High-strength steel sheet with excellent hole expandability and method for producing the same

Abstract

The present invention provides a high-strength steel plate and a method for manufacturing the same. The high-strength steel plate is a material suitable for automobile structural parts and the like, has a low yield ratio and high strength, and has excellent formability such as excellent hole expandability by improving ductility.

Description

Technical Field

[0001] The present invention relates to a steel suitable for automobile materials, and more particularly to a high-strength steel plate having excellent hole expandability and a method for manufacturing the same. Background Art

[0002] In recent years, in the automotive industry, due to various environmental regulations and energy use regulations, the use of high-strength steel is required to improve fuel efficiency and durability.

[0003] In particular, as regulations on impact stability of automobiles are expanded, high-strength steel having excellent strength is used as a material for structural parts such as members, seat rails, and pillars in order to improve the impact resistance of vehicle bodies.

[0004] Such automotive parts have complex shapes in terms of stability and design, and are mainly formed and manufactured using press dies, and therefore require high strength and a high level of formability.

[0005] As the strength of steel increases, it has the characteristic of absorbing impact energy, but generally, as the strength increases, the elongation decreases, so there is a problem of reduced formability. In addition, when the yield strength is too high, the material introduction in the mold during molding will decrease, so there is a problem of poor formability.

[0006] In addition, automobile parts have many parts that expand after processing holes, so hole expandability is required for smooth forming. However, high-strength steel has low hole expandability, and there are problems such as cracks during the forming process. As mentioned above, if the hole expandability is poor, cracks will be generated in the hole forming part when the car collides, making the parts easily damaged, so the safety of passengers may be threatened.

[0007] In addition, representative high-strength steels used as automotive materials include dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), complex phase steel (CP steel), ferrite bainite steel (FB steel), etc.

[0008] DP steel, as an ultra-high tensile steel, has a low yield strength ratio of about 0.5 to 0.6, so it is easy to process and has the advantage of high elongation second only to TRIP steel. Therefore, it is mainly used in outer doors, seat rails, seat belts, suspensions, arms, wheel discs, etc.

[0009] TRIP steel has a yield ratio in the range of 0.57 to 0.67, and thus has excellent formability (high ductility), and is therefore suitable for parts requiring high formability, such as frames, roofs, seat belts, and bumpers.

[0010] CP steel has a low yield strength ratio, high elongation and bending workability, so it is used in side panels, underbody reinforcements, etc., and FB steel has excellent hole expandability, so it is mainly used in suspension lower arms or wheel discs, etc.

[0011] Among them, DP steel is mainly composed of ferrite with excellent ductility and hard phases (martensite phase and bainite phase) with high strength, and there may be a trace of retained austenite. This DP steel has low yield strength and high tensile strength, thus having excellent properties such as low yield ratio (YR), high work hardening rate, high ductility, continuous yield behavior, room temperature resistance to aging, and bake hardening. In addition, when the fraction and morphology (shape) of the bainite phase in the organization are controlled, high-strength steel with high hole expandability can be manufactured.

[0012] However, in order to ensure ultra-high tensile strength of 980 MPa or more, the fraction of hard phases such as martensite phases that contribute to improving strength needs to be increased. In this case, the yield strength increases, so there is a problem of defects such as cracks occurring during stamping.

[0013] Generally, DP steel for automobiles is manufactured by making slabs through steelmaking and continuous casting processes, and then subjecting the slabs to [heating-rough rolling-hot finishing rolling] to obtain hot-rolled coils, which are then annealed to produce the final product.

[0014] Among them, the annealing process is mainly a process carried out when manufacturing cold-rolled steel sheets. The cold-rolled steel sheets are manufactured as follows: the hot-rolled coils are pickled to remove the surface oxide scale, and cold-rolled at a specified reduction rate at room temperature, and then annealed and further flattened as needed.

[0015] The cold-rolled steel sheet (cold-rolled material) obtained by cold rolling is in a very hardened state and is not suitable for making parts that require workability. Therefore, the workability can be improved by softening it through heat treatment in a continuous annealing furnace as a subsequent process.

[0016] As an example, the annealing process is to heat a steel plate (cold rolled material) to about 650-850° C. in a heating furnace and then hold the steel plate for a certain period of time, thereby reducing hardness and improving workability through recrystallization and phase transformation phenomena.

[0017] The steel sheet that has not been annealed has high hardness, especially surface hardness, and insufficient workability. However, the steel sheet that has been annealed has a recrystallized structure, so that the hardness, yield point, and tensile strength are reduced, which can help improve the workability.

[0018] As a representative method for reducing the yield strength of DP steel, in the heating process during continuous annealing, ferrite is completely recrystallized to form an equiaxed crystal form, so that austenite is formed and grown in the subsequent process in an equiaxed crystal form, which is conducive to the formation of an austenite phase with a small grain size and uniformity.

