CN101665888B - Thick steel board - Google Patents

Abstract

The invention provides a thick steel board excellent in brittle crack propagation-arresting property without causing increase in production cost. Even if the thickness of the steel board exceeds 50mm, the board still meets the requirement of high strength (tensile strength is over 490MPa), and Kca value under the temperature of -10 DEG C. is over 3500N/mm[3/2]. By appropriately modulating composition of chemical components, the thick steel board has an average area ratio of the quasi-polygonal ferrite of 30-85% in the microscopic constitution at the position t/4-t/2 deep from the surface, wherein t represents the thickness of the board.

Description

Plate Steel

Technical field

The present invention relates to mainly to be used as the Plate Steel of former material of the structured material of boats and ships and bridge, the propagation that particularly relates to the brittlement crevasse crack that will take place is stopped the Plate Steel that this characteristic (suppressing (arrest) characteristic) is improved.

Background technology

In the employed Plate Steels of works such as boats and ships, buildings, oil tank, marine structure, line pipe, in order to suppress the brittle rupture of works, require it to have rejection characteristic (hereinafter referred to as " brittlement crevasse crack propagation halt characteristic "), this characteristic is to suppress the destructive ability that the propagation owing to brittlement crevasse crack causes.In recent years, along with the maximization of works, use yielding stress as more than the 390MPa, thickness of slab is that the situation of the above high-strength steel plate of 50mm exists in a large number.But, above-mentioned such brittlement crevasse crack propagation halt characteristic, to its guarantee generally can be along with the high strength of steel plate, wall thickening and the difficulty that becomes.

On the other hand, in container-ship, advancing maximization for efficient activity, the consequent is to use heavy wall, high-intensity steel plate.If consider the breach security of hull, then primary is that brittle rupture is taken place, if but brittle rupture when having taken place for fear of comprehensive damage of hull, makes hull enbrittle crevasse crack propagation halt characteristic so that the propagation of be full of cracks stops particularly important.From this background, in upperdeck portion the brittlement crevasse crack that takes place from the hatch coaming board is stopped with regard to requiring.For brittlement crevasse crack is stopped, about the desired brittlement crevasse crack propagation halt characteristic of upperdeck portion, up to now also possess some special knowledge, even think that bearing strength test and brittlement crevasse crack progress length becomes big, and the stress sampling factor K value of slab steel plate is saturated, if the Kca value under-10 ℃ (as the numerical value of the index of brittlement crevasse crack propagation halt characteristic) is 3500N/mm ^3/2(MPamm ^1/2) about, brittlement crevasse crack is stopped.Therefore, particularly expect to have a kind of technology in container-ship, it can give above-mentioned brittlement crevasse crack propagation halt characteristic to high-strength steel plate.

In addition, above-mentioned Kca value also expects to access higher value (4500N/mm for example ^3/2More than, 5000N/mm ^3/2More than).

In addition, in recent years among container-ship etc., the fatigue that causes with repeated stress load is reason, the advance rate of the be full of cracks that takes place becomes problem, therefore except above-mentioned brittle rupture stop performance, can make the advance rate of the fatigue cracking that takes place delay such characteristic (hereinafter referred to as " fatigue cracking progression inhibiting characteristic ") as far as possible in case also require it to have.About fatigue cracking progression inhibiting characteristic, be to require specifically at specific stress $(\mathrm{\ΔK}=10\mathrm{MPa}\·\sqrt{\mathrm{}}m)$ Under the environment, the expression mechanical load exponential da/dn of round-robin progressivity (fatigue cracking advance rate) each time is 5.0 * 10 ^{- 6}Below the mm/cycle.

As the method that brittlement crevasse crack propagation halt characteristic is improved, the known method that has (a) to add alloying element; (b) make the method etc. of crystal grain diameter miniaturization.Wherein as the method for adding alloying element, for example propose to have the such technology of patent documentation 1.In this technology, contain Ni as alloying element, make the particle diameter miniaturization of bainite by the speed of cooling that suppresses in the process of cooling, thereby brittlement crevasse crack propagation halt characteristic is improved.But in this technology, also can cause the cost increase owing to adding alloying element.

On the other hand, as the method by making the crystal grain diameter miniaturization that brittlement crevasse crack propagation halt characteristic is improved, known have a such technology of patent documentation for example 2,3.In these technology, be parent phase with the ferrite, and make this ferritic particle diameter miniaturization, guarantee good brittlement crevasse crack propagation halt characteristic thus.But, in these technology, because be parent phase, so the application difficult on high-strength steel plate with soft ferrite.

In addition, as the technology of improving brittlement crevasse crack progression inhibiting characteristic, up to now also propose to have multiple, for example in patent documentation 4, propose to have a kind of steel plate of fatigue cracking progression inhibiting excellent, it is realized in the following way: when the normal direction of establishing surface of steel plate is ND, have (100) face of α iron orientation parallel with ND (100) //crystal grain of ND}, the orientation parallel with ND with (111) face with α iron (111) //boundary of the intergranule of ND} crosses more than 1 place at 30 μ m at least along the progress direction of be full of cracks, and at the mensuration face parallel with surface of steel plate, the ratio of the α of steel plate inside (111) face strength ratio and α (100) face strength ratio is 1.25～2.0.

At the heavily stressed steel plate that uses down, concern for fatigue characteristic is high more, but aforementioned techniques is because based on ferrite (for example more than the 70 area %), so particularly there is the problem that can not be suitable in the intensity level about can only corresponding 390～490MPa in the part that fatigue cracking constitutes problem.

In addition in aforementioned techniques,, be disclosed in the low-temperature region or the α temperature field that make ferrite separate out above γ-α two phase regions of 70 area % and force the worker for crystallization control orientation so as described above.For such ferritic structure, in the tissue (being referred to as " only this tissue of Bei Shi ") based on bainite, known and austenite have certain position relation and generate, by with the same method of aforementioned techniques can not the crystallization control orientation.

In patent documentation 5, proposition has a kind of steel plate of fatigue cracking progress excellent, it is under bainite structure or martensitic stucture, apply maximum tension, stress under compression is ± 0.012, toggle speed 0.5Hz, reach the increasing progressively to successively decrease and repeat load when being 15 times, the stress σ during by 1 maximum strain of wave number 12 of maximum strain ₁Stress σ during with 15 times maximum strain ₁₅Ratio σ ₁/ σ ₁₅Represented softening repeatedly parameter is more than 0.65, below 0.95.And in this technology, also show since the dislocation of be full of cracks front end move, disappear and bring softening, make that strain obtains relaxing, the be full of cracks progress is suppressed.In this technology, by under being, making with general manufacture method with the similar composition of Primary Steel, can access the steel plate of fatigue cracking progression inhibiting excellent, but the meaning of must be not clearly not distinguishing with general material, not to say that the characteristic of hope will obtain performance as long as stipulate aforementioned this softening parameter.In addition, also exist fracture transition temperature vTrs to surpass 0 ℃ embodiment, the possibility as the characteristic of works can not be fully satisfied in existence.

[patent documentation 1] spy opens the 2007-302993 communique

No. 3845113 communique of [patent documentation 2] patent

[patent documentation 3] spy opens the 2002-256374 communique

[patent documentation 4] spy opens the 2000-17379 communique

[patent documentation 5] spy opens the 2004-27355 communique

Summary of the invention

The present invention is conceived to above-mentioned such situation and does, its first purpose is, a kind of Plate Steel of excellent brittlement crevasse crack propagation halt characteristic is provided, it can not cause the increase of manufacturing cost, even when thickness of slab surpasses 50mm, still satisfy high strength (tensile strength is more than the 490MPa), and the Kca value under-10 ℃ satisfies 3500N/mm ^3/2More than.

In addition, second purpose is, provides a kind of brittlement crevasse crack propagation halt characteristic more excellent Plate Steel, and its Kca value under-10 ℃ satisfies 4500N/mm ^3/2More than.

In addition, the 3rd purpose is, provides a kind of brittlement crevasse crack propagation halt characteristic further excellent Plate Steel, and its Kca value under-10 ℃ satisfies 5000N/mm ^3/2More than.

In addition, the 4th purpose is, except the Kca value that has under-10 ℃ satisfies 3500N/mm ^3/2More than beyond the Plate Steel of this brittlement crevasse crack stop performance, provide a kind of fatigue cracking progression inhibiting characteristic also excellent Plate Steel again, it is at specific stress $(\mathrm{\ΔK}=10\mathrm{MPa}\·\sqrt{\mathrm{}}m)$ Under the environment, fatigue cracking advance rate (da/dn) is 5.0 * 10 ^-6Below the mm/cycle.

Can reach the Plate Steel of the present invention (hereinafter referred to as the steel plate of first invention) of described first purpose, have following main idea: it contains the C:0.03～0.10% (meaning of " quality % " respectively, relating to chemical ingredients forms then down together), Si:0.50% following (containing 0%), Mn:1.0～2.0%, P:0.015% following (not containing 0%), S:0.010% following (not containing 0%), Al:0.005～0.060%, Nb:0.020～0.060%, Ti:0.008～0.030%, N:0.0020～0.010% and O:0.010% following (not containing 0%), and solid solution B is suppressed at below 0.0005% and (contains 0%), (t represents thickness of slab at the dark t/4～t/2 in distance surface, in the microstructure of position down together), accurate polygon ferritic average area rate is 30～85%.

Can reach the Plate Steel of the present invention (hereinafter referred to as the steel plate of second invention) of described second purpose, it is the steel plate of above-mentioned first invention, wherein, have following main idea: will be apart from the surface average crystal grain diameter of position of dark t/4 and t/2 when being made as d (t/4) and d (t/2) respectively, by the K of following formula (1) regulation ₀Satisfy (K ₀＞6750) relation.

