JP7698043B2 - Ultra-high strength steel plate with excellent ductility and manufacturing method thereof - Google Patents

Description

The present invention relates to a steel sheet suitable as a material for automobiles, and more specifically, to an ultra-high strength steel sheet with excellent ductility.

Recently, the automobile industry has been demanding the use of high-strength steel sheets to improve fuel efficiency and durability in response to various environmental and energy use regulations.

However, it was discovered that when the strength of steel plate is increased, the ductility decreases relatively, and so much research has been carried out to improve the relationship between strength and ductility. As a result, transformation structure steels that utilize the low-temperature structures of martensite and bainite as well as the retained austenite phase have been developed and applied.

Transformation structure steels are divided into ferrite-martensite dual phase (DP) steels, which have a hard martensite phase formed in the ferrite matrix; TRIP (Transformation Induced Plasticity) steels, which use the transformation-induced plasticity of retained austenite; and CP (Complexed Phase) steels, which are composed of ferrite and hard bainite or martensite structures. Each of these steels has different mechanical properties, i.e., tensile strength and elongation, depending on the type and fraction of the parent phase and second phase.

In particular, TRIP steel, which contains a large amount of retained austenite, has the highest balance of tensile strength and elongation (TS x El).

As an example, Patent Document 1 discloses a steel that contains about 10% retained austenite phase in addition to ferrite and martensite, has a product of tensile strength and elongation of 21,000 MPa% or more, and can ensure a tensile strength of 780 MPa or more. However, this steel contains large amounts of carbon (C) of about 0.2% and silicon (Si) of about 1.5% or more, which may result in poor spot weldability and hot-dip galvanization properties. In addition, annealing is performed twice to achieve high physical properties, which raises the problem of increased manufacturing costs for the steel sheet.

Meanwhile, Patent Document 2 discloses a technology that reduces the Si content to the 1% level to ensure good plating and spot weldability, and that allows the microstructure to be composed of martensite, bainite, and ferrite without the presence of residual austenite phase, ensuring a tensile strength of 980 MPa or more and an elongation of 15% or more. However, recently, regulations on the impact stability of automobiles have been expanded, and high-strength steel with excellent yield strength is being used for structural members such as members, seat rails, and pillars to improve the impact resistance of the vehicle body. However, the yield strength of this steel is 700 MPa or less, so there are limitations to the applications.

Korean Patent Publication No. 2015-0130612 Korean Patent Publication No. 2013-0106142

One aspect of the present invention is to provide a steel sheet suitable for use in automotive structural components, which has excellent tensile strength as well as yield strength and improved ductility, and a method for manufacturing the same.

The object of the present invention is not limited to the above. The object of the present invention can be understood from the entire contents of this specification, and a person having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding the further object of the present invention.

One aspect of the present invention provides an ultra-high strength steel sheet with excellent ductility, containing, by weight percent, carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, the balance being Fe and other unavoidable impurities, and satisfying the following relational expressions 1 to 3.

[Relationship 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380

[Relationship 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300

[Relationship 3]
-29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≧-24
(In Relational Formulas 1 to 3, each element represents its weight content.)

Another aspect of the present invention is a method for producing a hot-rolled steel sheet by preparing a steel slab that satisfies the above-mentioned alloy composition and relational expressions 1 to 3, heating the steel slab at a temperature range of 1050 to 1300°C, hot-rolling the heated steel slab at a temperature range of 800 to 1000°C to produce a hot-rolled steel sheet, coiling the hot-rolled steel sheet at a temperature range of 400 to 700°C, and cold-rolling the coiled hot-rolled steel sheet at a total reduction rate of 20 to 70% to produce a cold-rolled steel sheet. The present invention provides a method for manufacturing an ultra-high strength steel sheet having excellent ductility, which includes a step of annealing the cold-rolled steel sheet in a temperature range of 800 to 900°C, a step of cooling the continuously annealed cold-rolled steel sheet to a temperature range of 250 to 400°C, and a step of reheating and maintaining the cooled cold-rolled steel sheet, the reheating and maintaining being performed for 0.1 to 60 minutes in a temperature range of the cooled temperature +50°C to the cooled temperature +200°C.

According to the present invention, it is possible to provide a steel sheet that has excellent tensile strength as well as yield strength and improved ductility. Such a steel sheet of the present invention has the advantage that it guarantees the formability and collision stability required for steel sheets for cold forming.

1 shows a photograph of the microstructure of an inventive steel according to an embodiment of the present invention, measured by SEM. 1 shows a photograph of the microstructure of a comparative steel according to an embodiment of the present invention, measured by SEM.

The inventors of the present invention have conducted extensive research to provide a steel sheet that is suitable for use as an automobile material, has excellent tensile strength, ductility, and yield strength, and also has guaranteed formability and crashworthiness, making it applicable to structural components that require processing into complex shapes.

