JPH11236642A - High fatigue strength thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain the increase of the fatigue strength of a steel plate without deteriorating its fracture toughness by arranging lamellar structures in which martensite as-quenched, residual austenite or their mixed structure occupies >=a specific ratio in a base phase contg. C, Si, Mn and Al of a specified compsn. and composed of ferrite or the like, at specified thickness and specified intervals in the plate thickness direction. SOLUTION: This steel plate is composed of, by weight, 0.02 to 0.35%, C, 0.02 to 2.0% Si, 0.30 to 2.5% Mn, 0.002 to 0.10% Al, the balance Fe with inevitable impurities. The base phase of the steel plate is composed of ferrite, pearlite, bainite and tempered martensite. In the base phase, lamellar structures in which martensite as-quenched, residual austenite or their mixed structure occupies >=50% are arranged at 2 to 30 μm thickness and 20 to 200 μm intervals in the plate thickness direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high fatigue strength steel plate used as a member in a structure.

[0002]

2. Description of the Related Art In structures receiving variable loads such as ships, marine structures, bridges, and land-based structures, fatigue cracks are generated and propagated from welds and stress-concentrated parts of the structure, causing the structures to break. Cause. Therefore, for example, Japanese Patent Publication No. 54-3
Various approaches have been taken to avoid stress concentration in design and construction as seen in Japanese Patent No. 0386. However, such a method has disadvantages such as causing great restrictions on the design and construction of the structure and increasing the weight of the structure.Therefore, a method of increasing the fatigue strength of the steel itself without relying on the design and construction. Has been desired.

[0003] As a method for increasing the fatigue strength of such a steel material, Japanese Patent Publication No. Hei 3-61748 and Japanese Patent Laid-Open Publication No. Hei 3-2913 are known.
No. 55 publication. However, the former technique is relatively C
Since application to steel having a high content is limited, its application is limited to strength members such as automobile parts, and application to the above-described structures has been difficult. Further, the latter technique has a drawback that the method of use is greatly restricted due to the inhomogeneous material in the thickness direction.

On the other hand, as an example of a mixed-structure steel containing martensite, as shown in JP-A-57-108241, there is a mixed-structure steel of ferrite and martensite. Although it is a so-called thin plate, it has been shown to be excellent in strength-elongation balance,
There is no effect of improving the fatigue strength. Examples of similar steels include JP-A-57-137422 and JP-A-58-69.
No. 37 publication.

Japanese Patent Application Laid-Open (JP-A) No. 58-93814 and Japanese Patent Application Laid-Open (JP-A) No. 62-174322 disclose examples of composite structure thick steel sheets containing martensite, all of which aim at a low yield ratio. There is no effect of improving fatigue strength. Regarding the improvement of the fatigue strength by the mixed structure of ferrite and martensite, as shown in, for example, Journal of the Japan Society of Mechanical Engineers Vol. 57, No. 544 (1991-12), p. There is a report that fatigue strength is improved when is distributed,
A thick steel plate having such a structure has an excessively large martensite ratio, and thus has a deteriorated fracture toughness.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to achieve high fatigue strength of a thick steel plate by controlling a metal structure without impairing fracture toughness.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a high fatigue strength steel sheet by defining the state of a metal structure including martensite. That is, the gist of the present invention is as follows. (1) By weight%, C: 0.02 to 0.35%, Si: 0.02 to 2.0%, Mn: 0.30 to 2.5%, Al: 0.002 to 0.10% The balance is steel composed of Fe and unavoidable impurities, and in a matrix composed of ferrite, pearlite, bainite, and tempered martensite, as-quenched martensite or residual austenite or a mixed structure of them occupying 50% or more. A high fatigue strength thick steel plate wherein the structure is arranged in the thickness direction within a thickness of 2 μm or more and 30 μm or less and an interval of 20 μm or more and 200 μm or less.

(2) The steel further comprises one or two of Nb: 0.002 to 0.10% and Ti: 0.002 to 0.10% by weight. 2. The high fatigue strength thick steel sheet according to the above item 1. (3) In addition to the above steel, Cu: 0.05 to 3.0%, Ni: 0.05 to 5.0%, Cr: 0.05 to 10.0%, Mo: 0.05 3 to 3.5%, Co: 0.05 to 10.0%, W: 0.05 to 2.0%. Fatigue strength steel plate. (4) In addition to the above steel, V: 0.002-
4. The high fatigue strength thick steel sheet according to any one of the above items 1 to 3, which contains 0.10%. (5) In the above steel, B: 0.0003-
5. The high fatigue strength thick steel sheet according to any one of the above items 1 to 4, which contains 0.0025%. (6) In the above steel, Rem: 0.002 by weight%.
The high-fatigue-strength steel sheet according to any one of the preceding items 1 to 5, wherein the steel sheet contains one or two of 0.1 to 0.10% and Ca: 0.0003 to 0.0030%. (7) In the above steel, Mg: 0.0003 by weight%.
1 to 6 characterized by the above-mentioned.
The high fatigue strength thick steel sheet according to any one of the above.

