JP5017782B2 - Steel with excellent toughness in weld heat affected zone - Google Patents

Description

  The present invention relates to a steel material excellent in toughness of a weld heat-affected zone, and in particular, has a precipitate that suppresses toughness deterioration of the weld heat-affected zone, and has a welded structure such as shipbuilding, building, bridge, offshore structure, line pipe, etc. The present invention relates to a steel material excellent in toughness of a welding heat-affected zone, which is suitable for use of an object.

  In recent years, as steel structures and ships are becoming larger, there is an increasing need for higher strength and thicker steel materials to be used. In the welding of thick-walled steel materials, in order to increase the welding efficiency, welding that can take a large amount of heat, such as submerged arc welding, electrogas welding, and electroslag welding, is often applied. In the heat affected zone (hereinafter also referred to as HAZ), there is a problem that the toughness deteriorates due to the coarsening of the structure accompanying the increase in heat input, and the toughness of the weld heat affected zone (hereinafter also referred to as HAZ toughness). There is a need for improved steel. Various proposals have been made according to this demand.

  For example, Patent Document 1 proposes a technique for improving the HAZ toughness by suppressing the coarsening of austenite crystal grains using a nitride such as TiN. However, even in the HAZ, at a portion heated to a high temperature of 1300 ° C. or higher, such as in the vicinity of the bond (the boundary between the HAZ and the weld metal), nitrides such as TiN dissolve and the austenite crystal grains become coarse. Lose ability to suppress. Furthermore, in the method using nitride such as TiN, addition of N is essential, so that HAZ toughness due to nitride such as TiN due to deterioration of toughness due to increase of N content in steel or adverse effect on weld metal. The improvement is diminished and its effect is limited.

On the other hand, Patent Document 2 proposes a method of increasing the HAZ toughness by promoting precipitation of intragranular ferrite using a Ti oxide or a composite of TiN and TiN. According to this method, coarsening of austenite crystal grains can be suppressed by the pinning effect of Ti oxide that does not dissolve even at a high temperature of 1300 ° C. or higher. However, in order to uniformly disperse Ti oxide in steel, a high steelmaking technique is required. For example, as disclosed in Patent Document 3, it is necessary to newly add Mg or the like as a dispersant.
JP 58-31065 A JP 60-245768 A Japanese Patent Laid-Open No. 6-179842

As described above, for steel materials for welded structures that require excellent HAZ toughness, it has been proposed to utilize the pinning effect due to various precipitates, but with precipitates conventionally used such as TiN However, the HAZ toughness cannot be improved with respect to various welding techniques, and oxides have problems such as difficulty in dispersing precipitates.
The present invention solves this problem, and provides a steel material having a weld heat-affected zone excellent in toughness having precipitates that are uniformly dispersed in steel and can exhibit a pinning effect even in the vicinity of a bond of high heat input welding. With the goal.

  In the following, “high temperature” means 1300 ° C. or higher unless otherwise specified, and the upper limit is substantially about 1450 ° C.

  The present inventors have made various studies on precipitates that are uniformly finely dispersed in steel and stably suppress coarsening of HAZ austenite crystal grains. As a result, by deoxidizing only Si or Mn without adding Al, it is possible to leave oxides in the steel even during super-high heat input welding where the welding heat input exceeds 800 kJ / cm, Found that there is an optimum range of REM (Rare Earth Metal) content in which HAZ austenite crystal grains become fine (Japanese Patent Application No. 2003-122618). This optimum range was a range in which the REM content was 0.0010 to 0.0100% by mass and equal to Mn content × O content × (0.2 to 1.2).

