阅读说明：本技术 屈强比优异的超高强度高延展性钢板及其制造方法 (Ultrahigh-strength and high-ductility steel sheet having excellent yield ratio and method for producing same ) 是由 柳朱炫 李圭荣 李世雄 于 2019-09-03 设计创作，主要内容包括：本发明的一个方面的屈强比优异的超高强度高延展性钢板,以重量％计,包含：碳(C)：0.1-0.3％、硅(Si)：2％以下、锰(Mn)：6-10％、磷(P)：0.05％以下、硫(S)：0.02％以下、氮(N)：0.02％以下、铝(Al)：0.5％以下(0％除外)、余量的Fe和不可避免的杂质,还包含选自钛(Ti)：0.1％以下、铌(Nb)：0.1％以下、钒(V)：0.2％以下和钼(Mo)：0.5％以下中的一种以上,所述钢板包含20面积％以上的残留奥氏体作为微细组织,所述残留奥氏体的平均纵横比为2.0以上。(An ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention includes, in wt%: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less (excluding 0%), the balance Fe and inevitable impurities, and further comprising titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or more, the steel sheet contains 20% or more by area of retained austenite as a fine structure, and the retained austenite has an average aspect ratio of 2.0 or more.)

1. An ultra-high strength and high ductility steel sheet having an excellent yield ratio, comprising, in wt%: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less and 0% or less excluding Fe and inevitable impurities, and further comprising titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or less of one or more,

the steel sheet contains 20 area% or more of retained austenite as a fine structure,

the retained austenite has an average aspect ratio of 2.0 or more.

2. The ultra-high strength and high ductility steel sheet excellent in yield ratio as claimed in claim 1, wherein said steel sheet further comprises a material selected from the group consisting of nickel (Ni): 1% or less, copper (Cu): 0.5% or less and chromium (Cr): 1% or less.

3. The ultra-high strength and high ductility steel sheet excellent in yield ratio according to claim 1, wherein the steel sheet contains one or more of ferrite, annealed martensite, and newly grown martensite as a residual structure.

4. An ultra-high strength and high ductility steel sheet excellent in yield ratio as claimed in claim 3, wherein said steel sheet comprises a balance structure of one or more of ferrite, annealed martensite and newly grown martensite in a total fraction of 50-80 area%.

5. The ultra-high strength and high ductility steel sheet excellent in yield ratio as claimed in claim 1, wherein the steel sheet has a tensile strength of 1400MPa or more,

the yield ratio of the steel plate is more than 0.7,

the product (TS) of the tensile strength and the elongation of the steel sheet is 22000 MPa% or more.

6. A method of manufacturing an ultra-high strength and high ductility steel sheet having an excellent yield ratio,

heating a slab to a temperature range of 1050 ℃ -: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less and 0% or less excluding Fe and inevitable impurities, and further comprising titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or less of one or more,

subjecting the heated slab to finish hot rolling at a temperature range of 800 ℃ -,

rolling the hot rolled steel sheet at a temperature of 50-750 ℃,

pickling the rolled hot rolled steel sheet, and then cold rolling at a reduction of 15% or more to provide a cold rolled steel sheet,

selectively subjecting the cold-rolled steel sheet to annealing heat treatment according to any one of a first annealing condition and a second annealing condition,

wherein the first annealing condition is to perform annealing heat treatment on the cold-rolled steel sheet within a temperature range of 600-720 ℃ for 10-3600 seconds,

the second annealing condition is to perform a primary annealing heat treatment within a temperature range of over 720 ℃ and below 900 ℃ for 10-3600 seconds, then cool to the normal temperature, and perform a secondary annealing heat treatment within a temperature range of 480-700 ℃ for 10-3600 seconds.

7. The method of manufacturing an ultra-high strength and high ductility steel sheet excellent in yield ratio as claimed in claim 6, wherein said slab further comprises a material selected from the group consisting of nickel (Ni): 1% or less, copper (Cu): 0.5% or less and chromium (Cr): 1% or less.

Technical Field

The present invention relates to an ultrahigh-strength and high-ductility steel sheet having an excellent yield ratio and a method for manufacturing the same, and more particularly, to an ultrahigh-strength and high-ductility steel sheet having an excellent yield ratio and a method for manufacturing the same, which is suitable for use as an automobile structural member for cold forming.

Background

Automobile manufacturers are continuously pursuing weight reduction of vehicles due to carbon dioxide emission regulations related to environmental issues. The most effective means for reducing the weight of the automobile body is to reduce the thickness of the steel plate, but simply reducing the thickness of the steel plate may cause a problem that the safety of passengers cannot be ensured due to the reduction in the rigidity of the automobile body. Therefore, it is necessary to use ultra-high strength steel to reduce the body weight of the automobile while ensuring the safety of passengers.

However, steel materials generally tend to have a reduced elongation with an increase in strength, and when ultrahigh-strength steels are applied to structural parts for automobiles requiring complicated formability, various limitations are imposed on workability.