There is a fatigue limit as an index representing the fatigue strength.
In general, the fatigue limit tends to increase uniformly as the tensile strength or yield strength obtained in a tensile test increases. The fatigue limit of ordinary steel has been reported as a function of the yield strength, for example, as shown below (Takahashi et al .: Transactions of the Japan Society of Mechanical Engineers, No. 38).
Vol. 310, No. 310, p. 1154). σ _o = 0.382 σ _y +23.9 (1) where σ _o : fatigue limit (kgf / mm ² ) σ _y : yield strength (kgf / mm ² ).

Therefore, if a fatigue limit equal to or greater than the value predicted from this equation can be obtained for the same steel, it can be considered that the present invention has increased the fatigue strength. However, the applicable range of this equation is σ _y ≧ 45 kgf / mm ² .

[0011] Further, the as-quenched martensite or retained austenite can be confirmed from the diffraction pattern of the lattice image by an electron microscope. As a simpler method, after polishing the surface of the steel material to a mirror surface, a modified Repeller reagent such as It is corroded by an etching method (a method of pre-corroding with hydrochloric acid and then corroding with a mixed solution of an aqueous solution of sodium pyrosulfite and an aqueous solution of picric acid) and can be recognized as being observed on the observation surface. 1 (a) and 1 (b) show examples of the metallographic structure in a 200 × magnification micrograph.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The technical idea underlying the present invention is as follows. Thick steel sheets having a tensile strength of 60 kg / mm ² or more are usually manufactured by quenching and tempering or direct quenching and tempering after rolling, and the metal structure is often martensite, bainite, or a mixed structure thereof. Further, steels having a tensile strength of 50 kg / mm ² or more and less than 60 kg / mm ² are usually produced as they are rolled or normalized or by accelerated cooling after rolling, and have a metal structure of ferrite-pearlite or ferrite-bainite. Often a mixed tissue. These steels have relatively high fatigue strength due to their high strength, but as expected from the above equation, the same yield strength results in almost the same fatigue strength regardless of the component system Therefore, even if the metal structure was controlled, it was not possible to obtain higher fatigue strength with the same yield strength.

However, the present inventors have found that a steel having a structure in which martensite and retained austenite, which are in a transformed state harder than the parent phase, are distributed in a layered manner so as to divide the parent phase, has remarkably high fatigue strength. Was. The metallographic structure of this steel is basically a steel composed of ferrite, pearlite, bainite or tempered martensite and a mixed structure thereof. When a metal structure with a relatively narrow width and high hardness is arranged in a row in this matrix, the growth of fatigue cracks is stopped by the hard phase, and the direction of crack growth is changed or the cracks branch and grow. I will be. This significantly increases the time to fracture and increases fatigue life. What is important here is that the metal structure having high hardness is repeatedly arranged in a layered manner in the thickness direction, and as a result, fatigue cracks that attempt to penetrate the thickness always hit the hard phase regularly, so the growth rate is Significantly reduced, resulting in longer fatigue life.

Based on the above-mentioned new findings regarding the high fatigue strength of thick steel plates, the inventors of the present invention, based on the chemical composition of steel,
As a result of investigating the metal structure of the steel in detail, a high fatigue strength steel as shown in the claims was invented. Hereinafter, the reasons for limiting the chemical components and the metal structure will be described in detail. First, the reasons for limiting the components of the steel of the present invention will be described.

C is an element effective for strengthening steel, and if it is less than 0.02%, sufficient strength cannot be obtained. On the other hand, when the content exceeds 0.35%, the weldability deteriorates.

Although Si is effective as a deoxidizing element and as a strengthening element for steel, it is not effective at a content of less than 0.02%. On the other hand, if it exceeds 2.0%, the surface properties of the steel are impaired.

Mn is an element effective for strengthening steel.
If it is less than 30%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 2.5%, the workability of steel deteriorates.

Al is added as a deoxidizing element. 0.0
If the content is less than 02%, the effect is not obtained, and if it exceeds 0.1%, the surface properties of the steel are impaired.

