CN101490295B - High-strength steel sheet excellent in stretch flangeability and fatigue property - Google Patents

High-strength steel sheet excellent in stretch flangeability and fatigue property Download PDF

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CN101490295B
CN101490295B CN2007800268244A CN200780026824A CN101490295B CN 101490295 B CN101490295 B CN 101490295B CN 2007800268244 A CN2007800268244 A CN 2007800268244A CN 200780026824 A CN200780026824 A CN 200780026824A CN 101490295 B CN101490295 B CN 101490295B
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inclusion
mns
steel plate
stretch flange
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CN101490295A (en
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笹井胜浩
大桥渡
山本研一
川崎薰
原田宽
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
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  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

The invention provides a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue properties. The steel sheet contains 0.03-0.20% C, 0.08-1.5% Si, 1.0-3.0% Mn, up to 0.05% P, at least 0.0005% S, 0.0005-0.01% N, up to 0.01% acid-soluble Al, less than 0.008% acid-soluble Ti, and 0.0005-0.04% at least either of Ce and La, the remainder being iron and unavoidable impurities. The steel sheet contains stretched inclusions which have a diameter, in terms of equivalent-circle diameter, of 1 mum or larger and in which the ratio of the major-axis length to the minor-axis length is 5 or higher, in a proportion of 20% or higher by number.

Description

The high tensile steel plate of stretch flange property and excellent in fatigue characteristics
Technical field
The present invention relates to be suitable as automobile with the raw-material stretch flange property of ground-engaging element and the high tensile hot rolled steel sheet of excellent in fatigue characteristics.
Background technology
Improve and relate to the viewpoint that the fuel cost of environmental protection improves from the security of automobile, to the HS of hot-rolled steel plates for automobiles light-weighted require increasingly high.Automobile is high with shared ratio in the vehicle body gross weight such as the weight of vehicle frame class that particularly is called as running gear in the parts and arm class etc., therefore reduces thickness through improving the employed raw-material intensity in these positions, can realize its lightweight.In addition, from viewpoint, require the employed material of this running gear to have the height fatigue characteristic to the weather resistance of the vibration going.
But, being accompanied by the raising of high strength, resistance to fatigue, all there are the tendency of decline in reaming property and ductility, and when high tensile steel plate was used for complex-shaped automobile traveling system etc., its reaming property became important research project.
Therefore, having proposed multiple is the steel plate of purpose to take into account mechanical strength property and fatigue characteristic and reaming property (processibility).For example, in japanese patent laid-open 11-199973 communique, proposed in the complex tissue steel plate of ferritic phase and martensitic phase, to separate out fine Cu or disperse sosoloid and the steel plate (being commonly referred to as the DP steel plate) that obtains.In the public technology shown in this japanese patent laid-open 11-199973 communique, find the Cu of solid solution or be that Cu precipitate below the 2nm is very effective to improving fatigue characteristic by the size of particles that Cu constitutes separately; And can not influence processibility, therefore the ratio of components to various compositions limits.
Though it is not known this DP steel plate is excellent on the balance of intensity and ductility and fatigue characteristic, still good through the stretch flange property that the drifiting test evaluation obtains.Can think that one of its reason is because the DP steel plate is a soft ferritic and the complex body of hard martensitic phase, therefore add man-hour biphase boundary portion and can't adapt to and be out of shape and become the starting point of fracture easily in reaming.
This has been proposed not only satisfy the high tensile hot rolled steel sheet (for example opening the 2001-200331 communique) of requirement of stretch flange property that fatigue characteristic also satisfy the desired harshness of material of nearest wheel and ground-engaging element with reference to Japanese Patent Laid.The major technique content that this Japanese Patent Laid is opened the public technology of 2001-200331 communique is: making principal phase through low as far as possible Cization is bainite structure; Simultaneously contain the ferritic structure after solution strengthening or the precipitation strength with suitable volumetric ratio; Thereby reduce the difference of hardness of above-mentioned ferritic and bainite, and avoid producing thick carbide etc.
Summary of the invention
Above-mentioned Japanese Patent Laid opens that disclosed in the 2001-200331 communique to make the steel plate tissue be main body with bainite mutually and suppress the high tensile hot rolled steel sheet that thick carbide generates and have outstanding stretch flange property really, but its fatigue characteristic not necessarily are superior to containing the DP steel plate of Cu.And, if only suppress the generation of thick carbide, then add and can't prevent the generation of chapping man-hour carrying out harsh reaming.Research according to the present inventor can know that it is the sulfide-based inclusion of the extension of main body that its reason is to exist with MnS in the steel plate.If receive repeated deformation; Then the periphery at the thick MnS type impurity that is positioned at top layer or near the extension it produces subsurface defect; Propagate and fatigue characteristic are descended with the form of be full of cracks, and the thick MnS type impurity that extends becomes reaming easily and adds and produce the disruptive starting point man-hour.For this reason, hope not make the MnS type impurity extension in the steel and that it is formed is fine spherical as far as possible.
But; Mn and C and Si are the element that helps effectively to improve the strength of materials equally, therefore in high tensile steel plate, set the concentration of Mn higher usually in order to ensure intensity, in addition; If the heavily processing of in the secondary refining operation, not implementing to take off S, then S concentration also can be more than 50ppm.Therefore, in the casting sheet, there is MnS usually.If the casting sheet is by hot rolling and cold rolling, then MnS is because easy deformation becomes the MnS type impurity of extension, and this is the reason that makes fatigue characteristic and stretch flange property (bore expanding workability) decline.But do not see stretch flange property and fatigue characteristic on the example of hot-rolled steel sheet of excellence of proposition from the angle of separating out and being out of shape of control MnS.
For this reason; The present invention is in view of the above problems and the invention that proposes; Its purpose is to provide the high tensile steel plate of a kind of stretch flange property and excellent in fatigue characteristics; It separates out fine MnS through making casting in sheet, and makes this MnS with not yielding when rolling and be difficult for becoming the break form of fine nodule of the starting point that takes place and be scattered in the steel plate, thereby improves stretch flange property and fatigue characteristic.
