KR101344620B1 - High strength steel sheet - Google Patents

Abstract

Disclosed is a high-strength steel sheet having a high strength of 980 MPa or more and having excellent burring property through controlling alloy components and controlling process conditions, and a method of manufacturing the same.
The method for producing a high strength steel sheet according to the present invention is (a) wt%, carbon (C): 0.04 to 0.1%, silicon (Si): 0.2 to 1.0%, manganese (Mn): 0.7 to 1.7%, phosphorus (P) : 0.01 ~ 0.1%, Sulfur (S): 0.001 ~ 0.01%, Chromium (Cr): 0.7 ~ 1.2%, Niobium (Nb): 0.01 ~ 0.09%, Titanium (Ti): 0.01 ~ 0.09%, Boron (B) : Hot rolling a slab plate made of 0.001% to 0.005% and the remaining iron (Fe) and unavoidable impurities at a finishing rolling temperature of 850 ° C to 950 ° C; (b) first cooling the hot rolled sheet to 750-600 ° C .; (c) secondary cooling the first cooled plate for 5 to 10 seconds at a cooling rate of 10 ° C./sec or less; And (d) cooling the secondary cooled plate to third degree to 400 to 550 ° C. at a cooling rate faster than the cooling rate of the secondary cooling.

Description

High Strength Steel Sheet {HIGH STRENGTH STEEL SHEET}

The present invention relates to a high-strength steel sheet manufacturing technology, and more specifically, through a process control, such as chromium (Cr) and other components, such as cooling after hot rolling, a high-strength steel sheet with excellent burring property while having a high strength of 980 MPa or more. The manufacturing method is related.

According to the high oil price era, weight reduction for automobiles has become a necessity. As a result, steelmakers are concentrating on the development of high-strength steels for lightweight materials.

In particular, the parts requiring high-strength steel in automobile parts may be a typical example of an automobile chassis part.

In the case of such chassis parts, high burring property is required with high strength.

The background art relating to the present invention is an ultra-high strength gorging hot-rolled steel sheet disclosed in Korean Patent Laid-Open Publication No. 10-2010-0047022 (published on May 28, 2010) and a method for manufacturing the same.

An object of the present invention is to provide a high-strength steel sheet manufacturing method having a high burring property while having a high strength of 980 MPa or more through the process control, such as chromium and other alloy components and hot rolling and cooling.

Another object of the present invention is to provide a high-strength steel sheet produced by the above method, excellent in burring properties with high strength, and can be used for applications such as automobile chassis parts.

High-strength steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is (a) wt%, carbon (C): 0.04 ~ 0.1%, silicon (Si): 0.2 ~ 1.0%, manganese (Mn ): 0.7 ~ 1.7%, Phosphorus (P): 0.01 ~ 0.1%, Sulfur (S): 0.001 ~ 0.01%, Chromium (Cr): 0.7 ~ 1.2%, Niobium (Nb): 0.01 ~ 0.09%, Titanium (Ti ): 0.01 ~ 0.09%, boron (B): 0.001 ~ 0.005% and hot-rolled slab plate material consisting of the remaining iron (Fe) and inevitable impurities at the finish rolling temperature conditions of 850 ~ 950 ℃; (b) first cooling the hot rolled sheet to 750-600 ° C .; (c) secondary cooling the first cooled plate for 5 to 10 seconds at a cooling rate of 10 ° C./sec or less; And (d) cooling the secondary cooled plate to third degree to 400 to 550 ° C. at a cooling rate faster than the cooling rate of the secondary cooling.

At this time, the primary cooling is preferably carried out at a cooling rate of 50 ~ 100 ℃ / sec. In addition, the third cooling is preferably carried out at a cooling rate of 50 ~ 100 ℃ / sec.

High strength steel sheet according to an embodiment of the present invention for achieving the other object by weight, carbon (C): 0.04 ~ 0.1%, silicon (Si): 0.2 ~ 1.0%, manganese (Mn): 0.7 ~ 1.7% Phosphorus (P): 0.01 ~ 0.1%, Sulfur: 0.001 ~ 0.01%, Chromium (Cr): 0.7 ~ 1.2%, Niobium (Nb): 0.01 ~ 0.09%, Titanium (Ti): 0.01 ~ 0.09% , Boron (B): 0.001 ~ 0.005% and the remaining iron (Fe) and inevitable impurities, comprising a needle-like ferrite 90vol% or more, characterized in that having a fine structure with a precipitate formed.

