KR101355725B1 - High strength hot-rolled steel sheet and method for manufacturing the hot-rolled steel sheet - Google Patents

Abstract

Disclosed is a high strength hot rolled steel sheet having a resistivity ratio and a high elongation and having a tensile strength of 700 MPa or more through controlling alloy components and controlling process conditions, and a method of manufacturing the same.
The method for producing a high strength hot rolled steel sheet according to the present invention is (a) wt%, carbon (C): 0.04 to 0.15%, silicon (Si): 0.6 to 1.0%, manganese (Mn): 1.0 to 2.5%, aluminum (Al) ): 0.1 ~ 0.3%, niobium (Nb): 0.03 ~ 0.09%, molybdenum (Mo): 0.1 ~ 0.3%, vanadium (V): 0.05 ~ 0.09% Reheating; (b) hot rolling the reheated sheet to a finish rolling temperature of 850 to 950 ° C .; And (c) cooling the hot rolled sheet to 550 to 650 ° C.

Description

High-strength hot rolled steel sheet and its manufacturing method {HIGH STRENGTH HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE HOT-ROLLED STEEL SHEET}

The present invention relates to a high strength hot rolled steel sheet production technology, and more particularly, to a high strength hot rolled steel sheet having a high strength of tensile strength of 700 MPa or more and excellent in formability and a method of manufacturing the same.

In the automotive industry, body weight reduction has become an essential part. Accordingly, steel companies are doing a lot of research to develop high strength steel to reduce the weight of the material.

In the automotive parts, the part requiring high strength steel may be an example of an automobile main frame. In the case of such a main frame, high tensile strength and formability are required as properties required for the material.

Therefore, it is required to develop a material having a high strength of 700 MPa or more and a low yield ratio and excellent elongation.

Background art related to the present invention is Korean Patent Publication No. 10-2001-0038132 (2001.05.15. Publication).

An object of the present invention is to provide a high-strength hot-rolled steel sheet having high strength of tensile strength of 700 MPa or more through control of alloy components and hot rolling process and excellent in moldability due to low resistance and high elongation, and a method for manufacturing the same.

High-strength hot-rolled steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is (a) wt%, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.6 ~ 1.0%, manganese (Mn) : 1.0 ~ 2.5%, Aluminum (Al): 0.1 ~ 0.3%, Niobium (Nb): 0.03 ~ 0.09%, Molybdenum (Mo): 0.1 ~ 0.3%, Vanadium (V): 0.05 ~ 0.09% and the remaining iron (Fe) Reheating the slab plate consisting of) and inevitable impurities; (b) hot rolling the reheated sheet to a finish rolling temperature of 850 to 950 ° C .; And (c) cooling the hot rolled sheet to 550 to 650 ° C.

At this time, in the step (c), the cooling is preferably carried out at a cooling rate of 50 ~ 200 ℃ / sec.

In addition, high-strength hot-rolled steel sheet according to an embodiment of the present invention for achieving the above object by weight, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.6 ~ 1.0%, manganese (Mn): 1.0 ~ 2.5%, aluminum (Al): 0.1-0.3%, niobium (Nb): 0.03-0.09%, molybdenum (Mo): 0.1-0.3%, vanadium (V): 0.05-0.09% and inevitable with the remaining iron (Fe) It is made of impurities and has a tensile strength of 700 to 900 MPa, a yield ratio of 0.8 or less, and an elongation of 20 to 30%.

The hot rolled steel sheet may include a weight%, phosphorus (P): 0.01 ~ 0.10%, sulfur (S): 0.001 ~ 0.01% and nitrogen (N): 10 ~ 50ppm.

On the other hand, the microstructure of the hot rolled steel sheet includes ferrite, martensite and fine precipitates having a particle diameter of 0.5㎛ or less.

In the method of manufacturing a high strength hot rolled steel sheet according to the present invention, the final microstructure may have a composite structure including ferrite, martensite, fine precipitates, and the like, by controlling alloy components and controlling process conditions.

Through this, the steel sheet produced by the method according to the present invention can exhibit a tensile strength of 700MPa or more, and can also exhibit a yield ratio of 0.8 or less and an elongation of 20% or more, thereby having excellent moldability.

