JP6101132B2 - Manufacturing method of steel materials with excellent resistance to hydrogen-induced cracking - Google Patents

Description

The present invention relates to a steel material excellent in hydrogen-induced crack resistance and a method for producing the same, and in a wet hydrogen sulfide environment containing a large amount of hydrogen sulfide (H ₂ S), it has good HIC (Hydrogen Induced Cracking), hydrogen The present invention relates to a steel material that exhibits (induced cracking) and a method for producing the same.

Equipment used in a wet hydrogen sulfide environment that contains a large amount of hydrogen sulfide (H ₂ S), for example, line pipes for transporting crude oil, natural gas, etc., petroleum refining equipment (hydrorefining equipment, hydrodesulfurization equipment, contact In the case of a decomposition apparatus, etc., the occurrence of hydrogen induced cracking (HIC) is concerned. This is because hydrogen generated by the electrochemical reaction between the water containing hydrogen sulfide contained in crude oil and natural gas and the steel material enters the steel, and the inclusions present in the steel (for example, sulfides such as MnS, Oxides such as Al ₂ O _3, carbonitrides such as Ti (CN) and Nb (CN)) and gasified by accumulating around the defects present at the interface between inclusions and steel, and the generated internal pressure yields the steel This is a phenomenon in which plastic deformation occurs and cracks and blisters occur when the strength is exceeded. As introduced in Non-Patent Document 1, the case of HIC began to become apparent in the 1950s, and in the wake of the massive oil spill caused by the submarine line pipe damage accident that occurred in the 1970s, the material and usage environment aspects Has been studied.

  A method for improving the HIC resistance of a steel material used in such an environment has been proposed from the viewpoints of the steel material and the surface treatment.

  First, the following proposals have been made from the viewpoint of materials. That is, the most commonly reported starting point of HIC is MnS that is greatly stretched by rolling. Accordingly, it has been proposed to reduce the amount of S in the steel and to add Ca to replace Mn in MnS with Ca and to make it spherical in order to suppress the formation of MnS that is easily stretched. In addition, carbon nitride is controlled by controlling the contents of carbon and nitrogen, as well as technology for suppressing oxide generation by controlling the amount of oxygen and Al, and carbon nitride forming elements such as Ti and Nb. A method has been proposed (for example, Non-Patent Document 2).

  Further, for the purpose of improving the corrosion resistance of the steel material, Non-Patent Document 1 proposes a technique such as adding an element such as Cu or suppressing the formation of a center segregation hardened layer that becomes a crack occurrence site. ing.

  Next, from the viewpoint of the surface treatment, in view of the fact that hydrogen that penetrates into the steel material is generated by an electrochemical reaction at the surface of the steel material with water containing hydrogen sulfide, the water containing the hydrogen sulfide and the steel Techniques for preventing contact have been proposed.

As a specific example, a method of forming a layer containing Fe ₃ O ₄ (magnetite) on the surface of a steel material has been proposed. For example, Patent Document 1 discloses a method for forming an oxide layer containing magnetite having a volume ratio of 50% or more on the surface of a steel material. In this method, an iron oxide layer containing magnetite is formed on the surface of the steel material by reducing iron hydroxide generated during a water-filling test of a pressure vessel, piping, or the like with water vapor at about 150 ° C.

  However, iron hydroxide as generated in the water-filling test is difficult to form uniformly on the steel surface (generally, it is formed at the location where water droplets remain), and the formed iron hydroxide is rough. Because of the structure (structure with a lot of gaps), even when magnetite is formed using such iron hydroxide as a raw material, the resulting oxide layer is uneven and rough so that the steel material is partially exposed. That is, it seems that the environmental barrier property is insufficient. Therefore, in sweet and light sour environments (environment of about pH 5 in the experimental example), the effect of improving HIC resistance can be expected rather than untreated steel, but it is sufficient in the severe sour environment (pH 3 or less) that is required recently. It seems that HIC resistance is hardly exhibited.

  A method of applying an epoxy resin to the surface of a steel material has been proposed. For example, Patent Document 2 proposes a technique for suppressing hydrogen intrusion into a steel material by applying an anticorrosion paint composed of a reaction product of an epoxy resin and a modified polyamine to the steel material.

  In heavy anti-corrosion coating, a coating film of several hundred to several thousand μm is usually formed on the surface of a steel material. However, the coating method represented by spray coating or the like as in Patent Document 2 described above completely eliminates coating film defects. It is difficult to suppress it. If there is a defect in the coating film, moisture containing hydrogen sulfide reaches the surface of the steel material through this defect, and a corrosion reaction occurs at the reaching portion, and there is a concern about HIC due to the generated hydrogen. In addition, the coating film (resin) deteriorates due to long-term use, and moisture containing hydrogen sulfide penetrates into the deteriorated coating film, resulting in the corrosion reaction as described above and the hydrogen generated by this reaction. Concern about HIC.

  Furthermore, a method of forming a plated layer of Cu or Ni on the steel material surface has been proposed. However, this method has a problem that a Cu or Ni plating layer is cracked or peeled off. Patent Document 3 proposes a technique in which a Cu plating layer is formed on a steel material surface, a Ni plating layer is formed thereon, and then a synthetic resin layer is further formed. However, when this technology is applied to the entire surface of steel for line pipes used in large quantities, the cost becomes very high and the reality is low.