In this case, the microstructure may include 10 vol% or less of a second phase including at least one of bainite and martensite.

In addition, the steel sheet may have a tensile strength of 980 to 1200 MPa and a hole expansion ratio of 50% or more. In addition, the steel sheet may have a yield strength of 800 to 1100 MPa and an elongation of 7 to 20%.

The method for manufacturing a high strength steel sheet according to the present invention is based on needle-shaped ferrite through control of components such as chromium (Cr) and boron (B) and cooling process after hot rolling, A high strength steel sheet having a high strength can be produced.

As a result, the manufactured high-strength steel sheet according to the present invention has a tensile strength of 980 MPa or more and a hole expanding rate of 50% or more, so that high strength and high ductility can be achieved.

1 is a flowchart showing a method of manufacturing a high strength steel sheet according to the present invention.
Fig. 2 is a microstructure photograph of a specimen prepared according to Example 1. Fig.
Figure 3 shows a microstructure photograph of the specimen prepared according to Comparative Example 1.

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. Like reference numerals refer to like elements throughout the specification.

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

Steel sheet according to the present invention by weight, carbon (C): 0.04 ~ 0.1%, silicon (Si): 0.2 ~ 1.0%, manganese (Mn): 0.7 ~ 1.7%, phosphorus (P): 0.01 ~ 0.1%, Sulfur (S): 0.001-0.01%, Chromium (Cr): 0.7-1.2%, Niobium (Nb): 0.01-0.09%, Titanium (Ti): 0.01-0.09% and Boron (B): 0.001-0.005% Include.

In addition to the above components, the remainder is composed of iron (Fe) and impurities inevitably included in the steelmaking process.

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

Carbon (C)

Carbon (C) is an element contributing to the increase in strength of steel.

The carbon is preferably added in an amount of 0.04 to 0.1% by weight based on the total weight of the steel sheet. When the amount of carbon added is less than 0.04% by weight, it may be difficult to secure a tensile strength of 980 MPa or more. Conversely, when the amount of carbon added exceeds 0.1% by weight, weldability may be lowered and it is difficult to exhibit a hole expansion ratio of 50% or more.

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 0.2 ~ 1.0% by weight of the total weight of the steel sheet. When the amount of silicon added is less than 0.2% by weight, the deoxidation effect and the strength improving effect due to the addition of silicon cannot be exhibited properly. On the contrary, when the addition amount of silicon exceeds 1.0 weight%, there exists a problem that weldability and plating property fall.

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 0.7 ~ 1.7% by weight of the total weight of the steel sheet. When the addition amount of manganese is less than 0.7% by weight, the effect of addition is insufficient. On the other hand, when the addition amount of manganese exceeds 1.7% by weight, MnS-based nonmetallic inclusions are excessively generated and the weldability may be lowered.

Phosphorus (P)

Phosphorus (P) is an element contributing to strength improvement.

The phosphorus is preferably controlled to be contained in an amount of 0.01 to 0.1% by weight based on the total weight of the steel sheet. When the phosphorus content is less than 0.01% by weight, the effect of improving the strength is insufficient. On the contrary, when the content of phosphorus exceeds 0.1 wt%, fine segregation is formed as well as center segregation, which adversely affects the steel material.

Sulfur (S)

Sulfur (S) contributes partly to the improvement of processability.

It is preferable that the sulfur is controlled to be contained in an amount of 0.001 to 0.01% by weight of the total weight of the steel sheet. If the content of sulfur is less than 0.001% by weight, the effect of improving the workability by sulfur is insufficient and the content of sulfur must be controlled to a minimum. On the other hand, when the content of sulfur exceeds 0.01% by weight, there is a problem that the weldability is greatly deteriorated.

Chromium (Cr)

Chromium (Cr) is a ferrite forming element, and serves to improve the strength of the steel through the formation of carbides and quenching properties.

The chromium is preferably added in 0.7 to 1.2% by weight of the total weight of the steel sheet. When the amount of chromium added is less than 0.7% by weight, the strength and burring properties are insufficient. In contrast, when the amount of chromium added exceeds 1.2% by weight, the elongation may be significantly lowered.

