JPWO2018020660A1 - High strength steel sheet - Google Patents

Abstract

This high-strength steel sheet has a predetermined chemical composition, a DI of 2.0 to 7.8, a Pcm of 0.189% or more, and a martensite having a total area ratio of 99% or more. The length in the major axis direction of cementite containing one or two of site and bainite, the aspect ratio of prior austenite grains being 2.0 or more, and the length in the major axis direction being 1.0 μm or more The number fraction with respect to cementite having a thickness of 0.1 μm or more is 5% or less, the sheet thickness is 4.5 mm to 20 mm, the yield strength is 885 MPa or more, the tensile strength is 950 MPa or more, and the elongation at break is 12% or more. Yes, Charpy absorbed energy at −20 ° C. is 59 J / cm 2 or more.

Description

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

Along with the increase in the number of buildings, construction machinery such as crane trucks and industrial machinery are being increased in size. However, for further enlargement, 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, it is required to increase the strength of steel materials used in construction machines and industrial machines.
However, if the strength of the steel sheet is increased to limit the thickness of the steel sheet in order to suppress an increase in the weight of the member, the elongation at break usually decreases. For example, when the plate thickness is limited to 25 mm or less, it is difficult to ensure a breaking elongation of 12% or more. When the plate thickness is limited to 8 mm or less, securing the elongation at break becomes even more difficult. When the elongation at break becomes small, the processing becomes difficult. Therefore, when the steel plate is used as a member of a construction machine or an industrial machine, the steel plate is required to have not only strength but also ductility such as elongation at break. Further, when used as a structural member, low temperature toughness is also necessary to prevent brittle fracture.

From such a background, a high-strength steel sheet having a tensile strength of 780 MPa or more, and further 950 MPa, and a method for producing the same have been proposed.
For example, in Patent Document 1, a steel sheet having high strength and excellent toughness obtained by hot rolling and quenching a steel to which an alloy is added so as to obtain an appropriate hardenability while reducing the amount of C, and its manufacture. A method has been proposed.
However, the technique of Patent Document 1 does not consider the workability of the steel sheet.

  In addition, for example, Patent Documents 2 to 4 include a high-strength hot-rolled steel sheet manufactured by winding a steel strip into a coil shape after hot rolling as a steel sheet used for construction machinery and the like, and a method for manufacturing the same. Proposed. Specifically, in Patent Documents 2 to 4, after the hot rolling, the martensite phase is rapidly cooled to the vicinity of the temperature (Ms) at which martensitic transformation starts, held for a predetermined time, and then wound into a coil shape. Alternatively, a method for producing a hot-rolled steel sheet having a tempered martensite phase as a main phase is disclosed. However, in these methods, it is necessary to wind in a coil shape, and in the steel sheet obtained by these methods, there is a difference between the characteristics in the rolling direction and the characteristics in the direction perpendicular to the rolling direction, and uniform characteristics. Cannot be obtained. Moreover, since the time held in the temperature range where fine carbides are generated becomes long, the yield strength increases and the workability decreases.

  Conventionally, when manufacturing high-strength steel sheets, hot-rolled steel slabs are hot-rolled, accelerated and cooled to room temperature to make the metal structure martensite, and then tempered (tempered heat treatment) to increase ductility and toughness. Had been given. When the metal structure of the steel sheet is martensite, the strength is increased. However, in order to ensure ductility and toughness, it is preferable to perform tempering after accelerated cooling to make the metal structure tempered martensite. However, if this tempering is omitted from the viewpoint of shortening the work period and reducing the manufacturing cost, the metal structure becomes martensite and high strength is obtained, but ductility and toughness are reduced.

Patent Document 5 discloses a high-strength steel sheet that suppresses the contents of Mn and Ni, while increasing the contents of Mo and V, suppresses the formation of martensite, and has a lower bainite-based structure, and a method for manufacturing the same. Has been proposed.
However, since the technique of Patent Document 5 is based on a structure obtained by setting the cooling stop temperature to 300 to 450 ° C., sufficient elongation at break cannot be obtained. When the present inventors made a steel plate according to the disclosure of Patent Document 5 and conducted a test, the elongation at break of 12% or more was not obtained.

As described above, it is difficult to ensure ductility and further toughness in the conventional high-strength steel plate whose thickness is limited and whose metal structure is mainly martensite.
Moreover, when applying a steel plate to the structural member mentioned above, generally welding is performed. At the time of welding, the welded joint is required to have a tensile strength (joint strength) that is greater than or equal to the required value for the base material from the viewpoint of the reliability of the structure. However, when a steel sheet whose main structure is martensite is welded, the tensile strength of the welded joint (joint strength) is lower than that of the base metal due to softening of the heat affected zone, which may not satisfy the required value. there were.

Japanese Unexamined Patent Publication No. 2009-287081 Japanese Unexamined Patent Publication No. 2011-52320 Japanese Unexamined Patent Publication No. 2011-52321 Japanese Unexamined Patent Publication No. 2012-77336 International Publication No. 2012/60405

This invention is made | formed in view of such a situation, and makes it a subject to provide the high strength steel plate used suitably for a construction machine or an industrial machine, and its manufacturing method. Specifically, the plate thickness is 4.5 to 20 mm, the yield strength is 885 MPa or more, the tensile strength is 950 MPa or more, the Charpy absorbed energy at −20 ° C. is 59 J / cm ² or more, and the elongation at break is 12%. As described above, it is an object of the present invention to provide a high-strength steel sheet and a method for manufacturing the same, in which the metal structure is mainly composed of martensite and, further, the welded joint can have a sufficient tensile strength when welded.

The inventors investigated the relationship between the ductility of the steel sheet and the accelerated cooling stop temperature. As a result, it has been found that the ductility is lowered when the accelerated cooling stop temperature is 300 ° C. or higher or higher than the temperature (Mf) at which the martensitic transformation is completed. As a result of further investigations, when accelerated cooling is stopped at a temperature of 300 ° C. or higher or higher than Mf, untransformed austenite undergoes bainite transformation in the metal structure, and coarse carbides (cementite) generated due to the bainite. As a starting point, it was found that ductility is reduced by excessive generation of voids.
The present inventors examined measures against such a decrease in ductility. As a result, in order to suppress the above-mentioned bainite transformation, a component capable of enhancing the hardenability is designed, and after hot rolling, the metal structure is martensite by accelerated cooling to less than 300 ° C. and below the Mf temperature. The present inventors have found a new finding that it can be a main component and can ensure the ductility of a high-strength steel sheet.
This invention is made | formed based on such knowledge, The summary is as follows.

