Steel sheet and method for manufacturing same

Abstract

The present invention relates to a material applied to automotive parts, and more particularly, to a cold-rolled steel sheet and a plated steel sheet having very excellent bending characteristics, and methods for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a material used for automobile parts, and more particularly, to a cold-rolled steel sheet (and plated steel sheet) having very excellent bending properties and a method for manufacturing the same. Background Art

[0002] In recent years, there has been a demand for improved fuel efficiency of automobiles in order to protect the global environment. In particular, automobile steel sheets have been required to have a higher level of strength in order to reduce the weight of the vehicle body and ensure safety.

[0003] So far, various steel sheets that have both strength and workability have been designed and put into practical use. For example, composite structure steel sheets in which ferrite phase and low temperature transformation phases such as martensite or bainite coexist are used as high strength steel sheets with excellent workability. The purpose of composite structure steel sheets is to improve both strength and workability by dispersing hard low temperature transformation phases in soft ferrite.

[0004] In addition, in the case of steel materials used as collision parts, when manufactured using cold forming technology, it is necessary to develop an ultra-high strength steel with higher processing characteristics, especially excellent bendability, and research related to a manufacturing method for ultra-high strength steel with a tensile strength of 1500MPa or above using a single phase of martensite is being actively carried out.

[0005] Generally, a hot press forming (HPF) process is being developed, which ensures the required strength by water cooling between the mold and the material after forming the material at a high temperature that is relatively easy to form. Since high strength can be ensured compared with the same thickness, the HPF process is widely used in the manufacture of parts, but there are problems in application due to excessively high equipment investment costs and increased process costs, so it is necessary to develop a material for cold stamping. Therefore, it is necessary to develop a cold-rolled steel sheet suitable for use as a material for cold stamping, having high strength and a high yield ratio to ensure collision performance and having excellent bending characteristics.

[0006] As a representative prior art of this method, there is Patent Document 1. Patent Document 1 uses a steel, wherein the steel composition includes: C: 0.25-0.4%, Si: 1.0% or less, Mn: 1.5-2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01-0.1%, N: 0.005% or less, B: 0.0005-0.005%, and Ti: 0.005-0.1% and Nb: 0.005-0.1% contain a total of 0.005-0.1%, and the metal structure is a martensitic single-phase structure. By using this steel, heating, storage and holding in a temperature range above the Ae3 transformation point and below 900°C, then rapidly cooling to below 200°C at an average cooling rate of 300°C / second or more, and then tempering at below 250°C, a steel plate is manufactured, but water cooling leads to poor shape (flatness), which has the disadvantage of causing defects during forming.

[0007] Patent document 2 is a technology for manufacturing a thin steel plate, wherein the thin steel plate has the following steel structure, wherein C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, N: 0.0060% or less, ferrite area ratio is 0% or more and 90% or less, bainite area ratio is 5% or less (including 0%), martensite and tempered martensite area ratio is 10% or more (including 100%), retained austenite area ratio is 2.0% or less (including 0%), the standard deviation of yield strength in the width direction is 30 MPa or less, and the maximum bending amount of the steel plate when sheared at a length of 1 m is 10 mm or less. However, there is also a problem of shape deterioration due to rapid cooling after annealing, so there are limitations that are unfavorable in practical use.

[0008] (Patent Document 1) Japanese Application No. 2009-098534

[0009] (Patent Document 2) Japanese Application No. 2018-143806 Summary of the invention

[0010] 1. Technical issues to be resolved

[0011] An object of one aspect of the present invention is to overcome the limitations of the above-mentioned prior art and to provide a steel plate having excellent shape and bending properties and an ultra-high strength of 1500 MPa or more by optimizing the steel composition and the manufacturing process.

[0012] The technical problems of the present invention are not limited to the above contents. Additional technical problems of the present invention are described in the overall content of the specification, and those skilled in the art can easily understand the additional technical problems of the present invention from the contents recorded in the specification of the present invention.

[0013] (II) Technical solution

[0014] One embodiment of the present invention provides a steel plate, which comprises, by weight%, carbon (C): 0.1-0.3%, silicon (Si): 0.5% or less (except 0%), manganese (Mn): 1.3-2.5%, chromium (Cr): 0.2% or less (except 0%), molybdenum (Mo): 0.01-0.1%, boron (B): 0.0005-0.003%, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.01% or less (except 0%), nitrogen (N ): less than 0.01% (except 0%), aluminum (Al): 0.01-0.1%, niobium (Nb): 0.01-0.05%, titanium (Ti): 0.01-0.05% and the balance Fe and inevitable impurities, in an area within 20μm from the surface, the ratio (a / b×100) of the total content of C and Mn (a) to the total content of C and Mn (b) at a distance of (1 / 4)×t (wherein t is the total thickness of the steel plate) from the surface is 75% or more (except 100%).

[0015] In the steel sheet, in a region within 20 μm from the surface, the microstructure may include 10% or less (excluding 0%) of one or more phases selected from ferrite and bainite in terms of area %.

[0016] In the steel plate, in a region within 20 μm from the surface, the balance of the microstructure may be martensite.

[0017] In the steel plate, in a region within 20 μm from the surface, the microstructure may include 90-99% of martensite in terms of area %.

[0018] The t may be 0.6-2.5 mm.

[0019] The steel plate may include a zinc-based coating on its surface.

