KR20230082600A - 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 manufacturing method thereof.

Description

High-strength steel sheet with excellent formability and its manufacturing method {HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT FORMABILITY AND METHOD FOR MANUFACTURING 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 manufacturing method thereof.

Recently, securing a technology capable of manufacturing a steel sheet having high strength has been promoted in order to reduce the weight of automobiles. Among them, in the case of high-strength steel sheet for cold forming that combines formability, productivity can be increased, which is excellent in terms of economy, and 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 supporting load until fracture occurs, a demand for a steel material having a high tensile strength of 980 MPa or more is increasing.

Accordingly, various attempts have been made to improve the strength of steel, but it has been found that ductility and bending properties are degraded when simply improving the strength.

As a method for improving the formability of steel materials, a method for increasing elongation is applied, and in particular, a method using a TRansformation Induced Plasticity (TRIP) phenomenon by introducing a retained austenite phase into 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 in that the plating property is deteriorated and the plating is peeled off.

Korean Patent Publication No. 10-2020-0128159

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

The object of the present invention is not limited to the above. Additional tasks of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional tasks of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention, in weight percent, carbon (C): 0.15 ~ 0.25%, silicon (Si): 0.3 ~ 2.3%, manganese (Mn): 1.9 ~ 3.0%, aluminum (Al): 0.01 ~ 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, the C, Si and Al satisfy the following relational expression 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 with an area fraction of 5% or less (excluding 0%), and the balance of tempered martensite and bainite This excellent high-strength steel sheet is provided.

[Relationship 1]

2.7 ≤ (9.1 × C) + Si + Al ≤ 4.5

(Here, each element means a weight content.)

Another aspect of the present invention, preparing a steel slab that satisfies the above-described alloy composition and relational expression 1; heating the steel slab in a temperature range of 1150 to 1250 °C; manufacturing a hot-rolled steel sheet by finish hot-rolling the heated steel slab at a temperature range of 880 to 980° C.; After the finish hot rolling, rapid cooling to 200 ° C. or less, and then winding; Heat-treating the coiled 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 the temperature range of 300 to 500° C. A manufacturing method is provided.

[Relationship 2]

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

(Here, each element means a weight content.)

According to the present invention, a steel sheet having high strength and high formability can be provided without excessively adding expensive elements.

The steel sheet of the present invention has an effect that can be suitably applied as an automobile material.

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

The inventors of the present invention have studied in depth a method capable of providing a steel sheet having excellent plating properties while securing 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 a structure advantageous to securing target physical properties could 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.

The high-strength steel sheet with excellent formability according to one aspect of the present invention contains, 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, 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): 0.15 to 0.25%

Carbon (C) is a useful element for securing the strength of steel through solid solution hardening and precipitation hardening. On the other hand, when the content exceeds 0.25%, there are problems in that arc weldability and laser weldability deteriorate, the risk of weld cracks due to low temperature brittleness increases, and hole expandability deteriorates.

Therefore, the C may be included in 0.15 ~ 0.25%. More advantageously, the C may be included at 0.17% or more, 0.19% or more, and may be included at 0.23% or less.

Silicon (Si): 0.3 to 2.3%

Silicon (Si) contributes to the stabilization of retained austenite by suppressing the precipitation of cementite in the region where bainite is formed, and serves to contribute to the improvement of ductility of steel. If the content of Si is less than 0.3%, there is a concern that the retained austenite phase in the steel is insufficient and the ductility is lowered. On the other hand, when the content exceeds 2.3%, the physical properties of the welded part 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 to 3.0%

Manganese (Mn) is an element added to secure strength of steel, and if its content is less than 1.9%, it is difficult to secure a target level of strength. On the other hand, when the content exceeds 3.0%, the bainite transformation rate slows down, resulting in excessive formation of fresh martensite phase, resulting in poor hole expandability.

Therefore, the Mn may be included in 1.9 to 3.0%. More advantageously, it may be included at 2.0% or more, 2.1% or more, and may be included at 2.8% or less.

