JP5151693B2 - Manufacturing method of high-strength steel - Google Patents

Description

The present invention relates to a method for producing high-strength steel such as thick steel plates that can be used in industrial fields such as construction, industrial machinery, construction machinery, and pressure vessels.

  As the performance and size of structures increase, the importance of developing high-strength steel sheets is increasing. The strength of the steel sheet can be achieved by increasing the amount of the alloying element to be added. However, the increase in the alloying element generally reduces the weld crack resistance during welding. For this reason, various investigations have been made on steel materials having a chemical composition capable of obtaining high strength without increasing the amount of alloying elements added and methods for producing the same.

For example, the addition of a small amount of B of about 0.0003 to 0.0025% significantly improves the hardenability of the steel, so that 780 MPa (780 N / mm ^{2) is} obtained without causing extreme deterioration of the weld crack resistance. It has been clarified that high strength steel having the above tensile strength can be produced.

  For example, Patent Document 1 listed below increases the rolling end temperature for B-containing steel to improve hardenability, and Patent Document 2 describes the chemical composition of B-containing steel with Mn, Cr, Mo, N, Ti, What is defined by the formula consisting of the Zr and Hf contents is Patent Document 3 that specifies the chemical composition of the B-containing steel by the formula consisting of Nb and V contents, and Patent Document 4 is the B-containing steel. Each of those specifying the upper limit of the content of B carbonitride is disclosed.

  Patent Document 5 defines the chemical composition by a formula comprising Mn, Ni, Cu, Mo, Nb, and V contents, and Patent Document 6 includes the chemical composition containing C, Mn, Cu, and Ni. Each of those defined by a formula consisting of a rate is disclosed.

Japanese Patent Laid-Open No. 9-256049 Japanese Unexamined Patent Publication No. 2000-160281 Japanese Patent Laid-Open No. 2003-160833 Japanese Patent Laid-Open No. 2004-156095 JP 2004-323917 Japanese Patent Laid-Open No. 2004-312916

  However, any of the high-tensile steels disclosed in the above-mentioned patent documents has no consideration on the effect on weld joint strength when the addition amount of alloy elements is reduced to reduce the hardenability.

  Furthermore, in Patent Document 1, the improvement in toughness due to the refinement of the structure due to the reduction of the unrecrystallized region is insufficient. In Patent Document 2, consideration for Cu and Ni effective for hardenability is insufficient. Patent Document 4 only discloses that the content of B carbonitride is suppressed, and no investigation has been made to dig into the effective B amount. And in patent document 5 and 6, since the dependence to Cu and Ni is high, it becomes high cost.

The object of the present invention is to suppress the amount of expensive alloy elements added, enable stable and large-scale production, and have excellent base material characteristics and resistance to weld cracking, as well as good weld joint characteristics. An object of the present invention is to provide a method for producing a high-tensile steel having a tensile strength of 780 MPa or more.

  The present inventors have intensively studied from various viewpoints including alloy components and production methods on the method of improving the weld heat affected zone toughness without degrading the base metal properties and weld crack resistance of high-strength steel. Went.

  First, various types of alloy elements of high-strength steel having a tensile strength of 780 MPa or more and the content thereof are changed, and multilayer welding with a welding heat input of 40 kJ / cm is performed. The toughness, strength, and The cross-sectional hardness was investigated. As a result, it was in the vicinity of the melting line that the toughness was most reduced in the weld zone. Further, the hardness was greatly reduced in the vicinity of the melting line. And when the fall of hardness was remarkable or the fall range of hardness was wide, the obtained welded joint was the tensile strength less than 780 Mpa. This is presumably because hardenability decreases in the vicinity of the melting line, and an upper bainite structure inferior in strength and toughness is generated.

  Next, as a result of intensive investigations on the relationship between the type of alloy element whose content was changed, its content, and the characteristics of the welded joint, it was possible to suppress the deterioration of hardenability near the melting line and to achieve excellent welded joint properties. In order to obtain the desired tensile strength and weld joint tensile strength by adjusting the Nb addition amount and the effective B amount in addition to increasing the hardenability index DI, which is an index of hardenability. Further, it was found that the desired weld joint toughness can be obtained by adjusting the amount of C, S and Nb.

