CN101899615A

CN101899615A - The high tensile steel of the low-temperature flexibility excellence of welding heat affected zone

Info

Publication number: CN101899615A
Application number: CN201010154697XA
Authority: CN
Inventors: 太田诚; 高桥祐二; 大垣诚一; 金子雅人; 泉学
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2007-03-23
Filing date: 2008-02-28
Publication date: 2010-12-01
Also published as: CN101942602A; KR100992254B1; KR20080086849A

Abstract

The invention provides a kind of high tensile steel of low-temperature flexibility excellence of welding heat affected zone, even when carrying out high heat energy input welding, the low-temperature flexibility of HAZ is also excellent, and the toughness of mother metal, fatigue cracking progress resistance, brittlement crevasse crack rejection characteristic or brittlement crevasse crack stop performance are also excellent.Steel of the present invention satisfy specified chemical composition, and satisfy following formula.-20≤(B-NT/1.3)≤10, { in the formula, B represents B content (quality ppm), what NT represented in addition is, when N (N content, unit: quality ppm) with Ti (Ti content, unit: pass quality ppm) is (N-Ti/3.4) 〉=0 o'clock, NT=(N-Ti/3.4), (N-Ti/3.4)＜0 o'clock, NT=0}.

Description

The high tensile steel of the low-temperature flexibility excellence of welding heat affected zone

The application is an application number: 200810081533.1, and the applying date: 2008.02.28, denomination of invention: the dividing an application of the application of " high tensile steel of the low-temperature flexibility excellence of welding heat affected zone ".

Technical field

Even The present invention be more particularly directed to a kind of being used in when exposing to the open air in cryogenic purposes, be subjected to the still excellent steel of low-temperature flexibility at the position (hereinafter referred to as " welding heat affected zone " or " HAZ ") of heat affecting during welding steel.Steel of the present invention also have yield tensile ratio high tensile, good fatigue cracking progress resistance, good brittlement crevasse crack generation inhibition or good brittlement crevasse crack stop performance except the low-temperature flexibility of excellence.

Also have, the present invention does not limit the welding process of above-mentioned steel, but can be applicable to submerged arc welding, electro-gas welding etc., but below, be to guarantee that with the toughness of implementing to be considered to welding heat affected zone the situation of the single face submerged arc welding of the high heat energy input of difficulty is that example describes especially.In addition,, be applied to the situation of different shape steel etc., but what enumerate explanation typically is the situation that is applicable to steel plate about the form of steel of the present invention.

Background technology

The desired characteristic of steel plate that bridge and boats and ships etc. use is increasingly stringent in recent years, and special requirement have good toughness.These steel plates generally are engaged by welding, but particularly HAZ is subjected to heat affecting when welding, has the such problem of the easy deterioration of toughness.This toughness deterioration is along with the input heat energy in when welding becomes big and manifests significantly all the more, and its reason is considered to, if the input heat energy during welding becomes big, then the speed of cooling of HAZ is slack-off, and hardenability reduces, and thick island martensite body generates.Therefore,, think that the input heat energy when doing one's utmost to suppress to weld gets final product, but improving on the welding operation efficient that then the high heat energy input welding process of for example electro-gas welding, electroslag welding, submerged arc welding etc. is adopted in expectation in order to improve HAZ toughness.

In recent years, for the low temperature of the liquefied gas of making marine structure and storage LPG etc. in a short time with storage tank etc., for example import the single face buried arc kitchen range construction that heat energy relates to the high heat energy of 50～200kJ/cm and be widely adopted.But though this welding can realize the high efficiency of constructing, it is negative to be to be difficult to the stable toughness of guaranteeing the welding heat affected zone that formed by welding, and must using the multi-layer welding that can import based on low-heat, to tap into the situation of capable manufacturing a lot.Therefore, in the manufacturing of above-mentioned low temperature with storage tank etc., just require a kind of steel plate, even it adopts the above-mentioned high heat energy input welding process that can carry out the high-level efficiency construction, even and the low temperature about-60 ℃, the toughness of HAZ (low-temperature flexibility) is also excellent.

Up to now, for the low-temperature flexibility of improving above-mentioned HAZ the whole bag of tricks has been proposed also.For example propose that a kind of method is arranged in the public clear 55-026164 communique of spy, No. 2950076 communique of special permission, it suppresses thickization of austenite crystal by the anchoring particle that utilizes TiN, Al oxide compound etc., thereby improves HAZ toughness.In addition, disclose a kind of technology in special fair 07-068577 communique, the special fair 05-017300 communique, there is ferrite transformation nuclear in it in a large number by making the austenite crystal intragranular, thereby realizes the miniaturization of crystal grain.Specifically, be that ferrite transformation nuclear is used with TiN, MnS, BN, Ti oxide compound etc., reach the miniaturization of crystal grain thus, with the improvement of the low-temperature flexibility that realizes HAZ.

But in above-mentioned any method, when carrying out the single face submerged arc welding of high heat energy input, solid solution considerably all can take place in the precipitate of TiN etc., be difficult to suppress thickization of crystal grain thereafter etc., therefore for the toughness of under the low temperature about-60 ℃, guaranteeing excellent HAZ (hereinafter referred to as " low-temperature flexibility of HAZ " or only be called " HAZ toughness "), just need further to improve.

In addition, in the tough property improvement technology of HAZ that proposes up to now, practical situation are as the multi-functional storage tank of using that mixes carrier fluid ammonium (ammonium) and liquid propane gas (propane gas), should not have desired low yield strength ratio (for example below 75%) to it and study.

On the other hand, in order to prevent stress corrosion cracking (SCC), require to have low yield strength YS below the 440MPa to being used for liquefied ammonia with the steel plate of storage tank, and, also require it to have the following tensile strength of 530MPa in order to reduce the steel gross weight.If mix the storage tank be loaded with liquid propane gas in the liquefaction ammonium, then require low-temperature flexibility also excellent as the characteristic of the steel plate that uses.Liquefied ammonia has been notified the stress corrosion cracking (SCC) that causes steel, thereby be defined as the characteristic of steel plate yield strength YS is suppressed at (IGC CODE 17.13 (International Code for theConstruction and Equipment of Ships Carrying Liquefied Gases in Bulk) version in 2002) below the 440MPa.

Yet, in mixing the multifunctional usage that carries above-mentioned liquefaction ammonium and liquid propane gas, both are met at desired characteristic, in addition along with the maximization of the marine structure of boats and ships etc., the high capacity of the storage tank that boats and ships carried also advances, this just also requires the high tensileization of consequent steel plate, but follows the upper limit regulation of yield strength YS and reach low yield strength ratio (bend and strengthen YR=YS/TS) simultaneously and just become great problem.

In addition, in above-mentioned various structured materials, because it is many to apply the situation of repeated stress, therefore in order to ensure the security of structured material, not only HAZ toughness wants good, and it is good that fatigue characteristic are also wanted, and this is very important in design.The fatigue process of steel plate is thought and is divided into generation and in case these 2 processes of progress of the be full of cracks that takes place in the be full of cracks of stress concentration portion substantially.And, in common mechanical component, macroscopic be full of cracks be considered to use boundary, almost allow the design of the progress of be full of cracks.Yet, in welded structure, even fatigue cracking takes place also can not cause immediately destroying, before reaching terminal stage, this be full of cracks is found by making regular check on etc., and the part that having be full of cracks to enter can obtain repairing, perhaps in the usage period, do not cause final destructive length if be full of cracks grows to, even be full of cracks is then arranged, works also can fully withstand use.

In welded structure, exist in a large number as the welding tape edge portion of stress concentration portion, prevent fully occurring in of fatigue cracking technical almost be also to be impossible, in addition neither very wise move on economy.That is, for the fatigue lifetime that makes welded structure good, compared with the generation that prevents to chap itself, more efficient methods is, significantly prolong the be full of cracks progress life-span from the state that existed of be full of cracks, for this reason, slow down as far as possible the advance rate of be full of cracks of steel such be designed to important item.

Technology as the speed that suppresses the fatigue cracking progress, up to now also have to plant and be suggested, for example in No. 3298544 communique of special permission, propose to have a kind of technology, it is by as hard phase and soft 2 phase constitutions mutually, utilizes bending, stop, the difference of the be full of cracks on soft phase/hard phase border to suppress the advance rate that chaps.Yet the HAZ toughness to steel plate is not considered in this technology, might can not guarantee sufficient security.Just expectation realizes the steel plate that a kind of HAZ of making toughness and this two specific character of fatigue cracking progress resistance all are met thus.

In addition, in order to ensure security, also be desirably in the be full of cracks that suppresses to cause in the steel [below, be called " brittlement crevasse crack rejection characteristic " or CTOD (Crack-Tip Opening Displacement) characteristic] takes place because of brittle rupture as works.If this is because brittlement crevasse crack takes place, then the works autoclasia.Both suppressed the generation (below, this specific character is called " brittlement crevasse crack generation rejection characteristic ") of brittlement crevasse crack, the high-tensile steel that the HAZ toughness when making high heat energy input welding again improves is unknown so far.

Even brittlement crevasse crack takes place, the propagation of brittlement crevasse crack is stopped, it also is important important document that the propagation regions of brittlement crevasse crack is suppressed at inferior limit (below, be called " brittlement crevasse crack stop performance ").If this is to propagate owing to the brittlement crevasse crack that takes place runs through vast scope, then cause the destruction of works self., about the propagation of the brittlement crevasse crack that both suppressed to take place, the high-tensile steel that the HAZ toughness when making high heat energy input welding again improves is also unknown so far.

Set out thus, truth is that expectation realizes the HAZ toughness after a kind of high heat energy input welding and the steel plate of brittlement crevasse crack generation rejection characteristic or brittlement crevasse crack stop performance excellence.

Summary of the invention

The present invention carries out in view of this situation, its purpose is, even provide a kind of when welding with high heat energy, the warm toughness of HAZ is also excellent, and for mother metal (steel plate), toughness, fatigue cracking progress resistance, brittlement crevasse crack generation rejection characteristic and all excellent high tensile steel of brittlement crevasse crack stop performance.

Can reach the steel of basis first invention of above-mentioned purpose, contain the C:0.03～0.09% (meaning of " quality % " respectively, relate under the chemical ingredients same), Si:0.01～0.25%, Mn:1.20～1.60%, below the P:0.010%, below the S:0.003%, Al:0.02～0.04%, Nb:0.005～0.016%, B:0.0006～0.0020%, N:0.0045～0.0090%, Ti:0.008～0.020%, and satisfy following formula (1), surplus is iron and unavoidable impurities, and all shared ferrite branch rate is 45～85 area % in the tissue, surplus is made of at least one side of bainite structure and martensitic stucture, and described ferritic average crystal grain diameter is below the 19 μ m.

-20≤(B-NT/1.3)≤10…(1)

{ in the formula, B represents B content (quality ppm).What NT represented in addition is that (N content, unit: quality ppm) (Ti content, unit: pass quality ppm) is with Ti as N

(N-Ti/3.4) 〉=0 o'clock, NT=(N-Ti/3.4)

(N-Ti/3.4)＜0 o'clock, NT=0}

According to this first invention, the yield strength YS of steel (mother metal) is below the 440MPa, tensile strength TS is more than the 530MPa, and the toughness of steel is also excellent, even in addition when implementing the welding of high heat energy input, HAZ also demonstrates excellent toughness under about-60 ℃, thus, help mix to carry the maximization of welded structure of the multi-functional storage tank etc. of liquefied ammonia and the liquidization third Ai gas, can adopt the single face submerged arc welding method of for example high heat energy input, can be during shorter in the above-mentioned welded structure of manufacturing.

