KR20030040150A - Steel material having improved fatigue crack driving resistance and manufacturing process thereof - Google Patents

Abstract

PURPOSE: Provided are a structural steel material with improved resistance against fatigue crack propagation, thus being suitable for use in applications where a steel material is expected to undergo repeated loads, and a method for manufacturing the same. CONSTITUTION: The structural steel material is characterized in that it has a cyclic softening parameter of at least 0.65 and at most 0.95, the cyclic softening parameter being represented by the ratio (sigma 15/sigma 1) of the stress at the maximum strain in the first cycle (sigma 1) to that in the 15 th cycle (sigma 15) measured when a waveform of incremental and decremental cyclic loads is applied 15 times with a maximum tensile and compressive strain of +/- 0.012, with a frequency of 0.5 Hz, and with the number of waves to the maximum strain being 12. The structural steel material comprises C 0.02 to 0.20 wt.%, Si 0.60 wt.% or less, Mn 0.50 to 2.0 wt.%, Al 0.003 to 0.10 wt.%, Cu 1.5 wt.% or less, Ni 1.5 wt.% or less, Cr 1.20 wt.% or less, Mo 1.0 wt.% or less, V 0.10 wt.% or less, Nb 0.10 wt.% or less, Ti 0.10 wt.% or less, B 0.0003 to 0.0020 wt.%, Ca 0.0005 to 0.010 wt.%, a balance of Fe and incidental impurities, wherein a value of carbon equivalent, Ceq, represented below is in the range of from 0.28 to 0.65 %, Ceq (%) = C + Si/24 + Mn/6 +Ni/40 + Cr/5 + Mo/4 + V/14.

Description

Steel material with excellent fatigue crack growth resistance and its manufacturing method {STEEL MATERIAL HAVING IMPROVED FATIGUE CRACK DRIVING RESISTANCE AND MANUFACTURING PROCESS THEREOF}

The present invention relates to structural steels having excellent fatigue crack propagation resistance suitable for use in applications in which repeated loads are expected, such as ships, offshore structures, bridges, buildings, tanks, and industrial and construction machinery. It is about a method.

Steel materials used in ships, offshore structures, bridges, buildings, tanks, industrial and construction machinery, etc. are rarely subjected to repeated loads, and attention to fatigue characteristics is therefore necessary to ensure their safety. It is known that the environmental conditions greatly affect the fatigue failure of steel materials, and that the strength of steel materials is reduced when cyclic stress is applied in a corrosive environment such as in seawater.

The fatigue process of steel can be thought of as two stages that differ in the characteristics of crack development and crack propagation. In normal mechanical parts, macro cracking is considered to be a limit of use, and most designs that allow for crack propagation are not performed. However, in a highly redundant structure, even if a fatigue crack occurs, it does not immediately lead to breakdown. Therefore, since the cracks are found by periodic inspection before the final fracture and the cracks are not repaired, or the cracks do not grow to the length of the final fracture within the service period, even if the fatigue crack is present, the structure can be sufficiently used. Endured.

In a welded structure, there are many weld ends as stress concentration portions, and it is technically impossible or impossible to completely prevent the occurrence of fatigue cracking, and there is no economic benefit. Therefore, it is important to make the propagation rate of the crack as slow as possible to significantly extend the fatigue life (preliminary life) from the state where the crack already exists.

Regarding a means for rectifying the fatigue crack propagation of steel materials and prolonging the fatigue life, Japanese Patent Laid-Open No. Hei 5-185441 discloses a method of generating microcracks at the fatigue crack tip. However, the effectiveness of this method is limited to the case where the ΔK (difference between the maximum stress expansion coefficient and the minimum stress expansion coefficient) region is small, i.e., when the crack is not long and the stress level is low, and it is generated from the welded part to some extent. It can be considered that the effect is small for cracks in the intermediate? K region with.

Japanese Patent Application Laid-Open No. 4-337026 is a method for producing a high strength hot rolled steel sheet having excellent fatigue strength and fatigue crack propagation resistance. It is proposed to make the fraction into the ideal structure of 10 to 30%. However, the fatigue crack propagation resistance in this publication is the lower limit stress expansion coefficient range (ΔK _th ) in the fatigue crack propagation, and there is an effect of increasing the stress expansion coefficient of the lower limit at which the fatigue crack propagates. Slowing fatigue crack growth is not an effective technique.

Japanese Patent No. 2692134 discloses a steel sheet composed of a base constituting the hard part and a soft part dispersed therein, wherein the steel sheet has a fatigue crack growth suppression effect of having a hardness difference of 150 or more at Vickers hardness. However, this patent does not reveal the mechanical properties of the steel. In addition, this technique is applicable only if the tissues of the hard and soft portions can be clearly distinguished. In general, it is necessary to refine the tissue in order to improve strength or toughness, but it is not necessary to clearly distinguish the hard and soft tissues, or to measure the hardness difference.

Japanese Patent Laid-Open No. 2001-41868 discloses a steel material having 20% or more of bainite structure based on a softening ratio (repeating softening variable) when a stress ratio of -4 to -0.25 is applied and a cyclic load of distortion control is applied at both vibration waveforms. A method of evaluating the fatigue crack growth rate is disclosed. According to this method, if the master curve which calculated | required the correlation of repeated softening amount and fatigue crack growth rate beforehand is created, the fatigue crack growth rate can be evaluated quickly and efficiently from the repeated softening amount.

