KR20150038664A - Thick steel sheet having excellent welding heat-affected part toughness - Google Patents

Abstract

The steel sheet of the present invention satisfies a predetermined chemical composition and contains constituent elements other than oxygen in an amount of 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40% Wherein the oxide satisfies the following conditions:% <REM <50%, 2% <Zr <30%, 1.0 ≤ REM / Zr, A diameter of circle equivalent diameter of 20 nm or more exists at a density of 1.0 x 10 ⁶ / mm 2 or more, and a relation of | da-df | / da? 0.35.

Description

{THICK STEEL SHEET HAVING EXCELLENT WELDING HEAT-AFFECTED PART TOUGHNESS}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate to be applied to a welded structure such as a bridge, a high-rise structure, a ship, a line pipe, and the like, and more particularly to a steel plate having excellent toughness of a heat affected zone (hereinafter also referred to as a HAZ) Steel plate.

In recent years, with the enlargement of welded structures such as bridges, high-rise buildings, ships, and line pipes, a post-steel plate having a thickness of 50 mm or more is often applied to such a welded structure, Welding of steel plates is inevitable. As described above, there is a demand for large-volume heat welding for the purpose of improving efficiency of welding construction.

However, since the HAZ at the time of welding with large heat is maintained for a long time in the high-temperature austenite (gamma) region by heating and then gradually cooled, the coarsening of the coarse ferrite (?) Particles It is likely to cause coarsening of the structure represented by the generation, which is a cause of the deterioration of the toughness of the HAZ at the time of heat welding. Therefore, it is a necessary task to develop a technique for stably maintaining the toughness (hereinafter also referred to as HAZ toughness) of the HAZ at a high level during welding at a large heat input.

As means for securing HAZ toughness, there has been proposed a technique relating to refinement of the structure by formation of alpha in the grains based on inclusion particles as a starting point by? -Incorporation pinning by inclusion particles such as oxides, nitrides, sulfides and the like . As a proposal of such a technique, there is a technique described in Patent Document 1 or Patent Document 2, in which a fine Ti nitride is dispersed and precipitated as? Grain growth pinning particles in a steel material, It is disclosed that suppressing conversation and suppressing deterioration of HAZ toughness. However, Ti nitride tends to disappear if the heat of welding is increased, and special design is required to obtain stable HAZ toughness.

The inventors of the present invention also proposed a technique for improving the HAZ toughness of the substitution heat by precisely controlling the size and the number of the fine Ti nitrides in Patent Document 3. However, the assumed heat input is only 55 kJ / mm, and further improvement is required in order to cope with an increase in the heat input of a further layer of welding.

In Patent Documents 4 to 7, there is proposed a technique of using an oxide inclusion stable at high temperature as a? -Growth fin pinned particle. However, since the number of oxide inclusions is smaller than that of the Ti-containing nitride, a sufficient pinning effect can not be obtained, and thus it is impossible to sufficiently cope with large-volume heat welding, and further improvement is required.

That is, Patent Document 4 discloses that good HAZ characteristics are obtained by the presence of an oxide including REM and Zr. However, since the assumed heat input is low and the good HAZ characteristics are necessarily obtained by the large heat welding I can not say. Patent Literature 5 discloses a technique using an oxide including REM or Zr as in Patent Document 4, and evaluates Charpy absorbed energy as HAZ toughness. However, from the viewpoint of reliability of the material, It is considered necessary to guarantee the minimum value at a high level.

Further, Patent Document 6 discloses a technique of obtaining high HAZ toughness by using both oxide-based inclusions and Ti-containing inclusions as? Grain growth pinning particles. However, in Patent Document 6, the HAZ toughness is evaluated by a heat cycle test simulating large heat welding, but the maximum heating temperature is 1400 deg. C and a part of the Ti-containing nitride remains. However, the maximum heating temperature of the HAZ becomes a high temperature exceeding a part of 1450 DEG C, and the disappearance of the Ti-containing nitride is further promoted. Therefore, in order to accurately evaluate the HAZ toughness of the substitution heat HAZ, it is preferable to actually perform the substitution heat welding test. Further, the inventors of the patent document 7 propose a technique utilizing the? -Dickening pin fixing effect of the fine oxide based inclusions, but this technique is a technique using the suppression of re-precipitation of the fine Mn sulfide and is a technique in which dissolved oxygen amount, The amount of the alloy to be added is determined based on the amount of the alloy.

In addition, as a technology relating to refinement of the structure by the generation of? In the particle starting from the inclusion particles, there is proposed a technique using the composite oxide containing Ti or REM and MnS described in Patent Document 8, Patent Document 9 proposes a technique for promoting the generation of? In the particle by controlling the inclusion shape. These techniques are based on the assumption that an inclusions having a low interfacial energy (particle in? / Inclusions in the particle) is effective for? Production in the particle. However, when α in the particle is produced, the contribution of the interfacial energy (α / γ in the particle) is large, and the generation of α in the particle can not be sufficiently obtained merely by lowering the interface energy (α in the particle / inclusion) It does not reach enough to insure sufficient heat of HAZ toughness in the heat of substitution.

