Art
[1]
The present invention relates to a high temperature elongation characteristics are excellent high-strength steel sheet, the warm press forming members and methods for their preparation.
[2]
BACKGROUND
[3]
Recently, the development of steel to satisfy the high strength and high-performance simultaneously formed for the purpose, such as car weight and improve fuel efficiency and passenger safety is required, has been carried out various studies relating to this.
[4]
[5]
The typical steel material satisfying the above requirements is a nitro-based austenite high-manganese steel. In order to secure the austenite single-phase tissue, it is common to add more than 0.5% by weight carbon and more than 15% by weight of Mn.
[6]
[7]
For example, Patent Document 1, the carbon (C) and manganese (Mn), such as austenite stabilizing much addition of the elements to secure the steel microstructure from room temperature to the austenite phase and excellent molding and high strength by using a twin crystal that occurs during deformation It discloses a method of securing the property at the same time.
[8]
However, in Patent Document 1, not only the manufacturing cost due to the large amount of alloy elements added to the steel sheet increases, the austenitic to the grain energy of microstructure due deseo high weld cracking due to private use dish liquid metal embrittlement of galvanized steel problems such as occur.
[9]
[10]
Further, Patent Document 2, as well as to the tensile strength to secure the 1500MPa or more ultra-high strength member by the hot forming and quench by then heating the Zn-plated steel sheet to more than 880 ℃ press, can secure an excellent formability at high temperatures have.
[11]
However, in Patent Document 2, not only the time of hot forming temperature can drop the welding point by the Zn oxide is formed on the Zn plating layer surface by more than 880 ℃, has a crack propagation resistance it may cause a problem for heat.
[12]
[13]
Therefore, a situation that is being developed a demand for the austenitic high-manganese steel and a steel sheet which can solve the problems of the hot forming.
[14]
(Prior art document)
[15]
[Patent Document 1] Korea Patent Laid-Open Publication No. 2007-0023831 No.
[16]
[Patent Document 2] Korea Patent Laid-Open Publication No. 2014-0035033 No.
[17]
Detailed Description of the Invention
SUMMARY
[18]
One aspect of the present invention is to provide a high strength steel sheet excellent in high-temperature elongation properties, the warm press forming members and methods for their preparation.
[19]
[20]
On the other hand, object of the present invention is not limited to the above-described information. An object of the present invention will be understood from the overall context of the present specification, Those having ordinary skill in the art will not have any difficulty to understanding an additional object of the present invention.
[21]
Problem solving means
[22]
In one aspect% by weight of the present invention, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01 % or less (excluding 0%), sol.Al: 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si: less than 1.6% (excluding 0% ) includes at least one of,
[23]
Microstructure comprising a ferrite of less than 80% perlite and 20% in area fraction, the pearlite is long in the major axis of the high strength steel sheet excellent high-temperature elongation properties containing not more than 200nm cementite.
[24]
[25]
Further, in another aspect of% by weight of the present invention, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (0% excluded), S: (excluding 0%) 0.01% or less, N: 0.01% or less (excluding 0%), sol.Al: 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si: less than 1.6% ( excluding 0%) heating a slab containing at least one kind of a 1100 ~ 1300 ℃;
[26]
Steps to finish hot-rolling the heated slab at a temperature of Ar3 + 10 ℃ ~ Ar3 + 90 ℃ to obtain a hot-rolled steel sheet;
[27]
Step for winding the hot-rolled steel sheet from 550 ~ 700 ℃; And
[28]
A method for producing excellent high-temperature elongation properties including a high-strength steel sheet; and the cold-coiling a hot-rolled steel sheet to a reduction ratio 40% to 80% rolling to obtain a cold-rolled steel sheet.
[29]
[30]
Further, another aspect of the invention relates to a warm press-forming member and a manufacturing method manufactured using the steel sheet according to the present invention.
[31]
[32]
In addition, solving means of the above-described problems is, it is not enumerate all features of the present invention. The various features and advantages and effects thereof of the present invention can be understood in more detail with reference to specific embodiments below.
[33]
Effects of the Invention
[34]
According to the present invention, it is possible to provide a steel sheet which can ensure elongation of 60% or more in a temperature range of tensile strength and 500 ℃ ~ Ac1 + 30 ℃ 1000MPa or more at the same time at room temperature.
[35]
In addition, conventional hot forming (HOT PRESS FORMING) by molding is possible at a temperature of lower 500 ℃ ~ Ac1 + 30 ℃ than the temperature galvanized steel or alloy galvanized steel sheet also effects that can suppress the micro-cracks when molding the a.
