CN100494463C

CN100494463C - Tenacity excellent high intensity steel for welding heat influenced part

Info

Publication number: CN100494463C
Application number: CNB2005101164512A
Authority: CN
Inventors: 高冈宏行; 冈崎喜臣; 畑野等
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2004-10-22
Filing date: 2005-10-21
Publication date: 2009-06-03
Anticipated expiration: 2025-10-21
Also published as: JP4279231B2; KR100714540B1; CN1763234A; KR20060049102A; JP2006118007A

Abstract

The present invention provides a steel product which comprises by mass, 0.010 to 0.080% C, 0.02 to 1.00% Si, 1.10 to 2.90% Mn, 0 to 0.030% P, 0 to 0.010% S, <=0.20% Al, 0.40 to 2.40% Ni, 0.50 to 1.95% Cr, 0.16 to 1.10% Mo, 0.002 to 0.030% Ti and 0.0058 to 0.0120% N also so as to satisfy 1.0<=[Ti]/[N]<4.0, and the balance Fe with inevitable impurities, and in which AS value and DL value defined by the following formulae satisfy AS>=3.60 and DL<=2.80, and has a structure mainly composed of bainitic ferrite: AS=[Mn]+[Ni]+2*[Cu], and DL=2.5*[Mo]+30*[Nb]+10*[V]; wherein, [X] denotes the content (mass%) of the element X.

Description

The High Strength Steel of the tenacity excellent of welding heat affected zone

Technical field

The present invention relates to a kind of for example bridge, building, boats and ships, steel penstock (penstock), oil tank (tank), other large structure is employed, the High Strength Steel of tensile strength more than 780MPa, the material that particularly relates to the tenacity excellent of welding heat affected zone after a kind of big heat input welding (below, be called " HAZ ").

Background technology

In the high tensile steel plate more than 780MPa, from guaranteeing the viewpoint of strength of parent, because alloying constituent is added in a large number, so little heat is imported when speed of cooling is fast under the welding conditions, solder cracks (low temperature breaks) takes place in HAZ sclerosis easily.In order to prevent that it from taking place, and carries out the preheating more than 100 ℃ when welding procedure.If can dispense this preheating, then operating efficiency can significantly improve, and can realize reducing cost.Therefore, urgently wish a kind of above high-tensile steel of 780MPa level of low temperature resistant disruptiveness excellence.

Index as low temperature resistant disruptiveness, parameter by the defined Pcm of following formula (%) has been proposed, always, such as the spy open flat 9-No. 3591 communiques (patent documentation 1) and spy open 2001-No. 200334 communiques (patent documentation 2) record, Pcm is limited and improves low temperature resistant disruptiveness.In addition, make Nb, V, the Mo that Pcm adds to the difficulties, micro-interpolation can improve hardenability by adding energetically, and guarantee strength of parent.

Pcm=[C]+[Si]/30+[Mn]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5 * [B] here, [C]～[B] represents the mass% of each element.

In addition, in recent years,, inevitably to carry out the welding (big heat input welding) of big section member, at this moment, thickization of organizing of HAZ just arranged, the problem that HAZ toughness descends along with the maximization of works.So far, as the HAZ flexible technology of improving steel, following motion is arranged, for example, in 9-No. 104949 communiques of Te Kaiping (patent documentation 3), Ti is applied flexibly, perhaps open in 2002-No. 121641 communiques (patent documentation 4), apply flexibly the oxide inclusion that contains Ti and HAZ toughness is improved the spy.

[patent documentation 1] spy opens flat 9-No. 3591 communiques

[patent documentation 2] spy opens communique 2001-No. 200334

[patent documentation 3] spy opens flat 9-No. 104949 communiques

[patent documentation 4] spy opens communique 2002-No. 121641

Summary of the invention

The high tensile steel plate of 780MPa level, as mentioned above, Nb, V, the Mo that can guarantee the low temperature disruptiveness have actively been added, but thus when bainitic transformation, as resisting block thickization of bainite that crack propagation is had an effect, MA (Martensite-Austenite Constituent: martensite and austenitic mixture) as the second mutually thick hard generates, so have the problem that base metal tenacity and HAZ toughness worsen.