K ₀＝5.68×10 ^-20{2.25×10 ^-2[-400·d(t/2) ^-1/2+313] ²-15[-400·d(t/2) ^-1/2+313]+6418} ^6.25/{1-3.288×10 ^-9(-92[-400·d(t/4) ^-1/2+313]+32700) ²} ...(1)

Also have,, be meant accurate polygon ferritic average crystal grain diameter at this said average crystal grain diameter d.

Can reach the Plate Steel of the present invention (hereinafter referred to as the steel plate of the 3rd invention) of described the 3rd purpose, it is the steel plate of above-mentioned first invention, wherein, has following main idea: in position apart from the dark t/4 in surface, if when the particle diameter of the crystal grain that 2 crystalline azimuth differencees of adjacency are the big angle crystal boundary more than 15 ° is surrounded was effective crystal grain diameter D (μ m), it satisfied following formula (2) in the relation of the yielding stress YS (MPa) of average equivalent circular diameter d of island martensite body (μ m) and steel plate.

-215+1.56×D+9.79×d+0.24×YS＜-60...(2)

Can reach the Plate Steel of the present invention (hereinafter referred to as the steel plate of the 4th invention) of described the 4th purpose, it is the steel plate of above-mentioned first invention, wherein, have following main idea: from the average crystal grain diameter apart from position to the zone of the position of described t/4 of the dark 2mm in surface is below the 50 μ m, and when the crystal grain diameter of the structure of steel in the position of the dark 2mm in distance surface was represented by histogram with the relation of the number of die of being counted by this crystal grain diameter, the difference of getting great 2 crystal grain diameters of number of die was 10～30 μ m.

In Plate Steel of the present invention (described the first～four invention steel plate), contain as inferior element also useful as required: (a) Cu:2% following (not containing 0%), Ni:2% following (not containing 0%) and below the Cr:2% among (not containing 0%) more than a kind; (b) Mo:0.5% following (not containing 0%); (c) V:0.1% following (not containing 0%); (d) Mg:0.005% following (not containing 0%); (e) Zr:0.1% following (not containing 0%) and/or Hf:0.05% following (not containing 0%); (g) Ca:0.0035% following (not containing 0%); (h) Co:2.5% following (not containing 0%) and/or W:2.5% following (not containing 0%); (i) rare earth element: (do not contain 0) below 0.010%, according to the composition that is contained, the characteristic of steel plate improves.

In steel plate of the present invention, realize suitableization with chemical constitution and interior tissue by tight regulation mother metal steel plate, can realize the Plate Steel of brittlement crevasse crack stop performance excellence, the perhaps Plate Steel of brittlement crevasse crack propagation halt characteristic and fatigue cracking progression inhibiting excellent, this steel plate is as useful with the former material of the various large structures headed by boats and ships, the buildings.

Embodiment

Present inventors are in order to reach above-mentioned problem, and the essential factor that all the time just influences brittlement crevasse crack propagation halt characteristic is studied repeatedly.The situation that its result shows is, from destroying the viewpoint of mechanics, on brittlement crevasse crack propagation halt characteristic improves, compared with the thickness of slab surface, the method that improves inner toughness value more has superiority, because recognized the meaning of this technology, so rate first to file (special hope 2007-262872).Promptly, for brittlement crevasse crack propagation halt characteristic, think the toughness of steel plate skin section is well got final product, but distinguish according to present inventors' research, (t: the toughness of position thickness of slab), brittlement crevasse crack can stop effectively apart from the dark t/4～t/2 of surface of steel plate by improving.

The conclusion that also obtains in addition has, and the brittlement crevasse crack propagation halt characteristic of steel plate is subjected to the influence of the form of bainite structure to a great extent, and this tissue can be controlled by suitableization of chemical ingredients, hot rolling, cooling conditions.

Therefore present inventors find, as thickness of slab inside, select (t: position thickness of slab) apart from expression t/4～t/2, if the accurate polygon ferrite of specified amount is formed, then can make the brittlement crevasse crack propagation halt characteristic of steel plate good, at first finish the steel plate of first invention.

Distinguish in addition, generate that at the composition of suitably adjusting steel plate, particularly suitably adjusted after the solid solution B amount (below 0.0005%), it is very important to adjust hot rolled condition and cooling conditions thereafter in order to make so accurate polygon ferrite.

Always, making transformation temperature low temperatureization is general improvement method with the bainite structure (bainite ferrite) that obtains fine lath-shaped (pencil), but has completely newly proposed effective form of bainite structure in the present invention, and this point becomes main points.Accurate polygon ferrite is granular (bulk), and its Vickers' hardness Hv is about 150～200.By such granulous is formed in the regulation zone, the brittlement crevasse crack propagation halt characteristic of steel plate is good.

But, form in order to make accurate polygon ferrite, thereby make brittlement crevasse crack propagation halt characteristic good, need guarantee that its average area rate is at least more than 30%, but surpass 85% that then intensity reduces if its amount is superfluous.Also have, except that accurate polygon ferrite, also can contain lath-shaped bainite, martensite, ferrite, cementite etc.

In the steel plate of first invention, by regulation chemical ingredients as above-mentioned and the tissue in the specific region, can realize that the Plate Steel of excellent brittlement crevasse crack propagation halt characteristic can realize, and such Plate Steel, the toughness of its welding heat affected zone (following table is shown " HAZ ") is also good substantially.That is, Plate Steel of the present invention is suitable for as the welded structure of boats and ships, buildings, oil tank, line pipe etc., even if the toughness of the HAZ when also requiring welding is good, and this HAZ toughness is also good.

The steel plate of first invention, it also is one of feature that its chemical ingredients composition is suitably adjusted.Below, illustrate that the scope of chemical ingredients limits reason.

[C：0.03～0.10％]

C is the needed element of intensity that is used to guarantee steel plate (welding base metal), in order to ensure desired intensity, need make it to contain more than 0.03%.But if contain C superfluously, then HAZ toughness reduces on the contrary.Thus, its upper limit need be 0.10%.Also have, the preferred lower limit of C content is 0.04%, more preferably 0.05%, and preferred upper limit is 0.09%, more preferably 0.08%.

[Si:0.50% following (containing 0%)]

Si is the intensity effective elements that is used to guarantee steel plate, contains as required.But if contain superfluously, then make and separate out island martensite body phase (M-A phase) in the steel (mother metal) in a large number, make HAZ toughness deterioration.Thus, make and be limited to 0.50% on it.Also have, the preferred lower limit of Si content is 0.1%, and preferred upper limit is 0.4%.

[Mn：1.0～2.0％]

Mn improves hardenability, is guaranteeing on the armor plate strength it is effective elements, in order to bring into play such effect, Mn is contained more than 1.0%.But if contain Mn superfluously, then therefore the HAZ toughness deterioration of steel plate makes and is limited to 2.0%.The preferred lower limit of Mn content is 1.3%, and preferred upper limit is 1.8%.

[P:0.015% following (not containing 0%)]

P is the impurity of unavoidably sneaking into, and brings disadvantageous effect for the toughness of steel plate and HAZ, and is therefore preferably the least possible.From this viewpoint, P can be suppressed at below 0.015%.The preferred upper limit of P content is 0.01%.

[S:0.010% following (not containing 0%)]

Alloying element chemical combination in S and the steel plate forms various inclusiones, is the impurity that ductility and toughness to steel plate play deleterious effect, and is therefore preferably the least possible, still, considers the degree of the purity of practical steel, can be suppressed at below 0.010%.Also have, S is the impurity that unavoidably can comprise in the steel, and it is 0% very difficult on industrial production making its amount.

[Al：0.005～0.060％]

Al is as the reductor effective elements, and is bringing into play the base metal tenacity that the microstructure miniaturization that makes steel plate brings and improving effect.In order to wave such effect, Al content need be for more than 0.005%.But, if surplus contains, make and separate out island martensite body phase (M-A phase) in the steel (mother metal) in a large number, make HAZ toughness deterioration.Thus, make and be limited to 0.060% on it.Also have, the preferred lower limit of Al content is 0.01% (more preferably 0.02%), and preferred upper limit is 0.04%.

[Nb：0.020～0.060％]

Nb improves hardenability, is bringing into play the effect that strength of parent is improved.But if contain in a large number, then the generation of carbide becomes many, and therefore the brittlement crevasse crack propagation halt characteristic deterioration is preferably (more preferably below 0.04%) below 0.060%.Also have, the content that is used for bringing into play effectively these effects is more than 0.020%.

[Ti：0.008～0.030％]

Ti makes TiN fine dispersion and prevent to add thickization of the austenite crystal of pining in steel, and the same with Nb, has the effect that suppresses austenitic recrystallize, so it is bringing into play and make the austenite crystal miniaturization, makes the effect of microstructure after phase transformation miniaturization.In addition, the austenite crystal miniaturization of the HAZ portion when TiN makes welding improves effectively the HAZ flexible.In order to bring into play such effect, Ti need contain more than 0.008%.But if the content surplus of Ti, then weldability is impaired, is below 0.030% therefore.

[N：0.0020～0.010％]

N combines with Al, Ti, Nb, B etc., forms nitride, has the effect that makes mother metal organize miniaturization, and makes the austenite crystal miniaturization when welding, and makes intergranular structure's miniaturization, and HAZ toughness is improved.In order to bring into play such effect, N is contained more than 0.0020%.But solid solution N becomes the reason that makes HAZ toughness deterioration.Because the increase of total nitrogen causes aforesaid nitride to increase, solid solution N is also superfluous and harmful, is below 0.010% therefore.Be preferably (more preferably below 0.006%) below 0.008%.

