JP3298665B2 - Manufacturing method of high fatigue strength steel sheet for welded joint - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of manufacturing a high fatigue strength steel sheet for welded joints.

[0002]

2. Description of the Related Art In response to demands for lighter structures and larger capacities of structures, the strength of structural steel sheets is rapidly increasing. However, in a structure that is subjected to a repeated load, not only the yield strength but also the fatigue strength must be taken into consideration, and it may not be possible to meet the needs for higher strength. In particular, in a welded structure, fatigue cracks often occur from the weld toe, and even if the strength of the steel material is improved, the fatigue strength hardly improves.

It is known that the fatigue strength of a welded structure is mainly governed by the shape of the toe of the weld.
In some cases, measures for improving fatigue strength such as treatment of a toe portion of a welded portion are applied. However, the toe treatment not only increases the number of man-hours for building the structure, but also often cannot be carried out depending on the welded portion, and there is a strong demand for improvement in fatigue strength from the steel surface.

[0004] Since fatigue fracture of a welded joint generally occurs from a weld toe where stress concentration is large, it is known that the occurrence characteristics are greatly affected by the shape of the weld toe and have little effect on the steel material composition and structure. ing. Therefore, in order to improve the fatigue properties by controlling the steel structure, it is effective to delay the propagation of fatigue cracks generated at the toe in the thickness direction.

On the other hand, the means for generating separation and delaying the propagation of fatigue causes separation only in a range of large stress, and this effect is effective only in a small range of stress. Difficult to improve.

The present inventors have disclosed in Japanese Patent Application No. 4-68497 a billet or a steel plate having a temperature of Ac ₃ or more.
Assuming that the sheet thickness during water-rolling during rolling is t ₀ , an area of 0.05 × t ₀ (mm) or more from the surface layer at least in the sheet thickness direction is 2
Rapid cooling to Ar ₁ point or less at a cooling speed of at least
The surface layer portion starts or resumes rolling from a temperature of more than ₃ points Ar, (Ac ₃ point -100) to exit the rolling in a temperature range of Ac ₃ point from ℃ point, be recuperation or more subsequent Ac ₃ point At least up to Ar ₁ point, and
/ sec or more at a cooling speed of not less than 5% of the plate thickness from the front and back surfaces of the steel plate, and the aspect ratio (long axis diameter / short diameter) of a colony composed of grains having the same crystal orientation between adjacent crystal grains. A method of manufacturing a steel sheet having excellent fatigue properties to obtain a structure having a ratio of a shaft diameter of 4 or more and a short axis diameter of 5 μm or less is proposed.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention, in order to improve the fatigue strength of the welded portion, it is an object to provide a manufacturing technique which can prevent the propagation of the initial course of fatigue cracks.

[0008]

The gist of the present invention is that Ac ₃
The steel strip or steel sheet of a point above the temperature, the plate thickness at the middle in the rolling water cooling t _0, when the product thickness was _{t, 2 × t 0 / t} (mm) or more at least in the thickness direction from the front surface , 0.3 × t ₀
And rapidly cooled to Ar ₁ point or less at 2 ° C. / sec or more cooling rate the surface area below, then, the surface area is resumed rolling from Ar ₃ temperature below, Ac from (Ac ₃ -50) ℃ ₃ Exit rolled in the range of ° C., after which the surface area to at least Ar ₁ point without recuperation _three or more points Ac 1
° C. and cooled in / sec or more cooling rate, further (Ac ₁ -100) ℃
A high fatigue strength steel sheet for welded joints, wherein the steel sheet is heated to a temperature of Ac ₃ or more while staying at a temperature of (Ac ₁ ) ° C. for 120 seconds or more and then rapidly cooled at a cooling rate of 10 ° C./sec or more. Is the way.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The structural steel to be used in the present invention is a steel containing the types and amounts of additional elements which are determined by ordinary welded structural steel to obtain the required material.

[0010] Each of these constituent elements, the reasons for their addition, and their amounts are shown below. C is added as an effective component for improving the strength of steel. However, an excessive content exceeding 0.20% not only increases the deformation resistance during two-phase rolling, but also makes rolling difficult. In addition, since island martensite precipitates in the welded portion and significantly deteriorates the toughness of steel, the content is restricted to 0.20% or less.