As a result, we confirmed that by optimizing the alloy composition and manufacturing conditions, we could provide steel sheets with a structure that is advantageous for ensuring the target physical properties, which led to the completion of this invention.

In particular, the present invention is characterized by providing a steel sheet having a composite structure in which soft and hard phases are appropriately dispersed by controlling the content relationship of specific elements among the alloy components and optimizing the process conditions of the steel sheet manufactured through a series of processes.

The present invention will be described in detail below.

The ultra-high strength steel plate with excellent ductility according to one embodiment of the present invention can contain, by weight, carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, and nitrogen (N): 0.02% or less.

The reasons for restricting the alloy composition of the steel plate provided in this invention as described above are explained in detail below.

Unless otherwise specified in the present invention, the content of each element is based on weight, and the proportion of the structure is based on area.

Carbon (C): 0.1-0.2%
Carbon (C) is an element that greatly contributes to strengthening the strength of steel sheet, and the C precipitates in the crystal grains of the steel sheet to induce solid solution strengthening and promote the formation of martensite in the steel, thereby strengthening the steel. In addition, the C is an austenite stabilizing element and plays an important role in the formation of retained austenite. Specifically, the more the amount of carbon (C) that dissolves in austenite increases, the higher the austenite stability becomes, and the higher the fraction of austenite in the steel becomes. This induces an increase in the fraction of martensite formed by the transformation of the austenite, which has the effect of improving the strength of the steel sheet, and some austenite remains at room temperature as retained austenite.

To fully obtain the above-mentioned effects, 0.1% or more of C can be added, but if the content exceeds 0.2%, the proportion of martensite phase increases excessively and the proportion of ferrite phase, which has relatively excellent elongation and impact absorption energy, decreases. This reduces the ductility of the steel sheet and increases the possibility of brittleness occurring.

Therefore, the above C can be contained at 0.1 to 0.2%, and more preferably at 0.12% or more and 0.18% or less.

Silicon (Si): 0.1-1.0%
Silicon (Si) is an element that suppresses the precipitation of carbides in ferrite, induces the diffusion of carbon in ferrite into austenite, and contributes to stabilizing retained austenite.

To obtain the above-mentioned effects, it is advantageous to contain 0.1% or more of Si, but if the content exceeds 1.0%, Si oxides are formed on the steel surface, which may hinder the effects of hot-dip galvanizing and chemical conversion coating.

Therefore, the above-mentioned Si content can be 0.1 to 1.0%, more preferably 0.2% or more, and even more preferably 0.4% or more. On the other hand, the above-mentioned Si content can more preferably be 0.9% or less.

Manganese (Mn): 2.0-3.0%
Manganese (Mn) can act as an austenite stabilizing element, similar to C. Specifically, Mn can reduce the critical cooling rate at which martensite is formed in a dual-phase steel, and contribute to increasing the fraction of martensite in the steel.

To fully obtain the above-mentioned effects, it is advantageous to contain 2.0% or more Mn, but if the content exceeds 3.0%, the weldability of the steel sheet may decrease and the hot rolling properties may deteriorate. In addition, there is a problem that striped bands called Mn-Bands are formed, which impair formability and increase the risk of processing cracks.

Therefore, the Mn content can be 2.0 to 3.0%, and more preferably 2.2% or more and 2.8% or less.

Aluminum (Al): 1.0% or less Aluminum (Al) is an element added for deoxidizing steel, and is a ferrite stabilizing element like Si. The above-mentioned Al is effective in distributing carbon in ferrite to austenite to improve martensite hardening ability, and is an element that effectively suppresses the precipitation of carbides in bainite when the bainite region is maintained, and is useful for improving the ductility of the steel sheet.

If the Al content exceeds 1.0%, continuous castability during continuous casting operations in steelmaking will decrease, and excessive inclusions will form, increasing the possibility of poor material properties in the annealed material.

Therefore, the above Al can be contained in an amount of 1.0% or less, excluding 0%. More preferably, the above Al can be contained in an amount of 0.01% or more.

In the present invention, Al means soluble aluminum (Sol.Al).

Chromium (Cr): 1.0% or less Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength, and plays an important role in the formation of martensite. It also minimizes the decrease in elongation rate with increasing strength, which is advantageous for manufacturing dual-phase steel with high ductility.

If the Cr content exceeds 1.0%, not only will the above-mentioned effects saturate, but there will also be problems with the hot-rolled strength increasing excessively, resulting in poor cold-rolling properties, and with the martensite fraction increasing significantly after annealing, resulting in reduced elongation.

Therefore, it is clear that the above Cr can be contained in an amount of 1.0% or less, and that it is not difficult to ensure the intended physical properties even if the above Cr is not intentionally added.