The following elements are selectively added according to the purpose. All of Ti and / or Nb function effectively in terms of crystal grain refinement and precipitation hardening with a small amount of addition, but the effect is not obtained with a small amount of addition, and excessive addition of In order to deteriorate toughness, the addition amount of both Ti and Nb is limited to the range of 0.002 to 0.10%.

At least one of Cu, Ni, Cr, Mo, Co and W is added, and all are elements that improve the hardenability of steel. In the case of the present invention, the addition of the steel can increase the strength of the steel.However, if the addition amount is small, the effect of improving the hardenability is not obtained, and the addition of an excessive amount impairs the weldability of the steel, so To Cu: 0.05-
3.0%, Ni: 0.05 to 5.0%, Cr: 0.05
１10.0%, Mo: 0.05 to 3.5%, Co: 0.
05 to 10.0%, W: limited to the range of 0.05 to 2.0%.

V is effective in increasing the strength of the steel by precipitation hardening, but the effect is not obtained if the addition amount is small, and the addition of an excessive amount impairs the toughness of the steel. Is limited to the range of 0.002 to 0.10%.

B is an element that improves the hardenability of steel. In the case of the present invention, the addition can increase the strength of the steel, but if the addition amount is small, hardenability will not be improved, and excessive addition will increase the precipitation of B and impair the toughness of the steel, The content is 0.0003-0.00
The range is 25%.

At least one of Rem, Ca and Mg is added as required. Rem and Ca are effective in detoxifying S, but if the added amount is small, S remains harmful, and excessive addition impairs toughness.
0.10%, Ca: 0.0003-O. Add in the range of 0030%. Mg forms an oxide and contributes to the refinement of crystal grains, but its effect is small when the addition amount is small, and excessive addition generates a coarse oxide and impairs the toughness of the steel. The amount ranges from 0.0003 to 0.01%.

Next, conditions for limiting the metallographic structure in the present invention will be described. As described above, the present invention basically consists of ferrite, pearlite, bainite or tempered martensite or a mixed structure thereof, in which as-quenched martensite or residual austenite or a mixed structure thereof contains a total of 50 or more. % Or more, the layered structure is characterized by being arranged in the plate thickness direction with a thickness of 2 μm or more and 30 μm or less and an interval of 20 μm or more and 200 μm or less. There is no need to limit.

The structure dispersed in the matrix is a structure in which as-quenched martensite or retained austenite or a mixed structure thereof accounts for 50% or more, and desirably, the hardness of as-quenched martensite is 400 in Vickers hardness. With the above, the effect of suppressing the growth of fatigue cracks growing from the parent phase is great. Further, the retained austenite is not expected to have high hardness, but rather transforms into martensite with the progress of fatigue cracks, thereby providing the same effect as the above-mentioned as-quenched martensite.

When the ratio of as-quenched martensite or residual austenite or a mixed structure thereof to the structure arranged in a layered structure is less than 50%, the fatigue cracks slip through and propagate between them, so that the fatigue cracks The effect of suppressing progress is reduced.

Further, it is necessary that these structures are arranged in layers in a direction close to the direction perpendicular to the thickness of the steel sheet, but this is because fatigue cracks that attempt to penetrate the steel sheet always hit these hard phases repeatedly, This is to significantly reduce the speed. When the thickness (width in the thickness direction) of the layered structure is less than 2 μm, the effect of suppressing the growth of fatigue cracks is small, and
If it exceeds 0 μm, the fracture toughness deteriorates. Further, if the interval between the layered structures (in the thickness direction) is less than 20 μm, the ratio of the hard structure in the entire structure is too large to degrade the fracture toughness, and if it is more than 200 μm, the fatigue crack hits the hard structure. The frequency is too low and the effect of suppressing the growth of fatigue cracks is small.

FIG. 2 shows an example of a method for measuring the ratio, thickness (width in the thickness direction), and interval (width in the thickness direction) of the layer structure of as-quenched martensite or retained austenite or a mixed structure thereof. 2A to 2E schematically show the layered structure of martensite or retained austenite or a mixed structure thereof as quenched.

FIG. 2 (b) shows the layered structure b of FIG. 2 (a). When quenched martensite and residual austenite are divided and arranged in layers as shown in FIG. (A) of the as-quenched martensite or retained austenite or a mixed structure thereof and the area (B) of the inside of a broken line are determined, and the ratio of the as-quenched martensite or retained austenite or a mixed structure thereof within the broken line = Measure A / B. Also,
The distance between the layered structures = the distance between the broken lines, and the thickness of the layered structure = the thickness of the broken lines, as shown in the drawings.

