JP5509923B2 - Method for producing high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding - Google Patents

Description

  The main structure of the present invention is martensite (tempered martensite) which is welded by laser welding or laser-arc hybrid welding combining laser and gas shielded arc welding used in the fields of construction machinery, industrial machinery and the like. In particular, the present invention relates to a method for producing a super high strength steel sheet having a tensile strength of 1100 MPa or more.

  In recent years, there has been a demand for an increase in size and performance of structures, and there has been an increasing demand for higher strength, particularly in thick steel plates. In addition, there is a strong demand for size increase and performance improvement in construction machines, and therefore, weight reduction of members is required. Therefore, the strength of steel used is also increasing. In particular, in construction machines such as cranes, what has been used up to 590 MPa class in the past has recently been used from 780 MPa class to 950 MPa class steel. Is progressing compared to other fields. Very recently, 1100 MPa class and 1200 MPa class ultra-high strength steel sheets having a tensile strength exceeding 950 MPa have begun to be used, and attempts have been made to use steel sheets having a tensile strength of 1400 MPa class.

On the other hand, these steel materials and steel plates are generally joined by welding, but up to a tensile strength of 950 MPa class, it is very commonly used for manual welding (SMAW), gas shielded arc welding (MAG welding, A so-called strength overmatching welding material in which the strength of the weld metal is higher than the strength of the steel material is provided industrially for CO ₂ welding, etc.) and submerged arc welding, and recently disclosed in Patent Document 1 As described above, welding wires for gas shielded arc welding for further high-strength steels are being developed. Ultra-high strength steel materials having a tensile strength exceeding 950 MPa, particularly steel materials having a tensile strength of 1100 MPa or more are manually welded ( SMAW), gas shielded arc welding (MAG welding, an attempt to weld a CO ₂ welding, etc.) and submerged arc welding, the weld metal toughness and ductility, or , Weld metal and HAZ which tissue by heat history is changed by welding the steel side (hereinafter, sometimes referred to as HAZ) low temperature cracking in, it is very difficult to ensure resistance to hot cracking at the same time. Therefore, although there is a strong demand for ultra-high-strength steel with a tensile strength of 1100 MPa or more, the high-strength steel is overmatched, has sufficient toughness, and can produce a sound welded joint that does not crack. Because there is no material or welding technology, its use has not progressed very easily. As for steel materials, for example, Patent Document 2 discloses a method for producing welding steel, but as it is assumed that the strength and toughness of the welding heat-affected zone are not disclosed at all, there are various methods. The present condition is that the steel material which can exhibit the characteristic outstanding as a joint corresponding to the welding method and the wide welding heat input range is not established.

  As described above, in the welding of ultra-high strength steel, ensuring strength, toughness, low temperature cracking, and suppression of high temperature cracking are not easy to solve with the above general arc welding method, which is multi-layer welding, In order to ensure toughness and avoid cracking, the number of welding passes cannot be increased because of the need to lower the heat input of welding, and the welding efficiency is also inferior. Therefore, a new welding technique and a high-tensile steel plate corresponding to this are required.

JP 2006-110581 A JP 2006-045644 A

  As a means for solving these problems, one-pass welding by laser-arc hybrid welding in which laser welding having a high energy density or a combination of laser and gas shielded arc welding is conceivable instead of arc welding. Laser arc or laser / arc hybrid welding that combines laser and gas shielded arc welding makes it easy to increase the strength because the weld metal is not reheated. Since the O content and the impurity content in the weld metal can be reduced, it is easy to ensure toughness. In addition, since the amount of impurity elements is small and high temperature cracking is unlikely to occur due to the specific solidification behavior, laser welding or laser and gas shielded arc welding can be used as a welding method to ensure the characteristics of the weld metal part. It is preferable to use laser / arc hybrid welding in combination. However, in these welding methods, unlike the conventional multi-pass welding by arc welding, since the reheating by the subsequent welding pass is not received at all, the heat history received by the weld heat affected zone of the steel material is different. If a steel material that is substantially premised on multi-pass arc welding as disclosed in Document 2 is used as it is, softening of the weld heat-affected zone is remarkable, and it is difficult to ensure the joint strength. There is a high possibility that the toughness of the affected area will deteriorate. That is, in order to use laser-arc or laser / arc hybrid welding in which laser and gas shielded arc welding are combined for welding ultra-high-strength steel sheets having a tensile strength of 1100 MPa or more, tensile strength suitable for the welding is required. It becomes a problem to provide an ultra-high strength steel plate of 1100 MPa or more.

  In the present invention, more specifically, as the base material characteristics, the tensile strength is 1100 MPa or more, the absorbed energy in the 2 mmV notch Charpy impact test at −40 ° C. is 27 J or more, and the welding heat affected zone is greatly softened. It is an object of the present invention to obtain a high-strength steel sheet that has no deterioration in toughness and has a tensile strength of 1100 MPa or more as a welded joint and an absorption energy in a 2 mmV notch Charpy impact test at −40 ° C. in a weld heat affected zone of 27 J or more. Needless to say, at the same time, it is assumed that the hot crack resistance and the cold crack resistance in the weld heat affected zone are sufficiently high.

  In a steel material or a steel plate, if a tensile strength of 1100 MPa or higher is achieved and at the same time an attempt is made to secure toughness and crack resistance, it is necessary to increase the strength with low C as much as possible. Must be a site organization. On the other hand, the martensite structure develops its strength due to the high amount of solute C and the dislocation density, so that the strength is greatly reduced by heat treatment at the level of tempering. Therefore, in martensitic steel that has been quenched and low-temperature tempered in order to achieve as high a strength as possible at low C, the strength of the welded heat-affected zone is significantly reduced. In both cases, since the fracture occurs at the weld heat affected zone where the softening is remarkable, the strength as a welded joint is significantly lower than that of the steel material. In general, the martensite structure has low toughness when quenched, and the toughness is better when reheated such as tempering. When a welded joint is manufactured by multi-layer welding, martensite is reheated in most of the heat affected zone, and the toughness is often maintained by this reheating effect. When such a steel material is subjected to laser welding, which is a one-pass welding, or laser / arc hybrid welding, which combines laser and gas shielded arc welding, the weld heat affected zone is only from a martensitic structure that remains untransformed and is not subject to reheating. Due to the construction, the toughness is lowered. The present invention assumes that the steel structure is martensite, but the martensite structure in the present invention is intended to increase the strength, and up to several percent as long as the strength is not adversely affected. In other words, those containing a microstructure such as ferrite, bainite, and retained austenite are also included.

  Therefore, the inventors of the present invention have made a rapid increase in temperature from the vicinity of the melting point to the tempering temperature range, that is, the heat history of the welding heat-affected zone of steel in laser welding or laser / arc hybrid welding combining laser and gas shielded arc welding. As a result of a detailed study of the relationship between strength, toughness, steel composition, and manufacturing conditions in the case of a thermal history that is reheated at a low temperature, cooled at a relatively high cooling rate, and not subjected to subsequent reheating, laser welding In laser / arc hybrid welding that combines laser and gas shielded arc welding, the weld heat-affected zone has high toughness and can exhibit strength higher than that of the base metal as a joint without causing extreme softening. Invented a new component composition and manufacturing method. The main points are as follows.