(1) The high-strength steel sheet according to one embodiment of the present invention has a chemical composition of mass%, C: 0.050 to 0.100%, Si: 0 to 0.50%, Mn: 1.20 to 1. 70%, P: 0.020% or less, S: 0.0050% or less, N: 0 to 0.0080%, B: 0.0003 to 0.0030%, Ti: 0.003 to 0.030% Nb: 0.003 to 0.050%, Cr: 0 to 2.00%, Mo: 0 to 0.90%, Al: 0 to 0.100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.100%, W: 0 to 0.50%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030 %, Balance: Fe and impurities, and one or both of Cr and Mo are contained in total of 0.20% or more, and the Mo content is more than 0.50%. The Cr content is 0.80% or less, the DI obtained by the following formula 1 is 2.0 to 7.8, and the Pcm obtained by the following formula 2 is 0.189% or more. In addition, the metal structure includes one or two 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, and the length in the major axis direction is 1 The number fraction of cementite having a length in the major axis direction of 0.1 μm or more with respect to cementite having a length of 0.1 μm or more is 5% or less, the plate thickness is 4.5 mm to 20 mm, and the yield strength is 885 MPa. As described above, the tensile strength is 950 MPa or more, the elongation at break is 12% or more, and the Charpy absorbed energy at −20 ° C. is 59 J / cm ² or more.
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 (Formula 1)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (formula 2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] in Formula 1 and Formula 2 are the values of each element. It is a content in mass%, and when not including, it calculates as 0.
(2) The high-strength steel sheet according to (1) may include martensite in which the metal structure has an area ratio of 90% or more.
(3) The high-strength steel sheet described in the above (1) or (2) may be% by mass and Cu: 0 to 0.25%.
(4) In the high-strength steel sheet according to any one of (1) to (3) above, Ni may be 0 to 0.25% by mass%.
(5) In the high-strength steel sheet according to any one of (1) to (4) above, the mass% may be V: 0 to 0.050%.
(6) In the high-strength steel sheet according to any one of (1) to (5) above, the mass% may be W: 0 to 0.05%.
(7) In the high-strength steel plate according to any one of (1) to (6) above, the plate thickness may be 4.5 mm to 15 mm.
(8) In the high-strength steel sheet according to any one of (1) to (7) above, when the Mo content is [Mo] and the Cr content is [Cr], [Mo] / [ Cr]) may be 0.20 or more.
(9) In the high-strength steel sheet of (8) above, Charpy absorbed energy at −40 ° C. may be 59 J / cm ² or more.
(10) In the high-strength steel sheet according to any one of (1) to (9), the 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 885 MPa or more, a tensile strength of 950 MPa or more, and a breaking elongation of 12% or more is provided without containing a large amount of expensive alloy elements. be able to. This steel sheet exhibits excellent toughness with Charpy absorbed energy at −20 ° C. of 59 J / cm ² or more. In addition, by setting Pcm, which is an index of hardenability, to 0.189% or more, preferably 0.196% or more, the high-strength steel sheet according to the present invention can be used as a base when welding is performed at a predetermined heat input or less. 950 MPa or more can be secured in the tensile strength of the welded joint used as the material.
Furthermore, by controlling together [Mo] / [Cr], which is the ratio of the Mo content [Mo] and the Cr content [Cr], the Charpy absorbed energy at −40 ° C. is 59 J / cm ^2. It is also possible to provide a high-strength steel sheet with superior toughness as described above.
Therefore, the present invention provides a high-strength steel plate that is suitably used as a structural member for construction machinery and industrial machinery, and contributes to increasing the size and weight of construction machinery and industrial machinery, without significantly increasing manufacturing costs. The industrial contribution is extremely remarkable.

It is a figure explaining the relationship between breaking elongation (Total elongation), the stop temperature Tcf of accelerated cooling, hardenability parameter | index DI, and C amount. It is a figure which shows the relationship between [Mo] / [Cr] and the Charpy absorbed energy (vE-40) in -40 degreeC. It is a figure which shows the relationship between the stop temperature of accelerated cooling, and breaking elongation. It is a SEM photograph which shows the influence on the shape of cementite by the stop temperature of accelerated cooling, and is an SEM photograph at the time of making an accelerated cooling stop temperature into 290 ° C. It is a SEM photograph which shows the influence on the shape of cementite by the stop temperature of accelerated cooling, and is an SEM photograph at the time of setting an accelerated cooling stop temperature to 400 ° C. It is the photograph of the void generated from the vicinity of coarse cementite.

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

  First, the chemical composition (component) of the high-strength steel sheet according to the present embodiment will be described. Hereinafter, the notation of “%” regarding the content means “% by mass” unless otherwise specified.

(C: 0.050-0.100%)
C is a useful element that increases the strength of the steel and is an extremely important element that determines the elongation at break of a steel having a martensite structure. In the high-strength steel plate according to this embodiment, the C content needs to be 0.050% or more in order to obtain sufficient strength. In order to further increase 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%, the ductility and toughness of the steel deteriorate due to the formation of excess carbides. 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)
When Si is contained excessively, the ductility and toughness of the steel are lowered. Therefore, the Si amount is limited to 0.50% or less. There is no need to particularly define the lower limit of the Si amount, and the lower limit of the Si amount is 0%. However, when Si is used for deoxidation, the Si content is preferably 0.03% or more in order to obtain sufficient effects. Si is also an element that suppresses the formation of carbides, and when obtaining this effect, the Si amount is preferably 0.10% or more, and more preferably 0.20% or more. When it is not necessary to obtain these effects, the upper limit of the Si amount may be 0.45%, 0.40%, or 0.35%.

(Mn: 1.20 to 1.70%)
Mn is an important element that improves the hardenability of steel. In order to increase the martensite area ratio in the metal structure and obtain high strength, the amount of Mn is set to 1.20% or more. The amount of Mn is preferably more than 1.20%, 1.25% or more or 1.30% or more, more preferably 1.35% or more or 1.39% or more. On the other hand, when the amount of Mn is excessive, ductility and toughness may be lowered. Therefore, the amount of Mn is made 1.70% or less. More preferably, the amount of Mn 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 and S are elements inevitably contained in the steel as impurities, and are elements that deteriorate the toughness of the steel. In addition, when welding is performed, the element deteriorates the 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 toughness, the P content may be 0.015% or less and the S content may be 0.0030% or less. The smaller the amount of P and S, the better. Therefore, it is not necessary to particularly define the lower limits of the P amount and the S amount, and the lower limits of the P amount and the S amount are 0%. However, from the viewpoint of dephosphorization and desulfurization costs, the P content may be 0.001% or more and S: 0.0001% or more.

(B: 0.0003 to 0.0030%)
B is an element that segregates at the grain boundary and enhances the hardenability of the steel, and is a useful element that exhibits its effect even when contained in a small amount. In the high-strength steel sheet according to the present embodiment, the B content is set to 0.0003% or more in order to increase martensite in the metal structure. Preferably, the B amount is 0.0005% or more. On the other hand, even if B is contained excessively, not only the effect of improving hardenability is saturated, but also precipitates such as nitrides and carbonitrides are formed, and ductility and toughness are rather lowered. Therefore, the B amount is set to 0.0030% or less. Preferably, the B amount is 0.0020% or less or 0.0015% or less.