As one method for overcoming these problems, a method using a hot-formed steel is proposed. In hot-formed steel, a steel sheet provided by a steel manufacturer is heated to a high temperature and formed, and then cooled to introduce a low-temperature transformation phase into the steel sheet, so that high strength can be secured while workability is secured in manufacturing structural parts for automobiles requiring formability. As one example, 1.5 GPa-grade hot formed steel is commercially used in structural parts for automobiles, such as a-pillar (a-pillar) of automobiles, which have complicated formability and require impact resistance. However, such hot-forming steel is accompanied by a problem of increasing investment in hot-forming equipment of automobile parts companies and manufacturing costs due to high-temperature heat treatment.

In order to solve the above-described problems, studies have been continuously made on a steel material which can be cold-formed while securing high strength. As an example, patent document 1 proposes an ultra-high strength steel having a yield strength of 1344MPa and a tensile strength of 1520MPa by adding, in weight%, 0.2 to 0.3% of carbon (C) and 2.0 to 3.5% of manganese (Mn). The steel material of patent document 1 has an advantage of having an excellent yield ratio and thus excellent impact resistance and excellent bending properties, but has a poor elongation at a level of less than 7%, and therefore its use in cold forming is limited to the production of members having a relatively simple shape.

Further, patent document 2 proposes an ultra-high strength steel sheet excellent in impact characteristics, wherein the ultra-high strength steel sheet has a tensile strength of 1300MPa or more and a yield strength of 1000MPa or more by adding 0.4 to 0.7% of carbon (C) and 12 to 24% of manganese (Mn) in weight%. However, the steel material of patent document 2 has an elongation of about 10% and a low elongation, and therefore, is limited in application to a member having a complicated shape in cold forming. Further, patent document 2 has a problem that a process flow and cost are increased because high strength is secured by the re-rolling after annealing.

Therefore, as a steel material for cold forming to replace hot formed steel, development of an ultra-high strength and high ductility steel sheet having an excellent yield ratio is currently required.

(Prior art document)

(patent document 1) Korean patent laid-open publication No. 10-1586933 (2016 publication on 01/19/2016)

(patent document 2) Korean laid-open patent publication No. 10-2013-0138039 (published 12 months and 18 days in 2013)

Disclosure of Invention

Technical problem to be solved

According to an aspect of the present invention, an ultra-high strength and high ductility steel sheet having an excellent yield ratio and a method for manufacturing the same can be provided.

The technical problem to be solved by the present invention is not limited to the above. It will be apparent to those skilled in the art that there is no difficulty in understanding the additional technical problems of the present invention from the entire contents of the present specification.

Technical scheme

An ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention includes, in wt%: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less (excluding 0%), the balance Fe and inevitable impurities, and further comprising titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or more, the steel sheet containing 20% or more by area of retained austenite as a fine structure, and the retained austenite having an average aspect ratio (aspect ratio) of 2.0 or more.

The steel sheet may further include, in wt%, a metal selected from the group consisting of nickel (Ni): 1% or less, copper (Cu): 0.5% or less and chromium (Cr): 1% or less.

The steel sheet may include one or more of ferrite, annealed martensite, and newly grown martensite as a residual structure.

The steel sheet may include a balance structure of one or more of the ferrite, the annealed martensite, and the newly grown martensite in a total fraction of 50 to 80 area%.

The steel sheet may have a tensile strength of 1400MPa or more, a yield ratio of 0.7 or more, and a product (TS × EL) of the tensile strength and the elongation of 22000 MPa% or more.

An ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention is manufactured by the following method: heating a slab to a temperature range of 1050 ℃ -: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less (excluding 0%), the balance Fe and inevitable impurities, and further comprising titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or less, hot finish rolling the heated slab at a temperature in the range of 800-, rolling the hot rolled steel sheet at a temperature of 50-750 ℃, pickling the rolled hot rolled steel sheet, and then cold rolling at a reduction ratio of 15% or more to provide a cold rolled steel sheet, the cold rolled steel sheet being selectively subjected to annealing heat treatment according to any one of a first annealing condition and a second annealing condition, wherein the first annealing condition is to perform annealing heat treatment on the cold-rolled steel sheet within a temperature range of 600-720 ℃ for 10-3600 seconds, the second annealing condition is that primary annealing heat treatment is carried out for 10-3600 seconds in a temperature range of over 720 ℃ and below 900 ℃, then cooling to normal temperature, and carrying out secondary annealing heat treatment within the temperature range of 480-700 ℃ for 10-3600 seconds.

The slab may further comprise, in weight%, a material selected from nickel (Ni): 1% or less, copper (Cu): 0.5% or less and chromium (Cr): 1% or less.

Advantageous effects

According to an aspect of the present invention, an ultra-high strength and high ductility steel sheet having an excellent yield ratio and a method for manufacturing the same can be provided.

According to a preferred aspect of the present invention, it is possible to provide an ultra-high strength steel sheet having a tensile strength of 1400MPa or more while satisfying a yield ratio of 0.7 or more and a product of the tensile strength and elongation of 22000 MPa% or more, and thus being particularly suitable for cold forming, and a method for manufacturing the same.

Further, according to a preferred aspect of the present invention, there is provided an ultra-high strength steel sheet for cold forming suitable for automotive structural parts and a method for manufacturing the same, so that equipment investment costs and component manufacturing costs can be effectively reduced.

