CN109154041A

CN109154041A - High-strength steel sheet

Info

Publication number: CN109154041A
Application number: CN201680085629.8A
Authority: CN
Inventors: 吉村仁秀; 皆川昌纪; 川端纪正; 都筑岳史
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-07-29
Filing date: 2016-07-29
Publication date: 2019-01-04
Anticipated expiration: 2036-07-29
Also published as: BR112018071948B1; BR112018071948A2; JPWO2018020660A1; CN109154041B; EP3492610A1; KR20180126591A; KR102142472B1; JP6501042B2; EP3492610A4; WO2018020660A1; EP3492610B1

Abstract

A kind of high-strength steel sheet, with defined chemical composition, DI is 2.0~7.8, Pcm is 0.189% or more, metal structure includes a kind or two kinds added up among 99% or more martensite and bainite of the area ratio, the asperratio of original austenite grains is 2.0 or more, the number point rate for the cementite that the length relative to the long axis direction for the cementite that the length of long axis direction is 1.0 μm or more is 0.1 μm or more is 5% or less, plate thickness is 4.5mm~20mm, yield strength is 885MPa or more, tensile strength is 950MPa or more, elongation at break is 12% or more, it can be 59J/cm in -20 DEG C of Charpy absorptions²More than.

Description

High-strength steel sheet

Technical field

The present invention relates to high-strength steel sheets.

Background technique

With the high stratification etc. of building, the enlargement of the building machineries such as derrick car, industrial machine is being promoted.But it is Further enlargement, needs the structural elements lightweight of building machinery, industrial machine.Therefore, in order to by structural elements light weight Change, it is desirable that steel high intensity used in building machinery, industrial machine.

But it is usually disconnected if improving the intensity of steel plate in order to inhibit the weight of component to increase, limiting the plate thickness of steel plate Elongation is split to become smaller.For example, in the case where plate thickness to be limited to 25mm situation below, it is difficult to ensure 12% or more extension at break Rate.In the case where plate thickness to be limited to 8mm situation below, it is more difficult to ensure elongation at break.If elongation at break becomes smaller, process It becomes difficult, therefore in the case where steel plate to be used for the component of building machinery, industrial machine, steel plate is not required nothing more than strong Degree, and require the ductility such as elongation at break.In addition, brittleness is broken in order to prevent in the case where using as structural elements It is bad, it is also desirable to low-temperature flexibility.

From such background, it is proposed that a kind of tensile strength is 780MPa or more and then is the high-strength of 950MPa Spend steel plate and its manufacturing method.

For example, in patent document 1, a kind of high-intensitive and excellent tenacity steel plate and its manufacturing method are proposed, it is described Steel plate is obtained and to reducing C amount, adding alloy so that obtaining the steel progress hot rolling, chilling of suitable hardenability 's.

But in the technology of patent document 1, the processability of steel plate is not accounted for.

In addition, for example, as the steel plate for building machinery etc., proposed after hot rolling in patent document 2~4, The high-intensitive hot rolled steel plate that steel strip coiling is manufactured at web-like, and propose the manufacturing method of the steel plate.Specifically, It is disclosed in patent document 2~4: by being quenched near the temperature (Ms) that martensitic traoformation starts after hot rolling, keeping rule After fixing time, web-like is coiled into, to manufacture with martensitic phase or tempered martensite mutually for the method for the hot rolled steel plate of main phase.It can In these methods, to need to coil into web-like, in the steel plate obtained using these methods, the characteristic of rolling direction and with The characteristic in the vertical direction of rolling direction generates difference, cannot obtain uniform characteristic.Separately fine carbon is generated due to being maintained at The time of the temperature region of compound is elongated, therefore yield strength is got higher, and processability reduces.

In the past, when manufacturing high-strength steel sheet, hot rolling is carried out to warmed-up steel billet, acceleration is cooled to room temperature to make Metal structure become martensite after, in order to improve ductility, toughness and implement tempering (Tempering and Quenching).If making the gold of steel plate Belonging to tissue becomes martensite, then intensity is got higher, but if ensuring ductility, toughness, preferably accelerate it is cooling after implement tempering, To make metal structure become tempered martensite.But from the viewpoint of shortening the construction period, inhibiting manufacturing cost, if omitting this time Fire, then metal structure becomes martensite, although high intensity can be obtained, ductility, toughness are reduced.

In patent document 5, a kind of content for inhibiting Mn and Ni is proposed, and increases the content of Mo and V, inhibits geneva Body generates, and is formed as the high-strength steel sheet and its manufacturing method of the tissue based on lower bainite.

But in the technology of patent document 5, as stopping group obtained from temperature is set as 300~450 DEG C will cool down It is woven to premise, therefore sufficient elongation at break cannot be obtained.Steel plate is made according to the disclosure of patent document 5 in the present inventor, And implement test as a result, not obtaining 12% or more elongation at break.

In this way, it is previous, plate thickness is limited and high-strength steel sheet of the metal structure based on martensite, it is difficult to ensure Ductility and toughness.

In addition, generally being welded in the case where steel plate is applied to above-mentioned structural elements.In welding, for Welding point, from the aspect of the reliability of works, as its tensile strength (strength of joint), it is desirable that wanted to base material It is more than evaluation.But in the case where the steel plate that the Main Tissues to metal structure are martensite is welded, due to welding The softening of heat affected area, the tensile strength (strength of joint) of welding point reduce compared with base material, are unsatisfactory for required value sometimes.

Citation

Patent document

Patent document 1: Japanese Laid-Open 2009-287081 bulletin

Patent document 2: Japanese Laid-Open 2011-52320 bulletin

Patent document 3: Japanese Laid-Open 2011-52321 bulletin

Patent document 4: Japanese Laid-Open 2012-77336 bulletin

Patent document 5: International Publication No. 2012/60405

Summary of the invention

The present invention is in view of such actual situation to complete, project be to provide be well suited for for building machinery, The high-strength steel sheet and its manufacturing method of industrial machine.It is 4.5~20mm, surrenders by force specifically, its project is to provide plate thickness Degree is 885MPa or more, tensile strength is 950MPa or more, can be 59J/cm in -20 DEG C of Charpy absorptions²More than, and break Splitting elongation is 12% or more, and metal structure can substantially ensure welding when having carried out welding based on martensite The high-strength steel sheet and its manufacturing method of the tensile strength of connector.

The present inventor investigates the ductility of steel plate and the cooling relationship for stopping temperature of acceleration.Its result hair It is existing, accelerate it is cooling stop temperature be 300 DEG C or more or be over higher than martensitic traoformation temperature (Mf) in the case where, prolong Malleability reduces.Further progress investigation as a result, it has been found that, at 300 DEG C or more or higher than Mf at a temperature of stopped accelerate cooling In the case of, in metal structure, bainitic transformation occurs for non-transformed austenite, to generate coarse due to the bainite Carbide (cementite) is starting point, excessively generates gap, thus ductility reduction.

Present inventors studied the countermeasures for being directed to such ductility reduction.Its result has obtained following new opinion: for Inhibition bainitic transformation above-mentioned, design can be improved the ingredient of hardenability, moreover, acceleration is cooled to low after hot rolling In 300 DEG C and Mf temperature is hereinafter, thus it enables that metal structure is based on martensite, it can be ensured that high-strength steel sheet prolongs Malleability.

The present invention is completed based on such opinion, and main idea is as follows.

