Specification
The title of the invention: high yield ratio high-strength ultra high-ductility steel sheet and a method for their preparation
Art
[1]
The present invention relates to, and more particularly, the yield ratio high-strength high-ductility steel sheet excellent in seconds and a method related to a second high-strength steel sheet for automobiles.
[2]
BACKGROUND
[3]
And to the safety regulations for automobiles it is reinforced to ensure the passenger's safety upon collision of the vehicle, this should improve the strength of the steel sheet for automobiles, or increasing the thickness.
[4]
However, the current car and the CO that strengthened 2 , and a high strength steel sheets for the automotive necessarily required, depending on the achievement of emission regulations and fuel efficiency for the vehicle body weight consistently.
[5]
However, when increasing the strength of automotive steel sheets tend to be ductile is lowered, has a restricted use in part is required if high strength steel formability.
[6]
[7]
Hot press molding to finally and implement the yield strength and tensile strength as the part by, by rapid cooling since the room temperature molding the part from good moldability and high temperature securing a low temperature organization to overcome this high strength steel disadvantages (Hot Press Forming ) the river was developed.
[8]
However, the increase in operating costs of the new investment and high-temperature heat treatment of automotive parts manufacturers in the hot press forming equipment is a problem that eventually caused the rising cost of automobile parts were found.
[9]
[10]
Thus, high strength and elongation and excellent, have been continuously made in research on the steel and the cooling press molding possible.
[11]
For example, Patent Document 1 has been proposed the C and Mn of 0.5% to 1.5%, respectively, was added to 10% to 25% to 700 ~ 900MPa of tensile strength and 50 to 90% level of very excellent with a second flexible high-strength steel sheet. However, the steel sheet has a disadvantage in that this limited use as a hot press forming steel than the yield strength and automotive structural member for the tensile strength is low impact properties to heat.
[12]
On the other hand, Patent Document 2 has been proposed the case of C and Mn of 0.4 ~ 0.7%, was added to 12 ~ 24% in tensile strength and impact properties, with excellent ultra-high strength steel plate having a yield strength of at least 1000MPa than 1300MPa. However, the steel sheet has an elongation is low then cold press is limited to the to the molding manufacture of parts of a complicated shape with, the process step of securing the ultra-high strength by the re-rolling after annealing possible bar, the process process to around 10%, and there is a disadvantage that the manufacturing cost increases.
[13]
[14]
Thus, while replacing the steel sheet for hot press forming, as well as additional strength and ductility without the process, the development of a steel sheet having the collision characteristics with excellent yield strength ratio is desired.
[15]
[16]
[Patent Document 1] International Patent Application Publication No. WO2011-122237
[17]
[Patent Document 2] Korea Patent Laid-Open Publication No. 10-2013-0138039 No.
[18]
Detailed Description of the Invention
SUMMARY
[19]
One aspect of the present invention, Steels by controlling the alloy composition and the production conditions at the same time to ensure the ultra-high strength and high ductility, yield strength ratio (yield ratio) is higher impact properties are excellent cold press forming ultra-high strength high-ductility steel sheet and its intended to provide a method of manufacturing the same.
[20]
Problem solving means
[21]
One aspect of the present invention is, by weight%, carbon (C): 0.4 ~ 0.9%, silicon (Si): 0.1 ~ 2.0%, manganese (Mn): 10 ~ 25%, of (P): not more than 0.05% ( excluding 0%), sulfur (S): 0.02% or less (excluding 0%), aluminum (Al): 4% or less (excluding 0%), vanadium (V): 0.7% or less (excluding 0%), molybdenum modem (Mo): 0.5% or less (excluding 0%), nitrogen (N): 0.02% or less (excluding 0%), the balance includes Fe and other unavoidable impurities,
[22]
If the X value represented by the following equation 1, less than 40 micro-organization is stable austenite made of a single-phase, metastable austenite and ferrite in the case X value is less than 40 Microstructure The area fraction of 50% or more (including 100%) the yield of high-strength high-ductility steel sheet providing excellent second ratio will made of.
[23]
[24]
[Expression 1]
[25]
X = (80×C) + (0.5×Mn) - (0.2×Si) -(0.4×Al) - 21
[26]
(In the above equation 1 C, Mn, Si, and Al means the content based on the weight of the respective elements.)
[27]
[28]
Another aspect of the invention includes the steps of: preparing a steel slab having the above-described alloy composition; Further comprising: reheating a steel slab to a temperature range of 1050 ~ 1300 ℃; Preparing a hot rolled steel sheet by finish hot rolling the reheated steel slab in a temperature range of 800 ~ 1000 ℃; Step for winding the hot-rolled steel sheet in a temperature range of 50 ~ 750 ℃; To prepare a cold-rolled steel sheet to pickling and cold rolling the hot-rolled steel sheet wound; And an annealing step of annealing the cold-rolled steel sheet,
[29]
The annealing heat treatment following relation not less than the X value represented by the 1 40 700 ℃ than ~ carried out at a temperature range of not more than 840 ℃ less than 10 minutes, and the X value of 40 is less than if the temperature of a range from 610 ℃ ~ 700 ℃ range for more than 30 seconds, the yield ratio superior exemplary ultra-high strength, characterized in that that in the high provides a process for the production of the flexible steel plate.
