JP2007204781A - Method for producing steel material excellent in fatigue crack propagating characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel excellent in fatigue crack propagating characteristic, suitable to a steel material used for welding to a large structural material which receives variable loading, such as ship, bridge, ocean structure. <P>SOLUTION: The steel composed by mass% of 0.05-0.2% C, 0.05-0.7% Si, 0.05-2.5% Mn, ≤0.03% P, ≤0.02% S, 0.005-0.1% Al, 0.005-0.03% Ti and if necessary, adding one or more kinds of Cu, Ni, Cr, Mo, Nb, V, B, Ca, Mg, REM and the balance Fe with inevitable impurities, is cooled to Ar<SB>1</SB>transformation point or lower after solution-treatment, and after again heating to 1,000-1,350°C, hot-rolling is applied and if necessary, accelerating-cooling and further, tempering are performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is used by welding to large structures such as ships, bridges, and offshore structures where fatigue cracks occur due to fluctuating loads applied to the structures depending on the conditions of use, and fatigue fracture is a concern. This is related to steels with excellent fatigue crack propagation characteristics, suitable for steel materials (thick plates, section steels).

  When a fluctuating load is continuously applied to the steel plate, fatigue cracks may occur. Especially in the weld zone, there are discontinuous shape change, structure change and residual stress change between the weld metal and steel material, so stress tends to concentrate and it tends to become the starting point of fatigue crack.

  Also, the generated fatigue cracks continue to propagate, and in the worst case, the structure itself breaks down. If the structure is a ship, a bridge, an offshore structure, etc., the social impact when it is destroyed is large, and in many cases it is expected to be accompanied by danger of human life.

  In these structures, in order to prevent the occurrence of fatigue failure from the weld, a design that does not cause stress concentration structurally is adopted, or stress concentration at the boundary between the weld metal and the base metal is selected by selecting appropriate welding conditions. Is avoided.

  When fatigue cracks occur in a structure, if the fatigue propagation rate of steel is slow, cracks can be found and repaired by periodic inspection before the structure breaks down. This is advantageous in terms of life cycle cost.

Regarding the fatigue crack propagation characteristics of steel, in the microstructure, hard pearlite, bainite, or martensite phase is dispersed in the soft ferrite phase, the hardness difference and the dispersion state and amount of the second phase are specified, and the micro Many proposals have been made regarding a method of manufacturing a steel sheet having a structure (for example, Patent Documents 1 to 5).
JP-A-6-271984 Japanese Patent No. 3037855 Japanese Patent No. 2962134 Japanese Patent Application Laid-Open No. 08-739100 JP 2001-316756 A

  However, when fatigue fracture is prevented by the structure design method, it is impossible to design efficiently because of the large constraints on the design. On the other hand, preventing stress concentration on the welding work side requires time to finish welding. This is inefficient and causes high costs.

  In addition, the steel sheets described in Patent Documents 1 to 5 can be used to add large Mo as an essential element in the component composition, or to heat treatment conditions and cooling methods in manufacturing conditions, and to large structures that require a large amount of steel sheets. Application was not always easy.

  Therefore, an object of the present invention is to provide a steel material for welded structure that has excellent fatigue crack propagation characteristics that can be manufactured without using special manufacturing conditions.

  In order to solve the above-described problems, the present inventors have proposed a microstructure with excellent fatigue crack propagation characteristics and a microstructure of ferrite / pearlite steel as a steel in which a hard second phase is dispersed in a soft ferrite phase. Various studies were conducted on the composition of components and production conditions to obtain solidification, focusing on solidification microsegregation of steel materials, which is completely different from the conventional technology.