[0019] like Figure 1 As shown, the continuous annealing process is carried out through the [heating zone-soaking zone-slow cooling zone-rapid cooling zone-overaging zone] in the annealing furnace. At this time, a fine ferrite phase is formed by sufficient recrystallization in the heating zone, and then a fine and uniform austenite phase is formed from the fine ferrite phase in the soaking zone. Then, during the cooling process, austenite forms fine bainite and martensite phases while the ferrite phase is recrystallized.

[0020] In addition, as a prior art for improving the workability of high-strength steel, Patent Document 1 proposes a method based on microstructure refinement, specifically, a method for dispersing fine precipitated copper particles with a particle size of 1-100 nm in the microstructure of a dual-phase steel plate mainly composed of a martensite phase. However, this technology requires the addition of 2-5% Cu to obtain good fine precipitated phase particles, so red hot brittleness caused by a large amount of Cu may occur, and there is a problem of excessive increase in manufacturing costs.

[0021] Patent document 2 discloses a high-strength steel plate, which has a ferrite matrix structure and a structure containing 2-10% pearlite, and precipitation strengthening and grain refinement are achieved by adding carbonitride-forming elements (e.g., Ti, etc.). In the case of this technology, although it has the advantage of being easy to achieve high strength relative to low manufacturing costs, the recrystallization temperature rises sharply due to fine precipitation. Therefore, in order to ensure high ductility caused by sufficient recrystallization, it is known that heating at a relatively high temperature is required during continuous annealing. In addition, the existing precipitation-strengthened steel that strengthens the steel by precipitating carbonitrides on the ferrite matrix has limitations in obtaining high strengths of more than 600 MPa.

[0022] Patent document 3 discloses the following technology: a steel material containing 0.18% or more carbon is continuously annealed and water-cooled to room temperature, and then aged at 120-300°C for 1-15 minutes to ensure a volume fraction of 80-97% martensite. Although this technology is beneficial for improving yield strength, the shape quality of the coil is deteriorated due to temperature deviations in the width and length directions of the steel plate during water cooling, so there are problems such as material defects in certain parts and reduced operability during roll forming and other processing.

[0023] In view of the above-mentioned prior art, in order to improve the formability such as hole expandability of high-strength steel, it is necessary to develop a method that can reduce the yield strength but improve the ductility.

[0024] (Patent Document 1) Japanese Patent Publication No. 2005-264176

[0025] (Patent Document 2) Korean Patent Publication No. 2015-0073844

[0026] (Patent Document 3) Japanese Patent Publication No. 1992-289120 Summary of the invention

[0027] Technical issues to be solved

[0028] One aspect of the present invention provides a high-strength steel sheet which has a low yield ratio and high strength and is excellent in formability such as hole expandability due to improved ductility, and a method for producing the same, as a material suitable for automobile structural parts and the like.

[0029] The technical problem of the present invention is not limited to the above content. The technical problem of the present invention can be understood from the overall content of this specification, and those skilled in the art of the present invention can easily understand the additional technical problem of the present invention.

[0030] Technical Solution

[0031] One aspect of the present invention provides a high-strength steel plate with excellent hole expandability, which comprises, in terms of weight percent, carbon (C): 0.05-0.15%, silicon (Si): 0.5% or less, manganese (Mn): 2.0-3.0%, titanium (Ti): 0.1% or less (except 0%), niobium (Nb): 0.1% or less (except 0%), chromium (Cr): 1.5% or less (except 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, and the remainder Fe and unavoidable impurities.

[0032] The microstructure is composed of ferrite with an area fraction of 35-60%, bainite with an area fraction of 40-50%, and the remainder of martensite and residual austenite, and the average aspect ratio (long diameter: short diameter) of the bainite phase is 1.5 to 2.3:1.

[0033] Another aspect of the present invention provides a method for manufacturing a high-strength steel plate with excellent hole expansion performance, characterized in that it includes the following steps: heating a steel billet having the above-mentioned alloy composition; hot finish rolling the heated steel billet at an outlet side temperature of Ar3 or more and 1000°C or less to manufacture a hot-rolled steel plate; coiling the hot-rolled steel plate within a temperature range of 400-700°C; cooling to room temperature after coiling; cold rolling at a reduction rate of 40-70% after cooling to manufacture a cold-rolled steel plate; continuously annealing the cold-rolled steel plate; primary cooling after the continuous annealing to a temperature range of 650-700°C; and secondary cooling after the primary cooling to a temperature range of 300-580°C.

[0034] The continuous annealing step is performed in a device provided with a heating zone, a soaking zone and a cooling zone, and the end temperature of the heating zone is higher than the end temperature of the soaking zone by more than 10°C.

