CN1986862A - High tension steel plate of good material less anisotropic low temperature toughness - Google Patents

Abstract

The present invention provides a kind of high tension steel plate with tensile strength over 490 MPa, excellent base metal toughness and HAZ toughness and suppressed material anisotropy. The steel plate contains C 0.01-0.08 wt%, Si less than 0.8 wt%, Mn 0.5-1.9 wt%, Ti 0.005-0.10 wt%, B 0.0006-0.0050 wt%, N 0.002-0.010 wt%, KP less than 2.4 wt%, with KP being [Mn]+1.5[Cr]+2[Mo], and TP more than 0.62 wt%, with TP being 4[Ti]/[C]. In addition, it has MA lower than 0.5 %, and average aspect ratio of old austenite crystal lower than 1.3. It may have Nb less than 0.030 % and in 2[Nb]/[Ti]<4.0 added.

Description

The high-tensile steel of good material less anisotropic low temperature toughness

Technical field

The present invention relates to a kind of tensile strength is the above high-tensile steel of 490MPa, particularly relates to a kind of base metal tenacity, HAZ tenacity excellent and the little high-tensile steel of material anisotropy.

Background technology

In recent years, for the steel plate that is used for boats and ships and marine structure, requirement is strength of parent high and low temperature toughness and the excellent high-tensile steel of weldability (HAZ toughness).

As this steel plate, for example open in the flat 9-111337 communique (patent documentation 1), proposed under the condition of low C, add energetically and compare the especially little Nb of toughness deterioration that follows precipitation strength with Ti the spy, realization is precipitation strength by NbC, and improves the high-tensile steel of hardenability.Also have, open in the 2000-178645 communique (patent documentation 2), improve toughness thereby proposed to purify by the amount of separating out of cutting down MnS, TiN the spy, and the high-tensile steel that by Mn hardenability is improved by making austenite (γ) thickization of crystal grain.

(patent documentation 1) spy opens flat 9-111337 communique

(patent documentation 2) spy opens the 2000-178645 communique

Recently, from improving operating accuracy and improving the viewpoint of security, the anisotropy (difference of the characteristic of the rolling direction in steel plate a part (L direction), rolling vertical direction (C direction)) that requires to reduce material and material difference (characteristic of steel plate different sites different).Particularly, owing to the sound equipment anisotropy is reduced, therefore require the low steel plate of material anisotropy by the anisotropy that reduces material.This is because if the sound equipment anisotropy is low, then when UT (UT (Ultrasonic Testing)) checks, can carry out the inspection of welding flaw expeditiously.

According to the technology of being put down in writing in the above-mentioned patent documentation,, particularly in the material anisotropy, do not improve though can realize the low-temperature flexibility of mother metal under the high strength, the raising of HAZ to a certain degree.Promptly, in the high-tensile steel of patent documentation 1, Nb is excessively added, though Nb has an effect effectively to hardenability, but because the element of microsegregation and macroscopic segregation takes place when solidifying Nb easily, so exist the material anisotropy to generate the problem that in material, is easy to generate difference greatly, in addition.Also having, in the steel plate of patent documentation 2, mainly is to guarantee hardenability by Mn, but because Mn is the element that is easy to generate microsegregation, therefore equally also is easy to generate the material anisotropy.Also have,, then when rolling, extend, be easy to generate the material anisotropy along rolling direction if microsegregation takes place.On the other hand, if microsegregation takes place, then at top, bottom or the position therebetween of steel plate, intensity, toughness are easy to generate difference (material difference).

The present invention is in view of these problems, and its purpose is to provide a kind of low-temperature flexibility, HAZ tenacity excellent, and the repressed high-tensile steel of material anisotropy.

Summary of the invention

The key points in design of composition of steel of the present invention is, can access structure of steel based on no carbon bainite and/or quasi-polygonal ferrite, except C amount is limited to low-level, by being added with the Ti that helps refinement HAZ tissue, guarantee HAZ toughness with the B that can form ferritic karyogenesis point, also having relatively can the non-generting element (Mn of deterioration HAZ flexible carbide by interpolation, also has Cu as required, Ni, Cr etc.) realize the high strength of mother metal, also have by suppressing martensite and austenite mixed thing (Martensite-Austenite Constituent as the hard phase, below be called MA) growing amount, and restriction KP=[Mn]+1.5[Cr]+2[Mo] value, and guarantee low-temperature flexibility.In addition, by serving as main the use, and guarantee hardenability, and reduce the material anisotropy thus to compare the difficult Ti that takes place of microsegregation and macroscopic segregation with Nb, Mn, Cu, Ni.