(Ti: 0.003-0.030%)
Ti is an element that forms nitride, and is an element that fixes N in steel as TiN and suppresses the generation of BN. As described above, B is an element that enhances hardenability, but if BN is formed, the effect 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 the hardenability. Preferably, the Ti content is 0.005% or more, more preferably 0.010% or more. On the other hand, when Ti is contained excessively, TiN becomes coarse and ductility and toughness may be reduced. Therefore, the Ti content is 0.030% or less. Preferably, the Ti amount is 0.020% or less.

(Nb: 0.003 to 0.050%)
Nb is an element that remarkably improves the hardenability of steel by being contained simultaneously with B. In the high-strength steel plate 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 metal structure. Nb is an element that forms fine nitrides, contributes to refinement of crystal grains, and increases toughness. When obtaining this effect, the Nb content is preferably 0.005% or more. More preferably, the Nb amount is 0.010% or more or 0.015% or more. On the other hand, when Nb is contained excessively, the nitride becomes coarse and ductility and toughness may be lowered. Therefore, the Nb content is 0.050% or less. Preferably, the Nb content is 0.040% or less, 0.035% or less, or 0.030% or less.

(Cr: 2.00% or less)
(Mo: 0.90% or less)
(When one or both of Cr and Mo is 0.20% or more in total and Mo content is more than 0.50%, Cr content is 0.80% or less)
Cr and Mo are important elements that improve the hardenability, and contain one or both. In the high-strength steel plate according to the present embodiment, the total amount of Cr and Mn is 0.20% or more in order to increase the martensite area ratio in the metal structure. Preferably, the total amount of Cr and Mn is 0.30% or more, and more preferably 0.40% or more. Considering the case of containing only Cr or Mo, the lower limit of the Cr content and the Mo content is 0%. If necessary, the lower limit of the Cr amount may be 0.20% or 0.30%. Similarly, the lower limit of the Mo amount may be 0.20% or 0.30%.
On the other hand, when the Cr content exceeds 2.00% or the Mo content exceeds 0.90%, fine carbides are generated and ductility and toughness are lowered. Therefore, the Cr amount and the Mo amount are set to 2.00% or less and 0.90% or less, respectively. The Cr content is preferably 1.50% or less or 1.00% or less, more preferably 0.90% or less or 0.80%. Further, the Mo amount is preferably 0.70% or less, more preferably 0.60% or less or 0.50%.
Further, when both Cr and Mo are contained, if the content is excessive, the toughness is lowered. Therefore, when the Mo amount exceeds 0.50%, the Cr amount needs to be 0.80% or less. There is. In this case, the Cr amount may be 0.70% or less. On the other hand, when the Cr content is more than 0.80%, the Mo content should be 0.50% or less, and when the Cr content is more than 1.20%, the Mo content should be 0.40% or less. Good. 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 and inhibits the hardenability improving effect of B. Therefore, the N content is limited to 0.0080% or less. Preferably, the N content is limited to 0.0060% or less, more preferably 0.0050% or less. The amount of N is preferably reduced as much as possible, and the lower limit is 0%. However, from the viewpoint of denitrification cost, the N amount may be 0.0001% or more. On the other hand, the N content may be 0.0020% or more in order to make the metal structure finer by the nitride.

  The above are the elements included as essential elements and impurities of the high-strength steel sheet according to the present embodiment, and the high-strength steel sheet according to the present embodiment includes the essential elements, the remaining Fe, and impurities (other than the above-mentioned impurity elements and in some cases In principle, it has a component composed of an impurity element). However, the high-strength steel sheet according to the present embodiment, in addition to the above components, is part of Fe for deoxidation, improvement of strength and / or ductility, refinement of the metal structure, and sulfide morphology control. Instead of 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% Hereinafter, one or more of Mg: 0.0030% or less and REM: 0.0030% or less may be further contained. However, these elements are not essential and may be 0%.

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

(Cu: 0.50% or less)
(Ni: 0.50% or less)
Cu and Ni are elements that improve the hardenability of the steel. When the hardenability is increased and the martensite area ratio in the metal structure is increased, the Cu content and the Ni content are each preferably set to 0.10% or more. On the other hand, since Cu and Ni are expensive elements, it is preferable that the Cu content and the Ni content be 0.50% or less, respectively, even when contained. More preferably, the amount of Cu and the amount of Ni are each 0.40% or less, more preferably 0.30% or less.

(V: 0.100% or less)
V is an element that forms carbide or nitride. When the crystal grains are refined by carbide or nitride to enhance toughness, the V content is preferably 0.005% or more. On the other hand, when V is contained excessively, ductility and toughness are lowered. However, since the adverse effect is small as compared with Nb and Ti, the upper limit of the V amount in the case of inclusion is 0.100%. Preferably, the V amount is 0.050% or less.

(W: 0.50% or less)
W is an element that improves the hardenability of steel. When obtaining this effect, the W amount is preferably 0.05% or more. On the other hand, when W is contained excessively, weldability deteriorates. Therefore, even when it contains, W amount shall be 0.50% or less or 0.30% or less. If necessary, the W content may be 0.02% or less or 0.01% or less.

(Ca: 0.0030% or less)
Ca is an element that controls the form of oxides and sulfides. When obtaining this effect, the Ca content is preferably 0.0001% or more. More preferably, the Ca content is 0.0005% or more, and further preferably 0.0010% or more. On the other hand, when Ca is excessively contained, not only the effect is saturated, but also ductility and toughness may be lowered due to the formation of inclusions. Therefore, even when contained, the Ca content is 0.0030% or less.

(Mg: 0.0030% or less)
Mg is an element having an action of increasing the toughness of steel by refining the structure. When obtaining this effect, the Mg content is preferably 0.0005% or more. On the other hand, when Mg is excessively contained, not only the effect is saturated, but also ductility and toughness may be lowered due to the formation of inclusions. Therefore, even when it contains, Mg amount shall be 0.0030% or less.

(REM: 0.0030% or less)
REM (rare earth element) is an element having an effect of increasing the toughness of steel by controlling the form of sulfide, particularly MnS. When obtaining this effect, the REM content is preferably 0.0001% or more. On the other hand, when REM is excessively contained, inclusions containing REM become coarse and ductility and toughness may decrease. Therefore, even when it contains, REM amount shall be 0.0030% or less.

  In addition to the above elements, other elements may be contained in a trace amount as long as they do not impair the effects.

  In the high-strength steel plate according to the present embodiment, each element should be within the above range, and DI and Pcm determined by the chemical composition must satisfy the following ranges.

