JP2005336541A - Low yr high tensile strength steel plate and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength steel plate having a tensile strength of ≥590 MPa, a low YR (yield ratio) of ≤80% and having excellent high heat input joint toughness without performing reheating/tempering after hot rolling, and to provide its production method. <P>SOLUTION: The low YR high tensile strength steel plate contains, by mass, 0.04 to 0.20% C, 0.0002 to 0.003% B, 0.0003 to 0.0025% Ca or the like, and the balance Fe with inevitable impurities, and in which the value of C+100×B is 0.1 to 0.32, and the fraction of bainite in a region other than range to 5 mm below the plate surface is ≥50%, and has a plate thickness of ≥70 mm. The steel plate comprising the above components is hot-rolled and is thereafter rapidly cooled from a temperature of an Ar<SB>3</SB>point or higher, thus the fraction of bainite in the steel can be controlled to ≥50% even if the plate thickness of the steel plate is ≥70 mm to increase its strength without causing a strength difference in the plate thickness direction even as-quenched, and, by the movable dislocation of the bainite structure, the YR can be reduced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a low YR high-tensile steel sheet having a plate thickness of 70 mm or more, a tensile strength of 590 MPa or more and excellent toughness of high heat input joints, and a method for producing the same.

  In recent years, the demand for material properties of steel materials for welding used in large-scale building structures such as medium- and high-rise buildings and bridges has increased. Furthermore, when constructing such a structure, in order to promote the efficiency of welding, the application of a high heat input welding method represented by a flux-copper backing welding method, an electrogas welding method, an electroslag welding method, etc. The demand for the toughness of the HAZ is becoming stricter as well as the toughness of the steel material itself. In addition, since large-scale building structures are required to have earthquake resistance, steel having a low YR (YP / TS) characteristic and a high tensile strength of 590 MPa or more is required.

  In order to achieve a tensile strength of 590 MPa or more under the microstructure condition, in the steel component system, it is particularly necessary to increase C and to add Cu / Ni and Nb. The problem that heat weld joint toughness falls occurs.

  Various methods have been proposed as methods for improving the HAZ toughness of steel materials during high heat input welding.

  A method to reduce HAZ austenite grains and improve toughness by securing fine Ti nitride in the steel, utilizing the composite precipitates of Ti nitride and MnS as ferrite transformation nuclei, and toughness of HAZ A method for improving the above has been proposed. However, since the Ti nitride is almost dissolved in the vicinity of the boundary (referred to as a weld bond portion) with the weld metal having a maximum temperature exceeding 1400 ° C. in the HAZ, the effect of improving toughness is lowered.

  As a method for improving the toughness in the vicinity of the weld bond, steel containing Ti oxide is used in various fields such as thick plates and shaped steels (Patent Documents 1 and 2). Steel containing Ti oxide is very effective for improving the toughness of high heat input welds. This principle is based on the fact that Ti oxide, which is stable even at the melting point of steel, is used as a site, Ti nitride, MnS, etc. are precipitated in the middle of the temperature drop after welding, and fine ferrite is generated using these as a site, resulting in improved toughness. Generation of harmful coarse ferrite is suppressed, and deterioration of toughness can be prevented.

  However, the number of such Ti oxides dispersed in the steel cannot be increased so much. The cause is coarsening or aggregation of Ti oxides. If the number of Ti oxides is increased, coarse Ti oxides of 5 μm or more, so-called inclusions increase. The inclusions of 5 μm or more are harmful as a starting point of destruction of the structure and cause a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it is necessary to utilize an oxide that is less likely to be coarsened or aggregated and is more finely dispersed than Ti oxide.

  Recently, in the construction industry and the like, it is required to weld large structural materials for construction, and a further increase in welding heat input of 500 kJ / cm or more, and 1000 kJ / cm or more for large ones is being promoted. There is a need for a high-tensile steel material having the HAZ toughness. At this time, it is necessary to improve toughness particularly in the vicinity of the welded fusion part. In addition, since large-scale building structural materials are also required to have earthquake resistance, it is also necessary to use steel materials having a low YR characteristic of 80% or less.

