JP2005068478A - Method for manufacturing thick steel plate with low yield ratio and high tension superior in toughness at heat-affected zone in super heavy-heat-input welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel plate with a low yield ratio and high tension, which has a TS of 590 MPa or higher, a superior toughness in a base metal, and a superior toughness at a heat-affected zone by welding with a super heavy heat input exceeding 400 kJ/cm. <P>SOLUTION: The base material of steel has a composition comprising 0.03-0.15% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.005-0.1% Al, 0.004-0.03% Ti, 0.0020-0.0070% N, 0.030% or less P, 0.0005-0.0030% S, 0.0005-0.0030% Ca, 0.0005-0.0030% B, 0.0050% or less O, and further one or more of 1.5% or less Cu and 2.0% or less Ni, while having Ceq so as to satisfy 0.35 or more and ACR satisfy 0.3 to 0.8. The manufacturing method comprises heating the base material to 1,000 to 1,300°C, hot rolling it, then air-cooling it to make a thick steel plate, subsequently reheating it to Ac3 or higher, quenching it at an average cooling rate of 1°C/s or higher, then heating it to the temperature of a two-phase region between (Ac1+10°C) and (Ac1+50°C), holding it, quenching it at an average cooling rate of 1°C/s or higher, and tempering it. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a thick steel plate suitable for use in construction, civil engineering, and bridges, and in particular, performs super-high heat input welding with a welding heat input exceeding 400 kJ / cm without causing toughness deterioration in the weld heat affected zone. The present invention relates to a low yield ratio high tensile steel plate having a tensile strength (TS) of 590 MPa or more and a yield ratio of 80% or less. In the present invention, the “thick steel plate” refers to a steel plate having a thickness of 30 mm or more.

  Steel materials used in various fields such as architecture, civil engineering, and bridges are generally finished into steel structures having a desired shape by welding. In such a steel structure, from the viewpoint of safety, the steel material used is required to have excellent weld heat affected zone toughness as well as base metal toughness.

  In recent years, with the increase in the size of building structures, improvement in welding efficiency is required from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, and the application range of large heat input welding is expanding. For example, for box columns for building structures, super-high heat input welding, such as submerged arc welding and electroslag welding, in which welding heat input exceeds 400 kJ / cm is applied.

  In recent years, improvement in earthquake resistance of building structures has been demanded, and the welded joints of building structures are also required to have high toughness. For example, the column-beam joint is required to have high toughness such that the Charpy absorbed energy at 0 ° C. exceeds 70 J. Moreover, the same request | requirement also exists in the welding part of a box pillar.

  In general, when high heat input welding is applied to a steel material, the most serious problem is toughness deterioration in the bond portion of the weld heat affected zone (hereinafter also referred to as HAZ). In the bond part, the austenite grains are most likely to be coarsened when exposed to a high temperature immediately below the melting point during high heat input welding, and are easily transformed into a fragile upper bainite structure upon subsequent cooling. Brittle structures such as island martensite structures are easily generated. Such a structure formation causes a decrease in toughness in the bond portion.

  For such a problem, for example, in Patent Document 1, TiN and REM oxysulfide are combined and finely dispersed in steel to suppress coarsening of austenite grains, and toughness of high heat input welding HAZ is improved. Technology has been proposed. Patent Document 2 discloses that rare earth elements (REM) and Ti are added in combination to disperse fine particles in steel and prevent austenite grain growth, and toughness of weld bond part with heat input of 230 kJ / cm. A technique for improving the above is disclosed.

  Patent Document 3 proposes a technique for finely dispersing Ti oxide to increase the toughness of high heat input welding HAZ. Patent Document 4 proposes a technique for increasing the toughness of high heat input welding HAZ by combining fine dispersion of Ti nitride and BN precipitation after reducing the amount of dissolved B.

  Patent Documents 5 and 6 propose a technique for improving the toughness of high heat input welding HAZ by finely dispersing Ti oxide and using it as a nucleation site for ferrite transformation.