(DI: 2.0-7.8)
DI is an index of hardenability and is obtained by the following (formula 1). Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] in the formula are the contents (mass%) of each element and include the element. If not, calculate as 0.
Qualitatively, as shown in FIG. 1, increasing the hardenability index DI suppresses the decrease in elongation at break even when the acceleration cooling stop temperature Tcf is increased (that is, moved to the right in FIG. 1). it can. When the stop temperature Tcf for accelerated cooling is increased, an excessive increase in strength is suppressed, and toughness and ductility can be increased. In order to achieve a good balance of strength, ductility, and toughness, it is preferable to set DI to 2.0 or more. More preferably, DI is 3.0 or more, More preferably, it is 4.0 or more. On the other hand, when the hardenability becomes excessively high, the strength becomes excessively high and the toughness may be lowered. Therefore, DI is preferably 7.8 or less. More preferably, DI is 7.0 or less, and more preferably 6.5 or less.

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 (Formula 1)

(Pcm: 0.189% or more)
A welded joint is usually required to have a tensile strength (joint strength) that is equal to or higher than the required tensile strength for the base material used for welding. In the case of welding a steel plate whose main structure is martensite, the present inventors have a case where the tensile strength of the welded joint (joint strength) is lower than the tensile strength of the base metal due to softening of the weld heat affected zone. Found that there is. Therefore, the present inventors manufactured and tested a welded joint using various high-strength steel plates with varying welding heat input. As a result, by increasing the hardenability of the steel sheet, specifically, by increasing the Pcm calculated by the following (Equation 2) to 0.189% or more, softening of the heat affected zone is suppressed, and construction machinery and industrial machinery It was found that the tensile strength of the welded joint can be increased to 950 MPa or more when welding is performed at 7.0 kJ / cm, which is the lower limit of the welding heat input range, which is often applied to the manufacture of structural members. .
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (formula 2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are the contents (mass%) of each element, When the element is not included, it is calculated as 0.

Further, the inventors have examined the welding heat input and the strength of the welded joint. The strength of the welded joint is determined by the above-described (Equation 2) from the component composition of the high-strength steel plate used for welding and the weld heat input Hi [ kJ / cm] can be evaluated by JS calculated by the following (formula a). It was found that if JS is 950 MPa or more, a joint strength of 950 MPa or more can be ensured even with 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, it is understood that it is preferable to make the welding heat input as small as possible in order to ensure the strength of the welded joint. However, in order to ensure the soundness of the welded joint, there is a lower limit for welding heat input. In order to ensure the productivity of welding work when manufacturing construction machines and industrial machines, it is not easy to reduce the welding heat input to less than 7.0 kJ / cm. In the case of this welding heat input of 7.0 kJ / cm, Pcm required to make JS 950 MPa or more is 0.189% from the above formula. That is, joint strength of 950 MPa or more can be secured by setting Pcm to 0.189% or more.
Moreover, if Pcm is 0.196% or more, a joint strength of 950 MPa or more can be secured even in the case of 10.0 kJ / cm, which is welding heat input that does not require special management during welding. That is, by making Pcm 0.196% or more, the strength of the welded joint can be made 950 MPa or more without performing special welding construction management.
Note that Pcm may be 0.200% or more, 0.205% or more, 0.210% or more, or 0.215% or more in order to ensure the strength of the welded joint even with larger welding heat input. A larger welding heat input is preferable because the number of welding passes can be reduced and productivity is improved. The upper limit of Pcm is not particularly required, but may be 0.250% or less or 0.240% or less for preventing weld cracking.

([Mo] / [Cr]: 0.20 or more)
Furthermore, the inventors investigated the influence of Cr and Mo, which are elements that enhance hardenability, on toughness, and proceeded with investigation. As a result, it was found that when the hardenability (DI) is constant, the ratio of Mo to Cr affects the toughness. Specifically, when the ratio ([Mo] / [Cr]) of the Mo content [Mo] and the Cr content [Cr] in mass% increases, the martensite substructure (packet, block) As a result, the toughness was improved. In order to further improve the toughness, this ratio may be 0.40 or more, 0.80 or more, or 1.00 or more.

  FIG. 2 is a diagram showing the relationship between [Mo] / [Cr] and Charpy absorbed energy at −40 ° C. In FIG. 2, “◯” indicates an actual measurement value, and “●” indicates an average value of the actual measurement values.

As shown in FIG. 2, as [Mo] / [Cr] increases, the Charpy absorbed energy at −40 ° C. tends to increase. When [Mo] / [Cr] is 0.20 or more, It can be seen that the Charpy absorbed energy at −40 ° C. is 59 J / cm ² or more. Therefore, when low temperature toughness is required, [Mo] / [Cr] is preferably 0.20 or more. On the other hand, Mo is an element that easily forms fine carbides and clusters as compared with Cr. Therefore, when Mo is contained in excess of Cr, toughness may be reduced, and [Mo] / [Cr] may be 2.00 or less or 1.50 or less.
The Charpy absorbed energy was measured by a Charpy test conducted according to JIS Z 2242. However, the plate thickness of the steel plate from which the test piece was taken was 8 mm, and the test piece taken from the central portion of the plate thickness with the longitudinal direction as the rolling direction was a subsize of 10 mm × 5 mm.

(Total area ratio of one or two of martensite and bainite: 99% or more and elongation at break: 12% or more)
The present inventors examined the hardenability of the high-strength steel sheet and the relationship between the metal structure and the elongation at break. As a result, when the hardenability is insufficient, the inventors reduce the elongation at break. Further, the cause of the decrease in elongation at break, that is, the decrease in ductility is due to bainite as shown in FIGS. 4A and 4B. It was found that voids originated from the generated coarse carbides. And in order to raise the ductility of a high-strength steel plate, the knowledge that it was necessary to suppress the production | generation of the bainite which causes the production | generation of coarse cementite was acquired. In order to suppress bainite which causes the formation of coarse cementite, it is preferable to use a martensite-based structure in which 90% or more of the metal structure is martensite. In order to increase the strength of the steel sheet, it is preferable that the martensite area ratio in the metal structure is 90% or more. More preferably, it is 92% or more, More preferably, it is 94% or more.
However, both martensite and bainite are continuously cooled transformation structures, and accurate discrimination may be difficult depending on the 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 is judged that bainite that causes the formation of coarse cementite is suppressed. it can.
Therefore, in the high-strength steel sheet according to the present embodiment, the total area ratio of one or two of martensite and bainite is 99% or more, and the elongation at break is 12% or more as a structure index. In the case where martensite and bainite can be sufficiently distinguished by structural observation, the martensite area ratio is preferably 90% or more.
In the case of the high-strength steel plate according to the present embodiment, the martensite having a metal structure is as-quenched and is different from the tempered martensite obtained by tempering treatment. Tempered martensite is not preferred because cementite grows by prolonged tempering.
The remainder other than the above may be one or more of ferrite, pearlite, and retained austenite.