[O:0.010% following (not containing 0%)]

O is contained as unavoidable impurities, but exists as oxide compound in steel.But if its content surpasses 0.010%, then thick oxide compound generates, HAZ toughness deterioration.Therefore, make O content on be limited to 0.010%.The preferred upper limit of O content is 0.005% (more preferably 0.003%).

[solid solution B:0.0005% following (containing 0%)]

The solid solution capacity of B is because can produce very big influence to the effectively accurate polygon ferritic generation of the raising of brittlement crevasse crack propagation halt characteristic, so need restriction.If the solid solution B ultrasonic crosses 0.0005%, then accurate polygon ferrite is difficult to generate, and brittlement crevasse crack propagation halt characteristic is reduced.Thus, make solid solution B on be limited to 0.0005%.Preferred range is suppressed at below 0.0003%, more preferably below 0.0001%.Solid solution B amount can be controlled by addition and heating, the rolling condition of B.The rolling draught that reduce the B addition, reduce Heating temperature, increases under the low temperature can both reduce solid solution B amount (5ppm is following).

In the steel plate of first invention, except that mentioned component, be made of iron and unavoidable impurities (for example Sb, Se, Te etc.), but also can contain trace ingredients (permission composition) in the degree of not destroying its characteristic, this steel plate is also contained in the scope of the present invention.

When making the steel plate of first invention, get final product as follows: satisfy the steel of above-mentioned chemical ingredients amount with common smelting process melting, this molten steel is cooled off and after becoming slab, carry out hot rolling after for example being heated to 950～1300 ℃ scope, then make Ar ₃Transient point+150 ℃～Ar ₃The accumulation draft of transient point+100 ℃ is 10～30% to be rolled, and makes Ar ₃Transient point+50 ℃～Ar ₃The accumulation draft of transient point is 10～20% to carry out finish to gauge, is cooled to 400 ℃ with 0.1～20 ℃/second average cooling rate thereafter.Also have, the temperature shown in above-mentioned manages with the temperature on surface.

[Heating temperature: 950～1300 ℃]

Organize this viewpoint of austenitizing from what all make steel plate quickly, need be for more than 950 ℃, but if Heating temperature surpasses 1300 ℃, then thickization of austenite is difficult to obtain desirable tissue in the operation of back.

[Ar ₃Transient point+150 ℃～Ar ₃The accumulation draft of transient point+100 ℃: 10～30%]

By the accumulation draft that makes this temperature range is 10～30%, and is combined with the operation of back, can spheroidizing (granular).Break away from this temperature range, the accumulation draft is lower than 10% or surpass at 30% o'clock, can not guarantee that accurate polygon ferrite is more than 30%.Also has so-called in the present invention " Ar ₃Transient point ", be the value of trying to achieve by following formula (3).

Ar ₃＝910-230×[C]+25×[Si]-74×[Mn]-56×[Cu]-16×[Ni]-9×[Cr]-5×[Mo]-1620×[Nb]…(3)

Wherein, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] represent the amount (quality %) of C, Si, Mn, Cu, Ni, Cr, Mo and Nb respectively.

[Ar3 transient point+50 ℃～Ar ₃The accumulation draft of transient point: 10～20%]

By the accumulation draft that makes this temperature range is more than 10%, can carry out suitable spheroidizing (granular).Break away from this temperature range, the accumulation draft is lower than 10% or surpass at 20% o'clock, can not guarantee that accurate polygon ferrite is more than 30%.Also have, above-mentioned accumulation draft is that following formula (4) is tried to achieve.

Accumulation draft=(t ₀-t ₁)/t ₀* 100 ... (4)

[in the formula (4), t ₀The rolling beginning thickness (mm) of representing the steel disc of this temperature province, t ₁The rolling end thickness (mm) of representing the steel disc of this temperature province.]

[Ar ₃The average cooling rate of transient point～400 ℃: 0.1～20 ℃/second]

Average cooling rate during cooling is lower than 0.1 ℃/second or when surpassing 20 ℃/second, can not the time row spheroidizing (granular).In addition, why being cooled to 400 ℃, is owing to the tissue inter-variable that can not occur under the temperature below this more than this.

Present inventors are 4500MPamm based on the discovery of the steel plate of relevant above-mentioned first invention for the Kca value ^1/2The above excellent brittlement crevasse crack propagation halt characteristic Plate Steel of bringing into play is further studied repeatedly.It found that, forms in the chemical ingredients of suitably controlling steel plate, suitably controls in the steel plate of the tissue (that is, making cold polygon ferritic average area rate is 30～85%) in the steel plate, if make the be full of cracks progress motivating force K that is given by preceding formula (1) ₀Value (following " the K that only is called ₀Value ") above 6750 (MPamm ^1/2), then can realize meeting the Plate Steel of above-mentioned purpose, thereby finish the steel plate of second invention.

As the standard of estimating rejection characteristic, the general Kca value (with reference to aftermentioned embodiment) of trying to achieve that adopts by the ESSO test.But, in order to try to achieve this Kca value, need carry out complicated experiment, therefore as the standard of being convenient to estimate rejection characteristic, regulation is by the K of preceding formula (1) regulation ₀Value.

The fact of trying to achieve for this formula describes.

In above-mentioned ESSO test,, suppose by the temperature T of brittlement crevasse crack is arranged at the situation of stress application on the test body ₀, be full of cracks progress motivating force at this moment is made as K ₀As make progress motivating force K with respect to this be full of cracks ₀Impedance, the impedance K that has the ductile failure (shear lip shear lip) that takes place at surface of steel plate (being t/4 portion) to cause _s(unit: MPamm ^1/2) and at the impedance K of thickness of slab central part (t/2 portion) _d(unit: MPamm ^1/2) 2.Wherein, think impedance K _sMake above-mentioned be full of cracks progress motivating force and K ₀Proportional, can be expressed as K thus _s=K ₀R (r: rate constant).These impedances K _s, K _dBring be full of cracks and stop very big influence.

At this moment, in temperature T ₀Down, stop, needing the relation of following formula (5) to set up in order to make be full of cracks.In addition, at this moment think be full of cracks progress motivating force K ₀With temperature T ₀Under the Kca value corresponding.Following formula (5) can be out of shape in the mode of following formula (6).

K ₀＝K _s+K _d＝K ₀·r+K _d (5)

K ₀＝K _d/(1-r) (6)

Secondly, according to technical literature " pressure techniques Vol.31, No.2 (1993), p2 " (hereinafter referred to as " reference 1 "), think near the dynamic destruction toughness value K that rate constant r and thickness of slab top layer are _{D (B)}Associated.Dynamically destroying toughness value is the corresponding destruction toughness value of be full of cracks that makes progress at a high speed, thinks different with general destruction toughness value (Kci).On the other hand, according to technical literature " the collection of thesis Vol.177 of Society of Naval Architects of Japan (1995), p243 " (below, be called " reference 2 "), even think the brittlement crevasse crack of progress at a high speed, in the time of near the top layer that shear lip takes place, the be full of cracks advance rate also can extremely reduce.At this, near the top layer because the be full of cracks advance rate extremely reduces, so its dynamically destroy toughness value also with common destruction toughness value equivalence.Thus, near the destruction toughness value Kci rate constant r and the top layer is relevant.

In addition, according to above-mentioned reference 2, think that destruction toughness value Kci is relevant with fracture transition temperature vTrs.Also have, so-called fracture transition temperature vTrs is the brittle rupture transition temperature vTrs that tries to achieve by pendulum impact test.Thus, near the fracture transition temperature vTrs rate constant r and the top layer is relevant.For example, near the material behavior the top layer can be by the material behavior representative of t/4 portion if think, then thinks the fracture transition temperature vTrs (following simply be designated as [vTrs (t/4)]) of rate constant r and t/4 portion.

On the other hand, the impedance K of thickness of slab central part _dBe the dynamic destruction toughness value at same position, different with common destruction toughness value.According to above-mentioned reference 2, dynamically destroying toughness value (is impedance K _d) and the stress σ of local criticality _FRelevant.The stress σ of local criticality _FBe the fail in tension stress of the atomic zonule of be full of cracks front end.This fail in tension is the continuity of the ductile failure of the cleavage fracture of crystallisate and crystal boundary.

At this, for the intensity (stress) of the ductile failure of crystal boundary, being considered to ductile failure portion, many more it is high more.It is many more that ductile failure portion is considered to crystal boundary, and promptly crystal grain diameter d is more little, high more for the intensity of ductile failure.That is local criticality's stress σ, _FBe inversely proportional to the crystal boundary diameter d.On the other hand, think d ^-1/2There is proportionlity in general and fracture transition temperature vTrs.As known from the above, local criticality's stress σ _FVTrs is relevant with fracture transition temperature.

For example, if near the material behavior the thickness of slab central part is considered to can be by the material behavior representative of t/2 portion, then K _dRelevant with the fracture transition temperature vTrs of t/2 portion.According to more than, as the destruction toughness motivating force K of the replacement parameter of Kca ₀Can be as shown in the formula (7) expression like this.

K ₀＝K _d/(1-r)＝f ₂(vTrs(t/2)/(1-f ₁(vTrs(t/4))))…(7)

The f of following formula (7) ₁(), f ₂() is respectively function, for example, if the temperature condition T that can grasp yielding stress σ y, the thickness of slab t under the room temperature (25 ℃) and obtain according to Design Designation ₀, then can formulism as the function of fracture transition temperature vTrs (t/4) and vTrs (1/2).This formulistic step is as follows.

At first, about aforementioned proportion constant r, can represent in the mode of following formula (8) (described reference 1).

r＝(4/π)(ts1/t)(σ _Y1/σ ₀)cos ^-1{(a-ls1)/a}...(8)

Wherein, ts1 represents the amplitude (mm) of the ductile failure (shear lip) of skin section, σ _Y1The expression temperature T ₀Under the steel plate top layer near high-speed tensile deformation the time yielding stress (MPa), σ ₀The yielding stress (MPa) in expression ESSO when test, a length (mm) of representing to chap, ls1 represents (sideligament) length (mm).