The effect of improving HAZ toughness by (REM, Mn) oxysulfide (: oxysulfide containing REM and Mn) formed by limiting the REM content to the above optimal range is that Al content is 0.01% by mass. It was expressed in the composition range limited to the following.
Therefore, the case where Al is further added in the range of Al content exceeding 0.01% by mass is examined, and as a result, (REM, Mn, Al) oxysulfide (: REM, Mn and Oxysulfide containing Al) is formed, and these are separated into REM oxide and MnS little by little like (REM, Mn) oxysulfide to precipitate sulfide at high temperature, and fine austenite grains of HAZ It was found to have the effect of

  In this study, a REM-Al-added steel material having various REM contents in the range of 0 to 0.02 mass% and Al content in the range of 0.01 to 0.08 mass% was applied to HAZ having a heat input of 10 to 800 kJ / cm. A reproducible thermal cycle test for applying a thermal cycle corresponding to the vicinity of the route was performed, and the grain size (austenite grain size) of the austenite crystal grains after the test was measured. In this thermal cycle, the test piece is heated from room temperature (25 ° C.) to the maximum heating temperature (1300 to 1450 ° C.) at a rate of 35 seconds (temperature increase rate 36 to 41 ° C./s) and held at the maximum heating temperature for 10 seconds. Then, it cools to 1000 degreeC and then water-cools. The cooling from the maximum heating temperature to 1000 ° C. is set to a cooling rate (for example, 100 ° C./s) slower than the water cooling. When the relationship among the austenite grain size, Al content, and REM content was arranged from the obtained results, a correlation as shown in FIG. 1 was obtained.

Since Al is a very strong deoxidizer, Al deoxidation is conventionally performed in the steelmaking process. Moreover, Al is known to form relatively large inclusions, and its form has been mainly composed of oxides. However, when an appropriate amount of REM is added to a steel material to which Al is added, fine austenite crystal grains are obtained as shown in FIG. 1, and when the precipitates of these steel materials are examined in detail, the following is clear. became.
(1) (REM, Mn, Al) oxysulfide is formed even in a steel material (for example, A in FIG. 1) having an Al content of 0.02% by mass or less.
(2) (REM, Mn, Al) oxysulfide is separated into (REM, Al) oxide and MnS.

Furthermore, although Si can be considered as an element that exhibits the same effect as Al, it has been confirmed that the effect of improving HAZ toughness is exhibited if the Si content is within a certain range as well as the Al content.
That is, the present inventor forms an oxysulfide containing REM, Mn and Al or Si if the amount of REM added is adjusted, and separates and precipitates into MnS, and the separated MnS maintains a precipitated state even at high temperatures. And it came to the mind that the austenite crystal grain coarsening is suppressed by the pinning effect of MnS.

This means that the effect of improving the HAZ toughness according to the present invention is exhibited even in the conventionally used deoxidized steels of Al and Si, and the applicable range is expanded industrially.
The present invention has been made based on these findings, and the gist thereof is as follows.
(Invention Item 1) C: 0.001 to 0.15% by mass, Si: 0.05 to 0.5% by mass, Mn: 0.5 to 2.0% by mass, Al: more than 0.010% by mass 0 0.08 mass% or less, O: 0.0005-0.0100 mass%, S: 0.0005-0.0100 mass%, REM: 0.0010-0.0200 mass%, the following formulas (1) to ( 3) It is contained within the formation range, and consists of the balance Fe and inevitable impurities . The Charpy absorbed energy at −20 ° C. of the welding heat-affected zone is 207 J or more, and the toughness of the welding heat-affected zone is excellent. Steel material.

0.2 ≦ REM / (Mn × O) ≦ 1.20 (1)
REM / (Al + Si) ≧ 0.012 (2)
Mn × S ≧ 0.0000294 (3)
Mn, O, REM, Al, Si, and S in the above formula represent the content (mass%) of the same element in the steel material.

(Invention Item 2) A steel material excellent in toughness of the weld heat affected zone according to Invention Item 1, wherein Y: 0.0010 to 0.0200 mass% is contained as the REM.
(Invention Item 3) Invention Item 1 containing one or two of Cu: 0.01 to 1% by mass and Ni: 0.01 to 2% by mass in place of part of Fe Or the steel material excellent in the toughness of the welding heat affected zone of 2 description.

  ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in the toughness of a welding heat affected zone which has the precipitate which can disperse | distribute uniformly in steel and can exhibit the pinning effect also in the bond vicinity of a high heat input welding is obtained.