[0020] The steel plate may have a tensile strength (TS) of 1500 MPa or more, and a bendability (R / t) of 3.7 or less.

[0021] Another embodiment of the present invention provides a method for manufacturing a steel plate, the manufacturing method comprising the following steps: reheating a steel billet at a temperature of 1100-1300° C., wherein the steel billet comprises, by weight%, carbon (C): 0.1-0.3%, silicon (Si): 0.5% or less (except 0%), manganese (Mn): 1.3-2.5%, chromium (Cr): 0.2% or less (except 0%), molybdenum (Mo): 0.01-0.1%, boron (B): 0.0005-0.003%, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.01% or less (except 0%), nitrogen (N): 0.01% or less (except 0%), aluminum (Al): 0.01-0 .1%, niobium (Nb): 0.01-0.05%, titanium (Ti): 0.01-0.05% and the balance Fe and inevitable impurities; hot rolling the reheated slab; coiling the hot rolled steel plate at 400-600°C; cold rolling the coiled steel plate at a reduction rate of 30-80%; heat treating and annealing the cold rolled steel plate at Ac3+10°C to Ac3+80°C; cooling the annealed steel plate once at an average cooling rate of less than 10°C / second to 680-749°C as the termination temperature range of the primary cooling; and secondary cooling the steel plate after the primary cooling at an average cooling rate of 60-160°C / second to a temperature of 100°C to Mf.

[0022] The annealing step may be performed by heat treatment at Ac3+10° C. to Ac3+80° C. for more than 30 seconds.

[0023] The method may further include reheating the secondary cooled steel plate to 150-240° C. and performing an aging heat treatment.

[0024] The overaging heat treatment step may be performed for 400-1000 seconds.

[0025] (III) Beneficial effects

[0026] According to the present invention, it is possible to provide a steel sheet which overcomes the limitations of the prior art, is excellent in shape and bending properties, and has an ultra-high strength of 1500 MPa or more in tensile strength.

[0027] The various advantages and effects of the present invention are not limited to the above contents, and the various advantages and effects of the present invention will be more easily understood in the process of describing the specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A photograph of a cross section in the thickness direction of the steel plate of Inventive Example 1 according to the present invention taken with a scanning electron microscope (SEM) is shown. Best Mode for Carrying Out the Invention

[0029] The terms used in this specification are used to illustrate the present invention, not to limit the present invention. In addition, as long as there is no clear contrary meaning in the relevant definition, the singular form used in this specification also includes the plural form.

[0030] The words “include” or “comprising” used in the specification are intended to specify a configuration, but do not exclude the existence or addition of other configurations.

[0031] Unless otherwise defined differently, all terms used in this specification, including technical terms and scientific terms, have the same meanings as those generally understood by those skilled in the art. Terms defined in dictionaries should be interpreted as having meanings consistent with relevant technical documents and currently disclosed contents.

[0032] In order to ensure the existing collision performance, research on steel plates with high strength and high yield ratio is continuously being conducted, but there are technical limitations in the existing technology, such as defects caused during forming or shape defects caused by rapid cooling after annealing during the process.

[0033] In order to solve the above-mentioned problems of the prior art, it is necessary to develop an ultra-high strength cold-rolled steel sheet and plated steel sheet having excellent shape and bending properties and a tensile strength of 1500 MPa or more, but a technology that meets such requirements has not been developed so far.

[0034] Therefore, the inventors have conducted intensive research to solve the above-mentioned problems of the prior art, and have confirmed through experiments that when the composition and operating conditions satisfy a specific relationship, the desired physical properties can be ensured, thereby completing the present invention. Therefore, according to the present invention, in order to overcome the limitations of the prior art, it is possible to provide an ultra-high strength steel sheet having excellent shape and bending properties and a tensile strength of 1500 MPa or more by optimizing the steel composition and the manufacturing process.

[0035] The present invention will be described in detail below. First, a specific embodiment of the steel sheet according to the present invention will be described in detail.

[0036] The alloy composition of the steel sheet according to the present invention includes, by weight%, carbon (C): 0.1-0.3%, silicon (Si): 0.5% or less (except 0%), manganese (Mn): 1.3-2.5%, chromium (Cr): 0.2% or less (except 0%), molybdenum (Mo): 0.01-0.1%, boron (B): 0.0005-0.003%, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.01% or less (except 0%), nitrogen (N): 0.01% or less (except 0%), aluminum (Al): 0.01-0.1%, niobium (Nb): 0.01-0.05%, titanium (Ti): 0.01-0.05% and the balance of Fe and inevitable impurities. In the present invention, unless otherwise specified, the content of each element is based on weight%.

[0037] Carbon (C): 0.1-0.3 wt%

[0038] Carbon is an interstitial solid solution element, and is the most effective and important element for improving the strength of steel, and is an element that must be added to ensure the strength of martensitic steel. In order to obtain ultra-high strength steel that meets the target yield ratio and tensile strength of the present invention, it is preferred to add more than 0.1% carbon, and more preferably more than 0.15% carbon. However, when the carbon content exceeds 0.3%, due to the increase in hardenability, too much martensite is formed during the cooling process, so the strength increases rapidly, resulting in the possible deterioration of elongation. In addition, the increase in the carbon content has the problem of damaging weldability, so it is preferred that the upper limit of the C content is limited to 0.3%, and more preferably it can be less than 0.28%. In addition, the lower limit of the C content can be 0.2%, or the upper limit of the C content can be 0.25%.