  As described above, various methods have been proposed, but in recent years, the drilling environment for crude oil and natural gas has been increasing in a severe environment containing a lot of hydrogen sulfide. However, the steel materials proposed from the viewpoints of the material surface and surface treatment described above are not always excellent in HIC resistance even in this severe sour environment.

JP 62-238378 A JP-A-1-172466 JP 63-161190 A

Matsuyama Junsaku, “Delayed Destruction”, published by Nikkan Kogyo Shimbun, August 31, 1989 “Pipeline” Materials, Journal of Welding Society, 2011, Vol. 2, p. 180

  The present invention has been made by paying attention to the above-mentioned circumstances, and its purpose is in a severe sour environment such as a line pipe for transporting crude oil or natural gas containing a large amount of hydrogen sulfide, a pressure vessel, or the like. It is to establish a steel material that exhibits excellent HIC resistance (hydrogen-induced crack resistance) and a method for producing the steel material even when used.

The steel material excellent in hydrogen-induced crack resistance of the present invention that has solved the above problems is a base steel material,
C: 0.01 to 0.20% (meaning mass%, the same applies to chemical components below),
Si: 0.10 to 0.50%,
Mn: 0.70 to 2.50%,
P: 0.03% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.005 to 0.10%, and O: 0.01% or less (excluding 0%)
And 90% or more of the surface of the base steel material is covered with an oxide layer having an average thickness of 1.0 μm or more and 100 μm or less and containing Fe ₃ O ₄ in a volume ratio of 70% or more. Have.

  In a preferred embodiment, the steel material further contains Ca: 0.0001 to 0.01%.

  In a preferred embodiment, the steel material is further selected from the group consisting of Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, and Cr: 0.05 to 1.0%. Containing one or more elements.

  In a preferred embodiment, the steel material is further selected from the group consisting of Mo: 0.05 to 0.5%, Nb: 0.001 to 0.1%, and Ti: 0.001 to 0.1%. Containing one or more elements.

  In a preferred embodiment, the steel material further contains 0.0001 to 0.01% of one or more elements selected from the group consisting of Zr, V, Co, REM, Y, Mg, and B.

  Examples of the steel materials include steel plates for line pipes and steel pipes for line pipes.

  The present invention also includes a method for manufacturing the above steel material (manufacturing method a), and the manufacturing method a is characterized in that descaling is performed at least once during hot rolling.

  Moreover, this invention also includes another manufacturing method (manufacturing method b) of the said steel materials, and this manufacturing method b is characterized by introducing a crack into the scale layer formed on the steel material surface after hot rolling. Have

  Examples of methods for introducing cracks in the scale layer in the production method b include performing descaling at least once after hot rolling, or cooling at an average cooling rate of 10 ° C./second or more after hot rolling. .

According to the present invention, most of the surface of the base steel material is covered with an oxide layer having a certain thickness or more, and the oxide layer contains a certain amount or more of Fe ₃ O ₄ (magnetite). Therefore, the surface of the base steel material is substantially covered with a dense Fe ₃ O ₄ layer, and the component composition of the steel material is also controlled, so in a severe sour environment (environment with high hydrogen sulfide concentration) Inevitable hydrogen intrusion is sufficiently suppressed, and it is possible to provide a steel material whose HIC resistance is significantly superior to that of conventional steel. Since the steel material of the present invention exhibits such excellent HIC resistance, transportation line pipes and petroleum refining equipment (hydrorefining equipment, hydrodesulfurization equipment) that are in contact with crude oil or natural gas having a high hydrogen sulfide concentration. And a pressure vessel used in a catalytic cracking apparatus.

In light of the fact that the recent drilling environment for crude oil and natural gas is a severe environment containing a lot of hydrogen sulfide, the present inventors have obtained a steel material exhibiting excellent HIC resistance even in such an environment. First, based on the mechanism of hydrogen-induced cracking in a hydrogen sulfide environment (sour environment),
・ Various studies were conducted from the viewpoint of hydrogen generation behavior on the steel surface and hydrogen penetration behavior into the steel material. As a result, it was first found that reducing the amount of hydrogen penetrating into the steel material is effective in ensuring excellent HIC resistance by suppressing blistering occurring in the steel material. Pay attention.

In the case of a normal corrosive environment other than the hydrogen sulfide environment, most of the hydrogen atoms adsorbed on the steel surface are
Recombination between hydrogen atoms (H _ad + H _ad → H ₂ ↑); or a bond between hydrogen ions, hydrogen atoms and electrons (H ⁺ + H _ad + e ⁻ → H ₂ ↑);
It escapes from the steel surface by the “hydrogen gasification reaction” that becomes hydrogen gas.

On the other hand, in a hydrogen sulfide environment, hydrogen ions are generated by decomposition of hydrogen sulfide (H ₂ S → H ⁺ + HS ⁻ ), and the generated hydrogen ions receive electrons on the steel surface, atomize and adsorb (H ⁺ + E ⁻ → H _ad ) and is thought to penetrate into the steel.