The width ts1 of the ductile failure of above-mentioned skin section (shear lip) can represent in the mode of following formula (9).Also have, in following formula (9), ks1 is the coefficient ratio of plastic region size (the shear lip width with), according to described reference 2, ks1=2.

ts1＝ks1·rp …(9)

In addition, in following formula (9), rp is plastic region size (mm), is represented by following formula (10).

rp＝1/6π·(K _D(B)/σ _Y1) ²…(10)

Wherein, K _{D (B)}Near the expression skin section dynamic destruction toughness value (MPamm ^1/2)

According to described reference literature 2,,, therefore can wait to be all common destruction toughness value Kci because the be full of cracks advance rate shows extremely low speed in the generating unit of shear lip.That is, as shown in the formula shown in (11).

K _D(B)＝Kci…(11)

Destroy toughness value kci, for example (northern Tian Bochong work, Ph D dissertation is " about the yield-point 40kgf/mm of TMCP according to document ²The research of the toughness reguirements benchmark during the real ship of grade steel plate is used " (1990), p32: hereinafter referred to as " reference 3 "), as shown in the formula (12)～(14), demonstrate relation with fracture transition temperature vTrs.

Kci＝3.81×(σ _y0/9.8)·exp{K ₀(1/iTK-1/T ₀)}…(12)

K ₀＝6.65·iTK-290…(13)

iTK＝(0.0032×σ _y0/9.8+0.391)vTrs+2.74(T) ^1/2+17.3…(14)

Wherein, σ _Y0Yielding stress under the room temperature (25 ℃) of expression steel

At this, specifically with stress σ ₀For thickness of slab=60mm, yielding stress σ _Y0When the steel of=500MPa is implemented the ESSO test, the temperature condition T that attempts making the Design Designation according to boats and ships etc. to obtain ₀Under K ₀Formulism.The situation of boats and ships is, the temperature condition T that obtains according to design conditions ₀Mostly be 0～10 ℃, therefore, be set at T here ₀=-10 ℃.In the ESSO test, though experimentize with various stress conditions, if stress is low excessively, the progress that then chaps amount is minimum, to temperature T in addition ₀=-10 ℃ temperature province, the possibility height that be full of cracks does not make progress.Therefore, need fully high stress.Under the situation of boats and ships, be mostly to depend on design stress that grasping be full of cracks under this design stress, to stop performance the most reasonable according to Design Designation.Therefore, use the pairing design service stress of ABS specification (American Bureau of Shipping's specification) ET40 (for example, " Japanese boats and ships ocean engineering lecture collection of thesis " Vol.3 (2006), p359 :) here, σ hereinafter referred to as " reference 4 " ₀=252MPa.

Under the situation of this example, vTrs and Kci demonstrate proportionlity.According to this relation, present inventors find, adopt the Kci (Kci of skin section _(B)), can access following formula (15).

K _D(B)＝Kci _(B)d＝-92vTrs+32700…(15)

Also has the σ of preceding formula (8) _Y1It is temperature T ₀Yielding stress during near the steel top layer 10 ℃ of (=-) high-speed tensile deformation.This yielding stress σ _Y1Depend near the be full of cracks advance rate the top layer, if based on aforementioned reference 2 with this speed as 100m/ second, then make yielding stress σ _Y1Fig1 (b) according to reference can access yielding stress σ _Y1=800MPa.In addition, a shown in the preceding formula (8) is be full of cracks length, and ls1 is (side ligament) length, but ls1 is about 10～20mm according to aforementioned reference literature 2.In addition, the be full of cracks length a in the common ESSO test mostly is about 300mm, and therefore (a-ls1)/a is about about 0.95.

As known from the above, rate constant r can be with the mode formulism of following formula (16).Also have, the vTrs here is near near the fracture transition temperature vTrs[vTrs (top layer) of the material the top layer].For example as described above, if think that near the vTrs the top layer is equal to the vTrs of t/4 portion, the then relation of following formula (17) establishment.

R=3.288 * 10 ^-9(near 92v[Trs (top layer)]+32700) ²... (16)

r＝3.288×10 ^-9(-92[v?Trs(t/4)]+32700) ²...(17)

More than be the concrete formulistic example of rate constant r, but can understand as follows for the formulism of rate constant r.That is, rate constant r is following such function.

r＝f ₁’(ts1、σ _Y1、σ ₀、(a-ls1)/a)

＝f ₁’(K _D(B)、ks1、σ _Y1、σ ₀(a-ls1)/a

=f ₁' ([vTrs (table Layer closely is close to)], σ _Y0, T ₀, t, ks1, σ _Y1, σ ₀, (a-ls1)/a)

At this, parameter is tried to achieve as follows.

Near vTrs (top layer): near the fracture transition temperature the top layer (℃) → extract from steel

σ _Y0: the yielding stress of the room temperature of steel (25 ℃) → extract from steel

T ₀: the temperature condition that obtains according to Design Designation.

T: the thickness of steel plate → extract from steel

Ks1: the ratio of shear lip width and plastic region size →, be presented in reference 2 grades with respect to general steel.

σ _Y1: temperature T ₀Under the steel top layer near high-speed tensile deformation the time yielding stress → be presented in reference 2 grades.

σ ₀: the bearing strength test condition that obtains according to Design Designation.

(a-ls1)/and a:a is a be full of cracks length, ls1 is (side ligament) length, all at general steel, can analogize according to reference 2.

As known from the above, remove value of trying to achieve and the value that obtains by document according to Design Designation, for

R=f ₁" ([vTrs (near the top layer)], σ _Y0, t),

With respect to certain yielding stress σ _Y0With the condition of thickness of slab t, can be expressed as r=f ₁([vTrs (near the top layer)]).

On the other hand, for K _dThe step of formulism be described in detail.According to aforementioned reference 2, K _dMode formulism with following formula (18).

σ _F＝σ _Y2·∑ _yy{(1-v ²)(K _d/σ _Y2)2/r _c} ^-s…(18)

In following formula (18), σ _Y2Be temperature T ₀Yielding stress (MPa) during near the thickness of slab central part 10 ℃ of (=-) high-speed tensile deformation, this yielding stress σ _Y2Also depending near the be full of cracks advance rate of thickness of slab central part, is 600m/ second (ESSO with standard tests the be full of cracks advance rate that obtains) if make speed equally, and then the Fig11 (b) according to aforementioned reference 2 is 800MPa.

Above-mentioned v is a Poisson's ratio, is 0.3.In addition, r _cIt is the constant (unit: mm), be 0.3mm of expression regional area according to aforementioned reference 2.-s is the index of expression stress specific intensity, at this ,-10 ℃, be full of cracks advance rate 600m/ under second-s is 0.08 according to aforementioned reference 2.∑ _YyBe the coefficient of the intensity of expression stress, according to aforementioned reference 2, ∑ _Yy=4.

σ _FBe local criticality's stress (MPa), if adopt crystal grain diameter d, then as described above, think with 1/d (=(AvTrs+B) ²) proportional.At this, A, B are according to document (" thermal treatment ", Vol.47, No.2 (2007), p66: hereinafter referred to as " reference 5 "), A=3, B=1000.The unit of fracture transition temperature vTrs is K.In addition, according to reference 2, make the stress σ F of local criticality become the value of 4000～4500MPa, (273～263K) is corresponding, then represents with following formula (19) as if supposing its variation with fracture transition temperature vTrs.

σ _F＝2.25×10 ^-2vTrs ²-15vTrs+6418…(19)

According to more than, K _dIn the mode of following formula (20) by formulism.K _dUnit be MPamm ^1/2

K _d＝5.68×10 ^-20(2.25×10 ^-2vTrs ²-15vTrs+6418) ^6.25…(20)

Also have, the fracture transition temperature vTrs here is near near the vTrs[vTrs (the thickness of slab central part) of the material the thickness of slab central part].For example, as described above, if think near the thickness of slab central part fracture transition temperature vTrs and the vTrs[vTrs (t/2) of t/2 portion] be equal to, then following formula (20) can be represented with following formula (21).

K _d=5.68 * 10 ^-20(2.25 * 10 ^-2[vTrs (near the thickness of slab central part)] ₂Near-15[vTrs (the thickness of slab central part)]+6418) 6.25

＝5.68×10 ^-20(2.25×10 ^-2[vTrs(t/ ²)]2-15[vTrs(1/2))]+6418) ^6.25

…(21)

More than be K _dConcrete formulistic example, but about K _dFormulism, can understand as follows.That is K, _dIt is the function shown in the following formula (22).

K _d＝f ₂’(σ _F、σ _Y2、r _c、v、-s、∑ _yy)

=f ₂' ([vTrs (near the thickness of slab central part)], σ _Y2, r _c, v ,-s, ∑ _Yy) ... (22)

At this, each parameter is tried to achieve as follows.

Near vTrs (the thickness of slab central part): near the fracture transition temperature the thickness of slab central part (℃) → extract by steel

σ _Y2: the yielding stress during near the thickness of slab central part under the temperature T 0 high-speed tensile deformation →, show reference 2 for general steel.

r _c: the constant of expression regional area →, be presented in the reference 2 for general steel.

V: Poisson's ratio → think to be 0.3 for general steel.

-s: the index of the specific intensity of expression stress → be presented in reference 2 grades for general steel.

∑ _Yy: the coefficient of the intensity of expression stress → be presented in reference 2 grades for general steel.

According to more than, remove value of trying to achieve and the value that obtains by document according to design requirements, can be expressed as K _d=f ₂([vTrs (near the thickness of slab central part)]).