Can reach the steel of basis second invention of above-mentioned purpose, satisfy the chemical ingredients of above-mentioned first invention and form, satisfy the important document of described formula (1), and tissue is the complex tissue that is made of mutually soft phase and hard, the Vickers' hardness Hv of hard phase ₁With soft Vickers' hardness Hv mutually ₂Ratio (Hv ₁/ Hv ₂) be 1.5～5.0, the particle diameter of soft phase is worked as the phase diameter with circle and is counted below the 20 μ m.

This second the invention steel in, described soft be mutually among ferrite, tempering bainite and the tempered martensite more than a kind, hard can be made of bainite and martensitic at least one side mutually.At this, comprise the island martensite body in the martensite of hard phase.

According to this second invention, even when steel are implemented the welding of high heat energy input, HAZ also demonstrates excellent toughness under about-60 ℃, therefore in the manufacturing of the low temperature of the liquefied gas of marine structure and storage LPG etc. with storage tank etc., can adopt the single face submerged arc welding method of for example high heat energy input, can make above-mentioned marine structure etc. with shorter time, because fatigue cracking progress resistance is also excellent, so also can improve the security of structural member.

Can reach the steel of basis the 3rd invention of above-mentioned purpose, satisfying the chemical ingredients of above-mentioned first invention forms, satisfy the important document of described formula (1), and rolling direction with the steel that are parallel to thickness t (mm), when observation is vertical a metal structure with respect to steel surface, satisfy following (a)～(c).

(a) the ferrite area occupation ratio is more than 75%,

(b) the leveled circular equivalent diameter of the ferrite crystal grain of t/2 position is below the 20.0 μ m,

(c) mean aspect ratio of the ferrite crystal grain of t/4 position is below 1.6.

According to this 3rd invention, can realize the low-temperature flexibility and all excellent high tensile steel of brittlement crevasse crack generation rejection characteristic of welding heat affected zone.

Can reach the steel of above-mentioned purpose in this 4th invention, satisfying the chemical ingredients of above-mentioned first invention forms, satisfy the important document of described formula (1), and during the metal structure of the steel of observation phase thickness t (mm), the median size of the ferrite crystal grain the zone of the position from steel surface to t/100 is below the 25 μ m.

According to this 4th invention, can realize the low-temperature flexibility and all excellent high tensile steel of brittlement crevasse crack stop performance of welding heat affected zone.

Promptly, according to this third and fourth invention, even when steel being implemented the welding of high heat energy input, HAZ still demonstrates excellent toughness under about-60 ℃, therefore in the manufacturing of the low temperature of the liquefied gas of marine structure and storage LPG etc., can adopt the single face submerged arc welding method of for example high heat energy input, can make above-mentioned marine structure etc. with shorter time with storage tank etc., and because brittlement crevasse crack suppresses, stop performance is all excellent, so can improve the security of structural member.

In steel of this first～four invention, as required, also can also contain among Cu:0.03～0.5%, Ni:0.03～0.8% and V:0.003～0.05% more than a kind, to satisfy following formula (2).Perhaps as required, also can also contain Ca:0.0003～0.003%.

(Cu+Ni+60Nb+20V)≤1.4…(2)

{ in the formula, Cu, Ni, Nb, V represent the content (quality %) of each element }

Description of drawings

Fig. 1 is the vE of expression (B-NT/1.3) and HAZ _-60The graphic representation of relation.

Fig. 2 is the graphic representation of the relation of expression ferrite branch rate and yield strength YS.

Fig. 3 is that the absorption of expression ferrite particle diameter and mother metal can vE _-60The graphic representation of relation.

Fig. 4 is the vE of expression (Cu+Ni+60Nb+20V) and HAZ _-60The graphic representation of relation.

Fig. 5 represents the sectional view of the groove shape in the welding of embodiment.

The mode chart of the electrode configuration when Fig. 6 represents the FCB welding.

Fig. 7 is that the leveled circular equivalent diameter of ferrite crystal grain of t/2 position of expression steel plate and long-width ratio are to CTOD characteristic (δ c _{-60 ℃}) graphic representation of the influence that causes.

Fig. 8 is that the leveled circular equivalent diameter of ferrite crystal grain of t/4 position of expression steel plate and long-width ratio are to CTOD characteristic (δ c _{-60 ℃}) graphic representation of the influence that causes.

Fig. 9 is relative position and CTOD characteristic (the δ c of expression from steel plate central part (t/2 position) _- _{60 ℃}) the graphic representation of relation.

Figure 10 is the graphic representation of the relation of the median size of ferrite crystal grain of t/100 position of expression steel plate and brittlement crevasse crack stop performance (60 ℃ Kca values).

Figure 11 is the true strain amount of expression in the zone of surface of steel plate t/100 position and the graphic representation of the relation of the median size of ferrite crystal grain.

Embodiment

At first, the high tensile steel for this 1st～4 invention low-temperature flexibility excellence common, HAZ describes.In the following description, this 1st～4 invention is referred to as " the present invention ".

About form of the present invention, be that example describes with the situation that is applied to steel plate.

The present inventor is in order to obtain when implementing the welding of high heat energy input, the high-tensile steel that the low-temperature flexibility of HAZ is excellent especially and carried out research with keen determination.

Consequently find following concrete grammar:

(a) if set C, Si lower, making C is below 0.09%, Si be below 0.25% after, make the balance optimizing of B, N and the Ti of specified amount, and add a certain amount of Nb, then the generation from the thick ferrite of austenite grain boundary (below, only be called " grain boundary ferrite ") is subjected to suppressing fully, can reach the crystal grain miniaturization of austenite crystal intragranular.

(b) in addition, when adding Cu, Ni, V,, then can suppress HAZ flexible deterioration if control the content of this Cu, Ni, V and Nb comprehensively for further high strength.

Some features that have at first in the present invention are, B, N by making each specified amount and the average optimizing of Ti, thereby realizing the optimizing of solid solution B amount closely, can make the crystal grain miniaturization of austenite crystal intragranular, as a result of is the low-temperature flexibility that can improve HAZ extraordinarily.

With 0.06%C-0.20%Si-1.4%Mn-0.03%Al-0.010%Nb is basal component, B, N and Ti are changed, { B represents B content (quality ppm) to adopt (B-NT/1.3), what NT represented is, when N (N content, unit: quality ppm) (Ti content, unit: pass quality ppm) is with Ti

(N-Ti/3.4) 〉=0 o'clock, NT=(N-Ti/3.4)

(N-Ti/3.4)＜0 o'clock, NT=0.

Below, it is also identical to relate to formula (1) }

Be the steel plate of various values, carry out thermal cycling test, measure the low-temperature flexibility (vE of HAZ by the mode of embodiment described later _-60), Fig. 1 has put these results in order.Also have, thermal cycling test supposition welding input heat energy: 60kJ/cm (thickness of slab 12mm), after heating remained to 1400 ℃ * 5 seconds, cooling was 150 seconds from 800 ℃ to 500 ℃.

By this Fig. 1 as can be known, as the HAZ low-temperature flexibility, in order to reach vE _-60: more than the 100J, shown in (1), the value of (B-NT/1.3) is in-the following scope of the above 10ppm of 20ppm.

-20≤(B-NT/1.3)≤10…(1)

As following formula (1), by making the balance optimizing of B, N and Ti, think and to bring into play following effect to greatest extent: thickization of the grain boundary ferrite that the solid solution B that is present in crystal boundary of inhibition austenite crystal intragranular causes, and can suppress generation, and as the effect of the ferrite transformation nuclear of BN from the ferrite side plate of crystal boundary.

As above-mentioned, for the balance optimizing that makes B, N and Ti positively improving the low-temperature flexibility of HAZ, and guarantee intensity of mother metal (steel plate) etc., the content that need make above-mentioned B, N, Ti is respectively in following ranges.

In high-tensile steel of the present invention, for the basic characteristic as this steel plate is met, except the basal component of C, Si, Mn, P, S, Al etc., also need suitably to be adjusted as B, the N of the composition that is related to following formula (1), Ti etc., but at first the scope of B, N, Ti etc. limits and the reasons are as follows.

(B：0.0006～0.0020％)

B fixes the deleterious solid solution N of HAZ toughness by generating NB, and has the effect that promotes the ferritic generation of intracrystalline.In addition, solid solution B also has thickization of inhibition grain boundary ferrite and the generation of ferrite side plate, makes the effect of the crystal grain miniaturization in the austenite crystal.In order to give full play to this action effect, B is contained more than 0.0006%.On the other hand, if B is too much, then under the effect of the solid solution B of surplus, crystallization is formed certain orientation, and HAZ toughness is deterioration on the contrary.Therefore B content is suppressed at below 0.0020%.Also have, the preferred lower limit of B content is 0.0008%, and preferred upper limit is 0.0018%.

(N：0.0045～0.0090％)

Elements such as N and Ti and Al form nitride, are the elements that HAZ toughness is improved, and therefore can contain more than 0.0045% (to be preferably more than 0.0060%).Also have, solid solution N becomes the reason of the toughness deterioration that makes HAZ.Because the increase of total nitrogen, though aforesaid nitride increases, it is superfluous that solid solution N also becomes, and therefore in the present invention N content is suppressed at below 0.0090%.

(Ti：0.008～0.020％)

Ti generates the TiN precipitate, promotes the ferritic generation of intracrystalline, and also is effective elements for thickization that suppresses austenite crystal.In addition, it also is the element that helps high strength.In order to bring into play such effect effectively, Ti is contained more than 0.008%, be preferably more than 0.012%.But, if contain Ti superfluously, then cause the HAZ flexible on the contrary and reduce, be below 0.020% therefore.

(Nb：0.005～0.016％)

In the present invention, as above-mentioned, make B, the Ti of each specified amount, the balance optimizing of N, and add a certain amount of Nb.Nb is for the thick grain boundary ferrite of abundant inhibition, and the crystal grain miniaturization of reaching the austenite crystal intragranular is useful element.In the present invention, in order to give full play to such effect Nb is contained more than 0.005%.But if its containing by surplus, then the island martensite body (MA:Martensite-Austenite constituent) as the hard phase generates easily, crystallization is formed certain orientation in addition, thereby causes HAZ flexible deterioration, therefore it is suppressed at below 0.016%.

(C：0.03～0.09％)

In order more positively to improve the low-temperature flexibility of HAZ, further reduce C, Si is effective.In the present invention, in order to suppress the generation of MA, the C amount is suppressed at below 0.09% with the HAZ toughness of guaranteeing-60 ℃ of pacts in HAZ portion.On the other hand, C also is the necessary element of intensity of guaranteeing steel plate, therefore makes it to contain more than 0.03%.

(Si：0.01～0.25％)

In addition, by Si is reduced to below 0.25%, also can suppress the generation of MA fully, thereby can easily guarantee the low-temperature flexibility of HAZ.On the other hand, Si is used to the deoxidation of molten steel, and raising also is the element that effectively plays a role for intensity, therefore can make it to contain more than 0.01%, preferably makes it to contain more than 0.05%.

Also have, in order positively to improve HAZ toughness, and make steel plate (mother metal) have other characteristic of intensity and toughness etc., the content of the composition beyond above-mentioned is in the following ranges as above-mentioned.