Niobium (Nb)

Niobium (Nb) improves the strength of the steel sheet produced by forming niobium-based precipitates in steel.

The niobium is preferably added in 0.01 to 0.09% by weight of the total weight of the steel sheet. When the addition amount of niobium is less than 0.01% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of niobium exceeds 0.09% by weight, the workability is lowered.

Titanium (Ti)

Titanium (Ti) contributes to grain refinement and strength enhancement.

The titanium is preferably added in 0.01 to 0.09% by weight of the total weight of the steel sheet. When the addition amount of titanium is less than 0.01% by weight, the addition effect is insufficient. On the contrary, when the added amount of titanium exceeds 0.09% by weight, it may cause surface defects of the steel sheet to be manufactured.

Boron (B)

Boron (B) serves to improve the strength through the hardenability of the steel.

The boron is preferably added in 0.001 ~ 0.005% by weight of the total weight of the steel sheet. When the addition amount of boron is less than 0.001% by weight, the addition effect is insufficient. Conversely, when the addition amount of boron exceeds 0.005% by weight, there is a problem that the burring property is greatly reduced.

The high strength steel sheet according to the present invention may have a final microstructure in which fine precipitates are formed, including 90 vol% or more of needle-like ferrite through the above-described composition and the process described below. In this case, the microstructure may include 10 vol% or less of a second phase including at least one of bainite and martensite.

Through the above microstructure, the high strength steel sheet according to the present invention may have a hole expandability of 50% or more with high strength of 980 ~ 1200MPa tensile strength. In addition, the high strength steel sheet according to the present invention may have a yield strength of 800 to 1100 MPa and an elongation of 7 to 20%.

High strength steel plate manufacturing method

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

Referring to FIG. 1, the method of manufacturing a high strength steel sheet according to the present invention includes a hot rolling step (S110), a first cooling step (S120), a second cooling step (S130), and a third cooling step (S140).

Hot rolling

In the hot rolling step (S110), the slab plate having the above composition is hot-rolled. At this time, the slab plate may be reheated at about 1150 to 1250 ° C for 1 to 3 hours.

Hot rolling is preferably carried out under the finish rolling temperature conditions of 850 ~ 950 ℃. When the finish rolling temperature exceeds 950 ° C, austenite grains are coarsened, and ferrite grains may not be sufficiently refined after transformation, thereby making it difficult to secure strength. On the other hand, if the finish rolling temperature is lower than 850 占 폚, a problem such as occurrence of blistering due to abnormal reverse rolling may occur.

Primary cooling

In the primary cooling step (S120), the hot rolled plate is first cooled to 750 to 600 ° C.

It is preferable that the completion | finish temperature of primary cooling is 750-600 degreeC. If the end temperature of the primary cooling is above 750 ° C. or below 600 ° C., there may be a problem that the desired secondary ferrite transformation does not occur in subsequent secondary cooling.

In addition, the primary cooling is preferably carried out at a cooling rate of 50 ~ 100 ℃ / sec. When the cooling rate of the primary cooling is less than 50 ° C / sec is coarse steel sheet structure can be difficult to secure a tensile strength of 980MPa or more. On the other hand, when the cooling rate of the primary cooling exceeds 100 캜 / sec, the process control may become difficult due to an excessively short cooling time.

Secondary cooling

In the second cooling step S130, the first cooled plate is secondarily cooled for a predetermined time. At this time, the secondary cooling is performed at a cooling rate slower than that of the primary cooling in order to generate sufficient ferrite transformation.

More specifically, the secondary cooling is preferably carried out for 5 to 10 seconds at a cooling rate of 10 ° C / sec or less, more preferably by air cooling rather than forced cooling.

If the cooling rate of the secondary cooling exceeds 10 ° C / sec, it is difficult to secure sufficient ferrite.

In addition, when the secondary cooling time is less than 5 seconds, sufficient ferrite transformation is difficult. On the contrary, when the secondary cooling time exceeds 10 seconds, it is difficult to secure a tensile strength of 980 MPa or more.

3rd cooling

In the third cooling step (S140), the plate cooled second to third to 400 ~ 550 ℃. At this time, the tertiary cooling is performed at a cooling rate faster than the cooling rate of the secondary cooling. After tertiary cooling, the manufactured steel sheet may be coiled.

