JP2010047826A - Steel material superior in fatigue-crack propagation resistance and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material superior in fatigue-crack propagation resistance. <P>SOLUTION: The steel material superior in the fatigue-crack propagation resistance includes, by mass%, 0.01 to 0.1% C, 0.04 to 0.6% Si, 0.5 to 2% Mn, 0.01% or less P, 0.003% or less S, more than 0.0007 to 0.01% B, less than 0.05% Al, 0.007% or less N, 0.003% or less O and the balance Fe with impurities; has a Bq value of 0.003 or less, which is determined by the expression (1); has a Ceq value of 0.15 to 0.35, which is determined by the expression (2), wherein each symbol of the elements in the expression represents the content of each element (mass%), but when the content of each element is an impurity level, 0 shall be substituted; and contains 5×10<SP>4</SP>pieces or less of oxides by the number in a region between the surface layer and a 2 mm deep plane. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a steel material suitable for use in a hull, a civil engineering structure, a construction machine, a hydraulic iron pipe, an offshore structure, a line pipe, and other welded structures that require fatigue crack growth resistance, and a manufacturing method thereof.

  In recent years, the tendency to increase the size of welded structures has become remarkable, and higher strength and lighter weight are desired. However, when using high-strength steel, the design stress increases, so that fatigue failure is likely to occur from the welded portion, and the improvement thereof is an important problem. In a thick steel plate such as a structural steel material, since welding is generally performed, fatigue cracks may occur from the welded portion. Therefore, if the fatigue cracks generated and propagated from the welded portion can be retained by the steel material, it is effective for extending the fatigue life of the structure. For this reason, various steel plates having a fatigue crack growth suppressing effect have been proposed.

  For example, Patent Document 1 proposes a technique for reducing the residual stress in the weld joint by reducing the high-temperature strength by utilizing a ferrite phase having a lower strength than the austenite phase at the same temperature. Yes. That is, since the welded portion is rapidly cooled after welding, the temperature range of the austenite single phase is wide, and a high level of residual stress is generated with the thermal contraction of the weld metal. Therefore, in the invention described in Patent Document 1, Al is contained as a ferrite-forming element in the steel in an amount of 0.5 to 2.0%, and ferrite is generated in a temperature range of 800 to 600 ° C. Residual stress is relaxed by plastic deformation of ferrite.

  Patent Document 2 discloses that the HAZ (welding heat affected zone) structure of a high-tensile steel sheet having a tensile strength of 490 to 780 MPa is mainly used as bainite and improves fatigue strength by suppressing grain boundary ferrite generated from austenite grain boundaries. Techniques to make it have been proposed. In this technique, B is added in an amount of 0.0005 to 0.01% to suppress the formation of grain boundary ferrite, and further, the carbon equivalent (Ceq) is limited to strengthen the entire structure including bainite and martensite. ing.

JP 2004-211150 A JP 2003-171731 A

  The technique proposed in Patent Document 1 requires high-concentration Al addition to make the ferrite phase exist in a wider temperature range. However, Al contributes to the formation of a ferrite phase, but is an element that significantly reduces toughness, which is one of the basic characteristics required for structural steel sheets. For this reason, according to this technique, although the residual stress of the welded portion can be suppressed and an improvement in fatigue strength can be expected, the toughness itself against a static load is insufficient. The design of the shape and dimensions of the structural material needs to be carried out not only from the viewpoint of fatigue strength but also from the viewpoint of preventing brittle fracture against static loads. With the technique proposed in Patent Document 1, the strength and soundness are improved. The balance cannot be improved.

  In the technique proposed in Patent Document 2, the formation of grain boundary ferrite is suppressed by adding B, which can enhance hardenability at the grain boundary and efficiently suppress grain boundary ferrite. However, since B is an element that lowers the toughness of the heat affected zone, its use requires caution. In welded joints, it is important to ensure toughness in order to prevent brittle fracture due to static loads as well as fatigue characteristics against repeated loads. In particular, the majority of member dimensions are determined by the toughness of the latter, and in the current fatigue design system in which fatigue investigation is conducted to confirm fatigue fracture prevention for the necessary parts, the same as the fatigue characteristics Toughness is also important. In this sense, considering the welding conditions such as the case where the welding heat input fluctuates, it can be said that it is extremely difficult to achieve both the formation of grain boundary ferrite and the weld heat affected zone toughness only by the addition of B. .

  The present invention has been made in order to solve such problems, and a steel material excellent in fatigue crack growth resistance without adding a large amount of elements that inhibit toughness such as Al and B, and a method for producing the same. The purpose is to provide.