(Mn：1.20～1.60％)

Mn catches S as MnS, is useful element for the HAZ flexible deterioration that suppresses to cause because of S.In addition, it improves hardenability, also is the element of the high strength (high-tensile TSization and high-yield strength YSization) that helps steel plate.In order to bring into play such effect effectively, Mn is contained more than 1.20%.But superfluous if the Mn quantitative change gets, then therefore HAZ toughness deterioration on the contrary is suppressed at it below 1.60%.

(P:0.010% is following)

Because P is the element that makes HAZ toughness deterioration,, be suppressed at below 0.010% in the present invention so need do one's utmost to reduce.

(S:0.003% is following)

S generates thick sulfide, is the element that makes HAZ toughness deterioration.Therefore need do one's utmost to reduce, be suppressed at below 0.003% in the present invention.

(Al：0.02～0.04％)

Al is used as reductor and uses, and to generate AlN be precipitate, is the element that the HAZ toughness when making high heat energy input welding improves, and makes it in the present invention to contain more than 0.02%.But superfluous if Al content becomes, then the oxide based inclusion of aluminum oxide etc. increases, and MA generates and promoted that therefore HAZ toughness deterioration is suppressed at it below 0.04%.

The present invention regulation contain element as mentioned above, surplus is iron and unavoidable impurities, as this unavoidable impurities, can allow sneaking into of the element that mixed up because of the situation of raw material, goods and materials, producing apparatus etc.In addition, also can contain following element energetically again.

(among Cu:0.03～0.5%, Ni:0.03～0.8% and V:0.003～0.05% more than a kind (but will in the scope of following formula (2)))

(Cu+Ni+60Nb+20V)≤1.4…(2)

Cu, Ni, V all are the elements to guaranteeing that intensity is useful.

Cu is an effective elements improving by solution strengthening and precipitation strength aspect the intensity (tensile strength TS and yield strength YS).In order to bring into play such effect effectively, preferably make it to contain more than 0.03%.But if making it surplus contains, hot workability is hindered, therefore it is suppressed at below 0.5%.

Ni makes the intensity of mother metal and the element that toughness improves simultaneously.In order to bring into play this effect effectively, preferably make it to contain more than 0.03%.More preferably more than 0.2%.Can cause cost to rise but add superfluously, therefore be suppressed at below 0.8%.

V guarantees improving hardenability aspect the high strength, and is being useful element aspect the impedance of raising temper softening.In order to bring into play such effect effectively, preferably make it to contain more than 0.003%.But if contained superfluously, then therefore HAZ toughness deterioration is suppressed at it below 0.05%.

In addition in the present invention, as described above, by Nb is suppressed at below 0.016%, and limit the content of Cu, Ni, Nb, V in the mode of following formula (2), though among containing Cu, Ni and V more than a kind the time, also can guarantee excellent HAZ toughness.

With 0.06%C-0.20%Si-1.4%Mn-0.03%Al-0.014%Ti-0.0014%B-0.0065%N is basal component, make (Cu+Ni+60Nb+20V) become various values, contain in this way that Cu:0.5% is following, Ni:0.8% following and V:0.05% among following more than a kind and the Nb of specified amount, such steel plate of collecting seed carries out thermal cycling test, measures the low-temperature flexibility (vE of HAZ by the mode of embodiment described later _-60), Fig. 4 has put these results in order.Also have, thermal cycling test supposition welding input heat energy: 60kJ/cm (thickness of slab 12mm), after heating remained to 1400 ℃ * 5 seconds, cooling was 150 seconds from 800 ℃ to 500 ℃.

By this Fig. 4 as can be known, contain among Cu:0.03～0.5%, Ni:0.03～0.8% and V:0.003～0.05% more than a kind the time, as the low-temperature flexibility of HAZ, in order to reach vE _-60: more than the 100J, shown in (2), the value that need make (Cu+Ni+60Nb+20V) is below 1.4.By Nb is suppressed at below 0.016%, and as the above-mentioned content that limits Cu, Ni, Nb, V comprehensively, can suppress generation as the MA of hard phase, guarantee excellent HAZ toughness.

(Cu+Ni+60Nb+20V)≤1.4…(2)

(Ca：0.0003～0.003％)

Ca will cause dysgenic S to make CaS for HAZ toughness and be fixed, and non-metallic inclusion is carried out morphology Control make it to become granular, be to improve effective elements on the toughness.In order to bring into play such effect fully, Ca is contained more than 0.0003%, more preferably make it to contain more than 0.0010%.Even but containing superfluously, these effects also are saturated, and HAZ toughness deterioration on the contrary.Therefore Ca content is preferably below 0.003%.

Secondly, describe for this first invention.

The present inventor has also found following this point: do not damaging above-mentioned HAZ toughness and mother metal characteristic; tensile strength TS is in the above high tensile steel plate of 530MPa; in order to reach yield strength YS is low yield strength ratio YR such below the 440MPa; as long as among the bainite structure of hard and/or martensitic stucture, soft ferritic phase is existed in right amount, and the particle diameter grain refined of this soft phase is got final product.

Fig. 2 is the graphic representation of the relation of expression ferrite branch rate and yield strength YS (lower yield point YP or 0.2% yield-point σ 0.2), tensile strength TS, the data of having put aftermentioned embodiment in order.By its result as can be known,, can satisfy more than the tensile strength TS:530MPa two specific characters that yield strength YS:440MPa is following by making the scope of ferrite branch rate at 45～85 area %.The preferable range of this ferrite branch rate is 50～80 area %.

Also have, in this first invention, so-called " surplus is bainite structure and/or martensitic stucture ", basically be except that ferrite be bainite and/or martensitic stucture the meaning (promptly, the branch rate of bainite and/or martensitic stucture is 15～55 area %), but there is an aim to be, in manufacturing processed, also can includes other elements (cementite and island martensite body) of inevitable formation.

Fig. 3 is expression ferrite particle diameter and base metal tenacity (60 ℃ pendulum shock absorption energy vE _-60) the graphic representation of relation.By its result as can be known, be below 19 μ by making ferritic median size, can become to reach good base metal tenacity (vE _-60Count more than the 100J).

For steel plate,, just can access the low yield strength ratio high-tensile steel of the low-temperature flexibility excellence of HAZ for example according to the method shown in following by this first invention of above-mentioned such tissue manufacturing.

Be heated to below 1050～1200 ℃ satisfying the former material of steel that aforementioned one-tenth is grouped into, implementing heat pricks to the thickness of slab of regulation, after 880～720 ℃ of end hot rollings, be cooled to 600～700 ℃ (coolings for the first time) with the speed of cooling more than 10 ℃/second, stop cooling and air cooling in the way more than 30 seconds, then be cooled to 550～350 ℃ with the speed of cooling more than 10 ℃/second from this temperature range.The scope of each condition is set and be the reasons are as follows in this method.

(Heating temperature: 1050～1200 ℃)

The former material of steel is heated to more than 1050 ℃ in order to make the casting flaw pressing.On the other hand, if Heating temperature surpasses 1200 ℃, thickization of austenite crystal then, the base metal tenacity deterioration therefore need be with 1050～1200 ℃ of heating.

(hot-rolled temperature: 880～720 ℃)

After being heated to said temperature, begin rollingly,, finishing hot rolling more than 720 ℃ carrying out hot rolling below 880 ℃.When rolling temperature is lower than 880 ℃, by the rolling austenite recrystallization that causes, also can be even perhaps do not make it crystallization in the inner defective that imports deformation bands of austenite crystal, the karyogenesis point of ferrite transformation increases thus, makes and organizes miniaturization, and base metal tenacity improves.In view of the above, rolling need carrying out below 880 ℃.But hot rolling end temp (FRT) is more than 720 ℃.If hot rolling is tied fast temperature and is lower than 720 ℃, then just analyse ferrite and be subject to processing, yield strength YS and yield tensile ratio YR will be risen.

(cooling conditions)

(a) speed of cooling for the first time: more than 10 ℃/second

With said temperature finish rolling after, be cooled to 600～700 ℃ temperature field with the speed of cooling of (preferred more than 15 ℃/second) more than 10 ℃/second, in this way, stop cooling, thereby fine ferrite is separated out from the supercooled austenite.With this temperature (for the first time cooling stop temperature) keep 30 second or more thus ferrite branch rate can be controlled at suitable scope in thereafter.At this moment hold-time, ferrite divided the rate deficiency when being lower than 30 seconds, and cooling stops temperature and is lower than 600 ℃ and surpass 700 ℃, and ferrite branch rate also can reduce.But, if this hold-time surpasses 150 seconds, then become pearlitic structure easily, be below 150 seconds therefore.

(b) speed of cooling for the second time: more than 10 ℃/second

Stop temperature from above-mentioned cooling, be cooled to (cooling for the second time stops temperature) below 550 ℃, second of hard is generated mutually with the speed of cooling more than 10 ℃/second.If speed of cooling is lower than 10 ℃/second, and that cooling stops temperature is higher than 550 ℃, then second tissue that becomes the perlite main body mutually.

After as above-mentioned, being cooled to below 550 ℃, reach room temperature, preferred air cooling (AC) without what special method of cooling.Also can carry out tempering with 500～600 ℃,, can carry out the adjustment of intensity by additional this operation.

Also have, the temperature shown in above-mentioned is that (t: the temperature of position thickness of slab) manages, and t/4 portion is the position as the average performance of performance steel plate to t/4 portion.In addition, the steel plate of this first invention can advantageously be applied as so-called Plate Steel.At this moment thickness of slab is about more than the 7mm, and its upper limit is not particularly limited, but is generally about 40mm.

Secondly, describe for this second invention.

Fatigue cracking is in common stabilized growth zone, and the right angle orientation of prolonging with respect to stress advances.Consider the mechanism of progression of such fatigue cracking, in order to improve resistance for the be full of cracks progress, the imagination that can access is, by making the complex tissue that is organized as of steel, with hard border mutually be full of cracks circuitous (bending) is stopped soft, the be full of cracks advance rate is reduced, thereby can prolong fatigue lifetime.And, in the bending of the be full of cracks in hard phase (hereinafter referred to as " second phase "), need certain rigidity poor.But if the difference of hardness is excessive, then brittle rupture takes place in hard mutually because be full of cracks can proceed to hard mutually in, so its effect reduces on the contrary.From this viewpoint, in steel of this second invention, by in soft phase and the complex tissue that hard constitutes mutually, the Vickers' hardness Hv of hard phase ₁With soft Vickers' hardness Hv mutually ₂Ratio (Hv ₁/ Hv ₂) need be controlled in 1.5～5.0 the scope.

That is, above-mentioned than (Hv by making ₁/ Hv ₂) value be more than 1.5, the plastic region of the soft and hard of dislocation when mobile of be full of cracks front end interface be full of cracks front end mutually changes, and bends, stop, difference, the advance rate that therefore chaps reduces.But, if the hardness of hard phase becomes too high,, make hard under the stress of be full of cracks front end, cause brittle rupture then as above-mentioned, be full of cracks progression inhibiting effect reduces, and therefore need make above-mentioned than (Hv ₁/ Hv ₂) value be below 5.0.The preferred lower limit of this ratio is 1.7, more preferably more than 2.0, is limited to 4.5 on preferred, more preferably below 4.0.In addition, need necessarily to guarantee hard and soft interface mutually abovely, for this reason, need suitably control hard phase and soft ratio mutually.From this viewpoint, the comparative optimization of soft phase is 20～90 area %.Also have, below with above-mentioned than (Hv ₁/ Hv ₂) be called " hardness ratio ".