In addition, the inventors have constructed a high HAZ toughness technique utilizing α generation in the particle at the starting point of the oxysulfide, and propose it as Patent Document 10. However, since it is necessary to disperse oxalic acid grains having a comparatively large size of 2 mu m or more as a target at a certain number of times, this technique does not sufficiently ensure the HAZ toughness of the substitution thermal HAZ. That is, in the technology described in Patent Document 8, the assumed heat input amount is small, and even in the techniques described in Patent Document 9 and Patent Reference 10, the average value of the Charpy absorbed energy is high, but there is room for improvement in the minimum value, to be.

In addition, the inventors of the present invention have proposed a technology that can obtain a high HAZ toughness by dispersing an oxide controlled by a tissue as Patent Document 11 and Patent Document 12. With this technique, it was possible to realize a steel sheet having excellent toughness of the weld heat affected zone, but there still remains a problem to be improved in manufacturing.

In the technique described in Patent Document 11, Ca addition amount is controlled based on the amount of dissolved oxygen before Ca addition in order to realize a predetermined oxide form, and at the same time, the time from Ti addition to Ca addition is required to be within 3 to 20 minutes There is a concern that the burden on the operator increases. On the other hand, in the technique described in Patent Document 12, since it is necessary to keep Ca addition for 40 to 90 minutes from initiation of injection, productivity is improved.

Japanese Patent Laid-Open No. 2001-98340 Japanese Patent Application Laid-Open No. 2004-218010 Japanese Patent Application Laid-Open No. 2010-95781 Japanese Patent Laid-Open No. 2001-20031 Japanese Patent Application Laid-Open No. 2007-247005 Japanese Patent Application Laid-Open No. 2008-223062 Japanese Patent Application Laid-Open No. 2009-179844 Japanese Patent Laid-Open No. 7-252586 Japanese Patent Application Laid-Open No. 2008-223081 Japanese Patent Application Laid-Open No. 2009-138255 Japanese Patent Application Laid-Open No. 2010-168644 Japanese Patent Laid-Open Publication No. 2011-219797

The present invention has been made in view of the above conventional circumstances, and it is an object of the present invention to provide a welding machine capable of improving not only the average value of HAZ toughness but also the minimum value thereof, And to provide a superior steel sheet.

The invention according to claim 1 is a ferritic stainless steel comprising, by mass%, 0.03 to 0.12% of C, 0.10 to 0.25% of Si, 1.0 to 2.0% of Mn, 0.03% 0.004 to 0.05%, Ti: 0.010 to 0.050%, REM: 0.0003 to 0.02%, Zr: 0.0003 to 0.02%, Ca: 0.0005 to 0.010%, N: 0.002 to 0.010 Ti <40%, 5% <Al <30%, 5% <Ca <40%, and the balance elements are iron and inevitable impurities. %, REM <50%, 2% <Zr <30%, 1.0 ≤ REM / Zr, and the oxides having a circle- Mm2 or more, oxides having a circle-equivalent diameter of 2 占 퐉 or more are present at 100 / mm2 or less, and among the contained Ti nitrides, the number of Ti nitrides having a circle equivalent diameter of 1 占 퐉 or more is 7 / mm2 or less, Or more of Ti nitride is 1.0 x 10 ⁶ / mm 2 or more and the size of the Ti nitride having the circle equivalent diameter of 20 nm or more is divided in the order of 5 nm from the one having the circle equivalent diameter of 20 nm, (Di = 25, 30, 35, ... 500), and the di in the region where the number density of Ti nitrides present in each region was the greatest was set to df And the average circle-equivalent diameter da of the Ti nitride having a circle-equivalent diameter of 20 nm or more and less than 500 nm is | da-df | / da? 0.35. It is a steel sheet with excellent toughness.

The circle equivalent diameter described in the present invention, including the above-described substrate, refers to a circle obtained by determining the diameter of a circle so that the area of the oxide is equal to the area of the oxide and Ti nitride, And can be obtained by observation with a scanning electron microscope (SEM).

The invention according to claim 2 is characterized in that the constituent elements other than oxygen contain 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50% And the oxide satisfying 2% < Zr < 30% and 1.5 REM / Zr is more than 300 pieces / mm2.

The invention according to claim 3 is characterized in that, in mass%, Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50%, Nb: 0.002 to 0.10% 0.10%, and B: 0.0005 to 0.0050%. The welded heat affected zone according to claim 1 or 2, further comprising at least one member selected from the group consisting of:

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to improve the average value and the minimum value of the HAZ toughness not only by small to medium heat input welding but also by large heat welding, and it is possible to improve the toughness of the weld heat affected zone, Can be obtained.

The inventors have searched for a means for improving the heat transfer HAZ toughness of the post-steel sheet under relatively highly productive manufacturing conditions. As a result, it is possible to secure the production of? In the grain at the oxide origin and to suppress the generation of coarse Ti nitride which is a HAZ toughness inhibiting factor and appropriately control the Ti nitride dispersed form to improve the productivity of the after- Can be compatible with each other. That is, by controlling the oxide composition appropriately, it is possible to secure the generation of? In the grain, and to control the size and the number of Ti nitrides appropriately and suppress the coarsening of the spherical? The intergranular ferrite can be made finer, so that it is possible to obtain a post-steel sheet having an excellent heat transfer HAZ toughness.