  As a result, 1. A granular pearlite structure having a specific volume fraction (volume fraction) and aspect ratio (extensibility) is excellent in fatigue crack propagation characteristics; It has been found that such a structure depends on the degree of segregation and the ferrite transformation behavior of the segregation part, and can be obtained by adjusting the component elements to be microsegregated and reducing the microsegregation generated in the raw material production stage by solution treatment.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. % By mass, C: 0.05 to 0.2%
Si: 0.05-0.7%
Mn: 0.05 to 2.5%
P: 0.03% or less S: 0.02% or less Al: 0.005-0.1%
Ti: 0.005 to 0.03%
After the solution treatment of the steel composed of the remaining Fe and the inevitable impurities satisfying the above, it is cooled to Ar ₁ or less, heated again to 1000 to 1350 ° C., hot-rolled, and at least one of pearlite, bainite, and martensite. A method for producing a steel material having excellent fatigue crack propagation characteristics, wherein the second phase of seeds or more has a volume ratio of 15% or more and an aspect ratio of 5 or less.
2. Furthermore, in mass%, the component composition of steel
Cu: 0.01 to 1%
Ni: 0.01-4%
Cr: 0.01-2%
Mo: 0.01 to 1%
Nb: 0.003 to 0.1%
V: 0.003-0.5%
B: 0.0005 to 0.004%
1 or 2 types or more are added, The manufacturing method of the steel materials excellent in the fatigue crack propagation characteristics of 1 characterized by the above-mentioned.
3. Furthermore, in mass%, the component composition of steel
Ca: 0.0005 to 0.006%
Mg: 0.0005 to 0.006%
REM: 0.0005 to 0.02%
1 or 2 or more types are added, The manufacturing method of the steel materials excellent in the fatigue crack propagation characteristics of 1 or 2 characterized by the above-mentioned.
The steel having the composition described in any one of 4.1 to 3 is solution-treated, cooled to Ar ₁ or less, heated again to 1000 to 1350 ° C., hot-rolled, and then accelerated cooled. A method for producing a steel material having excellent fatigue crack propagation characteristics.
5.1 After cooling the steel having the composition described in any one of 1 to 3 to Ar ₁ or less, heating again to 1000 to 1350 ° C., hot rolling, accelerated cooling, and Ac ₁ point A method for producing a steel material having excellent fatigue crack propagation characteristics, characterized by performing tempering at the following temperature.
6). The method for producing a steel material having excellent fatigue crack propagation characteristics according to any one of 1 to 5, wherein the solution treatment is held at 1200 ° C to 1350 ° C for 5 hours or more.

  According to the present invention, a hard second phase is dispersed in a microstructure, and a welded structural steel material having excellent fatigue crack propagation characteristics can be manufactured very easily, which is extremely useful industrially.

  The present invention is characterized by having a component composition with little microsolidification segregation and performing hot rolling after solution treatment. Hereinafter, the component composition, production conditions, and obtained microstructure will be described in detail. In the component composition,% is mass%.

[Ingredient composition]
C: 0.05 to 0.2%
C is a basic element that improves the strength and governs the fraction of pearlite, bainite or martensite, which is a hard second phase, and ensures the strength as a welded structural steel, and is a pearlite that is a hard second phase. In order to secure a volume fraction of bainite or martensite of 15% or more, a minimum of 0.05% or more is necessary.

  On the other hand, when C exceeds 0.2%, the upper limit is made 0.2% in order to lower the weldability that is fundamental for welded structural steel. Preferably, the C content is in the range of 0.08 to 0.18%.

Si: 0.05 to 0.7% or less Si is a basic element that dissolves into steel and increases the strength, but in order to expect such an effect, at least 0.05% or more is added. is required. On the other hand, addition exceeding 0.7% impairs the toughness of the base metal and the weld heat-affected zone, so the upper limit was made 0.7%. Preferably, the upper limit is 0.6%.

Mn: 0.05 to 2.5%
Since Mn is an element useful for increasing the strength of welded structural steel, 0.05% or more is added. However, since addition exceeding 2.5% inhibits weldability, the upper limit was made 2.5%.

P: 0.03% or less P is unavoidably present in steel and promotes embrittlement, so it is desirable that P be small. However, reducing P causes a significant cost increase in melting and impairs practicality, so 0.03% was made the upper limit. Preferably, the upper limit is 0.025%.

S: 0.02% or less S is also unavoidably present in the steel, and forms MnS, which becomes a transformation nucleus of ferrite and promotes banding of the hard second phase, so it is desirable that the content of S be small. However, reducing S causes a significant cost increase in melting and impairs practicality, so 0.02% was made the upper limit. Preferably, the upper limit is 0.015%.

Al: 0.005 to 0.1%
Al is most effective as a deoxidizing element and improves the toughness and ductility of steel by reducing oxide inclusions in the steel. However, if the content is less than 0.005%, the effect is small. Conversely, if the content exceeds 0.1%, the effect is saturated and the toughness is decreased. Therefore, it was made into the range of 0.005 to 0.1%.

