JP6169667B2 - Cast steel product for welded structure and method for producing cast steel product for welded structure - Google Patents

Description

  The present invention relates to a welded cast steel product and a method for producing a welded cast steel product.

  JIS-G5102 (2015) defines SCW550 and the like as a cast steel product for welded structure having a tensile strength of 550 MPa or more. In order to satisfy such strength, it is generally necessary to increase the amount of alloy. However, if the amount of alloy is increased, the weldability is reduced, and preheating to heat the welded part to a temperature higher than the temperature before welding is performed, and annealing (postheating) to heat the welded part and remove stress after welding is necessary. It has been known.

  However, in particular, in the case of large cast steel products used as ship hull parts such as ladder phones and neck bearings, welding is often performed outdoors, and it is difficult to control preheating and afterheating. It is. Therefore, it is expected to improve the weldability of a particularly large cast steel for welded structure and to simplify preheating and afterheating.

In general, strength and weldability are in a trade-off relationship that is mutually contradictory. Therefore, in order to improve weldability while minimizing the decrease in strength of the cast steel product for welded structure, the weld cracking susceptibility composition Pcm represented by the following formula (1) is used as an index of weldability. Has been proposed (see, for example, JP-A-2001-181783). Specifically, in the above publication, good weldability can be obtained by setting the weld cracking susceptibility composition Pcm to 0.3 or less.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)

  However, further improvements in strength and weldability are required for large cast steel products for welded structures. Specifically, there is a need for a cast steel product for welded structure that can be welded outdoors without performing preheating or afterheating in winter.

JP 2001-181783 A

  In view of the above situation, an object of the present invention is to provide a cast steel product for welded structure that is relatively excellent in strength and weldability.

The invention made in order to solve the above problems is as follows: C: 0.10% by mass to 0.17% by mass, Si: 0.01% by mass to 0.40% by mass, Mn: 0.7% by mass or more 1.4 mass% or less, Ni: 1.00 mass% or more and 2.00 mass% or less, Cr: 0.20 mass% or more and 0.50 mass% or less, Mo: 0.10 mass% or more and 0.30 mass% or less Hereinafter, V: 0.05% by mass or more and 0.20% by mass or less, and the balance: Fe and an inevitable impurity composition, and the weld crack susceptibility composition Pcm represented by the following formula (1) is 0.3 or less. It is a cast steel product for welded structure characterized by having a main structure of ferrite and bainite and a dislocation density of 1.35 × 10 ¹³ m ⁻² or more.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)

The cast steel product for welded structure has a content of C, Si, Mn, Ni, Cr, Mo and V within the above ranges, a weld cracking susceptibility composition Pcm of 0.3 or less, and a dislocation density of 1 When it is .35 × 10 ¹³ m ⁻² or more, it has a relatively excellent weldability that allows welding without preheating or afterheating even at 0 ° C., which is lower than room temperature, and relatively excellent strength. The “dislocation density” is a value measured by the Williamson-Hall method using an X-ray diffractometer.

Moreover, another invention made | formed in order to solve the said subject is C: 0.10 mass% or more and 0.17 mass% or less, Si: 0.01 mass% or more and 0.40 mass% or less, Mn: 0.00. 7 mass% to 1.4 mass%, Ni: 1.00 mass% to 2.00 mass%, Cr: 0.20 mass% to 0.50 mass%, Mo: 0.10 mass% to 0 .30% by mass or less, V: 0.05% by mass or more and 0.20% by mass or less, and the balance: Fe and an inevitable impurity composition, and the weld cracking susceptibility composition Pcm represented by the following formula (1) is A step of casting a cast steel for welded structure that is 0.3 or less and a step of tempering the cast product obtained in the casting step, wherein the tempering step heats the cast product to a temperature above the austenitizing temperature. And the casting is mainly composed of ferrite and bainite. It includes a step of cooling to be, and a step of tempering the casting at a temperature lower than the austenitizing temperature, in the cooling step, cooling at a rate that dislocation density is 1.35 × 10 ¹³ m ^-2 or more It is the manufacturing method of the cast-steel goods for welded structures characterized by doing.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)

