KR20230089785A - Ultra high strength steel sheet having excellent bendability, and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a steel sheet suitable for structural members of automobiles, etc., and more particularly, to an ultra-high strength steel sheet having excellent bending properties and a manufacturing method thereof.

Description

Ultra-high strength steel sheet with excellent bending properties and its manufacturing method {ULTRA HIGH STRENGTH STEEL SHEET HAVING EXCELLENT BENDABILITY, AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a steel sheet suitable for structural members of automobiles, etc., and more particularly, to an ultra-high strength steel sheet having excellent bending properties and a manufacturing method thereof.

Recently, in the field of automobiles, research to reduce the weight of a vehicle body is being actively conducted in developed countries, led by Europe, for reasons such as fuel economy regulations and performance improvements. In particular, in the case of steel, efforts are being made to further reduce the thickness and increase the strength of the same grade compared to competing materials (Mg, Al, carbon fiber reinforced plastic (CFRP), etc.) in order to respond to the demand for weight reduction by automobile companies. In other words, in addition to weight reduction, stability and strength of vehicle body materials are also required due to the strengthening of safety regulations for automobile passengers and pedestrians.

On the other hand, in order to improve the stability and crash characteristics of the car body, the adoption of high-strength steel with excellent yield strength for BIW (Body-In-White) structural members is increasing. / The higher the tensile strength, YR), the more advantageous it is to absorb impact energy.

Therefore, as a representative method for increasing the yield strength of steel, a method of utilizing water cooling during continuous annealing is mainly used. Specifically, after a two-phase or single-phase annealing of a cold-rolled steel sheet, it is rapidly cooled to about room temperature, and then subjected to a process such as tempering to manufacture ultra-high strength steel.

However, while the ultra-high strength steel manufactured by this method has a very high yield ratio, there is a problem in that the shape quality of the coil is deteriorated due to temperature deviation in the width direction and the longitudinal direction, and material defects depending on the part during parts processing by roll forming, etc. Problems such as deterioration of workability may occur. In addition, since the elongation of the steel generally decreases as the strength of the steel increases, there is a problem in that forming processability is lowered.

In order to overcome this, a hot press forming (HPF) method has been developed and applied to secure strength through water cooling between a die and a material after forming a material at a relatively easy high temperature.

When the HPF method is applied, it is possible to secure high strength compared to the same thickness, so parts using the HPF method are being developed mainly in Europe.

However, problems such as excessive capital investment required for the HPF method and an increase in process cost have emerged, and thus the development of a material for cold stamping is required.

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

International Publication No. WO 2021/084303

One aspect of the present invention is to provide a steel sheet suitable for cold stamping as well as a material suitable for automotive structural members, particularly ultra-high strength steel sheet having excellent bending properties, and a method for manufacturing the same.

The object of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention, in weight%, carbon (C): 0.1 ~ 0.3%, manganese (Mn): 1.0 ~ 2.3%, silicon (Si): 0.05 ~ 1.0%, phosphorus (P): 0.1% or less ( 0% excluded), Sulfur (S): 0.03% or less (excluding 0%), Aluminum (Al): 0.01 to 0.5%, Chromium (Cr): 0.01 to 0.2%, Molybdenum (Mo): 0.01 to 0.2 % and boron (B): at least two of 0.005% or less, titanium (Ti): at least 0.1% and niobium (Nb): at least one of 0.1% or less, the balance including Fe and unavoidable impurities, the following relational expression 1 Satisfactory, and provides an ultra-high strength steel sheet having excellent bending properties including martensite and/or tempered martensite phase in an area fraction of 99% or more as a microstructure.

[Relationship 1]

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

Another aspect of the present invention comprises the steps of heating a steel slab satisfying the above-described alloy composition and relational expression 1 in a temperature range of 1100 to 1300 ° C; Manufacturing a hot-rolled steel sheet by finish hot-rolling the reheated steel slab at Ar3 or higher; winding the hot-rolled steel sheet at a temperature of 700° C. or lower; manufacturing a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a total reduction ratio of 30 to 80%; Continuously annealing the cold-rolled steel sheet at Ac3 or higher for 30 seconds or more; 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; Secondary cooling at an average cooling rate of 20 to 80 ° C / s to a temperature of Ms -190 ° C or less after the first cooling; And reheating after the secondary cooling and then overaging treatment,

The reheating and overaging step provides a method for manufacturing an ultra-high strength steel sheet having excellent bending properties, characterized in that the heating is performed to a temperature range that satisfies the following relational expression 3.

[Relationship 3]

CT2 + 30℃ ≤ A ≤ 270℃

(Here, CT2 means the secondary cooling end temperature (℃), and A means the reheating and overaging temperature (℃).)

According to the present invention, it is possible to provide a steel sheet having improved workability by achieving a high yield ratio as well as ultra-high strength. In particular, the steel sheet of the present invention is not only a material that can be suitably applied for automotive structural members, but also has effects that can be advantageously applied to processing such as cold stamping.

