KR101467052B1 - Ultra-high strength cold-rolled steel sheet and method for manufacturing the same - Google Patents

Abstract

An ultra-high-strength cold-rolled steel sheet exhibiting a tensile strength of 1470 MPa or more and exhibiting a 5% elongation, and a method for producing the same.
The method for manufacturing an ultra high strength cold rolled steel sheet according to the present invention is characterized in that it comprises 0.15 to 0.25% of carbon (C), 0.2% or less of silicon (Si), 2.0 to 3.0% of manganese (Mn) (Al): not more than 0.05%, chromium (Cr): not more than 0.4%, molybdenum (Mo): not more than 0.1%, titanium (Ti): not more than 0.02% , Cold rolling a hot rolled steel sheet comprising 0.06% or less of niobium (Nb), 0.004% or less of boron (B), 0.006% or less of nitrogen (N), and the balance of iron (Fe) and unavoidable impurities; Annealing the cold-rolled steel sheet in a single phase of austenite by heating; Cooling the annealed steel sheet to an martensite temperature range at an average cooling rate of 20 to 50 DEG C / sec; And overcoating the primary cooled steel sheet at a temperature of the martensite temperature for 30 seconds or more.

Description

TECHNICAL FIELD [0001] The present invention relates to a super high strength cold rolled steel sheet and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to an ultra high strength cold rolled steel sheet, and more particularly to a super high strength cold rolled steel sheet having an ultra high strength cold rolled steel sheet having an ultrahigh strength of 1.5 GPa or more with a tensile strength of 1470 MPa or more and an elongation of 5% And a manufacturing method thereof.

In general, a steel plate of 1 GPa or more in grade is utilized as a material for collision members such as a bumper beam, a side sill, and a door impact beam for protecting a passenger in the event of a vehicle collision.

The reason why ultra high strength steel plates of 1 GPa or more are used for these collision members is to realize a vehicle with a high strength and a thinner thickness for the purpose of reducing carbon dioxide by global warming.

The production conditions of the ultra-high strength cold-rolled steel sheet of 1.5 GPa or more are classified into hot rolling, cold rolling and annealing. The hot rolled plate having ferrite and pearlite is cold rolled to obtain the final thickness of the product. It is heated to the annealing temperature of the austenite single phase and rapidly cooled.

At this time, the austenite formed in the annealing process is transformed into martensite or bainite by controlling the cooling rate during the cooling process. At this time, the austenite single phase is cooled rapidly (no diffusion phase transformation) Steel produced to have a full martensite single phase structure is called MS steel.

In the case of a 1.5G class MS steel, the strength is excellent, but the elongation is less than 5%.

As a prior art related to the present invention, there is a method of manufacturing an ultra-high strength cold rolled steel sheet disclosed in Korean Patent Laid-Open Publication No. 10-2003-0055530 (published on Apr. 4, 2003).

It is an object of the present invention to provide an ultra high strength cold rolled steel sheet manufacturing method having an ultra high strength of 1470 MPa or higher in tensile strength and an excellent elongation of 5% or higher through control of alloy components and process conditions.

Another object of the present invention is to provide an ultra-high strength cold rolled steel sheet produced by the above method and having an ultra high strength of 1470 MPa or higher in tensile strength and an excellent elongation.

According to an aspect of the present invention, there is provided a method of manufacturing an ultrahigh-strength cold rolled steel sheet, comprising the steps of: 0.15 to 0.25% of carbon (C); 0.2 to 0.2% (S): not less than 0% to not more than 0.003%, soluble aluminum (S-Al): not less than 0% to not more than 0.05% (Ti): more than 0% to 0.02%; niobium (Nb): more than 0% to 0.06%; (B): not less than 0% to not more than 0.004%, nitrogen (N): not less than 0% to not more than 0.006%, and the balance of iron (Fe) and unavoidable impurities; Annealing the cold-rolled steel sheet in a single phase of austenite by heating; Cooling the annealed steel sheet to an martensite temperature range at an average cooling rate of 20 to 50 DEG C / sec; And overcoating the primary cooled steel sheet at a temperature of the martensite temperature for 30 seconds or more.

At this time, it is more preferable that the annealing treatment is performed at 860 to 880 ° C.

It is more preferable that the overexposure treatment is performed at 250 to 300 ° C.

The hot-rolled steel sheet may be prepared by reheating the steel slab at 1150 to 1300 ° C, hot-rolling the reheated steel slab to a finish rolling temperature of Ar3 to Ar3 + 200 ° C, and then cooling the steel slab.