Promptly, high-tensile steel of the present invention, contain the composition that satisfies following condition: in mass%, C:0.01～0.08%, Si:0.8% are following, in the scope of Mn:0.5～1.9%, Ti:0.005～0.10%, B:0.0006～0.0050%, N:0.002～0.010%, and establishing KP=[Mn]+1.5[Cr]+2[Mo], TP=4[Ti]/during [C] (content (mass%) of [X] expression element X), satisfy KP＜2.4, TP＞0.62; The average area rate of MA is below 0.5% in addition, and the average aspect ratio of old austenite crystal (aspect ratio) is below 1.3.Described MA is preferably, and its diameter of equivalent circle is below the 1.0 μ m, and average aspect ratio is below 2.0.

In addition, in mentioned component, can be separately or compound interpolation from the optional element of following each group: (1) Nb: be lower than 0.030%, and 2[Nb]/[Ti]＜4.0; (2) below the Ni:2.0%, following any one or two kinds of of Cu:2.0%; (3) below the Cr:1.0%, below the Mo:0.30%, more than any below the V:0.10%; (4) below the Ca:0.0050%; (5) below the Mg:0.005%, below the REM:0.02%, more than any below the Zr:0.05%.Also have as deoxidant element and can contain Al 0.20% below,, can be made as below the P:0.020%, below the S:0.010% as impurity P, S.

According to high-tensile steel of the present invention, owing to have the C of making is low amount, particularly make Mn, Cr, Mo satisfy the KP value and be lower than 2.4, and make microsegregation and macroscopic segregation generation Ti difficult, that hardenability is excellent satisfy the TP value and surpass 0.62 composition, and the average area rate of MA is below 0.5%, the average aspect ratio of old austenite crystal is below 1.3, so 490MPa can be provided above high strength, and low-temperature flexibility, HAZ tenacity excellent, and material anisotropy and then the little high-tensile steel of sound equipment anisotropy.

Embodiment

At first, the composition qualification reason to high-tensile steel of the present invention describes.Unit is mass%.

C：0.01～0.08％

C is anti-welding crack and the strength of parent that is used for taking into account simultaneously the HAZ when welding, and improves big heat input HAZ flexible important element.If C surpasses 0.08%, then do not have carbon bainite and generate martensite, anti-welding crack reduction in high speed of cooling side.Also have, MA generates in a large number in low speed of cooling side (big heat input HAZ), and big heat input HAZ toughness can not improve.C content can be preferably below 0.06%, more preferably below 0.055%.On the other hand, be lower than at 0.01% o'clock, strength of parent descends.So can be preferably more than 0.02%, more preferably more than 0.03%.

Below the Si:0.8%

Si is as the useful element of reductor, but because if surpassing 0.8% adds, then weldability and base metal tenacity reduce, so the upper limit is made as 0.8%.Can be preferably below 0.6%, more preferably below 0.3%.

Mn：0.5～1.9％

Mn has the quench for improving effect, and has crystal grain thinning and improve the effect of base metal tenacity.Especially, if surpass 1.9% then the anti-welding crack reduction of HAZ.Can be preferably below 1.8%, more preferably below 1.6%.On the other hand, be lower than at 0.5% o'clock at Mn and just can not obtain sufficient strength of parent.So can be preferably more than 0.8%, more preferably more than 1.0%, more preferably more than 1.2%, the best is preferably more than 1.3%.

Ti：0.005～0.10％

Ti is very important in the present invention as the element that improves hardenability, also has with N to form nitride and the γ grain of refinement HAZ, and becomes the generation point of BN, promotes the ferritic generation of intracrystalline, has and significantly improves HAZ flexible effect.Be lower than at 0.005% o'clock, then can not guaranteeing this effect fully.Can be preferably more than 0.007%, more preferably more than 0.009%.On the other hand, if Ti surpasses 0.10%, then HAZ toughness, the common deterioration of base metal tenacity.Can be preferably below 0.025%, more preferably below 0.020%.Be elaborated about the hardenability of Ti qualification reason by the TP value.

B：0.0006～0.0050％

B has the effect that improves hardenability by solid solution, but under the too much situation of solid solution capacity, can diminish toughness on the contrary.Also have, in HAZ, become BN and act on, have the HAZ of raising flexible effect as ferritic karyogenesis point.Amount at B is lower than at 0.0006% o'clock, can not guarantee the additive effect of B fully.Can be preferably more than 0.0010%, more preferably more than 0.0012%.On the other hand,, can reduce hardenability on the contrary if the amount of B surpasses 0.005%, and base metal tenacity, HAZ deterioration.Can be preferably below 0.0030%, more preferably below 0.0025%.