(1) high-strength steel sheet that a mode of the invention is related to, chemical composition in terms of quality %, containing C:0.050~ 0.100%, Si:0~0.50%, Mn:1.20~1.70%, P:0.020% or less, S:0.0050% or less, N:0~ 0.0080%, B:0.0003~0.0030%, Ti:0.003~0.030%, Nb:0.003~0.050%, Cr:0~2.00%, Mo:0~0.90%, Al:0~0.100%, Cu:0~0.50%, Ni:0~0.50%, V:0~0.100%, W:0~ 0.50%, Ca:0~0.0030%, Mg:0~0.0030%, REM:0~0.0030%, surplus are Fe and impurity, contain conjunction 1 kind among 0.20% or more Cr and Mo or two kinds are calculated as, also, when above-mentioned Mo content is more than 0.50%, above-mentioned Cr contains Amount is 0.80% hereinafter, being 2.0~7.8 with the DI that following formula 1 acquires, and is 0.189% or more with the Pcm that following formula 2 acquires, gold Belong to a kind or two kinds added up among 99% or more martensite and bainite that tissue includes the area ratio, original austenite is brilliant The asperratio of grain is 2.0 or more, the cementite that the length of long axis direction is 1.0 μm or more relative to above-mentioned long axis direction Length be 0.1 μm or more the number point rate of cementite be 5% hereinafter, plate thickness is 4.5mm~20mm, yield strength is 885MPa or more, tensile strength are 950MPa or more, and elongation at break is 12% or more, can be in -20 DEG C of Charpy absorptions 59J/cm²More than.

DI=[C]^0.5×{0.34×(1+0.64×[Si])×(1+4.1×[Mn])×(1+0.27×[Cu])×(1+ 0.52 × [Ni]) × (1+2.33 × [Cr]) × (1+3.14 × [Mo]) } × 1.2 formulas 1

Pcm=[C]+[Si]/30+ [Mn]/20+ [Cu]/20+ [Ni]/60+ [Cr]/20+ [Mo]/15+ [V]/10+5 × [B] formula 2

Wherein, above-mentioned formula 1 and [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] in above-mentioned formula 2 are The content in terms of quality % of each element is calculated in the case where not containing as 0.

(2) high-strength steel sheet described in above-mentioned (1), above-mentioned metal structure may include the geneva that the area ratio is 90% or more Body.

(3) high-strength steel sheet described in above-mentioned (1) or (2), Cu content can be 0~0.25 mass %.

(4) described in any item high-strength steel sheets of above-mentioned (1)~(3), Ni content can be 0~0.25 mass %.

(5) described in any item high-strength steel sheets of above-mentioned (1)~(4), V content can be 0~0.050 mass %.

(6) described in any item high-strength steel sheets of above-mentioned (1)~(5), W content can be 0~0.05 mass %.

(7) described in any item high-strength steel sheets of above-mentioned (1)~(6), above-mentioned plate thickness can be 4.5mm~15mm.

(8) above-mentioned Mo content is being denoted as [Mo], above-mentioned Cr by described in any item high-strength steel sheets of above-mentioned (1)~(7) When content is denoted as [Cr], [Mo]/[Cr] can be 0.20 or more.

(9) high-strength steel sheet described in above-mentioned (8), absorbing in -40 DEG C of Charpy can be 59J/cm²More than.

(10) described in any item high-strength steel sheets of above-mentioned (1)~(9), above-mentioned Pcm can be 0.196% or more.

Aforesaid way according to the present invention can be provided under the premise of not alloying element containing high price in large quantities and be bent Take that intensity is 885MPa or more, tensile strength is 950MPa or more, and the high-strength steel sheet that elongation at break is 12% or more. The steel plate shows that in -20 DEG C of Charpy absorption can be 59J/cm²Above excellent toughness.In addition, by making as hardening Property index Pcm be 0.189% or more, preferably 0.196% or more, welded below with defined heat input When, about using high-strength steel sheet of the present invention as the tensile strength of the welding point of base material, it can be ensured that for 950MPa with On.

In turn, it by controlling ratio i.e. [Mo]/[Cr] of the content [Mo] of Mo and the content [Cr] of Cr simultaneously, is also capable of providing It can be 59J/cm in -40 DEG C of Charpy absorptions²The superior high-strength steel sheet of above toughness.

Therefore, the present invention can not be such that manufacturing cost greatly improves and provide and be used for building machinery, industry machine with being well suited for The structural elements of tool, and facilitate enlarged, the light-weighted high-strength steel sheet of building machinery, industrial machine, above-mentioned etc. Contribution in industry is extremely significant.

Detailed description of the invention

Fig. 1 is the stopping temperature for illustrating elongation at break (Total elongation (percentage of total elongation)) and accelerating cooling Tcf, hardenability index DI and C amount relationship figure.

Fig. 2 is to indicate that [Mo]/[Cr] absorbs the figure of the relationship of energy (vE-40) with the Charpy at -40 DEG C.

Fig. 3 is the figure for indicating to accelerate the relationship of cooling stopping temperature and elongation at break.

Fig. 4 A is to indicate by accelerating the SEM photograph of influence of the cooling stopping temperature bring to the shape of cementite, is Make the SEM photograph in the case where accelerating cooling to stop 290 DEG C of temperature.

Fig. 4 B is to indicate by accelerating the SEM photograph of influence of the cooling stopping temperature bring to the shape of cementite, is Make the SEM photograph in the case where accelerating cooling to stop 400 DEG C of temperature.

Fig. 5 is the photo in the gap occurred near coarse cementite.

Specific embodiment

The high-strength steel sheet being related to below to one embodiment of the present invention is (hereinafter sometimes referred to of the present embodiment High-strength steel sheet) it is described in detail.

Firstly, the chemical composition (ingredient) to high-strength steel sheet of the present embodiment is illustrated.Below in relation to containing The label of the % of amount means quality % in case of no particular description.

(C:0.050~0.100%)

C is the useful element for improving hardness of steel, and is the elongation at break for determining to have the steel of martensitic structure Extremely important element.High-strength steel sheet of the present embodiment, in order to obtain sufficient intensity need to make the C amount be 0.050% or more.In order to further increase intensity, C amount be preferably 0.060% or more, 0.065% or more or 0.070% with On.On the other hand, if C amount is more than 0.100%, cause the ductility of steel and toughness bad due to generating excessive carbide Change.Therefore, good elongation at break and toughness in order to obtain need that C amount is made to be 0.100% or less.In order to further make Ductility improves, and C amount is preferably set as 0.095% or less, 0.090% or less or 0.085% or less.

(Si:0.50% or less)

If excessively containing Si, ductility, the toughness of steel are reduced.Therefore, Si amount is limited to 0.50% or less.It is not required to Particularly to provide that the lower limit of Si amount, the lower limit of Si amount are 0%.But in the case where Si is used for deoxidation, in order to obtain its Si amount is preferably set as 0.03% or more by sufficient effect.In addition, Si is also the element for inhibiting carbide to generate, to obtain In the case where the effect, Si amount is preferably set to 0.10% or more, is more preferably set as 0.20% or more.It is not needing to obtain these effects In the case where fruit, the upper limit of Si amount can be set as 0.45%, 0.40% or 0.35%.

(Mn:1.20~1.70%)

Mn is the important element for improving the hardenability of steel.In order to increase martensite the area ratio in metal structure to High intensity is obtained, Mn amount is set as 1.20% or more.Mn amount be preferably set to more than 1.20%, 1.25% or more or 1.30% with It is upper, more preferable to be set as 1.35% or more or 1.39% or more.On the other hand, if Mn amount becomes excessive, sometimes ductility with And toughness reduces.Therefore, Mn amount is set as 1.70% or less.More preferably by Mn amount be set as 1.60% or less, 1.55% or less or Below person 1.50%.