[0035] Beneficial Effects

[0036] According to the present invention, it is possible to provide a steel sheet having excellent hole expandability even though having high strength and thus having improved formability.

[0037] As described above, the steel sheet of the present invention with improved formability can prevent processing defects such as cracks and wrinkles during press forming, and thus has the effect of being suitable for use in parts that need to be processed into complex shapes, etc. Furthermore, it is also effective to manufacture materials with improved collision resistance, so that when a car using such a part inevitably collides, defects such as cracks are not easily formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the heat treatment of the conventional continuous annealing process (CAL).

[0039] Figure 2 is a schematic diagram of a continuous annealing process (CAL) according to one aspect of the present invention, and Figure 1 The graph of (grey line) is shown together.

[0040] Figure 3 A microstructure photograph of an inventive example according to one embodiment of the present invention is shown.

[0041] Figure 4 A microstructure photograph of a comparative example according to one embodiment of the present invention is shown.

[0042] Figure 5 Schematic diagram showing the aspect ratio of bainite grains according to one embodiment of the present invention.

[0043] Best Mode for Carrying Out the Invention

[0044] The inventors of the present invention have conducted intensive studies to develop a material having a moldability at a level suitable for use in parts that need to be processed into a complex shape among automotive materials.

[0045] In particular, the present inventors have confirmed that the objective can be achieved by inducing sufficient recrystallization of the soft phase that affects the ductility of steel and controlling the refinement of the hard phase and the shape of the crystal grains that contribute to ensuring strength, thereby completing the present invention.

[0046] Hereinafter, the present invention will be described in detail.

[0047] In terms of weight %, a high-strength steel plate with excellent hole expandability according to one aspect of the present invention may contain: carbon (C): 0.05-0.15%, silicon (Si): 0.5% or less, manganese (Mn): 2.0-3.0%, titanium (Ti): 0.1% or less (except 0%), niobium (Nb): 0.1% or less (except 0%), chromium (Cr): 1.5% or less (except 0%), phosphorus (P): 0.1% or less, and sulfur (S): 0.01% or less.

[0048] Hereinafter, the reasons for limiting the alloy composition of the steel sheet provided in the present invention as described above will be described in detail.

[0049] In addition, unless otherwise specified, the content of each element in the present invention is based on weight, and the proportion of the structure is based on area.

[0050] Carbon (C): 0.05-0.15%

[0051] Carbon (C) is an important element added for solid solution strengthening. This C combines with precipitation elements to form fine precipitates, thereby contributing to improving the strength of steel.

[0052] When the C content exceeds 0.15%, the hardenability increases, and since martensite is formed during the cooling process when manufacturing the steel, the strength increases excessively, and there is also a problem of causing a decrease in elongation. In addition, due to poor weldability, there is a possibility of welding defects when processing into parts. In addition, when the C content is less than 0.05%, it is difficult to ensure the target level of strength.

[0053] Therefore, the C content may be 0.05-0.15%. More preferably, the C content may be 0.06% or more and 0.13% or less.

[0054] Silicon (Si): 0.5% or less

[0055] Silicon (Si) is a ferrite stabilizing element and helps to ensure the target level of ferrite fraction by promoting ferrite transformation. In addition, due to its excellent solid solution strengthening ability, it is effective in improving the strength of ferrite and is an element that effectively ensures strength without reducing the ductility of steel.

[0056] When the Si content exceeds 0.5%, the solid solution strengthening effect is excessive, which in turn reduces ductility and causes surface scale defects, thereby adversely affecting the quality of the plated surface. In addition, there is a problem of hindering chemical treatability.

[0057] Therefore, the content of Si may be 0.5% or less, and 0% may be excluded. The content of Si may more preferably be 0.1% or more.

[0058] Manganese (Mn): 2.0-3.0%

[0059] Manganese (Mn) is an element that precipitates sulfur (S) in steel as MnS, thereby preventing hot brittleness due to the formation of FeS and contributing to solid solution strengthening of steel.

[0060] When the content of such Mn is less than 2.0%, not only the above-mentioned effects cannot be obtained, but also it is difficult to ensure the strength at the target level. On the other hand, when the content of Mn exceeds 3.0%, the possibility of problems such as weldability and hot rolling is high, and martensite is more likely to be formed due to the increase in hardenability, so ductility may be reduced. In addition, too many Mn bands (Bands) (Mn oxide bands) are formed in the structure, so there is a problem of increased risk of defects such as processing cracks. In addition, Mn oxides are dissolved from the surface during annealing, so there is a problem of greatly hindering the plating property.

[0061] Therefore, the Mn content may be 2.0-3.0%, and more preferably may be 2.2-2.8%.