Ti: 0.005 to 0.03%
Ti is effective for fixing free N as TiN and can be expected to improve the toughness of the weld heat affected zone. However, if the content is less than 0.005%, the effect is small. With the precipitation of Ti carbide, fatigue crack propagation characteristics and toughness are reduced, so the upper limit was made 0.03%.

  The above is the basic component composition of the present invention, but when further improving the characteristics, one or more of Cu, Ni, Cr, Mo, Nb, V, B, Ca, Mg, and REM are added as selective elements. To do.

Cu: 0.01 to 1%
Cu, like Cr, is an effective element for granulation of bainite or martensite, even if added in large amounts to increase the strength of the base metal, and has little effect on Ar _{3 due} to microsegregation. .

  However, if the content is less than 0.05%, the effect is small. Conversely, addition exceeding 1% promotes embrittlement accompanying Cu precipitation, so the upper limit was made 1%. In addition, since Cu also promotes hot brittleness, it is preferable to use Ni together with addition.

Ni: 0.01-4%
Ni is an effective element for obtaining high strength without impairing the toughness of the base metal and the weld heat-affected zone, but if it is less than 0.05%, the effect cannot be obtained. On the other hand, even if added over 4%, the effect on fatigue properties is saturated, so the upper limit was made 4% in consideration of practicality.

Cr: 0.05-2%
Even if Cr is added in a large amount for increasing the strength of the base material, it has little influence on Ar ₃ due to microsegregation, and is an effective element for granulating bainite or martensite. However, if it is less than 0.05%, it is insufficient for increasing the strength. Conversely, if it exceeds 2%, the weldability is impaired, so the upper limit was made 2%.

Mo: 0.01 to 1%
Mo is a useful element that increases the hardenability and increases the strength of the thick-walled material. Addition of 0.01% or more is necessary to expect the effect. On the other hand, the increase in Mo decreases weldability, so the upper limit was made 1%.

Nb: 0.003 to 0.1%
Nb is a very effective element for increasing the strength of hot rolled welded structural steels by refining the structure and strengthening the precipitation of Nb by adding a small amount, but it must be added in an amount of 0.003% or more. . On the other hand, addition exceeding 0.1% adversely affects toughness, so the upper limit was made 0.1%.

V: 0.003-0.5%
V, like Nb, is a useful element that increases the strength by adding a small amount, and in order to obtain the effect, it is necessary to add 0.003% or more. Conversely, addition exceeding 0.5% lowers the HAZ toughness, so the upper limit was made 0.5%.

B: 0.0005 to 0.004%
B is a useful element that improves the hardenability, but 0.0005% or more must be added if the effect is expected. Conversely, the effect is saturated even if added over 0.004%, so the upper limit was made 0.004%.

Ca: 0.0005 to 0.006%
Ca is added to prevent nozzle clogging during casting and to control the form of inclusions, and to further improve HAZ toughness. However, if the amount is less than 0.0005%, there is no effect, and conversely 0.006% If added beyond the range, the cleanliness is impaired, so the upper limit was made 0.006%.

Mg: 0.0005 to 0.006%
Mg is an element effective for fine dispersion as a nucleus for precipitation of TiN by fixing S in steel and improving the toughness of the steel sheet, becoming a fine oxide, oxysulfide or a complex inclusion thereof. . This effect becomes effective when 0.0005% or more is added, and if added over 0.006%, the effect is saturated and the amount of inclusions in the steel becomes coarse and increases, which inhibits cleanliness and deteriorates toughness. .

REM: 0.0005 to 0.02%
REM is an element that works to improve the toughness of the steel sheet by fixing S in the steel, and is an effective element for fine dispersion as a core of precipitation of TiN that becomes a fine oxide, oxysulfide, or a composite inclusion thereof. . This effect becomes effective when added over 0.0005%, and when added over 0.02%, the effect is saturated and the amount of inclusions in the steel becomes coarse and increases, which impairs cleanliness and deteriorates toughness. .

[Production conditions]
Molten steel having the above composition is melted by a normal method such as a converter, electric furnace, vacuum melting furnace, or the like, and is used as a rolling material such as a slab by a generally known casting method such as a continuous casting method or an ingot forming method.
Solution Treatment In the present invention, the solution treatment is an important process for eliminating micro solidification segregation and improving fatigue crack propagation characteristics.