First, the reasons for limiting the chemical composition of the steel materials (present invention steel materials) described in Invention Items 1 to 4 will be described.
C: 0.001 to 0.15% by mass
If C is reduced to less than 0.001% by mass, the productivity is significantly reduced in the mass production process. On the other hand, if it exceeds 0.15% by mass, the workability and welding workability of the steel are impaired, and the HAZ toughness is reduced. Therefore, 0.001 to 0.15 mass% is set. The base material (steel material before welding) and the HAZ structure may be any of a bainite structure, a ferrite / pearlite structure, and a martensite structure. In the case of bainite, C is 0.001 to 0.03 mass% is preferable. On the other hand, in the structure other than bainite, C is an element that increases the strength of the steel material, and is preferably 0.01% by mass or more in order to ensure the strength as a structural member.

Si: 0.05-0.5 mass%
Si is an element that has a deoxidizing action and improves the strength of the steel material, but if it is less than 0.05% by mass, sufficient strength cannot be obtained as a structural steel material, and the deoxidizing action becomes insufficient. On the other hand, if it exceeds 0.5% by mass, an oxide or oxysulfide of Si and Mn is formed and the amount of oxysulfide containing REM is reduced, and the pinning effect is not manifested and the austenite crystal grains are coarse. Therefore, the content is set to 0.05 to 0.5% by mass.

Mn: 0.5 to 2.0% by mass,
Mn is an element that improves the strength of a steel material, similar to Si. However, if it is less than 0.5% by mass, the strength required as a structural steel material cannot be obtained, whereas if it exceeds 2.0% by mass, HAZ In order to deteriorate toughness, the content is set to 0.5 to 2.0 mass%.
Al: more than 0.010 mass% and 0.08 mass% or less Al is an element having a very strong deoxidizing action. Therefore, it is not necessary to secure a stable oxide at high temperature in the steel material. However, in the presence of REM, if it exceeds 0.08% by mass, the number of oxysulfides of REM and Mn decreases, and austenite crystals. Coarse grains and reduce HAZ toughness. On the other hand, when Al is 0.010% by mass or less, sufficient deoxidation cannot be performed. Therefore, deoxidation with Si, Mn, or the like is required, and the manufacturing cost increases. Therefore, Al is made more than 0.010 mass% and 0.08 mass% or less.

O: 0.0005-0.0100 mass%
O is usually an unavoidable impurity, but in the present invention, O is an important element involved in the formation of oxides and oxysulfides containing REM or Mn. Since the amount of crystallized oxide or oxysulfide is very small, the austenite crystal grains become coarse. On the other hand, if it exceeds 0.0100% by mass, coarse inclusions are generated due to crystallization in the initial stage of solidification, and the cleanliness of the steel material is lowered. This coarse inclusion does not contribute to the refinement of austenite crystal grains. Therefore, O is set to 0.0005 to 0.0100 mass%.

S: 0.0005-0.0100 mass%
S is an element required for precipitating MnS or (REM, Mn, Al) oxysulfide, which is the point of the present invention, to suppress the growth of austenite crystal grains, and less than 0.0005 mass%. , MnS does not precipitate and austenite crystal grains become coarse. On the other hand, when it exceeds 0.0100 mass%, coarse MnS precipitates in addition to (REM, Mn, Al) oxysulfide, and the toughness of the steel material decreases. Therefore, S is set to 0.0005 to 0.0100 mass%.

REM: 0.0010 to 0.0200 mass%
REM is an element necessary for facilitating the formation of (REM, Mn, Al, Si) oxysulfide and suppressing the growth of austenite crystal grains. Since no austenite or oxide is formed and the austenite crystal grains become coarse, the HAZ toughness decreases. On the other hand, if it exceeds 0.0200% by mass, it will crystallize from a high temperature during solidification, resulting in coarse inclusions, coarsening of austenite crystal grains, and occurrence of various defects inside the steel material. Accordingly, the REM is set to 0.0010 to 0.0200 mass%.

Note that REM refers to an element from Sc to Y and La to Lu by atomic number. From the viewpoint of availability, one of Ce (cerium), La (lanthanum), and Nd (neodymium) or 2 or more types.
Furthermore, in this invention, content of O, Mn, and REM satisfies Formula (1), content of REM, Al, and Si satisfies Formula (2), and content of S and Mn is Formula (3). ) Must be satisfied.
0.2 ≦ REM / (Mn × O) ≦ 1.20 (1)
REM / (Al + Si) ≧ 0.012 (2)
Mn × S ≧ 0.0000294 (3)
Mn, O, REM, Al, Si, and S in the above formula represent the content (mass%) of the same element in the steel material.