Figure 1 shows a picture of the microstructure of the surface layer of the inventive example and the comparative example in one embodiment of the present invention.
2 is a microstructure photograph of a 1/4t region of an inventive example and a comparative example according to an embodiment of the present invention.

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

Hereinafter, the present invention will be described in detail.

Ultra-high strength steel sheet according to one aspect of the present invention, by weight, 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 to 0.5%.

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

Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.

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, in martensitic steel, it is an element that must be added in order to secure strength.

In order to obtain a steel sheet having strength, yield ratio, etc. targeted in the present invention, it is preferable to add the above C in an amount of 0.1% or more. However, when the content exceeds 0.3%, the martensite strength is increased, while carbide is easily generated in the continuous annealing process, and it is easy to be coarsened, resulting in poor ductility as well as poor bending properties. In addition, excessive increase in carbon content has a problem of inhibiting weldability.

Therefore, in the present invention, the C may be included in 0.1 to 0.3%, more advantageously, 0.12% or more and 0.28% or less.

Manganese (Mn): 1.0 to 2.3%

Manganese (Mn) is an element that is easy to finally secure the martensite phase by suppressing the formation of ferrite and promoting the formation of austenite in composite steel.

When the content of Mn exceeds 2.3%, Mn is segregated in the thickness direction of the steel, so that a Mn band is easily formed in the slab, which increases the occurrence of defects during rolling along with playing cracks. On the other hand, if the content is less than 1.0%, the target level of strength cannot be secured.

Therefore, in the present invention, the Mn may be included in 1.0 to 2.3%, more advantageously, 1.2% or more and 2.1% or less. More advantageously, it may be included at 1.4% or more.

Silicon (Si): 0.05 to 1.0%

Silicon (Si) plays a role of suppressing the generation of carbides and controlling the size of carbides in the reheating and overaging treatment steps performed after continuous annealing and cooling during the process of manufacturing a steel sheet to be obtained in the present invention.

In order to sufficiently obtain the above-mentioned effects, it is preferable to include the Si at 0.05% or more. However, if the content exceeds 1.0%, there is a concern that ferrite is generated during cooling in the continuous annealing furnace to weaken the strength of the steel. In addition, Si-based oxides may be generated during reheating and overaging after cooling, resulting in surface oxidation of the steel.

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

Phosphorus (P): 0.1% or less (excluding 0%)

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

Sulfur (S): 0.03% or less (excluding 0%)

Sulfur (S) is an impurity unavoidably contained in steel, similar to P, and is an element that inhibits ductility and weldability of steel, so it is advantageous to manage its content as low as possible. In the present invention, even if the S is contained at a maximum of 0.03%, there is no difficulty in securing the target physical properties, etc., so the upper limit can be limited to 0.03%, and 0% is excluded in consideration of the level inevitably added during the steel manufacturing process. can do.

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

Aluminum (Al): 0.01 to 0.5%

Aluminum (Al) may be added to remove oxygen in molten steel, and is an element that stabilizes ferrite similarly to Si. In addition, Al is a component that increases the carbon content in austenite to improve the hardenability of the final martensitic steel.

In order to sufficiently obtain the above effects, the Al may be contained in an amount of 0.01% or more. However, when the content exceeds 0.5%, ferrite is generated during cooling in the continuous annealing furnace, and there is a concern that the strength is weakened. In addition, there is a risk of causing cracks in the cast steel by forming AlN by combining with N, which is unavoidably present in the steel to an impurity level, and there is a problem of impairing hot rolling properties.

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

On the other hand, the steel sheet of the present invention may further include elements advantageous to securing the physical properties of the steel in addition to the above-described alloy composition. Specifically, the steel sheet of the present invention preferably further includes at least two selected from chromium (Cr), molybdenum (Mo) and boron (B), and at least one selected from titanium (Ti) and niobium (Nb).

Chromium (Cr): 0.01~0.2%

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

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

Therefore, when the Cr is added, it may be added at 0.01 to 0.2%.

Molybdenum (Mo): 0.01 to 0.2%

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

In order to sufficiently obtain the hardenability effect, the Mo may be added in an amount of 0.01% or more, but when the content exceeds 0.2%, the amount of alloy added is excessive, resulting in an increase in the cost of ferroalloy.

Therefore, when the Mo is added, it may be added at 0.01 to 0.2%.

Boron (B): 0.005% or less

Boron (B) is an element that suppresses the transformation of austenite into ferrite during the continuous annealing process, and is an element that is effective in improving hardenability, such as Cr and Mo, even when added in a very small amount. However, when the content exceeds 0.005%, there is a concern that the Fe ₂₃ (B, C) ₆ precipitated phase may act to promote the production of ferrite as the precipitated phase is precipitated at the austenite grain boundary.

Therefore, when adding the B, it may be added at 0.005% or less.

