KR20220086233A - High-strength steel sheet having excellent formability and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a steel sheet suitable as a material for automobiles, and more particularly, to a high-strength steel sheet having excellent formability and a method for manufacturing the same.

Description

High-strength steel sheet with excellent formability and manufacturing method thereof

The present invention relates to a steel sheet suitable as a material for automobiles, and more particularly, to a high-strength steel sheet having excellent formability and a method for manufacturing the same.

In recent years, for the purpose of reducing the weight of automobiles, securing of a technology capable of manufacturing a steel sheet having high strength is being promoted. Among them, high-strength steel sheet for cold forming with formability is excellent in terms of economical efficiency as productivity can be increased, and it is more advantageous in terms of safety of final parts. In particular, since a steel sheet having a high tensile strength (TS) has a high bearing load until fracture occurs, there is a growing demand for a steel material having a tensile strength of 980 MPa or higher.

Accordingly, various attempts have been made to improve the strength of the steel, but when simply improving the strength, the disadvantage of lowering the ductility and bending properties was found.

As a method for improving the formability of steel, a method for increasing the elongation is applied, and in particular, a method using the TRIP (TRansformation Induced Plasticity) phenomenon by introducing a retained austenite phase into the steel is widely used. However, in the case of TRIP steel, it is common to add a large amount of elements such as Si, Mn, and Al to the steel to introduce retained austenite. There is a problem of making plating properties inferior and causing plating peeling.

Korean Patent Publication No. 10-2020-0128159

One aspect of the present invention is to provide a steel sheet having high strength and high formability and a method for manufacturing the same.

The subject of the present invention is not limited to the above. Additional problems of the present invention are described in the overall content of the specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention, by weight, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 2.3%, manganese (Mn): 1.9 to 3.0%, aluminum (Al): 0.01 to 2.0% , phosphorus (P): 0.04% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), balance Fe and other unavoidable impurities Including, wherein C, Si and Al satisfy the following relation 1,

Formability in which the microstructure contains ferrite with an area fraction of 10% or less (excluding 0%), retained austenite of 5 to 15%, martensite of 5% or less (excluding 0%), and the remainder tempered martensite and bainite To provide this excellent high-strength steel sheet.

[Relational Expression 1]

2.7 ≤ (9.1×C) + Si + Al ≤ 4.5

(Here, each element means a weight content.)

Another aspect of the present invention comprises the steps of: preparing a steel slab that satisfies the above-described alloy composition and Relational Equation 1; heating the steel slab in a temperature range of 1150 to 1250 °C; manufacturing a hot-rolled steel sheet by finishing hot rolling the heated steel slab in a temperature range of 880 to 980°C; After the finish hot rolling, the step of rapidly cooling to 200 ℃ or less, and then winding; heat-treating the wound hot-rolled steel sheet at a heat treatment temperature (HT) ± 5° C. expressed by the following relational expression 2; manufacturing a cold-rolled steel sheet by cold-rolling the heat-treated hot-rolled steel sheet at a reduction ratio of 22% or less (excluding 0%); continuously annealing the cold-rolled steel sheet in a temperature range of 780 to 860°C; and cooling the continuously annealed cold-rolled steel sheet to a temperature range of 200 to 400° C. at a cooling rate of 10° C./s or more, and then maintaining it in a temperature range of 300 to 500° C. A manufacturing method is provided.

[Relational Expression 2]

Heat treatment temperature (HT, ℃) = (140×C) + (10×Al) + (3×Si) + 360

(Here, each element means a weight content.)

According to the present invention, it is possible to provide a steel sheet having high strength and high formability without excessively adding expensive elements.

The steel sheet of the present invention has the effect of being suitably applicable as an automobile material.

1 is a graph showing a change in elongation according to a cold reduction ratio according to an embodiment of the present invention.

The inventors of the present invention have studied in depth a method for providing a steel sheet having excellent plating properties while ensuring excellent formability by improving ductility as well as strength suitable as an automobile material.

As a result, it was confirmed that a steel sheet having an advantageous structure for securing target physical properties by optimizing the alloy composition system and manufacturing conditions can be provided, and the present invention has been completed.

