JP2023547102A - Ultra-high strength steel plate with excellent ductility and its manufacturing method - Google Patents

Abstract

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

Description

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

Recently, in the automobile industry, there has been a demand for the use of high-strength steel sheets in order to improve fuel efficiency or durability in accordance with various environmental regulations and energy usage regulations.

However, it has been discovered that when increasing the strength of a steel plate, the ductility relatively decreases, so much research has been conducted to improve the relationship between strength and ductility. As a result, in addition to martensite and bainite, which are low-temperature structures, transformed steels that utilize retained austenite phases have been developed and are now being applied.

Transformation structure steels include ferrite-martensite dual phase (DP) steel in which a hard martensite phase is formed in a ferrite base, TRIP (Transformation Induced Plasticity) steel that uses transformation-induced plasticity of retained austenite, It is classified into CP (Complexed Phase) steel, which is composed of ferrite and hard bainite or martensitic structures, and each of these steels has mechanical properties, such as tensile strength and elongation, depending on the type and fraction of the parent phase and second phase. There are different levels of rates.

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

As an example, Patent Document 1 describes a steel that contains about 10% retained austenite phase in addition to ferrite and martensite, has a product of tensile strength and elongation of 21000 MPa% or more, and can secure a tensile strength of 780 MPa class or higher. Disclosed. However, this steel has a large content of carbon (C) of approximately 0.2% and silicon (Si) of approximately 1.5% or more, which results in poor spot weldability and hot-dip galvanizing properties. There is a risk. Furthermore, since annealing is performed twice in order to achieve high physical properties, there is a problem in that the manufacturing cost of the steel sheet increases.

On the other hand, in Patent Document 2, in order to ensure good plating properties and spot weldability, the Si content is lowered to the 1% level, and the microstructure is composed of martensite, bainite, and ferrite even if it does not contain a retained austenite phase. It discloses a technology that can ensure a tensile strength of 980 MPa or more and an elongation rate of 15% or more. However, recently, regulations regarding the impact stability of automobiles have been expanded, and in order to improve the impact resistance of automobile bodies, structural members such as members, seat rails, and pillars are required to have yield strength. Although high-strength steels with excellent properties are currently being used, the yield strength of these steels is 700 MPa or less, which limits their applicability.

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

One aspect of the present invention is to provide a steel plate that is suitable for structural members of automobiles, has excellent not only tensile strength but also yield strength, and has improved ductility, and a method for manufacturing the same. It is.

The object of the present invention is not limited to the above-mentioned content. The problems to be solved by the present invention can be understood from the contents of this specification as a whole, and a person having ordinary knowledge in the technical field to which the present invention pertains will not have any difficulty in understanding further problems to be solved by the present invention. do not have.

One embodiment of the present invention has carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, and manganese (Mn): 2.0 to 3.0% by weight. 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, containing residual Fe and other unavoidable impurities, and satisfying the following relational expressions 1 to 3. To provide an ultra-high strength steel plate with excellent ductility. .

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

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

[Relational expression 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 expressions 1 to 3, each element means the weight content.)

Another aspect of the present invention includes the steps of preparing a steel slab that satisfies the alloy composition and relational expressions 1 to 3 described above, heating the steel slab in a temperature range of 1050 to 1300°C, and a step of hot rolling a steel slab at a temperature range of 800 to 1000°C to produce a hot rolled steel plate; a step of winding the hot rolled steel plate at a temperature range of 400 to 700°C; and a step of manufacturing the hot rolled steel plate. a step of manufacturing a cold rolled steel sheet by cold rolling a steel sheet at a total reduction rate of 20 to 70%; a step of annealing the cold rolled steel sheet at a temperature range of 800 to 900°C; The steps include cooling the rolled steel sheet to a temperature range of 250 to 400° C., and reheating and maintaining the cooled cold rolled steel sheet, and the reheating and maintaining step includes the step of cooling the rolled steel sheet to a temperature range of 250 to 400° C. Provided is a method for producing an ultra-high strength steel sheet with excellent ductility, which is carried out for 0.1 to 60 minutes in a temperature range of 0.1 to 200° C. to the cooled temperature.