(1) A method for producing a high-strength steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding combining laser welding and gas shielded arc welding,
% By mass
C: 0.12 to 0.20%
S i: 0.03 to 1.0%,
Mn: 0.5 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
Ni: 0.6-6%,
N: 0.001 to 0.01%
V: 0.12-0.50%,
Each containing
Cr: 0.1 to 3%,
Mo: 0.05-2%,
W: 0.10 to 2 %,
Nb : 0.005 to 0.20%
Ta: 0.005 to 0.50%,
Zr: 0.005 to 0.50%
Nb equivalent defined by the following formula [1] is 0.09 to 0.80%, and a carbon equivalent defined by the following formula [2] ( Ceq.) Is 0.45 to 1.2%, and the balance of Fe and unavoidable impurities is heated to a temperature range of Ac3 transformation point to 1250 ° C, and the cumulative rolling reduction at 900 ° C or less is 20 ~ 23.1%, rolling end temperature is 800 ~ 860 ° C, hot rolled into steel plate, reheating temperature is Ac3 transformation point ~ 1000 ° C reheat quenching, and more than 550 ° C ~ A method for producing a high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding, wherein tempering is performed at a temperature lower than the Ac1 transformation point.
Nb equivalent = Nb% + 0.35 Mo% + 0.20 W% + 0.75 V% + 0.30 Ta%
+ 0.20Zr% + 0.05Cr% [1]
Ceq. = C% + Si% / 24 + Mn% / 6 + Ni% / 40 + Cr% / 5 + Mo% / 4 + W% / 8 + V% / 14 [2]
In the above formulas [1] and [2],% of each element represents mass% of each element in the weld metal.

(2) The steel slab described in (1) above is heated to a temperature range of Ac3 transformation point to 1300 ° C, the rolling end temperature is 650 ° C or higher and 800 ° C or lower , and the hot rolling with a cumulative rolling reduction of 30 to 95%. Followed by accelerated cooling at a cooling rate of 2 to 100 ° C./s starting at 600 ° C. or higher and ending at 300 ° C. or lower, and further tempering at a temperature higher than 550 ° C. and lower than the Ac1 transformation point. A method for producing a high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding.

(3) The steel is further in mass%,
Cu: 0.005 to 1.5%,
Co: 0.01 to 6%
Ti: 0.002 to 0.05%,
B: 0.0002 to 0.0030%
A high-tensile steel plate having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding as described in (1) or (2) above, comprising one or more of Manufacturing method.

(4) The steel is further mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%,
REM: 0.0002 to 0.01%
The tensile strength for laser welding or laser arc hybrid welding according to any one of the above (1) to (3), characterized by containing one or more of The manufacturing method of the above high-tensile steel plate.

  According to the present invention, as the base material characteristics, the tensile strength is 1100 MPa or more, the absorbed energy in the 2 mmV notch Charpy impact test at −40 ° C. is 27 J or more, and there is no significant softening or toughness deterioration of the weld heat affected zone. Also, the tensile strength as a welded joint is 1100 MPa or more, the absorbed energy in the 2 mm V notch Charpy impact test at −40 ° C. in the weld heat affected zone is 27 J or more, and the hot crack resistance and cold crack resistance of the weld heat affected zone are ensured. It is possible to produce a high-tensile steel plate with sufficiently high properties, and the industrial effect is extremely large.

  In the present invention, regardless of whether the microstructure of the steel sheet is as-quenched martensite structure or tempered martensite, the steel having the martensite structure is effective regardless of the strength level. In the sense that the effect of suppressing the softening of the weld heat-affected zone appears clearly, the effect is certain with an ultra-high strength steel having a tensile strength of 1100 MPa or more, and further, an effect is obtained with an ultra-high strength steel having a tensile strength of 1150 MPa or more. Is more prominent.

The present invention has a chemical composition of C: 0.12 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.5 to 2.5%, P: 0.02% or less, S : 0.01% or less, Al: 0.002 to 0.1%, Ni: 0.6 to 6%, N: 0.001 to 0.01%, V: 0.12 to 0.50% , respectively In addition, Cr: 0.1 to 3%, Mo: 0.05 to 2%, W: 0.10 to 2 %, Nb: 0.005 to 0.20%, Ta: 0.005 to 0 .50%, Zr: 0.005 to 0.50% or more, and Nb equivalent defined by the formula [1] satisfies 0.09 to 0.80% And the carbon equivalent (Ceq.) Defined by the formula [2] is 0.45 to 1.2%, and further, if necessary, Cu: 0.005 to 1.5%, Co: 0. 1 to 6%, Ti: 0.002 to 0.05%, B: 0.0002 to 0.0030% of one kind or two or more kinds are contained, and if necessary, Ca: 0.0002 to Appropriate steel containing one or two of 0.01%, Mg: 0.0002 to 0.01%, REM: 0.0002 to 0.01%, the balance being Fe and inevitable impurities The gist is to make a steel plate by a simple manufacturing method. Hereinafter, the reasons for limiting the component composition of steel and the reasons for limiting the manufacturing method will be sequentially described in detail.

[Component composition of steel]
The chemical composition of steel is: C: 0.12 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.5 to 2.5%, P: 0.02% or less, S: 0 0.01% or less, Al: 0.002-0.1%, Ni: 0.6-6%, N: 0.001-0.01%, V: 0.12-0.50% , Furthermore, Cr: 0.1 to 3%, Mo: 0.05 to 2%, W: 0.10 to 2 %, Nb: 0.005 to 0.20%, Ta: 0.005 to 0.50 %, Zr: 0.005 to 0.50% of 1 type or 2 or more types are essential requirements. First, this essential requirement will be described.

“C: 0.12 to 0.20%”
C is the most important element for strength development in the martensite structure, and in order to make the tensile strength 1100 MPa or more, it is necessary to contain at least 0.12% or more. As the C content increases, the strength can be increased, but the toughness, hot cracking resistance, and low temperature cracking are significantly deteriorated. Therefore, in the present invention, the C content is set to 0.12 to 0.20%. limit.

"Si: 0.03-1.0%"
Si is a deoxidizing element and needs to be at least 0.03% in order to maintain the soundness of steel. However, if the content exceeds 1.0%, the hardening of the weld heat affected zone increases, which is not preferable because it deteriorates the toughness of the weld heat affected zone and the low temperature cracking property. The content is limited to 0.03 to 1.0%.

"Mn: 0.5-2.5%"
Mn is an essential element for ensuring hardenability and increasing the strength as a martensite structure. In order to reliably exert the structure control and strength improvement effects, it is necessary to contain 0.5% or more in the steel sheet. On the other hand, if the content exceeds 2.5%, the susceptibility to grain boundary embrittlement increases, and the possibility of deterioration in toughness and cold cracking resistance increases. Therefore, in the present invention, the Mn content in the steel sheet is 0.00. Limited to 5-2.5%.

“P: 0.02% or less”
P is an impurity element, which inhibits the toughness of the base metal and the weld heat affected zone, and deteriorates the hot crack resistance and cold crack resistance of the weld heat affected zone. If the content is 0.02% or less, adverse effects on toughness can be tolerated. Therefore, in the present invention, the P content in the steel sheet is 0.02% or less.

“S: 0.01% or less”
S is also an impurity element, and if it is excessively present in the steel sheet, it deteriorates both toughness and ductility, so it is preferable to reduce it as much as possible. If the content in the steel sheet is 0.01% or less, adverse effects on toughness and ductility can be tolerated. Therefore, in the present invention, the S content in the steel sheet is 0.01% or less.