In order to address the above problem, the present inventor is so that separate out fine MnS in the casting sheet and make it with not yielding when rolling and to be difficult for becoming the illustrating of interpolation element that the form of fine nodule of the starting point that takes place is scattered in the method in the steel plate and fatigue characteristic are descended of breaking be to concentrate on studies in the center.The result finds: separating out MnS because of adding on Ce oxide compound that Ce, La carry out the fine and hard that deoxidation produces, La oxide compound, sulphur cerium oxide, the sulphur lanthanum trioxide; The MnS that this is separated out when rolling is not yielding; Therefore the thick MnS that in steel plate, extends significantly reduces; When repeated deformation and reaming add man-hour, these MnS type impurities are difficult for becoming the starting point and the path of crevasse crack propagation of breaking and taking place, and have explained the reason of raisings such as above-mentioned resistance to fatigue.
The main contents of the stretch flange property that the present invention relates to and the high tensile steel plate of excellent in fatigue characteristics are described below.
(1) steel plate of the present invention is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; The number ratio of extending inclusion is below 20%, and said extension inclusion is that the diameter of equivalent circle that in this steel plate, exists is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.
(2) steel plate of the present invention is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; In this steel plate, contain and separating out the inclusion that MnS is arranged by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide more than 10% with the number proportional meter.
(3) steel plate of the present invention is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; A volume number density of extending inclusion is 1.0 * 10 4Individual/mm 3Below, said extension inclusion is that the diameter of equivalent circle that in this steel plate, exists is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.
(4) steel plate of the present invention is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; In this steel plate, be 1.0 * 10 by a volume number density of separating out the inclusion of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide 3Individual/mm 3More than.
(5) steel plate of stretch flange property of the present invention and excellent in fatigue characteristics is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; The average equivalent circular diameter that extends inclusion is below the 10 μ m, and said extension inclusion is that the diameter of equivalent circle that in this steel plate, exists is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.
(6) steel plate of stretch flange property of the present invention and excellent in fatigue characteristics is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; In this steel plate, exist in by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and separate out the inclusion that MnS is arranged, in this inclusion, contain the Ce of 0.5～50 quality % or a kind or 2 kinds total among the La in average composition.
(7) steel plate of stretch flange property of the present invention and excellent in fatigue characteristics is characterised in that; In quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; (Ce+La)/the S ratio is 0.1～70.
(8) high tensile steel plate of each described stretch flange property and excellent in fatigue characteristics in basis (1)～(7); It is characterized in that; In quality % contain in Nb:0.01～0.10%, V:0.01～0.05%, Cr:0.01～0.6%, Mo:0.01～0.4%, B:0.0003～0.03% any or more than 2 kinds, remainder is made up of iron and unavoidable impurities.
Description of drawings
Fig. 1 is the figure of the relation of expression Ce+La (%) and S (%).
Embodiment
Below, as the best mode of embodiment of the present invention, the high tensile steel plate of stretch flange property and excellent in fatigue characteristics is elaborated.Quality % in the following composition simply is expressed as %.
At first, describe accomplishing experiment of the present invention.
The present inventor to contain C:0.07%, Si:0.2%, Mn:1.2%, below the P:0.01%, S:0.005%, N:0.003% and remainder be that the molten steel of Fe uses various elements to carry out deoxidation, makes bloom.The bloom that obtains is rolled into the hot-rolled steel sheet of 3mm.The hot-rolled steel sheet of these manufacturings is supplied drifiting test and fatigue test, and to analyzing the number density of the inclusion in the steel plate, form and average the composition.
Can know from its result: use the Al deoxidation hardly, add add Ce, La behind the Si at least and the steel plate of deoxidation best aspect stretch flange property and the fatigue characteristic.Its reason is; Separating out MnS because of adding on Ce oxide compound that Ce, La carry out the fine and hard that deoxidation produces, La oxide compound, sulphur cerium oxide, the sulphur lanthanum trioxide; The MnS that this is separated out when rolling is not yielding yet, so the thick MnS that extends in the steel plate significantly reduces.Consequently, when repeated deformation and reaming add man-hour, these MnS type impurities are difficult for becoming the break starting point that takes place and the path of crevasse crack propagation, this makes raisings such as above-mentioned resistance to fatigue.
In addition, the reason of Ce oxide compound, La oxide compound, sulphur cerium oxide and the miniaturization of sulphur lanthanum trioxide is, the SiO that generates because of the Si deoxidation at first 2Type impurity by after the Ce, La reduction decomposition that add and form fine Ce oxide compound, La oxide compound, sulphur cerium oxide and sulphur lanthanum trioxide; And Ce oxide compound, La oxide compound, sulphur cerium oxide and the sulphur lanthanum trioxide self and the interfacial energy between the molten steel that generate are low, thereby the agglomeration after also suppressing to generate.
According to the discovery that obtains from these experimental researches, the present inventor is described below the chemical ingredients condition of steel plate is studied, thereby has accomplished the present invention.
Below, the reason that limits chemical ingredients is in the present invention described.
C：0.03～0.20％
C is hardenability and the most basic element of intensity of control steel, helps to improve effectively hardness and the degree of depth of laser quenching hardened layer and improves fatigue strength.That is, this C is a necessary element of guaranteeing armor plate strength, in order to obtain high tensile steel plate, must be at least 0.03%.But, if contain too much this C, then can because of generating the Ti carbide C be fixed as in the past, even apply cooling conditions, still can generate the cementite phase.This cementite brings out the work hardening of steel plate mutually, is unfavorable for the raising of stretch flange characteristic.Therefore, the present invention is decided to be below 0.20% from the viewpoint that the improves processibility concentration with C.