According to the present invention, it is possible to provide a steel plate that has excellent tensile strength and yield strength, and improved ductility, and the steel plate of the present invention has the formability and collision stability required for cold forming steel plates. It has the advantage of being guaranteed.

1 is a photograph showing a microstructure of an inventive steel according to an example of the present invention measured by SEM. 1 is a photograph showing a microstructure of a comparative steel according to an example of the present invention measured by SEM.

The inventors of the present invention have developed a structural member that requires processing into complex shapes as a material for automobiles because it has excellent yield strength as well as tensile strength and ductility, and ensures formability and collision stability. We conducted extensive research to provide a steel plate that can be used in other applications.

As a result, it was confirmed that by optimizing the alloy composition system and manufacturing conditions, it was possible to provide a steel plate having a structure that is advantageous in securing the target physical properties, and the present invention was completed.

In particular, the present invention controls the content relationship of specific elements among the alloy components and optimizes the process conditions of the steel plate manufactured through a series of processes, thereby separating the soft phase and hard phase. ) is characterized in that it provides a steel plate having a composite structure in which these are appropriately dispersed.

The present invention will be explained in detail below.

The ultra-high strength steel sheet with excellent ductility according to one embodiment of the present invention has 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, and nitrogen (N): 0.02% or less.

Below, the reason why the alloy composition of the steel plate provided by the present invention is limited as described above will be explained in detail.

On the other hand, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of 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 sheets, and the above C precipitates in the crystal grains of steel sheets, induces solid solution strengthening, promotes the formation of martensite within the steel, and strengthens the steel. strengthen. Moreover, the above-mentioned C is an austenite stabilizing element and plays an important role in the formation of retained austenite. Specifically, as the amount of carbon (C) dissolved in austenite increases, the austenite stability becomes higher and the fraction of austenite in the steel becomes higher. This induces an increase in the fraction of martensite formed by the transformation of austenite, which has the effect of improving the strength of the steel sheet, and some austenite remains at room temperature and remains as retained austenite.

In order to fully obtain the above-mentioned effects, 0.1% or more of C can be added, but if the content exceeds 0.2%, the fraction of martensitic phase will increase excessively and the relative The fraction of ferrite phase, which has excellent elongation and impact absorption energy, decreases. This reduces the ductility of the steel sheet and increases the possibility of brittleness.

Therefore, the above C can be contained in an amount of 0.1 to 0.2%, more preferably 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 carbon in ferrite to diffuse into austenite, and contributes to stabilizing retained austenite.

In order to obtain the above-mentioned effects, it is advantageous to contain 0.1% or more of Si. However, if the content exceeds 1.0%, Si oxide is formed on the surface of the steel, so hot-dip plating is difficult. Also, there is a possibility that the effect of chemical conversion coating may be inhibited.

Therefore, the above-mentioned Si can be contained in an amount of 0.1 to 1.0%, more preferably 0.2% or more, and still more preferably 0.4% or more. On the other hand, the above-mentioned Si can be contained more preferably at 0.9% or less.

Manganese (Mn): 2.0-3.0%
Manganese (Mn) can act as an austenite stabilizing element similarly to the above-mentioned C. Specifically, the Mn can contribute to reducing the critical cooling rate at which martensite is formed in a composite structure steel and increasing the fraction of martensite within the steel.

In order to fully obtain the above-mentioned effects, it is advantageous to contain Mn in an amount of 2.0% or more; however, if the content exceeds 3.0%, the weldability of the steel plate decreases, making hot rolling difficult. There is a risk that performance may deteriorate. In addition, there is a problem in that striped bands called Mn-Bands are formed, which impairs formability and increases the risk of processing cracks.