In the case of a hydrogen sulfide environment, it is said that the hydrogen gasification reaction is remarkably suppressed. In particular, in hydrogen hydrogen embrittlement evaluation tests for high strength steels and the like, when introducing hydrogen into steel by cathodic charging in an acid solution, catalyst poisons added for the purpose of increasing the amount of hydrogen introduced (P, As, Group 15 elements such as Sb and Group 16 elements such as S and Se) are said to suppress recombination (H _ad + H _ad → H ₂ ↑) between hydrogen atoms. Therefore, since S of H ₂ S in the sour environment is considered to exhibit the same action as the above catalyst poison, in the sour environment, it is difficult to escape hydrogen by suppressing the recombination reaction between hydrogen atoms. . Therefore, in the hydrogen gasification reaction, attention was focused on promoting the bonding reaction (H ⁺ + H _ad + e ⁻ → H ₂ ↑) between hydrogen ions, hydrogen atoms, and electrons. Furthermore, the present inventors pay attention to the fact that the “bonding reaction between hydrogen ions, hydrogen atoms and electrons” is a rate-limiting reaction on the oxide, and that the oxide has a low hydrogen diffusion coefficient, and the surface of the steel plate is oxidized. We have intensively studied how to suppress hydrogen intrusion by forming objects.

As a result, a steel material which becomes a hydrogen generation site by covering most of the surface of the base steel material with an oxide layer having a certain thickness or more and making the structure of the oxide layer mainly Fe ₃ O ₄ (magnetite). Not only can the surface be isolated from the corrosive environment, but also hydrogen gas by causing a bond reaction (cathode reaction) of hydrogen atoms, hydrogen ions, and electrons in the oxide layer, particularly on Fe ₃ O ₄ present on the steel surface. As a result, it has been found that the amount of hydrogen intrusion into the steel can be reduced.

As described above, since the bonding reaction between hydrogen ions, hydrogen atoms, and electrons is a rate-determining process on Fe ₃ O ₄ in the oxide layer, hydrogen atoms stay on Fe ₃ O ₄ . In addition, the hydrogen diffusion coefficient of oxide is significantly lower than that of steel. Therefore, when the thickness of the oxide layer and the volume ratio of Fe ₃ O _{4 in} the oxide layer are not above a certain level, not only the steel material surface that becomes a hydrogen generation site cannot be isolated from the corrosive environment, but also Fe that exists on the steel material surface. _The hydrogen atoms staying on ₃ O ₄ intrude into the steel and increase the amount of hydrogen intrusion.

Therefore, in order to obtain the effect of reducing the amount of hydrogen in steel using Fe ₃ O ₄ present in the above oxide layer, most of the surface of the base steel material was covered with an oxide layer having a certain thickness or more. It is necessary to be in a state. Therefore, in this invention, the average thickness of an oxide layer shall be 1.0 micrometer or more. The thickness is preferably 2 μm or more, more preferably 5 μm or more. However, if the oxide layer is too thick, it is easy to peel off, so the average thickness of the oxide layer is 100 μm or less. The thickness is preferably 90 μm or less, more preferably 80 μm or less.

In order to obtain the effect of reducing the amount of hydrogen in steel using Fe ₃ O ₄ , the oxide layer needs to contain Fe ₃ O ₄ in a volume ratio of 70% or more. The volume ratio of the Fe ₃ O ₄ is preferably 80% or more, more preferably 90% or more. As described above, when the oxide layer contains Fe ₃ O ₄ at a high ratio, substantially the surface of the base steel material is substantially covered with the dense Fe ₃ O ₄ layer. As a result, hydrogen gasification by the bonding reaction of hydrogen atoms, hydrogen ions and electrons can be promoted on the steel material surface, and as a result, the amount of hydrogen intrusion into the steel can be reduced. The oxide layer may also contain FeO (wustite), Fe ₂ O ₃ (hematite), etc. as impurities.

  Furthermore, if there is a portion where the oxide layer is not formed on the surface of the steel material, the corrosion reaction is concentrated in the portion, which may cause an increase in the amount of hydrogen penetration. Therefore, 90% (area)% or more of the surface of the base steel material needs to be covered with the oxide layer. Preferably, it is 95% or more of the surface of the base steel material, and most preferably 100%.

The thickness of the oxide layer can be measured by cutting a steel material and using an optical microscope or SEM (scanning electron microscope) from a cross section. The volume ratio of the Fe ₃ O _{4 in} the oxide layer can be measured using a technique such as X-ray diffraction.

  When the steel material is a steel plate, the surface of the base steel material includes not only the rolled surface / back surface but also the side surface (plate thickness surface).

Next, the reason for defining the component composition in the present invention will be described below. In the present invention, in particular, the amount of Al involved in the formation of oxides such as clustered Al ₂ O _3, which is responsible for the formation of coarse MnS causing hydrogen-induced cracking, and also the cause of hydrogen-induced cracking. By controlling the amount of O and O, excellent HIC resistance can be ensured even in an environment with a high hydrogen sulfide concentration.

[Ingredient composition]
[C: 0.01-0.20%]
C is an element necessary for improving the strength of steel. In the present invention, the C content is 0.01% or more. Preferably it is 0.03% or more. On the other hand, if the amount of C is excessive, the formation of carbides is promoted and the HIC resistance is lowered or the weldability is deteriorated, so the upper limit of the amount of C is 0.20%. Preferably it is 0.15% or less.