“Al: 0.002 to 0.1%”
Al is a deoxidizing element and, like Si, is an effective element for reducing the oxygen content of steel and ensuring soundness. For that purpose, it is necessary to contain 0.002% or more. On the other hand, if the content exceeds 0.1% excessively, a coarse oxide may be formed and the toughness may be inhibited. Therefore, in the present invention, the Al content is 0.002 to 0.1%. Limited to.

"Ni: 0.6-6%"
Ni is the only element that enhances hardenability and contributes to strength improvement while also improving toughness. In particular, in steel with the martensite structure as a basic structure in the present invention, the toughness improvement by the structure refinement technique such as acicular ferrite cannot be basically used, so the martensite transformation remains with the base material toughness. In order to ensure the toughness of the weld heat affected zone, it is an essential element. In order to exhibit this effect reliably, the Ni content in the steel sheet needs to be 0.6% or more. The higher the Ni content, the more effective is to improve toughness. However, Ni is an expensive element. If excessively contained, the amount of retained austenite increases and the yield strength is excessively increased to around 50% in some cases. Therefore, even if the tensile strength is within the range of the present invention, it is not preferable. In addition, due to the presence of retained austenite and the behavior of dislocations during plastic deformation, the ductile fracture characteristics in Charpy impact tests are deteriorated, and even if brittle fracture characteristics (for example, fracture surface transition temperature) are improved, the absorbed energy is rather lowered. After all, it is not preferable. Therefore, in the present invention, the upper limit of the Ni content is set to 6% as the content that can achieve practically sufficient toughness.

“N: 0.001 to 0.01%”
In a small amount, N forms fine nitrides when the steel slab is heated to refine the heated austenite grain size and contribute to toughness. For that purpose, 0.001% or more is necessary as content in steel. On the other hand, if the content exceeds 0.01%, the nitride becomes coarse or the amount of solute N increases to deteriorate the toughness. Therefore, in the present invention, the N content is 0.001 to 0. Limited to 01%.

"V: 0.12-0.50%"
V is effective for improving the strength of steel having a tempered martensite structure mainly by precipitation strengthening. Also, in the heat affected zone, it is possible to suppress softening without inducing significant toughness in a region heated below the tempering temperature range below the Ac1 transformation point, which is extremely effective in securing joint strength. It is an element. In order to exhibit the effect reliably, it is necessary to contain 0.12% or more in steel. However, if the content exceeds 0.50%, the deterioration of the toughness of the base material and the weld heat-affected zone increases, which is not preferable. Therefore, when V is contained in the steel in the present invention, the content is limited to 0.12 to 0.50%. In addition, as the strength-toughness balance of the weld heat affected zone in laser welding and laser / arc hybrid welding, Cr, Mo, Nb, Ta, which have the same effect as V and can be used selectively with V, are used. Since the effectiveness of V is the greatest compared to Zr, it is more preferable to consider the use of V first.
In addition to the above requirements, Cr: 0.1 to 3%, Mo: 0.05 to 2%, W: 0.10 to 2 %, Nb : 0.005 to 0.2%, Ta: 0.005 It is essential to contain at least one of ˜0.5% and Zr: 0.005 to 0.5% in order to increase strength at low C as much as possible without impairing toughness. As described later, the total content of the element group needs to be limited as Nb equivalent, but the individual content also needs to be limited for the following reason.

"Cr: 0.1-3%"
Cr is an element effective for improving the strength of the base metal by improving hardenability and precipitation strengthening, and is also effective for suppressing softening of the weld affected part, but 0.1% or more is necessary for producing a clear effect. On the other hand, if the content exceeds 3%, the toughness and cold cracking resistance of the base metal and the weld heat-affected zone tend to be deteriorated. The range is ˜3%.

"Mo: 0.05-2%"
Mo is an element effective for improving the strength of the base metal by improving hardenability and precipitation strengthening. In particular, Mo is an extremely effective element for suppressing softening of the heat affected zone due to its strong temper softening resistance. In addition, it has a certain effect on the suppression of hot cracking in the weld heat affected zone. In order to exert a clear effect, 0.05% or more is necessary. On the other hand, if it exceeds 2%, the toughness of the base metal and the weld heat affected zone, and further the low temperature cracking resistance deteriorates. When it is contained, the content is made 0.05 to 2%.

"W: 0.10 to 2%"
W is an element effective for improving the hardenability and precipitation strengthening similar to Mo, and is effective for improving the strength of the base metal. In order to exert its effect, 0.10% or more is necessary, while 2% If it exceeds 1, the toughness of the base metal and the weld heat affected zone and further the low temperature cracking resistance deteriorate, so the range of 0.10 to 2% is limited as the range in which the material does not deteriorate.

“Nb: 0.005 to 0.20%”
Nb is an element effective for improving the base metal strength and suppressing the softening of the heat affected zone by improving the hardenability together with precipitation strengthening. In order to clearly show the effect, at least 0.005% is necessary. On the other hand, if the content exceeds 0.20%, the toughness deterioration of the base material and the weld heat affected zone increases, which is not preferable. Therefore, in the present invention, when Nb is contained in the steel, the content is limited to 0.005 to 0.20%.

"Ta: 0.005-0.50%"
Ta is an element that exhibits qualitatively almost the same effect as Nb. In order to clearly demonstrate the effect of ensuring the strength of the base metal and the strength of the joint, it is necessary to contain 0.005% or more in the steel. On the other hand, if the content exceeds 0.50%, coarse precipitates are formed and the toughness of the base metal and the weld heat-affected zone is deteriorated. The amount is 0.005 to 0.50%.

"Zr: 0.005-0.50%"
Zr is also an element that exhibits qualitatively almost the same effect as Nb. In order to clearly demonstrate the effect of ensuring the strength of the base metal and the strength of the joint, it is necessary to contain 0.005% or more in the steel. On the other hand, if the content exceeds 0.50%, coarse precipitates are formed and the toughness of the base metal and the weld heat affected zone is deteriorated. Therefore, when Zr is contained in the steel in the present invention, the inclusion The amount is 0.005 to 0.50%.

  The above is the basic requirement regarding the chemical composition of the steel sheet of the present invention. For adjusting the mechanical properties, one or more of Cu, Co, Ti, and B can be contained as necessary. .

"Cu: 0.005-1.5%"
Cu is an element effective in increasing the strength of the base metal mainly by improving hardenability, solid solution strengthening, and precipitation strengthening. In order to exert the effect, it is necessary to contain 0.005% or more. However, if it exceeds 1.5%, a problem arises in hot workability. Therefore, when Cu is contained in the present invention as a range that exhibits the effect and does not cause problems such as hot workability, 0.005 to Limited to a range of 1.5%.

"Co: 0.01-6%"
Co is an element capable of controlling the formation of retained austenite by preventing the transformation point from being lowered excessively in a high alloy component steel. Due to this effect, the element is useful for adjusting the strength and toughness of the base material. In order to exhibit the effect reliably, it is necessary to contain at least 0.01%. On the other hand, the content exceeding 6% is not preferable because the alloy cost is excessive for the effect, and the possibility of deterioration of toughness also occurs. Therefore, when using Co in the present invention, the content in the steel is set in the range of 0.01 to 6%.