Drawings

Fig. 1 is a photograph showing a cross section of invention example 1 observed with a Transmission Electron Microscope (TEM).

Fig. 2 is a photograph showing a cross section of invention example 1 observed with a Scanning Electron Microscope (SEM).

Best mode for carrying out the invention

The present invention relates to an ultra-high strength and high ductility steel sheet having an excellent yield ratio and a method for manufacturing the same, and preferred embodiments of the present invention will be described below. The embodiments of the present invention may be changed into various embodiments, and it should not be construed that the scope of the present invention is limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

An ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention includes, in wt%: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less (excluding 0%), the balance Fe and inevitable impurities, and comprises titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or more, the steel sheet may contain 20 area% or more of retained austenite as a fine structure, and the average aspect ratio of the retained austenite may be 2.0 or more.

In addition, the ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention may further include, in wt%, a material selected from the group consisting of nickel (Ni): 1% or less, copper (Cu): 0.5% or less and chromium (Cr): 1% or less.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified,% indicating the content of each element is based on weight.

The ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention may include, in wt%: carbon (C): 0.1-0.3%, silicon (Si): 2% or less, manganese (Mn): 6-10%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, aluminum (Al): 0.5% or less (excluding 0%), and the balance Fe and inevitable impurities.

Carbon (C): 0.1 to 0.3 percent

Carbon (C) is an effective element for reinforcing steel, and in the present invention, carbon (C) is an important element added to control the stability of austenite and to ensure strength. In the present invention, in order to obtain the above-described effects, 0.1% or more of carbon (C) may be added. A preferable lower limit of the carbon (C) content may be 0.11%, and a more preferable lower limit may be 0.12%. However, since weldability may be reduced when a large amount of carbon (C) is added, the present invention may limit the upper limit of the carbon (C) content to 0.3%. A preferable upper limit of the content of carbon (C) may be 0.27%, and a more preferable upper limit may be 0.25%.

Silicon (Si): 2% or less

Silicon (Si) is an element that suppresses precipitation of carbides in ferrite and promotes diffusion of carbon in ferrite into austenite, and is an element that contributes to stabilization of retained austenite. However, when a large amount of silicon (Si) is added, hot rolling and cold rolling properties may become very poor, and silicon (Si) oxide is formed on the steel surface, thereby possibly deteriorating hot dip galvanizability, and therefore, the present invention may limit the upper limit of the silicon (Si) content to 2%. The upper limit of the preferable silicon (Si) content may be 1.9%, and the upper limit of the more preferable silicon (Si) content may be 1.7%.

In addition, the silicon (Si) content of the present invention may be referred to as including 0%. That is, the present invention can exclude intentionally added silicon (Si). As described below, since the present invention contains a large amount of manganese (Mn), the stability of retained austenite can be easily ensured even without adding silicon (Si). However, the lower limit of the content of silicon (Si) in the present invention may be 0.03%, 0.05% or 0.1% in consideration of the content of silicon (Si) inevitably introduced.

Manganese (Mn): 6 to 10 percent

Manganese (Mn) is an element effective for formation and stabilization of retained austenite while suppressing transformation of ferrite, and is an element effective for ensuring mechanical physical properties of steel. In the present invention, in order to obtain the above-described effects, 6% or more of manganese (Mn) may be added. The lower limit of the preferred manganese (Mn) content may be 6.2%, and the lower limit of the more preferred manganese (Mn) content may be 6.5%. However, when a large amount of Mn is added, there is a possibility that an increase in alloy cost and a decrease in spot weldability are caused, so the present invention may limit the upper limit of the Mn content to 10%. The upper limit of the preferred manganese (Mn) content may be 9.8%, and the upper limit of the more preferred manganese (Mn) content may be 9, 5%.

Phosphorus (P): less than 0.05%

Phosphorus (P) is a solid solution strengthening element, but when a large amount of phosphorus (P) is added, weldability may be reduced and the risk of brittleness of the steel may increase. Therefore, the present invention may limit the upper limit of the content of phosphorus (P) to 0.05%, and preferably, may limit the upper limit of the content of phosphorus (P) to 0.02%. However, the lower limit of the content of phosphorus (P) in the present invention may be 0.001% or 0.002% in consideration of the content of phosphorus (P) inevitably introduced.

Sulfur (S): less than 0.02%

Sulfur (S) is an impurity element inevitably contained in steel, and is an element that impairs ductility and weldability of steel. Therefore, in the present invention, in order to secure ductility and weldability of steel, the upper limit of the content of sulfur (S) may be limited to 0.02%, and preferably, the upper limit of the content of sulfur (S) may be limited to 0.015%. However, the lower limit of the sulfur (S) content of the present invention may be 0.001% or 0.002% in consideration of the inevitably introduced sulfur (S) content.

Nitrogen (N): less than 0.02%

Nitrogen (N) is a solid solution strengthening element. However, when a large amount of nitrogen (N) is added, there is a high risk of brittleness, and nitrogen (N) is bonded to aluminum (Al) to precipitate too much AlN, which may impair the quality of continuous casting. Therefore, the present invention may limit the upper limit of the nitrogen (N) content to 0.02%, and preferably, may limit the upper limit of the nitrogen (N) content to 0.015%. However, the lower limit of the nitrogen (N) content of the present invention may be 0.001% or 0.002% in consideration of the content of nitrogen (N) inevitably introduced.

