KR101778404B1

KR101778404B1 - Clad steel sheet having excellent strength and formability, and method for manufacturing the same

Info

Publication number: KR101778404B1
Application number: KR1020150184305A
Authority: KR
Inventors: 송태진; 진광근; 조원태
Original assignee: 주식회사 포스코
Priority date: 2015-12-22
Filing date: 2015-12-22
Publication date: 2017-09-14
Also published as: KR20170075138A

Abstract

In one aspect of the present invention, there is provided a clad sheet comprising a base material and a clad material provided on both sides of the base material, wherein the base material contains 0.3 to 0.8% of C, 13 to 25% of Mn, Wherein the clad material is a ferritic carbon steel containing 0.03 to 0.3% of C, 0.3 to 3.5% of C, the balance of Fe and unavoidable impurities, in terms of weight%, and The present invention relates to a clad sheet having excellent moldability.

Description

TECHNICAL FIELD [0001] The present invention relates to a clad steel sheet having excellent strength and formability, and a method of manufacturing the clad sheet. [0002]

The present invention relates to a clad steel sheet excellent in strength and moldability and a method for producing the same. And more particularly to a clad steel sheet excellent in strength and moldability which can be used for structural members of automobiles by press molding.

In recent years, carbon dioxide regulations for reducing global warming have strongly demanded the weight reduction of automobiles, and at the same time, the strength of automotive steel sheets has been steadily increasing to improve the collision stability of automobiles. In order to produce such high-strength steel sheets, it is common to utilize low-temperature transformed structures. However, it is difficult to obtain an elongation of 30% or more at a tensile strength of 600 MPa or more when a low-temperature transformed structure is used to achieve high strength. Therefore, it is difficult to apply to cold- There was a problem that the parts were difficult to design.

On the other hand, when a ferrite based low carbon steel or a low carbon steel is used to produce a complicated shape component by cold press forming, the required formability can be secured, but it is difficult to secure a tensile strength of 400 MPa, There is a problem in that the weight of the steel is increased and the weight of the automobile is not achieved.

On the other hand, in Patent Document 1, a large amount of austenite stabilizing elements such as carbon (C) and manganese (Mn) are added to maintain the steel structure as austenite single phase, and simultaneously strength and formability are secured Method. In order to secure austenite single phase structure, it is general to add at least 0.5 wt% of carbon and at least 15 wt% of Mn.

However, in this case, there is a problem that the production cost of the steel sheet is increased by adding a large amount of Mn, and there is a problem that it is difficult to secure the plating property by the Mn oxide.

Therefore, it is necessary to develop a steel sheet for automobiles which is excellent in strength and formability and is excellent in plating property.

Korean Patent Publication No. 2007-0023831

An aspect of the present invention is to provide a clad sheet having high strength and excellent elongation, and excellent in plating ability, 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.

One aspect of the present invention is a clad sheet including a base material and a clad material provided on both sides of the base material,

The base material is an austenitic high manganese steel containing 0.3 to 0.8% of C, 13 to 25% of Mn, the balance of Fe and unavoidable impurities,

Wherein the clad material is a ferritic carbon steel containing 0.03 to 0.3% of C, 0.3 to 3.5% of Mn, and the balance of Fe and unavoidable impurities in weight percent.

According to another aspect of the present invention, there is provided a method for manufacturing a high-strength steel sheet, comprising: preparing a base material which is an austenitic high-manganese steel containing 0.3 to 0.8% of C, 13 to 25% of Mn, Fe and unavoidable impurities;

Preparing a clad material which is a ferritic carbon steel containing 0.03 to 0.3% of C, 0.3 to 3.5% of Mn, and the balance of Fe and unavoidable impurities in terms of% by weight;

Disposing the base material between the two clad materials to obtain a laminate;

Welding the rim of the laminate to a temperature range of 1050 to 1350 캜;

Finishing rolling the heated laminate to a temperature range of 750 to 1050 캜 to obtain a hot-rolled steel sheet;

Winding the hot-rolled steel sheet at 50 to 700 ° C;

Cold rolling the rolled hot-rolled steel sheet at a reduction ratio of 35 to 90% after pickling to obtain a cold-rolled steel sheet; And

And annealing the cold-rolled steel sheet at a temperature of 550 ° C or higher and a temperature of A3 + 50 ° C or lower of the clad material, and a method of manufacturing the clad sheet.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a clad steel sheet which can be suitably applied to a steel sheet for automobiles and which can be applied preferably to cold press forming, having a yield strength of 400 MPa or more and a product of tensile strength and elongation of 30000 MPa% There is an effect that the manufacturing method thereof can be provided.

1 is a schematic view of a clad steel sheet in which austenitic high manganese steel is used as a base material (B) and ferrite-based carbon steels are used as clad materials (A and C).
Fig. 2 is a photograph of a wavefront after the tensile test of Inventive Example 16 taken by a scanning electron microscope.
3 is a graph showing tensile strengths and elongation ratios of abnormal texture steels 1 to 4, composite structure steels 1 to 3, high manganese steels 1 to 3, and inventive examples 1 to 39 in Table 1.
Fig. 4 is a photograph of the appearance of the hot-dip galvanizing material of Inventive Examples 1, 3, 5, 6, 8 and 10. Fig.
5 is a photograph of the appearance of the hot-dip galvanizing material of the high manganese steel 1 in Table 1;

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The present inventors have found that the addition of a large amount of manganese and carbon in the conventional high manganese steel sheet allows the steel microstructure to be maintained at austenite at room temperature to secure strength and moldability. And that there is a problem that it is difficult to secure the plating property by the Mn oxide, and it is deeply studied to solve this problem.