[0011] Si is required as a deoxidizing element of molten steel,
Further, it is useful as a strength increasing element, but if added in excess of 1.0%, the workability of the steel is reduced, and the toughness of the weld is deteriorated. Further, if the content is less than 0.01%, the deoxidizing effect is insufficient, so the addition amount is restricted to 0.01 to 1.0%.

Mn is also required as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is reduced and the workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, since Mn lowers the transformation temperature, the excessive addition lowers the two-phase rolling temperature too much,
Since deformation resistance increases, the upper limit is 2.0%.

[0013] Al and N are added to refine the steel by Al nitrides, and also to provide a solid solution and precipitation during the rolling process, thereby making the crystal orientation of the steel effective and effective for recrystallization. However, when the addition amount is small, the effect is not obtained, and when the addition amount is excessive, the toughness of the steel is deteriorated.
01 to 0.20%, N: 0.020% or less.
The above are the basic components of the steel targeted by the present invention, but preferably contain Nb 0.2% or less and Ceq 0.4% or less. Further, when alloying elements are added in accordance with the required properties for the purpose of increasing the base material strength or improving the joint toughness, if the transformation temperature is too low, the deformation resistance in the two-phase region increases, and the rolling resistance increases. , It is difficult to add Ni, Cr, Mo, Cu, W, Co, V,
Ti, Zr, Ta, Hf, rare earth element, Y, Ca, M
One or more of g, Te, Se, and B may be added, but the total amount is regulated within 4.5%.

[0014] It is known that a fatigue crack is generated from a weld toe in a weld. This cannot completely deny the existence of micro defects in the weld, and it is difficult to prevent the occurrence of fatigue cracks. In addition, when fillet welding is applied to the surface layer of the steel sheet, the structure and hardness of the steel sheet change due to heat effects.However, if there is a layer harder than the weld metal outside the heat affected area, the plastic zone at the tip of the fatigue crack is prevented from spreading. Therefore, fatigue crack propagation is greatly delayed. This crack propagation delay is determined by the surrounding hardness distribution.

According to the findings of the inventors, as a result of repeating the fatigue crack propagation test with the hardness distribution shown in FIG. 1 at the fatigue crack tip, the surface layer portion (at least in the thickness direction from the surface).
2 × t ₀ / t (mm) or more and 0.3 × t ₀ or less)
The difference between hardness and internal hardness is Vickers hardness Hv
It was found that the fatigue crack propagation can be greatly delayed when the number of points is 50 or more in (10) .

In the present invention, the difference in hardness is provided over a range of not less than 2 mm and not more than 30% of the plate thickness. According to the experiment, the heat-affected width of the target fillet weld toe was 1 mm to 1.5 mm, so it was set to 2 mm or more.

Further, up to 30% of the thickness of the sheet, the life of the fatigue fracture which propagates in the direction of the thickness is reduced, so that it is not necessary to harden the sheet by 30% or more. Further, when the sheet thickness at the center of the sheet is 30% or more, the toughness is remarkably deteriorated. Therefore, the thickness is set to 30% or less.

In order to form the structure of the present invention on the surface layer of the steel sheet, for example, the surface of the steel sheet is water-cooled during rolling, and Ar ₁
If it is below the point, it will be transformed into ferrite once,
Utilizing the sensible heat at the center of the sheet thickness where the temperature hardly decreases even by cooling, further rolling is performed while raising the temperature of the ferrite structure of the surface layer, and the surface layer is cooled at a cooling rate of 1 ° C / sec or more, A modified structure having a high residual dislocation density is formed in the surface layer.

Further, (Ac ₁ -100) ° C .- (Ac ₁ ) ° C.
When the temperature is kept at a temperature of Ac ₃ or more while staying for 120 seconds or more in the range, cooling is performed at a cooling rate of 10 ° C./sec or more.
The processed ferrite grains grow by the residence for more than one second and become larger in grain size, further obtain a γ phase, and are transformed into martensite by rapid cooling at 10 ° C./sec or more, thereby improving the hardenability.