(P:0.020% or less)

(S:0.0050% or less)

P, S is the element inevitably contained in steel as impurity, is the element for deteriorating the toughness of steel.In addition, It is the element for deteriorating the toughness of welding heat affected zone in the case where being welded.Therefore, P amount is limited to 0.020% Below, S amount is limited to 0.0050% or less.In order to further increase toughness, P amount can be set as to 0.015% or less, S amount and set It is 0.0030% or less.The fewer P amount, S amount the better, therefore preferably reduces within the bounds of possibility.It therefore, there is no need to particularly advise Determine the lower limit of P amount and S amount, the lower limit of P amount and S amount is 0%.But from the viewpoint of dephosphorization, desulfurization cost, can be by P Amount is set as 0.001% or more, S and is set as 0.0001% or more.(B:0.0003~0.0030%)

B is in cyrystal boundary segregation to improve the element of the hardenability of steel, is just to embody its effect by micro containing Useful element.B amount is set as by high-strength steel sheet of the present embodiment in order to increase the martensite in metal structure 0.0003% or more.It is preferred that B amount is set as 0.0005% or more.On the other hand, it if excessively containing B, not only improves and hardens Property effect saturation, and form nitride, precipitates, ductility, the toughness such as carbon boride reduce instead.Therefore, B amount is set It is 0.0030% or less.It is preferred that B amount is set as 0.0020% or less or 0.0015% or less.

(Ti:0.003~0.030%)

Ti is the element to form nitride, is to fix the N in steel in the form of TiN, the element for inhibiting BN to generate.As above It stating like that, B is the element for improving hardenability, if but form BN, it cannot get its effect.High strength steel of the present embodiment Plate ensures hardenability to inhibit BN to be formed, needs Ti amount being set as 0.003% or more.It is preferred that Ti amount is set as 0.005% More than, Ti amount is more preferably set as 0.010% or more.On the other hand, if excessively containing Ti, TiN is become thick, and is existed and is prolonged The case where malleability, toughness reduce.Therefore, Ti amount is set as 0.030% or less.It is preferred that Ti amount is set as 0.020% or less.

(Nb:0.003~0.050%)

Nb is by simultaneously containing the element significantly improve the hardenability of steel with B.Height of the present embodiment Nb amount is set as 0.003% or more to increase the area ratio of the martensite in metal structure by strength steel sheet.Nb is also shape At fine nitride, facilitate the miniaturization of crystal grain, improves the element of toughness.In the case where to obtain the effect, preferably Nb amount is set as 0.005% or more.Nb amount is more preferably set as 0.010% or more or 0.015% or more.On the other hand, if Excessively contain Nb, then nitride becomes thick, the case where reduction there are ductility, toughness.Therefore, Nb amount is set as 0.050% or less.It is preferred that Nb amount is set as 0.040% or less, 0.035% or less or 0.030% or less.

(Cr:2.00% or less)

(Mo:0.90% or less)

(a kind containing the Cr and Mo for adding up to 0.20% or more or two kinds, also, when Mo content is more than 0.50%, Cr Content is 0.80% or less)

Cr and Mo is the important element for improving hardenability, contains its one side or both.Height of the present embodiment Cr amount and the total of Mn amount are set as 0.20% or more to increase the martensite the area ratio in metal structure by strength steel sheet. Cr amount and the total of Mn amount are preferably set to 0.30% or more, are more preferably set as 0.40% or more.Also allow for only containing Cr or The lower limit of the case where person Mo, Cr amount and Mo amount is 0%.As needed, the lower limit of Cr amount can be set as 0.20% or 0.30%, likewise it is possible to which the lower limit of Mo amount is set as 0.20% or 0.30%.

In addition, if Cr amount is more than 2.00%, alternatively, generating fine carbide if Mo amount is more than 0.90%, prolonging Malleability, toughness reduce.Therefore, Cr amount and Mo amount are set to 2.00% or less and 0.90% or less.Cr amount is preferably 1.50% or less or 1.00% hereinafter, more preferably 0.90% or less or 0.80%.In addition, Mo amount be preferably 0.70% with Under, more preferably 0.60% or less or 0.50%.

Moreover, if content becomes excessive, toughness reduces, therefore in Mo amount in the case where containing this two side of Cr and Mo When more than 0.50%, need Cr amount being set as 0.80% or less.In this case, Cr amount can also be set as 0.70% or less. On the other hand, when Cr amount is more than 0.80%, Mo amount is set as 0.50% or less preferably, when Cr amount is more than 1.20%, by Mo Amount is set as 0.40% or less preferably.The total of Cr amount and Mo amount can be set as 2.50% hereinafter, but also can be set to 2.00% Below, 1.50% or less, 1.30% or less or 1.10% or less.

(N:0.0080% or less)

N is impurity, is inevitably contained.N forms BN, the effect for the raising hardenability that damage B has.Therefore, by N amount It is limited to 0.0080% or less.It is preferred that N amount is limited to 0.0060% hereinafter, N amount is more preferably limited to 0.0050% or less. N amount preferably reduces within the bounds of possibility, and lower limit is set as 0%.It, can be by N amount but from the viewpoint of the cost of denitrogenation It is set as 0.0001% or more.It on the other hand, can also be by N for the miniaturization for realizing metal structure using nitride Amount is set as 0.0020% or more.

It is above indispensable element and impurity as high-strength steel sheet of the present embodiment and the element contained, this The high-strength steel sheet that embodiment is related to, to have Fe and impurity (including the above-mentioned impurity by above-mentioned indispensable element and surplus Element and the according to circumstances impurity element other than the above that contains) composition ingredient based on.But present embodiment relates to And high-strength steel sheet, in addition to the above ingredients, in order to the raising of deoxidation, intensity and/or ductility, metal structure it is fine The form of change and sulfide control etc., also can replace a part of Fe and also contains Al:0.100% or less, Cu:0.50% Below, Ni:0.50% or less, V:0.100% or less, W:0.50% or less, Ca:0.0030% or less, Mg:0.0030% with Under, a kind in REM:0.0030% or less or two or more.But these elements are not required, therefore may be 0%.

(Al:0.100% or less)

Al is deoxidant element, and in the case where Al is used for deoxidation, Al amount, is preferably set as by sufficient effect in order to obtain 0.010% or more.On the other hand, if excessively containing Al, due to oxide, nitride formation and lead to ductility, tough Property reduce.Therefore, even the case where containing, Al amount is also limited to 0.100% or less.Be preferably limited to 0.080% hereinafter, 0.050% is more preferably limited to hereinafter, being further preferably limited to 0.030% or less.

(Cu:0.50% or less)

(Ni:0.50% or less)

Cu and Ni is the element for improving the hardenability of steel.Improve hardenability come make in metal structure geneva dignity In the product increased situation of rate, Cu amount, Ni amount are preferably set to 0.10% or more.On the other hand, Cu and Ni is high price Element, even therefore containing the case where it is also preferred that Cu amount, Ni amount are set to 0.50% or less.More preferably by Cu amount with And Ni amount is set to 0.40% hereinafter, being further preferably set to 0.30% or less.