Aluminum (Al): below 0.5% (except 0%)

Aluminum (Al) is an element added for deoxidation of steel, and contributes to stabilization of retained austenite by suppressing formation of carbide in ferrite. In the present invention, in order to obtain the above-described effects, aluminum (Al) may be added. The lower limit of the preferable aluminum (Al) content may be 0.005%, and the lower limit of the more preferable aluminum (Al) content may be 0.01%. However, when a large amount of aluminum (Al) is added, the tensile strength of the steel is reduced, and the integrity of the slab is deteriorated and surface oxides are formed by reaction with the mold flux at the time of casting, so that the plating property may be deteriorated, and thus the present invention may limit the upper limit of the aluminum (Al) content to 0.5%. The upper limit of the preferable aluminum (Al) content may be 0.45%, and the upper limit of the more preferable aluminum (Al) content may be 0.4%.

The remainder of the composition of the present invention is iron (Fe). However, undesirable impurities may be inevitably mixed from the raw materials or the surrounding environment in a general manufacturing process, and thus cannot be completely excluded. These impurities are well known to those skilled in the art of typical manufacturing processes and therefore not all of them are specifically mentioned in this specification.

The ultra-high strength and high ductility steel sheet having an excellent yield ratio according to an aspect of the present invention may further include titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, vanadium (V): 0.2% or less and molybdenum (Mo): 0.5% or less.

Titanium (Ti): less than 0.1%

Titanium (Ti) is a fine carbide-forming element, and is an element that contributes to securing yield strength and tensile strength. Titanium (Ti) is a nitride-forming element, and combines with nitrogen (N) in steel to form TiN precipitates, and therefore titanium (Ti) is an element that contributes to reducing the risk of cracking during continuous casting by suppressing the formation of AlN precipitates. In the present invention, titanium (Ti) may be added in order to obtain the above-described effects. However, when a large amount of titanium (Ti) is added, coarse carbides are precipitated, and strength and elongation may be reduced due to a decrease in carbon content in steel, and clogging of a nozzle may be caused at the time of continuous casting, and therefore, the present invention may limit the upper limit of the titanium (Ti) content to 0.1%. A preferable content of titanium (Ti) may be 0.09%, and a more preferable content of titanium (Ti) may be 0.08%. Further, the present invention does not particularly limit the lower limit of the content of titanium (Ti), but the lower limit of the content of titanium (Ti) may be 0.005% or 0.01%.

Niobium (Nb): less than 0.1%

Niobium (Nb) is an element that segregates at austenite grain boundaries to suppress coarsening of austenite grains during annealing heat treatment, and forms fine carbides to contribute to strength increase. In the present invention, niobium (Nb) may be added in order to obtain the above-described effects. However, when a large amount of niobium (Nb) is added, coarse carbides are precipitated, and strength and elongation may be reduced due to a reduction in the carbon content in the steel, and there is a problem in that the manufacturing cost is increased, so the present invention may limit the upper limit of the niobium (Nb) content to 0.1%. The preferable content of niobium (Nb) may be 0.09%, and the more preferable content of niobium (Nb) may be 0.08%.

Vanadium (V): less than 0.2%

Vanadium (V) is an element that reacts with C or N in steel to form carbonitride, and is an element that forms fine precipitates at low temperature and plays an important role in increasing the yield strength of steel. In the present invention, V may be added in order to obtain the effects as described above. However, when a large amount of V is added, coarse carbides are precipitated, and strength and elongation may be reduced due to a reduction in the carbon content in the steel, and there is a problem of an increase in manufacturing cost, so the upper limit of the V content of the present invention may be limited to 0.2%. The upper limit of the preferable content of vanadium (V) may be 0.18%.

Molybdenum (Mo): less than 0.5%

Molybdenum (Mo) is an element that forms carbides, and when added in a mixed manner with carbonitride forming elements such as titanium (Ti), niobium (Nb), and vanadium (V), it acts to increase the yield strength and tensile strength by keeping the size of precipitates fine. In the present invention, molybdenum (Mo) may be added in order to obtain the effects as described above. However, when a large amount of molybdenum (Mo) is added, the above effects are saturated, and there is a problem that the manufacturing cost is increased, so the present invention can limit the upper limit of the content of molybdenum (Mo) to 0.5%. The upper limit of the preferable content of molybdenum (Mo) may be 0.4%.

The ultra-high strength and high ductility steel sheet according to an aspect of the present invention may further include, in wt%, one or more elements selected from the group consisting of nickel (Ni): 1% or less, copper (Cu): 0.5% or less, chromium (Cr): 1% or less.

Nickel (Ni), copper (Cu), and chromium (Cr) are elements that contribute to stabilization of austenite by acting in combination with the above-described carbon (C), silicon (Si), aluminum (Al), and the like. However, when nickel (Ni), copper (Cu), and chromium (Cr) are added in excess of a certain level, an excessive increase in manufacturing cost is caused, and thus the upper limits of the contents of nickel (Ni), copper (Cu), and chromium (Cr) of the present invention may be limited to 1%, 0.5%, and 1%, respectively. Further, since chromium (Cr) may cause brittleness during hot rolling, it is more preferable to add chromium (Cr) together with nickel (Ni).