[30]
Effects of the Invention
[31]
According to the invention, this has the effect of providing a steel sheet that can satisfy the formability and impact the stability required for the automobile steel sheet for cold forming.
[32]
In addition, this has the effect of reducing the production cost by replacing a conventional steel sheet for hot press forming.
[33]
Brief Description of the Drawings
[34]
Figure 1 according to one embodiment of the invention, EBSD (Electron Backscatter Diffraction) of the steel sheet microstructure of the X value of the relational expression 1 shows a phase map analysis result (a: Inventive Example 5 Annealing tissue, b: Inventive Example 5 after the tissue deformation, c: to honor 17 organizations annealing, d: to honor 17 after deformation tissue).
[35]
Here, the red color means the FCC (austenite), green BCC (α'- martensite or ferrite), white HCP (ε- martensite) organization.
[36]
Best Mode for Carrying Out the Invention
[37]
The inventors being able to replace the conventional steel sheet for hot press forming, having mechanical properties at least equal to its preparation, and depth research to develop a reducing possible cold press forming steel sheet for use in manufacturing costs. The result confirmed that by optimizing the steel composition and manufacturing conditions, the yield strength with good mechanical properties and microstructure in the cold press forming can provide excellent ultra-high strength high-ductility steel sheet, thus completing the present invention.
[38]
[39]
Hereinafter, a detailed description of the present invention.
[40]
[41]
Yield strength and high-strength high early ductility steel sheet according to an aspect of the present invention in weight%, carbon (C): 0.4 ~ 0.9%, silicon (Si): 0.1 ~ 2%, manganese (Mn): 10 ~ 25% , the (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), aluminum (Al): 4% or less (excluding 0%), vanadium (V): 0.7% or less (excluding 0%), molybdenum modem (Mo): less than 0.5% (excluding 0%), nitrogen (N): it is preferred to include 0.02% or less (excluding 0%).
[42]
[43]
Hereinafter will be described in detail the reasons for the control as described above, the alloy composition of the ultra-high strength steel sheet provided by the present invention. In this case, one, the content of each component unless otherwise specified refers to% by weight.
[44]
[45]
C: 0.4~0.9%
[46]
Carbon (C) is an effective element to strengthen the steel, is an important element to be added to the austenite stability control and securing strength of the present invention. In order to obtain the effect described above it is preferred to add a C with at least 0.4%, and the content is if it exceeds 0.9%, the stability of the austenite or stacking fault energy significantly increased production variations organic martensitic transformation or twin (twin) this is reduced to make it difficult that the same time ensuring a high strength and high ductility, there is a fear that the weldability decreases and electrical resistivity increases.
[47]
Therefore, the content of C in the present invention is preferably limited to 0.4 ~ 0.9%.
[48]
[49]
Si: 0.1 ~ 2.0%
[50]
Silicon (Si) is an element used as a deoxidizer, but the normal teaching, the present invention is added to improve yield strength and tensile strength of the river in order to obtain a beneficial solid solution strengthening effect. For this purpose a preferable to add Si to 0.1% or more, when the content exceeds 2.0%, the silicon oxide formed in a large amount at the time of hot rolling surface and decrease the pickling performance, by increasing the electrical resistivity is the weldability is inferior there is a problem.
[51]
Therefore, in the present invention it is preferred to limit the content of Si to 0.1 to 2.0%.
[52]
[53]
Mn: 10 ~ 25%
[54]
Manganese (Mn), while inhibiting the transformation of ferrite, an effective element for the formation and stabilization of the retained austenite. When adding such a Mn of less than 10% is insufficient in stability of the retained austenite, and results in a decrease of mechanical properties, whereas the problem that the content is when it is greater than 25% results in an increase in alloy cost and that the weldability decreases of a.
[55]
Therefore, in the present invention it is preferable to limit the content of Mn of 10 to 25%.
[56]
[57]
P: 0.05% or less (excluding 0%)
[58]
Phosphorus (P) is a solid solution strengthening element and, when its content exceeds 0.05%, weldability is degraded, so the problem increases the risk of embrittlement caused this course is preferably limited to the upper limit as 0.05%. And more preferably it is preferably limited to not more than 0.02%.
[59]
[60]
S: 0.02% or less (excluding 0%)
[61]
Sulfur (S) is an impurity element contained inevitably in steel, is an element which inhibits the ductility and weldability of the steel sheet. When the content of S exceeds 0.02%, this becomes a high potential to inhibit the ductility and weldability of the steel sheet, it is preferable to limit the upper limit as 0.02%.
[62]
[63]
Al: 4% or less (excluding 0%)
[64]
Aluminum (Al), but is an element that is added for deoxidation of ordinary river, in the present invention serves to increase the stacking fault energy and improves the ductility lecture in delayed fracture properties. When the content of the Al exceeds 4%, the tensile strength is lowered river, the sound is difficult to slabs produced by a reaction with a casting mold when the positive and there is a problem that to form surface oxides inhibit the plating property.