Patent Document 3 describes an invention capable of realizing a 600 MPa class steel with low YR characteristics (YR ≦ 80%) that realizes excellent HAZ toughness even during welding with an ultra-high heat input of 500 kJ / cm or more. Yes. In order to achieve YR ≦ 80%, the microstructure is two-phase of ferrite and bainite and the ferrite fraction is 25 to 75%. In order to achieve TS ≧ 590 MPa, C is increased as a component of steel, and addition of Cu, Ni, and Nb is required. However, these elements reduce the toughness of the super high heat input welded joint. This reduction in toughness of the super high heat input welded joint is prevented by oxide dispersion. Therefore, the steel contains oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm in a number density of 100 to 3000 / mm ² per unit area. Ca is used to form a large amount of oxide.

In Non-Patent Document 1, after hot rolling is completed, a method of performing water-cooling quenching from Ar ₃ temperature or higher, and then reheating to a ferrite + austenite two-phase region temperature and quenching again is used. A method for obtaining a low yield ratio by obtaining a two-phase structure of + bainite is described.

JP-A-61-79745 JP-A-62-103344 JP 2002-256377 A Yukio Tomita et al., “Effects of various processes on the yield point of architectural HT60”, CAMP-ISIJ, Vol. 1 (1988), p. 88

  As a steel material for welding used in large building structures such as high-rise buildings, steel materials having a plate thickness exceeding 70 mm are required. Even in such a steel material, it is necessary to have a low YR with a tensile strength of 590 MPa or more and a yield ratio of 80% or less, and an excellent large heat input joint toughness.

  If an attempt is made to manufacture a steel plate having a thickness of 70 mm or more by the method described in Patent Document 3, the strength at the central portion of the thickness is insufficient and it is difficult to obtain a predetermined quality.

As described in Non-Patent Document 1, after hot rolling is completed, water-cooled quenching is performed from the Ar ₃ temperature or higher, and then re-heating to the two-phase temperature of ferrite + austenite and rapid cooling is performed at room temperature. Thus, a ferrite and bainite two-phase structure can be obtained to achieve a low yield ratio, and even a steel sheet having a thickness of 70 mm or more can produce a steel that satisfies the low yield ratio, high tension, and joint toughness. However, since reheating and tempering are performed after hot rolling, productivity deteriorates and an increase in manufacturing cost is inevitable.

  The present invention relates to a high-tensile steel sheet having a low YR with a tensile strength of 590 MPa or more and a yield ratio of 80% or less without reheating and tempering after hot rolling, and a method for producing the same. The purpose is to provide.

Even if the steel sheet thickness is 70 mm or more, B is contained in the steel, the component range of C and B is specified, and the steel is rapidly cooled after the hot rolling from the temperature of the Ar ₃ point or higher. The fraction can be 50% or more. If the bainite fraction of the steel is 50% or more, the strength can be increased without causing a difference in strength in the thickness direction even when quenched, and at the same time, YR can be reduced by the movable dislocation of the bainite structure. .

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.35%, Mn: 0.50 to 2.50%, Al: 0.003 to 0.03% Ti: 0.001 to 0.02%, B: 0.0002 to 0.003%, Ca: 0.0003 to 0.0025%, N: 0.006% or less, the balance Fe and It consists of inevitable impurities, the value of C + 100 × B is 0.1% or more and 0.32% or less, and the bainite fraction is 50% or more in the region excluding up to 5 mm below the plate surface in the plate thickness direction. A low YR high-tensile steel sheet having a thickness of 70 mm or more.
(2) Further, by mass%, Ni: 0.001 to 0.9%, Cr 0.001 to 0.3%, Mo: 0.001 to 0.04%, V: 0.001 to 0.05%, Nb: The low YR high-tensile steel sheet according to (1) above, containing one or more of 0.001 to 0.05%.
(3) Oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm are contained in the steel in a number density of 100 to 3000 / mm ² per unit area, and the composition of the oxide particles is at least Ca, Al The low YR high-tensile steel plate according to (1) or (2) above, wherein the elements containing O and excluding O contain, by mass ratio, Ca: 5% or more and Al: 5% or more .
(4) The low YR high tension according to any one of (1) to (3) above, which has excellent large heat input joint toughness with a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less. steel sheet.
(5) The steel sheet having the component according to any one of (1) to (3) above is subjected to water-cooling and quenching from the Ar ₃ temperature or higher after the hot rolling is completed, and is water-cooled to 150 ° C. or lower. The manufacturing method of the low YR high-tensile steel sheet.