[36]
Accordingly, there can be preferably applied to a member for a car collision to the inside plate form a high strength and high-performance is required at the same time.
[37]
Brief Description of the Drawings
[38]
Figure 1 is a picture taking a microstructure with a scanning electron microscope (SEM) after hot rolling of the specimen No. 1-1.
[39]
Figure 2 is a picture taken with a transmission electron microscope (TEM) of the microstructure after the cold rolling of the specimen No. 2-1.
[40]
Figure 3 is a schematic view showing a molded part.
[41]
Figure 4 is a photograph taken after the micro-crack length warm compaction of the sample No. 2-1.
[42]
Best Mode for Carrying Out the Invention
[43]
Hereinafter, the preferred embodiments of the present invention. However, embodiments of the present invention can be modified in many different forms and is not limited to the embodiments and the scope of the present invention described below. In addition, embodiments of the present invention is provided in order to explain more fully the present invention to those having ordinary skill in the art.
[44]
[45]
The present inventors have found that conventional austenitic and increase manufacturing costs manganese steel having, Spot plate problems such as weld cracking due to liquid metal embrittlement, and a high molding temperature due to crack propagation resistance, and becomes a spot weldability disadvantage problems of the conventional hot forming It was a depth research to solve.
[46]
[47]
As a result, the elongation at to obtain the pearlite having a cementite (cementite) segment by properly controlling the alloy composition and the production method (pearlite) strength and high temperature (500 ℃ ~ Ac1 + 30 ℃) excellent, conventional It confirmed that this can provide a steel sheet capable of forming at a temperature range of hot forming (hOT fORMING PRESS) low 500 ℃ ~ Ac1 + 30 ℃ than the temperature, and completed the present invention.
[48]
[49]
High-temperature elongation properties are high-strength steel sheet excellent
[50]
Will now be described in detail with respect to the high temperature elongation characteristics are excellent high-strength steel sheet according to an aspect of the invention.
[51]
[52]
A high temperature is wt% elongation characteristics superior high-strength steel sheet according to an aspect of the invention, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (0% excluded), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), sol.Al: 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si: 1.6% or less include at least one of (excluding 0%), the microstructure comprises a ferrite of less than 80% perlite and 20% in area fraction, wherein the pearlite is a tight long in the major axis 200nm or less cementite It includes.
[53]
[54]
First, a detailed description of the alloy composition according to the invention. Hereinafter, the unit of each element content is a% by weight unless otherwise specified.
[55]
[56]
C: 0.4~0.9%
[57]
Carbon (C) is an important component for the production of steel having a pearlitic microstructure consisting of ferrite and cementite after hot rolling in the present invention, in general, the more the C content is increased to obtain a higher pearlite structure fraction and ensure the strength of the steel is an essential element to be added to.
[58]
If the C content is less than 0.4%, there is a difficult to sufficiently ensure the pearlite problems. On the other hand, if the C content exceeds 0.9%, the My carbide pearlite is formed over by reducing the phase matching of the precipitate, as well as a hot rolling resistance and room temperature ductility can be reduced, by drastically increasing the halitosis strength decrease the ductility can.
[59]
Therefore, the C content is preferably 0.4 ~ 0.9%, may be more preferably 0.5 ~ 0.65%.
[60]
[61]
Cr: 0.01~1.5%
[62]
Cr, like Mn, and it serves to lower the carbon content required for the vacancy composition. In addition, the promotion of the formation of cementite and the cementite spheroidization promoting got a characteristic of reducing the lamellar spacing of the pearlite. It also has the property of further improving the corrosion resistance of the steel sheet by the addition of a trace amount of
[63]
If the Cr content exceeds 1.5%, there may adversely affect the mechanical properties, there is a problem that may be a disadvantage when pickling the surface-scale pickling performance.
[64]
If the Cr content is less than 0.01% is because the higher the C content for the vacancy pearlite formation in the hot rolled condition only is the point weldability by C significant disadvantage as not exerts no effect on the corrosion resistance required for the steel sheet basically Cr content of 0.01% it is desirable to add more than and preferably at least more preferably 0.05%.
[65]
[66]
(Excluding 0%) 0.1% or less: sol.Al
[67]
Acid soluble Aluminum (sol.Al) as is the element added for deoxidation and the miniaturization lecture particle size, the content is likely to occur when it exceeds 0.1%, steel-making operation performance during hot-dip galvanized steel sheet with excessive formation of inclusions surface defects as well as higher, there is the problem that caused the rise in production costs.
[68]
Need not be limited to the particular lower limit, taking into account the level inevitably added in the manufacturing process is other than 0%.