Hereinafter, the present invention will be described in detail.

The high-strength steel sheet having excellent formability according to an aspect of the present invention is, by weight, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 2.3%, manganese (Mn): 1.9 to 3.0%, aluminum ( Al): 0.01 to 2.0%, phosphorus (P): 0.04% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less.

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

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

Carbon (C): 0.15 to 0.25%

Carbon (C) is a useful element for securing the strength of steel through solid solution strengthening and precipitation strengthening. If the content is less than 0.15%, it becomes difficult to secure not only the target level of strength but also high ductility. On the other hand, if the content exceeds 0.25%, arc weldability and laser weldability are deteriorated, the risk of welding cracks occurring due to low-temperature brittleness increases, and there is a problem in that the hole expandability is inferior.

Accordingly, the C may be included in an amount of 0.15 to 0.25%. More advantageously, C may be included in 0.17% or more, 0.19% or more, and may be included in 0.23% or less.

Silicon (Si): 0.3-2.3%

Silicon (Si) contributes to stabilization of retained austenite by suppressing precipitation of cementite in a region where bainite is formed, and plays a role in improving ductility of steel. When the content of Si is less than 0.3%, the retained austenite phase in the steel becomes insufficient, and there is a fear that the ductility may be lowered. On the other hand, when the content exceeds 2.3%, the physical properties of the welded portion are deteriorated due to the formation of LME cracks, and the surface properties and plating properties of the steel are deteriorated.

Accordingly, the Si may be included in an amount of 0.3 to 2.3%.

Manganese (Mn): 1.9~3.0%

Manganese (Mn) is an element added to secure the strength of steel, and if the content is less than 1.9%, it is difficult to secure the strength of the target level. On the other hand, when the content exceeds 3.0%, there is a problem in that the bainite transformation rate is slowed, so that a fresh martensite phase is excessively formed and the hole expandability is inferior.

Accordingly, the Mn may be included in an amount of 1.9 to 3.0%. More advantageously, it may be included in 2.0% or more, 2.1% or more, and may be included in 2.8% or less.

Aluminum (Al): 0.01~2.0%

Aluminum (Al) is an element added for deoxidation of steel, and is effective in stabilizing retained austenite by suppressing precipitation of cementite. If the content of Al is less than 0.01%, the deoxidation effect is insufficient and the cleanliness of the steel is impaired. On the other hand, when the content exceeds 2.0%, there is a problem in that the castability of the steel is deteriorated.

Accordingly, the Al may be included in an amount of 0.01 to 2.0%.

Phosphorus (P): 0.04% or less (excluding 0%)

Since phosphorus (P) is an impurity that is unavoidably contained in steel, it is advantageous to control the content as low as possible, but it is also intentionally added to increase the strength of the steel. However, since the toughness of steel deteriorates when the content of P is excessive, in the present invention, P is limited to 0.04% or less. More advantageously, it may be 0.02% or less, and even more advantageously it may be 0.015% or less, and 0% may be excluded in consideration of the unavoidably added level.

Sulfur (S): 0.01% or less (excluding 0%)

Since sulfur (S) is an impurity that is unavoidably contained in steel, it is advantageous to control the content as low as possible. In addition, when the content of S is excessive, there is a risk that the ductility and impact properties of steel may be inferior. In consideration of this, in the present invention, the S may be included in an amount of 0.01% or less. More advantageously, it may be included in an amount of 0.008% or less, and even more advantageously, 0.005% or less, and 0% may be excluded in consideration of the unavoidably added level.

Nitrogen (N): 0.01% or less (excluding 0%)

Nitrogen (N) is an impurity that is unavoidably contained in steel, and when its content exceeds 0.01%, it combines with Al in the steel to form AlN, thereby compromising the playing quality.

Therefore, the N may include 0.01% or less, more advantageously 0.007% or less, even more advantageously 0.005% 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 the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

In the steel sheet of the present invention having the above-described alloy composition, it is preferable that the content relation of C, Si and Al among the above-described alloy composition satisfies the following relational expression (1).

[Relational Expression 1]

2.7 ≤ (9.1×C) + Si + Al ≤ 4.5

(Here, each element means a weight content.)