Hereinafter, the microstructure of the ultra-high strength and high ductility steel sheet according to one aspect of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the fraction of the fine structure and the aspect ratio (aspect ratio) refer to values measured with reference to a steel sheet section.

The ultra-high strength and high ductility steel sheet according to an aspect of the present invention may include retained austenite as a fine structure. The retained austenite is a structure effective for securing strength characteristics and elongation characteristics of steel, and therefore the fraction of the retained austenite may be limited to 20 area% or more based on the steel sheet cross section.

The ultra-high strength and high ductility steel sheet according to an aspect of the present invention may include one or more of ferrite, annealed martensite, and newly grown martensite as a residual structure, and a total fraction of the residual structure may be 50 to 80 area% on the basis of a cross-section of the steel sheet. The ultra-high strength and high ductility steel sheet according to an aspect of the present invention is a transformation induced plasticity (TRIP), i.e., a steel in which the retained austenite is transformed into martensite to increase elongation upon application of external deformation, and the mechanical stability of the retained austenite and the fraction thereof are important factors for an optimal combination of strength and elongation. When the fraction of the retained austenite exceeds 50 area%, the mechanical stability of the retained austenite is lowered, and therefore the total fraction of the residual microstructure may be limited to 50 area% or more. Further, when the residual microstructure exceeds 80 area%, a desired fraction of retained austenite cannot be secured, so the total fraction of the residual microstructure may be limited to 80 area% or less.

The retained austenite may have an average aspect ratio of 2.0 or more. The aspect ratio is a value obtained by dividing the length of the major axis of the crystal grain by the length of the minor axis, and the average aspect ratio of austenite in the present invention is an average value of the aspect ratios of austenite crystal grains observed in the cross section. When the average aspect ratio of the retained austenite is 2.0 or more, the retained austenite exists in a needle-like shape, so that stability is high, and propagation of cracks is inhibited at the time of fracture, so that elongation can be effectively secured.

The ultrahigh-strength and high-ductility steel sheet according to one aspect of the present invention has a tensile strength of 1400MPa or more and is excellent in yield strength because it satisfies both a fraction of retained austenite of 20 area% or more and an average aspect ratio of retained austenite of 2.0 or more, and can ensure a yield ratio (yield strength/tensile strength) of 0.7 or more and further ensure a product of the tensile strength and elongation of 22000 MPa% or more.

The tensile strength after hot forming of the steel for hot forming which is most widely used at present is about 1470MPa, but the ultra-high strength and high ductility steel sheet according to one aspect of the present invention has a tensile strength of 1400MPa or more and a yield ratio of 0.7 or more, and therefore, a steel for cold forming which can replace the steel for hot forming can be provided. In addition, in the case of structural members for automobiles, particularly B-pillars (B-pilars), the steel for hot forming is currently manufactured from steel for hot forming for reasons of structural complexity, impact stability, and the like, but the ultra-high-strength and high-ductility steel sheet according to one aspect of the present invention ensures a product of tensile strength and elongation of 22000 MPa% or more, and thus can provide a steel for cold forming which is particularly suitable for manufacturing structural members for automobiles.

In addition, the ultra-high strength and high ductility steel sheet according to an aspect of the present invention may further have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

The production method of the present invention will be described in more detail below.

The ultra-high strength and high ductility steel sheet having an excellent yield ratio according to one aspect of the present invention may be manufactured by the following method: heating a slab having the above composition to a temperature range of 1050-, then cooling to normal temperature, and carrying out secondary annealing heat treatment within the temperature range of 480-700 ℃ for 10-3600 seconds.

Heating slab

The slab composition of the present invention corresponds to the composition of the ultra-high strength and high ductility steel sheet described above, and therefore the description of the composition of the slab is replaced with the description of the composition of the ultra-high strength and high ductility steel sheet described above.

In the present invention, the homogenization treatment may be performed by heating the slab before the hot rolling. At this time, when the heating temperature of the slab is less than 1050 ℃, there is a problem that the rolling load may be sharply increased at the time of the subsequent hot rolling. On the other hand, when the heating temperature of the slab exceeds 1300 ℃, not only energy cost is increased but also the amount of surface scale is increased, thus possibly causing loss of material, and when a large amount of manganese (Mn) is contained, a liquid phase may exist. Thus, the heating temperature range of the slab of the present invention may be 1050-1300 ℃.

Hot rolling

The heated slab may be hot rolled to manufacture a hot rolled steel sheet. In this case, when the finish hot rolling temperature is less than 800 ℃, there is a possibility that a rolling load is sharply increased. On the other hand, when the finish hot rolling temperature exceeds 1000 ℃, surface defects due to surface scale and shortening of the life of the roll may become problems. Therefore, the finish hot rolling temperature of the invention can be 800-1000 ℃.