More specifically, oxides having a circle-equivalent diameter of less than 2 占 퐉 are dispersed in an oxide of 300 / mm2 or more, and oxides having a circle-equivalent diameter of 2 占 퐉 or more are suppressed to 100 / It was confirmed that excellent HAZ toughness was obtained.

The present invention has been completed on the basis of the above-described findings, but the reason for defining each constituent requirement is as follows.

(Constituent elements other than oxygen are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30 %, 1.0? REM / Zr, and the number of oxides whose circle equivalent diameter is less than 2 占 퐉 is 300 pieces / mm2 or more)

When the circle-equivalent diameter of the oxide is less than 2 mu m, HAZ toughness can be promoted by alpha promoting in the grain. When the circle equivalent diameter of the oxide is 2 占 퐉 or more, the barrier energy when the coarse Ti nitride is crystallized is lowered and the amount of coarse Ti nitride is increased. The composition of the oxide preferably includes 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30% , And if it is deviated from the range of 1.0 ≤ REM / Zr, sufficient α generation in the particle can not be obtained. Further, by setting the REM / Zr ratio (mass%) in the oxide to 1.5 or more, the amount of coarse-pit Ti nitride formed on the surface of the oxide in the molten steel is further reduced, and further excellent HAZ toughness is realized.

(100 pieces of oxide having a circle-equivalent diameter of 2 탆 or more / mm 2 or less)

Of the oxides satisfying the above-mentioned composition, oxides having a circle equivalent diameter of 2 탆 or more induce brittle fracture and deteriorate HAZ toughness, so that it is preferable that the oxide is as small as possible. From this viewpoint, in the present invention, the number of oxides having a circle-equivalent diameter of 2 탆 or more is defined as 100 pieces / mm 2 or less.

In the present invention, the shape of the Ti nitride is also defined in detail. Ti nitride suppresses coarsening of y particles at the time of HAZ high temperature heating and contributes to improvement of HAZ toughness by reducing the size of intergranular ferrite produced at the time of cooling. In order to sufficiently suppress the γ-particle coarsening, it is naturally necessary to disperse a large amount of Ti nitride particles. However, the inventors of the present invention have found that as the size of the Ti nitride particles is uniformly closer, the dissolution rate of the Ti nitride at the HAZ high- , And by controlling the size and the number of the Ti nitrides appropriately, it is made clear that the effect of suppressing coarsening of the? Grain is obtained even by the heat welding. Concretely, by satisfying the following two conditions, a high substitution heat HAZ toughness is realized.

(7 pieces / mm &lt; 2 &gt; or less of Ti nitride having a circle equivalent diameter of 1 mu m or more)

If the number of Ti nitrides having a circle-equivalent diameter of 1 탆 or more exceeds 7 / mm 2, brittle fracture is promoted and HAZ toughness is deteriorated. Such a Ti nitride has not only a rectangular parallelepiped shape but also a characteristic of remarkably deteriorating HAZ toughness due to stress concentration because it is remarkably harder than steel. Therefore, the coarse Ti nitride needs to be more strictly controlled than the coarse oxide.

(The Ti nitride having a circle equivalent diameter of 20 nm or more is 1.0 x 10 ⁶ / mm 2 or more)

When the Ti nitride having a circle-equivalent diameter of 20 nm or more is less than 1.0 x 10 ⁶ pieces / mm 2, the Ti nitride particles necessary for suppressing coarsening of the 粒 particles are not secured. Further, the ultrafine Ti nitride particles having a circle-equivalent diameter of less than 20 nm are lost in a short time in high-temperature heating at the time of high-temperature heat welding, and do not contribute to suppression of? Particle coarsening, and therefore no particular control is required.

(| da-df | / da? 0.35)

The smaller the size of the Ti nitride particle, the more unstable it is in terms of energy. Specifically, the smaller the size of the Ti nitride particle than the average size of the whole particles, the easier the Ti nitride particle is lost during the HAZ high temperature heating. Therefore, the larger the number of the Ti nitride particles having a size relatively close to the average size, the larger the number of particles contributing to the suppression of the coarsening of the? Particles, the larger the average size, or the smaller the average size.

In the present invention, by controlling the difference between the size and the average size of the recorded maximum number of Ti nitrides in the size-number histogram of the Ti nitride, the substantial number of the Ti nitride particles increases, Is realized.

Specifically, the size of the Ti nitride having a circle-equivalent diameter of 20 nm or more is divided into regions each having an interval of 5 nm from the region having a circle-equivalent diameter of 20 nm, and the range of the circle- -5) and not more than di (where, di = 25, 30, 35, ... 500), and the di in the region where the number density of Ti nitrides existing in each region is the largest is df , And the average circle equivalent diameter da of the Ti nitride having a circle equivalent diameter of 20 nm or more and less than 500 nm. By this control, the substantial number of Ti nitride particles increases, and a high? -Particle coarsening suppressing effect is realized.

Further, at the time of calculating the average circle equivalent diameter of the Ti nitride, transmission electron microscope (TEM) observation was carried out under the conditions described in the column of Examples to be described later, and the area of each Ti nitride in this observation field And the circle equivalent diameter of each Ti nitride was calculated from the area, and then the arithmetic mean of the circle equivalent diameters was calculated for Ti nitride having a circle equivalent diameter of 20 nm or more and less than 500 nm.

