JP3509634B2 - Low alloy cast steel and its heat treatment method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a low alloy cast steel material having excellent balance of strength, weldability and shrinkage cavity resistance, which is used as a large thick member such as a crank throw of a large marine diesel engine, and the like. The present invention relates to a heat treatment method.

[0002]

2. Description of the Related Art As a cast steel product for welded structures, the JIS standard defines SCW620 as having a tensile strength of 620 MPa or more in G5102. Quenching was required for cooling during the austenitizing treatment (quenching) as a heat treatment for satisfying this strength. Further, in the case of the high-strength steel and the heat treatment method thereof (about 160 kg) described in JP-A-59-136451, it is necessary to perform quenching in oil cooling in order to obtain a tensile strength of 560 MPa or more. There was (a strength of only 534 MPa was obtained by air cooling).

[0003]

The above-mentioned conventional technique cannot be applied to a super-large member having a weight of several tons to several tens of tons. That is, in the case of a large-sized thick member having a diameter of 1 m or more, it is facility and operational to perform the heat treatment (quenching or normalizing: austenitizing treatment) for obtaining a predetermined strength with water cooling or oil cooling. Difficult to
In addition, it is uneconomical, and even if it is possible, it is difficult to secure a high cooling rate to the central portion.

Therefore, in such a super-large member,
Although air cooling is most suitable, in the above-mentioned conventional technology,
If the cooling rate during the austenitizing treatment is slow such as air cooling, it is impossible to obtain a predetermined strength. Therefore, the present invention, in the large-sized thick member, even when the cooling rate during the austenitizing process is slow as in air cooling, high strength can be obtained, and the main structure is the bainite structure. (Intensity difference between inside and outside)
It is a first object of the present invention to provide a low alloy cast steel material and a heat treatment method therefor capable of obtaining a high strength member which is uniform in the inside and the outside by reducing the above.

Another object of the present invention is to provide a low alloy cast steel material which can be used as a welded structural member and can be repaired by welding. Still another object is to produce shrinkage cavities during casting. It is an object of the present invention to provide a low alloy cast steel material that is not used.

[0006]

[Means for Solving the Problems] In order to achieve the first object, the present invention takes the following means. That is, the feature of the low alloy cast steel material of the present invention is, in mass%, 0.15
≤ C ≤ 0.22 , Si ≤ 0.4 , 0.7 ≤ Mn ≤ 1.4 , 1.2 ≤ Ni ≤ 2.5 , 0.
1 ≤ Cr ≤ 1.1 , 0.1 ≤ Mo ≤ 0.7 , V ≤ 0.3 , balance
A low alloy cast steel material that is qualitatively Fe and has a hardenability multiple α
There is a point of satisfying ≧ 560. However, α = 750C + 88Si
+ 162Mn + 81Ni + 34Cr + 301Mo + 454V + 20, and each component is mass %. The hardenability of steel significantly changes depending on the amount of alloying elements. Therefore, "hardenability multiple" using the amount of alloying element as a parameter is used as an index showing the hardenability.

By the way, when high strength is applied to a large thick cast steel material, the cooling rate at the center of the wall thickness becomes slow even with water cooling or oil cooling, high strength is difficult to obtain, and cracking may occur during quenching. There is. Therefore, a component (bainite structure with stable strength) that uniformly quenches the inside and outside of the member during air cooling (slow cooling rate) is required. Therefore, in the present invention, α which is a “hardenability multiple” that can secure high strength in the ordinary heat treatment (normalizing and tempering) is introduced. This α is the average cooling rate of 1 ° C /
It is a prediction formula (experimentally obtained formula) obtained from the tensile strength after tempering at 660 ° C. for min.