Titanium (Ti): 0.1% or less

Titanium (Ti) is an element that forms fine carbides and contributes to securing yield strength and tensile strength. In addition, since Ti is an element that scavengs by precipitating N, which is unavoidably present in an impurity level in steel, as TiN, it can be added in an amount equal to or higher than 48/(14×N) based on chemochemical standards.

When the content of Ti exceeds 0.1%, rather coarse carbides are precipitated, and as the amount of carbon in the steel is reduced, there is a problem in that strength and elongation are lowered. In addition, since it may cause nozzle clogging during playing, when adding the Ti, it may be added in an amount of 0.1% or less.

On the other hand, when adding B, it is advantageous to add Ti together in order to maximize the addition effect.

Niobium (Nb): 0.1% or less

Niobium (Nb) is an element that is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during continuous annealing and to form fine carbides to contribute to strength improvement.

When the Nb content exceeds 0.1%, the precipitation of coarse carbonitrides increases, and there is a concern that strength and elongation may be lowered due to a decrease in the amount of carbon in steel. In addition, there is a problem that the workability of the base material is lowered and the manufacturing cost is increased.

Therefore, when the Nb is added, it may be added in an amount of 0.1% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

In the steel sheet of the present invention that satisfies the above-described alloy composition, it is preferable that the content relationship of specific elements satisfies the following relational expression 1.

[Relationship 1]

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

Relational Equation 1 is a complex relational expression of Ceq1 and Ceq2 for the effect of the content of alloying elements added in steel on the welding characteristics. When the range is 0.12 to 0.28, the basic welding characteristics are satisfied, and properties can be obtained advantageously.

Specifically, if the value of relational expression 1 is less than 0.12, it is not possible to secure the level of strength targeted in the present invention, whereas if the value exceeds 0.28, among physical properties, particularly welding characteristics may be significantly deteriorated.

The value of relational expression 1 may be more preferably 0.15 or more and 0.27 or less, and even more preferably 0.17 or more.

The steel sheet of the present invention that satisfies the above-described alloy composition and relational expression 1 preferably includes a martensite phase as a main phase as a microstructure.

Specifically, the steel sheet may include martensite and/or tempered martensite phase in an area fraction of 99% or more. At this time, the fraction may be 100%.

When the fraction of the martensite and/or tempered martensite phase is 99%, the remaining 1% may be a ferrite and/or bainite phase.

As will be described later, the steel sheet of the present invention has a surface layer portion of a specific region, and the main structure in the remaining region (eg, the center region) excluding the surface layer portion is preferably martensite and/or tempered martensite phase.

On the other hand, in the steel sheet of the present invention, a region corresponding to a minimum of 50 μm to a maximum of 70 μm in the thickness direction from the surface may be defined as a surface layer portion, and the surface layer portion includes a soft phase.

Preferably, the surface layer portion includes a tempered martensite phase at an area fraction of 70% or less, and may include at least one of ferrite and bainite having softer properties than the tempered martensite as a remaining structure. In this way, the effect of further improving the bending properties can be obtained by softening the surface layer portion of the steel sheet.

In addition, the surface layer portion including a certain soft phase is characterized by including a decarburized layer containing C at a lower C content than the C content contained in the steel sheet.

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

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

The decarburized layer in the surface layer is advantageous for improving the bending properties of the steel sheet. Target bending properties are achieved when the carbon (C) content ratio of specific regions (A and B) in the surface layer exceeds 0.6 and 0.9, respectively. will not be able to

Here, the decarburized layer may be formed to a thickness corresponding to the surface layer portion, or may be formed thinner than the thickness of the surface layer portion.

In the present invention, the decarburization layer can be formed by controlling the continuous annealing process during the steel sheet manufacturing process, and this will be described in detail below.

As described above, while the microstructure is composed of a hard phase, the steel sheet of the present invention including a decarburization layer in the surface layer has ultra-high strength with a tensile strength of 1300 MPa or more, a high yield ratio of 0.72 or more, and bending characteristics (R /t) can have an effect of 3 or less.

In addition, in the steel sheet of the present invention, the relationship between tensile strength and bending properties, specifically, the relationship between tensile strength (TS), which is a basic tensile property, and the maximum bending angle after a 3-point bending test according to the VDA238-100 standard, satisfies the following relational expression 2 can

[Relationship 2]

(tensile strength (TS)/maximum bending angle) ≤ 25

If the value of the relational expression 2 is 25 or less, excellent bending properties can be secured in ultra-high strength steel having a tensile strength of 1300 MPa or more, whereas when the value exceeds 25, the strength is high but the bending properties are inferior.

Hereinafter, a method for manufacturing an ultra-high strength steel sheet having excellent bending properties according to another aspect of the present invention will be described in detail.

Briefly, the present invention can manufacture a desired steel sheet through the processes of [steel slab heating - hot rolling - winding - cold rolling - continuous annealing], and each process will be described in detail below. Meanwhile, the continuous annealing process includes a cooling process as well as a reheating and overaging process, which means that the process is performed collectively in a continuous annealing line.

