KR20130110646A - High strength steel sheet and method of manufacturing the steel sheet - Google Patents

Abstract

PURPOSE: A high-strength steel sheet and a manufacturing method thereof are provided to prevent the weldability and productivity of the high-strength steel sheet from deteriorating due to a large amount of alloy elements added to the same, and to improve the tensile strength of the high-strength steel sheet over 1180 MPa. CONSTITUTION: A manufacturing method of a high-strength steel sheet includes: a step (S110) of reheating a slab sheet material; a step (S120) of hot-rolling the reheated sheet material at a finish rolling temperature of 750 to 950°C; and a step (S130) of cooling the hot-rolled sheet material to a martensitic temperature range at an average cooling rate of 200-500°C/sec, and then coiling the cooled sheet material. The sheet material includes 0.1 to 0.35 wt% of C, 0.01 to 0.5 wt% of Si, 0.3 to 2.0 wt% of Mn, 0.002 to 0.1 wt% of Al, 0.02 wt% or less of P, 0.005 wt% or less of S, and 0.01 wt% or less of N, and further includes at least one among 0.01 to 0.1 wt% of Cr and 0.0005 to 0.004 wt% of B, and the remaining amount of Fe and inevitable impurities. [Reference numerals] (AA) Start; (BB) End; (S110) Reheating of a slab (SRT: 1200 ± 50°C); (S120) Hot-rolling (FDT: 850 ± 100°C); (S130) Cooling/coiling (300-500°C/sec, CT: 50-200°C)

Description

High strength steel sheet and its manufacturing method {HIGH STRENGTH STEEL SHEET AND METHOD OF MANUFACTURING THE STEEL SHEET}

More particularly, the present invention relates to a high-strength steel sheet excellent in abrasion resistance and having a tensile strength of 1180 MPa or more by using a hot-rolling process as a low-alloy component, and a method of manufacturing the same. .

The automotive industry is demanding a lightweight material for improved fuel efficiency and CO2 reduction. Accordingly, the steel sheet applied to automobile parts has been intensified in order to reduce weight.

Among them, attention is focused on a martensitic structure exhibiting ultra-high strength properties while reducing the content of alloy elements. Martensitic steels are very high hardness phases in which the austenite phase at high temperature (above about Ar3) is formed as a non-diffusive transformation upon cooling at a very high rate.

The background art related to the present invention is an ultra-high strength hot-dip galvanized steel sheet having excellent processability and a tensile strength of 1180 MPa or more and a method of manufacturing the same, disclosed in Korean Patent Registration No. 10-0723204 (published on May 29, 2007).

It is an object of the present invention to solve the problem of lowering of the weldability and productivity due to the addition of a large amount of alloying elements and the increase of the manufacturing cost due to the addition of alloying elements to produce ultra high strength steel sheets, Strength hot-rolled steel sheet having a tensile strength of 1,180 MPa or more, and a method of manufacturing the same.

High-strength steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object by weight, carbon (C): 0.1 ~ 0.35%, silicon (Si): 0.01 ~ 0.5%, manganese (Mn): 0.3 ~ 2.0 %, Aluminum (Al): 0.002 to 0.1%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.01% or less, and chromium (Cr): 0.01 to 0.1 % And boron (B): 0.0005 ~ 0.004% further comprising at least one, reheating the slab plate consisting of the remaining iron (Fe) and inevitable impurities; Hot rolling the reheated slab sheet at a finishing rolling temperature of 750 to 950 캜; And cooling the hot-rolled sheet material to an martensite temperature range at an average cooling rate of 200 to 500 ° C / sec, and then winding the hot-rolled sheet material.

At this time, the reheating of the slab plate is preferably performed at 1150 to 1250 ° C for 3 to 5 hours.

The coiling temperature is preferably 50 to 250 ° C.