  Patent Document 7 proposes a technique for improving the toughness of high heat input welding HAZ by using BN precipitated on TiN or the like in the cooling process during welding as a core of ferrite transformation.

  Further, Patent Document 8 contains Ti and a sufficient amount of Al (0.05 to 0.10% by mass) in order to thoroughly reduce solid solution N, and further utilizes Ca oxide as a fine oxide, which is extremely large. Techniques for improving HAZ toughness in heat input welding have been proposed.

  Furthermore, Patent Document 9 proposes a technique for improving the toughness of the high heat input welding HAZ by controlling the form of sulfide by adding Ca. Patent Document 10 proposes a technique for improving toughness of high heat input welding HAZ by adding REM and controlling the form of sulfide.

  However, the conventional technology using the Ti oxide described above involves considerable difficulty in uniformly and finely dispersing the oxide. For this reason, various studies have been made to improve the dispersibility by combining oxides. However, in high heat input welding where the heat input exceeds 400 kJ / cm, it has been difficult to control the growth of austenite grains so far, and the super high heat input welding HAZ is made stable and highly tough. It was difficult.

  In addition, when high heat input welding exceeding 400 kJ / cm is applied to the steel materials manufactured by the prior art mainly using TiN, the HAN is exposed to the high temperature region where TiN dissolves for a long time. There is a problem that the effect of controlling the coarsening of crystal grains is lost, and the super-high heat input welding HAZ cannot be toughened. In addition, the above-described conventional technique has a problem that an embrittled structure is generated due to an increase in solute Ti and solute N, and the HAZ toughness may be significantly reduced.

In addition, the technique described in Patent Document 8 uses ultra-high heat input welding by utilizing an oxide having a reduction effect of solid solution N that adversely affects toughness and a grain coarsening control effect even in a high temperature region near the melting point. HAZ toughness is improved and is characterized by containing Al excessively. However, a large amount of Al added to the weld metal affects the deoxidation reaction, resulting in a problem that the weld metal part toughness is lowered.
Japanese Patent Publication No. 3-53367 JP-A-6-184663 JP 57-51243 A JP-A-62-170459 JP-A-60-245768 JP 61-79745 JP 61-253344 JP Japanese Patent Laid-Open No. 2001-107177 JP 60-204863 A Japanese Patent Publication No. 4-14180

  Recently, with further increase in size of building structures, steel materials have been made thicker and stronger, and steel plates with a tensile strength of 590 MPa or more are being demanded. On the other hand, steel materials having a low yield ratio are required from the viewpoint of safety of steel structures, that is, prevention of embrittlement failure. By reducing the yield ratio, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

  The steel plate targeted by the present invention is a so-called low alloy steel for welding. For example, as a 590 MPa class high strength steel (SA440) for building structures, the chemical composition is mass%, C: 0.18% or less, Si: 0.55% or less, Mn: 1.60% or less, P: 0.035% or less, S: 0.008% or less, tensile strength of 590 to 740 MPa, yield ratio of 80% or less is the standard strength of the steel structure evaluation committee It is requested.

  However, in general, the yield ratio tends to increase as the strength of the steel sheet is increased, and thus there is a problem that it is extremely difficult to manufacture a thick steel sheet having a high strength at a low yield ratio. Furthermore, to date, it is difficult to say that a technology has been established that makes a super high heat input weld with a welding heat input exceeding 400 kJ / cm stable and tough.

  Therefore, tensile strength: 590 MPa or more, low yield ratio of 80% or less, and low yield ratio and high tensile strength with excellent HAZ toughness even when super high heat input welding exceeding 400 kJ / cm is applied. The development of thick steel plates has been desired.

  For example, Japanese Patent Application Laid-Open No. 59-211528 proposes a technique for obtaining a two-phase mixed structure of ferrite and hard phase by precipitating a part of ferrite by slow cooling after hot rolling is completed and then rapidly cooling. Yes.

  JP-A-5-125438 discloses that after the hot rolling is completed, controlled cooling is performed at a rate of 5 ° C./s or higher from the temperature above the Ac3 transformation point to a temperature range of 550 to 700 ° C. Has been proposed in which ferrite precipitation is suppressed and transformed in the subsequent air cooling process to obtain a ferrite + pearlite + bainite structure suitable for a low yield ratio.