[Heating of steel slabs]

First, after preparing a steel slab satisfying the aforementioned alloy composition, it may be heated.

This process is performed in order to smoothly perform the subsequent hot rolling process and sufficiently obtain target physical properties of the steel sheet. In the present invention, there is no particular restriction on the conditions of such a heating process, and it may be a normal condition. As an example, the heating process may be performed in a temperature range of 1100 to 1300 °C. If the heating temperature is less than 1100 ° C, there is a problem that the load increases rapidly during subsequent hot rolling, whereas if the temperature exceeds 1300 ° C, the amount of surface scale increases and the yield of the material decreases.

[Hot rolling]

The steel slab heated according to the above may be hot-rolled to produce a hot-rolled steel sheet, and at this time, finish hot-rolling may be performed in a temperature range of Ar3 or higher.

When the temperature during the finish hot rolling is less than Ar3, two-phase or ferrite reverse rolling of ferrite + austenite is performed to form a mixed texture, and there is a risk of malfunction due to fluctuations in the hot rolling load.

More specifically, the finish hot rolling may be performed in a temperature range of 800 ~ 1000 ℃.

[wind up]

The hot-rolled steel sheet manufactured according to the above may be wound into a coil shape.

The winding may be performed in a temperature range of 700° C. or less. If the coiling temperature exceeds 700° C., excessive oxide film is formed on the surface of the steel sheet, which may cause defects.

On the other hand, since the strength of the hot-rolled steel sheet increases as the coiling temperature decreases, there is a disadvantage in that the rolling load increases in the subsequent cold rolling process. Accordingly, the lower limit of the coiling temperature may be limited to 100°C.

[Cold Rolling]

The hot-rolled steel sheet wound according to the above may be cold-rolled to produce a cold-rolled steel sheet, and in the present invention, the cold rolling may be performed at a cold rolling reduction of 30 to 80%.

If the cold rolling reduction ratio is less than 30% during the cold rolling, it is not possible to secure the target thickness, and hot-rolled crystal grains remain, which may affect the creation of austenite and the securing of final physical properties during subsequent continuous annealing. there is On the other hand, when the cold rolling reduction ratio exceeds 80%, the rolling reduction in the length and width directions becomes non-uniform due to work hardening occurring during cold rolling, which may cause material deviation of the final steel sheet. In addition, it may be difficult to secure the target thickness due to the rolling load.

Meanwhile, prior to the cold rolling, a pickling process may be additionally performed for the purpose of removing an oxide layer formed on the surface of the hot rolled steel sheet obtained by hot rolling. Conditions for the pickling process are not particularly limited, and may be performed under commonly used conditions.

[Continuous Annealing]

It is preferable to subject the cold-rolled steel sheet manufactured according to the above to continuous annealing. The continuous annealing treatment may be performed in a continuous annealing furnace (CAL), for example.

The continuous annealing treatment may be performed by a process of heat treatment at a temperature of Ac3 or higher for 30 seconds or more. This is to secure the austenite fraction to 100% through austenite single phase annealing.

Here, Ac3 can be calculated from the formula below.

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

(Each element in the formula is a weight content.)

In the present invention, it is preferable to control the dew point temperature in the annealing furnace to 0 to 20 ° C. during the continuous annealing treatment under the above-described conditions.

Normally, the dew point in the continuous annealing furnace is about -50℃, but when the dew point temperature is raised to 0℃ or higher by adding moist nitrogen (N ₂ +H ₂ O), the oxygen partial pressure increases, and the carbon (C) of the steel As oxygen (O) in the annealing furnace meets and is released as CO gas, decarburization occurs in the surface layer.

If the dew point temperature in the annealing furnace is less than 0 ° C, the decarburization layer cannot be sufficiently formed on the steel surface, whereas if it exceeds 20 ° C, there is a problem of equipment life and productivity decrease.

In this way, by forming a decarburization layer on the surface of the steel in the continuous annealing process to soften only the surface layer, there is an effect of further improving the bending characteristics of the steel having ultra-high strength.

[Gradual Cooling]

As mentioned above, it is possible to form a target structure by cooling the continuously annealed cold-rolled steel sheet according to the above, and at this time, it is preferable to perform cooling in a stepwise manner.

In the present invention, the stepwise cooling may be composed of primary cooling - secondary cooling, specifically, after the first 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, Secondary cooling can be performed at an average cooling rate of 20 to 80 ° C / s up to a temperature range of Ms-190 ° C or less.

If the end temperature of the primary cooling is less than 550 ° C, phases such as ferrite and bainite may be formed and the strength may decrease. On the other hand, if the temperature exceeds 750 ° C, the durability of the annealing furnace may be shortened. In addition, excessive cooling is required during subsequent secondary cooling, which may cause problems in the actual production line, such as defective plate shape and difficulty in meandering control.