So-called soft phase in steel of this second invention, can enumerate among ferrite, tempering bainite and the tempered martensite more than a kind, as the hard phase, can enumerate bainite and/or martensite (containing the island martensite body).In addition, the tissue of steel of this second invention, comprise as the soft of first phase with get final product mutually as second mutually the hard, but may not to leave no choice but be two phase constitutions, also can be the complex tissue more than 3 kinds or 4 kinds that comprises each above-mentioned phase.But perlite is that soft ferrite is the tissue that strip exists with the cementite that the hard of brittle rupture easily takes place in microcosmic, because be difficult to obtain above-mentioned effect, so anyly all do not contain in mutually.In addition, soft phase and hard mutually its total be preferably 95 area % above (surplus is " perlite " etc., also can be its hetero-organization).

Be full of cracks progress bends in above-mentioned hard phase/soft phase border, crystal boundary, stop, difference, thereby the be full of cracks advance rate is reduced.If the particle diameter of soft phase becomes thick, then with the hard of the impedance that constitutes the be full of cracks progress mutually/frequency of soft phase border, crystal boundary conflict reduces, the advance rate that therefore chaps can not reduce.In the steel of this second invention, by the manufacture method of showing after using (for example carrying out cold), karyogenesis point increases, and along with the ferrite miniaturization, hard is disperseed mutually imperceptibly.In view of the above, when the be full of cracks progress, with the probability equalization that hard meets with mutually, the frequency of experience rises, and therefore can access the be full of cracks advance rate and reduce such effect.From this viewpoint, in the steel of this second invention, also need the particle diameter of hard phase to count (about particle size determination method aftermentioned) below the 20 μ m with equivalent circle diameter.The particle diameter of this soft phase is preferably below the 15 μ m.

For the steel plate of basis second invention that creates above-mentioned this tissue, according to (1), the method for (2) shown in for example following, just can suitably control hard with mutually soft, obtain fatigue cracking and make progress the steel plate of low-temperature flexibility excellence of resistance and HAZ.

(1) will have the steel billet that above-mentioned this chemical ingredients forms be heated to more than 950 ℃, below 1250 ℃, at (Heating temperature～Ar ₃Transformation temperature) temperature range finishes rolling, carry out the heating cooling first time with the speed of cooling more than 10 ℃/second, guaranteeing that austenite (γ) branch rate is under the state more than the 90 area %, after making γ become supercooled state with 600～700 ℃, keep 30～100 seconds (also can put cold) at this temperature field, implement to quicken for the second time to be cooled to below 400 ℃ with the speed of cooling more than 5 ℃/second thereafter with the speed of cooling below 0.5 ℃/second.The scope of each condition limits and the reasons are as follows in this method.

Heating temperature: when being lower than 950 ℃, rolling temperature is low excessively, can't generate sufficient austenite, and cast structure is remaining, and the possibility of deterioration in characteristics is arranged.On the other hand, if surpass 1250 ℃, thickization of austenite crystal then, the base metal tenacity deterioration therefore need be with 950～1250 ℃ of heating.

Rolling temperature: if rolling temperature is lower than Ar ₃Transformation temperature, then anisotropy takes place in tissue, and shock absorption can might reduce, and rolling load height in this external manufacturing reduces productivity.

Quicken for the first time speed of cooling: by quickening cooling, γ becomes supercooled state, and phase transformation is suppressed until low temperature.Thereafter, by carrying out phase transformation at low temperatures, the motivating force height of phase transformation generates and organizes the fine ferrite of homogeneous.When speed of cooling is lower than 20 ℃/second, in quickening cooling a part of phase transformation, the homogeneous miniaturization that can't reach tissue can take place.

Cooling stops temperature: when stopping temperature and being lower than 600 ℃, ferrite becomes needle-like, and perhaps organizing only becomes bainite or the such hard phase of martensite.Though acicular ferrite toughness is good, hardness is higher with respect to polygon ferrite, reduces with second mutually the difference of hardness, and is therefore little in the be full of cracks progression inhibiting effect on phase border.On the other hand, surpass 700 ℃ if cooling stops speed, then phase transformation is slow under the maintenance temperature of regulation, can not guarantee sufficient ferrite branch rate (for example more than the 20 area %), and it is thick that crystal grain becomes, and makes the toughness deterioration.

The cooled hold-time: this hold-time is when being lower than 30 seconds, and is in a disguised form insufficient, and ferrite divides rate insufficient, and C is concentrated among the unreacted γ and does not remain in addition.In addition, if the hold-time surpasses 100 seconds, then productivity reduces, and near equilibrium state, visible pearlitic generation.Though this perlite is the lamellar structure that ferrite and cementite constitute, cementite is crisp, and at be full of cracks front end generation brittle rupture, the progression inhibiting effect that therefore chaps is little.

For the second time quicken speed of cooling: when this speed of cooling was lower than 5 ℃/second, at cooling stages, the austenite of phase transformation never generated ferrite+perlite, and the hardness of hard phase is insufficient.

Final cooling stops temperature: if the temperature that stops at this moment surpassing 400 ℃, then because self-tempering, hard is softening mutually, can not fully guarantee hardness, and therefore cooling stops temperature and need be preferably below 300 ℃ for below 400 ℃.

(2) can reach in the following way: will have this state steel billet that this chemical ingredients forms be heated to more than 950 ℃, below 1250 ℃, at Heating temperature～Ar ₃After the temperature range end of transformation temperature was rolling, reheat was to Ac ₃The temperature that transformation temperature is above is carried out quench treatment, and reheat arrives (Ac once again thereafter ₃Transformation temperature+30 ℃)～(Ac ₃Transformation temperature-30 ℃) temperature field carries out the quench treatment second time with the speed of cooling more than 5 ℃/second thereafter.Also have, online from Ar ₃More than quicken cooling with the speed of cooling more than 10 ℃/second, to replace Ac ₃The above reheat of transformation temperature quenches, and also can access same sclerosis.

In this method, become quenching structure by making the tissue before the reheat, can make organization unit fine, by reheat to Ac ₁Become high tempering bainite or tempered martensite+austenite structure more than the transformation temperature.Carbide spreads to the reverse transformation austenite from tempering bainite, tempered martensite, tempering bainite, martensitic hardness reduce greatly, and because the cooling of heating thereafter, concentrating has the γ of C to become the hard phase mutually, thereby can become hard phase and soft complex tissue mutually.The scope of each condition limits and the reasons are as follows in this method.

Heating temperature: when being lower than 950 ℃, rolling temperature is low excessively, if surpass 1250 ℃, and thickization of austenite crystal then, the base metal tenacity deterioration therefore need be with 950～1250 ℃ of heating.

Speed of cooling, cooling stop temperature: if speed of cooling is lower than 10 ℃/second, cooling stops temperature and surpasses 400 ℃, and then tissue can't become quenching structure, so particle diameter becomes thick, and toughness and fatigue cracking make progress resistance and all reduce.

Heating temperature: be lower than (Ac ₁Transformation temperature+30 ℃) time, α → γ phase transformation can take place hardly, thereby can not guarantee sufficient hard phase.If surpass (Ac ₃Transformation temperature+30 ℃), then most of behind the reheat α → γ phase transformation takes place, the quenching through thereafter all becomes the hard phase.

For the second time quicken speed of cooling: when this speed of cooling was lower than 5 ℃/second, the hardness of hard phase was insufficient.

Final cooling stops temperature: if the temperature that stops at this moment surpassing 400 ℃, then because self-tempering causes hard softening mutually, can not fully guarantee hardness, therefore needing cooling to stop temperature is below 400 ℃, is preferably below 300 ℃.

Also have, the temperature shown in above-mentioned is that (t: the management that the temperature of position thickness of slab) is carried out, t/4 portion is the position as the average performance of performance steel plate to t/4 portion.In addition, the steel plate of this second invention can advantageously be applied as so-called Plate Steel.At this moment thickness of slab is about more than the 7mm, and its upper limit is not particularly limited, but is generally about 40mm.

Secondly, describe for this 3rd invention.

Though it is good to satisfy the steel plate HAZ toughness that chemical ingredients forms as above-mentioned,, for can not making this HAZ toughness deterioration, and the important document that is used to improve the characteristic of the generation that suppresses brittlement crevasse crack is also studied.Its result shows, steel plate for thick t (mm), observe it and be parallel to rolling direction and when being vertical metal structure with respect to surface of steel plate, if (a) the ferrite area occupation ratio is more than 75%, (b) the leveled circular equivalent diameter of the ferrite crystal grain of t/2 position is below the 20.0 μ m, (c) mean aspect ratio of the ferrite crystal grain of t/4 position is below 2.0, then can improve the characteristic of the generation of the brittlement crevasse crack that suppresses steel plate, and also can not make above-mentioned HAZ toughness deterioration.Below, described in detail for the reason of so stipulating.

The metal structure of high-tensile steel of this 3rd invention, in order to ensure the intensity of steel plate based on ferrite.So-called ferrite main body, the meaning is that ferrite branch rate shared in steel plate is more than the 75 area %, when observing the metal structure in steel plate cross section, ferritic area occupation ratio is to get final product more than 75%.Ferritic area occupation ratio is preferably more than 80%, more preferably more than 85%.

The surplus of above-mentioned metal structure generates perlite, bainite and martensite etc. mutually as second and gets final product, and its kind is not particularly limited.The area occupation ratio of second phase is lower than 25% and gets final product, and preferably is lower than 20%, more preferably less than 15%.

The metal structure of above-mentioned steel plate except based on the ferrite, in order to improve the CTOD characteristic, is importantly suitably adjusted equivalent circle diameter and this two side of long-width ratio of ferrite crystal grain.That is, present inventors carry out various result of experiment repeatedly and distinguish, the leveled circular equivalent diameter that need make the ferrite crystal grain of t/2 position is below the 20.0 μ m, and the mean aspect ratio that makes the ferrite crystal grain of t/4 position is below 2.0.

This is illustrated by embodiment described later, and Fig. 7 represents the leveled circular equivalent diameter of ferrite crystal grain of t/2 position of steel plate and the influence that long-width ratio causes the CTOD characteristic.Among Fig. 7, X-axis is represented the leveled circular equivalent diameter of the ferrite crystal grain of t/2 position, and Y-axis is represented CTOD characteristic (δ c _{-60 ℃}), represents the mean aspect ratio of ferrite crystal grain of t/2 position more than 1.6, and the mean aspect ratio of the ferrite crystal grain of zero expression t/2 position surpasses 1.6 and below 2.0, and △ represents to surpass with mean aspect ratio 2.0 o'clock result.

Fig. 8 represents the leveled circular equivalent diameter of ferrite crystal grain of t/4 position of steel plate and the influence that long-width ratio causes the CTOD characteristic.Among Fig. 8, X-axis is represented the leveled circular equivalent diameter of the ferrite crystal grain of t/4 position, and Y-axis is represented CTOD characteristic (δ c _{-60 ℃}), the mean aspect ratio of the ferrite crystal grain of zero expression t/4 position is below 2.0, ● the mean aspect ratio of the ferrite crystal grain of expression t/4 position surpasses 2.0 o'clock result.