  These technologies attempt to secure a low yield strength by the soft phase and a high tensile strength by the hard phase by making a mixed structure of a soft phase such as ferrite and a hard phase such as bainite. is there.

  However, each of the above-described techniques has a problem that the temperature difference in the plate thickness direction of the thick steel plate becomes large and the variation in material becomes large.

  The present invention advantageously solves the above-mentioned problems of the prior art, eliminates the non-uniformity of the mechanical properties in the thickness direction, has a low yield ratio of 80% or less, and a tensile strength (TS of 590 MPa or more). ), Excellent toughness of the base metal, and super high heat input welding with excellent weld heat affected zone toughness even when super high heat input welding with a heat input exceeding 400 kJ / cm is applied. It aims at proposing the manufacturing method of the low yield ratio high tension thick steel plate excellent in the heat affected zone toughness.

  The inventors of the present invention have intensively studied to achieve the above-mentioned problems, and as a result, it is possible to effectively suppress the coarsening of austenite grains by controlling the finely dispersed form of TiN by strict component adjustment, and at the time of welding. By stably dispersing transformation nuclei that promote ferrite transformation during the cooling process of steel, excellent toughness can be stably secured in the heat affected zone when high heat input welding with a heat input exceeding 400 kJ / cm is applied. I found out that I can do it. Furthermore, the present inventors contain appropriate amounts of Cu and Ni, which are effective elements for solid solution strengthening, and are heated to a two-phase temperature of about (Ac1 transformation point + 30 ° C.), held, and quenched. It has been found that by performing heat treatment, the tensile strength (TS) can be increased to 590 MPa or more and the yield ratio can be reduced to 80% or less without causing deterioration of the super high heat input welding HAZ toughness.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.15%, Si: 0.05-0.5%, Mn: 0.5-3.0%, Al: 0.005-0.1%, Ti: 0.004-0.03%, N: 0.0020-0.0070%, P : 0.030% or less, S: 0.0005 to 0.0030%, Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0030%, O: 0.0050% or less, Cu: 1.5% or less, Ni: 2.0% or less Contains one or two selected, and the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
The carbon equivalent Ceq defined by is satisfied with 0.35 or more, and the following formula (2)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
A steel material having a composition satisfying an ACR defined by 0.3 to 0.8 is heated to 1000 to 1300 ° C. and hot-rolled, and then subjected to a hot rolling step of air cooling to obtain a thick steel plate. A reheating and quenching step of reheating to a heating temperature above the Ac3 transformation point, holding at that heating temperature and quenching at an average cooling rate of 1 ° C / s or higher, then (Ac1 transformation point + 10 ° C) to (Ac1) A two-phase region heating and quenching process in which the steel is heated to a temperature in the two-phase region (transformation point + 50 ° C.) and held at the temperature, and then quenched at an average cooling rate of 1 ° C./s or more, and a temperature below the Ac1 transformation point. A low-yield ratio high-tensile steel plate with excellent tensile strength: 590 MPa and yield ratio: 80% or less Production method.
(2) In (1), in addition to the above composition, in addition to mass, one selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, V: 0.2% or less Or the manufacturing method of the low yield ratio high-tensile thick steel plate characterized by containing 2 or more types.
(3) In the above (1) or (2), in addition to the above composition, the composition further comprises one or two selected from REM: 0.02% or less and Mg: 0.005% or less. Yield ratio high tension steel plate manufacturing method.

  According to the present invention, a high-tensile thick steel plate having a tensile strength of 590 MPa or more, a low yield ratio of 80% or less, and an excellent weld heat-affected zone toughness of super-high heat input welding exceeding 400 kJ / cm, It can be manufactured stably without material variations, greatly contributes to increasing the size of steel structures, improving the earthquake resistance of steel structures and improving construction efficiency, and has a remarkable industrial effect.

