JP4497009B2 - Thick steel plate with excellent fatigue crack propagation characteristics and toughness and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate excellent in fatigue crack propagation characteristics and toughness suitable as a material for a hull, an offshore structure, a civil engineering building structure, a construction machine, an industrial machine, and the like, and a method for manufacturing the same.

    The application of high-strength steel is expanding with the demand for larger and lighter steel structures. As design stress increases due to higher strength, the occurrence of brittle fracture and fatigue fracture becomes a problem. In addition, from the viewpoint of reducing the life cycle cost of steel structures, it is desired to extend the life by suppressing fatigue fracture.

  In order to prevent brittle fracture, methods have conventionally been adopted to increase the toughness of steel by adjusting the composition and microstructure. In order to suppress fatigue fracture, there are a method of suppressing the occurrence of fatigue cracks from stress-concentrated parts such as welds and a method of suppressing the growth of generated fatigue cracks. Since effective measures may not always be taken due to constraints such as the design of the structure, it is effective to apply steel plates that can suppress fatigue crack growth by reducing the propagation speed of fatigue cracks. Therefore, the development of steel sheets with excellent toughness and fatigue crack propagation characteristics is desired.

  In order to reduce the fatigue crack propagation rate in a thick steel plate, there is a method of dispersing hard second phases such as bainite and martensite in the ferrite matrix as in Patent Document 1 and Patent Document 2, for example. When the hardness of the hard phase was increased for strengthening, there was a problem that caused toughness deterioration.

  Patent Document 3 discloses a method for reducing the crack propagation speed in the plate thickness direction by controlling the crystal orientation of ferrite. This method has a problem that fatigue crack propagation characteristics in directions other than the thickness direction cannot be improved.

  Patent Document 4 discloses a technique for improving fatigue characteristics by reducing the ferrite grain size to 1 to 3 μm. In this method, the toughness can be improved at the same time by refining the crystal grains, but rolling with a large rolling reduction of 50% or more in the austenite / ferrite two-phase region at a lower temperature than the normal hot rolling temperature. There is a problem that the load on the rolling mill becomes large and the occupation time of the rolling mill becomes long and the rolling efficiency decreases.

  Patent Document 5 discloses a technique for improving fatigue crack propagation characteristics by increasing the content of Si or Al to contain retained austenite in steel. However, increasing the content of Si and Al has the problem of deteriorating the toughness of the base metal and the weld heat affected zone.

In any of the above-described techniques, it is difficult to improve the fatigue crack propagation characteristics of the weld because the microstructure changes greatly in the weld heat-affected zone.
Japanese Patent Laid-Open No. 10-60575 JP-A-11-310846 Japanese Patent Application Laid-Open No. 08-199286 JP 2002-363644 A JP 2004-76156 A

  In view of the above-mentioned problems of the prior art, the present invention achieves excellent fatigue crack propagation characteristics in the base metal and welded part without applying a high load to the rolling mill and does not depend on the crack propagation direction. The object is to obtain a thick steel plate with excellent crack propagation characteristics and toughness.

  The gist of the present invention is as follows.

  (1) 1st invention is the mass%, contains C: 0.02-0.16%, Si: 0.05-0.5%, Al: 0.005-0.060%, Furthermore, Mn: 0.1-2.5%, Cu: 0.1- 2.0%, Ni: One or more selected from 0.1 to 6.0%, containing 2Mn + Cu + Ni with a value of 3.5 to 6.0%, the balance being Fe and inevitable impurities. It is a steel plate with excellent fatigue crack propagation characteristics and toughness characterized by containing 5 to 20% of retained austenite with an average diameter of 0.1 to 0.5 μm.

  (2) 2nd invention is further mass%, Cr: 0.05-3.0%, Mo: 0.05-2.0%, Nb: 0.003-0.10%, V: 0.003-0.10%, Ti: 0.003-0.05%, B: A thick steel plate excellent in fatigue crack propagation characteristics and toughness according to the first invention, characterized by containing one or more selected from 0.0003 to 0.0030%.

(3) In the third invention, after hot rolling the steel having the component composition described in the first invention or the second invention, direct quenching or reheating quenching is performed, and subsequently, 650 ° C or higher. It is a method for producing a thick steel plate excellent in fatigue crack propagation characteristics and toughness, characterized by performing a two-phase region heat treatment by heating to a temperature of less than Ac ₃ and cooling.