Specifically, if the value obtained from | da-df | / da exceeds 0.35, even if the number of Ti nitride particles is large,? Grain coarsening can not be sufficiently suppressed, and high substitution thermal HAZ toughness can not be obtained.

(Manufacturing method)

The steel sheet according to the present invention satisfying the above requirements, that is, the constituent elements other than oxygen, contains 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40% Wherein the oxide containing oxides satisfying the following relationships:% <REM <50%, 2% <Zr <30%, 1.0 ≤ REM / Zr, The number of oxides having a circle-equivalent diameter of 2 탆 or more is 100 / mm 2 or less, and among the contained Ti nitrides, Ti nitride having a circle equivalent diameter of 1 탆 or more is 7 / mm 2 or less, Is not less than 1.0 x 10 &lt; ⁶ &gt; / mm &lt; 2 &gt;, and further satisfies the relational expression | da-df | / da? 0.35, There is a need.

The production requirements are as follows: the amount of dissolved oxygen in the molten steel is made 0.002 to 0.01% by mass% by deoxidation using Mn, Si or the like in the solvent, and then the order of Al → Ti → (REM, Zr) → Ca The time t1 from the addition of REM or Zr to the addition of Ca is controlled to be 5 minutes or more while adding the respective elements and the cooling time t2 in the temperature range of 1500 to 1450 DEG C 300 seconds or less, and the cooling time t3 in the temperature range of 1300 to 1200 ° C at the time of casting may be set to 680 seconds or less. Further, by setting the ratio [REM] / [Zr] of the REM addition amount [REM] / Zr addition amount [Zr] to 1.8 or more and the t1 to 10 minutes or more, a more appropriate oxide form can be realized, By further decreasing the coarsely crystallized Ti nitride to be produced, a further excellent HAZ toughness can be obtained. Next, reasons for defining these manufacturing requirements will be described in detail. Here, (REM, Zr) means that both REM and Zr may be simultaneously added, and either of them may be added first.

The amount of dissolved oxygen in the molten steel is reduced to 0.002 to 0.01% by deoxidation using Mn, Si,

If the dissolved oxygen amount is less than 0.002%, it becomes impossible to secure a required amount of an oxide having an appropriate composition, which is a starting point of α generation in the particle. When the dissolved oxygen amount exceeds 0.01%, coarse oxides having a circle equivalent diameter of 2 탆 or more are increased, and HAZ toughness is deteriorated.

ㆍ Time t1 from addition of REM or Zr to Ca addition is 5 minutes or more

The oxide specified by the present invention has a feature that it has a function of promoting the generation of? In the grain and hardly functions as a starting point of coarse Ti nitride. In particular, in order to set the REM / Zr ratio (mass%) in the oxide to 1.0 or more, it is necessary to sufficiently advance the oxide formation reaction of REM or Zr prior to the addition of Ca as the strong acid element. Specifically, by controlling the time t1 from the addition of REM or Zr to the addition of Ca to 5 minutes or more, it is possible to obtain an oxide satisfying REM / Zr? 1.0 of a predetermined number density. If the time t1 from the addition of REM or Zr to the addition of Ca is less than 5 minutes, the oxide satisfying REM / Zr? 1.0 becomes insufficient. In addition, in addition to this, by setting the ratio [REM] / [Zr] as the mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr] to 1.8 or more and t1 to 10 minutes or more, oxides satisfying REM / The number density of which can be obtained.

Further, in the case of the solvent, the addition of Al → Ti → (REM, Zr) → Ca is carried out in the order of Al → Ti → Ca. When each element is added in a sequence other than the order of addition, an appropriate composition The necessary amount of oxide can not be secured. Particularly, since Ca is a strong acid element having a strong deoxidizing ability, when oxygen is added before Ti or Al, the amount of oxygen associated with Ti or Al is remarkably reduced.

ㆍ Cooling time t2 at 1500 to 1450 ℃ during casting is less than 300 seconds

When the cooling time t2 at 1500 to 1450 DEG C during casting exceeds 300 seconds, the coarse oxide increases. Or, the coarse Ti nitride is crystallized by segregation of components during solidification, and HAZ toughness is deteriorated.

ㆍ When the cooling time t3 in the temperature range of 1300 to 1200 ℃ during casting is less than 680 seconds

If the cooling time t3 in the temperature range of 1300 to 1200 占 폚 during casting exceeds 680 seconds, the relationship | da-df | / da? 0.35 can not be satisfied. The cause can be thought as follows.

In the Ti nitride to be formed during casting, there are: Ti nitride to be crystallized in molten steel; B. Ti nitride produced in the solidification segregation portion of the solidified steel; and Ti nitride produced in the non-solidified segregation portion of the solidified steel. → B → C, and the size (particle diameter) is in the order of A> B> C. On the other hand, the number of particles is in the order of A < B < C, and most of the size Ti nitride corresponding to the df is Ti nitride of C. Further, since the number of particles of Ti nitride of A is smaller than that of B and C of Ti nitride, the average circle equivalent diameter da of the Ti nitride is hardly affected. Therefore, in order to enter | da-df | / da within a predetermined range, it can be said that it is necessary to control the generation of B and C Ti nitrides. If the cooling time t3 in the temperature range of 1300 to 1200 占 폚 at the time of casting exceeds 680 seconds, since the Ti nitride of B grows prior to the generation of the Ti nitride of C, the average circle equivalent da diameter It is supposed that | da-df | / da exceeds 0.35.