[65]
Therefore, in the present invention it is preferred to limit the content of Al to less than 4%, 0% is excluded.
[66]
[67]
V: 0.7% or less (excluding 0%)
[68]
Vanadium (V) is as an element that forms a carbon-nitride react with carbon or nitrogen, in the present invention plays an important role in increasing the yield strength of the river to form a fine precipitate at a low temperature. When the content of V exceeds 0.7%, this is burnt, this action of the nitride at high temperature is formed, there is a problem that the hot workability is reduced, lowering the yield strength of the river.
[69]
Thus, the present invention, preferable to limit the content of the V to less than 0.7%, 0% is excluded.
[70]
[71]
Mo: 0.5% or less (excluding 0%)
[72]
Molybdate modem (Mo) acts as an element for forming a carbide, by minutely maintain the carbon, the size of the nitride forming elements and the composite addition of precipitates such as V improves the yield strength and tensile strength. However, if its content exceeds 0.5%, the effect is saturated and the above-described, but rather there is a problem that causes an increase in manufacturing costs.
[73]
Thus, the present invention, preferable to limit the content of Mo to 0.5% or less, 0% is excluded.
[74]
[75]
N: 0.02% or less (excluding 0%)
[76]
Nitrogen (N) is the slowing or a solid solution strengthening element, that when the content exceeds 0.02%, large and the risk of embrittlement occurs, the performance by excessive precipitation of AlN in combination with Al quality.
[77]
Therefore, in the present invention, it is preferable to limit the upper limit of N to 0.02%.
[78]
[79]
The present invention may further comprise the following components in addition to the aforementioned components.
[80]
[81]
The present invention specifically is titanium (Ti): may further include at least one selected from 0.005 ~ 0.5% 0.005 to 0.1% of niobium (Nb): 0.005% to 0.1%, and tungsten (W).
[82]
The titanium (Ti), niobium (Nb), tungsten (W), in combination with the carbon of the steel as an element effective for precipitation strengthening of the steel sheet, and grain refinement, it is preferable that each added in 0.005% or more in order to sufficiently ensure this. However, if the Ti and Nb exceed 0.1%, respectively, or, in the case of W is saturated and the effect described above when it exceeds 0.5%, there is a problem that increases the alloy cost, the precipitate is formed in the over-the C concentration in the steel as the reduction has a problem that the strength and ductility disadvantage.
[83]
[84]
Of 1% or less (excluding 0%) In addition, the present invention is nickel (Ni): 1% or less (0% is excluded), copper (Cu): 0.5% or less (0% excluded), chromium (Cr) It may further include a selected one or more of them.
[85]
The nickel complex acting as (Ni), copper (Cu) and chromium (Cr) is an element contributing to the stabilization of retained austenite, the above-described C, Si, Mn, Al and contributes to the stabilization of austenite.
[86]
However, there is a problem that the content is the case of Ni and Cr exceed 1% respectively, and increased manufacturing costs are excessive When more than 0.5% of Cu. Among them, Cu as it may result in embrittlement during hot rolling, it is more preferable to be added together upon addition of Cu Ni.
[87]
[88]
The remaining components of the invention are iron (Fe). However, in the conventional manufacturing process it is not intended from the raw material or the environment, because impurities may be unavoidably incorporated, it is impossible to exclude them. These impurities are, because you know if anyone skilled in the normal manufacturing process does not specifically mention that all the contents herein.
[89]
[90]
Steel sheet of the present invention having the above-described alloy composition preferably comprises as a main phase to the austenite phase in the microstructure.
[91]
More preferably, made up of the steel sheet of the invention is not less than the value of X represented by the following relation 140 stable austenite single-phase, metastable of the X value is 40, the area fraction of at least 50% less than (including 100%) it is made of austenite and ferrite are preferable.
[92]
Here, the stable austenite phase is stable organizations that do not have a (phase) transformation occurs for external modification (e.g., processing and tensile strain and the like), the metastable austenite phase is the structural transformation is taking place the (phase) to the outer deformation . Preferably, the metastable austenite phase can be transformed into lesser organization such as α'- martensite or martensite ε- against external deformation. The stability is advantageous in the austenite phase and the metastable austenite phase both ultra-high strength obtained.
[93]
In the present invention, if the X value is less than 40, the metastable austenite phase the night mechanical properties aimed by securing a 50% or more fraction (ultra-high strength, ductility and impact properties, etc.) can be excellently secured both. The metastable austenite phase is preferably a phase transformation occurs at least 10% external strain.
[94]
[95]
[Expression 1]
[96]
X = (80×C) + (0.5×Mn) - (0.2×Si) -(0.4×Al) - 21
[97]
(In the above equation 1 C, Mn, Si, and Al means the content based on the weight of the respective elements.)
[98]
[99]
In this way, the steel sheet of the present invention as well as the tensile strength is very high, more than 1400MPa, by including a stable austenite as microstructures, contain the metastable austenite phase and a ferrite composite structure of the transformation is made of as a hard segment during processing , in excellent yield strength it can be secured for a yield ratio (yield strength (YS) / tensile strength (TS)) to more than 0.65. In other words, it has a conflicting characteristics can provide superior steel.