In the alloy composition, both Si and Al are elements that suppress the precipitation of cementite and stabilize retained austenite by moving C to austenite with high solubility. If the value of the relationship between these contents (Relational Expression 1) is less than 2.7, it is difficult to secure high elongation and high hole expandability at the same time, and if the value exceeds 4.5, the surface properties and plating properties of the steel deteriorate.

Therefore, it is preferable that the value of Relation 1 representing the content relationship of Si, Al and C satisfies the range of 2.7 to 4.5.

The high-strength steel sheet having excellent formability according to the present invention may further include the following components in addition to the above-described components.

The steel sheet of the present invention includes at least one selected from the group consisting of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and chromium (Cr): 0.2% or less. may include

The Cu, Ni, Mo and Cr are elements that increase the strength of steel. The above elements are advantageous for increasing the strength and hardenability of steel, but when the content is excessive, the target strength is exceeded, and there is a problem in that the manufacturing cost is greatly increased with expensive elements. On the other hand, since Cu, Ni, Mo and Cr all act as solid solution strengthening elements, it is advantageous to contain each of 0.03% or more at the time of addition in order to sufficiently obtain the effect thereof.

In addition, the steel sheet may further include at least one of niobium (Nb), titanium (Ti), and vanadium (V) in an amount of 0.1% or less in total.

The Nb, Ti and V can improve the yield strength of steel even with a small amount of addition, but the effect is insignificant in improving ductility. have.

Furthermore, the steel sheet may further include boron (B) in an amount of 0.005% or less.

The boron (B) serves to increase the hardenability of the steel by segregation in an elemental state at grain boundaries in the steel. When the content of B exceeds 0.005%, BC precipitates are formed at the grain boundaries, and there is a problem in that hardenability is rather deteriorated.

Therefore, when the B is added, it may be included in an amount of 0.005% or less, and more advantageously, it may be included in an amount of 0.003% or less. However, in order to sufficiently obtain the effect of improving the hardenability by the addition of B, it may be included in an amount of 0.0008% or more, and more advantageously, it may be included in an amount of 0.0013% or more.

On the other hand, when adding B together with Ti, if the content of B is 0.0008% or more, it is advantageous to add the Ti in an amount of 0.015% or more. This is to sufficiently obtain the effect of B, and since the effect of B cannot be obtained when B is combined with N in steel and is lost to BN, it is advantageous to induce precipitation of TiN by adding Ti in a certain amount or more. do. However, when the content of Ti exceeds 0.04%, defects such as nozzle clogging may occur due to the formation of coarse TiN, thereby deteriorating continuous castability.

The steel sheet of the present invention satisfying the above-described alloy composition system may include tempered martensite and bainite phase as main phases, but may include ferrite, retained austenite phase, and martensite phase in a certain proportion.

The present invention may include a ferrite phase in an area fraction of 10% or less in order to obtain excellent hole expandability of the steel sheet. When the fraction of the ferrite exceeds 10%, there is a fear that the hole expandability is inferior. The minimum fraction of the ferrite phase is not particularly limited, but may be included in an amount of 3% or more in consideration of the aspect of securing ductility of the steel sheet.

The retained austenite phase is transformed into martensite during processing such as punching, and thus there is a concern that the hole expandability may be inferior. However, in order to secure the elongation, it is preferable to include 5% or more.

The martensite phase is an advantageous structure for improving strength, but when the fraction exceeds 5%, there is a risk that the hole expandability is rather inferior. Here, the martensite phase means a fresh martensite phase.

It is advantageous to include a tempered martensite phase and a bainite phase as the remaining structures other than the ferrite, retained austenite, and martensite phases. According to the present invention, by forming the tempered martensite phase and the bainite phase as main structures, it is possible to secure the elongation with a target level of strength.

In the present invention, the respective fractions of the tempered martensite phase and the bainite phase are not particularly limited, and in a state in which the above-described ferrite, retained austenite and martensite phases are formed in appropriate fractions, the remainder is the tempered martensite phase and the It turns out that there will be no difficulty in securing the intended physical properties in the present invention by being composed of the bainite phase.