The determination of the metal structure and the measurement of the martensite area ratio are performed with an optical microscope. Specifically, a cross section parallel to the rolling direction in the vicinity of a 1/4 t part (part of 1/4 of the plate thickness t in the plate thickness direction from the steel plate surface) is subjected to Nital corrosion and 500 times using an optical microscope, Two fields of view in the range of 120 μm × 100 μm are photographed, and the area ratio of the tissue in which the acicular lath structure is developed is measured. Moreover, after electrolytically polishing the cross section of the steel plate in the needle-like structure, the vicinity of a 1/4 t portion of the cross section of the steel plate is observed with a scanning electron microscope (SEM). Here, the magnification is 5000 times, and a range of 50 μm × 40 μm is photographed. When the long axis direction of cementite is oriented in two or more directions in the block, the acicular structure is assumed to be martensite, and the area ratio of the region is obtained. The product of the needle-like structure area ratio in the optical microscope and the martensite area ratio in the SEM is defined as the area ratio of the martensite structure of the steel type.
In the structure observation by the above-described scanning electron microscope, it may be impossible to clearly determine that the long axis direction of cementite is oriented in two or more directions in the block. In this case, the area ratio of the structure in which the acicular lath structure is developed by the optical microscope is defined as the total area ratio of martensite and bainite.

(Number fraction of cementite having a length in the major axis direction of 1.0 μm or more to cementite having a length in the major axis direction of 0.1 μm or more: 5% or less)
As described above, in order to increase the ductility of the steel sheet, it is important to suppress the formation of bainite that causes the formation of coarse cementite and to form a metal structure mainly composed of martensite. However, in order to further improve the ductility, it is effective to suppress the formation of voids starting from coarse carbides (particularly cementite).
By controlling the stop temperature of accelerated cooling, the present inventors can reduce the number fraction of coarse carbides (particularly cementite) whose length in the major axis direction is 1.0 μm or more, and as a result, It has been found that the formation of voids can be suppressed and the elongation at break can be improved. Specifically, among the cementite having a length in the major axis direction of 0.1 μm or more, the number fraction of cementite having a length in the major axis direction of 1.0 μm or more is set to 5% or less. It has been found that the elongation at break can be improved.
As will be described in detail later, in the present invention, the accelerated cooling is stopped at a temperature equal to or lower than Mf and lower than 300 ° C., whereby a martensite-based structure in which the formation of coarse carbides is suppressed can be obtained. That is, by controlling the acceleration cooling stop temperature, it is possible to suppress generation of voids starting from coarse cementite having a length in the major axis direction of 1.0 μm or more.
The number density of cementite is measured by a scanning electron microscope (SEM). Specifically, after electrolytically polishing the cross section of the steel plate, a region of 50 μm × 40 μm is photographed with a scanning electron microscope (SEM) at a magnification of 5000 times around a 1/4 t portion of the cross section of the steel plate. From the contrast of the obtained image, image analysis software is used to count the number of precipitates having an aspect ratio of 2.0 or more and a length in the major axis direction of 0.1 μm or more as being cementite. Similarly, the number of cementites having an aspect ratio of 2.0 or more and a length in the major axis direction of 1.0 μm or more is counted. Then, by dividing the number of obtained precipitates of 1.0 μm or more by the number of cementite of 0.1 μm or more, a cementite number fraction (%) of 1.0 μm or more is obtained. The shape of the carbide is not particularly limited. For example, in the case of a spherical shape, the “length in the major axis direction” indicates the major axis.

(The aspect ratio of prior austenite grains is 2.0 or more)
In the high-strength steel sheet according to the present embodiment, the aspect ratio of the prior austenite grains is set to 2.0 or more. When the aspect ratio is less than 2.0, there is a concern that the toughness decreases.
In addition, when accelerated cooling (direct quenching) is performed online after rolling in the non-recrystallized region, the aspect ratio of the prior austenite grains can be 2.0 or more. On the other hand, after re-heating and quenching after rolling and cooling, the processed structure by rolling is not inherited, and the aspect ratio of the prior austenite grains is less than 2.0.
The aspect ratio of prior austenite grains is measured by the following method. That is, the cross section parallel to the rolling direction in the vicinity of the 1/4 t portion, which is 1/4 of the thickness t from the surface in the thickness direction, is corroded with nital, and the magnification is 500 times with an optical microscope, 120 μm × 100 μm Shoot two fields of view. From the obtained image, for at least 50 or more prior austenite grains, measure the length of the major axis and the length of the minor axis, and divide the major axis length by the minor axis length to determine the aspect ratio for each grain. Ask. And the average value of the aspect ratio of these prior austenite grains is obtained.

Next, the plate thickness and mechanical characteristics of the high-strength steel plate according to this embodiment will be described.
(Thickness: 4.5-20mm)
The thickness of the high-strength steel plate used for a crane etc. is generally 4.5-20 mm. Therefore, the plate | board thickness of the high strength steel plate which concerns on this embodiment shall be 4.5-20 mm. However, in terms of contribution to weight reduction, the thickness is preferably 4.5 to 15 mm.
(Yield strength: 885 MPa or more)
(Tensile strength: 950 MPa or more)
In order to contribute to the increase in size and weight of construction machinery and industrial machinery, high strength is required. To obtain a remarkable economic effect, the yield strength is 885 MPa or more, and the tensile strength is It is necessary to be 950 MPa or more. The upper limit of the yield strength is not particularly required, but may be 1100 MPa or less. The upper limit of the tensile strength is not particularly required, but may be 1300 MPa or less or 1250 MPa or less.
(Elongation at break: 12% or more)
In order to apply a high-strength steel sheet to a member of a construction machine or an industrial machine, workability such as bendability is required, so the elongation at break is set to 12% or more. In addition, as described above, the elongation at break is also an index of the structure as to whether bainite, which causes the formation of coarse cementite, is suppressed.
Yield strength, tensile strength, and elongation at break are measured by performing a tensile test in accordance with JIS Z 2241. However, the value of the elongation at break in the tensile test depends on the shape of the test piece. The above-mentioned limitation on elongation at break (12% or more) is that, as a tensile test piece, a JIS Z2241 No. 5 test piece (the distance between original marks is 50 mm, the width of the parallel part is 25 mm, and the thickness of the test piece is the thickness of the steel plate. It is a value when using a flat test piece as it is.
The elongation conversion formula based on the difference in the shape of the test piece is also defined in ISO2566-1. The 12% elongation of the JIS Z2241 No. 5 test piece is the tensile test piece of the JIS Z2241 No. 13B test piece (original The distance between the gauge points is 50 mm, the width of the parallel part is 12.5 mm, and the thickness of the test piece is a flat type test piece with the thickness of the steel plate, which is 10.4%, and the tensile test piece is JIS Z2241 No. 13A In the test piece (the distance between original marks is 80 mm, the width of the parallel portion is 20 mm, and the thickness of the test piece is the same as the thickness of the steel plate), it can be converted to 9.5%.
(Charpy absorbed energy at −20 ° C .: 59 J / cm ² or more)
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 absorbed energy at −20 ° C. is 59 J / cm ² . More preferably, the Charpy absorbed energy at −40 ° C. is 59 J / cm ² or more.
The Charpy absorbed energy is measured by collecting a test piece whose rolling direction is the longitudinal direction from the center of the plate thickness, and performing a Charpy test in accordance with JIS Z 2242 at -20 ° C or -40 ° C. Depending on the plate thickness of the steel sheet, it is difficult to collect a full-size test piece of 10 mm × 10 mm. The Charpy absorbed energy is J / cm ² obtained by dividing the absorbed energy by the cross-sectional area (cm ² ) of the test piece at the bottom of the V notch. For example, in the case of a full-size test piece of 10 mm × 10 mm and a sub-size test piece of 10 mm × 5 mm, the measured Charpy absorbed energy value (J) is 1 cm × 0.8 cm = 0. in 8 cm ^2, and dividing the case of sub-sized specimens 0.5cm × 0.8cm = 0.4cm ^2.