N：0.002～0.010％

N and Ti form nitride and the γ particle diameter of refinement HAZ, and promote the ferritic generation of HAZ with B formation nitride, have the HAZ of improvement flexible effect.If the amount of N surpasses 0.010%, then base metal tenacity and the common deterioration of HAZ toughness.Also have, make intensity reduce owing to solid solution B amount descends.Can be preferably below 0.0060%.On the other hand, the amount of N is lower than at 0.002% o'clock, then improves HAZ flexible effect with Ti formation nitride and becomes insufficient.Can be preferably more than 0.0030%.

Kp＝〔Mn〕+1.5〔Cr〕+2〔Mo〕＜2.4

KP is the index of the generation easness of the MA of expression in the low-carbon bainite, if KP more than 2.4, then the growing amount of MA becomes too much and makes HAZ toughness (particularly low temperature HAZ toughness) deterioration.From the viewpoint of the tough property improvement of HAZ, wish that the value of KP is little, can be preferably below 2.0, more preferably below 1.7.

TP＝4〔Ti〕/〔C〕＞0.62

Solid solution Ti amount in (Ti)/(C) the expression austenite is promptly represented by bringing into play the hardenability that Ti produces effectively, can improve material anisotropy, material difference by the index of the hardenability of Ti generation, and is suppressed the generation of MA.From embodiment described later as can be known, the TP value helps the solid solution Ti quantity not sufficient of hardenability 0.62 when following, and the growing amount of MA increases, and also has the material anisotropy to become remarkable.That is, if solid solution Ti quantity not sufficient, Mn, the Cu, Ni, the Nb that then compare easy segregation with Ti have an effect to hardenability, and these elements segregation become cause, and material anisotropy and material difference easily take place.

Steel plate of the present invention is formed by remainder Fe and impurity outside above basal component.Impurity P, S are few more good more, and P is below 0.020%, can be preferably to be limited to below 0.010%, and also having S is below 0.010%, can be preferably to be limited to below 0.005%.Also have, Al is to the deoxidation effective elements, though can contain as composition of steel, but because of surpassing the tendency that 0.20% base metal tenacity and HAZ toughness have reduction, so can be limited to below 0.20%, be preferably below 0.10%, more preferably below 0.05%.

In addition, in the scope of the effect that does not damage above-mentioned basal component, effect, can add the element of further raising characteristic.For example, can independent or compound interpolation from the optional element of following each group, Cr, the Mo of any one or two kinds of, (3) following ranges among Ni, the Cu of the Nb of (1) following ranges, (2) following ranges, among the V any are above, Mg, the REM of the Ca of (4) following ranges, (5) following ranges, more than among the Zr any.

Nb: be lower than 0.030%, 2 (Nb)/(Ti)＜4.0

Nb has the effect that improves hardenability, but 0.030% when above, then makes thickization of crystal grain, reduction base metal tenacity and HAZ toughness.Can be preferably below 0.025%, more preferably below 0.018%.On the other hand, in order to bring into play the additive effect of Nb effectively, be preferably more than 0.005%, more preferably more than 0.008%.In addition, if 2 (Nb)/(Ti) is more than 4.0, then the hardenability that produces of Nb has more ascendant than Ti, because macroscopic segregation, the microsegregation of the Nb when solidifying become greatly material difference and material anisotropy.For this reason, establish 2 (Nb)/(Ti), can be preferably below 3.0 for being lower than 4.0.

Below the Ni:2.0%

Ni is the element useful to the raising of strength of parent and base metal tenacity, if but surpassing 2.0% contains, then have deterioration HAZ flexible tendency on the contrary, so its upper limit is preferably 2.0%.More preferably below 1.5%, more preferably below 0.9%.

Below the Cu:2.0%

Cu improves strength of parent by solution strengthening and precipitation strength, and has the effect that improves hardenability.If surpass 2.0% then have the tendency that big heat input HAZ toughness descends, so its upper limit is preferably 2.0%.Can be more preferably below 1.2%, more preferably below 0.9%.

Below the Cr:1.0%

Cr has by improving the effect that hardenability improves strength of parent, if but surpass 1.0% then the growing amount increase of MA has the tendency of HAZ toughness deterioration, so its upper limit is preferably 1.0%.Can be more preferably below 0.5%, more preferably below 0.3%.