  That is, (a) In addition to increasing the hardenability index DI, adding an appropriate amount of Nb and securing an effective B amount increases the strength of the base material. (b) In addition to increasing the hardenability index DI, adding a suitable amount of Nb increases the joint tensile strength. (c) In addition to increasing the hardenability index DI, suppressing the C content and S content and adjusting the Nb content increases the toughness at the position of the melt line.

  Let us now consider each of these findings in detail.

(a) In addition to increasing the hardenability index DI, adding an appropriate amount of Nb and securing the effective B amount increases the strength of the base metal. The hardenability index DI is an approximation of the ideal critical diameter. By increasing this, it is possible to improve the hardenability in direct quenching after hot rolling. Nb has the effect of lowering the transformation point and improving hardenability, particularly in direct quenching after hot rolling, and this can be compensated when the hardenability index DI is insufficient. Effective B refers to solid solution B that is not bonded to N, and is effective in improving the hardenability in direct quenching after hot rolling.

(b) In addition to increasing the hardenability index DI, adding an appropriate amount of Nb increases the joint tensile strength. For the hardenability index DI and Nb, in addition to the effects described for the base metal strength, Has the following effects.

  Increasing the hardenability index DI is effective in improving the hardenability even when the cooling rate is relatively low in the melted line or weld heat affected zone. Nb is effective in increasing the softening resistance of the weld heat affected zone by the recovery / recrystallization delay effect, and can compensate for the lack of the hardenability index DI.

(c) In addition to increasing the hardenability index DI, suppressing the amount of C and S and adjusting the amount of Nb to increase the toughness at the position of the melt line is the same as in the case of improving the joint tensile strength The deterioration of toughness can be suppressed by increasing the hardenability index DI and increasing the hardenability. C and S are elements that deteriorate toughness. In particular, in order to increase toughness at the position of the melting line, it is preferable to suppress the amounts of C and S. Nb has the effect of suppressing coarsening and deterioration of toughness due to the recovery and recrystallization delay effect, but excessive addition precipitates as carbide on the grain boundary and deteriorates toughness. It is good to adjust. Here, the reason why the hardenability improvement by the effective B is not effective is that TiN is re-dissolved during welding in the melting line, and as a result, the effective B is lost and the hardenability is lowered.

  The present invention has been completed based on these findings, but the target characteristic is a high-tensile steel having a tensile strength of 780 MPa or more. Specifically, multilayer butt welding with a Charpy absorbed energy at -20 ° C of 70 J or more and a welding heat input of 40 kJ / cm using a 2 mm V-notch Charpy impact test specimen taken in parallel to the rolling direction from a 1/4 thickness position of steel. The joint tensile strength of the No. 1 tensile test piece specified in JIS Z 3121, taken from the joint obtained by JIS Z 3121, is 780 MPa or more. The Charpy absorbed energy at 0 ° C. obtained by sampling according to the melting line was 70 J or more.

And as will be described later, the high-tensile steel having this target property is in mass%, C: 0.03 to 0.1%, Si: 0.5% or less, Mn: 0.4 to 2.5%, P: 0.03% or less, S: 0.008% or less, Cr: 0.1-2%, Nb: 0.005-0.06%, Ti: 0.005-0.03%, B: 0 .0003 to 0.0025%, Al: 0.005 to 0.08%, N: 0.006% or less, and the balance Fe and impurities are included, and the weld crack sensitivity index Pcm, which is an index indicating weld crack sensitivity, is 0. It was found that it was obtained by satisfying any of the following formulas (1) to (3).
680 × Nb (%) + 12000 × [B (%)-10.8 / 14.1 × (N (%) − Ti (%) / 3.4)] + DI ≧ 82 (1)
3390 × Nb (%) + DI ≧ 125 (2) formula
5800 × C (%) + 76400 × S (%) + 3190 × | Nb (%) − 0.013 | −DI ≦ 410 (3) However, when calculating (1), (N (%) − Ti If the value of (%) / 3.4) is negative, substitute zero instead of the negative value, and [B (%)-10.8 / 14.1 × (N (%)-Ti (%) / 3.4 If the value of]) is negative, substitute zero instead of the negative value.

DI and Pcm are defined by the following equations (4) and (5), respectively.
DI = 0.311 x √C (%) x [1 + 0.64 x Si (%)] x [1 + 4.1 x Mn (%)] x [1 + 0.27 Cu (%)] x [1 + 0.52 x Ni (%) ] X [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] x 25.4 (4)
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) (5) Formula Moreover, each element symbol (%) in Formula (1)-Formula (5) shows content (mass%) of each element.