  FIG. 2 is a diagram schematically illustrating the mechanism of pearlite stretching in a band shape in high Mn steel. In the case of high Mn steel, micro solidification segregation (C / Co) of Mn increases, and rolling in the γ region is performed. Among them, a micro-segregation part in which Mn is positively segregated and negatively segregated is formed in a layer shape (a).

In the region where Mn is negatively segregated, since the Ar ₃ point is high, the ferrite transformation precedes in the cooling process, and carbon is discharged to the Mn positive segregation portion along with this ferrite transformation (b). As a result, carbon is concentrated in the Mn positive segregation part to cause pearlite transformation, and pearlite is banded (c).

That is, when the added alloy is microsegregated and the difference in ferrite transformation temperature (Ar ₃ ) between the positive and negative microsegregations is excessively increased, band-like pearlite is formed. It is effective to reduce the Ar ₃ temperature difference in the micro-segregation part.

  FIG. 3 is a diagram showing the effect of solution treatment on the microstructure, where (a) shows the case of Si-Mn steel and (b) shows the case of Nb-added steel.

  The microstructure was 1250 ° C-50h after solution treatment, reheated to 1100 ° C, hot-rolled to a sheet thickness of 22mm at a rolling finish temperature of 920 ° C, air cooled, and observed for L, C, and Z cross sections. . By performing the solution treatment, the hard second phase is granulated and the anisotropy of the microstructure is extremely reduced.

  FIG. 4 shows the fatigue crack propagation characteristics of the steels having the respective microstructures shown in FIG. 3, wherein (a) shows the case of Si—Mn steel and (b) shows the case of Nb-added steel. Fatigue crack propagation properties are improved in solution treated steel.

  Hereinafter, the solution treatment conditions of the present invention will be specifically described. It is desirable to heat and hold at least 5 hours or more in the temperature range of 1200 to 1350 ° C. in the material having the component composition according to the present invention.

  When the heating temperature is less than 1200 ° C., solutionization is insufficient and the second phase is banded, and conversely, heating above 1350 ° C. increases scale loss and surface defects. Therefore, the heating temperature is desirably in the range of 1200 to 1350 ° C. The holding time is preferably 5 hours or more because the effect of solution treatment is small if it is less than 5 hours. On the other hand, the upper limit is not particularly specified, but is preferably 100 hours or less in consideration of productivity.

Reheating conditions After the solution treatment, after cooling to Ac1 point or less, reheat to 1000 to 1350 ° C. Reheating is a process necessary for refining γ coarsened by solution treatment. When reheated from a temperature higher than the Ac1 point, γ recrystallization remains insufficient and coarse, and the toughness of the base metal In order to lower the temperature, it is cooled to the Ac1 point or less.

  Reheating temperature shall be 1000-1350 degreeC. If it is less than 1000 ° C., the additive element does not sufficiently dissolve, and the strength toughness required for welded structural steel cannot be obtained. On the other hand, when heated to 1350 ° C. or higher, γ grains become coarse, and the toughness required for welded structural steel Since it cannot ensure, it shall be 1000-1350 degreeC.

After hot-rolling reheating, hot-rolling is performed to obtain a desired sheet thickness. In the present invention, however, controlled rolling that appropriately defines the reduction rate and rolling temperature is performed in hot-rolling according to the desired strength and toughness. Or after hot rolling, accelerated cooling, and further tempering. Hot rolling, accelerated cooling, and tempering after accelerated cooling may be conventional methods and are not particularly defined in the present invention. Hereinafter, the effect of the present invention will be shown in Examples.

  In the steel according to the present invention, a volume ratio of 15% or more and an aspect ratio of 5 or less of at least one second phase of pearlite, bainite, and martensite is obtained as a microstructure.

  In a microstructured steel having at least one second phase of pearlite, bainite, martensite, when the aspect ratio of the second phase is 5 or less, the fatigue crack propagation rate when the volume fraction is 15% or more Will drop significantly. The aspect ratio (extension) of the second phase is the average aspect ratio in the cross-sectional microstructure in the rolling direction.