Rolling-up device

After hot rolling, the hot rolled steel sheet may be wound. When the coiling temperature is too high, excessive surface scale of the steel sheet is formed, and thus the plating property may be deteriorated, so the coiling temperature of the present invention may be 750 ℃ or less. Further, since the steel sheet containing 5% or more of manganese (Mn) has increased hardenability, there is no transformation of ferrite when cooled to normal temperature after hot rolling and coiling, and therefore, the lower limit of the coiling temperature is not particularly limited. However, when the coiling temperature is less than 50 ℃, a cooling process accompanied by spraying cooling water is required in order to lower the temperature of the steel sheet, and thus the process cost is inevitably increased. Therefore, the rolling temperature range of the invention can be 50-750 ℃.

In addition, as manganese (Mn) is added, the transformation start temperature (Ms) of martensite may be lowered, and martensite may also be formed at normal temperature. In this case, the hardness of the hot-rolled steel sheet becomes very high due to the martensite structure, and thus the load of the cold rolling may increase, so that it is possible to selectively perform further heat treatment on the hot-rolled steel sheet before the cold rolling.

Pickling and cold rolling

The rolled hot rolled steel sheet is unwound and then subjected to an acid washing process to remove an oxide layer, and cold rolling may be performed to manufacture a cold rolled steel sheet in order to adjust the thickness and shape of the steel sheet according to conditions required by a client company. When the cold rolling reduction does not reach a certain level, it is difficult to secure the fraction of the retained austenite and the average aspect ratio of the retained austenite, which are targeted in the present invention. This is because when the cold rolling reduction is low, the driving force for reverse transformation and growth of austenite at the time of final annealing is insufficient. Therefore, the cold rolling reduction of the present invention may be 15% or more. In addition, the present invention contains a large amount of manganese (Mn) so that the hot rolled steel sheet has relatively high strength, and thus, when the cold rolling reduction exceeds a certain level, an excessive load of a cold rolling apparatus may be induced. Therefore, the cold rolling reduction of the present invention may be 50% or less, and the upper limit of the more preferable cold rolling reduction may be 45%.

Annealing heat treatment

After the cold rolling, annealing heat treatment may be performed under certain conditions. In particular, in order to secure the physical properties required for the present invention, it is necessary to control the area fraction of the retained austenite and the average aspect ratio of the retained austenite at target levels, which can be achieved by strictly controlling the annealing heat treatment conditions. The annealing heat treatment of the present invention may be performed by selecting either one of a first annealing condition in which the annealing heat treatment is performed at a relatively low annealing temperature or a second annealing condition in which the annealing heat treatment is performed at a relatively high annealing temperature and then a subsequent heat treatment is further performed.

That is, in the case of the first annealing condition, the cold-rolled steel sheet may be subjected to the annealing heat treatment within the temperature range of 600-.

The reason why the first annealing condition is limited among the annealing heat treatment conditions of the present invention will be described in more detail below.

The annealing heat treatment according to the first annealing condition may be performed at a temperature range of 600-720 deg.c for 10-3600 seconds.

For the steel composition of the present invention, the temperature range of 600-720 ℃ belongs to the two-phase region temperature range. When the two-phase region annealing is performed, elements such as carbon (C) and manganese (Mn) are enriched in austenite, thereby increasing the stability of austenite and remaining at normal temperature. Thereafter, when deformation is applied to the steel sheet, the retained austenite is transformed into martensite, and a necking phenomenon of the steel sheet is delayed, thereby contributing to improvement of elongation and strength of the steel sheet.

When the annealing temperature under the first annealing condition is less than 600 ℃, the austenite fraction finally remaining in the steel sheet cannot be sufficiently secured due to the small austenite fraction in the two-phase region, and thus desired mechanical physical properties cannot be secured. In addition, when the annealing temperature under the first annealing condition exceeds 720 ℃, the stability of austenite in the two-phase region and the single-phase region is insufficient, so it is difficult to secure the residual austenite fraction of the final steel sheet to 20 area% or more, and desired mechanical and physical properties cannot be secured. Therefore, the annealing heat treatment temperature range of the first annealing condition of the present invention may be 600-720 ℃.

When the annealing heat treatment is performed under the first annealing condition, it is preferable to perform the heat treatment for a heat treatment time of at least 10 seconds in consideration of the phase transition mechanism (mechanism) and the driving force (driving force). As the annealing heat treatment time increases, the equilibrium phase is approached, and thus a uniform structure can be obtained, but a problem of increasing the process cost may occur. Further, when the annealing heat treatment time exceeds 3600 seconds, an average aspect ratio of austenite of 2.0 or more cannot be achieved due to grain growth and recrystallization of austenite. Therefore, the annealing heat treatment time of the first annealing condition of the present invention may be 10 to 3600 seconds.

The reason for limiting the second annealing condition among the annealing heat treatment conditions will be described in more detail below.

The annealing heat treatment according to the second annealing condition may be performed by performing the primary annealing heat treatment for 10 to 3600 seconds within a temperature range of more than 720 ℃ and 900 ℃ or less, cooling to a normal temperature, and performing the secondary annealing heat treatment for 10 to 3600 seconds within a temperature range of 480-.

