CN109154041B

CN109154041B - High-strength steel plate

Info

Publication number: CN109154041B
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 and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-07-29
Filing date: 2016-07-29
Publication date: 2020-07-31
Anticipated expiration: 2036-07-29
Also published as: KR102142472B1; WO2018020660A1; EP3492610B1; EP3492610A1; JP6501042B2; JPWO2018020660A1; BR112018071948A2; BR112018071948B1; CN109154041A; EP3492610A4; KR20180126591A

Abstract

HeightA strength steel sheet having a predetermined chemical composition, a DI of 2.0 to 7.8, a Pcm of 0.189% or more, a microstructure including 1 or two of martensite and bainite having a total area ratio of 99% or more, an aspect ratio of prior austenite grains of 2.0 or more, a number fraction of cementite having a length of 0.1 [ mu ] m or more relative to a length of 1.0 [ mu ] m or more in a major axis direction of cementite having an aspect ratio of 5% or less, a sheet thickness of 4.5mm to 20mm, a yield strength of 885MPa or more, a tensile strength of 950MPa or more, an elongation at break of 12% or more, and a Charpy absorption energy at-20 ℃ of 59J/cm²The above.

Description

High-strength steel plate

Technical Field

The present invention relates to a high-strength steel sheet.

Background

With the increase in the number of stories in buildings and the like, the size of construction machines and industrial machines such as crane cars is increasing. However, in order to further increase the size, it is necessary to reduce the weight of structural members of construction machines and industrial machines. Therefore, in order to reduce the weight of the structural member, the steel material used for the construction machine and the industrial machine is required to have high strength.

However, when the strength of the steel sheet is increased to suppress an increase in the weight of the member and the thickness of the steel sheet is limited, the elongation at break is generally decreased. For example, when the sheet thickness is limited to 25mm or less, it is difficult to ensure a breaking elongation of 12% or more. When the plate thickness is limited to 8mm or less, it is more difficult to ensure the elongation at break. Since the work becomes difficult when the elongation at break is small, when the steel sheet is used as a member of a construction machine or an industrial machine, the steel sheet is required to have not only strength but also ductility such as elongation at break. In addition, when used as a structural member, low-temperature toughness is also required to prevent brittle fracture.

Against this background, high-strength steel sheets having a tensile strength of 780MPa or more, and further 950MPa, and a method for producing the same have been proposed.

For example, patent document 1 proposes a high-strength and excellent-toughness steel sheet obtained by hot rolling and rapid cooling a steel in which the amount of C is reduced and an alloy is added so as to obtain an appropriate hardenability, and a method for producing the same.

However, in the technique of patent document 1, workability of the steel sheet is not considered.

Further, for example, patent documents 2 to 4 propose a high-strength hot-rolled steel sheet produced by winding a steel strip into a coil after hot rolling as a steel sheet used for construction machinery and the like, and a method for producing the steel sheet. Specifically, patent documents 2 to 4 disclose: a hot-rolled steel sheet having a martensite phase or a tempered martensite phase as a main phase is produced by rapidly cooling the steel sheet to a temperature (Ms) near the martensite transformation initiation temperature after hot rolling, holding the steel sheet for a predetermined time, and then coiling the steel sheet into a coil. However, these methods require winding into a coil shape, and the steel sheets obtained by these methods have different properties in the rolling direction and properties in the direction perpendicular to the rolling direction, and thus uniform properties cannot be obtained. Further, since the time for holding the steel in the temperature range in which fine carbide is formed is long, the yield strength is high, and the workability is deteriorated.

Conventionally, in the production of high-strength steel sheets, a heated billet is hot-rolled, cooled to room temperature at an accelerated speed to form martensite in the microstructure, and then tempered (heat treatment for tempering) is performed to improve ductility and toughness. The strength is increased when the microstructure of the steel sheet is made martensite, but in order to ensure ductility and toughness, it is preferable to perform tempering after accelerated cooling to make the microstructure into tempered martensite. However, from the viewpoint of shortening the working period and suppressing the production cost, omitting this tempering causes the metal structure to become martensite, and although high strength can be obtained, ductility and toughness deteriorate.

Patent document 5 proposes a high-strength steel sheet having a structure mainly composed of lower bainite, in which martensite formation is suppressed by increasing the contents of Mo and V while suppressing the contents of Mn and Ni, and a method for manufacturing the same.

However, in the technique of patent document 5, since the structure obtained by setting the cooling stop temperature to 300 to 450 ℃ is assumed, a sufficient elongation at break cannot be obtained. The present inventors produced a steel sheet according to the disclosure of patent document 5 and carried out a test, and as a result, did not obtain an elongation at break of 12% or more.

As described above, it is difficult to ensure ductility and toughness in conventional high-strength steel sheets whose thickness is limited and whose metallic structure is mainly composed of martensite.

When a steel sheet is applied to the above-described structural member, welding is generally performed. In welding, a welded joint is required to have a tensile strength (joint strength) equal to or higher than a required value for a base material in view of reliability of a structure. However, when a steel sheet having a martensite main structure as a metallic structure is welded, the tensile strength (joint strength) of the welded joint is lowered as compared with the base material due to softening of the weld heat affected zone, and the required value may not be satisfied.

Prior art documents

Patent document

Patent document 1 Japanese patent application laid-open No. 2009-287081

Patent document 2 Japanese patent application laid-open No. 2011-52320

Patent document 3 Japanese patent application laid-open No. 2011-52321

Patent document 4 Japanese unexamined patent application publication No. 2012-77336

Patent document 5 International publication No. 2012/60405

Disclosure of Invention

The present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength steel sheet suitably used for construction machines and industrial machines, and a method for manufacturing the same. Specifically, the object is to provide a steel sheet having a thickness of 4.5 to 20mm, a yield strength of 885MPa or more, a tensile strength of 950MPa or more, and a Charpy absorption energy at-20 ℃ of 59J/cm²As described above, the high-strength steel sheet has a breaking elongation of 12% or more, has a metal structure mainly containing martensite, and can sufficiently ensure the tensile strength of a welded joint when welded, and a method for producing the same.

The inventors investigated the relationship between the ductility of the steel sheet and the accelerated cooling stop temperature. As a result, it was found that when the accelerated cooling stop temperature is 300 ℃ or higher than the temperature (Mf) at which the martensitic transformation is completed, the ductility is lowered. As a result of further investigation, it was found that when accelerated cooling is stopped at a temperature of 300 ℃ or higher than Mf, the non-transformed austenite undergoes bainitic transformation in the metal structure, and voids are excessively generated from coarse carbides (cementite) generated due to the bainite, thereby reducing ductility.

The present inventors have studied a countermeasure against such a reduction in ductility. As a result, the following new findings were obtained: in order to suppress the bainite transformation, a component capable of improving hardenability is designed, and after hot rolling, accelerated cooling is performed to a temperature lower than 300 ℃ and equal to or lower than the Mf temperature, so that the metal structure can be mainly martensite, and ductility of the high-strength steel sheet can be ensured.

The present invention has been completed based on such findings, and the gist thereof is as follows.