Molybdenum (Mo): 0.5% or less Molybdenum (Mo) is an element that forms carbides in steel and can contribute to improving yield strength and tensile strength by forming fine carbides in steel by combining with Ti, Nb, etc. in the steel. If the Mo content exceeds 0.5%, there is a problem that the elongation of the steel decreases and the manufacturing cost increases.

Therefore, it is clear that the above Mo can be contained in an amount of 0.5% or less, and that it is not difficult to ensure the intended physical properties even if the above Mo is not intentionally added.

Titanium (Ti): 0.1% or less Titanium (Ti) forms fine carbides in steel, similar to Mo, and can contribute to ensuring the yield strength and tensile strength of steel. In addition, Ti forms nitrides, which causes N contained in steel to precipitate as TiN and inhibits the N from combining with Al to precipitate as AlN, which has the effect of reducing the risk of cracks occurring during the continuous casting process.

If the Ti content exceeds 0.1%, coarse carbides will precipitate, and the strength of the steel plate may decrease due to a reduction in C in the steel. Furthermore, the coarse carbides may cause nozzle clogging during the continuous casting process.

Therefore, it is clear that the Ti content can be 0.1% or less, and that it is not difficult to ensure the intended physical properties even if the Ti content is not intentionally added.

Niobium (Nb): 0.1% or less Niobium (Nb) segregates at austenite grain boundaries, suppresses coarsening of austenite crystal grains during annealing heat treatment, and precipitates fine carbides in the crystal grains, thereby contributing to an increase in the strength of the steel sheet.

If the Nb content exceeds 0.1%, the carbon content in the steel is reduced due to the formation of coarse carbides, resulting in problems such as reduced strength and elongation of the steel plate, and increased manufacturing costs.

Therefore, it is clear that the above Nb can be contained in an amount of 0.1% or less, and that it is not difficult to ensure the intended physical properties even if the above Nb is not intentionally added.

Antimony (Sb): 0.1% or less Antimony (Sb) is distributed at grain boundaries and retards the diffusion of oxidizing elements such as Mn, Si, and Al in the steel through the grain boundaries, thereby suppressing the surface concentration of oxides and having an advantageous effect of suppressing the coarsening of surface concentrated oxides due to temperature rise and changes in the hot rolling process.

If the Sb content exceeds 0.1%, not only will workability be poor, but manufacturing costs will also increase.

Therefore, the Sb content may be 0.1% or less, excluding 0%. More preferably, the Sb content may be 0.01% or more.

Phosphorus (P): 0.05% or less Phosphorus (P) segregates at grain boundaries and is the main cause of temper brittlement, which impairs weldability and toughness. Therefore, it is advantageous to control the content of P as low as possible to as close to 0%, but since P is inevitably contained in the steel manufacturing process, and the process for reducing the P content is difficult and the production cost increases due to the additional process, it is effective to control the upper limit.

Therefore, the above P can be limited to 0.05% or less, and more preferably to 0.03% or less. However, it is clear that 0% can be excluded taking into account the level of unavoidable addition.

Sulfur (S): 0.02% or less Sulfur (S) is an impurity that is inevitably contained in steel along with the above-mentioned P, and has the problem of impairing the ductility and weldability of the steel sheet. Therefore, it is advantageous to control the content of S as low as possible so as to approach 0%, but considering the cost and time consumed in the process of reducing the S content, it is effective to manage its upper limit.

Therefore, the above S can be limited to 0.02% or less, and more preferably to 0.01% or less. However, it is clear that 0% can be excluded taking into account the level of unavoidable addition.

Nitrogen (N): 0.02% or less Nitrogen (N) can combine with Al in steel to form alumina-based nonmetallic inclusions (AlN). The AlN reduces the quality of continuous casting and increases the brittleness of steel sheets, thereby increasing the risk of fracture defects.

Therefore, the above N can be limited to 0.02% or less, and more preferably to 0.01% or less. However, 0% can be excluded taking into account the level of unavoidable inflow.

The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may be unavoidably mixed in from the raw materials or the surrounding environment, and it is not possible to eliminate these. These impurities are known to any technician in normal manufacturing processes, so the entire contents of them will not be mentioned in this specification.

In the steel sheet of the present invention having the above-mentioned alloy composition, it is preferable that the content relationships between specific elements in the steel satisfy all of the following relational expressions 1 to 3.

[Relationship 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380

[Relationship 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300

[Relationship 3]
-29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≧-24
(In Relational Formulas 1 to 3, each element represents its weight content.)

The above formulas 1 and 2 are compositional formulas derived by quantifying the degree to which the phase fraction of the microstructure that constitutes the steel plate is controlled and the solid solution strengthening effect is improved, thereby contributing to the strengthening of the yield strength and tensile strength of the steel plate.