As in the present invention having the above structure, this α is 56
If it is 0 or more, the tempering temperature (60
At 0 ° C. to 700 ° C.), the ferrite ratio is about 40% or less, and the tensile strength of 560 MPa or more is stably obtained inside and outside the member. Further, in the present invention, in order to improve the weldability, the weldability index β ≦ 0.8 is satisfied. However, β = C + Mn / 6 + Ni / 15 + (Cr + Mo + V) / 5 Difficulty of welding of metallic materials (whether welding cracks occur, etc.)
The term "weldability" is used to describe Weld cracks are evaluated by using carbon equivalent as an index of weldability. When the carbon equivalent is large, the hardness of the heat-affected zone becomes high, which promotes the occurrence of cracks. Therefore, when the carbon equivalent is high, it is necessary to raise the preheating temperature during welding. In the case of a large cast steel product, it is difficult to preheat at a high temperature, so it is necessary to enable welding at a preheating temperature of 50 ° C or lower.

Therefore, in the present invention, as a result of various studies,
Find the relationship between the preheating temperature and the carbon equivalent, and
We have found a weldability index β ≦ 0.8 that is less than or equal to ℃. That is, according to the present invention having the above configuration, the preheating temperature is 50
Good welding is possible even at temperatures below ℃. Further, in the present invention, the castability index γ ≧ 16 is satisfied. However, γ = 39.9C + 10.0Si-0.8Mn-1.5Ni + 0.2Cr + 0.
6Mo-6.2V + 14.3 It is difficult to achieve directional solidification in the case of large thick cast steel products.
If the difference between the temperature at which the solid fraction is 0.7 and the liquidus temperature, that is, the solidification temperature range is narrow, a large shrinkage cavity tends to occur at the center of the wall thickness, and a certain solidification temperature range or higher is required. It turned out to be.

Therefore, the solid fraction of various multi-component components is 0.
The prediction formula γ representing the shrinkage cavity property obtained from the difference between the temperature of 7 and the liquidus temperature was found. That is, the castability index γ is 16
If it is above, it was found that the defect length was 5 mm or less, which satisfied the level 2 of BS (UK standard). Therefore, according to the present invention having the above structure, the length of the defect due to the shrinkage cavity can be set to 5 mm or less.

The reason for limiting the numerical values will be described below. C
Is an element that improves strength and hardenability, and if the content is less than 0.15%, it becomes difficult to obtain a predetermined strength.
Further, if it exceeds 0.22%, the susceptibility to weld cracking becomes high, so 0.15 to 0.22% is desirable. Si is
It is an element that improves the use as a deoxidizing agent and hardenability, but if the content is high, segregation becomes large, so 0.4
% Or less is preferable. Mn is an element that improves strength and hardenability, and if the content is less than 0.7%, it becomes difficult to obtain a predetermined strength. Further, if it exceeds 1.4%, temper embrittlement becomes remarkable, so 0.7 to 1.4% is preferable.

[0012] Ni is an element that improves the strength and hardenability and is an element that causes a relatively small decrease in weldability, so it is preferable to add Ni as much as possible, but if the content is less than 1.2%, it is predetermined. Is difficult to obtain. Also,
Since it is an expensive element, 1.2 to 2.5% is preferable.
Cr and Mo are elements that improve the strength and hardenability, and enhance the temper softening resistance, so if it is less than 0.1%, it becomes difficult to obtain a predetermined strength. Further, since it is an element that causes a decrease in weldability, Cr is 0.1 to 1.1%,
Mo is preferably 0.1 to 0.7%.

V is an element that enhances the temper softening resistance, but it is an element that hinders weldability and, even if added in a certain amount or more, a large effect of the softening resistance cannot be expected, so V.
3% or less is desirable. A feature of the heat treatment method of the present invention is that when the above-mentioned low alloy cast steel material is hardened or normalized, its cooling is air cooling, and then 600 ° C to 7 ° C.
It is at the point of tempering at 00 ° C. The air cooling is 900
This is a process in which the average cooling rate from 0 ° C to 400 ° C is slow (about 1 ° C / min at the center of a material corresponding to a diameter of 1000 mm).

