High-Yield Ratio High-Strength Steel Plate Having Excellent Impact Resistance After Cold Forming and Manufacturing Method Thereof

The present disclosure relates to high-strength steel and a manufacturing method therefor and, more specifically, to high-strength steel having excellent impact resistance after cold forming and having a high yield ratio, and a manufacturing method therefor.

Conventional chassis components of a commercial vehicle may be mainly formed of a steel material having a thickness of 8 mm or more due to characteristics of the vehicle. High-strength hot-rolled steel plates having a tensile strength of about 600 MPa may be used for members, and hot-rolled steel plates having a tensile strength of about 440 MPa may be used for wheel disks. However, recently, in order to reduce weight, there may be a trend to apply high-strength hot-rolled steel plates having a tensile strength of 700 MPa or more to the members, apply high-strength hot-rolled steel plates having a tensile strength of 590 MPa or more to the wheel disks, and reduce thicknesses of the steel plates, or change designs of the components. In addition, a wheel has been manufactured using a press molding process in the past, but recently there may be a trend to manufacture the same by a spinning process and a flow forming process. The forming process requires a hot-rolled steel plate having superior elongation because a greater amount of deformation is given to the hot-rolled steel plate, and formed components are required to secure durability and impact resistance during use.

However, when conventional high-strength steel is applied to the spinning and flow forming processes, durability of components should be equal or better than that of an existing one, but when the components are formed, there are difficulties in application, e.g., fine cracks occur in a shear surface, durability is inferior in regions in which forming volume is high, or the like.

Conventional high-strength steel may undergo hot-rolling in a normal austenite zone, may be then coiled at a high temperature to use ferrite as a matrix structure, and may form fine precipitates, as in Patent Documents 1 and 2, and a technique in which a coiling temperature is cooled to a temperature at which a bainite matrix structure is formed, to prevent formation of coarse pearlite from forming, and is then coiled, may be applied, as in Patent Document 3. In addition, as in Patent Document 4, a technique for refining austenite grains by pressure reduction ratio of 40% or more in a non-recrystallized region during hot-rolling using Ti, Nb, or the like has also been proposed. Recently, as in Patent Document 5, a technique for improving uniformity of a microstructure between an external portion and an internal portion in a steel plate and suppressing formation of coarse carbides has been proposed, and as in Patent Document 6, a technique for simultaneously suppressing formation of pearlite and a martensite-austenite (MA) phase, and formation of martensite, adversely affecting durability, has been proposed.

However, Patent Documents 1 to 4 do not take into account occurrence of cracks on and around a shear surface during shear forming of high-strength thick materials, and configure cooling rate conditions and pressure reduction ratio conditions, difficult to be secured in manufacturing for thick materials having a thickness of 8 mm or more. When precipitate-forming elements such as Ti, Nb, V, or the like, are used to refine crystal grains of thick materials and secure strength at the same time, and coiling is performed at a high temperature of 500 to 700° C., in which precipitates are easily formed, there may be problems in that ferrite is excessively grown to decrease yield strength, and coarse pearlite is formed. In addition, even when manufacturing at a low coiling temperature to utilize a bainite matrix structure, and when a cooling rate in a width direction of a steel plate is not uniformly controlled during cooling after hot-rolling, a non-uniform microstructure may be formed in a hot-rolled steel plate, making it difficult to obtain a high elongation rate, and making it difficult to secure stable high yield ratio characteristics, and sensitivity to molding cracks such as cracks in a shear surface during molding or the like may also increase. Moreover, these technologies target hot-rolled steel plates less than 5 mm thick, which may be used for passenger cars, and the required cooling rate may be too high, making them unsuitable for manufacturing thick materials. In addition, applying high pressure reduction ratio of 40% in the non-recrystallization zone may deteriorate a shape quality of a rolled plate and may cause load on the equipment, making it difficult to apply to thick materials having a thickness of 8 mm or more. Patent Documents 5 and 6 may be inventions targeting thick materials. First, in order to improve durability of thick high-strength steel, Patent Document 5 discloses a manufacturing technology to ensure that a grain shape in an internal portion (¼to ½) is equiaxed and has fine grains, and to suppress formation of MA phase and martensite. Patent Document 6 proposes a technique for manufacturing a hot-rolled coil by dividing it into three parts in a longitudinal direction through a relational formula derived for a specific component, cooling a head portion, an intermediate portion, and a tail portion under constant cooling rate conditions to different cooling end temperatures, and then coiling them. These technologies may be technologies for producing a uniform microstructure by controlling the cooling rate after hot-rolling through a relationship derived for a specific component in consideration of the quality of the cross-section of the part, may include a large number of punching holes, and may be subjected to continuous load. Although it may be effective in improving durability of commercial vehicle wheels, impact resistance after molding was not considered. In addition, it may be difficult to control cooling of the steel plate uniformly across an entire width after hot-rolling, and when the hot-rolled steel plate is thicker than 8 mm, it may be difficult to control the actual cooling rate.