Therefore, the above-mentioned Mn can be contained in an amount of 2.0 to 3.0%, 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 the above-mentioned Si. The above Al is effective in distributing carbon in ferrite to austenite and improving martensite hardening ability, and when maintained in the bainite region, effectively suppresses precipitation of carbides in bainite, improving the ductility of the steel sheet. It is a useful element for improvement.

When the content of Al exceeds 1.0%, continuous castability decreases during continuous casting operations in steelmaking, and there is a high possibility that inclusions will be excessively formed and material defects will occur in the annealed material.

Therefore, the above-mentioned Al can be contained in an amount of 1.0% or less, and 0% is excluded. More preferably, the Al content may be 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. In addition, the reduction in elongation rate is minimized with respect to the increase in strength, which is advantageous for producing a composite structure steel having high ductility.

If the content of Cr exceeds 1.0%, not only will the above-mentioned effects become saturated, but there will also be a problem that the hot rolling strength will increase excessively and the cold rolling properties will be poor. There is a problem in that the growth rate increases significantly, leading to a decrease in the growth rate.

Therefore, it is clear that the above-mentioned 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 without intentionally adding the above-mentioned Cr.

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

Therefore, it is clear that the above-mentioned 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 without intentionally adding the above-mentioned Mo.

Titanium (Ti): 0.1% or less Titanium (Ti), like the above Mo, forms fine carbides in steel and can contribute to ensuring the yield strength and tensile strength of steel. Furthermore, by forming nitrides, Ti can cause the N contained in the steel to precipitate as TiN, and can suppress the above N from combining with Al and precipitating as AlN. It has the effect of reducing the risk of cracks occurring during the casting process.

When the content of Ti exceeds 0.1%, coarse carbides are precipitated and C is reduced in the steel, which may reduce the strength of the steel sheet. Furthermore, the coarse carbides may cause clogging of nozzles during the continuous casting process.

Therefore, it is clear that the Ti can be contained in an amount of 0.1% or less, and that it is not difficult to secure the intended physical properties even if the Ti 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 improving the strength of steel sheets. It can contribute to increased strength.

When the Nb content exceeds 0.1%, the C content in the steel is reduced due to the formation of coarse carbides, which causes a problem of decreasing the strength and elongation of the steel sheet, which increases the manufacturing cost of the steel. There is a problem with rising.

Therefore, it is clear that the Nb can be contained in an amount of 0.1% or less, and that it is not difficult to secure the intended physical properties even without intentionally adding the Nb.

Antimony (Sb): 0.1% or less Antimony (Sb) is distributed at grain boundaries and prevents oxidation by slowing the diffusion of oxidizing elements such as Mn, Si, and Al in steel through grain boundaries. It has an advantageous effect in suppressing the surface thickening of the material and suppressing the coarsening of the surface concentrated material due to temperature rise and changes in the hot rolling process.

If the content of Sb exceeds 0.1%, there is a problem that not only the processability is poor but also the manufacturing cost increases.

Therefore, the above-mentioned Sb can be contained at 0.1% or less, and 0% is excluded. More preferably, the above-mentioned Sb can be contained in an amount of 0.01% or more.

Phosphorus (P): 0.05% or less Phosphorus (P) segregates at grain boundaries and becomes the main cause of temper brittlement, which poses a problem of impairing weldability and toughness. Therefore, it is advantageous to control the P content as low as possible to approach 0%, but it is inevitably contained in the steel manufacturing process, and there are Since the process is difficult and the production cost increases due to additional steps, it is effective to control the upper limit.

Therefore, the above P can be limited to 0.05% or less, more preferably 0.03% or less. However, it will be clarified that 0% can be excluded in consideration of the level that is unavoidably added.

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

Therefore, the above S can be limited to 0.02% or less, more preferably 0.01% or less. However, it will be clarified that 0% can be excluded in consideration of the level that is unavoidably added.