The ultra high strength cold rolled steel sheet according to an embodiment of the present invention includes 0.15 to 0.25% carbon (C), more than 0% to 0.2% silicon (Si), manganese (Mn) : More than 0% to 0.03%; sulfur (S): more than 0% to 0.003%; soluble aluminum (S-Al) (Ti): more than 0% to 0.02%, niobium (Nb): more than 0% and not more than 0.06%, and more than 0% Boron (B): having a tensile strength of 1470 MPa or more and an elongation of 5% or more, consisting of more than 0% to less than 0.004%, nitrogen (N): more than 0% to less than 0.006% .

At this time, the ultrahigh strength cold rolled steel sheet may have a microstructure of full lath martensite.

According to the method for manufacturing an ultra-high strength cold rolled steel sheet according to the present invention, it is possible to control the alloy components such as carbon (C), manganese (Mn), titanium (Ti) and niobium (Nb), annealing, cooling, The ultra-high strength cold rolled steel sheet having excellent mechanical properties with a tensile strength of 1470 MPa or more and an elongation of 5% or more can be produced through condition control.

1 is a flowchart schematically showing a method for manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.
2 is a photograph showing the microstructure of a cold-rolled specimen according to Example 3. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Hereinafter, an ultrahigh-strength cold-rolled steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

The ultra-high strength cold-rolled steel sheet according to the present invention comprises 0.15 to 0.25% of carbon (C), more than 0 to 0.2% of silicon (Si), 2.0 to 3.0% of manganese (Mn) : More than 0% to 0.03%, sulfur (S): more than 0% to less than 0.003%, soluble aluminum (S-Al): more than 0% to less than 0.05%, chromium (Cr) More than 0% and not more than 0.1% of molybdenum (Mo), more than 0% and not more than 0.02% of titanium (Ti), more than 0 and 0.06% of niobium (Nb) % Or less and nitrogen (N): 0% to 0.006% or less.

The rest of the above components are composed of iron (Fe) and impurities inevitably included in the steelmaking process.

Hereinafter, the role and content of each component included in the ultra high strength cold rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to securing a tensile strength of 1470 MPa or more through formation of martensite.

The carbon is preferably added in an amount of 0.15 to 0.25% by weight based on the total weight of the steel sheet. If the addition amount of carbon is less than 0.15% by weight, the carbon content is insufficient and it is difficult to secure a tensile strength of 1470 MPa or more. On the other hand, when the carbon content exceeds 0.25% by weight, weldability is deteriorated.

Silicon (Si)

Silicon (Si) is a solid solution strengthening element, which promotes the purification of steel and carbon enrichment in austenite.

The silicon is preferably added in an amount of more than 0 wt% to 0.2 wt% of the total weight of the steel sheet. When the addition amount of silicon is more than 0.2% by weight, the weldability and the plating ability are lowered, and the formation of full-length martensite is difficult.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element, stabilizing austenite and making martensite easier to be produced.

The manganese is preferably added in an amount of 2.0 to 3.0% by weight based on the total weight of the steel sheet. When the content of manganese is less than 2.0% by weight, the effect of addition thereof is insufficient. On the contrary, when the content of manganese exceeds 3.0% by weight, a manganese band develops at the center of the material thickness and the elongation rate is lowered.

In (P)

Phosphorus (P) contributes to the improvement of the strength of the steel sheet by solid solution strengthening, and as an element effective in suppressing the formation of carbide, it plays a role of preventing elongation decrease due to formation of carbide during annealing and cooling. It is also effective for obtaining martensite by improving manganese equivalence. However, when phosphorus is added in excess, Fe3P stddite is formed, which causes hot brittleness.

Therefore, in the present invention, the content of phosphorus is limited to more than 0 wt% and not more than 0.03 wt% of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits toughness and weldability, and increases MnS nonmetallic inclusions, which hinders the effect of incombustibility of Mn and causes cracks. Also, if sulfur is contained excessively, the coarse inclusions are increased to deteriorate the fatigue characteristics.

Therefore, in the present invention, the content of sulfur is limited to more than 0 wt% to 0.003 wt% or less of the total weight of the steel sheet.

Soluble Aluminum (S-Al)

Soluble Aluminum (S-Al) is an element mainly used as a deoxidizing agent. It contributes to stabilize the austenite by improving the elongation rate of the ferrite and enhancing the amount of carbon enrichment in the austenite.

The aluminum is preferably added in an amount of more than 0 wt% to 0.05 wt% or less of the total weight of the steel sheet. If the addition amount of aluminum exceeds 0.05 wt%, the performance is deteriorated and AlN is formed in the slab, which causes hot cracking.

Chromium (Cr)

Cr (Cr) is an austenite stabilizing element and contributes to improving the ingotability.

The chromium is preferably added in an amount of more than 0 wt% to 0.4 wt% or less based on the total weight of the steel sheet. If the chromium is added in an amount exceeding 0.4% by weight, plating performance is deteriorated.

Molybdenum (Mo)

Molybdenum (Mo) is an austenite stabilizing element, which together with chromium contributes to improving the ingotability.

