Ultra-high-strength steel sheet with excellent bending properties and method for manufacturing the same

Abstract

The present invention relates to a steel sheet suitable for use in automobile structural members and the like, and more particularly to an ultra-high strength steel sheet having excellent bending properties and a method for producing the same.

Description

Technical Field

[0001] The present invention relates to a steel sheet suitable for use in automotive structural members, and more particularly, to an ultra-high strength steel sheet having excellent bending characteristics and a method for producing the same.

Background Art

[0002] Recently, in the automotive field, research is actively underway in developed countries led by Europe to reduce the weight of vehicle bodies for reasons such as fuel consumption regulations and performance improvement. In particular, in the steel industry, in order to meet the requirements of such automotive manufacturers for weight reduction, efforts have been made to increase the strength and further reduce the thickness of steel sheets compared to competing materials (such as Mg, Al, CFRP (carbon fiber reinforced plastic), etc.) at the same grade. That is, along with weight reduction, due to the strengthening of safety regulations for vehicle passengers and pedestrians, there is a tendency to also require stability and high strength of vehicle body materials.

[0003] On the other hand, in order to improve the stability and impact characteristics of the vehicle body, the adoption of high-strength steel with excellent yield strength in BIW (Body-In-White) structural members has been increasing. Such structural members have the characteristic that the higher the yield ratio (yield strength / tensile strength, YR), that is, the ratio of the yield strength to the tensile strength, the more advantageous it is for absorbing impact energy.

[0004] Therefore, as a typical method for increasing the yield strength of steel, a method that utilizes water cooling during continuous annealing is mainly used. Specifically, it is a method for producing ultra-high strength steel after cold-rolled steel sheets are annealed in the two-phase or single-phase region and then rapidly cooled to approximately room temperature, followed by processes such as tempering.

[0005] However, while the ultra-high-strength steel produced in this way has an extremely high yield ratio, it suffers from a problem where the shape quality of the coil deteriorates due to temperature variations in the width and length directions. This can lead to problems such as material defects and reduced workability depending on the part during parts processing by roll forming, etc. In addition, since the elongation rate generally decreases as the strength of steel increases, there is a problem of reduced formability.

[0006] To overcome this, a hot press forming (HPF) method has been developed and applied, in which the material is formed at a relatively high temperature where molding is easier, and then strength is ensured through water cooling between the die and the material (see Patent Document 1).

[0007] Because the HPF method allows for higher strength compared to the same thickness, component development using the HPF method is being carried out, mainly in Europe.

[0008] However, the HPF method requires excessive capital investment and has led to problems such as increased process costs, so there is a real need for the development of materials for cold stamping.

[0009] In other words, there is a need to develop steel sheets that are suitable for use as materials for cold stamping, possess high strength and a high yield ratio to ensure collision performance characteristics, and have excellent formability and other properties. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] International Publication No. 2021 / 084303 [Overview of the project] [Problems that the invention aims to solve]

[0011] One aspect of the present invention is to provide a steel sheet that is suitable for cold stamping, particularly an ultra-high-strength steel sheet with excellent bending properties, and a method for manufacturing the same, while being suitable for automotive structural members.

[0012] The problems addressed by the present invention are not limited to those described above. The problems addressed by the present invention can be understood from the entirety of this specification, and any person with ordinary skill in the art to which the present invention belongs will have no difficulty in understanding the additional problems addressed by the present invention. [Means for solving the problem]

[0013] One aspect of the present invention provides an ultra-high-strength steel sheet with excellent bending properties, comprising, by weight percent, carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01-0.5%, two or more of chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.01-0.2%, and boron (B): 0.005% or less, one or more of titanium (Ti): 0.1% or less, and niobium (Nb): 0.1% or less, with the remainder being Fe and unavoidable impurities, satisfying the following relational formula 1, and containing martensite and / or tempered martensite phases in an area fraction of 99% or more as a microstructure.

[0014] [Relationship 1]

number

[0015] (Here, Ceq1=C+(Mn / 20)+(Si / 30)+(2P)+(4S), Ceq2=C+(Mn / 6)+(Si / 30)+(Cr+Mo+V+Nb) / 5+(Cu+Ni) / 15.)

[0016] Another aspect of the present invention includes the steps of heating a steel slab having the above-described alloy composition and satisfying the relational expression 1 in a temperature range of 1100 to 1300°C; finish hot rolling the reheated steel slab at a temperature above Ar3 to produce a hot-rolled steel sheet; winding the hot-rolled steel sheet at a temperature of 700°C or lower; cold rolling the wound hot-rolled steel sheet with a total reduction rate of 30 to 80% to produce a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a temperature above Ac3 for 30 seconds or more; 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 lower after the primary cooling; and performing overaging treatment after reheating after the secondary cooling, The reheating and overaging steps are characterized by heating to a temperature range that satisfies the following relational expression 3, and provide a method for manufacturing a high-strength steel sheet having excellent bending properties.