As a result, it has been found that strength and elongation rate are ensured by producing a composite steel sheet made of a ferrite-based carbon steel having high strength and low moldability and austenitic high manganese steel as a base material and having excellent plating ability and low manufacturing cost It has been confirmed that it is possible to produce a steel sheet for automobiles excellent in plating ability, and the present invention has been accomplished.

Excellent in strength and formability Clad Steel plate

Hereinafter, a clad sheet having excellent strength and formability according to one aspect of the present invention will be described in detail.

A clad sheet having excellent strength and formability according to one aspect of the present invention is a clad sheet including a base material and a clad material provided on both sides of the base material,

Hereinafter, the base material and the clad material of the present invention will be described, respectively, and then the clad sheet including the clad material provided on both sides of the base material will be described.

Base material ( Austenitic system High manganese steel )

Hereinafter, the alloy composition of the austenitic high manganese steel constituting the base material of the clad steel sheet, which is one aspect of the present invention, will be described in detail. Units of each elemental content are by weight unless otherwise specified.

Carbon (C): 0.3 to 0.8 wt%

Carbon is an element contributing to the stabilization of the austenite phase, and as the content thereof increases, there is an advantageous aspect in securing the austenite phase. Carbon also increases the energy of lamination defects in the steel, thereby increasing the tensile strength and elongation at the same time. When the content of carbon is less than 0.3%, there is a problem that the α '(alpha re-) -martensite phase is formed on the surface layer due to decarburization at the time of high-temperature processing of the steel sheet, resulting in poor delayed fracture and fatigue performance. There is a problem that is difficult to secure. On the other hand, if the content exceeds 0.8%, the electrical resistivity increases and the weldability may decrease. Therefore, in the present invention, it is preferable to limit the carbon content to 0.3 to 0.8%.

Manganese (Mn): 13 to 25 wt%

Manganese is an element which stabilizes the austenite phase together with carbon. When the content is less than 13%, it is difficult to secure a stable austenite phase due to the formation of α '(alpha re-) martensite phase during deformation, There is a problem that the further improvement with respect to the increase of the strength, which is a concern of the present invention, does not occur substantially and the manufacturing cost rises. Therefore, the content of Mn in the present invention is preferably limited to 13 to 25%.

The remaining component of the base material 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.

In addition to the above-mentioned composition, the austenitic high manganese steel constituting the base material contains 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.0005 to 0.005% of B, 0.04% or less of N, P: not more than 0.03%, and S: not more than 0.03%.

Aluminum (Al): 0.3 to 2.5 wt%

Aluminum is usually added for deoxidation of steel, but the present invention enhances the ductility and delayed fracture characteristics of steel by suppressing the formation of ε (entrance run) -martensite by increasing the stacking fault energy. If the aluminum content is less than 0.3%, there is a problem that the ductility of the steel is deteriorated due to the rapid work hardening phenomenon and the delayed fracture resistance is poor. On the other hand, when the aluminum content exceeds 2.5% by weight, The main composition is heated, and the oxidation of the steel surface is deepened during the hot rolling, thereby deteriorating the surface quality. Therefore, in the present invention, the aluminum content is preferably limited to 0.3 to 2.5 wt%.

Thinatium (Ti): 0.01 to 0.5 wt%

Titanium reacts with nitrogen in the steel to precipitate nitrides, which improves the formability of hot rolling. In addition, the titanium reacts with carbon in some steel to form precipitation phases, thereby increasing the strength. It is preferable that titanium is contained in an amount of 0.01% or more, but if it exceeds 0.5%, precipitates are formed excessively and deteriorate the fatigue characteristics of the parts. Accordingly, the titanium content is preferably 0.01 to 0.5%.

Boron (B): 0.0005 to 0.005 wt%

When boron is added in a small amount, the grain boundary of the cast steel is strengthened to improve the hot rolling property. However, when the content of boron is less than 0.0005%, the above effect is not sufficiently exhibited. If the content of boron exceeds 0.005%, further performance improvement can not be expected and the cost is increased. Therefore, the content of boron is preferably 0.0005 to 0.005%.

Nitrogen (N): 0.04% by weight or less (excluding 0%)

It reacts with Al during the solidification process in the nitrogen (N) austenite crystal grains to precipitate fine nitrides to promote the generation of twin, thereby improving the strength and ductility of the steel sheet during molding. However, when the content exceeds 0.04%, excessive nitrides are precipitated and the hot workability and elongation can be lowered. Therefore, in the present invention, the nitrogen content is preferably limited to 0.04% or less

Phosphorus (P): 0.03% by weight or less

The phosphorus is an impurity which is inevitably contained and is an element which is a main cause of deteriorating the processability of steel by segregation. Therefore, it is preferable to control the content as low as possible. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.03% by weight.