  First, the reasons for limiting the composition of the steel material used in the present invention will be described. In addition, hereinafter, the “%” regarding the composition means “% by mass” unless otherwise specified.

C: 0.03-0.15%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In the present invention, it is necessary to contain 0.03% or more in order to obtain the effects described above. On the other hand, a content exceeding 0.15% deteriorates the toughness and weld crack resistance of HAZ. For this reason, C was limited to the range of 0.03-0.15%. In addition, Preferably, it is 0.03 to 0.12%.

Si: 0.05-0.50%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. However, if it exceeds 0.50%, the toughness of the base metal deteriorates, and island martensite is present in the super large heat input welding HAZ toughness. And HAZ toughness is significantly degraded. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably, it is 0.05 to 0.40%.

Mn: 0.5-3.0%
Mn has the effect of increasing the strength of the steel, and in the present invention, it is necessary to contain 0.5% or more in order to ensure a tensile strength of 590 MPa or more. On the other hand, if the content exceeds 3.0%, the base material toughness and the weld heat affected zone toughness deteriorate significantly. For this reason, Mn was limited to the range of 0.5 to 3.0%. In addition, Preferably it is 0.6 to 1.6%.

Al: 0.005-0.1%
Al acts as a deoxidizer and requires 0.005% or more in steelmaking. However, if it exceeds 0.1%, the toughness of the base metal is reduced and mixed into the weld metal during welding. , Deteriorate the weld metal toughness. For this reason, Al was limited to 0.005 to 0.1%. In addition, Preferably, it is 0.005-0.05%.

Ti: 0.004 to 0.03%
Ti has a strong affinity for N and precipitates as TiN during solidification, thereby suppressing the coarsening of austenite grains in the HAZ or contributing to the toughening of the HAZ as a ferrite transformation nucleus. Such an effect is recognized when the content is 0.004% or more. However, if the content exceeds 0.03%, the TiN particles become coarse, and the above-described effects cannot be expected. For this reason, Ti was limited to the range of 0.004 to 0.03%. In addition, Preferably, it is 0.005-0.020%.

N: 0.0020-0.0070%
N combines with Ti and precipitates as TiN to suppress coarsening of austenite grains in the HAZ, or contributes to high toughness of the HAZ as a ferrite transformation nucleus. In order to secure the necessary amount of TiN having such an effect, N needs to be contained in an amount of 0.0020% or more. On the other hand, if the content exceeds 0.0070%, in the region heated to a temperature at which TiN dissolves during welding, the amount of solute N increases and the toughness significantly decreases. For this reason, N was limited to 0.0020-0.0070%.

P: 0.030% or less P is an element that increases the strength of steel and degrades toughness. In particular, P is desirably reduced as much as possible in order to degrade the toughness of welds. Since this tendency becomes conspicuous when P exceeds 0.030%, it was set as the upper limit. In addition, since excessive reduction of P raises refining costs and is economically disadvantageous, it is preferable to make it 0.005% or more.

S: 0.0005-0.0030%
S combines with Ca and crystallizes finely as CaS particles in the solidification stage. S further precipitates as MnS on the CaS particles during welding, acts as a ferrite transformation nucleus, and has an effect of improving weld toughness. Such an effect is recognized when the content is 0.0005% or more. On the other hand, when it contains exceeding 0.0030%, the toughness of a base material and a welding part will deteriorate. For this reason, S was limited to 0.0005 to 0.0030%.

Ca: 0.0005 to 0.0030%
Ca is an element that contributes to improving the ductility of steel by controlling the form of sulfide. In order to exhibit such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0030%, the effect is saturated. For this reason, in this invention, Ca was limited to 0.0005 to 0.0030% of range. In the present invention, as will be described later, after adjusting the amount of dissolved oxygen immediately before Ca addition to 0.0050% or less, Ca is added to suppress Ca oxide and crystallize CaS. Since CaS crystallizes in molten steel at a lower temperature than oxides, it enables fine and uniform dispersion in the steel. Such fine CaS particles are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness.