[100]
In addition, and it is possible to secure the ductility is excellent over 25,000MPa% product of tensile strength and elongation.
[101]
[102]
On the other hand, the steel sheet as referred to in this invention can be a cold-rolled steel sheet, as well as hot-dip galvanized steel sheet or galvannealed steel sheet obtained by plating the cold-rolled steel sheet.
[103]
[104]
Or less, and the other one side of the yield ratio is excellent ultra-high strength of the present invention will be described in detail a manufacturing method of the flexible steel plate.
[105]
[106]
First, it will be described in detail below with respect to the method for producing a cold-rolled steel sheet according to the present invention.
[107]
[108]
After preparing a steel slab of the aforementioned components the composition satisfy the cold-rolled steel sheet according to the present invention, re-heating it - hot rolling - winding - cold rolling - may be prepared by the annealing heat treatment process by performing, will now be described in detail for each process conditions .
[109]
[110]
Steel slab reheating process
[111]
In the present invention, preferably it undergoes a process of homogenizing treatment by reheating the prepared steel slab prior to the hot rolling line, and wherein it is preferred to re-heating step at 1050 ~ 1300 ℃ deeds.
[112]
And a problem that a load during hot rolling that follows is that the reheating temperature is below 1050 ℃ rapidly increased, whereas when it exceeds 1300 ℃ as well as energy costs increase, the amount of surface scale increase leads to loss of the material , there may be present if the liquid phase contains a large amount of Mn.
[113]
Therefore, it is preferably carried out in the temperature range at the time of steel slab re-heating 1050 ~ 1300 ℃.
[114]
[115]
Hot rolling
[116]
Wherein the re-heated steel slab is preferably for producing a hot rolled steel sheet by hot rolling, at this time it is preferable to carry out the finish hot rolling in the temperature range of 800 ~ 1000 ℃.
[117]
The finish hot rolling, and a problem that the temperature is the rolling load increases significantly if it is less than 800 ℃, while that when the temperature exceeds 1000 ℃ results in a reduced life of the surface defect and the rolling rolls according to the scale.
[118]
Therefore, it is preferable to carry out the finish hot rolling, the temperature range of 800 ~ 1000 ℃.
[119]
[120]
Winding process
[121]
The hot-rolled steel sheet manufactured in preferably wound at a temperature of 50 ~ 750 ℃.
[122]
When the coiling temperature exceeds 750 ℃ ​​the scale of the steel sheet surface is formed excessively and cause defects, which is a cause of deteriorating the plating property. If contained in the other hand, the steel composition is 10% or more of Mn is so greatly increased curing ability is no transformation of ferrite is also cooled to room temperature after hot rolling take-up. Therefore, it is not necessary to particularly limit the lower limit of the coiling temperature. However, if less than 50 ℃, the cooling by water injection is required to reduce the temperature of the steel sheet, it is preferable that it causes increase in unnecessary step ratio, limiting the coiling temperature above 50 ℃.
[123]
[124]
If the martensitic transformation starting temperature or higher temperature according to the addition amount of the composition of the steel components Mn, may be the martensite is produced at room temperature. In these cases, because due to the martensitic structure has a very high strength of the hot-rolled sheet it can be carried out in addition to heat treatment prior to cold rolling in order to reduce the load during the subsequent cold rolling. On the other hand, in the Mn addition amount increases, so that the transformation starting temperature maintaining the austenite phase at room temperature is less than room temperature, in this case, it can be directly cold-rolled.
[125]
[126]
Pickling and cold rolling
[127]
To remove the oxide layer by a conventional pickling process a winding hot-rolled steel sheet according to the embodiment Next, the cold rolling in order to secure the shape and thickness as required by the customers of the steel sheet is preferable.
[128]
When the cold rolling reduction ratio is one that does not suggest particular, it is best carried out as a cold rolling reduction of 25% or more preferable in order to suppress the formation of coarse ferrite grains during re-crystallization in the subsequent heat treatment step of annealing.
[129]
[130]
Annealing heat treatment step
[131]
The present invention, as well as strength and ductility, as for manufacturing a steel sheet having particularly superior to the yield strength ratio is preferably performed according to the conditions for the annealing heat treatment process for this.
[132]
Specifically, the following the invention is more than 610 ℃ ~ 700 ℃ when conducted not less than the X value represented by the first to the annealing heat treatment equation 40 below 840 exceeds 700 ℃ ~ ℃ in 10 minutes or less, and is less than the value of X 40 it is preferred to carry at least 30 seconds in a.
[133]
[134]
[Expression 1]
[135]
X = (80×C) + (0.5×Mn) - (0.2×Si) -(0.4×Al) - 21
[136]
(In the above equation 1 C, Mn, Si, and Al means the content based on the weight of the respective elements.)
[137]
[138]
The relational expression 1 is that the limited amount of relationship affects the austenite stabilizing elements, to the relative representation of the size or the austenite stability of the stacking fault energy (Stacking Fault Energy) of austenite.