Sulfur (S): 0.03 wt% or less

Sulfur is an inevitably contained impurity, which forms a coarse manganese sulfide (MnS) to generate defects such as flange cracks and greatly reduces the hole expandability of the steel sheet. Therefore, it is preferable to control the content as low as possible. The theoretical sulfur content is advantageous to be limited to 0% but it is inevitably contained in the manufacturing process normally. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.03% by weight.

In addition to the above composition, the austenitic high manganese steel constituting the base material may contain at least one of 0.03 to 2.0% of Si, 0.2 to 3.0% of Cr, 0.01 to 0.5% of Nb and 0.05 to 0.7% of V, May be further included.

Silicon (Si): 0.03 to 2.0 wt%

Silicon is a component that can be added to improve the yield strength and tensile strength of steel by solid solution strengthening. Since silicon is used as a deoxidizer, it can be included in the steel in an amount of more than 0.03%. When the content of silicon exceeds 2.0%, a large amount of silicon oxide is formed on the surface during hot rolling to lower the acidity and increase the electrical resistivity There is a problem that the weldability is lowered. Therefore, the content of silicon is preferably limited to 0.03 to 2.0%.

Cr (Cr): 0.2 to 3.0 wt%

Chromium is an effective element to increase strength. In order to obtain such an effect, the chromium content is preferably 0.2% or more. On the other hand, if the content of chromium exceeds 3.0%, coarse carbide is formed on the grain boundary during hot rolling to deteriorate hot workability, so the addition amount is limited to 3.0%. Therefore, the content of chromium in the present invention is preferably limited to 0.2 to 3.0 wt%.

Niobium (Nb): 0.01 to 0.5 wt%

Niobium is an element that reacts with carbon to form carbides, and can be added to increase the yield strength of steel by grain refinement and precipitation strengthening. In order to obtain such an effect, the content of niobium is preferably 0.01% by weight or more. On the other hand, when the content of niobium exceeds 0.5%, coarse carbide is formed at a high temperature, which causes surface cracking of the cast steel. Therefore, the content of niobium in the present invention is preferably limited to 0.01 to 0.5 wt%

Vanadium (V): 0.05 to 0.7 wt%

Vanadium is an element that reacts with carbon or nitrogen to form a carbonitride. It is a component that can be added to increase the yield strength by refining the crystal grains and strengthening the precipitation. In order to obtain such an effect, the vanadium content is preferably 0.05 wt% or more. On the other hand, when the content of vanadium exceeds 0.7% by weight, coarse carbonitrides are formed at a high temperature, thereby deteriorating hot workability. Therefore, the content of vanadium in the present invention is preferably limited to 0.05 to 0.7 wt%.

On the other hand, in the present invention, it is preferable that the austenitic high manganese steel constituting the base material not only satisfies the above-mentioned component system but also secures the austenite single phase structure with the microstructure of the steel sheet. By securing the microstructure as described above, strength and elongation can be secured at the same time. The austenite single phase means that all the microstructures except carbide are made of austenite. However, some unavoidable impure tissue may be included.

Clad material ( Ferritic Carbon steel)

Hereinafter, the alloy composition of the ferrite-based carbon steel constituting the clad material of the clad steel sheet as one aspect of the present invention will be described in detail. Units of each elemental content are by weight unless otherwise specified.

Carbon (C): 0.03 to 0.3 wt%

Carbon is an element that forms carbides and serves to improve the strength of steel. In addition, carbon diffuses into the austenite during the annealing process and serves to disperse martensite, bainite and retained austenite in the ferrite matrix during the cooling process after annealing to improve the strength of the steel. When the content is less than 0.03%, it can not be expected to form secondary phases such as carbide, martensite, bainite and retained austenite to improve the strength of steel. On the other hand, if the content exceeds 0.3%, the weldability of the steel sheet may be lowered. Therefore, in the present invention, the carbon content is preferably limited to 0.03 to 0.3%.

Manganese (Mn): 0.3 to 3.5 wt%

Manganese is an element that increases the hardenability and improves the strength of the steel sheet. In order to obtain such effects, the content thereof is preferably 0.3% or more. On the other hand, if it exceeds 3.5%, there is a fear of decreasing the formability of the steel sheet due to the segregation layer structure. Therefore, in the present invention, the content of Mn is preferably limited to 0.3 to 3.5%.

The remaining component of the clad material 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.

In addition to the above composition, the ferrite-based carbon steel constituting the clad material may further contain 0.02 to 1.0% of Al, 0.04% or less of N, 0.03% or less of P, 0.03% or less of S, .

Aluminum (Al): 0.02 to 1.0%

Aluminum (Al), like silicon, prevents the growth of iron carbide and secures retained austenite, thereby improving the elongation of steel. Silicon is an element to be added for deoxidation. Excessive cost is required to control the content to less than 0.02%. When the content exceeds 1.0%, silicon is generated and the plating ability is weakened by annealing. Therefore, the content of aluminum is preferably 0.02 to 1.0%.

Nitrogen (N): 0.04% by weight or less (excluding 0%)

Nitrogen (N) is an element that is inevitably contained, and AlN produced by reacting with aluminum that remains in the steel may cause surface cracking during performance. Therefore, it is preferable to control the content as low as possible, but it is inevitably contained in the manufacturing process. It is important to control the upper limit of nitrogen, and in the present invention, the upper limit of the nitrogen content is controlled to 0.04% by weight.