However, this is a simple evaluation method and cannot provide excellent steel equivalent to this evaluation. In addition, there is no disclosure as to whether the structural steel has sufficient strength, toughness, and weldability.

While the annealing material hardens under conditions subject to cyclic distortion, it has been conventionally known to soften steel materials containing a hardened structure (that is, exhibit "repeated softening"). In the steel material exhibiting such repeated softening, the nature of fatigue resistance is not elucidated, and therefore, industrial design criteria are not established. As a result, such a material was not used for the purpose of utilizing the fatigue resistance.

An object of the present invention is to provide a structural steel material and a method of manufacturing the same, which are capable of designing materials in which quantitatively handling fatigue resistance is particularly exemplified in materials exhibiting repeated softening.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an increase and decrease distortion waveform.

2 is a graph showing the relationship between cyclic softening parameters and fatigue crack growth rate.

3 is a graph showing the amount of distortion at the fatigue crack tip obtained by finite element analysis.

According to the present invention, a steel material excellent in fatigue crack growth resistance and a method for producing the same are provided on the basis of the study of fatigue resistance of steel materials including softening behavior under conditions that impart cyclic distortion. This steel combines strength, toughness and weldability that are most suitable for structural steel used in ships, offshore structures, bridges, buildings, tanks, etc.

The steel material excellent in fatigue crack growth resistance according to the present invention has the first tensile stress when the maximum tensile and compressive distortion is ± 0.012, the repetition speed is 0.5 GPa, and the wave number up to the maximum distortion is 15 times. The cyclic softening parameters represented by sigma ₁₅ / sigma ₁ , which is the ratio between the stress sigma ₁ at the maximum distortion and the stress sigma ₁₅ at the 15th maximum distortion, are 0.65 or more and 0.95 or less.

This steel material preferably has the following composition.

Mass% contains C: 0.02 to 0.20%, Si: 0.60% or less, Mn: 0.50 to 2.0%, Al: 0.003 to 0.10%, and optionally one selected from the following (A) to (D) or Two or more types of elements may further be included and the carbon equivalent Ceq value represented by following Formula is 0.28 to 0.65%.

Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

(A) Cu: 1.5% or less, Ni: 1.5% or less, Cr: 1.20% or less, Mo: 1.0% or less and V: 0.10% or less, 1 type, or 2 or more types,

(B) 1 or 2 types of Nb: 0.10% or less and Ti: 0.10% or less,

(C) B: 0.0003 to 0.0020% and

(D) Ca: 0.0005 to 0.010%.

The steel material excellent in the fatigue crack growth resistance which concerns on this invention can be manufactured by either of the following methods.

(a) a method of performing one or more heat treatments for reheating the steel material of the above-mentioned composition to at least _one Ac and cooling it to 550 ° C. or less at a cooling rate of 5 ° C./s or more,

(b) The method of cooling the steel material of the said composition which was hot worked to 550 degrees C or less at the cooling rate of 5 degrees C / s or more from the temperature range of (Ar ₃ points-100) degrees C or more and (Ar ₃ points +150) degrees C or less.

(c) The steel material of the above-mentioned composition which was hot worked was cooled to 550 ° C. or less at a cooling rate of 5 ° C./s or more from a temperature range of (Ar ₃ point-100) ° C. or higher and (Ar ₃ point + 150) ° C. or lower Furthermore, the method of reheating to Ac ₁ or more point, and performing the heat processing more than once to cool to 550 degrees C or less at the cooling rate of 5 degrees C / s or more.

In any of the above methods, the heat treatment may be performed by heating to a temperature of Ac ₁ or lower and tempering.

EMBODIMENT OF THE INVENTION Below, this invention is demonstrated in detail. In the following description,% is mass% unless otherwise specified.

Since this invention relates to evaluating the softening degree of steel materials on condition to give a cyclic distortion, the steel object of this invention has the structure (namely hardened structure) which softens when it receives a cyclic distortion.

Steel having a hardened structure is softened by cyclic distortion, but the fatigue crack growth rate in the stress expansion coefficient range (ΔK) = 20 MPa · m ^0.5 at which the cyclic softening parameter σ ₁₅ / σ ₁ is usually used (da / dN), and it turned out that it can be used for evaluation of the propagation rate of a fatigue crack.

If the cyclic softening variable is less than 0.65, the crack propagation rate is delayed, but the toughness and weldability of the steel material deteriorate, and the use of the structural steel is significantly limited. On the other hand, when the cyclic softening variable exceeds 0.95, not only the crack propagation speed is increased but also the strength is lowered. Therefore, the repetitive softening variable is set to 0.65 or more and 0.95 or less. The repeat softening parameter is preferably 0.70 or more and 0.90 or less.

The repeated softening variable σ ₁₅ / σ ₁ employed in the present invention will be described.