  In order to achieve the above object, the inventors first conducted a study focusing on the correlation between the fatigue characteristics of a welded joint and the cleanliness of inclusions present in the steel material. It was found that there was no correlation between the joint fatigue characteristics.

  Therefore, focusing on the surface of the steel sheet with a large amount of displacement, and conducting a more detailed investigation, it was found that the joint fatigue characteristics greatly depend on the characteristics of the steel sheet surface, and that improving the cleanliness improves the joint fatigue characteristics. did. More specifically, the inclusion analysis was limited to the region from the steel sheet surface to a depth of 2 mm in the thickness direction, the cleanliness was obtained for each steel sheet, and the correlation with the joint fatigue characteristics was examined. It was recognized that there was. The reason why such a correlation is recognized is that the surface of the steel sheet has a large amount of displacement and is likely to start a fatigue crack.

  By the way, inclusions do not deform even under high stress because of their high hardness. On the other hand, since the steel plate surface has a large amount of displacement, it is considered that a crack occurs at the interface between the inclusion and the base structure, and the fatigue characteristics deteriorate. Therefore, the cleanliness of inclusions is usually a problem at the center of the thickness of the steel material, but the cleanliness of the steel sheet surface is a problem with regard to fatigue characteristics.

  The present invention has been completed based on such findings, and the gist of the present invention is the steel material having excellent fatigue crack growth characteristics shown in the following (a), and the following (c) and (d). It is in the manufacturing method of the steel material excellent in the fatigue crack growth characteristic shown.

ただし、上記式中の各元素記号は、各元素の含有量（質量％）を意味する。 (1) By mass%, C: 0.01 to 0.10%, Si: 0.04 to 0.6%, Mn: 0.5 to 2%, P: 0.01% or less, S: 0.00. 003% or less, B: more than 0.0007% and 0.005% or less, Al: less than 0.05%, N: 0.007% or less and O: 0.003% or less, with the balance being Fe and impurities Oxidation in a region where the Bq value obtained from the following equation (1) is 0.003 or less, the Ceq value obtained from the following equation (2) is 0.15 to 0.35, and within 2 mm from the surface layer A steel material excellent in fatigue crack growth resistance, characterized in that the number of objects is 5 × 10 ⁴ or less per square mm.

However, each element symbol in the above formula means the content (% by mass) of each element.

In addition to the above chemical composition, the steel material excellent in fatigue crack growth characteristics shown in (1) above further contains one or more elements listed in the following [1] to [5] in mass%. be able to.
[1] One or more selected from Cu: 1.5% or less, Mo: 1% or less, V: 0.1% or less, and Nb: 0.1% or less [2] Ni: 1.5% or less [ 3] Cr: 1.2% or less [4] Ti: 0.05% or less [5] Ca: 0.003% or less and Mg: 0.003% or less

ただし、上記（３）式中の記号の定義は、下記のとおりである。
Ｇ₁：溶鋼内に吹き込まれる不活性ガス流量（ＮＬ／ｍｉｎ）
Ｈ₁：不活性ガス吹き込みノズルの先端から溶鋼湯面までの距離（ｍ）
ｔ₁：不活性ガス吹き込み時間（ｍｉｎ）
Ｓ₁：取鍋溶鋼量（ｔｏｎ）
Ｄ₁：取鍋内径（ｍ） (B) A method for producing a steel material having excellent fatigue crack growth characteristics, comprising the following steps A to D and having a recuperation temperature width of 70 ° C. or less after completion of cooling in the step D.
Step A: A step of blowing an inert gas into the molten steel under conditions that satisfy the following formula (3):
Step B: Continuously casting the obtained molten steel to obtain a steel slab having the chemical composition shown in (a) above,
Step C: After the obtained steel slab is heated to 900 to 1180 ° C., hot rolling is performed under conditions where the finishing temperature is 650 to 1000 ° C., and a hot rolled material is obtained, and
Process D: The obtained hot-rolled material is accelerated and cooled from a temperature range of 620 to 950 ° C. under a condition that an average cooling rate in the temperature range of 620 to 500 ° C. is 5 to 50 ° C./second, and is 500 ° C. or less. The process of finishing cooling in the temperature range.