The manufacturing method of the cast steel product for welded structure uses cast steel in which the contents of C, Si, Mn, Ni, Cr, Mo, and V are within the above ranges, respectively, and the weld cracking susceptibility composition Pcm is 0.3 or less. As a result, the dislocation density of the resulting cast steel for welded structure can be increased, and weldability can be improved until welding can be performed without preheating or afterheating even at 0 ° C., which is lower than normal temperature. And the manufacturing method of the said cast steel product for welded structures improves the intensity | strength of the cast steel product for welded structure obtained by having the process cooled so that a dislocation density may become 1.35 * 10 ^{< 13} > m ^<-2 > or more. Can do.

  As described above, the cast steel product for welded structure and the cast steel product for welded structure obtained by the method for producing a welded cast steel product for welded structure according to the present invention are relatively excellent in strength and weldability.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[Cast steel products for welded structures]
The cast steel product for welded structure is C (carbon): 0.10% by mass to 0.17% by mass, Si (silicon): 0.01% by mass to 0.40% by mass, Mn (manganese): 0 0.7 mass% or more and 1.4 mass% or less, Ni (nickel): 1.00 mass% or more and 2.00 mass% or less, Cr (chromium): 0.20 mass% or more and 0.50 mass% or less, Mo ( Molybdenum): 0.10% by mass to 0.30% by mass, V (vanadium): 0.05% by mass to 0.20% by mass, and the balance: Fe (iron) and an inevitable impurity composition .

Further, the lower limit of the weld cracking susceptibility composition Pcm represented by the following formula (1) of the cast steel product for welded structure is inevitably 0.17, preferably 0.2, more preferably 0.22 due to the above composition. preferable. On the other hand, the upper limit of the weld cracking susceptibility composition Pcm of the cast steel product for welded structure is 0.3, preferably 0.27, and more preferably 0.26. If the weld cracking susceptibility composition Pcm is less than the lower limit, the strength may be insufficient. Conversely, if the weld cracking susceptibility composition Pcm exceeds the above upper limit, welding at normal temperature may cause weld cracking.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(However, the symbol of each element in the formula is the value representing the mass content of each element in%)

  Further, the cast steel product for welded structure has ferrite and bainite as a main structure. “The main structure is ferrite and bainite” means that the total area ratio of the ferrite structure and the bainite structure in the cross section is 50% or more, preferably 70% or more, more preferably 90% or more.

Moreover, as a minimum of the dislocation density in the said cast steel for welded structures, it is 1.35 * 10 ^{< 13} > m ^<-2 >, 1.80 * 10 ^{< 13} > m ^<-2> is preferable, and 2.00 * 10 ^{< 13} > m ^<-2> is. More preferred. On the other hand, the upper limit of the dislocation density in the cast steel product for welded structure is not particularly limited, but in reality, 1.00 × 10 ¹⁵ m ⁻² is considered the limit. If the dislocation density is less than the lower limit, the strength may be insufficient.

  Hereinafter, each component of the cast steel product for welded structure will be described.

<C (carbon)>
C is an element necessary for ensuring the strength of the welded cast steel product. The lower limit of the C content is 0.10% by mass, preferably 0.11% by mass, and more preferably 0.12% by mass. On the other hand, the upper limit of the C content is 0.17% by mass, preferably 0.15% by mass, and more preferably 0.14% by mass. When content of C is less than the said minimum, there exists a possibility that the intensity | strength of the said cast steel for welded structures may become inadequate. On the other hand, when the C content exceeds the above upper limit, island martensite is generated in the cast steel product for welded structure, and this is a starting point for HIC (Hydrogen Induced Cracking), resulting in a decrease in HIC resistance. Doing so may result in insufficient weldability. When the C content exceeds the above upper limit, the martensite hardness increases in the vicinity of the bond portion in the HAZ (Heat Affected Zone) during welding, and SSCC resistance (sulfide stress corrosion cracking: Sulfied). The weldability may be insufficient due to a decrease in stress corrosion cracking properties.