[139]
When the austenite present in the steel after annealing heat treatment, varies depending on the deformation mode value of stacking fault energy. For example, if the stacking fault energy is relatively low austenite appears the transformation of organic plastic (transformation induced plasticity) symptoms that are relative to the outer deformation transformation to martensite α'- or ε- martensite, than a value (approximately 10 ~ 40mJ / m 2 , if having a) an organic plastic has a twin crystal (Twining induced plasticity) appears and the developer, a larger value (approximately 40mJ / m 2 and has the potential to form a cell without a specific phase transformation when having more). In the tensile properties according to the same transformation mode, such as tensile strength and elongation of Steel it becomes different. Therefore, in the present invention, by controlling the stacking fault energy of austenite within the steel of steel composition and the annealing heat treatment conditions, to obtain the mechanical properties of the target level.
[140]
[141]
The content in the composition of the steel components C and Mn is relatively high as the value of X is 40 or more cold-rolled steel sheet is composed of mostly austenite phase in the annealing heat treatment temperature, wherein the austenite is shown the symptoms twin organic plastic (Twining induced plasticity) It has a degree of stacking fault energy. Accordingly, the scope relatively high temperature that is, at a temperature range of not more than 840 more than 700 ℃ ~ ℃ for more than 30 seconds, 10 in order to minimize the X value is sufficiently austenite grain size (grain size) of the knight being up and recrystallization of the cold-rolled steel sheet less than 40 minute, it is advantageous in ensuring tensile properties for heat-treated below. At this time, annealing if the time is less than 30 seconds do not occur sufficiently recrystallized and is a risk of elongation is inferior, while when it exceeds 10 minutes can not becomes crystal grain coarse obtain the target degree of rigidity, by increasing the formation of annealing oxide a problem that the plating property inferior.
[142]
Further, if the annealing temperature is below 700 ℃ if it is difficult to secure not happen enough the recrystallization of the cold-rolled steel sheet elongation, whereas exceeding 840 ℃ or exceeds 10 minutes, the annealing time is more than 1400MPa the austenitic grain to coarse growing tension it is impossible to ensure the strength.
[143]
[144]
On the other hand, of the composition of the steel components C and the content of Mn it is contained relatively low value of X is 40 less than the case than stations annealing and to take advantage of the distribution behavior of the element performing a heat treatment to ensure the retained austenite at room temperature, or austenite even if performing the heat treatment in a night phase inverse it is preferred to conduct a heat treatment at a temperature range below the austenite of the bar which heat treatment is required to increase the stability and minimize the grain size, a relatively low temperature range, i.e., more than 610 ℃ ~ 700 ℃ .
[145]
At this time, the annealing temperature is low, the recrystallization retardation is less than 610 ℃ or to secure an appropriate fraction austenite during heat treatment, or annealing temperature has a detrimental drawback to secure elongation. On the other hand not that the temperature when it exceeds 700 ℃ becomes the crystal grains of the austenite coarse austenite stability and mechanical reduction of the so obtained the excellent strength and ductility at the same time. In this manner, when performing an annealing heat treatment at a relatively low temperature range is preferred in consideration of the phase transformation kinetic to a heat treatment to at least 30 seconds. One not particularly limited in its upper limit, it is deeds preferably within 60 minutes in consideration of productivity.
[146]
[147]
On the other hand, the present invention can be prepared by treating the plated steel sheet coated with the annealing heat treatment of cold-rolled steel sheet according to the above.
[148]
At this time, it is possible to use the electrical plating method, plating method or melt alloyed hot-dip plating method, it is possible to specifically, the galvanized cold-rolled steel sheet immersed in the plating bath to produce a hot-dip galvanized steel sheet. Further, the galvannealed steel sheet to alloying heat treatment of the hot-dip galvanized steel sheet can be produced.
[149]
The plating process when the conditions are merely exemplary and are not particularly limited, and may generally be performed as is done in the conditions.
[150]
[151]
The present invention the following examples will be described in detail. However, the following examples are to be noted in that not intended to limit the may make the scope of the present invention to be described in more detail by illustrating the invention. The scope of the present invention is because it is determined by what is a reasonable inference from this information and set forth in the claims.
[152]
Mode for the Invention
[153]
( Example )
[154]
To a steel having the composition components shown in Table 1 was kept in vacuum it was dissolved ingot (ingot) of 30Kg, 1 sigan them at a temperature of 1200 ℃. By later, ronaeng then prepare a hot-rolled steel sheet subjected to finish hot rolling at 900 ℃ then maintained for 1 hour by the hot-rolled steel sheets pre-charged to the furnace heated to 600 ℃ simulated coiling the hot-rolled. Then, each specimen was cooled to room temperature by acid pickling and cold rolling to prepare a cold-rolled steel sheet. The cold rolling was performed with cold rolling reduction of 40% or more.
[155]
After the heat treatment annealing to the respective cold-rolled steel sheets manufactured in the conditions shown in Table 2 below, the measurement of mechanical properties for each specimen, and to a result of observing the microstructure measured tissue-specific fraction shown in Table 3 It was.
[156]
The mechanical properties, a tensile test was performed using a universal tensile testing machine after processing with JIS5 tensile specimen No. standard.