B: 0.0005-0.0030%
B is an element that contributes to increasing the strength of steel in a small amount through improvement of hardenability. In order to obtain such an effect, the content of 0.0005% or more is required, but the content exceeding 0.0030% deteriorates the toughness of the weld heat affected zone. For this reason, B was limited to the range of 0.0005 to 0.0030%.

O: 0.0050% or less O is contained as an unavoidable impurity and exists as an oxide in steel, which lowers the cleanliness. For this reason, in this invention, it is preferable to reduce as much as possible. When the O content exceeds 0.0050%, CaO inclusions become coarse and adversely affect toughness. Further, in the present invention, in order to crystallize Ca as CaS, O having a strong binding force with Ca reinforces degassing or introduces a deoxidizer before adding Ca to add O in molten steel. It is preferable to reduce it to 0.0050% or less.

One or two selected from Cu: 1.5% or less, Ni: 2.00% or less
Cu and Ni are important alloying elements in the present invention, both of which are elements that contribute to increasing the strength of the base material, and optionally contain one or two kinds.

  Cu contributes to the strengthening of the base metal by solid solution strengthening and is an austenite stabilizing element. It is preferentially concentrated in the austenite phase during heating and holding at a temperature in the two-phase region, improving the hardenability of the austenite phase. Let On the other hand, since the Cu concentration in the ferrite phase decreases, the ferrite phase becomes softer. For this reason, by rapidly cooling (quenching) from the temperature in the two-phase region, a mixed structure of ferrite and martensite is obtained, and it is possible to achieve both high strength and a low yield ratio. Such an effect becomes remarkable when the Cu content is 0.1% or more. However, when the Cu content exceeds 1.5%, hot brittleness occurs and the surface properties of the steel sheet are deteriorated. For this reason, Cu was limited to 1.5% or less. In addition, Preferably it is 0.1 to 1.0%.

  Ni, like Cu, is a useful element that contributes to increasing the strength of the base metal by solid solution strengthening and improves the low temperature toughness of the base material. Further, Ni is an austenite stabilizing element, and is preferentially concentrated in the austenite phase during heating and holding at a temperature in a two-phase region, thereby increasing the hardenability of the austenite phase. Along with this, the Ni concentration in the ferrite phase decreases and the ferrite phase becomes softer. For this reason, by rapidly cooling (quenching) from the temperature in the two-phase region, a mixed structure of ferrite and martensite is obtained, and it is possible to achieve both high strength and a low yield ratio. Such an effect becomes remarkable by containing 0.1% or more of Ni. However, even if the Ni content exceeds 2.0%, the effect is saturated, so Ni is expensive and economically disadvantageous. For this reason, Ni was limited to 2.0% or less. In addition, Preferably it is 0.1 to 1.5%.

Ceq: 0.35 or more In the present invention, within the above component composition range, the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
The content of each component is adjusted so that the carbon equivalent Ceq defined by is 0.35% or more. If Ceq is less than 0.35%, the desired tensile strength: 590 MPa or more cannot be secured. Ceq is preferably 0.50% or less from the viewpoint of securing HAZ toughness. In the calculation of Ceq in equation (1), when there is an element not contained, the content of the element is assumed to be zero.

ACR: 0.3-0.8
In this invention, after adjusting the amount of dissolved oxygen in molten steel at the time of Ca addition to 0.0050% or less, the following formula (2)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
Ca and S are added and adjusted so that the ACR defined by the above satisfies 0.3 to 0.8. When ACR is less than 0.3, CaS does not crystallize, so S precipitates in the form of MnS alone. This MnS is stretched by rolling at the time of manufacturing the steel sheet and does not disperse uniformly and finely, so that the toughness of the base metal is reduced and does not contribute to the improvement of the welded HAZ toughness. On the other hand, when the ACR exceeds 0.8, S is fixed by Ca and S which becomes MnS is insufficient, and MnS does not precipitate on CaS which functions as a ferrite nuclei. Therefore, improvement in HAZ toughness cannot be expected. Only when the ACR satisfies 0.3 to 0.8 is a composite sulfide in which MnS is deposited on CaS. The presence of this composite sulfide functions as a ferrite transformation nucleus, refines the HAZ structure, and improves the HAZ toughness.