The distortion waveform loaded on the steel to obtain the cyclic softening variable is a vibration waveform on both sides of which tension and compression are alternately applied to evaluate the amount of cyclic softening of the steel. This waveform has an increasing and decreasing shape, and the repetition speed is 0.5 Hz, and the distortion range after increasing the distortion is 0.024 (maximum tensile / compression distortion = ± 0.012). The repetition rate of 0.5 Hz was set in consideration of suppression of internal heat generation. Further, the distortion range of 0.024 is set to be the normal use of the stress intensity factor range (ΔK) is the 20 ㎫ and ^0.5 m. In the process of increasing the distortion, 12 waves reached the maximum distortion, and in the process of reducing the distortion, the 12 waves had a distortion amount of zero. This increment / decrement process is assumed to be one set, and this one set is represented by the unit of a "block" below.

Fig. 1 shows a distortion waveform when time (sec) is taken on the horizontal axis and distortion amount is taken on the vertical axis. Although only the 1st and 2nd blocks are shown, the number of blocks is 15, ie, the same incremental and deterioration distortion waveform is repeated 15 times. The number of times of 15 times is set from what is considered that the softening effect by repetitive distortion is about saturated until 15 times.

The value of sigma ₁₅ / sigma ₁ is defined as a repetitive softening parameter, with the stress corresponding to the maximum distortion of the first block being sigma ₁ and the stress corresponding to the maximum distortion of the fifteenth block being sigma ₁₅ .

2 is a graph showing the relationship between the cyclic softening parameter and the fatigue crack growth rate defined above, and as described above, a constant correlation can be seen between these factors.

This mechanism can be thought of as:

When the steel is subjected to repeated loads of alternating tension and compression, the reverse cracking of dislocations occurs at the tip of the fatigue crack and softens because dislocations move and disappear. The softening mitigates the distortion of the fatigue crack tip and reduces the driving force of fatigue crack growth.

The distortion relaxation phenomenon of the fatigue crack tip was analyzed by the finite element method. The cyclic softener was assumed to have element divisions in which the circumference of the fatigue crack was covered by the softening portion, and a model was applied to a load having a stress expansion coefficient range of 20 MPa · m ^0.5 at the crack tip.

Fig. 3 shows the result of comparing the amount of distortion at the fatigue crack tip with the homogeneous material and the repeated softening material. It was confirmed that the distortion at the fatigue crack tip of the repeated softener was smaller than the distortion at the homogeneous material. As described above, in the repeated softening material, it can be considered that the relaxation of the distortion applied to the crack tip portion contributes to the suppression of the fatigue crack growth.

In the fatigue test in the atmosphere In the present invention, the stress intensity factor range of the steel the fatigue crack growth rate in the 20 ㎫ and ^0.5 m (da / dN) is aimed at 4.0 × 10 ^-5 ㎜ / cycle or less do.

In a suitable state, the steel material which concerns on this invention has the said composition. The reason for this is as follows.

C: 0.02 to 0.20%

Carbon (C) is an element effective for securing the strength of the structural member. If the content is less than 0.02%, it is difficult to obtain the effect of improving strength. On the other hand, when C content exceeds 0.20%, weldability will fall, and welding construction becomes difficult, and the use area | region as structural steel is remarkably limited. In order to ensure a great strength and to ensure weldability, it is preferable to make C content into 0.04 to 0.15%.

Si: 0.60% or less

Silicon (Si) has a deoxidation action. However, when the content exceeds 0.60%, toughness deteriorates. Si content becomes like this. Preferably it is 0.05 to 0.5%.

Mn: 0.50% to 2.0%

Manganese (Mn) is an element effective for securing strength. If the Mn content is less than 0.50%, the effect is not sufficient. On the other hand, when Mn content exceeds 2.0%, toughness will deteriorate. It is preferable to make Mn content into 0.70 to 1.8%.

Al: 0.003 to 0.10%

Aluminum (Al) has a deoxidation action. If the Al content is less than 0.003%, the effect is not sufficient, and since the oxide in the steel increases, the toughness deteriorates. On the other hand, even if Al content exceeds 0.10%, toughness will fall. It is preferable to make Al content into 0.010 to 0.050%.

The steel material of the present invention may further contain a predetermined amount of at least one element selected from the following group in addition to the above-described component elements.

(A) at least one of Cu, Ni, Cr, Mo and V,

(B) Nb and / or Ti,

(C) B,

(D) Ca.

Cu: 1.5% or less

Copper (Cu) is an element effective in securing strength and improving corrosion resistance, but when Cu content exceeds 1.5%, the toughness is deteriorated. The preferable Cu content in the case of adding is 0.10 to 1.0%.

Ni: 1.5% or less

Nickel (Ni) is an effective element for securing strength and improving toughness. However, even if an amount of Ni content exceeding 1.5% is contained, the effect is not only saturated but also causes an increase in cost. Preferable Ni content in the case of adding is 0.05 to 1.3%.

Cr: 1.2% or less

Chromium (Cr), like Cu, is an effective element for securing strength and improving corrosion resistance. However, when Cr content exceeds 1.2%, toughness will deteriorate. Preferable Cr content in the case of adding is 0.10 to 1.0%.

Mo: 1.0% or less

Molybdenum (Mo) is an effective element to improve the strength by increasing the hardenability. However, when Mo content exceeds 1.0%, not only will it cause toughness deterioration, but will also raise a cost. When adding, preferable Mo content is 0.050 to 0.80%.

V: 0.10% or less

Vanadium (V) has the effect of increasing the strength, and may be contained for the purpose of securing a large strength as a structural steel. However, containing V in excess of 0.10% will cause toughness deterioration. Preferable V content in the case of adding is 0.010 to 0.080%.