The steel sheet of the present invention composed of the above-described microstructure has high strength and high formability, and specifically, it has a tensile strength of 980 MPa or more, a yield strength of 600 to 850 MPa, a hole expandability (HER) of 20% or more, and an elongation of 20% or more. can have

Meanwhile, the steel sheet of the present invention may be a cold rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plated layer on at least one surface of the cold rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

Although not particularly limited, the zinc-based plating layer may be a zinc plating layer mainly containing zinc or a zinc alloy plating layer containing aluminum and/or magnesium in addition to zinc.

Hereinafter, a method for manufacturing a high-strength steel sheet having excellent formability provided by the present invention, which is another aspect of the present invention, will be described in detail.

Briefly, the present invention can manufacture a desired steel sheet through the process of [steel slab reheating - hot rolling - winding - heat treatment - cold rolling - continuous annealing - cooling].

The conditions for each step will be described in detail below.

[Heating of steel slabs]

First, after preparing a steel slab that satisfies all of the above-described alloy composition systems, it can be heated. This process is performed in order to smoothly perform the subsequent hot rolling process and sufficiently obtain the target physical properties of the steel sheet.

The heating process may be performed in a temperature range of 1150 to 1250 °C. When the heating temperature is less than 1150° C., there is a problem in that the friction between the steel sheet and the rolling mill increases, so that the load applied to the roller during hot rolling rapidly increases. On the other hand, when the temperature exceeds 1250° C., not only the energy cost required for temperature increase increases, but also the amount of surface scale increases, which may lead to material loss.

Accordingly, the heating process may be performed in a temperature range of 1150 to 1250°C. More advantageously, it can be carried out at 1170°C or higher and 1180°C or higher, and can be carried out at 1230°C or lower and 1220°C or lower.

[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, the finish hot rolling may be performed in a temperature range of 880 to 980°C.

By performing the finish hot rolling in the above temperature range, the effect of simultaneously improving the rigidity and formability of the steel sheet can be obtained. However, if the temperature is less than 880 ℃, there is a problem in that the rolling load increases and the shape defect increases, so that the productivity deteriorates. On the other hand, if the temperature exceeds 980 ℃, the surface quality is inferior due to an increase in oxides due to excessively high temperature operation.

Therefore, the finish hot rolling during the hot rolling may be performed in a temperature range of 880 to 980 ℃. More advantageously, it can be carried out at 890°C or higher, and it can be carried out in a temperature range of 950°C or lower and 930°C or lower.

[Cooling and Winding]

The hot-rolled steel sheet manufactured according to the above may be wound, and at this time, it may be performed after rapid cooling to the coiling temperature. Preferably, after cooling to 200° C. or less at a cooling rate of 20 to 100° C./s, the winding process can be performed at that temperature.

That is, in the present invention, by performing the winding process in a relatively low temperature region, the martensite phase can be formed in the steel during the winding process. As such, the martensite phase formed in the winding process is transformed into fine needle-shaped austenite and ferrite in the subsequent annealing process, thereby playing an advantageous role in improving the formability of steel. In particular, through the winding process of the present invention, it is possible to secure the martensite phase in the hot-rolled steel sheet to an area fraction of 95% or more.

When the winding temperature exceeds 200° C., the martensite phase cannot be sufficiently formed, and thus the target formability cannot be improved. More advantageously, the winding process may be limited to Ms or less so that the martensite phase can be advantageously formed.

On the other hand, it is advantageous to carry out at a fast cooling rate when cooling to the coiling temperature after completing the above-described finish hot rolling, and preferably, it may be performed at a cooling rate of 20 to 100° C./s.

When the cooling rate during cooling is less than 20° C./s, the hot rolling productivity is lowered, and there is a disadvantage in that a cooling medium having a low cooling capacity must be intentionally adopted during actual production. On the other hand, when the cooling rate exceeds 100° C./s, the temperature deviation inside the steel is not uniform, resulting in poor shape and excessively high strength.