[69]
[70]
P: 0.03% or less (excluding 0%)
[71]
Phosphorus (P) of the steel, but beneficial element for securing strength, and likely occurs brittle fracture greatly increased if the excessive addition, and increased possibility of problems such as the hot rolling during the slab is broken, acts as an element to inhibit the plating surface properties there is a problem.
[72]
Therefore, in the present invention it is P, it is important to control the upper limit as an impurity is preferably limited to 0.03% or less. However, considering the levels inevitably added in the manufacturing process is other than 0%.
[73]
[74]
S: (excluding 0%) 0.01%
[75]
Sulfur (S), so the problem to increase the potential to generate an element which is unavoidably added, S is glowing embrittlement in steel as an impurity element in steel, it is desirable to control its content to 0.01% or less. However, considering the levels inevitably added in the manufacturing process is other than 0%.
[76]
[77]
N: 0.01% or less (excluding 0%)
[78]
Nitrogen (N) is an element which is unavoidably added as an impurity element in steel, it is preferable to control to not more than 0.01% by operating conditions is possible. However, considering the levels inevitably added in the manufacturing process is other than 0%.
[79]
[80]
In addition to the aforementioned ingredients Mn: 1.6% or less and comprising at least one of (0% is not included): 2.1% or less (excluding 0%), and Si.
[81]
[82]
(Excluding 0%) 2.1% or less: Mn
[83]
Mn, like Cr, plays a role to lower the carbon content required for the vacancy composition. In addition, an element which serves to inhibit the production of pro-eutectoid ferrite.
[84]
If the Mn content exceeds 2.1%, there is a problem that may cause a low-temperature cooling of the tissue.
[85]
[86]
(Excluding 0%) 1.6% or less: Si
[87]
Si serves to with the solid solution strengthening effect, by stabilizing the pearlite structure within the layer structure suppressing reduction in strength.
[88]
If the Si content exceeds 1.6%, there can be, and can reduce the elongation, the reduction of the river surface and the plating quality.
[89]
[90]
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.
[91]
[92]
In this case, the above-described, as well as satisfy the respective element contents, may satisfy the C, Cr, Mn and Si content to the equation 1.
[93]
Equation 1: 0.7 ≤ C + Cr / 2 + Mn / 3 + Si / 4 ≤ 3.0
[94]
(Each element symbol in the equation 1 is a value showing the content of each element in weight percent, does not include calculated to zero.)
[95]
[96]
The relational expression 1 is designed in consideration of the impact of the elements for the manufacture of a steel having a composition and a vacancy corresponding in composition required in the present invention.
[97]
When the first relational expression is less than 0.7, it is difficult to secure a more than 80 area% pearlite after hot rolling. On the other hand, if the value exceeds 3.0, the elongation has a due to a large amount of alloy elements added may be lowered, and a crack propagation resistance during hot molding may be inferior.
[98]
[99]
The microstructure of the steel sheet according to the invention comprises a ferrite of less than 80% perlite and 20% in area fraction, the perlite comprises a long in the major axis 200nm or less cementite.
[100]
When pearlite is less than 80%, because it is difficult to secure a high strength, the elongation may decline at a high temperature molding.
[101]
The higher the fraction of pearlite in the glass, so ensuring high-strength and high-temperature elongation, the upper limit is not particularly limited, and it is more preferable that pearlite single phase.
[102]
Pearlite is spheroidized by including tight long in the major axis 200nm or less cementite, to the segment of cementite in the warm-compacting or annealing step can easily be excellently ensure the high-temperature elongation and the final ductility.
[103]
[104]
In this case, cementite in the pearlite is tight, this may be at least 60% N value by the following relation 2.
[105]
Equation 2: N (%) = Nx / (Nx + Ny) * 100
[106]
(In the equation 2, Nx is a number of the length of the major axis 200nm or less cementite, Ny is the number of the cementite length of the major axis than 200nm.)
[107]
[108]
Because in the above equation 2, the number of Nx, i.e. cementite which the length in the major axis segment to 200nm or less more to the segmented cementite are easily been spheroidized excellently ensure the high-temperature elongation and the final soft in warm compaction or annealing step to be.
[109]
Therefore, the N value must be 60% or more preferably, may be more preferably 75% or more.
[110]
[111]
On the other hand, the steel sheet of the present invention to provide a tensile strength greater than or equal to 60% elongation and at least 1000MPa, a high temperature (500 ℃ ~ Ac1 + 30 ℃).
[112]
By securing such physical properties even when molded at a low range of 500 ℃ ~ Ac1 + 30 ℃ than conventional hot forming temperature it is possible to manufacture a high-strength warm press forming member does not cause breakage of the cosmetic.