Nb: 0.10% or less

Niobium (Nb) is an effective element for securing toughness. However, when Nb is contained exceeding 0.10%, toughness will fall rather. The preferable Nb content in the case of adding is 0.020 to 0.050%.

Ti: 0.10% or less

Titanium (Ti) is also an effective element for securing toughness like Nb. However, when Ti content exceeds 0.10%, toughness will fall rather. Preferable Ti content in the case of adding is 0.010 to 0.050%.

B: 0.00030 to 0.020%

Boron (B) is an effective element for controlling ferrite amount by increasing hardenability. However, if the content is less than 0.00030%, the effect is not sufficient. On the other hand, when B content exceeds 0.0020%, toughness will deteriorate. Preferable B content in the case of adding is 0.00080 to 0.0015%.

Ca: 0.00050 to 0.010%

Calcium (Ca) is an element effective in spheroidizing nonmetallic inclusions and improving toughness. However, when the content is less than 0.00050%, the effect is not sufficient. On the other hand, when the Ca content exceeds 0.010%, a large amount of inclusions such as CaO, CaS, etc. are generated to deteriorate the toughness. When adding, preferable Ca content is 0.0010 to 0.0050%.

Carbon equivalent (Ceq): 0.28 to 0.65%

Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14:

The carbon equivalent Ceq value shown by said Formula is an index which evaluates the hardenability and weldability of steel materials, and is generally used widely. However, Ceq is used only as an index for obtaining steel with desired mechanical properties or weldability, and studies on the relationship with fatigue crack growth resistance are not noticeable.

In order to suppress fatigue crack growth and to satisfy general strength characteristics of structural steel: tensile strength TS of 500 N / mm 2 or more and Charpy absorbed energy value (vE ₀ ) at 0 ° C. to satisfy 27 J or more. Not coarsened). The present inventors found that in steels having a microstructure, the Ceq value is related not only to mechanical properties and weldability but also to fatigue crack growth rate.

In other words, if Ceq is less than 0.28%, not only the strength decreases but also the amount of repeated softening decreases, and thus the fatigue crack growth rate does not sufficiently decrease. On the other hand, when Ceq exceeds 0.65%, fatigue crack growth is suppressed, but weldability deteriorates and welding construction becomes difficult and the use is remarkably limited. Therefore, the range of Ceq is made 0.28 to 0.65%. The range of Ceq is preferably 0.30 to 0.63%.

Next, the manufacturing method of the steel material which concerns on this invention is demonstrated.

A steel piece having the steel composition as described above is prepared by, for example, a continuous casting method and hot rolled thereto. The hot rolled steel sheet thus obtained is obtained by controlling the cooling rate after hot rolling and / or performing heat treatment to obtain a steel material having a cyclic softening parameter of 0.65 or more and 0.95 or less.

The hot rolling itself is not particularly limited and may be performed under conventional conditions. Although the steel material which concerns on this invention is generally a form of a hot rolled sheet steel, it may be another form according to a use, such as a shaped steel, a rod-shaped material, a pipe material, etc. Therefore, you may perform another hot working instead of hot rolling.

After the hot rolling, as described below, if the heat treatment for reheating and rapid cooling to Ac ₁ or more is performed, or if the cooling after the hot rolling is rapid cooling, or both are performed, the hardened structure is repeated. Steels with softening parameters of 0.65 to 0.95 can be obtained. Thereafter, tempering described later may be performed.

Heat treatment (reheating and rapid cooling after hot rolling):

The reheating temperature of the steel material after hot rolling shall be Ac ₁ point or more. If the reheating temperature is less than ₁ AC, austenite transformation does not occur, and thus steel materials having a desired repeat softening parameter cannot be obtained, resulting in deterioration of fatigue crack growth characteristics. Re-heating temperature is preferably in the (Ac ₁ point + 100-300) temperature range ℃.

The cooling rate after reheating is made into rapid cooling of 5 degrees C / s or more. If the cooling rate is less than 5 ° C / s, cooling is delayed, causing an increase in the fatigue crack growth rate and a decrease in strength and toughness. The cooling rate after reheating becomes like this. Preferably it is 10 degreeC or more. The upper limit of the cooling rate is not particularly limited, but also depends on the size of the steel (thickness in the case of steel sheet). For example, if plate | board thickness is 10 mm or less, it is also possible to set it as the cooling rate of 50 degreeC / s or more.

The stop temperature of rapid cooling shall be 550 degreeC or less. When the cooling stop temperature is higher than 550 ° C., the fatigue crack growth rate is increased, that is, the cyclic softening property is lowered. Cooling stop temperature becomes like this. Preferably it is 450 degrees C or less.

The heat treatment by the above reheating and rapid cooling may be performed two or more times as necessary. In order to hold | maintain the hardened structure produced | generated thereby, after this heat processing, heat processing is not performed except the tempering mentioned later.

Rapid cooling after hot rolling:

The steel after hot rolling - simply by rapid cooling from (Ar ₃ point 100) ℃ above, (Ar ₃ point + 150) ℃ temperature below also, the cyclic softening parameter can be obtained a steel material of 0.65 to 0.95.

For the same reasons as the cooling after the reheating described above, rapid cooling immediately after the hot rolling causes the cooling rate to be 5 ° C / s or more and the cooling stop temperature to 550 ° C or less.