[Heat treatment]

According to the present invention, the hot-rolled steel sheet wound to form a columnar martensite in the steel can be heat-treated under specific conditions. This is to smoothly perform the subsequent cold rolling process, and the hot-rolled steel sheet wound according to the above has low cold rolling properties due to the presence of a large amount of martensite in the steel. Accordingly, as a softening process for subsequent cold rolling, it is preferable to heat-treat within the range of the heat treatment temperature (HT) ± 5°C expressed by the following relational formula (2).

If the heat treatment temperature (HT) during the heat treatment is less than -5°C, the softening of the hot-rolled tissue is insufficient and subsequent cold rolling becomes difficult, whereas when the temperature exceeds the heat treatment temperature (HT)+5°C, the martensitic structure is completely destroyed, There is a difficulty in securing the target level of ductility after annealing.

On the other hand, during the heat treatment of the hot-rolled steel sheet wound within the above-mentioned temperature range, the hot-rolled structure can be sufficiently softened to a level capable of subsequent cold rolling. make it clear Furthermore, a person skilled in the art will not have any particular difficulty in arbitrarily selecting the time.

[Relational Expression 2]

Heat treatment temperature (HT, ℃) = (140×C) + (10×Al) + (3×Si) + 360

(Here, each element means a weight content.)

In the present invention, the heat treatment process softens the hot-rolled martensite structure so that the subsequent cold rolling can be performed, but it is necessary to control the martensite main structure not to be completely destroyed.

The martensitic structure formed through hot rolling and low temperature winding is a supersaturated quenching structure of carbon (C), and the structure can be decomposed by the movement of C during heat treatment, and Al and Si in the steel affect the movement of carbon (C). Therefore, it is preferable to limit it to the temperature range according to Relational Expression 2 (±5° C.) due to the content relationship between them.

[Cold Rolling]

Thereafter, the hot-rolled steel sheet wound according to the above can be cold-rolled to manufacture a cold-rolled steel sheet, and in this case, it can be performed at a cold rolling reduction (CR%) of 22% or less.

In general, in cold rolling to obtain a cold rolled steel sheet, it is common to apply a cold reduction ratio exceeding 22%, but in the present invention, the martensitic structure formed in the winding process is not destroyed in the subsequent annealing process, and fine along the martensite interface Cold rolling is performed at a reduction ratio of 22% or less so that needle-shaped austenite and ferrite can be formed.

When the cold rolling reduction ratio exceeds 22%, the recrystallization driving force increases and the hot-rolled structure is destroyed, and finally, coarse austenite and ferrite are formed, resulting in poor formability.

In the present invention, the cold rolling is performed within the suggested cold rolling reduction ratio, and the lower limit can be arbitrarily set to obtain a cold rolled steel sheet of a desired thickness, which can be applied without particular difficulty to those skilled in the art.

[Annealing]

The cold-rolled steel sheet manufactured according to the above may be annealed, and as an example, may be performed by a continuous annealing process, but is not limited thereto, and any known annealing method may be used.

The annealing process is to increase the temperature of the steel sheet above the austenite transformation temperature to form an austenite phase in a sufficient fraction, and to cause carbon diffusion into the austenite.

The present invention can be carried out in the temperature range of 780 ~ 860 ℃ during annealing of the cold-rolled steel sheet. When the annealing temperature is less than 780° C., sufficient transformation into austenite cannot be achieved, and after completing the annealing, it is impossible to finally secure the martensite and bainite phases to the target level. On the other hand, when the temperature exceeds 860° C., productivity may decrease and coarse austenite may be formed to deteriorate the material. In addition, the size of retained austenite in the final structure also becomes coarse.

[Cooling and holding]

The cold-rolled steel sheet subjected to the continuous annealing treatment according to the above can be cooled, and at this time, it can be cooled to a temperature range where transformation of the bainite phase occurs, and then the holding process can be performed.

Preferably, the cooling may be maintained in a temperature range of 300 to 500° C. after cooling to a temperature range of 200 to 400° C. at a cooling rate of 10° C./s or more.