[0039] Silicon (Si): 0.5 wt% or less (except 0 wt%)

[0040] Silicon is a well-known core element of the transformation induced plasticity (TRIP) steel with the effect of improving the fraction and elongation of retained austenite. In addition, in the present invention, adding Si can suppress the precipitation of cementite and play the effect of suppressing the generation of cracks during bending. Therefore, in order to obtain the above-mentioned effect, 0 weight % is excluded from the content of Si. However, when the content of the Si exceeds 0.5%, not only the weldability deteriorates, but also the surface characteristics and the coating property of the steel sheet deteriorate, so the content of Si includes 0.5 weight %. In addition, in terms of further improving the above-mentioned effect, the lower limit of the content of the Si can be 0.01%, or the upper limit of the content of the Si can be 0.3%.

[0041] Manganese (Mn): 1.3-2.5 wt%

[0042] Manganese is an element added to ensure strength. When the content of Mn is less than 1.3%, the hardenability is low, and when the cooling rate is not fast enough during cooling after annealing, martensite cannot be formed, so it is difficult to ensure the strength of the level required by the present invention. On the other hand, when the content of manganese exceeds 2.5%, the Ms temperature decreases during cooling after annealing, and as the final cooling temperature decreases, the shape of the steel plate deteriorates, and it is difficult to ensure the initial martensitic structure. In addition, during steelmaking / continuous casting operations, a segregation band is generated in the length direction of the Mn-based slab, which is a factor that reduces the bendability, so the upper limit of the manganese content is set. That is, since manganese is easy to segregate in the thickness direction and form a manganese band (Mn band) in the slab, in addition to continuous casting cracks, the problem of increased occurrence of defects during the rolling process may also occur, so the content of Mn is preferably 1.3-2.5%. In addition, in terms of further improving the above-mentioned effects, the lower limit of the content of Mn can be 1.5%, or the upper limit of the content of Mn can be 2.1%.

[0043] Chromium (Cr): 0.2 wt% or less (except 0 wt%)

[0044] Chromium is an alloying element that easily ensures low-temperature phase transformation structure by suppressing ferrite phase transformation. When using a continuous annealing process with slow cooling as described in the present invention, it has the advantage of suppressing the formation of ferrite. In order to obtain the above effect, 0 weight% is excluded from the Cr content. However, when the Cr content exceeds 0.2%, the delayed fracture resistance may deteriorate. In addition, due to the formation of carbides such as CrC, the hole expansion and bending processability are hindered, and the cost may increase due to excessive alloy addition. Therefore, the Cr content preferably has a range of less than 0.2%. In addition, in terms of further improving the above effect, the upper limit of the Cr content can be 0.15% or 0.1%. However, if it can be manufactured by optimizing the composition and operating conditions, there is no need to specifically set the lower limit of the Cr content, but as an example, the lower limit of the Cr content can be 0.01%.

[0045] Molybdenum (Mo): 0.01-0.1 wt%

[0046] Molybdenum has the effect of improving the hardenability of steel, the effect of generating fine carbides containing Mo that become hydrogen traps, and the effect of improving delayed fracture resistance by miniaturizing martensite. However, when the content of Mo exceeds 0.1%, its effect is not significant compared with the cost increase caused by adding high-cost alloying elements, so the upper limit is preferably set to 0.1% or less. On the other hand, when the content of Mo is less than 0.01%, the basic characteristics of Mo are not exhibited at all, so it was confirmed through experiments that there is no improvement effect on delayed fracture, and the lower limit of the content of Mo is set to 0.01% or more. In addition, in terms of further improving the above-mentioned effects, the lower limit of the content of Mo can be 0.012%, or the upper limit of the content of Mo is more preferably 0.08%.

[0047] Boron (B): 0.0005-0.003 wt%

[0048] The boron is an element that inhibits the formation of ferrite, and therefore, in the present invention, it has the advantage of inhibiting the formation of ferrite during cooling after annealing. However, when the content of the B exceeds 0.003%, the ductility may be greatly reduced. On the other hand, when the content of the B is less than 0.0005%, there is no hardenability effect at all, so not only can the desired strength not be ensured, but also ferrite is formed in the surface layer, and there is a tendency for the bendability to deteriorate, so its lower limit is limited. In addition, in order to further improve the above-mentioned effect, the lower limit of the content of the B can be 0.0008%, or the upper limit of the content of the B can be 0.0022%.

[0049] Phosphorus (P): 0.1% by weight or less (except 0%)

[0050] Phosphorus is an impurity element contained in steel. The lower the amount added to the steel, the better. However, considering the situation that it is inevitably included in the manufacturing process, the content is excluded to be 0%. However, when the content of P exceeds 0.1%, weldability may deteriorate and brittleness of the steel may be caused, so the upper limit of the content of P may be limited to 0.1% or less. In addition, in terms of further improving the above-mentioned effect, the lower limit of the content of P may be 0.0001%, or the upper limit of the content of P may be 0.03%.

[0051] Sulfur (S): 0.01% by weight or less (except 0%)

[0052] Like P, sulfur is an impurity inevitably contained in steel and is an element that impairs the ductility and weldability of the steel plate, so it is preferred to control the sulfur content to a low level as much as possible, and therefore the sulfur content in the present invention is preferably limited to 0.01% or less. However, considering the case where it is inevitably contained during the manufacturing process, 0% is excluded. In addition, in terms of further improving the above-mentioned effect, the lower limit of the S content may be. In addition, in order to minimize the MnS precipitates in the steel to further contribute to improving the bendability, the upper limit of the S content may be 0.008%, or the upper limit may be 0.005%.