Below the Mo:0.30%

Mo improves hardenability and has the effect that improves strength of parent, but on the other hand, because have the effect that significantly makes HAZ toughness deterioration, its upper limit is preferably below 0.30%.Can be more preferably below 0.15%, more preferably below 0.10%.

Below the V:0.10%

V adds by a small amount of, has the effect that improves hardenability and anti-temper softening, if but surpass 0.10% then have and reduce base metal tenacity and HAZ flexible tendency, so its upper limit is preferably 0.10%.Can be more preferably below 0.06%, more preferably below 0.02%.

Below the Ca:0.0050%

Ca makes the MnS balling, thereby has the anisotropic effect that reduces inclusion.In order to bring into play this effect fully, preferably adding Ca is more than 0.0005%.Can be more preferably more than 0.001%.On the other hand, if Ca amount surpasses 0.0050% then have the tendency that base metal tenacity descends, so its upper limit is preferably 0.0050%.Can be more preferably below 0.0030%.

Below the Mg:0.005%, below the REM:0.02%, below the Zr:0.05%

These elements have and improve the effect of HAZ flexible, if but make it excessively contain then have the tendency of HAZ toughness deterioration on the contrary, therefore, be preferably below the Mg:0.005%, below the REM:0.02%, below the Zr:0.050%.More preferably below the Mg:0.003%, below the REM:0.01%, below the Zr:0.03%.Also have, the REM that is contained in the steel plate of the present invention can use any of element of the scandium (Sc), yttrium (Y) and the lanthanide rare dvielement (ordination number 51～71) that belong to periodic table of elements three races.

Steel plate of the present invention has above-mentioned composition, by the organizational composition based on no carbon bainite and/or quasi-polygonal ferrite, in addition, also has the tissue that some contain MA, and the average aspect ratio of old γ crystal grain is below 1.3.Under the big situation of the aspect ratio of old γ crystal grain, the organization unit of L direction and C direction is the size difference of block bainite (crystal orientation identical no carbon bainite bunch), also has the orientation right and wrong of bainite to form randomly, and the material anisotropy is very remarkable.Therefore, the average aspect ratio of old γ crystal grain is made as below 1.3, is preferably and is made as below 1.2.Also have,,, make mother metal performance deterioration significantly, therefore on average be located at below the 0.5 area % so become the starting point that be full of cracks takes place because MA is thick hard phase.In addition, to the influence of the low-temperature flexibility of MA, also special relevant with aspect ratio with its size except its amount, above 2.0, then the deterioration of low-temperature flexibility becomes remarkable to the average equivalent circular diameter of MA above 1.0 μ m, average aspect ratio.Therefore, the diameter of equivalent circle that is preferably MA is below the 1.0 μ m, and average aspect ratio is below 2.0.

Then, the recommendation of steel plate of the present invention is created conditions describe.High-tensile steel of the present invention, for Ti, B, Nb solid solution in austenite, and after being heated to steel disc about 1050～1150 ℃, be rolled in basic recrystallize zone.Because recrystallization temperature is subjected to the influence of Ti in the alloying element, Nb, B largely, therefore be necessary to set final final rolling temperature FRT with reference to the concentration of these elements.According to present inventor's research,, be to approach 1.0 value thereby can access aspect ratio by FRT being set at the temperature that satisfies following formula.Also have, if be rolled with 100% recrystallization temperature for each groove (pass) substantially, then aspect ratio is roughly 1 in theory.Especially, be to be lower than under 830 ℃ the situation on the right of following formula, establishing it is more than 830 ℃.This is because find can become the rolling of part recrystallization temperature zone when being lower than 830 ℃, because the material anisotropy that influences of the remaining non-recrystallization grain in rolling back becomes big.

FRT(℃)≥(4〔Nb〕+〔Ti〕+20〔B〕)×1000+770

During cooling after hot rolling, speed of cooling is preferably more than 3 ℃/second, also has cooling to stop temperature and is preferably below 500 ℃.Be lower than 3 ℃/second, cooling in speed of cooling and stop temperature and surpass under 500 ℃ the situation, surpass allowed band and generate as the MA of infringement toughness major cause.Specifically, can carry out water-cooled after the hot rolling,, also its speed of cooling can be controlled to be about 4 ℃/second even be about 100mm by the water-cooled thickness of slab.