[157]
[158]
TABLE 1
Grades Composition (% by weight)
C Mn And V Al you P S N
Invention steel 1 0.8 16 0.5 0.5 0.021 0.019 0.010 0.008 0.005
Invention steel 2 0.8 20 0.5 0.5 0.025 0.022 0.015 0.007 0.006
Invention steel 3 0.5 12 1 0.5 1.5 0.3 0.008 0.008 0.006
Invention steel 4 0.4 13.5 1 0.5 1.0 0.3 0.009 0.006 0.004
Inventive steels 5 0.4 12 1 0.5 1.5 0.3 0.012 0.009 0.007
Invention steel 6 0.5 12 1 0.5 3.0 0.3 0.009 0.008 0.009
Invention steel 7 0.5 15 1 0.5 1.0 0.3 0.007 0.007 0.008
Comparative Steel 1 0.3 15 1 0.5 1.0 0.3 0.011 0.007 0.007
Comparative Steel 2 0.2 15 1 0.5 1.0 0.3 0.011 0.004 0.005
Comparative Steel 3 0.1 15 1 0.5 1.0 0.3 0.009 0.009 0.004
Comparative Steel 4 0.5 12 0 0.3 3.0 0.3 0.008 0.007 0.006
Comparative Steel 5 0.7 12 1 0.5 5.0 0.3 0.010 0.009 0.009
Comparative Steel 6 0.7 12 0 0.3 5.0 0.3 0.011 0.005 0.005

[159]
[160]
TABLE 2
Grades Value X (equation 1) Annealing conditions Mechanical Properties Microstructure division
Temperature (℃) Time (min) YS(MPa) TS(MPa) The(%) YR TS×El(MPa%) F(%) c (%)
Invention steel 1 51 600 60 1131 1278 4 0.88 5112 0 100 Comparative Example 1
51 620 1 1135 1590 13 0.71 20670 0 100 Comparative Example 2
51 650 60 1004 1238 10 0.81 12380 0 100 Comparative Example 3
51 700 1 1107 1577 16 0.70 25232 0 100 Examples 1
51 775 15 818 1327 35 0.62 46445 0 100 Comparative Example 4
51 800 1 1119 1495 20 0.75 29900 0 100 To honor two
51 810 1 995 1431 31 0.70 44361 0 100 To honor 3
51 830 1 1007 1455 37 0.69 53835 0 100 To honor four
51 850 1 699 1387 13 0.50 18031 0 100 Comparative Example 5
51 850 5 733 1348 55 0.54 74140 0 100 Comparative Example 6
51 850 15 629 1260 38 0.50 47880 0 100 Comparative Example 7
Invention steel 2 53 600 60 1105 1360 17 0.81 23120 0 100 Comparative Example 8
53 620 1 1311 1586 12 0.83 19032 0 100 Comparative Example 9
53 650 60 948 1313 25 0.72 32825 0 100 Comparative Example 10
53 700 1 1195 1522 25 0.79 38050 0 100 To honor five
53 775 15 787 1303 40 0.60 52120 0 100 Comparative Example 11
53 800 1 1096 1461 34 0.75 49674 0 100 To honor 6
53 810 1 1130 1462 30 0.77 43860 0 100 To honor 7
53 830 1 1065 1433 34 0.74 48722 0 100 To honor eight
53 850 5 748 1392 47 0.54 65424 0 100 Comparative Example 12
53 850 1 791 1325 52 0.60 68900 0 100 Comparative Example 13
53 850 15 612 1226 49 0.50 60074 0 100 Comparative Example 14
Invention steel 3 24 600 10 1376 1590 8 0.87 12720 0 100 Comparative Example 15
24 650 10 1336 1529 20 0.87 30580 0 100 To honor nine
24 700 10 1150 1409 23 0.82 32407 0 100 To honor 10
24 750 10 937 1160 15 0.81 17400 0 100 Comparative Example 16
Invention steel 4 17 700 3 1122 1495 27 0.75 40365 0 100 To honor 11
17 750 10 822 1308 26 0.63 34008 0 100 Comparative Example 17
Inventive steels 5 16 600 10 1242 1473 11 0.84 16203 5 95 Comparative Example 18
16 650 10 1235 1497 25 0.82 37425 4 96 To honor 12
16 700 10 1046 1605 38 0.65 60990 2 98 To honor 13
16 750 10 866 1147 19 0.76 21793 0 100 Comparative Example 19
Invention steel 6 24 650 10 1390 1521 49 0.91 74529 41 59 To honor 14
24 650 30 1270 1434 46 0.89 65964 38 62 To honor 15
24 675 10 1322 1450 44 0.91 63800 28 72 To honor 16
24 700 10 1189 1404 54 0.85 75816 24 76 To honor 17
24 750 10 900 1074 22 0.84 23628 8 92 Comparative Example 20
24 800 10 760 973 18 0.78 17514 1 99 Comparative Example 21
Invention steel 7 26 600 10 1298 1534 12 0.85 18408 0 100 Comparative Example 22
26 650 10 1237 1475 26 0.84 38350 0 100 To honor 18
26 700 10 1067 1411 30 0.76 42330 0 100 To honor 19
26 750 10 915 1232 39 0.74 48048 0 100 Comparative Example 23
Comparative Steel 1 10 600 10 1373 1607 8 0.85 12856 16 84 Comparative Example 24
10 650 10 1279 1494 7 0.86 10458 5 95 Comparative Example 25
10 700 10 1006 1366 45 0.74 61470 2 98 Comparative Example 26
10 750 10 817 1289 52 0.63 67028 0 100 Comparative Example 27
Comparative Steel 2 2 600 10 1431 1650 8 0.87 13200 14 86 Comparative Example 28
2 650 10 1250 1474 10 0.85 14740 11 89 Comparative Example 29
2 700 10 940 1332 45 0.71 59940 2 98 Comparative Example 30
2 750 10 778 1287 47 0.60 60489 1 99 Comparative Example 31
Comparative Steel 3 -6 600 10 1443 1445 1 1.00 1445 26 74 Comparative Example 32