In addition, if the average cooling rate during the first cooling is less than 1 ° C / s, a ferrite phase is formed during cooling, making it impossible to secure the target level of strength. As the cooling rate decreases, the fraction of other low-temperature transformation phases other than martensite increases, and finally, the target level of strength cannot be secured.

After the primary cooling is completed as described above, rapid cooling (secondary cooling) may be performed at a predetermined average cooling rate or more.

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

Specifically, by cooling at a temperature below Ms-190° C., a sufficiently hard martensitic structure can be formed, and an effect of increasing yield strength due to carbide precipitation can be obtained during the subsequent reheating (tempering) process. If the temperature at which the cooling ends exceeds Ms-190 ° C, it may be difficult to secure the desired level of strength in the present invention, and there is a risk that the subsequent reheating temperature will be excessively high, in which case the bendability of the steel will be deteriorated. there is In addition, the fraction of the intended structure (martensite and/or tempered martensite) may not be sufficiently secured.

Therefore, in the present invention, by limiting the end temperature of the secondary cooling, it is possible to sufficiently induce the tempering effect without excessively increasing the subsequent reheating temperature, thereby securing bending characteristics.

The lower limit of the end temperature of the secondary cooling is not particularly limited, but may be limited to about 50° C. in consideration of facility characteristics.

Here, Ms (martensite transformation start temperature) can be calculated from the formula below.

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

(Each element in the formula is a weight content.)

If the average cooling rate during the secondary cooling is less than 20 ° C / s, there is a concern that some bainite structures may be generated during the secondary cooling process, whereas if it exceeds 80 ° C / s, the rapid martensite transformation rate at the time of secondary cooling Due to this, there is a problem that the surface shape of the steel sheet is inferior and material deviation occurs in the width direction.

[Reheating and Overaging]

In the present invention, the toughness of the steel can be improved by changing the hard martensite phase having a high dislocation density formed during secondary cooling to tempered martensite through reheating and overaging treatment.

Specifically, the reheating and overaging treatment is preferably a process of heating the cold-rolled steel sheet cooled step by step according to the above to a temperature range that satisfies the following relational expression 3, and then maintaining the temperature at that temperature for 1 to 20 minutes.

[Relationship 3]

CT2 + 30℃ ≤ A ≤ 270℃

(Here, CT2 means the secondary cooling end temperature (℃), and A means the reheating and overaging temperature (℃).)

That is, in order to sufficiently secure the tempering effect, the lower limit of the reheating temperature is limited to a temperature of 30° C. or higher relative to the secondary cooling end temperature (CT2). The yield strength of the steel increases due to the fine carbides formed in the reheating process of the present invention, and if the temperature at this time is less than CT2 + 30 ° C, the tempering effect becomes insufficient. On the other hand, when the temperature exceeds 270 ℃, there is a problem that the carbide is coarsened and the bending properties are inferior.

In addition, if the holding time in the overaging treatment after reheating to the above-described temperature range is less than 1 minute, martensite is not sufficiently changed to tempered martensite, making it difficult to obtain the desired tempering effect. On the other hand, if the time exceeds 20 minutes, the carbide produced by overaging may become coarse, resulting in deterioration of bending properties and adverse effects on the material.

The steel sheet of the present invention manufactured as described above not only has a tensile strength of 1300 MPa or more as the microstructure is composed of martensite and/or tempered martensite, but also has high temperature in the continuous annealing process, cooling process, reheating process, etc. By controlling the yield ratio can be excellently secured. Moreover, excellent bending properties may be obtained by forming a decarburization layer on the surface layer during the continuous annealing process.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for exemplifying the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

The steel slabs having the alloy components shown in Table 1 below were heated at 1100 to 1300 ° C, and then hot-rolled by finish hot rolling at 850 to 950 ° C, which is a temperature of Ar3 or higher, to prepare a hot-rolled steel sheet. Thereafter, each hot-rolled steel sheet was wound at 300 to 700° C., and then cold-rolled at a cold rolling reduction ratio of 45 to 65% to prepare a cold-rolled steel sheet.

Each of the cold-rolled steel sheets manufactured according to the above was continuously annealed for 100 to 400 seconds in a temperature range of 800 to 900 ° C., and then gradual cooling was performed under the conditions shown in Table 2 below. Thereafter, reheating and overaging were performed under the conditions shown in Table 2 below to prepare final steel sheets. The dew point temperature in the annealing furnace during the continuous annealing treatment is also shown in Table 2 below.

Subsequently, the C content of the surface layer region of the manufactured steel sheet was measured by GDS, and the physical properties were measured through material evaluation. At this time, the yield strength, tensile strength, yield ratio, total elongation, and uniform elongation were determined by processing each steel sheet according to JIS standards (gauge length width × length: 25 × 50 mm, total length of specimen: 200 ~ 260 mm), and then testing at a speed of 28 mm It was measured by performing a tensile test under the conditions of /min.