(4) In the fourth invention, instead of the direct quenching described in the third invention, after accelerated cooling from a temperature of Ar ₃ -20 ° C or higher to 400 ° C or lower at an average cooling rate of 1 ° C / s or higher , A method for producing a thick steel plate having excellent fatigue crack propagation characteristics and toughness, characterized by performing a two-phase heat treatment by heating to 650 ° C. or more and less than an Ac ₃ point.

(5) The fifth invention relates to a temperature of 600 ° C. or lower and 400 ° C. or higher at a cooling rate of 1 ° C./s or more on average after hot rolling the steel having the component composition described in the first or second invention. This is a method of manufacturing a thick steel plate with excellent fatigue crack propagation characteristics and toughness, characterized by controlling the temperature so that it stays in the temperature range of Ac ₃ -50 ° C or lower and 500 ° C or higher for 300 seconds or longer.

  (6) The sixth invention is a thickness excellent in fatigue crack propagation characteristics and toughness characterized by further tempering to a temperature of 700 ° C. or less after the heat treatment described in the third to fifth inventions. It is a manufacturing method of a steel plate.

    Thus, the present invention optimizes the stability of residual γ by dispersing fine residual γ that is moderately enriched with γ-stabilizing elements, and provides excellent fatigue crack propagation characteristics and toughness only for the base metal. It can also be achieved in the weld heat affected zone.

  As a result of examining the conditions for maximizing the effect of retained austenite (residual γ), the present inventors have optimized the stability of residual γ, and at the same time achieved high fatigue crack propagation characteristics and toughness. It was found that it can be obtained.

Residual γ with low stability refers to those that readily undergo martensitic transformation when subjected to plastic deformation or impact loads. In steel materials with a large amount of residual γ with low stability, martensitic transformation is induced by slight plastic deformation and load. The volume expansion associated with the martensitic transformation generates compressive stress around it, which significantly suppresses fatigue crack propagation, but reduces the toughness because a large amount of martensite with low toughness is generated.
On the other hand, stable residual γ refers to those that do not readily undergo martensitic transformation even when subjected to plastic deformation or impact load. An excessively stable residual γ improves the toughness, but the residual γ itself hardly hinders fatigue crack growth, so the fatigue crack propagation characteristics do not improve much. Therefore, to improve both toughness and fatigue crack propagation characteristics, it is important to optimize the stability of residual γ.

  The chemical composition of steel dominates the chemical stability of residual γ. C, Mn, Cu, and Ni, which are gamma stabilizing elements, increase the stability of residual gamma. In addition, the size of the residual γ affects the stability, and the stability increases by making the residual γ particles finer. In the present invention, the stability of residual γ was optimized by controlling the chemical composition of steel and the size of residual γ particles, and excellent fatigue crack propagation characteristics and toughness were achieved.

  In order to control the size of the residual γ, it is necessary to control the formation site density of the residual γ. The formation sites of residual γ are boundaries such as ferrite grain boundaries, old γ grain boundaries, martensite and bainite packet boundaries, block boundaries, lath boundaries, and so on. It is effective to reduce the size as much as possible. Since the martensite and bainite structures contain high density lath boundaries, a quenching structure mainly composed of martensite and bainite or a structure formed by accelerated cooling is more advantageous than a structure mainly composed of ferrite.

  In addition, when fine residual γ is generated, substitution-type elements can be distributed by diffusion at a short distance, so that substitution-type γ-stabilizing elements such as Mn, Cu, and Ni are easily concentrated in the residual γ. Concentration of Mn, Cu, Ni, etc. in the residual γ increases the amount of residual γ in the heat affected zone or the amount of island martensite and improves the fatigue crack propagation characteristics of the weld. This is because in the heat-affected zone where the maximum heating temperature is about Ac3 or less, diffusion does not occur sufficiently, so that the chemical composition of the original residual γ tends to be maintained. This is because it becomes island-shaped martensite. Such a phenomenon is unlikely to occur in a region heated to a high temperature such as in the vicinity of the melting line or in the coarse grain region because the γ-stabilizing element diffuses.