Fig. 9 represents from the relative position of steel plate central part (t/2 position) and CTOD characteristic (δ c _-60 _℃) relation.Among Fig. 9, X-axis represents that steel plate central part (t/2 position) is 0% o'clock a relative position, and for example so-called relative position is 25% expression t/4 position.

Shown as can be known that by these results the leveled circular equivalent diameter of the ferrite crystal grain of (1) t/2 position is directly little, CTOD demonstrates improved tendency (δ c more _{-60 ℃}Numerical value become big tendency), (2) if the leveled circular equivalent diameter of the ferrite crystal grain of t/4 position is below the 20.0 μ m, and mean aspect ratio is below 2.0, then δ c _{-60 ℃}More than 0.20mm, can improve the CTOD characteristic, and (3) because the CTOD characteristic there is the tendency of step-down at the steel plate central part with guaranteeing, get final product or the like so the CTOD characteristic is managed at the steel plate central part.

Consider as follows about the reason that produces this phenomenon.Promptly, in brittle rupture, because the border of crystal grain and crystal grain (crystal grain boundary) becomes the impedance of crevasse crack propagation, if so intensive existence of crystal grain boundary, then brittle rupture itself is difficult to take place, even there is small brittle rupture to take place, if on be full of cracks progress direction the intensive crystal grain boundary that exists, then also can prevent the propagation of chapping., because ferrite extends along rolling direction in rolling process, so the long-width ratio of ferrite crystal grain becomes greatly.Therefore ferritic major diameter is consistent easily on rolling direction, and consistent easily at thickness of slab direction minor axis.

Therefore, though make the intensive existence of crystal grain boundary on the thickness of slab direction, because the crystal grain boundary on the rolling direction becomes sparse, so the density of crystal grain boundary easily fluctuates, brittle rupture takes place easily.With respect to this, reduce the leveled circular equivalent diameter of ferrite crystal grain, and dwindle mean aspect ratio, the fluctuation of the density of grain boundary is almost disappeared, so brittle rupture is difficult to take place, even if taken place, crystal grain boundary also can become impedance, can prevent the propagation of chapping.

In the steel plate of this 3rd invention, the leveled circular equivalent diameter that makes ferrite crystal grain is below the 20.0 μ m, and the mean aspect ratio that makes ferrite crystal grain is below 2.0.The position of control leveled circular equivalent diameter is the t/2 position of the thickness with steel plate when being made as t.Because (with reference to earlier figures 9) takes place in general known brittle rupture near the central part of thickness of slab, so by suitably controlling the generation that just can suppress brittle rupture of organizing of t/2 position.In addition, the position of control mean aspect ratio is thickness with the steel plate t/4 position when being made as t.This selection be the position that shows the average characteristic of steel plate.

Thickness of slab is thick more, the temperature of strain that is imported on the temperature of the t/2 position of steel plate and the t/2 position and the near surface of steel plate (for example t/4 position) and the strain differential that is imported into are big more, therefore by managing the temperature of t/2 position, suitably the tissue of control t/2 position can suppress the generation of brittlement crevasse crack.

The leveled circular equivalent diameter of the ferrite crystal grain of above-mentioned t/2 position is preferably below the 17.5 μ m, more preferably below the 16 μ m.The lower limit of the leveled circular equivalent diameter of ferrite crystal grain does not limit especially, and more little is preferred more, has boundary but dwindle, and therefore is generally 7 μ m above (especially more than 10 μ m).Also have, so-called equivalent circle diameter, the meaning is the circular diameter that ferrite crystal grain is scaled the bowlder of same area.

On the other hand, the mean aspect ratio of the ferrite crystal grain of above-mentioned t/4 position is preferably below 1.9, more preferably below 1.8.Also have, the long-width ratio of so-called ferrite crystal grain, the meaning are that ferrite crystal grain is at the particle diameter (Dl) of rolling direction and the ratio (Dl/Dt) of the particle diameter (Dt) of thickness of slab direction.

The leveled circular equivalent diameter and the mean aspect ratio of above-mentioned ferrite crystal grain for example can be calculated by step as follows.At first, comprise steel plate ground front and back, and make and be parallel to rolling direction and that cutting sample down grinds this and exposes face and carry out mirror finish in this way with respect to vertical the showing out of surface of steel plate (front of steel plate).

The Ginding process that exposes face limits especially, for example adopts the wet type silicon carbide pouncing paper of #150～#1000 to grind, or adopts the Ginding process that has equal function with it to grind to get final product.In addition, when carrying out mirror finish, abrasives such as use diamond lap slurry get final product.

Sample through mirror finish corrodes with 3% nital, the crystal grain boundary of ferritic structure is manifested after, make multiplying power be 100 times or 400 times and take pictures the input picture analytical equipment.The mode of input picture is all to make the zone be equivalent to 1mm * 1mm under any multiplying power.

Secondly, in image analysis apparatus, the zone (area) of the ferrite crystal grain that crystal boundary surrounded is converted into the circle with equal area, the circular diameter that is converted is defined as the equivalent circle diameter of ferrite crystal grain, and measures equivalent circle diameter.In whole field of view it is measured, average result calculates the leveled circular equivalent diameter.

On the other hand, about the long-width ratio of ferrite crystal grain, be the ferrite crystal grain that surrounds for above-mentioned crystal boundary, measure the particle diameter Dl of its rolling direction and the particle diameter Dt of thickness of slab direction, the ratio (Dl/Dt) that calculates Dl and Dt is as long-width ratio.This carries out in whole field of view, and average result calculates mean aspect ratio.

The metal structure of the steel plate of the 3rd invention is the ferrite main body in order to make originally, and the leveled circular equivalent diameter that makes the ferrite crystal grain of t/2 position is below the 20.0 μ m, the mean aspect ratio of the ferrite crystal grain of t/4 position is below 2.0, need only and carry out roughing after the steel billet that casting is obtained is heated to 1000～1200 ℃, then carry out finish rolling and get final product at austenite non-recrystallization temperature field.Below explanation in order.

The temperature of heating steel billet is preferably 1000～1200 ℃.In order to make the ferritic structure miniaturization that (putting cold or Forced water cooling naturally) obtains after roughing and the cooling subsequently, effective means is a rolling austenite structure and make it recrystallize.Though the chemical ingredients of the lower limit basis steel sheet of austenitic recrystallization temperature is formed, because of being generally 850～900 ℃, so at this rolling austenite and make it recrystallize more than lower limit temperature, Heating temperature can be for more than 1000 ℃.But if heating surpasses 1200 ℃, the austenite structure at initial stage thickization too then is even therefore rolling such austenite and make it recrystallize also is difficult to make fully the austenite structure miniaturization.Therefore, Heating temperature is below 1200 ℃.

The steel billet of heating makes the accumulation draft in austenitic recrystallization temperature territory be to carry out roughing more than 40% to get final product.Making the accumulation draft by the recrystallization temperature territory at body difficult to understand is to carry out roughing more than 40%, can under recrystallize and the effect of depressing, make the ferritic structure miniaturization, and become near an austenite structure that waits, as consequently after rolling, become fine and near a ferritic structure that waits.If the accumulation draft under the recrystallization temperature territory is lower than 40%, then the miniaturization that is brought by depressing under the recrystallization temperature territory is insufficient, can be mixed with thick austenite crystal after therefore rolling.Therefore, the metal structure that finally obtains also becomes the mixed granular attitude that is mixed with coarse ferrite grains and fine ferrite crystal grain easily.Mix granular attitude if metal structure so becomes, then the brittlement crevasse crack rejection characteristic has the tendency of easy generation fluctuation.Therefore, in order to make austenite structure miniaturization fully in austenitic recrystallization temperature territory, recommending the accumulation draft in the austenite recrystallization temperature territory is more than 40%.Though the austenite recrystallization temperature territory is formed according to chemical ingredients some variations being arranged, is about 900～1000 ℃ in this 3rd invention.

Preferably increase above-mentioned accumulation draft as far as possible, along with the increase of accumulation draft, the equivalent circle diameter of ferrite crystal grain can miniaturization to about 25～30 μ m.But, surpassing 70% even increase the accumulation draft in the austenitic recrystallization temperature territory, its effect also is saturated substantially, therefore this accumulation draft is to get final product below 70%.

Above-mentioned accumulation draft, the thickness the when temperature (calculated value) of the t/2 position of steel plate is 1000 ℃ is made as t ₀, the thickness the when temperature (calculated value) of the t/2 position of steel plate is 900 ℃ is made as t ₁The time, can calculate by following formula (8).

Accumulation draft (%)=[(t ₀-t ₁)/t ₀] * 100 ... (8)

But roughing begins temperature when being lower than 1000 ℃, and establishing the steel plate thickness of roughing when beginning is t ₀, roughing begins temperature when surpassing 1000 ℃, and the temperature of then establishing the t/2 position of steel plate is t at 1000 ℃ steel plate thickness ₀, and calculate above-mentioned accumulation draft.On the other hand, when roughing stops not reaching 900 ℃ (when surpassing 900 ℃), the steel plate thickness when establishing the roughing termination is t ₁, be t at 900 ℃ steel plate thickness ₁, and calculate above-mentioned accumulation draft.

Temperature during roughing is to use process control computer to calculate the temperature of t/2 position, and gets final product as benchmark with the temperature that calculates.For being in order suitably to control the metal structure of t/2 position.Also have, compare with the temperature (calculated value) of t/2 position, the temperature of surface of steel plate (measured value) is at the thickness of steel plate 50～70 ℃ of about step-downs during for 150mm, at steel plate thickness 40～50 ℃ of about step-downs during for 100mm.Therefore, carrying out the temperature of above-mentioned roughing and consider so temperature head, also can be benchmark employing carrying out temperature treatment with the temperature (measured value) of surface of steel plate.

Austenitic Tc territory make the accumulation draft be 40% carry out roughing after, be cooled to austenite non-recrystallization temperature field, be recommended in this austenite non-recrystallization temperature field and straight strain is reached more than 0.5 and carry out finish rolling.This is owing to by carrying out finish rolling at austenite non-recrystallization temperature field, can make the further miniaturization of ferrite crystal grain.Promptly, be rolled and the metal structure that obtains at austenite non-recrystallization temperature field, it is the austenite structure of about 25～30 μ m of median size, even therefore steel plate is directly carried out air cooling or forces cooling, the leveled circular equivalent particle diameter of the ferrite crystal grain that obtains also only can reach about 25 μ m at most.Therefore, the CTOD characteristic can not be fully improved.With respect to this, if carry out finish rolling, then be imported into strain in the ferrite crystal grain at austenite non-recrystallization temperature field, therefore can make the further miniaturization of ferrite crystal grain.

In this finish rolling, can make the true strain amount is to be rolled more than 0.5.The true strain amount is lower than at 0.5 o'clock, and the miniaturization of ferrite crystal grain is insufficient, can not substantially improve the CTOD characteristic.The true strain amount is many more to be preferred more, if increase then can dwindle ferrite crystal grain.

Also have, above-mentioned so-called austenite non-recrystallization temperature field is even be meant the temperature field people that the rolled sheet material austenite structure can recrystallize yet.This temperature field is formed and how many variations to some extent according to the chemical ingredients of steel plate, but in this 3rd invention, the temperature of the t/2 position that makes at steel plate is that the true strain amount that the zone imported below 850 ℃ is to carry out finish rolling more than 0.5.But if the temperature field of finish rolling is low excessively, then the flat ratio of ferrite crystal grain (being long-width ratio) significantly becomes big easily, so the CTOD characteristic has the tendency of deterioration.Therefore, the finish rolling end temp can be " Ar ₃Transformation temperature+10 ℃ " more than.Ar ₃The temperature of transformation temperature can be calculated by following formula (9) based on the content of chemical ingredients contained in the steel plate.Wherein, [] represents the content (quality %) of each element.