  FIG. 1 shows the effect of the volume fraction of the second phase having an aspect ratio (extension) of 5 or less on the fatigue crack propagation rate. For comparison, the figure also shows the results of investigation on a steel having an aspect ratio (elongation) of 10.

  The test material is a steel having a component composition including C: 0.04 to 0.23%, Mn: 0.1 to 1.6%, and YP: 300 to 350 MPa, and the second phase is at least pearlite, bainite, martensite. Adjusted to one or more.

  In the case of the second phase having an aspect ratio (extension) of 5 or less, the fatigue crack propagation rate is greatly reduced when the volume ratio is 15% or more, and when the aspect ratio (extension) is 10, the volume ratio is increased. The decrease in the fatigue crack propagation rate is small.

  Using steel materials melted by the regular process, thick steel plates and sections were produced by the regular process, and their strength, toughness and fatigue crack propagation characteristics were investigated. Table 1 shows the chemical composition and production conditions of the specimen, and Table 2 shows the microstructure, strength, toughness, and fatigue crack propagation rate at ΔK = 25 MPa√m of the obtained steel.

In Table 2, the solution treatment conditions of the developed steel are within the preferable range. In steels 2, 5, 7 to 11, a steel material having a fatigue crack propagation rate of 8 × 10 ⁻⁸ mm / cycle or less and a very slow fatigue crack propagation rate was obtained. Steels Nos. 3 and 6 had a fatigue crack propagation rate of ^{8 to} 9 × 10 ⁻⁸ mm / cycle.

On the other hand, in Table 2, No. 1 is a steel that does not undergo solution treatment and deviates from the scope of the invention. The steels Nos. 1 and 4 have a fatigue crack propagation speed as high as 9.5 × 10 ⁻⁸ mm / cycle or more due to the extension of the hard second phase, and are inferior in fatigue crack propagation characteristics.

The figure which shows the influence of the volume fraction and aspect ratio of a 2nd phase which have on fatigue crack propagation characteristics. The schematic diagram which shows the formation mechanism of a band-shaped hard 2nd phase. The figure which shows the influence which solution treatment has on a microstructure. The figure which shows the influence which solution treatment has on the fatigue crack propagation characteristic.

Claims (6)

% By mass, C: 0.05 to 0.2%
Si: 0.05-0.7%
Mn: 0.05 to 2.5%
P: 0.03% or less S: 0.02% or less Al: 0.005-0.1%
Ti: 0.005 to 0.03%
After the solution treatment of the steel composed of the remaining Fe and the inevitable impurities satisfying the above, it is cooled to Ar ₁ or less, heated again to 1000 to 1350 ° C., hot-rolled, and at least one of pearlite, bainite, and martensite. A method for producing a steel material having excellent fatigue crack propagation characteristics, wherein the second phase of seeds or more has a volume ratio of 15% or more and an aspect ratio of 5 or less. Furthermore, in mass%, the component composition of steel
Cu: 0.01 to 1%
Ni: 0.01-4%
Cr: 0.01-2%
Mo: 0.01 to 1%
Nb: 0.003 to 0.1%
V: 0.003-0.5%
B: 0.0005 to 0.004%
1 or 2 types or more of these are added, The manufacturing method of the steel materials excellent in the fatigue crack propagation characteristics of Claim 1 characterized by the above-mentioned. Furthermore, in mass%, the component composition of steel
Ca: 0.0005 to 0.006%
Mg: 0.0005 to 0.006%
REM: 0.0005 to 0.02%
The method for producing a steel material having excellent fatigue crack propagation characteristics according to claim 1 or 2, wherein one or more of the above are added. The steel having the component composition according to any one of claims 1 to 3 is solution-treated, cooled to Ar ₁ or less, heated again to 1000 to 1350 ° C, hot-rolled, and then subjected to accelerated cooling. A method for producing a steel material having excellent fatigue crack propagation characteristics. The steel having the component composition according to any one of claims 1 to 3 is solution-treated, cooled to Ar ₁ or less, heated again to 1000 to 1350 ° C, hot-rolled, accelerated cooled, and Ac ₁ point. A method for producing a steel material having excellent fatigue crack propagation characteristics, characterized by performing tempering at the following temperature. The method for producing a steel material having excellent fatigue crack propagation characteristics according to any one of claims 1 to 5, wherein the solution treatment is held at 1200 ° C to 1350 ° C for 5 hours or more.

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