(V:0.100% or less)

V is the element to form carbide, nitride.Crystal grain is set to miniaturize to improve using carbide, nitride In the case where toughness, V amount is preferably set as 0.005% or more.On the other hand, if excessively containing V, ductility, toughness drop It is low.But compared with Nb, Ti, adverse effect is small, therefore the upper limit of the V amount in the case where containing is set as 0.100%.It is preferred that V amount is set as 0.050% or less.

(W:0.50% or less)

W is the element for improving the hardenability of steel.In the case where to obtain the effect, W amount is preferably set as 0.05% More than.On the other hand, if excessively containing W, weldability deterioration.Therefore, even W amount is also set as by the case where containing 0.50% or less or 0.30% or less.W amount can be set as to 0.02% or less or 0.01% or less as needed.

(Ca:0.0030% or less)

Ca is the element of control oxide, the form of sulfide.In the case where to obtain the effect, preferably Ca amount is set It is 0.0001% or more.Ca amount is more preferably set as 0.0005% or more, is further preferably set as 0.0010% or more.Another party Face, if excessively containing Ca, not only effect be saturated, but also sometimes due to the formation of field trash and cause ductility, toughness drop It is low.Therefore, even Ca amount is also set as 0.0030% or less by the case where containing.

(Mg:0.0030% or less)

Mg is that have the function of improving the element of the toughness of steel by that will organize miniaturization.In the feelings that obtain the effect Under condition, Mg amount is preferably set as 0.0005% or more.On the other hand, if excessively containing Mg, not only effect is saturated, Er Qieyou When due to field trash formation and cause ductility, toughness reduce.Therefore, even Mg amount is also set as by the case where containing 0.0030% or less.

(REM:0.0030% or less)

REM (rare earth element) is that have the function of that the form by control sulfide, particularly MnS improves the toughness of steel Element.In the case where to obtain the effect, REM amount is preferably set as 0.0001% or more.On the other hand, if excessively containing There is REM, then the field trash coarsening containing REM, ductility, toughness reduce sometimes.Therefore, even the case where containing also will REM amount is set as 0.0030% or less.

In addition, can also contain as long as in the range of not detrimental effect effect other than above-mentioned element microly Other elements.

High-strength steel sheet of the present embodiment determines that each element in above-mentioned range, and by chemical composition DI and Pcm need to meet range below.

(DI:2.0~7.8)

DI is the index of hardenability, is acquired by following formula 1.Here, [C], [Si], [Mn], [Cu], [Ni] in formula, [Cr], [Mo] are the content (quality %) of each element, are calculated in the case where being free of the element as 0.

As shown in figure 1 qualitatively illustrated by, if improving hardenability index DI, accelerate cooling stopping temperature even if improving Tcf (that is, mobile to the right of Fig. 1) is spent, the reduction of elongation at break is also able to suppress.Accelerate cooling stopping temperature if improving Tcf, then the excessive rising of intensity is suppressed, and can be improved toughness, ductility.Keep the balance of intensity, ductility, toughness good If good, DI is preferably set as 2.0 or more.More preferable DI is 3.0 or more, and further preferred DI is 4.0 or more.On the other hand, The case where if hardenability is exceedingly got higher, intensity is exceedingly got higher, and there are toughness reductions.Therefore DI is preferably 7.8 or less.More It is preferred that DI is 7.0 hereinafter, further preferably DI is 6.5 or less.

(Pcm:0.189% or more)

It is the tensile strength to the base material for welding that welding point, which is usually required to its tensile strength (strength of joint), It is more than required value.The inventors discovered that the case where the steel plate that the Main Tissues to metal structure are martensite has carried out welding Under, there is the softening due to welding heat affected zone and causes the tensile strength (strength of joint) of welding point and the tension of base material strong Degree is compared to reduction situation.Therefore, the present inventor uses various high-strength steel sheets, and weld heat input is made to change and manufacture and connect Head, and tested.Its result is it is found that pass through the hardenability for improving steel plate, specifically, by making to be acquired by following formula 2 Pcm be 0.189% or more, in the softening to inhibit welding heat affected zone, and be applied to building machinery, industrial machine mostly In the case that the lower limit value 7.0kJ/cm of the weld heat input range of the manufacture of structural elements is welded, it can make to connect The tensile strength of head is 950MPa or more.

Wherein, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], the content (quality %) that [B] is each element, It is calculated in the case where being free of the element as 0.

In turn, inventor studies the intensity of weld heat input and welding point, and the intensity of welding point can adopt Evaluated with JS, the JS be using according to the high-strength steel sheet for welding at being grouped as, the Pcm that is acquired by above-mentioned formula 2, And weld heat input Hi [kJ/cm], it is calculated by following formula a, it is known that if JS is 950MPa or more, even actual Welding point can also ensure that the strength of joint of 950MPa or more.

JS=(4.3/Hi+3.4) × (1680.7 × Pcm-81.5) formula a

By above-mentioned formula it is found that in order to ensure welding point intensity, as far as possible reduction weld heat input preferably.It can It is that, in order to ensure the viability of welding point, there are the lower limits of weld heat input.In order to ensure making building machinery, industry machine The productivity etc. of weld job when tool, decreases below 7.0kJ/cm for weld heat input and is not easy to.In the sealing wire energy It is 0.189% according to above-mentioned formula to make the Pcm needed for JS 950MPa or more in the case where measuring 7.0kJ/cm.That is, By making 0.189% or more Pcm, it can be ensured that the strength of joint of 950MPa or more.

In addition, if making 0.196% or more Pcm, even as not needing particularly to manage in welding procedure Weld heat input 10.0kJ/cm the case where, can also ensure that the strength of joint of 950MPa or more.That is, by making Pcm is 0.196% or more, even if can also make the intensity 950MPa of welding point without special welding procedure management More than.

Furthermore even Pcm can be set as the intensity that bigger weld heat input also ensures that welding point 0.200% or more, 0.205% or more, 0.210% or more or 0.215% or more.When weld heat input is big, weldering can reduce The number of plies connect, productivity improve, therefore it is preferred that.The upper limit of special provision Pcm is not needed, but weld crack etc. in order to prevent, it can be with It is set as 0.250% or less or 0.240% or less.

([Mo]/[Cr]: 0.20 or more)

In turn, the influence that inventor's investigation gives toughness as Cr, Mo of the element for improving hardenability, and ground Study carefully.It is found that in the case where hardenability (DI) is certain, the ratio of Mo and Cr give toughness to be influenced its result.Specifically, if The ratio ([Mo]/[Cr]) of the content [Cr] of the content [Mo] and Cr of Mo in terms of quality % becomes larger, then the lower part tissue of martensite (lath beam (packet), lath block (block)) becomes fine, as a result, knowing that toughness improves.It is tough in order to more improve Property, it can be by this than being set as 0.40 or more, 0.80 or more or 1.00 or more.

Fig. 2 is to indicate that [Mo]/[Cr] and the Charpy at -40 DEG C absorb the figure of the relationship of energy."○" in Fig. 2 indicates real Measured value, "●" indicate the average value of above-mentioned measured value.

As shown in Figure 2 it is found that becoming larger in the presence of with [Mo]/[Cr], the tendency that can increase is absorbed in -40 DEG C of Charpy, if [Mo]/[Cr] becomes 0.20 or more, then the Charpy absorption at -40 DEG C can become 59J/cm²More than.Therefore, requiring low temperature tough In the case where property, [Mo]/[Cr] is preferably set as 0.20 or more.On the other hand, Mo is the fine carbon easy to form compared with Cr The element of compound and cluster (cluster).It therefore, the case where there are toughness reductions, can be with if excessively containing Mo compared with Cr [Mo]/[Cr] is set as 2.00 or less or 1.50 or less.