<Si (silicon)>
Si is an element necessary for deoxidation of the cast steel product for welded structure. The lower limit of the Si content is 0.01% by mass, preferably 0.10% by mass, and more preferably 0.20% by mass. On the other hand, the upper limit of the Si content is 0.40 mass%, preferably 0.36 mass%, and more preferably 0.34 mass%. When content of Si is less than the said minimum, there exists a possibility that the deoxidation of the said cast steel product for welded structures may become inadequate. Conversely, when the Si content exceeds the above upper limit, casting defects such as reverse V segregation are likely to occur, and there is a risk of variation in toughness, and island martensite that is the starting point of HIC is generated. The HIC resistance may be reduced.

<Mn (manganese)>
Mn is an element necessary for securing the strength of the welded cast steel product. The lower limit of the Mn content is 0.7% by mass, preferably 0.9% by mass, and more preferably 1.0% by mass. On the other hand, the upper limit of the Mn content is 1.4% by mass, preferably 1.3% by mass, and more preferably 1.2% by mass. When content of Mn is less than the said minimum, there exists a possibility that the intensity | strength of the said cast steel for welded structures may become inadequate. Conversely, when the Mn content exceeds the above upper limit, forming MnS together with S in the unavoidable impurities may reduce the HIC resistance and make the weldability insufficient.

<Ni (nickel)>
Ni is an element that contributes to improving the strength of the cast steel for welded structure. The lower limit of the Ni content is 1.00% by mass, preferably 1.10% by mass, and more preferably 1.20% by mass. On the other hand, the upper limit of the Ni content is 2.00% by mass, preferably 1.95% by mass, and more preferably 1.90% by mass. When content of Ni is less than the said minimum, there exists a possibility that the intensity | strength of the said cast steel for welded structures may fall. Conversely, if the Ni content exceeds the upper limit, the SSCC resistance may decrease due to an increase in hard martensite in the vicinity of the bond part in the HAZ during welding of the cast steel product for welded structure. There is a possibility that the structural cast steel product is unnecessarily expensive.

<Cr (chrome)>
Cr is an element that contributes to improving the strength of the cast steel product for welded structure. As a minimum of content of Cr, it is 0.20 mass%, 0.23 mass% is preferable, and 0.25 mass% is more preferable. On the other hand, the upper limit of the Cr content is 0.50 mass%, preferably 0.35 mass%, and more preferably 0.30 mass%. When content of Cr is less than the said minimum, there exists a possibility that the intensity | strength of the said cast steel for welded structures may fall. On the other hand, when the Cr content exceeds the above upper limit, hard martensite increases in the vicinity of the bond portion in the HAZ during welding of the welded structural cast steel product, resulting in poor weldability due to decreased SSCC resistance. May be sufficient.

<Mo (molybdenum)>
Mo is an element that contributes to improving the strength of the welded cast steel product and increases the temper softening resistance. As a minimum of content of Mo, it is 0.10 mass%, 0.12 mass% is preferable, and 0.13 mass% is more preferable. On the other hand, the upper limit of the Mo content is 0.30 mass%, preferably 0.25 mass%, and more preferably 0.20 mass%. If the Mo content is less than the above lower limit, the strength of the cast steel product for welded structure may be insufficient, or it may not be easy to perform tempering while maintaining the strength. On the other hand, when the Mo content exceeds the above upper limit, hard martensite increases in the vicinity of the bond portion in the HAZ during welding of the cast steel product for welded structure, resulting in poor weldability due to decreased SSCC resistance. May be sufficient.

<V (Vanadium)>
V is an element that contributes to improving the strength of the cast steel for welded structure and increases the temper softening resistance. The lower limit of the V content is 0.05% by mass, preferably 0.07% by mass, and more preferably 0.08% by mass. On the other hand, the upper limit of the V content is 0.20 mass%, preferably 0.17 mass%, and more preferably 0.15 mass%. If the V content is less than the above lower limit, the strength of the cast steel product for welded structure may be insufficient, and it may not be easy to perform tempering while maintaining the strength. On the other hand, when the V content exceeds the above upper limit, hard martensite is increased in the vicinity of the bond portion in the HAZ during welding of the welded structural cast steel product, resulting in poor weldability due to decreased SSCC resistance. May be sufficient.