(1) The high-strength steel sheet according to one aspect of the present invention has a chemical composition containing, in mass%, C: 0.050 to 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 to 0.030%, Nb: 0.003-0.050%, Cr: 0-2.00%, Mo: 0-0.90%, Al: 0 to 0.100%, Cu: 0-0.50%, Ni: 0-0.50%, V: 00.100%, W: 0-0.50%, Ca: 0-0.0030%, Mg: 0-0.0030%, REM: 0 to 0.0030%, the balance being Fe and impurities, and containing at least 0.20% in total of 1 or two of Cr and Mo, and, when the Mo content exceeds 0.50%, the Cr content is 0.80% or less, DI obtained by the following formula 1 is 2.0 to 7.8, Pcm obtained by the following formula 2 is 0.189% or more, the metal structure contains 1 or two kinds of martensite and bainite having a total area ratio of 99% or more, the aspect ratio of the prior austenite grains is 2.0 or more, the number fraction of cementite having a length of 0.1 μm or more relative to the length in the long axis direction of cementite having a length in the long axis direction of 1.0 μm or more is 5% or less, the sheet thickness is 4.5mm to 20mm, the yield strength is 885MPa or more, the tensile strength is 950MPa or more, the elongation at break is 12% or more, and the Charpy absorption energy at-20 ℃ is 59J/cm.²The above.

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]) × 1.2.2. formula 1

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

Wherein [ C ], [ Si ], [ Mn ], [ Cu ], [ Ni ], [ Cr ], [ Mo ], [ V ], [ B ] in the above formulas 1 and 2 are the contents of the respective elements in mass%, and are calculated as 0 when not contained.

(2) The high-strength steel sheet according to item (1) above, wherein the microstructure may contain martensite in an area ratio of 90% or more.

(3) The high-strength steel sheet described in the above (1) or (2), wherein the Cu content may be 0 to 0.25 mass%.

(4) The high-strength steel sheet according to any one of (1) to (3) above, wherein the Ni content may be 0 to 0.25 mass%.

(5) The high-strength steel sheet according to any one of (1) to (4) above, wherein the content of V is 0 to 0.050% by mass.

(6) The high-strength steel sheet according to any one of (1) to (5) above, wherein the W content may be 0 to 0.05 mass%.

(7) The high-strength steel sheet according to any one of (1) to (6), wherein the sheet thickness may be 4.5mm to 15 mm.

(8) The high-strength steel sheet according to any one of (1) to (7), wherein [ Mo ]/[ Cr ] is 0.20 or more when the Mo content is [ Mo ] and the Cr content is [ Cr ].

(9) The high-strength steel sheet described in the above (8), which has a Charpy absorption energy of 59J/cm at-40 ℃²The above.

(10) The high-strength steel sheet according to any one of (1) to (9) above, wherein Pcm may be 0.196% or more.

According to the above aspect of the present invention, a high-strength steel sheet having a yield strength of 885MPa or more, a tensile strength of 950MPa or more, and an elongation at break of 12% or more can be provided without containing a large amount of expensive alloying elements. The steel sheet showed a Charpy absorption energy at-20 ℃ of 59J/cm²The above excellent toughness. Further, by setting Pcm, which is an index of hardenability, to 0.189% or more, preferably 0.196% or more, the tensile strength of a welded joint using the high-strength steel sheet according to the present invention as a base material can be ensured to be 950MPa or more when welding is performed at a predetermined heat input or less.

Further, by controlling the Mo content [ Mo ] at the same time]Content of Cr and [ Cr ]]Ratio of (i.e. [ Mo ]]/[Cr]Also, it can provide a Charpy absorption energy of 59J/cm at-40 deg.C²The high-strength steel sheet having more excellent toughness.

Therefore, the present invention can provide a high-strength steel sheet which is suitably used for structural members of construction machines and industrial machines without significantly increasing the manufacturing cost, and contributes to upsizing and weight reduction of the construction machines and industrial machines, and the industrial contribution thereof is extremely remarkable.

Drawings

Fig. 1 is a graph illustrating a relationship between a fracture elongation (Total elongation) and a stop temperature Tcf of accelerated cooling, a hardenability index DI, and a C amount.

FIG. 2 is a graph showing the relationship between [ Mo ]/[ Cr ] and Charpy absorption energy at-40 ℃ (vE-40).

Fig. 3 is a graph showing a relationship between the stop temperature of accelerated cooling and the elongation at break.

Fig. 4A is an SEM photograph showing the influence of the accelerated cooling stop temperature on the shape of the cementite, and is an SEM photograph when the accelerated cooling stop temperature is set to 290 ℃.

Fig. 4B is an SEM photograph showing the influence of the accelerated cooling stop temperature on the shape of the cementite, and is an SEM photograph when the accelerated cooling stop temperature is set to 400 ℃.

Fig. 5 is a photograph showing voids generated from the vicinity of coarse cementite.

Detailed Description

Hereinafter, a high-strength steel sheet according to an embodiment of the present invention (hereinafter, may be referred to as a high-strength steel sheet according to the present embodiment) will be described in detail.

First, the chemical composition (components) of the high-strength steel sheet according to the present embodiment will be described. The following notation of% of content means% by mass unless otherwise specified.

(C：0.050～0.100％)

C is an element useful for improving the strength of steel, and is an extremely important element for determining the fracture elongation of steel having a martensite structure. In order to obtain sufficient strength, the high-strength steel sheet according to the present embodiment needs to have a C content of 0.050% or more. In order to further improve the strength, the C content is preferably 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if the C content exceeds 0.100%, excessive carbide is produced, which deteriorates the ductility and toughness of the steel. Therefore, in order to obtain good elongation at break and toughness, the C content needs to be 0.100% or less. In order to further improve ductility, the C content is preferably 0.095% or less, 0.090% or less, or 0.085% or less.

(Si: 0.50% or less)

If Si is contained excessively, ductility and toughness of the steel decrease. Therefore, the Si content is limited to 0.50% or less. The lower limit of the amount of Si is not particularly limited, and is 0%. However, when Si is used for deoxidation, the amount of Si is preferably 0.03% or more in order to obtain sufficient effects. In addition, Si is also an element that suppresses the formation of carbide, and in order to obtain this effect, the amount of Si is preferably 0.10% or more, and more preferably 0.20% or more. When these effects are not necessarily obtained, the upper limit of the Si amount may be set to 0.45%, 0.40%, or 0.35%.

(Mn：1.20～1.70％)

Mn is an important element for improving the hardenability of steel. In order to increase the area ratio of martensite in the microstructure and obtain high strength, the Mn content is set to 1.20% or more. The Mn content is preferably more than 1.20%, 1.25% or more, or 1.30% or more, and more preferably 1.35% or more, or 1.39% or more. On the other hand, if the Mn amount becomes excessive, ductility and toughness may be reduced. Therefore, the Mn content is 1.70% or less. More preferably, the Mn content is 1.60% or less, 1.55% or less, or 1.50% or less.