Next, a preferable method in manufacturing the high-strength steel plate according to the present embodiment will be described.
The high-strength steel sheet according to the present embodiment melts molten steel having the chemical composition in the above-described range by a conventional method, heats a steel piece obtained by casting the molten steel, performs hot rolling, and accelerated cooling. And after accelerating cooling stop, it can cool to room temperature as it is and can manufacture. However, in the production of the high-strength steel sheet according to the present embodiment, tempering heat treatment such as tempering is not performed after the accelerated cooling is stopped or after cooling to room temperature. When tempering is performed, martensite becomes tempered martensite. That is, the high-strength steel sheet according to the present embodiment is manufactured in a so-called non-tempered manufacturing process in which the tempering heat treatment is omitted for the purpose of shortening the work period and reducing the manufacturing cost. The high-strength steel plate according to this embodiment manufactured by the non-tempered manufacturing process may be referred to as a non-tempered high-strength steel plate.
Below, the preferable conditions of each process are demonstrated.

(Heating temperature of steel slab: 1100 to 1250 ° C.)
In the high-strength steel sheet according to the present embodiment, it is necessary to contain a predetermined amount of alloy element in order to improve hardenability. For this reason, carbides and nitrides of alloy elements are generated in the steel slab subjected to hot rolling. When the steel slab is heated, it is necessary to decompose these carbides and nitrides and dissolve them in the steel, and the heating temperature is set to 1100 ° C. or higher. On the other hand, if the heating temperature of the steel slab is too high, the crystal grain size becomes coarse and the toughness may decrease, so the heating temperature is set to 1250 ° C. or lower.

(Finish temperature: Ar3 (° C) or higher)
(Accelerated cooling start temperature: Ar3 (° C) or higher)
Hot rolling is performed on the heated steel slab. After hot rolling, in order to obtain a metal structure mainly composed of martensite by accelerated cooling, it is necessary to start accelerated cooling at a temperature at which the metal structure is austenite. Therefore, the hot rolling must be finished at a temperature at which the metal structure is austenite. Therefore, the finishing temperature of hot rolling is set to Ar3 (° C.) or higher. Ar3 (° C.) is a temperature at which ferrite transformation starts from austenite during cooling, and can be obtained from thermal expansion behavior. Ar3 (° C.) can be easily obtained by, for example, the 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], and [Mo] are the contents (% by mass) of each element, and 0 when no element is included. Calculate as

  The hot rolling may be performed by a conventional method, but the recrystallization zone rolling in which the cumulative rolling reduction in the temperature range of 1050 ° C. or higher is 50 to 80%, and the cumulative rolling reduction in the temperature range of Ar 3 to 950 ° C. is 50. It is preferable to perform non-recrystallized region rolling to ˜90%.

(Cooling rate of accelerated cooling: 30 to 200 ° C./s)
In accelerated cooling performed subsequent to hot rolling, martensite is generated. The cooling rate of accelerated cooling needs to be 30 ° C./s or more in order to increase the area ratio of martensite. If it is less than 30 ° C./s, a sufficient martensite area ratio cannot be obtained. In order to promote martensitic transformation, it is preferable to increase the cooling rate, but the upper limit may be set to 200 ° C./s or less because of restrictions on the plate thickness and equipment. The cooling rate is 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.

(Accelerated cooling stop temperature: Mf (° C) or less and less than 300 ° C)
The present inventors examined the relationship between hardenability and accelerated cooling stop temperature, metal structure, and elongation at break. Here, when the steel sheet is rapidly cooled after hot rolling, the temperature Ms (° C.) at which martensitic transformation starts is obtained by the following (formula 3). Further, the temperature Mf (° C.) at which the martensitic transformation is completed is a temperature lower by about 150 ° C. than Ms (° C.), and is obtained by the following (formula 4). [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], and [Al] in the following (Formula 3) are the contents (mass%) of each element, When the element is not included, it is calculated 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 change the metal structure to martensite, it is necessary to cool to a temperature of at least Ms (° C.) or less, and when cooled (quenched) to a temperature of Mf (° C.) or less, 90% or more of the metal structure is martensite. Become a site. However, when the cooling stop temperature is 300 ° C. or higher, cooling may become unstable and part of martensite may be bainite. Therefore, the cooling stop temperature is set to Mf (° C.) or less and lower than 300 ° C.
As described above, the accelerated cooling stop temperature is extremely important, and it is a precondition that the accelerated cooling stop temperature is lower than the temperature Ms (° C.) at which the martensitic transformation starts. When accelerated cooling is performed to a temperature Mf (° C.) or less at which the martensite transformation is completed and less than 300 ° C., the metal structure becomes a martensite-based structure in which the formation of carbides is suppressed.

  On the other hand, when the accelerated cooling stop temperature is between Ms (° C.) and Mf (° C.) (between Ms and Mf), the ductility of the high-strength steel plate is affected by the hardenability. That is, when the hardenability is increased, the generation of bainite is suppressed, the generation of cementite coarse carbides is suppressed, the elongation at break is improved, and the variation is reduced.

  Then, the relationship between the accelerated cooling stop temperatures Tcf and Mf and the elongation at break, and the influence of the DI and C amounts on the elongation at break can be schematically shown in FIG. Here, the vertical axis in FIG. 1 is the elongation at break (Total elongation), the horizontal axis is the stop temperature Tcf for accelerated cooling, and DI is an index of the hardenability obtained by the above (Equation 1).