If the onset temperature of rapid cooling is lower than (Ar ₃ point-100) ° C, a decrease in cyclic softening characteristics and a decrease in strength occur. On the other hand, when this cooling start temperature is higher than (Ar ₃ point + 150) degreeC, austenite particle diameter will coarsen and cause toughness deterioration. Preferable cooling start temperature is (Ar ₃ point-50) degreeC or more, and (Ar ₃ point + 100) degreeC or less.

After performing the above-mentioned rapid cooling after hot rolling, you may repeat the reheating to AC ₁ or more point mentioned above, and the heat processing by rapid cooling once or more.

Tempering:

As described above, tempering may be finally performed on the steel material rapidly cooled after hot rolling and / or after reheating. In particular, in the case where the cooling rate in the treatment is large (for example, 50 ° C. or more), since the toughness improvement obtained by tempering is large, tempering is preferable. Tempering temperature shall be Ac ₁ or less. When the tempering temperature exceeds Ac ₁ point, austenite transformation occurs, causing a decrease in cyclic softening characteristics and a decrease in strength and toughness. Tempering temperature becomes like this. Preferably it is 550 degrees C or less.

The steel material of the present invention has a repeat softening parameter in the range of 0.65 to 0.95, but the repetitive softening parameter can be adjusted by the conditions of heat treatment and cooling and / or Ceq, as will be understood by those skilled in the art from the foregoing description.

(First embodiment)

Steels having the chemical composition and Ceq shown in Table 1 were dissolved in a test furnace by a conventional method. Table 1 also shows the values of the Ar ₃ point and Ac ₁ point of each steel.

Each steel was made into steel with a thickness of 150 mm by normal hot forging. The steel was heated to 1150 ° C. to hot roll to obtain a steel sheet having a plate thickness shown in Table 1.

These steel plates were subjected to the following treatment.

In steels (1, 2, and 13) having a sheet thickness of 10 mm or less, the heat-rolled hot rolled steel sheet is reheated to (Ac ₁ point + 200) ° C., and then subjected to one heat treatment for cooling to room temperature at a cooling rate of 60 ° C./s. After performing it, tempering was performed at 400 degreeC.

If the river (3 to 12) and the steel (14 to 28) after hot rolling, (Ar ₃ point + 50) if from ℃, the thickness 15 ㎜ is 30 ℃ / s, thickness 25 ㎜ which is 20 ℃ / s and a plate thickness of 40 mm were cooled to 450 ° C at a cooling rate in the range of 5 ° C to 8 ° C / s in the case of 10 ° C / s and 50 mm in the plate thickness. No tempering was performed.

In order to evaluate the sheet thickness center part from these steel plates, the test member was appropriately taken and used for the cyclic softening test, the fatigue crack propagation test, the tensile test, and the Charpy impact test.

The cyclic softening test was performed using a round bar test member having a diameter of 6 to 8 mm and a parallel part length of 15 mm taken from the center of the plate thickness so that the longitudinal direction of the test member coincided with the rolling direction. Was carried out. An extensometer having a gauge length of 12.5 mm was attached to the parallel portion of the test member, and the axial force load of the distortion control was applied to the test member using the extensometer as a sensor. The test machine used was an electro-hydraulic closed loop type fatigue test machine, and the distortion waveform was an incremental step waveform of increasing and decreasing type. The repetition rate was 0.5 Hz, the distortion range was 0.024, and the maximum distortion was reached with 12 waves in the process of increasing the distortion, and the distortion returned to 0 with 12 waves in the process of reducing the distortion (see FIG. 1).

As described above, one set of this increase and decrease is referred to as "block". The stress corresponding to the maximum distortion of the first block was measured as σ ₁ and the stress corresponding to the maximum distortion of the 15th block as σ ₁₅ , and the value of σ ₁₅ / σ ₁ was determined as a repetitive softening variable.

The fatigue crack propagation test was performed using a CT (compact) test member taken so that the crack propagation direction was perpendicular to the rolling direction, in a room temperature atmosphere under a load condition of 25 kV repetition rate and a stress ratio (minimum stress / maximum stress) of 0.1. It carried out according to ASTM specification (E647).

The fatigue crack growth rate was calculated as the growth rate in the stress expansion coefficient range (ΔK) at the crack tip portion at 20 MPa · m ^0.5 . Since the fatigue crack growth rate in the conventional material was 5-6 * 10 <^-5> mm / cycle, the target fatigue crack growth rate was made into 4.0 * ^0--5 mm / cycle or less.

The tensile test was performed using the 4th test member of JISZ2201 (1998) extract | collected from the plate thickness center part so that the longitudinal direction of a test member might be perpendicular to a rolling direction.

The Charpy impact test was conducted using the V notch impact test member described in JIS Z2202 (1998), which was taken from the sheet thickness center so that the longitudinal direction of the test member coincided with the rolling direction. The test was performed three times at each test temperature and the brittle-ductile wavefront transition temperature (vTrs) was determined.

Table 2 shows each test result described above. In the table, the meanings of the marks "O", "Δ", and "x" in the tensile test and the Charpy impact test are as follows.

In the tensile test, the case where the tensile strength was 500 MPa or more was "O" and the case below 500 MPa was made "△."