High strength steel sheet according to an embodiment of the present invention for achieving the above object by weight, carbon (C): 0.1 ~ 0.35%, silicon (Si): 0.01 ~ 0.5%, manganese (Mn): 0.3 ~ 2.0%, Aluminum (Al): 0.002 to 0.1%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.01% or less, chromium (Cr): 0.01 to 0.1% and Boron (B): at least one of 0.0005 ~ 0.004%, consisting of the remaining iron (Fe) and inevitable impurities, martensite in the microstructure has an area ratio of 95% or more, having a tensile strength of 1180MPa or more It is characterized by.

At this time, the steel sheet may have a yield strength of 1000 MPa or more, an elongation of 5% or more, and a Vickers hardness Hv of 400 or more.

According to the method of manufacturing a high-strength steel sheet according to the present invention, the hot-rolled condition can be precisely controlled while using a low alloy, and particularly, the reduction amount in hot rolling can be increased to make the microstructure crystal grains smaller than 5 탆 without adding alloying elements, It is possible to manufacture a hot-rolled steel sheet having an ultra-high strength which is greatly increased in tensile strength of 1180 MPa or more by maximizing the amount of cooling during cooling to produce a martensite microstructure.

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

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with 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, a high strength 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.

High strength steel plate

High strength steel sheet according to the present invention in weight%, carbon (C): 0.1 ~ 0.35%, silicon (Si): 0.01 ~ 0.5%, manganese (Mn): 0.3 ~ 2.0%, aluminum (Al): 0.002 ~ 0.1% Phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, nitrogen (N): 0.01% or less.

In order to produce a sufficient martensite structure by improving the hardenability, the high-strength steel sheet according to the present invention may contain one or more of chromium (Cr): 0.01 to 0.1% and boron (B): 0.0005 to 0.004% .

In addition to the above components, the remainder consists of iron (Fe) and inevitable impurities.

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

Carbon (C)

Carbon (C) is an element contributing to the strength increase of the steel, and the hardness of martensite is determined at the time of transformation into martensite by concentrating the carbon in the austenite at high temperature during formation of the martensite microstructure.

The carbon is preferably added in an amount of 0.1 to 0.35% by weight based on the total weight of the steel sheet. When the amount of carbon added is less than 0.1 wt%, it is difficult to secure the desired strength without increasing the strength of solid solution through addition of alloying elements. On the other hand, when the amount of carbon added exceeds 0.35% by weight, weldability and toughness are deteriorated. In order to have a martensite structure of 1180 MPa class, it is more preferable to control it in the range of 0.15 to 0.25%.

Silicon (Si)

Silicon (Si) contributes to securing strength and also acts as a deoxidizer to remove oxygen in the steel.

The silicon is preferably added in an amount of 0.01 to 0.5% by weight based on the total weight of the steel sheet. If the addition amount of silicon is less than 0.01% by weight, the deoxidation effect due to the addition of silicon is insufficient. On the contrary, when the addition amount of silicon exceeds 0.5% by weight, there is a problem that the weldability and plating ability are deteriorated. On the other hand, in the case of hot dip galvanizing, the addition amount of silicon is preferably 0.3% by weight or less in terms of non-plating prevention, and more preferably 0.3% by weight or more from the viewpoint of contributing to the improvement of strength in the case of nonplating material.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the ingotability of steel. Addition of manganese causes less deterioration of ductility when strength is increased than that of carbon.

The manganese is preferably added in an amount of 0.3 to 2.0% by weight based on the total weight of the steel sheet. When the addition amount of manganese is less than 0.3% by weight, the effect of addition thereof is insufficient. On the other hand, when the addition amount of manganese exceeds 2.0% by weight, MnS-based nonmetallic inclusions are excessively generated, and weldability such as cracking is lowered.

Aluminum (Al)

Aluminum (Al) is added together with silicon to deoxidize the steel.

The aluminum is preferably added in an amount of 0.002 to 0.1% by weight based on the total weight of the steel sheet. When the addition amount of aluminum is less than 0.002% by weight, the effect of addition is insufficient. On the other hand, if the addition amount of aluminum exceeds 0.1% by weight, performance may be deteriorated and the fraction of alumina in the steel may increase, thereby causing voids in the bending process in martensite, thereby deteriorating the characteristics.