  In the present invention, in addition to the basic components described above, one or two selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less as necessary. One or two selected from the above and / or REM: 0.02% or less and Mg: 0.005% or less can be contained.

One or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, V: 0.2% or less
Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel, and can be selected and contained as necessary.

  Cr is preferably contained in an amount of 0.05% or more, but the content exceeding 0.7% deteriorates the HAZ toughness. For this reason, it is preferable to limit Cr to 0.7% or less.

  Mo is preferably contained in an amount of 0.05% or more, but the content exceeding 0.7% adversely affects the base material toughness and the HAZ toughness. For this reason, it is preferable to limit Mo to 0.7% or less.

  Nb is preferably contained in an amount of 0.005% or more, but the content exceeding 0.05% deteriorates the base metal toughness and the HAZ toughness. For this reason, it is preferable to limit Nb to 0.05% or less.

  V is preferably contained in an amount of 0.01% or more, but containing over 0.2% deteriorates toughness. For this reason, it is preferable to limit V to 0.2% or less.

One or two selected from REM: 0.02% or less and Mg: 0.005% or less
REM and Mg are both elements that contribute to the improvement of toughness, and can be selected and contained as necessary.

  REM is preferably contained in an amount of 0.002% or more, but the effect is saturated even if it exceeds 0.02%, so 0.02% was made the upper limit.

  Mg is a useful element that improves toughness through refinement of crystal grains, and it is preferable to contain 0.001% or more, but the effect is saturated even if it exceeds 0.005%, so 0.005% is the upper limit. It was.

  The balance other than the components described above is Fe and inevitable impurities.

  Control of inclusions by adding CaSi wire after controlling the gas components of molten steel melted by known methods such as converters, electric furnaces, vacuum melting furnaces, etc. using deoxidation and degassing processes In addition, the steel material (slab) having the above composition is obtained by a casting method such as a continuous casting method. In addition, in order to crystallize Ca as CaS during melting, O having a strong binding force with Ca reinforces degassing before adding Ca, or a deoxidizer is added, so that O in molten steel is removed. It is preferable to reduce it to 0.0050% or less.

  Next, preferably, the steel material (slab) obtained through the above-described steps is heated to 1000 to 1300 ° C., hot-rolled, and then subjected to a hot rolling step of air cooling to obtain a thick steel plate having a predetermined thickness.

  If the heating temperature of the steel material is less than 1000 ° C., the deformation resistance of the steel material is high, and a large amount of reduction per pass cannot be obtained. Therefore, the number of hot rolling passes increases, and the rolling efficiency decreases. In addition, when the heating temperature of the steel material is low, the amount of reduction per pass cannot be increased, and thus casting defects in the steel material may not be able to be crimped, increasing the risk of occurrence of internal defects. . On the other hand, when the heating temperature is higher than 1300 ° C, TiN precipitated during the solidification process becomes coarse due to Ostwald growth, and the effect of suppressing the coarsening of austenite grains near the weld joint during super-high heat input welding is lost. The toughness of the heat input weld HAZ decreases. For this reason, the heating temperature of a steel raw material shall be the temperature of the range of 1000-1300 degreeC.

  In the hot rolling step, air cooling is performed after the hot rolling is completed. In addition, it is preferable that the completion | finish temperature of hot rolling shall be more than Ac3 transformation point from a viewpoint of rolling efficiency and the yield ratio reduction of a base material. When the hot rolling end temperature is lower than the Ac3 transformation point, since ferrite is processed, the yield point rises and the yield ratio increases, which is not preferable.

