Ultra-High Strength Steel Sheet Having Excellent Bendability, and Manufacturing Method Therefor

The present invention relates to a steel sheet suitable for automobile chassis members, etc., and, more particularly, to an ultra-high strength steel sheet having excellent bendability, and a manufacturing method therefor.

Recently, in the automobile field, research to reduce the weight of a vehicle body is being actively conducted in developed countries, including Europe, due to fuel efficiency regulations and performance improvement.

Specifically, in the case of the steel field, efforts are being made to produce a high strength steel further reduce the thickness of the steel sheet in the same grade compared to competitive materials (Mg, Al, CFRP (carbon fiber reinforced plastic), etc.) in order to meet the demand for weight reduction of automakers. In other words, in addition to weight reduction, stability and high strength of vehicle body materials are also required due to the strengthening of safety regulations for automobile passengers and pedestrians.

On the other hand, in order to improve the stability and collision characteristics of the vehicle body, the use of high-strength steel with excellent yield strength in Body-In-White (BIW) structural members is increasing, and these structural members have the characteristic that as the yield strength compared to the tensile strength, that is, a yield ratio (yield strength/tensile strength, YR) increases, this is advantageous for absorbing impact energy.

Thus, as a representative method to increase the yield strength of steel, a method using water cooling during continuous annealing is mainly used. Specifically, the representative method may include a method of manufacturing ultra-high strength steel through processes such as tempering after annealing a cold-rolled steel sheet with a two-phase region or a single-phase region and quenching to approximately room temperature.

However, while ultra-high strength steel produced thereby has a very high yield ratio, there may be a problem that a shape quality of a coil may deteriorate due to a temperature deviation in a width direction and a length direction, and further, problems such as poor material and deterioration of workability depending to components may occur during component processing due to roll forming or the like. In addition, in general, as the strength of the steel increases, the elongation decreases, which may cause a decrease in molding processability.

To overcome these problems, a hot press forming (HPF) method has been developed and applied to secure strength through water cooling between a die and a material after molding a material at a high temperature at which molding may be relatively easily performed (see Patent Document 1).

When the HPF method is applied, high strength compared to the same thickness may be secured, and components using the HPF method are being developed in Europe.

However, the HPF method requires excessive facility investment costs and problems such as increased process costs are emerging, and accordingly, the development of materials for cold stamping is required.

In other words, it is necessary to develop a steel sheet that is suitable for use as a cold stamping material, has high strength and a high yield ratio to secure collision performance characteristics, and has excellent formability.

An aspect of the present invention is to provide a steel sheet suitable for automobile chassis members while being suitable for cold stamping, and particularly, to a steel sheet having excellent bendability, and a manufacturing method therefor.

An object of the present invention is not limited to the description above. An object of the present invention may be understood from the overall contents of the present specification, and it may be understood by those of ordinary skill in the art that there would be no difficulty in understanding the additional problems of the present invention.

According to an aspect of the present invention, provided is an ultra-high strength steel sheet having excellent bendability, including: by wt %, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.5%, two or more types of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2% and boron (B): 0.005% or less, one or more types of titanium (Ti): 0.1% or less and niobium (Nb): 0.1% or less, and a balance of Fe and unavoidable impurity elements, and satisfying the following relational expression 1, wherein a microstructure comprises, in area fraction, 99% or more of martensite and/or tempered martensite phases.

According to an aspect of the present invention, provided is a manufacturing method for an ultra-high strength steel sheet having excellent bendability, including: heating a steel slab including, by wt %, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.5%, two or more types of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2% and boron (B): 0.005% or less, one or more types of titanium (Ti): 0.1% or less and niobium (Nb): 0.1% or less, and a balance of Fe and unavoidable impurity elements, and satisfying the following relational expression 1 in a temperature range of 1100 to 1300° C.; manufacturing a hot-rolled steel sheet by finish hot rolling the reheated steel slab at Ar3 or higher; coiling the hot-rolled steel sheet at a temperature of 700° C. or less; manufacturing a cold-rolled steel sheet by cold rolling the coiled hot-rolled steel sheet with a total reduction ratio of 30 to 80%; continuously annealing the cold-rolled steel sheet for 30 seconds or more at Ac3 or higher; performing primary cooling at an average cooling rate of 1 to 10° C./s to a temperature range of 550 to 750° C. after the continuous annealing; performing secondary cooling at an average cooling rate of 20 to 80° C./s to a temperature of Ms−190° C. or less after the performing primary; and performing reheating after the secondary cooling and then performing an over-aging treatment,