Phosphorus (P)

Although phosphorus (P) contributes partly to the strength improvement, it is an element with a high possibility of segregation in the production of steel sheet. It forms fine segregation as well as center segregation, which adversely affects the material and can deteriorate the weldability.

Therefore, in the present invention, the phosphorus content is limited to 0.02% by weight or less of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) combines with manganese to form nonmetallic inclusions such as MnS, which hinders weldability and hinders workability during molding. Especially, in the martensite microstructure, linear MnS is minimized by minimizing the bending property by generating a large amount of voids in grain boundaries with a large difference in hardness between phases.

Therefore, in the present invention, the content of sulfur is limited to 0.005% by weight or less of the total weight of the steel sheet.

Nitrogen (N)

Nitrogen (N) is an inevitable impurity. If it is contained in a large amount, nitrogen nitrogen is increased and the impact property and elongation rate of the steel sheet are lowered and the toughness of the welded portion is greatly lowered.

Thus, in the present invention, the content of nitrogen was limited to 0.01% by weight or less of the total weight of the steel sheet.

Chromium (Cr)

Chromium (Cr) is an element for improving hardenability and is effective for producing martensite.

When chromium is added, the addition amount thereof is preferably 0.01 to 0.1% by weight based on the total weight of the steel sheet. When the addition amount of chromium is less than 0.01% by weight, the effect of improving the hardenability by the addition of chromium may become insufficient. On the contrary, if the amount of chromium is more than 0.1 wt%, it may cause a drastic decrease in ductility to strength due to generation of chromium carbide having high brittleness at the end of low temperature cooling.

Boron (B)

Boron (B) is a strong incipient element and can contribute greatly to the formation of martensite even by adding a small amount.

When boron is added, the addition amount thereof is preferably 0.0005 to 0.004% by weight based on the total weight of the steel sheet. When the addition amount of boron is less than 0.0005% by weight, the effect of addition thereof may be insufficient. On the contrary, when the addition amount of boron exceeds 0.004% by weight, the toughness and ductility of the steel are deteriorated.

The high strength steel sheet according to the present invention may have a microstructure in which the phase fraction of martensite is 95% or more in area ratio through the composition and the hot rolling process described below. At this time, the low temperature transformation phase such as bainite can be formed at an area ratio of 5% or less. However, when a low temperature transformation phase such as bainite is included, workability may be lowered due to the difference in hardness between phases, and therefore it is more preferable to form a 100% martensite structure.

High strength steel plate manufacturing method

Hereinafter, a high strength steel sheet manufacturing method according to the present invention having the above composition will be described.

1 is a flow chart showing a method for manufacturing a high strength steel sheet according to an embodiment of the present invention, showing a method for manufacturing a hot rolled steel sheet.

Referring to FIG. 1, the method for manufacturing a high strength steel sheet includes a slab reheating step (S110), a hot rolling step (S120), and a cooling / winding step (S130).

The slab reheating step (S110) re-uses the components and precipitates segregated during casting through reheating of the slab plate of the semi-finished state having the above-described composition.

The slab reheating is preferably carried out at 1150 to 1250 ° C for 3 to 5 hours. If the slab reheating is conducted at less than 1150 占 폚 or the reheating time is less than 3 hours, the reuse or homogenizing effect may become insufficient. Conversely, if the reheating is performed at a temperature exceeding 1250 占 폚 or the reheating time exceeds 5 hours, it may become difficult to secure strength due to crystal grain coarsening and the economical efficiency due to excessive heating may be lowered.

Next, in the hot rolling step (S120), the reheated slab plate is hot-rolled.

