JP6492793B2 - Steel material, steel structure for embedding in soil, and method for manufacturing steel material - Google Patents

Description

  The present invention relates to a steel material excellent in corrosion resistance used in a highly corrosive soil environment, a steel structure for embedding in the soil using the steel material, and a method for producing the steel material.

  In recent years, in Southeast Asia and subtropical regions, demand for steel materials for civil engineering and construction has been strong along with infrastructure development. As an example of steel for civil engineering and construction, steel sheet piles and steel pipe piles that are used by partially burying them in the soil as the foundation of civil engineering and construction can be mentioned. The soil distributed in these areas is clayey soil containing acidic sulfates that are considered to be highly corrosive to steel.

  On the other hand, the average corrosion rate of steel in general soil in Japan is as small as 0.01 mm / y (the unit of average corrosion rate “mm / y” is the corrosion depth per 365 days, that is, 8760 h (mm)). The same shall apply hereinafter.) Therefore, in Japan, as a steel material for civil engineering and construction used in soil, a rolled steel material for general structure (SS material) defined by JIS G 3101 and a rolled steel material for welded structure (SM material) defined by JIS G 3106 Common steel materials such as are used.

  However, when steel materials similar to those used in civil engineering and construction used in Japan are used in soils in Southeast Asia and subtropical regions, corrosion of steel materials proceeds rapidly, and the safety of structures using these steel materials and Reliability may be reduced. Therefore, anti-corrosion measures are necessary for steel materials used in highly corrosive soil.

  For example, Patent Documents 1 and 2 are disclosed as anticorrosion measures for steel materials used in a highly corrosive soil environment.

  In Patent Document 1, the surface of a steel material is coated with a water-soluble butyral resin, and the resin contains sodium vanadate, sodium metavanadate, sodium molybdate, ammonium molybdate, sodium tungstate, sodium niobate and potassium tantalate. There is provided a steel material for embedding in soil that is provided with a resin layer containing at least one of the above, and has at least one of V, Mo, W, Nb, and Ta in the base material and is excellent in corrosion resistance in an embedding environment. It is disclosed.

  Patent Document 2 shows an underground steel structure containing Cr: 0.5 to 2.0%, and when emphasizing strength and weldability, further contains Mo, Cu, and Ni. Is disclosed. It is said that the corrosion resistance of 0.02 mm / y can be obtained by backfilling this structure with soil having a pH of 9 or more.

JP 2010-144210 A JP 2001-011569 A

Kazuyuki Kashima and 4 others, “Effect of Cr addition on the weather resistance of steel in chloride-induced atmospheric corrosion environment”, Journal of the Japan Institute of Metals, Japan Institute of Metals, March 1, 2013, Vol. 77, No. 3, p. 107-113

  Although the steel material disclosed in Patent Document 1 has a resin layer or contains V, Mo, or the like, the corrosion resistance is inferior in the welded portion formed when welding is performed. In addition, the formation of the resin layer is expensive.

  Further, according to Non-Patent Document 1, corrosion of steel containing Cr like the steel material disclosed in Patent Document 2 may be promoted in an acidic soil environment.

  The present invention has been made in order to solve these problems, and does not require a resin layer, has excellent corrosion resistance in welds, and has excellent corrosion resistance in an acidic soil environment, and a structure for embedding in the soil. It aims at providing the steel material suitable as a raw material, and the manufacturing method of the steel material.

  The present inventors have intensively studied to solve the above problems. As a result, the following findings (a) and (b) were obtained.

(A) The scale formed on the surface of the steel material containing Cr, Si and Sn increases the adhesion with the steel material by adjusting the composition ratio of the oxide constituting the scale to an appropriate range.

  The reason is considered as follows.

The scale formed on the surface of the steel material is composed of magnetite, firelite, wustite, and hematite. Magnetite, firelite and wustite improve the adhesion between steel and scale. On the other hand, hematite is likely to occur on the surface layer of the scale and has little influence on the adhesion to the steel material. When the value Fn1 of the composition ratio of the oxide represented by the following formula (i) is a specific condition (0.15 <Fn1 <0.35), the adhesion of the scale formed on the steel material surface is enhanced. In this case, the scale has a structure in which magnetite, firelite, wustite, and hematite are laminated or partially mixed from the steel material side.
Fn1 = (Mm + Mf) / (Mw + Mh) (i)
However, the meaning of each symbol in the above formula (i) is as follows.
Mm: Magnetite content (% by mass) in the scale
Mf: Firelight content (% by mass) in the scale
Mw: Wustite content (% by mass) in the scale
Mh: hematite content (% by mass) in the scale

(B) The corrosion resistance of the steel material in the soil can be maintained by adjusting the contents of Cr, Sn and Si in the scale to an appropriate range.

  The reason is considered as follows.