In recent years, improve shock resistance etc., strengthen day by day for the requirement of the security that improves works, in the high tensile steel plate more than 780MPa equally except guaranteeing base metal tenacity and low temperature resistant disruptiveness, the HAZ toughness when also requiring to improve big heat input welding.But, in the existing HAZ of improvement flexible technology, can't reach and satisfy this demand.

The present invention is in view of these problems, and its purpose is to provide a kind of high strength of the 780MPa of having level, and is guaranteeing outside base metal tenacity and the low temperature resistant disruptiveness, HAZ flexible High Strength Steel, the steel plate of excellence in the time of can also obtaining big heat input welding.

One of main points of the present invention, when the composition of design steel, be not subjected at present restriction as the Pcm of the index of low temperature resistant disruptiveness, but consider that structure of steel is carried out to the branch design, promptly limiting C for extremely hanging down outside the amount, inhibition has Nb, the V of bad influence, the interpolation of Mo to base metal tenacity, HAZ toughness, add element M n and the Ni that improves hardenability energetically, or Cu, though the speed of cooling thus after forge hot be at a high speed, low speed is in any, all can make the tissue generation based on no carbon bainite.

Also have, other main points of the present invention, absorb the result that the reason of energy decreases is investigated near to big heat input welding the time weld seam the HAZ, draw following understanding, its reason is thickization in old austenite grain (γ grain) footpath, because HAZ organizes all thickization, so HAZ toughness worsens, based on this understanding, make can fine dispersive TiN till the high temperature stabilization, thereby can form the composition that old γ grain obtains refinement.

That is, High Strength Steel of the present invention or high tensile steel plate, in mass%,

Contain C:0.010～0.080%,

Si：0.02～1.00％，

Mn：1.10～2.90％，

P：0～0.030％，

S：0～0.010％，

Below the Al:0.20%,

Ni：0.40～2.40％，

Cr：0.50～1.95％，

Mo：0.16～1.10％，

Ti：0.002～0.030％，

N：0.0058～0.0120％，

And 1.0 ≦ [Ti]/[N]＜4.0,

Remainder is made of Fe and unavoidable impurities, and the defined AS value of following formula and DL value be AS ≧ 3.60, DL ≦ 2.80, and tissue is based on no carbon bainite (bainitic ferrite), wherein,

AS＝[Mn]+[Ni]+2×[Cu]

DL＝2.5×[Mo]+30×[Nb]+10×[V]

Here, the content (mass%) of [X] expression X element.

Steel of the present invention or high tensile steel plate, outside above-mentioned chemical ingredients, can also the independent or compound optional elements that contains in following each group, (1) below the Cu:1.60%, (2) B:0.0050% is following, Nb:0.100% following, V: be lower than more than in 0.060% any, (3) one or both among Ca, the REM add up to below 0.0050%, (4) below the Mg:0.0050%, (5) following, the Zr:0.100% of Hf:0.050% any one or two kinds of in following, (6) W:5.0% is following, Co:5.0% any one or two kinds of in following.

Also have, above-mentioned high tensile steel plate, the steel that will contain mentioned component is heated to the austenite region temperature, and when carrying out hot rolling, cooling manufacturing, the hot rolled final forging temperature is being good below 870 ℃.Below 870 ℃, can make the old γ grain refinement after the welding by the control final forging temperature, thus the HAZ toughness can further improve big heat input welding the time.

Steel, steel plate according to the present invention, because the content of C is extremely low, Mn and Ni or also have Cu are to add energetically more than 3.60 with the AS value, in addition, suppress the interpolation of Mo, Nb, V, make the DL value below 2.80, thus be not subjected to the restriction of the height of the speed of cooling after the hot rolling, when also having thickness of slab thick too, can generate tissue, strength of parent, base metal tenacity excellence based on no carbon bainite.In addition, because a large amount of relatively N adds in the scope of fixed [Ti]/[N] ratio, thus can be TiN in high-temperature stableization, near the old γ grain thus can the refinement weld seam, even, also can access excellent HAZ toughness so carry out the welding of big heat input.

Description of drawings

Fig. 1 is the CCT figure for the pattern of the relation of speed of cooling after the hot rolling that existing steel and steel of the present invention are described and tissue.