A cooling process after hot-rolling may be usually carried out within tens of seconds at a run-out table (ROT) having a length of 100 to 120 m, but may be difficult to manufacture hot-rolled steel by cooling it to a cooling end temperature or a coiling temperature while satisfying a proposed range for a cooling rate of an internal portion. Therefore, in the prior art, hot-rolled steel plates having a thickness of 8 mm or more have problems in that it may be difficult to achieve an effect of suppressing formation of coarse carbides and it may be insufficient to secure a high level of impact resistance.

According to an aspect of the present disclosure, an object of the present disclosure is to provide high-strength steel having excellent impact resistance after cold forming and having a high yield ratio, and a manufacturing method therefor.

An object of the present disclosure is not limited to those described above. A person skilled in the art will have no difficulty in understanding the additional problems of the present disclosure from the overall content of the present specification.

An aspect of the present disclosure may provide a steel plate including, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, a remainder of Fe and inevitable impurities, and Nb+Ti: 0.04 to 0.1%,

The steel plate may have a thickness of 8 to 25 mm.

The steel plate may have a tensile strength of 590 MPa or more, a fracture elongation of 25% or more, a yield ratio of 0.75 to 0.9, and an impact toughness of 70 J or more at −20° C. after cold forming.

In the steel plate, a ratio (E/YS) of impact toughness (E) at −20° C. after cold forming and yield strength (YS) before cold forming may be 0.15 or more.

The steel plate may include edge portions corresponding to 30% regions from both ends, and a central portion of a central 40% region corresponding to a region excluding both of the edge portions, in a width direction, wherein, in the edge portions and the central portion, a difference in tensile strength is 10 MPa or less, a difference in elongation at break is 8% or less, and a difference in impact toughness at −20° C. after cold forming is 20 J or less.

Another aspect of the present disclosure may provide a method of manufacturing a steel plate, including, reheating a steel slab including, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, a remainder of Fe and inevitable impurities, and Nb+Ti: 0.04 to 0.1%,

A temperature of the reheating may be 1100 to 1350° C., and

The method may further include air-cooling the coiled coil to a temperature range of 200° C. or lower.

The steel plate may have a thickness of 8 to 25 mm.

According to an aspect of the present disclosure, high-strength steel having excellent impact resistance after cold forming and having a high yield ratio, and a manufacturing method therefor may be provided.

According to an aspect of the present disclosure, high-strength steel that can be applied as a steel material used in chassis members, wheels, or the like of medium-to-large commercial vehicles, and a manufacturing method therefor may be provided.

Hereinafter, preferred embodiments of the present disclosure will be described. Embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as limited to embodiments described below. The present embodiments may be provided to explain the present disclosure in more detail to those skilled in the art.

In order to solve the above-described conventional problems and ensure excellent formability and impact resistance, the present inventor researched a change in impact resistance after cold forming according to characteristics of a microstructure of a steel plate. Therefore, it was confirmed that desired properties could be secured by controlling the microstructure in thickness and width directions of the steel plate by optimizing an alloy composition and manufacturing conditions, to complete the present disclosure.