Si：0.08～1.5％
Si is that the present invention does not add so the main deoxidant element in the molten steel of Al and Ti as far as possible, so extremely important in the present invention.And Si makes austenitic nucleus formation number of sites increase when quenching heating, suppresses austenitic particle growth, and undertakes the effect of the particle diameter miniaturization that makes laser quenching hardened layer.This Si suppresses carbide and generates, the decline of the grain-boundary strength that the inhibition carbide causes.In addition, this Si is also very effective for the generation of bainite structure, from the viewpoint of the intensity of guaranteeing material monolithic, is undertaking important effect.In order to reduce dissolved oxygen concentration in the molten steel, temporarily to generate SiO 2Type impurity (this SiO 2Type impurity is through being made the inclusion miniaturization by the Ce that adds afterwards, La reduction), must add the Si more than 0.08%.Therefore, the present invention is decided to be 0.08% with the lower limit of Si.Relative therewith, if the excessive concentration of Si, the then SiO in the inclusion 2Concentration improves and generates large-scale inclusion easily, and tough ductility extreme difference, because of surface decarburization and surface spots increase cause fatigue characteristic decline on the contrary.In addition, if too much add Si, also can produce detrimentally affect to weldability and ductility.Therefore, the present invention is with being defined as 1.5% on the Si.
Mn：1.0～3.0％
Mn is the useful element of deoxidation to the system steel stage, is with C, the Si high strength effective elements to steel plate.In order to obtain this effect, must contain this Mn more than 1.0%.But, surpass 3.0% Mn if contain, then because the increase of the segregation of Mn and solution strengthening makes ductility decline.In addition, because weldability and base material toughness also can descend, so with being defined as 3.0% on this Mn.
Below the P:0.05%
P can be effective as than the little displaced type solution strengthening element of Fe atom and work, but owing at austenitic grain boundary segregation, make grain-boundary strength descend, causes torsional fatigue strength to descend, and processibility is descended, and therefore is decided to be below 0.05%.In addition, if need not solution strengthening, then needn't add P, so the lower value of P comprises 0%.
More than the S:0.0005%
S owing to the thick extension inclusion that forms MnS makes the decline of stretch flange property, therefore hopes for trying one's best low concentration as impurity and segregation.In the past, in order to ensure stretch flange property, must carry out extremely low sulfur " and make the concentration of S be lower than 0.0005%.But in the present invention, not yielding when rolling owing on Ce oxide compound, La oxide compound, sulphur cerium oxide, sulphur lanthanum trioxide, separate out MnS yet, prevented the extension of inclusion, therefore the higher limit of the concentration of S is not done special stipulation.
In addition; In order to make the S density loss to being lower than 0.0005% level with equal in the past; Just must in secondary refining, handle by certain degree ground reinforcing desulfuration; Because be to realize that the desulfurization processing cost of this concentration is too high, and be difficult for appearing the effect of MnS being carried out morphology Control, therefore the lower value with S concentration is decided to be 0.0005%.
N：0.0005～0.01％
N sneaks into the element in the steel inevitably owing in molten steel is handled, take in airborne nitrogen.Formation nitride such as N and Al, Ti and promote the grain refined of base material tissue.But,,, cause stretch flange decline even then trace of Al and trace Ti also can generate thick precipitate if add this too much N.Therefore, the present invention is with being defined as 0.01% on the concentration of N.On the other hand, if the concentration of N is lower than 0.0005%, then cost increases, therefore with 0.0005% as lower limit.
Below the solvable Al:0.01% of acid
The solvable Al of acid is big because of its oxide compound is prone to bunch change chap, causes stretch flange property and fatigue characteristic to descend, and therefore preferably does one's utmost control.But, allow to use below 0.01% as the preliminary deoxidation material.This is because surpass 0.01% as if sour solvable Al concentration, then the Al in the inclusion 2O 3Content surpasses 50%, causes bunch change of inclusion.From preventing the viewpoint of bunch change, sour solvable Al concentration is low more good more, and lower value comprises 0%.In addition, sour solvable Al concentration is meant measures the concentration that dissolved Al concentration obtains in the acid, is to utilize to dissolve to deposit Al and in acid, dissolve and Al 2O 3Insoluble analysis of phenomenon method in acid.Here, as acid, the nitration mixture that can illustration for example mixes in the ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, water 2.Use this acid can distinguish in acid soluble Al and insoluble Al in acid 2O 3, can measure sour solvable Al concentration.
The solvable Ti of acid: be lower than 0.008%
The solvable Ti of acid is also big because of easy bunch of change chap of its oxide compound, and the thick TiN inclusion of the combination of the N among Yi Yugang generation, therefore makes sour solvable Ti for being lower than 0.008%, and lower value comprises 0%.In addition, sour solvable Ti concentration is meant the concentration that the concentration of measuring dissolved Ti in the acid obtains, and is to utilize to dissolve to deposit Ti and in acid, dissolve and Ti oxide compound insoluble analysis of phenomenon method in acid.Here, as acid, the nitration mixture that can illustration for example mixes in the ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, water 2.Use this acid can distinguish in acid soluble Ti and in acid insoluble Ti oxide compound, can measure sour solvable Ti concentration.
1 kind or 2 kinds total among Ce or the La: 0.0005～0.04%
Ce, La will be through the SiO of Si deoxidation generation 2Reduction is prone to become the site of separating out of MnS, and has formation with hard, on-deformable Ce oxide compound when fine and rolling (Ce for example 2O 3, CeO 2), sulphur cerium oxide (Ce for example 2O 2S), La oxide compound (La for example 2O 3, LaO 2), sulphur lanthanum trioxide (La for example 2O 2S), Ce oxide compound-La oxide compound or sulphur cerium oxide-sulphur lanthanum trioxide are the effect of the inclusion of principal phase (to be standard more than 50%).
Here, in above-mentioned inclusion,, contain some MnO, SiO sometimes because of the difference of deoxygenation conditions 2Or Al 2O 3, but, can give full play to the effect of separating out the site as MnS as long as principal phase is above-mentioned oxide compound, and can not damage the effect of fine, the hardization of inclusion.In order to obtain above-mentioned inclusion, must make among Ce or the La a kind or 2 kinds total concn is more than 0.0005% and below 0.04%.If among Ce or the La a kind or 2 kinds total concn are lower than 0.0005%, then can't be with SiO 2The inclusion reduction if surpass 0.04%, then can generate sulphur cerium oxide, sulphur lanthanum trioxide in a large number, forms thick inclusion, causes stretch flange property and fatigue characteristic to descend.