1 is a flow chart showing a high strength hot rolled steel sheet manufacturing method according to an embodiment of the present invention.
2 shows a strain-stress curve of specimens prepared according to Example 1 and Comparative Example 1. FIG.
Figure 3 shows the mechanical properties of the specimen prepared according to Example 1 and Comparative Example 1.
Figure 4 is a microstructure photograph of the specimen prepared according to Comparative Example 1.
5 is a microstructure photograph of a specimen prepared according to 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 hot rolled steel sheet according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

Hot-rolled steel sheet

High-strength hot-rolled steel sheet according to the present invention by weight%, carbon (C): 0.04 ~ 0.15%, silicon (Si): 0.6 ~ 1.0%, manganese (Mn): 1.0 ~ 2.5%, aluminum (Al): 0.1 ~ 0.3 %, Niobium (Nb): 0.03 to 0.09%, molybdenum (Mo): 0.1 to 0.3% and vanadium (V): 0.05 to 0.09%.

In addition, the hot rolled steel sheet may include phosphorus (P): 0.01 ~ 0.10%, sulfur (S): 0.001 ~ 0.01% and nitrogen (N): 10 ~ 50ppm.

The remainder other than the alloying components are made of iron (Fe) and impurities which are inevitably included in a steelmaking process.

Hereinafter, the role and content of each component included in the high strength hot rolled 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 at 0.04 to 0.15% by weight of the total weight of the steel sheet, and more preferably added at 0.12 to 0.15% by weight in terms of securing strength. If the carbon content is less than 0.04% by weight, it may be difficult to secure the desired strength. On the contrary, when the content of carbon exceeds 0.15% by weight, there is a problem in that moldability and toughness are lowered.

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 0.6 ~ 1.0% by weight of the total weight of the steel sheet. When the amount of silicon added is less than 0.6% by weight, the deoxidation effect and strength improvement effect due to the addition of silicon may not be properly exhibited. 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 an amount of 1.0 to 2.5% by weight based on the total weight of the steel sheet. When the amount of manganese added is less than 1.0% by weight, the amount added is insignificant and the above effects cannot be properly exhibited. On the contrary, when the content of manganese exceeds 2.5% by weight, there is a problem in that excessive generation of MnS-based non-metallic inclusions.

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% by weight, a central segregation zone may be formed to deteriorate the material of the steel sheet and also deteriorate weldability.

Sulfur (S)

Sulfur (S) is an element contributing to improvement of processability.

The sulfur is preferably added in 0.001 to 0.01 weight% of the total weight of the steel sheet. If the content of sulfur is less than 0.001% by weight, the content of sulfur must be controlled to the minimum, and thus there is a problem in that steel production costs increase. On the other hand, when the content of sulfur exceeds 0.01% by weight, there is a problem that the weldability is greatly deteriorated.

Aluminum (Al)

Aluminum (Al) reacts with nitrogen (N) to form fine AlN precipitates, thereby contributing to improvement of strength by precipitation strengthening as well as grain refinement.

Aluminum (Al) is preferably added at 0.1 to 0.3% by weight of the total weight of the steel sheet. If the content of aluminum (Al) is less than 0.1% by weight, the amount of AlN precipitates may be reduced, thereby making it difficult to secure sufficient strength. On the contrary, when the content of aluminum (Al) exceeds 0.3% by weight, there is a problem of inhibiting toughness and excessively increasing yield strength.

Niobium (Nb)

Niobium (Nb) forms fine precipitates in the form of Nb (C, N), (Nb, V) (C, N) in steel and contributes to the improvement of tensile strength of structural steels.

The niobium is preferably added at 0.03 to 0.09% by weight of the total weight of the steel sheet. If the content of niobium is less than 0.03% by weight, the above-mentioned effects cannot be properly exhibited because the added amount is insignificant. On the contrary, when the content of niobium exceeds 0.09% by weight, the yield strength is also greatly increased along with the improvement in tensile strength, making it difficult to secure a resistance ratio, and there is a problem in deteriorating low temperature impact characteristics.

Molybdenum (Mo)

Molybdenum (Mo) contributes to improvement of strength and toughness, and also contributes to ensuring stable strength at room temperature or high temperature.

The molybdenum (Mo) is preferably added in 0.1 to 0.3% by weight of the total weight of the steel sheet. When the amount of molybdenum (Mo) is less than 0.1% by weight, the effect of adding molybdenum is insufficient. On the other hand, when the content of molybdenum exceeds 0.3% by weight, the weldability is lowered and the yield ratio is increased.