Aluminum (Al): 0.01 to 2.0%

Aluminum (Al) is an element added for deoxidation of steel, and is effective in stabilizing retained austenite by suppressing the 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 lowered.

Therefore, the Al may be included in 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 steel. However, since the toughness of steel deteriorates when the content of P is excessive, in the present invention, the P is limited to 0.04% or less. More advantageously, it may be 0.02% or less, and even more advantageously, 0.015% or less, and 0% may be excluded in consideration of an inevitably 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 concern that the ductility and impact properties of the steel may be inferior. Considering this, in the present invention, the S may be included at 0.01% or less. More advantageously, 0.008% or less, and even more advantageously, 0.005% or less may be included, and 0% may be excluded in consideration of an unavoidably added level.

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

Nitrogen (N) is an impurity inevitably contained in steel, and when its content exceeds 0.01%, it combines with Al in the steel to form AlN, which may impair performance quality.

Therefore, the N may be included in 0.01% or less, more advantageously 0.007% or less, and 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 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 having the above-described alloy composition, it is preferable that the relation between the contents of C, Si, and Al in the above-described alloy composition satisfies the following relational expression 1.

[Relationship 1]

2.7 ≤ (9.1 × C) + Si + Al ≤ 4.5

(Here, each element means a weight content.)

Among the alloy compositions, both Si and Al are elements that suppress the precipitation of cementite and stabilize retained austenite by moving C to austenite having a high solid 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 relational expression 1 representing the relationship between the contents 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 components.

The steel sheet of the present invention further contains 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. can include

The Cu, Ni, Mo and Cr are elements that increase the strength of steel. The above elements are advantageous in increasing the strength and hardenability of steel, but when the content thereof is excessive, the target strength is exceeded, and manufacturing costs are greatly increased due to expensive elements. On the other hand, since the Cu, Ni, Mo, and Cr all act as solid solution strengthening elements, it is advantageous to contain them at 0.03% or more when added in order to sufficiently obtain their effects.

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

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

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

The boron (B) serves to increase the hardenability of the steel by segregating in an elemental state at the grain boundaries in the steel. When the content of B exceeds 0.005%, BC precipitates are formed at the grain boundaries, resulting in poor hardenability.

Therefore, when the B is added, it may be included in 0.005% or less, more advantageously, it may be included in 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, more advantageously, it may be included in an amount of 0.0013% or more.

On the other hand, in the case of complex addition of B together with the Ti, when 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 when the B is combined with N in the steel and lost to BN, the effect of B cannot be obtained. Therefore, it is advantageous to induce precipitation of TiN by adding Ti in a certain amount or more do. However, if the content of Ti exceeds 0.04% at this time, defects such as clogging of nozzles may occur due to the formation of coarse TiN, resulting in deterioration in continuous castability.

The steel sheet of the present invention that satisfies the above-described alloy component system includes tempered martensite and bainite phase as main phases, but may include ferrite, retained austenite phase, and martensite phase at a predetermined fraction.

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

Since the retained austenite phase transforms into martensite during processing such as punching, and there is a concern that hole expandability may be deteriorated due to this, the retained austenite phase may be controlled to 15% or less. However, in order to secure the elongation rate, it is preferable to include it at 5% or more.

The martensite phase is an advantageous structure for improving strength, but when its fraction exceeds 5%, there is a concern that hole expandability may be 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 structure other than the ferrite, retained austenite, and martensite phase. In the present invention, by forming the tempered martensite phase and the bainite phase as main structures, it is possible to secure a target level of strength and elongation.

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 phase are formed in appropriate fractions, the remainder is the tempered martensite phase and By being composed of bainite phase, it is revealed that there will be no difficulty in securing the intended physical properties in the present invention.

The steel sheet of the present invention composed of the above-described microstructure is characterized by high strength and high formability, and specifically, 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, and may be a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, and 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 excellent in 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].

Conditions for each step are described in detail below.