  In addition, this high-tensile steel is mass%, and further, Ni: 2% or less, Cu: 0.6% or less, Zr: 0.05% or less, Mo: 1% or less, V: 0.1% or less, One or more components of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less may be contained.

  Next, in the high-tensile steel having this target characteristic, the steel slab having the above chemical composition is heated to 1000 to 1200 ° C. so that the cumulative rolling reduction in the temperature range of 900 ° C. or less is 50% or more. After hot rolling, flattening is performed with a hot multistage constraining roller and immediately cooled rapidly. When the surface temperature reaches 300 ° C. or lower, the rapid cooling is stopped, and then the product is allowed to cool to room temperature. it can. Or after standing to cool to room temperature, you may further temper at the temperature of 600 degrees C or less. In addition to increasing the hardenability index DI, using steel slabs with adjusted ingredients, it is possible to obtain the necessary tensile strength by controlling the structure by performing quenching after hot rolling under certain conditions. it can.

The present invention has been completed based on these findings. The gist of the present invention is the following (1) to (6) of a method for producing a high-strength steel having a tensile strength of 780 MPa or more.

(1) By mass%, C: 0.03-0.1%, Si: 0.5% or less, Mn: 0.4-2.5%, P: 0.03% or less, S: 0.008 %: Cr: 0.1-2%, Nb: 0.005-0.06%, Ti: 0.005-0.03%, B: 0.0003-0.0025%, Al: 0.005 ~ 0.08%, N: 0.006% or less Remaining Fe and impurities are included, weld cracking sensitivity index Pcm is 0.23 or less, and any of the following formulas (1) to (3) Steel strip with a satisfactory chemical composition is heated to 1000-1200 ° C, hot-rolled so that the cumulative reduction in the temperature range of 900 ° C or lower is 50% or more, and then flattened with a hot multistage constraining roller , Immediately quenching, stopping rapid cooling when the surface temperature reaches 300 ° C. or lower, and then allowing to cool to room temperature, 780 Method for producing a high tensile steel having a tensile strength of at least Pa.
680 × Nb (%) + 12000 × [B (%)-10.8 / 14.1 × (N (%) − Ti (%) / 3.4)] + DI ≧ 82 (1)
3390 × Nb (%) + DI ≧ 125 (2) formula
5800 × C (%) + 76400 × S (%) + 3190 × | Nb (%) − 0.013 | −DI ≦ 410 (3) However, when calculating (1), (N (%) − Ti If the value of (%) / 3.4) is negative, substitute zero instead of the negative value, and [B (%)-10.8 / 14.1 × (N (%)-Ti (%) / 3.4 If the value of]) is negative, substitute zero instead of the negative value.
DI and Pcm are defined by the following equations (4) and (5), respectively.
DI = 0.311 x √C (%) x [1 + 0.64 x Si (%)] x [1 + 4.1 x Mn (%)] x [1 + 0.27 Cu (%)] x [1 + 0.52 x Ni (%) ] X [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] x 25.4 (4)
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) (5) Formula Moreover, each element symbol (%) in Formula (1)-Formula (5) shows content (mass%) of each element.

  (2) In addition to the component specified in (1) above, a steel piece containing Ni: 2% or less in mass% is used, and the tensile strength of 780 MPa or more of (1) above is used. Manufacturing method of high-strength steel.

  (3) In addition to the component specified in the above (1) or (2), by mass%, further, one or two of Cu: 0.6% or less and Zr: 0.05% or less A method for producing high-tensile steel having a tensile strength of 780 MPa or more as described in (1) or (2) above, wherein the steel piece is contained.

  (4) In addition to the component specified in any one of the above (1) to (3), in mass%, Mo: 1% or less and V: 0.1% or less of one or two components A method for producing a high-tensile steel having a tensile strength of 780 MPa or more according to any one of the above (1) to (3), characterized by using a steel slab containing bismuth.

  (5) In addition to the components specified in any one of (1) to (4) above, in mass%, Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less A method for producing a high-strength steel having a tensile strength of 780 MPa or more according to any one of (1) to (4), wherein a steel piece containing one or more components is used.

  (6) The method for producing high-tensile steel having a tensile strength of 780 MPa or more according to any one of (1) to (5) above, wherein the steel is allowed to cool to room temperature and then tempered at a temperature of 600 ° C. or lower. .