(P: 0.020% or less)

(S: 0.0050% or less)

P, S is an element that is inevitably contained in steel as an impurity, and is an element that deteriorates the toughness of steel. In addition, when welding is performed, the element deteriorates toughness of the weld heat affected zone. Therefore, the P amount is limited to 0.020% or less and the S amount is limited to 0.0050% or less. In order to further improve the toughness, the P content may be 0.015% or less and the S content may be 0.0030% or less. The smaller the P content and the S content, the better, and therefore, the lower the content is preferably within a possible range. Therefore, the lower limits of the P amount and the S amount do not need to be particularly specified, and the lower limits of the P amount and the S amount are 0%. From the viewpoint of the cost of dephosphorization and desulfurization, the amount of P may be 0.001% or more and the amount of S may be 0.0001% or more. (B: 0.0003-0.0030%)

B is an element which segregates in grain boundaries to improve hardenability of steel, and is an element useful for exhibiting its effect by containing a trace amount of B. In the high-strength steel sheet according to the present embodiment, the amount B is set to 0.0003% or more in order to increase martensite in the metal structure. The amount of B is preferably 0.0005% or more. On the other hand, if B is excessively contained, not only the effect of improving hardenability is saturated, but also precipitates such as nitrides and carboborides are formed, and ductility and toughness are rather lowered. Therefore, the amount of B is set to 0.0030% or less. The amount of B is preferably 0.0020% or less or 0.0015% or less.

(Ti：0.003～0.030％)

Ti is an element that forms nitrides, and is an element that fixes N in steel in the form of TiN and suppresses the generation of BN. As described above, B is an element that improves hardenability, but if BN is formed, the effect thereof cannot be obtained. In the high-strength steel sheet according to the present embodiment, the Ti content needs to be 0.003% or more in order to suppress the formation of BN and ensure hardenability. The amount of Ti is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if Ti is excessively contained, TiN becomes coarse, and ductility and toughness may be reduced. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.020% or less.

(Nb：0.003～0.050％)

Nb is an element that significantly improves the hardenability of steel by being contained together with B. In the high-strength steel sheet according to the present embodiment, the Nb content is set to 0.003% or more in order to increase the area ratio of martensite in the microstructure. Nb is also an element that forms a fine nitride, contributes to grain refinement, and improves toughness. In order to obtain this effect, the Nb content is preferably 0.005% or more. More preferably, the Nb content is 0.010% or more or 0.015% or more. On the other hand, if Nb is excessively contained, the nitride becomes coarse, and ductility and toughness may be reduced. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less, 0.035% or less, or 0.030% or less.

(Cr: 2.00% or less)

(Mo: 0.90% or less)

(containing 1 or two kinds of Cr and Mo in an amount of 0.20% or more in total, and when the Mo content exceeds 0.50%, the Cr content is 0.80% or less.)

Cr and Mo are important elements for improving hardenability, and one or both of them are contained. In the high-strength steel sheet according to the present embodiment, the total of the Cr content and the Mn content is 0.20% or more in order to increase the area ratio of martensite in the microstructure. The total of the Cr content and the Mn content is preferably 0.30% or more, and more preferably 0.40% or more. Considering the case where only Cr or Mo is contained, the lower limit of the amount of Cr and Mo is 0%. The lower limit of the amount of Cr may be set to 0.20% or 0.30% as required, and similarly, the lower limit of the amount of Mo may be set to 0.20% or 0.30%.

If the amount of Cr exceeds 2.00% or if the amount of Mo exceeds 0.90%, fine carbides are formed, resulting in a decrease in ductility and toughness. Therefore, the Cr content and the Mo content are set to 2.00% or less and 0.90% or less, respectively. The amount of Cr is preferably 1.50% or less or 1.00% or less, more preferably 0.90% or less or 0.80%. The Mo content is preferably 0.70% or less, more preferably 0.60% or less or 0.50%.

In addition, when both Cr and Mo are contained, the toughness decreases if the content is excessive, so when the Mo amount exceeds 0.50%, the Cr amount needs to be 0.80% or less. In this case, the Cr content may be 0.70% or less. On the other hand, when the Cr content exceeds 0.80%, the Mo content is preferably 0.50% or less, and when the Cr content exceeds 1.20%, the Mo content is preferably 0.40% or less. The total of the Cr content and the Mo content may be 2.50% or less, but may be 2.00% or less, 1.50% or less, 1.30% or less, or 1.10% or less.

(N: 0.0080% or less)

N is an impurity and is inevitably contained. N forms BN, impairing the effect of B in improving hardenability. Therefore, the N content is limited to 0.0080% or less. The amount of N is preferably limited to 0.0060% or less, and more preferably limited to 0.0050% or less. The amount of N is preferably reduced within a possible range, with the lower limit set to 0%. However, from the viewpoint of the cost of denitrification, the N content may be set to 0.0001% or more. On the other hand, in order to miniaturize the metal structure with nitride, the N amount may be 0.0020% or more.

The above are elements contained as essential elements and impurities in the high-strength steel sheet according to the present embodiment, and the high-strength steel sheet according to the present embodiment is based on a composition containing the essential elements, the balance Fe, and impurities (including the impurity elements and, if necessary, other impurity elements than those described above). However, the high-strength steel sheet according to the present embodiment may contain, in addition to the above components, Al: 0.100% or less, Cu: 0.50% or less, Ni: 0.50% or less, V: 0.100% or less, W: 0.50% or less, Ca: 0.0030% or less, Mg: 0.0030% or less, REM: 0.0030% or less, and 1 or 2 or more. However, these elements are not essential, and thus may be 0%.

(Al: 0.100% or less)

Al is a deoxidizing element, and when Al is used for deoxidation, the amount of Al is preferably 0.010% or more in order to obtain a sufficient effect. On the other hand, if Al is excessively contained, ductility and toughness are reduced due to formation of oxides and nitrides. Therefore, even when contained, the Al content is limited to 0.100% or less. The content is preferably limited to 0.080% or less, more preferably limited to 0.050% or less, and still more preferably limited to 0.030% or less.

(Cu: 0.50% or less)

(Ni: 0.50% or less)

Cu and Ni are elements that improve the hardenability of steel. When the hardenability is improved to increase the area ratio of martensite in the metal structure, the Cu content and the Ni content are preferably 0.10% or more, respectively. On the other hand, since Cu and Ni are expensive elements, it is preferable to set the Cu content and the Ni content to 0.50% or less, respectively, even when they are contained. The Cu content and the Ni content are preferably 0.40% or less, and more preferably 0.30% or less, respectively.

(V: 0.100% or less)

V is an element forming carbide or nitride. When the carbide or nitride is used to refine the crystal grains and improve the toughness, the V content is preferably 0.005% or more. On the other hand, if V is contained excessively, ductility and toughness are reduced. However, since the adverse effect is smaller than that of Nb and Ti, the upper limit of the V amount in the case of inclusion is set to 0.100%. The V content is preferably 0.050% or less.