Comparative Steel 3 -6 650 10 1263 1392 5 0.91 6960 16 84 Comparative Example 33
-6 700 10 851 1228 32 0.69 39296 5 95 Comparative Example 34
-6 750 10 559 1147 31 0.49 35557 0 100 Comparative Example 35
Comparative Steel 4 24 650 10 1161 1288 36 0.90 46368 38 62 Comparative Example 36
24 650 10 1041 1188 26 0.88 30888 19 81 Comparative Example 37
24 700 10 846 1065 31 0.79 33015 21 79 Comparative Example 38
Comparative Steel 5 39 650 10 1518 1620 4 0.94 6480 5 95 Comparative Example 39
39 700 10 1403 1480 11 0.95 16280 0 100 Comparative Example 40
39 750 10 764 814 16 0.94 13024 0 100 Comparative Example 41
Comparative Steel 6 39 650 10 1444 1542 9 0.94 13878 0 100 Comparative Example 42
39 700 10 1258 1321 12 0.95 15852 0 100 Comparative Example 43
39 750 10 971 1094 31 0.89 33914 0 100 Comparative Example 44

[161]
(In Table 2 the YS: yield strength, TS: tensile strength, El: elongation, YR: means the yield ratio (YS / TS), and, F: means the austenite: ferrite, γ.)
[162]
[163]
As shown in Table 1 and 2, Examples 1 to 19 which satisfy the component composition and production conditions proposed by the present invention is not only a tensile strength of ultra-high strength above 1400MPa, while the yield ratio is 0.65 or more elongation is excellent and it is possible to secure the value of tensile strength × elongation over 25000MPa%. Therefore, the steel sheet according to the present invention can be confirmed that extremely favorable as a conventional hot press steel sheet for cold press forming which can be substituted for the steel sheet for forming.
[164]
In particular, the value of X greater than 40 Examples 1 to 8 were all formed with a stable austenite phase tissue. In addition, the value of X is less than 40 Inventive Example 9 to 19 is a single-phase austenite organization formed or ferrite + austenite was night composite structure is formed, wherein the austenite phase was all the metastable austenite.
[165]
[166]
On the other hand, if satisfied, even if the composition does not meet the components of the present invention the production conditions (annealing heat treatment step) The present invention has been difficult to secure mechanical properties aimed.
[167]
Among them, in Comparative Examples 1-3, in the case of 8-10 it has mothayeo the annealing heat treatment temperature of the recrystallization take place sufficiently to less than 700 ℃ elongation were inferior, in Comparative Examples 4 and 5 to 7, 11, 12 to 14, the annealing ratio and yield strength were inferior to coarse crystal grains to grow, as the heat treatment time exceeds 10 minutes, or if the annealing heat treatment temperature exceeds 840 ℃.
[168]
In the case of the annealing heat treatment temperature is less than 610 ℃ comparison examples 15, 18 and 22, the elongation was inferior, in Comparative exceeding 700 ℃ Examples 16, 17, 19-21 and 23, the securing of the ultra-high strength difficult .
[169]
[170]
Furthermore, even if the steel manufacturing conditions satisfying the steel composition of this invention in this case does not meet the present invention, that is, strength in the case of Comparative Examples 25-26, 29-30, 33-34, 37-40, 42-43 or the elongation were inferior.
[171]
[172]
Figure 1 shows the microstructure of the steel sheet according to the X value of the relational expression 1, and observed by EBSD (Electron Backscatter Diffraction) analysis phase map, shows the results. The microstructure is a microstructure observed after the microstructure (annealing tissue) of a steel sheet is completed up to the annealing heat treatment, and applying a tensile strain with respect to the steel sheet.
[173]
It can be seen that as shown in FIG. 1, in the invention example 5 or higher X value is 40 is composed of the annealing tissue to austenite single phase (a), the austenite is stable austenite because there is no phase change even after modification (b) . On the other hand, X if the value is less than 40 Inventive Example 17 Annealing tissue is composed of more than 50% austenite, and the balance of ferrite (c), wherein austenitic transformation, by the phase transformation to martensite α'- or ε- martensite It gave a stable austenite takes place (d).