[0017] [Relational Expression 3] CT2 + 30°C ≤ A ≤ 270°C

[0018] (Here, CT2 means the secondary cooling end temperature (°C), and A means the reheating and overaging temperature (°C).) [Advantages of the Invention]

[0019] ​​​​​​​​​​​​​​​​

[0021] The inventors of the present invention have conducted in-depth research to provide a steel sheet that is suitable as a material for automotive structural members and is advantageous for processing such as cold stamping. As a result, by optimizing the alloy component system and manufacturing conditions, it was confirmed that a steel sheet having the desired structure, physical properties, etc. can be provided, and the present invention was completed.

[0022] Hereinafter, the present invention will be described in detail.

[0023] The ultra-high strength steel sheet according to one aspect of the present invention contains, by weight%, 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%.

[0024] Hereinafter, the reasons for limiting the alloy composition of the ultra-high strength steel sheet provided by the present invention as described above will be described in detail.

[0025] On the other hand, unless otherwise specified in the present invention, the content of each element is based on weight, and the ratio of the structure is based on area.

[0026] Carbon (C): 0.1 to 0.3% Carbon (C) is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. In particular, it is an element that must be added as essential for ensuring strength in martensitic steel.

[0027] In order to obtain a steel sheet having the strength, yield ratio, etc. targeted in the present invention, it is preferable to add the above C at 0.1% or more. However, when the content exceeds 0.3%, while the martensite strength increases, carbide formation becomes easy during the continuous annealing process and it tends to coarsen, resulting in not only a decrease in ductility but also a problem of deterioration of bending characteristics. In addition, an excessive increase in carbon content has a problem of inhibiting weldability.

[0028] Therefore, in the present invention, C can be included in an amount of 0.1 to 0.3%, and more preferably in an amount of 0.12% or more and 0.28% or less.

[0029] Manganese (Mn): 1.0~2.3% Manganese (Mn) is an element that readily ensures the final martensite phase in composite steels by suppressing ferrite formation and promoting austenite formation.

[0030] When the Mn content exceeds 2.3%, Mn segregates in the thickness direction of the steel, easily forming manganese bands within the slab. This leads to increased defects during rolling, along with continuous casting cracks. On the other hand, if the content is less than 1.0%, it becomes impossible to achieve the target level of strength.

[0031] Therefore, in the present invention, the above Mn can be contained in an amount of 1.0 to 2.3%, more preferably in an amount of 1.2% or more and 2.1% or less. Even more preferably, it can be contained in an amount of 1.4% or more.

[0032] Silicon (Si): 0.05~1.0% In the process of manufacturing the steel sheet to be obtained in this invention, silicon (Si) plays a role in suppressing the formation of carbides and controlling the size of the carbides during the reheating and overaging treatment stages that are performed after continuous annealing and cooling.

[0033] To fully obtain the effects described above, it is preferable to include 0.05% or more of the above-mentioned Si. However, if the content exceeds 1.0%, ferrite may form during cooling in a continuous annealing furnace, potentially weakening the strength of the steel. Furthermore, Si-based oxides may form during reheating and overaging after cooling, potentially leading to surface oxidation problems in the steel.

[0034] Therefore, in the present invention, the above Si can be contained in an amount of 0.05 to 1.0%, more preferably in an amount of 0.09% or more and 0.8% or less. Even more preferably in an amount of 0.6% or less.

[0035] Phosphorus (P): 0.1% or less (excluding 0%) Phosphorus (P) is an impurity element contained in steel, and if its content exceeds 0.1%, the weldability of the steel deteriorates and brittleness may occur. Therefore, the amount of P is limited to 0.1% or less, and 0% can be excluded considering the level that is inevitably added during the steel manufacturing process. More advantageously, the amount of P can be 0.05% or less, and even more advantageously, 0.03% or less.

[0036] Sulfur (S): 0.03% or less (excluding 0%) Sulfur (S), like P mentioned above, is an impurity that is inevitably contained in steel and is an element that inhibits the ductility and weldability of steel, so it is advantageous to control its content to be as low as possible. In this invention, even if the above S is contained at a maximum of 0.03%, it is not difficult to ensure the target physical properties, and the upper limit can be limited to 0.03%, and 0% can be excluded considering the level that is inevitably added during the steel manufacturing process.

[0037] On the other hand, in order to more advantageously secure the bending properties targeted in the present invention, the content of S can be limited to 0.01% or less, and more advantageously to 0.005% or less.