[0062] Titanium (Ti): 0.1% or less (except 0%)

[0063] Titanium (Ti) is an element that forms fine carbides and helps to ensure yield strength and tensile strength. In addition, Ti precipitates N in steel as TiN, thereby having the effect of suppressing the formation of AlN caused by Al that inevitably exists in steel, and thus has the effect of reducing the possibility of cracking during continuous casting.

[0064] When the Ti content exceeds 0.1%, coarse carbides are precipitated, and the strength and elongation may be reduced due to the reduction in the carbon content in the steel. In addition, there is a possibility of causing nozzle clogging during continuous casting, and there is a problem of increased manufacturing cost.

[0065] Therefore, the Ti content may be 0.1% or less, except for 0%.

[0066] Niobium (Nb): 0.1% or less (except 0%)

[0067] Niobium (Nb) is an element that segregates at austenite grain boundaries to suppress the coarsening of austenite grains during annealing heat treatment and forms fine carbides to contribute to improving strength.

[0068] When the Nb content exceeds 0.1%, coarse carbides are precipitated, and strength and elongation may be poor due to the reduction of carbon content in steel, and there is a problem of increased manufacturing cost. Therefore, the Nb content may be 0.1% or less, and 0% is excluded.

[0069] Chromium (Cr): 1.5% or less (except 0%)

[0070] Chromium (Cr) is an element that easily forms a bainite phase, and is an element that suppresses the formation of a martensite phase during annealing heat treatment and forms fine carbides to contribute to improving strength.

[0071] When the Cr content exceeds 1.5%, too much bainite phase is formed, elongation is reduced, and when carbides are formed on the grain boundaries, strength and elongation may be deteriorated. In addition, there is a problem of increased manufacturing cost.

[0072] Therefore, the Cr content may be 1.5% or less, except for 0%.

[0073] Phosphorus (P): 0.1% or less

[0074] Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect, and is an element that improves in-plane anisotropy without significantly reducing formability while ensuring strength. However, when too much P is added, the possibility of brittle failure increases significantly, resulting in an increased possibility of plate breakage during hot rolling, and there is a problem of hindering the surface properties of the plated surface.

[0075] Therefore, in the present invention, the P content may be controlled to be less than 0.1%, except for 0% in consideration of the level that is inevitably added.

[0076] Sulfur (S): 0.01% or less

[0077] Sulfur (S) is an impurity element in steel and is an element that is inevitably added. Since it hinders ductility, it is preferable to control the sulfur content to a minimum. In particular, S has the problem of increasing the possibility of red-hot brittleness, so it is preferable to control the sulfur content to 0.01% or less. However, considering the level that is inevitably added during the manufacturing process, 0% may be excluded.

[0078] The remaining component of the present invention is iron (Fe). However, undesirable impurities may inevitably be mixed from the raw materials or the surrounding environment during the conventional manufacturing process, so this impurity cannot be excluded. These impurities are well known to technicians in the conventional manufacturing process, so all of them are not specifically described in this specification.

[0079] The steel sheet of the present invention having the above alloy composition may be composed of ferrite and a bainite phase and a martensite phase as hard phases as a microstructure.

[0080] Specifically, the steel sheet of the present invention may include a ferrite phase with an area fraction of 35-60%, and may include a bainite phase of 40-50%, and the remainder may include a martensite phase and a trace amount of retained austenite phase.

[0081] When the fraction of the bainite phase is too high, the fraction of the relatively soft phase becomes low, and the target level of formability cannot be ensured. On the other hand, when the fraction of the bainite phase is less than 40%, the hole expandability may be deteriorated.

[0082] In addition, with respect to the retained austenite phase, it is favorable for the fraction to be not more than 3%, and even if the fraction is 0%, there is no problem in ensuring the desired physical properties.

[0083] The steel sheet of the present invention includes a bainite phase in the above-mentioned fraction range, and by controlling the shape of the bainite phase, it is possible to more advantageously ensure desired formability.

[0084] Specifically, the average aspect ratio (longer diameter:shorter diameter) of the bainite phase is preferably 1.5 to 2.3:1.

[0085] When the average aspect ratio of the bainite exceeds 2.3, there is a problem that local deformation and stress are concentrated on the bainite distributed in the rolling direction, resulting in poor ductility. Although there is no need to particularly limit the lower limit of the average aspect ratio of the bainite phase, the lower limit of the average aspect ratio can be set to 1.5 or more in consideration of the shape of the bainite phase caused by processing.

[0086] In the present invention, the aspect ratio refers to the ratio of the longitudinal direction (longer diameter) to the transverse direction (shorter diameter) of the crystal grain size relative to the rolling direction (longer diameter:shorter diameter). For example, Figure 5 shown. Figure 5(a) is a schematic diagram showing the crystal grain size of bainite having an aspect ratio of about 1:1. Figure 5 (b) is a schematic diagram showing the crystal grain size of bainite having an aspect ratio at a level limited by the present invention. In the present invention, the aspect ratio value refers to the average aspect ratio value of bainite grains.