In a hot-rolled steel plate, which may be usually manufactured in the form of a coil, coarse carbides and pearlite may be likely to be formed, when maintained for a long time at a high temperature range of about 500 to 700° C. In particular, when ferrite phase transformation that begins during a cooling process after completion of hot-rolling progresses slowly, a solid content of carbon increases in an untransformed phase, creating conditions in which coarse carbides or pearlite is easy to be formed. Moreover, since a central portion of the coil in a width direction has a slower cooling rate, as compared to an edge portion thereof, this type of tissue may develop further. Therefore, in order to suppress formation of such coarse carbides and pearlite in the central portion of the coil in a width direction, it may be necessary to cool a coiled coil to room temperature by forced cooling such as water cooling, but in this case, in the edge portion in which the cooling rate is fast, a martensite phase or a martensite and austenite (MA) phase may be formed excessively, making the microstructure non-uniform, making it difficult to secure high elongation, and cracks in a shear surface may also increase, which may be undesirable. Therefore, the present disclosure proposes a method for suppressing formation of coarse carbides and pearlite without forcibly cooling the coil.

Hereinafter, the present disclosure will be described in detail.

Hereinafter, a steel composition of the present disclosure will be described in detail.

In the present disclosure, unless otherwise specified, % indicating a content of each element is based on weight.

Steel according to an aspect of the present disclosure includes, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, a remainder of Fe and inevitable impurities, and Nb+Ti: 0.04 to 0.1%.

Carbon (C) may be the most economical and effective element in strengthening steel, and as an addition amount increases, a precipitation strengthening effect or a bainite phase fraction may increase, making it easier to secure strength. However, as a thickness of a hot-rolled steel plate increases, since a cooling rate in a core portion in a thickness direction may slow down during cooling after hot-rolling, coarse carbides or pearlite may be likely to be formed when an amount of the carbon (C) is large high. Therefore, when an amount of the carbon (C) is less than 0.05%, it may be difficult to obtain a sufficient strengthening effect, and when an amount thereof exceeds 0.15%, there may be a problem that impact resistance may be reduced due to formation of pearlite or coarse carbides in the core portion in the thickness direction, and weldability may be also inferior. A more preferred lower limit may be 0.055%, and a more preferred upper limit may be 0.12%.

Silicon (Si) may be an element that may be effective in deoxidizing molten steel and solid-solution strengthening steel, and may be also effective in improving formability by delaying formation of coarse carbides. However, when an amount thereof is less than 0.01%, an effect of delaying solid solution strengthening and carbide formation may not be maximized, and when an amount thereof exceeds 0.5%, red scale may be formed on a surface of the steel plate during hot-rolling, which not only deteriorates the quality. In addition, there may be problems with decreased ductility and weldability. More preferably, it may be included in an amount of 0.05% or more, and more preferably, it may be included in an amount of 0.3% or less.

Manganese (Mn), like Si, may be an effective element in solid solution strengthening steel, and may increase hardenability of steel, facilitating formation of bainite during cooling after hot-rolling. However, when an amount thereof is less than 1.0%, the above effect due to addition may not be obtained, and when an amount thereof exceeds 2.0%, hardenability may increase significantly, martensite phase transformation may be likely to occur, and a segregation zone may develop significantly in the core portion in the thickness direction when casting slabs in the casting process. More preferably, it may be included in an amount of 1.3% or more, and more preferably, it may be included in an amount of 1.8% or less.

Aluminum (Al) may be an ingredient mainly added for deoxidation, and when an amount thereof is less than 0.01%, the effect of addition may be insufficient. On the other hand, when an amount thereof exceeds 0.1%, AlN may be formed by combining with nitrogen, which easily causes corner cracks in a slab during continuous casting and defects due to formation of inclusions. More preferably, it may be included in an amount of 0.015% or more, more preferably, it may be included in an amount of 0.06% or less.