The temperature range of the steel composition of the present invention exceeding 720 ℃ and not more than 900 ℃ falls within the temperature range of the two-phase region or austenite single-phase region in which the austenite fraction is too large. Therefore, when the annealing heat treatment is performed in a temperature range of more than 720 ℃ and 900 ℃ or less, the stability of austenite is greatly lowered, and therefore, most of the austenite is transformed into martensite in the cooling step, and only a part of the austenite remains. The retained austenite has low stability and a low fraction thereof as described above, and therefore, the stability and the fraction of austenite can be ensured by further annealing heat treatment. However, even if the primary annealing heat treatment temperature of the second annealing condition exceeds 900 ℃, the physical properties desired in the present invention can be secured, but problems such as a shortened equipment life of the annealing furnace due to the high-temperature heat treatment and poor plating property due to an increase in oxide on the surface of the steel sheet occur, and therefore the primary annealing heat treatment temperature of the second annealing condition of the present invention can be limited to 900 ℃ or less.

The annealing heat treatment according to the second annealing condition of the present invention is performed by performing the primary annealing heat treatment for 10 to 3600 seconds in a temperature range of more than 720 ℃ and 900 ℃ or less, then cooling to a normal temperature, and performing the secondary annealing heat treatment for 10 to 3600 seconds in a temperature range of 700 ℃ or less, so that the stability and the fraction of austenite can be secured. The second annealing condition of the present invention can be also interpreted as a supplementary annealing heat treatment condition for ensuring stability and fraction of austenite when the annealing heat treatment is performed at an annealing temperature range exceeding the limit of the first annealing condition, which is originally intended to perform the annealing heat treatment according to the first annealing condition.

The reason why the temperature range and the annealing time of the secondary annealing heat treatment in the second annealing condition are limited to 480-700 ℃ and 10-3600 seconds is as follows.

At 480-600 c, which is a relatively low temperature range in the temperature range of the secondary annealing heat treatment, carbon supersaturated in martensite after the primary annealing heat treatment is redistributed to a part of the retained austenite, and thus has the effect of increasing the stability of austenite. When the phase transformation mechanism and the driving force are considered, the above-described effects can be achieved when the annealing heat treatment is performed at this temperature for 10 seconds or more. When the time of the secondary annealing heat treatment at this temperature exceeds a certain level, carbide precipitates rather than redistribution of carbon between phases occurs, and thus the elongation tends to decrease instead. Therefore, the time of the secondary annealing heat treatment at this temperature is preferably limited to 3600 seconds or less.

At 600-700 c, which is a relatively high temperature range in the temperature range of the secondary annealing heat treatment, reverse transformation from the primary annealed structure to austenite occurs, thus having the effect of increasing the fraction of austenite. When the secondary annealing heat treatment is performed at this temperature for 10 seconds or more in consideration of the phase transformation mechanism and the driving force, the effects as described above can be achieved. As the time of the secondary annealing heat treatment at this temperature increases, a uniform structure close to the equilibrium phase can be obtained, but a problem that an excessive process cost is required may occur. Therefore, the time of the secondary annealing heat treatment at this temperature is preferably limited to 3600 seconds or less.

When the secondary annealing heat treatment temperature exceeds 700 ℃, the stability of the finally remaining austenite is lowered due to the increase of the fraction of austenite in the two-phase region, or the average aspect ratio of austenite is shown to be less than 2.0, and thus it may be difficult to secure the desired physical properties in the present invention. Further, when the primary annealing heat treatment is performed before the secondary annealing heat treatment as in the second annealing condition, the reverse transformation of austenite is accelerated at the same annealing temperature at the time of the secondary annealing heat treatment, and thus a phenomenon of increasing the fraction of austenite in the two-phase region occurs. Therefore, the upper limit of the secondary annealing temperature of the second annealing condition is preferably limited to 700 ℃, which is slightly lower than the upper limit of the annealing temperature of the first annealing condition of 720 ℃.

The method of manufacturing an ultra-high strength and high ductility steel sheet according to an embodiment of the present invention may perform hot-dip galvanizing or alloying hot-dip galvanizing on the cold-rolled steel sheet.

Detailed Description

The present invention will be described more specifically with reference to examples.

(examples)

Steels having the composition of the following table 1 were vacuum melted into ingots of 30kg, which were then maintained at a temperature of 1200 c for 1 hour and hot rolled, and finish rolling was completed at 900 c, thereby manufacturing hot rolled steel sheets. The hot rolled steel sheet was charged into a furnace previously heated to 600 ℃ and held for 1 hour, and then wound up by furnace cooling simulating hot rolling, and then cooled to normal temperature and then pickled, and cold rolled and annealed according to the conditions of table 2 below. In table 2, the test piece in which only the primary annealing condition is described indicates the case where the one-stage annealing condition is applied, and the test piece in which the primary annealing condition and the secondary annealing condition are described indicates the case where the two-stage annealing condition is applied. The results of observation of the microstructure and evaluation of the mechanical physical properties of the cold-rolled steel sheets manufactured as described above are shown in table 3 below. The austenite fraction of each test piece was measured by XRD, and the physical properties of each test piece were evaluated by measuring the physical properties in the rolling direction and the perpendicular direction of a tensile test piece in JIS standard.