[0053] Nitrogen (N): 0.01% by weight or less (except 0%)

[0054] Nitrogen is an impurity element. When the content of N exceeds 0.01%, the risk of cracking during continuous casting due to the formation of AlN and the like is greatly increased, so the upper limit of the content of N is preferably limited to 0.01%. However, considering the situation that it is inevitably included in the manufacturing process, 0% is excluded. In addition, in terms of further improving the above-mentioned effect, the lower limit of the content of N may be 0.0001%, or the upper limit of the content of N is more preferably 0.008%, and more preferably 0.006%.

[0055] Aluminum (Al): 0.01-0.1 wt%

[0056] Aluminum can be added to remove oxygen from molten steel. Like Si, aluminum is an element that inhibits the precipitation of cementite during the reheating and overaging steps and is effective in stabilizing retained austenite. When the Al content is less than 0.01%, the deoxidation of the steel is insufficient and the cleanliness of the steel is impaired. On the other hand, when the Al content exceeds 0.1%, not only the castability of the slab deteriorates, but also the temperature required for heating the single-phase region during annealing increases, so there may be production and equipment problems. In addition, in order to further improve the above-mentioned effects, the lower limit of the Al content may be 0.02%, or the upper limit of the Al content may be 0.05%.

[0057] Niobium (Nb): 0.01-0.05 wt%

[0058] Niobium is an element that segregates at austenite grain boundaries and inhibits the growth of austenite grains during annealing heat treatment, thereby contributing to improving strength through precipitation strengthening effects. However, when the Nb content exceeds 0.05%, the precipitation of carbonitrides and the like increases, the processability of the base material decreases, and as the amount of alloy addition is too much, the cost increases. On the other hand, when the Nb content is less than 0.01%, it does not contribute to improving strength at all, so the lower limit of the Nb content is limited to 0.01%. In addition, in order to further improve the above-mentioned effects, the lower limit of the Nb content may be 0.02%, or the upper limit of the Nb content may be 0.04%.

[0059] Titanium (Ti): 0.01-0.05 wt%

[0060] Titanium is a nitride-forming element, and is an element that removes (scavenging) N in steel by precipitating N in the form of TiN. When the Ti is not added, cracks may occur during continuous casting due to the formation of AlN. However, when the Ti content exceeds 0.05%, in addition to removing the solid solution N, the further precipitation of carbides may reduce the strength of martensite, and the formation of carbonitrides such as TiC and TiN may hinder the hole expansion and bending workability. On the other hand, when the Ti content is less than 0.01%, similar to the Nb element, it has no contribution to improving the strength, so the lower limit of the Ti content is set. In addition, in terms of further improving the above-mentioned effect, the lower limit of the Ti content can be 0.02%, or the upper limit of the Ti content can be 0.04%.

[0061] The rest includes iron (Fe), and in the usual manufacturing process, undesirable impurities are inevitably mixed from the raw materials or the surrounding environment, so these impurities cannot be completely eliminated. These impurities are well known to those skilled in the art, so all of them are not particularly mentioned in this specification.

[0062] Hereinafter, the features of the steel sheet according to the present invention will be described.

[0063] In an area within 20 μm from the surface of the steel plate, a ratio (a / b) of the total content of C and Mn (a) to the total content of C and Mn (b) at a distance of (1 / 4)×t (where t is the total thickness of the steel plate) from the surface is 75% or more (excluding 100%).

[0064] As a result of repeated studies, the inventors have found that when the total content of C and Mn in the region within 20 μm from the surface of the steel sheet and the total content of C and Mn at a position (1 / 4)×t (where t is the total thickness of the steel sheet) from the surface (along the thickness direction of the steel sheet) meet a specific ratio, the bending properties tend to be improved, and thus the present invention has been completed. Therefore, according to the present invention, when the ratio of the total content of C and Mn in the region within 20 μm from the surface of the steel sheet to the total content of C and Mn at a position (1 / 4)×t (where t is the total thickness of the steel sheet) from the surface is less than 75%, ferrite or bainite is excessively formed in clusters in the surface layer, cracks are generated on the grain boundaries between the phases of ferrite and martensite, and thus the problem of reduced bendability may occur.

[0065] In order to further improve the above-mentioned effects, the lower limit of the ratio (a / b) may be 78%, or the upper limit of the ratio (a / b) may be 87%.

[0066] In addition, according to one embodiment of the present invention, in the region within 20 μm from the surface, the microstructure may contain 10% or less (except 0%) of one or more phases selected from ferrite and bainite in terms of area%. In the region within 20 μm from the surface, when the ratio (A) of one or more phases selected from ferrite and bainite exceeds 10%, too much soft ferrite or bainite is formed around the martensite as a hard phase, so cracks may be generated during bending. At this time, the lower limit of the ratio (A) is not particularly limited, but it is advantageous to control it as low as possible within the range of manufacturability.

[0067] At this time, according to one embodiment of the present invention, in the region within 20 μm from the surface, the remainder of the fine structure other than the ferrite and bainite may be martensite.