(W: 0.50% or less)

W is an element for improving the hardenability of steel. In order to obtain this effect, the amount of W is preferably 0.05% or more. On the other hand, if W is excessively contained, weldability deteriorates. Therefore, the W amount is set to 0.50% or less or 0.30% or less even when contained. The W amount may be 0.02% or less or 0.01% or less as necessary.

(Ca: 0.0030% or less)

Ca is an element that controls the form of oxides and sulfides. In order to obtain this effect, the amount of Ca is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, if Ca is excessively contained, not only the effect is saturated, but also ductility and toughness may be reduced due to the formation of inclusions. Therefore, even when it is contained, the Ca content is set to 0.0030% or less.

(Mg: 0.0030% or less)

Mg is an element having an effect of improving the toughness of steel by refining the structure. In order to obtain this effect, the Mg content is preferably 0.0005% or more. On the other hand, if Mg is excessively contained, not only the effect is saturated, but also ductility and toughness may be reduced due to the formation of inclusions. Therefore, even when the Mg is contained, the Mg content is set to 0.0030% or less.

(REM: 0.0030% or less)

REM (rare earth element) is an element having an action of improving toughness of steel by controlling the form of sulfide, particularly MnS. In order to obtain this effect, the REM amount is preferably set to 0.0001% or more. On the other hand, when REM is excessively contained, REM-containing inclusions coarsen, and ductility and toughness may be reduced. Therefore, even when the REM content is contained, the REM content is set to 0.0030% or less.

In addition, other elements may be contained in a trace amount within a range not impairing the action and effect.

The high-strength steel sheet according to the present embodiment is such that the respective elements are in the above ranges, and DI and Pcm determined by the chemical composition need to satisfy the following ranges.

(DI：2.0～7.8)

DI is an index of hardenability and is obtained by the following formula 1. In the formula, [ C ], [ Si ], [ Mn ], [ Cu ], [ Ni ], [ Cr ], [ Mo ] represent the content (mass%) of each element, and the content is calculated as 0 when the element is not contained.

As shown qualitatively in fig. 1, if the hardenability index DI is increased, the reduction in the elongation at break can be suppressed even if the stop temperature Tcf of the accelerated cooling is increased (i.e., moved to the right in fig. 1). When the stop temperature Tcf of the accelerated cooling is increased, an excessive increase in strength is suppressed, and the toughness and ductility can be improved. To achieve a good balance among strength, ductility, and toughness, DI is preferably 2.0 or more. DI is more preferably 3.0 or more, and DI is still more preferably 4.0 or more. On the other hand, if the hardenability is excessively increased, the strength may be excessively increased, and the toughness may be decreased. Therefore, DI is preferably 7.8 or less. DI is more preferably 7.0 or less, and still more preferably 6.5 or less.

(Pcm: 0.189% or more)

A welded joint is generally required to have a tensile strength (joint strength) equal to or higher than a required value for the tensile strength of a base material used for welding. The present inventors have found that, when a steel sheet having a martensitic structure as a main structure of a metal structure is welded, the tensile strength (joint strength) of a welded joint may be reduced from the tensile strength of a base material due to softening of a welding heat affected zone. Therefore, the present inventors produced a welded joint by changing the welding line energy using various high-strength steel sheets, and conducted tests. As a result, it was found that the hardenability of the steel sheet is improved, and specifically, when Pcm obtained by the following formula 2 is 0.189% or more, the tensile strength of the welded joint can be set to 950MPa or more when welding is performed so as to suppress softening of the welding heat affected zone and to obtain a lower limit value of 7.0kJ/cm of the welding line energy range which is often used for manufacturing structural members of construction machines and industrial machines.

Wherein [ C ], [ Si ], [ Mn ], [ Cu ], [ Ni ], [ Cr ], [ Mo ], [ V ], [ B ] are the contents (mass%) of the respective elements, and 0 is calculated when the element is not contained.

Further, the inventors studied the weld line energy and the strength of the welded joint, and the strength of the welded joint was evaluated by using JS calculated from the following equation a using Pcm obtained by the above equation 2 and the weld line energy Hi [ kJ/cm ] composed of components based on the high-strength steel sheet used for welding, and found that if JS is 950MPa or more, the joint strength of 950MPa or more was secured even in an actual welded joint.

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

As can be seen from the above equation, in order to secure the strength of the welded joint, it is preferable to reduce the weld line energy as much as possible. However, there is a lower limit of the weld line energy to ensure the soundness of the welded joint. In order to ensure productivity of welding work in manufacturing construction machines and industrial machines, it is not easy to reduce the welding line energy to less than 7.0 kJ/cm. When the weld heat input was 7.0kJ/cm, Pcm required for JS to be 950MPa or more was 0.189% by the above formula. That is, when Pcm is 0.189% or more, the joint strength of 950MPa or more can be ensured.

In addition, if Pcm is 0.196% or more, even when the weld line energy is 10.0kJ/cm, which does not require special management during welding, the joint strength of 950MPa or more can be ensured. That is, by setting Pcm to 0.196% or more, the strength of the welded joint can be set to 950MPa or more without special welding work management.

Further, Pcm may be set to 0.200% or more, 0.205% or more, 0.210% or more, or 0.215% or more in order to secure the strength of the welded joint even with a larger welding line energy. When the energy of the bonding wire is large, the number of layers to be bonded can be reduced, and productivity is improved, which is preferable. The upper limit of Pcm is not particularly required, but may be 0.250% or less or 0.240% or less to prevent weld cracking or the like.

([ Mo ]/[ Cr ]: over 0.20)

Further, the inventors investigated the influence of Cr and Mo, which are elements for improving hardenability, on toughness, and studied them. As a result, it was found that the ratio of Mo to Cr affects the toughness when the hardenability (DI) is constant. Specifically, when the ratio ([ Mo ]/[ Cr ]) of the Mo content [ Mo ] to the Cr content [ Cr ] in mass% is increased, the lower structure of martensite (a packet or a block) becomes fine, and as a result, the toughness is improved. In order to further improve toughness, the ratio may be 0.40 or more, 0.80 or more, or 1.00 or more.

FIG. 2 is a graph showing the relationship between [ Mo ]/[ Cr ] and Charpy absorption energy at-40 ℃, wherein "○" in FIG. 2 represents measured values, and "●" represents an average value of the measured values.

As shown in FIG. 2, the existence of the following [ Mo ]]/[Cr]Larger, and the Charpy absorption energy at-40 ℃ tends to be larger, [ Mo ]]/[Cr]When the temperature is 0.20 or more, the absorption energy in summer at-40 ℃ is 59J/cm²The above. Therefore, when low-temperature toughness is required, [ Mo ] is preferably used]/[Cr]Is set to 0.20 or more. On the other hand, Mo is an element which is more likely to form fine carbides and clusters (clusters) than Cr. Therefore, if Mo is contained in an excess amount compared with Cr, toughness may be lowered, and [ Mo ] may be used]/[Cr]Is set to 2.00 or less or 1.50 or less.

The Charpy absorption energy was measured by the Charpy impact test according to JIS Z2242, however, the steel sheet from which the test piece was made had a thickness of 8mm, the longitudinal direction was the rolling direction, and the test piece from the center of the thickness of the sheet was made to have a small size of 10mm × 5 mm.