  According to the present invention, a high strength steel having a tensile strength of 780 MPa or more, which has excellent base material characteristics and resistance to weld cracking, and exhibits good weld joint characteristics, suppresses the addition amount of expensive alloy elements. Thus, stable and mass production is possible.

(A) Chemical composition of high-strength steel Below, the chemical composition and content of the high-strength steel according to the present invention, Pcm, and the reasons for defining the expressions (1) to (3) are explained together with their effects. To do. In addition, "%" regarding content means "mass%".

C: 0.03-0.1%
C is added for the purpose of securing the strength of the steel sheet. If the content is less than 0.03%, hardenability is insufficient, and it is difficult to secure a desired tensile strength (TS) of 780 MPa, and toughness is also deteriorated. In order to secure strength and toughness with TS of 780 MPa or more and absorbed energy of 70 J or more in an impact test at −20 ° C. using a 2 mm V notch Charpy impact test piece, C needs to be contained by 0.03% or more. It is. On the other hand, if the content exceeds 0.1%, not only the toughness of the base material is lowered, but also the hardness of the weld heat affected zone is increased, and the weld crack sensitivity is increased. Therefore, the content of C is set to 0.03 to 0.1%. Note that the upper limit of the C content is preferably 0.07%. The lower limit of the C content is desirably 0.05%.

Si: 0.5% or less Si may not be added. However, if added, it has the effect of deoxidizing the steel. However, when its content exceeds 0.5%, local formation of island martensite that significantly increases the hardness of the base metal and the weld heat affected zone is induced, leading to deterioration of toughness. Therefore, the upper limit of the Si content needs to be 0.5%. The upper limit of the Si content is desirably 0.3%. In addition, in order to acquire this effect reliably, it is preferable to make Si content 0.01% or more. More preferably, it is 0.03% or more.

Mn: 0.4 to 2.5%
Mn is an element added to improve the hardenability of the steel sheet and increase the strength. If the content is less than 0.4%, it is difficult to ensure the desired strength. On the other hand, if the content exceeds 2.5%, the frequency of occurrence of welding cold cracks increases. Therefore, the Mn content is set to 0.4 to 2.5%. The Mn content is preferably 0.7-2%.

P: 0.03% or less P is unavoidably present in steel as an impurity. If it exceeds 0.03%, not only does it segregate at the grain boundary to lower the toughness, but also cause hot cracking during welding, so it is necessary to make it 0.03% or less.

S: 0.008% or less S is unavoidably present in steel as an impurity. If the amount is too large, center segregation is promoted or stretched MnS is produced in a large amount, so that the mechanical properties of the base material and the HAZ are deteriorated. For this reason, the upper limit is made 0.008%. Since the smaller S is, the lower limit is not particularly specified.

Cr: 0.1 to 2%
Cr has the effect of increasing the strength of the steel sheet mainly through improving hardenability. In order to acquire this effect, it is necessary to make Cr content 0.1% or more. However, if the content exceeds 2%, the base metal toughness and weldability are deteriorated. Therefore, the content of Cr is set to 0.1 to 2%.

Nb: 0.005 to 0.06%
Nb refines austenite grains by forming fine Nb carbonitrides in the low temperature region of austenite. Further, the precipitated Nb carbonitride has an effect of suppressing the recovery and recrystallization of unrecrystallized austenite grains that have been processed by rolling, and is effective in securing the base material toughness. If the content is less than 0.005%, these effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.06%, the cracking property during welding deteriorates, so the upper limit of the Nb content needs to be 0.06%. The upper limit of the Nb content is preferably 0.03%. Therefore, the Nb content is set to 0.005 to 0.06%.

Ti: 0.005 to 0.03%
Ti is an indispensable element for securing an effective B amount by refining austenite grains and fixing solute N. Moreover, the addition is indispensable also in preventing the lateral crack of a continuous cast slab. However, if the content is less than 0.005%, the effect of addition becomes insufficient. On the other hand, if the content exceeds 0.03%, the base material toughness and the toughness of the weld heat affected zone are significantly impaired. Therefore, the content of Ti is set to 0.005 to 0.03%.

B: 0.0003 to 0.0025%
Since B improves hardenability by dissolving in a small amount in steel, it is possible to ensure sufficient strength up to the center of the plate thickness. However, if the content is less than 0.0003%, this effect cannot be sufficiently obtained, and if it exceeds 0.0025%, the base material toughness and the weld heat affected zone toughness are significantly deteriorated. Therefore, the content of B is set to 0.0003 to 0.0025%.