[0068] According to one embodiment of the present invention, in the region within 20 μm from the surface, the microstructure may contain 90-99% martensite in terms of area %. Alternatively, as a more preferred range, in the region within 20 μm from the surface, the lower limit of the fraction of martensite in terms of area % may be 95%, or in the region within 20 μm from the surface, the upper limit of the fraction of martensite in terms of area % may be 98%.

[0069] According to one embodiment of the present invention, the t may be 0.6-2.5 mm.

[0070] In addition, the steel sheet of the present invention may further include a coating. There is no particular limitation on the coating, and thus there is no limitation on the type of zinc-based coating, aluminum-based coating, etc., and there is no limitation on the method of hot-dip coating, electroplating, etc. That is, any coating that can be used in the technical field to which the present invention belongs may be used. However, as a preferred example of the present invention, the coating may be a zinc-based coating.

[0071] According to one embodiment of the present invention, an ultra-high strength steel sheet having excellent bending properties, i.e., a tensile strength (TS) of 1500 MPa or more and a bendability (R / t) of 3.7 or less, can be provided.

[0072] In addition, according to one embodiment of the present invention, a steel plate having a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 3% or more (or, a more preferred range of elongation is 5% or more, and in particular, the higher the upper limit, the more advantageous it is, so the upper limit is not calculated.), and a bendability (R / t) of 3.7 or less can be provided.

[0073] Next, a preferred method for producing a steel sheet in the present invention will be described.

[0074] First, the slab of the steel having the above-mentioned composition system is reheated at a temperature of 1100-1300°C. This process is performed in order to smoothly carry out the subsequent hot rolling process and fully obtain the desired physical properties of the steel plate. At this time, when the reheating temperature is lower than 1100°C, the problem of rapid increase in hot rolling load occurs. On the other hand, when the reheating temperature exceeds 1300°C, the amount of surface oxide scale increases and the yield of the material decreases, so the reheating temperature is limited.

[0075] The reheated slab is hot rolled. At this time, the hot rolling can be performed at Ar3 to 1000°C. The hot finishing rolling temperature of the reheated slab is limited to Ar3 (the temperature at which ferrite begins to appear when austenite is cooled) or above. This is because, at a temperature lower than Ar3, a dual phase region of ferrite + austenite or ferrite region rolling is formed, a mixed crystal structure is generated, and fluctuations in hot rolling load may cause failures, so the above hot rolling temperature is limited.

[0076] Next, the hot-rolled steel sheet is coiled at a temperature range of 400-600°C. When the coiling temperature exceeds 600°C, too much oxide film is generated on the surface of the steel sheet, which may cause defects and the surface properties of the plated material may deteriorate, so the upper limit of the coiling temperature is limited. In addition, in order to make the structure of the hot-rolled sheet as single-phase structure as possible rather than composite structure and to ensure the uniformity of the material over the entire length and width, it is preferred to maintain a low coiling temperature. However, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, so that the rolling load of the cold rolling as a post-process increases, and there may be factors that make it impossible to actually produce, so its lower limit is limited to above 400°C. In addition, more preferably, the lower limit of the coiling temperature can be 420°C, or the upper limit of the coiling temperature can be 520°C, and it can be cooled by water cooling after coiling.

[0077] Next, the oxide layer formed on the surface of the coiled hot-rolled steel sheet after the hot rolling is removed by a pickling process, and then cold rolling is performed at a reduction rate of 30-80%. When the reduction rate of the cold rolling is less than 30%, it is not only difficult to ensure the target thickness, but also due to the residual hot-rolled grains, the formation of austenite and the final physical properties may be affected during the annealing heat treatment. In addition, when the reduction rate of the cold rolling exceeds 80%, due to the processing solidification generated during the cold rolling, the rolling reduction in the length and width directions is uneven, so there is a problem of material deviation in the final steel plate, and due to the rolling load, it may be difficult to ensure the target thickness.

[0078] After cold rolling, heat treatment may be performed for more than 30 seconds within an annealing temperature range of Ac3+10°C to Ac3+80°C. The Ac3 temperature varies depending on the composition and is therefore determined by the following formula 1. When the annealing temperature is lower than Ac3+10°C, a mixed crystal structure may be formed through dual-phase annealing rather than single-phase annealing over the entire length of the coil, thereby having a negative impact on the material quality, and therefore the lower limit of the annealing temperature is defined as Ac3+10°C. On the other hand, when the annealing temperature exceeds Ac3+80°C, equipment failure may occur due to overloading of the annealing furnace, and therefore the upper limit of the annealing temperature is set to Ac3+80°C. In addition, more preferably, the lower limit of the annealing temperature may be 823°C, or the upper limit of the annealing temperature may be 916°C.

[0079] [Formula 1]

[0080] Ac3=910-203√[C]-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0081] (In Formula 1, [C], [Ni], [Si], [V], [Mo], and [W] show the weight % content of each element in parentheses.)

[0082] According to one embodiment of the present invention, the heat treatment during annealing can be performed (maintained) for more than 30 seconds in a temperature range of Ac3+10°C to Ac3+80°C. When the heat treatment time at Ac3+10°C to Ac3+80°C is less than 30 seconds, the problem that the structure cannot be sufficiently heat-treated in a single phase may occur, and thus it may be difficult to ultimately ensure a martensitic structure.