If the cooling temperature (cooling end temperature) is less than 200°C or exceeds 400°C, the amount of bainite transformation at the time the subsequent maintenance process is finished decreases, and a fresh martensite phase, which is unfavorable to securing elongation and hole expandability, is excessively formed there is a risk of becoming

When cooling to the above-mentioned temperature range, it is advantageous to perform the critical cooling rate to minimize the generation of ferrite, pearlite, etc. If the cooling rate is less than 10 °C / s, even when the target cooling temperature is reached, high-temperature phase transformation (eg, ferrite, pearlite, etc.) occurs during cooling, so that high-strength steel using the low-temperature transformation structure cannot be manufactured.

The cooling process after the continuous annealing may use a common quenching facility, but is not particularly limited, but as an example, it is pointed out that a quenching facility using mist or hydrogen may be used.

The cold-rolled steel sheet cooled to the above-mentioned temperature range can be maintained at a specific temperature to cause a target level of bainite phase transformation, and in this case, it can be maintained at a temperature slightly lower than the cooling temperature, or can be maintained after reheating to a rather high temperature.

When an excessively large amount of martensite phase is formed due to a small amount of phase transformation of bainite during maintenance of the cooled hot-rolled steel sheet, there is a problem in that elongation and hole expandability are increased.

In consideration of this, in the present invention, the holding process may be performed in a temperature range of 300 to 500°C. If the temperature is less than 300° C., the amount of cooling during cooling is excessive and the temperature deviation within the plate is increased, so that the shape may be deteriorated. In addition, the strength is excessively increased, so that it is impossible to secure a target level of elongation. On the other hand, when the temperature exceeds 500 ℃, the bainite phase transformation is slowed and finally, as martensite is excessively formed, elongation and hole expandability cannot be obtained.

When the temperature is maintained in the above-described temperature range, if the temperature is higher than Mf, tempering occurs, and from this, a tempered martensite structure may be included in the final structure.

The cold-rolled steel sheet obtained by completing a series of processes according to the above may be further subjected to a process of [hot-dip galvanizing-alloying heat treatment] if necessary.

First, a plated steel sheet having a plating layer on at least one surface can be manufactured by plating the cold-rolled steel sheet as described below.

[Hot dip galvanizing]

A hot-dip galvanized steel sheet may be manufactured by immersing the steel sheet manufactured through the above-described series of processes in a hot-dip galvanizing bath.

In this case, the hot-dip galvanizing may be performed under normal conditions, but for example, may be performed in a temperature range of 450 to 470°C. In addition, the composition of the hot-dip zinc-based plating bath is not particularly limited during the hot-dip galvanizing, and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, or the like.

[alloying heat treatment]

If necessary, an alloyed hot-dip galvanized steel sheet can be obtained by performing alloying heat treatment on the hot-dip galvanized steel sheet.

In the present invention, the alloying heat treatment process conditions are not particularly limited, and may be under normal conditions. As an example, the alloying heat treatment process may be performed in a temperature range of 480 to 540 °C.

Meanwhile, if necessary, after completing the alloying heat treatment, a temper rolling treatment may be further performed to correct the shape of the steel sheet and adjust the yield strength.

The temper rolling treatment may be performed by cooling the alloyed hot-dip galvanized steel sheet obtained by the alloying heat treatment to room temperature, and then performing at a reduction ratio of less than 1%.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

The steel slab having the alloy composition shown in Table 1 below was heated in a temperature range of 1150 to 1200 ° C, and then the [hot rolling - coiling - heat treatment - cold rolling - annealing - cooling and maintenance] process according to the conditions shown in Table 2 below. Through this process, each cold rolled steel sheet was manufactured.

The microstructure, physical properties, etc. were measured for the cold-rolled steel sheet manufactured as described above, and the results are shown in Table 3 below.

At this time, the microstructure of each steel sheet was measured by a point counting method after observing the tissue photograph with a scanning electron microscope (SEM). However, the fraction of the retained austenite phase was measured by XRD.

The tensile strength (TS), yield strength (YS), and elongation (El) of each steel sheet were evaluated through a tensile test in the direction perpendicular to rolling. was used.