CT2+30° C.≤≤270° C.  <Relational Expression 3>

According to the present invention, a steel sheet with improved processability may be provided by achieving a high yield ratio in addition to ultra-high strength. In particular, a steel sheet of the present invention is not only a material that may be suitably applied to automobile chassis members, but is also advantageously applicable to processing such as cold stamping.

The inventors of the present invention conducted in-depth research to provide a steel sheet that is suitable for automobile chassis members and is advantageous for processing such as cold stamping. Accordingly, the inventors have confirmed that it was possible to provide a steel sheet with a desired structure, physical properties, etc. by optimizing an alloy composition and manufacturing conditions, and have completed the present invention.

Hereinafter, the present invention will be described in detail.

An ultra-high strength steel sheet according to an aspect of the present invention may include, by wt %, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), and aluminum (Al): 0.01 to 0.5%.

Hereinafter, the reason for limiting the alloy composition of the ultra-high strength steel sheet provided by the present invention as described above will be explained in detail.

Meanwhile, in the present invention, unless specifically stated, the content of each element is based on weight, and the ratio of a structure is based on area.

Carbon (C) is an interstitial solid solution element and is the most effective and important element in improving the strength of steel. Specifically, in martensitic steel, Carbon (C) is an element that must be added to secure strength.

In order to obtain a steel sheet with a target strength, yield ratio, and the like, in the present invention, Carbon (C) may be added in an amount of 0.1% or more. However, when the content thereof exceeds 0.3%, the martensite strength increases, but during a continuous annealing process, carbides may be easily generated and coarsening may be facilitated, which causes not only reduced ductility but also poor bendability. Additionally, an excessive increase in carbon content has the problem of deteriorating weldability.

Therefore, in the present invention, Carbon (C) may be included in an amount of 0.1 to 0.3%, and more advantageously, 0.12% or more and 0.28% or less.

Manganese (Mn) is an element that suppresses the formation of ferrite in composite steel and promotes the formation of austenite, making it easy to ultimately secure a martensite phase.

When the Mn content exceeds 2.3%, Mn is segregated in a thickness direction of steel and a manganese band is easily formed in the slab, which increases an occurrence of defects during rolling as well as continuous casting cracks. On the other hand, when the content thereof is less than 1.0%, a target level of strength may not be secured.

Accordingly, in the present invention, Manganese (Mn) may be included in an amount of 1.0 to 2.3%, and may be included in an amount of, more advantageously, 1.2% or more and 2.1% or less. Manganese (Mn) may be included in an amount of, more advantageously, 1.4% or more.

Silicon (Si) serve to suppress the formation of carbides and controlling the size of carbides in reheating and over-aging treatment steps performed after continuous annealing and cooling during a process of manufacturing a steel sheet to be obtained in the present invention.

In order to sufficiently obtain the above-described effect, Si may be included in an amount of 0.05% or more. However, when the content exceeds 1.0%, there is a risk that ferrite may be generated during cooling in a continuous annealing furnace, which may weaken the strength of the steel. Furthermore, Si-based oxides are generated during reheating and over-aging after cooling, which may cause surface oxidation problems in the steel.

Accordingly, in the present invention, Si may be included in an amount of 0.05 to 1.0%, and may be included in an amount of, more advantageously, 0.09% or more and 0.8% or less. Si may be included in an amount of even more advantageously, 0.6% or less.

Phosphorus (P) is an impurity element contained in steel, and when a content thereof exceeds 0.1%, the weldability of the steel may deteriorate and brittleness may occur. Accordingly, Phosphorus (P) is limited to 0.1% or less, and the amount of 0% may be excluded in consideration of a level of unavoidable inclusion during the steel manufacturing process. More advantageously, Phosphorus (P) may be included in an amount of 0.05% or less, and even more advantageously, 0.03% or less.