Embodiment

The main points of first on the composition of steel plate of the present invention are, on the extremely low basis of C content, according to can guarantee AS ≧ 3.60 of fixed strength of parent add element M n, Ni, the Cu that improves hardenability energetically, on the other hand, suppress Nb, V, Mo energetically according to the DL that can guarantee base metal tenacity ≦ 2.80.At first, to the tissue that is generated, the characteristic after the composition hot rolling of the steel of steel plate,, describe with reference to CCT figure according to the present invention.

Fig. 1 represents is that the extremely low C that the present invention has added Mn, Ni, Cu energetically is steel (A) and existing high C are the CCT figure that steel (B1), low C are steel (B2).Represent among the figure that BF is no carbon bainite, GBF is granular no carbon bainite, and M is a martensite, and B is a bainite, and F is a ferrite.According to this figure, steel plate of the present invention, among the high speed of cooling (CR1) of hot rolling postcooling, low speed of cooling (CR2) arbitrary, the BF that generates is all at area occupation ratio more than 85%, preferably, all can access the second phase MA fine bainite of dispersive (no carbon bainite) tissue imperceptibly more than 90%.According to tissue,, possesses excellent toughness in addition even the slab of thickness of slab more than about 50mm also can access as the intensity more than the mechanical properties 780MPa of mother metal based on this BF.And, among the high speed of cooling (CR1) of hot rolling postcooling, low speed of cooling (CR2) arbitrary, as mentioned above, owing to all organize and become the low BF of the speed of cooling of hardness susceptibility basically, so in little heat input welding conditions (heat input: about number kJ/cm), can make the hardness of HAZ reduce (improving low temperature resistant disruptiveness), when also having the low speed cooling under big heat input welding conditions (heat input: about hundreds of kJ/cm), also can access better HAZ toughness.In addition, existing high C is steel (B1), in high speed of cooling (CR1), owing to generate ferrite and thick bainite, along with generating thick and blocky MA, thus strength of parent and toughness descend, also be difficult to guarantee with ining addition above-mentioned in the HAZ toughness of heat input when welding.

Then, the main points to second on the composition of steel plate of the present invention describe.

According to the tissue that forms based on above-mentioned BF, can improve base metal tenacity, HAZ toughness, but in the big heat input welding about 800kJ/cm, in order to ensure sufficient HAZ toughness, with the above-mentioned prerequisite that is organized as, at the ratio of [Ti]/[N] is 1.0～4.0 scope, and it is important that interpolation can suppress the old γ particle diameter N thickization, a large amount of relatively ten minutes of HAZ.

Because the heavy addition of above-mentioned N, thus the HAZ flexible reason that improves under the big heat input welding is also indeterminate, but carried out following supposition.At first, think that the motivating force when increasing the TiN generation is even common manufacturing also can make TiN fine disperse by high Nization.Think in addition, meanwhile carry out above-mentioned control, so can increase the stability under the high temperature of TiN by interpolation balance to N and Ti.Promptly, be speculated as by carrying out high Nization and the interpolation equilibrated of N and Ti being controlled, the refinement that reaches near the old γ grain of weld seam that can be stable, thereby significantly improve the fluctuation of (reduction) HAZ flexible, and by further to the suitable adjustment of AS, DL, make the tissue (no carbon bainite) of the γ intragranular after the phase transformation can carry out refinement,, also can guarantee excellent HAZ toughness even thus in big heat input welding.

Also have, research according to the present inventor, at existing parent phase is in the ferritic-pearlitic tissue, because solid solution N exists in the parent phase, toughness worsens, so can not carry out high Nization fully, but as the present invention, parent phase is the tissue based on BF, thus solid solution N can be in the second phase MA denseization, so can not worsen even can learn high Nization toughness yet.

Here, the composition qualification reason to High Strength Steel of the present invention, steel plate describes.Unit all is mass%.

C：0.010～0.080％

C is in order to ensure the necessary element of strength of parent.Be lower than 0.010%, even add the strength of parent that to guarantee more than the 780MPa energetically as the element that improves hardenability.In addition, surpass 0.080%, then MA generates in a large number, and base metal tenacity, HAZ toughness worsen.Therefore, the following of C content is limited to 0.01%, be preferably more than 0.020%, and more preferably be good more than 0.030%, on the other hand, be limited to 0.080% on it, be preferably 0.070%, more preferably 0.060% is good.