Ar ₃Transformation temperature (℃)=868-369 * [C]+24.6 * [Si]-68.1 * [Mn]-36.1 * [Ni]-20.7 * [Cu]-24.8 * [Cr]+190 * [V] ... (9)

Above-mentioned true strain amount, the thickness the when temperature (calculated value) of the t/2 position of steel plate is 850 ℃ is made as t ₂, the temperature (calculated value) of the t/2 position of steel plate is made as t for the thickness under the finish rolling end temp ₃The time, can calculate by following formula (10).

True strain amount=ln (t ₂/ t ₃) ... (10)

But finish rolling begins temperature when being lower than 850 ℃, and establishing the steel plate thickness of finish rolling when beginning is t ₂, finish rolling begins temperature when surpassing 850 ℃, and the temperature of establishing the t/2 position of steel plate is t at 850 ℃ steel plate thickness ₂Thereby, calculate above-mentioned true strain.On the other hand, the finish rolling end temp does not reach " Ar ₃Transformation temperature+10 ℃ " time (surpass " Ar ₃Transformation temperature+10 ℃ ") time, the finish rolling end temp established than " Ar ₃Transformation temperature+10 ℃ " when low, establish " Ar ₃Transformation temperature+10 ℃ " under steel plate thickness be t ₃And calculate above-mentioned true strain.

Temperature during above-mentioned finish rolling is to adopt process control computer to calculate the temperature of t/2 position respectively, and is benchmark with the temperature that calculates.

Temperature during finish rolling when the thickness of steel plate is made as t (mm), adopts process control computer to calculate the temperature of t/2 position, and is that benchmark gets final product with the temperature that calculates.This is in order suitably to control the metal structure of t/2 position.Also have, when the thickness of steel plate is 40～80mm left and right sides, the temperature head of the surface temperature of steel plate temperature inside (temperature of t/2 position) and steel plate is at most only about 10～40 ℃, therefore consider such temperature head, even with the surface temperature (measured value) of steel plate is that benchmark manages and also do not have what influence (for example " 850 ℃-temperature head ", " Ar ₃Transformation temperature+10 ℃-temperature head ").

After finish rolling finishes, cool off according to conventional methods and get final product.Method of cooling is not specially limited, can air cooling, also can force cooling.At this moment speed of cooling also is not particularly limited, but present inventors confirm, if below 4 ℃/second, impact then can not for the size of ferrite crystal grain.

Secondly, describe for this 4th invention.

Present inventors, for can not making HAZ toughness deterioration, and the important document that is used to improve the brittlement crevasse crack stop performance has also carried out research repeatedly.It found that, when the metal structure of the steel plate of observing thickness t (mm), if the median size apart from the interior ferrite crystal grain in the zone of surface of steel plate t/100 position is below the 25 μ m, then can improve the brittlement crevasse crack stop performance of steel plate, can not make above-mentioned HAZ toughness deterioration yet.

This is illustrated by embodiment described later, particularly in Figure 10, has shown the median size of the ferrite crystal grain in the zone of surface of steel plate t/100 position and the relation of brittlement crevasse crack stop performance (60 ℃ Kca values).According to this Figure 10 as can be known, the median size of the ferrite crystal grain in the zone of the t/100 position of surface of steel plate steel plate is more little, brittlement crevasse crack stop performance can improve more (60 ℃ Kca value is big more).

So,, then can improve the brittlement crevasse crack stop performance, think as follows about this reason if reduce the median size of the ferrite crystal grain of surface of steel plate.That is, the biography of brittlement crevasse crack is turned over and is considered to, because the border of crystal grain and crystal grain (crystal grain boundary) becomes the impedance of be full of cracks, if so intensive existence of crystal grain boundary, then brittle rupture self is difficult to take place, even there is small brittlement crevasse crack to take place, the propagation of be full of cracks also is stopped.Therefore, if make the ferrite crystal grain miniaturization, the then propagation of the brittlement crevasse crack that can stop to take place.

In this 4th invention, be below the 25 μ m by making above-mentioned median size, then (in detail with reference to embodiment) can guarantee that-60 ℃ Kca is 5900N/mm in brittle rupture propagation stopping test ^1.5More than, can improve the brittlement crevasse crack stop performance.Above-mentioned median size is preferably below the 20 μ m.

The median size of above-mentioned ferrite crystal grain can be calculated according to aforesaid method (measuring method of ferritic leveled circular equivalent diameter).Also have, the median size of above-mentioned ferrite crystal grain is to observe in the zone of surface of steel plate t/100 position.If the particle diameter of the ferrite crystal grain in the zone of surface of steel plate t/100 position is suitably controlled, then be not only surface of steel plate portion, and the brittlement crevasse crack stop performance of steel plate integral body increases all, this research by present inventors has obtained confirmation.

Metal structure in the zone of surface of steel plate t/100 position is based on ferrite.So-called ferrite main body, the meaning is basic the same with aforesaid steel plate, but ferritic minute rate also can be more than 50%.The surplus of above-mentioned metal structure generates perlite, bainite and martensite etc. mutually as second and gets final product, and its kind is not particularly limited.The area occupation ratio of second phase is lower than 50% and gets final product, and preferably is lower than 45%, more preferably less than 40%.

For the median size that makes the ferrite crystal grain of surface of steel plate to the zone of t/100 position below 25 μ m, after heating is cast the steel billet that obtains and is carried out roughing, carry out air cooling for the temperature of adjusting finish rolling or force cooling, then in the austenite recrystallization temperature territory, austenite non-recrystallization temperature field or two-phase temperature field, to make the true strain amount be more than 0.5 and carry out finish rolling gets final product.This is owing to being suitable temperature field by the temperature field that makes finish rolling, can make the ferrite crystal grain miniaturization.That is, not carrying out temperature treatment, but after following ordinary method and being rolled, carry out air cooling or force cooling and the metal structure that obtains, is the above ferritic structures of the about at the most 35 μ m of median size, therefore can not substantially improve the brittlement crevasse crack stop performance.With respect to this,, then can further make the ferrite crystal grain miniaturization if carry out finish rolling at suitable temperature field.If particularly carry out finish rolling at the two-phase temperature field, ferrite crystal grain directly is out of shape, therefore straight strain is imported in a large number, can further make the ferrite crystal grain miniaturization.

Making the true strain amount in finish rolling is more than 0.5, is that the miniaturization of ferrite crystal grain is insufficient, can not substantially improve the brittlement crevasse crack stop performance because the true strain amount is lower than at 0.5 o'clock.The true strain amount is many more to be preferred more, if increase then ferrite crystal grain diminishes.

The temperature field that carries out above-mentioned finish rolling is formed according to the chemical ingredients of steel plate some variations.Therefore in this 4th invention, be that to make the true strain amount of importing be to carry out finish rolling more than 0.5 for zone below 900 ℃ preferably in the surface temperature of steel plate.But if the temperature of finish rolling is low excessively, then the processing embrittlement of ferritic structure is remarkable, and the brittlement crevasse crack stop performance has the tendency of reduction.Therefore, the finish rolling end temp can be " Ar ₃Transformation temperature-40 ℃ " more than.Ar ₃The temperature of transformation temperature can be calculated by aforementioned formula (4) based on the content of chemical ingredients contained in the steel plate.Said temperature gets final product in the temperature of above-mentioned scope control apart from surface of steel plate t/100 position.

Above-mentioned true strain amount is that 900 ℃ steel billet thickness is made as t with the surface temperature of steel plate ₄, the steel plate thickness under the finish rolling end temp is made as t ₅The time, can calculate by following formula (11).

True strain amount=ln (t ₄/ t ₅) ... (11)

But finish rolling begins temperature when being lower than 900 ℃, and establishing the steel plate thickness of finish rolling when beginning is t ₄And calculate above-mentioned true strain amount.Also have, finish rolling begins temperature when surpassing 900 ℃, and the thickness that the surface of establishing steel plate is crossed at 900 ℃ steel billet is t ₄

After finish rolling finishes, cool off according to ordinary method and to get final product.Method of cooling is not specially limited, can air cooling, also can force cooling.

The various steel plates of third and fourth invention of basis that as above-mentioned, obtains, for example can use as the works of bridge, high-rise and boats and ships etc., much less little～middle heat energy input welding, even in high heat energy input welding (for example more than the 40kJ/mm), also can prevent the toughness deterioration of welding heat affected zone, and brittlement crevasse crack rejection characteristic or brittlement crevasse crack stop performance are also excellent.In addition, the steel plate of this third and fourth invention can advantageously be applied as so-called Plate Steel.At this moment thickness of slab is about more than the 7mm, though the upper limit is not particularly limited, is generally the following degree of 40mm.

Below, enumerate embodiment and be described more specifically the present invention, but the present invention is not subjected to the restriction of following embodiment certainly, also can in the scope that meets the forward and backward aim of stating, suitably be changed enforcement, these all are included in the technical scope of the present invention.

[embodiment 1]

Displaying is about this first inventive embodiment.

The steel billet that chemical ingredients shown in the following table 1,2 is formed is heated to 1050～1200 ℃, enforcement is hot-rolled down to the thickness of slab (12mm or 30mm) of regulation, after 880～720 ℃ temperature ranges finish hot rolling, be cooled to 700～600 ℃ with the speed of cooling more than 10 ℃/second, stop cooling in this way, air cooling is more than 30 seconds.From this temperature range with speed of cooling 10 ℃/second or more be cooled to 550 ℃ below (cooling stop temperature) thereafter.Creating conditions at this moment is presented in the table 3,4.

For each steel plate that obtains in the above described manner, the main points of pressing are respectively implemented mother metal tissue (ferrite branch rate, ferrite particle diameter), mother metal characteristic [thickness of slab, yield strength TS, toughness (vE _-60)] and HAZ toughness (vE _-60) evaluation.

(ferrite divides rate, the mensuration of ferrite particle diameter)

The branch rate of ferrite (polygon ferrite), be the (t: thickness of slab) of t/4 portion for each steel plate, observe 1 visual field with opticmicroscope with 200 times multiplying power: the zone of 300 μ m * 300 μ m, use image analysis software to measure, try to achieve the mean value in 5 visuals field.In addition, ferritic average crystal grain diameter be at each steel plate t/4 (t: position thickness of slab), observe 10 visuals field with 400 times, measure according to the relative method of JIS G 0551 defined.

(evaluation of mother metal characteristic)

From the total thickness of each steel plate, along extracting the 1B test film of JIS Z 2201, carry out tension test by the main points of JIS Z 2241 with the rectangular direction of rolling direction, (being lower yield point YP when yield-point is arranged, is 0.2% yield-point σ when not having to measure yield strength YS _0.2) and tensile strength (TS).Then, yield strength: below the 440MPa, tensile strength: more than the 530MPa, yield tensile ratio (YP/TS) is to be evaluated as the low yield strength ratio high-tensile steel below 75%.

In addition, from the position of having pruned 1mm by the face side of each steel plate, the V nick-break test sheet along rolling direction extraction JIS Z2202 carries out pendulum impact test by the main points of JIS Z 2242, and the determination test temperature is that-60 ℃ absorption can (vE _-60).Then, this absorbs energy (vE _-60) be that above being evaluated as of 100J has excellent base metal tenacity.