By carrying out the heat treatment, a cast steel product larger than the conventionally known high-strength cast steel product,
It has a tensile strength of 560 MPa or more and can reduce the mass effect.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. In cast steel members, not only high strength but also weldability is a necessary item. The strength can be improved by increasing the alloy addition, but the weldability deteriorates as the added element increases. Therefore, the present invention balances both high strength and weldability. Further, in the case of a cast steel material, it is necessary to prevent shrinkage cavities that cause deterioration in strength, and the present invention is to improve shrinkage cavity characteristics in terms of components.

The low alloy cast steel material of the present invention is used for a super-large crank throw or the like. This cast steel material contains at least the following components ( mass %). C:
0.15-0.22, Si: 0.4 or less, Mn: 0.7
.About.1.4, Ni: 1.2 to 2.5, Cr: 0.1 to 1.
1, Mo: 0.1 to 0.7, V: 0.3 or less , the hardenability multiple α, the weldability index β, and the castability index γ are represented by the following formulas: α = 750C + 88Si + 162Mn + 81Ni + 34Cr + 301Mo + 454V + 2
0 , β = C + Mn / 6 + Ni / 15 + (Cr + Mo + V) / 5 , γ = 39.9C + 10.0Si−0.8Mn−1.5Ni + 0.2Cr + 0.6Mo−6.2
V + 14.3 , and α ≧ 560, β ≦ 0.8, and γ ≧ 16 are satisfied.

In the heat treatment method for the low alloy cast steel material, quenching or normalizing is performed by air cooling. 100 at that time
The average cooling rate at the center of the 0 mm diameter material is 900 ° C.
In the austenitizing process from 1 to 400 ℃
/ Min. Then, it tempers at 600-700 degreeC. According to the first embodiment, even in the case where the cooling rate of the austenitizing process is slow in the large thick member, the strength is satisfied, and the main structure is the bainite structure, and the mass effect (the center part and the outer peripheral part of the large member are The strength difference) can be made extremely small, and a stable high-strength member can be obtained, and at the same time, it can be used as a welded structural member and weld repaired. Further, shrinkage cavity defects during casting are prevented.

[0018]

[Example] "Example 1": Strength evaluation The components shown in Table 1 (excluding comparative steel 10) and other components were melted in an electric furnace. And with sand mold, 200mm × 5
It was cast into a size of 50 mm × 600 mm. The ingot was cut into a predetermined size to prepare a test piece. After holding this test piece at 920 ° C., it was controlled and cooled at a cooling rate of 1 ° C./min. This cooling rate corresponds to the slow cooling rate of thick castings and simulates air cooling. Then, tempering was performed at 660 ° C. Then, a tensile test piece was cut out from the test piece and a tensile test was performed.

[0019]

[Table 1]

The hardenability multiple α, the weldability index β, and the castability index γ in Table 1 are as follows. In addition, each component is mass %. α = 750C + 88Si + 162Mn + 81Ni + 34Cr + 301Mo + 454V + 2
0 , β = C + Mn / 6 + Ni / 15 + (Cr + Mo + V) / 5 , γ = 39.9C + 10.0Si−0.8Mn−1.5Ni + 0.2Cr + 0.6Mo−6.2
V + 14.3 , according to Table 1 above, Comparative Steels 7 to 10 do not have sufficient strength because α is less than 560. Comparative steel 7
Is a material equivalent to JIS SCW620. Comparative steel 10
Are disclosed in Japanese Patent Laid-Open No. 59-136451.
This is the data described in the publication.

FIG. 1 is a graph showing the results of the tensile test, showing the relationship between the hardenability multiple α and the measured value of tensile strength. From the above figure, α is 560 or more and high temperature is 660.
It was confirmed that even in tempering at ℃, a strength of 560 MPa or more was obtained on average. FIG. 2 shows the result of measuring the ferrite ratio of the test piece after the heat treatment. Figure 2
As shown in, the tensile strength is up to 560 MPa, the ferrite ratio is up to about 40%, the mass effect is small, and it is suitable for large-sized thick cast steel products. On the other hand, when the ferrite ratio exceeds 40% and increases, the tensile strength sharply decreases, and the inner and outer strengths become very large due to the mass effect.