In the above Relational Equations 1 and 2, the C has a relatively large coefficient compared to the Si and Mn, which is due to the fact that the C dissolves in the crystal grains of the steel sheet and contributes greatly to improving the strength. On the other hand, the Si has a relatively small coefficient compared to the C, which is due to the fact that it has a smaller effect of contributing to solid solution strengthening than the C. Furthermore, the Al has a negative coefficient value, which is due to the fact that although it contributes to solid solution strengthening, it has a greater effect of leaving dual phase region ferrite during annealing or promoting ferrite transformation during subsequent cooling, resulting in a decrease in strength. On the other hand, the Cr and Mo are representative hardening elements that have the effect of suppressing ferrite transformation during cooling after annealing, thereby improving strength, and are expressed as positive values.

Ti and Nb are elements that form fine carbides and contribute to improving strength, so they can have positive coefficient values in the strength relationship equation for the component elements. However, as they form fine carbides, the amount of dissolved carbon decreases, and the solid solution strengthening effect of carbon decreases. Therefore, Ti and Nb have positive coefficient values when the precipitation strengthening effect of their addition is dominant, but can be expressed as negative coefficient values when the solid solution strengthening effect of carbon due to the precipitation of carbides is dominant.

The above formula 3 is a chemical formula derived by quantifying the degree to which specific elements contribute to improving the solid solution strengthening effect and improving the elongation of steel sheet.

In general, when the strength of a steel sheet increases, the elongation tends to decrease, so the coefficients of each element in the above formula 3 tend to be opposite to those in the above formulas 1 and 2.

Specifically, the above C and Mn are advantageous in improving strength due to their solid solution strengthening effect, but such an improvement in strength tends to reduce elongation, resulting in a negative coefficient value. In contrast, Al is effective in increasing elongation, so it has a positive coefficient value. On the other hand, in the case of Si, in addition to improving strength through solid solution strengthening, it also contributes to securing retained austenite, so it also has a positive coefficient value in Relational Formula 3.

If any one of the above relational expressions 1 to 3 proposed in this invention is not satisfied, there is a problem that the physical properties of the steel sheet, particularly one or more of the tensile strength, yield strength, and elongation percentage, will be deteriorated. This will be clearly demonstrated in the examples described below.

The steel sheet of the present invention having the above-mentioned alloy component system has a microstructure in which soft and hard phases are appropriately dispersed, and is characterized by particularly containing an area fraction of 3-20% ferrite, 1-10% retained austenite, 1-30% bainite, 30-70% tempered martensite, and the remainder fresh martensite.

The ferrite is an allotrope of iron (Fe) with a body-centered cubic (BCC) structure, and is a soft structure different from martensite and bainite. Therefore, it has the advantage of having a higher elongation rate and excellent impact energy absorption compared to the bainite and martensite phases.

If the proportion of such ferrite exceeds 20%, excessive soft tissue may form in the steel plate, promoting plastic deformation, which may lead to a decrease in the yield strength of the steel plate. On the other hand, if the proportion of the above ferrite is less than 3%, there is a problem in that the elongation of the steel plate decreases, resulting in a decrease in formability.

Therefore, the above ferrite can be contained at an area fraction of 3 to 20%, and more preferably at 5 to 15%.

The term "retained austenite" refers to the austenite structure that remains in the steel and cannot transform into martensite or bainite during a series of heat treatment processes (which corresponds to the annealing-cooling-reheating and maintaining process in this invention) during the manufacturing process of the steel sheet, and plays a role in adjusting the balance between the strength and elongation of the steel sheet.

Generally, when the strength of a steel plate increases, the elongation decreases and formability deteriorates, and when the elongation of a steel plate increases, the strength decreases and it becomes difficult to ensure the physical properties required for a structural member. However, the above-mentioned retained austenite phase increases the value of the tensile strength (TS) x elongation (El) of the steel plate, making it useful for improving the balance between strength and elongation.

To fully obtain the above-mentioned effects, the area fraction of the retained austenite phase can be 1% or more, but if the fraction exceeds 10%, there is a problem that the sensitivity to liquid metal embrittlement increases and the spot weldability becomes poor.

Therefore, the above-mentioned retained austenite can be contained at an area fraction of 1 to 10%, and more preferably at 3 to 9%.

The bainite reduces the strength difference between structures within the steel and contributes to improving workability. In other words, it plays a role in preventing cracks, defects, and fractures from occurring in the steel sheet due to the difference in hardness between the relatively low hardness ferrite and retained austenite phases and the relatively high hardness tempered martensite and fresh martensite.

To fully obtain the above-mentioned effects, the area fraction should be 1% or more, and more preferably 5% or more. However, if the fraction exceeds 30%, the fraction of fresh martensite decreases, making it difficult to ensure the target level of strength.

Therefore, the above bainite can be contained at an area fraction of 1 to 30%.