According to by r and K with upper type formulism _d, the replacement evaluating K of Kca ₀Value can be represented in the mode of following formula (23), (24).

K ₀=K _d(1-r)=5.68 * 10 ^-20{ 2.25 * 10 ^-2[vTrs (near the thickness of slab central part)] ²Near-15[vTrs (the thickness of slab central part)]+6418} ^6.25/ { 1-3.288 * 10 ^-9(near 92[vTrs (top layer)]+32700) ²... (23)

＝5.68×10 ^-20{2.25×10 ^-2[vTrs(t/2)] ²-15[vTrs(t/2)]+6418} ^6.25/{1-3.288×10 ^-9(-92[vTrs(t/4)]+32700) ²}…(24)

At this, brittle rupture transition temperature vTrs, it is generally acknowledged and average crystal grain diameter d between, have 1/d=(A * vTrs+B) ²The relation of (A, B are constant).Above-mentioned relation can be rewritten as $\mathrm{vTrs}=-1/A\·\sqrt{\mathrm{}}(1/2)+B/A.$ Therefore, be full of cracks progress motivating force K ₀Not only can show by vTrs, and can be by [representational is apart from surperficial t/4 (t: position thickness of slab) near the top layer, below only be called " t/4 portion "] and the thickness of slab central part near [representational be apart from surperficial t/2 (t: position thickness of slab) below only is called " t/2 portion "] the average crystal grain diameter performance.And for example according to the experimental result shown in the document (forum of iron and steel institute " intensity of building material and destruction " in November, 2006 distribution), brittle rupture transition temperature vTrs can represent in the mode of following formula (25).

$\mathrm{vTrs}=-400\·\left[\sqrt{\mathrm{}}(1/d)\right]+313\·\·\·\left(25\right)$

If by average crystal grain diameter d, launch above-mentioned be full of cracks progress motivating force K again ₀Formula [preceding formula (24)], then following formula (1) is tried to achieve.

K ₀＝5.68×10 ^-20{2.25×10 ^-2[-400·d(t/2) ^-1/2+313] ²-15[-400·d(t/2) ^-1/2+313]+6418} ^6.25/{1-3.288×10 ^-9(-92[-400·d(t/4)-1/2+313]+32700) ²}…(1)

Make the method for the steel plate of second invention, in most operation with the steel plate of described first invention be fabricated to identical condition, but different points is, is not to finish hot rolling to cool off immediately from this temperature, in a single day but arrive finishing temperature, make it to wait for 30～150 seconds and cool off afterwards.In the method, under finishing temperature, wait for 30～150 seconds the reasons are as follows.Also have the temperature shown in above-mentioned to manage with the temperature on surface.

[under finishing temperature, waiting for 30～150 seconds]

Waiting time after rolling, the temperature height of thickness of slab central part, the average crystal grain diameter of thickness of slab central part became big, can not make described K when being lower than 30 seconds ₀Value satisfies above 6750MPamm ^1/2If standby time surpasses 150 seconds, then crystal grain-growth carries out in addition, no matter the central part of plate, skin section, it is big that average crystal grain diameter becomes, and can not make described K ₀Value satisfies above 6750MPamm ^1/2

Present inventors are 5000MPamm for bringing into play the Kca value again based on the discovery of the steel plate of relevant described first invention ^1/2The Plate Steel of the brittlement crevasse crack propagation halt characteristic of above excellence is research repeatedly further.

Its result obtains as drawing a conclusion: the brittlement crevasse crack propagation halt characteristic of steel plate, the median size of the crystal grain that is subjected to the form of bainite structure, surrounds by the big angle crystal boundary, the size of island martensite body, the influences such as yielding stress YS of steel plate, and this tissue can be controlled by suitableization of chemical ingredients and rolling condition.

Therefore present inventors find, if steel plate based on described first invention, make again [promptly apart from effective crystal grain diameter D (μ m) of surperficial t/4 position, accurate polygon ferritic effective crystal grain diameter D], the average equivalent circular diameter d (μ m) of island martensite body (MA) and the yielding stress YS (MPa) of steel plate satisfy following formula (2), controlled so evenly, then can make the brittlement crevasse crack propagation halt characteristic of steel plate good, thereby finish the steel plate of the 3rd invention.

-215+1.56×D+9.79×d+0.24×YS＜-60?(2)

The fact of trying to achieve following formula (2) is as follows.At first, as the essential factor that the brittlement crevasse crack propagation halt characteristic of steel plate is exerted one's influence, present inventors have selected apart from effective crystal grain diameter D (μ m), the average equivalent circular diameter d (μ m) of island martensite body (MA) of surperficial t/4 position and the yielding stress YS (MPa) of steel plate.

Among the above-mentioned important document, have the then high more tendency of yielding stress YS of steel plate of high strength, it is contemplated that so this yielding stress YS is low more, brittlement crevasse crack propagation halt characteristic is also just good more.And this yielding stress YS degree of correlation general and hardenability index (Pcm) is a lot, so hardenability index (Pcm) is considered to the controlling index of yielding stress YS.But, in common hardenability index (Pcm), do not consider the influence of alloying elements such as Nb, thus among the present invention after the influence of also considering such alloying element, study with regard to the relation of brittlement crevasse crack propagation halt characteristic and yielding stress.

Based on above-mentioned discovery, the influence that the average equivalent circular diameter d of just accurate polygon ferritic effective crystallization particle diameter D and island martensite body MA causes yielding stress YS, study based on experiment (regression analysis), its result distinguishes, when satisfying the concerning of following formula (1), the brittlement crevasse crack propagation halt characteristic of steel plate is good.

Also have, " crystal orientation is poor " of so-called regulation big angle crystal boundary is also referred to as " drift angle " or " inclination angle ", and be poor in order to measure this crystal orientation, adopts EBSP method (Electron BackscatteringPattern method) to get final product (with reference to aftermentioned embodiment).

When making the steel plate of the 3rd invention, in most operation with the steel plate of described first invention be fabricated to identical condition, but different points are, before 950～1300 ℃ scopes are carried out hot rolling, to once carry out heat treated (950～1100 ℃), and in the end carry out temper with the temperature more than 500 ℃.In the method, before hot rolling, once carry out heat treated, and in the end carrying out the reasons are as follows of temper.Also have the temperature shown in above-mentioned to manage with the temperature on surface.

[Heating temperature: 950～1100 ℃]

Before rolling heating, carry out heat treated, can carry out the control of effective crystal grain, the brittle rupture stop performance is improved.By making initial stage γ crystal grain grain refined, effective crystal grain diameter of steel plate is grain refined further, thus the raising that brings characteristic, but to can not realize this miniaturization heat-treating the person more than 950 ℃ before the rolling heating quickly.Being lower than under 950 ℃ the temperature, can't become austenite fully, therefore invalid.On the other hand, if Heating temperature surpasses 1100 ℃, then temperature is too high, adds the reheat of back, causes crystal grain thick, deterioration in characteristics.

[temper]

By carrying out temper, can make the miniaturization of MA size, the brittle rupture stop performance is improved.If low temperature too, then its effect is little, and MA can't miniaturization, therefore need be for more than 500 ℃.

In addition, in addition present inventors also study for the also excellent steel plate of fatigue cracking progression inhibiting characteristic repeatedly based on the discovery of the steel plate of relevant above-mentioned first invention.

Its result also obtains following conclusion: the fatigue cracking progression inhibiting characteristic of steel plate is subjected near average crystal grain diameter and the size-grade distribution influence the top layer, and this tissue can be controlled by suitableization of chemical ingredients and rolling, cooling conditions.

Therefore, present inventors find, steel plate based on described first invention, in addition, make position apart from the position (top layer) of the dark 2mm in surface to described t/ [apart from surperficial t/4 (t: position thickness of slab)] the zone in average crystal grain diameter (promptly, accurate polygon ferritic average crystal grain diameter) for the value of regulation following after, represent near the crystal grain diameter of the structure of steel of (apart from the position of the dark 2mm in the surface) top layer and during with histogram with the concerning of the crystal grain number of this crystal grain diameter counting, the difference that makes great 2 crystal grain diameters of crystal grain number within the limits prescribed, thereby can make the brittlement crevasse crack propagation halt characteristic of steel plate and fatigue cracking progression inhibiting characteristic all good, thereby finish the steel plate of the 4th invention.

Also have, when making steel plate with common manufacture method, if with near the crystal grain diameter of the structure of steel the histogram table presentation surface with the relation of the crystal grain number of this crystal grain diameter counting, be 1 place (size distribution is near so-called regular distribution) under the greatly such most situation of crystal grain diameter of crystal grain number then, perhaps, even a plurality of maximal points are arranged, also be that both are closer particle diameter (difference that constitutes great 2 crystal grain diameters is extremely about most μ m).

With respect to this, in Plate Steel of the present invention, at the near surface of steel plate, initiatively making the great this crystal grain diameter of crystallization particle diameter is more than 2 places, and making the difference of the particle diameter at 2 places that show maximal point is that 10～30 μ m control tissue.In addition in the present invention, be " apart from the position of the dark 2mm in surface " as near the position (top layer) that shows this size distribution, it is for following reason.

Fatigue cracking enters from the top layer, will take a long time on the top layer to make progress.And as if the center of progress with plate, then advance rate becomes quite fast, if like this, then the influence of structure of steel diminishes.Therefore, the influence that near the aspect of the structure of steel the top layer brings fatigue cracking is very big, thereby as representing near the tissue in top layer, selects " apart from the position of the dark 2mm in surface ".Also have, the meaning of " degree of depth of 2mm " is, on top layer, it is in the majority that zone of oxidation etc. are subjected to the surperficial distinctive situation that influences, and it is " degree of depth of 2mm " of avoiding this influence.