The molybdenum is preferably added in an amount of more than 0 wt% to 0.1 wt% or less of the total weight of the steel sheet. If the amount of the molybdenum added exceeds 0.1 wt%, the toughness of the steel sheet is deteriorated.

Titanium (Ti)

The titanium (Ti) element is added to remove nitrogen (N) which is a nitride forming element such as BN or AlN.

The content of titanium is preferably more than 0 wt% to 0.02 wt% of the total weight of the steel sheet. Even if the amount of titanium exceeds 0.02% by weight or more, no further effect occurs.

Niobium (Nb)

Niobium (Nb) is a precipitate-forming element and contributes to the improvement of steel strength. Also, niobium disperses the manganese band layer and contributes to improvement of the bending processability. However, if niobium is added in an amount exceeding 0.06% by weight, the roll force is increased due to the formation of excessive precipitates, which may significantly reduce the rolling property.

Therefore, in the present invention, the content of niobium is limited to more than 0 wt% and not more than 0.06 wt% of the total weight of the steel sheet.

Boron (B)

Boron is a strong ingenious element and contributes greatly to the improvement of hardenability of steel.

The boron is preferably contained in an amount of more than 0 wt% to 0.004 wt% of the total weight of the steel sheet. If the addition amount of boron exceeds 0.004% by weight, the brittleness of the steel may be sharply lowered.

Nitrogen (N)

When nitrogen (N) is contained in an excess amount, it causes a factor of lowering the elongation. Therefore, in the present invention, the content of nitrogen is limited to more than 0 wt% and 0.006 wt% or less of the total weight of the steel sheet.

The ultra high strength cold rolled steel sheet according to the present invention having the above alloy composition may have a tensile strength of 1470 MPa or more and an elongation of 5% or more by the process conditions to be described later. In addition, the ultrahigh strength cold rolled steel sheet according to the present invention may have a microstructure of full lath martensite.

Hereinafter, a method for producing an ultra-high strength cold rolled steel sheet comprising the above alloy components and having excellent mechanical properties with a tensile strength of 1470 MPa or more and an elongation of 5% or more will be described.

1 is a flowchart schematically showing a method for manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated method of manufacturing an ultra-high strength cold rolled steel sheet includes a cold rolling step (S110), an annealing step (S120), a cooling step (S130), and an overpass treatment step (S140).

Cold rolling

In the cold rolling step (S110), the hot-rolled steel sheet is cold-rolled and processed to the final thickness of the steel sheet.

The hot-rolled steel sheet to be subjected to cold rolling can be produced by hot-rolling and cooling the steel slab having the above-mentioned composition. The hot-rolled steel sheet may be prepared by various known process conditions, and more preferably, the following process may be employed.

First, the steel slab is reheated at 1150 to 1300 占 폚. If the slab reheating temperature is less than 1150 ° C, the segregated components may become difficult to reuse during casting. On the other hand, when the slab reheating temperature exceeds 1300 ° C, the austenite grain size increases and it becomes difficult to secure strength.

Next, the reheated steel slab is hot-rolled to a finish rolling temperature of Ar3 to Ar3 + 200 占 폚. When the finishing rolling temperature exceeds Ar 3 + 200 ° C, strength and ductility may be reduced due to increase in grain size. On the other hand, if the finishing rolling temperature is lower than the Ar3 temperature, a problem may arise, such as the occurrence of blistering due to abnormal reverse rolling.

Next, the hot-rolled steel is cooled. Various methods such as natural cooling and forced cooling can be applied to the cooling. When winding is required, winding can be carried out at about 500 to 600 ° C.

On the other hand, in the cold rolling step (S110), the reduction rate of the cold rolling can be set to about 50 to 70% depending on the thickness of the hot-rolled steel sheet and the final steel sheet thickness to be targeted.

And performing acid pickling to remove the scale of the hot-rolled steel sheet before cold rolling.

Annealing treatment

In the annealing step S120, the cold-rolled steel sheet is heated to austenite single phase and annealed for about 30 to 150 seconds. The microstructure can be austenitized through the annealing treatment, and the target microstructure and strength can be ensured by the cooling described later.

The annealing temperature is more preferably 860 to 880 캜. If the annealing temperature is less than 860 캜, it may become difficult to secure complete austenite. On the other hand, if the annealing temperature exceeds 880 DEG C, the strength of an ultra-high strength cold rolled steel sheet produced due to an increase in the austenite grain size may be reduced.

Cooling

In the cooling step (S130), the annealed steel sheet is cooled to the martensite temperature.

The cooling method may be a roll quenching method, a gas jet method, or the like.

At this time, the cooling rate at the time of cooling is preferably 20 to 50 DEG C / sec. If the average cooling rate is less than 20 占 폚 / sec, the strength may be lowered by undesired phase transformation. Conversely, when the average cooling rate exceeds 50 캜, a problem of material imbalance may occur.

