the group consisting of: Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1% and V: 0.025 to 0.5%, Reheating step to homogenize by heating to 1050 ~ 1300 ℃; Hot rolling step of producing a steel sheet by finishing hot rolling at 850 ~ 1000 ℃; And A winding step of winding the steel sheet at a temperature of 700 ° C. or less; Method for producing a high manganese hot rolled steel sheet comprising a. By weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.04% or less For the steel slab which further comprises one or two or more components selected from the group consisting of: Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1% and V: 0.025 to 0.5%, Reheating step to homogenize by heating to 1050 ~ 1300 ℃; Hot rolling step of producing a steel sheet by finishing hot rolling at 850 ~ 1000 ℃; A winding step of winding the steel sheet at a temperature of 700 ° C. or less; A cold rolling step of cold rolling the wound steel sheet at a reduction ratio of 30 to 80%; And Continuous annealing step of continuous annealing at 700 ℃ or more; Method for producing a high manganese cold rolled steel sheet comprising a. By weight%, C: 0.3-0.9%, Mn: 15-30%, Al: 0.01-4.0%, Si: 0.1-1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.04% or less For the steel slab which further comprises one or two or more components selected from the group consisting of: Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1% and V: 0.025 to 0.5%, Reheating step to homogenize by heating to 1050 ~ 1300 ℃; Hot rolling step of producing a steel sheet by finishing hot rolling at 850 ~ 1000 ℃; A winding step of winding the steel sheet at a temperature of 700 ° C. or less; A cold rolling step of cold rolling the wound steel sheet at a reduction ratio of 30 to 80%; Continuous annealing step of continuous annealing at 700 ℃ or more; And Plating treatment for plating the steel sheet; Method for producing a high manganese plated steel sheet comprising a. 8. The method of claim 7, wherein the plating treatment is an electroplating treatment or an alloyed hot dip plating treatment.

Images