Sulfur (S), similar to Phosphorus (P), is an impurity inevitably contained in steel and is an element that impairs the ductility and weldability of steel, and it may be advantageous to keep a content thereof as low as possible. In the present invention, there is no difficulty in securing target physical properties even if Sulfur (S) is contained at a maximum of 0.03%, and thus, an upper limit thereof may be limited to 0.03%, and the amount of 0% may be excluded in consideration of a level of unavoidable inclusion during the steel manufacturing process.

Meanwhile, in order to more advantageously secure the bendability targeted in the present invention, the content of S may be limited to 0.01% or less, and even more advantageously, 0.005% or less.

Aluminum (Al) may be added to remove oxygen in molten steel and, similarly to Si, Aluminum (Al) is an element that stabilizes ferrite. Furthermore, Al is an element that improves the hardenability of final martensitic steel by increasing the carbon content in austenite.

In order to sufficiently obtain the above-described effect, Al may be contained in an amount of 0.01% or more. However, when the content exceeds 0.5%, there is a risk that ferrite may be formed during cooling in a continuous annealing furnace, thereby weakening the strength. Furthermore, there may be a risk of causing cast piece cracks by combining with N, which is inevitably present as an impurity element in the steel, to form AlN, and there may be a problem of impairing hot rolling properties.

Therefore, in the present invention, Al may be included in an amount of 0.01 to 0.5%.

Meanwhile, the steel sheet of the present invention may further include elements advantageous for securing the physical properties of steel in addition to the alloy composition described above. Specifically, the steel sheet of the present invention may further include two or more types selected from chromium (Cr), molybdenum (Mo), and boron (B), and one or more types of titanium (Ti) and niobium (Nb)

Chromium (Cr) may be added to improve the hardenability of steel and ensure high strength. Specifically, it is useful for manufacturing an ultra-high strength steel sheet including pure martensite phase by suppressing the formation of bainite during cooling in a continuous annealing furnace.

In order to fully obtain the above-mentioned effect, Cr may be added in an amount of 0.01% or more, but when the content thereof exceeds 0.2%, the cost of ferroalloy increases, which may become economically disadvantageous.

Therefore, when adding Cr, Cr may be added in an amount of 0.01 to 0.2%.

Molybdenum (Mo), similar to Cr, is an element that improves the hardenability of steel.

In order to obtain sufficient hardening effect, Mo may be added in an amount of 0.01% or more, but when the content exceeds 0.2%, an alloy input amount becomes excessive, which may cause a problem in that costs of iron alloy may increases.

Therefore, when adding Mo, Mo may be added in an amount of 0.01 to 0.2%.

Boron (B) is an element that suppresses the transformation of austenite into ferrite during the continuous annealing process, and is an element that is effective in improving hardenability, like Cr and Mo, even when Boron (B) is added in very small amounts. However, when the content thereof exceeds 0.005%, as Fe(B,C)precipitated phase precipitates at a austenite grain boundary, there may a risk that Boron (B) may act to promote the formation of ferrite.

Therefore, when adding B, B may be added in an amount of 0.005% or less.

Titanium (Ti) is an element that forms fine carbides and contributes to securing yield strength and tensile strength. Furthermore, Ti is an element that performs scavenging by precipitating N which is inevitably present at an impurity level in steel, as TiN, and Ti may be added in an amount of 48/(14×N) or more based on chemical equivalent

When the content of Ti exceeds 0.1%, there may be a problem in that coarse carbides are precipitated and the strength and elongation are lowered as the amount of carbon in the steel decreases. Furthermore, since Ti may cause nozzle clogging in a continuous casting process, Ti may be added in an amount of 0.1% or less.

Meanwhile, in order to maximize the effect of addition of B, it may be advantageous to add Ti together.

Niobium (Nb) is an element that segregates at austenite grain boundaries, suppresses coarsening of austenite grains during the continuous annealing process, and forms fine carbides, thus contributing to strength improvement.