(Chemical composition)

Next, the chemical composition of the steel sheet of the present invention will be described. The post-steel sheet of the present invention is capable of satisfactory properties and HAZ of the base material (post-steel sheet) unless the content of each chemical component (element) is within an appropriate range, although the above-described dispersion state of oxides and the like are appropriate. Therefore, in the post-steel sheet of the present invention, the content of each chemical component is also within the range described below. Of these chemical components, the content of Al, Ca, Ti, etc. constituting the oxide includes an amount constituting the oxide as apparent from the action and effect thereof. Incidentally, the content (%) of the following chemical components all represent% by mass.

C: 0.03 to 0.12%

C is an essential element for securing the strength of the steel sheet. If the content of C is lower than 0.03%, the necessary strength can not be secured. On the other hand, if the content of C is excessive, a large amount of hard island-shaped martensite (MA) is produced, resulting in toughness deterioration of the base material. Therefore, the content of C should be 0.12% or less. The preferable lower limit of the C content is 0.04%, and the preferable upper limit is 0.10%.

Si: 0.10 to 0.25%

Si is an element which improves the activity of Ti and needs to be appropriately added in order to realize a predetermined Ti nitride form. When the addition amount is less than 0.10%, the number density of Ti nitrides having a circle equivalent diameter of 20 nm or more can not be ensured to be 1.0 x 10 ⁶ / mm 2. On the other hand, if it exceeds 0.25%, coarse Ti nitride tends to be formed, and a hard MA structure is formed, and a predetermined HAZ toughness can not be obtained. A preferable lower limit is 0.12%, a more preferable lower limit is 0.14%, a preferable upper limit is 0.22%, and a more preferable upper limit is 0.20%.

Mn: 1.0 to 2.0%

Mn is an element useful for securing the strength of the steel sheet. In order to effectively exhibit such effects, it is necessary to contain not less than 1.0%. However, if it is contained in an amount exceeding 2.0%, the strength of the HAZ is excessively increased and the toughness is deteriorated, so that the content of Mn is 2.0% or less. The preferable lower limit of the Mn content is 1.4%, and the preferable upper limit is 1.8%.

P: not more than 0.03% (not including 0%)

P is an impurity element which tends to cause grain boundary destruction and adversely affects the toughness, and therefore the content thereof is preferably as small as possible. From the viewpoint of ensuring HAZ toughness, the content of P must be suppressed to 0.03% or less, preferably 0.02% or less. However, it is difficult to industrially set P in the steel to 0%.

S: 0.015% or less (not including 0%)

S is an element that forms a Mn sulfide on the old austenite grain boundaries in the HAZ and deteriorates the HAZ toughness. Therefore, the content of S is preferably as small as possible. From the viewpoint of ensuring HAZ toughness, the content of S needs to be suppressed to 0.015% or less, preferably 0.010% or less. However, it is difficult to industrially make S in steel 0%.

Al: 0.004 to 0.05%

Al is an element that forms an oxide which is a starting point of? In the particle. If the content is less than 0.004%, the desired oxide form can not be obtained and the transformation in the particles is not sufficiently promoted, so that the HAZ toughness is deteriorated. On the other hand, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated. Therefore, it is required to be suppressed to 0.05% or less. The lower limit of the Al content is preferably 0.007% and the upper limit is 0.04%.

Ti: 0.010 to 0.050%

Ti is added in advance of REM, Zr, and Ca together with the element forming the Ti nitride, thereby enabling the fine dispersion of the oxide having the action of promoting the generation of? In the grain. In order to realize a predetermined Ti nitride and oxide form, it is necessary to contain Ti in an amount of 0.010% or more. However, if the content is excessive, coarse Ti nitride is largely crystallized to deteriorate HAZ toughness, so that it is required to be suppressed to 0.050% or less. The preferable lower limit of the Ti content is 0.012%, the preferable upper limit is 0.035%, and the more preferable upper limit is 0.025%.

REM: 0.0003 to 0.02%, Zr: 0.0003 to 0.02%

REM (rare-earth element) and Zr are added prior to the addition of Ca after addition of Ti to form oxides effective for generation of? In the grain. These oxides are more suitable for the precipitation of Ti nitride, resulting in a more suitable α-producing site in the grain. These effects increase with an increase in the content thereof. However, in order to effectively exhibit such effects, it is necessary to add them all at 0.0003% or more. However, if these are contained in excess, the oxides become coarse and the HAZ toughness deteriorates, so that all of them should be suppressed to 0.02% or less. A more preferable lower limit of these contents is 0.0005%, and a more preferable upper limit is 0.015%.