In the present invention, C: 0.04 to 0.20% in steel, B: 0.0002 to 0.003%, the value of C + 100 × B is 0.1% to 0.32%, and hot rolling After completion, water cooling quenching is started from Ar ₃ temperature or higher, and water cooling is performed to 150 ° C. or lower, so that the bainite fraction is 50% or higher at any position of the plate thickness except directly below the plate surface, and the plate thickness is 70 mm or more. However, it is possible to provide a low YR high-tensile steel sheet having excellent large heat input joint toughness with a proof stress of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less.

  In a conventional as-quenched steel with a ferrite and bainite two-phase structure by quenching using strong cooling after hot rolling and a ferrite fraction of 25 to 75%, when the plate thickness is 70 mm or more, There was a difference in strength in the thickness direction, and the strength at the central portion of the plate thickness could not be secured, while the hardness of the surface layer portion tended to be excessive. Moreover, when trying to suppress the yield ratio while quenching, it is used to increase the C to cause stress concentration around cementite, which is a hard phase, and to induce the yield there. It is difficult to ensure high heat input weld joint toughness.

In the present invention, the C in the steel 0.04% or more, B 0.0002% or more, the value of C + 100 × B 0.32% or less than 0.1%, after the end of hot rolling, Ar ₃ temperature By starting water cooling and cooling to 150 ° C. or less, the bainite fraction is 50% at any position of the plate thickness except directly below the plate surface even if the plate thickness is 70 mm or more. As described above, these problems have been solved.

  The bainitic structure contains many mobile dislocations in the state of transformation, and the yield ratio can be reduced because the yield starts before such mobile dislocations under stress loading. In the bainite structure, carbides are finely dispersed under the condition of C + 100 × B ≦ 0.32%, and the stress concentration on these also contributes to the reduction of the yield ratio.

  The present invention has a yield strength of 440 MPa or more by making the steel structure a bainitic structure with C in the steel being 0.04% or more, B being 0.0002% or more, and C + 100 × B being 0.1% or more. The tensile strength is set to 590 MPa or more. In particular, even a thick material having a plate thickness exceeding 70 mm can ensure strength to the center of the plate thickness as it is quenched.

  On the other hand, both C and B increase the hardenability of the weld heat affected zone (HAZ), so C + 100 × B is limited to 0.32% or less, thereby reducing HAZ hardness and ensuring HAZ toughness. can do. In particular, C forms a large amount of coarse cementite which is an embrittled phase, so that the toughness is ensured by limiting to C ≦ 0.20%.

  The present invention improves the HAZ toughness by achieving reheated austenite refinement of the HAZ region heated to 1400 ° C. or higher by using an oxide.

  In order to refine the reheated austenite grains, it is necessary to suppress austenite grain growth at high temperatures. The most effective method for this is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. Conventionally, Ti nitrides and oxides have been considered effective as one of the dispersed particles having such an action. However, since the ratio of solid solution of Ti nitride increases at a high temperature of 1400 ° C. or higher, the pinning effect is reduced. On the other hand, it is necessary to use an oxide that is stable at high temperatures as pinning particles. Further, the pinning effect of the crystal grain boundaries by the dispersed particles is larger as the volume ratio of the dispersed particles is larger and the size of one particle is larger. However, since the volume fraction of dispersed particles has an upper limit depending on the concentration of the elements constituting the particles contained in the steel, if the volume fraction is assumed to be constant, a smaller particle diameter is more effective for pinning. .

As a means for increasing the volume fraction of the oxide, an element having a small solubility product with oxygen can be used. Generally, Al is used as a small product of solubility with oxygen, that is, as a strong deoxidizing element. However, Al alone is insufficient to sufficiently use oxygen, and a deoxidizing element stronger than Al is required, and it is important to utilize Ca that is widely used in the deoxidation process of steel. Since Ca has a small solubility product with oxygen, a larger amount of oxide than Al can be generated for the same amount of oxygen. As a composition of oxide particles generated in steel, Ca is contained at 5% or more and Al is contained at 5% or more, whereby oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm per unit area in steel. It is possible to increase the volume fraction of oxide, that is, the amount of oxide, by containing 100 to 3000 / mm ² in number density.

