JPH09310116A - Production of high strength member excellent in delayed fracture characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength member capable of meeting the demand for >=1500MPa tensile strength and excellent in delayed fracture resistance. SOLUTION: The member has a composition containing, by weight 0.06-0.15% C, 0.08-0.5% Si, <=0.03% N, <=0.03% S, and <=0.03% P, satisfying 3.5%<=Mn+Cr+Mo+(Cu+Ni)/2+(V+W)/5+10Nb+XB<=5.5% (where1.0%<=Mn<=3.0%, 0.5<=Cr<=3.0%, <=4.0% Ni, <=1.0% Cu, <=2.0% Mo, <=0.5% W, <=0.3% V, and <=0.06% Nb are satisfied and XB=0.5% is satisfied when B is added by the amount in the range of 0.0008 to 0.005%, and further, 0.01-0.10% Ti is added in order to fix N when B is added), and having the balance essentially Fe with inevitable impurities. This alloy is heated temporarily up to a temp. not lower than the Ac3 point to undergo austenitizing, cooled by air cooling, etc., forged at 900-550 deg.C, and immediately formed into martensite by means of oil quenching or water cooling, or, after that, in order to release hydrogen in the material or dessipate the trap site of hydrogen, tempering is done 250-350 deg.C or 420-550 deg.C for >=10min.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for processing a member made of high strength steel having excellent delayed fracture characteristics.

[0002]

2. Description of the Related Art Usually, when a high strength steel having a tensile strength of more than 1200 MPa is used in an environment such as a bolt which is always under a high stress field, an accident due to delayed fracture becomes serious. Delayed destruction is a destruction phenomenon mainly due to internal diffusion of hydrogen. Various compositional trials are conducted to improve delayed fracture resistance. For example, reduction of C amount, Mo
Addition of V or V or increasing the tempering temperature to control the shape of grain boundary carbides, addition of B to suppress hydrogen diffusion at grain boundaries, and addition of Y or Ti to form a compound with diffused hydrogen. Attempts such as monkeys are being made.

However, the tensile strength is limited to a level of about 1400 MPa only by improving these components. This is because even if the amount of C is reduced or the shape of the carbide is controlled, a high tempering temperature is premised, so the amount of C added must be high.

In the case where both the reduction of C content and the enhancement of strength are achieved by utilizing the secondary hardening by Mo or V,
The material cost becomes extremely high, which poses a problem in terms of practicality.

In the manufacturing process as well, since the annealing and tempering processes are necessarily included, these relatively high temperature heat treatments are not desirable from the viewpoint of performance variation due to the influence of surface roughness and heat treatment cost.

[0006]

DISCLOSURE OF THE INVENTION The present invention has hitherto been carried out by using a material having a low C content and high hardenability to perform diffusion heat release treatment by ausforming and tempering, which are one of the thermomechanical treatments. Was impossible in 15
The present invention proposes a method for manufacturing a high-strength member having excellent delayed fracture resistance capable of handling a tensile strength of 00 MPa or more.

[0007]

[Means for Solving the Problems] The conditions for excellent delayed fracture are that the structure is fine, that segregation of carbides and the like at grain boundaries is small, that there are few hydrogen trap sites, and that diffusible hydrogen is sufficient. Are emitted, and the precipitated carbides are spherical and fine.

In order to satisfy these conditions, the strength should be increased with a smaller amount of C, the martensite lath obtained after quenching should be fine, and the trap sites of hydrogen due to strain should be eliminated to release diffusible hydrogen. Letting
It is effective to precipitate fine carbides in the grains.

When processing was performed in a metastable austenite state during quenching, a uniform and high dislocation density was inherited by the martensite transformation from work-hardened austenite, and the martensite lath was extremely fine and fragmented. It becomes a state. The tensile strength in this state is in the range of 0.06% ≤ C ≤ 0.15%, and is about 100 to 200 as compared with the state of just quenching without processing.
As high as 1 MPa, it reaches from 1300 to 1600 MPa.