<Remainder>
The cast steel product for welded structure contains Fe (iron) and inevitable impurities as the balance in addition to the elements described above. Examples of such inevitable impurities include P (phosphorus) and S (sulfur). The total content of inevitable impurities in the cast steel product for welded structure is not particularly limited as long as various properties are not impaired. As an upper limit of the total content of inevitable impurities in the concrete cast steel for welded structure, 0.1% by mass is preferable, 0.05% by mass is more preferable, and 0.02% by mass is more preferable. By making the total content of inevitable impurities not more than the above upper limit, the strength and weldability of the cast steel product for welded structure can be suppressed.

<P (phosphorus)>
P is an element inevitably contained in the cast steel product for welded structure, and decreases the HIC resistance and SSCC resistance of the cast steel product for welded structure. As a minimum of content of P, 0.001 mass% is preferable and 0.002 mass% is more preferable. On the other hand, the upper limit of the P content is preferably 0.012% by mass, more preferably 0.010% by mass, and still more preferably 0.008% by mass. When the content of P is less than the lower limit, there is a possibility that the effect of improving the HIC resistance and SSCC resistance sufficient to meet the increase in the manufacturing cost of the welded structural cast steel product may not be obtained. Conversely, when the P content exceeds the above upper limit, the HIC resistance and SSCC resistance of the welded structural cast steel product may be reduced.

<S (sulfur)>
S is an element that is inevitably included in the cast steel product for welded structure, forms MnS together with Mn, and lowers the HIC resistance of the cast steel product for welded structure. As a minimum of content of S, 0.0001 mass% is preferable and 0.0003 mass% is more preferable. On the other hand, the upper limit of the S content is preferably 0.010% by mass, more preferably 0.008% by mass, and still more preferably 0.006% by mass. When the content of S is less than the above lower limit, there is a possibility that the effect of improving the HIC resistance sufficient to meet the increase in the manufacturing cost of the welded structural cast steel product may not be obtained. On the other hand, when the S content exceeds the upper limit, the HIC resistance of the welded cast steel product may be lowered.

<Cu (copper)>
Cu is a metal that is often added to cast steel in order to generally improve strength. However, since Cu is a metal that increases the weld cracking susceptibility composition Pcm, it is preferable that the cast steel product for welded structure does not contain Cu, but the cast steel product for welded structure is manufactured in the same factory as other cast steels. When it does, there is a possibility that Cu may be mixed in a trace amount. That is, the cast steel product for welded structure does not intentionally add Cu, and only allows inclusion as an inevitable impurity. The upper limit of the Cu content in the cast steel for welded structure is preferably 0.02% by mass, and more preferably 0.01% by mass. When the Cu content exceeds the above upper limit, the weld cracking susceptibility composition Pcm is increased, and the weldability may be insufficient.

<B (boron)>
B is a metal that can generally be added to cast steel to improve strength. However, since B is a metal that increases the weld cracking susceptibility composition Pcm, the cast steel product for welded structure does not intentionally add B, and only allows inclusion as an inevitable impurity. As an upper limit of content of B, 0.003 mass% is preferable and 0.002 mass% is more preferable. When the content of B exceeds the above upper limit, the weld cracking susceptibility composition Pcm is increased, and the weldability may be insufficient.

[Method of manufacturing cast steel product for welded structure]
The welded structure cast steel product includes a step of casting the welded structure cast steel and a step of tempering the cast product obtained in the casting process. It is preferable that the manufacturing method of the cast steel product for welded structure further includes a step of heating and annealing the cast product to an austenitizing temperature or higher after the casting step and before the tempering step.

<Casting process>
In the casting process, C: 0.10% by mass to 0.17% by mass, Si: 0.01% by mass to 0.40% by mass, Mn: 0.7% by mass to 1.4% by mass, Ni : 1.00 mass% or more and 2.00 mass% or less, Cr: 0.20 mass% or more and 0.50 mass% or less, Mo: 0.10 mass% or more and 0.30 mass% or less, V: 0.05 mass % To 0.20% by mass and the balance: Fe and a composition which is an inevitable impurity, and a weld cracking susceptibility composition Pcm represented by Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B Cast a welded structural cast steel of 0.3 or less.