[0038] Aluminum (Al): 0.01-0.5% Aluminum (Al) can be added to remove oxygen from molten steel and, like Si mentioned above, is an element that stabilizes ferrite. Furthermore, Al increases the carbon content in austenite and improves the hardening ability of the final martensitic steel.

[0039] To fully obtain the effects described above, the Al content can be 0.01% or more. However, if the content exceeds 0.5%, ferrite may form during cooling in a continuous annealing furnace, potentially weakening the strength. Furthermore, it may combine with N, which is inevitably present as an impurity in the steel, to form AlN, which may induce slab cracking and hinder hot rolling.

[0040] Therefore, in the present invention, the above Al can be contained in an amount of 0.01 to 0.5%.

[0041] On the other hand, the steel sheet of the present invention may further contain elements that are advantageous for ensuring the physical properties of steel, in addition to the alloy composition described above. Specifically, it is preferable that the steel sheet of the present invention further contains two or more elements selected from chromium (Cr), molybdenum (Mo), and boron (B), and one or more elements selected from titanium (Ti) and niobium (Nb).

[0042] Chromium (Cr): 0.01-0.2% Chromium (Cr) can be added to improve the hardening ability of steel and ensure high strength. In particular, it is useful in suppressing bainite formation during cooling in a continuous annealing furnace and in producing ultra-high-strength steel sheets composed of a pure martensite phase.

[0043] To fully obtain the effects described above, the above-mentioned Cr can be added at a concentration of 0.01% or more. However, if the content exceeds 0.2%, the cost of the ferroalloy increases, which becomes economically disadvantageous.

[0044] Therefore, when adding Cr as described above, it can be added in a concentration of 0.01 to 0.2%.

[0045] Molybdenum (Mo): 0.01-0.2% Molybdenum (Mo), like Cr mentioned above, is an element that improves the hardening ability of steel.

[0046] To obtain sufficient hardening effect, the above-mentioned Mo can be added at a concentration of 0.01% or more. However, if the content exceeds 0.2%, the amount of alloy added becomes excessive, leading to a problem of increased cost for the ferroalloy.

[0047] Therefore, when adding Mo as described above, it can be added at a concentration of 0.01 to 0.2%.

[0048] Boron (B): 0.005% or less Boron (B) is an element that suppresses the transformation of austenite to ferrite during the continuous annealing process, and even in very small amounts, it is an effective element for improving hardening ability, similar to Cr and Mo. However, if its content exceeds 0.005%, Fe 23 (B, C)6 The precipitated phase may promote ferrite formation by precipitation at the austenite grain boundaries.

[0049] Therefore, when adding B as described above, it can be added at a concentration of 0.005% or less.

[0050] Titanium (Ti): 0.1% or less Titanium (Ti) is an element that forms fine carbides and contributes to ensuring yield strength and tensile strength. Furthermore, since Ti is an element that scavenges by precipitating nitrogen (N), which is inevitably present in steel at impurity levels, into TiN, it can be added in a content of 48 / (14×N) or more based on chemical equivalent standards.

[0051] If the Ti content exceeds 0.1%, coarse carbides will precipitate, leading to problems such as decreased strength and elongation as the carbon content in the steel decreases. Furthermore, it may cause nozzle clogging during continuous casting; therefore, the Ti should be added at a concentration of 0.1% or less.

[0052] On the other hand, in order to maximize the effect of adding B as described above, it is advantageous to add Ti together with it.

[0053] Niobium (Nb): 0.1% or less Niobium (Nb) is an element that segregates at austenite grain boundaries, suppressing the coarsening of austenite grains during the continuous annealing process, and contributing to improved strength by forming fine carbides.

[0054] When the Nb content exceeds 0.1%, the precipitation of coarse carbonitrides increases, potentially reducing the strength and elongation due to a decrease in the carbon content in the steel. Furthermore, the workability of the base material decreases, leading to increased manufacturing costs.

[0055] Therefore, when adding Nb as described above, it can be added at a concentration of 0.1% or less.

[0056] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from the raw materials or the surrounding environment, and these cannot be eliminated. Since these impurities are easily recognizable to any technician in the normal manufacturing process, their details are not specifically mentioned in this specification.

[0057] The steel sheet of the present invention that satisfies the alloy composition described above preferably satisfies the following relational expression 1 regarding the content of specific elements.

[0058] [Relationship 1]

number

[0059] (Here, Ceq1=C+(Mn / 20)+(Si / 30)+(2P)+(4S), Ceq2=C+(Mn / 6)+(Si / 30)+(Cr+Mo+V+Nb) / 5+(Cu+Ni) / 15.)