More specifically, it is preferable that tertiary cooling is performed to 400-550 degreeC. When the cooling end temperature of the tertiary cooling is less than 400 ℃, it is advantageous to secure the strength, but there is a problem that the formability of the steel sheet to be produced is lowered. On the contrary, when the cooling end temperature of tertiary cooling exceeds 550 degreeC, it is difficult to ensure sufficient strength.

In addition, the tertiary cooling is preferably carried out at a cooling rate of 50 ~ 100 ℃ / sec. If the cooling rate of the third cooling is less than 50 ° C / sec it may be difficult to secure the strength. On the contrary, when the cooling rate of the tertiary cooling exceeds 100 ° C./sec, the structure of the steel sheet is hardened, so the toughness may be drastically reduced, and the process control may be difficult due to the excessively short cooling time.

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 the description can be inferred by those skilled in the art.

1. Preparation of specimens

The specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were produced under 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 each of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 to 3.

In Table 3, tensile strength (TS), yield strength (YS) and elongation (EL) were measured through a tensile test based on JIS No. 5 test piece.

The burring property was determined by forming a perforation hole having an initial diameter (d ₀ : 10 mm), then expanding it with a 60-pound cone punch to measure a hole expansion ratio ((d)) obtained from the hole diameter d at the time when the crack penetrated the plate dd ₀ ) / d ₀ X 100).

 [Table 3]

Referring to Tables 1 to 3, in the case of specimens prepared according to Examples 1 to 3, it was confirmed that the tensile strength (TS), yield strength (YS), elongation (EL) and the burring property satisfies all target values Can be.

On the other hand, the same process conditions as in Example 1, but in the composition is only 0.3% by weight of chromium, titanium is not added, the specimen according to Comparative Example 1 containing 0.02% by weight of aluminum, the tensile strength target It was less than 980 MPa.

In addition, the composition proposed in the present invention is satisfactory, but in the case of the specimen according to Comparative Example 2 in which cooling was performed at a time up to 450 ° C. without secondary cooling, the strength was sufficient, but the burring property did not reach the target value.

In addition, a part of the composition is outside the range set forth in the present invention, the specimen according to Comparative Example 3, the third cooling rate is relatively slow, the elongation and burring properties were sufficient, but the strength, in particular yield strength was significantly lower.

Figure 2 shows a microstructure picture of the specimen prepared according to Example 1, Figure 3 shows a microstructure picture of the specimen prepared according to Comparative Example 1.

Referring to FIG. 2, in the case of the specimen prepared according to Example 1, it can be confirmed that the specimen has a final microstructure in which fine precipitates are formed in the needle-like ferrite structure. On the other hand, referring to Figure 3, in the case of the specimen prepared according to Comparative Example 1, it can be seen that the final microstructure consists of only ferrite and bainite.

According to the needle-like ferrite structure and fine precipitates, in the case of the specimen according to Example 1, it may exhibit high burring property with excellent strength.

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: Hot rolling step
S120: primary cooling step
S130: second cooling stage
S140: 3rd cooling stage

Claims (7)

delete delete delete By weight%, carbon (C): 0.04 to 0.1%, silicon (Si): 0.2 to 1.0%, manganese (Mn): 0.7 to 1.7%, phosphorus (P): 0.01 to 0.1%, sulfur (S): 0.001 ~ 0.01%, Chromium (Cr): 0.7 ~ 1.2%, Niobium (Nb): 0.01 ~ 0.09%, Titanium (Ti): 0.01 ~ 0.09%, Boron (B): 0.001 ~ 0.005% and the rest of iron (Fe) Made of inevitable impurities,
A high strength steel sheet comprising a needle-like ferrite 90vol% or more, having a microstructure in which fine precipitates are formed.
5. The method of claim 4,
The microstructure
A high strength steel sheet comprising 10 vol% or less of a second phase including at least one of bainite and martensite.
5. The method of claim 4,
The steel sheet
A high strength steel sheet having a tensile strength of 980 to 1200 MPa and a hole expansion ratio of 50% or more.
The method according to claim 6,
The steel sheet
A high strength steel sheet having a yield strength of 800 to 1100 MPa and an elongation of 7 to 20%.

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