“Ti: 0.002 to 0.05%”
Ti is an element that contributes to improving the strength of the base metal by precipitation strengthening, and is also an effective element for refining the heated austenite grain size by forming TiN that is stable even at high temperatures, and manufactures steel sheets by thermomechanical treatment and re-quenching and tempering. In the present invention, it is an element useful for improving toughness. In order to exhibit the effect when contained in steel, 0.002% or more is necessary. On the other hand, if it exceeds 0.05%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.05%. That is, when Ti is contained in the present invention, it is limited to a range of 0.002 to 0.05%.

“B: 0.0002 to 0.0030%”
B segregates at the austenite grain boundary in a solid solution state, and can improve the hardenability in a very small amount. Therefore, B is an extremely effective element for increasing the strength as a martensite structure. In order to exhibit the effect of improving hardenability, the content of 0.0002% or more is necessary. On the other hand, if the content exceeds 0.0030% and is excessively contained in the steel, BN and Fe23 (C, B ) 6 and the like, and the toughness deteriorates remarkably. Therefore, when B is contained in the present invention, the content is limited to 0.0002 to 0.0030%.

  Furthermore, in order to improve the ductility of the steel plate, the ductility of the weld heat affected zone, and the toughness, one or more of Ca, Mg, and REM can be contained as necessary.

“Ca: 0.0002 to 0.01%”
Ca is effective in improving ductility by suppressing the expansion of the sulfide in steel during hot rolling. Moreover, it becomes effective also in the improvement of the toughness of a heat affected zone by making the oxide finer and having an appropriate oxide composition. In order to exhibit this effect reliably, 0.0002% or more is necessary. On the other hand, if it exceeds 0.01% and is excessively contained in the steel, coarse inclusions are formed and, on the contrary, it becomes a cause of toughness deterioration. Therefore, in the present invention, when Ca is contained in steel, the content is limited to 0.0002 to 0.01%.

“Mg: 0.0002 to 0.01%”
Since Mg has substantially the same effect as Ca, for the same reason, when Mg is contained in the steel in the present invention, it is limited to 0.0002 to 0.01%.

“REM: 0.0002 to 0.01%”
Since REM has almost the same effect as Ca and Mg, for the same reason, when REM is contained in steel in the present invention, it is limited to 0.0002 to 0.01%.

  The above is the reason for limiting individual elements. In order to optimize the balance of strength and toughness by making the steel sheet structure a martensite structure, not only the limitation of individual elements but also the formula [1], [2 It is an essential requirement to adjust the Nb equivalent and the carbon equivalent shown in the formula to an appropriate range.

“Nb equivalent: 0.09 to 0.80%”
When the base material structure is a martensite structure, it is naturally C that has the greatest influence on the strength. However, if it is attempted to secure the strength with only C, the deterioration of toughness increases, which is not preferable. Therefore, in the present invention, a temper softening resistance element and a precipitation strengthening element are used in order to ensure strength while the amount of C is as small as possible. These elements are also important for suppressing softening of the weld heat affected zone. Specifically, it is Cr, Mo, W, V, Nb, Ta, and Zr, and for each of the above reasons, the content of each element is limited, but in order to ensure the strength of the base material and to suppress the softening of the heat affected zone. As an effect of the whole element having these similar effects, it is necessary to simultaneously limit the Nb equivalent calculated by the following formula [1]. Even if the content of each element is within the range of the present invention, if the Nb equivalent is less than 0.09%, the strength improvement and the softening suppressing effect are not sufficient. The larger the Nb equivalent, the more advantageous the improvement of the base metal strength and the suppression of the softening of the weld heat affected zone. However, if the Nb equivalent exceeds 0.80%, the sensitivity to hot cracking and cold cracking of the weld heat affected zone is ignored. Therefore, in the present invention, the Nb equivalent is limited to a range of 0.09 to 0.80%.
Nb equivalent = Nb% + 0.35 Mo% + 0.20 W% + 0.75 V% + 0.30 Ta%
+ 0.20Zr% + 0.05Cr% [1]
However,% of each element in said Formula [1] shows the mass% of each element in a weld metal, respectively.

“Carbon equivalent: 0.45 to 1.2%”
In the present invention, it is assumed that the steel structure is a martensite structure. However, in order to ensure the martensite structure in the production method of the present invention, it is necessary to limit the carbon equivalent in addition to the limitation of individual elements. There is. In the present invention, the carbon equivalent formula of the following formula [2] derived from the relationship between experimentally determined martensite hardenability and chemical composition is used, and the lower limit is set to 0.45%. This is because if the carbon equivalent according to the formula [2] is less than 0.45%, there is no possibility that a structure other than martensite appears in the steel structure depending on the thickness of the steel sheet and the cooling conditions. is there. The higher the carbon equivalent is, the more advantageous for securing the strength. However, if it exceeds 1.2%, the deterioration of cold cracking resistance and toughness increases, so in the present invention the upper limit of carbon equivalent is 1.2%. .
Ceq. = C% + Si% / 24 + Mn% / 6 + Ni% / 40 + Cr% / 5 + Mo% / 4
+ W% / 8 + V% / 14 [2]
However,% of each element in said Formula [2] shows the mass% of each element in a weld metal, respectively.

[Steel plate manufacturing method]
The above is the reason for limiting the chemical composition in the present invention. However, in order to obtain a steel plate having desired characteristics in the steel plate and the weld heat affected zone, it is necessary to further define the manufacturing method after limiting the chemical composition for the above reason. Hereinafter, the contents of the present invention related to the manufacturing method will be described in detail.

  In the present invention, the requirements relating to the production method are as follows. When a steel having a chemical composition satisfying the present invention is produced by <1> reheating quenching and tempering treatment, after hot rolling, the heating temperature is the Ac3 transformation point. When reheating and quenching at ˜1000 ° C. and further tempering above 550 ° C. to less than Ac1 transformation point, or <2> produced by thermomechanical treatment, heating to Ac3 transformation point to 1300 ° C. Hot rolling is performed at a temperature of 650 ° C. or higher and a cumulative rolling reduction of 30 to 95%, followed by accelerated cooling at a cooling rate of 2 to 100 ° C./s starting from 600 ° C. or higher and ending at 300 ° C. or lower. Further, tempering is performed at a temperature higher than 550 ° C. and lower than the Ac1 transformation point. Hereinafter, the reason for limitation of the manufacturing method will be described in detail by dividing into reheating quenching and tempering and thermomechanical treatment.

<Reheat quenching and tempering>
"Hot rolling conditions"
When manufacturing by a reheating process, in order to adjust the shape of a steel plate, generally a steel piece or an ingot is first made into a steel plate by hot rolling. When manufacturing by reheating quenching and tempering, the structure formed by hot rolling is almost eliminated at the time of reheating, so the hot rolling conditions have little effect on the final steel plate and weld heat affected zone properties. Absent. Therefore, in the present invention, it is not necessary to particularly limit the conditions relating to the steel plate production prior to the production of the steel plate by the reheating quenching and tempering treatment. However, depending on the chemical composition, the fineness of the microstructure formed during hot rolling may not affect the final fineness of the steel plate structure. More preferably, the heating temperature is from Ac3 transformation point to 1250 ° C., the total cumulative rolling reduction ratio of hot rolling ≧ 50%, the cumulative rolling reduction ratio below 900 ° C. ≧ 20%, and the rolling finish temperature ≧ 650 ° C.