Phosphorus (P): 0.03% by weight or less

Sulfur (S): 0.03 wt% or less

In addition to the above composition, the ferrite-based carbon steel constituting the clad material may contain at least one of Si: 0.03 to 2.5%, Mo: 0.01 to 2%, Ti: 0.005 to 0.05%, and Nb: 0.005 to 0.05% May be further included.

Silicon (Si): 0.03 to 2.5%

Silicon (Si) is dissolved in the steel sheet to improve the strength of the steel and to inhibit the growth of iron carbide, thereby securing the retained austenite, thereby improving the elongation of the steel. Silicon is an element present in the molten steel as an impurity and an excessive cost is incurred to control it to less than 0.03%. When the content exceeds 2.5%, silicon is generated and the plating ability is weakened by annealing. Therefore, the silicon content is preferably 0.03 to 2.5%.

Molybdenum (Mo): 0.01 to 2.0%

Molybdenum is an element which improves the hardenability of steel and improves the strength of steel by promoting the formation of low-temperature transformation phase, and improves the strength of steel by forming carbide in steel. In order to obtain such an effect, the content thereof is preferably 0.01% or more, and if the content exceeds 2.0%, an excessive increase in manufacturing cost may be caused as compared with an intended strength improvement effect. Therefore, the molybdenum content is preferably 0.01 to 2.0%.

Tin (Ti): 0.005 to 0.05 wt%

The content of titanium (Ti) is preferably 0.005 to 0.05%. Titanium reacts with nitrogen and carbon in the steel to form carbonitride and increase the strength. For this purpose, it is preferable that titanium is contained in an amount of 0.005% or more, but if it exceeds 0.05%, precipitates are formed excessively and the main composition is deteriorated. Therefore, the titanium content is preferably 0.005 to 0.05%.

Niobium (Nb): 0.005 to 0.05 wt%

Niobium (Nb) is preferably 0.005 to 0.05%. Niobium is a carbonitride-forming element, such as titanium, which reacts with nitrogen and carbon in the steel to increase its strength. For this purpose, it is preferable that the niobium is contained in an amount of 0.005% or more, but if it exceeds 0.05%, precipitates are formed excessively and the main composition is deteriorated. Therefore, the content of niobium is preferably 0.005 to 0.05%.

On the other hand, in the present invention, the ferrite-based carbon steels constituting the cladding satisfy not only the above-mentioned composition but also the ferritic microstructure having a ferrite content of 40 to 95% by area and the remainder being retained in the retained austenite, ferrite, martensite, bainite, Or more. By securing the microstructure as described above, strength and excellent plating adhesion can be ensured.

At this time, the ferrite-based carbon steel may be an abnormal texture steel or a composite texture steel.

Clad Steel plate

The clad sheet according to one aspect of the present invention includes the base material and the clad material provided on both sides of the base material.

A clad steel sheet is defined as a laminate type composite material in which two or more surfaces of metal materials are metallurgically bonded and integrated. In general, a clad steel sheet has been used for a special purpose such as extreme corrosive environment by using a noble metal such as nickel (Ni) or copper (Cu) as a clad material. However, in the present invention, in order to excel in strength, elongation and plating A clad steel sheet comprising the aforementioned austenitic high-manganese steel as a base material and ferrite-based carbon steel as a clad material on both sides of the base material is proposed.

The base material, which is an inner steel material of the present invention, is characterized by being an austenitic high manganese steel excellent in strength and elongation rate by a high alloy amount. However, due to the high cost of manufacturing due to the large amount of alloying elements, and due to the Mn oxide produced on the surface of the steel, the galvanizing property is disadvantageously inadequate for use as automobile steel. On the other hand, the clad material, which is an outer steel, is composed of a ferrite-based carbon steel excellent in plating ability.

By including the above-mentioned base material and the clad material provided on both sides of the base material, an effect of excellent plating strength and excellent formability can be obtained.

At this time, the thickness of the base material may be 30 to 90% of the thickness of the clad sheet.

When the thickness of the base material is more than 90% of the thickness of the clad steel sheet, the manufacturing cost is increased. On the other hand, if it is less than 30%, there is a problem that the strength of the clad steel sheet is lowered and the formability is lowered.

The clad steel sheet may have a tensile strength of 400 MPa or more and a product of tensile strength and elongation of 30000 MPa% or more. By securing such yield strength, tensile strength and elongation, the clad steel sheet can be preferably applied to automotive structural members.

The cladding steel sheet may further include a plating layer, and the plating layer may be a Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg-based, Zn- Si-based, and Al-Si-Mg-based.

Excellent in strength and formability Clad Method of manufacturing steel sheet

Hereinafter, a method of manufacturing a clad sheet having excellent strength and formability, which is another aspect of the present invention, will be described in detail.