However, the definitions of the symbols in the above formula (3) are as follows.
G ₁ : Inert gas flow rate (NL / min) blown into molten steel
H ₁ : Distance from the tip of an inert gas blowing nozzle to the molten steel surface (m)
t ₁ : inert gas blowing time (min)
S ₁ : Ladle molten steel amount (ton)
D ₁ : Ladle inner diameter (m)

ただし、上記（４）式中の記号の定義は、下記のとおりである。
Ｇ₂：溶鋼環流に使用される不活性ガス流量（ＮＬ／ｍｉｎ）
Ｄ₂：浸漬管内径（ｍ）
ｔ₂：真空処理時間（ｍｉｎ）
Ｓ₂：取鍋溶鋼量（ｔｏｎ） (C) A method for producing a steel material having excellent fatigue crack growth characteristics, comprising the following steps A1 to D and having a recuperation temperature width of 70 ° C. or less after completion of cooling in step D.
Step A1: A step of subjecting the molten steel to a vacuum refining treatment under the conditions satisfying the following formula (4):
Step B: Continuously casting the obtained molten steel to obtain a steel slab having the chemical composition shown in (a) above,
Step C: After the obtained steel slab is heated to 900 to 1180 ° C., hot rolling is performed under conditions where the finishing temperature is 650 to 1000 ° C., and a hot rolled material is obtained, and
Process D: The obtained hot-rolled material is accelerated and cooled from a temperature range of 620 to 950 ° C. under a condition that an average cooling rate in the temperature range of 620 to 500 ° C. is 5 to 50 ° C./second, and is 500 ° C. or less. The process of finishing cooling in the temperature range.

However, the definitions of the symbols in the above formula (4) are as follows.
G ₂ : Inert gas flow rate used for molten steel recirculation (NL / min)
D ₂ : inner diameter of dip tube (m)
t ₂ : Vacuum processing time (min)
S ₂ : Ladle molten steel amount (ton)

  Since the steel material of the present invention is excellent in fatigue crack propagation characteristics, such as hulls, civil engineering structures, construction machinery, hydraulic iron pipes, offshore structures, line pipes and other welded structures that require fatigue crack propagation characteristics. Suitable for use in.

  First, the chemical composition of the steel material of the present invention will be described. In the following description, “%” for the content means “% by mass”.

C: 0.01 to 0.10%
C is an element necessary for ensuring strength. If the content is less than 0.01%, the required strength cannot be ensured. However, when the content exceeds 0.10%, it becomes difficult to secure toughness in both the heat affected zone (hereinafter referred to as “HAZ”) and the base material when welding. Therefore, the content of C is set to 0.01 to 0.10%.

Si: 0.04 to 0.6%
Si has a deoxidizing action and contributes to an increase in strength of the steel sheet. In order to obtain these effects, it is necessary to contain Si by 0.04% or more. However, if its content exceeds 0.6%, toughness is reduced. Therefore, the Si content is set to 0.04 to 0.6%.

Mn: 0.5-2%
Mn has an effect of enhancing the hardenability of steel and is an effective component for securing the strength. If the content is less than 0.5%, the hardenability is insufficient and the desired strength and toughness cannot be obtained. However, if Mn is contained in excess of 2%, segregation increases and hardenability increases too much, and the toughness of both the HAZ and the base material decreases during welding. Therefore, the Mn content is set to 0.5 to 2%.

P: 0.01% or less P is unavoidably present in steel as an impurity. If its content exceeds 0.01%, it not only segregates at the grain boundaries and lowers toughness, but also causes hot cracking during welding. Therefore, the content of P needs to be limited to 0.01% or less. The smaller the P, the better.

S: 0.003% or less S is unavoidably present in steel as an impurity. If the content is too large, the center segregation is promoted or a large amount of stretched MnS is generated, which deteriorates the mechanical properties of the base material and the HAZ. Therefore, the S content needs to be limited to 0.003% or less. The smaller the S, the better.

B: More than 0.0007% and 0.005% or less B is an element having an effect of improving hardenability and increasing strength. In order to acquire this effect, it is necessary to make it contain exceeding 0.0007%. However, if the content exceeds 0.005%, the fatigue characteristics deteriorate. Therefore, the B content is more than 0.0007% and 0.005% or less.

Al: less than 0.05% Al is an element having a deoxidizing action. However, when the content is 0.05% or more, toughness tends to deteriorate mainly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, the Al content is less than 0.05%. However, when deoxidizing with Si having a deoxidizing action, it is not particularly necessary to add it. In order to exhibit the deoxidation effect by Al, it is preferable to contain 0.001% or more.

N: 0.007% or less N is an element unavoidably present in steel as an impurity. When present in a large amount, it causes deterioration of the toughness of the base material and the HAZ. Therefore, the N content is 0.007% or less. The smaller N, the better.

O: 0.003% or less O is an element unavoidably present in steel as an impurity. When the content exceeds 0.003%, the base material toughness and ductility such as stretch drawing are adversely affected. Therefore, the O content is limited to 0.003% or less.