[0087] In addition, although there is no specific limitation on the fraction of the martensitic phase in the phase constituting the hard phase, in order to ensure an ultra-high strength of a tensile strength of more than 980 MPa, the total microstructure fraction may contain a maximum of 15 area % of the martensitic phase, and preferably may contain less than 15 area % (excluding 0%) of the martensitic phase.

[0088] The steel plate of the present invention having the above-mentioned fine structure has a tensile strength of 980 MPa or more, a yield strength of 680 MPa or less, an elongation (total elongation) of 13% or more, and a yield ratio of 0.7 or less, and thus can have the characteristics of high strength, high ductility and low yield ratio.

[0089] Furthermore, the steel plate has a hole expansion ratio (HER) of 30% or more, and thus can also have an effect of excellent hole expandability.

[0090] Hereinafter, a method for producing a high-strength steel sheet having excellent hole expandability according to another aspect of the present invention will be described in detail.

[0091] In short, the present invention can manufacture the desired steel plate through the process of [steel billet heating-hot rolling-coiling-cold rolling-continuous annealing], and each process is described in detail below.

[0092] [Heating of steel billet]

[0093] First, a steel billet satisfying the above-mentioned alloy composition may be prepared and then heated.

[0094] This process is carried out in order to smoothly carry out the subsequent hot rolling process and fully obtain the desired physical properties of the steel plate. In the present invention, there is no particular restriction on the conditions of this heating process, as long as it is a common condition. As an example, the heating process can be carried out in a temperature range of 1100-1300°C.

[0095] [Hot rolling]

[0096] The steel slab heated as described above may be hot rolled to produce a hot rolled steel sheet, and at this time, hot finish rolling may be performed at an outlet side temperature of Ar3 or more and 1000° C. or less.

[0097] When the outlet temperature during the hot finish rolling is lower than Ar3, the thermal deformation resistance increases sharply, and the top, tail and edge of the hot rolled coil become single-phase regions, so the in-plane anisotropy increases, which may deteriorate the formability. In addition, when the outlet temperature during the hot finish rolling exceeds 1000°C, the rolling load is relatively reduced, which is beneficial to productivity, but there is a possibility of generating thick oxide scale.

[0098] More specifically, the hot finish rolling may be performed within a temperature range of 760-940°C.

[0099] [Collect]

[0100] The hot-rolled steel sheet manufactured as described above can be rolled into a coil shape.

[0101] The coiling can be performed at a temperature range of 400-700° C. When the coiling temperature is lower than 400° C., excessive martensite or bainite is formed, causing excessive increase in the strength of the hot-rolled steel sheet, so that problems such as shape defects caused by load may occur during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 700° C., there is a problem of deterioration in pickling properties due to the increase in surface oxide scale.

[0102] [cool down]

[0103] Preferably, the coiled hot-rolled steel sheet is cooled to room temperature at an average cooling rate of 0.1°C / s or less (except 0°C / s). At this time, the coiled hot-rolled steel sheet may be cooled after being transported, placed, etc., and the process before cooling is not limited thereto.

[0104] As described above, by cooling the coiled hot-rolled steel sheet at a predetermined speed, a hot-rolled steel sheet in which carbides serving as nucleation sites of austenite are finely dispersed can be obtained.

[0105] [Cold Rolling]

[0106] The hot-rolled steel sheet coiled as described above may be cold-rolled to manufacture a cold-rolled steel sheet.

[0107] At this time, the cold rolling can be performed at a cold rolling reduction rate of 40-70%. When the cold rolling reduction rate is less than 40%, the recrystallization driving force is weakened, and it is difficult to obtain good recrystallized grains. On the other hand, when the cold rolling reduction rate exceeds 70%, there is a high possibility of cracks at the edge of the steel plate, and there is a possibility that the rolling load increases sharply.

[0108] The present invention can perform pickling treatment on the hot-rolled steel plate before the cold rolling, and the pickling treatment process can be performed by a conventional method.

[0109] [Continuous annealing]

[0110] Preferably, the cold rolled steel sheet manufactured as described above is subjected to a continuous annealing process. As an example, the continuous annealing process can be performed in a continuous annealing furnace (CAL).

[0111] Generally, a continuous annealing furnace (CAL) may be composed of [a heating zone - a soaking zone - a cooling zone (a slow cooling zone and a rapid cooling zone) - an aging zone], and undergoes the following process: a cold-rolled steel sheet is loaded into the continuous annealing furnace as described above, and then heated to a specific temperature in the heating zone, and maintained in the soaking zone for a specified time after reaching the target temperature.

[0112] In the present invention, in order to obtain recrystallized ferrite and fine bainite and martensite phases as the final fine structure, it is desired to establish a method for applying sufficient heat input to the steel sheet in the heating zone consisting of [heating zone-soaking zone] during continuous annealing.