[113]
[114]
Here, the Ac1 temperature can be defined by the following equation 3.
[115]
Equation 3: Ac1 (℃) = 723 - 10.7 * Mn - 16.9 * Ni + 29.1 * Si + 16.9 * Cr + 290 * As + 6.38 * W
[116]
(Each element symbol in the above equation 3 is the value showing the content of each element in weight percent, does not include calculated to zero.)
[117]
[118]
[119]
Further, the steel sheet of the present invention may be one of an aluminum layer on the surface, a zinc alloy plating layer and the zinc plated layer is further formed.
[120]
[121]
Method of producing a high-strength steel sheet having excellent high-temperature elongation properties
[122]
Will now be described in detail a manufacturing method of another aspect of the high temperature elongation characteristics are excellent high-strength steel sheet of the present invention.
[123]
[124]
Production method of another aspect of a high temperature high-strength steel sheet having excellent stretch properties of the present invention includes the steps of heating a slab having the above-described alloy composition to 1100 ~ 1300 ℃; Steps to finish hot-rolling the heated slab at a temperature of Ar3 + 10 ℃ ~ Ar3 + 90 ℃ to obtain a hot-rolled steel sheet; Step for winding the hot-rolled steel sheet from 550 ~ 700 ℃; It includes; and by cold rolling a hot-rolled steel sheet to the take-up by the reduction rate 40% to 80% to obtain a cold-rolled steel sheet.
[125]
[126]
Slab heating step
[127]
Heat the slab having the above-described alloy composition to 1100 ~ 1300 ℃ to hot rolling.
[128]
When the When the heating temperature is less than 1100 ℃ it is difficult to equalize the process organization and composition of the slab, greater than 1300 ℃ may cause a problem of surface oxidation and degradation equipment.
[129]
[130]
Hot rolling step
[131]
Finishing hot-rolling the heated slab at a temperature of Ar3 + 10 ℃ ~ Ar3 + 90 ℃ to obtain a hot-rolled steel sheet.
[132]
Finishing the hot-rolling temperature is less than Ar3 + 10 ℃ there may give rise to difficulties in honrip tissue and the plate-like control to the surface it is more than likely to reverse rolling of the ferrite and austenite steel, may also result in a material non-uniformity.
[133]
On the other hand, if the hot rolling finish temperature of Ar3 + 90 ℃ than there tends to occur the grain coarsening phenomenon of the column series.
[134]
Therefore, it is preferable that the finish of Ar3 + 10 ℃ ~ Ar3 + 90 ℃ temperature austenitic phase inverse case of hot rolling. By the finish hot rolling in the temperature range because the increase in the uniformity of tissue was added to more uniform strain in the microstructure consisting of a single phase austenite grains.
[135]
[136]
Here, the Ar3 temperature can be defined by the following equation 4.
[137]
관계식 4: Ar3 (℃) = 910-95 * (C ^ 0.5) * 44.7 -15.2 * Si + Ni + V + 104 * 31.5 * Mo (15 * Mn + 11 * Cr + 700 * 20 * Cu-P 400 * -400 * Al-Ti)
[138]
(Each element symbol in the above equation 4 is a value showing the content of each element in weight percent, does not include calculated to zero.)
[139]
[140]
Winding steps
[141]
And winding the hot-rolled steel sheet from 550 ~ 700 ℃.
[142]
By the coiling temperature results in less than 550 ℃ If the low-temperature transformation organization that is, bainite or martensite is generated and excessive strength rise of the hot-rolled steel sheet and may cause problems, such as shape defects caused by the excessive load in cold rolling, which is the object of the present invention it is difficult to obtain pearlite microstructure.
[143]
On the other hand, if the coiling temperature be in excess of 700 ℃ easy to excessive heat extending grain boundary oxidation occurred, so that there may occur a problem pickling performance becomes inferior.
[144]
[145]
In this case, it may further include the step of performing (batch annealing) based on the 200 ~ 700 ℃ appeal after step taking the volume to reduce the cold rolling before rolling load as needed.
[146]
If placed appeal temperature is lower than 200 ℃ there does not have a significant impact in reducing the rolling load mothayeo hot rolled tissue is not sufficiently softened, when it exceeds 700 ℃ the pearlite decomposed by high-temperature annealing is caused the required pearlite visualization features of the invention It can not be sufficiently exhibited.
[147]
On the other hand, the heat treatment time before the appeal does not have to be particularly limited in the present invention because it does not affect significantly.