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

Therefore, the above N can be limited to 0.02% or less, more preferably 0.01% or less. However, in consideration of the level that inevitably flows in, 0% can be excluded.

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

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

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

[Relational expression 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 expressions 1 to 3, each element means the weight content.)

The above relational expressions 1 and 2 were derived by quantifying the extent to which controlling the phase fraction of the microstructure constituting the steel sheet and improving the solid solution strengthening effect contributes to strengthening the yield strength and tensile strength of the steel sheet. This is a component relational expression.

In the above relational expressions 1 and 2, the above C has a relatively large coefficient compared to the above Si and Mn, and this is because the above C dissolves into the crystal grains of the steel sheet and greatly contributes to improving the strength. to cause. On the other hand, the coefficient of Si is relatively smaller than that of C, and this is because the effect of contributing to solid solution strengthening is smaller than that of C. Furthermore, the Al has a negative coefficient value, which contributes to solid solution strengthening, but may leave dual phase region ferrite during annealing or promote ferrite transformation during subsequent cooling. This is due to the fact that the effect of reducing strength is greater. On the other hand, the above-mentioned Cr and Mo are typical hardenable elements, and since they suppress ferrite transformation during cooling after annealing, they have the effect of improving strength, and are expressed as positive values.

Note that Ti and Nb are elements that form fine carbides and contribute to improving the strength, so they can have positive coefficient values in the strength relationship equation based on the component elements. However, as fine carbides are formed, the amount of solid solution carbon decreases, and the solid solution strengthening effect of carbon decreases. Therefore, Ti and Nb have a positive coefficient value when the precipitation strengthening effect is dominant due to their addition, whereas they have a negative coefficient value when the solid solution strengthening effect of carbon due to carbide precipitation is dominant. It can be expressed by the coefficient value of

The above relational expression 3 is a component relational expression derived by quantifying the extent to which a specific element contributes to improving the solid solution strengthening effect and improving the elongation rate of the steel plate.

In general, considering that the elongation rate tends to decrease as the strength of a steel plate increases, the coefficients of each element in the above relational expression 3 tend to be contradictory to the above relational expressions 1 and 2.

Specifically, C and Mn are advantageous in improving strength due to their solid solution strengthening effect, but such improvement in strength tends to reduce elongation, so it has a negative coefficient value. . On the other hand, since Al is effective in increasing the elongation rate, it has a positive coefficient value. On the other hand, in the case of Si, since it contributes to securing retained austenite as well as improving strength through solid solution strengthening, it also has a positive coefficient value in relational expression 3.

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

The steel sheet of the present invention having the above-mentioned alloy composition system contains a soft phase and a hard phase appropriately dispersed as a microstructure, and in particular contains ferrite with an area fraction of 3 to 20%, retained austenite with an area fraction of 1 to 10%, It is characterized by containing 1 to 30% bainite, 30 to 70% tempered martensite, and the remainder fresh martensite.

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

When the fraction of such ferrite exceeds 20%, soft structures within the steel sheet may be formed excessively and promote plastic deformation, which causes a decrease in the yield strength of the steel sheet. On the other hand, if the ferrite fraction is less than 3%, there is a problem that the elongation rate of the steel sheet decreases and the formability decreases.

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

The above-mentioned retained austenite can be transformed into martensite or bainite during a series of heat treatment processes (corresponding to the [annealing-cooling-reheating and maintenance] process in the present invention) in the steel sheet manufacturing process. This refers to the austenite structure that remains within the steel and plays a role in adjusting the balance between the strength and elongation of the steel plate.

In general, when the strength of a steel plate increases, its elongation rate decreases and its formability decreases, and when the elongation rate of a steel plate increases, its strength decreases, making it difficult to secure the physical properties required for structural members. The retained austenite phase increases the value of tensile strength (TS) x elongation (El) of the steel plate, and is therefore useful for improving the balance between strength and elongation.