Si：0.02～1.00％

Si has the effect of solution strengthening, but excessive interpolation, then MA generates in a large number among mother metal, the HAZ, worsens base metal tenacity, HAZ toughness.Therefore, the following of Si content is limited to 0.02%, is preferably 0.10%, is limited to 1.00% on it, is preferably 0.80%.

Mn：1.10～2.90％

Mn improves hardenability, is to guaranteeing intensity, toughness effective elements.But excessive interpolation, then intensity is excessive, and opposite base metal tenacity, HAZ toughness descend.Therefore, be limited to 1.10% under the Mn content, be preferably 1.40%, more preferably 1.70%, be preferably 1.90% especially for good.

P:0～0.030% is following, S:0～below 0.010%

These elements are impurity elements that segregation takes place easily, because base metal tenacity, HAZ toughness are had bad influence, thus few more good more, be defined as among the present invention below the P:0.030%, below the S:0.010%.

Below the Al:0.20%

Al adds as deoxidant element, but excessive interpolation then MA generate in a large number, worsen base metal tenacity, HAZ toughness.Therefore, the Al upper content limit is 0.20%, is preferably 0.15%, and more preferably 0.10% is good.

Ni：0.40～2.40％

Make intensity enhancing thereby Ni improves the low-temperature flexibility of steel and improves hardenability, and have the thermal rupture of preventing and welding high temperature disruptive effect.But, excessive interpolation, then pit takes place easily.Therefore, the following of Ni content is limited to 0.40%, is preferably 0.60%, and more preferably 0.80%, be preferably especially more than 1.00%, be limited to 2.40% on it.

Cr：0.50～1.95％

Cr improves the intensity of mother metal, weld part, but excessive interpolation then can make base metal tenacity, HAZ toughness worsen on the contrary.Therefore, the following of Cr content is limited to 0.50%, is preferably 0.70%, and more preferably 1.00%, be limited to 1.95% on it, be preferably 1.70%, more preferably 1.50%.

Mo：0.16～1.10％

Mo is improved hardenability, and is effective to guaranteeing high strength, is to preventing the temper brittleness effective elements, but excessive interpolation then can reduce base metal tenacity, HAZ toughness on the contrary.Therefore, the following of Mo content is limited to 0.16%, is preferably 0.22%, and more preferably 0.25%, be preferably 0.40 especially, be limited to 1.10% on it, be preferably 0.80%, more preferably 0.60%.

Ti：0.002～0.030％

Ti combines with N and forms nitride, and the austenite crystal of the HAZ during the refinement welding is to improving HAZ toughness effective elements.Because to be lower than 0.002% grain refining effect too small for Ti content, so be limited to 0.002% under it, is preferably 0.007%, more preferably 0.010%, be preferably 0.012% especially.On the other hand, excessive interpolation, thickization of TiN then might make base metal tenacity, HAZ toughness worsen on the contrary, thus on be limited to 0.030%, be preferably 0.025%, more preferably 0.020%.

N：0.0058～0.0120％

N is the important element of HAZ flexible when importing welding with the big heat of the common raising of Ti, combines with Ti, forms TiN, and the austenite crystal of the HAZ when the big heat input of refinement is welded has raising HAZ flexible effect.But the excessive interpolation of N has bad influence to base metal tenacity, HAZ toughness.So in order to bring into play the effect of above-mentioned N effectively, be limited to 0.0058% under the N content, be preferably 0.0060%, more preferably 0.0070%, be preferably 0.0080 especially for good, be limited to 0.0120% on it, be preferably 0.0100%, more preferably 0.0090% is good.

[Ti]/[N]：1.0～4.0

It is excessive that the ratio of [Ti]/[N] is lower than 1.0 solid solution N, worsens base metal tenacity, HAZ toughness.On the other hand, surpass 4.0 TiN and be difficult to disperse imperceptibly, base metal tenacity, HAZ toughness descend.Therefore, the following of above-mentioned ratio is limited to 1.0, is limited to 4.0 on it, is preferably 3.0, and more preferably 2.0.