[0060] Relational equation 1 expresses the influence of the alloying element content added to steel on welding properties as a combined relation between Ceq1 and Ceq2. When the range satisfies 0.12 to 0.28, the basic welding properties can be satisfied while advantageously securing the physical properties targeted by the present invention.

[0061] Specifically, if the value of the above relational expression 1 is less than 0.12, it becomes impossible to secure the strength level targeted by the present invention, while if the value exceeds 0.28, there is a possibility that the physical properties, particularly the welding properties, will deteriorate significantly.

[0062] The value of the above relational expression 1 can more preferably be 0.15 or more and 0.27 or less, and even more preferably 0.17 or more.

[0063] The steel sheet of the present invention that satisfies the alloy composition and relational formula 1 described above preferably contains a martensite phase as the main phase in its microstructure.

[0064] Specifically, the steel sheet may contain martensite and / or tempered martensite phases in an area fraction of 99% or more. In this case, the above fraction may be 100%.

[0065] If the above fraction of martensite and / or tempered martensite phase is 99%, the remaining 1% may be ferrite and / or bainite phase.

[0066] The steel sheet of the present invention preferably has a surface layer in a specific region, as described later, and the main structure in the remaining region excluding the surface layer (for example, the central region) is martensite and / or tempered martensite phase.

[0067] On the other hand, the steel sheet of the present invention has the characteristic that the region corresponding to a minimum of 50 μm to a maximum of 70 μm in the thickness direction from the surface can be defined as the surface layer, and the above surface layer contains a soft phase.

[0068] Preferably, the surface layer contains tempered martensite phase in an area fraction of 70% or less, and the remaining structure may include one or more of ferrite and bainite, which have softer properties than the tempered martensite. By softening the surface layer of the steel sheet in this way, it is possible to further improve the bending properties.

[0069] Furthermore, the surface layer containing a certain soft phase is characterized by containing a decarburized layer with a lower carbon content than that contained in the steel sheet.

[0070] Specifically, it is preferable that the ratio of carbon content in the region (A) of 1 to 3 μm in the thickness direction based on the surface is 0.6 or less relative to the carbon content of the steel sheet of the present invention. Here, the carbon content ratio in region (A) means [average carbon content in region (A) / carbon content of the steel sheet].

[0071] Furthermore, it is preferable that the carbon content ratio of the region (B) in the thickness direction of 0.2 to 30 μm relative to the carbon content of the steel sheet is 0.9 or less. Here, the carbon content ratio of region (B) means [average carbon content of region (B) / carbon content of the steel sheet].

[0072] While the decarburized layer within the surface is advantageous for improving the bending properties of the steel sheet, if the carbon (C) content ratio in specific regions (A and B) within the surface exceeds 0.6 and 0.9, respectively, the target bending properties cannot be achieved.

[0073] Here, the decarburized layer may be formed to a thickness corresponding to the surface layer, or it may be formed to be thinner than the thickness of the surface layer.

[0074] In the present invention, the decarburized layer can be formed by controlling the continuous annealing process during the steel sheet manufacturing process, which will be explained in detail later.

[0075] As described above, the steel sheet of the present invention, in which the microstructure is composed of a hard phase while the surface layer includes a decarburized layer, has ultra-high strength with a tensile strength of 1300 MPa or more, a high yield ratio of 0.72 or more, and a bending characteristic (R / t) of 3 or less.

[0076] Furthermore, the steel sheet of the present invention can satisfy the following relational equation 2 in terms of the relationship between tensile strength and bending characteristics, specifically the relationship between tensile strength (TS), which is a basic tensile property, and the maximum bending angle after a three-point bending test according to the VDA238-100 standard.

[0077] [Relationship 2] (Tensile strength (TS) / Maximum bending angle) ≤ 25

[0078] When the value of relational equation 2 above is 25 or less, excellent bending properties can be ensured in ultra-high-strength steel with a tensile strength of 1300 MPa or more. However, when the value exceeds 25, although the strength is high, the bending properties deteriorate.

[0079] The following describes in detail a method for manufacturing an ultra-high-strength steel sheet with excellent bending properties according to another aspect of the present invention.

[0080] In short, the present invention allows for the production of a target steel sheet through the processes of [steel slab heating - hot rolling - coiling - cold rolling - continuous annealing], and each process will be described in detail below. On the other hand, the continuous annealing process includes a cooling process as well as a reheating and overaging process, which means that the above processes are carried out all at once on a continuous annealing line.

[0081] [Heating steel slabs] First, a steel slab satisfying the aforementioned alloy composition can be prepared and then heated.