Charpy absorbs energy, is measured by the Charpy-type test carried out according to JIS Z 2242.But produce test specimen The plate thickness of steel plate is 8mm, makes length direction rolling direction, and the test specimen produced from plate thickness central part is the small ruler of 10mm × 5mm It is very little.

(a kind or 2 kinds of total the area ratio in martensite and bainite: 99% or more, also, elongation at break: 12% More than)

The present inventor grinds the hardenability and metal structure of high-strength steel sheet and the relationship of elongation at break Study carefully.As a result, present inventor have discovered that if hardenability is insufficient, elongation at break is reduced, moreover, the elongation at break reduces, i.e. The reason of ductility reduction, which is as shown in Fig. 4 A, Fig. 4 B, with the coarse carbide generated due to bainite, is The generation in the gap of point.Moreover, obtaining following opinion: if the ductility that improve high-strength steel sheet, needing to inhibit to become thick The generation of the bainite of the generational verctor of big cementite.It is excellent in order to inhibit the bainite of the generational verctor as coarse cementite Form slection become metal structure 90% the above are the martensite subject organizations of martensite.In addition, in order to improve the intensity of steel plate, It is preferred that the martensite the area ratio in metal structure is set as 90% or more.More preferably 92% or more, further preferably 94% More than.

But martensite and bainite are all continuous cooling transformation tissues, and it is accurate to be difficult to carry out sometimes through structure observation Differentiation.In this case, if total the area ratio of martensite and bainite is 99% or more and elongation at break It is 12% or more, then can be judged as the bainite for inhibiting the generational verctor as coarse cementite.

Therefore, for high-strength steel sheet of the present embodiment, by a kind or 2 kinds in martensite and bainite Total the area ratio be set as 99% or more, also, the elongation at break of the index as tissue is set as 12% or more.Passing through In the case that structure observation can fully differentiate martensite and bainite, preferably by the area ratio of martensite be set as 90% with On.

In the case where high-strength steel sheet of the present embodiment, the martensite of metal structure is quenching state, and is passed through Tempered martensite obtained from tempering is different.If it is tempered martensite, then by being tempered for a long time, cementite It can grow, therefore not preferably.

Rest part other than the above, as long as ferrite, pearlite, a kind among retained austenite or two or more ?.

The measurement of the area ratio of the differentiation and martensite of metal structure is carried out using optical microscopy.Specifically, By section parallel with rolling direction of the portion 1/4t (1/4 part for being plate thickness t apart from surface of steel plate on plate thickness direction) nearby Face nital corrodes, and using optical microscopy, 2 views of the range of 100 μm of 120 μ m are shot under 500 times , and determine the area ratio of the tissue of acicular battened construction prosperity.In addition, among its acicular structure, by the section of steel plate After electrolytic polishing, the portion 1/4t in steel plate section is nearby observed using scanning electron microscope (SEM).Here, multiplying power It is 5000 times, shoots the range of 40 μm of 50 μ m.Cementite long axis direction more than lath block interior orientation is at 2 directions In the case of, acicular structure is considered as martensite, finds out the area ratio in the region.By under optical microscopy acicular structure the area ratio with The area ratio of the product of the area ratio of martensite under SEM as the martensitic structure of the steel grade.

In the above-mentioned structure observation carried out using scanning electron microscope, cementite can not clearly be differentiated by having Long axis direction is in lath block interior orientation at situations more than 2 directions.It in this case, will acicular plate under an optical microscope Total the area ratio of the area ratio of the tissue of structure prosperity as martensite and bainite.

(length relative to long axis direction for the cementite that the length of long axis direction is 1.0 μm or more is 0.1 μm or more The number of cementite divides rate: 5% or less)

As described above, in order to improve the ductility of steel plate, inhibit the bainite for becoming the generational verctor of coarse cementite Generation, be formed as the metal structure based on martensite and be important.But in order to further increase ductility, inhibit Generation with the gap that coarse carbide (especially cementite) is starting point is effective.

The inventors discovered that accelerating cooling stopping temperature by control, the length that can reduce long axis direction is 1.0 μm The number of above coarse carbide (especially cementite) divides rate, as a result, it is possible to inhibit the generation in gap, fracture is sought to stretch The raising of long rate.Specifically, discovery: passing through among 0.1 μm of length or more that makes long axis direction of cementite, long axis side To length be 1.0 μm or more the number point rate of cementite be 5% hereinafter, elongation at break can be made to improve.

It can be described in detail, in the present invention, be accelerated by the at a temperature of stopping in Mf temperature below and lower than 300 DEG C cold below But, it is capable of forming the tissue based on martensite to inhibit coarse carbide to generate.That is, being accelerated by control Cooling stopping temperature being able to suppress the coarse cementite for being 1.0 μm or more using the length of long axis direction as the gap of starting point It generates.

The measurement of a number density of cementite is carried out using scanning electron microscope (SEM).Specifically, by steel After the section of plate carries out electrolytic polishing, near the portion 1/4t in steel plate section, multiplying power is set as 5000 times, utilizes sweep type electricity Sub- microscope (SEM) shoots the range of 40 μm of 50 μ m.From the contrast (comparison: contrast) of obtained image, figure is used As analysis software, the precipitate that asperratio is 2.0 or more and the length of long axis direction is 0.1 μm or more is considered as carburizing Body, to count number.It similarly, is 2.0 or more to asperratio and carburizing that the length of long axis direction is 1.0 μm or more The number of body is counted.Then, by the number of obtained 1.0 μm or more of precipitate divided by 0.1 μm or more of carburizing Body number, come find out 1.0 μm or more cementite number point rate (%).The shape of carbide is not particularly limited, but is, for example, In the case where spherical, " length of long axis direction " refers to major diameter.

(asperratio of original austenite grains is 2.0 or more)

For high-strength steel sheet of the present embodiment, the asperratio of original austenite grains is set as More than 2.0.In the case where asperratio is less than 2.0, toughness reduction can be worried.

Furthermore in the case where having carried out accelerating cooling (direct quenching) online after the rolling of non-recrystallization region, it can make The asperratio of original austenite grains is 2.0 or more.On the other hand, after rolling and cooling, reheated into It has gone in the case where quenching, the worked structure as obtained from rolling is not inherited, the asperratio of original austenite grains Become smaller than 2.0.

The asperratio of original austenite grains is measured using the following method.That is, will as on plate thickness direction away from The section nital parallel with rolling direction near the portion 1/4t from 1/4 position that surface is plate thickness t is rotten Erosion, be set as 500 times using optical microscopy, and by multiplying power, come shoot 100 μm of 120 μ m range 2 visual fields.From gained The image arrived, the length of length and short axle at least 50 or more original austenite grains measurement long axis, long axis length Divided by minor axis length, to find out asperratio for each crystal grain.Then, the ruler in length and breadth of these original austenite grains is found out The average value of very little ratio.

Then, the plate thickness of high-strength steel sheet of the present embodiment, mechanical property are illustrated.

(plate thickness: 4.5~20mm)

The plate thickness of high-strength steel sheet for crane etc. is generally 4.5~20mm.Therefore, by height of the present embodiment The plate thickness of strength steel sheet is set as 4.5~20mm.But in terms of facilitating lightweight, preferably 4.5~15mm.