(evaluation of HAZ flexible)

Adopt above-mentioned steel plate to implement the single face submerged arc welding with the FCB method.The FCB method be at liner plate on copper coin and apply soldering flux, be crimped onto the groove back side, one-sided formation back bead limit makes the method completely of welding from the surface on the limit, generally is useful in the plate joint welding of shipbuilding etc.The groove shape is presented among Fig. 5 and [(a) is the situation of thickness of slab 12mm, (b) is the situation of thickness of slab 20mm].Welding material uses following Cryogenic Steel welding material (Kobe Steel is made), makes welding joint with the welding conditions of Fig. 6 and table 5.

＜welding material 〉

Welding wire: US-255

Surface welding-assistant: PFI-50LT

Liner plate soldering flux: MF-1R

[table 5]

Then,,,, extract the V nick-break test sheet of 3 such JIS Z 2202 respectively, carry out pendulum impact test by the main points of JIS Z 2242 at plate Surface Vertical incision otch in the position of HAZ (fusion portion, fusion portion+1mm[HAZ1mm]) from face side cutting 1mm.Then, the determination test temperature is-60 ℃ of absorption energy (vE down _-60).Then, this absorbs energy (vE _-60) its absorb can mean value be the above low-temperature flexibility excellence that is evaluated as HAZ of 100J.

These results are presented in the following table 6,7 in the lump with actual welding execution conditions (construction method, input heat energy).

Can carry out following investigation (also having the experiment No. during following No. represents) by these results.

Satisfy the steel plate of No.1～17 of this first invention regulation important document, the low-temperature flexibility excellence of HAZ, and be the mother metal characteristic (toughness, below the yield strength YS:440MPa, more than the tensile strength TS:530MPa, yield tensile ratio YR:75% is following) also excellent high-tensile steel, even weld this steel plate, and when being used for the purposes of cold condition, still given play to excellent characteristic with high heat energy input single face submerged arc welding method.

With respect to this, No.18～40 of the regulation of discontented unabridged version first invention have following problem respectively.That is, though the low-temperature flexibility excellence of No.18～20 HAZ, the ferrite of mother metal divides rate low, the mother metal characteristic that can't obtain to wish (below the yield strength YS:440MPa, yield tensile ratio YR:75% is following).

Its C content of No.21 surpasses the upper limit, and its Si content of No.22 surpasses the upper limit, so the HAZ poor toughness.In No.22, ferrite crystal grain is also big in addition, and base metal tenacity is also poor.In addition, the ferrite of any mother metal divides rate low, all can not get the mother metal characteristic (below the yield strength YS:440MPa, more than the tensile strength TS:530MPa, yield tensile ratio YR:75% is following) of wishing.

No.23 is because Mn contains quantity not sufficient, so the HAZ poor toughness.The mother metal characteristic that can not get in addition wishing (more than the tensile strength TS:530MPa, yield tensile ratio YR:75% is following).On the other hand, No.24 is because of Mn content surplus, so can not guarantee excellent HAZ toughness.

No.25 is because of P content surplus, and No.26 is because of S content surplus, so HAZ toughness inequality in addition.In addition, as long as the ferrite of mother metal divides rate low, all can not get the mother metal characteristic (below the yield strength YS:440MPa, yield tensile ratio YR:75% is following) of wishing.

No.27 is because Al contains quantity not sufficient, and No.28 is because of Al content surplus, so the HAZ poor toughness.No.29 is because Nb contains quantity not sufficient in addition, and No.30 is because of Nb content surplus, so HAZ toughness inequality.Wherein among No.29, ferrite crystal grain is also big, the base metal tenacity deterioration.

No.31 is because Ti contains quantity not sufficient, and No.32 is because of Ti content surplus, so the HAZ poor toughness.No.33 is because B contains quantity not sufficient, and No.34 is because of B content surplus, so HAZ toughness inequality.No.35 is because N contains quantity not sufficient in addition, and No.36 is because of N content surplus, so HAZ toughness inequality on the other hand.

No.37 is because (B-NT/1.3) be higher than the lower limit of formula (1), and No.38 is because (B-NT/1.3) be lower than the upper limit of formula (1), so HAZ toughness inequality in addition.

Though No.39,40 contain among Cu, Ni and the V more than a kind, because be higher than the upper limit of formula (2), so the HAZ poor toughness.

[embodiment 2]

Show this second inventive embodiment.

With the former material of steel that the chemical ingredients shown in the following table 8 of converter melting, 9 is formed, make various steel plates by continuous casting and hot rolling.Transformation temperature (Ar shown in the table 8,9 ₃, Ar ₁, Ac ₁, Ac ₃) be value according to following formula (3)～try to achieve (6).Creating conditions at this moment is presented in the table 10,11.Also having, about temperature at this moment, is to manage under the temperature of the position of t/4 (t is a thickness of slab), and the step of detailed temperature treatment is as follows.

Ar ₃＝868-369·[C]+24.6·[Si]-68.1·[Mn]-36.1·[Ni]-20.7·[Cu]-24.8·[Cr]+190×[V] …(3)

Ar ₁＝630.5+51.6·[C]+122.4·[Si]-64.8·[Mn] …(4)

Ac ₁＝723-14·[Mn]+22·[Si]-14.4·[Ni]+23.3·[Cr]…(5)

Ac ₃＝908-223.7·[C]+43.85·[P]+30.49·[Si]-34.3·[Mn]+37.92·[V]-23.5·[Ni] …(6)

Wherein, [C], [Si], [Mn], [N], [Cu], [Cr], [V] and [P] represent the content (quality %) of C, Si, Mn, Ni, Cu, Cr, V and P respectively.

(step of temperature treatment)

1. use control computer, based on the atmosphere temperature and the time inside furnace that begin from heating to finish to heating, calculating is from the Heating temperature of the position arbitrarily (for example t/4 position) of surface to the inside of steel billet.

2. adopt the Heating temperature that calculates, based on the data of pass sequence in rolling and the method for cooling between mill train (water-cooled or air cooling), adopt Calculation Method such as being suitable for method of finite difference, the rolling temperature of the position arbitrarily of thickness of slab direction is calculated on the limit, and the limit is implemented rolling.

3. the surface temperature of steel plate uses the radial pattern thermometer that is arranged on the rolling line to survey.But, also with process control computer theory of computation value in advance.

4. when roughing being begun, when roughing finishes, the surface temperature of the steel plate of surveying respectively when finish rolling begins, contrast with the accounting temperature of calculating by process control computer.

5. the difference of accounting temperature and observed temperature is more than ± 30 ℃ the time, make the gauging surface temperature consistent with observed temperature, calculate so again and as the accounting temperature on the process control computer, when be lower than ± 30 ℃ the time, then directly adopt the accounting temperature that calculates by process control computer.

6. the accounting temperature that adopts aforementioned calculation to go out is managed the rolling temperature as the zone of controlled member.

For each steel plate that obtains in the above described manner, implement the hardness ratio (Hv of fatigue cracking advance rate, (hard phase/soft phase) respectively by following main points ₁/ Hv ₂) with the flexible evaluation of soft particle diameter, steel plate (mother metal) and HAZ mutually.

(fatigue cracking advance rate)

Cut off hot rolling material,, adopt the small-scale test sheet, implement the test of fatigue cracking progress, try to achieve the fatigue cracking advance rate thus according to ASTM E647.At this moment, the stabilized growth zone that will set up by the PARIS rule of following formula (7) regulation

Value estimate as typical value.Also have, the evaluation of fatigue cracking advance rate is about benchmark, because common steel plate is 4.0～6.0 * 10 ^-5Advance rate about mm/cycle (during Δ K=20) is therefore with 3.5 * 10 ^-5Mm/cycle is following to be benchmark.

da/dn＝C(ΔK) ^m …(7)

Wherein, a: expression be full of cracks length; N: the expression number of occurrence; C, m: the constant that expression is determined by conditions such as material, load.

[hardness ratio of (hard phase/soft phase)]

Measure the Vickers' hardness Hv of hard phase with the micro Vickers of 10gf ₁With soft Vickers' hardness Hv mutually ₂, try to achieve each mean value of 5, calculate hardness ratio (Hv ₁/ Hv ₂).

(measuring method of the particle diameter of soft phase)

(a) rolling direction and the mean direction at steel plate cuts off, and prepares to comprise the table of thickness of slab, the sample of back side portion.

(b) adopt the wet type silicon carbide pouncing paper of #150～#1000 to grind, or adopt the Ginding process that has equal function with it to grind, use grinding material enforcement mirror finish such as diamond lap slurry.

(c) will corrode with 3% nital (corrosive fluid) through the sample that grinds, the crystal grain boundary of soft phase is manifested.

(d) with the multiplying power of 100 times or 400 times the tissue that shows is taken pictures by (photo of taking 6cm * 8cm) input picture analytical equipment (100 times time be equivalent to 600 μ m * 800 μ m, be equivalent to 150 μ m * 200 μ m in the time of 400 times).When carrying out this input, under any multiplying power, all import the number (100 times is 6 visuals field at least, and be 35 visuals field) that is equivalent to 1mm * 1mm at 400 o'clock.

(e) in image analysis apparatus, be converted into the circle that has equal area with a crystal boundary institute area surrounded, the circular diameter that is converted is defined as the particle diameter of the soft phase of round equivalent.

(f) mean value of the value that will measure for the whole visuals field is as the soft phase particle diameter of leveled circular equivalent and calculate.

(evaluation of base metal tenacity)

Estimate by the method identical with embodiment 1.

The measurement result of fatigue test advance rate, hardness ratio and soft particle diameter mutually (equivalent circle diameter) are presented in the following table 12,13 with base metal tenacity, HAZ toughness and welding conditions (construction method, input heat energy).

Satisfy No.1～8,9～14 of the important document of this second invention regulation, 18～22 steel plate, it is the low-temperature flexibility excellence of HAZ, and the steel plate that mother metal characteristic (toughness, fatigue cracking progression inhibiting characteristic) is also excellent, weld this steel plate with high heat energy input single face submerged arc welding method, and when being used for the purposes of cold condition, still given play to excellent characteristic.

With respect to this, the No.9,15～17,23～42 of the regulation of discontented unabridged version second invention has following problem respectively.That is, No.9 is the single phase structure of tempering bainite, and No.17 is the single phase structure of bainite, fatigue cracking progression inhibiting characteristic inequality.Its hardness ratio of No.15 is 1.3, and its soft phase particle diameter of No.16 is thick, fatigue cracking progression inhibiting characteristic inequality.

No.23 is because the C amount surpasses the upper limit, and No.24 is because Si surpasses the upper limit, so the HAZ poor toughness in addition.No.25 is because of the Mn quantity not sufficient, thus the HAZ poor toughness, and the value of the discontented unabridged version second invention defined of hardness ratio, so fatigue cracking progression inhibiting characteristic is poor.No.26 is because the Mn amount is superfluous, so can not guarantee excellent HAZ toughness.

No.27 is because the P amount is superfluous, and No.28 is because the S amount is superfluous, so HAZ toughness inequality in addition.No.29 is because of the Al quantity not sufficient, and No.30 is because the Al amount is superfluous, so the HAZ poor toughness.In addition, No.31 is because of the Nb quantity not sufficient, and No.32 is because the Nb amount is superfluous, so HAZ toughness inequality.