In order to fully obtain the above-mentioned effects, it is possible to include retained austenite phase with an area fraction of 1% or more, but if the fraction exceeds 10%, the sensitivity to liquid metal embrittlement increases and spot welding becomes difficult. There is a problem with being inferior in gender.

Therefore, the retained austenite can be contained in an area fraction of 1 to 10%, more preferably 3 to 9%.

Bainite can reduce the strength difference between structures within steel and contribute to improving workability. In other words, it plays a role in preventing cracks, defects, and destruction in the steel sheet due to the difference in hardness between the ferrite and retained austenite phases, which have relatively low hardness, and the tempered martensite and fresh martensite, which have relatively high hardness. .

In order to fully obtain the above-mentioned effects, it can be contained in an area fraction of 1% or more, more preferably 5% or more. However, if the fraction exceeds 30%, the fraction of fresh martensite decreases, making it difficult to secure the target level of strength.

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

The tempered martensite refers to a structure obtained by softening a martensite phase obtained by quenching austenite at a temperature of about 500°C. Since such a tempered martensitic phase has higher strength than the above-mentioned structure, it greatly contributes to improving the yield strength and tensile strength of the steel sheet. In addition, the 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, improving the elongation rate of the steel plate. This has the effect of achieving

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

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

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

The above-mentioned fresh martensite phase is a structure obtained during the final cooling process to room temperature and has the highest strength, so it greatly contributes to improving the yield strength and tensile strength of the steel sheet. The fraction of such fresh martensite phase is not particularly limited, but as an example, it is clarified that it can be included at an area fraction of 3% or more.

As mentioned above, the steel sheet of the present invention is characterized by excellent tensile strength, yield strength, and elongation due to the appropriate formation of a soft phase and a hard phase. It can have a tensile strength of 980 MPa or more and an elongation rate of 13% or more.

On the other hand, the steel sheet of the present invention may be a cold-rolled steel sheet, such as a hot-dip galvanized steel sheet containing 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. It may also be a steel plate.

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 an ultra-high strength steel plate with excellent ductility provided by the present invention, which is another embodiment of the present invention, will be described in detail.

Simply put, the present invention can produce a target steel plate through the steps of [steel slab reheating - hot rolling - winding - cold rolling - continuous annealing - cooling - reheating and maintenance]. After that, a process of "hot dip galvanizing-alloying heat treatment" can be further performed.

The conditions for each stage will be explained in detail below.

[Steel slab heating]
First, a steel slab satisfying all of the above-mentioned alloy composition systems is prepared and then heated. This step is performed in order to smoothly perform the subsequent hot rolling step and to sufficiently obtain the target physical properties of the steel sheet.

The above heating step can be performed at a temperature range of 1050 to 1300°C. If the heating temperature is less than 1050° C., there is a problem in that friction increases between the steel plate and the rolling mill, and the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300° C., not only will the energy cost required due to the temperature increase increase, but also the amount of surface scale will increase, which may lead to material loss.

Therefore, the above heating step can be carried out at a temperature range of 1050 to 1300°C, more preferably 1090 to 1250°C.

[Hot rolling]
The heated steel slab as described above can be hot rolled to produce a hot rolled steel plate, and at this time, finish hot rolling can be performed at a temperature range of 800 to 1000°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 800° C., rolling is performed in the ferrite region, which increases friction between the steel plate and the rolling mill, causing a problem that the load due to rolling increases significantly. This causes the formation of excessive dislocations and the formation of coarse grains on the surface of the steel sheet during the subsequent winding or cold rolling process, resulting in a decrease in strength. On the other hand, if the temperature exceeds 1000° C., the size of ferrite crystal grains increases, resulting in a problem that the strength also decreases. Furthermore, scale may be generated on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolling rolls.