  Next, the chemical component, microstructure and production conditions of the present invention will be specifically described.

・ Explain why chemical components are limited. In addition, all content of each element in a chemical component means the mass%.
C: 0.02-0.16%
C is one of the most powerful γ-stabilizing elements and is an essential element for forming residual γ. However, if it is less than 0.02%, its effect is poor, and if it exceeds 0.16%, excessive residual γ is generated. Or toughness significantly deteriorated, so it was limited to 0.02 to 0.20%. In addition, Preferably it is 0.03%-0.12%.
Si: 0.05-0.5%
Si is a useful element for deoxidation of steel, and increases the strength of the steel by solid solution strengthening. For this reason, it can be added as necessary. However, if it is less than 0.05%, these effects cannot be expected, and since Si becomes easy to form cementite due to Si reduction and prevents C concentration in γ, residual γ From the viewpoint of promoting the formation of Si, it is not desirable to make Si less than 0.05%. Also, if it exceeds 0.5%, the toughness of the steel material is impaired, so it is limited to 0.05 to 0.5%.
Al: 0.005-0.060%
Al is an element that acts as a deoxidizer for molten steel. To obtain a sufficient deoxidation effect, Al must be added in an amount of 0.005% or more. On the other hand, if it exceeds 0.060%, the cleanliness of the steel decreases and the toughness decreases, so it was limited to the range of 0.005 to 0.060%.
One or more of Mn: 0.1-2.5%, Cu: 0.1-2.0%, Ni: 0.1-6.0%
2Mn + Cu + Ni: 3.5-6.0%
Mn, Cu, and Ni are typical substitution-type γ-stabilizing elements and play an important role in the formation of residual γ. Similarly, C, which is a γ-stabilizing element, has a high diffusion rate, so that it diffuses during heating in the heat affected zone and hardly contributes to the formation of residual γ. In order to produce a relatively stable residual γ even in the heat-affected zone, substitutional elements such as Mn, Cu, Ni, etc. that are slow in diffusion are essential. The effects of these elements are hardly obtained when the content is less than 0.1%. If the Mn content exceeds 2.5%, the toughness of the base metal and the weld heat-affected zone deteriorates. Therefore, if the Mn content exceeds 0.1 to 2.5% and Cu exceeds 2.0%, surface cracks during hot rolling become prominent. The Cu content is 0.1-2.0%, and the Ni content is 0.1-6.0% to prevent an excessive increase in alloy costs.

  The stability of residual γ depends largely on the value of 2Mn + Cu + Ni. If 2Mn + Cu + Ni is less than 3.5%, a sufficient amount of residual γ cannot be obtained, and the stability of residual γ is too low, so excellent fatigue crack propagation characteristics and toughness cannot be obtained. On the other hand, if 2Mn + Cu + Ni exceeds 6.0%, the amount of residual γ increases and the toughness is improved, but the stability is too high to provide excellent fatigue crack propagation characteristics. Therefore, 2Mn + Cu + Ni was set to 3.5-6.0%.

One or more of Cr: 0.05 to 3.0%, Mo: 0.05 to 2.0%, Nb: 0.003 to 0.10%, V: 0.003 to 0.10%, Ti: 0.003 to 0.05%, B: 0.0003 to 0.0030% These These elements have the effect of increasing the strength of the steel material and also affect the formation of residual γ, so they can be added as necessary. Elements such as Cr, Mo, Nb, and B increase the formation sites of residual γ by increasing the hardenability and contribute to the refinement of residual γ. Elements such as Nb, V, and Ti refine γ grains during heating by forming carbonitrides, refine the transformation structure during subsequent cooling, and increase the sites of residual γ formation. Nb and V have the effect of increasing the strength of steel by precipitation strengthening. If any element is added more than necessary, the toughness of the base metal and the weld heat-affected zone is impaired.