[Heating of steel slabs]

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

The heating process may be performed in a temperature range of 1150 to 1250 °C. If the heating temperature is less than 1150 ° C., there is a problem in that the load applied to the roller increases rapidly during hot rolling due to increased friction between the steel sheet and the rolling mill. On the other hand, when the temperature exceeds 1250 ° C., energy costs required for temperature rise increase, and the amount of surface scale increases, which may lead to material loss.

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

By performing finish hot rolling in the above-mentioned 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 ° C., the rolling load increases and shape defects increase, resulting in poor productivity. On the other hand, when the temperature exceeds 980 ° C., the surface quality deteriorates due to the increase in oxide due to excessively high temperature work.

Therefore, during the hot rolling, the finish hot rolling may be performed in a temperature range of 880 to 980 °C. More advantageously, it can be carried out at 890 ° C or higher, and 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 rapidly cooling to the coiling temperature. Preferably, after cooling to 200 ° C or less at a cooling rate of 20 ~ 100 ° C / s, the winding process may be performed at that temperature.

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

When the coiling temperature exceeds 200° C., the martensite phase is not sufficiently formed, and therefore, it is impossible to achieve a target formability improvement. More advantageously, the winding process can be limited to Ms or less so that the martensite phase can be advantageously formed.

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

If the cooling rate during the 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, there is a problem that the temperature deviation inside the steel is not uniform, and the shape is deteriorated and the strength is excessively increased.

[Heat treatment]

According to the present invention, the coiled hot-rolled steel sheet may be heat-treated under specific conditions so that martensite is formed in a columnar shape in the steel according to the above. 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. Therefore, as a softening process for subsequent cold rolling, heat treatment is preferably performed in the heat treatment temperature (HT) ± 5 ° C range represented by the following relational expression 2.

If the heat treatment temperature (HT) is less than -5 ° C during the heat treatment, the softening of the hot-rolled structure is insufficient, making subsequent cold rolling difficult. It is difficult to secure 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 that allows subsequent cold rolling. The time is not particularly limited, but it can be performed for up to 5 hours. let it out Furthermore, those skilled in the art will not have any particular difficulty in arbitrarily selecting the time.

[Relationship 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 cold rolling, which is a subsequent process, needs to be controlled so that the martensitic main structure is not completely destroyed.

The martensitic structure formed through hot-rolled low-temperature coiling 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 steel affect this movement of carbon (C). Therefore, it is preferable to limit it to the temperature range according to the relational expression 2 (±5 ° C) due to the content relationship between them.

[Cold Rolling]

Thereafter, the cold-rolled steel sheet may be manufactured by cold-rolling the hot-rolled steel sheet wound according to the above, and at this time, the cold-rolled steel sheet may be performed at a cold rolling reduction ratio (CR%) of 22% or less.

In general, cold rolling to obtain a cold-rolled steel sheet generally applies a cold rolling reduction of more than 22%, but in the present invention, the martensitic structure formed in the coiling process is not destroyed in the subsequent annealing process, and fine grains along the martensitic interface It is characterized by performing cold rolling at a reduction ratio of 22% or less so that acicular austenite and ferrite can be formed.

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

In the present invention, the cold rolling may be performed within the proposed cold rolling reduction ratio, and the lower limit may be arbitrarily set so as to obtain a cold-rolled steel sheet having a desired thickness, which is applicable 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 one example, it may be performed in a continuous annealing process (Continuous Annealing Process), but is not limited thereto, and any of the known annealing methods may be used.

The purpose of the annealing process is to raise 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. If the annealing temperature is less than 780 ° C., sufficient transformation to austenite is not achieved, and thus martensite and bainite phases cannot be finally secured at target levels after annealing is completed. On the other hand, when the temperature exceeds 860 ° C., productivity may decrease and coarse austenite may be formed, resulting in material deterioration. 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 process according to the above may be cooled, and at this time, after cooling to a temperature range in which transformation of the bainite phase occurs, a holding process may be performed.