(the total area ratio of 1 or 2 of martensite and bainite: 99% or more, and the elongation at break: 12% or more)

The present inventors have studied the relation between the hardenability and the elongation at break of the high-strength steel sheet and the metal structure. As a result, the present inventors found that: if the hardenability is insufficient, the elongation at break is reduced, and the reason why the elongation at break is reduced, that is, the ductility is reduced is the generation of voids starting from coarse carbides generated by bainite as shown in fig. 4A and 4B. Further, the following findings were obtained: in order to improve the ductility of the high-strength steel sheet, it is necessary to suppress the formation of bainite, which causes the formation of coarse cementite. In order to suppress berms which cause the generation of coarse cementite, it is preferable to form a martensite main structure in which 90% or more of the metal structure is martensite. In order to increase the strength of the steel sheet, it is also preferable that the area ratio of martensite in the metal structure is 90% or more. More preferably 92% or more, and still more preferably 94% or more.

However, martensite and bainite are both continuously cooled transformation structures, and it is sometimes difficult to accurately distinguish them by structure observation. In such a case, if the total area ratio of martensite and bainite is 99% or more and the elongation at break is 12% or more, it can be determined that bainite that causes the formation of coarse cementite is suppressed.

Therefore, the high-strength steel sheet according to the present embodiment has a total area ratio of 1 or 2 of martensite and bainite of 99% or more, and a fracture elongation as an index of the structure of 12% or more. When martensite and bainite can be sufficiently distinguished by observing the structure, the area ratio of martensite is preferably 90% or more.

In the high-strength steel sheet according to the present embodiment, martensite in the metal structure is in a quenched state, and is different from tempered martensite obtained by tempering treatment. In the case of tempered martensite, it is not preferable because a carburized body grows by long-time tempering.

The remainder other than the above may be 1 or 2 or more kinds of ferrite, pearlite, and retained austenite.

Specifically, a cross section parallel to the rolling direction in the vicinity of 1/4t (a portion 1/4 of the thickness t from the surface of the steel sheet in the thickness direction) was etched with a nital etching solution, 2 fields in the range of 120 μm × 100 μm at 500 times were imaged using an optical microscope, and the area ratio of a structure in which the needle-like lath structure was developed was measured.A cross section of the steel sheet was electrolytically ground, and then the vicinity of 1/4t of the cross section of the steel sheet was observed using a Scanning Electron Microscope (SEM). The magnification of this is 5000 times and the range of 50 μm × 40 μm was imaged.when the long axis direction of cementite was oriented in 2 or more directions within the lath, the needle-like structure was considered as martensite, and the area ratio of this region was determined.

In the above-described observation of the structure by the scanning electron microscope, it may not be possible to clearly distinguish that the longitudinal direction of the cementite is oriented in 2 or more directions in the lath block. In this case, the area ratio of the structure in which the needle-like lath structure is developed under an optical microscope is defined as the area ratio of the sum of martensite and bainite.

(the number fraction of cementite having a length of 1.0 μm or more in the longitudinal direction to cementite having a length of 0.1 μm or more in the longitudinal direction: 5% or less)

As described above, in order to improve the ductility of the steel sheet and to suppress the formation of bainite, which is a cause of the formation of coarse cementite, it is important to form a microstructure mainly composed of martensite. However, in order to further improve the ductility, it is effective to suppress the generation of voids starting from coarse carbide (particularly cementite).

The present inventors have found that the number fraction of coarse carbides (particularly cementite) having a length in the long axis direction of 1.0 μm or more can be reduced by controlling the stop temperature of accelerated cooling, and as a result, the generation of voids can be suppressed, and the elongation at break can be improved. Specifically, it was found that: the elongation at break can be improved by setting the number fraction of cementite having a length in the long axis direction of 1.0 μm or more, among cementites having a length in the long axis direction of 0.1 μm or more, to 5% or less.

As will be described in detail later, in the present invention, by stopping the accelerated cooling at a temperature of Mf or less and less than 300 ℃, a structure mainly composed of martensite can be formed in which the generation of coarse carbides is suppressed. That is, by controlling the stop temperature of accelerated cooling, it is possible to suppress the generation of voids starting from coarse cementite having a length of 1.0 μm or more in the longitudinal direction.

Specifically, the number density of cementite is measured by a Scanning Electron Microscope (SEM), after electropolishing the cross section of the steel sheet, the magnification is 5000 times for the vicinity of the 1/4t portion of the cross section of the steel sheet, and a Scanning Electron Microscope (SEM) is used to photograph a range of 50 μm × 40 μm.from the contrast (contrast: contast) of the obtained image, the number is counted by using image analysis software, using precipitates having an aspect ratio of 2.0 or more and a length in the long axis direction of 0.1 μm or more as cementite.similarly, the number fraction (%) of cementite having an aspect ratio of 2.0 or more and a length in the long axis direction of 1.0 μm or more is counted, and then, the shape (%) of carbide is determined by dividing the number of the obtained precipitates having a size of 1.0 μm or more by the number of cementite having a size of 0.1 μm or more, but, for example, the "long axis direction" means a long axis length.

(aspect ratio of prior austenite grains of 2.0 or more)

In the high-strength steel sheet according to the present embodiment, the longitudinal/lateral size ratio of the prior austenite grains is 2.0 or more. In the case where the aspect ratio is less than 2.0, there is a fear that toughness is lowered.

Further, in the case where accelerated cooling (direct quenching) is performed in-line after non-recrystallization zone rolling, the aspect ratio of prior austenite grains can be made 2.0 or more. On the other hand, when the steel is quenched by reheating after rolling and cooling, the worked structure obtained by rolling is not retained, and the aspect ratio of the prior austenite grains is less than 2.0.

The aspect ratio of the prior austenite grains was measured by etching a cross section parallel to the rolling direction near a portion 1/4t, which is a position distant from the surface at 1/4 of the sheet thickness t, in the sheet thickness direction with a nital etchant, and taking 2 fields of view in the range of 120 μm × 100 μm by using an optical microscope with a magnification of 500 times, and the aspect ratio was determined for each grain by measuring the length of the major axis and the length of the minor axis for at least 50 or more prior austenite grains from the obtained images, and dividing the length of the major axis by the length of the minor axis.

Next, the plate thickness and mechanical properties of the high-strength steel sheet according to the present embodiment will be described.

(plate thickness: 4.5 to 20mm)

The thickness of a high-strength steel sheet used for cranes and the like is generally 4.5 to 20 mm. Therefore, the thickness of the high-strength steel sheet according to the present embodiment is set to 4.5 to 20 mm. However, it is preferably 4.5 to 15mm in terms of contributing to weight reduction.

(yield strength: 885MPa or more)

(tensile Strength: 950MPa or more)

Further, in order to contribute to the increase in size and weight of construction machines and industrial machines, the strength is required to be increased, and in order to obtain a remarkable economic effect, it is necessary to set the yield strength to 885MPa or more and the tensile strength to 950MPa or more. The upper limit of the yield strength is not particularly required to be specified, but may be 1100MPa or less. The upper limit of the tensile strength is not particularly required, but may be 1300MPa or less or 1250MPa or less.