ただし、上記式中の各元素記号は、各元素の含有量（質量％）を意味する。 The chemical composition of the steel material of the present invention contains each of the above elements, the balance is Fe and impurities, and the Bq value obtained from the following equation (1) is 0.003 or less, and is obtained from the following equation (2). It is necessary that the obtained Ceq value is 0.15 to 0.35.

However, each element symbol in the above formula means the content (% by mass) of each element.

Bq: 0.003 or less In order to exert the effect of improving the hardenability by B, it is necessary to eliminate the influence of N in the steel. This is because B is easily bonded to N, and if free N is present in the steel, it is bonded to N and BN is easily generated. For this reason, Ti is added according to the N content and fixed as TiN, so that B is present in the steel. However, if the Bq value obtained from the equation (1) exceeds 0.003, a coarse ferrocarbon boride is formed, leading to deterioration of fatigue characteristics. Therefore, the Bq value needs to be 0.003 or less.

  In order to surely obtain the above effect, it is preferable that the Bq value defined by the above equation (1) is 0.0001 or more. In addition, the Bq value is more preferably 0.0005 or more and even more preferably 0.001 or more because the hardenability is improved as the Bq value is larger in the range of 0.003 or less.

Ceq: 0.15-0.35
Ceq calculated | required from said Formula (2) is what is called a carbon equivalent, and is an parameter | index which evaluates the hardenability and weldability of a steel plate, and is generally used widely.

The inventors of the present invention have improved the fatigue characteristics of welded joints, have a tensile strength (TS) generally used as structural steel of 500 MPa or more, and a Charpy absorbed energy value vE _{0 at} 0 ° C. of 27 J or more. We searched for the necessary conditions to meet the requirements. As a result, when the Ceq value is less than 0.15%, the strength is lowered. On the other hand, when the Ceq value exceeds 0.35%, the hardenability of the steel sheet is increased, the joint hardness distribution is uneven, and the joint fatigue is increased. It was found to have an adverse effect on strength. Moreover, when Ceq exceeds 0.35, deterioration of weldability is caused, welding work becomes difficult, and there is a demerit that the use of the steel sheet is remarkably limited. Therefore, the Ceq value is set to 0.15 to 0.35%. In addition, the preferable minimum of Ceq is 0.20%. Moreover, the preferable upper limit of Ceq is 0.30%.

For the purpose of improving various performances, the steel material of the present invention may further contain one or more elements listed in the following [1] to [5].
[1] One or more selected from Cu: 1.5% or less, Mo: 1% or less, V: 0.1% or less, and Nb: 0.1% or less [2] Ni: 1.5% or less [ 3] Cr: 1.2% or less [4] Ti: 0.05% or less [5] Ca: 0.003% or less and Mg: 0.003% or less

  First, the preferable range of the content of each optional additive element and the reason for limitation will be described.

Cu: 1.5% or less Cu has an effect of further improving strength and corrosion resistance, and may be added as necessary. In particular, when quenching and tempering is performed on a steel material containing Cu, the strength is further increased by the age hardening effect of Cu. These effects become significant when the content is 0.5% or more. However, even if the content exceeds 1.5%, the performance improvement commensurate with the cost increase is not observed. Therefore, when Cu is contained, the content is preferably 1.5% or less.

Mo: 1% or less Mo has an effect of improving the strength and toughness of the base material, and may be added as necessary. This effect becomes remarkable when the content is 0.05% or more. However, if the content exceeds 1%, the hardness of the HAZ mainly increases, and the toughness and SSC resistance are impaired. Therefore, when Mo is contained, the content is preferably 1% or less.

V: 0.1% or less V has an effect of improving the strength of the base material mainly by precipitation of carbonitride during tempering, and may be added as necessary. This effect becomes significant when the content is 0.005% or more. However, if the content exceeds 0.1%, the performance improvement effect of the base material is saturated, leading to toughness deterioration. Therefore, when V is contained, the content is preferably 0.1% or less.

Nb: 0.1% or less Nb has an effect of improving the strength and toughness of the base material by refining and precipitation of carbides, and may be added as necessary. This effect becomes significant when the content is 0.005% or more. However, if the content exceeds 0.1%, the above effects are saturated, while the toughness of the HAZ is significantly impaired. Therefore, when Nb is contained, the content is preferably 0.1% or less.

Ni: 1.5% or less Ni has the effect of increasing the toughness of the steel matrix (material) in the solid solution state, and may be added as necessary. This effect becomes remarkable when the content is 0.05% or more. However, if the content exceeds 1.5%, improvement in characteristics commensurate with the increase in alloy cost cannot be obtained. Therefore, when Ni is contained, the content is preferably 1.5% or less.