[0113] Specifically, in a conventional continuous annealing process, the final temperature of the heating zone and the temperature of the soaking zone are controlled to be the same temperature, whereas the present invention has a feature of independently controlling the temperature of the heating zone and the temperature of the soaking zone.

[0114] That is, in a conventional continuous annealing process, the starting temperature and the ending temperature of the soaking zone are controlled to be the same temperature, which means that the ending temperature of the heating zone and the starting temperature of the soaking zone are the same.

[0115] In contrast, the present invention can further promote the recrystallization of ferrite in the heating zone by controlling the temperature of the heating zone to be higher than the temperature of the soaking zone, thereby inducing the formation of fine ferrite, so that the austenite formed at the ferrite grain boundary can also be formed finely and uniformly.

[0116] Preferably, in the present invention, the termination temperature of the heating zone is controlled to be 10° C. or higher than the termination temperature of the soaking zone, and more preferably, the following relationship can be satisfied.

[0117] [Relational formula]

[0118] 10≤End temperature of heating zone - End temperature of soaking zone (℃)≤40

[0119] That is, the present invention controls the termination temperature of the heating zone to be higher than the termination temperature of the soaking zone, but when the temperature difference is lower than 10°C, the recrystallization of ferrite is delayed, so it is difficult to obtain a fine and uniform austenite phase. On the other hand, when the temperature difference exceeds 40°C, the subsequent cooling process cannot be fully carried out due to the excessive temperature difference, so there is a possibility of forming a coarse bainite phase or a coarse martensite phase in the final structure.

[0120] In the present invention, the termination temperature of the heating zone can be 790-830°C. When the termination temperature of the heating zone is lower than 790°C, sufficient heat input for recrystallization cannot be applied. On the other hand, when the termination temperature of the heating zone exceeds 830°C, productivity decreases and excessive austenite phase is formed, so that the fraction of hard phase after subsequent cooling is greatly increased, and there is a possibility that the ductility of the steel will deteriorate.

[0121] In addition, the end temperature of the soaking zone may be 760-790°C. When the end temperature of the soaking zone is lower than 760°C, excessive cooling is required at the end temperature of the heating zone, which is disadvantageous in economy and the heat for recrystallization may be insufficient. On the other hand, when the end temperature of the soaking zone exceeds 790°C, the fraction of austenite is too large, and the fraction of the hard phase exceeds during cooling, so there is a possibility that the formability is reduced.

[0122] In addition, in the present invention, the temperature difference between the termination temperature of the heating zone and the termination temperature of the soaking zone can be achieved by turning off the heating means from the time point when the heating zone process is completed to the time point when the soaking zone process is completed. As an example, furnace cooling treatment can be performed in this interval. However, it is not limited to this.

[0123] [Stage cooling]

[0124] The desired structure is formed by cooling the cold-rolled steel sheet that has been subjected to the continuous annealing treatment as described above. In this case, it is preferable to perform cooling in steps.

[0125] In the present invention, the segmented cooling may consist of primary cooling-secondary cooling. Specifically, after the continuous annealing, primary cooling may be performed at an average cooling rate of 1-10°C / second to a temperature range of 650-700°C, and then secondary cooling may be performed at an average cooling rate of 5-50°C / second to a temperature range of 300-580°C.

[0126] At this time, the primary cooling is performed more slowly than the secondary cooling, so that defects in the sheet shape caused by a rapid temperature drop during the secondary cooling which is the subsequent relatively rapid cooling period can be suppressed.

[0127] When the final temperature during the primary cooling is lower than 650°C, the carbon diffusion activity is low due to the too low temperature, and the carbon concentration in the ferrite increases. On the other hand, since the carbon concentration in the austenite becomes low, the fraction of the hard phase is too large, and the yield ratio increases, thereby increasing the tendency of cracks to occur during processing. In addition, since the cooling speed in the soaking zone and the slow cooling zone is too fast, the shape of the plate becomes uneven.

[0128] When the end temperature exceeds 700°C, there is a disadvantage that an excessively fast cooling rate is required during the subsequent cooling (secondary cooling). In addition, when the average cooling rate during the primary cooling exceeds 10°C / second, carbon diffusion may not be fully performed. In addition, considering productivity, the primary cooling process may be performed at an average cooling rate of 1°C / second or more.

[0129] As described above, after the completion of the primary cooling, rapid cooling (secondary cooling) can be performed at a cooling rate above a certain level. At this time, when the secondary cooling termination temperature is lower than 300°C, cooling deviations occur in the width and length directions of the steel plate, and there is a possibility that the plate shape may deteriorate. On the other hand, when the secondary cooling termination temperature exceeds 580°C, the hard phase cannot be fully ensured, and the strength may be reduced. In addition, when the average cooling rate during the secondary cooling is less than 5°C / second, there is a possibility that the fraction of the hard phase is too high. On the other hand, when the average cooling rate during the secondary cooling exceeds 50°C / second, there is a possibility that the hard phase is opposite and insufficient.