(yield strength: 885MPa or more)

(tensile strength: 950MPa or more)

In addition, in order to facilitate the enlargement of building machinery, industrial machine and lightweight, it is desirable that high intensity, in order to aobvious It lands to obtain the effect of economy, needs that yield strength 885MPa or more, tensile strength is made to be 950MPa or more.Spy is not needed Not Gui Ding yield strength the upper limit, but can be set to 1100MPa or less.The upper limit of special provision tensile strength is not needed, but can To be set as 1300MPa or less or 1250MPa or less.

(elongation at break: 12% or more)

In order to which high-strength steel sheet to be applied to the component of building machinery, industrial machine, it is desirable that the processabilities such as bendability, therefore Elongation at break is set as 12% or more.In addition, as described above, elongation at break is also whether to inhibit as coarse carburizing The tissue markers of the bainite of the generational verctor of body.

Yield strength, tensile strength, elongation at break carry out tension test according to JIS Z 2241 to measure.But it draws The value for stretching the elongation at break of test depends on the shape of test specimen.The restriction (12% or more) of above-mentioned elongation at break is to make Having used No. 5 test specimens of JIS Z2241 for tensile test specimen, (distance is 50mm between former punctuate, the width of parallel portion is 25mm, test specimen The thickness with a thickness of steel plate state plate test specimen) in the case where value.

Different, elongation conversion formula based on Specimen Shape, is also prescribed, using JIS in ISO2566-1 12% elongation when No. 5 test specimens of Z2241, when the 13B test specimen (distance between former punctuate that tensile test specimen is JIS Z2241 Width for 50mm, parallel portion is 12.5mm, the plate test specimen of the state of the thickness with a thickness of steel plate of test specimen) when can change It is counted as being 10.4%, when (distance is the width of 80mm, parallel portion to the 13A test specimen that tensile test specimen is JIS Z2241 between former punctuate For the plate test specimen of the state of 20mm, the thickness with a thickness of steel plate of test specimen) when can convert as 9.5%.

(energy: 59J/cm is absorbed in -20 DEG C of Charpy²More than)

In building machinery, industrial machine in the case where cold district use, exist tough to high-strength steel sheet requirement low temperature The case where property.It is therefore preferable that the Charpy absorption at -20 DEG C can be 59J/cm².More preferably -40 DEG C Charpy absorption can be 59J/cm²More than.

Charpy absorbs energy, the test specimen for making length direction rolling direction is produced from plate thickness central part, at -20 DEG C or -40 DEG C measure based on the Charpy-type test of JIS Z 2242.According to the plate thickness of steel plate, be difficult to produce sometimes 10mm × The full-scale test specimen of 10mm uses the test specimen of small size in this case.Charpy absorbs energy, is that absorption can be divided by V notch Test specimen sectional area (the cm of bottom²) obtained value (J/cm²).For example, in the case where the full-scale test specimen of 10mm × 10mm, In the case where the small-size test piece of 10mm × 5mm, the Charpy determined absorb can value (J) removed in the case where full-scale test specimen With 1cm × 0.8cm=0.8cm², divided by 0.5cm × 0.8cm=0.4cm in the case where small-size test piece², to acquire.

Then, method preferred in the manufacture of high-strength steel sheet of the present embodiment is illustrated.

Chemical composition of the conventional method melting with above range can be used in high-strength steel sheet of the present embodiment Molten steel casts the molten steel, by obtained heating steel billet, carries out hot rolling, carries out acceleration cooling, accelerates cooling stop Naturally cool to room temperature as former state afterwards to manufacture.But when manufacturing high-strength steel sheet of the present embodiment, accelerate cooling After stopping or after naturally cooling to room temperature, do not implement the Tempering and Quenchings such as tempering.If carrying out modifier treatment, martensite becomes back Fiery martensite.That is, high-strength steel sheet of the present embodiment, uses for the purpose of shortening the construction period, cut down manufacturing cost and saves The so-called Micro Alloying manufacturing process of Tempering and Quenching has been omited to manufacture high-strength steel sheet.Using Micro Alloying, manufacturing process is produced High-strength steel sheet of the present embodiment, sometimes referred to as high-strength non-hardened and non-tempered steel plate.

It is illustrated below for the preferred condition of each process.

(heating temperature of steel billet: 1100~1250 DEG C)

High-strength steel sheet of the present embodiment needs the alloying element containing specified amount to improve hardenability.Cause This, generates carbide, the nitride of alloying element in the steel billet for hot rolling.When by heating steel billet, need to make this A little carbide, nitride decompose to be solid-solution in steel, and heating temperature is set as 1100 DEG C or more.On the other hand, if steel billet Heating temperature is excessively high, then crystallization particle diameter becomes thick, and toughness reduces sometimes, therefore heating temperature is set as 1250 DEG C or less.

(finishing temperature: Ar3 (DEG C) or more)

(accelerating cooling start temperature: Ar3 (DEG C) or more)

Hot rolling is carried out to warmed-up steel billet.In order to after carrying out hot rolling, by accelerate it is cooling come be formed as with Metal structure based on martensite needs to start at a temperature of metal structure is austenite to accelerate cooling.Therefore, it is necessary to Metal structure is hot rolling of ending at a temperature of austenite.Therefore, the finishing temperature of hot rolling is set as Ar3 (DEG C) or more.Ar3 (DEG C) is the temperature of the ferrite transformation since austenite while cooling, can be found out according to thermal expansion behavior.In addition, Ar3 (DEG C) can simply be found out using such as following formula b.

Ar3=868-396 × [C]+24.6 × [Si] -68.1 × [Mn] -36.1 × [Ni] -20.7 × [Cu] - 24.8 × [Cr]+29.6 × [Mo] formula b

Here, [C], [Si], [Mn], [Ni], [Cu], [Cr], [Mo] are the contents (quality %) of each element, without member It is calculated in the case where element as 0.

Hot rolling is carried out using conventional method, but is preferably made the accumulation pressure in 1050 DEG C or more of temperature range The rolling of recrystallization region that lower rate is 50~80% and make the temperature range of Ar3~950 DEG C accumulation reduction ratio 50~ 90% non-recrystallization region rolling.

(the cooling cooling velocity of acceleration: 30~200 DEG C/s)

The acceleration for connecting and carrying out with hot rolling is cooling, generates martensite.Accelerate cooling cooling velocity, in order to make horse The area ratio of family name's body increases, and needs to be set as 30 DEG C/s or more.If cannot get sufficiently martensitic the area ratio less than 30 DEG C/s. In order to promote martensitic traoformation, preferably quickening cooling velocity, but the restriction due to having plate thickness, equipment can be set the upper limit For 200 DEG C/s or less.About cooling velocity, the temperature change on the surface of the steel plate after measuring hot rolling, water cooling start before table The poor of surface temperature after face temperature and just water cooling stop calculates divided by the cooling required time.

(accelerate cooling stopping temperature: Mf (DEG C) is below and less than 300 DEG C)

The present inventor is for hardenability and accelerates cooling stopping temperature and the pass of metal structure and elongation at break System is studied.Here, in the case where carrying out chilling to steel plate after hot rolling, temperature Ms (DEG C) that martensitic traoformation starts It is found out with following formula 3.In addition, the temperature Mf (DEG C) that martensitic traoformation is ended is about 150 DEG C of temperature lower than Ms (DEG C), use is following Formula 4 is found out.[C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [Al] of following formula 3 are the contents (quality %) of each element, It is calculated in the case where being free of the element as 0.