Vanadium (V)

Vanadium (V) is an element that forms a fine precipitate together with the niobium by contributing to the improvement of strength by acting as a pinning (pinning) to the grain boundary.

The vanadium is preferably added in a content ratio of 0.05 to 0.09% by weight of the total weight of the steel sheet. If the vanadium content is less than 0.05% by weight, the vanadium addition effect may not be properly exhibited. On the contrary, when the content of vanadium (V) is more than 0.09% by weight, there is a problem of increasing the manufacturing cost of the steel sheet and also greatly increasing the yield strength.

Nitrogen (N)

Nitrogen (N) may be contained in a content ratio of 10 to 50 ppm of the total weight of the steel sheet. Nitrogen is combined with niobium to form a precipitate, but when the nitrogen content is less than 10ppm it is difficult to form a precipitate. On the contrary, when the content of nitrogen (N) exceeds 50 ppm, the plating property and the like may be greatly reduced by solid solution nitrogen.

The high strength hot rolled steel sheet according to the present invention may have a microstructure including ferrite and martensite through the alloy composition described above and the hot rolling process described later. At this time, the microstructure further includes fine precipitates such as Nb (C, N), (Nb, V) (C, N). These precipitates have a particle diameter of 0.5 μm or less, contributing to the improvement of strength.

Through this, the hot rolled steel sheet according to the present invention may exhibit a tensile strength of 700 ~ 900MPa, a yield ratio of 0.8 or less and 20 to 30% elongation.

Steel plate manufacturing method

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

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

Reheating slabs

In the slab reheating step (S110), the slab plate having the alloy composition described above is reheated. By reheating the slab sheet, the segregated components are cast again.

It is preferable to perform slab reheating at 1150-1250 degreeC. If the slab reheating temperature is less than 1150 ℃ there is a problem that the reheating temperature is low to increase the rolling load. In addition, there is a problem that does not reach the solid solution temperature, such as NbC, NbN, Nb-based precipitates as a fine precipitate during hot rolling.

On the other hand, when the slab reheating temperature exceeds 1250 ℃ it may be difficult to secure the strength and low-temperature toughness of the steel sheet produced due to the coarsening of austenite grains.

Hot rolling

In the hot rolling step (S120), the slab plate is hot rolled.

The finish rolling temperature in the hot rolling step (S120) is preferably 850 ~ 950 ℃. When hot rolling is finished in the above temperature range, the structure of the steel sheet before cooling after hot rolling may become an austenite phase. If the finish rolling temperature exceeds 950 ° C, the austenite grains are coarsened, so that the ferrite grains may not be sufficiently refined after the transformation, thereby making it difficult to secure the strength. On the contrary, when the finishing temperature is lower than 850 ° C., problems such as a mixed structure caused by abnormal reverse rolling may occur.

Cooling

In the cooling step S130, in order to secure sufficient strength and toughness, the hot rolled sheet is forcedly cooled.

Cooling is preferably carried out at a cooling rate of 50 ~ 200 ℃ / sec. If the cooling rate is less than 50 ° C / sec, martensite is difficult to produce, it is difficult to secure a tensile strength of 700MPa or more. On the other hand, if the cooling rate exceeds 200 ℃ / sec, it is difficult to secure the moldability of 20% or more.

It is preferable that the winding temperature corresponding to cooling end temperature is 550-650 degreeC. If the winding temperature is less than 550 ° C., there is a problem in that a large amount of low temperature transformation tissue is formed and the low temperature impact toughness is sharply lowered. On the contrary, when the coiling temperature exceeds 650 ° C., there is a problem in that strength is insufficient due to coarse microstructure formation.

After the cooling step (S130), the microstructure of the produced hot rolled steel sheet includes ferrite, martensite and fine precipitates. At this time, the fine precipitate may include a form of Nb (C, N), (Nb, V) (C, N).

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 specimens

Specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were prepared under the compositions shown in Table 1 and the process conditions described in Table 2.

[Table 1] (unit:% by weight)

2. Evaluation of mechanical properties

Table 2 shows the evaluation of mechanical properties of the specimens according to Examples 1 to 3 and Comparative Examples 1 to 3.