In addition, for the bending characteristics (R/t), a 90° bending test was performed under the condition of a test speed of 100 mm/min after processing a specimen of the same steel sheet into a width of 100 mm × length of 30 mm. Thereafter, cracks in the bent portion were confirmed using a microscope, and the R/t value was obtained by dividing the minimum bending radius (R value of the mold) in which no cracks occurred by the thickness (t, mm) of the test piece. The maximum bending angle of the 3-point bending test is obtained by processing the same steel sheet into a width of 60 mm × length of 30 mm, and then testing at a test speed of 20 mm/min and a punching radius of 0.4 R, which is the VDA238-100 standard, at the maximum load at which cracks occur. The maximum bending angle of was measured.

And, the microstructure of each steel plate was observed using SEM, and each fraction was measured.

steel grade Alloy composition (% by weight) Relation 1 C Si Mn P S Cr Mo Al Ti V B Nb Ceq1 Ceq2 calculated value invent
river 1 0.24 0.10 1.9 0.01 0.003 0.10 0.05 0.025 0.025 0 0.002 0.03 0.370 0.596 0.25 invent
river 2 0.17 0.10 1.9 0.01 0.003 0 0.05 0.025 0.025 0 0.002 0.04 0.300 0.508 0.17 comparison
river 1 0.18 0.05 3.6 0.01 0.003 0.05 0 0.025 0.020 0.004 0.002 0.04 0.394 0.800 0.40 comparison
river 2 0.17 1.50 2.5 0.01 0.003 0 0.05 0.025 0.025 0 0.002 0.04 0.377 0.655 0.29 comparison
river 3 0.22 0.10 0.6 0.01 0.003 0.05 0 0.025 0.025 0.005 0.002 0 0.285 0.334 0.10

steel grade continuous annealing staged cooling Reheating and Overaging division Annealing
temperature
(℃) dew point
temperature
(℃) Primary
Cooling down
Temperature (℃) Primary
cooling rate
(℃/s) Secondary
Cooling down
Temperature (℃) Secondary
cooling rate
(℃/s) reheat
temperature
(℃) overage
hour
(minute) Invention Steel 2 850 -50 700 2 100 47 210 10 Comparative Example 1 Invention Steel 2 850 -50 700 2 130 44 210 10 Comparative Example 2 Invention Steel 2 850 -50 700 2 150 43 210 10 Comparative Example 3 Invention Steel 2 850 -50 700 2 100 47 230 10 Comparative Example 4 Invention Steel 2 850 -50 700 2 130 44 230 10 Comparative Example 5 Invention Steel 2 850 -50 700 2 150 43 230 10 Comparative Example 6 Invention Steel 2 850 -50 700 2 100 47 180 10 Comparative Example 7 Invention Steel 2 850 -50 700 2 100 47 270 10 Comparative Example 8 Invention Steel 2 850 -50 700 2 150 43 270 10 Comparative Example 9 Invention Steel 2 850 -50 700 2 150 43 300 10 Comparative Example 10 Invention Steel 2 850 -50 700 2 220 37 270 10 Comparative Example 11 Invention Steel 2 850 5 700 2 200 39 210 10 Comparative Example 12 Invention Steel 1 850 -50 700 2 100 47 210 10 Comparative Example 13 Invention Steel 1 850 -50 700 2 150 43 210 10 Comparative Example 14 Invention Steel 1 850 5 700 2 100 47 210 10 Invention Example 1 Invention Steel 1 850 5 700 2 150 43 210 10 Invention example 2 Invention Steel 2 850 5 700 2 100 47 210 10 Inventive example 3 Invention Steel 1 850 5 700 2 200 39 210 10 Comparative Example 15 Comparative Lecture 1 850 5 700 2 150 43 210 10 Comparative Example 16 comparative steel 2 850 5 700 2 150 43 210 10 Comparative Example 17 comparative lecture 3 850 5 700 2 150 43 210 10 Comparative Example 18