The hot rolling is preferably carried out at a finishing rolling temperature (FDT) of 750 to 950 占 폚. Under the above-mentioned finish rolling temperature condition, the structure of the steel sheet prior to cooling may be an austenite phase. If the finishing rolling temperature exceeds 950 DEG C, it may become difficult to secure strength due to crystal grain coarsening. On the other hand, if the finish rolling temperature is lower than 750 占 폚, the material of the steel sheet may deteriorate due to abnormal reverse rolling.

Next, in the cooling / winding step (S130), the hot-rolled plate material is cooled to an martensite temperature region at an average cooling rate of 200 to 500 DEG C / sec in order to secure a target martensite phase fraction, and then wound.

The cooling is preferably carried out at an average cooling rate of 200 to 500 DEG C / sec. Such a high cooling rate can be achieved by installing a cooling nozzle which is applied in water cooling twice as much as usual. When the cooling rate applied at the time of cooling is less than 200 ° C / sec, transformation such as ferrite and pearlite occurs, and it is difficult to secure a martensite structure of 95% or more. On the other hand, when the cooling rate exceeds 500 ° C / sec, it is difficult to secure an elongation of 5% or more, and restrictions on the plate-type control on the apparatus may be observed.

On the other hand, the coiling temperature is preferably 50 to 250 ° C. If the coiling temperature exceeds 250 캜, the cooling may be insufficient and it may become difficult to ensure a martensite fraction of 95% or more in area ratio. On the other hand, if the coiling temperature is less than 50 캜, surface hardness and spots may occur due to residual cooling water on the material surface.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. 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.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of hot-rolled specimens

Hot-rolled specimens were prepared according to the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the tensile test results of the specimens 1 to 6 and the Vickers hardness measurement results.

[Table 3]

Referring to Table 3, the tensile strength and the Vickers hardness of the specimens 1 to 3 according to the embodiment of the present invention both exceeded the target value, and the martensite area ratio was also 95% or more.

On the other hand, in the case of specimens 4 to 6, in which the cooling rate was less than 200 ° C./sec, the martensite area ratio was less than 95%, so that the tensile strength or abrasion resistance did not meet the target value.

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.

S110: Slab reheating step
S120: Hot rolling step
S130: cooling / winding step

Claims (5)

By weight%, carbon (C): 0.1 to 0.35%, silicon (Si): 0.01 to 0.5%, manganese (Mn): 0.3 to 2.0%, aluminum (Al): 0.002 to 0.1%, phosphorus (P): 0.02 % Or less, sulfur (S): 0.005% or less, nitrogen (N): 0.01% or less, and at least one of chromium (Cr): 0.01 to 0.1% and boron (B): 0.0005 to 0.004% And reheating the slab plate made of the remaining iron (Fe) and unavoidable impurities;
Hot rolling the reheated slab sheet at a finishing rolling temperature of 750 to 950 캜; And
Cooling the hot-rolled plate to an martensite temperature range at an average cooling rate of 200 to 500 ° C / sec, and then winding the hot-rolled plate.
The method of claim 1,
The reheating of the slab plate
Wherein the heat treatment is carried out at 1150 to 1250 占 폚 for 3 to 5 hours.
The method of claim 1,
The coiling temperature
50 to < RTI ID = 0.0 > 250 C. < / RTI >
By weight%, carbon (C): 0.1 to 0.35%, silicon (Si): 0.01 to 0.5%, manganese (Mn): 0.3 to 2.0%, aluminum (Al): 0.002 to 0.1%, phosphorus (P): 0.02 % Or less, sulfur (S): 0.005% or less, nitrogen (N): 0.01% or less, and at least one of chromium (Cr): 0.01 to 0.1% and boron (B): 0.0005 to 0.004% It is made of the remaining iron (Fe) and inevitable impurities,
Martensite in the microstructure is 95% or more by area ratio,
A high strength steel sheet having a tensile strength of 1180 MPa or more.
5. The method of claim 4,
The steel sheet
A high strength steel sheet having a yield strength of at least 1000 MPa, an elongation of at least 5%, and a Vickers hardness of Hv 400 or more.

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