[0082] This process is carried out to ensure a smooth subsequent hot rolling process and to obtain the desired physical properties of the steel sheet. In this invention, there are no particular restrictions on the conditions of this heating process; ordinary conditions are acceptable. For example, the heating process can be carried out in a temperature range of 1100 to 1300°C. If the heating temperature is below 1100°C, there is a problem of a rapid increase in load during the subsequent hot rolling process. On the other hand, if the temperature exceeds 1300°C, there is a problem of an increase in the amount of surface scale and a decrease in the yield of the material.

[0083] [Hot rolling] The heated steel slab can be hot-rolled to produce a hot-rolled steel sheet, and in this process, finish hot rolling can be performed in a temperature range of Ar3 or higher.

[0084] If the temperature during the finish hot rolling described above is below Ar3, rolling will occur in the ferrite + austenite two-phase region or the ferrite region, resulting in the formation of a mixed grain structure. Furthermore, fluctuations in the hot rolling load may lead to defects.

[0085] More specifically, the above-mentioned finish hot rolling can be carried out in a temperature range of 800 to 1000°C.

[0086] [Rewind] The hot-rolled steel sheet produced by the above method can be wound into a coil shape.

[0087] The above winding process can be performed in a temperature range of 700°C or lower. If the winding temperature exceeds 700°C, an excessive oxide film may form on the steel sheet surface, potentially causing defects.

[0088] On the other hand, the lower the winding temperature, the higher the strength of the hot-rolled steel sheet, which has the disadvantage of requiring a higher rolling load in the subsequent cold rolling process. Therefore, the lower limit of the winding temperature can be limited to 100°C.

[0089] [Cold rolling] The hot-rolled steel sheet wound up as described above can be cold-rolled to produce a cold-rolled steel sheet, and in this invention, the cold rolling can be performed at a cold reduction ratio of 30 to 80%.

[0090] If the cold reduction ratio is less than 30% during the cold rolling process described above, it may not only fail to achieve the target thickness, but hot-rolled grains may remain, potentially affecting the formation of austenite and the securing of final physical properties during the subsequent continuous annealing process. On the other hand, if the cold reduction ratio exceeds 80%, the amount of reduction rolled in the length and width directions will become uneven due to work hardening that occurs during cold rolling, which may result in variations in the material properties of the final steel sheet. Furthermore, it may become difficult to achieve the target thickness due to the rolling load.

[0091] On the other hand, prior to the cold rolling process, a pickling process may be carried out to remove the oxide layer formed on the surface of the hot-rolled steel sheet obtained by hot rolling. The conditions for the pickling process are not particularly limited and can be carried out under commonly used conditions.

[0092] [Continuous annealing] It is preferable to perform continuous annealing on the cold-rolled steel sheet produced as described above. This continuous annealing can be carried out, for example, in a continuous annealing furnace (CAL).

[0093] The above continuous annealing process can be carried out by heat treatment at a temperature of Ac3 or higher for 30 seconds or more. This is to ensure 100% austenite fraction through annealing in the austenite single-phase region.

[0094] Here, Ac3 can be calculated using the following formula.

[0095] [Formula] Ac3=910-203√C-15.2Ni+44.7Si+104V+31.5Mo+13.1W

[0096] (In the formula, each element is represented by its weight content.)

[0097] In the present invention, it is preferable to control the dew point temperature inside the annealing furnace to 0 to 20°C when performing continuous annealing under the above-described conditions. By controlling the dew point temperature in this way, a decarburized layer can be formed on the steel surface during the continuous annealing process.

[0098] Normally, the dew point in a continuous annealing furnace is around -50°C. However, when humid nitrogen (N2 + H2O) is introduced to raise the dew point temperature above 0°C, the partial oxygen pressure increases. This causes the carbon (C) in the steel to react with the oxygen (O) in the annealing furnace, releasing CO gas, which then causes decarburization to occur in the surface layer.

[0099] If the dew point temperature inside the annealing furnace is below 0°C, a sufficient decarburized layer will not form on the steel surface. On the other hand, if it exceeds 20°C, there will be problems with the equipment's lifespan and reduced productivity.

[0100] Thus, by forming a decarburized layer on the steel surface during the continuous annealing process, and softening only the surface layer, it is possible to further improve the bending properties of ultra-high-strength steel.

[0101] [Gradual cooling] As mentioned earlier, the target microstructure can be formed by cooling the cold-rolled steel sheet that has undergone continuous annealing treatment as described above, and it is preferable to perform this cooling stepwise.

[0102] In the present invention, the above-mentioned stepwise cooling can be performed in a primary-secondary cooling manner. Specifically, after the above-mentioned continuous annealing, primary cooling can be performed to a temperature range of 550-750°C at an average cooling rate of 1-10°C / s, followed by secondary cooling to a temperature range of Ms-190°C or lower at an average cooling rate of 20-80°C / s.