AS value: more than 3.60

The addition of Mn, Ni, Cu has confidential relation with base metal tenacity, HAZ toughness, and Cu compares about 2 times with Mn, Ni, improves the effect height of intensity.After the hot rolling, in order to make the strength of parent that arrives the scope of hanging down speed of cooling in high speed of cooling more than 780MPa, embodiment clearly shows as described later, and the AS value is very necessary more than 3.6.Thus, the BF of parent phase amount also can access more than the 85 area %.Base metal tenacity, HAZ toughness are improved more because BF amount is many more, so the BF amount is preferably more than the 90 area %, and more preferably 95 area % are above be good, therefore in order to make above-mentioned AS value rising, need adjust the addition of Mn, Ni, Cu described later.The AS value is high, the BF of phase transformation under the low temperature in the time of then accessing low speed of cooling (big heat input welding), thus increase the BF amount.Therefore, the AS value is preferably more than 4.00, more preferably more than 4.50, is preferably especially more than 5.00 to good.

DL value: below 2.80

Mo has the effect that improves hardenability as mentioned above.Nb described later, V also have same effect.Its on the other hand, these elements of excessive interpolation then can generate thick bainite structure, worsen base metal tenacity, HAZ toughness.The effect of this deterioration flexible, each element and different, according to present inventor's etc. experiment, Mo is 1 o'clock, and Nb is about 12 times, and V is about 4 times.Embodiment described later clearly shows, in order to ensure v E _-20The good base metal tenacity that=200J is above, making the DL value is below 2.80, is preferably below 2.50, more preferably below 2.00, be preferably below 1.50% especially, further be preferably especially below 1.00%, be restricted to good the interpolation of Mo, Nb, V.

Steel plate of the present invention, outside above composition, remainder is formed by Fe and unavoidable impurities, in the scope of the effect that does not damage mentioned component, effect, can add the element of further raising characteristic.For example, can the independent or compound optional elements that contains in following each group, (1) below the Cu:1.60%, (2) B:0.0050% is following, Nb:0.100% following, V: be lower than more than in 0.060% any, (3) one or both among Ca, the REM add up to below 0.0050%, (4) below the Mg:0.0050%, (5) Hf:0.050% is following, Zr:0.100% any one or two kinds of in following, and (6) W:5.0% is following, Co:5.0% any one or two kinds of in following.Below, the qualification reason of these auxiliary elements is described.

Below the Cu:1.60%

Cu improves strength of parent, the effect that has Mo, Mn, the unexistent raising hardenability of Ni, Cr in addition by solution strengthening and precipitation strength.In order to realize described effect effectively, wish that its interpolation is preferably more than 0.30%, more preferably more than 0.50%.But, surpass 1.60%, then base metal tenacity, HAZ toughness descend, thus Cu content on be limited to 1.60%, be preferably 1.40%, more preferably 1.20%, be preferably 1.00% especially for good.

Below the B:0.0050%

B improves hardenability and has the effect of the HAZ of improvement flexible.Its effect was big when particularly, heat was imported big welding.In order to realize described effect effectively, being added to more than 0.0005% is good.But excessive interpolation then can worsen base metal tenacity, HAZ toughness on the contrary.Therefore, be limited to 0.0050% on the B content, be preferably 0.0030%, more preferably 0.0020% is good.

Below the Nb:0.100%

Nb and B are same, and hardenability is improved.That is, solid solution Nb improves the hardenability of mother metal, thereby has the effect that improves strength of parent, strength of welded joint, but excessive interpolation, and then intensity is excessive, and base metal tenacity, HAZ toughness are worsened.Therefore, be limited to 0.100% on the NB content, be preferably 0.040%, more preferably 0.020%.

V: be lower than 0.060%

V is also same with B, Nb, and a small amount of interpolation can improve hardenability.Also have, have the effect that improves anti-temper softening.But, excessive interpolation, then intensity is excessive, can worsen base metal tenacity, HAZ toughness.Therefore, the V upper content limit is 0.060%, is preferably 0.050%, more preferably 0.040%.

Ca, REM: be aggregated in below 0.0050%

These elements by making the control to inclusion morphology of MnS balling, have the anisotropic effect of minimizing, thereby have the HAZ of raising flexible effect.But excessive interpolation then can worsen base metal tenacity on the contrary.Therefore, these elements add up to and are limited to 0.0050% on it, are preferably 0.0030%.