[ Table 1]

[ Table 2]

[ Table 3]

As shown in tables 1 to 3, it was confirmed that, in the case of invention examples 1 to 13, which all satisfy the alloy composition and the production conditions of the present invention, not only the Tensile Strength (TS) of 1400MPa or more but also the Yield Ratio (YR) of 0.7 or more and the product (TS × El) of the tensile strength and the elongation of 22000 MPa% or more were satisfied. That is, it is understood that in the cases of invention examples 1 to 13, since the steel material for cold forming which can replace the hot formed steel has suitable physical properties because the steel material ensures an ultra high strength and excellent yield strength and elongation.

The excellent physical properties of invention examples 1 to 13 are based on the characteristics of the fraction and aspect ratio of the retained austenite structure, and the ultra-fine refinement of crystal grains and precipitates. Fig. 1 is a photograph showing a cross section of invention example 1 observed with a Transmission Electron Microscope (TEM), and it is understood that the size of most of the microstructures is 1 μm or less and very fine, and therefore, strength and elongation can be effectively secured. Fig. 2 is a photograph showing a cross section of invention example 1 observed with a Scanning Electron Microscope (SEM), and it can be confirmed that the retained austenite is formed in a needle shape and the average aspect ratio has a value of 2.0 or more.

On the other hand, in the case of comparative examples 1 to 13 which do not satisfy any one or more of the alloy composition and the production conditions of the present invention, it was confirmed that any one or more of the fraction of the retained austenite structure and the average aspect ratio of the retained austenite structure of the present invention was not satisfied, and the desired physical properties of the present invention could not be secured.

In the case of comparative examples 1 and 2, although the alloy compositions of the present invention were satisfied, the annealing heat treatment temperatures were 550 ℃ and 780 ℃, respectively, out of the range of the present invention when the first annealing condition was applied, and thus it was confirmed that the retained austenite fraction was less than 20 area%. Further, it is found that the comparative examples 1 and 2 do not satisfy the range of the fraction of retained austenite of the present invention, and therefore the yield ratio is less than 0.7 or the product value of the tensile strength and the elongation is less than 22.000MPa, and the desired physical properties cannot be secured.

In the case of comparative examples 3 and 11, the alloy composition of the present invention was satisfied, and the temperature of the primary annealing heat treatment exceeded 720 ℃, and the second annealing condition for performing the secondary annealing heat treatment was applied, but since the temperature of the secondary annealing heat treatment was 460 ℃ and did not fall within the range of the present invention, it was confirmed that the retained austenite fraction was less than 20 area%. Further, it is found that the product of the tensile strength and the elongation is less than 22000MPa and the desired physical properties cannot be secured because comparative example 3 and comparative example 11 do not satisfy the range of the retained austenite fraction of the present invention.

In the case of comparative examples 7 and 8, the alloy composition of the present invention was satisfied, and the primary annealing heat treatment temperature exceeded 720 ℃, and the second annealing condition for performing the secondary annealing heat treatment was applied, but the secondary annealing heat treatment temperatures were 710 ℃ and 740 ℃, respectively, which were out of the range of the present invention, and therefore it was confirmed that the average aspect ratio of the retained austenite was less than 2.0. Further, it is found that the average aspect ratio of the retained austenite of the present invention is not satisfied in comparative examples 7 and 8, and therefore the yield ratio is less than 0.7, and the product of the tensile strength and the elongation is less than 22.000MPa, and the desired physical properties cannot be secured.

In the case of comparative examples 4 and 5, the alloy composition and the annealing conditions of the present invention were satisfied, but the cold rolling reduction was 11% and the range of the present invention was not satisfied, and therefore the fraction of the retained austenite was less than 20 area%, and it was confirmed that the average aspect ratio of the retained austenite was less than 2.0. Further, it is found that the product of the tensile strength and the elongation is less than 22000MPa, and the desired physical properties cannot be secured, since comparative examples 4 and 5 do not satisfy the fraction of retained austenite and the average aspect ratio of the present invention, the yield ratio is less than 0.7.

In the case of comparative examples 6 and 10, the alloy composition of the present invention was satisfied, and the primary annealing heat treatment temperature exceeded 720 ℃, and the second annealing condition for performing the secondary annealing heat treatment was applied, but the secondary annealing heat treatment time was 7200 seconds and was out of the range of the present invention, and therefore, it was confirmed that the fraction of retained austenite was less than 20 area%. Further, it is found that the product of the tensile strength and the elongation is less than 22000MPa because comparative examples 6 and 10 do not satisfy the retained austenite fraction of the present invention, and the desired physical properties cannot be secured.

In the case of comparative examples 12 and 13, although the cold rolling condition and the annealing condition of the present invention were satisfied, since the carbon (C) content was not within the range of the present invention, it was confirmed that the fraction of the retained austenite was less than 20 area%. Further, it is found that the yield ratio is less than 0.7 and the desired physical properties cannot be secured because comparative example 12 and comparative example 13 do not satisfy the retained austenite fraction of the present invention.

The present invention has been described in detail with reference to the examples, but other embodiments are possible. Therefore, the technical spirit and scope of the claims of the present invention are not limited to the embodiments.