[Si: 0.10 to 0.50%]
Si is an element necessary for deoxidation of steel and further acts as a strength improving element. In order to obtain these effects, the Si content is set to 0.10% or more in the present invention. Preferably it is 0.15% or more. On the other hand, if the Si amount is excessive, the toughness of the weld heat affected zone (HAZ) is lowered, so the upper limit of the Si amount is 0.50%. Preferably it is 0.40% or less.

[Mn: 0.70 to 2.50%]
Mn is an element that improves strength and toughness. In order to exhibit this effect, the amount of Mn is 0.70% or more, preferably 0.8% or more. On the other hand, if the amount of Mn is excessive, the HAZ toughness decreases or the amount of MnS that combines with S in steel and decreases the HIC resistance increases, so the upper limit of the amount of Mn is set to 2.50%. Preferably it is 2.0% or less.

[P: 0.03% or less (excluding 0%)]
P (phosphorus) is an unavoidable impurity and is an element that causes a decrease in HIC resistance and HAZ toughness, so it is suppressed to 0.03% or less. Preferably it is 0.020% or less. Note that the lower the amount of P, the better. The lower limit of the amount of P is not particularly defined, but the lower limit is about 0.001% from the viewpoint of manufacturing cost.

[S: 0.003% or less (excluding 0%)]
S (sulfur) is an element that forms MnS that is stretched in the rolling direction during hot rolling by combining with Mn, and this MnS causes HIC and reduces HIC resistance. Therefore, the amount of S should be reduced as much as possible, and is suppressed to 0.003% or less. Preferably it is 0.0020% or less, More preferably, it is 0.0010% or less. The lower limit is not particularly defined because the smaller S is, but it is difficult to make it less than 0.0001% from the viewpoint of manufacturing cost. Therefore, the lower limit of the amount of S is about 0.0001%.

[Al: 0.005 to 0.10%]
Al is a deoxidizing element, and in order to reduce the amount of oxygen in the molten steel, the amount of Al is made 0.005% or more. Preferably it is 0.01% or more. However, when Al is contained in a large amount, it combines with oxygen in the steel to form cluster-like Al ₂ O ₃ that becomes the starting point of HIC. Therefore, the upper limit is made 0.10%. Preferably it is 0.05% or less.

[O: 0.01% or less (excluding 0%)]
If O (oxygen) is present in a large amount in steel, a large amount of oxides such as Al ₂ O ₃ that will be the starting point of HIC will be formed, so the upper limit is made 0.01%. Preferably it is 0.005% or less, More preferably, it is 0.0020% or less. Although the lower limit is not particularly defined, it is about 0.0005% in actual operation.

  The component composition of this invention steel material is as above-mentioned, and what consists of iron and an unavoidable impurity for remainder is mentioned. As an unavoidable impurity, N (nitrogen) or H (hydrogen) may be contained in addition to the above-described P, S, and O to the extent that they do not impair the properties of the steel material. The amount of H is preferably smaller from the viewpoint of suppressing cracking of the steel ingot during ingot formation, and the upper limit is preferably 0.0005%. The lower limit is not particularly defined, but is about 0.0001% in actual operation. On the other hand, when N is excessive, it becomes a starting point of cracks and causes the formation of cluster-like nitrides that lower the HIC resistance. Therefore, the upper limit is preferably 0.0050%.

  The steel material of the present invention may further contain other elements shown below as long as the effects of the present invention are not adversely affected to further improve the characteristics.

[Ca: 0.0001 to 0.01%]
Ca is an element that contributes to improving the HIC resistance by binding to S instead of Mn and suppressing the formation of MnS extending in the rolling direction. Further, Ca oxide present steel material surface is dissolved in water, OH ^- acidity of the steel surface is relaxed by generating also a element contributing to a reduction in the amount of hydrogen generation. In order to obtain these effects, it is preferable to contain 0.0001% or more of Ca, and more preferably 0.0010% or more. On the other hand, when the amount of Ca is excessive, it combines with oxygen in the steel to form an oxide, and the HIC resistance decreases due to the accumulation of the oxide. Therefore, the upper limit of the Ca content is preferably 0.01%. More preferably, it is 0.005% or less.

[One or more elements selected from the group consisting of Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, and Cr: 0.05 to 1.0%]
Cu, Ni and Cr are elements that contribute to further improvement in strength. Of these, Cu is also effective in improving toughness, and in an environment close to neutral pH of about 5, it reacts with hydrogen sulfide to form a stable CuS film, thereby reducing the amount of hydrogen intrusion into the steel. It is also an element that contributes to In order to obtain these effects, the Cu content is preferably 0.05% or more. More preferably, it is 0.10% or more. On the other hand, when the amount of Cu is excessive, the hot workability is lowered, so the upper limit of the amount of Cu is preferably 1.0%. More preferably, it is 0.8% or less.

  Ni is an element effective for improving toughness as well as strength. In order to obtain these effects, the Ni content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the amount of Ni is excessive, the cost increases and the economic efficiency decreases, so the upper limit of the amount of Ni is preferably 1.0%. More preferably, it is 0.8% or less.

  Cr is an effective element for obtaining sufficient strength even at low C. In order to obtain this effect, the Cr content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the amount of Cr is excessive, the weldability is lowered, so the upper limit is preferably made 1.0%, more preferably 0.8% or less.