Overflow treatment

In the overexposure treatment step (S150), the cooled steel sheet is treated at 250 to 300 DEG C for over 30 seconds.

The overflow treatment time is preferably 30 seconds or more, more preferably 30 to 200 seconds. If the time of overexposure treatment is less than 30 seconds, it may be difficult to secure sufficient elongation.

In addition, the over-treatment temperature, that is, the cooling termination temperature is preferably 250 to 300 占 폚. If the over-treatment temperature exceeds 300 ° C, sufficient tensile strength may be difficult to obtain. If the over-treatment temperature is lower than 250 ° C, the elongation may become insufficient.

After the overexposure treatment, the steel sheet can be finally cooled to room temperature by water cooling, air cooling, or the like.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

The ingot having the composition shown in Table 1 was prepared, reheated at 1220 ° C for 2 hours, hot rolled at a finish rolling temperature of 880 ° C, cooled to 540 ° C at a cooling rate of 5 ° C / sec, Hot - rolled specimens were prepared.

Each of the manufactured hot-rolled specimens was pickled, cold-rolled at a reduction ratio of 60%, annealed at 870 ° C for 100 seconds, cooled to 270 ° C at an average cooling rate of 30 ° C / For 120 minutes. Thereafter, the over-treated specimen was air-cooled to prepare cold-rolled specimens 1 to 8.

For specimen 7, cooling was carried out at an average cooling rate of 15 ° C / sec. The rest of the conditions were the same as for specimens 1 to 6 and 8.

[Table 1] (unit:% by weight)

2. Evaluation of mechanical properties

Tensile tests and Vickers hardness measurements were carried out for cold-rolled specimens 1 to 8, and the results are shown in Table 2.

[Table 2]

Referring to Table 2, specimens 1 to 5 satisfying the conditions of the present invention exhibited an elongation of 5% or more while exhibiting an ultrahigh strength of tensile strength of 1470 MPa or more.

On the other hand, in the case of specimen 6 in which carbon was excessively added and manganese content was insufficient, the strength was sufficient but the elongation was less than 5%.

In addition, although the composition was satisfactory, the strength of the cold-rolled specimen 2 to which the average cooling rate was applied was relatively low. This is because, in the case of specimen 7, phase transformation such as ferrite or pearlite occurred during cooling.

Also, in the case of specimen 8 to which carbon was added in an undesirable manner, the tensile strength was less than 1470 MPa.

Fig. 2 is a photograph showing the microstructure of the cold-rolled specimen 3. Fig.

Referring to FIG. 2, in the case of the cold-rolled specimen 3, it can be seen that the microstructure is made of full-length martensite. These full-martensite microstructures contribute greatly to the improvement of strength and hardness.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims (6)

(C): 0.15 to 0.25%, silicon (Si): more than 0% to 0.2%, manganese (Mn): 2.0 to 3.0%, phosphorus: more than 0% to 0.03% (S): more than 0% to less than 0.003%, soluble aluminum (S-Al): more than 0% to less than 0.05%, chromium (Cr): more than 0% to less than 0.4%, molybdenum (N): more than 0% to 0.06%, boron (B): more than 0% to less than 0.004%, nitrogen (N): 0 to 0.1% % To less than 0.006%, and the remaining iron (Fe) and unavoidable impurities;
Annealing the cold-rolled steel sheet in a single phase of austenite by heating;
Cooling the annealed steel sheet to an martensite temperature range at an average cooling rate of 20 to 50 DEG C / sec; And
And subjecting the cooled steel sheet to aggressive treatment for at least 30 seconds in a martensitic temperature range.
The method according to claim 1,
The annealing treatment
Lt; RTI ID = 0.0 > 860 ~ 880 C. < / RTI >
The method according to claim 1,
The cooling termination and over-
Wherein the cold rolling is carried out at a cooling rate of 250 to 300 占 폚.
The method according to claim 1,
The hot-
And reheating the steel slab at 1150 to 1300 캜 and hot-rolling the reheated steel slab to a finish rolling temperature of Ar 3 to Ar 3 + 200 캜, and then cooling the cold rolled steel slab.
delete (C): 0.15 to 0.25%, silicon (Si): more than 0% to 0.2%, manganese (Mn): 2.0 to 3.0%, phosphorus: more than 0% to 0.03% (S): more than 0% to less than 0.003%, soluble aluminum (S-Al): more than 0% to less than 0.05%, chromium (Cr): more than 0% to less than 0.4%, molybdenum (N): more than 0% to 0.06%, boron (B): more than 0% to less than 0.004%, nitrogen (N): 0 to 0.1% % To less than 0.006% and the balance of iron (Fe) and unavoidable impurities,
The microstructure consists of Full Lath Martensite,
A tensile strength of 1470 MPa or more, and an elongation of 5% or more.

Images