"Reheating temperature"
The reheating temperature in reheating and quenching is in the range of Ac3 transformation point to 1000 ° C. When the reheating temperature is lower than the Ac3 transformation point, the entire structure is not a martensite structure because the pre-quenching structure is not the austenite single phase structure. For this reason, the strength of the steel sheet is not sufficiently high, and an extremely non-uniform structure is formed, so that the toughness of the steel sheet often deteriorates, which is not preferable. On the other hand, when the reheating temperature exceeds 1000 ° C., depending on the chemical composition of the steel sheet, the heated austenite grains become extremely mixed grains or the grain size becomes coarse, which adversely affects toughness. Absent.

"Quenching conditions"
After reheating from Ac3 transformation point to 1000 ° C., quenching is performed to obtain a martensite structure. In the present invention, as a method of reheating and quenching, as usual, it may be rapidly cooled to near room temperature by being immersed in a coolant such as water or oil, or may be heat-treated. It is also possible to use a method of performing accelerated cooling with a controlled cooling rate and ending the accelerated cooling at an appropriate temperature in the middle.

  When performing the quenching treatment, as long as martensite is obtained, the welding means, the type of the refrigerant, and the like are not limited. However, in the chemical composition of the present invention, in order to ensure the entire martensite structure, 800 ° C to 300 ° C It is preferable that the average cooling rate at 0 ° C. is 2 ° C./s or more. In the present invention, in the case of normal quenching treatment, quenching rapid cooling needs to be performed to a temperature at which a substantially 100% martensite structure is ensured, but in the chemical composition of the present invention, If quenching is rapidly cooled until the average temperature becomes 200 ° C. or less, a substantial effect is exhibited.

"Tempering conditions"
In order to stabilize the martensite structure and more reliably suppress the large softening of the weld heat affected zone, it is necessary to perform appropriate tempering after reheating and quenching. Tempering temperature shall be more than 550 degreeC and less than Ac1 transformation point. This is because when the tempering temperature is 550 ° C. or less, the softening suppression of the weld heat-affected zone is not sufficient, and when the tempering temperature is equal to or higher than the Ac1 transformation point, reverse transformation to austenite occurs during tempering heating, and the strength This is because there is a greater possibility that the deterioration of the steel and the deterioration of toughness will occur.

<Processing heat treatment>
Another manufacturing method in the present invention is based on a thermomechanical processing method in which accelerated cooling is performed at the time of cooling after hot rolling. As in the case of reheating quenching and tempering, accelerated cooling conditions and tempering conditions are important for suppressing reheat softening of the weld heat affected zone, but in order to ensure good strength and toughness in the steel sheet, It is necessary to regulate the rolling conditions of the steel slab and the ingot within an appropriate range.

"Heating conditions"
The heating temperature of the steel slab or ingot prior to hot rolling is in the range of Ac3 transformation point to 1300 ° C. When the heating temperature is less than the Ac3 transformation point, the heated structure does not become an austenite single phase structure, and therefore the final structure does not become a full martensite structure. For this reason, the strength of the steel sheet is not sufficiently high, and an extremely uneven structure is formed, so that the toughness of the steel sheet often deteriorates, which is not preferable. On the other hand, when the heating temperature exceeds 1300 ° C., the heated austenite grain size becomes extremely coarse, and subsequent rolling does not make the grain fine enough and adversely affects the steel sheet toughness.

"Rolling conditions"
The conditions for hot rolling are important mainly for refining the austenite grain size before transformation and for improving toughness by introducing processing strain. In the present invention, the total cumulative rolling reduction is 30 to 95% and the rolling end temperature is 650 ° C. or higher. If the total cumulative rolling reduction is less than 30%, the austenite grain size is not sufficiently refined by recrystallization, so that the toughness may be inferior, which is not preferable. The larger the total cumulative rolling reduction, the more advantageous the refinement of austenite. However, when the content exceeds 95%, the effect is saturated, and problems such as deterioration of the steel sheet shape and difficulty in securing the rolling end temperature become obvious. In this case, the upper limit of the cumulative rolling reduction is set to 95%. The rolling temperature is premised on the heating temperature range of the present invention, and it is possible to achieve the toughness level of the steel sheet intended by the present invention even if not specified as long as it is within the condition range that can keep the rolling end temperature. However, if you want to maximize the refinement effect by recrystallization of austenite, effectively use rolling in non-recrystallized region, introduce processing strain, and maximize the toughness improvement effect Of the total rolling, it is preferable that the cumulative reduction ratio of rolling performed in a temperature range of 900 ° C. to 700 ° C. is 30 to 80%.

  The rolling end temperature is an important requirement for making the transformation structure a martensite structure. That is, if the rolling end temperature is excessively low, a structure other than martensite may be generated during rolling or after rolling and before the start of accelerated cooling, resulting in deterioration of strength and toughness. In the chemical composition range of the present invention, the rolling end temperature needs to be 650 ° C. or higher in order to ensure an untransformed austenite single phase during accelerated cooling. If the rolling end temperature is 650 ° C. or higher, there is no problem in keeping the accelerated cooling start temperature of the present invention.

`` Accelerated cooling conditions ''
After the hot rolling, accelerated cooling is continued, and the condition is that the cooling rate is 2 to 100 ° C./s, the temperature starts from 600 ° C. or higher and ends at 300 ° C. or lower. A cooling rate of less than 2 ° C./s is not preferable because a 100% martensite structure may not be obtained depending on the chemical composition of the steel. The larger the cooling rate, the more advantageous for obtaining the martensite structure. However, in the chemical composition of the present invention, the effect is saturated even if the cooling rate is increased beyond 100 ° C./s, and the steel plate thickness is increased. When becomes larger, it becomes industrially difficult to accelerate cooling at over 100 ° C./s. Therefore, in this invention, the upper limit of the cooling rate in accelerated cooling shall be 100 degrees C / s. The cooling rate refers to the average cooling rate from the start to the stop of accelerated cooling. The accelerated cooling at the cooling rate needs to start at 600 ° C. or higher and end at 300 ° C. or lower. If the start temperature of accelerated cooling is less than 600 ° C., even in the chemical composition range of the present invention, transformation other than martensite may occur before accelerated cooling, which may deteriorate both strength and toughness. Absent. On the other hand, regarding the end temperature of accelerated cooling, when the tempering step after the thermomechanical treatment is essential, the lower the temperature, the more advantageous for securing the base material strength and joint strength. Assuming the tempering temperature range of the present invention, high strength can be sufficiently achieved by accelerated cooling to 300 ° C. or lower. The lower limit of the accelerated cooling temperature is not particularly limited, but it is sufficient to set it to room temperature. In addition, although cooling after stopping accelerated cooling is normally air cooling, since the structure stability is ensured by the tempering process, the cooling method is not ask | required.

"Tempering conditions"
In order to suppress softening of the weld heat affected zone, it is an essential requirement to reliably impart a tempering effect by tempering treatment. If the tempering temperature is 550 ° C. or lower, depending on the chemical component, a sufficient tempering effect may not be obtained. If the tempering temperature is equal to or higher than the Ac1 transformation point, reverse transformation to austenite may occur during tempering heating. Therefore, there is a high possibility that the strength is lowered and the toughness is deteriorated.