  In order to produce (REM, Mn) oxysulfide or (REM, Mn, Al, Si) oxysulfide, REM / (Mn × O) should be within the range of 0.2 to 1.20. It is necessary to adjust the contents of Mn, O, and REM. When REM / (Mn × O) is less than 0.2, oxysulfide containing REM is not generated and austenite crystal grains become coarse. On the other hand, when REM / (Mn × O) exceeds 1.20, Mn does not form a solid solution in the oxysulfide and becomes REM sulfide alone, and when subjected to a thermal cycle, the REM / (Mn × O) is converted into REM oxide and MnS. Precipitation due to separation does not occur, and austenite crystal grains are also coarsened.

  Furthermore, the REM content is also limited by the Al content and the Si content, and in the present invention containing a large amount of Al exceeding 0.01% by mass, REM and (Al + Si) are set to 0.010 or more. It is necessary to adjust the contents of Al and Si. When REM / (Al + Si) is less than 0.010, since the Al and Si contents are larger than the REM content, oxysulfides containing REM are not generated, and austenite crystal grains become coarse.

Moreover, in order to ensure the precipitation state of MnS even at high temperatures, it is necessary to adjust the contents of Mn and S so that Mn × S is 0.00044 or more. When Mn × S is less than 0.0000294, the amount of MnS precipitated at high temperature is insufficient, the pinning effect is poor, and austenite crystal grains are coarsened.
In addition to the above elements, it is preferable to add the following elements in order to achieve further increase in tension and thickness of the steel material as necessary.

Y: 0.0010-0.0200 mass%
Y is a kind of REM and is an easily available REM element after Ce, La and Nd, and generates oxysulfide ((Y, Mn) oxysulfide) of Y and Mn during solidification in the steelmaking stage. Separated during the thermal cycle test, MnS is present at a high temperature. However, when Y is less than 0.0010 mass%, (Y, Mn) oxysulfide is not generated, and the effect of refining austenite crystal grains cannot be obtained. On the other hand, when it exceeds 0.0200 mass%, the toughness of steel material and HAZ will fall. Therefore, Y is preferably 0.0010 to 0.0200% by mass.

Cu: 0.01 to 1% by mass, Ni: 0.01 to 2% by mass One or two of Cu and Ni are elements that improve the strength and toughness of the steel material. Like Mn, it is an element that forms sulfides, and contributes to austenite grain refinement like REM and Mn. However, both Cu and Ni are less effective at less than 0.01% by mass. On the other hand, if Cu exceeds 1% by mass or Ni exceeds 2% by mass, the toughness of the steel material or HAZ decreases. Accordingly, Cu is preferably 0.01 to 1% by mass, and Ni is preferably 0.01 to 2% by mass.

As described above, coarsening of austenite crystal grains can be suppressed by appropriately adjusting the contents of S and REM according to the contents of O, Mn, Al, and Si. In order to suppress the coarsening of the austenite crystal grains, a sufficient number of precipitates is necessary, but the number of precipitates can be confirmed as follows. That is, the steel material used as a sample was heated from 25 ° C. to 1450 ° C. at 41 ° C./s, held at this temperature for 10 seconds, cooled to 1000 ° C. at 100 ° C./s, and then water-cooled. The number of precipitates per 1 mm ² of the cross section of the steel material is counted with an electron microscope. As shown in Table 2, all the steel materials of the present invention have 20 or more precipitates.