As shown in the graph of FIG. 1, when the accelerated cooling stop temperature Tcf is lowered, martensitic transformation is promoted and the formation of bainite is suppressed, so that the breaking elongation is improved, and when the Tcf is Mf or less, the breaking elongation is constant. become. When Tcf is less than or equal to Mf, the elongation at break is almost determined by the C content, and the elongation at break improves as the C content decreases.
On the other hand, when the accelerated cooling stop temperature Tcf is between Ms and Mf, the elongation at break increases with a decrease in Tcf. At this time, when an alloying element is added to improve the hardenability, DI increases and bainite Generation | occurrence | production is suppressed and breaking elongation improves by the production | generation of a coarse carbide | carbonized_material being suppressed.

The lower limit of the accelerated cooling stop temperature is not particularly limited, and accelerated cooling may be performed to room temperature. In order to increase the yield strength by an action such as fixing carbon atoms to dislocations, the stop temperature of accelerated cooling is preferably 100 ° C. or higher.
After stopping the accelerated cooling, it is allowed to cool to room temperature without any tempering heat treatment such as tempering.

  Examples of the present invention will be described below. The conditions in the following examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel pieces obtained by melting the chemical components shown in Table 1 (the balance being Fe and impurities) were made into steel plates having a thickness of 4.5 to 20 mm according to the manufacturing conditions shown in Table 2. “Heating temperature” is the reheating temperature of the steel slab, “Rolling end temperature” is the end temperature of hot rolling, “Water cooling start temperature” is the surface temperature of the steel plate at the start of accelerated cooling (water cooling), “Cooling rate” "" Means the average cooling rate at the central part of the plate thickness in the temperature range from Ar3 (° C) to the accelerated cooling stop temperature, and "water cooling stop temperature" represents the surface temperature of the steel plate when water cooling is stopped. The surface temperature of the steel plate was measured with a radiation thermometer, and the “cooling rate” was calculated by obtaining the temperature at the center of the plate thickness from the surface temperature by heat conduction calculation. None of the steel sheets was tempered.
The obtained steel sheet was evaluated for the metal structure and mechanical properties (yield strength, tensile strength, elongation at break, toughness, joint strength).

The determination of the metal structure and the measurement of the area ratio of martensite and bainite were performed by the following methods.
After mirror polishing the cross section of the steel plate, the cross section parallel to the rolling direction in the vicinity of the 1/4 t portion is corroded by nital, and two optical fields of 120 μm × 100 μm are photographed with an optical microscope at a magnification of 500 times, and are needle-shaped. The area ratio of the tissue in which the lath structure was developed was measured. Moreover, about the acicular structure, after carrying out the electrolytic polishing of the cross section of a steel plate, the 1/4 t part vicinity of the steel plate cross section was observed with the scanning electron microscope (SEM). Here, the magnification was 5000 times, and a range of 50 μm × 40 μm was photographed. When the long axis direction of cementite is oriented in two or more directions in the block, the acicular structure is assumed to be martensite, and the area ratio of the region was obtained. The product of the needle-like structure area ratio in the optical microscope and the martensite area ratio in the SEM was defined as the martensite area ratio of the steel type. Moreover, the acicular structure other than martensite was used as bainite.
In the above-mentioned structure observation with a scanning electron microscope, when it is impossible to clearly determine that the long axis direction of cementite is oriented in two or more directions in the block, a needle-like lath structure is observed with an optical microscope. The area ratio of the structure in which the developed was defined as the total area ratio of martensite and bainite.
The total area ratio of martensite and bainite was 99% or more, or when the martensite can be clearly determined, the martensite area ratio was set to 90% or more as a target value.
The structure (remainder) other than “martensite and bainite” described in Table 3 was one or more of ferrite, pearlite, bainite, and retained austenite.

  Furthermore, after electrolytically polishing the cross section of the steel sheet, the vicinity of a 1/4 t portion of the cross section of the steel sheet was observed with a scanning electron microscope (SEM), and the number density of cementite was measured. Specifically, after the cross section of the steel plate was electrolytically polished, the range of 50 μm × 40 μm was photographed with a scanning electron microscope (SEM) at a magnification of 5,000 in the vicinity of a ¼ t portion of the cross section of the steel plate. From the contrast of the obtained image, using image analysis software, the number of precipitates having an aspect ratio of 2.0 or more and a length in the major axis direction of 0.1 μm or more was counted as being cementite. Similarly, the number of cementite having an aspect ratio of 2.0 or more and a length in the major axis direction of 1.0 μm or more was counted. Then, by dividing the number of the obtained precipitates of 1.0 μm or more by the number of mentites of 0.1 μm or more, a cementite number fraction (%) of 1.0 μm or more was obtained. In addition, it was judged that the cementite number fraction of 1.0 μm or more was 5% or less.

  Furthermore, the aspect ratio of prior austenite grains was measured. Specifically, the cross section parallel to the rolling direction in the vicinity of the 1/4 t part is corroded with nital, the magnification is 500 times with an optical microscope, and two fields of view in the range of 120 μm × 100 μm are photographed, and from the obtained image The major axis length and minor axis length of at least 50 prior austenite grains were measured, and the major axis length was divided by the minor axis length to determine the aspect ratio for each grain. And the average value of the aspect-ratio of these prior austenite grains was calculated | required, and it was set as the aspect-ratio of prior austenite grains. In addition, it was judged that it was favorable if the aspect ratio of the prior austenite grains was 2.0 or more.

Furthermore, a test piece (total thickness) was taken from the steel plate, and the tensile strength, yield strength, and elongation at break were measured according to JIS Z 2241. Further, Charpy absorbed energy at −20 ° C. and −40 ° C. was measured according to JIS Z 2242. The tensile test piece is a No. 5 test piece (total thickness) taken with the longitudinal direction perpendicular to the rolling direction, and the yield strength is 0.2% proof stress. The Charpy test piece is a 10 × 5 mm subsize sampled from the center of the plate thickness with the longitudinal direction as the rolling direction.
As a result of these tests, when the yield strength is 885 MPa or more, the tensile strength is 950 MPa or more, the elongation at break is 12% or more, and the absorbed energy value (vE- ₂₀ ) at −20 ° C. is 59 J / cm ² or more. The mechanical properties were evaluated as good.

A welded joint was prepared using a steel plate (steel numbers 1 to 16) having good mechanical properties and a steel plate number 32 having a Pcm of less than 0.189%.
The welding method was MAG welding, and the welding heat input was 7.0 kJ / cm or 10.0 kJ / cm. When the heat input is 7.0 kJ / cm, the welding conditions are current 280 A, voltage 27 V, welding speed 65 cm / min, and when 10.0 kJ / cm, current 305 A, voltage 29 V, welding speed 53 cm / min. did.
The tensile strength (joint strength) of the welded joint was evaluated by a tensile test specified in JIS Z 3121, and 950 MPa or more was evaluated as good.

  The above evaluation results are shown in Table 3. In Table 3, an underlined numerical value indicates that the value is outside the scope of the present invention, or the target characteristics are not obtained.