Nb：0.01～0.10％
Nb and C or N form carbide, nitride, carbonitride and promote the grain refined of base material tissue.In order to obtain this effect, must be at least 0.01%.But even surpass 0.10% and contain in a large number, effect is also saturated, and cost increases, therefore with 0.10% as the upper limit.
V：0.01～0.05％
V and C or N form carbide, nitride, carbonitride and promote the grain refined of base material tissue.In order to obtain this effect, must be at least 0.01%.But even surpass 0.05% and contain in a large number, effect is also saturated, and cost increases, therefore with 0.05% as the upper limit.
Cr：0.01～0.6％
Cr can contain with the hardenability that improves steel as required, guarantee the intensity of steel plate in order to obtain this effect, must be at least 0.01%.But, contain the balance that can make strength-ductility on the contrary in a large number and descend.Therefore, with 0.6% as the upper limit.
Mo：0.01～0.4％
Mo can contain with the hardenability that improves steel as required, guarantee the intensity of steel plate in order to obtain this effect, must be at least 0.01%.But, contain the balance that can make strength-ductility on the contrary in a large number and descend.Therefore, with 0.4% as the upper limit.
B：0.0003～0.003％
B can contain with the hardenability that improves steel as required, strengthens crystal boundary, improve processibility, in order to obtain this effect, must be at least 0.0003%.But, contain the detergency that diminishes steel on the contrary in a large number, ductility is descended.Therefore, with 0.003% as the upper limit.
Then, the existence condition to the inclusion in the steel plate of the present invention describes.In addition, steel plate is meant through hot rolling or the further plate behind cold rolling obtain rolling.
In order to obtain the steel plate of stretch flange property and excellent in fatigue characteristics, importantly reduce as far as possible and be prone to become the thick MnS type impurity of extension that starting point and rupture propagation path take place of breaking in the steel plate.The present inventor is lower than 1 μ m through experiment discovery diameter of equivalent circle MnS type impurity does not have harm as the generation starting point of breaking; Can not make the decline of stretch flange property and fatigue characteristic; And diameter of equivalent circle is that the above inclusion of 1 μ m can easily utilize scanning electron microscope (SEM) to wait to observe; Therefore be that inclusion more than the 1 μ m is an object with diameter of equivalent circle in the steel plate, analyze its form and composition, estimate the distribution of MnS type impurity.Here, diameter of equivalent circle be defined as the inclusion that obtains according to cross-section major diameter and minor axis by (major diameter * minor axis) 0.5The value of trying to achieve.
In addition, the upper limit of the diameter of equivalent circle of MnS type impurity is not done special stipulation, can observe the MnS type impurity about 1mm in fact sometimes.
About extending the number ratio of inclusion, through being that a plurality of (for example about 50) inclusion more than the 1 μ m carries out compositional analysis and measures the major diameter and the minor axis of inclusion from SEM figure to the diameter of equivalent circle that uses SEM to select at random.Here; When extending inclusion is that major diameter/minor axis (extension ratio) is when being the inclusion more than 5; Total inclusion number (is that example is about 50 with above-mentioned example) that the number of the above-mentioned extension inclusion that detection is obtained obtains divided by analysis can be obtained the number ratio of above-mentioned extension inclusion.
In addition, why the extension ratio with inclusion is decided to be more than 5, is because the extension ratio is that inclusion more than 5 all is the MnS type impurity in not adding the comparison steel plate of Ce, La.In addition, the upper limit of the extension ratio of MnS type impurity is not done special stipulation, can observe the extension ratio in fact sometimes and be about 50 MnS type impurity.
Its result shows: making the extension ratio through morphology Control is that the number ratio of the extension inclusion more than 5 is the steel plate below 20%, and its stretch flange property and fatigue characteristic improve.Promptly; If being the number ratio of the extension inclusion more than 5, the extension ratio surpasses 20%; Then be prone to become and break that to extend the number ratio of inclusion too much for MnS class that starting point takes place; Stretch flange property and fatigue characteristic descend, and are that the number ratio of the extension inclusion more than 5 is decided to be below 20% with the extension ratio in the present invention therefore.Because the MnS type impurity that extends stretch flange property more at least is good more with fatigue characteristic, so should the extension ratio be that the lower value of the number ratio of the extension inclusion more than 5 comprises 0%.
Here; Diameter of equivalent circle is that the above and extension ratio of 1 μ m is that the lower value of the number ratio of the extension inclusion 5 or more is 0%, though be that not have the extension ratio be that situation or the extension ratio of the inclusion more than 5 is the extension inclusion more than 5 but diameter of equivalent circle all is lower than the situation of 1 μ m for inclusion more than the 1 μ m even be meant diameter of equivalent circle.
In addition, making the extension ratio through morphology Control is that the number ratio of the extension inclusion more than 5 is that steel plate below 20% correspondingly is formed on by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and separates out the form that MnS is arranged.Form as this inclusion; As long as separating out MnS by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide; Not having special stipulation, in most cases is to be core and around it, separate out MnS by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide.
In addition, by separating out inclusion that MnS is arranged on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide because not yielding when rolling yet, therefore be that the shape of not extending promptly is similar to the globular inclusion in steel plate.
Here, become the nodule that does not extend not have special stipulation to judging, refer to that the extension ratio in the steel plate is the inclusion below 3, preferably extension ratio is the inclusion below 2.This is because casting sheet stage before rolling, is below 3 in the extension ratio by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide.In addition, the nodule that is judged as not extension is if be spherical fully, and then extension ratio is 1, and therefore extending the following of ratio is limited to 1.
Use with the same method of the number proportion grading that extends inclusion the number ratio of above-mentioned inclusion is analyzed.Its result shows: to make in the number ratio by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide be the steel plate 10% or more through separating out control, its stretch flange property and fatigue characteristic raising.If the number ratio by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide is lower than 10%, then therewith correspondingly the MnS class to extend the number ratio of inclusion too much, stretch flange property and fatigue characteristic decline.Therefore, will be decided to be more than 10% in number ratio by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide.In addition, then stretch flange property and fatigue characteristic are good more separated out many more MnS by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide, so the higher limit of this number ratio comprises 100%.