The term "tempered martensite" refers to a structure in which the martensite phase obtained by quenching austenite is softened by tempering at a temperature of about 500°C. This tempered martensite phase has higher strength than the above-mentioned structures, and therefore contributes greatly to improving the yield strength and tensile strength of the steel plate. In addition, carbon in the martensite obtained by quenching is distributed to the surrounding austenite during the tempering process, increasing the thermal stability of the austenite and allowing it to remain at room temperature, which has the effect of improving the elongation of the steel plate.

In order to fully obtain the above-mentioned effects, it is preferable that the tempered martensite phase has an area fraction of 30% or more. However, if the area fraction exceeds 70%, there is a problem that the fraction of the retained austenite phase decreases relatively.

Therefore, the above tempered martensite can be contained at an area fraction of 30 to 70%.

The remaining structure excluding the above ferrite, retained austenite, bainite and tempered martensite phases may include fresh martensite phase.

The above-mentioned fresh martensite phase is a structure obtained during the final cooling process to room temperature, and since it has the highest strength, it contributes greatly to improving the yield strength and tensile strength of the steel plate. There are no particular limitations on the fraction of such fresh martensite phase, but as an example, it is clear that it can be contained at an area fraction of 3% or more.

As described above, the steel plate of the present invention is characterized by excellent tensile strength, yield strength, and elongation due to the appropriate formation of soft and hard phases, and specifically, can have a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more.

On the other hand, the steel sheet of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plating 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 that contains mainly zinc, or a zinc alloy plating layer that contains aluminum and/or magnesium in addition to zinc.

The following is a detailed description of another aspect of the present invention, namely, the method for producing the ultra-high strength steel plate with excellent ductility provided by the present invention.

In simple terms, the present invention can produce the desired steel sheet through the process of [steel slab reheating - hot rolling - coiling - cold rolling - continuous annealing - cooling - reheating and maintaining], after which the further process of "hot-dip galvanizing - alloying heat treatment" can be carried out.

The conditions for each stage are explained in detail below.

[Steel slab heating]
First, a steel slab that satisfies all of the above-mentioned alloy composition systems is prepared, and then heated. This process is performed to smoothly carry out the subsequent hot rolling process and to sufficiently obtain the desired physical properties of the steel sheet.

The heating process can be carried out at a temperature range of 1050 to 1300°C. If the heating temperature is less than 1050°C, there is a problem that friction between the steel plate and the rolling mill increases, and the load on the rollers during hot rolling increases sharply. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for increasing the temperature increase, but the amount of surface scale increases, which may lead to material loss.

The heating process can therefore be carried out in a temperature range of 1050 to 1300°C, and more preferably in a temperature range of 1090 to 1250°C.

[Hot rolling]
The steel slab heated as described above can be hot rolled to produce a hot rolled steel sheet, and in this case, finish hot rolling can be performed in the temperature range of 800 to 1000°C.

By performing the 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 800°C, the rolling is performed in the ferrite region, which increases friction between the steel sheet and the rolling mill, and the rolling load increases significantly. This causes excessive dislocations to form, which induces the formation of coarse crystal grains on the surface of the steel sheet during the subsequent coiling or cold rolling process, resulting in a decrease in strength. On the other hand, if the temperature exceeds 1000°C, the size of the ferrite crystal grains increases, which also causes a decrease in strength. In addition, scale is generated on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolling roll.

Therefore, during the above hot rolling, the finish hot rolling can be performed in the temperature range of 800 to 1000°C, and more preferably in the temperature range of 850 to 950°C.

[Winding]
The hot-rolled steel sheet produced as described above can be coiled, and this can be done at a temperature in the range of 400 to 700°C.

If the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet becomes excessively high, which may induce rolling load during the subsequent cold rolling. In addition, the cost and time required to cool the hot-rolled steel sheet to the coiling temperature is excessive, which causes an increase in process costs. On the other hand, if the temperature exceeds 700°C, excessive scale is likely to occur on the surface of the hot-rolled steel sheet, which may induce surface defects and weaken the galvanizing ability.

Therefore, the winding process can be carried out at a temperature range of 400 to 700°C, and more preferably at a temperature range of 500 to 700°C.

[cooling]
The coiled hot-rolled steel sheet can be cooled to room temperature. At this time, the cooling rate is not particularly limited, but air cooling can be performed.

[Cold rolling]
Thereafter, the hot rolled steel sheet can be cold rolled to produce a cold rolled steel sheet, and the cold rolling reduction can be in the range of 20 to 70%.

If the cold reduction rate during the cold rolling is less than 20%, it is difficult to obtain a steel sheet of the target thickness and it is difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, there is a high possibility that cracks will occur at the edge of the steel sheet, which will cause a problem of cold rolling load. Furthermore, there is a risk that coarse ferrite will form on the surface of the steel sheet during the subsequent continuous annealing due to excessive load.