Cr: 1.2% or less Since Cr has the effect of enhancing the corrosion resistance of carbon dioxide gas and enhancing the hardenability, it may be added as necessary. This effect becomes remarkable when the content is 0.05% or more. However, if the content exceeds 1.2%, it becomes difficult to suppress the hardening of HAZ even if other component conditions are satisfied, and the effect of improving the corrosion resistance of carbon dioxide gas is saturated. Therefore, when Cr is contained, the content is preferably set to 1.2% or less.

Ti: 0.05% or less Ti acts as a deoxidizing element and forms an oxide phase composed of Ti and Mn. Especially, it refines the structure in the heat-affected zone of high heat input welding, and improves fatigue characteristics. May be added as necessary. In order to form this oxide phase in steel, the total amount of Ti in the steel is preferably 0.003% or more. However, if the content exceeds 0.05%, the formed oxide becomes Ti oxide or Ti-Al oxide, and the dispersion density decreases, and the structure in the heat-affected zone of the high heat input weld zone is reduced. The ability to refine is lost. For this reason, when Ti is contained, the content is preferably 0.05% or less. More preferred is less than 0.02%. Furthermore, it is preferable to set it as 0.018% or less.

Ca: 0.003% or less Ca reacts with S in steel to form oxysulfide (oxysulfide) in molten steel. Unlike MnS, etc., this oxysulfide does not extend in the rolling direction during rolling and is spherical after rolling, so it suppresses weld cracks and hydrogen-induced cracks starting from cracks at the ends of stretched inclusions. Has the effect of Therefore, you may add as needed. This effect becomes remarkable when the content is 0.0005% or more. However, if its content exceeds 0.003%, toughness may be deteriorated. Therefore, when Ca is contained, the content is preferably 0.003% or less.

Mg: 0.003% or less Mg forms an Mg-containing oxide, serves as a generation nucleus of TiN, and has the effect of finely dispersing TiN. Therefore, Mg may be added as necessary. This effect becomes remarkable when the content is 0.0005% or more. However, when the content exceeds 0.003%, the amount of oxide becomes excessive and ductility is reduced. Therefore, when Mg is contained, the content is preferably 0.003% or less.

Number of oxides in a region within 2 mm from the surface layer: 5 × 10 ⁴ or less per square mm The number of oxides in a region within 2 mm from the surface layer is 5 × 10 ⁴ or less per square mm. This is because when more than 5 × 10 ⁴ oxides are present, the number of sources of fatigue cracks increases and the fatigue characteristics deteriorate.
Here, the number of oxides was measured by the following procedure.
(1) A small piece is cut out using a cross section perpendicular to the rolling direction of the manufactured steel as an observation surface, and the observation surface is corroded with a night solution to prepare a test piece.
(2) Set the above test piece on a scanning electron microscope (SEM) equipped with an energy dispersive X-ray fluorescence spectrometer (EDX), and use a 0.05 mm square area as one field of view, and an area within 2 mm from the surface layer. 5 fields of view are observed at a magnification of 2000 times, and the number of oxides in each field is measured. At this time, the distinction between oxides and other inclusions is made by composition analysis by EDX. Further, the number of oxides is measured by changing the depth in a region from the surface layer to a depth of 2 mm in order to avoid variations in the visual field.
(3) The number of oxides in each visual field is averaged to obtain the number of oxides in a region within 2 mm from the surface layer.

ただし、上記（３）式中の記号の定義は、下記のとおりである。
Ｇ₁：溶鋼内に吹き込まれる不活性ガス流量（ＮＬ／ｍｉｎ）
Ｈ₁：不活性ガス吹き込みノズルの先端から溶鋼湯面までの距離（ｍ）
ｔ₁：不活性ガス吹き込み時間（ｍｉｎ）
Ｓ₁：取鍋溶鋼量（ｔｏｎ）
Ｄ₁：取鍋内径（ｍ） In producing the steel material excellent in fatigue crack growth resistance of the present invention, adjustment from the refining stage is necessary. That is, in the refining stage, the oxide in the surface layer can be reduced by devising an inert gas blowing process or a vacuum refining process. Specifically, in performing the inert gas blowing process, it is effective to blow the inert gas into the molten steel under the condition that satisfies the following expression (3).