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

When the cooling temperature (cooling end temperature) is less than 200 ° C or exceeds 400 ° C, the amount of bainite transformation is reduced at the end of the subsequent holding process, resulting in excessive formation of fresh martensite phase, which is disadvantageous in securing elongation and hole expandability. There is a risk of becoming

When cooling to the above-described temperature range, it is advantageous to perform the cooling at a critical cooling rate capable of minimizing the generation of ferrite and pearlite in the cooling process, and in the present invention, it is preferable to perform at least 10° C./s. If the cooling rate is less than 10 ° C / s, even if the target cooling temperature is reached, high-temperature phase transformation (eg, ferrite, pearlite, etc.) occurs during cooling, making it impossible to manufacture high-strength steel using a low-temperature transformation structure.

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

The cold-rolled steel sheet cooled to the above-described temperature range may be maintained at a specific temperature to cause bainite phase transformation at a target level, and at this time, it may be maintained at a temperature slightly lower than the cooling temperature or after reheating to a slightly higher temperature.

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

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

When maintained in the above-described temperature range, tempering occurs when the temperature exceeds Mf, and from this, a tempered martensitic 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 zinc-based plating bath.

At this time, the hot-dip galvanizing may be performed under normal conditions, but may be performed in a temperature range of 450 to 470 ° C as an example. 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, and the like.

[Alloy heat treatment]

If necessary, an alloyed hot-dip galvanized steel sheet may be obtained by subjecting the hot-dip galvanized steel sheet to alloying heat treatment.

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

On the other hand, if necessary, after completing the alloying heat treatment, 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 at a reduction ratio of less than 1% after cooling the alloyed hot-dip galvanized steel sheet obtained by alloying heat treatment to room temperature.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating 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 the matters reasonably inferred therefrom.

(Example)

After heating the steel slabs having the alloy compositions shown in Table 1 in the temperature range of 1150 to 1200 ° C, according to the conditions shown in Table 2 below, [hot rolling - winding - heat treatment - cold rolling - annealing - cooling and holding] process After that, each cold-rolled steel sheet was manufactured.

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 plate was measured by a point counting method after observing a photograph of the tissue 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 plate were evaluated through a tensile test in the direction perpendicular to rolling, and the gauge length is 50 mm, and the width of the tensile specimen is 25 mm. was used.

On the other hand, to evaluate the hole expandability of each steel plate, it was measured according to the ISO 16330 standard, and at this time, the hole was evaluated by shearing with a clearance of 12% using a punch with a diameter of 10 mm.

steel grade Alloy Composition (wt$%) relational expression
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 Inventive 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 Inventive 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 Inventive 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 Inventive 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

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

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

In Comparative Example 2, the reduction ratio was excessive during cold rolling, and coarse ferrite was excessively formed, resulting in poor yield strength, and ductility and hole expandability could not be secured.

Comparative Example 3 is a case in which a conventional coiling process is applied and the heat treatment process of the present invention is not applied, and ductility and hole expandability are inferior due to a retained austenite phase not being properly formed in the final structure.

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

In Comparative Example 5, the cooling end temperature during cooling after annealing was excessively high, and finally, the bainite phase was not sufficiently formed, resulting in poor ductility.

Comparative Example 6 is a case in which a normal winding process is applied, the heat treatment process of the present invention is not applied, and the reduction ratio during cold rolling is excessive, and the target level of strength cannot be secured because the ferrite phase is excessively formed.

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 an elongation of 20% or more can be secured when the cold rolling reduction is applied at 22% or less.

Claims (6)

In % 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 balance including Fe and other unavoidable impurities, and the above C, Si And Al satisfies the following relational expression 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 with an area fraction of 5% or less (excluding 0%), and the balance of tempered martensite and bainite This excellent high-strength steel plate.

[Relationship 1]
2.7 ≤ (9.1 × C) + Si + Al ≤ 4.5
(Here, each element means a weight content.)
According to claim 1,
The steel sheet contains 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%. High-strength steel sheet with excellent formability further comprising.
According to 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 a total content of 0.1% or less in weight%.
According to claim 1,
The steel sheet is a high-strength steel sheet with excellent formability further comprising boron (B): 0.005% or less in weight%.
According to 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.
According to claim 1,
The steel sheet is a high-strength steel sheet having excellent formability, which is one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.

Images