[148]
[149]
Cold rolling step
[150]
Cold-rolling the hot-rolled steel sheet to a take-up reduction rate 40% to 80% to obtain a cold-rolled steel sheet.
[151]
It is difficult to ensure a thickness that when the rolling reduction is less than 40% of the target, the length of the major axis may be difficulties to 200nm or less cementite sufficiently. In the case of hot-rolled steel sheet having a pearlite is generally elongated shape of the lamellar cementite, if transformation during the growth time is sufficient. However, the hot rolling after the coiling process, unless a sufficient time pearlite transformation is given according to the conditions in the hot-rolled steel sheet 1, but may receive the cementite portion of the segment type as shown, it is not possible to sufficiently secure the segment pearlite. Therefore, in the present invention, obtained by performing a reduction ratio of 40% or more cold rolling the fully tight long in the major axis 200nm or less cementite. After the cold rolling cementite in the lamellar form can be displayed is stretched or segment in the rolling direction, the layer becomes closer distances between cementite.
[152]
On the other hand, if the reduction ratio exceeds 80%, cracks are likely to occur on cold-rolled steel sheet edge (edge) portion, may increase the load of cold rolling.
[153]
[154]
At this time, the cold rolling can be carried out at room temperature.
[155]
[156]
In the present invention, it is possible to ensure the properties required in the present invention in the case of performing the direct warm compaction does not have a special annealing after cold rolling.
[157]
[158]
However, may include a cold-rolled steel sheet by adding a step of performing continuous annealing or appeal, placed in a temperature range of Ac1 + 70 ~ Ac1-70 ℃ ℃ to ensure a more stable material properties.
[159]
By carrying out the continuous annealing or appeal, placed in the temperature range lamellar formed during hot rolling (lamellar) in the form of cementite it may be spheroidized by a spherical shape. Spheroidizing annealing method of the cementite has the two main methods of performing annealing Intercritical between Subcritical annealing method and ~ Ac1 Ac3 Ac1 temperature performed by the temperature directly below. The visualization begins with a density gradient according to the radius of curvature of the car, etc. When Subcritical annealing, the lamellar structure in cementite defect portion. On the other hand, the ferrite of Intercritical annealing during, certain fraction the transformation is started in the austenite, pearlite within the cementite particles maintain the fine solid solution state that is, to cosmetic and austenite is composed of cementite tissue, these undissolved cementite the visualization goes to the nucleus.
[160]
If the annealing temperature is less than Ac1-70 ℃ may if the cementite spheroidization is performed becomes difficult than, Ac1 + 70 ℃ as desired, because of the un-dissolved cementite becomes non-uniform, such as in the form of cementite. Therefore, in the temperature range of Ac1 + 70 ~ Ac1-70 ℃ ℃ it is preferable to put a continuous annealing or appeal.
[161]
[162]
On the other hand, it may further comprise the step of plating the cold-rolled steel sheet. Plating method and a plating type does not have great influence material properties by the conventional operating conditions are not particularly limited.
[163]
For example, aluminum, zinc, aluminum alloys, can be coated with a zinc alloy or the like, can be carried out by using a plating melt plating method, electroplating method.
[164]
[165]
At this time, it may further comprise the step of alloying the galvanized cold-rolled steel sheet. Material properties by the ordinary operating condition as in the step of the plating does not have a large impact is not particularly limited.
[166]
For example, it is possible to perform alloying treatment in a temperature range of 400 ~ 600 ℃.
[167]
[168]
Warm press forming member
[169]
Or less, by using the another aspect of the above-described steel sheet of the present invention of the present invention will be described in detail in the warm press forming member made.
[170]
[171]
Another aspect of the warm press forming member of the present invention is the same, so prepared by molding the warm high-strength steel sheet of the present invention described above, the alloy composition and microstructure is not changed. Therefore, it is possible to secure a tensile strength of high strength than 1000MPa. However, the N value is more than 70% because by warm compaction relation to the value N is higher than the steel sheet according to the second.
[172]
Equation 2: N (%) = Nx / (Nx + Ny) * 100
[173]
(In the equation 2, Nx is a number of the length of the major axis 200nm or less cementite, Ny is the number of the cementite length of the major axis than 200nm.)
[174]
[175]
On the other hand, the molding member can be can be formed by adding the aluminum layer on the surface, it is formed by adding a zinc plating layer or zinc alloy plating layer.
[176]
[177]
Further, even if the zinc plating layer or zinc alloy plating layer is further formed it may be less than a member within the fine crack length 10㎛.
[178]
This is because since the production through the warm compaction in the range of a conventional low 500 ℃ ~ Ac1 + 30 ℃ than the hot forming temperature can reduce the length of the micro-crack (micro crack) that occur during molding.