FIG. 2 shows the processing heat history in the present invention.
FIG. 3 schematically shows the change in the particle size of the surface layer portion and the inside of the steel sheet of the present invention. 10 indicates an outer layer, and 11 indicates an inner layer.

[0021]

EXAMPLES The components of the test steel are shown in Table 1, and the manufacturing conditions and the obtained materials are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

[Table 6]

[0028]

[Table 7]

Test Nos. 1 to 12 in Table 2 show examples of the present invention, and Test Nos. 13 to 24 show comparative examples. According to the Invention Example The interior and the surface layer portion average hardness of each site, Vickers
It has a difference of 50 points or more in hardness Hv (10) , and is excellent in fatigue strength and joint toughness.

On the other hand Comparative Example 13 given the way the cooling, but through a rolling process, Ac ₁ since -100~Ac residence time between ₁ took short, prior austenite grain size is small enough hardness is obtained I couldn't.

In Comparative Examples 14, 17 to 24, since the rolling was not performed during the heating, the residual dislocation density in the surface layer did not change to the inside. The effect of granulation could not be utilized. In Comparative Example 14, the hardness of the surface layer was increased due to the high holding temperature of the heat treatment, but the inside was hardened, and the toughness and elongation were not sufficient.

[0032] Comparative Example 15 is the maximum recuperation temperature after heated rolling becomes Ac ₃ or more, the residual dislocation density has been reduced, the grain growth in the retention of the subsequent Ac ₁ -100~Ac ₁ pronounced In addition, the prior austenite particle size could not be coarsened, and sufficient surface hardness could not be obtained.

In Comparative Example 16, although the steel sheet was subjected to a predetermined temperature-increasing rolling, the holding temperature of the heat treatment was low, so that the martensite ratio was insufficient and did not reach the predetermined hardness level.

In Comparative Examples 17, 18, 21 to 24, no heat treatment was applied, so that the hardness of the surface layer was at a general fatigue strength level regardless of the inside.

In Comparative Example 19, since the heat treatment was applied, the surface layer was hardened, but the thickness was 30% or more, and the required load during processing was not improved.

Further, in Comparative Example 20, martensite was not obtained because the cooling rate after the heat treatment was low, and the surface layer was not cured. In addition, although the surface layer is hardened, it is not hardened by a chemical component, so that there is no deterioration in weld joint toughness.

[0037]

According to the present invention, if a layer harder than the weld metal is present on the surface layer, the extension of the plastic zone at the tip of the fatigue crack is prevented, and the propagation of the fatigue crack is greatly delayed. Its use is great.

[Brief description of the drawings]

FIG. 1 is a table showing a relationship between a propagation delay and a difference in hardness.

FIG. 2 is a chart of a processing heat history of the present invention.

FIG. 3 is a schematic view of a steel sheet structure of the present invention.

Continuation of the front page (56) References JP-A-5-271856 (JP, A) JP-A-5-271860 (JP, A) JP-A-6-49596 (JP, A) JP-A-6-49595 (JP) JP-A-3-187738 (JP, A) JP-A-4-141517 (JP, A) JP-A-3-104820 (JP, A) JP-A-53-80317 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/02 C22C 38/00-38/60

Claims (1)

(57) [Claims] The steel strip or steel sheet according to claim 1] Ac ₃ point or higher, the thickness at the middle in the rolling water cooling t _0, when the product thickness was t, 2 × t in at least the thickness direction from the front surface ₀ / t (m
m) The surface layer area of not less than 0.3 × t _{0 is} rapidly cooled at a cooling rate of not less than 2 ° C./sec to Ar ₁ point or less.
Rolling is resumed from a temperature of the region Ar ₃ or less, and (Ac ₃ −
50) to exit the rolling within a range of Ac ₃ ° C. from ° C., then,
The surface layer region is cooled to at least Ar ₁ point without recuperation to the Ac ₃ point or more at 1 ° C. / sec or more cooling rate, 12 in further (Ac ₁ -100) from ° C. (Ac ₁₎ of ° C. range
After keeping the temperature of Ac ₃ or more while keeping it for 0 second or more,
A method for producing a high fatigue strength steel sheet for a welded joint, wherein the steel sheet is quenched at a cooling rate of 10 ° C./sec or more.