[0030]

Next, the present invention will be described in detail with reference to examples. First, for steels having the components shown in Table 1, mechanical tests were performed on steels having the metal structures of the present invention steel and comparative steel shown in Table 2,
When a fatigue test is performed, strength, toughness and fatigue strength characteristics as shown in Table 2 are obtained. However, in both the tensile test and the impact test, a JIS No. 4 test piece was used. The fatigue test piece used had a parallel part diameter of 10 mm, a parallel part length of 22 mm, and a surface polished in the tensile direction. This was subjected to repeated tensile loads of different magnitudes at a stress ratio of 0 to obtain an SN diagram. Table 2 shows the fatigue limit obtained from the test results. The thickness, spacing, etc. of the layered structure in the metallographic structure were determined by mirror-polishing the cross section of the sample in the rolling direction x plate thickness direction and then corroding with a modified Repeller reagent to reveal a hard phase, which was taken from a photograph taken with an optical microscope. Was measured.

According to Table 2, the fatigue strength was clearly increased by the present invention, and it was confirmed that the present invention was effective. FIG. 3 shows the yield strength of the example and (fatigue limit / yield strength).
The relationship is shown below. The steel of the present invention and the comparative steel can be clearly separated,
Comparing with the same yield strength, it is understood that the steel of the present invention has significantly higher (fatigue limit / yield strength) than the comparative steel. The curve in the figure shows the boundary between the steel of the present invention and the comparative steel.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

In general, when the yield strength is determined, the fatigue limit is also determined. However, as shown in Table 2, when the same steel type is compared, the (fatigue limit / yield strength) of the steel of the present invention all increases. You can see that it is doing. Further, even if the fatigue strength is improved, there is almost no deterioration in toughness. Looking at the relationship between the yield strength and the (fatigue limit / yield strength) described in the example of FIG. 3, the steel of the present invention and the comparative steel are clearly separated from each other at the curve in the figure. The steel of the present invention (marked with ○) has significantly higher (fatigue limit / yield strength) than the comparative steel (marked with △). As described above, according to the present invention, high fatigue strength of a thick steel plate can be achieved without deteriorating fracture toughness, and a great industrial effect is achieved.

[Brief description of the drawings]

FIGS. 1A and 1B are micrographs (× 200) showing examples of the metal structure of the steel of the present invention.

FIGS. 2 (a) and 2 (b) are methods for measuring the ratio, thickness (width in the thickness direction), and interval (width in the thickness direction) of the layered structure of as-quenched martensite or retained austenite or a mixed structure thereof FIG.

FIG. 3 is a view showing the relationship between the yield strength and the (fatigue limit / yield strength) of the steel of the present invention and the comparative steel.

Claims (7)

[Claims] C .: 0.02 to 0.35%, Si: 0.02 to 2.0%, Mn: 0.30 to 2.5%, Al: 0.002 to 0. 10%, with the balance being Fe and unavoidable impurities, in a parent phase composed of ferrite, pearlite, bainite, tempered martensite, as-quenched martensite or residual austenite or a mixed structure thereof accounts for 50% or more. A high-fatigue-strength steel plate wherein the layered structure is arranged in the thickness direction within a thickness of 2 μm or more and 30 μm or less and an interval of 20 μm or more and 200 μm or less. 2. The steel according to claim 1, further comprising one or two of Nb: 0.002 to 0.10% and Ti: 0.002 to 0.10% by weight. Item 1
2. A high fatigue strength thick steel sheet according to item 1. 3. The steel further contains, in weight%, Cu: 0.05 to 3.0%, Ni: 0.05 to 5.0%, Cr: 0.05 to 10.0%, Mo: 0. 3. The composition according to claim 1, wherein the composition contains one or more of 0.05 to 3.5%, Co: 0.05 to 10.0%, and W: 0.05 to 2.0%. The high fatigue strength thick steel plate described. 4. The high-fatigue-strength steel plate according to claim 1, wherein the steel further contains V: 0.002 to 0.10% by weight. . 5. The high-fatigue-strength steel sheet according to claim 1, wherein the steel further contains B: 0.0003 to 0.0025% by weight. . 6. The steel according to claim 1, further comprising one or two of Rem: 0.002 to 0.10% and Ca: 0.0003 to 0.0030% by weight. Item 1
6. A high fatigue strength thick steel sheet according to any one of items 1 to 5. 7. The high fatigue strength steel sheet according to claim 1, wherein the steel further contains Mg: 0.0003 to 0.01% by weight. .