[179]
[180]
The method of the warm press forming member
[181]
It will now be described in detail a manufacturing method of another aspect of the warm press forming member of the present invention.
[182]
[183]
Another aspect of the warm production method of a press molded part of the present invention is a press at a temperature of heating the steel sheet produced by the process according to the excellent high-temperature elongation properties above high strength steel sheet after, 500 ℃ ~ Ac1 + 30 ℃ and a step of forming a.
[184]
[185]
When the warm molding temperature is lower than 500 ℃ has a high-temperature elongation properties may be insufficient cementite are mothayeo not sufficiently spheroidized. On the other hand, if the the warm molding temperature is higher than Ac1 + 30 ℃ has been oxide is generated on the surface of the steel sheet, and may require additional shot-blasting (Shot blast) process to then warm compaction, of shaping the steel sheet with zinc plating or alloyed galvanizing layer formed If the final is likely to cause fine cracks tend to carry higher iron grain boundaries movement is spreading Zn is liquefaction.
[186]
[187]
In the case of the conventional HPF (HOT PRESS FORMING) or PHS hot forming member known as (Press Hardening Steel) product, the annealing temperature by heating to obtain a final microstructure to martensite, and the heat treatment above the austenite single-phase reverse Ac3 required, critical characterized by a final cooling is made of an organization of martensite under cooling conditions or more cooling rate, however, this may impact resistance properties can be inferior accordingly.
[188]
Further, the molten Zn plating layer in the surface of the steel sheet according to the high-temperature annealing at least Ac3 possession iron grain boundaries during hot forming to easily transport through the diffusion finally possibility is very high that fine cracks having a length of a hard disadvantages premier 10㎛ below have.
[189]
[190]
One, since the steel sheet according to the present invention has excellent elongation properties at high temperature (500 ℃ ~ Ac1 + 30 ℃) forming even if the molding at a range of low 500 ℃ ~ Ac1 + 30 ℃ than conventional hot forming temperature, as described above not fracture does not occur it is possible to manufacture a warm press-forming member.
[191]
In addition, it is not necessary to heat up to austenite single phase reverse can be secured to a non-pearlite after forming martensite as a main phase is excellent in impact resistance properties.
[192]
[193]
Furthermore, fine cracks that occur during forming because the production through the warm compaction in the range of forming around the lower 500 ℃ ~ Ac1 + 30 ℃ even if it is formed of the surface of the steel sheet added to the zinc plating layer or alloyed zinc plating layer in the conventional than hot forming temperature it is possible to reduce the length of the (micro crack).
[194]
Turning to the fine crack generation mechanism according to the Zn plating layer of zinc or zinc alloy plating layer in detail, generally in liquid Zn Fe-Zn phase diagram is generated from the peritectic temperature (about 780 ℃). If there is more than a conventional heat the heat treatment temperature Ac3 side of the peritectic becomes higher than the temperature of the liquid Zn in the zinc plating layer or alloyed zinc plating layer of the steel sheet surface is formed it is easy to austenite grain boundary diffusion of the Zn subsequent hot fine cracks in molding the molded part parts and prone (Fig microcracks observed surface of the second), its length has also been raised difficult issues to get to the top 10㎛ below.
[195]
On the other hand, the warm molding temperature range in the present invention is 500 ℃ ~ Ac1 + at 30 ℃ the Fe-Zn peritectic temperature than the lower liquid phase and to minimize the grain boundary diffusion of the solid phase Zn got the amount of micro-cracks occurring after hot forming length the will can be reduced.
[196]
[197]
In this case, the molding can be carried out in more than 0.001 / s strain rate.
[198]
If the strain rate is less than 0.001 / s, it is preferable to be more advantageous. However, the workability in the field very off because the productivity can be performed by jeohal least 0.001 / s strain rate in a high-temperature elongation side.
[199]
Mode for the Invention
[200]
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.
[201]
[202]
(Example 1)
[203]
To prepare a cold-rolled steel sheet under the conditions described in the heat-treated for 1 hour, the slab having the composition components shown in Table 1 in a 1180 ℃ heating, Table 2 below. To the annealing temperature in Table 2 refers to the annealing temperature after cold-rolling, and - means that it is marked as not subjected to the annealing after the cold rolling ".
[204]
To measure the microstructure, N value, tensile strength and high-temperature elongation of said cold-rolled steel sheets produced are shown in Table 2.