[0083] Next, the annealed steel sheet is subjected to primary cooling at an average cooling rate of 10°C / sec or less (exceeding 0°C / sec) to a primary cooling end temperature range of 680-749°C. When the primary cooling end temperature is lower than 680°C, according to experimental results, in the surface layer portion within 20 μm from the surface, the ratio of other phase structures (one or more phases selected from ferrite and bainite) other than the martensite structure exceeds 10% by area %, and / or when comparing the total content (a) of C and Mn within 20 μm from the surface and the total content (b) of C and Mn at a position (1 / 4)×t from the surface (where t is the total thickness of the steel sheet), the ratio (a / b) is less than 75%, and the final bendability (R / t) evaluation value may exceed 3.7, so that the formability may be deteriorated. On the other hand, when the primary cooling end temperature exceeds 749° C., not only can it not be reproduced due to the equipment configuration, but the structure becomes coarse, which may cause a decrease in strength, so the primary cooling end temperature is limited to 680-749° C. In addition, more preferably, the lower limit of the primary cooling end temperature can be 700° C., or the upper limit of the primary cooling end temperature can be 730° C.

[0084] In addition, when the average cooling rate of the primary cooling exceeds 10°C / second, the shape of the plate may cause problems, so the upper limit of the average cooling rate of the primary cooling is set to 10°C / second. In addition, in particular, the lower limit of the average cooling rate of the primary cooling can be as low as the range that can be achieved according to the equipment configuration, so the lower limit of the average cooling rate of the primary cooling can not be set separately, so the lower limit of the average cooling rate of the primary cooling is set to more than 0°C / second (or, more than 1°C / second). In addition, more preferably, the lower limit of the average cooling rate of the primary cooling can be 4.3°C / second, or the upper limit of the average cooling rate of the primary cooling can be 7.7°C / second.

[0085] Next, the steel sheet that has been cooled once is cooled twice (rapidly) at an average cooling rate of 60-160°C / sec to a temperature of 100°C to Mf. Here, Mf refers to the martensitic transformation finish temperature (Finish Temperature; Mf), and is measured using a dilatometer.

[0086] In order to ensure the strength level required by the present invention, it is preferred to maintain rapid cooling conditions during the secondary cooling. Specifically, when the average cooling rate of the secondary cooling is less than 60°C / second, a partial bainite structure may be formed during cooling, so it may be difficult to ensure the desired strength. On the other hand, when the average cooling rate of the secondary cooling exceeds 160°C / second, the shape of the steel plate deteriorates due to the rapid martensitic phase transformation rate during the secondary cooling, and material deviation problems may occur in the width direction. In addition, in order to further improve the above-mentioned effects, the lower limit of the average cooling rate of the secondary cooling may be 80°C / second, or the upper limit of the average cooling rate of the secondary cooling may be 153°C / second.

[0087] In addition, the cooling end temperature of the secondary cooling is 100°C to Mf temperature. When the cooling end temperature of the secondary cooling exceeds the Mf temperature, it may be difficult to ensure the fine structure expected by the present invention because the martensitic phase transformation is not fully achieved. On the other hand, when the cooling end temperature of the secondary cooling is lower than 100°C, it is disadvantageous in shape due to cooling to too low a temperature and exceeds the manufacturing process range of the equipment, so the lower limit of the cooling end temperature of the secondary cooling is limited to 100°C. In addition, in terms of further improving the above-mentioned effect, the lower limit of the cooling end temperature of the secondary cooling can be 106°C, or the upper limit of the cooling end temperature of the secondary cooling can be 152°C.

[0088] Although not particularly limited, the method for manufacturing a steel plate according to one embodiment of the present invention may satisfy the following Relationship 1. As a result of repeated studies by the present inventors, it has been confirmed that excellent properties in terms of strength, bendability, etc. can be ensured by satisfying a specific relationship such as the following Relationship 1 between the cooling end temperatures during primary cooling and secondary cooling.

[0089] [Equation 1]

[0090] 4.5≤T1 / T2≤7

[0091] (In the above relational expression 1, T1 refers to the cooling end temperature (°C) of the primary cooling, and T2 refers to the cooling end temperature (°C) of the secondary cooling.)

[0092] Next, according to one embodiment of the present invention, the steel plate that has undergone the secondary cooling is reheated at 150-240°C for overaging heat treatment. Through the reheating and overaging heat treatment, the martensite obtained by the above-mentioned rapid cooling secondary cooling process is transformed into tempered martensite, so that the yield strength can be improved. When the overaging heat treatment temperature is lower than 150°C, the yield strength is low due to insufficient tempering, and there is a disadvantage that sufficient toughness cannot be ensured. On the other hand, when the overaging heat treatment temperature exceeds 240°C, there is a disadvantage that a large amount of carbide precipitation and coarsening lead to poor bending workability.

[0093] In addition, the lower the lower limit of the overaging heat treatment temperature, the more favorable it is for bendability, but considering the equipment characteristics, it is recommended to be above 150°C, and more preferably, the lower limit of the overaging heat treatment temperature may be above 170°C. In addition, the upper limit of the overaging heat treatment temperature is preferably 200°C, and more preferably 198°C.

[0094] Although not particularly limited, the method for manufacturing a steel plate according to one embodiment of the present invention may satisfy the following Relationship 2. As a result of repeated studies by the present inventors, it has been further discovered that by satisfying a specific relationship such as the following Relationship 2 between the cooling end temperature of the secondary cooling and the overaging heat treatment temperature, a steel plate having more excellent characteristics in terms of strength and bendability can be ensured.