In another aspect of the present invention, there is provided a method of manufacturing a clad sheet having excellent strength and formability, comprising the steps of: preparing a base material that is an austenitic high manganese steel satisfying the alloy composition; Preparing a clad material which is a ferritic carbon steel satisfying the alloy composition described above; Disposing the base material between the two clad materials to obtain a laminate; Welding the rim of the laminate to a temperature range of 1050 to 1350 캜; Finishing rolling the heated laminate to a temperature range of 750 to 1050 캜 to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 50 to 700 ° C; Cold rolling the rolled hot-rolled steel sheet at a reduction ratio of 35 to 90% after pickling to obtain a cold-rolled steel sheet; And annealing the cold-rolled steel sheet in a temperature range of 550 ° C or higher and A3 + 50 ° C or lower of the clad material.

Base material Clad material Preparation step and lamination step

After the base material and the clad material satisfying the alloy composition described above are prepared, the base material is disposed between the two clad materials to obtain a laminate. At this time, the surface of the base material and the clad material can be cleaned before lamination.

The production method of the base material and the clad material is not particularly limited because it can be produced by applying a general manufacturing process. However, as a preferable example, the base material can be manufactured by casting molten steel produced in an electric furnace or a blast furnace, and the clad material is manufactured by refining and casting molten steel produced in a blast furnace to control an impurity content which can inevitably be contained .

Welding and heating stage

The edge of the laminate is welded and then heated to a temperature range of 1050 to 1350 ° C.

By welding the edges of the laminate, it is possible to prevent oxygen from entering between the base material and the clad material, thereby preventing the generation of oxides during heating.

When the heating temperature is lower than 1050 占 폚, it is difficult to ensure the finish rolling temperature during hot rolling, and there is a problem that the rolling load due to the temperature decrease increases and rolling to a predetermined thickness is difficult. On the other hand, when the heating temperature is higher than 1350 DEG C, crystal grain size increases and surface oxidation tends to occur to decrease the strength or surface disadvantage. In addition, since the liquid phase film is formed on the columnar phase boundary of the performance slab, there is a fear that cracks may occur during the subsequent hot rolling.

Hot rolling step

The heated laminate is subjected to finish rolling in a temperature range of 750 to 1050 DEG C to obtain a hot-rolled steel sheet.

If the finishing rolling temperature is less than 750 캜, there is a problem that the rolling load increases and the rolling mill becomes difficult. On the other hand, when the finish rolling temperature exceeds 1050 DEG C, surface oxidation may occur during rolling.

Coiling step

The hot-rolled steel sheet is wound at 50 to 700 ° C. If the coiling temperature is less than 50 캜, cooling by cooling water injection is required to reduce the temperature of the steel sheet, which causes an unnecessary increase in the process ratio. On the other hand, when the coiling temperature exceeds 700 ° C, a thick oxide film is formed on the surface of the hot-rolled steel sheet, which makes it difficult to control the oxide layer during the pickling process. Therefore, the winding temperature is preferably limited to 50 to 700 캜.

Cold rolling step

The rolled hot-rolled steel sheet is cold-rolled at a reduction ratio of 35 to 90% after pickling to obtain a cold-rolled steel sheet.

When the reduction rate is less than 30%, the recrystallization of the ferrite-based carbon steel constituting the clad material does not occur smoothly and the workability is poor. On the other hand, when the reduction rate exceeds 90%, there is a problem that the possibility of occurrence of plate fracture due to the rolling load is increased.

Annealing step

The cold-rolled steel sheet is annealed at a temperature of 550 ° C or higher and A3 + 50 ° C or lower of the clad material. Strength and recrystallization to ensure workability.

When the annealing temperature is lower than 550 占 폚, recrystallization of the austenite-type high manganese steel as the base material does not occur and sufficient workability can not be ensured. On the other hand, when the cladding material is annealed at a temperature exceeding A3 + 50 占 폚, the crystal grains of the cladding material are coarsened, so that the strength of the steel can be lowered.

Therefore, it is preferable that the annealing is carried out at a temperature ranging from 550 캜 to A 3 + 50 캜 of the clad material.

The plating layer may further include a Zn-based, Zn-Fe based, Zn-Al based, Zn-Mg based, Zn-Mg-Al based , A Zn-Ni-based alloy, an Al-Si-based alloy, and an Al-Si-Mg-based alloy.

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 )

A steel ingot of ultra-low carbon steel, unstructured steel and composite steel having high manganese steel and carbon steel having the composition shown in the following Table 1 was prepared, reheated in a heating furnace at 1150 ° C for 1 hour, and then rolled at a finish rolling temperature of 900 ° C Hot-rolled steel sheets were produced. Thereafter, the hot-rolled steel sheet was rolled at 450 ° C, cold-rolled at a cold-reduction rate of 50% after pickling to produce a cold-rolled steel sheet. Then, continuous annealing was performed at an annealing temperature of 810 캜. The tensile strength (YS), the tensile strength (TS), the uniform elongation (UEL) and the total elongation (TEL) of each of the prepared specimens were measured using a universal tensile tester, . The area fraction of each phase constituting the microstructure by observing the microstructure with an optical microscope is shown in Table 2 below.