The thick steel plate obtained by the hot rolling process is then subjected to a reheating and quenching process.
In the reheating and quenching step, the thick steel plate is first reheated to a heating temperature equal to or higher than the Ac3 transformation point. By reheating and holding the thick steel plate at a heating temperature equal to or higher than the Ac3 transformation point, a uniform austenite phase is formed even inside the thick steel plate. The upper limit of the heating temperature is not particularly specified, but is preferably 1100 ° C. or lower. If the temperature exceeds 1100 ° C., the surface properties of the steel sheet deteriorate. The holding time is preferably 1 h or less from the viewpoint of deterioration of the toughness of the base metal due to coarsening of austenite. Further, if the soaking in the heat treatment furnace is good, it may be held for a short time of about 10 minutes. The Ac3 transformation point is approximately the following formula: Ac3 (° C.) = 910−273C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb
(However, C, Mn, Ni, Cr, Mo, Cu, Nb: Content of each element (mass%))
Can be organized.

  The thick steel plate reheated and held at the heating temperature above the Ac3 transformation point is then quenched at an average cooling rate of 1 ° C./s or higher. Thereby, homogenization and refinement | miniaturization of the structure | tissue in a thick steel plate can be achieved. When the quenching cooling rate is less than 1 ° C./s, the above-described effects cannot be expected. The upper limit of the quenching cooling rate is not particularly limited, but is preferably 40 ° C./s or less from the viewpoint of the toughness of the base material. In addition, the cooling rate in hardening shall mean the average cooling rate to 300 degreeC in the 1 / 2T position of a thick steel plate.

  Next, the two-phase region heat quenching step is performed on the thick steel plate subjected to the reheating quenching step.

  In the two-phase region heating and quenching process, the thick steel plate is first heated to a temperature in the two-phase region of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 50 ° C.). By heating to a temperature in the two-phase region from (Ac1 transformation point + 10 ° C) to (Ac1 transformation point + 50 ° C), strengthening elements such as C, Mn, Cu and Ni are concentrated in the austenite phase, and the ferrite phase is diluted and softened. This phenomenon is efficiently promoted. Thereby, the structure after quenching becomes a mixed structure of a hard martensite phase and a soft ferrite phase, and a thick steel plate having both high strength and a low yield ratio can be obtained. On the other hand, when the heating temperature is less than (Ac1 transformation point + 10 ° C.), the austenite phase fraction is low, the martensite phase fraction in the structure after quenching is small, and the structure is mainly composed of ferrite, and the desired high strength cannot be secured. In addition, when the heating temperature exceeds (Ac1 transformation point + 50 ° C), the austenite phase fraction becomes too high, the concentration of alloy elements such as Cu and Ni in the austenite phase is diluted, and the structure after quenching becomes bainite. It becomes a main structure, and the desired high strength and low yield ratio cannot be secured. For this reason, the heating temperature in the two-phase region heating and quenching step is limited to a temperature in the range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 50 ° C.). The holding time at the heating temperature is not particularly specified, but it is preferably 10 min or more and 1 h or less in order to make the temperature uniform in the steel sheet and to suppress variation in characteristics.

  The thick steel plate heated and held at the above-described two-phase heating temperature is then quenched at an average cooling rate of 1 ° C./s or more. When the quenching cooling rate is less than 1 ° C./s on average, the austenite phase is transformed into a phase other than the martensite phase, and a desired high strength cannot be ensured. The upper limit of the quenching cooling rate is not particularly defined, but is preferably 40 ° C./s or less from the viewpoint of the toughness of the base material.

  The thick steel plate that has been subjected to the two-phase region heating and quenching step is then subjected to a tempering step.

In the tempering process, the thick steel plate is tempered at a temperature below the Ac1 transformation point. By tempering below the Ac1 transformation point, toughening of the brittle hard phase produced by quenching can be achieved. In addition, it is preferable that tempering temperature shall be 300-650 degreeC. If the tempering temperature is less than 300 ° C, a long-time heat treatment is required and the efficiency is poor, and if it exceeds 650 ° C, the strength of the base material is significantly reduced. The holding time at the tempering temperature is not particularly limited as long as it can achieve toughening of the brittle hard phase generated by quenching and ensure a desired strength, but it is preferably about 60 min. Incidentally, Ac1 transformation point, Ac ₃ transformation point shall be determined by measuring the transformation expansion curve.

  Hereinafter, the present invention will be described in more detail based on examples.