Al: 0.005 to 0.08%
Al has an action of deoxidation and fine structure. In order to acquire this effect, it is necessary to make Al content 0.005% or more. However, if its content exceeds 0.08%, the toughness of the weld heat-affected zone deteriorates, and even if the structure is refined by heat treatment, the toughness is reduced. Therefore, the content of Al is set to 0.005 to 0.08%. The Al content is preferably 0.03 to 0.06%. Here, the “Al” amount in the present invention refers to a so-called “sol.Al (acid-soluble Al)” amount.

N: 0.006% or less N is present as an inevitable impurity in steel, and reduces the toughness of the base metal and the weld heat affected zone. In particular, when the content exceeds 0.006%, the toughness of the base metal and the weld heat affected zone is significantly reduced. Therefore, the N content is set to 0.006% or less.

Pcm: 0.23% or less
Pcm is an index indicating weld cracking sensitivity and is expressed by equation (5). The lower this value is, the lower the welding preheating temperature is, and welding can be performed without causing cracks. In general, it is practically impossible to perform preheating when assembling a steel building structure by welding at a construction site. If Pcm is suppressed to 0.23% or less, welding can be performed without causing weld cracking even without preheating.
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) (5) Formula In addition, each element symbol (%) in Formula (5) shows content (mass%) of each element.

Equation (1): 82 or more Equation (1) shown below is an equation determined by comprehensively investigating the contribution of each alloy element with respect to the effect of developing the strength of the base material of the steel. In this case, the base material strength and joint strength are 780 MPa or more. Here, the term related to B specifically shows the degree of contribution to the base material strength by deriving an effective amount of B by calculating the amount not bonded as B nitride.
680 x Nb (%) + 12000 x [B (%)-10.8 / 14.1 x (N (%)-Ti (%) / 3.4)] + DI ≥ 82 (1) However, calculation of formula (1) At this time, if the value of (N (%) − Ti (%) / 3.4) is negative, substitute zero instead of the negative value, and [B (%) − 10.8 / 14.1 × (N ( %) − Ti (%) / 3.4)] is negative, substitute zero instead of the negative value.
In addition, each element symbol (%) in the formula (1) indicates the content (% by mass) of each element.

Formula (2): 125 or more Formula (2) below shows the contribution of each alloying element to the effect of suppressing the softening of the weld heat affected zone of the joint that has been welded to the steel material and expressing the joint strength. The formula is determined by comprehensive investigation, and when this value is 125 or more, the base material strength and joint strength are 780 MPa or more.
3390 × Nb (%) + DI ≧ 125 (2) Formula Here, DI is defined by the following formula (4).
DI = 0.311 x √C (%) x [1 + 0.64 x Si (%)] x [1 + 4.1 x Mn (%)] x [1 + 0.27 Cu (%)] x [1 + 0.52 x Ni (%) ] X [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] x 25.4 (4) Equations (2) and (4) %) Indicates the content (mass%) of each element.

Equation (3): 410 or less Equation (3) shown below is for each of the alloy elements in terms of impact absorption energy at 0 ° C. of a 2 mm V notch Charpy impact specimen taken from the melting line of a joint that has been welded to steel. Is determined by comprehensively investigating the contribution, and when this value is 410 or less, the impact absorption energy is 70 J or more.
5800 × C (%) + 76400 × S (%) + 3190 × | Nb (%) − 0.013 | −DI ≦ 410 (3) Here, DI is defined by the following equation (4).
DI = 0.311 x √C (%) x [1 + 0.64 x Si (%)] x [1 + 4.1 x Mn (%)] x [1 + 0.27 Cu (%)] x [1 + 0.52 x Ni (%) ] X [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] x 25.4 (4) Equations (3) and (4) %) Indicates the content (mass%) of each element.

  The high-tensile steel according to the present invention may further contain one or more selected from Ni, Cu, Zr, Mo, V, Ca, Mg, and REM in addition to the above alloy components as necessary. Good.