On the other hand, high-manganese steel having the composition shown in the following Table 1 and ultra-low carbon steel, ultra-fine steel and carbon steel composite steels were prepared, and the surface of the steel ingot was cleaned, A three-ply laminate was prepared so as to have a lamination ratio shown in Table 2. Thereafter, arc welding was performed using a welding rod along the interface of the laminate. The welded interface was reheated in a heating furnace at 1150 캜 for one hour and then rolled at a finish rolling temperature of 900 캜 to produce a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was rolled at 450 ° C, cold-rolled at a cold-reduction rate of 50% after pickling to produce a cold-rolled steel sheet. Then, continuous annealing and hot-dip galvanizing were performed at an annealing temperature of 810 캜. The yield strength (YS), tensile strength (TS), elongation (EL) and TS * EL values of the prepared specimens were measured using a universal tensile tester and the results are shown in Table 3 below.

Steel grade C Si Mn P S Al Cr Mo Ti Nb V B N Ultra-low carbon steel 0.039 0.25 0.009 0.0051 0.03 0.0028 Abnormal tissue 1 0.07 1.6 0.011 0.0022 0.03 0.05 0.004 Abnormal tissue layer 2 0.08 0.3 1.8 0.013 0.0015 0.03 0.02 0.004 Abnormal texture steel 3 0.07 One 2.3 0.011 0.0016 0.03 0.02 0.02 0.0045 Abnormal tissue 4 0.1 1.1 2.8 0.012 0.0023 0.03 0.0048 Composite Steel 1 0.09 1.2 1.7 0.025 0.0015 0.04 0.0042 Composite structure steel 2 0.14 One 2.1 0.012 0.0027 0.04 0.025 0.0044 Composite structure steel 3 0.2 1.4 2.5 0.009 0.0014 0.12 0.025 0.0046 High manganese steel 1 0.51 16.800 0.013 0.013 1.340 0.210 0.067 0.0018 0.009 High manganese steel 2 0.60 1.510 17.300 0.010 0.001 1.400 0.068 0.250 0.0020 0.003 High manganese steel 3 0.62 0.990 16,900 0.010 0.001 1.280 0.200 0.072 0.021 0.392 0.0024 0.006

In Table 1, the unit of each element content is% by weight.

Steel grade Tension material Microstructure area fraction (area%) YS
(MPa) TS
(MPa) UEL
(%) TEL
(%) Austenite ferrite Martensite Bay knight Carbide Ultra-low carbon steel 219 341 25 44 100 Abnormal tissue 1 332 532 16 31 93 7 Abnormal tissue layer 2 378 654 18 28 88 12 Abnormal texture steel 3 505 861 12 20 80 20 Abnormal tissue 4 643 1066 10 15 45 45 10 Composite Steel 1 439 674 21 32 5 80 15 Composite structure steel 2 541 864 14 21 7 70 23 Composite structure steel 3 602 1008 16 24 9 41 5 45 High manganese steel 1 534.4 1007.1 50.3 60.2 99 One High manganese steel 2 789.0 1131.2 42.5 48.1 98 2 High manganese steel 3 897.6 1183.9 33.5 37.4 97 3