In addition, by the inclusion of separating out the form that MnS is arranged on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide because not yielding when rolling, so its diameter of equivalent circle do not have special stipulation, can be for more than the 1 μ m.But, then might become the generation starting point of breaking, so the upper limit is preferably about 50 μ m if excessive.
On the other hand, this inclusion is not only not yielding when rolling, and when diameter of equivalent circle is lower than 1 μ m, can not become the generation starting point of breaking, and therefore the lower limit of diameter of equivalent circle is not done special stipulation.
Then, as the existence condition of the inclusion in the steel plate of the invention described above, stipulate with the individual number density of the unit volume of inclusion.
About the size distribution of inclusion, the SEM of the electrolysis face through adopting tachometric method estimates and implements.Adopt the SEM evaluation of the electrolysis face of tachometric method to be meant, after the surface grinding with print, utilize tachometric method to carry out electrolysis, directly sample surfaces is carried out SEM and observe a size and a number density of estimating inclusion.In addition, tachometric method is meant, uses 10% methyl ethyl diketone-1% tetramethyl ammonium chloride-methyl alcohol that the method for inclusion is extracted in the sample surfaces electrolysis, as the electrolysis amount, with respect to the area 1cm of sample surfaces 2Carry out electrolysis with 1C.The SEM figure on the surface that electrolysis like this is obtained carries out picture processing, and the frequency of obtaining with respect to diameter of equivalent circle (number) distributes.Calculate the average equivalent circular diameter by the frequency distribution of this particle diameter, and with frequency divided by the area of field of view and the degree of depth of trying to achieve by the electrolysis amount, thereby calculate the individual number density of the unit volume of inclusion.
The evaluation result that the diameter of equivalent circle that becoming breaks starting point takes place and make stretch flange property to descend with fatigue characteristic is more than the 1 μ m, the extension ratio is a volume number density of the inclusion more than 5 shows: if 1.0 * 10 4Individual/mm 3Below, then stretch flange property and fatigue characteristic improve.If to be 1 μ m above and the extension ratio is that the volume number density of the extension inclusion 5 or more is above 1.0 * 10 for diameter of equivalent circle 4Individual/mm 3, then be prone to become and break that to extend individual number density of inclusion too much for MnS class that starting point takes place, stretch flange property and fatigue characteristic decline, so be more than the 1 μ m and the extension ratio is that the volume number density of the extension inclusion more than 5 is decided to be 1.0 * 10 with diameter of equivalent circle 4Individual/mm 3Below.In addition since the MnS type impurity that extends stretch flange property is good more with fatigue characteristic more at least, so diameter of equivalent circle is more than the 1 μ m and the extension ratio is that the lower value of a volume number density of the extension inclusion more than 5 comprises 0%.
Here, diameter of equivalent circle is that the above and extension ratio of 1 μ m is that the lower value of a volume number density of the extension inclusion more than 5 is that 0% the meaning is the same.
In addition, in that make diameter through morphology Control be more than the 1 μ m and unit elongation is that the volume number density of the extension inclusion more than 5 is 1.0 * 10 4Individual/mm 3Below steel plate in, corresponding therewith, the MnS type impurity that does not extend is for separating out the form that MnS is arranged by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide, it is shaped as approximate nodule.
Form as this inclusion; With above-mentioned same; As long as separating out MnS by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide; Not having special stipulation, in most cases is to be core and around it, separate out MnS by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide.
As nodule, there are not special stipulation, refer to that the extension ratio in the steel plate is the inclusion below 3, preferably extension ratio is the inclusion below 2.Here, spherical as if being entirely, then extension ratio is 1, and therefore extending the following of ratio is limited to 1.
The analytical results of the volume of an above-mentioned inclusion number density shows: make by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide to be core and the volume number density of inclusion of around it, separating out the form of MnS is 1.0 * 10 through separating out control 3Individual/mm 3Above steel plate, its stretch flange property and fatigue characteristic improve.If the volume number density by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide is lower than 1.0 * 10 3Individual/mm 3Then correspondingly the number ratio of the extension inclusion of MnS is too much; Stretch flange property and fatigue characteristic descend, and therefore will be defined as 1.0 * 10 in the volume number density by the inclusion of the form of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide 3Individual/mm 3More than.And, with by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide be the MnS that separates out of core stretch flange property and fatigue strength are good more more at most, therefore the higher limit of this volume number density is not done special stipulation.
In addition,, there are not special stipulation by the diameter of equivalent circle of the inclusion of separating out the form that MnS is arranged on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and above-mentioned same, can be for more than the 1 μ m.But,, then might become the generation starting point of breaking, so the upper limit is preferably about 50 μ m if this equivalent circular diameter is excessive.
On the other hand, when the diameter of equivalent circle of this inclusion is lower than 1 μ m, have no problem, therefore lower limit is not had special stipulation.
Then, as the existence condition of the extension inclusion in the steel plate of the invention described above, stipulate with the higher limit of diameter of equivalent circle.Particularly; Starting point takes place and make stretch flange property and the diameter of equivalent circle of fatigue characteristic decline is more than the 1 μ m and the extension ratio is that the average equivalent circular diameter of the inclusion more than 5 is estimated becoming to break; The result shows: when the average equivalent circular diameter of this extension inclusion is 10 μ m when following, stretch flange property and fatigue characteristic improve.This is to be conceived to along with diameter of equivalent circle is more than the 1 μ m and the extension ratio is the increase of the number ratio of the extension inclusion more than 5, and this average equivalent circular diameter that extends inclusion increases, and is that index is stipulated with the average equivalent circular diameter that extends inclusion.This can infer and be the increase along with the amount of Mn in the molten steel and S that the number of the MnS of generation increases, and the size chap of the MnS that generates is big.
Therefore; If to be 1 μ m above and the extension ratio is that extension inclusion more than 5 is greater than 10 μ m for diameter of equivalent circle; The number ratio that then correspondingly should extend inclusion surpasses 20%; Therefore be prone to become and break that to extend the number ratio of inclusion too much for thick MnS class that starting point takes place, stretch flange property descends with fatigue characteristic, so is more than the 1 μ m and the extension ratio is that the average equivalent circular diameter of the extension inclusion more than 5 is set at below the 10 μ m with diameter of equivalent circle.