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

[Winding]
The hot-rolled steel sheet produced as described above can be rolled up at a temperature 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 subsequent cold rolling. Further, it takes excessive cost and time to cool the hot rolled steel plate to the coiling temperature, which causes an increase in process costs. On the other hand, if the temperature exceeds 700° C., there is a high possibility that scale will be generated excessively on the surface of the hot rolled steel sheet and induce surface defects, which will cause the plating properties to become weak.

Therefore, the above-mentioned winding step can be performed at a temperature range of 400 to 700°C, more preferably at a temperature range of 500 to 700°C.

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

[Cold rolling]
Thereafter, the hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet, and this can be done at a cold rolling reduction rate of 20 to 70%.

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

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

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

[Annealing]
The cold-rolled steel sheet manufactured as described above can be annealed. As an example, a continuous annealing process may be performed, but the present invention 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 through the annealing process is recrystallized, and the fraction 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 fraction of each phase formed at this time, and generally speaking, the higher the austenite fraction, the more the austenite transforms. As the fraction of martensite or bainite increases, the strength of the steel sheet tends to improve. However, in the present invention, the strength can be further controlled by a series of heat treatment conditions described below.

Additionally, carbon (C) within the steel can be distributed through the above annealing process, thereby increasing the amount of carbon (C) contained within the austenite, allowing the steel to have an austenite phase of up to 10% by area even at room temperature. I can do it.

The above annealing step can be performed at a temperature range of 800 to 900°C.

If the temperature during the above annealing is less than 800°C, the fraction of austenite formed in the annealing process will decrease, and the fraction of tempered martensite, bainite and fresh martensite formed during the heat treatment described below may not be sufficient. There is. This can cause the yield strength and tensile strength of the final steel plate to decrease. On the other hand, if the temperature exceeds 900°C, there is a problem that the fraction of austenite in the steel sheet becomes excessively high, and some austenite transforms into ferrite during the heat treatment process described below. Furthermore, there is a possibility that the carbon enrichment of the retained austenite becomes low and the mechanical stability decreases, which causes a decrease in the elongation rate of the steel sheet. Furthermore, during the annealing process, moisture generated while Fe in the steel is oxidized reacts with Si, Mn, and Al in the steel, increasing the possibility that an oxide film will be formed on the steel sheet. The above oxide film may inhibit the wettability of Zn during hot-dip galvanizing, and the surface quality of the steel sheet may deteriorate.

Therefore, the annealing step can be performed at a temperature range of 800 to 900°C, more preferably 820 to 870°C.

[cooling]
The cold-rolled steel sheet that has undergone the annealing process can be cooled by the above process.

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

During the cooling, the lower the temperature, the higher the fraction of quenched martensite, which can induce an improvement in the strength of the steel plate. In addition, supersaturated carbon in martensite is distributed to surrounding austenite in the subsequent heat treatment process, increasing the stability of retained austenite and, as a result, improving elongation.

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

When cooling to the above-mentioned temperature range, it can be performed at an average cooling rate of 2 to 50° C./s. If the cooling rate is less than 2° C./s, the ferrite is further transformed during cooling, leading to a decrease in strength. On the other hand, if the cooling speed exceeds 50° C./s, there is a problem that the shape of the steel sheet deteriorates due to variations in cooling depending on the position of the steel sheet. When performing cooling at the above-mentioned cooling rate, there are no particular limitations on the cooling method. As an example, the above-mentioned cooling may be a single cooling method in which cooling is performed at an initially set cooling rate until the cooling end temperature is reached.As another example, after slow cooling is performed up to a certain section, the cooling end temperature is A step-by step cooling method may be used, but is not limited to this method.

On the other hand, it is possible to undergo a step of maintaining the above-mentioned cooled temperature for a certain period of time, and in this step, an isothermal transformation phase is further introduced, and it is possible to obtain the effect of promoting the transformation of bainite in the subsequent step. To this end, the above maintenance step can be performed for 0.1 to 60 minutes.