Ms=550-361 × [C] -39 × [Mn] -35 × [V] -20 × [Cr] -17 × [Ni] -10 × [Cu] -5 × [Mo]+30 × [Al] formula 3

Mf=Ms-150 formula 4

In order to make metal structure become martensite, need at least to be cooled to Ms (DEG C) temperature below, if cooling (chilling) To Mf (DEG C) temperature below, then 90% or more of metal structure becomes martensite.It but is 300 DEG C in the cooling temperature that stops In the case where above, cooling the case where becoming unstable, becoming bainite there are a part of martensite, therefore cooling stopping temperature Degree is set as Mf (DEG C) below and less than 300 DEG C.

Accelerate cooling stopping temperature, it is particularly important as described above, in the temperature Ms (DEG C) started than martensitic traoformation Low temperature stops becoming precondition.Moreover, if accelerating, to be cooled to the temperature Mf (DEG C) that martensitic traoformation is over below and small In 300 DEG C of temperature, then metal structure becomes the tissue for the geneva phosphor bodies for inhibiting carbide to generate.

On the other hand, in the case where accelerating cooling stopping temperature between Ms (DEG C) and Mf (DEG C) (between Ms-Mf), The ductility of high-strength steel sheet is influenced by hardenability.That is, inhibiting the generation of bainite if improving hardenability, along with this, seeping The generation of the coarse carbide of carbon system is suppressed, and elongation at break improves, and deviation also becomes smaller.

Moreover, ought qualitatively arrange the relationship for accelerating cooling stopping temperature Tcf and Mf and elongation at break, DI and It, can schematically as shown in Figure 1 when the influence that C amount gives elongation at break.Here, the longitudinal axis of Fig. 1 is that elongation at break is (total Elongation), horizontal axis be that accelerate cooling stopping temperature Tcf, DI be the index of the hardenability found out with above-mentioned formula 1.

As shown in the plot of figure 1, if cooling stopping temperature Tcf is accelerated to reduce, martensitic traoformation is promoted, it is suppressed that The generation of bainite, thus elongation at break improve, if Tcf become Mf hereinafter, if elongation at break become constant.If Tcf becomes For Mf hereinafter, then elongation at break is substantially determined that improving reduces C content by C content, elongation at break is improved.

On the other hand, in the case where accelerating cooling stopping temperature Tcf between Ms-Mf, with the reduction of Tcf, fracture Elongation improves, if but add alloying element at this time in order to improve hardenability, DI increases, to inhibit the life of bainite At, inhibit the generation of coarse carbide, thus elongation at break improve.

Accelerate the lower limit of cooling stopping temperature being not specially limited, can also accelerate to be cooled to room temperature.If using making The effect of fixed carbon atom of dislocation etc. is come if improving yield strength, accelerating cooling stopping temperature is preferably 100 DEG C or more.

After accelerating cooling stop, not implementing the Tempering and Quenchings such as tempering and naturally cool to room temperature as former state.

Embodiment

The embodiment of the present invention is illustrated below.Condition in embodiment described below is to confirm this hair Bright exploitativeness and effect and the condition example used, the present invention are not limited by the condition example.In addition, energy of the present invention It is enough that various conditions are used under the limit for not departing from the gist of the invention and reaching the purpose of the present invention.

By steel billet obtained from chemical component shown in melting table 1 (surplus is Fe and impurity), made using shown in table 2 Condition is made the steel plate of 4.5~20mm of plate thickness is made." heating temperature " indicates the relation reheating temperature of steel billet, " rolling finishing temperature " Indicate that the finishing temperature of hot rolling, " water cooling start temperature " indicate the surface temperature of steel plate when cooling (water cooling) being accelerated to start, " cooling velocity " indicates in the cooling temperature range for stopping temperature of Ar3 (DEG C)~acceleration, plate thickness central part average cooling Speed, " water cooling stopping temperature " indicate the surface temperature of steel plate when water cooling stops.The surface temperature of steel plate uses radiation temperature Meter measurement, " cooling velocity " calculate the temperature for finding out plate thickness central part by heat transfer according to surface temperature to calculate.No matter Which kind of steel plate is not all tempered.

To the metal structure of obtained steel plate, mechanical property (yield strength, tensile strength, elongation at break, toughness, Strength of joint) it is evaluated.

The measurement of the area ratio of the differentiation of metal structure and martensite and bainite is carried out using the following method.

After the section of steel plate is carried out mirror ultrafinish, by the section parallel with the rolling direction nitric acid second near the portion 1/4t Alcohol corrosion corrosion is shot 2 visual fields of the range of 100 μm of 120 μ m under 500 times, is determined needle using optical microscopy The area ratio of the tissue of the battened construction prosperity of shape.In addition, about its acicular structure, it, will after the section electrolytic polishing of steel plate It is nearby observed using scanning electron microscope (SEM) in the portion 1/4t in steel plate section.Here, multiplying power is 5000 times, shooting The range of 40 μm of 50 μ m.It is needle-shaped in the long axis direction of cementite in the case where lath block interior orientation is at more than 2 directions Tissue is considered as martensite, and finds out the area ratio in the region.By the horse under the acicular structure the area ratio and SEM under optical microscopy The area ratio of the product of the area ratio of family name's body as the martensite of the steel grade.In addition, the acicular structure other than martensite is considered as shellfish Family name's body.

Furthermore in the above-mentioned structure observation carried out using scanning electron microscope, it can not clearly differentiate infiltration The long axis direction of carbon body, will acicular lath under an optical microscope in the case where lath block interior orientation is at more than 2 directions Total the area ratio of the area ratio of the tissue of structure prosperity as martensite and bainite.

It is 99% or more or in the feelings that can clearly judge martensite by total the area ratio of martensite and bainite Martensite the area ratio is set as target value for 90% or more under condition.

Tissue (rest part) other than " martensite and the bainite " recorded in table 3 is ferrite, pearlite, bayesian 1 kind in body, retained austenite or two or more.

In turn, by after the section electrolytic polishing of steel plate, nearby utilize scanning electron micro- in the portion 1/4t in steel plate section Mirror (SEM) is observed, and a number density of cementite is determined.Specifically, by after the section electrolytic polishing of steel plate, for steel Near the portion 1/4t in plate section, multiplying power is set as 5000 times, 50 μ ms 40 are had taken using scanning electron microscope (SEM) μm range.From the contrast of obtained image, image solution is used from the contrast of obtained image using image analysis software Software is analysed, the precipitate that asperratio is 2.0 or more and the length of long axis direction is 0.1 μm or more is considered as cementite, is come Count number.It similarly, is 2.0 or more to asperratio and of cementite that the length of long axis direction is 1.0 μm or more Number is counted.Then, by the number of obtained 1.0 μm or more of precipitate divided by 0.1 μm or more of cementite number, Come find out 1.0 μm or more cementite number point rate (%).Furthermore if the number point rate of 1.0 μm or more of cementite is 5% hereinafter, be then judged as good.

In turn, the asperratio of original austenite grains is determined.Specifically, by near the portion 1/4t with The parallel section of rolling direction is corroded with nital, using optical microscopy, multiplying power is set as 500 times, to shoot 2 visual fields of the range of 120 100 μm of μ ms.From obtained image, at least 50 or more original austenite grains are surveyed The length of fixed length axis and the length of short axle, long axis length is divided by minor axis length, to find out asperratio for each crystal grain.So Afterwards, the average value of the asperratio of these original austenite grains, and the size in length and breadth as original austenite grains are found out Than.Furthermore if the asperratio of original austenite grains is 2.0 or more, it is judged as good.