In steel plate of the present invention, needn't necessarily carry out temper, but under the situation of implementing temper, wish tempering temperature is made as 400～600 ℃.When tempering temperature was lower than 400 ℃, MA did not decompose, and can not expect only to improve toughness by reducing intensity.On the other hand, if surpass 600 ℃, then crystal grain-growth and thickization are understood the deterioration base metal tenacity on the contrary.

In the present invention, as composition of steel,, can bring into play the hardenability of Ti to greatest extent, and reduce amount and the size of MA by making Ti/C or Nb/Ti optimizing.Therefore, offset influence, thereby when can make the material anisotropy that suppresses mother metal, material difference, made the mother metal high strength, had the steel plate of excellent low-temperature flexibility and weldability concurrently because of thickization of the high caused bainite crystal grain of rolling temperature.

As mentioned above, steel plate of the present invention, by in hot rolling, to having limited the KP value, the TP value decide the steel disc of composition, carry out main high temperature rolling energetically in the recrystallization temperature zone, and carry out chilling, thereby form tissue based on no carbon bainite and/or quasi-polygonal ferrite, and form the MA amount is suppressed as far as possible, the aspect ratio of old γ grain is similar to 1 tissue, even for example thickness of slab is the above steel plate of 50mm, also can reach the above intensity of 490MPa, thereby can make base metal tenacity, the HAZ tenacity excellent, and reduce material anisotropy and material difference.

Next, the present invention is carried out more specific description, but the present invention is not limited to the explanation of embodiment for embodiment.

(embodiment)

Steel by the chemical constitution shown in the following table 1 of usual method melting, after forming steel ingot, with the condition shown in the table 2 heat and soaking after, finish hot rolling by the final final rolling temperature (FRT) shown in this table, and by cooling off with the speed of cooling shown in the table (the thickness of slab average cooling rate of expression steel plate).Also have,, after the cooling, carried out under with the tempering temperature shown in the table, keeping about 15 minutes tempering heat treatment the test portion of a part.Also have, after the hot rolling, steel plate is cooled off by water-cooled, the average cooling rate of this moment is approximately 5 ℃/s, 12 ℃/s, 5 ℃/s, 4 ℃/s, the expression in table 3 of these values respectively with respect to thickness of slab 30mm, 50mm, 80mm, 100mm.

The test portion hot-rolled sheet that forms for manufacturing like this, take three structure observation test films from 1/4 position of hot-rolled sheet thickness, when carrying out observation by light microscope (400 times of multiplying powers), discovery has formed based on no carbon bainite and quasi-polygonal ferrite, and the tissue of a little formation of MA is arranged at the interface of no carbon bainite and quasi-polygonal ferrite.

To described structure observation test film by after the nital corrosion, 1000 times tissue is taken with multiplying power with SEM (scanning electronic microscope), use image analysis software that captured image is resolved, obtain centre plane integration rate, average equivalent circular diameter and the average aspect ratio of MA.In addition, use opticmicroscope tissue to be taken, measure the size of old γ crystal grain, and obtain its average aspect ratio with 100～400 times.These measurement results are represented in table 2.

Also have, carry out tension test, shock test, the mechanical properties of investigation mother metal by following main points.

In three positions (front portion of the length direction of steel plate, central part, rear portion) of each steel plate along L direction and C direction, taked the JIS4 test film from 1/4 position of thickness of slab, use this test film to carry out tension test, measured 0.2% yield strength (YS), tensile strength (TS).Also have, shock test is identical with the situation of tension test sheet, has taked the JIS4 test film, and carries out pendulum impact test with it, obtains-60 ℃ absorption energy (vE-60).In the purposes of steel plate of the present invention, if tensile strength is more than the 490MPa, vE-60 is more than the 250J, and then practical going up is qualified level.In YS, the TS of table 3, vE-60, expression be minimum value in the value of being measured.

Also have,, in the characteristic difference of the L direction at each sampling position and C direction, obtain the maximum (anisotropy Δ TS, anisotropy Δ YS, anisotropy Δ vE-60) for the anisotropy of evaluating material.Also have, in order to estimate material difference, and it is poor to obtain the characteristic minimax of the characteristic minimax difference of the L directions between different samplings position or C direction, and obtains wherein the greater (otherness Δ TS, otherness Δ YS, otherness Δ vE-60).