In the Charpy impact test, the case where the brittle-ductile wavefront transition temperature (vTrs) is -20 ° C. or less exceeds "0", -20 ° C., and the case where it is 0 ° C. or less exceeds "Δ" and 0 ° C. ”. If vTrs of a material is -20 degrees C or less, the absorption energy in 0 degreeC will be 150 J or more in average value, and the absorption energy in -20 degreeC will be 100 J or more in average.

As shown in Table 2, the steels 15, 18, 25, and 27 exceeded the cyclic softening parameters defined in the present invention, and the fatigue crack growth rate in the atmosphere was large. The steel 14 had too low a cyclic softening variable, so the impact characteristics were not good. The steels 13, 16, 17, 19-24, 26, and 28 had good cyclic softening parameters and good fatigue crack propagation rates, but were slightly inferior in strength or impact properties of the steel.

(2nd Example)

Effect of manufacturing conditions using steel (4) (Ac ₁ point: 688 ° C., Ar ₃ point: 774 ° C.) of the first embodiment in which any of cyclic softening parameters, fatigue crack growth rate, tensile strength, and vTrs reached the target Was investigated.

A steel lump having a sheet thickness of 150 mm was heated to 1150 ° C, hot rolled to obtain a steel sheet having a sheet thickness of 25 mm. Cooling after hot rolling and / or heat treatment (reheating and cooling) after the end of hot rolling was performed under the conditions shown in Table 3. The test similar to the case of 1st Example was performed using each test member extract | collected from the steel plate obtained in this way. Table 4 shows the results of cyclic softening parameters, fatigue crack growth rate, tensile strength and Charpy impact test (vTrs).

sign Manufacture conditions A Reheat to 900 ° C. and cool to room temperature at a cooling rate of 20 ° C./s. B After reheating to 900 ° C., cooling to room temperature at a cooling rate of 20 ° C./s, and tempering at 500 ° C. C Reheat to 900 ° C. and cool to room temperature at a cooling rate of 20 ° C./s, then reheat to 750 ° C. to cool to room temperature at a cooling rate of 15 ° C./s. D After reheating to 900 ° C and cooling to room temperature at a cooling rate of 20 ° C / s, reheating to 780 ° C, cooling to room temperature at a cooling rate of 20 ° C / s, and then tempering to 450 ° C. E After hot rolling, cooling from 830 ° C. to 500 ° C. at a cooling rate of 20 ° C./s. F After hot rolling, cooling was carried out at 800 ° C to 450 ° C at a cooling rate of 25 ° C / s, and then tempered at 400 ° C. G After hot rolling, cooling from 750 ° C. to room temperature at a cooling rate of 30 ° C./s. H After hot rolling, it cooled to room temperature at the cooling rate of 30 degree-C / s at 800 degreeC, then reheated to 750 degreeC, and cooled to room temperature at the cooling rate of 25 degree-C / s. I Reheat to 650 ° C. and cool to room temperature at a cooling rate of 20 ° C./s. J After reheating to 900 ° C., cooling to room temperature at a cooling rate of 3 ° C./s, and tempering to 450 ° C. K Reheat to 900 ° C. and cool to room temperature at a cooling rate of 20 ° C./s, then reheat to 660 ° C. and cool to room temperature at a cooling rate of 15 ° C./s. L After reheating to 900 ° C., cooling to room temperature at a cooling rate of 20 ° C./s, reheating to 780 ° C., cooling to room temperature at a cooling rate of 3 ° C./s, and then tempering to 400 ° C. M After hot rolling, cooling from 830 ° C. to 600 ° C. at a cooling rate of 20 ° C./s. N After hot rolling, cooling was carried out at 800 ° C to 450 ° C at a cooling rate of 25 ° C / s, and then tempered at 700 ° C. O After hot rolling, cooling from 650 ° C to room temperature at a cooling rate of 30 ° C / s. P After hot rolling, it cooled to room temperature at the cooling rate of 30 degree-C / s at 750 degreeC, then reheated to 650 degreeC, and cooled to room temperature at the cooling rate of 25 degree-C / s.

Manufacture conditions Repeat Softening Variable σ ₁₅ / σ ₁ Fatigue Crack Growth Rate (× 10 ^-5 mm / cycle) Tensile test Charpy Impact Test A 0.852 2.42 O O B 0.911 3.21 O O C 0.831 1.98 O O D 0.912 3.05 O O E 0.875 2.61 O O F 0.891 2.97 O O G 0.860 2.85 O O H 0.855 2.47 O O I 1.105 6.24 △ × J 1.011 5.03 △ △ K 0.976 4.98 O × L 0.987 5.01 △ × M 0.979 4.95 O O N 0.981 4.96 △ × O 0.993 5.02 △ O P 0.984 4.88 O ×

As can be seen from Table 4, the steel sheets of A to H, in which the manufacturing conditions were in the range defined by the present invention, had a repeat softening variable within an appropriate range of 0.65 to 0.95, and the fatigue crack growth rate was 4.0 × 10 ^{− of the} target. ⁵ mm / cycle or less. Moreover, both tensile strength and impact characteristics were also favorable.

On the other hand, the steel sheet of I to P whose manufacturing conditions were out of the range prescribed | regulated by this invention had a cyclic softening parameter exceeding 0.95, and the fatigue crack growth rate did not reach the target value. Except for M, at least one of tensile strength and impact strength was not sufficient.