[0095] [Equation 2]

[0096] 1.2≤T3 / T2≤1.8

[0097] (In the relational expression 2, T2 indicates the cooling end temperature (° C.) of the secondary cooling, and T3 indicates the temperature (° C.) of the overaging heat treatment.)

[0098] According to one embodiment of the present invention, the overaging heat treatment may be performed for more than 400 seconds. The overaging heat treatment time refers to the time kept within the overaging heat treatment temperature range.

[0099] When the overaging heat treatment time is less than 400 seconds, tempering is insufficient, so the yield strength is reduced. On the other hand, there is no particular limit on the upper limit of the overaging heat treatment time, but due to the characteristics of the continuous annealing equipment, it is difficult to exceed 1000 seconds. Therefore, the overaging heat treatment time can be 400-1000 seconds. In terms of further improving the above effect, the lower limit of the overaging heat treatment time can be 428 seconds, or the upper limit of the overaging heat treatment time can be 600 seconds.

[0100] Next, if necessary, in order to improve the plate shape, it can be subjected to temper rolling or tension leveling.

[0101] In addition, as required, a step of forming a coating on the surface of the steel sheet may be further included. The coating may be a hot dip coating method in which a coating bath is provided and the steel sheet is immersed in a molten coating solution (dipping) or a method in which electroplating is performed in an electrolyte after annealing. The coating conditions are not particularly limited as long as they are generally well-known coating conditions in the technical field to which the present invention belongs.

[0102] According to the above-mentioned manufacturing method, an ultra-high strength steel sheet having an ultra-high strength of a tensile strength of 1500 MPa class or higher and a bendability (R / t) of 3.7 or less and excellent shape and bendability can be efficiently obtained. DETAILED DESCRIPTION

[0103] The following are examples of the present invention. Those skilled in the art may make various modifications to the following examples without departing from the scope of the present invention. The following examples are provided for understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be determined by the claims and their equivalents.

[0104] (Example)

[0105] Molten steel having the alloy composition shown in Table 1 below is cast into an ingot, and then a steel billet is manufactured by sizing rolling. The steel billet is heated to a temperature of 1200°C, and after being held for 1 hour, hot finish rolling is performed at 900°C, and then loaded into a heated furnace set with various conditions, and after being held for 1 hour, hot rolling and coiling are simulated by furnace cooling. After the hot-rolled steel sheet is pickled, it is cold rolled at a cold reduction rate of 50%, and then annealing heat treatment is performed under the conditions shown in Table 2 below, primary cooling (slow cooling), secondary cooling (rapid cooling), reheating and over-aging heat treatment are performed to manufacture the cold-rolled steel sheet, and electrogalvanizing treatment is performed using normal conditions.

[0106] The cold-rolled steel sheet manufactured as described above was evaluated for microstructure by optical microscopy and SEM microstructure observation. In particular, the microstructure observation of the surface layer corresponding to the area within 20 μm from the surface was performed using a 3000-fold SEM microstructure, and the fraction of each microstructure phase was analyzed by image analysis of each phase to analyze the area ratio, and the average value of the three analyses was used as a representative value.

[0107] Furthermore, in order to measure the total content ratio of each of C and Mn at a position of (1 / 4)×t from the surface and in a region within 20 μm from the surface, quantitative analysis was performed using a microanalyzer (FE-EPMA) using a TEM apparatus.

[0108] That is, for the local concentration of C and Mn, the quantitative concentration ratio (%) of C and Mn is analyzed for 5 points within a diameter of about 20 μm, and their arithmetic mean is used as a representative value. In addition, in order to compare the overall concentration ratio of C and Mn, the line profile technology is used, and through relative comparison, the ratio of the average total content of C and Mn in the area within 20 μm from the surface to the average total content of C and Mn at a position (1 / 4)×t from the surface can be accurately evaluated.

[0109] The tensile strength (TS) and the yield strength (YS) were measured by taking a tensile test piece of JIS No. 5 size in a direction perpendicular to the rolling direction and then conducting a tensile test at a strain rate of 0.01 / sec.

[0110] For R / t (bending properties), the cold-rolled steel sheet is processed into a test piece with a width of 100 mm and a length of 30 mm, and then a 90° bending test is performed at a test speed of 100 mm / min. The cracks in the bent part are then confirmed using a microscope to improve the reliability of the result value.

[0111] The flatness is measured by cutting 200 mm in the length direction, scanning the shape of the entire width using a 3D scanner, and then measuring each section to evaluate the flatness. Generally, in the present invention, when the flatness value is 3 mm or less, it is judged to be a satisfactory level.

[0112] Table 1 below shows the composition ranges for making the invention steel and comparative steel, and Table 2 lists the operating conditions for the invention steel and comparative steel. The operating conditions for the invention steel that are out of the scope of the invention are indicated by The comparison steel is marked with mark.

[0113] [Table 1]

[0114]

[0115] [Table 2]

[0116]

[0117] [Table 3]

[0118]

[0119] In addition, in the above Table 3, ① to ③ represent the following values.