  For example, when welding is performed when processing a steel material into a steel pipe, the scale formed on the surface of the steel material is removed at the welded portion, and the base material of the steel material is exposed. In addition, although the scale described above is excellent in adhesion, when a steel sheet pile and steel pipe pile using a steel material with this scale formed on the surface as a material are driven into the soil, the soil and the steel sheet pile and steel pipe pile A shearing force acts between the surface and a part of the scale may peel off to expose the base material. When the steel base material is exposed from the scale formed on the steel surface, a potential difference is generated between the scale and the exposed base material in the soil, so that local corrosion of the steel material is likely to occur.

However, by heating and processing a steel material containing appropriate amounts of Cr and Sn under specific conditions, a scale enriched with Cr and Sn is formed on the surface of the steel material. When the content of Cr, Sn, and Si in the scale formed on the steel material surface satisfies 9.0 <Fn2 <23.0 for Fn2 represented by the following formula (ii), The potential difference generated during the period becomes smaller. Therefore, even if the base material of the steel material is exposed from the scale formed on the surface of the steel material, the corrosion of the steel material is suppressed and the corrosion resistance of the steel material in the soil can be maintained. In addition, this scale has high adhesion because Cr and Sn are concentrated.
Fn2 = ([Cr] + [Sn]) / [Si] (ii)
However, the meaning of each symbol in the above formula (ii) is as follows.
[Cr]: Cr content (% by mass) in the scale
[Sn]: Sn content (% by mass) in the scale
[Si]: Si content (% by mass) in the scale

  The present invention has been made on the basis of the above knowledge, and includes a steel material shown in the following (1), a steel structure for buried in the earth shown in the following (2), and a method for manufacturing the steel material shown in the following (3). The gist.

(1) A steel material with a scale formed on its surface,
The chemical composition of the steel material is mass%,
C: 0.01-0.20%
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.5%
P: 0.01% or less,
S: 0.01% or less,
Cr: 3.0-5.0%,
Sn: 0.01-0.30%,
Al: 0.005 to 0.05%,
N: 0.0005 to 0.01%,
Mo: 0 to 0.5%,
W: 0 to 0.5%
Ti: 0 to 0.5%,
Balance: Fe and impurities,
The scale is a structure in which magnetite, firelite, wustite and hematite are laminated or partially mixed,
The scale has a thickness of 5 to 50 μm,
Steel materials in which Fn1 and Fn2 represented by the following formulas (i) and (ii) are 0.15 <Fn1 <0.35 and 9.0 <Fn2 <23.0, respectively.
Fn1 = (Mm + Mf) / (Mw + Mh) (i)
Fn2 = ([Cr] + [Sn]) / [Si] (ii)
However, the meaning of each symbol in the above formulas (i) and (ii) is as follows.
Mm: Magnetite content (% by mass) in the scale
Mf: Firelight content (% by mass) in the scale
Mw: Wustite content (% by mass) in the scale
Mh: hematite content (% by mass) in the scale
[Cr]: Cr content (% by mass) in the scale
[Sn]: Sn content (% by mass) in the scale
[Si]: Si content (% by mass) in the scale

(2) A steel structure for embedding in soil using the steel material according to (1) above.

(3) A method for manufacturing a steel material, including the following steps (P1) to (P6).
(P1) A step of heating the slab having the chemical composition described in (1) above to 1100 to 1300 ° C,
(P2) removing the scale formed on the surface of the heated slab;
(P3) When the slab is hot-rolled at least at a rough rolling start temperature of 1000 ° C. and a finish rolling completion temperature of 950 to 800 ° C. by a rolling mill group including at least a rough rolling mill and a finish rolling mill, The scale formed on the surface of the steel material during rolling is removed by injecting water having a water pressure of 10 MPa or more and an injection flow rate of 20 to 300 L / min in the initial stage of rolling on the delivery side and the finishing mill. The process of
(P4) The process of starting cooling of the obtained hot-rolled steel material within 5 s after completion of finish rolling and cooling to a temperature T1 in the temperature range of 650 to 500 ° C. at an average cooling rate of 25 ° C./s or more in the water injection zone ,
(P5) a step of winding the cooled hot rolled steel material in a coil shape at a temperature T2 in a temperature range of 650 to 500 ° C;
(P6) The coiled hot-rolled steel material is subjected to an average cooling rate of 0.005 to 0.1 ° C / s from a temperature T3 in a temperature range of 650 to 500 ° C to a temperature T4 in a temperature range of 300 to 150 ° C. The process which cools over 10 hours or more.

  The steel material of the present invention has excellent corrosion resistance in a soil corrosive environment (particularly in an acidic soil environment) because the scale formed on the surface has adhesion. Even if the scale is peeled off and the steel base material is exposed, the potential difference generated between the scale and the base material is small, so that excellent corrosion resistance can be maintained. The steel material of the present invention can be suitably used for steel structures for underground use such as steel sheet piles and steel pipe piles. Moreover, according to the manufacturing method of the steel material of this invention, the steel material of this invention can be manufactured efficiently.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting the chemical composition of the steel material of the present invention are as follows. In the following description, “%” for the content of each element means “mass%”.