  In the present invention, in order to make the tensile strength of the steel sheet and the joint 1100 MPa or more, the chemical composition and the steel sheet manufacturing method are optimized to make the steel sheet structure a martensitic structure. As described above, the “martensite” structure in the present invention does not strictly indicate only a structure composed of 100% martensite, and most of the structure is a martensite structure and does not cause a significant decrease in strength. A structure other than martensite having an area ratio of about 10% or less is also referred to as a martensite structure. For example, both the reheating quenching and the thermomechanical processing may include a bainite structure when the accelerated cooling rate is relatively low or the accelerated cooling end temperature is relatively high. If so, the characteristics as a “martensite” structure can be substantially exhibited, and desired high strength and high toughness can be achieved at the same time.

  Hereinafter, the present invention will be described more specifically with reference to examples of a method for producing a high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding according to the present invention. However, the present invention is not limited to the following examples, and can be carried out with appropriate modifications within a range that can be adapted to the purpose described above and below, all of which fall within the technical scope of the present invention. It is included.

In the examples, welding was performed by producing a joint by laser-arc hybrid welding, and the joint characteristics were evaluated, but the joint tension when affected by the toughness of the welded heat affected zone on the steel plate side and the strength of the welded heat affected zone was measured. There is no essential difference in characteristics even in the case of laser welding alone. For this reason, in the examples, laser-arc hybrid welding that facilitates maintaining the integrity of the joint and can clearly evaluate the contribution of the steel sheet in the joint tensile test was used. In this embodiment, a CO ₂ laser is used as the laser, but the effect of the present invention does not change at all even if the type of laser is changed.

The steel plates used in the examples were prepared from steel pieces having chemical compositions shown in Table 1-1 and Table 1-2. As a steel piece, a converter melting slab, a laboratory small vacuum melting ingot, or the like was used. Some steel slabs are used while being melted, but some steel slabs include those whose thickness is adjusted by cutting the surface for the convenience of subsequent hot rolling or heat treatment. Steel bill symbols SA1 , SA8, SA10, SA12-13, SA16-21, and SA24-26 in Table 1-1 are invention examples in which the chemical composition satisfies the present invention , and the others are reference examples. On the other hand, billet symbols SB1 to SB28 in Table 1-2 are comparative examples in which the chemical composition deviates from the scope of the present invention.

  Using steel slabs having the component compositions shown in Table 1-1 and Table 1-2, steel sheets are produced by various production methods that are examples of the production method of the present invention or comparative examples thereof, and steel plate (base material) characteristics. In addition, laser-arc hybrid welding was performed using the steel sheet, and the joint characteristics were also investigated.

Table 2-1 and Table 2-2, Table 3-1 and Table 3-2 show steel sheet production conditions and mechanical properties of the steel sheet and welded joint. Tables 2-1 and 2-2 are invention examples of reheat quenching and tempering according to claim 1, comparative examples and reference examples . Tables 3-1 and 3-2 are invention examples of thermomechanical treatment with tempering treatment according to claim 2 and comparative examples and reference examples thereof.

  Most of the steel plates were manufactured with a thickness of 10 mm, but some steel plates with a thickness of 25 mm were also manufactured. In producing the welded joint, due to the problem of the ability of laser / arc hybrid welding, the front and back surfaces were cut by the same amount by machining, and all were welded to a plate thickness of 8 mm.

  The mechanical properties of the steel plate were obtained by collecting test pieces from a plate thickness of 8 mm. For the tensile test piece, a plate-shaped tensile test piece having a total thickness (8 mm thickness) is taken so that the direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and yield strength (YP) and tensile strength (TS) at room temperature. Was measured. Toughness was evaluated by a 2 mm V notch Charpy impact test, but the test piece was a sub-size test piece with a plate thickness of 5 mm from the center of the plate thickness in the direction in which the longitudinal direction of the test piece was perpendicular to the rolling direction, like the tensile test piece. The sample was collected and tested at −40 ° C. to determine the average absorbed energy. The sub-size test piece with a plate thickness of 5 mm has a ligament area halved compared to the standard test piece with a plate thickness of 10 mm. Therefore, in principle, the absorbed energy at 100% ductile fracture is that of the standard test piece. It becomes almost 1/2. Absorbed energy at any temperature, especially the relationship between absorbed energy at test temperatures that include brittle fracture surfaces, and the relationship between absorbed energy between sub-size specimens and standard specimens is complex and complex However, in a sub-size test piece with a thickness of 5 mm, if the absorbed energy at −40 ° C. is 14 J or more, which is more than half of 27 J, the standard test piece can substantially satisfy 27 J or more as the absorbed energy at −40 ° C. it is conceivable that. In this example, in order to evaluate safety, the target is that the absorbed energy at −40 ° C. is 27 J or more even in a sub-size test piece having a thickness of 5 mm.

  Table 4 shows the laser-arc hybrid welding conditions. It was carried out by butt welding using an I groove where the steel plate ends were butted. The gap was set to 0.5 mm, and welding was performed with laser preceding. The shielding gas was He for all of the center gas, the side gas, and the back gas, and the arc welding wire with a diameter of 1.2 mm was used. In this embodiment, He is used as the shielding gas, but the effect of the present invention does not change at all depending on the type of gas.

アーク溶接に使用した溶接ワイヤを表５に示す。継手における鋼板側の評価ができるよう、再熱部を含まない１パス溶接となるレーザ・アークハイブリッド溶接における溶接金属の強度が引張強さで１１００ＭＰａを十分上回り、靭性が、−４０℃におけるシャルピー吸収エネルギーで２７Ｊを十分上回るように選定した６種類のソリッドワイヤである。本発明による鋼板であれば、溶接熱影響部の特性や溶接熱影響部に支配される継手特性が、溶接金属の特性が十分良好となる溶接ワイヤであれば、溶接ワイヤの種類によらず確保できることを確認するために、鋼板と溶接ワイヤとの組み合わせを様々変化させた。

Table 5 shows the welding wires used for arc welding. The strength of the weld metal in laser-arc hybrid welding, which is a one-pass welding that does not include a reheat zone, is sufficiently higher than the tensile strength of 1100 MPa, and the toughness is Charpy absorption at −40 ° C. so that the steel plate side of the joint can be evaluated. Six types of solid wires selected to sufficiently exceed 27J in energy. If it is a steel plate according to the present invention, the characteristics of the weld heat-affected zone and the joint characteristics governed by the weld heat-affected zone are ensured regardless of the type of weld wire as long as the weld metal has sufficiently good weld metal properties. In order to confirm that this was possible, various combinations of steel plates and welding wires were changed.

  For the welded joint, a joint tensile test and a 2 mmV notch Charpy impact test of the weld heat affected zone were performed. The joint tension was a full thickness (8 mm thickness) tensile test in accordance with JIS Z3121 “Tensile test method for butt weld joints”, and No. 1 test piece was used as the test piece. The 2 mm V notch Charpy impact test was the same 5 mm thickness subsize test as that of the steel plate, and was taken from the center of the plate thickness so that the longitudinal direction of the test piece was perpendicular to the longitudinal direction of the weld bead. The notch position is a cross-sectional notch at the center of the weld heat affected zone with the intention of evaluating the toughness of the weld heat affected zone. The test temperature was set to −40 ° C. like the steel plate.