Therefore, the cold rolling can be performed at a cold reduction rate of 20 to 70%, and more preferably at a cold reduction rate of 30 to 60%.

Meanwhile, before the cold rolling, the hot-rolled steel sheet may be subjected to pickling. The pickling is a process for removing scale formed on the surface of the hot-rolled steel sheet using hydrochloric acid (HCl) or the like, and can be performed under normal conditions, so the conditions are not particularly limited.

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

In the present invention, the ferrite formed in the cold-rolled steel sheet is recrystallized by the annealing process, and the proportions of ferrite and austenite in the steel can be adjusted. The strength of the steel sheet produced after the final heat treatment (referring to the reheating process described below) is determined by the proportion of each phase formed at this time, and generally, the higher the proportion of austenite, the higher the proportion of martensite or bainite transformed from austenite, and the greater the strength of the steel sheet tends to be. However, in the present invention, the strength can be further controlled by a series of heat treatment conditions described below.

The annealing process also distributes the carbon (C) in the steel, thereby increasing the amount of carbon (C) contained in the austenite, making it possible to have up to 10% by area of austenite phase even at room temperature.

The above annealing process can be carried out at a temperature range of 800 to 900°C.

If the annealing temperature is less than 800°C, the percentage of austenite formed by the annealing process may decrease, and the percentage of tempered martensite, bainite, and fresh martensite formed during the heat treatment described below may be insufficient. This may cause the yield strength and tensile strength of the final steel sheet to decrease. On the other hand, if the temperature exceeds 900°C, the percentage of austenite in the steel sheet may become excessively high, resulting in a problem that some of the austenite may transform into ferrite during the heat treatment process described below. In addition, the carbon concentration of the residual austenite may decrease, reducing the mechanical stability, which may cause a decrease in the elongation of the steel sheet. Furthermore, moisture generated as the Fe in the steel oxidizes during the annealing process may react with Si, Mn, and Al in the steel, increasing the possibility of forming an oxide film on the steel sheet. The oxide film may inhibit the wettability of Zn during hot-dip galvanization, thereby deteriorating the surface quality of the steel sheet.

Therefore, the annealing process can be carried out at a temperature range of 800 to 900°C, and more preferably at a temperature range of 820 to 870°C.

[cooling]
The cold-rolled steel sheet that has completed the annealing process as described above can be cooled.

In the present invention, quenched martensite can be formed by cooling the annealed cold-rolled steel sheet, and for this purpose, the cooling is preferably performed below the martensitic transformation start temperature (Ms). More preferably, it can be performed up to a temperature range of 250 to 400°C.

During the above cooling process, the lower the temperature, the higher the proportion of hardened martensite, which can lead to improved strength of the steel plate. In addition, the carbon supersaturated in the martensite is distributed to the surrounding austenite during the subsequent heat treatment process, increasing the stability of the retained austenite, and as a result, the elongation rate can be improved.

However, if the cooling temperature is less than 250°C, the proportion of hardened martensite increases excessively, and the proportion of retained austenite decreases, resulting in a problem of deterioration of the shape of the steel sheet. On the other hand, if the temperature exceeds 400°C, hardened martensite is not sufficiently formed, making it difficult to expect the above-mentioned effects.

When cooling to the above-mentioned temperature range, the average cooling rate can be 2 to 50°C/s. If the cooling rate is less than 2°C/s, ferrite will further transform during cooling, resulting in a decrease in strength. On the other hand, if the rate exceeds 50°C/s and the steel plate is rapidly cooled, the shape of the steel plate will deteriorate due to variations in cooling depending on the position of the steel plate. There is no particular limit to the cooling method when cooling at the above-mentioned cooling rate. As an example, the cooling may be a single cooling method in which the steel plate is cooled to the cooling end temperature at an initially set cooling rate, or as another example, a step-by-step cooling method in which the steel plate is slowly cooled to a certain section and then strongly cooled to the cooling end temperature, but it is not limited thereto.

Meanwhile, a process of maintaining the cooled temperature for a certain period of time can be performed, during which an isothermal transformation phase is further introduced, which has the effect of promoting the transformation of bainite in the subsequent process. For this reason, the maintaining process can be performed for 0.1 to 60 minutes.

[Reheating and Maintenance]
The cooled cold-rolled steel sheet and the cold-rolled steel sheet that has been cooled and maintained can be reheated to a temperature range of about 50 to 200° C. higher than the cooling temperature, and then maintained for a certain period of time, thereby performing a tempering treatment.

By reheating the cooled cold-rolled steel sheet, the quenched martensite phase formed during the cooling process is tempered and transformed into tempered martensite, which has the advantage of having high yield strength due to carbon being fixed to dislocations. In addition, carbon (C) supersaturated in the quenched martensite during the tempering process is redistributed to the surrounding austenite or induces bainite transformation, improving the stability of the retained austenite and improving the elongation rate.