division Surface layer (from the surface to 50㎛ in the thickness direction) Excluding the surface layer decarburized layer
depth
(μm) Area A
average
C content Area A
C
content ratio Area B
average
C content Area B
C
content ratio F and/or B
(area%) TM
(area%) M/TM
(area%) balance Comparative Example 1 - 0.183 1.076 0.174 1.024 5 95 99 One Comparative Example 2 - 0.175 1.029 0.194 1.141 5 95 99 One Comparative Example 3 - 0.189 1.112 0.185 1.088 5 95 99 One Comparative Example 4 - 0.211 1.241 0.196 1.153 5 95 99 One Comparative Example 5 - 0.164 0.965 0.205 1.206 5 95 99 One Comparative Example 6 - 0.232 1.365 0.186 1.094 5 95 99 One Comparative Example 7 - 0.178 1.047 0.193 1.135 5 95 99 One Comparative Example 8 - 0.190 1.118 0.235 1.382 5 95 99 One Comparative Example 9 - 0.225 1.324 0.173 1.018 5 95 99 One Comparative Example 10 - 0.198 1.165 0.169 0.994 5 95 99 One Comparative Example 11 - 0.167 0.982 0.227 1.335 5 95 99 One Comparative Example 12 40 0.088 0.518 0.122 0.718 65 35 99 One Comparative Example 13 - 0.313 1.304 0.274 1.142 5 95 99 One Comparative Example 14 - 0.345 1.438 0.283 1.179 5 95 99 One Invention example 1 50 0.047 0.196 0.142 0.592 53 47 99 One Invention example 2 50 0.099 0.413 0.169 0.704 47 53 99 One Inventive example 3 40 0.097 0.571 0.127 0.747 54 46 99 One Comparative Example 15 50 0.094 0.392 0.151 0.629 51 49 99 One Comparative Example 16 40 0.097 0.539 0.139 0.772 58 42 95 5 Comparative Example 17 50 0.037 0.218 0.095 0.559 75 25 80 20 Comparative Example 18 40 0.065 0.295 0.156 0.709 99 One One 99

(In Table 3, the depth of the decarburized layer means the depth measured from the surface in the thickness direction.)

division yield strength
(MPa) tensile strength
(MPa) yield ratio uniform elongation
(%) total elongation
(%) bendability
(R/t) bending angle Relation 2 Comparative Example 1 1079 1345 0.80 4.5 7.5 3< 74 18 Comparative Example 2 1072 1344 0.80 4.3 7.4 3< 76 18 Comparative Example 3 1060 1335 0.79 4.7 8.4 3< 78 17 Comparative Example 4 1112 1349 0.82 4.5 7.5 3< 75 18 Comparative Example 5 1105 1347 0.82 4.4 7.7 3< 77 17 Comparative Example 6 1098 1345 0.82 4.1 7.2 3< 78 17 Comparative Example 7 1061 1364 0.78 4.3 7.0 3< 73 19 Comparative Example 8 1188 1325 0.90 2.5 4.7 4< 79 17 Comparative Example 9 1174 1319 0.89 2.7 5.8 4< 78 17 Comparative Example 10 1203 1288 0.93 2.2 4.7 4< 80 16 Comparative Example 11 1017 1271 0.80 4.1 7.3 4< 78 16 Comparative Example 12 936 1284 0.73 5.9 9.8 <2.5 101 13 Comparative Example 13 1208 1556 0.78 4.5 7.9 <4 55 28 Comparative Example 14 1191 1557 0.76 4.6 8.1 <4 57 27 Invention Example 1 1156 1516 0.76 4.5 8.5 <2.5 84 18 Invention example 2 1125 1508 0.75 4.5 8.1 <2.5 80 19 Inventive example 3 1013 1318 0.77 4.8 8.6 <2.5 103 13 Comparative Example 15 1067 1495 0.71 4.8 8.7 <2.5 88 17 Comparative Example 16 1005 1514 0.66 4.4 7.8 <2.5 99 15 Comparative Example 17 926 1174 0.79 8.5 11.4 <2.5 115 10 Comparative Example 18 331 588 0.56 15.5 27.3 <2.5 140 4

As shown in Tables 1 to 4, Inventive Examples 1 to 3 satisfying both the alloy composition and manufacturing conditions proposed in the present invention had a sufficiently decarburized layer formed in the surface layer portion, and thus had excellent bending properties. In addition, it can be confirmed that the steel sheet has ultra-high strength because the main structure of the steel sheet is formed of martensite/tempered martensite.

On the other hand, Comparative Examples 1 to 11 in which the alloy composition of the present invention was satisfied, but the manufacturing conditions, particularly the annealing conditions or the reheating conditions did not satisfy the present invention, had inferior bending properties as the decarburization layer was not formed in the surface layer portion.

In Comparative Example 12, the secondary cooling end temperature was high during cooling after continuous annealing, and the temperature was not sufficiently raised during reheating, so a decarburized layer was formed, but the tempering effect was insufficient, resulting in low yield strength and tensile strength.

In Comparative Examples 13 and 14, although the alloy composition of the present invention was satisfied, the annealing condition (dew point temperature condition) did not satisfy the present invention, so the decarburization layer was not formed in the surface layer portion, and the bending property was inferior to about 4, and the bending test It can be seen that the maximum angle is not sufficient and deviate from relational expression 2.

Comparative Example 15 had a high secondary cooling end temperature during cooling after continuous annealing, and a decarburized layer was formed due to the temperature not being sufficiently raised during reheating, but the yield strength was low and the yield ratio was poor due to insufficient tempering effect.

Comparative Example 16 was a case in which the alloy component system of the present invention was not satisfied, and the yield strength and yield ratio were inferior.