No.33 is because of the Ti quantity not sufficient, and No.34 is because the Ti amount is superfluous, so the HAZ poor toughness.No.35 is because of the B quantity not sufficient, and No.36 is because the B amount is superfluous, so HAZ toughness inequality.No.37 is because of the N quantity not sufficient in addition, and No.38 is because the N amount is superfluous, so HAZ toughness inequality on the other hand.

No.39 is because (B-NT/1.3) be lower than the lower limit of formula (1), and No.40 is because (B-NT/1.3) be higher than the upper limit of formula (1), so HAZ toughness inequality in addition.

Though No.41,42 contain among Cu, Ni and the V more than a kind, because be higher than the upper limit of formula (2), so the HAZ poor toughness.

[embodiment 3-1]

Show this 3rd inventive embodiment.

Various steel billet [the Ar that use the chemical ingredients shown in the following table 14,15 to form ₃Transformation temperature is calculated based on aforementioned formula (9)], make various steel plates with creating conditions shown in the following table 16,17 (billet heating temperature, roughing condition, finish rolling condition).Also having, about temperature at this moment, is to manage under the temperature of t/2 position, t/4 (t is a thickness of slab), and the step of detailed temperature treatment is as follows.

(step of temperature treatment)

Identical with the step among the embodiment 2.

[table 16]

[table 17]

For each steel plate that obtains in the above described manner, implement steel plate (mother metal) and the evaluation of HAZ flexible by following main points respectively.

(evaluation of base metal tenacity)

Identical with the evaluation method of embodiment 2.

(evaluation of HAZ flexible)

Identical with the evaluation method of embodiment 1.

In addition, press the observation (equivalent circle diameter of ferrite crystal grain and long-width ratio) and the brittlement crevasse crack rejection characteristic of the metal structure of following each steel plate of step measurements.

[observation of metal structure (determination step of equivalent circle diameter and long-width ratio)]

Comprise steel plate ground front and back, and make and be parallel to rolling direction and that cutting sample down grinds this and exposes face and carry out mirror finish in this way with respect to vertical the showing out of surface of steel plate (front of steel plate).In the grinding of exposing face, after the wet type silicon carbide pouncing paper of use #150～#1000 grinds, use the diamond lap slurry to carry out mirror finish as abrasive.

Sample through mirror finish corrodes with 3% nital, the crystal grain boundary of ferritic structure is manifested after, take with 400 times multiplying power, as the photo of 6cm * 8cm (that is, 400 times time be equivalent to 150 μ m * 200 μ m).The corresponding thickness of slab direction in the limit of the 6cm of photo, the corresponding rolling direction in the limit of 8cm.The mode that is input to image analysis apparatus is all to make the zone be equivalent to 1mm * 1mm under any multiplying power.

Also have, the locating of the equivalent circle diameter of ferrite crystal grain and long-width ratio is thickness with steel plate when being made as t (mm), is t/2 position, t/4 position.In addition, the field of view number is 35.When the leveled circular equivalent diameter of calculating ferrite crystal grain and long-width ratio, also measure the shared area occupation ratio of ferrite in the metal structure in the lump.

(evaluation of brittlement crevasse crack rejection characteristic)

The brittle rupture occurrence features, be based on " the front opening displacement (CTOD test) of chapping " of WES1108 (formulations on the February 1 nineteen ninety-five) defined that Corporation Japan welding association (WES) takes place and the front opening racking test that chaps, measure the unstable aperture displacement (δ c) that destroys when beginning, the result estimates in view of the above.Also have, when chapping the front opening racking test, also with reference to " about the guide of the welding heat affected k CTOD of portion test method " of WES1109 (formulation on April 1 nineteen ninety-five) defined.

Test film adopts P.6 " the standard three point bending test sheet " shown in Figure 6 of WES1108 (formulation on February 1 nineteen ninety-five).Test temperature is-60 ℃, measures δ c _{-60 ℃}(mm).This first bright in, δ c _- _{60 ℃}For 0.20mm is qualified when above.

The t/2 position of each steel plate or the tissue of t/4 position [the leveled circular equivalent diameter and the long-width ratio of ferrite (α)] and ferritic minute rate are presented in the following table 18 mother metal characteristic (thickness of slab, vE _-60With δ c _{-60 ℃}) and HAZ toughness be presented in the following table 19,20 with actual welding execution conditions (construction method, input heat energy).

[table 18]

[table 19]

[table 20]

Can carry out following investigation (also having the experiment No. during following No. represents) according to these results.

Satisfy the No.3,5,6,12,14,15,18,20 of the important document of this 3rd invention regulation, 21 steel plate, it is the low-temperature flexibility excellence of HAZ, and the steel plate that mother metal characteristic (toughness, brittlement crevasse crack generation rejection characteristic) is also excellent, weld this steel plate with high heat energy input single face submerged arc welding method, and when being used for the purposes of cold condition, still given play to excellent characteristic.

With respect to this, the No.1,2,4,7～11,13,16,17,19,22～41 of the regulation of discontented unabridged version the 3rd invention, a certain important document in the important document of discontented unabridged version the 3rd invention regulation, having at least of HAZ toughness, brittlement crevasse crack generation rejection characteristic is a kind of poor.

[embodiment 3-2]

Show this 4th inventive embodiment.

With the former material of each steel of the composition of the chemical ingredients shown in the following table 14 of converter melting, 15, adopt the various steel billets of making by continuous casting (steel grade No.1～38), carry out roughing after the heating, carry out finish rolling after air cooling or the pressure cooling, make various steel plates.The end temp of finish rolling (surface temperature), the cooling conditions (method of cooling, speed of cooling) after rolling and be presented in the following table 21,22 in the true strain amount below 900 ℃.Also have, the speed of cooling shown in the table 21,22 is to begin mean value (average cooling rate) to 500 ℃ from cooling.

[table 21]

[table 22]

For each steel plate that obtains in the above described manner, carry out the evaluation of HAZ flexible equally with embodiment 3-1, and estimate the brittlement crevasse crack stop performance in the following manner.

(evaluation of brittlement crevasse crack stop performance)

The steel grade that the brittlement crevasse crack stop performance takes place based on Corporation's Japan's welding association (WES) assert that test method (formulation on March 31st, 2003) defined " brittle rupture is propagated and stopped test " carries out.Use brittle rupture to propagate the test film of the shape shown in Figure 7 that stops test method, in the temperature range of selecting from-190 ℃～+ 60 ℃ scope arbitrarily,, divide 4 test bodies to carry out this test film design temperature gradient.By following formula (12) calculating K ca value.In the following formula (12), c represents the enter the mouth length of brittlement crevasse crack front end from propagation portion, and T represents the temperature (unit K) of brittlement crevasse crack front end, and σ represents the total stress approach of shear strength of propagation portion, and W represents propagation portion width.X-axis is 1/T, and Y-axis is the Kca value that calculates, and make to show the graphic representation of the correlationship of 1/T and Kca value, with the intersection point of 4 curve of approximation and the 213K Kca value as-60 ℃.In this 4th invention ,-60 ℃ Kca value is 5900N/mm ^1.5The time be qualified (brittlement crevasse crack stop performance excellence).

Kca = \sqrt{2 W \tan (πc / 2 W)} - - - (12)

In addition, the observation of the metal structure of each steel plate (to the t/100 position, follow the step shown in the embodiment 1 from skin section, also measure) for ferritic median size.When calculating the median size of ferrite crystal grain, observe the metal structure the zone from surface of steel plate to the t/100 position, also measure the ferrite area occupation ratio simultaneously.Consequently, shared ferritic area occupation ratio is more than 50% in the metal structure.Also have, when multiplying power was 100 times, the field of view number was at least 6, and the field of view number is at least 35 in the time of 400 times.

These results are presented in the following table 23,24.Also have, the example that satisfies HAZ toughness and this two side's of brittlement crevasse crack stop performance characteristic is example of the present invention, and synthetic determination is (zero), and the example that does not satisfy a side characteristic at least is a comparative example, and synthetic determination is (*).

[table 23]

[table 24]

Can carry out following investigation by these results.The important document of this 4th invention regulation is satisfied in No.42～46,48～50,52,54,56～58,60～66,68,70～73, it is the low-temperature flexibility excellence of HAZ, and the steel plate that mother metal characteristic (brittlement crevasse crack stop performance) is also excellent, weld this steel plate with high heat energy input single face submerged arc welding method, and when being used for the purposes of cold condition, still given play to excellent characteristic.

With respect to this, the No.47,51,53,55,59,67,69,74～93 of the important document of discontented unabridged version the 4th invention regulation, any one important document of the important document of discontented unabridged version the 4th invention regulation, at least one characteristic of HAZ toughness, brittlement crevasse crack stop performance is poor.

The median size of the ferrite crystal grain in the zone of surface of steel plate t/100 position and the relation of brittlement crevasse crack stop performance are presented among Figure 11, if but the true strain amount is controlled at more than 0.5, then the median size of the ferrite crystal grain of surface of steel plate to the zone of t/100 position can be controlled at below the 25 μ m as can be known.

Claims

1. steel, it is characterized in that, contain C:0.03～0.09% in quality %, Si:0.01～0.25%, Mn:1.20～1.60%, below the P:0.010%, below the S:0.003%, Al:0.02～0.04%, Nb:0.005～0.016%, B:0.0006～0.0020%, N:0.0045～0.0090%, Ti:0.008～0.020%, and satisfy following formula (1), surplus is iron and unavoidable impurities, and observing the parallel of thickness t (mm) and during with the metal structure of the perpendicular face of steel surface with the Steel Rolling direction, satisfy following (a)～(c)

(a) the ferrite area occupation ratio is more than 75%,

(c) mean aspect ratio of the ferrite crystal grain of t/4 position is below 2.0,

-20≤(B-NT/1.3)≤10…(1)

In the formula, B represents the content of B in quality ppm,

In addition, when the pass of N and Ti be

(N-Ti/3.4) 〉=0 o'clock, NT=(N-Ti/3.4)

(N-Ti/3.4)＜0 o'clock, NT=0

Wherein, N represents the content of N in quality ppm, and Ti represents the content of Ti in quality ppm.

2. steel, it is characterized in that, contain C:0.03～0.09% in quality %, Si:0.01～0.25%, Mn:1.20～1.60%, below the P:0.010%, below the S:0.003%, Al:0.02～0.04%, Nb:0.005～0.016%, B:0.0006～0.0020%, N:0.0045～0.0090%, Ti:0.008～0.020%, and satisfy following formula (1), surplus is iron and unavoidable impurities, and when observing the steel metal structure of thickness t (mm), the median size of the ferrite crystal grain the zone from steel surface to the t/100 position is below the 25 μ m

-20≤(B-NT/1.3)≤10…(1)

In the formula, B represents the content of B in quality ppm,

In addition, when the pass of N and Ti be

(N-Ti/3.4) 〉=0 o'clock, NT=(N-Ti/3.4)

(N-Ti/3.4)＜0 o'clock, NT=0

3. steel according to claim 1 and 2 is characterized in that, also in the mode that satisfies following formula (2) with quality % contain select in Cu:0.03～0.5%, Ni:0.03～0.8% and V:0.003～0.05% more than a kind,

(Cu+Ni+60Nb+20V)≤1.4…(2)

In the formula, Cu, Ni, Nb, V represent the mass percentage content of each element.

4. steel according to claim 1 and 2 is characterized in that, also contain Ca:0.0003～0.003%.