In addition; With diameter of equivalent circle is more than the 1 μ m and the extension ratio is that the average equivalent circular diameter of the extension inclusion more than 5 is set at the regulation below the 10 μ m; Mean that in steel plate, having diameter of equivalent circle is the inclusion more than the 1 μ m, so the lower value of diameter of equivalent circle is 1 μ m.
On the other hand; As in the steel plate of the invention described above by the existence condition of separating out the inclusion of the form that MnS is arranged on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide, stipulate with separating out by the average content of forming of Ce in the inclusion of MnS or La.
Particularly, as stated, improve stretch flange property and fatigue characteristic aspect, make separating out MnS by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide, prevent the extension of MnS, this is very important.
Form as this inclusion; As stated; As long as separating out MnS by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide; Not having special stipulation, in most cases is to be core and around it, separate out MnS by a kind among Ce or the La or 2 kinds of oxide compounds that form or oxysulfide.
In addition, as nodule, do not have special stipulation, refer to that the extension ratio in the steel plate is the inclusion below 3, preferably extension ratio is the inclusion below 2.Here, spherical as if being entirely, then extension ratio is 1, and therefore extending the following of ratio is limited to 1.
Therefore, in order clearly effectively to suppress the composition that the MnS type impurity extends, to carrying out compositional analysis by the inclusion of separating out the form of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide.
But, if the diameter of equivalent circle of this inclusion is more than the 1 μ m, then be easy to observe, therefore for simplicity, be to be object more than the 1 μ m with diameter of equivalent circle.But,, preferably also comprise the inclusion that diameter of equivalent circle is lower than 1 μ m if can observe.
In addition, separating out the inclusion of the form that MnS is arranged owing to extend by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide, thereby confirming it is that the extension ratio is all at the inclusion below 3.Therefore, be more than the 1 μ m with diameter of equivalent circle and the extension ratio is that inclusion below 3 is that object carries out compositional analysis.
Its result shows: when being more than the 1 μ m at diameter of equivalent circle and the extension ratio is that stretch flange property and fatigue characteristic improve when containing 0.5～50% by a kind among average composition Ce or the La or 2 kinds of totals in the inclusion below 3.When being more than the 1 μ m at diameter of equivalent circle and the extension ratio is that the average content of the total of among Ce or the La in the inclusion below 3 a kind or 2 kinds is when being lower than 0.5 quality %; Significantly reducing by the inclusion number ratio of separating out the form of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide; Therefore correspondingly be prone to it become and break that to extend the number ratio of inclusion too much for MnS class that starting point takes place, stretch flange property and fatigue characteristic decline.
On the other hand; If more than diameter of equivalent circle is 1 μ m and the extension ratio be that the average content of total of among Ce or the La in the inclusion below 3 a kind or 2 kinds was above 50% o'clock; Then can generate a large amount of sulphur cerium oxide, sulphur lanthanum trioxide; Forming diameter of equivalent circle is thick inclusion above about 50 μ m, causes stretch flange property and fatigue characteristic to descend.
As in the steel plate of the present invention by the existence condition of separating out the inclusion of the form that MnS is arranged on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide, recently stipulate with chemical ingredients (the Ce+La)/S of steel plate.
Particularly, as stated, improve stretch flange characteristic and fatigue characteristic aspect, be the chemical ingredients ratio that makes prevent the MnS extension by separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide.
Therefore, in order clearly to suppress the chemical ingredients ratio that the MnS type impurity extends effectively, recently form, stretch flange property and the fatigue characteristic of inclusion are estimated (Fig. 1) through (the Ce+La)/S that changes steel plate.Its result shows: when (Ce+La)/S ratio was 0.1～70, stretch flange property and fatigue characteristic improved.If (Ce+La)/the S ratio is lower than 0.1; Then significantly reducing by the inclusion number ratio of separating out the form of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide; Therefore correspondingly be prone to it become and break that to extend the number ratio of inclusion too much for MnS class that starting point takes place, stretch flange property and fatigue characteristic decline.
On the other hand, if (Ce+La)/S is than surpassing 70, then can generate sulphur cerium oxide, sulphur lanthanum trioxide in a large number, and forming diameter of equivalent circle is thick inclusion above about 50 μ m, causes the decline of stretch flange property and fatigue characteristic.
Below, the tissue of steel plate is described.
The present invention improves stretch flange property and fatigue characteristic through control MnS type impurity, and the microstructure of steel plate is not had particular determination.To have with the bainite ferrite be the steel plate of the tissue of principal phase, with the ferritic phase be principal phase and be the complex tissue steel plate of the 2nd phase mutually with martensitic phase, bainite, all can obtain effect of the present invention in arbitrary steel plate in the complex tissue steel plate that constitutes by ferritic, retained austenite and low temperature phase change phase (martensite or bainite); But in order to obtain excellent stretch flange property, being preferably formed with the bainite ferrite is the tissue of principal phase.Preferred bainite ferrite or bainite in area than the phase that is necessary for maximum.The area occupation ratio of the bainite ferrite phase in the steel plate is preferably more than 50%, more preferably more than 80%, further is preferably 100%.In addition, remainder can contain 20% with upper bainite phase or polygonal ferrite phase.
Then explanation is created conditions.In the present invention; Blow and decarburization in converter; Or further use vacuum degasser to carry out decarburization, and, C concentration adds alloys such as Si, Mn, P in being 0.03～0.1% molten steel, and carry out deoxidation and composition and regulate; And do not add Al and Ti or when needs carry out the oxygen adjusting, add a small amount of Al and Ti with the degree of minimal residue solvable Al of acid and sour solvable Ti, a kind or 2 kinds that adds then among Ce or the La is carried out the composition adjusting.With the molten steel continuous casting that melting like this is processed, make the casting sheet.
About continuous casting, be not only applicable to the continuous casting of slab of common 250mm about thick, the casting mold thickness that also can fully be applicable to bloom or steel billet and continuous casting of slab machine is than the bar strip continuous casting below the thin usually for example 150mm.