Below the Mg:0.0050%

Mg forms MgO, thickization of the austenite crystal by suppressing HAZ, thus have the HAZ of raising flexible effect.But excessive interpolation then can worsen base metal tenacity on the contrary.Therefore, be limited to 0.0050% on it, be preferably 0.0035%.

Below the Zr:0.100%

Below the Hf:0.050%

Zr, Hf and Ti are same, form nitride with N, and the austenite crystal of the HAZ during the refinement welding is to improving HAZ toughness effective elements.But excessive interpolation then can make base metal tenacity, HAZ toughness descend on the contrary.Therefore, be limited to 0.100% on the Zr content, be preferably 0.050%, be limited to 0.050% on the Hf content, be preferably 0.030%.

Below the W:5.0%

Below the Co:5.0%

W, Co can improve hardenability on a small quantity, and be effective to easily guaranteeing intensity.W has the effect that improves anti-temper softening in addition concurrently.On the other hand, excessive interpolation, then intensity is too high, and base metal tenacity, HAZ toughness are descended.Therefore, the upper limit of these elements is respectively 5.0%, is preferably 2.5%.

High-intensity high-tenacity steel plate of the present invention can be made by usual method, and steel billet is heated to the austenitic temperature zone, preferably is heated to AC ₃After about～1350 ℃, carry out hot rolling, after the hot rolling, by air cooling or directly cooling cool off with average cooling rate following about 60 ℃/sec and get final product.In order to suppress the generation of MA and GBF as much as possible, can be preferably average cooling rate more than about 5 ℃/sec quicken cooling.This quickens cooling, can proceed to the following temperature province of BF transformation temperature (about 650～400 ℃).In order positively to arrive below the BF transformation temperature, can proceed to about 200 ℃ below.Also have, quicken cooling, because at high temperature speed of cooling is fast, so can carry out below 800 ℃ at least.

The hot rolled final forging temperature can still by being controlled at below 870 ℃, can further improve HAZ toughness below 1000 ℃ as usual method.Its reason be because, it is rolling to carry out the non-recrystallization zone when mother metal is rolled mostly, when then being subjected to welding heat affected generation reverse transformation, exist because phase transformation nuclear (strain of rolling processing) is a large amount of, so old its result of γ particle diameter after the welding obtains refinement with comparing of finish-forging of high temperature (above 870 ℃).From this reason, by final forging temperature is controlled at below 870 ℃, be preferably below 800 ℃, more preferably below 750 ℃, can more effectively improve HAZ toughness.Also have, rolling by carrying out such low temperature finish-forging, to compare with the hot rolling of usual method, the lower limit of AS can be extended to about 3.20 downwards.

By above-mentioned manufacture method, can obtain hot rolling after, contain from high speed of cooling to low speed of cooling BF in area % more than 85%, be preferably more than 90%, remainder is by the formed high strength of GBF, MA, high tenacity tissue.Because MA generates imperceptibly at the interface of BF and GBF, worsen toughness so can not resemble block MA, but because few more toughness that can access excellence more so be preferably below the 5.0 area %, more preferably is good below 3.0%.

Steel plate of the present invention, as mentioned above, because the cooling after the hot rolling all can access the tissue based on BF from high speed of cooling to low speed of cooling, so thicker steel plate, for example thickness of slab also can have intensity more than the 780MPa at the steel plate about 50mm, has good base metal tenacity, HAZ toughness, low temperature resistant disruptiveness simultaneously.

Then, the present invention is carried out more specific description, but the present invention also not only is confined to the explanation of embodiment for embodiment.

[embodiment]

Steel shown in the following table 1～3 carries out melting, the casting of its molten steel and slab (thickness 250mm) 1150 ℃ heat after, carry out hot rolling, finish hot rolling with final forging temperature shown in table 4 and 5, directly cool off (online water-cooled) with the average cooling rate of 10 ℃/sec 800～200 ℃ temperature provinces.Also have, the steel of table 4 and 5 test portion, the test portion numbering is corresponding to the steel of same steel numbering system (table 1～3).