[Reheating and maintenance]
The above-mentioned cooled cold-rolled steel sheet and the cooled and maintained cold-rolled steel sheet can be reheated to a temperature range approximately 50 to 200°C higher than the above-mentioned cooling temperature, and then tempered by maintaining the temperature for a certain period of time. can.

By reheating the cooled cold-rolled steel sheet, the hardened martensite phase formed in the cooling process is tempered and transformed into tempered martensite, and the tempered martensite yields due to carbon fixation at dislocations. It has the advantage of high strength. In addition, in the above tempering process, supersaturated carbon (C) in the hardened martensite is redistributed to the surrounding austenite or induces bainite transformation, which improves the stability of the retained austenite and has the effect of improving the elongation rate. is obtained.

The above fixation of dislocations and carbon distribution into austenite occur more smoothly as the tempering temperature is higher, so it is necessary to reheat at a temperature 50° C. or more higher than the cooling temperature (cooled temperature + 50° C. or more). However, if the temperature is too high, cementite will be generated within the quenched martensite, which will become coarser and reduce the strength of the steel sheet, reducing the effect of carbon redistribution to austenite, which is expected to improve the elongation rate. It becomes difficult to do. Considering this, the reheating can be limited to below the cooled temperature +200°C.

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

During the above maintenance, if the time exceeds 60 minutes, ferrite and cementite, which are equilibrium phases, will be formed at the maintenance temperature, resulting in a decrease in the strength of the steel plate. No effect is obtained.

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

Specifically, it is composed of ferrite with an area fraction of 3 to 20%, retained austenite of 1 to 10%, bainite of 1 to 30%, tempered martensite of 30 to 70%, and the remainder fresh martensite. The steel plate of the present invention has excellent yield strength and tensile strength, and can have the effect of improved ductility.

The step of cooling to room temperature is not particularly limited, but can be performed by air cooling, for example. However, it is obvious that known cooling methods such as water cooling, oil cooling, and furnace cooling can be used instead.

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

[Hot dip galvanizing]
A hot-dip galvanized steel sheet can be manufactured by immersing a steel sheet manufactured through the series of steps described above in a hot-dip galvanizing bath.

At this time, hot-dip galvanizing can be performed under normal conditions, and as an example, it can be performed at a temperature range of 430 to 490°C. Furthermore, during the hot-dip galvanizing, the composition of the hot-dip zinc plating bath is not particularly limited, and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc. .

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

In the present invention, the alloying heat treatment process conditions are not particularly limited, and any normal conditions may be used. As an example, the alloying heat treatment step can be performed at a temperature range of 480-600°C.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, it should be noted that the following examples are for illustrating and explaining the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of rights to the present invention is determined by the matters stated in the claims and matters reasonably inferred from the claims.

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

Each of the cold-rolled steel sheets manufactured as described above was continuously annealed for 1 minute at the temperature T1 (°C) shown in Table 2 below, then cooled to a temperature T2 (°C) and maintained for 10 seconds, and then maintained at a temperature T3 (°C). ℃), maintained for 1 minute, and then cooled to room temperature (air cooling) to produce a final steel plate. After the annealing treatment, cooling to temperature T2 was uniformly performed at a cooling rate of 15° C./s.

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

As for the mechanical properties, yield strength (YS), tensile strength (TS), and elongation rate (El) were measured using a universal tensile tester using an ASTM tensile test piece.

For the internal structure, the test piece was polished and nital etched, and the area of each phase was calculated 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 that satisfy all of the alloy composition systems and manufacturing conditions proposed in the present invention ensured the target physical properties by forming the intended structure. .

On the other hand, in Comparative Examples 1 and 2, which do not satisfy at least one of Relational Expressions 1 and 2 of the component relational expressions proposed in the present invention, one or more of the physical properties of yield strength and tensile strength cannot be secured at the target level. I understand. Furthermore, it can be confirmed that Comparative Example 7, which does not satisfy Relational Expression 3 among the component relational expressions, has a significantly inferior elongation rate.