Ni: 2% or less Ni is an optional additive element. If added, it has the effect of improving weldability and toughness. Therefore, when it is desired to obtain these effects, Ni may be added as necessary. However, since Ni is an expensive element and adding it causes an increase in the price of the steel sheet, it is preferable to keep it low from the viewpoint of economy, and its content is set to 2% or less. From the economical point of view, the upper limit of the Ni content is desirably 0.6%, more preferably 0.4%. In addition, in order to acquire the addition effect of Ni reliably, it is preferable to make content of Ni 0.1% or more.

Cu: 0.6% or less Cu is an optional additive element. If added, it has the effect of improving hardenability and increasing strength. Therefore, when it is desired to obtain this effect, Cu may be added as necessary. However, if the content exceeds 0.6%, the effect of degrading toughness and weldability becomes more significant than the effect of increasing strength, so the upper limit of the Cu content needs to be 0.6%. There is. The upper limit of the Cu content is desirably 0.4%. In addition, in order to acquire the addition effect of Cu reliably, it is desirable that content of Cu shall be 0.15% or more.

Zr: 0.05% or less Zr is an optional additive element. If added, fine nitride is dispersed and precipitated in the steel, and the strength is improved. Therefore, in order to obtain this effect, Zr may be added as necessary. However, if its content exceeds 0.05%, coarse precipitates are formed and toughness is deteriorated, so the upper limit of the Zr content needs to be 0.05%. In order to reliably obtain the effect of adding Zr, the content of Zr is preferably set to 0.0005% or more.

Mo: 1% or less Mo is an optional additive element. If added, there is an effect of improving hardenability and temper softening resistance. Therefore, when it is desired to obtain this effect, Mo may be added as necessary. However, if the content exceeds 1%, the strength becomes too high, the base material toughness decreases, and the weldability is significantly deteriorated. Therefore, the upper limit of the Mo content needs to be 1%. In addition, in order to acquire the addition effect of Mo reliably, it is desirable that content of Mo shall be 0.2% or more.

V: 0.1% or less V is an optional additive element. If added, the temper softening resistance is increased to enable tempering at a high temperature, thereby contributing to improvement of strength and toughness. Therefore, if this effect is desired, V may be added as necessary. However, since the toughness deteriorates when the content exceeds 0.1%, the upper limit of the V content needs to be 0.1%. The upper limit of the V content is preferably 0.05%. In order to surely obtain the effect of addition of V, the V content is preferably 0.01% or more.

Ca: 0.004% or less Ca is an optional additive element. If added, Ca reacts with S in the steel to form acid / sulfide (oxysulfide) in the molten steel, and this acid / sulfide does not extend in the rolling direction during rolling unlike MnS. Since it is spherical after rolling, it has the effect of suppressing weld cracks and hydrogen-induced cracks starting from the ends of the stretched inclusions. Therefore, in order to obtain these effects, Ca may be added as necessary. However, if the content exceeds 0.004%, the toughness may be deteriorated, so the upper limit of the Ca content needs to be 0.004%. In addition, in order to acquire the addition effect of Ca reliably, it is desirable to make content of Ca 0.0005% or more.

Mg: 0.002% or less Mg is an optional additive element. When added, Mg forms an Mg-containing oxide, serves as a TiN generation nucleus, and has the effect of finely dispersing TiN. Therefore, when it is desired to obtain these effects, Mg may be added as necessary. However, if the content exceeds 0.002%, the amount of oxides increases and ductility is reduced, so the upper limit of the Mg content needs to be 0.002%. In order to reliably obtain the effect of adding Mg, the Mg content is desirably 0.0005% or more.

REM: 0.002% or less REM (rare earth element) is an optional additive element. If added, REM has the effect of contributing to refinement of the structure of the weld heat affected zone and fixation of S. Therefore, when it is desired to obtain these effects, REM may be added as necessary. REM, on the other hand, has the effect of reducing the cleanliness as an inclusion, but the inclusion formed by the addition of REM has a relatively small influence on toughness deterioration. Even if it is contained, the lowering of the toughness of the base material is acceptable, so the upper limit of the content of REM is made 0.002%. In order to surely obtain the effect of adding REEM, the content of REM is preferably 0.0005% or more.

  Here, “REM” as used in the present invention is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM indicates the total content of the above elements.

(B) About the manufacturing conditions of high-tensile steel Below, the reason which prescribes | regulates the manufacturing conditions of the high-tensile steel which concerns on this invention as mentioned above is demonstrated with an effect.