[Embodiment 2]: The method for producing the weldability evaluation material is the same as that in Embodiment 1. The heat treatment was performed only at 920 ° C. The materials used were invention steel 1, comparative steels 1, 2, 3, 8, and 9, and the weldability index β was in the range of 0.59 to 0.88. Regarding the weld crack test, based on the JIS Z3158 y-type weld crack test method (Tig welding was performed and common metal was used for the welding rod), the relationship between the preheating temperature and the presence or absence of cracks was examined, and the cross-section crack ratio was The relationship between the critical preheat temperature of 0 and the weldability index β was determined.

FIG. 3 shows the test results. According to the figure, when β is 0.8 or more, the preheating temperature rises rapidly, and
It can be seen that a temperature of 0 ° C. or higher is required. "Example 3": Weldability evaluation For materials other than the materials used in Example 2 shown in Table 1 above (inventive steels 2 to 7 and comparative steels 4 to 7), the preheating temperature was 25.
The test was conducted at ℃, and only the presence or absence of cracks was confirmed. Cracking occurred in Comparative Steels 4 to 6 having a large weldability index β, and no cracking occurred in Invention Steel and other comparative steels.

"Example 4": Castability evaluation The invention steel 6 and the comparative steels 2, 5 and 8 shown in Table 1 were melted in an electric furnace and weighed about 10 tons (thickness: about 400 mm) in a sand mold. ) Cast steel products were cast. However, it does not use a chill to forcibly solidify. The center portion of the cast steel product was subjected to an ultrasonic flaw detection test to confirm the defect size and to determine the relationship with the castability index γ. FIG. 4 shows the relationship between the γ and the defect length.

According to the figure, the larger the γ, the smaller the defect. In the usual casting method, it is desirable that γ be 16 or more in order to reduce the defect length componentally (BS standard is 5 mm or less).

[0026]

According to the present invention, strength can be satisfied in a large-sized thick member. Further, it can be used as a welded structural member. Furthermore, it is possible to prevent defects in shrinkage cavities.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between hardenability multiple α and measured strength.

FIG. 2 is a graph showing the relationship between ferrite ratio and strength.

FIG. 3 is a graph showing the relationship between the weldability index β and the preheating temperature.

FIG. 4 is a graph showing the relationship between the castability index γ and the defect length.

─────────────────────────────────────────────────── ─── Continued Front Page (72) Koichi Sakamoto 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Prefecture Kobe Steel Works, Ltd. Kobe Research Institute (72) Inventor, Hitoshi Ishida Nishi-ku, Kobe-shi, Hyogo Prefecture Takatsukadai 1-5-5 Kobe Steel, Ltd., Kobe Research Institute (56) Reference JP-A-8-176728 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) ) C22C 38/00 C21D 6/00

Claims (3)

(57) [Claims] 1. In mass%, 0.15 ≤ C ≤ 0.22 , Si ≤ 0.4 , 0.7
≤ Mn ≤ 1.4 , 1.2 ≤ Ni ≤ 2.5 , 0.1 ≤ Cr ≤ 1.1 , 0.1 ≤ Mo ≤ 0.
A low alloy cast steel material containing 7 , V ≤ 0.3 and the balance being substantially Fe, wherein the hardenability multiple α satisfies α ≧ 560. However, α = 750C + 88Si + 162Mn + 81Ni + 34Cr + 301Mo +
454V + 20 , each component is% by weight. 2. The weldability index β is β ≦ 0.8 , and
Or / or the castability index γ satisfies γ ≧ 16.
The described low alloy cast steel material. However, β = C + Mn / 6 + Ni / 15 + (Cr + Mo + V) / 5 γ = 39.9C + 10.0Si − 0.8Mn − 1.5Ni + 0.2Cr + 0.6Mo − 6.2
V + 14.3 3. A low alloy cast steel material according to claim 1 or 2,
When quenching or normalizing, the cooling should be air cooling.
After that, it is characterized by tempering at 600 ° C to 700 ° C.
Heat treatment method for low alloy cast steel.