[0103] If the end temperature during the primary cooling is below 550°C, phases such as ferrite and bainite may form, potentially reducing the strength. On the other hand, if the temperature exceeds 750°C, not only will the lifespan of the annealing furnace be shortened, but excessive cooling will be required during the subsequent secondary cooling, which could lead to problems in the actual production line, such as defects in the shape of the sheet metal and difficulty in controlling meandering.

[0104] Furthermore, if the average cooling rate during the primary cooling is less than 1°C / s, a ferrite phase will form during cooling, making it impossible to secure the target level of strength. On the other hand, if it exceeds 10°C / s, the average cooling rate during the subsequent secondary cooling will decrease, increasing the proportion of other low-temperature transformation phases besides martensite, ultimately making it impossible to secure the target level of strength.

[0105] As mentioned above, after the primary cooling is complete, rapid cooling (secondary cooling) can be performed at an average cooling rate above a certain level.

[0106] In particular, in the present invention, it is advantageous to rapidly cool the material to a temperature below Mf (martensitic transformation completion temperature) during secondary cooling in order to secure the martensite and / or tempered martensite phase as the main structure.

[0107] Specifically, by cooling to a temperature of Ms-190°C or lower, a sufficiently hard martensite structure can be formed, and the effect of increasing yield strength due to carbide precipitation during the subsequent reheating (tempering) process can be obtained. If the temperature at which the above cooling ends exceeds Ms-190°C, it becomes difficult to secure the desired level of strength in this invention, and the subsequent reheating temperature may become excessively high, in which case the bendability of the steel may deteriorate. In addition, the fraction of the intended structure (martensite and / or tempered martensite) may not be sufficiently secured.

[0108] Therefore, in this invention, by limiting the termination temperature during secondary cooling, the tempering effect can be sufficiently induced without excessively raising the subsequent reheating temperature, thereby ensuring bending characteristics.

[0109] While there are no specific limitations on the lower limit of the termination temperature for the secondary cooling described above, it can be limited to around 50°C, taking into consideration the characteristics of the equipment.

[0110] Here, Ms (the transformation onset temperature of martensite) can be calculated from the following formula.

[0111] [Formula] Ms=539-423C-30.4Mn-7.5Si+30Al-17.7Ni-12.1Cr-7.5Mo

[0112] (In the formula, each element is represented by its weight content.)

[0113] If the average cooling rate during the secondary cooling described above is less than 20°C / s, there is a risk that a bainite structure may be partially formed during the secondary cooling process. On the other hand, if it exceeds 80°C / s, the rapid transformation rate of martensite at the time of secondary cooling can cause deterioration of the surface shape of the steel sheet, resulting in material variation in the width direction.

[0114] [Reheating and over-aging] In this invention, the toughness of steel can be improved by transforming the martensite phase, which has a high potential density and is hard, formed during secondary cooling into tempered martensite through reheating and overaging treatment.

[0115] Specifically, the above reheating and overaging treatment preferably involves heating the cold-rolled steel sheet, which has been cooled in stages as described above, to a temperature range that satisfies the following relational equation 3, and then maintaining that temperature for 1 to 20 minutes.

[0116] [Relationship Equation 3] CT2 + 30℃ ≤ A ≤ 270℃

[0117] (Here, CT2 represents the secondary cooling end temperature (°C), and A represents the reheating and overaging temperature (°C).)

[0118] In other words, in order to ensure a sufficient tempering effect, the lower limit of the reheating temperature is restricted to a temperature of 30°C or higher relative to the secondary cooling end temperature (CT2). In the reheating process of this invention, the yield strength of the steel increases due to the formation of fine carbides, but if the temperature at this time is less than CT2 + 30°C, the tempering effect will be insufficient. On the other hand, if the temperature exceeds 270°C, the carbides become coarser, which leads to a deterioration of the bending properties.

[0119] Furthermore, if the holding time during the overaging treatment is less than 1 minute after reheating to the temperature range mentioned above, the martensite will not be sufficiently transformed into tempered martensite, making it difficult to obtain the desired tempering effect. On the other hand, if the holding time exceeds 20 minutes, the carbides formed by overaging will become coarse, reducing the bending properties and potentially adversely affecting the material.

[0120] The steel sheet of the present invention manufactured as described above has a microstructure composed of martensite and / or tempered martensite, resulting in ultra-high strength with a tensile strength of 1300 MPa or more. Furthermore, by controlling the temperature in the continuous annealing process, the cooling process, and the reheating process, an excellent yield ratio can be ensured. In addition, a decarburized layer is formed on the surface during the continuous annealing process, resulting in excellent bending properties. [Examples]

[0121] The present invention will be described in more detail below with reference to examples. However, such examples are provided to illustrate the implementation of the present invention and do not limit the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0122] (Examples) Hot-rolled steel sheets were produced by heating steel slabs having the alloy composition systems shown in Table 1 below to 1100-1300°C, followed by finish hot rolling at 850-950°C, a temperature above Ar3. Subsequently, each hot-rolled steel sheet was wound at 300-700°C, and then cold-rolled at a cold reduction ratio of 45-65% to produce cold-rolled steel sheets.