(Third Embodiment)

Corrosion fatigue crack growth test and corrosion fatigue test were done about nine steels of 3, 4, 5, 7, 12, 15, 18, 25, and 27 shown in Table 2.

The corrosion fatigue crack propagation test was conducted in seawater at room temperature. In this test, a CT test member having the same shape as that described in Example 1 was used. The difference from the fatigue crack propagation test in the atmosphere is that the repetition rate is set to 0.17 kPa for the wave period in the ocean. The stress ratio was set to 0.1 as in the fatigue crack growth test in air.

The corrosion fatigue test was carried out in five different environments: seawater at room temperature, seawater at 60 ° C, saturated aqueous chlorine solution at room temperature, 1% saline at room temperature, and 3% saline at room temperature. Seawater as used herein means artificial seawater shown in the ASTM standard. In addition, room temperature means what was formulated without performing temperature control in particular, and 60 degreeC means controlling so that this temperature may be maintained by a thermostat. Two types of saline environments were prepared to clarify the effect of a single member of sodium chloride on corrosion fatigue strength, with a 3% saline environment corresponding to a seawater environment containing about 3.5% sodium chloride.

In seawater at room temperature selected as the standard test environment for the corrosion fatigue test, a test in which the load mode or the test surface processing conditions were changed was also performed to examine their effects.

Similar to the corrosion fatigue crack growth rate, the corrosion fatigue strength is very strongly dependent on the repetition rate, and the lower the repetition rate is, the lower the corrosion fatigue strength is. For this reason, the load repetition rate of the corrosion fatigue test was set to 0.17 kW in all test environments in accordance with the cycle of the wave load in the marine environment. The stress ratio was set to 0.1, which is the standard value most widely adopted in the fatigue test.

The load modes of the corrosion fatigue test are three modes of axial force, rapid force and torsion, and axial force load is the standard load mode.

The corrosion fatigue test member used in the axial force and the bending load mode was a plate with a width of 80 mm of the handle and a width of 25 mm of the test, and the reduction from the handle to the test was smoothly connected by a curve of R100. In addition, the plate | board thickness of the handle part was 12 mm and the plate | board thickness of the test part was 6 mm, and the reduced thickness from the handle part to the test part was gently connected by the curve of R40.

The corrosion fatigue test member used in the torsional load mode was an axisymmetric round bar shape having a handle diameter of 12 mm and a test diameter of 6 mm.

The test surface machining conditions are three kinds of machining, plasma cutting, and laser cutting, and machining is standard.

In the case where fatigue cracks were generated on the machined surface to evaluate the corrosion fatigue strength, the final finish of the test surface was performed so that the maximum peak height of the surface roughness was between 1.6 and 6.3 μm between 8 mm in length.

In the case of evaluating the corrosion fatigue strength starting from the plasma cut surface, the plasma cutting method was applied when the planar shape of the test member was cut out. At this time, the chamfer of R1 was previously performed in the corner part of the cross section of a test part using a baby grinder so that a fatigue crack might arise reliably from a plasma cutting surface. Plasma cutting was performed under the following conditions.

Plasma Cutting Conditions:

Current: 240 A, Voltage: 110 V, Cutting speed: 1000 mm / min, Electrode: Tungsten electrode, Gas: H ₂ -N ₂ -Ar mixed gas.

In the case of evaluating the corrosion fatigue strength of the laser cut portion, the cut surface was prepared by laser cutting under the following conditions. Also in this case, the same chamfering method as that of the plasma cut surface was adopted so that fatigue cracking occurred reliably from the cut surface.

Laser cutting conditions

CO ₂ laser, output: 40 ㎾ (continuous), posture: transverse, cutting speed: 2.5 m / min,

Focal length: 381 mm (parabolic condensing), focal removal amount: + 8 mm.

In the corrosion fatigue test, six to eight test members are supplied for one test condition, that is, a combination of a steel sheet and a test condition, and an S-N curve (fatigue strength diagram) is generated. The time intensity Δσ (stress range: maximum stress min minimum stress) at a repetition number of 1 × 10 ⁶ was determined as the corrosion fatigue strength.

Table 5 shows the results of the corrosion fatigue crack propagation test and Table 6 shows the results of the corrosion fatigue test. For reference, the repeat softening parameters of Table 2 and the values of fatigue crack propagation rate in air are also listed in Table 5.

River number Repeat Softening Variable σ ₁₅ / σ ₁ Fatigue Crack Growth Rate (× 10 ^-5 mm / cycle) Acceleration of Fatigue Crack Growth Rate by Environment Remarks Waiting Underwater 3 0.889 3.16 17.38 5.5 Inventive Example 4 0.863 2.87 17.79 6.2 〃 5 0.820 1.92 9.22 4.8 〃 7 0.811 1.72 9.98 5.8 〃 12 0.712 0.83 4.23 5.1 〃 15 0.980 * 4.95 24.26 4.9 Comparative example 18 1.092 * 5.95 36.30 6.1 〃 25 1.120 * 6.32 34.76 5.5 〃 27 1.052 * 5.67 33.45 5.9 〃

* Indicates that it is outside the scope of the present invention.