[Table 2]

Referring to Table 2, in the case of specimens prepared according to Examples 1 to 3, the target tensile strength (TS) 700 to 900 MPa Yield ratio (YP / TS): 0.8 or less and elongation (EL) All 20-30% were satisfied.

On the other hand, in the specimen according to Comparative Example 1 containing 0.2 wt% of carbon (C) and not adding niobium (Nb) and vanadium (V), the tensile strength did not reach 700 MPa.

In addition, in Comparative Example 2, in which vanadium was not added and the aluminum content was 0.05% by weight, the strength and the resistivity ratio were satisfied, but the elongation did not reach the target value of 20%.

In addition, in the case of the specimen according to Comparative Example 3 in which a relatively large amount of niobium and vanadium were added, and the cooling end temperature was relatively low, the yield ratio exceeded 0.8 and the elongation was less than 20%.

Figure 2 shows the strain-stress curve of the specimen prepared according to Example 1 and Comparative Example 1, the same process conditions are applied, Figure 3 shows the mechanical properties of the specimen prepared according to Example 1 and Comparative Example 1 .

2 and 3, even when the same process conditions are applied, the specimen according to Example 1, it can be seen that the tensile strength is significantly improved compared to the specimen according to Comparative Example 1. This difference in strength may be because niobium and vanadium were included in the specimen according to Example 1 to form fine precipitates in the form of Nb (C, N), (Nb, V) (C, N).

4 is a microstructure photograph of the specimen prepared according to Comparative Example 1, Figure 5 is a microstructure photograph of the specimen prepared according to Example 1.

Referring to FIG. 4, in Comparative Example 1 in which niobium (Nb) and vanadium (V) were not added, it can be seen that the microstructure is made of ferrite and martensite.

On the other hand, referring to FIG. 5, the microstructure of the specimen according to Example 1 including niobium (Nb) and vanadium (V) includes not only ferrite and martensite, but also fine precipitates having a particle diameter of 0.5 μm or less. Such fine precipitates may be in the form of Nb (C, N), (Nb, V) (C, N) as described above, and may be considered to contribute to an increase in tensile strength compared to Comparative Example 1.

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 step

Claims (7)

(a) By weight%, carbon (C): 0.04 to 0.15%, silicon (Si): 0.6 to 1.0%, manganese (Mn): 1.0 to 2.5%, aluminum (Al): 0.1 to 0.3%, niobium (Nb) ): Reheating the slab plate consisting of 0.03% to 0.09%, molybdenum (Mo): 0.1% to 0.3%, vanadium (V): 0.05% to 0.09%, and the remaining iron (Fe) and unavoidable impurities;
(b) hot rolling the reheated sheet to a finish rolling temperature of 850 to 950 ° C .; And
(C) cooling the hot-rolled sheet to 550 ~ 650 ℃; high strength hot rolled steel sheet manufacturing method comprising a.
The method of claim 1,
The slab plate
By weight%, phosphorus (P): 0.01 ~ 0.10%, sulfur (S): 0.001 ~ 0.01% and nitrogen (N): 10 ~ 50ppm characterized in that it comprises a high strength hot rolled steel sheet manufacturing method.
The method of claim 1,
In the step (a), the slab reheating is
High strength hot rolled steel sheet manufacturing method characterized in that carried out at 1150 ~ 1250 ℃.
The method of claim 1,
In step (c), cooling
High strength hot rolled steel sheet manufacturing method characterized in that carried out at a cooling rate of 50 ~ 200 ℃ / sec.
By weight%, carbon (C): 0.04-0.15%, silicon (Si): 0.6-1.0%, manganese (Mn): 1.0-2.5%, aluminum (Al): 0.1-0.3%, niobium (Nb): 0.03 ~ 0.09%, molybdenum (Mo): 0.1 ~ 0.3%, vanadium (V): 0.05 ~ 0.09% and the remaining iron (Fe) and inevitable impurities,
High strength hot rolled steel sheet characterized by having a tensile strength of 700 ~ 900MPa, a yield ratio of 0.8 or less and an elongation of 20 to 30%.
The method of claim 5,
The hot-
By weight%, phosphorus (P): 0.01 ~ 0.10%, sulfur (S): 0.001 ~ 0.01% and nitrogen (N): high strength hot rolled steel sheet, characterized in that it contains 10 to 50ppm.
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