When making the steel plate of the 4th invention, with the identical condition that is fabricated to of the steel plate of described first invention, but different points is, makes Ar in most operation ₃Transient point+50 ℃～Ar ₃The accumulation draft of transient point is 10～20% to be rolled, and make accumulation draft in the finish to gauge mill train be finish below 3% rolling.In the method, the accumulation draft in the regulation finish to gauge mill train the reasons are as follows.

[the mill train accumulation draft of finish to gauge: below 3%]

By making the accumulation draft in the finish to gauge mill train is below 3%, can control surface near the crystal grain diameter of (apart from the position of the dark 2mm in surface) distribute.Finish to gauge mill train when common steel plate is made carries out about 5% depress mostly, but under such draft, the great this crystal grain diameter of crystal grain number is 1 place, even a plurality of maximal points are perhaps arranged, both also are closer particle diameters.With respect to this, by initiatively reducing the draft in the finish to gauge mill train, can make the great crystal grain diameter of crystal grain number that 2 places are arranged, the difference of the particle diameter at 2 places of demonstration maximal point can be 10～30 μ m.So, near the crystal grain diameter by (apart from the position of the dark 2mm in surface) control surface distributes the raising of fatigue cracking progression inhibiting characteristic.

In described steel plate of the present invention (steel plate of the first～four invention), as required, contain as required as inferior element also useful: (a) Cu:2% following (not containing 0%), Ni:2% following (not containing 0%) and below the Cr:2% among (not containing 0%) more than a kind; (b) Mo:0.5% following (not containing 0%); (c) V:0.1% following (not containing 0%); (d) Mg:0.005% following (not containing 0%); (e) Zr:0.1% following (not containing 0%) and/or Hf:0.05% following (not containing 0%); (g) Ca:0.0035% following (not containing 0%); (h) Co:2.5% following (not containing 0%) and/or W:2.5% following (not containing 0%); (i) rare earth element: (do not contain 0) below 0.010%.Scope when containing these compositions limits and the reasons are as follows.

[Cu:2% following (not containing 0%), Ni:2% following (not containing 0%) and below the Cr:2% among (not containing 0%) more than a kind]

Cu, Ni and Cr improve hardenability, improve effective elements for the intensity that makes steel plate, contain as required.But if the content surplus of these elements, then HAZ reduces on the contrary, therefore is (more preferably below 1%) below 2%.The preferred lower limit that is used to bring into play above-mentioned effect is 0.20% (more preferably 0.40%).

[Mo:0.5% following (not containing 0%)]

Mo improves hardenability, effectively, suits to utilize in order to prevent temper brittleness on intensity is guaranteed.Such effect increases along with the increase of its content, but superfluous if Mo content becomes, and then therefore HAZ toughness deterioration is preferably below 0.5%.More preferably below 0.30%.

[V:0.1% following (not containing 0%)]

V is bringing into play hardenability is improved, the effect that strength of parent is improved, and V also has the effect that improves the temper softening impedance in addition.But if contained in a large number, then therefore the toughness deterioration of HAZ is (more preferably below 0.05%) below 0.1%.Also have, the content that is used for bringing into play effectively these effects is that V is more than 0.01%.

[Mg:0.005% following (not containing 0%)]

Mg suppresses thickization of the austenite crystal among the HAZ by forming MgO, thereby has the effect that HAZ toughness is improved, and therefore contains as required.But, if the content surplus of Mg, thickization of inclusion then, therefore HAZ toughness deterioration is (more preferably below 0.0035%) below 0.005%.

[Zr:0.1% following (not containing 0%) and/or Hf:0.05% following (not containing 0%)]

Zr is the same with Ti with Hf, forms nitride with N, and the austenite crystal miniaturization of the HAZ when making welding is to the tough property improvement effective elements of HAZ.But if surplus makes it to contain, then HAZ toughness reduces on the contrary.Therefore, when containing these elements, Zr is below 0.1%, and Hf is below 0.05%.

[Ca:0.0035% following (not containing 0%)]

The form of Ca control sulfide is the element that helps the HAZ flexible to improve.But, making it to contain superfluously and surpass 0.0035%, HAZ toughness is deterioration on the contrary.Also have, the preferred upper limit of Ca content is 0.020% (more preferably 0.0015%).

In addition, be limited to the steel plate of second invention, when containing Ca, make it to satisfy following formula (26), can realize that the HAZ flexible further improves by the content of controlling Ca, O and S.

2.0≤1000×([Ca]+2×[S]+3×[O])≤10.0…(26)

Wherein, [Ca], [S] and [O] represent the content (quality %) of Ca, S and O respectively.

In solidification stages, if CaO and CaS are disperseed imperceptibly, then ferrite transformation is examined fine dispersion, can suppress the generation of thick tissue, and HAZ toughness can further improve.In order to make the fine dispersion of CaO and CaS, effectively satisfy the relation of following formula (26) as can be known.The fine dispersive degree of each composition pairing " coefficient " expression.Under composition compositing range shown in the present, demonstrate by the order high-density dispersive tendency of Ca, S and O strong.By satisfying such relation,, also can guarantee the effective ferrite product nucleus of good HAZ flexible even can import in a large number when large-line energy welds.Specifically, as described later shown in the embodiment, even heat input surpasses the such large-line energy welding of 50kJ, the V pendulum impact value (vE under-55 ℃ _-55) also be more than the 100J.

[Co:2.5% following (not containing 0%) and/or W:2.5% following (not containing 0%)]

Co and W improve hardenability, have the effect that improves strength of parent, therefore contain as required.But, if contain superfluously, HAZ toughness deterioration then, so the upper limit is 2.5%.

[rare earth element (REM): (do not contain 0) below 0.010%]

Rare earth element (REM) makes shape miniaturization, the balling of the inclusion (oxide compound and sulfide etc.) of sneaking into inevitably in the steel, from but the element that helps the toughness of HAZ to improve make it as required to contain.This effect increases and increases along with its content, still, if the content surplus of REM, thickization of inclusion then, therefore HAZ toughness deterioration preferably is suppressed at below 0.010%.Also have, in the present invention, so-called REM is the meaning that contains lanthanon (15 kinds of elements from La to Ln) and Sc (scandium) and Y (yttrium).

Also have, the steel plate (steel plate of described the first～four invention) as object among the present invention is assumed to the above Plate Steel of thickness of slab 50mm basically, but the thickness of slab below this also has equal characteristic, is included in the object of the present invention.In addition, the heat input when welding steel plate of the present invention is assumed to more than the 2kJ/mm, and it demonstrates good HAZ toughness when welding under such heat input.

Below, by the embodiment the present invention that said so in more detail, but following embodiment does not limit character of the present invention, carries out all being included in the technical scope of the present invention of design variations according to preceding aim described later.

[embodiment]

Experimental example 1 (experimental example of the steel plate of first invention)

By common smelting process, the various molten steel that chemical ingredients shown in the following table 1 of melting, 2 is formed cool off this molten steel and after becoming slab, carry out hot rolling and cooling with the condition shown in the following table 3,4, obtain various steel plate (thickness: 60mm).Also have, in following table 1, REM is about 50% and to contain Ce be that the form of about 25% mishmetal is added to contain La.Also have, element is not added in "-" expression in the following table 1,2.

[table 3]

[table 4]

For each steel plate that obtains, measure mother metal tissue (accurate polygon ferritic hardness, average area rate), solid solution B amount, mechanical characteristics (tensile properties of mother metal, the impact characteristics of mother metal, brittlement crevasse crack stop performance) by following method, and also estimate for HAZ toughness.Measurement result is presented in the following table 5,6.

[mensuration of accurate polygon ferritic hardness]

Being used for differentiating accurate polygon ferritic hardness (Hv) measures as follows.T/4, t/2 (t: the test film on extraction 2cm limit, position thickness of slab) from each steel plate, after it is carried out mirror ultrafinish, after nital (2% nitric acid-ethanolic soln) etching, use nano-hardness tester (NanoIndenter) (Elionix corporate system " ENT-1100 "), the TD face is measured with load 250mg.At the range of observation of 150 μ m * 200 μ m, at first select granular structure (long-width ratio is below 3), carry out the measurement of hardness of this granular structure, the granular structure that Hv is in 150～200 the scope is differentiated the polygon ferrite that is as the criterion.

[accurate polygon ferrite divides the mensuration of rate]

In order to measure the accurate polygon ferrite branch rate that obtains according to above-mentioned measurement of hardness result, at first, carry out photograph taking with opticmicroscope for the test portion sightingpiston that has carried out above-mentioned measurement of hardness, thereafter, use image analysis software (Media Cybernetics system: Image-Pro Plus), carry out differentiating the quantification of the face integration rate (mean value) of the polygon ferritic tissue that is as the criterion according to measurement of hardness result hundred.

[mensuration of solid solution B amount]

Solid solution B amount is carried out quantification by the chemical analysis test of extraction residue.Test film is that (t: 10mm * 10mm limit is extracted in position thickness of slab), is electrolytic solution with 10 quality % methyl ethyl diketone-1 quality % tetramethyl ammonium chloride-methanol solutions, at 200mA/m from t/4 portion and t/2 portion ²Extract under the following electric current, use the strainer of 0.1 μ m.

[evaluation of the tensile properties of mother metal]

(t: the JIS4 test film is extracted in position thickness of slab), follows JIS Z2241 and carries out tension test, measures yielding stress YS (yield-point YP) and tensile strength TS thus, calculating yield tensile ratio YR from the t/4 portion of each steel plate.