Chromium (Cr), similar to Mn, may play a role in solid-solution strengthening steel and delaying ferrite phase transformation upon cooling to aid bainite formation. However, when an amount thereof is less than 0.001%, the above effect due to addition may not be obtained, and when an amount thereof exceeds 1.0%, ferrite transformation may be excessively delayed and elongation may be inferior due to excessive formation of martensite. In addition, excessive addition of Cr may cause a segregation zone in the core portion in the thickness direction to develop significantly and make the microstructure in the thickness direction non-uniform, resulting in poor impact resistance. A more preferable lower limit may be 0.01%, and a more preferable upper limit may be 0.5%.

Phosphorus (P), like Si, may have both solid solution strengthening and ferrite transformation promotion effects. However, to manufacture in an amount of phosphorus (P) of less than 0.001%, excessive manufacturing costs may be required, which may be economically disadvantageous and may be insufficient to obtain strength. On the other hand, when an amount thereof exceeds 0.05%, brittleness may occur due to grain boundary segregation, and fine cracks may be likely to occur during molding, which may greatly deteriorate impact resistance.

S may be an impurity present in steel, and when an amount thereof exceeds 0.01%, it may combine with Mn or the like to form non-metallic inclusions. Therefore, there may be a problem in that fine cracks may be likely to occur during forming, greatly reducing impact resistance. However, to manufacture in an amount of at less than 0.001%, a lot of time may be required during steelmaking, which reduces productivity.

Nitrogen (N) may be a representative solid solution strengthening element along with C, and may form coarse precipitates together with Ti, Al, or the like. In general, the solid solution strengthening effect of nitrogen (N) may be better than that of C, but there may be a problem in that toughness decreases significantly as an amount of nitrogen (N) in steel increases. Therefore, an upper limit thereof is limited to 0.01%. On the other hand, to manufacture in an amount of less than 0.001%, a lot of time may be required during steelmaking operations, resulting in lower productivity.

Titanium (Ti) may be a representative precipitation strengthening element, and may form coarse TiN in steel due to its strong affinity with N. TiN may have an effect of suppressing grain growth during a heating process for hot-rolling. In addition, titanium (Ti) remaining after reacting with N may be dissolved in steel, and may combine with C to form TiC precipitates, which may be a useful ingredient in improving the strength of steel. When an amount of titanium (Ti) is less than 0.03%, the above effect may not be obtained, and when an amount thereof exceeds 0.08%, there may be a problem of poor collision resistance during molding due to generation of coarse TiN and coarsening of precipitates. More preferably, it may be included in an amount of 0.04% or more, and more preferably, it may be included in an amount of 0.075% or less.

Niobium (Nb) may be a representative precipitation strengthening element along with Ti, and may be effective in improving strength and impact toughness of steel through an effect of grain refinement by precipitating during hot-rolling and delaying recrystallization. When an amount of niobium (Nb) is less than 0.01%, the above effect may not be obtained, and when an amount thereof exceeds 0.05%, formability may be inferior due to formation of elongated grains and coarse composite precipitates due to excessive recrystallization delay during hot-rolling. More preferably, a lower limit thereof may be 0.015%, and more preferably, an upper limit thereof may be 0.04%.

The steel of the present disclosure may contain remaining iron (Fe) and inevitable impurities, in addition to the composition described above. Since unavoidable impurities may be unintentionally introduced during the normal manufacturing process, and, thus, may not be excluded. Since these impurities may be known to anyone skilled in the field of steel manufacturing, all of them may not be specifically mentioned in this specification.

Steel according to an aspect of the present disclosure may have a sum of niobium (Nb) and titanium (Ti) of 0.04 to 0.1%.

Niobium (Nb) and titanium (Ti) may be precipitated as (Ti,Nb) (C,N)-based composite precipitates, and when they precipitate during hot-rolling, an effect of grain refinement due to delayed recrystallization may greatly increase. However, when formation of composite precipitates may be excessive, there may be a problem in that coarse composite precipitates increase, which reduces the effect of improving strength and deteriorates formability. When the sum of niobium (Nb) and titanium (Ti) is less than 0.04%, effects of grain refinement and strength improvement may be small. On the other hand, when the sum exceeds 0.1%, moldability becomes inferior and it may be economically disadvantageous. A more preferred lower limit may be 0.045%, and a more preferred upper limit may be 0.09%.