In turn, test specimen (complete thick) is produced from steel plate, tensile strength is determined according to JIS Z 2241, yield strength, is broken Split elongation.In addition, determined according to JIS Z 2242-20 DEG C and-40 DEG C at a temperature of Charpy absorb energy.It draws Stretching test specimen is No. 5 test specimens (complete thick) for keeping length direction vertical with rolling direction and producing, and yield strength refers to that offset yield is strong Spend σ_0.2(0.2% endurance).Charpy impact test specimen, be make length direction rolling direction and produced from plate thickness central part 10 × The small-size test piece of 5mm.

These test as a result, yield strength is 885MPa or more, tensile strength is 950MPa or more, elongation at break (vE can be worth for 12% or more, in -20 DEG C of absorptions_{- 20}) it is 59J/cm²In the case where above, it is good to be evaluated as mechanical property.

The steel plate number 32 less than 0.189% using the good steel plate of mechanical property (steel plate number 1~16) and Pcm To have made welding point.

Welding method is welded using MAG, and weld heat input is set as 7.0kJ/cm or 10.0kJ/cm.It is in heat input In the case where 7.0kJ/cm, welding condition is set as: electric current 280A, voltage 27V, speed of welding 65cm/min are in heat input In the case where 10.0kJ/cm, welding condition is set as electric current 305A, voltage 29V, speed of welding 53cm/min.

The tensile strength (strength of joint) of welding point, is evaluated using the tension test of 3121 defined of JIS Z, will 950MPa or more is evaluated as well.

Will be above evaluation result is shown in table 3.Furthermore in table 3, the numerical value with underscore indicates the value in this hair Bright range is outer or does not obtain target property.

Steel plate number 1~16 is example of the present invention, has obtained excellent intensity, ductility, toughness.In addition, strength of joint The intensity of 950MPa or more is obtained.Moreover, the example for being 0.20 or more for Mo/Cr, even -40 DEG C of test temperature Also excellent toughness has been obtained.

On the other hand, steel plate number 17~35 is comparative example, yield strength, tensile strength, elongation at break, vE_{- 20}In 1 or more discontented foot-eye.

Steel plate number 17,26,29 respectively due to C amount or the few therefore strength reduction of Mn amount.About steel plate number 26,29, Martensite divides rate also insufficient.

In addition, steel plate number 20, since Mn amount is few, hardenability is also low, therefore other than martensite, generates ferrite And bainite, the production quantity of martensite is unsatisfactory for the scope of the present invention, as a result, intensity is significantly low.

Steel plate number 18,19,21,22,23,27,28,30, C amount, Si amount, Mn amount, Cr amount or Mo amount are excessive respectively, prolong Malleability and toughness are low.

Steel plate number 24, it is low due to rolling finishing temperature and water cooling start temperature, other than martensite, generate Ferrite is processed, martensite point rate is unsatisfactory for the scope of the present invention, as a result, intensity is low.

In addition, steel plate number 33, low due to water cooling start temperature, other than martensite, processing ferrite is generated, Martensite point rate is unsatisfactory for the scope of the present invention, as a result, intensity is low.

Steel plate number 25,34, since water cooling stops temperature height, bainitic transformation, therefore horse is had occurred in non-transformed austenite Family name's body divides rate low.In addition, due to gap that the coarse carbide (cementite) that generates due to the bainite is starting point Excessive generation, elongation at break are lower.In addition, the yield strength of steel plate number 34 is also low.

Steel plate number 31, since the content of Cr and Mo is high, DI is excessively high, therefore toughness, elongation at break are low.

Steel plate number 32, since Pcm is low, strength of joint is lower than 950MPa.

Steel plate number 35, since the reduction ratio in non-recrystallization region is low, the asperratio of original austenite grains is small In 2.0, therefore toughness is low.

Table 2

Industrial availability

In accordance with the invention it is possible to provide yield strength under the premise of not largely alloying element containing high price and be The high-strength steel sheet that 885MPa or more, tensile strength are 950MPa or more and elongation at break is 12% or more.In addition, should Steel plate shows that in -20 DEG C of Charpy absorption can be 59J/cm²Above excellent toughness.Therefore, industrially useful.

Claims

1. a kind of high-strength steel sheet, which is characterized in that chemical composition is contained in terms of quality %

C:0.050~0.100%,

Si:0~0.50%,

Mn:1.20~1.70%,

P:0.020% or less,

S:0.0050% or less,

N:0~0.0080%,

B:0.0003~0.0030%,

Ti:0.003~0.030%,

Nb:0.003~0.050%,

Cr:0~2.00%,

Mo:0~0.90%,

Al:0~0.100%,

Cu:0~0.50%,

Ni:0~0.50%,

V:0~0.100%,

W:0~0.50%,

Ca:0~0.0030%,

Mg:0~0.0030%,

REM:0~0.0030%,

Surplus be Fe and impurity,

It containing a kind or two kinds added up among 0.20% or more Cr and Mo, also, is more than 0.50% in the Mo content When, the Cr content be 0.80% hereinafter,

It is 2.0~7.8 with the DI that following formula 1 acquires,

It is 0.189% or more with the Pcm that following formula 2 acquires,

Metal structure includes a kind or two kinds added up among 99% or more martensite and bainite of the area ratio,

The asperratio of original austenite grains is 2.0 or more,

The length relative to the long axis direction for the cementite that the length of long axis direction is 1.0 μm or more is 0.1 μm or more The number of cementite divide rate be 5% hereinafter,

Plate thickness is 4.5mm~20mm,

Yield strength is 885MPa or more, and tensile strength is 950MPa or more, and elongation at break is 12% or more, at -20 DEG C Charpy absorption can be 59J/cm²More than,

DI=[C]^0.5×{0.34×(1+0.64×[Si])×(1+4.1×[Mn])×(1+0.27×[Cu])×(1+0.52 × [Ni]) × (1+2.33 × [Cr]) × (1+3.14 × [Mo]) } × 1.2 formulas 1

Wherein, the formula 1 and [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] in the formula 2 are each members The content in terms of quality % of element is calculated in the case where not containing as 0.

2. high-strength steel sheet according to claim 1, which is characterized in that the metal structure include the area ratio be 90% with On martensite.

3. high-strength steel sheet according to claim 1 or 2, which is characterized in that Cu content is 0~0.25 mass %.

4. described in any item high-strength steel sheets according to claim 1~3, which is characterized in that Ni content is 0~0.25 matter Measure %.

5. high-strength steel sheet according to any one of claims 1 to 4, which is characterized in that V content is 0~0.050 matter Measure %.

6. high-strength steel sheet according to any one of claims 1 to 5, which is characterized in that W content is 0~0.05 matter Measure %.

7. described in any item high-strength steel sheets according to claim 1~6, which is characterized in that the plate thickness be 4.5mm~ 15mm。

8. high-strength steel sheet according to any one of claims 1 to 7, which is characterized in that be denoted as by the Mo content When [Mo], the Cr content are denoted as [Cr], [Mo]/[Cr] is 0.20 or more.

9. high-strength steel sheet according to claim 8, which is characterized in that absorbing in -40 DEG C of Charpy can be 59J/cm²With On.

10. described in any item high-strength steel sheets according to claim 1~9, which is characterized in that the Pcm be 0.196% with On.