(elongation at Break: 12% or more)

In order to apply high-strength steel sheets to members of construction machines and industrial machines, workability such as bendability is required, and therefore the elongation at break is set to 12% or more. As described above, elongation at break is also an index of the structure of bainite that causes the formation of coarse cementite.

The yield strength, tensile strength and elongation at break were measured by a tensile test according to JIS Z2241. However, the value of the elongation at break in the tensile test depends on the shape of the test piece. The above-mentioned limit of the elongation at break (12% or more) is a value in the case of using a No. 5 test piece (a flat plate type test piece in a state where the distance between the original points is 50mm, the width of the parallel portion is 25mm, and the thickness of the test piece is the thickness of the steel plate) of JIS Z2241 as a tensile test piece.

The conversion equation of the elongation based on the difference in the shape of the test piece is also specified in ISO 2566-1, and the elongation of 12% when the test piece No. 5 of JISZ2241 is used can be converted to 10.4% when the tensile test piece is the test piece No. 13B of JIS Z2241 (a flat plate test piece in a state where the distance between the original points is 50mm, the width of the parallel portion is 12.5mm, and the thickness of the test piece is the thickness of the steel plate), and can be converted to 9.5% when the tensile test piece is the test piece No. 13A of JIS Z2241 (a flat plate test piece in a state where the distance between the original points is 80mm, the width of the parallel portion is 20mm, and the thickness of the test piece is the thickness of the steel plate).

(Charpy absorption energy at-20 ℃ C.: 59J/cm²Above)

When construction machines and industrial machines are used in cold regions, low-temperature toughness may be required for high-strength steel sheets. Therefore, it is preferable that the Charpy absorption energy at-20 ℃ is 59J/cm². More preferably, the Charpy absorption energy at-40 ℃ is 59J/cm²The above.

The Charpy absorption energy was measured by preparing a test piece having the longitudinal direction as the rolling direction from the center of the plate thickness and performing the Charpy impact test in accordance with JIS Z2242 at-20 ℃ or-40 ℃ in some cases, it was difficult to prepare a full-size test piece of 10mm × 10mm depending on the plate thickness of the steel plate, and in such a case, the test piece was prepared so that the rolling direction was the longitudinal direction, and the Charpy absorption energy was adjusted so that the rolling direction was the longitudinal directionSmall size test pieces were used. Charpy absorption energy, which is the absorption energy divided by the cross-sectional area (cm) of the specimen at the bottom of the V notch²) The obtained value (J/cm)²) For example, in the case of a full-size test piece of 10mm × 10mm and a small-size test piece of 10mm × 5mm, the measured charpy absorption energy value (J) is 0.8cm to 0.8cm in the case of the full-size test piece divided by 1cm × 0.8 to 0.8cm²0.4cm in the case of small-sized test pieces divided by 0.5cm × 0.8cm²Thus, it is obtained.

Next, a method preferable for manufacturing the high-strength steel sheet according to the present embodiment will be described.

The high-strength steel sheet according to the present embodiment can be produced by melting a molten steel having a chemical composition in the above-described range by a conventional method, casting the molten steel, heating the obtained slab, hot rolling, performing accelerated cooling, and naturally cooling to room temperature as it is after the accelerated cooling is stopped. However, in the production of the high-strength steel sheet according to the present embodiment, after the accelerated cooling is stopped or after the steel sheet is naturally cooled to room temperature, the quenching and tempering heat treatment such as tempering is not performed. When the heat treatment is performed, the martensite is changed to tempered martensite. That is, the high-strength steel sheet according to the present embodiment is manufactured by a so-called non-heat treatment manufacturing process in which the heat treatment for heat treatment is omitted for the purpose of shortening the working period and reducing the manufacturing cost. The high-strength steel sheet according to the present embodiment manufactured in the non-heat-treated manufacturing process may be referred to as a non-heat-treated high-strength steel sheet.

Preferred conditions for each step will be described below.

(heating temperature of billet: 1100 to 1250 ℃ C.)

The high-strength steel sheet according to the present embodiment needs to contain a predetermined amount of alloying elements in order to improve hardenability. Therefore, carbides and nitrides of the alloying elements are generated in the billet used for hot rolling. When a billet is heated, it is necessary to decompose and dissolve these carbides and nitrides in steel, and the heating temperature is set to 1100 ℃ or higher. On the other hand, when the heating temperature of the billet is too high, the crystal grain size becomes coarse and the toughness may be reduced, so the heating temperature is 1250 ℃.

(finishing temperature: Ar3 (. degree. C.) or higher)

(accelerated Cooling Start temperature: Ar3 (. degree. C.) or more)

The heated billet is hot rolled. In order to form a microstructure mainly containing martensite by accelerated cooling after hot rolling, it is necessary to start accelerated cooling at a temperature at which the microstructure is austenite. Therefore, the hot rolling must be finished at a temperature at which the microstructure is austenite. Therefore, the finishing temperature in hot rolling is set to Ar3 (DEG C) or higher. Ar3 (c) is the temperature at which ferrite transformation starts from the austenite during cooling, and can be determined from the thermal expansion behavior. Ar3 (. degree. C.) can be easily determined by, for example, the following formula b.

Ar3 ═ 868-

Here, [ C ], [ Si ], [ Mn ], [ Ni ], [ Cu ], [ Cr ], [ Mo ] are the content (mass%) of each element, and when no element is contained, it is calculated as 0.

The hot rolling may be performed by a conventional method, but it is preferable to perform recrystallization zone rolling in which the cumulative reduction ratio in a temperature range of 1050 ℃ or higher is 50 to 80%, and non-recrystallization zone rolling in which the cumulative reduction ratio in a temperature range of Ar3 to 950 ℃ is 50 to 90%.

(Cooling rate of accelerated Cooling: 30 to 200 ℃/s)

The martensite is formed by accelerated cooling performed subsequent to hot rolling. The cooling rate of the accelerated cooling needs to be set to 30 ℃/s or more in order to increase the area ratio of martensite. If the ratio is less than 30 ℃/s, a sufficient martensite area ratio cannot be obtained. The cooling rate is preferably increased to promote the martensitic transformation, but the upper limit may be set to 200 ℃/s or less because of restrictions on the sheet thickness and facilities. The cooling rate was calculated by measuring the temperature change of the surface of the steel sheet after hot rolling and dividing the difference between the surface temperature before the start of water cooling and the surface temperature immediately after the stop of water cooling by the time required for cooling.

(stop temperature of accelerated Cooling: Mf (. degree. C.) or lower and lower than 300 ℃ C.)