Hot-rolled condition to being used to make high tensile hot rolled steel sheet describes.About the Heating temperature of the slab before the hot rolling,, be preferably more than 1150 ℃ in order to make the solid solutions such as carbonitride in the steel.Through making their solid solutions, be suppressed in the process of cooling after rolling and generate polygonal ferrite, obtain the tissue that is the main body mutually with the good bainite ferrite of stretch flange property.On the other hand; If the Heating temperature of the slab before the hot rolling surpasses 1250 ℃, then the oxidation of steel slab surface is remarkable, and the wedge-shaped surface defective that is particularly caused by the oxidation of being selected property of crystal boundary is residual after de-scaling; Its surface quality after can influence of rolled, therefore preferably will on be defined as 1250 ℃.
After being heated to the said temperature scope, carry out common hot rolling, but the finish rolling in this operation is accomplished temperature and is being carried out under the situation of organizational controls of steel plate very important.If accomplishing temperature, finish rolling is lower than Ar 3Point+30 ℃, then the easy chap of the crystal particle diameter of skin section is big, is unfavorable for fatigue characteristic.On the other hand, if surpass Ar 3Point+200 ℃ then generate the polygonal ferrite phase that is unfavorable for stretch flange property easily, therefore preferably will on be defined as Ar 3Point+200 ℃.
In addition, the average cooling rate that makes the steel plate after the finish rolling is more than 40 ℃/second and the process that is cooled to 300～500 ℃ scope can suppress the generation of polygonal ferrite phase effectively, obtains the tissue that is the main body mutually with bainite ferrite.
If above-mentioned average cooling rate is lower than 40 ℃/second, then generate polygonal ferrite easily, not preferred.On the other hand, though in organizational controls, need not speed of cooling is provided with the upper limit, too fast speed of cooling might make steel plate inhomogeneous cooling one, and makes that to carry out the required expense of this refrigerative equipment big, thereby causes the price of steel plate to rise.Based on this kind viewpoint, the upper limit of speed of cooling is preferably 100 ℃/second.
In addition, if stopping cooling temperature being lower than 300 ℃, then can generating the martensitic phase that is unfavorable for stretch flange property, thereby lower limit is decided to be 300 ℃.Therefore, in order to suppress to make the generation of the martensitic phase that stretch flange property extremely worsens, the coiling temperature of coils of hot rolled is preferably more than 300 ℃.
On the other hand,, then can't suppress the generation of polygonal ferrite phase, and in containing the steel of Cu, the part is separated out Cu and might be made fatigue characteristic improve effect decline in ferritic phase, thereby preferably coiling temperature is decided to be below 500 ℃ if surpass 500 ℃.Therefore, through batching below 500 ℃, after process of cooling in separate out carbonitride, make solid solution C, N in the ferritic phase measure minimizing, make the raising of stretch flange property.
Embodiment
Below, embodiments of the invention are described with comparative example.
Slab hot rolling under the condition shown in the table 2 with chemical ingredients shown in the table 1 obtains the hot-rolled sheet that thickness is 3.2mm.
Figure G2007800268244D00171
Table 2
Condition Heating temperature (℃) Finish rolling completion temperature (℃) Speed of cooling after the finish rolling (℃/second) Coiling temperature (℃)
A 1250 845 75 450
B 1200 825 45 450
In this table 1; Steel numbering system (below be called grade of steel) 1,3,5,7,9,11,13 constitutes with the composition in the scope of high tensile steel plate of the present invention, and grade of steel 2,4,6,8,10,12,14 constitutes as the comparative steel of the scope that breaks away from high tensile steel plate of the present invention.Grade of steel 2,4,6 constitutes to contain the slab that surpasses 0.01% sour solvable Al, and in addition, grade of steel 8,10,12,14 is so that total metering of among Ce or the La a kind or 2 kinds is reduced to the slab that is lower than till 0.0005% constitutes.
That is, in this table 1, can compare respectively, constitute, and make sour solvable Al etc. different with mutual much at one composition in order to make between grade of steel 1 and grade of steel 2, grade of steel 3 and grade of steel 4, grade of steel 5 and grade of steel 6, grade of steel 7 and the grade of steel 8.In addition, can compare respectively, constitute, and make Ce+La etc. different with mutual much at one composition for making between grade of steel 9 and grade of steel 10, grade of steel 11 and grade of steel 12, grade of steel 13 and the grade of steel 14.
In table 2; As condition A; Making Heating temperature is that to accomplish temperature be that speed of cooling behind 845 ℃, finish rolling is that 75 ℃/second, coiling temperature are 450 ℃ for 1250 ℃, finish rolling; As condition B, making Heating temperature is that to accomplish temperature be that speed of cooling behind 825 ℃, finish rolling is that 45 ℃/second, coiling temperature are 450 ℃ for 1200 ℃, finish rolling.
Grade of steel 1 adopts condition A with grade of steel 2; Grade of steel 3 adopts condition B with grade of steel 4; Grade of steel 5 and grade of steel 6 adopt condition A, and grade of steel 7 adopts condition B with grade of steel 8, grade of steel 9 and grade of steel 10, grade of steel 11 and grade of steel 12, grade of steel 13 and grade of steel 14, thus can be in the identical influence of comparative chemistry composition down of creating conditions.
To the fundamental characteristics of the steel plate that so obtains is that intensity, ductility, stretch flange property, limit of fatigue ratio are analyzed.
In addition, as the existence of the extension inclusion in the steel plate, be object with the inclusion that is more than the 1 μ m, be that number ratio, a volume number density, the average equivalent circular diameter of the inclusion more than 5 analyzed to the extension ratio.
In addition; Existence as the inclusion that does not extend in the steel plate; Inclusion to be more than the 1 μ m is an object, to being that the MV of the total content of among Ce or the La in the inclusion below 3 a kind or 2 kinds is analyzed by the number ratio of the inclusion of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and a volume number density and in the extension ratio.
In addition, be object with the inclusion more than the 1 μ m, be not only because observe easily, also can be not influential to the decline of stretch flange property and fatigue characteristic because be lower than the inclusion of 1 μ m.
Its result is shown in table 3 by the combination of steel and rolling condition respectively.