  In the present invention, by containing 0.0003% or more of Ca in the steel, oxide particles are dispersed in the steel as described above, thereby greatly improving the HAZ toughness.

  Hereinafter, the reasons for limitation including the components of the present invention will be described. % Means mass%.

  C is an effective component for improving the strength of the steel and is necessary for making the quenched structure a bainitic structure. If C is less than 0.04%, it is difficult to make the bainite fraction 50% or more, and the base material strength is insufficient, so the lower limit is made 0.04%. On the other hand, excessive addition significantly reduces the weldability and HAZ toughness of the steel material, so the upper limit was made 0.20%.

  Si is a component necessary for securing the strength of the base material, deoxidation, and the like, and its effect can be exhibited by containing 0.05% or more. On the other hand, the upper limit was made 0.35% in order to prevent the toughness from being lowered by the hardening of HAZ.

  Mn is contained in an amount of 0.50% or more as an effective component for ensuring the strength of the base material and the HAZ toughness. On the other hand, if the Mn content is too high, the HAZ toughness decreases, so the upper limit was made 2.50%.

  Al is an important deoxidizing element, and the lower limit was set to 0.003%. Further, when a large amount of Al is present, the surface quality of the slab deteriorates, so the upper limit was made 0.03%. Moreover, HAZ toughness is securable by making Al content into this range.

  Ti is added in an amount of 0.001% or more in order to combine with N to form Ti nitride. However, since the HAZ toughness decreases as the amount of dissolved Ti increases, the upper limit was made 0.02%. Moreover, HAZ toughness is securable by making Ti content into this range. Furthermore, containing Ti is also effective in securing the bainite fraction.

  B is necessary to make the quenched structure a bainite structure. When B is less than 0.0002%, it is difficult to make the bainite fraction 50% or more particularly in a steel sheet having a thickness of 70 mm or more, and the base material strength is low. Since it is insufficient, the lower limit is made 0.0002%. On the other hand, excessive addition reduces the HAZ toughness, so the upper limit was made 0.003%.

  In the present invention, the value of C + 100 × B is set to 0.1% or more and 0.32% or less. Here, C means C content (mass%), and B means B content (mass%). When the value of C + 100 × B is less than 0.1%, the quenched structure cannot be a bainite structure, and the target strength of yield strength of 440 MPa or more and tensile strength of 590 MPa or more cannot be achieved. In particular, in the case of a thick steel plate exceeding 70 mm as in the present case, the strength of the thickness center portion is insufficient. On the other hand, both C and B increase the hardenability of the weld heat affected zone (HAZ). Therefore, if the value of C + 100 × B exceeds 0.32%, the hardness of the HAZ becomes a problem, and the toughness decreases. The value of C + 100 × B is more preferably 0.30% or less. More preferably, it is 0.24% or less.

  Ca needs to be added in an amount of 0.0003% or more in order to form a Ca-based oxide. However, excessive addition generates coarse inclusions, so 0.0025% was made the upper limit. By setting the Ca content within this range, fine Ca-containing oxides can be dispersed, whereby reheated austenite in the HAZ region can be made finer and HAZ toughness can be ensured.

  N dissolves in the matrix and lowers its toughness, and also prevents the effect of B by bonding with B, so the upper limit is made 0.006%.

  Next, a preferable content range of the selective element will be described.

  Ni is effective for improving the strength and toughness of the steel material, and the effect can be obtained with a content of 0.001% or more. On the other hand, when the Ni content exceeds 0.9%, the YR increases, and the increase in the Ni content increases the manufacturing cost, so 0.9% was made the upper limit.

  Since Cr, Mo, and V have the effect of improving the strength and toughness of the steel, each is contained by 0.001% or more as necessary. On the other hand, excessive addition significantly decreases the HAZ toughness, so 0.3%, 0.04% and 0.05% were made the upper limits, respectively.