  After melting the steel having the composition shown in Table 1, it was made into a slab by continuous casting, and this was hot-rolled to produce a steel plate (thickness 40-60 mm). Thermal cycle test pieces cut out from these steel plates were heated from 25 ° C. to 1450 ° C. in 35 seconds, then held at 1450 ° C. for 10 seconds, then cooled to 1000 ° C. at 100 ° C./s, and then water-cooled thermal cycle A test was conducted. After this test, the observation surface on which the austenite grain boundary appeared by picric acid corrosion was observed with an optical microscope, and the austenite grain size was measured according to JIS G 0551. Moreover, the number of precipitates was measured by electron microscope observation. In addition, a Charpy impact test piece (JIS Z 2202 V notch standard size) was taken from the same heat cycle test piece, and a HAZ Charpy impact test was performed at a test temperature of −20 ° C. according to JIS Z 2242 in the vicinity of the HAZ bond. Energy was measured. The results are summarized in Table 2. The austenite particle size indicates the maximum austenite particle size.

In invention examples (steel materials 1 to 9, 17), the austenite grain size was as small as 300 μm or less, the Charpy absorbed energy was as large as 207 J or more, and it was confirmed that excellent HAZ toughness was exhibited.
On the other hand, referring to the comparative examples (steel materials 10 to 16, 18 to 22), since the REM / (Mn × O) of the steel material 10 exceeded 1.20, separation and precipitation into REM oxide and MnS did not occur. The austenite crystal grains coarsened and the HAZ toughness decreased. In addition, since the steel material 11 has too little S, no oxysulfide was formed, and austenite crystal grains became coarse due to thermal cycling. Further, since the steel material 12 contained excessive S, a large amount of oxysulfide or sulfide was precipitated, and although the austenite particle size after the thermal cycle was small, the HAZ toughness was lowered because the cleanliness was lowered.

Since the steel 13 has an excess of O, the cleanliness is lowered, and REM / (Mn × O) is also less than 0.2. Therefore, oxysulfide containing REM is not generated, austenite crystal grains are coarsened, and HAZ toughness is reduced. Declined. Since REM / (Mn × O) of steel material 14 is less than 0.2, oxysulfide containing REM was not generated, austenite crystal grains were coarsened, and HAZ toughness was reduced.
Since the steel material 15 is too REM and the steel material 16 is too REM, the austenite crystal grains are coarsened and the HAZ toughness is lowered. Since the steel material 18 had a Mn × S of less than 0.000029, the precipitation amount of MnS was insufficient at a high temperature, the austenite crystal grains were coarsened, and the HAZ toughness was lowered.

Since the steel material 19 was excessive in Si, oxides or oxysulfides of Si and Mn were formed, the amount of oxysulfide containing REM was reduced, austenite crystal grains were coarsened, and HAZ toughness was reduced. Since the steel material 20 has an excessive amount of Al, an oxysulfide containing REM was not formed, the austenite crystal grains were coarsened, and the HAZ toughness was lowered.
The steel materials 21 and 22 have appropriate contents of Si, Al, and REM, but because REM / (Al + Si) is less than 0.010, oxysulfide containing REM is not formed, and austenite crystal grains are coarsened. HAZ toughness decreased.

  The present invention can be used in the industrial field of building steel structures and ships.

It is a graph which shows the relationship between content of Al and REM, and the austenite particle size after a reproduction thermal cycle test.

Claims (3)

C: 0.001 to 0.15% by mass, Si: 0.05 to 0.5% by mass, Mn: 0.5 to 2.0% by mass, Al: more than 0.010% by mass and 0.08% by mass or less , O: 0.0005 to 0.0100 mass%, S: 0.0005 to 0.0100 mass%, REM: 0.0010 to 0.0200 mass%, and the establishment range of the following formulas (1) to (3) A steel material excellent in toughness of the weld heat affected zone , wherein the Charpy absorbed energy at −20 ° C. of the weld heat affected zone is 207 J or more .
0.2 ≦ REM / (Mn × O) ≦ 1.20 (1)
REM / (Al + Si) ≧ 0.012 (2)
Mn × S ≧ 0.0000294 (3)
Mn, O, REM, Al, Si, and S in the above formula represent the content (mass%) of the same element in the steel material.   The steel material excellent in toughness of the weld heat affected zone according to claim 1, characterized in that Y: 0.0010 to 0.0200 mass% is contained as the REM.   3. The welding according to claim 1, wherein one or two of Cu: 0.01 to 1 mass% and Ni: 0.01 to 2 mass% are contained instead of a part of Fe. Steel material with excellent heat-affected zone toughness.

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