However, the definitions of the symbols in the above formula (3) are as follows.
G ₁ : Inert gas flow rate (NL / min) blown into molten steel
H ₁ : Distance from the tip of an inert gas blowing nozzle to the molten steel surface (m)
t ₁ : inert gas blowing time (min)
S ₁ : Ladle molten steel amount (ton)
D ₁ : Ladle inner diameter (m)

If the inert gas blowing process is performed under the conditions satisfying the above expression (3), blowing can be performed while sufficiently stirring the bath. That is, at the beginning of blowing, the silicon in the hot metal is oxidized to become silica, which reacts with burned lime and iron oxide added in the furnace, and begins to form CaO—SiO ₂ —FeO slag. At the same time, the furnace temperature rises and scrap melting begins to progress. In the initial stage of blowing, since the carbon concentration in the hot metal is high, the injected pure oxygen gas reacts efficiently with the carbon to become carbon monoxide and decarburization proceeds. At this stage, the supply rate of pure oxygen gas determines the decarburization. As the decarburization progresses, the temperature of the bath further increases. As decarburization progresses and the carbon concentration decreases, the decarburization reaction is controlled by the movement of carbon in the molten steel. Insufficient carbon movement due to stirring of molten steel, the injected pure oxygen gas is used to oxidize iron rather than react with carbon, and iron oxide increases in the slag, reducing the iron yield. To do. To prevent this, gas blowing from the furnace bottom is activated.

ただし、上記（４）式中の記号の定義は、下記のとおりである。
Ｇ₂：溶鋼環流に使用される不活性ガス流量（ＮＬ／ｍｉｎ）
Ｄ₂：浸漬管内径（ｍ）
ｔ₂：真空処理時間（ｍｉｎ）
Ｓ₂：取鍋溶鋼量（ｔｏｎ） On the other hand, in carrying out the vacuum refining treatment, it is effective to blow an inert gas into the molten steel under conditions that satisfy the following expression (4).

However, the definitions of the symbols in the above formula (4) are as follows.
G ₂ : Inert gas flow rate used for molten steel recirculation (NL / min)
D ₂ : inner diameter of dip tube (m)
t ₂ : Vacuum processing time (min)
S ₂ : Ladle molten steel amount (ton)

  When performing vacuum refining treatment, it is effective to remove the gas component in the molten steel by putting the molten steel in a decompressed container and lowering the equilibrium partial pressure as a condition that satisfies the above-mentioned formula (4). .

  Furthermore, in order to increase the cleanliness of the steel, it should be avoided that Al deoxidation is mostly advanced in the initial stage of refining. It is desirable to adjust the composition other than Al together with Mn, Si, and the like, and after deoxidation proceeds with Ti or the like, Al is introduced into a small amount of molten steel immediately before the outgoing steel, and the obtained molten steel is cast.

In the case of ingot casting, prior to hot rolling, an extra step of producing a steel slab (slab) by split rolling must be passed, and the yield is also reduced. Therefore, casting is preferably performed by continuous casting. In the case of continuous casting, segregation of the steel slab also adversely affects the toughness of the HAZ, and therefore preferably C is 0.29% or less, P is 0.30% or less, and Mn is 3.5% or less in the segregated part. It is better to perform such management.

  In addition to the above conditions, electromagnetic brakes are applied at 1000 to 5000 gauss for discharge flow rate management during casting, electromagnetic stirring treatment is performed on unsolidified molten steel at 250 to 1000 gauss, and the final solidified part is about 1 mm / m. The steel sheet may be squeezed with a gradient and the concentrated segregated molten steel may be squeezed out from the final solidified portion. By appropriately combining the above management items, a steel slab having excellent cleanliness and little central segregation can be obtained.

  Then, it is good to heat the steel piece manufactured in this way to the temperature range of 900-1180 degreeC, and to perform hot rolling. At this time, the steel slab once cooled to room temperature may be reheated, and maintained or heated to the above temperature through a soaking furnace without cooling to room temperature after continuous casting by a so-called direct feed rolling process. Also good. Here, when the heating temperature is less than 900 ° C., the reverse transformation to austenite becomes insufficient at the time of slab heating, and the subsequent characteristics deteriorate. On the other hand, when the heating temperature exceeds 1180 ° C., the austenite crystal grains become coarse during the heating of the steel slab, and the toughness of the entire base metal as well as the center portion of the plate thickness decreases.

  The hot rolling conditions are preferably a hot rolling finishing temperature of 650 to 1000 ° C. When the finishing temperature is less than 650 ° C., the deformation resistance of the steel increases, so that it becomes difficult to finish the shape of the steel sheet after hot rolling into a target shape. If the finishing temperature is high, the effect of crystal grain refinement by controlled rolling cannot be obtained, and the toughness of the base material cannot be ensured. Therefore, the upper limit of the finishing temperature is limited to 1000 ° C.