[evaluation of the impact characteristics of mother metal (toughness)]

The impact characteristics of mother metal (toughness) is to carry out the pendulum impact test of V otch, tries to achieve vTrs (brittle rupture transition temperature) according to transition curve.(t: the JIS4 test film is extracted in thickness of slab) position, follows JIS Z2242 and implements test, at this moment about the mensuration of each temperature (minimum 4 more than the temperature), implements test with n=3 from t/4 portion.Describe brittle rupture transformation transition curve and make it by 3 the highest points of middle brittle rupture rate, the temperature of calculating brittle rupture rate 50% is as brittle rupture transition temperature vTrs (make vTrs become the top temperature side and carry out setting-out like this).

[brittlement crevasse crack propagation halt characteristic]

Brittlement crevasse crack propagation halt characteristic (rejection characteristic) assert that according to the steel grade of Corporation's Japan's welding association (WES) distribution " brittlement crevasse crack propagation stops test " of test method (formulation on March 31st, 2003) defined carries out.Test the test film of the shape shown in Fig. 7 .2 that adopts brittlement crevasse crack propagation to stop test method, under the temperature range of from-190 ℃～+ 60 ℃ scope, selecting arbitrarily, give thermograde, divide 4 test bodies to carry out this test film.The Kca value is calculated by following formula (27).In the following formula (27), c represents to enter the mouth to the length of brittlement crevasse crack front end from propagation portion, and σ represents to enter the mouth to the length of brittlement crevasse crack end from propagation portion, and W represents propagation portion width.

[numerical expression 1]

$\mathrm{Kca}=\mathrm{\σ}\sqrt{2W\tan (\mathrm{\πc}/2W)}\·\·\·\left(27\right)$

With T is the temperature (unit is K) of brittlement crevasse crack front end, and establishing X-axis is 1/T, establishes Y-axis and be the value of the Kca that calculates, makes the graphic representation of the correlationship of expression 1/T and Kca value, and the intersection point of 4 curve of approximation and 263K is the Kca value under-10 ℃.Kca value under-10 ℃ is presented in the following table 5,6.Kca value in the present invention ,-10 ℃ is at 3500N/mm ^3/2When above qualified (excellent brittlement crevasse crack propagation halt characteristic).

[HAZ toughness test]

The HAZ toughness evaluation assessment of the thermal cycling when carrying out submerged arc welding (2kJ/mm) as simulation, with Heating temperature: 1400 ℃ kept 5 seconds down, be cooled to 800～500 ℃ cooling time (Tc): 25 seconds thermal cycling thereafter, after steel plate heat-treats for examination to each, measure temperature-15 pendulum ℃ down and absorb energy (V otch).Also have, as test film, use be that (t: size 10mm * 10mm * 55mm's that extract position thickness of slab) is bar-shaped, is formed with the test film of the V otch of dark 2mm at the central part single face from thickness of slab t/4 portion.At this moment V pendulum impact value (vE _-15) be qualified more than 50J.

By these results as can be known, test No.1～22nd, satisfy the example of the important document of the present invention's regulation, can be met high strength, and the Plate Steel of excellent brittlement crevasse crack propagation halt characteristic.With respect to this, in experiment No.23～45, be the example that breaks away from a certain important document of the present invention's regulation, can't obtain certain specific character as can be known.

Experimental example 2 (experimental example of the steel plate of second invention)

By common smelting process, the various molten steel that chemical ingredients shown in the following table 7 of melting, 8 is formed cool off this molten steel and after becoming slab, carry out hot rolling and cooling with the condition shown in the following table 9,10, obtain various steel plate (thickness: 60mm).Also have, in following table 7,8, REM is about 50% and to contain Ce be that the form of about 25% mishmetal is added to contain La.Also have, element is not added in "-" expression in the following table 7,8.In table 7,8, also show the value (hereinafter referred to as " A " value) of [1000 * ([Ca]+2 * [S]+3 * [O])] simultaneously in addition.

For each steel plate that obtains,, measure with regard to accurate polygon ferritic hardness, average area rate, solid solution B amount, mechanical characteristics (tensile properties of mother metal, the impact characteristics of mother metal, brittlement crevasse crack stop performance) with the method identical with described experimental example 1.According to following method, measure the t/4 portion of each steel plate, the average crystal grain diameter of t/2 portion in addition, and also estimate for HAZ toughness.The K of measurement result and aforementioned (1) defined ₀Value is presented in the following table 11,12 together.Also have, the accurate polygon ferrite of "-" expression does not generate in the table 11, therefore measures.

[average crystal grain diameter]

The t/4 portion of steel plate, the average crystal grain diameter of t/2 portion use EBSP analytical equipment (TexSEMLaboratries corporate system) and Philips corporate system FE-SEM (field emission type scanning electronic microscope) " XL30S-FEG " to measure.With crystal orientation poor (oblique angle) is that boundary (big angle crystal boundary) more than 15 ° is as crystal grain boundary and be defined as " crystal grain ", the average crystal grain diameter of the accurate polygon ferritic crystal grain of decision.At this moment mensuration zone is 250 μ m, and measuring step is interval 0.4 μ m, and the confidence index (Confidence Index) of the reliability that expression fixes the position is that the measuring point below 0.1 is deleted from analytic target.In addition, when crystal grain diameter is lower than 2.0 μ m, be judged as the mensuration noise, from the object of the mean value calculation of crystal grain diameter except.

[HAZ toughness test 1]

The HAZ toughness evaluation assessment of the thermal cycling when carrying out submerged arc welding (2kJ/mm) as simulation, with Heating temperature: 1400 ℃ kept 5 seconds down, be cooled to 800～500 ℃ cooling time (Tc): 25 seconds thermal cycling thereafter, after steel plate heat-treats for examination to each, measure temperature-15 pendulum ℃ down and absorb energy (V otch).Also have, as test film, use be that (t: size 10mm * 10mm * 55mm's that extract position thickness of slab) is bar-shaped, is formed with the test film of the V otch of dark 2mm at the central part single face from t/4 portion.At this moment V pendulum impact value (vE _-15) be qualified more than 100J.

[HAZ toughness test 2]

The HAZ toughness evaluation assessment of the thermal cycling when carrying out electroslag welding (60kJ/mm) as simulation, with Heating temperature: 1400 ℃ kept 50 seconds down, be cooled to 800～500 ℃ cooling time (Tc): 500 seconds thermal cycling thereafter, after steel plate heat-treats for examination to each, measure temperature-55 pendulum ℃ down and absorb energy (V otch).Also have, as test film, use be that (t: size 10mm * 10mm * 55mm's that extract position thickness of slab) is bar-shaped, is formed with the test film of the V otch of dark 2mm at the central part single face from t/4 portion.At this moment, about V pendulum impact value (vE _-55), only estimate for the embodiment that satisfies important document of the present invention (containing Ca's), then do not estimate for comparative example.

By these results as can be known, experiment No.1～22,48～50th, the example of the important document of the steel plate regulation of satisfied second invention can be met high strength, and the Kca under-10 ℃ is 4500N/mm ^3/2The Plate Steel of above excellent brittlement crevasse crack propagation halt characteristic.With respect to this, in experiment No.23～43,45～47, be the example that breaks away from a certain important document of the second steel plate regulation of inventing, can't obtain certain specific character as can be known.Also have, experiment No.44 is a reference example, though it satisfies the condition of the steel plate defined of first invention,, the condition of making the steel plate of second invention because of the ratio of the waiting time after rolling is short, so, although the Kca under-10 ℃ satisfies 3500N/mm ^3/2More than, but the Kca under-10 ℃ is lower than 4500N/mm ^3/2

Experimental example 3 (experimental example of the steel plate of the 3rd invention)

By common smelting process, the various molten steel that chemical ingredients shown in the following table 13 of melting, 14 is formed cool off this molten steel and after becoming slab, carry out hot rolling and cooling with the condition shown in the following table 15,16, obtain various steel plate (thickness: 60mm).Also have, in following table 13, REM is about 50% and to contain Ce be that the form of about 25% mishmetal is added to contain La.Also have, element is not added in "-" expression in the following table 13,14.

For each steel plate that obtains,, measure with regard to accurate polygon ferritic hardness, average area rate, solid solution B amount, mechanical characteristics (tensile properties of mother metal, the impact characteristics of mother metal, brittlement crevasse crack stop performance) with the method identical with described experimental example 1.According to following method, measure accurate polygon ferritic effective crystal grain diameter (μ m), MA average equivalent circular diameter (μ m) in addition, and also estimate for HAZ toughness.Measurement result is presented in the following table 17,18.Also have, in the table 17,18, also show the value [215+1.56 * D+9.79 * d+0.24 * YS] on the left side of (2) formula simultaneously.In addition, "-" expression can not be measured in the following table 17,18.

[evaluation of the tensile properties of mother metal]

(t: the JIS4 test film is extracted in position thickness of slab), follows JIS Z2241 and carries out tension test, measures yielding stress YS (yield-point YP) and tensile strength TS thus from the t/4 portion of each steel plate.

[effectively crystal grain diameter D]

Effectively crystal grain diameter D uses EBSP analytical equipment (TexSEM Laboratries corporate system) and Philips corporate system FE-SEM (field emission type scanning electronic microscope) " XL30S-FEG " to measure.The crystal grain diameter that decision crystal orientation poor (oblique angle) is the crystal grain in the accurate polygon ferrite that the big angle crystal boundary is surrounded more than 15 °.At this moment mensuration zone is 250 μ m, and measuring step is interval 0.4 μ m, and the confidence index (Confidence Index) of the reliability that expression fixes the position is that the measuring point below 0.1 is deleted from analytic target.In addition, crystal grain diameter is judged as the mensuration noise below the 2.0 μ m, from the object of the mean value calculation of crystal grain diameter except.

[the average equivalent circular diameter d of island martensite body (MA)]

Carry out the test film of minute surface research for the t/4 position of each steel plate and implemented レ ペ ラ one (LePra) corrosion, by the observation by light microscope tissue, with 1000 times of multiplying powers, take the zone at 50 μ m angles with n=10, utilize image analysis apparatus (Media Cybernetics system: Image-Pro Plus), measure diameter of equivalent circle d.