[0120] ① The area ratio of one or more phases selected from ferrite and bainite within 20 μm from the surface (%)

[0121] ② The ratio of the total C and Mn content (a) within 20 μm from the surface to the total C and Mn content (b) at a distance of (1 / 4) × t (where t is the total thickness of the steel plate) from the surface (a / b × 100)

[0122] As can be seen from Table 1, it is confirmed that the inventive examples of the present invention satisfy the tensile strength of 1500 MPa or more and the bendability (R / t) of 3.7 or less, the flatness satisfies 3 mm or less, has ultra-high strength, and has excellent shape and bendability.

[0123] In particular, Figure 1 2 shows a photograph of a cross section in the thickness direction of the steel plate according to Inventive Example 1 of the present invention, which was taken using a scanning electron microscope (SEM).

[0124] On the other hand, in the case of the comparative examples of the present invention, since the conditions required by the present invention are not met, it is confirmed that one or more characteristics among the above-mentioned strength, shape and bendability are inferior.

[0125] Specifically, Comparative Examples 1 and 2 are cases where the first cooling termination temperature, the second cooling termination temperature and the cooling rate are out of the range required by the present invention, and the flatness or bendability is out of the range required by the present invention. This is because, within 20 μm from the surface, in addition to the martensite structure, the proportion of one or more mixed crystal structures selected from ferrite and bainite exceeds 10%, or the total content ratio of C and Mn is out of the range required by the present invention, and therefore the bendability is poor.

[0126] Comparative Examples 3 and 4 were manufactured under conditions where the coiling temperature and annealing temperature were outside the range of the present invention, and it was found that the bendability was also poor. In Comparative Examples 5 to 8, the steel components were outside the target range, so it was found that the bendability was also poor.

[0127] That is, by controlling the components and operating conditions required by the present invention, a 1500 MPa grade annealed and electro-galvanized steel sheet having a target shape (flatness) and excellent bendability can be manufactured in the same manner as in the inventive example.

Claims

1. A steel plate, comprising, by weight%, carbon (C): 0.1-0.3%, silicon (Si): 0.5% or less and excluding 0%, manganese (Mn): 1.3-2.5%, chromium (Cr): 0.2% or less and excluding 0%, molybdenum (Mo): 0.01-0.1%, boron (B): 0.0005-0.003%, phosphorus (P): 0.1% or less and excluding 0%, sulfur (S): 0.01% or less and excluding 0%, nitrogen (N): 0.01% or less and excluding 0%, aluminum (Al): 0.01-0.1%, niobium (Nb): 0.01-0.05%, titanium (Ti): 0.01-0.05% and the balance of Fe and unavoidable impurities, In the region within 20 μm from the surface, the ratio of the total content of C and Mn a to the total content of C and Mn b at a position (1 / 4)×t from the surface, i.e., a / b×100, is 75% or more and 100% is excluded, wherein: t is the total thickness of the steel plate.

2. The steel plate according to claim 1, wherein: In the region within 20 μm from the surface, the fine structure includes, in terms of area%, 10% or less and excluding 0% of one or more phases selected from ferrite and bainite.

3. The steel plate according to claim 2, wherein: In the region within 20 μm from the surface, the balance of the fine structure is martensite.

4. The steel plate according to claim 1, wherein: In the region within 20 μm from the surface, the microstructure contains 90-99% martensite in terms of area %.

5. The steel plate according to claim 1, wherein: The t is 0.6-2.5 mm.

6. The steel plate according to claim 1, wherein: The steel plate further includes a zinc-based coating on its surface.

7. The steel plate according to claim 1, wherein: The steel plate has a tensile strength (TS) of 1500 MPa or more and a bendability (R / t) of 3.7 or less.

8. A method for manufacturing a steel plate, comprising the following steps: The steel slab is reheated at a temperature of 1100-1300° C., wherein the steel slab comprises, in terms of weight %, carbon (C): 0.1-0.3%, silicon (Si): 0.5% or less and excluding 0%, manganese (Mn): 1.3-2.5%, chromium (Cr): 0.2% or less and excluding 0%, molybdenum (Mo): 0.01-0.1%, boron (B): 0.0005-0.003%, phosphorus (P): 0.1% or less and excluding 0%, sulfur (S): 0.01% or less and excluding 0%, nitrogen (N): 0.01% or less and excluding 0%, aluminum (Al): 0.01-0.1%, niobium (Nb): 0.01-0.05%, titanium (Ti): 0.01-0.05%, and the balance of Fe and inevitable impurities; hot rolling the reheated slab; The hot rolled steel plate is coiled at 400-600°C; The coiled steel plate is cold rolled at a reduction rate of 30-80%; The cold rolled steel sheet is heat treated and annealed at Ac3+10°C to Ac3+80°C; The annealed steel sheet is subjected to primary cooling at an average cooling rate of 10°C / s or less, to a temperature within a range of 680°C to 749°C as a termination temperature of the primary cooling; and The steel plate that has been subjected to the primary cooling is subjected to secondary cooling at an average cooling rate of 60-160° C. / sec to a temperature of 100° C. to Mf.

9. The method for manufacturing a steel plate according to claim 8, wherein: The heat treatment at Ac3+10° C. to Ac3+80° C. in the annealing step is performed for more than 30 seconds.

10. The method for manufacturing a steel plate according to claim 8, wherein: The manufacturing method further comprises the step of reheating the secondary cooled steel plate to 150-240° C. and performing an over-aging heat treatment.

11. The method for manufacturing a steel plate according to claim 10, wherein: The over-aging heat treatment step is performed for 400-1000 seconds.

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