  Subsequently, the obtained hot-rolled material is accelerated and cooled from a temperature range of 620 to 950 ° C. under a condition that the average cooling rate in the temperature range of 620 to 500 ° C. is 5 to 50 ° C./second. Cooling should be terminated in the temperature range. Furthermore, the recuperation temperature range after the cooling is preferably 70 ° C. or less.

By cooling under such conditions, fatigue characteristics can be improved.
That is, when the average cooling rate in the temperature range of 620 to 500 ° C. is less than 5 ° C./sec, a bainite structure with coarse carbides is likely to be generated, so that sufficient yield strength is ensured particularly in the center of the steel sheet. Can not do it. On the other hand, when the cooling rate in the temperature range exceeds 50 ° C./sec, the toughness of the surface layer may be deteriorated because the steel layer is easily baked near the surface layer portion. Therefore, in the present invention, the average cooling rate in the temperature range of 620 to 500 ° C. is set to 5 to 50 ° C./sec.
When the cooling stop temperature in this cooling exceeds 500 ° C., the strength cannot be ensured because the formation of martensite or lower bainite becomes insufficient not only in the central part of the steel sheet but also in the surface layer part. Therefore, the cooling stop temperature is set to 500 ° C. or lower. Such heat treatment makes it easier to obtain a martensite or bainite structure. In the case of the steel material having the chemical composition of the present invention, it mainly has a bainite structure.

  A slab obtained by melting steel having a chemical composition shown in Table 1 in a converter and performing an inert gas blowing process or a vacuum refining process shown in Table 2 and then performing continuous casting is appropriately used. The steel plate for test was obtained by hot rolling and cooling to the plate thickness under the conditions shown in Table 3.

The fatigue fracture life, the tensile strength and toughness of HAZ, and the number of oxides were measured by the following methods using the above test steel plates.
<Fatigue test>
Using the test steel plate, a load non-transmission type cross welded joint was produced under the welding conditions shown in Table 4 and subjected to a fatigue test. In addition, the shape and dimension of a joint test body are shown in FIG. The joint was manufactured by fillet welding. In FIG. 1, 1 and 2 are base metal steel plates, and 5 is a welded portion. A repeated axial load was applied to each joint specimen, and the fatigue crack initiation life at the weld toe, that is, fatigue fracture life, was measured. Table 5 shows the fatigue test conditions.

<HAZ tensile strength>
From the test steel plate, a test piece was taken in a direction parallel to the rolling surface and perpendicular to the rolling direction at ¼ t part of the plate thickness, and subjected to a tensile test according to the method defined in JIS Z 2241 (1998). And tensile strength (TS) was determined.

<HAZ toughness>
From the test steel plate, a test piece was taken in a direction parallel to the rolling surface and perpendicular to the rolling direction at ¼ t of the plate thickness, and subjected to an impact test according to the method specified in JIS Z 2242 (1998). And the absorbed energy (vE0) at 0 ° C. was determined.

<Number of oxides>
The number of oxides in a region within 2 mm from the surface layer was determined by the following procedure.
(1) A small piece was cut out using the cross section perpendicular to the rolling direction of the manufactured steel as an observation surface, and the observation surface was corroded with a night solution to prepare a test piece.
(2) Set the above-mentioned test piece on SEM with EDX, observe 0.05mm square area as 1 field of view, and observe 5 fields (5 fields at almost equal intervals) within 2mm from the surface layer at a magnification of 2000 times. The number of oxides in each field of view was measured. At this time, the oxides were distinguished from other inclusions by composition analysis using EDX. The number of oxides was measured by changing the depth in a region from the surface layer to a depth of 2 mm in order to avoid variation in the visual field.
(3) The number of oxides in each field of view was averaged to obtain the number of oxides in a region within 2 mm from the surface layer.

  Table 6 shows the chemical composition and manufacturing method of the steel material and various test results.

As shown in Table 6, in Invention Examples 1 to 10, which satisfy the conditions of the present invention for both the chemical composition and the production method, the number of oxides in a region within 2 mm from the surface layer is 5 × 10 ⁴ pieces / mm ² or less, In any of the examples, the fatigue fracture life (number of repetitions) exceeded 5 × 10 ⁶ times, and da / dn was 5 × 10 ⁻⁵ or less, so that it had sufficient fatigue crack propagation characteristics.