Table In 2-1, Ri invention examples der steel plate and its joint steel symbol 1-A1, 1-A8~9,1- A11~12,1-A15,1-A8~20 satisfy the present invention The others are reference examples. In Table 3-1, Inventive Example der steel plate and its joint steel symbol 2-A6~8,2-A10~11,2-A13~16 satisfy the present invention is, others are reference examples. In the steel sheet satisfying the present invention and its joint, the tensile strength of the steel sheet is sufficiently higher than 1100 MPa, and the joint does not cause significant softening of the weld heat affected zone. The material breaks, and as a result, the tensile strength as a joint also shows a sufficiently high strength almost the same as the base material. As for toughness, both the steel plate and the weld heat affected zone have sub-size test pieces with a thickness of 5 mm, the absorbed energy at −40 ° C. is well above 27 J, and the steel plate produced according to the present invention is a laser-arc hybrid. It is clear that it exhibits very good properties for welding.

  On the other hand, the steel plate symbols 1-B1 to 1-B19 in Table 2-2 and the steel plate symbols 2-B1 to 2-B24 in Table 3-2 satisfy the present invention in terms of the chemical composition, the manufacturing method, or both. Therefore, the mechanical properties of the steel plate or / and the joint are extremely inferior to those of the present invention, and therefore, the steel plate has sufficient characteristics as a super-high strength steel plate having good toughness and its laser-arc hybrid welded joint. It is a comparative example that is difficult to say.

  That is, first, about the comparative example concerning Claim 1, since steel plate symbol 1-B1 has too little C content of a steel plate, the tensile strength of a steel plate does not reach 1100 MPa. In addition, since the base material breaks even in the joint tension, the strength is almost the same as that of the steel plate.

  Steel plate symbol 1-B2 is not preferable because the C content of the steel plate is excessive, and the toughness of the steel plate and the weld heat affected zone is poor.

  Steel plate symbol 1-B3 is not preferable because the Si content of the steel plate is excessive, and the toughness of the steel plate and the weld heat affected zone is poor.

  Since the steel plate symbol 1-B4 has an excessively low Mn content in the steel plate, the hardenability is insufficient, and a coarse bainite structure is generated slightly. The toughness of the steel sheet is inferior.

  In steel plate symbol 1-B5, since the Mn content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  In steel plate symbol 1-B6, since the P content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  In steel plate symbol 1-B7, since the S content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  Steel plate symbol 1-B8 has an excessively low Mn content in the steel plate, but the tempering temperature is kept low, so the strength of the steel plate is high, but softening of the weld heat affected zone in the joint is large, so the joint tension Then, it fractures at the weld heat affected zone (HAZ), its tensile strength is greatly reduced, and has not reached 1100 MPa. Moreover, it cannot be said that the toughness of a steel plate is sufficient.

  Steel plate symbol 1-B9 satisfies the present invention in terms of the steel plate composition, but the tempering temperature is too low, so the function of suppressing the softening of the weld heat affected zone is not sufficient. Therefore, although the strength of the steel sheet is sufficient, in the joint tensile test, it breaks at the weld heat affected zone where softening is remarkable, and the tensile strength is also lower than 1100 MPa.

  Steel plate symbol 1-B10 satisfies the present invention in terms of steel plate composition, but the tempering temperature is excessive and exceeds the Ac1 transformation point, so that the steel plate is inferior to the present invention in both strength and toughness. Also, the joint has a large softening of the heat affected zone and has a low tensile strength.

  Steel plate symbol 1-B11 satisfies the present invention in the steel plate composition, but the quenching temperature is as low as less than the Ac3 transformation point, so that uniform austenitization is not achieved during heating, so 100% martensite. Not organized. As a result, the strength and toughness of the steel sheet are inferior to those of the present invention. Further, if the quenching temperature is too low, the precipitation strengthening element effective for suppressing the softening of the weld heat affected zone does not work effectively, so that the joint strength is reduced due to the softening of the weld heat affected zone.

  Steel plate symbol 1-B12 is not preferable because the steel plate composition satisfies the present invention, but the quenching temperature is excessive, so that the heated austenite grain size becomes coarse and the toughness of the steel plate is significantly deteriorated.

  In steel plate symbol 1-B13, since the Al content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  In steel plate symbol 1-B14, since the Ni content of the steel plate is too small, the toughness of the weld heat affected zone is inferior. The toughness of the steel sheet is not sufficient.

  Steel plate symbol 1-B15 is an example in which the Ni content of the steel plate exceeds the upper limit of the present invention and is excessive. The tensile strength and toughness satisfy the objectives of the present invention, but the absorbed energy of the Charpy impact test is not so much improved for a large amount of Ni, and the yield strength is very low. Depending on the application, it may be undesirable. That is, the production cost of the steel sheet becomes very expensive by containing a large amount of Ni, but no improvement in the characteristics commensurate with it is recognized.

  In steel plate symbol 1-B16, since the carbon equivalent of the steel plate is too small, the hardenability is not sufficient, and the steel plate and the joint have a tensile strength slightly exceeding 1000 MPa.

  In steel plate symbol 1-B17, since the carbon equivalent of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  Steel plate symbol 1-B18 does not contain all the elements related to Nb equivalent that are effective in suppressing the softening of the weld heat affected zone. Therefore, the weld heat affected zone is significantly softened. The joint has very low tensile strength because it breaks early. The tensile strength of the steel sheet does not reach 1100 MPa.

  In the steel plate symbol 1-B19, only Mo as a precipitation strengthening element related to the Nb equivalent is contained in the steel plate, but its content is too small. Therefore, the Nb equivalent is too small. The amount of softening is large, and in the joint tensile test, fracture occurred at the weld heat affected zone, and the tensile strength did not reach the target.

  Moreover, about the comparative example concerning Claim 2, since steel plate symbol 2-B1 has too much Mo content of a steel plate, the toughness of a steel plate and a welding heat affected zone is inferior.

  In the steel plate symbol 2-B2, only V is contained in the steel plate as a precipitation strengthening element related to the Nb equivalent, but its content is too small. Therefore, the Nb equivalent is too small. The amount of softening is large, and in the joint tensile test, fracture occurred at the weld heat affected zone, and the tensile strength did not reach the target.

  In steel plate symbol 2-B3, since the V content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  Steel plate symbol 2-B4 contains Cr, W, and Nb as precipitation strengthening elements related to the Nb equivalent, but each content is too small and the Nb equivalent is too small. The softening amount of the part is large, and in the joint tensile test, the fracture occurred at the weld heat affected part, and the tensile strength did not reach the target. The strength of the steel plate is also low.

  Steel plate symbol 2-B5 contains V, Ta, and Zr as precipitation strengthening elements related to Nb equivalent +, but, as with steel plate symbol 2-B4, each content is too small, and Nb Since the equivalent is too small, the amount of softening of the weld heat affected zone is large, and in the joint tensile test, the weld heat affected zone is broken and the tensile strength does not reach the target. The strength of the steel plate is similarly low.

  Steel plate symbol 2-B6 contains Cr, Mo, Ta as precipitation strengthening elements related to the Nb equivalent, and the respective contents are within the scope of the present invention. The softening amount of the part is large, and in the joint tensile test, the fracture occurred at the weld heat affected part, and the tensile strength did not reach the target. The strength of the steel plate is similarly low.