[205]
Microstructure is a scanning electron microscope (SEM) or the use was observed after application leaving the etching method, in the following Table 2 and Table 3 are P pearlite, F is ferrite, B is bainite, M means martensite. The number of cementite in the microstructure along the length of the major axis of the cold-rolled steel sheet are respectively a scanning electron microscope as shown in Figure 1 (SEM) and transmission electron microscopy (TEM) was measured using a micro-structure observation photograph.
[206]
High-temperature elongation was described by the average value of the total elongation was measured each three times under 0.001 / s strain rate conditions in the following Table A 2 each different set according to the test temperature over the then processing the high-temperature tensile test specimens, the high temperature tensile testing machine.
[207]
To a unit of each element content of Table 1 is% by weight.
[208]
[209]
TABLE 1
[210]
[211]
TABLE 2
[212]
[213]
If invention examples that satisfy all of the alloy compositions and manufacturing conditions provided by the present invention, the microstructure comprises a ferrite of less than 80% perlite and 20% in area fraction, N values ​​are tensile strength, and high temperature tensile by at least 60% the elongation can be seen that good.
[214]
[215]
While the tensile strength or a high temperature tensile elongation with the invention the alloy composition or under the nail, or N value is not sufficient 60% pearlite case it does not meet the production conditions were presented in the disadvantage.
[216]
[217]
(Example 2)
[218]
Examples of cold-rolled steel sheet prepared from 1 (Specimen No. identical) to a single side coating weight is 60g / m 2 formed by the press at a molding temperature described later subjected to electro-galvanizing so that, to heat the contents to heat to, and Table 3 and It was prepared in a HAT-shaped molded part, such as a 3 by carrying out the cooling.
[219]
To the tensile strength, microstructure, N values, whether breakage of the member within the fine crack length and molding of the molded part are shown in Table 3. However, it did not measure the tensile strength and the micro-crack length when rupture occurs, N values ​​were measured only when towing invention.
[220]
[221]
Tensile test using a JIS No. 5 specimen size was carried out by a test speed of 10mm per minute.
[222]
[223]
Microstructures were observed after application and release etching method using a scanning electron microscope (SEM), when the microstructure the same before and after the molding, the molding was expressed by '='.
[224]
Further, the members within the micro-crack length was measured for the average depth of the crack 10 microcrack through an optical image analysis, as shown in FIG. 4 to the depth of the micro-cracks which penetrate the member from the member and the surface coating layer.
[225]
[226]
TABLE 3
[227]
[228]
The present invention was an alloy composition and cold-rolled steel sheet which satisfies both the production conditions presented when the molding in a temperature range of 500 ℃ ~ Ac1 + 30 ℃, the breakage did not occur during forming in, the micro-crack length observed below 10㎛ you can see that.
[229]
[230]
However, the molded part of the alloy composition, and even with a cold rolled steel sheet that satisfies both the production conditions are a molding temperature lower Specimen No. 2-5 and 4-3 provided in this invention are generated is broken.
[231]
In addition, the molding member of the 5-3 alloy composition and even with a cold rolled steel sheet that satisfies both the production conditions are a molding temperature higher number samples presented in this invention is a micro-crack length was observed in 10㎛ exceeded.
[232]
[233]
When using a cold-rolled steel sheet or the alloy composition does not meet the production conditions in the present invention, the breakage of the molding occurs, whether or not a satisfactory forming temperature in the present invention, or exceeded the fine crack length 10㎛.
[234]
[235]
Above embodiment has been with reference to describe, understand that without departing from the spirit and scope of the invention defined in the claims below are those skilled in the art can make various modifications and variations to the present invention It will be.

Claims
[Claim 1]
By weight%, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (0% is excluded), S: 0.01% or less (excluding 0%), N: negative than 0.01% (0% ), sol.Al: the at least one of 1.6% or less (excluding 0%): 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si and including, the microstructure comprises a ferrite of less than 80% perlite and 20% in area fraction, the perlite has a high-strength steel sheet long in the major axis having excellent high-temperature elongation properties containing not more than 200nm cementite.
[Claim 2]
According to claim 1, wherein said steel sheet is a high-strength steel sheet having excellent high-temperature elongation properties satisfying the following relational expression: 1. Equation 1: 0.7≤C Cr + / 2 + Mn / Si 3 + / 4≤3.0 (each element in the equation 1 is a value showing the symbol of each element content by weight%, it does not include calculated to zero.)
[Claim 3]
The method of claim 1, wherein cementite in the pearlite to the N value is 60% or higher high-temperature elongation properties are excellent high-strength steel sheet according to equation 2. Equation 2: N (%) = Nx / (Nx + Ny) * 100 (in the equation 2, Nx is a number of the length of the major axis 200nm or less cementite, Ny is the number of the cementite long in the major axis 200nm exceeds the means).