Ca: 0.0005 to 0.010%

Ca is added after 5 minutes or more from the addition of REM and Zr to form an oxide which is effective for the generation of? In the grain and inhibits the crystallization of coarse Ti nitride. In order to exhibit such effect effectively, it is necessary to contain 0.0005% or more. However, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated, so that it is required to be suppressed to 0.010% or less. The lower limit of the Ca content is preferably 0.0008%, and the upper limit is preferably 0.008%.

N: 0.002 to 0.010%

N is an element useful for securing toughness of HAZ by forming fine Ti nitride. By setting the content to 0.002% or more, a desired Ti nitride can be obtained. However, if the content is excessive, the coarse Ti nitride is promoted to crystallize, and therefore, it is required to be suppressed to 0.010% or less. The preferable lower limit of the N content is 0.003%, and the preferable upper limit is 0.008%.

The above is an essential containing element defined by the present invention, and the remaining part is iron and inevitable impurities. As the inevitable impurities, incorporation of elements such as Sn, As, Pb, etc. brought in according to the conditions of raw materials, materials, and manufacturing facilities is permitted. It is also more effective to positively contain the following elements, and the characteristics of the steel after the steel sheet are further improved by the kind of the chemical components (elements) contained therein.

At least one selected from the group consisting of Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, and Mo: 0.05 to 1.50%

Ni, Cu, Cr, and Mo are all effective elements for increasing the strength of the steel sheet, and the effect thereof increases as their content increases. In order to effectively exhibit such effects, it is preferable that all of them are contained in an amount of 0.05% or more. However, if they are contained in an excess amount, the strength is excessively increased, and the HAZ toughness is deteriorated. Therefore, it is preferable to suppress all of them to 1.50% or less. A more preferable lower limit of the content thereof is 0.10%, and a more preferable upper limit is 1.20%.

Nb: 0.002 to 0.10% and / or V: 0.002 to 0.10%

Nb and V are effective elements for improving the toughness of the base material by precipitating as carbonitride and inhibiting coarsening of? Grains. The effect increases as the content thereof increases. However, in order to effectively exhibit such effect, it is preferable that the content is 0.002% or more. However, if they are contained excessively, it causes coarsening of the HAZ structure and deteriorates HAZ toughness, so that it is preferable to suppress all of them to 0.10% or less. A more preferable lower limit of the content thereof is 0.005%, and a more preferable upper limit is 0.08%.

B: 0.0005 to 0.0050%

B has an effect of improving the HAZ toughness by suppressing the generation of coarse grain boundary alpha. Such an effect increases with an increase in the content, but it is preferably contained in an amount of 0.0005% or more in order to exhibit the effect effectively. It is more preferably 0.0010% or more, and still more preferably 0.0015% or more. However, when the B content is excessive, the coarse bainite bucket from the old austenite grain boundary is promoted, and the HAZ toughness is rather lowered. A preferable upper limit is 0.0045%, a more preferable upper limit is 0.0040%, and a more preferable upper limit is 0.0035%.

In the description of the chemical composition, it has been described that it is effective to contain at least one element selected from the group consisting of Ni, Cu, Cr, and Mo. In that case, the content (mass% Cu] + [Cr] + [Mo] < 2.5% [wherein, [] represents the content (mass%) of each element].

The coarse Ti nitride is crystallized into a liquid phase in which Ti and N are concentrated by solidification segregation at the solidification stage of molten steel. When the content of [Ni] + [Cu] + [Cr] + [Mo] exceeds 2.5%, the solidification temperature is lowered and the liquid phase remains at a low temperature at which driving force for coarse Ti- .

(| da-df | / da? 0.35)

| da-df | / da is a parameter related to the number of Ti nitrides contributing to the suppression of coarsening of? particles during HAZ high-temperature heating. If it is more than 0.35, coarsening of the? Grains is not sufficiently suppressed, and the predetermined HAZ toughness can not be ensured. The preferred upper limit is 0.30, and the more preferred upper limit is 0.25.

The present invention relates to a post-steel sheet, but generally, a post-steel sheet refers to a steel sheet having a sheet thickness of 3.0 mm or more as defined in JIS. On the other hand, the rear steel plate of the present invention was invented with respect to rear plate steel welding with a plate thickness of 50 mm or more, and it is considered that the steel plate to be a target is a steel plate having a plate thickness of 50 mm or more, However, it is not excluded to apply the present invention to a steel plate having a plate thickness of less than 50 mm.

<Examples>

Hereinafter, the present invention will be described in more detail with reference to examples. However, it should be understood that the present invention is not limited to the following examples, but may be appropriately modified within the scope of the present invention. , All of which are included in the technical scope of the present invention.

In the examples of the present invention, first, the steels having the respective compositional compositions shown in Tables 1 and 2 were melted with a vacuum melting furnace (VIF: 150 kg), and then the cast steel 250 mm) was cast and hot-rolled by using the cast piece to obtain a hot rolled plate having a thickness of 80 mm. The hot rolling conditions were: heating before rolling: 1100 占 폚 for 3 hours; finish rolling temperature: 780 占 폚 or more; average cooling rate up to 450 占 폚: 6 占 폚 / s; cooling stopping temperature:

Table 3 and Table 4 show the conditions controlled in the production of the hot-rolled plate (post-steel plate). The conditions are as follows: the amount of dissolved oxygen [Of] (mass%) in the molten steel before addition of Al (Ti), the order of addition of Al, Ti, REM, Zr and Ca, the time t1 from addition of REM or Zr to Ca addition, [REM] / [Zr] (referred to as REM / Zr in the table), the cooling time t2 at 1500 to 1450 DEG C during casting, and the ratio of the amount of Zr added [ Cooling time t3.