[One or more elements selected from the group consisting of Mo: 0.05 to 0.5%, Nb: 0.001 to 0.1%, and Ti: 0.001 to 0.1%]
Mo, Nb, and Ti are elements that contribute to further improvement in strength. Among these elements, Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. It also contributes to improved toughness. In order to acquire these effects, it is preferable to contain 0.05% or more of Mo, and more preferably 0.10% or more. On the other hand, if the Mo content is excessive, the steel strength is excessively improved, the HIC resistance is lowered, and the cost is increased, so the upper limit is preferably made 0.5%. More preferably, it is 0.3% or less.

  Nb is an element effective for improving the strength by forming carbonitride. In order to obtain this effect, the Nb content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the Nb content is excessive, weldability deteriorates, so the upper limit is preferably made 0.1%, more preferably 0.08% or less.

  Ti is an element that combines with N in steel to form a nitride and contribute to strength improvement by refining crystal grains. In order to obtain this effect, the Ti content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the Ti content is excessive, coarse TiN is generated and the HIC resistance is lowered, so the upper limit is preferably made 0.1%, more preferably 0.05% or less.

[One or more elements selected from the group consisting of Zr, V, Co, REM (rare earth element), Y, Mg and B: 0.0001 to 0.01%, respectively]
In the present invention, in order to further improve the characteristics, one or more elements selected from the group consisting of Zr, V, Co, REM, Y, Mg, and B may be contained.

  Of these, Zr is an element that contributes to the improvement of strength by forming nitrides. To obtain this effect, the amount of Zr is preferably 0.0001% or more. On the other hand, if the amount of Zr is excessive, the toughness is reduced or coarse nitrides are generated and the HIC resistance is lowered, so the upper limit is preferably made 0.01%.

  V is an element that contributes to the improvement of strength by forming carbides and nitrides. In order to obtain this effect, the V amount is preferably 0.0001% or more. On the other hand, if the amount of V is excessive, the toughness decreases, so the upper limit is preferably made 0.01%.

  Co is an element that improves the corrosion resistance of the steel material, and in order to obtain the effect, the Co content is preferably 0.0001% or more. On the other hand, if the amount of Co is excessive, it is disadvantageous in terms of cost, so the upper limit is preferably made 0.01%.

  REM is an element added as a deoxidizing agent and a desulfurizing agent, and in order to obtain the effect, the REM amount is preferably 0.0001% or more. On the other hand, if the amount of REM is excessive, REM and oxygen in the steel are combined to produce a coarse oxide, resulting in a decrease in HIC resistance and a decrease in base metal toughness and HAZ toughness. Therefore, the upper limit is preferably 0.01%.

  In the present invention, the REM means a lanthanoid element (15 elements from La to Lu) and Sc (scandium).

  Y is an element added as a deoxidizing agent and a desulfurizing agent, and, like Ca and the like, combines with S instead of Mn to form YS, and suppresses the formation of MnS extending in the rolling direction. It is also an element that contributes to improvement in HIC resistance. In order to obtain this effect, the Y content is preferably 0.0001% or more. On the other hand, if the amount of Y is excessive, Y and oxygen in the steel are combined to produce a coarse oxide, resulting in a decrease in HIC resistance and a decrease in base metal toughness and HAZ toughness. Therefore, the upper limit is preferably 0.01%.

Mg is an element added as a deoxidizing agent and a desulfurizing agent, and is also an element that contributes to improving HAZ toughness by forming a fine oxide. Further Mg may, Mg oxides present steel material surface is dissolved in water, OH ^- acidity of the steel surface is reduced by generating is also an element it is possible to reduce the amount of hydrogen generation. In order to obtain these effects, the Mg content is preferably 0.0001% or more. On the other hand, when the amount of Mg is excessive, aggregated and coarsened oxides are likely to be generated, resulting in deterioration of HIC resistance and reduction of base metal toughness and HAZ toughness. Therefore, it is preferable that the upper limit of the Mg amount is 0.01%.

  B is an element effective for improving the strength and strengthening the grain boundary. To obtain these effects, the B content is preferably 0.0001% or more. On the other hand, if the amount of B is excessive, the toughness deteriorates, so the upper limit is preferably made 0.01%.

[Organization / Characteristics]
In the present invention, the structure is not particularly limited. For example, the microstructure of the steel material may be a structure mainly composed of ferrite and mixed with pearlite and the like, or may be composed mainly of bainite and mixed with ferrite and the like. However, it is better not to contain hard martensite with high cracking sensitivity as much as possible.

As precipitates present in steel materials, sulfides (eg MnS etc.), oxides (eg Al ₂ O ₃ etc.), carbides (eg NbC etc.), nitrides (eg TiN etc.) etc. generally contained Is mentioned. When fine carbides (for example, submicron order) are dispersed in the steel, the decrease in HIC resistance is small, but coarse MnS, agglomerated clustered Al ₂ O ₃ , NbC, TiN, etc. are not cracked. Since it becomes a starting point and reduces the HIC resistance, it is better to reduce as much as possible. Therefore, in this invention, it is good to make C amount in steel into a regulation range, and to reduce S amount, O amount, and N amount in steel as much as possible as mentioned above.

  The steel material of the present invention may be a strength class having a tensile strength of about 460 to 850 MPa and a yield strength of about 360 to 700 MPa (API standard X52 to X80) or higher.