Claims
[Claim 1]
By weight%, carbon (C): 0.4 ~ 0.9%, silicon (Si): 0.1 ~ 2.0%, manganese (Mn): 10 ~ 25%, of (P): not more than 0.05% (excluding 0%), sulfur ( S): 0.02% or less (excluding 0%), aluminum (Al): 4% or less (excluding 0%), vanadium (V): 0.7% or less (excluding 0%), molybdenum modem (Mo): 0.5% or less (0% included), nitrogen (N): 0.02% or less (excluding 0%), the balance to the Fe, and other or more X values ​​are 40 micro-organization is stable austenite single phase represented by the unavoidable inclusion of impurities, and the relational expression 1 becomes done, the X value is less than 40, if the microstructure is an area fraction of 50% or more (100% included) metastable austenite and ferrite to the yield ratio high-strength high-ductility steel sheet excellent early consisting of. [Expression 1] X = (80 × C) + (0.5 × Mn) - (0.2 × Si) - (0.4 × Al) - 21 (In the above equation 1 C, Mn, Si and Al based on the weight of the respective element It refers to the content.)
[Claim 2]
The method of claim 1, wherein the steel sheet is by weight%, titanium (Ti): 0.005 ~ 0.1%, niobium (Nb): 0.005 ~ 0.1%, and tungsten (W): further comprising 0.005 ~ 0.5% at least one selected from the yield ratio high-strength and ultra high ductility steel sheet.
[Claim 3]
3. A method according to claim 1 or 2, wherein the steel sheet is by weight%, nickel (Ni): 1% or less (0% is excluded), copper (Cu): (excluding 0%) 0.5% or less, chromium (Cr ): 1% or less (excluding 0%), further comprising a selected one or more of the yield ratio high-strength high-ductility steel sheet excellent seconds to.
[Claim 4]
The method of claim 1, wherein the metastable austenitic phase transformation when external α'- martensite or ε- martensite phase transformation to take place is that the yield ratio high-strength high-ductility steel sheet excellent in seconds.
[Claim 5]
According to claim 1, wherein said steel sheet is cold-rolled steel sheet, hot-dip galvanized steel sheet and a galvannealed steel sheet, one of the high yield ratio high-strength high-ductility steel sheet of the second.
[Claim 6]
By weight%, carbon (C): 0.4 ~ 0.9%, silicon (Si): 0.1 ~ 2.0%, manganese (Mn): 10 ~ 25%, of (P): not more than 0.05% (excluding 0%), sulfur ( S): 0.02% or less (excluding 0%), aluminum (Al): 4% or less (excluding 0%), vanadium (V): 0.7% or less (excluding 0%), molybdenum modem (Mo): 0.5% or less (excluding 0%), nitrogen (N): preparing a steel slab containing not more than 0.02% (excluding 0%), the balance Fe and other unavoidable impurities; Further comprising: reheating a steel slab to a temperature range of 1050 ~ 1300 ℃; Preparing a hot rolled steel sheet by finish hot rolling the reheated steel slab in a temperature range of 800 ~ 1000 ℃; Step for winding the hot-rolled steel sheet in a temperature range of 50 ~ 750 ℃; To prepare a cold-rolled steel sheet to pickling and cold rolling the hot-rolled steel sheet wound; And not less than the X value represented by the first to during the annealing heat treatment equation 40 carried out at a temperature range of not more than 840 more than 700 ℃ ~ ℃ to less than 10 min, and comprising the step of annealing, heat-treating the cold-rolled steel sheet wherein the value of X the method of 40 over 30 seconds at a temperature range between 610 ℃ ~ 700 ℃ embodiments characterized in the yield ratio high-strength high-ductility steel sheet excellent in seconds which is less than that. [Expression 1] X = (80 × C) + (0.5 × Mn) - (0.2 × Si) - (0.4 × Al) - 21 (In the above equation 1 C, Mn, Si and Al based on the weight of the respective element It refers to the content.)
[Claim 7]
The method of claim 6, wherein the steel slab is by weight%, titanium (Ti): The one or more selected from 0.005 ~ 0.5%, about 0.005 to 0.1% of niobium (Nb): 0.005% to 0.1%, and tungsten (W) method of producing a high yield ratio, including ultra-high strength and ductility of steel.
[Claim 8]
Claim 6 according to any one of claims 7, wherein the steel slab is by weight%, nickel (Ni): 1% or less (0% is excluded), copper (Cu): (excluding 0%) 0.5% or less, chromium ( Cr): 1% or less (excluding 0%) the process for producing a high yield ratio of the ultra-high strength high ductility steel sheet further comprises at least one member selected.
[Claim 9]
The method of claim 6, wherein the annealing the cold-rolled steel sheet annealing zinc plating bath is immersed in a method of manufacturing a step further comprises the yield ratio high-strength high-ductility steel sheet excellent in seconds that for producing a hot-dip galvanized steel sheet.
[Claim 10]
10. The method of claim 9, wherein the production of the hot-dip galvanized steel sheet for alloying heat treatment to alloy the hot-dip galvanizing step of manufacturing a plated steel sheet further comprises the yield ratio high-strength high-ductility steel sheet excellent seconds to.