Further, in Table 1 and Table 2, REM was added in the form of mischmetal containing about 50% of Ce and about 25% of La in terms of mass%. In Table 1 and Table 2, &quot; - &quot; indicates that the corresponding element is not added.

In Table 3 and Table 4, the order of addition of Al, Ti, REM, Zr and Ca is "O" when Al → Ti → (REM, Zr) → Ca, Quot; x &quot;.

[REM] / [Zr] was used as the hot-rolled steel sheet (post-steel sheet) produced in the above requirements, The number density of oxides of [REM] / [Zr] &gt; = 1.5, and the number density of oxides containing Ti, Al, Ca, REM and Zr in a predetermined concentration range and having a circle- The number density N2 of oxides having a circle-equivalent diameter of 2 占 퐉 or more, the number density N3 of Ti nitrides having a circle equivalent diameter of 1 占 퐉 or more, the number density N4 of the Ti nitrides having a circle equivalent diameter of 20 nm or more, | da-df | / da and HAZ toughness was obtained by measurement. The results of these measurements are shown in Tables 5 and 6.

(Measurement of number density of oxide having circle-equivalent diameter of less than 2 占 퐉)

The test pieces were cut out from the surface of the steel sheet after each time at a depth of t / 4 (t: sheet thickness) (the axial center of the test piece passed through the position of t / 4), and the cross section parallel to the rolling direction and the sheet thickness direction , And a field emission scanning electron microscope &quot; SUPRA35 (trade name) &quot; (hereinafter referred to as FE-SEM) manufactured by Carl Zeiss. The observation condition was a magnification of 5000 times and an observation area of 0.048 mm 2. The area of each oxide in the observation field was measured by image analysis, and the circle equivalent diameter of each oxide was calculated from the area. It was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described composition of the components. The acceleration voltage at the time of measuring the component composition by EDX is 15 kV, and the measurement time is 100 seconds. The number of oxides (N1, NA) having a circle-equivalent diameter of less than 2 占 퐉 was obtained by converting the number of oxides to a number density corresponding to 1 mm2. However, since the reliability of EDX is not sufficient for an oxide having a circle-equivalent diameter of 0.2 탆 or less, it is excluded from the analysis.

(Measurement of number density of oxides having a circle equivalent diameter of 2 탆 or more)

The test pieces were cut out from the surface of the steel sheet after each time at a depth of t / 4 (t: sheet thickness) (the axial center of the test piece passed through the position of t / 4), and the cross section parallel to the rolling direction and the sheet thickness direction , And observed using FESEM. The observation conditions were a magnification of 1,000 times, an observation field of view: 0.06 mm 2, and observation points of 20 points. The area of each oxide in the observation field was measured by image analysis, and the circle equivalent diameter of each oxide was calculated from the area. It was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described composition of the components. The acceleration voltage at the time of measuring the component composition by EDX is 15 kV, and the measurement time is 100 seconds. Then, the number of oxides (N2) having a circle-equivalent diameter of 2 占 퐉 or more was obtained by converting the number of oxides to a number density corresponding to 1 mm2.

(Measurement of the number density of Ti nitride having a circle equivalent diameter of 1 탆 or more)

The test pieces were cut out from the surface of the steel sheet after each time at a depth of t / 4 (t: sheet thickness) (the axial center of the test piece passed through the position of t / 4), and the cross section parallel to the rolling direction and the sheet thickness direction , And photographed at a magnification of 200 times at an angle of 20 degrees using an optical microscope. The number of coarse Ti nitrides was counted and converted into the number density (N3) equivalent to 1 mm2. The area of the measured image is 0.148 mm2 per field of view and 2.96 mm2 per sample. The identification of the Ti nitride was performed on the basis of the shape and the color, and the inclusive shape and the clear orange inclusions were regarded as the Ti nitride. The circle equivalent diameter of the Ti nitride was calculated by the analysis software. In many cases, the coarse Ti nitride is crystallized from an oxide as a starting point. In this case, the oxide inside the coarse Ti nitride is excluded from the measurement of the circle equivalent diameter.

(Measurement of the number density of Ti nitride having a circle equivalent diameter of 20 nm or more and calculation of | da-df | / da)

A specimen was cut from the t / 4 position of each steel sheet after each test, and a replica TEM specimen was prepared from the cross section parallel to the rolling direction and the thickness direction. The specimen was observed with a transmission electron microscope (TEM) at an observation magnification of 15,000,占 8.05 占 퐉, Ti and N-containing particles were identified by EDX (energy dispersive X-ray fluorescence spectrometry) to obtain a Ti-containing nitride. Further, by image analysis, the area of the Ti-containing nitride in the visual field was measured, and the number of Ti-containing nitrides of 20 nm or more in terms of circle equivalent diameter was measured and converted to 1 mm 2 to obtain the number density (N4). From the obtained data, the sizes of the Ti nitrides having a circle equivalent diameter of 20 nm or more were divided into regions in 5 nm intervals in a small order from the ones having a circle equivalent diameter of 20 nm, and the range of circle equivalent diameters di-5) or more and less than di, and di in the region where the number density of Ti nitrides present in each region is the largest is df, and df of the region having a circle equivalent diameter of less than 20 nm and less than 500 nm The average circle-equivalent diameter da of the nitride was found and | da-df | / da was calculated.