As a result, relational expression 1, which is a feature of the present invention, contributes to strengthening the yield strength due to the fraction of the microstructure of the steel plate and the solid solution strengthening effect, and relational expression 2 contributes to improving the tensile strength of the steel plate, and the relational expression 3 was proven to contribute to improving the ductility of steel sheets.

That is, when relational expressions 1 and 2 of the present invention are not satisfied, the strength of the steel plate is inferior, and when relational expression 3 is not satisfied, it means that 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 by the present invention is satisfied but the heat treatment conditions deviate from the present invention, the soft phase and hard phase were not formed appropriately as intended, and as a result, In all examples, it was not possible to ensure both excellent strength and ductility.

FIG. 1 shows a photograph of the structure of Invention Example 1, in which ferrite, retained austenite, tempered martensite, and bainite are formed within the target fraction range, and the remaining structure is a fresh martensite phase. It can be confirmed that it has been formed.

FIG. 2 shows a microstructure photograph of Comparative Example 6, in which the tempered martensite phase was not formed in the target fraction, the retained austenite phase could not be sufficiently secured, and the fresh martensite phase fraction was reduced. It can be confirmed that it was formed relatively high.

Claims (10)

In weight%, 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, including remaining Fe and other unavoidable impurities,
An ultra-high strength steel plate with excellent ductility, characterized by satisfying the following relational expressions 1 to 3.
[Relational expression 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380
[Relational expression 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300
[Relational expression 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 expressions 1 to 3, each element means the weight content.) The steel plate has a microstructure having an area fraction of 3 to 20% ferrite, 1 to 10% retained austenite, 1 to 30% bainite, 30 to 70% tempered martensite, and the remainder fresh martensite. The ultra-high strength steel plate with excellent ductility according to claim 1, comprising: The ultra-high strength steel plate with excellent ductility according to claim 2, wherein the steel plate contains a fresh martensite phase in an area fraction of 3% or more. The ultra-high strength steel plate with excellent ductility according to claim 1, wherein the steel plate has a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation rate of 13% or more. The ultra-high strength steel sheet with excellent ductility according to claim 1, wherein the steel sheet is one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet. In weight%, 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) : preparing a steel slab containing 0.02% or less, remaining Fe and other unavoidable impurities, and satisfying the following relational expressions 1 to 3;
heating the steel slab to a temperature range of 1050-1300°C;
hot rolling the heated steel slab at a temperature range of 800 to 1000°C to produce a hot rolled steel plate;
Coiling the hot rolled steel sheet at a temperature range of 400 to 700°C;
manufacturing a cold rolled steel sheet by cold rolling the wound hot rolled steel sheet at a total reduction rate of 20 to 70%;
annealing the cold rolled steel plate at a temperature range of 800 to 900°C;
cooling the continuously annealed cold rolled steel sheet to a temperature range of 250 to 400°C;
reheating and maintaining the cooled cold rolled steel sheet;
The method for producing an ultra-high strength steel sheet with excellent ductility, wherein the reheating and maintaining step is performed for 0.1 to 60 minutes at a temperature ranging from the cooled temperature +50°C to the cooled temperature +200°C.
[Relational expression 1]
1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380
[Relational expression 2]
2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300
[Relational expression 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 expressions 1 to 3, each element means the weight content.) 7. The method for producing an ultra-high strength steel sheet with excellent ductility according to claim 6, 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 6, 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 method of manufacturing an ultra-high strength steel sheet with excellent ductility according to claim 6, further comprising the step of hot-dip galvanizing after the reheating and maintenance. The method for producing an ultra-high strength steel sheet with excellent ductility according to claim 6, further comprising the step of performing alloying heat treatment after the hot-dip galvanizing.

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