In this state, however, since there are many hydrogen trap sites, tempering treatment for releasing hydrogen is required in the range of 250 ° C. to 350 ° C. or 420 ° C. to 550 ° C. By tempering at such a low temperature, fine carbide is deposited without segregation from a fine martensite structure with uniform intragranular strain to a grain boundary.

When processing with metastable austenite during quenching, diffusion transformations such as ferrite and bainite transformation are remarkably promoted. Therefore, existing low-alloy structural steels defined by JIS have sufficient hardness after quenching. I can't get it. In particular, it becomes remarkable in the region where the amount of C added is low.

Therefore, since the hardenability must be sufficiently enhanced, it is necessary to define the total amount of the elements that enhance the hardenability. The total amount of these hardenability improving elements is Mn + Cr +
Mo + (Cu + Ni) / 2 + (V + W) / 5 + 10Nb
When = H is defined, stable strength can be obtained by defining the lower limit as 3.5. However, if the total amount of the hardenability improving elements is too large, the forgeability is impaired, so the upper limit of H was set to 5.5.

Of the hardenability improving elements, Mn and Cr significantly enhance the hardenability and are excellent in cost.
Therefore, 1.0%, 0.5% or more are added respectively. However, even if Mn is added in excess of 3%, the ability to improve hardenability does not increase, so the upper limit is made 3%. Further, Cr tends to increase the deformation resistance, so the upper limit is made 3%.

Ni is usually added in the range of 1.5% to 4.0% to improve the hardenability without increasing the deformation resistance, but the hardenability improving ability is not so high, so the upper limit is 4%. 0.0%.

Although Cu has the effect of improving the hardenability, it deteriorates the hot workability, so its upper limit is defined as 1.0%.

Mo, W, V, and Nb have the effect of improving hardenability and the effect of secondary hardening, but at the same time, they form stable hard carbides and increase the deformation resistance. 2.0%, 0.5%, 0.3%, 0.06
%.

B, like Mn, Cr and Mo, also has the effect of remarkably enhancing the hardenability. However, since the effect of adding B is in the range of 0.0008 ≦ B ≦ 0.005,
Specified within this range. Further, at this time, it is necessary to fix N. Therefore, when B is added, 0.01 ≦ Ti ≦ 0.10
Must be added within the range.

C is an element that determines the strength of martensite. In the present invention, the strength range is 1200 MPa to 1
Since it is considered to be 600 MPa, the addition amount is specified to be 0.06% to 0.15%.

Although Si has the effect of improving hardenability,
At the same time, it has the effect of greatly increasing the deformation resistance, so the upper limit was defined as 0.5%. The lower limit of 0.08% is due to manufacturing restrictions.

Since S and P are elements that embrittle grain boundaries and deteriorate delayed fracture characteristics, their upper limits are 0.03%.
And

[0021]

Next, embodiments of the present invention will be described in detail. Table 1
Forging processing and quenching were performed according to the process shown in FIG. FIG. 2 shows the relationship between the hardenability index H, the forging temperature, and the hardness after quenching. The compressibility during forging is 50%.

[0022]

[Table 1]

From FIG. 2, it can be confirmed that when the hardenability index H is around 3.5, stable hardness after quenching can be obtained. Further, it can be seen that those having a hardenability index H of 3.5 or more have obviously increased strength as compared with the as-baked ones without processing.

FIG. 3 shows the relationship between the forging temperature and the tensile strength when the steel type E was once heated to 1030 ° C. and then forged during cooling.

From the results shown in FIG. 3, the forging temperature is 550 to 9
It can be confirmed that in the range of 00 ° C, extremely high tensile strength can be obtained as compared with the heat-quenched material.

Next, examples of delayed fracture characteristic evaluation will be described in detail. The test machine used is a cantilever bending type accelerated delayed fracture test machine, the outline of which is shown in FIG. With this method, when the bending stress / static bending stress is 0.7, it can be judged that the bolt or the like can be used unless it breaks for 30 hours or more.