[Production method]
The steel material of this invention can be manufactured as follows, for example. Using steel adjusted to the specified component composition, a slab produced by continuous casting is heated and held at, for example, about 1000 to 1200 ° C., and then hot-rolled at a temperature of about 750 to 950 ° C. (finish rolling temperature), Then, it cools to the temperature range of 300-650 degreeC with arbitrary cooling rates. After hot rolling, it may be cooled (air cooled) to a temperature range of 300 to 650 ° C. at an average cooling rate of 10 ° C./second or more as described later. Moreover, after cooling to the temperature range of 300-650 degreeC, you may hold | maintain for a fixed time in the temperature range of 300-650 degreeC other than continuing cooling to room temperature as below-mentioned.

In order to increase the volume ratio of Fe ₃ O ₄ in the oxide layer formed on the steel material surface, it is most important to actively supply oxygen in the atmosphere to the steel material surface.

Examples of the method include the following methods.
(A) During hot rolling, descaling (for example, high pressure water descaling, mechanical descaling, blasting, etc.) is performed at least once (for example, descaling at a rate of 1 pass or more for 5 passes of hot rolling) A method of facilitating the arrival of oxygen to the steel surface by thinning the scale (FeO) formed during hot rolling (b) Scale (FeO) formed during hot rolling after hot rolling In addition, the following (b-1) to (b-3) may be mentioned as specific methods of (b) above by introducing oxygen to the steel material surface by introducing cracks.
(B-1) Method of adding a light reduction pass after hot rolling (after the final pass of hot rolling) (b-2) Descaling at least 1 after hot rolling (after the final pass of hot rolling) (B-3) After the hot rolling (after the final pass of hot rolling) (preferably within 5 seconds after the final pass of hot rolling), an average cooling rate of 10 ° C./second or more is used. Method of introducing cracks into scale layer from difference in linear expansion coefficient between scale layer and steel material by cooling to 650 ° C. These methods may be appropriately selected or applied in combination.

In addition, after hot rolling, after cooling to a temperature range of 300 to 650 ° C. at an arbitrary cooling rate, the temperature range (retention temperature, 300 to 650 ° C.) is 1 minute under an oxidizing atmosphere (for example, air atmosphere). It is also possible to hold the above. By keeping the holding temperature lower than about 570 ° C. (more preferably 500 ° C. or less), FeO in the scale layer formed on the steel material surface is changed to Fe ₃ O ₄ (4FeO = Fe ₃ O ₄ + Fe ) Is preferable. Further, by this method, oxygen in the cooling atmosphere can be sufficiently diffused on the surface of the steel material, so that the oxide layer defined in the present invention can be formed on the surface of the steel material. In this case, the longer the holding time at the holding temperature, the thicker the oxide layer is. However, if the holding time is too long, the productivity is lowered.

[Usage]
By applying this technology, steel materials, particularly thick steel plates, in which HIC resistance becomes a problem, for example, used in oil refineries, such as steel plates for pressure vessels (JIS G3103: for boilers and pressure vessels) Carbon steel and molybdenum steel plate, JIS G3115: Steel plate for pressure vessel, JIS G3119: Manganese molybdenum and manganese molybdenum nickel steel plate for boiler and pressure vessel, JIS G4109: Chrome molybdenum steel plate for boiler and pressure vessel, JIS G4110: High temperature pressure The HIC resistance of the container can be improved.

In addition, in the present invention, in addition to forming a specified oxide layer on the surface of the steel material with a certain area or more, the steel component (particularly, the amount of S involved in the formation of coarse MnS causing cracking, clustering, The amount of Al and O involved in the oxidized oxides such as Al ₂ O ₃ is also controlled. In particular, strict HIC resistance is required to transport crude oil and natural gas with higher hydrogen sulfide concentration. It can be applied to steel plates for line pipes and steel pipes for line pipes.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

(Production of test materials)
Steel having the chemical components shown in Table 1 (using Ce and La as REM) is melted and heated to 1100 to 1200 ° C as a slab, followed by hot rolling at 750 to 900 ° C (finish rolling temperature), and Experiment No. 4 in Table 4 In 1, 4 and 6-31, descaling (during hot rolling) was performed at least once to obtain a plate thickness of 40 mm. After hot rolling, it is subsequently cooled to a temperature range of 300 to 650 ° C. at a cooling rate of 3 ° C./second (s), and the atmosphere in this temperature range (300 to 650 ° C.) is changed to 10 to 300 as an oxidizing atmosphere (air atmosphere). The sample material was produced by holding for 5 minutes. In addition, the experiment No. In 2, 4, and 6-31, descaling was performed at least once after the final pass of hot rolling. In Table 3, the experiment No. In 2-0 to 2-3, after hot rolling, after cooling to 650 ° C. at a cooling rate of 3 ° C./sec, the holding temperature was changed between 650 and 500 ° C. as shown in Table 3. Experiment No. 4 in Table 4 In No. 3, after the hot rolling was completed, the temperature range of 300 to 650 ° C. was cooled with water (rapid cooling), that is, cooled at an average cooling rate of 10 ° C./second or more. Experiment No. 4 in Table 4 In No. 6, after the hot rolling, after cooling to 680 ° C. at a cooling rate of 3 ° C./second, the atmosphere was maintained at this temperature as an oxidizing atmosphere (air atmosphere) for 600 minutes.