2. Next, the reason for limiting the microstructure will be described.
Average diameter of retained austenite: 0.1 to 0.5 μm
The size of the residual γ affects the stability, and the stability increases by making the residual γ particles finer. When the average diameter of residual γ exceeds 0.5 μm, the diffusion distance becomes longer, and substitutional γ-stabilizing elements such as Mn, Cu, and Ni are not sufficiently concentrated in the residual γ. Increasingly, martensitic transformation is induced by slight plastic deformation and loading. The volume expansion associated with the martensitic transformation generates compressive stress around it, which significantly suppresses fatigue crack propagation, but reduces the toughness because a large amount of martensite with low toughness is generated. On the other hand, if the average diameter of residual γ is less than 0.1 μm, the required diffusion distance is shortened, so that substitutional γ-stabilizing elements such as Mn, Cu, and Ni can be easily concentrated, and the stability of residual γ is remarkable. And the toughness is improved. However, if the stability of residual γ becomes excessively high, the martensitic transformation will not occur, compressive stress will not be generated to close the fatigue crack tip, and fatigue crack propagation characteristics will not be improved. Therefore, the average diameter of the residual γ is set in the range of 0.1 to 0.5 μm.

In the present invention, the average diameter of residual γ is the square root of the average area of residual γ particles measured from a structural photograph by SEM or TEM.
Volume ratio of retained austenite: 5-20%
If the volume fraction of residual γ is less than 5%, the effect of improving the fatigue crack propagation characteristics due to the residual γ described above is not recognized. On the other hand, if it exceeds 20%, the stability of residual γ decreases and the toughness of the steel deteriorates. Further, when the steel material is cold worked, the residual γ undergoes martensite transformation and a lot of hard martensite is generated, but undesirable material changes such as an excessive increase in strength and a decrease in toughness are remarkable. Therefore, the volume ratio of residual γ was set to 5-20%.

  In the present invention, the volume fraction of residual γ is obtained from the peak intensity ratio by X-ray diffraction.

3. Regarding the manufacturing conditions, the reasons for limiting the manufacturing conditions will be described below.

(1) After hot rolling, direct quenching or reheating quenching is performed, and two-phase region heat treatment. To generate a sufficient amount of residual γ, heat to 650 ° C or more and less than Ac ₃ point 2 Perform phase region heat treatment. The γ-stabilizing element is concentrated in γ while maintaining the temperature in the two-phase region, and residual γ is generated. Since the final distribution of residual γ depends on the microstructure before the two-phase region heat treatment, the residual γ formation site density can be increased by direct quenching or reheating quenching in advance.
(2) After hot rolling, accelerated cooling from Ar ₃ -20 ° C or higher to 400 ° C or lower at an average cooling rate of 1 ° C / s or higher, followed by two-phase heat treatment
Instead of direct quenching or reheating quenching in (1), accelerated cooling may be performed. The purpose is the same as (1) quenching, but if the cooling rate is less than 1 ° C / s, the microstructure becomes coarse and the effect of increasing the density of residual γ-forming sites cannot be obtained. Desirably, it is 5 ° C / s or more. In order to make most of the transformation proceed during accelerated cooling, the temperature range for accelerated cooling was set from Ar ₃ -20 ° C to 400 ° C.
(3) After hot rolling, after cooling to a temperature of 600 ° C or lower and 400 ° C or higher at an average cooling rate of 1 ° C / s or higher, control the temperature so that it stays in the temperature range of Ac3-50 ° C or lower and 500 ° C or higher for 100s or longer. After hot rolling, by maintaining the α + γ two-phase state during cooling from the austenite single-phase region, the γ-stabilizing element can be concentrated in the untransformed γ, and residual γ can be generated even without heat treatment in the two-phase region. it can.

  First, in order to obtain a fine α + γ mixed structure, cooling is performed at an average cooling rate of 1 ° C / s or higher until the temperature during transformation. At 600 ° C or higher, the α fraction is small, so the γ-stabilizing element does not concentrate sufficiently in γ. At 400 ° C or lower, the transformation is almost complete and the γ fraction is insufficient. It was supposed to be cooled. If the cooling rate is less than 1 ° C / s, the structure becomes coarse and fine residual γ cannot be obtained, so the cooling rate was set to 1 ° C / s or more. Desirably, it is 5 ° C./s or more.