For resulting hot-rolled sheet (thickness of slab 50mm), take the structure observation test film from 1/4 position of the thickness of slab of hot-rolled sheet, carried out the observation (multiplying power: 400 times) of opticmicroscope, to find to have formed based on BF, remainder is formed by GBF and MA.Also have, in order to measure the face integration rate of BF, structure observation uses test film after the nital corrosion, use SEM (sweep electron microscope) tissue to be taken with 1000 times multiplying power, captured image is by image analysis software (title Image-pro, プラネトロ Application system) resolves, obtain the area occupation ratio of BF.Its result represents in table 4 and 5.Also have, in example (test portion No.1～59), to the mensuration of MA amount, all below 3.0 area %.

Carry out tension test, shock test by following main points in addition, the mechanical properties of mother metal is investigated.Criterion of acceptability be tensile strength more than 780MPa, toughness is absorbing energy (v E _-20) more than 200J.

Tension test

The JIS4 test film that obtains from thickness of slab 1/4 position of each steel plate has carried out tension test, has measured 0.2% yield strength, tensile strength.

Shock test

The JIS4 test film that obtains from thickness of slab 1/4 position of each steel plate has carried out pendulum impact test, has obtained-40 ℃ absorption energy (v E _-20), base metal tenacity is estimated.

In addition, by following main points HAZ toughness is investigated.

Carry out the big heat input welding (electroslag welding) of heat input 800kJ/cm one welding bead, take the JIS4 test film, carry out the V-notch pendulum impact test, measured-40 ℃ absorption energy (v E from the HAZ that leaves weld seam (solidus curve) 0.5mm _-20).At this moment, the sample number is 5, obtains its mean value, and HAZ toughness is estimated.Criterion of acceptability is for absorbing energy (v E _-20) mean value more than 150J.

Also have, in example of the present invention, y-shaped solder cracks test method according to defined among the JISZ3158, be cooled to the state (prevent root from breaking preheating temperature=0 ℃ ,-20 ℃) of 0 ℃ and-20 ℃ at the steel plate that is for experiment, 1.7kJ/mm wraps the agent weldering with the heat input, low temperature resistant disruptiveness is investigated, but in arbitrary temperature, the generation of all not breaking.

Above-mentioned investigation result is represented in table 4 and 5.From same table, example, in base metal tenacity, tensile strength all more than 780MPa, also has v E _-20All more than 200J, high strength and base metal tenacity excellence.Also have, in the HAZ toughness when the big heat input of 800J/cm is welded, have v E _-40Be the absorption energy more than the 150J, HAZ toughness also shows excellence in big heat input welding even confirm.

In addition, any equal comparative example (table 5 outside the scope of the invention of alloy composition, [Ti]/[N], AS value, DL value, No.81～115), same with example, after the hot rolling, no matter whether carry out the acceleration cooling about 10 ℃/sec, HAZ toughness major part does not reach about 60J, also has the v E of mother metal _-20All being lower than 200J, is the material of base metal tenacity difference.

[table 1]

[table 2]

[table 3]

[table 4]

Annotate: test portion No.1～59th, example

[table 5]

Annotate: what have * among the test portion No. is comparative example

Claims

1. the High Strength Steel of the tenacity excellent of a welding heat affected zone, it is characterized in that, in mass%, contain C:0.010～0.080%, Si:0.02～1.00%, Mn:1.10～2.90%, P:0～0.030%, S:0～0.010%, below the Al:0.20%, Ni:0.40～2.40%, Cr:0.50～1.95%, Mo:0.16～1.10%, Ti:0.002～0.030%, N:0.0058～0.0120%, and 1.0 ≦ [Ti]/[N]＜4.0, remainder is made of Fe and unavoidable impurities, and AS value and DL value by the following formula definition are AS ≧ 3.60, DL ≦ 2.80, organize mainly and constitute by no carbon bainite

AS＝[Mn]+[Ni]+2×[Cu]，

DL＝2.5×[Mo]+30×[Nb]+10×[V]，

Here, [X] expression X element is in the amount of mass%.

2. according to the described High Strength Steel of claim 1, it is characterized in that, also contain below the Cu:1.60%.

3. according to the described High Strength Steel of claim 1, it is characterized in that, contain also that B:0.0050% is following, Nb:0.100% following, V: be lower than more than in 0.060% any.

4. according to the described High Strength Steel of claim 1, it is characterized in that, also contain among Ca, the REM one or both and add up to below 0.0050%.