  Nb is an effective element for improving the strength and toughness of steel by improving hardenability, and is contained in an amount of 0.001% or more as necessary. On the other hand, excessive addition significantly reduces the HAZ toughness, so 0.05% was made the upper limit.

  Next, the metal structure which the steel plate of this invention has is demonstrated.

  The steel sheet of the present invention has a bainite fraction of 50% or more in a region excluding up to 5 mm below the plate surface in the plate thickness direction. If the bainite fraction of steel is 50% or more, even if it is quenched, it will not cause a difference in strength in the thickness direction, and even if the thickness is 70 mm or more, the strength can be increased over the entire thickness range. At the same time, YR can be reduced by the movable dislocation of the bainite structure. More preferably, the bainite fraction is 60% or more. For this purpose, by adjusting C and B so that the value of C + 100 × B is 0.15% or more, the bainite fraction can be 60% or more. More preferably, the bainite fraction is 70% or more. For this purpose, the value of C + 100 × B is set to 0.20% or more in the same manner as described above. In the region up to 5 mm below the plate surface, a structure mainly composed of bainite + martensite is formed.

In the present invention, the C in the steel 0.04% or more, B 0.0002% or more, the value of C + 100 × B 0.32% or less than 0.1%, after the end of hot rolling, Ar ₃ temperature By starting water cooling and quenching to 150 ° C. or less from the above, the bainite fraction can be 50% or more at any position in the plate thickness except directly below the plate surface. Thereby, even if the plate thickness is 70 mm or more, it has excellent large heat input joint toughness with a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less without quenching after tempering heat treatment. It becomes possible to provide a low YR high strength steel sheet.

  About the water cooling quenching of the steel plate after hot rolling, the cooling rate may be 4 to 7.5 ° C / sec, preferably 5 to 6.5 ° C / sec. About the steel plate temperature at the time of completion | finish of water cooling, what is necessary is just 150 degrees C or less, and what is necessary is just to normally water-cool to room temperature.

  Finally, the preferable size and number of oxide particles necessary for realizing reheated austenite refinement in the HAZ region and improving the HAZ toughness will be described.

  When the equivalent circle diameter of the oxide particles is smaller than 0.1 μm, the pinning effect gradually decreases, and when it is smaller than 0.005 μm, the pinning effect is hardly exhibited. Moreover, although oxide particles larger than 2.0 μm have a pinning effect, they may be a starting point for brittle fracture, and are therefore unsuitable for the properties of steel materials. From this result, the required particle size was set to 0.005 to 2.0 μm, and in particular, 0.1 to 2.0 μm.

As the number of oxide particles increases, the structural unit becomes finer, and as the number of particles increases, the HAZ toughness improves. However, the HAZ toughness required for steel materials differs in a complicated manner depending on the application, the welding method used, and the like. In order to satisfy the HAZ toughness required for high heat input welding with high strength shipbuilding steel, which is considered to have particularly strict requirements, for example, to satisfy the absorbed energy of 50 J or more at a test temperature of −40 ° C. The number of oxide particles having an equivalent diameter of 0.005 to 2.0 μm is preferably 100 / mm ² or more. However, as the number of particles increases, the effect of improving toughness decreases, and considering that increasing the number of particles more than necessary increases the possibility of generating coarse particles harmful to toughness, the upper limit of the number of particles Is suitably 3000 pieces / mm ² .

Measurement of the size and number of the oxide particles is performed, for example, in the following manner. An extracted replica is prepared from a steel plate as a base material, and the size and number of the oxides are measured by observing at least 1000 μm ² in an observation area with an electron microscope at 10,000 magnifications for 20 fields of view or more. For the measurement of the size, for example, the equivalent circle diameter is obtained based on a photograph of particles. At this time, an extraction replica collected from any part from the surface layer part to the center part of the steel sheet may be used. If the particles can be observed properly, the observation magnification may be lowered.

  According to the present invention, a steel sheet having a thickness of 70 mm or more has a low YR and high toughness with excellent heat input joints, with a proof stress of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less, without quenching. A tensile steel plate can be realized. For a steel plate having a thickness of at least 100 mm, a steel plate having the above-described quality can be realized by the present invention.