division Base material Clad material Lamination ratio Thickness ratio Clad steel sheet properties High manganese steel Carbon steel Clad material Base material Clad material Base material Clad material YS (MPa) TS (MPa) EL (%) TS * EL (Mpa *%) Comparative Example 1 High manganese steel 1 Ultra-low carbon steel One One One 0.33 0.67 324 563 54 30212 Comparative Example 2 High manganese steel 1 Abnormal tissue 1 One 0.5 One 0.20 0.80 376 627 40 25319 Comparative Example 3 High manganese steel 1 Abnormal tissue 4 One 0.5 One 0.20 0.80 621 1054 24 24917 Inventory 1 High manganese steel 1 Abnormal tissue 1 One 6 One 0.75 0.25 484 888 56 49594 Inventory 2 High manganese steel 1 Abnormal tissue 1 One One One 0.33 0.67 403 690 45 31204 Inventory 3 High manganese steel 1 Abnormal tissue layer 2 One 6 One 0.75 0.25 495 919 54 50049 Honorable 4 High manganese steel 1 Abnormal tissue layer 2 One One One 0.33 0.67 430 772 42 32417 Inventory 5 High manganese steel 1 Abnormal texture steel 3 One 6 One 0.75 0.25 527 971 51 49776 Inventory 6 High manganese steel 1 Abnormal tissue 4 One 6 One 0.75 0.25 562 1022 48 49468 Honorable 7 High manganese steel 1 Abnormal tissue 4 One One One 0.33 0.67 607 1046 30 30869 Honors 8 High manganese steel 1 Composite Steel 1 One 6 One 0.75 0.25 511 924 55 50863 Proposition 9 High manganese steel 1 Composite Steel 1 One One One 0.33 0.67 471 785 44 34588 Inventory 10 High manganese steel 1 Composite structure steel 2 One 6 One 0.75 0.25 536 971 51 50007 Exhibit 11 High manganese steel 1 Composite structure steel 2 One One One 0.33 0.67 539 912 35 32305 Inventory 12 High manganese steel 1 Composite structure steel 3 One 6 One 0.75 0.25 551 1007 51 51519 Inventory 13 High manganese steel 1 Composite structure steel 3 One One One 0.33 0.67 579 1008 36 36337 Inventory 14 High manganese steel 2 Abnormal tissue 1 One 6 One 0.75 0.25 675 981 46 44931 Honorable Mention 15 High manganese steel 2 Abnormal tissue layer 2 One 6 One 0.75 0.25 686 1012 45 45386 Inventory 16 High manganese steel 2 Abnormal tissue layer 2 One 2 One 0.50 0.50 584 893 41 36361 Inventory 17 High manganese steel 2 Abnormal texture steel 3 One 6 One 0.75 0.25 718 1064 42 45113 Inventory 18 High manganese steel 2 Abnormal texture steel 3 One 2 One 0.50 0.50 647 996 36 35815 Evidence 19 High manganese steel 2 Abnormal tissue 4 One 6 One 0.75 0.25 753 1115 40 44806 Inventory 20 High manganese steel 2 Abnormal tissue 4 One 2 One 0.50 0.50 716 1099 32 35200 Inventory 21 High manganese steel 2 Composite Steel 1 One 6 One 0.75 0.25 702 1017 45 46200 Inventory 22 High manganese steel 2 Composite Steel 1 One 2 One 0.50 0.50 614 903 42 37989 Inventory 23 High manganese steel 2 Composite structure steel 2 One 6 One 0.75 0.25 727 1064 43 45344 Honors 24 High manganese steel 2 Composite structure steel 2 One 2 One 0.50 0.50 665 998 36 36277 Honors 25 High manganese steel 2 Composite structure steel 3 One 6 One 0.75 0.25 742 1100 43 46856 Evidence 26 High manganese steel 2 Composite structure steel 3 One 2 One 0.50 0.50 696 1070 37 39301 Honors 27 High manganese steel 3 Abnormal tissue 1 One 6 One 0.75 0.25 756 1021 37 37331 Evidence 28 High manganese steel 3 Abnormal tissue layer 2 One 6 One 0.75 0.25 768 1051 36 37786 Evidence 29 High manganese steel 3 Abnormal tissue layer 2 One 2 One 0.50 0.50 638 919 34 31295 Inventory 30 High manganese steel 3 Abnormal texture steel 3 One 6 One 0.75 0.25 799 1103 34 37513 PROPERTIES 31 High manganese steel 3 Abnormal texture steel 3 One 2 One 0.50 0.50 701 1022 30 30749 Exhibit 32 High manganese steel 3 Abnormal tissue 4 One 6 One 0.75 0.25 834 1154 32 37206 PROPERTIES 33 High manganese steel 3 Composite Steel 1 One 6 One 0.75 0.25 783 1056 37 38600 Honors 34 High manganese steel 3 Composite Steel 1 One 2 One 0.50 0.50 668 929 35 32923 Practice 35 High manganese steel 3 Composite structure steel 2 One 6 One 0.75 0.25 808 1104 34 37744 EXPERIENCE 36 High manganese steel 3 Composite structure steel 2 One 2 One 0.50 0.50 719 1024 30 31211 Honors 37 High manganese steel 3 Composite structure steel 3 One 6 One 0.75 0.25 824 1140 34 39256 Honors 38 High manganese steel 3 Composite structure steel 3 One 2 One 0.50 0.50 750 1096 31 34235 EVENTS 39 High manganese steel 3 Composite structure steel 3 One One One 0.33 0.67 701 1067 29 30887

The inventive Inventions 1 to 39 satisfying both the composition and the microstructure of the present invention can confirm that the yield strength of 400 MPa or more and the product of the tensile strength and the elongation can be 30000 MPa% or more.

On the other hand, in Comparative Example 1, the thickness ratio of the base material and the clad material proposed in the present invention is satisfied, but the microstructure of the clad material is composed of a ferrite single phase, so that sufficient yield strength can not be secured.

On the other hand, in Comparative Examples 2 and 3, the microstructure of the base material and the clad material satisfies the conditions proposed in the present invention, but the yield ratio of the base material is made to be 30% or less so that the yield strength is 400 MPa or more or the tensile strength and elongation Can not be ensured.

Fig. 2 is a photograph of a wavefront after the tensile test of Inventive Example 16 taken by a scanning electron microscope. It can be confirmed that both the base material and the clad material exhibit soft fracture behavior, and it can be confirmed that the interface between the base material and the clad material is soundly bonded at the time of tensile fracture.

3 is a graph showing tensile strengths and elongation ratios of abnormal texture steels 1 to 4, composite structure steels 1 to 3, high manganese steels 1 to 3, and inventive examples 1 to 39 in Table 1. It can be confirmed that various tensile strengths and elongation ratios can be manufactured by controlling the composition and thickness ratio of the high manganese steel and the carbon steel which is the clad material and the strength and formability of the steel according to the present invention are excellent. Can be fabricated as a structural member for an automobile having a product of 30,000 MPa or more.

4 is a photograph of the appearance of the hot-dip galvanized material of Inventive Example 1, Inventive Example 3, Inventive Example 5, Inventive Example 6, Inventive Example 8, Inventive Example 10, and the like. It can be confirmed that a high manganese steel-carbon steel clad excellent in plating ability can be manufactured because the plating ability of the abnormal texture steel and the composite texture steel constituting the outer steel material is excellent.