As an experiment for verifying the prior art, an experiment No. in Table 2 described later is performed. In 0-1 and 0-2, for each of steel symbol A and steel symbol i, iron hydroxide is generated by applying water to the surface of the steel material and then drying, and then reducing with water vapor at 150 ° C. Then, an oxide layer containing Fe ₃ O ₄ was formed. Experiment No. 2 in Table 2 described later. In No. 0-3, with respect to the steel symbol A, application of water to the steel surface and drying were repeated twice to produce iron hydroxide. In 0-4, with respect to the steel symbol A, the application of water to the steel surface and drying were repeated five times to produce iron hydroxide, both of which were subsequently reduced with steam at 150 ° C. to reduce Fe ₃ O _4. An oxide layer containing was formed.

  Tensile test pieces were collected from the obtained steel plates and subjected to a tensile test to determine yield strength (YS) and tensile strength (TS). Separately, a test piece (macro test piece) for observing and identifying the oxide layer and an HIC test piece were collected from the obtained steel sheet and subjected to the following measurement and evaluation.

[Measurement of average thickness and coverage of oxide layer and measurement of Fe ₃ O ₄ content in oxide layer]
The average thickness of the oxide layer is obtained by observing the cross section of the test material with five fields of view by SEM (magnification: 500 times), measuring the thickness of five points in each field of view, and calculating the average value of the total thickness of 25 points. Calculated and determined. Moreover, the coverage of the oxide layer with the average thickness specified in the present invention (the area ratio of the oxide layer covering the surface of the base steel material) was evaluated from the area ratio of the Cu non-adhered portion by the Cu plating method. In Table 2, for the examples where the coverage of the oxide layer does not meet the requirements (Experiment Nos. 0-1 to 0-3), the volume ratio of Fe ₃ O ₄ in the oxide layer was measured. There wasn't. The above experiment No. In 0-1 to 0-3, it was confirmed that Fe ₃ O ₄ was present in the oxide layer.

Further, the volume ratio of Fe ₃ O ₄ in the oxide layer having the average thickness specified in the present invention was confirmed by X-ray diffraction.

[Evaluation of HIC resistance]
For the evaluation of the HIC resistance of the steel material, a method standardized by NACE (National Association of Corrosion and Engineer) TM0177 was applied. That is, an HIC test piece (30 mm × 20 mm × 100 mm) was immersed for 96 hours in a solution in which hydrogen sulfide gas was saturated (about pH 3) in a 5% -NaCl solution + 0.5% -acetic acid solution.

  And after completion | finish of a test, the measurement of the crack area ratio (CSR), the number of blisters, and the amount of hydrogen in steel were performed.

  In addition, the experiment No. In 0-1 to 0-3, as a verification of the prior art, the environment is milder than the method specified in NACE TM0177; the method specified in NACE TM0284 (hydrogen sulfide gas is saturated in artificial seawater The test piece is immersed in the solution (about pH 5) and a test is conducted to determine whether cracks are generated after 96 hours). After the test is completed, the crack area ratio (CSR) is measured, the number of blisters, And the amount of hydrogen in steel was measured.

  The crack area ratio (CSR) was determined by determining the area of the cracked portion by ultrasonic flaw detection (C scan) and determining the area ratio of the cracked portion relative to the entire test piece. And the case where a crack area ratio was 2.0% or less was evaluated as (circle).

The number of swelling is 10 cm ² at an arbitrary position of the test piece, observed with a magnifying glass, and the number of blisters with a circle equivalent diameter of 0.1 mm or more existing in 10 cm ² is 20 or less. evaluated. The amount of hydrogen in the steel was measured by an inert gas melting method, and the case where the amount of hydrogen in the steel was 2.0 ppm or less was evaluated as ◯. These results are shown in Tables 2-4. In Tables 2 to 4, the average thickness and coverage of the oxide layer, the requirement for the amount of Fe ₃ O ₄ in the oxide layer, and the numerical values for which the above test results are not ○ are underlined.

  As a comprehensive evaluation, in Table 2, “◎” indicates that all six items in the NACE TM0284 and three measurement items in the NACE TM0177 are “◯”, and “○” indicates that the five items are ○. In other cases, “x” was assigned. In Tables 3 and 4, “◎” indicates that all three items of CSR, the number of blisters, and the amount of hydrogen (in steel) are ○, “○” indicates that the two items are ○, and “o” indicates the other cases. It was set as “x”. When the overall evaluation was “◎” or “◯”, it was evaluated that the HIC resistance was excellent.

First, the results in Table 2 will be considered. Experiment No. 2 in Table 2 From the results of 0-1 to 0-3, when the conventional technique was used, formation of Fe ₃ O ₄ was confirmed in the oxide layer, but the thickness of the oxide layer was insufficient, A portion where the oxide layer was not formed was observed. The steel plate was exposed in the place where the oxide layer was not formed on the steel surface. In addition, Experiment No. In 0-4, formation of Fe ₃ O ₄ was confirmed in the oxide layer, but the amount of Fe ₃ O _{4 in} the oxide layer was insufficient. Therefore, the HIC resistance of these steel materials is cracked and swollen in a mild environment (NACE TM0284) and the amount of intrusion hydrogen is suppressed, but in a severe environment (NACE TM0177), the crack area ratio (CSR) is high and swollen. The number and the amount of hydrogen were also large. From this, it can be understood that the oxide layer is not sufficiently formed by the conventional method, and it is difficult to ensure the HIC resistance in a severe environment.