(B-1) About the heating temperature of a steel piece The heating temperature of a steel piece shall be 1000-1200 degreeC. This is a temperature necessary for uniformly austenitizing the structure of the entire steel slab, and if it is less than 1000 ° C., uniform austenite grains cannot be obtained during heating. However, when heated above 1200 ° C, the austenite grains become extremely large and the base material toughness deteriorates, so the heating temperature needs to be 1200 ° C or lower.

(B-2) About hot rolling In order to refine the structure by “direct quenching after rolling”, it is necessary to apply an appropriate amount of reduction (processing) in the non-recrystallization temperature range. This is because no lattice defects are accumulated in the austenite grains even when the austenite is recrystallized in the recrystallization temperature range, so that the structure is not refined even if quenched after rolling. In addition, if the cumulative reduction amount is small even if the reduction is performed in the non-recrystallization temperature range, the lattice defects accumulated in the austenite grains are reduced, so that the structure is not refined even if rapidly cooled after rolling. .

  In the case of the steel having the chemical composition described in the item (A), which is the subject of the present invention, the non-recrystallization temperature range is 900 ° C. or less, and the reduction at which the cumulative reduction rate is 50% or more in this temperature range. When this is done, the structure that has been “hardened directly after rolling” can be refined for the first time. Therefore, when the steel slab heated to the temperature range described in the above (B-1) section is hot-rolled into a steel sheet, the cumulative rolling reduction in the temperature range of 900 ° C. or lower is set to 50% or more. The lower limit temperature of the non-recrystallization temperature range to which the cumulative reduction of 50% or more is applied is preferably 750 ° C. from the viewpoint of securing strength by rapid cooling after rolling. In addition, since it is only necessary that the cumulative rolling reduction in the temperature range of 900 ° C. or less is 50% or more, the rolling reduction in the recrystallization temperature range exceeding 900 ° C. does not have to be specified.

  In the present invention, flattening is performed with a hot multistage constraining roller before the subsequent “direct quenching after rolling”. The reason why such a process is performed is to increase the hardenability in the subsequent “direct quenching” by applying strain in this temperature range to regulate excessively flat crystal grains immediately after hot rolling. In addition, because it is a high-strength steel, the steel material immediately after hot rolling is kept at a higher temperature than the room temperature after the subsequent “direct quenching after rolling” or further “tempering” after that, This is because the mechanical load can be remarkably suppressed and is extremely efficient.

(B-3) About rapid cooling for “direct quenching after rolling” Immediately after hot rolling, rapid cooling is performed, but it is extremely important to appropriately control the quenching stop temperature. When the surface temperature of the material to be treated that stops the rapid cooling exceeds 300 ° C., the martensitic transformation necessary for securing the strength is insufficient and the strength is lowered and the toughness is also deteriorated. Accordingly, the rapid cooling is stopped when the surface temperature of the material to be treated reaches a temperature in the temperature range of 300 ° C. or less, and then it is allowed to cool to room temperature.

  It should be noted that “immediately cooling the steel sheet after hot rolling” means “direct quenching after rolling” with a coolant such as water or oil without reheating the steel sheet after rolling. Further, “cooling” refers to natural cooling in the atmosphere.

(B-4) Tempering After “direct quenching after rolling”, the steel sheet may be tempered as necessary to ensure a good balance between strength and toughness. However, when the tempering temperature exceeds 600 ° C., the strength is remarkably lowered, and it becomes difficult to secure a desired strength of a tensile strength of 780 MPa or more. Therefore, the tempering temperature when tempering the steel sheet was set to 600 ° C. or lower.

  Steel pieces having the chemical composition shown in Table 1 were melted and subjected to hot forging and controlled rolling to produce a steel plate having a width of 160 mm and a length of 1000 mm, and some steel types were tempered. Table 2 shows the heating temperature of the steel slab, the cumulative reduction ratio of 900 ° C. or less, the finished plate thickness, the direct quenching stop temperature, and the tempering temperature.

  For each steel plate, a round bar tensile test piece with a parallel part diameter of 14 mm and a mark distance of 50 mm, a test steel with a plate thickness of 25 mm, from a 1/2 plate thickness position, a test steel with a plate thickness of 60 mm (Test No. 28) , 2mmV notch Charpy impact test specimens were taken from the 1/4 thickness position in parallel to the rolling direction from the 1/4 thickness position, and the mechanical properties of the base metal were investigated.