[0130] In addition, as required, after the staged cooling is completed, an aging treatment may be performed.

[0131] The over-aging treatment is a process of maintaining the temperature for a certain period of time after the secondary cooling termination temperature, and uniformly heat treating the coil along the width and length directions, thereby having the effect of improving the shape quality. To this end, the over-aging treatment can be carried out for 200-800 seconds.

[0132] The overaging treatment may be performed after the secondary cooling is terminated, so the temperature of the overaging treatment may be the same as the secondary cooling termination temperature, or the overaging treatment may be performed within the secondary cooling termination temperature range.

[0133] The high-strength steel sheet of the present invention manufactured as described above is composed of a hard phase and a soft phase as a microstructure, and in particular, the recrystallization of ferrite is maximized through an optimized annealing process, so that a structure in which a bainite phase and a martensite phase as hard phases are uniformly distributed on a finally recrystallized ferrite matrix can be obtained.

[0134] Therefore, the steel sheet of the present invention can ensure excellent hole expandability and formability by ensuring a low yield ratio and high ductility even if it has a high strength of 980 MPa or more in tensile strength.

[0135] Hereinafter, the present invention will be described in more detail by way of examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited to the description of these examples. This is because the scope of the present invention is determined by the contents described in the claims and the contents reasonably derived therefrom. DETAILED DESCRIPTION

[0136] (Example)

[0137] Steel billets having the alloy composition shown in Table 1 below are manufactured, and then each steel billet is heated at 1200°C for 1 hour, and then hot-rolled at a finishing temperature of 880-920°C to manufacture hot-rolled steel sheets. Afterwards, each hot-rolled steel sheet is coiled at 650°C and cooled to room temperature at a cooling rate of 0.1°C / second. Afterwards, the coiled hot-rolled steel sheet is cold-rolled at a reduction rate of 50% to manufacture a cold-rolled steel sheet. Under the temperature conditions shown in Table 2 below, each cold-rolled steel sheet is continuously annealed, then staged cooled (primary cooling-secondary cooling), and then aged at 360°C for 520 seconds to manufacture the final steel sheet.

[0138] At this time, the primary cooling in the step cooling was performed at an average cooling rate of 3° C. / sec, and the secondary cooling was performed at an average cooling rate of 20° C. / sec.

[0139] The microstructure of each of the steel sheets produced as described above was observed, and the mechanical properties and the plating properties were evaluated, and the results are shown in Table 3 below.

[0140] At this time, the tensile test on each test piece was performed by sampling a tensile test piece of JIS No. 5 size in a direction perpendicular to the rolling direction and performing a tensile test at a strain rate of 0.01 / sec.

[0141] In addition, hole expansion (Hole Expanding Ratio, HER) is a test to measure the ultimate strain capacity that can be sustained when a large strain exceeding the uniform elongation of the steel plate is applied during the process of expanding and stretching a hole or a cross-section. The HER value can be calculated by measuring the diameter value (df) at the time when a crack is generated during the hole expansion process (refer to the following formula), which is performed according to the ISO 16630 standard method.

[0142] HER=(Df-Do) / Do×100(%)

[0143] (Do: initial punched hole diameter, Df: inner hole diameter after fracture)

[0144] The bainite in the microstructure phase was observed by SEM at a magnification of 5000 after etching with nitric acid alcohol solution (nital). At this time, the aspect ratio (long diameter: short diameter) of the bainite grains was measured from the grain shape of the observed bainite phase, with the elongated direction regarded as the longitudinal direction, and the fraction was measured.

[0145] The other phases were also etched with nitric acid alcohol solution and then measured for each fraction using SEM and image analyzer.

[0146] [Table 1]

[0147]

[0148] [Table 2]

[0149]

[0150]

[0151] [Table 3]

[0152]

[0153] As shown in Tables 1 to 3, it can be confirmed that in Invention Examples 1 to 9 in which the alloy composition and manufacturing conditions of the steel, especially the continuous annealing process, meet the conditions proposed in the present invention, the desired fine structure is formed, so that it has high strength and excellent elongation, thereby having excellent hole expandability, which shows that the target level of formability can be ensured.

[0154] On the other hand, in Comparative Examples 1 to 6, in which the same heating zone termination temperature and soaking zone termination temperature as the conventional continuous annealing process were applied in the continuous annealing process in the process of manufacturing the steel sheet, the bainite phase was excessively elongated, and the aspect ratio (longer diameter: shorter diameter) exceeded 2.3:1, and the physical properties expected in the present invention were not satisfied. Among them, in Comparative Examples 1 to 2 and Comparative Examples 4 to 5, in which the annealing temperature was relatively low, the elongation was low and the hole expandability was poor, and in Comparative Examples 3 and 6 other than these, the yield strength exceeded the target level.