(Evaluation of HAZ toughness)

Test specimens for welding joints were taken from each steel plate after being subjected to V-line machining, and then subjected to electro-arc arc welding at an input heat amount of 50 kJ / mm. Three Charpy impact test pieces (V-notch test pieces of JIS Z 2242) were prepared by cutting the HAZ near the weld line (bond) at a depth of t / 4 (t: plate thickness) from the surface of each steel sheet after each test piece Charpy impact test was carried out at -40 DEG C to measure absorbed energy (vE _{- 40} ), and the average value and the minimum value thereof were determined. From the measurement results, it was evaluated that the average value of vE- ₄₀ exceeded 180 J and the minimum value exceeded 120 J, indicating excellent HAZ toughness.

The Charpy impact test was carried out under the same conditions as those described above except that the electric arc welding was carried out at an input heat amount of 60 kJ / mm. The absorption energy (vE _{- 40} ) of the three test pieces was measured, Respectively. From this measurement result, it was evaluated that the average value of vE- ₄₀ exceeded 120 J, and that the HAZ toughness was excellent. In addition, it was evaluated that the average value of vE- ₄₀ exceeded 150 J, particularly the excellent HAZ toughness.

Nos. 1 to 35 are inventive examples satisfying the requirements of the present invention, and they are suitably made of chemical composition, oxides, Ti nitride, and the like. The HAZ toughness (average value and the like) when the heat input amount is 50 kJ / (The minimum value) and the HAZ toughness (average value) when the heat input amount is 60 kJ / mm. In other words, Nos. 1 to 35 are post-steel plates having excellent toughness of the weld heat affected zone.

Further, the steel sheet satisfying the requirements of claim 2 can be evaluated as a steel sheet having an average value of vE- ₄₀ of more than 150 J, particularly excellent in toughness of the weld heat affected zone.

On the other hand, Nos. 36 to 55 are comparative examples which do not satisfy the requirements of any one of the requirements of the present invention, and the HAZ toughness (average value and minimum value) and the heat input amount when the heat input amount is 50 kJ / Mm, it can be understood that any one of the HAZ toughness (average value) in the case of satisfying the evaluation criteria does not satisfy the evaluation criteria.

Although the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2012-205840) filed on September 19, 2012, the contents of which are incorporated herein by reference.

The post-steel sheet of the present invention is excellent in toughness of the heat-affected zone after heat input, and is useful for welded structures such as bridges, high-rise buildings, ships, and line pipes.

Claims (3)

The steel sheet contains 0.03 to 0.12% of C, 0.10 to 0.25% of Si, 1.0 to 2.0% of Mn, 0.03% or less of P (does not include 0%), 0.015% or less of S 0.004 to 0.05% of Al, 0.0010 to 0.050% of Ti, 0.0003 to 0.02% of REM, 0.0003 to 0.02% of Zr, 0.0005 to 0.010% of Ca and 0.002 to 0.010% of N, Iron and inevitable impurities,
%, &Lt; 30%, 5% < Ca < 40%, 5% < REM 50%, 2% < Zr < 30% , And 1.0? REM / Zr, wherein the oxides having a circle-equivalent diameter of less than 2 占 퐉 are 300 / mm2 or more, the oxides having a circle-equivalent diameter of 2 占 퐉 or more are 100 / Along with being present,
The number of Ti nitrides having a circle equivalent diameter of at least 1 mu m is 7 / mm 2 or less and the number of Ti nitrides having a circle equivalent diameter of 20 nm or more is 1.0 x 10 ⁶ /
Further, the size of the Ti nitride having a circle equivalent diameter of 20 nm or more is divided into regions each having an interval of 5 nm from the region having a circle-equivalent diameter of 20 nm, and the range of the circle- ) Of the region where the number density of Ti nitrides existing in each region is the largest is set to be df,
An average circle-equivalent diameter da of the Ti nitride having a circle-equivalent diameter of 20 nm or more and less than 500 nm,
| da-df | / da | 0.35, wherein the welded heat affected zone has excellent toughness. The method according to claim 1,
%, &Lt; 30%, 5% < Ca < 40%, 5% < REM 50%, 2% < Zr < 30% , 1.5? REM / Zr, and more than 300 pieces / mm2 of oxide having a circle equivalent diameter of less than 2 占 퐉 are present. 3. The method according to claim 1 or 2,
0.05 to 1.50% of Cr, 0.05 to 1.50% of Cr, 0.05 to 1.50% of Mo, 0.002 to 0.10% of Nb, 0.002 to 0.10% of V, 0.002 to 0.10% of V, 0.0005 to 0.10% of B, 0.0050%. The steel sheet according to any one of claims 1 to 3, further comprising at least one member selected from the group consisting of titanium oxide, titanium oxide and titanium oxide.