  In addition, for some of the test steels, multilayer butt welding was performed under the conditions shown in Table 3, and No. 1 tensile test piece specified in JIS Z 3121, A2 tensile test piece specified in JIS Z 2111 from weld metal, A 1 / 4mm thickness 2mmV notch Charpy impact test piece was taken with its notch position aligned with the melt line, and the mechanical properties of the joint were investigated. Table 3 shows the welding conditions.

  Table 4 shows the results of investigating the mechanical properties of the base metal and the joint.

  As can be seen from the results shown in Table 4, when the chemical composition deviates from the conditions defined in the present invention, at least one of the base material strength, joint strength, and joint toughness does not satisfy the target characteristics. In particular, steel G does not satisfy the target joint strength of 780 MPa even though it satisfies the target base material strength of 780 MPa, but the strength of the weld metal at this time sufficiently satisfies the target joint strength of 780 MPa, and the base metal is welded heat. This shows that the joint strength cannot be satisfied because it is softened by the influence.

  The high-tensile steel according to the present invention has excellent base material characteristics and resistance to weld cracking, and not only exhibits good weld joint characteristics, but also has a tensile strength of 780 MPa or more.

Claims (6)

In mass%, C: 0.03-0.1%, Si: 0.5% or less, Mn: 0.4-2.5%, P: 0.03% or less, S: 0.008% or less, Cr: 0.1-2%, Nb: 0.005-0.06%, Ti: 0.005-0.03%, B: 0.0003-0.0025%, Al: 0.005-0. 08%, N: 0.006% or less, remaining Fe and impurities, weld cracking sensitivity index Pcm is 0.23 or less, and satisfies any of the following formulas (1) to (3) A steel slab having a chemical composition is heated to 1000 to 1200 ° C., hot-rolled so that the cumulative rolling reduction in a temperature range of 900 ° C. or less is 50% or more, and then flattened with a hot multistage constraining roller. 780MP, characterized by immediately cooling immediately, stopping the rapid cooling when the surface temperature reaches 300 ° C or lower, and then allowing to cool to room temperature. Method for producing a high tensile steel having a tensile strength of at least.
680 × Nb (%) + 12000 × [B (%)-10.8 / 14.1 × (N (%) − Ti (%) / 3.4)] + DI ≧ 82 (1)
3390 × Nb (%) + DI ≧ 125 (2) formula
5800 × C (%) + 76400 × S (%) + 3190 × | Nb (%) − 0.013 | −DI ≦ 410 (3) However, when calculating (1), (N (%) − Ti If the value of (%) / 3.4) is negative, substitute zero instead of the negative value, and [B (%)-10.8 / 14.1 × (N (%)-Ti (%) / 3.4 If the value of]) is negative, substitute zero instead of the negative value.
DI and Pcm are defined by the following equations (4) and (5), respectively.
DI = 0.311 x √C (%) x [1 + 0.64 x Si (%)] x [1 + 4.1 x Mn (%)] x [1 + 0.27 Cu (%)] x [1 + 0.52 x Ni (%) ] X [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] x 25.4 (4)
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5 × B (%) (5) Formula Moreover, each element symbol (%) in Formula (1)-Formula (5) shows content (mass%) of each element.   The high tensile steel having a tensile strength of 780 MPa or more according to claim 1, wherein a steel piece containing, in addition to the components defined in claim 1, mass% and further containing Ni: 2% or less is used. Production method.   In addition to the components defined in claim 1 or 2, a steel slab containing, in mass%, one or two of Cu: 0.6% or less and Zr: 0.05% or less is used. The manufacturing method of the high strength steel which has the tensile strength of 780 Mpa or more of Claim 1 or 2 characterized by the above-mentioned.   A steel slab containing, in addition to the components defined in any one of claims 1 to 3, by mass%, and further containing one or two components of Mo: 1% or less and V: 0.1% or less. The method for producing a high-strength steel having a tensile strength of 780 MPa or more according to claim 1 or 2, wherein:   In addition to the components specified in any one of claims 1 to 4, in mass%, Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less Or the steel slab containing 2 or more types of components is used, The manufacturing method of the high strength steel which has the tensile strength of 780 Mpa or more of Claim 1 or 2 characterized by the above-mentioned. The method for producing high-tensile steel having a tensile strength of 780 MPa or more according to any one of claims 1 to 5, wherein the steel is further cooled to room temperature and further tempered at a temperature of 600 ° C or lower.