The higher the tempering temperature, the smoother the dislocation fixation and carbon distribution to the austenite occurs, so it is necessary to reheat at a temperature 50°C higher than the cooling temperature (cooled temperature + 50°C or more). However, if the temperature is too high, cementite is generated in the quenched martensite, which coarsens and reduces the strength of the steel plate, and the carbon redistribution effect to the austenite is reduced, making it difficult to expect an improvement in elongation. In consideration of this, the reheating can be limited to be performed at a temperature of the cooled temperature + 200°C or less.

It is preferable to reheat the cold-rolled steel sheet that has been cooled to the above-mentioned temperature range and then maintain that temperature for 0.1 to 60 minutes to fully realize the above-mentioned effects.

If the above-mentioned maintenance time is excessive and exceeds 60 minutes, there is a problem that the equilibrium phases of ferrite and cementite are formed at the maintained temperature, resulting in a decrease in the strength of the steel plate; if it is maintained for less than 0.1 minutes, the intended effect will not be obtained.

After completing the reheating and maintaining process of the cold-rolled steel sheet cooled as described above, it can be cooled to room temperature under normal conditions, and finally, a steel sheet having a structure in which a certain proportion of soft phase and hard phase are appropriately distributed is obtained.

Specifically, a steel sheet can be obtained that has a microstructure consisting of an area fraction of 3-20% ferrite, 1-10% retained austenite, 1-30% bainite, 30-70% tempered martensite, and the remainder fresh martensite. Such a steel sheet of the present invention has excellent yield strength and tensile strength, and has the effect of improved ductility.

The process of cooling to room temperature is not particularly limited, but as an example, it can be performed by air cooling. However, it is self-evident that this can be substituted with known cooling methods such as water cooling, oil cooling, and furnace cooling.

On the other hand, by subjecting the cold-rolled steel sheet that has completed the series of heat treatment steps described above to a plating process as described below, a plated steel sheet having a plating layer on at least one surface can be produced.

[Hot-dip galvanizing]
The steel sheet produced through the above-mentioned series of steps can be immersed in a hot-dip galvanized bath to produce a hot-dip galvanized steel sheet.

At this time, hot-dip galvanizing can be performed under normal conditions, and as an example, it can be performed in the temperature range of 430 to 490°C. Furthermore, there are no particular limitations on the composition of the hot-dip galvanizing bath during the above-mentioned hot-dip galvanizing, and it may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[Alloying heat treatment]
If necessary, the hot-dip galvanized steel sheet can be subjected to an alloying heat treatment to obtain an alloyed hot-dip galvanized steel sheet.

In the present invention, there are no particular limitations on the conditions for the alloying heat treatment process, and normal conditions are acceptable. As an example, the alloying heat treatment process can be carried out in the temperature range of 480 to 600°C.

The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the invention. The scope of the invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

(Example)
30 kg of a slab having the alloy composition shown in Table 1 below was heated at 1200°C for 1 hour, and the heated slab was finish hot rolled at 900°C to produce hot rolled steel sheets. Then, each hot rolled steel sheet was charged into a furnace preheated to 600°C and maintained for 1 hour, and then cooled in the furnace to simulate hot rolling and coiling. Then, the steel sheet was cooled to room temperature (air cooled) and cold rolled at a cold reduction of 45% to produce cold rolled steel sheets.

Each cold-rolled steel sheet produced as described above was subjected to continuous annealing treatment at temperature T1 (°C) shown in Table 2 below for 1 minute, then cooled to temperature T2 (°C) and maintained for 10 seconds, then reheated to temperature T3 (°C) and maintained for 1 minute, and then cooled (air-cooled) to room temperature to produce the final steel sheet. After the above annealing treatment, cooling to temperature T2 was performed at a uniform cooling rate of 15°C/s.

The mechanical properties and internal structure of each steel plate manufactured through all of the above-mentioned processes were measured, and the results are shown in Table 3 below.

The mechanical properties measured were yield strength (YS), tensile strength (TS) and elongation (El), and were measured using ASTM tensile test pieces with a universal tensile testing machine.

The above internal structure was measured by polishing the test specimens, then nital etching them, and calculating the area of each phase using a scanning electron microscope (SEM).

（表２において、鋼９、１０及び１１は、合金成分系が本発明から外れるため、比較例として分類したものである。）

(In Table 2, steels 9, 10 and 11 are classified as comparative examples because their alloy composition systems are outside the scope of the present invention.)

As shown in Tables 1 to 3 above, Invention Examples 1 to 11, which satisfy all of the alloy component systems and manufacturing conditions proposed in this invention, formed the intended structure and ensured the target physical properties.