<Annealing process>
In the annealing step, the structure is homogenized by reheating the cast product of the welded structural cast steel to an austenitizing (γ) temperature or higher. As a minimum of this annealing temperature, 900 degreeC is preferable and 910 degreeC is more preferable. On the other hand, as an upper limit of annealing temperature, 1000 degreeC is preferable and 980 degreeC is more preferable. If the annealing temperature is less than the above lower limit, homogenization of the cast product becomes insufficient, and the quality of the cast steel product for welded structure may vary. Conversely, when the annealing temperature exceeds the above upper limit, the cast steel product may be deformed.

  As a minimum of the holding time of the said annealing temperature, 1 hour is preferable and 3 hours is more preferable. On the other hand, the upper limit of the annealing temperature holding time is preferably 15 hours and more preferably 10 hours. When the annealing temperature holding time is less than the lower limit, homogenization of the cast steel for welded structure may be insufficient. On the other hand, when the annealing temperature holding time exceeds the above upper limit, the production cost of the welded structural cast steel product may be unnecessarily increased.

<Refining process>
The tempering step is a step of heating the cast product obtained in the casting step above the austenitizing temperature, a step of cooling the cast product so that ferrite and bainite are the main composition, and the casting product from the austenitizing temperature. Tempering at a low temperature.

(Heating process)
In the heating step, the structure is austenitized by heating the cast product of the welded structural cast steel to a temperature above the austenitizing temperature. The lower limit of the heating temperature in this heating step is preferably 860 ° C., more preferably 880 ° C., although it depends on the composition of the welded structural cast steel. On the other hand, as the heating temperature in the heating step, 1000 ° C. is preferable, and 980 ° C. is more preferable. When the heating temperature in the heating step is less than the above lower limit, the crystal grains cannot be increased, and the grain interfacial area is increased, so that the formation of bainite may be insufficient or the dislocation density cannot be increased sufficiently. There is a fear. Conversely, if the heating temperature in the heating step exceeds the above upper limit, the toughness may be insufficient.

  The lower limit of the holding time at the heating temperature is preferably 1 hour and more preferably 2 hours. On the other hand, the upper limit of the holding time at the heating temperature is preferably 10 hours, and more preferably 7 hours. If the holding time at the heating temperature is less than the lower limit, homogenization of the cast steel for welded structure may be insufficient. On the other hand, when the holding time at the heating temperature exceeds the upper limit, the manufacturing cost of the cast steel product for welded structure may increase unnecessarily.

(Cooling process)
In the cooling step, the dislocation density is 1.35 × 10 ¹³ m ⁻² or more, preferably 1.80 × 10 ¹³ m ⁻² or more, more preferably 2.00 × 10 ¹³ m ⁻² or more. The casting is cooled. Specific examples of the cooling method include forced air cooling, water cooling, and oil cooling.

  The lower limit of the cooling rate in the cooling step is preferably 2 ° C / min on the surface of the cast product, more preferably 5 ° C / min, and even more preferably 8 ° C / min. On the other hand, the upper limit of the cooling rate in the cooling step is preferably 250 ° C./min on the surface of the cast product. If the cooling rate in the cooling step is less than the lower limit, the dislocation density cannot be increased sufficiently, and the strength may be insufficient. Conversely, when the cooling rate in the cooling step exceeds the above upper limit, the cast steel product for welded structure may be inhomogeneous.

  As a minimum of the ultimate temperature in the above-mentioned cooling rate in a cooling process, 250 ° C is preferred and 280 ° C is more preferred. On the other hand, the upper limit of the reached temperature in the cooling step is preferably 450 ° C, more preferably 400 ° C. When the ultimate temperature in the cooling step is less than the lower limit, a bainite structure may not be sufficiently formed. On the contrary, there is a possibility that a bainite structure cannot be sufficiently formed even when the temperature reached in the cooling step exceeds the upper limit.

  In the cooling step, the ratio of the bainite structure can be increased by holding the temperature reached after cooling to the temperature reached at the cooling rate. It is not necessary to strictly hold at the above-mentioned temperature, and after cooling to the above-mentioned temperature at the cooling rate, it may be gradually cooled to room temperature.

