TECHNICAL FIELD

The present disclosure relates to a high-strength steel sheet mainly used for members of an automobile chassis component, a lower arm, a wheel disc, and the like. More particularly, the present disclosure relates to a high-strength steel sheet having excellent burring properties in a low-temperature region.

BACKGROUND ART

A high-strength steel sheet (a hot-rolled steel sheet) for a vehicle chassis component according to a related art was manufactured using ferrite-bainite two-phase composite structure steel to improve stretch flangeability or using a ferrite-martensite two-phase composite structure steel to improve strength and ductility simultaneously.

For example, in Patent Document 1, a steel sheet is maintained in a ferrite transformation region for several seconds immediately after hot-rolling the steel sheet under specific cooling conditions, and then coiled in a bainite formation temperature region in order to form bainite. Thus, a metallic structure (a microstructure) is formed as a mixed ferrite and bainite structure to secure strength and stretch flangeability simultaneously.

In a method disclosed in Patent Documents 2 to 4, a steel slab is hot-rolled based on a Si—Mn or Mn—P—Cr composition system. The hot-rolled steel sheet is maintained in a ferrite transformation region for several seconds, and is then controlled to be a martensite transformation initiation temperature (Ms) or lower. In a method disclosed in Patent Document 5, a steel sheet is also maintained in a ferrite transformation region for several seconds by using a Si—Mn—Cr or Si—Mn—Cr—Mo based composition system, and is then coiled at a temperature of a martensite transformation initiation temperature (Ms) or higher. According to the above methods, ferrite-martensite complex structure steel, having excellent strength and ductility, was manufactured.

However, such ferrite-martensite two-phase composite structure steel is actually deteriorated in stretch flangeability to be limited for use in a vehicle chassis component required to have the stretch flangeability.

It is known that silicon (Si), manganese (Mn), aluminum (Al), molybdenum (Mo) and the like, alloy components mainly used to manufacture steel having a ferrite-bainite two-phase composite structure, are effective in improving strength and stretch flangeability of the steel. However, when a large amount of the above alloying components is added to improve such physical properties, segregation of the alloying component and inhomogeneous of a microstructure occurs.

In detail, hardenability of steel is rapidly increased during cooling to sensitively change a microstructure depending on cooling conditions. Thus, a second-phase fraction is significantly increased depending on a position (or a portion) of a steel sheet, and stretch flangeability is significantly deteriorated in a low-temperature region.

Moreover, when an alloying component is inappropriately used to improve physical properties of steel even if the steel includes a ferrite single phase rather than a second-phase, a precipitation particle is excessively formed on grain boundary or a fine carbide is formed on the grain boundary to significantly reduce stretch flangeability in a low-temperature region. Accordingly, temperature dependency is increased during forming and productivity is lowered to cause economical disadvantages.(Patent Document 1) Japanese Laid-Open Patent Publication No. 1994-293910(Patent Document 2) Japanese Laid-Open Patent Publication No. 1995-278731(Patent Document 3) Japanese Laid-Open Patent Publication No. 1997-241790(Patent Document 4) Japanese Laid-Open Patent Publication No. 1994-049591(Patent Document 5) U.S. Pat. No. 4,502,897

DISCLOSURE

Technical Problem

An aspect of the present disclosure is to provide a high-strength steel sheet having excellent burring properties through optimization of an alloying composition and manufacturing conditions, and a method of manufacturing the same.

Technical Solution

According to an aspect of the present disclosure, a high-strength steel sheet having excellent burring properties in a low temperature range, includes, by wt %, carbon (C): 0.02 to 0.10%, silicon (Si): 0.01 to 0.30%, manganese (Mn): 1.0 to 1.7%, aluminum (Al): 0.01 to 0.10%, chromium (Cr): 0.005 to 0.2%, molybdenum (Mo): 0.005 to 0.3%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.001 to 0.01%, nitrogen (N): 0.001 to 0.01%, niobium (Nb): 0.005 to 0.06%, titanium (Ti): 0.005 to 0.13%, vanadium (V): 0.005 to 0.2%, boron (B): 0.0003 to 0.002%, and a balance of iron (Fe) and inevitable impurities,

[C]*, expressed by Relational Expression 1, satisfies −0.015 to 0.015,

Mn, Mo, Cr, and B satisfy Relational Expression 2,
[C]*=[C]−([C]×A)  (Relational Expression 1)

where A=(([Nb]/93)+([Ti]/48)+([V]/51)+([Mo]/96))/([C]/12), and each alloying component denotes a content by wt %,
[Mn]+(2.8[Mo])+(1.5[Cr])+(500[B])≤2.5  (Relation Expression 2)

where each alloying component denotes a content by wt %,

a microstructure comprises, by area fraction, 97% or more (including 100%) of a ferrite phase,

the number of coarse complex precipitates, containing at least one of Ti, Nb, Mo, and V, each having a diameter of at least 10 μm, observed within a unit area of 1 cm2, is 1×107or less, and

the number of austenitic phases, each having a diameter of 10 μm or more, observed from grain boundaries within a unit area of 1 cm2, is 1×104or less.

According to another aspect of the present disclosure, a method of manufacturing a high-strength steel sheet, having excellent burring properties in a low temperature range, includes reheating a steel slab, satisfying the above-described alloying composition and Relational Expressions 1 and 2, within a temperature range from 1200 to 1350° C., finish-hot-rolling the reheated steel slab within a temperature range from 850 to 1150° C. to manufacture a hot-rolled steel sheet, cooling the hot-rolled steel sheet to a temperature ranging from 550 to 700° C. at an average cooling rate of 10 to 70° C./s, coiling the cooled hot-rolled steel sheet within a temperature range from 550 to 700° C., and forced-cooling the hot-rolled steel sheet at a cooling rate of 0.01 to 10° C./s after coiling.

Advantageous Effects

According to the present disclosure, a steel sheet having excellent burring properties not only in a room-temperature region but also in a low-temperature region may be provided. The steel sheet may be appropriately applied to a material, having excellent strength and excellent stretch flangeability, such as a vehicle chassis component, or the like.