[Claim 4]
The method of claim 1, wherein the steel sheet has a tensile strength of not less than 1000MPa, 500 ℃ ~ Ac1 + 30 ℃ elongation of 60% or higher high-temperature elongation properties are high-strength steel sheet excellent in a temperature range of.
[Claim 5]
The method of claim 1, wherein the steel sheet is an aluminum plated layer on the surface, a high-strength steel sheet having excellent high-temperature elongation properties that one of the zinc alloy plating layer and zinc plated layer is further formed.
[Claim 6]
By weight%, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (0% is excluded), S: 0.01% or less (excluding 0%), N: negative than 0.01% (0% ), sol.Al: the at least one of 1.6% or less (excluding 0%): 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si heating the slab containing in 1100 ~ 1300 ℃; Steps to finish hot-rolling the heated slab at a temperature of Ar3 + 10 ℃ ~ Ar3 + 90 ℃ to obtain a hot-rolled steel sheet; Step for winding the hot-rolled steel sheet from 550 ~ 700 ℃; The method of excellent high-temperature elongation properties including a high-strength steel sheet; and the cold to the hot-rolled steel sheet to a take-up reduction rate 40% to 80% rolling to obtain a cold-rolled steel sheet.
[Claim 7]
The method of claim 6, wherein the slab is method of producing a high-strength steel sheet having excellent high-temperature elongation properties satisfying the following relational expression: 1. Equation 1: 0.7≤C Cr + / 2 + Mn / Si 3 + / 4≤3.0 (each element in the equation 1 is a value showing the symbol of each element content by weight%, it does not include calculated to zero.)
[Claim 8]
The method of manufacturing the high-temperature elongation properties volume that takes the high strength steel sheet further comprises the step of performing the appeal based on the 200 ~ 700 ℃ after the step of claim 6.
[Claim 9]
7. The method of claim 6, wherein the production of the cold-rolled steel sheet Ac1-70 ℃ ~ Ac1 + 70 ℃ high temperature elongation characteristics are excellent high-strength steel sheet in a temperature range including the further step of performing continuous annealing or the appeal placed.
[Claim 10]
The method of claim 6 wherein the method for manufacturing a high-strength steel sheet having excellent high-temperature elongation properties further including the step of plating the cold-rolled steel sheet.
[Claim 11]
11. The method of claim 10, wherein the production of the plated high strength steel sheet having excellent high-temperature elongation properties, including cold-rolled steel sheet additionally the step of the alloying treatment.
[Claim 12]
7. The method of claim 6, wherein the production of the cold rolling has a high strength steel sheet excellent high-temperature elongation properties is performed at room temperature.
[Claim 13]
By weight%, C: 0.4 ~ 0.9%, Cr: 0.01 ~ 1.5%, P: 0.03% or less (0% is excluded), S: 0.01% or less (excluding 0%), N: negative than 0.01% (0% ), sol.Al: the at least one of 1.6% or less (excluding 0%): 0.1% or less (excluding 0%), the rest Fe and unavoidable impurities, and containing, Mn: 2.1% or less (excluding 0%), and Si and including, the microstructure comprises a ferrite of less than 80% perlite and 20% in area fraction, N the value is 70% greater than the warm press forming member according to the cementite is the following relation 2 in the pearlite. Equation 2: N (%) = Nx / (Nx + Ny) * 100 (in the equation 2, Nx is a number of the length of the major axis 200nm or less cementite, Ny is the number of the cementite long in the major axis 200nm exceeds the means).
[Claim 14]
14. The method of claim 13, the warm press-forming member for forming the member satisfies the following relational expression: 1. Equation 1: 0.7≤C Cr + / 2 + Mn / Si 3 + / 4≤3.0 (each element in the equation 1 is a value showing the symbol of each element content by weight%, it does not include calculated to zero.)
[Claim 15]
The method of claim 13 wherein the member is warm press forming member is formed by adding the aluminum layer on the surface.
[Claim 16]
The method of claim 13 wherein the member is formed by adding a zinc plating layer or zinc alloy plating layer on the surface, and not more than a member within the fine crack length 10㎛ warm press forming member.
[Claim 17]
Claim 6 to 12 The method of the warm press forming member comprises the step of forming by the press in any one of after heating the steel sheet produced by the method, a temperature range of 500 ℃ ~ Ac1 + 30 ℃ of the claims.
[Claim 18]
18. The method of claim 17 wherein the molding is a method of producing a member for performing the warm press forming with a deformation rate of at least 0.001 / s.