[HAZ toughness test]

The HAZ toughness evaluation assessment of the thermal cycling when carrying out submerged arc welding (2kJ/mm) as simulation, with Heating temperature: 1400 ℃ kept 5 seconds down, be cooled to 800～500 ℃ cooling time (Tc): 25 seconds thermal cycling thereafter, after steel plate heat-treats for examination to each, measure temperature-15 pendulum ℃ down and absorb energy (V otch).Also have, as test film, use be that (t: size 10mm * 10mm * 55mm's that extract position thickness of slab) is bar-shaped, is formed with the test film of the V otch of dark 2mm at the central part single face from thickness of slab t/4 portion.At this moment V pendulum impact value (vE _-15) be qualified more than 50J.

By these results as can be known, experiment No.1～24th, the example of the important document of the invention regulation of satisfied the 3rd invention can be met high strength, and the Kca under-10 ℃ is 5000N/mm ^3/2The Plate Steel of above excellent brittlement crevasse crack propagation halt characteristic.With respect to this, in experiment No.25～47, be the example that breaks away from a certain important document of the 3rd steel plate regulation of inventing, can't obtain certain specific character as can be known.Also have, experiment No.48, the 49th, reference example, though it satisfies the condition of the steel plate defined of first invention,, because of tempering temperature is lower than the condition of the steel plate of making the 3rd invention, so, although the Kca under-10 ℃ satisfies 3500N/mm ^3/2More than, but the Kca under-10 ℃ is lower than 5000N/mm ^3/2

Experimental example 4 (experimental example of the steel plate of the 4th invention)

By common smelting process, the various molten steel that chemical ingredients shown in the following table 19 of melting, 20 is formed cool off this molten steel and after becoming slab, carry out hot rolling and cooling with the condition shown in the following table 21,22, obtain various steel plate (thickness: 60mm).Also have, in following table 19, REM is about 50% and to contain Ce be that the form of about 25% mishmetal is added to contain La.Also have, element is represented not add in "-" hurdle in the following table 19,20.

For each steel plate that obtains,, measure with regard to accurate polygon ferritic hardness, average area rate, solid solution B amount, mechanical characteristics (tensile properties of mother metal, the impact characteristics of mother metal, brittlement crevasse crack stop performance) with the method identical with described experimental example 1.According to following method, measure accurate polygon ferritic average crystal grain diameter, crystal grain distribution, fatigue cracking progression inhibiting characteristic in addition, and also estimate for HAZ toughness.The measurement result of steel plate tissue and solid solution B amount is presented in the following table 23,24, and mechanical characteristics (tensile properties of mother metal, the impact characteristics of mother metal, brittlement crevasse crack progression inhibiting characteristic) is presented in the following table 25,26.Also have, the meaning on "-" hurdle in following table 24 is, because there is not (0%) in accurate polygon ferrite, so can't measure, the meaning on "-" hurdle in the following table 26 is, because the Kca value does not reach the value of expectation, so the mensuration of the advance rate that do not chap.

[accurate polygon ferritic average crystal grain diameter and crystal grain diameter distribute]

Average crystal grain diameter and crystal grain diameter distribute,, use EBSP analytical equipment (TexSEMLaboratries corporate system) and Philips corporate system FE-SEM (field emission type scanning electronic microscope) " XL30S-FEG " to measure.With crystal orientation poor (oblique angle) is that boundary (big angle crystal boundary) more than 15 ° is as crystal grain boundary and be defined as " crystal grain ", the average crystal grain diameter of the accurate polygon ferritic crystal grain of decision.At this moment mensuration zone is 250 μ m, and measuring step is interval 0.4 μ m, and the confidence index (Confidence Index) of the reliability that expression fixes the position is that the measuring point below 0.1 is deleted from analytic target.In addition, when crystal grain diameter is lower than 2.0 μ m, be judged as the mensuration noise, from the object of the mean value calculation of crystal grain diameter except.And, with crystal grain diameter more than 2.0 μ m and determined crystal grain, with the per 1 μ m of the unit of crystal grain diameter (for example initial scope be 2.0 μ m above, be lower than 3.0 μ m, then be 3.0 μ m above, be lower than 4.0 μ m ... such order) counting, make the histogram of crystal grain diameter and grain number thereof, try to achieve the crystal grain diameter [maximum value (1), maximum value (2)] that is judged as maximum value.

[HAZ toughness test]

The HAZ toughness evaluation assessment of the thermal cycling when carrying out submerged arc welding (2kJ/mm) as simulation, with Heating temperature: 1400 ℃ kept 5 seconds down, be cooled to 800～500 ℃ cooling time (Tc): 25 seconds thermal cycling thereafter, after steel plate heat-treats for examination to each, measure temperature-15 pendulum ℃ down and absorb energy (V otch).Also have, as test film, use be that (t: size 10mm * 10mm * 55mm's that extract position thickness of slab) is bar-shaped, is formed with the test film of the V otch of dark 2mm at the central part single face from thickness of slab t/4 portion.At this moment V pendulum impact value (vE _-15) be qualified more than 50J.

[fatigue cracking progression inhibiting characteristic (fatigue cracking advance rate)]

According to ASTM E647, use the small-scale test sheet, implement the test of fatigue cracking progress, try to achieve the fatigue cracking advance rate thus.Also have, (t: extract with 12mm thickness, aligns the back of the body in be full of cracks progress portion and all implement mirror ultrafinish by position thickness of slab) from t/4 for test film.At this moment, the stabilized growth zone that the PARIS rule of following formula (28) defined is set up $\mathrm{\ΔK}=(10\mathrm{MPa}\·\sqrt{\mathrm{}}m)$ Value estimate as typical value.Also have, the judgement criteria of fatigue cracking advance rate shows according to above-mentioned aim, 5.0 * 10 ^-6Mm/cycle is following to be qualified.

da/dn＝C(ΔK) ^m…(28)

[in the formula (28), a: expression be full of cracks length (mm), n: the expression number of occurrence (number of times), C, m: expression is by the constant of the conditional decision of material, load etc.]

By these results as can be known, experiment No.1～22nd, the example of the important document of the steel plate defined of satisfied the 4th invention can be met high strength, and the Plate Steel of brittlement crevasse crack propagation halt characteristic and fatigue cracking progression inhibiting excellent.With respect to this, in experiment No.23～45, be the example that breaks away from a certain important document of the 4th steel plate regulation of inventing, can't obtain certain specific character as can be known.Also have, experiment No.46, the 47th, reference example, though it satisfies the condition of the steel plate defined of first invention,, because of the accumulation draft in the final mill train is bigger than the condition of the steel plate of making the 4th invention, so, although the Kca under-10 ℃ satisfies 3500N/mm ^3/2More than, but fatigue cracking progression inhibiting characteristic is insufficient.

Claims (5)

1. Plate Steel, it is characterized in that, contain C:0.03～0.10% in quality %, Si:0.50% is following and contain 0%, Mn:1.0～2.0%, P:0.015% is following but do not contain 0%, S:0.010% is following but do not contain 0%, Al:0.005～0.060%, Nb:0.020～0.060%, Ti:0.008～0.030%, N:0.0020～0.010% and O:0.010% are following but do not contain 0%, and solid solution B is suppressed at below 0.0005% and contains 0%, at the distance case depth is in the microstructure of position of t/4～t/2, accurate polygon ferritic average area rate is 30～85%, wherein, t represents thickness of slab.

2. Plate Steel according to claim 1 is characterized in that, will be that the average crystal grain diameter of the position of t/4 and t/2 is when being made as d (t/4) and d (t/2) respectively, by the K of following formula (1) regulation apart from case depth ₀Satisfy K ₀＞6750 relation,

K ₀＝5.68×10 ^-20{2.25×10 ^-2[-400·d(t/2) ^-1/2+313] ²-15[-400·d(t/2) ^-1/2+313]+6418} ^6.25/{1-3.288×10 ^-9(-92[-400·d(t/4) ^-1/2+313]+32700) ²} ...(1)。

3. Plate Steel according to claim 1, it is characterized in that, at the distance case depth is the position of t/4, if by 2 crystalline azimuth differencees of adjacency is that the particle diameter of the crystal grain that surrounded of the big angle crystal boundary more than 15 ° is when being effective crystal grain diameter D μ m, in the relation of the yielding stress YS MPa of the average equivalent circular diameter d μ m of island martensite body and steel plate, satisfy following formula (2)

-215+1.56×D+9.79×d+0.24×YS＜-60...(2)。

4. Plate Steel according to claim 1, it is characterized in that, from the distance case depth is that average crystal grain diameter the zone of position to the position of described t/4 of 2mm is below the 50 μ m, and apart from case depth is the crystal grain diameter of the structure of steel in the position of 2mm when being represented by histogram by the relation of the number of die of this crystal grain diameter counting, and number of die is that the difference of great 2 crystal grain diameters is 10～30 μ m.

5. according to each described Plate Steel in the claim 1～4, it is characterized in that, except that described composition, in quality % also contain following (a)～(h) group at least 1 group:

(a) Cu:2% following but do not contain 0%, Ni:2% is following but do not contain 0% and Cr:2% following but do not contain among 0% more than a kind;

(b) Mo:0.5% is following but do not contain 0%;

(c) V:0.1% is following but do not contain 0%;

(d) Mg:0.005% is following but do not contain 0%;

(e) Zr:0.1% following but do not contain 0% and/or Hf:0.05% following but do not contain 0%;

(f) Ca:0.0035% is following but do not contain 0%;

(g) Co:2.5% following but do not contain 0% and/or W:2.5% following but do not contain 0%;

(h) rare earth element: below 0.010% but do not contain 0%.