Test environment Underwater Underwater Underwater Underwater Underwater Underwater Chlorine Saturation 1% saline 3% saline Test temperature Room temperature Room temperature Room temperature 60 ℃ Room temperature Room temperature Room temperature Room temperature Room temperature Test surface Machining Machining Machining Machining Plasma cutting Laser cutting Machining Machining Machining Load form Axial force flex torsion Axial force Axial force Axial force Axial force Axial force Axial force River number 3 450 630 360 440 430 410 400 460 450 4 420 - - 410 410 410 410 440 420 5 410 - - - 400 400 410 - - 7 420 - - - 400 410 - - - 12 430 - - - 420 430 - - - 15 * 290 - - - 250 - - - - 18 * 290 - - - 240 - - - - 25 * 270 380 220 260 220 210 230 280 270 27 * 300 - - - 240 220 - - -

* Indicates that the steel deviated from the scope of the present invention. (Unit: MPa)

As can be seen from Table 5, the corrosion fatigue crack growth rate (fatigue crack growth rate in seawater) of any steel material is faster than the fatigue crack growth rate in the air, but the corrosion fatigue cracking rate against the growth rate in this atmosphere. The degree of acceleration of the propagation rate (that is, the degree of acceleration of the fatigue crack propagation rate due to the environment) had little dependence on the type of steel and was substantially constant. Therefore, by suppressing the fatigue crack growth in the atmosphere of the steel, it was confirmed that the corrosion fatigue crack growth is also suppressed.

As can be seen from the results of the corrosion fatigue test shown in Table 6, the axial corrosion fatigue strength based on the machined surface in room temperature seawater is superior to 400 MPa or more in the steels (3, 4, 5, 7) of the present invention. On the contrary, in the steels 15, 18, 25, and 27 of the comparative example in which the cyclic softening variable exceeded 0.95, all were low values of 310 MPa or less. In addition, when comparing the examples (steels 3 and 25) in which data were prepared, the corrosion fatigue strength of the steel 3 according to the present invention in any test environment, any test surface, and any load mode was compared to that of the comparative example ( It was clearly superior to the corrosion fatigue strength of 25).

When the wavefront of the test member after the corrosion fatigue test and its vicinity were observed, the present invention and the comparative example could not recognize a clear difference in the shape and size of the corrosion pit. By the way, when the micro hardness in the vicinity of the corrosion pit bottom was measured, the hardness of all the steel materials of this invention example was lower than the steel materials of a comparative example. In the steel of the present invention, it is considered that the hardness decreases due to the cyclic softening property, which is advantageously acting on the occurrence of fatigue cracking in the corrosive environment.

According to the present invention, since fatigue resistance of steel can be quantitatively evaluated, material design using this evaluation becomes possible, and it is also possible to provide a steel having excellent fatigue crack growth resistance by using it. In addition, this steel exhibits excellent performance even in a water environment containing chlorine or chloride ions. Therefore, it is suitable for use in various structures such as ships, offshore structures, bridges, buildings, tanks, and industrial and construction machinery.

Claims (9)

Stress σ ₁ and the 15th maximum at the first maximum distortion when 15 incremental / decrease repeated loads with a maximum tensile / compression distortion of ± 0.012, a repetition speed of 0.5 Hz, and a wavenumber up to a maximum distortion of 12 were applied. A structural steel material characterized by having a repetitive softening parameter of 0.65 or more and 0.95 or less, represented by σ ₁₅ / σ ₁ , which is a ratio to the stress σ ₁₅ at the time of distortion. The method according to claim 1, in mass% C: 0.02 to 0.20%, Si: 0.60% or less, Mn: 0.50% to 2.0%, Al: 0.003% to 0.10%, Balance: Fe and unavoidable impurities The structural steel material which has a composition and whose carbon equivalent Ceq value represented by a following formula is 0.28 to 0.65%. Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 The method according to claim 1, in mass% Cu: 1.5% or less, Ni: 1.5% or less, Cr: 1.20% or less, Mo: 1.0% or less, V: 0.10% or less, Nb: 0.10% or less, Ti: 0.10% or less, B: 0.0003 to 0.0020%, Ca : Structural steel further containing 0.0005 to 0.010%, and carbon equivalent Ceq value represented by a following formula is 0.28 to 0.65%. Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 The heat-treated steel having the composition according to claim 2 or 3 is reheated to at least _one Ac and subjected to at least one heat treatment for cooling to 550 ° C. or lower at a cooling rate of 5 ° C./s or more. Method of manufacturing steels. Claim 2 or claim 3, wherein the hot working of the steel having the composition according to (Ac ₃ point - 100) ℃ above, (Ac ₃ point + 150) from the temperature range of not more than ℃ below 550 ℃ in more than 5 ℃ / s cooling rate Method for producing a structural steel, characterized in that the cooling to. The method for producing a structural steel according to claim 5, wherein after the cooling, a heat treatment for reheating to Ac ₁ or more and cooling to 550 ° C or less at a cooling rate of 5 ° C / s or more is performed at least once. The method for producing a structural steel material according to claim 4, wherein tempering at a temperature equal to or less than Ac _{1 is performed} . The method for producing a structural steel material according to claim 5, wherein tempering at a temperature of Ac ₁ or less is carried out. The method for producing a structural steel material according to claim 6, wherein tempering at a temperature of Ac ₁ or less is carried out.