  A steel plate (slab) adjusted to the composition shown in Table 1 by a converter-ladder refining-continuous casting method was used to form a thick steel plate having a thickness shown in Table 2 by a hot rolling process under the conditions shown in Table 2. Subsequently, these thick steel plates were subjected to a reheating quenching process, a two-phase region heating quenching process, and a tempering process under the conditions shown in Table 2. In addition, the tempering process was not given to some thick steel plates.

  JIS No. 4 tensile test specimens were sampled from the position of the thickness ¼ of each thick steel plate obtained, and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to investigate the tensile characteristics. In addition, V-notch test specimens were collected from ¼ position of each thick steel plate obtained according to JIS Z 2202, and Charpy impact test was conducted according to JIS Z 2242. The absorbed energy (vE0) at 0 ° C. was determined, and the base metal toughness was evaluated.

  In addition, a test plate for joints (size: 400 x 600 mm) was sampled from each thick steel plate obtained, and electroslag welding (welding heat input: 1000 kJ / cm) with a groove shape as shown in Fig. 1 was performed. A welded joint was prepared.

  From the obtained welded joint, a V-notch test piece having a notch position as a bond portion as shown in FIG. 2 is collected and subjected to a Charpy impact test at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 2242. Then, the absorbed energy (vE0) at 0 ° C. of the welded joint bond part was obtained and the joint toughness was evaluated.

  The obtained results are shown in Table 3.

  All of the examples of the present invention have a tensile strength of 590 MPa or more, a yield ratio of 80% or less, a high strength of vE0 of 100 J or more, a low yield ratio, and a tough base material, and a heat input of welding of 1000 kJ / cm. Even when super-high heat input welding is applied, vE0 of the bond portion is 100 J or more and has excellent weld heat affected zone toughness. On the other hand, in a comparative example that is out of the scope of the present invention, any of the base material strength, base material yield ratio, base material toughness, weld heat affected zone toughness, or a plurality of characteristics do not satisfy the target value.

It is explanatory drawing which shows the groove shape of the electroslag welded joint used in the Example. It is explanatory drawing which shows the extraction | collection point of the Charpy impact test piece from the electroslag welded joint part in an Example.

Claims (3)

% By mass
C: 0.03-0.15%, Si: 0.05-0.5%,
Mn: 0.5-3.0%, Al: 0.005-0.1%,
Ti: 0.004 to 0.03%, N: 0.0020 to 0.0070%,
P: 0.030% or less, S: 0.0005 to 0.0030%,
Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0030%,
O: 0.0050% or less, Cu: 1.5% or less, Ni: 2.0% or less, one or two selected from the following, and the carbon equivalent Ceq defined by the following formula (1) A steel material having a composition satisfying 0.35 or more and having an ACR defined by the following formula (2) satisfying 0.3 to 0.8 is heated to 1000 to 1300 ° C., hot-rolled, and then air-cooled. A reheating and quenching step in which the thick steel plate is reheated to a heating temperature equal to or higher than the Ac ₃ transformation point and held at the heating temperature and then quenched at an average cooling rate of 1 ° C./s or higher. Then, the mixture is heated to a temperature in the two-phase region of (Ac ₁ transformation point + 10 ° C.) to (Ac ₁ transformation point + 50 ° C.), held at the temperature, and then quenched at an average cooling rate of 1 ° C./s or more. A phase region heating and quenching step, and a tempering step of further tempering at a temperature below the Ac ₁ transformation point are sequentially performed, A method for producing a high-tensile steel plate with a low yield ratio and high tensile strength that has excellent tensile strength: 590 MPa or more, yield ratio: 80% or less, and excellent heat-affected zone toughness.
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%) In addition to the above composition, the composition further contains one or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less in terms of mass%. The method for producing a low-yield ratio high-tensile thick steel plate according to claim 1. 3. The low content according to claim 1, further comprising one or two kinds selected from REM: 0.02% or less and Mg: 0.005% or less in mass% in addition to the composition. Yield ratio high tension steel plate manufacturing method.

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