(Tempering process)
In the tempering step, the cooled cast product is tempered at a temperature lower than the austenitizing temperature. Thereby, the toughness of the cast steel product for welded structure can be improved.

  As a minimum of tempering temperature, 400 degreeC is preferable and 500 degreeC is more preferable. On the other hand, as an upper limit of tempering temperature, 650 degreeC is preferable and 630 degreeC is more preferable. When the tempering temperature is less than the lower limit, the toughness may not be sufficiently improved. On the other hand, when the tempering temperature exceeds the above upper limit, the dislocation density is lowered by tempering, and the strength may be insufficient.

  The lower limit of the tempering time (the holding time at the tempering temperature) is preferably 2 hours, and more preferably 3 hours. On the other hand, the upper limit of the tempering time is preferably 15 hours, and more preferably 10 hours. If the tempering time is less than the above lower limit, the toughness may not be sufficiently improved. On the other hand, when the tempering time exceeds the above upper limit, the strength may be insufficient, or the manufacturing cost may increase unnecessarily.

<Advantages>
The cast steel product for welded structure has a content of C, Si, Mn, Ni, Cr, Mo and V within the above ranges, a weld cracking susceptibility composition Pcm of 0.3 or less, and a dislocation density of 1 When it is .35 × 10 ¹³ m ⁻² or more, it has a relatively excellent weldability that allows welding without preheating or afterheating even at 0 ° C., which is lower than room temperature, and relatively excellent strength.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

  For example, the cast steel product for welded structure may contain inevitable impurities other than P, S, and Cu, and may contain, for example, O (oxygen), H (hydrogen), etc. as inevitable impurities.

  EXAMPLES Hereinafter, although this invention is explained in full detail based on an Example, this invention is not interpreted limitedly based on description of this Example.

(Examples 1 to 4 and Comparative Examples 1 to 5)
Examples 1 to 4 and Comparative Examples 1 to 5 of the cast steel product for welded structure include C, Si, Mn, Ni, Cr, Mo and V (Comparative Example 5 further added Cu), with the balance being iron and A prototype of welded cast steel was obtained by casting, annealing and tempering cast steel for welded structure, which is an inevitable impurity.

  Table 1 shows the measured values of the contents of P and S as unavoidable impurities in addition to the measured values of the contents of each metal in Examples 1 to 4 and Comparative Examples 1 to 5. Table 1 also shows the value of the weld crack susceptibility composition Pcm calculated from the content of each metal. The metal and P contents were measured using an emission analyzer “PDA-1017” manufactured by Shimadzu Corporation, and the C and S contents were measured using a carbon / sulfur analyzer “EMIA-920V” manufactured by Horiba. And measured.

  In the trial production of Examples 1 to 4 and Comparative Examples 1 to 5 of cast steel products for welded structures, first, cast steel for welded structures having the above composition was cast. Subsequently, this cast product was cut, and a rectangular parallelepiped test piece of 25 mm × 25 mm × 180 mm (for tensile test and impact test described later) and a rectangular parallelepiped test piece of 220 mm × 170 mm × 60 mm (weld crack test described later) Created).

  First, the two types of test pieces were heated to 920 ° C. and held for 5 hours to austenite, then gradually cooled to 300 ° C. in the furnace, and then annealed by air cooling from 300 ° C. to room temperature. This annealed test piece is fully austenitized by heating to 890 ° C. and holding for 3 hours, followed by cooling to 300 ° C. at a cooling rate of 2 ° C./min or 10 ° C./min, and then from 300 ° C. to room temperature. The main structure was made into ferrite and bainite by air cooling. Further, this test piece was tempered by heating to 590 ° C., 620 ° C. or 640 ° C. and holding for 6 hours to obtain Examples 1 to 4 and Comparative Examples 1 to 5 of cast steel products for welded structure. The cooling rate and tempering temperature applied to each example are shown in Table 1. Each temperature is the surface temperature of the test piece.

(Dislocation density)
Examples 1 to 4 and Comparative Examples 1 to 5 of welded structural cast steel cut into cubes with a side of 15 mm were measured by the Williamson-Hall method using an Rigaku X-ray diffractometer “RINT-1500”. did.