On the other hand, the chemical composition satisfies the range defined by the present invention, but the production methods deviate from the conditions of the present invention, and Comparative Examples 1 and 2 and the chemical composition deviates from the range defined by the present invention. In each case, the fatigue rupture life was extremely poor at 10 ⁴ units.

  Since the steel material of the present invention is excellent in fatigue crack propagation characteristics, such as hulls, civil engineering structures, construction machinery, hydraulic iron pipes, offshore structures, line pipes and other welded structures that require fatigue crack propagation characteristics. Suitable for use in.

Diagram showing shape and dimensions of joint specimen

Explanation of symbols

1. Base steel plate 2. 4. Base steel plate welded part

Claims (8)

In mass%, C: 0.01 to 0.10%, Si: 0.04 to 0.6%, Mn: 0.5 to 2%, P: 0.01% or less, S: 0.003% or less B: more than 0.0007% and 0.005% or less, Al: less than 0.05%, N: 0.007% or less and O: 0.003% or less, with the balance being Fe and impurities, The Bq value obtained from the following formula (1) is 0.003 or less, the Ceq value obtained from the following formula (2) is 0.15 to 0.35, and the number of oxides in the region within 2 mm from the surface layer is A steel material excellent in fatigue crack growth resistance, characterized in that it is 5 × 10 ⁴ or less per square mm.

However, each element symbol in the above formula means the content (% by mass) of each element. When the content of each element is at the impurity level, 0 is substituted.   Furthermore, it contains at least one element selected from Cu: 1.5% or less, Mo: 1% or less, V: 0.1% or less, and Nb: 0.1% or less in mass%. The steel material excellent in fatigue crack growth characteristics according to claim 1.   The steel material having excellent fatigue crack growth characteristics according to claim 1 or 2, further comprising Ni: 1.5% or less by mass.   The steel material having excellent fatigue crack growth characteristics according to any one of claims 1 to 3, further comprising, by mass%, Cr: 1.2% or less.   The steel material having excellent fatigue crack growth resistance according to any one of claims 1 to 4, further comprising, by mass%, Ti: 0.05% or less.   The fatigue crack growth resistance according to any one of claims 1 to 5, further comprising one or both of Ca: 0.003% or less and Mg: 0.003% or less in mass%. Excellent steel material. A method for producing a steel material having excellent fatigue crack growth characteristics, comprising the following steps A to D and having a recuperation temperature width of 70 ° C. or less after completion of cooling in step D.
Step A: A step of blowing an inert gas into the molten steel under conditions that satisfy the following formula (3):
Step B: Continuously casting the obtained molten steel to obtain a steel slab having the chemical composition according to any one of claims 1 to 6,
Step C: After the obtained steel slab is heated to 900 to 1180 ° C., hot rolling is performed under conditions where the finishing temperature is 650 to 1000 ° C., and a hot rolled material is obtained, and
Process D: The obtained hot-rolled material is accelerated and cooled from a temperature range of 620 to 950 ° C. under a condition that an average cooling rate in the temperature range of 620 to 500 ° C. is 5 to 50 ° C./second, and is 500 ° C. or less. The process of finishing cooling in the temperature range.

However, the definitions of the symbols in the above formula (3) are as follows.
G ₁ : Inert gas flow rate (NL / min) blown into molten steel
H ₁ : Distance from the tip of an inert gas blowing nozzle to the molten steel surface (m)
t ₁ : inert gas blowing time (min)
S ₁ : Ladle molten steel amount (ton)
D ₁ : Ladle inner diameter (m) A method for producing a steel material having excellent fatigue crack growth characteristics, comprising the following steps A1 to D and having a reheat temperature width of 70 ° C. or less after completion of cooling in step D.
Step A1: A step of subjecting the molten steel to a vacuum refining treatment under the conditions satisfying the following formula (4):
Step B: Continuously casting the obtained molten steel to obtain a steel slab having the chemical composition according to any one of claims 1 to 6,
Step C: After the obtained steel slab is heated to 900 to 1180 ° C., hot rolling is performed under conditions where the finishing temperature is 650 to 1000 ° C., and a hot rolled material is obtained, and
Process D: The obtained hot-rolled material is accelerated and cooled from a temperature range of 620 to 950 ° C. under a condition that an average cooling rate in the temperature range of 620 to 500 ° C. is 5 to 50 ° C./second, and is 500 ° C. or less. The process of finishing cooling in the temperature range.

However, the definitions of the symbols in the above formula (4) are as follows.
G ₂ : Inert gas flow rate used for molten steel recirculation (NL / min)
D ₂ : inner diameter of dip tube (m)
t ₂ : Vacuum processing time (min)
S ₂ : Ladle molten steel amount (ton)

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