Meanwhile, for the evaluation of the hole expandability of each steel sheet, it was measured according to the ISO 16330 standard, and the hole was evaluated by shearing it with a clearance of 12% using a punch with a diameter of 10 mm.

steel grade Alloy composition (weight $%) relation
One C Si Mn P S Al Mo Ti B N One 0.190 0.988 1.96 0.011 0.0022 0.039 0.102 0.027 0.0015 0.006 2.76 2 0.228 0.630 2.00 0.012 0.0022 0.40 0.11 0.024 0.0017 0.005 3.10 3 0.221 0.710 2.75 0.011 0.0020 0.70 0.20 0.021 0 0.005 3.42 4 0.151 1.250 2.40 0.011 0.0020 0.03 0.10 0.024 0 0.006 2.65

steel grade rolled and wound heat treatment cold rolled continuous annealing Cooling maintain division FDT
(℃) CT
(℃) temperature
(HT,℃) hour
(Hr) reduction rate
(%) temperature
(℃) end temperature
(℃) speed
(℃/s) temperature
(℃) One 940 130 393 5 10 800 300 11 390 Invention Example 1 One 940 160 393 5 10 810 300 5 390 Comparative Example 1 One 940 120 393 5 48 805 300 10 390 Comparative Example 2 One 940 600 - - 45 820 400 10 390 Comparative Example 3 2 920 110 398 5 11 840 350 11 350 Invention Example 2 2 920 100 398 5 21 840 340 11 340 Invention example 3 2 920 110 430 5 20 830 310 11 300 Comparative Example 4 2 920 50 400 5 20 850 510 11 440 Comparative Example 5 3 940 98 400 5 15 805 350 12 450 Invention Example 4 4 940 600 - - 48 810 400 8 390 Comparative Example 6

division Microstructure (fraction%) mechanical properties F R-A M balance YS(MPa) TS(MPa) El (%) HER(%) Invention Example 1 8 10 4 T-M and B 767 1018 23 29 Comparative Example 1 13 2 17 T-M and B 510 1146 14 8 Comparative Example 2 12 3 11 T-M and B 574 1178 15 11 Comparative Example 3 45 One 12 T-M and B 511 950 19 10 Invention Example 2 7 6 3 T-M and B 655 1068 21 25 Invention example 3 8 7 4 T-M and B 683 1079 21 26 Comparative Example 4 15 2 16 T-M and B 561 1132 14 11 Comparative Example 5 11 One 13 T-M and B 779 1162 12 28 Invention Example 4 7 8 3 T-M and B 720 1035 23 26 Comparative Example 6 35 4 12 T-M and B 430 840 21 20 F: ferrite, RA: retained austenite, M: martensite, TM: tempered martensite, B: bainite
YS: Yield strength, TS: Tensile strength, El: Elongation, HER: Hole expansion rate

As shown in Tables 1 to 3, Inventive Examples 1 to 4, which satisfy both the alloy component system and manufacturing conditions proposed in the present invention, have high strength by forming a microstructure as intended, as well as ductility and holes It can be seen that it has excellent extensibility and high formability.

On the other hand, Comparative Example 1, in which the cooling rate was applied slowly at 5° C./s during cooling after annealing, finally had an excessive ferrite phase, so the yield strength was inferior, and it was difficult to secure ductility and hole expandability.

Comparative Example 2 was a case in which the reduction ratio during cold rolling was excessive. Coarse ferrite was excessively formed, so that the yield strength was inferior, and ductility and hole expandability could not be secured.

Comparative Example 3 was a case in which the conventional winding process was applied and the heat treatment process of the present invention was not applied, and the ductility and hole expandability were inferior because the retained austenite phase was not properly formed in the final structure.

In Comparative Example 4, the heat treatment temperature during the heat treatment after winding was higher than the level suggested in the present invention, and the retained austenite phase in the final structure was insufficient, resulting in inferior ductility and hole expandability.

Comparative Example 5 was a case in which the cooling termination temperature was excessively high during cooling after annealing, and finally, the bainite phase was not sufficiently formed, and thus the ductility was inferior.

Comparative Example 6 was a case in which a conventional winding process was applied, the heat treatment process of the present invention was not applied, and the reduction ratio during cold rolling was excessive, and the ferrite phase was excessively formed, so that the target level of strength could not be secured.

1 is a graph showing the change in elongation according to the cold rolling reduction.