(Tensile test)
The tensile strengths of Examples 1 to 4 and Comparative Examples 1 to 5 of the welded cast steel products were measured at room temperature using a 14A test piece in accordance with JIS-Z2241 (2015).

(Charpy impact test)
As an index of toughness in Examples 1 to 4 and Comparative Examples 1 to 5 of welded cast steel products, the absorbed energy in the Charpy impact test was measured several times in accordance with JIS-Z2242 (2005), and the average value and standard Deviation was calculated.

(Weld crack test)
For the weldability of Examples 1 to 4 and Comparative Examples 1 to 5 of the cast steel product for welded structure, a y-type weld crack test was performed in accordance with JIS-Z3158 (1993). The welding was performed by semi-automatic CO ₂ welding with a test piece temperature of 0 ° C. and a heat input of 16 kJ / cm. The evaluation of weldability is “A” when the crack rate in the y-type weld cracking test is 0%, “B” when the crack rate is over 0% and 1% or less, and “B” when the crack rate is over 1%. C ”.

  Table 2 shows the results of the dislocation density measurement, tensile test, Charpy impact test, and weld crack test. In addition, "-" in a table | surface means that the measurement is not implemented.

  As described above, it was confirmed that the cast steel for welded structure having the composition and dislocation density described in the embodiment has a sufficient tensile strength of 550 MPa or more and has good weldability.

  However, in Example 4 where the C content is 1.7% by mass and the substantially upper limit value, the weldability is evaluated to be slightly low although it is within the allowable range.

On the other hand, Comparative Examples 1, 2, 3 and 5 having a dislocation density smaller than 1.35 × 10 ¹³ m ⁻² have insufficient tensile strength.

  In Comparative Example 4 in which the Si content is greater than 0.40% by mass, the standard deviation of absorbed energy, that is, the variation in toughness is large.

  Further, Comparative Example 5 in which the weld cracking susceptibility composition Pcm is larger than 0.3 has poor weldability, and the weld cracking susceptibility composition Pcm is smaller than 0.3 but close to 0.29 and 0.28. In Example 4 and Comparative Example 4, the weldability is evaluated to be slightly low.

  The cast steel product for welded structure and the cast steel product for welded structure of the present invention are suitably used for a large cast steel product for welded structure.

Claims (2)

C: 0.10% by mass to 0.17% by mass,
Si: 0.01 mass% or more and 0.40 mass% or less,
Mn: 0.7% by mass or more and 1.4% by mass or less,
Ni: 1.00 mass% or more and 2.00 mass% or less,
Cr: 0.20% by mass or more and 0.50% by mass or less,
Mo: 0.10% by mass to 0.30% by mass,
V: 0.05% by mass or more and 0.20% by mass or less, and the balance: Fe and a composition that is an inevitable impurity,
The weld cracking sensitive composition Pcm represented by the following formula (1) is 0.3 or less,
The main structure is ferrite and bainite,
A cast steel product for welded structure having a dislocation density of 1.35 × 10 ¹³ m ⁻² or more.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1) C: 0.10% by mass to 0.17% by mass,
Si: 0.01 mass% or more and 0.40 mass% or less,
Mn: 0.7% by mass or more and 1.4% by mass or less,
Ni: 1.00 mass% or more and 2.00 mass% or less,
Cr: 0.20% by mass or more and 0.50% by mass or less,
Mo: 0.10% by mass to 0.30% by mass,
V: 0.05% by mass or more and 0.20% by mass or less, and the balance: Fe and a composition that is an inevitable impurity,
Casting a welded structural cast steel having a weld cracking susceptibility composition Pcm represented by the following formula (1) of 0.3 or less;
Tempering the cast product obtained in the casting process ,
After the casting step and before the tempering step, a step of heating the cast product to an austenitizing temperature or higher and annealing , and
The tempering process is
Heating the casting above the austenitizing temperature; and
Cooling the cast product such that ferrite and bainite are the main composition;
Tempering the cast product at a temperature lower than the austenitizing temperature,
In the cooling step, cooling is performed at a speed at which a dislocation density is 1.35 × 10 ¹³ m ⁻² or more.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)