On the other hand, it can be seen that in Comparative Examples 1 and 2, which do not satisfy at least one of the component relations 1 and 2 proposed in the present invention, one or more of the physical properties of the yield strength and tensile strength are not secured at the target level. In addition, it can be seen that Comparative Example 7, which does not satisfy the component relations 3, has a significantly inferior elongation rate.

This proves that the relational expression 1, which is a feature of the present invention, contributes to strengthening the yield strength of the steel plate due to the fraction of the microstructure and the solid solution strengthening effect, the relational expression 2 contributes to improving the tensile strength of the steel plate, and the relational expression 3 contributes to improving the ductility of the steel plate.

In other words, if the relationship formulas 1 and 2 of the present invention are not satisfied, the strength of the steel plate is inferior, and if the relationship formula 3 is not satisfied, the ductility of the steel plate is inferior.

On the other hand, in Comparative Examples 3 to 6, in which the alloy composition system proposed in this invention was satisfied but the heat treatment conditions were outside the scope of this invention, the soft and hard phases were not properly formed as intended, and as a result, it was not possible to ensure both excellent strength and ductility in all examples.

Figure 1 shows a microstructure photograph of Example 1, which confirms that ferrite, retained austenite, tempered martensite, and bainite were formed within the target fraction range, and that fresh martensite phase was formed as the remaining structure.

Figure 2 shows a microstructure photograph of Comparative Example 6, which confirms that the tempered martensite phase was not formed in the target fraction, the retained austenite phase was not sufficiently secured, and the fraction of fresh martensite phase was relatively high.

Claims (9)

In weight percent, it is composed of carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, the remainder being Fe and other unavoidable impurities.
The following relations 1 to 3 are satisfied,
The microstructure is composed of ferrite with an area fraction of 3 to 20% and the remainder being a hard phase (including retained austenite, bainite, and tempered martensite),
An ultra-high strength steel plate with excellent ductility , having a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more .
[Relationship 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380
[Relationship 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300
[Relationship 3]
-29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≧-24
(In Relational Formulas 1 to 3, each element represents its weight content.) 2. The ultra-high strength steel plate according to claim 1, wherein the hard phase of the steel plate is composed of an area fraction of 1 to 10% retained austenite, 1 to 30% bainite, 30 to 70% tempered martensite, and the remainder fresh martensite. 2. The ultra-high strength steel plate having excellent ductility according to claim 1 , wherein the steel plate contains a fresh martensite phase at an area fraction of 3% or more. The ultra-high strength steel sheet with excellent ductility according to claim 1, wherein the steel sheet is any one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet. preparing a steel slab consisting of, by weight percent, 0.1-0.2% carbon (C), 0.1-1.0% silicon (Si), 2.0-3.0% manganese (Mn), 1.0% or less aluminum (Al) (except 0%), 1.0% or less chromium (Cr), 0.5% or less molybdenum (Mo), 0.1% or less titanium (Ti), 0.1% or less niobium (Nb), 0.1% or less antimony (Sb), 0.05% or less phosphorus (P), 0.02% or less sulfur (S), 0.02% or less nitrogen (N), with the remainder being Fe and other unavoidable impurities, and satisfying the following relations 1 to 3;
heating the steel slab to a temperature range of 1050 to 1300°C;
Finish hot rolling the heated steel slab at a temperature range of 800 to 1000° C. to produce a hot-rolled steel sheet;
coiling the hot-rolled steel sheet at a temperature in the range of 400 to 700°C;
cold rolling the coiled hot-rolled steel sheet at a total rolling reduction of 20 to 70% to produce a cold-rolled steel sheet;
Annealing the cold-rolled steel sheet at a temperature range of 800 to 900° C.;
cooling the annealed cold-rolled steel sheet to a temperature range of 250 to 400° C.;
reheating and maintaining the cooled cold rolled steel sheet;
2. The method of claim 1, wherein the reheating and maintaining step is performed at a temperature in the range of the cooled temperature + 50° C. to the cooled temperature + 200° C. for 0.1 to 60 minutes.
[Relationship 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380
[Relationship 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300
[Relationship 3]
-29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≧-24
(In Relational Formulas 1 to 3, each element represents its weight content.) The method for producing an ultra-high strength steel sheet having excellent ductility according to claim 5 , wherein the cold-rolled steel sheet is cooled at a cooling rate of 2 to 50 ° C./s. The method for producing an ultra-high strength steel sheet with excellent ductility according to claim 5 , further comprising the step of maintaining the cooled cold-rolled steel sheet in the cooled temperature range for 0.1 to 60 minutes before reheating the cooled cold-rolled steel sheet. The method for producing an ultra-high strength steel sheet having excellent ductility according to claim 5 , further comprising the step of hot-dip galvanizing after the reheating and maintaining. The method for producing an ultra-high strength steel sheet having excellent ductility according to claim 8 , further comprising a step of performing an alloying heat treatment after the hot dip galvanizing.

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