[0155] In Comparative Example 7 in which the end temperature of the soaking zone during continuous annealing in the process of manufacturing the steel sheet was too high compared to the end temperature of the heating zone, the bainite phase was formed to exceed 50 area %, which was advantageous for ensuring strength but poor in hole expandability.

[0156] Figure 3 The microstructure photograph of Invention Example 4 is shown. Figure 4 A microstructure photograph of Comparative Example 6 is shown.

[0157] In Inventive Example 4, it was confirmed that a fine bainite phase and a certain fraction of a martensite phase were formed in a relatively sufficient fraction of a recrystallized ferrite matrix.

[0158] On the other hand, in Comparative Example 6, it was confirmed that since ferrite was elongated in the rolling direction, bainite was formed in the same shape, and since the bainite fraction was increased, the yield strength and the yield ratio were high, and thus the formability was poor.

Claims

1. A high-strength steel sheet having excellent hole expandability, comprising, in weight percent, carbon (C): 0.05-0.15%, silicon (Si): 0.5% or less, manganese (Mn): 2.0-3.0%, titanium (Ti): 0.1% or less and excluding 0%, niobium (Nb): 0.1% or less and excluding 0%, chromium (Cr): 1.5% or less and excluding 0%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, and the balance Fe and unavoidable impurities, The microstructure is composed of ferrite with an area fraction of 35-60%, bainite with an area fraction of 40-50%, and the remainder of martensite and residual austenite. The average aspect ratio of the bainite phase, i.e., major diameter:minor diameter, is 1.5 to 2.3:

1.

2. The high-strength steel sheet having excellent hole expandability according to claim 1, wherein The area fraction of the martensite phase is 15% or less, excluding 0%.

3. The high-strength steel sheet having excellent hole expandability according to claim 1, wherein The steel plate has a tensile strength of 980 MPa or more, a yield strength of 680 MPa or less, and an elongation of 13% or more.

4. The high-strength steel sheet having excellent hole expandability according to claim 1, wherein The steel plate has a yield ratio of less than 0.7 and a hole expansion ratio of more than 30%.

5. A method for producing a high-strength steel plate having excellent hole expandability, characterized in that: The following steps are involved: The steel slab is heated, wherein the steel slab comprises, in terms of weight percent, carbon (C): 0.05-0.15%, silicon (Si): 0.5% or less, manganese (Mn): 2.0-3.0%, titanium (Ti): 0.1% or less and excluding 0%, niobium (Nb): 0.1% or less and excluding 0%, chromium (Cr): 1.5% or less and excluding 0%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, and the balance of Fe and unavoidable impurities; The heated steel slab is subjected to hot finish rolling at an outlet side temperature of Ar3 or higher and 1000° C. or lower to produce a hot rolled steel sheet; The hot rolled steel sheet is coiled at a temperature ranging from 400 to 700° C. Cooling to room temperature after winding; After cooling, cold rolling is performed at a reduction rate of 40-70% to produce a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet; After the continuous annealing, cooling is performed once to a temperature range of 650-700° C.; and After the primary cooling, secondary cooling is performed to cool the mixture to a temperature range of 300-580°C. The continuous annealing step is carried out in a device provided with a heating zone, a soaking zone and a cooling zone, and the termination temperature of the heating zone is higher than the termination temperature of the soaking zone by more than 10°C.

6. The method for producing a high-strength steel sheet having excellent hole expandability according to claim 5, wherein: The termination temperature of the heating zone and the termination temperature of the soaking zone satisfy the following relationship: [Relational formula] 10≤the termination temperature of the heating zone-the termination temperature of the soaking zone≤40, wherein the unit of temperature is ℃.

7. The method for producing a high-strength steel sheet having excellent hole expandability according to claim 5, wherein: The termination temperature of the heating zone is 790-830°C, and the termination temperature of the soaking zone is 760-790°C.

8. The method for producing a high-strength steel sheet having excellent hole expandability according to claim 5, wherein: The cooling after the coiling is performed at an average cooling rate of 0.1° C. / sec or less and excluding 0° C. / sec.

9. The method for producing a high-strength steel plate having excellent hole expandability according to claim 5, wherein: The primary cooling is performed at an average cooling rate of 1-10°C / sec, and the secondary cooling is performed at an average cooling rate of 5-50°C / sec.

10. The method for producing a high-strength steel plate having excellent hole expandability according to claim 5, wherein: After the secondary cooling, the method further includes performing an aging treatment, wherein the aging treatment is performed for 200-800 seconds.

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