[0123] Each cold-rolled steel sheet produced as described above was subjected to continuous annealing at a temperature range of 800-900°C for 100-400 seconds, followed by stepwise cooling under the conditions shown in Table 2 below. Subsequently, the final steel sheets were produced by reheating and overaging under the conditions shown in Table 2 below. The dew point temperature inside the annealing furnace during the continuous annealing process is also shown in Table 2 below.

[0124] Subsequently, the carbon content in the surface layer of the manufactured steel sheets was measured using GDS, and the physical properties were determined through material evaluation. At this time, yield strength, tensile strength, yield ratio, total elongation, and uniform elongation were measured by processing each steel sheet to JIS standards (gauge length width × length: 25 × 50 mm, total length of test piece: 200~260 mm) and then performing a tensile test at a test speed of 28 mm / min.

[0125] Furthermore, the bending properties (R / t) were determined by processing the same steel plate into a 100mm wide x 30mm long specimen and performing a 90° bending test at a test speed of 100mm / min. After that, cracks in the bent area were checked using a microscope, and the R / t value was calculated by dividing the minimum bending radius (R value of the die) at which no cracks occurred by the thickness of the specimen (t, mm). For the three-point bending test, the maximum bending angle was measured by processing the same steel plate into a 60mm wide x 30mm long specimen and performing a test at a test speed of 20mm / min and a punching radius of 0.4R, as per the VDA238-100 standard, at which cracks occurred.

[0126] The microstructure of each steel sheet was then observed using a scanning electron microscope (SEM), and the fractions were measured.

[0127] [Table 1]

[0128] [Table 2]

[0129] [Table 3]

[0130] [Table 4]

[0131] As shown in Tables 1-4 above, Invention Examples 1-3, which satisfy all of the alloy compositions and manufacturing conditions proposed in this invention, have a sufficiently formed decarburized layer within the surface layer, thereby exhibiting excellent bending properties. Furthermore, it can be confirmed that the steel sheet has ultra-high strength because its main structure is formed from martensite / tempered martensite.

[0132] On the other hand, Comparative Examples 1 to 11, which satisfy the alloy composition of the present invention but whose manufacturing conditions, particularly the annealing conditions or reheating conditions, do not satisfy the present invention, exhibited inferior bending properties because a decarburized layer was not formed within the surface layer.

[0133] In Comparative Example 12, after continuous annealing, the secondary cooling end temperature was high, and the temperature during reheating was not sufficiently raised. As a result, although a decarburized layer was formed, the tempering effect was insufficient, leading to low yield strength and tensile strength.

[0134] Although Comparative Examples 13 and 14 both satisfy the alloy composition requirements of the present invention, the annealing conditions (dew point temperature conditions) do not meet the requirements of the present invention. As a result, a decarburized layer is not formed within the surface layer, the bending properties are poor (around 4), and the maximum angle during the bending test is insufficient. Therefore, it can be confirmed that they deviate from relational equation 2.

[0135] In Comparative Example 15, after continuous annealing, the secondary cooling end temperature was high, and the temperature during reheating was not sufficiently raised. As a result, although a decarburized layer was formed, the tempering effect was insufficient, leading to low yield strength and a poor yield ratio.

[0136] Comparative Example 16 did not satisfy the alloy composition system of the present invention, and therefore had inferior yield strength and yield ratio.

[0137] Comparative Example 17 is also an example that does not satisfy the alloy composition system of the present invention, and due to insufficient martensite (+tempered martensite) phase in the microstructure of the steel sheet, it showed inferior yield strength and tensile strength.

[0138] Comparative Example 18 is an example that deviates from relational formula 1 of the present invention. Despite the application of the annealing conditions of the present invention, the strength was extremely poor because the martensite (+tempered martensite) phase was hardly formed not only in the surface layer but also in the core.

[0139] Figure 1 shows photographs of the microstructure of the surface layer cross-section (up to approximately 80 μm in the thickness direction) of Invention Example 1 and Comparative Example 1, measured by SEM.

[0140] As shown in Figure 1, in the case of Invention Example 1, it can be confirmed that a decarburized layer containing a soft phase is formed in the surface layer, whereas in Comparative Example 1, it can be seen that a hard phase is densely formed.