FIG. 5 shows the manufacturing process of the test piece used in the delayed fracture test. FIG. 6 shows the relationship between the breaking time and the tensile strength when the bending stress / static bending stress is 0.7 in the cantilever bending type delayed fracture test. H is 3.5
With the above steel types, the test pieces manufactured according to Process 3 were hardly affected by the additional elements, and when the bending stress / static bending stress was 0.7, each showed a breaking time of 30 hours or more. On the other hand, in the test piece of Process 4 which is not processed, both the strength and the delayed fracture property are low, and the breaking time is less than 30 hours when the bending stress / static bending stress is 0.7. JIS
-Quenching of SCM435-The tempered material also has a delayed fracture property at such a high strength.

FIG. 7 shows the tempering temperature, tensile strength, and bending stress / bending stress of the test pieces manufactured by the processes 3 and 4.
The relationship with the breaking time when the static bending stress is 0.7 is shown. It can be seen that the breaking time extends when the tempering temperature is 250 ° C. to 350 ° C. and 420 ° C. or higher.

At 250 ° C. to 350 ° C., diffusible hydrogen solid-dissolved in the matrix is released, and at 420 ° C. or higher, diffusible hydrogen and non-diffusible hydrogen are released, and carbides are finely precipitated. The delayed fracture resistance is improved. However, in a region where the tempering temperature exceeds 550 ° C, the strength is significantly reduced, so the upper limit of the tempering temperature is set to 550 ° C.

The embodiment of the present invention has been described in detail above, but this is merely an example, and the present invention can be implemented in various modified forms without departing from the spirit of the invention.

[0031]

Industrial Applicability According to the present invention, it is possible to manufacture a member having a very high strength exceeding 1200 MPa, excellent toughness and delayed fracture characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing a forging history and a heat history used in a quenching test.

FIG. 2 is a diagram showing a relationship between a forging temperature and a tensile strength when the material is once heated to 1030 ° C. and then forged while being cooled.

FIG. 3 is a graph showing the relationship between the hardenability index H of the samples obtained according to Process 1 and Process 2, the forging temperature, and the hardness after quenching.

FIG. 4 is a diagram showing an outline of a delayed fracture evaluation test used in an experiment.

FIG. 5 is a diagram showing a forging history and a heat history of a test piece used in a delayed fracture test.

FIG. 6: Quenching index H, tensile strength and bending stress / static bending stress = 0.
It is the figure which showed the relationship of the fracture | rupture time at the time of 7.

FIG. 7: Quenching index H, tempering temperature and bending stress / static bending stress = 0.
It is the figure which showed the relationship of the breakage time at the time of 7.

Claims (1)

[Claims] 1. In weight%, 0.06 ≦ C ≦ 0.15,
0.08 ≦ Si ≦ 0.5%, N ≦ 0.03%, S ≦ 0.
03%, P ≦ 0.03%, Mn + Cr + Mo +
(Cu + Ni) / 2 + (V + W) / 5 + 10Nb + XB
= H, which is defined as the hardenability index H, is 3.5 ≦
H ≦ 5.5 (however, 1.0 ≦ Mn ≦ 3.0%, 0.5 ≦
Cr ≦ 3.0%, Ni ≦ 4.0%, Cu ≦ 1.0%, M
o ≦ 2.0%, W ≦ 0.5%, V ≦ 0.3%, Nb ≦
0.06%, B is 0.0008 ≦ B ≦ 0.005%
XB = 0.5 when added in the range. Also B
When N is added, 0.01 ≦ Ti ≦ 0.
It is assumed to be 10. ), And the unavoidable impurities and the balance are essentially Fe.
The alloy consisting of is once austenitized by heating at Ac 3 or higher, and then cooled by air cooling or the like,
Forging is performed in the range of 0 to 550 ° C, and then martensite is made by oil cooling or water cooling, and further 250 ° C to 350 ° C, or 420 ° C to 550 ° C.
A method for producing a high-strength member having excellent delayed fracture characteristics, which comprises performing tempering for 10 minutes or more in the range of