Steel plate numbers 1 to 16 are examples of the present invention, and excellent strength, ductility, and toughness are obtained. Also, the joint strength is 950 MPa or more. Furthermore, in the case where Mo / Cr is 0.20 or more, excellent toughness is obtained even at a test temperature of −40 ° C.
On the other hand, steel plate numbers 17 to 35 are comparative examples, and one or more of yield strength, tensile strength, breaking elongation, and vE- ₂₀ do not satisfy the target.
Steel plates Nos. 17, 26, and 29 have low strength because they each have a small amount of C or Mn. For steel plate numbers 26 and 29, the martensite fraction was not sufficient.
Steel plate No. 20 had a small amount of Mn and low hardenability, so ferrite and bainite were generated in addition to martensite, and the amount of martensite generated did not satisfy the scope of the present invention, resulting in strength. Was significantly lower.
Steel plate numbers 18, 19, 21, 22, 23, 27, 28, and 30 had excessive amounts of C, Si, Mn, Cr, or Mo, respectively, and had low ductility and toughness.
Steel plate number 24, due to the fact that the rolling end temperature and the water cooling start temperature were low, processed ferrite was generated in addition to martensite, and the martensite fraction did not satisfy the scope of the present invention. The strength was low.
Steel plate No. 33 had a low water cooling start temperature, so that processed ferrite was generated in addition to martensite, and the martensite fraction did not satisfy the scope of the present invention. It was low.
Steel plates Nos. 25 and 34 had a high water cooling stop temperature, and the untransformed austenite was bainite transformed, so the martensite fraction was low. Moreover, the elongation at break was lowered by the excessive generation of voids starting from coarse carbide (cementite) generated due to the bainite. Moreover, the yield strength was also low in the steel plate number 34.
Steel plate number 31 had high Cr and Mo contents and DI was too high, so the toughness and elongation at break were low.
Steel plate number 32 had a low Pcm, so the joint strength was below 950 MPa.
Steel plate number 35 had a low rolling reduction in the non-recrystallized region, and the aspect ratio of the prior austenite grains was less than 2.0, so the toughness was low.

According to the present invention, a high-strength steel sheet having a yield strength of 885 MPa or more, a tensile strength of 950 MPa or more, and a breaking elongation of 12% or more can be provided without containing a large amount of expensive alloy elements. . Further, this steel sheet exhibits excellent toughness with Charpy absorbed energy at −20 ° C. of 59 J / cm ² or more. Therefore, it is useful industrially.

The determination of the metal structure and the measurement of the area ratio of martensite and bainite were performed by the following methods.
After mirror polishing the cross section of the steel plate, the cross section parallel to the rolling direction in the vicinity of the 1/4 t portion is corroded by nital, and two optical fields of 120 μm × 100 μm are photographed with an optical microscope at a magnification of 500 times, and are needle-shaped. The area ratio of the tissue in which the lath structure was developed was measured. Moreover, about the acicular structure, after carrying out the electrolytic polishing of the cross section of a steel plate, the 1/4 t part vicinity of the steel plate cross section was observed with the scanning electron microscope (SEM). Here, the magnification was 5000 times, and a range of 50 μm × 40 μm was photographed. When the long axis direction of cementite is oriented in two or more directions in the block, the acicular structure is assumed to be martensite, and the area ratio of the region was obtained. The product of the needle-like structure area ratio in the optical microscope and the martensite area ratio in the SEM was defined as the martensite area ratio of the steel type. Moreover, the acicular structure other than martensite was used as bainite.
In the above-mentioned structure observation with a scanning electron microscope, when it is impossible to clearly determine that the long axis direction of cementite is oriented in two or more directions in the block, a needle-like lath structure is observed with an optical microscope. The area ratio of the structure in which the developed was defined as the total area ratio of martensite and bainite.
The total area ratio of martensite and bainite was 99% or more, or when the martensite can be clearly determined, the martensite area ratio was set to 90% or more as a target value.
Table 3 other than "martensite and bainite" described in the tissue (the remainder) is ferrite, pearlite, it was one or more of residual austenite.

Claims (10)

Chemical composition is mass%,
C: 0.050 to 0.100%,
Si: 0 to 0.50%,
Mn: 1.20 to 1.70%,
P: 0.020% or less,
S: 0.0050% or less,
N: 0 to 0.0080%,
B: 0.0003 to 0.0030%,
Ti: 0.003-0.030%,
Nb: 0.003 to 0.050%,
Cr: 0 to 2.00%
Mo: 0-0.90%
Al: 0 to 0.100%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
V: 0 to 0.100%,
W: 0 to 0.50%,
Ca: 0 to 0.0030%,
Mg: 0 to 0.0030%,
REM: 0 to 0.0030%,
Balance: Fe and impurities,
And
When the total content of one or both of Cr and Mo is 0.20% or more and the Mo content is more than 0.50%, the Cr content is 0.80% or less,
DI calculated | required by following formula 1 is 2.0-7.8,
Pcm calculated | required by following formula 2 is 0.189% or more,
Including one or two of martensite and bainite having a metal structure with 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 in the major axis direction of 1.0 μm or more with respect to cementite having a length in the major axis direction of 0.1 μm or more is 5% or less,
The plate thickness is 4.5 mm to 20 mm,
A high strength steel plate having a yield strength of 885 MPa or more, a tensile strength of 950 MPa or more, a breaking elongation of 12% or more, and a Charpy absorbed energy at -20 ° C of 59 J / cm ² or more.
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 (Formula 1)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (formula 2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] in Formula 1 and Formula 2 are the values of each element. It is a content in mass%, and when not including, it calculates as 0.   The high-strength steel sheet according to claim 1, wherein the metal structure includes martensite having an area ratio of 90% or more. % By mass
Cu: 0 to 0.25%,
The high-strength steel sheet according to claim 1 or 2, wherein % By mass
Ni: 0 to 0.25%,
The high-strength steel sheet according to any one of claims 1 to 3, wherein the steel sheet is high-strength steel sheet. % By mass
V: 0 to 0.050%,
The high-strength steel sheet according to any one of claims 1 to 4, wherein % By mass
W: 0 to 0.05%,
The high-strength steel sheet according to any one of claims 1 to 5, wherein the steel sheet has high strength.   The said plate | board thickness is 4.5 mm-15 mm, The high strength steel plate as described in any one of Claims 1-6 characterized by the above-mentioned.   The [Mo] / [Cr]) is 0.20 or more when the Mo content is [Mo] and the Cr content is [Cr]. The high-strength steel sheet according to item. The high-strength steel sheet according to claim 8, wherein Charpy absorbed energy at −40 ° C. is 59 J / cm ² or more.   The high-strength steel sheet according to any one of claims 1 to 9, wherein the Pcm is 0.196% or more.