[0141] Figure 2 shows photographs of the cross-sectional microstructure of the 1 / 4t region (t: steel plate thickness (mm), based on 1.4 mm) of Invention Example 1 and Comparative Example 1, measured by SEM.

[0142] As shown in Figure 2, it can be confirmed that both Invention Example 1 and Comparative Example 1 have a martensite (or tempered martensite) phase as the main structure.

Claims

1. In weight percent, it consists of carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01-0.5%, and two or more of the following: chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.01-0.2%, and boron (B): 0.005% or less, and one or more of the following: titanium (Ti): 0.1% or less, and niobium (Nb): 0.1% or less, with the remainder being Fe and unavoidable impurities, and satisfies the following relational formula 1. The microstructure contains martensite and / or tempered martensite phases in an area fraction of 99% or more. An ultra-high-strength steel sheet with excellent bending properties, having a bending characteristic (R / t) of 3 or less. [Relationship 1] [Math 1] (Here, Ceq1=C+(Mn / 20)+(Si / 30)+(2P)+(4S), Ceq2=C+(Mn / 6)+(Si / 30)+(Cr+Mo+V+Nb) / 5+(Cu+Ni) / 15.)

2. The steel sheet is an ultra-high-strength steel sheet with excellent bending properties as described in claim 1, wherein the ratio of the carbon content in a region (A) of 1 to 3 μm in the thickness direction based on the surface (average carbon content in region (A) / carbon content of the steel sheet) is 0.6 or less relative to the carbon content.

3. The steel sheet is an ultra-high-strength steel sheet with excellent bending properties as described in claim 1, wherein the ratio of the carbon content in a region (B) from 0.2 to 30 μm in the thickness direction based on the surface (average carbon content in region (B) / carbon content of the steel sheet) is 0.9 or less relative to the carbon content.

4. The ultra-high-strength steel sheet with excellent bending properties according to claim 1, wherein the content of ferrite and / or bainite phase in the steel sheet is 1% or less (including 0%) by area fraction.

5. The steel sheet is an ultra-high-strength steel sheet with excellent bending properties as described in claim 1, wherein the microstructure of the surface layer, which is a region from the surface in the thickness direction from a minimum of 50 μm to a maximum of 70 μm, is composed of tempered martensite with an area fraction of 70% or less (excluding 0%) and the remainder being one or more of ferrite and bainite.

6. The steel plate is an ultra-high-strength steel plate with excellent bending properties as described in claim 1, having a tensile strength of 1300 MPa or more and a yield ratio of 0.72 or more.

7. The steel plate is an ultra-high-strength steel plate with excellent bending properties as described in claim 1, satisfying the following relational equation 2. [Relationship Equation 2] (Tensile strength (TS) / Maximum bending angle) ≤ 25

8. A steel slab containing, by weight percent, carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01-0.5%, two or more of chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.01-0.2%, and boron (B): 0.005% or less, one or more of titanium (Ti): 0.1% or less, and niobium (Nb): 0.1% or less, with the remainder being Fe and unavoidable impurities, and satisfying the following relational formula 1, is heated in a temperature range of 1100-1300°C; The step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the heated steel slab with Ar3 or higher; The step of winding the hot-rolled steel sheet at a temperature of 700°C or lower; The step of cold-rolling the aforementioned wound hot-rolled steel sheet at a total reduction ratio of 30-80% to produce cold-rolled steel sheet; The step of continuously annealing the cold-rolled steel sheet at Ac3 or higher and a dew point temperature of 0 to 20°C for 30 seconds or more; Following the continuous annealing, a primary cooling step is performed to a temperature range of 550 to 750°C at an average cooling rate of 1 to 10°C / s; The first step is to perform a secondary cooling after the first cooling, at an average cooling rate of 20 to 80°C / s, to a temperature of Ms -190°C or lower; and The process includes a step of reheating after the secondary cooling, followed by an overaging treatment. The method for manufacturing an ultra-high-strength steel sheet with excellent bending properties, as described in claim 1, characterized in that the reheating and overaging steps are performed by heating to a temperature range that satisfies the following relational expression 3. [Relationship 1] [Math 2] (Here, Ceq1=C+(Mn / 20)+(Si / 30)+(2P)+(4S), Ceq2=C+(Mn / 6)+(Si / 30)+(Cr+Mo+V+Nb) / 5+(Cu+Ni) / 15.) [Relationship Equation 3] CT2+30℃≦A≦270℃ (Here, CT2 represents the secondary cooling completion temperature (°C), and A represents the reheating and overaging temperature (°C).)

9. The method for manufacturing an ultra-high-strength steel sheet with excellent bending properties, as described in claim 8, wherein the overaging treatment is performed for 1 to 20 minutes.

Images