  In steel plate symbol 2-B7, since the Cr content of the steel plate is excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  In steel plate symbol 3-B8, since the W and Nb contents of the steel plate are excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  The steel plate symbol 2-B9 satisfies the present invention in terms of the steel plate composition, but because the heating temperature of the steel slab is excessive in the thermomechanical treatment, the austenite is not sufficiently refined after hot rolling and before transformation. The toughness of the steel sheet is inferior. The weld heat affected zone is also slightly inferior in toughness due to the influence of the roughness of the steel sheet structure.

  The steel plate symbol 2-B10 satisfies the present invention in terms of the steel plate composition, but in the heat treatment, the heating temperature of the steel slab is too low and less than the Ac3 transformation point. Therefore, a uniform martensite structure is not obtained, and the strength and toughness of the steel sheet are inferior to those of the present invention. As a result, the joint strength is also poor.

  Steel plate symbol 2-B11, although the steel plate composition satisfies the present invention, the total cumulative rolling reduction is too small in the thermomechanical treatment, so the austenite before hot transformation is not sufficiently refined after hot rolling, The toughness of the steel sheet is inferior.

  In steel plate symbol 2-B12, the steel plate composition satisfies the present invention, but in the thermomechanical treatment, the rolling end temperature is lower than the lower limit of the present invention, so that transformations other than martensite before the start of accelerated cooling. As a result, the strength and toughness of the steel sheet are inferior to those of the present invention. The joint strength is also poor.

  Steel plate symbol 2-B13, although the steel plate composition satisfies the present invention, in the thermomechanical processing, the accelerated cooling start temperature is lower than the lower limit of the present invention, so that other than martensite before the start of accelerated cooling. A transformation structure is generated, and as a result, the strength and toughness of the steel sheet are inferior to those of the present invention. The joint strength is also poor.

  Steel plate symbol 2-B14 satisfies the present invention in terms of the steel plate composition, but since the cooling after rolling is not accelerated cooling but air cooling, a sufficient martensitic structure cannot be obtained. Both steel sheets and joints are very low in strength and do not meet the target. The toughness of the steel sheet is low, which is not preferable.

  Although the steel plate symbol 2-B15 satisfies the present invention in terms of the steel plate composition, the cooling rate of accelerated cooling after rolling is too low in the manufacturing method, so that a sufficient martensite structure cannot be obtained. Both joints have very low strength and do not meet the target. The toughness of the steel sheet is also poor.

  Steel plate symbol 2-B16 satisfies the present invention in terms of steel plate composition, but is not subjected to the tempering treatment that is essential in the work heat treatment in which accelerated cooling is performed to 300 ° C. or less, and therefore the toughness of the steel plate is inferior. The weld heat-affected zone is greatly softened, so that the tensile strength is insufficient in joint tension.

  Although the steel plate symbol 2-B17 is tempered, the tempering temperature is too low, so the toughness and joint strength of the steel plate are not sufficient for the same reason as the steel plate symbol 2-B16.

  Steel plate symbol 2-B18, on the contrary, has an excessive tempering temperature exceeding the Ac1 transformation point, so that the steel plate is inferior to the present invention in both strength and toughness. Also, the joint has a large softening of the heat affected zone and has a low tensile strength.

  In steel plate symbol 2-B19, since the contents of both Ta and Zr in the steel plate are excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  Steel plate symbol 2-B20 contains Cr, Mo, V as precipitation strengthening elements related to the Nb equivalent, and the respective contents are within the scope of the present invention. The toughness of the heat affected zone is inferior.

  Steel sheet symbol 2-B21 contains W, Nb, and V as precipitation strengthening elements related to the Nb equivalent, and the respective contents are within the scope of the present invention. The toughness of the heat affected zone is inferior.

  In steel plate symbol 2-B22, the content of elements related to Nb equivalent is individually within the scope of the present invention, but the Nb equivalent is excessive, so the toughness of the steel plate and the weld heat affected zone is inferior.

  In the steel plate symbol 2-B23, both the carbon equivalent and the Nb equivalent of the steel plate are too low, so the strength of the steel plate and the joint is low.

  In steel plate symbol 2-B24, since the carbon equivalent and Nb equivalent of the steel plate are both excessive, the toughness of the steel plate and the weld heat affected zone is inferior.

  Also from the above examples, according to the method for producing a high-tensile steel plate having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding of the present invention, both the steel plate and the joint have a tensile strength of 1100 MPa or more and toughness. However, it is clear that the absorption energy of the 2 mm V notch Charpy impact test at −40 ° C. can sufficiently satisfy 27 J or more sufficiently.

Claims (4)

A method for producing a high-tensile steel plate having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding combining laser welding and gas shielded arc welding,
% By mass
C: 0.12 to 0.20%
S i: 0.03 to 1.0%,
Mn: 0.5 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
Ni: 0.6-6%,
N: 0.001 to 0.01%
V: 0.12-0.50%,
Each containing
Cr: 0.1 to 3%,
Mo: 0.05-2%,
W: 0.10 to 2 %,
Nb : 0.005 to 0.20%
Ta: 0.005 to 0.50%,
Zr: 0.005 to 0.50%
Nb equivalent defined by the following formula [1] is 0.09 to 0.80%, and a carbon equivalent defined by the following formula [2] ( Ceq.) Is 0.45 to 1.2%, and the balance of Fe and unavoidable impurities is heated to a temperature range of Ac3 transformation point to 1250 ° C, and the cumulative rolling reduction at 900 ° C or less is 20 ~ 23.1%, rolling end temperature is 800 ~ 860 ° C, hot rolled into steel plate, reheating temperature is Ac3 transformation point ~ 1000 ° C reheat quenching, and more than 550 ° C ~ A method for producing a high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding, wherein tempering is performed at a temperature lower than the Ac1 transformation point.
Nb equivalent = Nb% + 0.35 Mo% + 0.20 W% + 0.75 V% + 0.30 Ta%
+ 0.20Zr% + 0.05Cr% [1]
Ceq. = C% + Si% / 24 + Mn% / 6 + Ni% / 40 + Cr% / 5 + Mo% / 4 + W% / 8 + V% / 14 [2]
In the above formulas [1] and [2],% of each element represents mass% of each element in the weld metal. The steel slab according to claim 1 is heated to a temperature range of Ac3 transformation point to 1300 ° C, the rolling end temperature is 650 ° C or higher and 800 ° C or lower , and hot rolling is performed at a cumulative reduction ratio of 30 to 95%. Laser welding characterized in that accelerated cooling starts at 600 ° C. or higher and ends at 300 ° C. or lower, and further performs tempering at temperatures higher than 550 ° C. and lower than the Ac1 transformation point. For producing high-tensile steel sheets having a tensile strength of 1100 MPa or more for use in laser or arc-arc hybrid welding. The steel is further mass%,
Cu: 0.005 to 1.5%,
Co: 0.01 to 6%
Ti: 0.002 to 0.05%,
B: 0.0002 to 0.0030%
A method for producing a high-tensile steel sheet having a tensile strength of 1100 MPa or more for laser welding or laser-arc hybrid welding according to claim 1 or 2, characterized by containing one or more of them . The steel is further mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%,
REM: 0.0002 to 0.01%
The high tensile strength for laser welding or laser-arc hybrid welding according to any one of claims 1 to 3, wherein the tensile strength is 1100 MPa or more. A method for producing a tension steel sheet.