As shown in FIG. 1 , it can be confirmed that when the cold rolling reduction is applied to 22% or less, it is possible to secure an elongation of 20% or more.

Claims (12)

By weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 2.3%, manganese (Mn): 1.9 to 3.0%, aluminum (Al): 0.01 to 2.0%, phosphorus (P): 0.04 % or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), the remainder including Fe and other unavoidable impurities, the above C, Si And Al satisfies the following relation 1,
Formability in which the microstructure contains ferrite with an area fraction of 10% or less (excluding 0%), retained austenite of 5 to 15%, martensite of 5% or less (excluding 0%), and the remainder tempered martensite and bainite This excellent high-strength steel plate.

[Relational Expression 1]
2.7 ≤ (9.1×C) + Si + Al ≤ 4.5
(Here, each element means a weight content.)
The method of claim 1,
The steel sheet includes at least one selected from the group consisting of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and chromium (Cr): 0.2% or less by weight in weight% A high-strength steel sheet with excellent formability.
The method of claim 1,
The steel sheet is a high-strength steel sheet with excellent formability, further comprising at least one of niobium (Nb), titanium (Ti) and vanadium (V) in an amount of 0.1% or less by weight.
The method of claim 1,
The steel sheet is a high-strength steel sheet with excellent formability further comprising boron (B): 0.005% or less by weight%.
The method of claim 1,
The steel sheet is a high-strength steel sheet having excellent formability having a tensile strength of 980 MPa or more, a yield strength of 600 to 850 MPa, a hole expandability (HER) of 20% or more, and an elongation of 20% or more.
The method of claim 1,
The steel sheet is a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a high-strength steel sheet with excellent formability, which is one of alloyed hot-dip galvanized steel sheet.
By weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 2.3%, manganese (Mn): 1.9 to 3.0%, aluminum (Al): 0.01 to 2.0%, phosphorus (P): 0.04 % or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), the remainder including Fe and other unavoidable impurities, the above C, Si and Al preparing a steel slab that satisfies the following Relation 1;
heating the steel slab in a temperature range of 1150 to 1250 °C;
manufacturing a hot-rolled steel sheet by finishing hot rolling the heated steel slab in a temperature range of 880 to 980°C;
After the finish hot rolling, the step of rapidly cooling to 200 ℃ or less, and then winding;
heat-treating the wound hot-rolled steel sheet at a heat treatment temperature (HT) ± 5° C. expressed by the following relational expression 2;
manufacturing a cold-rolled steel sheet by cold-rolling the heat-treated hot-rolled steel sheet at a reduction ratio of 22% or less (excluding 0%);
continuous annealing of the cold-rolled steel sheet in a temperature range of 780 to 860°C; and
After cooling the cold-rolled steel sheet subjected to continuous annealing to a temperature range of 200 to 400° C. at a cooling rate of 10° C./s or more, and maintaining it in a temperature range of 300 to 500° C. Way.

[Relational Expression 1]
2.7 ≤ (9.1×C) + Si + Al ≤ 4.5
(Here, each element means a weight content.)

[Relational Expression 2]
Heat treatment temperature (HT, ℃) = (140×C) + (10×Al) + (3×Si) + 360
(Here, each element means a weight content.)
8. The method of claim 7,
A method of manufacturing a high-strength steel sheet having excellent formability, wherein the rapid cooling after the finish hot rolling is performed at a cooling rate of 20 to 100° C./s.
8. The method of claim 7,
The method of manufacturing a high-strength steel sheet having excellent formability, further comprising the step of immersing the cooled and maintained cold-rolled steel sheet in a zinc-based plating bath to perform hot-dip galvanizing.
8. The method of claim 7,
The steel slab is at least one selected from the group consisting of copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.3% or less, and chromium (Cr): 0.2% or less by weight% by weight A method of manufacturing a high-strength steel sheet with excellent formability further comprising a.
8. The method of claim 7,
The method of manufacturing a high-strength steel sheet with excellent formability, wherein the steel slab further contains 0.1% or less of one or more of niobium (Nb), titanium (Ti), and vanadium (V) by weight.
8. The method of claim 7,
The steel slab is boron (B): 0.005% or less in weight %.

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