On the other hand, FIG. 5, which is a photograph of the appearance of the hot dip galvanizing material of the high manganese steel 1 in Table 1, shows that the plating ability is heated by the Mn oxide.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims

A clad sheet comprising a base material and a clad material provided on both sides of the base material,
The base material is an austenitic high manganese steel containing 0.3 to 0.8% of C, 13 to 25% of Mn, the balance of Fe and unavoidable impurities,
Wherein the clad material is a ferritic carbon steel containing 0.03 to 0.3% of C, 0.3 to 3.5% of Mn, and the balance of Fe and unavoidable impurities in weight percent.

The method according to claim 1,
The above-mentioned austenitic high-manganese steel according to the present invention is characterized in that it contains 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.0005 to 0.005% of B, 0.04% or less of N (excluding 0% And S: 0.03% or less.

3. The method of claim 2,
Wherein the austenitic high manganese steel further comprises at least one of 0.03 to 2.0% of Si, 0.2 to 3.0% of Cr, 0.01 to 0.5% of Nb and 0.05 to 0.7% of V in terms of% by weight A clad sheet having excellent strength and formability.

The method according to claim 1,
Characterized in that the ferritic carbon steel further comprises 0.02 to 1.0% of Al, 0.04% or less of N (excluding 0%), P of 0.03% or less, and S of 0.03% or less in weight% And excellent moldability.

5. The method of claim 4,
Characterized in that said ferrite-based carbon steel further comprises at least one of 0.03 to 2.5% of Si, 0.01 to 2% of Mo, 0.005 to 0.05% of Ti and 0.005 to 0.05% of Nb in weight% And excellent moldability.

The method according to claim 1,
Wherein the base material has a thickness of 30 to 90% of the thickness of the clad steel sheet.

The method according to claim 1,
Wherein the clad steel sheet has a yield strength of 400 MPa or more and a product of tensile strength and elongation of 30000 MPa% or more.

The method according to claim 1,
Wherein the microstructure of the austenitic high manganese steel is an austenite single phase.

The method according to claim 1,
Characterized in that the microstructure of the ferrite-based carbon steel is composed of at least one of ferrite, ferrite, martensite, bainite and carbide in an amount of from 40 to 95% by area and the remainder being composed of austenite, ferrite, martensite, bainite and carbide.

10. The method of claim 9,
Wherein the ferrite-based carbon steel is an ideal structure steel or a composite structure steel.

The method according to claim 1,
Wherein the clad steel sheet further comprises a plating layer.

12. The method of claim 11,
Wherein the plating layer is one selected from the group consisting of Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg based, Zn-Mg-Al based, Zn-Ni based, Al-Si based and Al- Wherein the clad steel sheet has excellent strength and moldability.

Preparing a base material which is an austenitic high manganese steel containing 0.3 to 0.8% of C, 13 to 25% of Mn, the balance of Fe and unavoidable impurities, in weight%;
Preparing a clad material which is a ferritic carbon steel containing 0.03 to 0.3% of C, 0.3 to 3.5% of Mn, and the balance of Fe and unavoidable impurities in terms of% by weight;
Disposing the base material between the two clad materials to obtain a laminate;
Welding the rim of the laminate to a temperature range of 1050 to 1350 캜;
Finishing rolling the heated laminate to a temperature range of 750 to 1050 캜 to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at 50 to 700 ° C;
Cold rolling the rolled hot-rolled steel sheet at a reduction ratio of 35 to 90% after pickling to obtain a cold-rolled steel sheet; And
And annealing the cold-rolled steel sheet at a temperature of 550 ° C or higher and A3 + 50 ° C or lower of the clad material.

14. The method of claim 13,
The above-mentioned austenitic high-manganese steel according to the present invention is characterized in that it contains 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.0005 to 0.005% of B, 0.04% or less of N (excluding 0% And S: 0.03% or less, based on the total weight of the cladding steel sheet.

15. The method of claim 14,
Wherein the austenitic high manganese steel further comprises at least one of 0.03 to 2.0% of Si, 0.2 to 3.0% of Cr, 0.01 to 0.5% of Nb and 0.05 to 0.7% of V in terms of% by weight A method for producing a clad sheet excellent in strength and moldability.

14. The method of claim 13,
Characterized in that the ferritic carbon steel further comprises 0.02 to 1.0% of Al, 0.04% or less of N (excluding 0%), P of 0.03% or less, and S of 0.03% or less in weight% And a process for producing a clad sheet excellent in moldability.

17. The method of claim 16,
Characterized in that said ferrite-based carbon steel further comprises at least one of 0.03 to 2.5% of Si, 0.01 to 2% of Mo, 0.005 to 0.05% of Ti and 0.005 to 0.05% of Nb in weight% And a process for producing a clad sheet excellent in moldability.

14. The method of claim 13,
Wherein the thickness of the base material is 30 to 90% of the thickness of the clad steel sheet.

14. The method of claim 13,
And further comprising the step of forming a plating layer by plating after the annealing step.

20. The method of claim 19,
Wherein the plating layer is one selected from the group consisting of Zn-based, Zn-Fe-based, Zn-Al-based, Zn-Mg based, Zn-Mg-Al based, Zn-Ni based, Al-Si based and Al- Wherein the strength of the clad steel sheet is less than the tensile strength of the clad sheet.