  Steel was melted in a converter, and deoxidized at the time of vacuum degassing by an RH vacuum degassing apparatus, and a steel having the components shown in Table 1 was made as a trial. After casting into a 280 mm-thick slab by continuous casting, after heating at 1150 ° C., it was cooled to room temperature through thick plate rolling under the conditions shown in Table 2 to obtain a steel plate having a thickness of 100 mm. S1 to S29 are steels of the present invention, and S30 to S51 are comparative steels. The obtained steel sheet was subjected to 1-pass electroslag welding using a general-purpose welding material. The heat input is about 900 kJ / cm.

  Table 3 shows the base material characteristics, bainite fraction, and HAZ toughness. The measurement of the base material characteristics and the bainite fraction was typically evaluated by the thickness center and the position of 1/4 of the thickness. For Charpy absorbed energy for HAZ toughness evaluation, six tests were conducted at a test temperature of 0 ° C. at the fusion line site, and the average value was entered.

  As is apparent from Table 3, the steels of the present invention of S1 to S29 have an excellent HAZ with a bainite fraction of 50% or more at both the center of the plate thickness and the position of 1/4 of the plate thickness, and an absorbed energy of 70 J or more. It can be seen that the steel is a high-strength steel having 0.2% proof stress (0.2% PS) of 440 MPa or more and tensile strength (TS) of 590 MPa or more, which has excellent low YR characteristics with toughness and YR of 80% or less.

  On the other hand, in S30 of the comparative example, C was too small, the bainite fraction was insufficient, and the base material strength was insufficient. In S31 and 34, C + 100 × B was excessive, and HAZ toughness was insufficient. In S32, C + 100 × B was too small, the bainite fraction was insufficient, and the base material strength was insufficient. In S33, B was too small, the bainite fraction was insufficient, and the HAZ toughness was insufficient. In S35, C was excessive and HAZ toughness was insufficient. S36 was deficient in Si, S38 was deficient in Mn, and both the base metal strength and HAZ toughness were deficient. S37 has excessive Si, S39 has excessive Mn, S40 has excessive Al, S41 has excessive Al, S42 has excessive Ti, S43 has excessive Ti, S44 has excessive Ca, S45 , Ca was excessive and both had insufficient HAZ toughness. In S46, Ni was excessive and YR was excessive. S47 had excessive Cr, S48 had excessive Mo, S49 had excessive V, and S50 had excessive Nb, both of which lacked HAZ toughness. In S51, the water cooling start temperature after rolling was too low, and the base metal strength and the HAZ toughness were insufficient.

Claims (5)

  In mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.35%, Mn: 0.50 to 2.50%, Al: 0.003 to 0.03%, Ti: 0.001 to 0.02%, B: 0.0002 to 0.003%, Ca: 0.0003 to 0.0025%, N: 0.006% or less, from the remaining Fe and inevitable impurities The value of C + 100 × B is 0.1% or more and 0.32% or less, the bainite fraction is 50% or more in the region excluding up to 5 mm below the plate surface in the plate thickness direction, and the plate thickness is 70 mm. A low YR high-tensile steel sheet characterized by the above.   Further, by mass%, Ni: 0.001 to 0.9%, Cr 0.001 to 0.3%, Mo: 0.001 to 0.04%, V: 0.001 to 0.05%, Nb: 0 The low YR high-tensile steel sheet according to claim 1, containing 0.001 to 0.05% of one kind or two or more kinds. The steel contains oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm in a number density per unit area of 100 to 3000 / mm ^2, and the composition of the oxide particles is at least Ca, Al, and O. The low YR high-tensile steel sheet according to claim 1 or 2, wherein the elements excluding O contain Ca: 5% or more and Al: 5% or more by mass ratio.   The low YR high-tensile steel sheet according to any one of claims 1 to 3, which has excellent large heat input joint toughness at a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less. A low YR high-tensile steel sheet, characterized in that after the hot rolling, the steel sheet having the component according to any one of claims 1 to 3 is subjected to water-cooling quenching from Ar ₃ temperature or higher and water-cooled to 150 ° C or lower. Manufacturing method.