Figure G2007800268244D00201
Intensity and rolling tension test through the JIS5 test film obtained abreast with rolling direction are tried to achieve.Stretch flange property is estimated with λ; This λ is through using 60 ° the circular cone drift will be in the punching expansion as 10mm of the diameter of the central opening of the steel plate of 150mm * 150mm; The aperture D (mm) that thickness of slab runs through when be full of cracks appears in mensuration, by reaming value λ=(D-10)/10 try to achieve.In addition, as the expression fatigue characteristic index limit of fatigue than with will according to the method for JIS Z 2275 try to achieve 2 * 10 6Inferior time intensity (σ W) is estimated divided by the value (σ W/ σ B) that the intensity (σ B) of steel plate obtains.
In addition, test film is No. 1 test film of stipulating in the same specification, and the employing parallel portion is that 25mm, radius of curvature R are the test film of the thick 3.0mm of two sides after equal grinding of 100mm, raw sheet (hot-rolled sheet).
In addition, inclusion is carried out SEM observe, major diameter and minor axis that to measure 50 diameter of equivalent circle of selecting at random be the inclusion more than the 1 μ m.In addition, with the quantitative analysis function of SEM, be that inclusion more than the 1 μ m carries out compositional analysis to 50 diameter of equivalent circle of selecting at random.Use these results, number ratio, the extension ratio of obtaining the extension ratio and be the inclusion more than 5 is the average equivalent circular diameter of the inclusion more than 5, is being the MV of the total of among Ce or the La in the inclusion below 3 a kind or 2 kinds by the number ratio of the inclusion of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and in the extension ratio.In addition, the SEM that carries out electrolysis face with tachometric method estimates, and calculates a volume number density of each form of inclusion.
As shown in table 3, the grade of steel 1,3,5,7,9,11,13 that uses method of the present invention can reduce the MnS type impurity that extends in the steel plate through separating out MnS by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide.That is, through make in the steel plate by the number ratio of separating out the inclusion of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide be more than 10%, the volume number density of this inclusion is 1.0 * 10 3Individual/mm 3More than, the extension ratio that exists in the steel plate is that the average content of the total of among Ce or the La in the inclusion below 3 a kind or 2 kinds is 0.5%～50%, can make diameter of equivalent circle is that the above and extension ratio of 1 μ m is that the number ratio of the extension inclusion more than 5 is, a volume number density of this inclusion is 1.0 * 10 below 20% 4Individual/mm 3Below, the average equivalent circular diameter of this inclusion is below the 10 μ m.Its result shows: compare with comparative steel, can obtain the steel plate of stretch flange property and excellent in fatigue characteristics as the grade of steel 1,3,5,7,9,11,13 of steel of the present invention.But; Comparative steel (grade of steel 2,4,6,8,10,12,14) since the MnS type impurity that extends with different by the distribution of stipulating among the distribution of the inclusion of separating out MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and the present invention; Therefore the MnS type impurity that when steel plate is processed, extends becomes the starting point of breaking and taking place, and stretch flange property and fatigue characteristic descend.
According to the present invention; Separate out fine MnS through making in the casting sheet; And make its as indeformable when rolling, be difficult for becoming the fine nodule that starting point takes place that breaks and be scattered in the steel plate, can obtain the high tensile hot rolled steel sheet of stretch flange property and excellent in fatigue characteristics.

Claims (7)

1. the high tensile hot rolled steel sheet of stretch flange property and excellent in fatigue characteristics; It is characterized in that; This steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce and the La: 0.0005～0.04%, remainder is made up of iron and unavoidable impurities; The number ratio of extending inclusion is below 20%; Said extension inclusion is that the diameter of equivalent circle that in this steel plate, exists is the inclusion more than the 1 μ m; And major diameter/minor axis is more than 5; And, in this steel plate, contain and separating out the inclusion that MnS is arranged by on a kind among Ce and the La or 2 kinds of oxysulfides that form more than 10% with the number proportional meter.
2. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and excellent in fatigue characteristics is characterized in that, the volume of a said extension inclusion number density is 1.0 * 10 4Individual/mm 3Below.
3. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and 2 and excellent in fatigue characteristics is characterized in that, a said volume number density of on oxysulfide, separating out the inclusion of MnS is 1.0 * 10 3Individual/mm 3More than.
4. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and 2 and excellent in fatigue characteristics is characterized in that, the average equivalent circular diameter of said extension inclusion is below the 10 μ m.
5. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and 2 and excellent in fatigue characteristics; It is characterized in that, said on oxysulfide, separate out by in the inclusion of MnS in the average Ce that contains 0.5～50 quality % and a kind or 2 kinds the total among the La formed.
6. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and 2 and excellent in fatigue characteristics is characterized in that, is 0.1～70 as (the Ce+La)/S ratio in the chemical ingredients ratio of quality %.
7. the high tensile hot rolled steel sheet of stretch flange property according to claim 1 and 2 and excellent in fatigue characteristics; It is characterized in that; In quality % also contain in V:0.01～0.05%, Cr:0.01～0.6%, Mo:0.01～0.4%, B:0.0003～0.03% any or more than 2 kinds, remainder is made up of iron and unavoidable impurities.
CN2007800268244A 2006-07-14 2007-03-02 High-strength steel sheet excellent in stretch flangeability and fatigue property Active CN101490295B (en)

Applications Claiming Priority (3)

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JP193893/2006 2006-07-14
JP2006193893A JP4901346B2 (en) 2005-11-07 2006-07-14 High-strength steel sheet with excellent stretch flangeability and fatigue characteristics
PCT/JP2007/054614 WO2008007477A1 (en) 2006-07-14 2007-03-02 High-strength steel sheet excellent in stretch flangeability and fatigue property

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KR20090018167A (en) 2009-02-19
EP2048254A1 (en) 2009-04-15
TW200804607A (en) 2008-01-16
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AU2007273767A1 (en) 2008-01-17
TWI424069B (en) 2014-01-21
CA2657587A1 (en) 2008-01-17
US20090317285A1 (en) 2009-12-24
EP2048254A4 (en) 2012-11-07
EP2048254B1 (en) 2020-08-19
WO2008007477A1 (en) 2008-01-17
CA2657587C (en) 2013-11-26

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