Hereinafter, a steel microstructure of the present disclosure will be described in detail.

In the present disclosure, unless specifically stated otherwise, % indicating a fraction of microstructure is based on area.

In steel according to an aspect of the present disclosure, a microstructure of a surface portion ranging from a surface to a 50 μm thickness may include, by area, 95% or more equiaxed ferrite and 3% or less pearlite, and may include, by area, 5% or less one or more of bainitic ferrite, bainite, a martensite-austenite constituent (MA) phase, or martensite, in total, and a microstructure of a core portion ranging from a ¼ thickness to a ¾ thickness may include, by area, 80 to 95% bainitic ferrite, 10% or less bainite, 3% or less pearlite, 5 to 10% one or two of a martensite-austenite constituent (MA) phase or martensite, and a remainder including equiaxed ferrite.

In the present disclosure, when the equiaxed ferrite in the surface portion is less than 95%, ductility may be insufficient during spinning and flow forming molding applied, in manufacturing commercial vehicle wheels, and work hardening in the surface portion becomes severe to have a risk of fine cracks occurring during forming. In particular, when 3% or more of highly brittle pearlite is formed, or more than 5% one or more of the bainitic ferrite, the bainite, the MA phase, or the martensite, having high hardness, is included, there may be problems in that cracks easily propagate along an interface with a matrix phase. Therefore, to suppress occurrence of fine cracks formed in the surface portion during molding and prevent propagation of cracks, 5% or less of one or more of the bainitic ferrite, the bainite, the MA phase, or the martensite, in total, may be included. In the present disclosure, as the microstructure of the surface portion, the equiaxed ferrite may be 100%, and the sum of the pearlite, the bainitic ferrite, the bainite, the MA phase, and the martensite may be 0%.

In addition, when an amount of the bainitic ferrite in the core portion is less than 80%, there may be a problem that cracks easily occur on a shear surface during a punching and shear forming process in manufacturing the wheel, and there may be a problem that impact resistance deteriorates after forming. In addition, when manufacturing a steel plate, in a core portion of a rolled sheet in a thickness direction during a cooling process of the steel plate after hot-rolling, bainitic ferrite, which is a matrix structure, may be formed, and then a high concentration of residual C may remain in an untransformed austenite, making it easy to form pearlite. In this case, when pearlite is formed in excess of 3%, cracks may occur in a shear surface during a molding process and impact resistance after molding may be poor. When the pearlite fraction is 3% or less, there may be no cracking caused by molding such as shear, and impact resistance at low temperatures may be excellent. In the present disclosure, carbides and nitrides, having a diameter of 1 μm or more, may be included as pearlite.

In comparison, when the MA phase or the martensite is included in an amount of 5 to 10%, it may not affect occurrence of cracks and may be advantageous in securing impact resistance and high strength after forming. The MA phase may have an advantage of securing high strength by forming a dislocation density therearound, and when formed together with a matrix structure including ferrite and bainite, it may have excellent impact resistance even though a dislocation density increases after cold forming. However, when the MA phase or the martensite is less than 5%, yield strength and tensile strength may be insufficient, and when the MA phase or the martensite is included in excess of 10%, ductility may be insufficient and formability may be poor. When the bainite also exceeds 10%, there may be a problem of insufficient ductility.

In the present disclosure, the microstructure of the core portion may include 0% each of the bainite and the pearlite, and may inevitably include the equiaxed ferrite in addition to the bainitic ferrite, the bainite, the pearlite, the MA phase, and the martensite.

In the present disclosure, an area fraction of the microstructure may be analyzed using an optical microscope and a scanning electron microscope (SEM), and an area fraction of the phase may be measured from an image observed at 3,000 times at a location corresponding to the core portion of the rolled cross-section in the thickness direction.

Hereinafter, a method of manufacturing steel of the present disclosure will be described in detail.