Next, in order to concentrate the γ-stabilizing element in γ, Ac ₃ -50 ° C or less and 500 ° C or more is kept for 100s or more. When Ac ₃ exceeds -50 ° C, the α fraction increases and the γ-stabilizing element does not concentrate sufficiently in γ. Below 500 ° C, the substitutional elements Mn, Cu, and Ni hardly diffuse and in γ. Does not thicken. Moreover, a certain amount of time is required for concentration of the γ-stabilizing element, and if it is less than 100 s, the time is insufficient _.
(4) After (1) to (3), the volume ratio of residual γ can be changed by adjusting the tempering temperature to a temperature below 700 ° C. When the tempering temperature is increased to around 650, the amount of residual γ increases, but when it exceeds 700 ° C, the stability of the residual γ suddenly decreases, so the tempering temperature was set to 700 ° C or less.

  A steel slab having the composition shown in Table 1 was used as a raw material, and a steel plate having a thickness of 20 mm was manufactured under the manufacturing conditions shown in Table 2. A round bar tensile test piece (JIS No. 4) and a 2 mm V notch Charpy test piece were taken in the rolling direction from the center of the thickness of the obtained steel sheet, and the tensile characteristics and Charpy impact characteristics were evaluated. In addition, using a 12.5 mm thick CT specimen taken from the center of the plate thickness, a fatigue crack propagation test in accordance with ASTM E647 was performed at room temperature under the conditions of stress ratio R = 0.1 and excitation frequency f = 15 Hz. The fatigue crack propagation rate da / dN at ΔK = 15 MPa√m was measured. Furthermore, weld joints were produced by downward MAG welding with a welding heat input of 20 to 30 kJ / cm, and Charpy specimens with the notch position 1 mm away from the Bond part were collected, and the Charpy of the weld heat affected zone (HAZ) was collected. Toughness was evaluated. The results are shown in Table 3.

In the example of the present invention, a base metal toughness of vE-40> 50 J and a fatigue crack propagation rate of da / dN <1.5 × 10 ⁻⁷ m / cycle were obtained.

  Since the steel material of the present invention has excellent fatigue crack propagation characteristics and toughness, it can be applied as a wide range of steel materials such as hulls, marine structures, civil engineering and construction structures, construction machines, and industrial machines.

Claims (6)

In mass%, C: 0.02-0.16%, Si: 0.05-0.5%, Al: 0.005-0.060%,
One or more selected from Mn: 0.1-2.5%, Cu: 0.1-2.0%, Ni: 0.1-6.0%
2Mn + Cu + Ni value is 3.5-6.0%, the steel is composed of the balance Fe and inevitable impurities, and contains 5-20% by volume of retained austenite with an average diameter of 0.1-0.5μm. Thick steel plate with excellent fatigue crack propagation characteristics and toughness.   Furthermore, in mass%, Cr: 0.05-3.0%, Mo: 0.05-2.0%, Nb: 0.003-0.10%, V: 0.003-0.10%, Ti: 0.003-0.05%, B: 0.0003-0.0030% The thick steel plate excellent in fatigue crack propagation characteristics and toughness according to claim 1, comprising one or more selected. A steel having the composition of claim 1 or claim 2 is hot-rolled and then subjected to direct quenching or reheating quenching, followed by heating to a temperature of 650 ° C. or more and less than Ac ₃ point for cooling. A method for producing a thick steel plate having excellent fatigue crack propagation characteristics and toughness, characterized by performing two-phase region heat treatment. Instead of direct quenching according to claim 3, after accelerating cooling from a temperature of Ar ₃ -20 ° C or higher to 400 ° C or lower at an average cooling rate of 1 ° C / s or higher, a temperature of 650 ° C or higher and lower than Ac ₃ point A method for producing a thick steel plate having excellent fatigue crack propagation characteristics and toughness, characterized by performing two-phase region heat treatment that is heated and cooled at a low temperature. After hot rolling the steel having the component composition according to claim 1 or claim 2, after cooling to a temperature of 600 ° C or lower and 400 ° C or higher at an average cooling rate of 1 ° C / s or higher, Ac ₃ -50 ° C or lower A method for producing a thick steel plate having excellent fatigue crack propagation characteristics and toughness, characterized by controlling the temperature so as to remain in a temperature range of 500 ° C. or higher for 300 seconds or longer.   A method for producing a thick steel plate having excellent fatigue crack propagation characteristics and toughness, characterized by further tempering to a temperature of 700 ° C. or lower after the heat treatment according to claim 3.