The present inventors have studied the relationship between hardenability and the stop temperature of accelerated cooling, and the metal structure and elongation at break. Here, when the steel sheet is quenched after hot rolling, the temperature Ms (c) at which martensitic transformation starts is obtained by the following formula 3. The temperature Mf (. degree. C.) after the martensitic transformation is a temperature lower than Ms (. degree. C.) by about 150 ℃ and is determined by the following formula 4. [ C ], [ Mn ], [ V ], [ Cr ], [ Ni ], [ Cu ], [ Mo ], [ Al ] of the following formula 3 are the contents (mass%) of the respective elements, and when the element is not contained, it is calculated as 0.

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

Mf-Ms-150. cndot. formula 4

In order to make the metal structure martensite, it is necessary to cool the metal structure to at least a temperature of Ms (DEG C) or less, and when the metal structure is cooled (quenched) to a temperature of Mf (DEG C) or less, 90% or more of the metal structure becomes martensite. However, when the cooling stop temperature is 300 ℃ or higher, the cooling becomes unstable, and some of martensite may become bainite, so the cooling stop temperature is Mf (c) or lower and less than 300 ℃.

The stop temperature of accelerated cooling is extremely important as described above, and it is a prerequisite to stop at a temperature lower than the temperature Ms (c) at which the martensitic transformation starts. When the cooling is accelerated to a temperature of not more than the temperature Mf (c) at which the martensite phase is completely changed and less than 300 c, the metallic structure becomes a martensite-based structure in which the formation of carbide is suppressed.

On the other hand, in the case where the stop temperature of accelerated cooling is between Ms (. degree. C.) and Mf (. degree. C.) (between Ms-Mf), the ductility of the high-strength steel sheet is affected by hardenability. That is, if hardenability is improved, the formation of bainite is suppressed, and along with this, the formation of coarse carbides in the carburized system is suppressed, and the elongation at break is improved and the variation is also reduced.

Further, when the relationship between the stop temperature Tcf and Mf of accelerated cooling and the elongation at break and the influence of DI and C amount on the elongation at break are qualitatively arranged, it can be schematically shown in fig. 1. Here, the vertical axis of fig. 1 represents elongation at break (total elongation) and the horizontal axis represents stop temperature Tcf of accelerated cooling, and DI represents an index of hardenability obtained by the above equation 1.

As shown in the graph of fig. 1, when the stop temperature Tcf of accelerated cooling is lowered, martensite transformation is promoted and the formation of bainite is suppressed, so that the elongation at break is improved, and when Tcf is equal to or less than Mf, the elongation at break is constant. When Tcf is equal to or less than Mf, the elongation at break is determined by the C content, and the C content is increased or decreased to increase the elongation at break.

On the other hand, when the stop temperature Tcf of accelerated cooling is between Ms and Mf, the elongation at break increases as Tcf decreases, but when an alloying element is added to improve hardenability at this time, DI increases to suppress the formation of bainite and suppress the formation of coarse carbides, thereby improving the elongation at break.

The lower limit of the stop temperature of the accelerated cooling is not particularly limited, and the accelerated cooling may be performed to room temperature. If the yield strength is improved by the action of fixing carbon atoms or the like to dislocations, the stop temperature of accelerated cooling is preferably 100 ℃ or higher.

After the accelerated cooling is stopped, the steel sheet is naturally cooled to room temperature without performing a quenching and tempering heat treatment such as tempering.

Examples

Examples of the present invention are explained below. The conditions in the examples shown below are conditions employed for confirming the feasibility and the effects of the present invention, and the present invention is not limited to the conditions. The present invention can employ various conditions within the limits of achieving the object of the present invention without departing from the gist of the present invention.

Steel sheets having a thickness of 4.5 to 20mm were produced from billets obtained by melting the chemical components (balance Fe and impurities) shown in Table 1 under the production conditions shown in Table 2. "heating temperature" represents a reheating temperature of a steel slab, "rolling end temperature" represents an end temperature of hot rolling, "water cooling start temperature" represents a surface temperature of a steel sheet at the start of accelerated cooling (water cooling), "cooling rate" represents an average cooling rate at the center portion of the sheet thickness in a temperature range of Ar3 (deg.c) to an accelerated cooling stop temperature, and "water cooling stop temperature" represents a surface temperature of a steel sheet at the stop of water cooling. The surface temperature of the steel sheet was measured with a radiation thermometer, and the "cooling rate" was calculated by obtaining the temperature at the center of the sheet thickness from the surface temperature by heat conduction calculation. Neither steel sheet was tempered.

The microstructure and mechanical properties (yield strength, tensile strength, elongation at break, toughness, joint strength) of the obtained steel sheet were evaluated.

The discrimination of the metal structure and the measurement of the area ratios of martensite and bainite were performed by the following methods.

A cross section of a steel sheet was mirror-polished, and a cross section parallel to the rolling direction in the vicinity of 1/4t part was etched with a nital etchant, and the area ratio of a structure with a developed needle-like lath structure was measured by imaging 2 fields in the range of 120 μm × 100 μm at 500 times using an optical microscope.A cross section of the steel sheet was electrolytically polished, and the vicinity of 1/4t part of the cross section of the steel sheet was observed by a Scanning Electron Microscope (SEM). The magnification is 5000 times, and a range of 50 μm × 40 μm was imaged.A needle-like structure was regarded as martensite when the longitudinal direction of cementite was oriented in 2 directions or more in the lath block, and the area ratio of this region was determined.

In the above-described observation of the structure by the scanning electron microscope, when it cannot be clearly determined that the long axis direction of the cementite is oriented in 2 or more directions in the lath block, the area ratio of the structure in which the needle-like lath structure is developed under the optical microscope is defined as the area ratio of the sum of martensite and bainite.

The area ratio of the sum of martensite and bainite is 99% or more, or when martensite can be clearly judged, the area ratio of martensite is 90% or more, which is set as a target value.

The structure (the remainder) other than "martensite and bainite" described in table 3 is 1 or 2 or more types of ferrite, pearlite, and retained austenite.

Specifically, after the steel sheet was electropolished, the area around the portion 1/4t of the steel sheet cross section was observed with a Scanning Electron Microscope (SEM) and the number density of cementite was measured.A magnification was set to 5000 times and a Scanning Electron Microscope (SEM) was used to capture an image of 50 μm × 40 μm around the portion 1/4t of the steel sheet cross section.from the contrast of the obtained image, image analysis software was used to count the number of precipitates having an aspect ratio of 2.0 or more and a length in the long axis direction of 0.1 μm or more as the cementite fraction, and similarly, the number of cementite having an aspect ratio of 2.0 or more and a length in the long axis direction of 1.0 μm or more was counted, and then the number of cementite having a number of 1.0 μm or more was divided by the number of cementite of 0.1 μm or more, and the number of cementite of 1.0 μm or more was found to be satisfactory, if not less than 5 μm.

Specifically, a cross section parallel to the rolling direction in the vicinity of the 1/4t portion was etched with a nital solution, and the magnification was 500 times by an optical microscope, and 2 fields in the range of 120 μm × 100 μm were photographed, from the obtained images, the length of the major axis and the length of the minor axis were measured for at least 50 or more prior austenite grains, and the length of the major axis was divided by the length of the minor axis to determine the aspect ratio for each grain, and then the average value of the aspect ratios of these prior austenite grains was found and used as the aspect ratio of the prior austenite grains, and if the aspect ratio of the prior austenite grains was 2.0 or more, it was judged to be good.