Comparative Example 17 is also an example that does not satisfy the alloy component system of the present invention, and as the martensite (+ tempered martensite) phase is insufficient in the steel sheet microstructure, both yield strength and tensile strength showed poor results.

Comparative Example 18 is an example that deviate from relational expression 1 of the present invention, and despite the application of the annealing conditions of the present invention, the martensite (+ tempered martensite) phase was hardly formed in the center as well as the surface layer, resulting in very poor strength. did

Figure 1 shows a photograph of the microstructure of the surface layer section (up to about 80 μm in the thickness direction) of Inventive Example 1 and Comparative Example 1 measured by SEM.

As shown in FIG. 1, in the case of Inventive Example 1, it can be confirmed that a decarburization layer including a soft phase is formed in the surface layer portion, whereas in Comparative Example 1, it can be seen that the hard phase is formed densely.

FIG. 2 shows photographs of cross-sectional microstructures of 1/4t regions (t: steel plate thickness (mm), based on 1.4 mm) of Inventive Example 1 and Comparative Example 1 measured by SEM.

As shown in FIG. 2 , it can be seen that a martensite (or tempered martensite) phase is formed as a main structure in both Inventive Example 1 and Comparative Example 1.

Claims (10)

In % 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%), aluminum (Al): 0.01 to 0.5%, chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): At least two of 0.005% or less, titanium (Ti): 0.1% or less and niobium (Nb): at least one of 0.1% or less, the balance including Fe and unavoidable impurities, and satisfying the following relational expression 1,
An ultra-high strength steel sheet with excellent bending properties, containing martensite and/or tempered martensite phase in a microstructure in an area fraction of 99% or more.

[Relationship 1]

(Where Ceq1 = C + (Mn/20) + (Si/30) + (2P) + (4S), Ceq2 = C + (Mn/6) + (Si/30) + (Cr+Mo+V+ Nb)/5 + (Cu+Ni)/15.)
According to claim 1,
The steel sheet has excellent bending properties, wherein the C content ratio (average C content in the area (A) / C content in the steel sheet) in the area (A) of 1 to 3 μm in the thickness direction relative to the surface is 0.6 or less. Ultra-high strength steel sheet.
According to claim 1,
The steel sheet is an ultra-high-strength steel sheet having excellent bending properties in which the C content ratio (average C content in area (B)/C content in steel sheet) in the area (B) of 0.2 to 30 μm in the thickness direction based on the surface is 0.9 or less compared to the C content. .
According to claim 1,
The steel sheet is an ultra-high-strength steel sheet having excellent bending properties including 1% or less (including 0%) of ferrite and/or bainite phase.
According to claim 1,
In the steel sheet, the microstructure of the surface layer, which is a region from the surface to a minimum of 50 μm and a maximum of 70 μm in the thickness direction, has an area fraction of 70% or less (excluding 0%) of tempered martensite and the remainder consisting of one or more of ferrite and bainite. Ultra-high-strength steel sheet with excellent bending properties.
According to claim 1,
The steel sheet has a tensile strength of 1300 MPa or more, a yield ratio of 0.72 or more, and a bending property (R / t) of 3 or less.
According to claim 1,
The steel sheet is an ultra-high strength steel sheet having excellent bending properties that satisfies the following relational expression 2.

[Relationship 2]
(tensile strength (TS)/maximum bending angle) ≤ 25
In % 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%), aluminum (Al): 0.01 to 0.5%, chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): At least two of 0.005% or less, titanium (Ti): 0.1% or less and niobium (Nb): at least one of 0.1% or less, the balance Fe and unavoidable impurities, and a steel slab that satisfies the following relational expression 1 heating in a temperature range of 1300°C;
Manufacturing a hot-rolled steel sheet by finish hot-rolling the reheated steel slab at Ar3 or higher;
winding the hot-rolled steel sheet at a temperature of 700° C. or lower;
manufacturing a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a total reduction ratio of 30 to 80%;
Continuously annealing the cold-rolled steel sheet at Ac3 or higher for 30 seconds or more;
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;
Secondary cooling at an average cooling rate of 20 to 80 ° C / s to a temperature of Ms -190 ° C or less after the first cooling; and
Comprising the step of reheating after the secondary cooling and then overaging treatment,
The reheating and overaging step is a method for producing an ultra-high strength steel sheet having excellent bending properties, characterized in that heating to a temperature range satisfying the following relational expression 3.

[Relationship 1]

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

[Relationship 3]
CT2 + 30℃ ≤ A ≤ 270℃
(Here, CT2 means the secondary cooling end temperature (℃), and A means the reheating and overaging temperature (℃).)
According to claim 8,
The continuous annealing treatment step is a method for producing an ultra-high strength steel sheet having excellent bending properties that is performed in a continuous annealing furnace having a dew point temperature of 0 to 20 ° C.
According to claim 8,
The overaging treatment is a method for producing an ultra-high strength steel sheet having excellent bending properties, which is performed for 1 to 20 minutes.

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