  In addition, in Experiment No. 2 of Table 2, 0-1 and 0-3 (using steel symbol A) and Experiment No. From the contrast of 0-2 (using steel symbol A), by controlling the steel components as specified in the present invention, a certain degree of improvement in HIC resistance is recognized even in harsh environments (NACE TM0177). I understand.

Experiment No. 1 in Table 3 2-0 to 2-3 and No. 2-4. 2 (this experiment No. 2 is the same as experiment No. 2 in Table 4), the lower the holding temperature of the oxidizing atmosphere (atmospheric atmosphere) after the hot rolling is completed, It can be seen that the Fe ₃ O ₄ fraction is increased and excellent HIC resistance can be secured.

Next, the results of Table 4 will be considered. From Table 4, the experiment No. 1 satisfying the chemical component composition, the average thickness and coverage of the oxide layer, and the amount of Fe ₃ O ₄ in the oxide layer, which are the requirements of the present invention. 1 to 4 and 7 to 26 are all excellent in HIC resistance.

  In contrast, Experiment No. 5, 6 and 27 to 31 have the following problems.

That is, Experiment No. No. 5 did not employ a process of increasing the oxygen supply to the steel surface, so that Fe ₂ O ₃ and FeO were formed instead of Fe ₃ O ₄ , and a desired amount of Fe ₃ O ₄ could not be secured. Hydrogen generation could not be suppressed, resulting in poor HIC resistance. Experiment No. No. 6 was inadequate (high temperature and long time) in the holding conditions in an oxidizing atmosphere (atmosphere), so a thick oxide layer having low adhesion to the base steel material was formed. As a result, hydrogen generation could not be suppressed, resulting in poor HIC resistance. Experiment No. In No. 27, the amount of C was excessive, and carbides were excessively generated in the steel, so that the HIC resistance was inferior. Experiment No. In No. 28, the amount of Mn was excessive, and coarse MnS was produced in the steel, so that the HIC resistance was inferior. Experiment No. In No. 29, S was excessive and coarse MnS was formed in the steel, so that the HIC resistance was inferior. Experiment No. No. 30 had an excessive amount of Al and produced Al ₂ O ₃ clustered in the steel, resulting in poor HIC resistance. Experiment No. No. 31 had an excessive amount of O, and a large number of oxides (Al ₂ O ₃ and CaO) were produced in the steel, so that the HIC resistance was inferior.

Claims (10)

The base steel is
C: 0.01 to 0.20% (meaning mass%, the same applies to chemical components below),
Si: 0.10 to 0.50%,
Mn: 0.70 to 2.50%,
P: 0.03% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.005-0.10%, and
O: 0.01% or less (excluding 0%)
And satisfy
90% or more of the surface of the base steel material has an average thickness of 1.0 μm or more and 100 μm or less, and is covered with an oxide layer containing Fe ₃ O ₄ in a volume ratio of 70% or more . A manufacturing method comprising:
During hot rolling, descaling is performed at least once, and after hot rolling, after cooling to a temperature range of 300 to 650 ° C., holding in the temperature range for 1 to 300 minutes in an oxidizing atmosphere. A method for producing a steel material. The base steel is
C: 0.01-0.20%
Si: 0.10 to 0.50%,
Mn: 0.70 to 2.50%,
P: 0.03% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.005-0.10%, and
O: 0.01% or less (excluding 0%)
And satisfy
90% or more of the surface of the base steel material has an average thickness of 1.0 μm or more and 100 μm or less, and is covered with an oxide layer containing Fe ₃ O ₄ in a volume ratio of 70% or more . A manufacturing method comprising:
After hot rolling, cracks are introduced into the scale layer formed on the surface of the steel material, and further cooled to a temperature range of 300 to 650 ° C., and then kept in the temperature range for 1 to 300 minutes in an oxidizing atmosphere. A method for producing a steel material, characterized in that: The said base steel material is a manufacturing method of the steel materials of Claim 1 or 2 which contains Ca: 0.0001-0.01% further. The base steel material is further one or more selected from the group consisting of Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, and Cr: 0.05 to 1.0%. The manufacturing method of the steel materials in any one of Claims 1-3 containing these elements. The base steel material is further one or more selected from the group consisting of Mo: 0.05 to 0.5%, Nb: 0.001 to 0.1%, and Ti: 0.001 to 0.1%. The manufacturing method of the steel materials in any one of Claims 1-4 containing these elements. The base steel material further contains 0.0001 to 0.01% of one or more elements selected from the group consisting of Zr, V, Co, REM, Y, Mg, and B, respectively. The manufacturing method of the steel materials in any one of. The said steel material is a steel plate for line pipes, The manufacturing method of the steel materials in any one of Claims 1-6. The said steel material is a steel pipe for line pipes, The manufacturing method of the steel materials in any one of Claims 1-6. The manufacturing method according to claim 2 , wherein cracks are introduced into the scale layer by performing descaling at least once after hot rolling. The manufacturing method according to any one of claims 2 to 8 , wherein cracks are introduced into the scale layer by cooling at an average cooling rate of 10 ° C / second or more after hot rolling.