Further, a test piece (full thickness) was prepared from the steel sheet) Tensile strength, yield strength and elongation at break were measured according to JIS Z2241. The Charpy absorption energy at-20 ℃ and-40 ℃ was measured in accordance with JIS Z2242. The tensile test piece was a No. 5 test piece (full thickness) produced so that the longitudinal direction was perpendicular to the rolling direction, and the yield strength was the conditioned yield strength σ_0.2Charpy impact test pieces were small-sized test pieces of 10 × 5mm, which were produced from the plate thickness center portion with the longitudinal direction being the rolling direction.

As a result of these tests, the yield strength was 885MPa or more, the tensile strength was 950MPa or more, the elongation at break was 12% or more, and the energy absorption value (vE) at-20 ℃ was_－20) Is 59J/cm²In the above case, the mechanical properties were evaluated to be good.

A welded joint was produced using steel sheets (steel sheet Nos. 1 to 16) having good mechanical properties and a steel sheet No. 32 having a Pcm of less than 0.189%.

The welding method adopts MAG welding, and the welding line energy is set to be 7.0kJ/cm or 10.0 kJ/cm. In the case where the in-line energy was 7.0kJ/cm, the welding conditions were set as follows: the current was 280A, the voltage was 27V, the welding speed was 65cm/min, and when the on-line energy was 10.0kJ/cm, the welding conditions were 305A, the voltage was 29V, and the welding speed was 53 cm/min.

The tensile strength (joint strength) of the welded joint was evaluated by a tensile test defined in JIS Z3121, and good results were evaluated at 950MPa or more.

The evaluation results are shown in table 3. In table 3, underlined values indicate that the values are out of the range of the present invention or that the target characteristics are not obtained.

Steel plate Nos. 1 to 16 are examples of the present invention, and excellent strength, ductility and toughness were obtained. In addition, the joint strength was 950MPa or more. In the case where Mo/Cr is 0.20 or more, excellent toughness was obtained even at a test temperature of-40 ℃.

On the other hand, steel sheet Nos. 17 to 35 are comparative examples, and yield strength, tensile strength, elongation at break, vE _－201 or more of the items do not satisfy the object.

The steel sheet numbers 17, 26 and 29 each had a low C content and Mn content, and therefore had low strength. The martensite fractions were also insufficient for the steel sheet numbers 26 and 29.

Further, steel sheet No. 20 had a small Mn content and low hardenability, and therefore, ferrite and bainite were formed in addition to martensite, and the amount of martensite formed did not satisfy the range of the present invention, and as a result, the strength was significantly low.

Steel plate Nos. 18, 19, 21, 22, 23, 27, 28 and 30 had excessive amounts of C, Si, Mn, Cr and Mo, and had low ductility and toughness.

Steel sheet No. 24 had low strength because the rolling completion temperature and the water cooling initiation temperature were low, worked ferrite was generated in addition to martensite, and the martensite fraction did not satisfy the range of the present invention.

Further, steel sheet No. 33 had low strength because the water cooling start temperature was low, worked ferrite was generated in addition to martensite, and the martensite fraction did not satisfy the range of the present invention.

The steel sheet nos. 25 and 34 had a low martensite fraction because the non-transformed austenite was transformed into bainite due to the high water cooling stop temperature. Further, the elongation at break is lowered by excessive generation of voids starting from coarse carbides (cementite) generated due to the bainite. In addition, the steel plate number 34 also had low yield strength.

Steel sheet No. 31 had too high DI due to high contents of Cr and Mo, and therefore had low toughness and elongation at break.

The steel plate No. 32 had a low Pcm, and therefore had a joint strength of less than 950 MPa.

Steel sheet No. 35 had a low rolling reduction in the unrecrystallized region, and had a low toughness because the longitudinal/transverse size ratio of prior austenite grains was less than 2.0.

TABLE 2

Industrial applicability

According to the present invention, a high-strength steel sheet having a yield strength of 885MPa or more, a tensile strength of 950MPa or more, and an elongation at break of 12% or more can be provided without containing a large amount of expensive alloying elements. In addition, the steel sheet showed a Charpy absorption energy at-20 ℃ of 59J/cm²The above excellent toughness. Therefore, it is industrially useful.

Claims

1. A high-strength steel sheet characterized by comprising a chemical composition containing, in mass%

C：0.050～0.100％、

Si：0～0.50％、

Mn：1.20～1.70％、

P: less than 0.020%,

S: less than 0.0050%,

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％，

The balance of Fe and impurities,

contains 1 or two of Cr and Mo in an amount of 0.20% or more in total, and when the Mo content exceeds 0.50%, the Cr content is 0.80% or less,

DI obtained by the following formula 1 is 2.0 to 7.8,

pcm obtained by the following formula 2 is 0.189% or more,

when the Mo content is [ Mo ] and the Cr content is [ Cr ], the ratio of [ Mo ]/[ Cr ] is 0.20 or more,

the microstructure includes 1 or two kinds of martensite and bainite whose area ratio is 99% or more in total,

the aspect ratio of the prior austenite grains is 2.0 or more,

the number fraction of cementite having a length of 1.0 [ mu ] m or more in the longitudinal direction to cementite having a length of 0.1 [ mu ] m or more in the longitudinal direction is 5% or less,

the thickness of the plate is 4.5 mm-20 mm,

the yield strength is more than 885MPa, the tensile strength is more than 950MPa, the elongation at break is more than 12 percent, and the Charpy absorption energy at the temperature of minus 20 ℃ is 59J/cm²In the above-mentioned manner,

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]) × 1.2.2. formula 1

2. The high-strength steel sheet according to claim 1, wherein the microstructure includes martensite at an area ratio of 90% or more.

3. The high-strength steel sheet according to claim 1, wherein the Cu content is 0 to 0.25 mass%.

4. The high-strength steel sheet according to claim 1, wherein the Ni content is 0 to 0.25 mass%.

5. The high-strength steel sheet according to claim 1, wherein the content of V is 0 to 0.050% by mass.

6. The high-strength steel sheet according to claim 1, wherein the W content is 0 to 0.05 mass%.

7. The high-strength steel sheet according to any one of claims 1 to 6, wherein the sheet thickness is 4.5mm to 15 mm.

8. The high-strength steel sheet according to any one of claims 1 to 6, wherein a Charpy absorption energy at-40 ℃ is 59J/cm²The above.

9. The high-strength steel sheet according to any one of claims 1 to 6, wherein Pcm is 0.196% or more.

10. The high-strength steel sheet according to claim 7, wherein the Charpy absorption energy at-40 ℃ is 59J/cm²The above.

11. The high-strength steel sheet according to claim 7, wherein Pcm is 0.196% or more.

12. The high-strength steel sheet according to claim 8, wherein Pcm is 0.196% or more.

13. The high-strength steel sheet according to claim 10, wherein Pcm is 0.196% or more.