Also have, for estimating HAZ toughness, with each steel plate be heated to 1400 ℃ and kept for 5 seconds after, in 500 seconds, carried out being cooled to 500 ℃ thermal cycling and handled (thermal process of the HAZ when being equivalent to the heat input union-melt weld with 70kJ/mm) from 800 ℃, thereafter, take the JIS4 test film from thickness of slab 1/4 position, carry out pendulum impact test, measured absorption energy (vE-60).In steel plate purposes of the present invention,, then be the practical qualified level that if HAZ toughness is counted more than the 100J with vE-60.These measurement results have been represented in the table 3.

[table 1]

Annotate: having the * target in steel No. is the comparison composition, and other are the invention composition

KP＝[Mn]+1.5[Cr]+2[Mo]，TP＝4[Ti]/[C]，NP＝2[Nb]/[Ti]，minFDT(℃)＝(4[Nb]+[Ti]+20[B])*1000+770

[table 2]

Annotate: having the * target in test portion No. is comparative example, and other are example

[table 3]

Annotate: having the * target in test portion No. is comparative example, and other are example

From table 3, the whole tensile strength of example are all more than 490MPa, mother metal low-temperature flexibility (vE-60) is all more than 250J, HAZ toughness (vE-60) is more than the 100J, and AYS≤50MPa, Δ TS≤30MPa, Δ vE-60≤50MPa, material anisotropy and material otherness all are in the good scope.

To this because the C of the test portion No.5 of comparative example amount too much, the Si of No.8 amount too much, so the MA quantitative change is many, base metal tenacity, HAZ flexible deterioration are remarkable.Also have, so because the very few tensile strength reduction of the Mn of test portion No.9 amount.On the other hand, because the Mn of No.12 amount is too much, so tensile strength is higher, but the base metal tenacity deterioration.Also have because the Nb of test portion No.14 amount too much, so and the dysequilibrium of Ti amount, material anisotropy and material otherness worsen jointly.Because the Ti of test portion No.15 amount is very few, therefore particularly the material harmony is lower.On the other hand, because the Ti of No.18 amount too much, though therefore hardenability is improved and can accesses high strength, base metal tenacity and HAZ toughness deterioration also have because the non-recrystallization grain becomes many, so also deterioration of material anisotropy.Also have because the B amount of test portion No.19 and No.22 is improper, so tensile strength or base metal tenacity reduction.Also have, because the N of test portion No.23 and 26 amount is improper, so base metal tenacity and HAZ toughness or tensile strength deterioration.Also have, though test portion No.38,42,45 composition of steel are suitable, because rolling temperature is lower, therefore generally speaking, the average aspect ratio of old γ crystal grain is excessive, the reduction of material anisotropy.

Claims (10)

1, a kind of high-tensile steel, its material anisotropy is little, and the low-temperature flexibility excellence is characterized in that, and institute contains composition and satisfies following condition:

In mass%,

C：0.01～0.08％，

Below the Si:0.8%,

Mn：0.5～1.9％，

Ti：0.005～0.10％，

B：0.0006～0.0050％，

N：0.002～0.010％

Scope in, and establishing KP=[Mn]+1.5[Cr]+2[Mo], TP=4[Ti]/when [C], satisfy KP＜2.4, TP＞0.62,

Wherein, the degree of [X] expression element X;

In addition, the average area rate of martensite and austenitic mixture is below 0.5%, and the average aspect ratio of old austenite crystal is below 1.3.

2, according to the described high-tensile steel of claim 1, it is characterized in that, also contain Nb: be lower than 0.030%, and satisfy 2[Nb]/[Ti]＜4.0.

3, according to the described high-tensile steel of claim 1, it is characterized in that, contain also that Ni:2.0% is following, Cu:2.0% one or both in following.

4, according to the described high-tensile steel of claim 1, it is characterized in that, contain also that Cr:1.0% is following, Mo:0.30% following, more than V:0.10% any in following.

5, according to the described high-tensile steel of claim 1, it is characterized in that, also contain below the Al:0.20%.

6, according to the described high-tensile steel of claim 1, it is characterized in that, also establish below the P:0.020%, below the S:0.010%.

7, according to the described high-tensile steel of claim 1, it is characterized in that, also contain below the Ca:0.0050%.

8, according to the described high-tensile steel of claim 1, it is characterized in that, contain also that Mg:0.005% is following, REM:0.02% following, more than Zr:0.05% any in following.

According to the described high-tensile steel of claim 1, it is characterized in that 9, the average equivalent circular diameter of described martensite and austenitic mixture is that 1.0 μ m are following, average aspect ratio is below 2.0.

According to each described high-tensile steel in the claim 1 to 9, it is characterized in that 10, thickness of slab is more than the 50mm.