C: 0.01 to 0.20%
C is an element necessary for ensuring the strength of the steel material, and needs to be contained by 0.01% or more. However, when the C content exceeds 0.20%, the weldability is significantly reduced. Moreover, the amount of cementite produced increases with an increase in the C content. Cementite acts as a cathode in an environment where the pH is lowered and promotes corrosion of the steel material, so that the corrosion resistance of the steel material when used in soil containing acidic sulfates is reduced. For this reason, C content shall be 0.01 to 0.20%. The C content is preferably 0.05% or more. Further, the C content is preferably 0.18% or less, and more preferably 0.16% or less.

Si: 0.01 to 1.0%
Si is an element necessary for securing the strength of the steel material and forming the firelight. When the Si content is less than 0.01%, the strength of the steel material is lowered and firelight is hardly formed. On the other hand, when the Si content exceeds 1.0%, the firelite in the primary scale formed when the steel material is heated in the heating furnace and the secondary scale formed during hot rolling after heating ( The amount of Fe ₂ SiO ₄ ) becomes excessive. Although the primary scale and the secondary scale are mostly removed by the scale removal process (hereinafter sometimes referred to as “descaling”), the scale adhered to the steel surface remains without being removed. When the formation amount of is excessive, the value of Fn2 described later does not satisfy the definition of the present invention. Moreover, when Si content exceeds 1.0%, Si will concentrate on the interface of a scale and steel materials, and the adhesiveness of a scale will fall. For this reason, Si content is made into 0.01 to 1.0%. The Si content is preferably 0.05% or more, and preferably 0.35% or less.

Mn: 0.3 to 2.5%
Mn is an element necessary for increasing the strength of steel. If the Mn content is less than 0.3%, it is difficult to ensure sufficient strength. On the other hand, when the Mn content exceeds 2.5%, it becomes difficult to maintain the workability of the steel material. For this reason, the Mn content is set to 0.3 to 2.5%. The Mn content is preferably 0.4% or more, and more preferably 2.0% or less.

P: 0.01% or less P is an element that is effective for increasing the strength of steel and is useful for improving corrosion resistance. Therefore, conventionally, P has been utilized in corrosion resistant steel materials. However, in an environment where the pH is lowered, such as acidic soil, if P is contained alone in steel, the corrosion resistance is conversely reduced. In addition, P is an element that does not contain as much as possible because it causes embrittlement of the slab during production of the steel material and causes cracking. However, P improves the corrosion resistance of steel materials in an acidic soil environment by coexisting with Sn. This is considered to be due to the fact that, by containing Sn, elution of Fe is suppressed, so that P imparts protection to the rust layer. However, since the embrittlement becomes significant when the P content exceeds 0.01%, the P content is set to 0.01% or less. The P content is preferably 0.008% or less. On the other hand, when obtaining the effect of imparting protection to the rust layer, the P content is preferably 0.001% or more.

S: 0.01% or less S combines with Mn to form MnS which is a sulfide. MnS is easily deformed, and when a steel material is rolled, it exists in a stretched state in the steel material and degrades the bendability and workability of the steel material. In particular, in high-strength steel materials, S is preferably reduced as much as possible in order to increase crack sensitivity. For this reason, S content shall be 0.01% or less. The S content is preferably 0.005% or less.

Cr: 3.0-5.0%
Cr is an element that improves the corrosion resistance of steel in a neutral environment such as the atmospheric environment. However, as described above, according to Non-Patent Document 1, in an acidic soil environment containing acidic sulfate, Cr promotes the dissolution reaction of iron, so that corrosion may be promoted in steel materials containing Cr. . In a high-temperature environment, the scale formed on the surface of the steel material containing Cr is concentrated on the Cr and has high adhesion to the steel material. Therefore, after descaling, the scale remains on the surface of the steel material. . This scale is highly corrosion resistant in neutral environments. In addition, when the scale on the surface of the steel material is partially peeled and the base material is exposed, the potential difference generated between the scale and the exposed base material becomes smaller as the Cr content in the steel is higher. Therefore, the occurrence of local corrosion of the steel material can be suppressed by containing Cr. Below, the local corrosion of the steel material caused by the potential difference between the scale and the base material exposed by partial peeling of the scale is referred to as “local corrosion”, and the potential difference between the scale and the base material is expressed as “ It is called “corrosion potential difference”.

  If the Cr content is less than 3.0%, Cr does not concentrate on the scale remaining on the surface of the steel material after descaling, and the adhesion of the scale decreases. On the other hand, if the Cr content is less than 3.0%, local corrosion tends to proceed. On the other hand, when the Cr content exceeds 5.0%, a scale that is firmly adhered to the surface of the steel material and difficult to scale is formed during the heating of the steel material in the heating furnace and during the rolling after the heating. In order to suppress the formation of a scale that is difficult to descal, it is necessary to lower the feed rate of the steel material during rolling, which lowers the production efficiency. For this reason, Cr content shall be 3.0 to 5.0%. The Cr content is preferably 3.5% or more, and more preferably 4.5% or less.

Sn: 0.01-0.30%
Sn is an element that improves the corrosion resistance of steel in an atmospheric environment containing chloride and an acidic environment. If the Sn content is less than 0.01%, the occurrence of local corrosion in an acidic environment cannot be suppressed. On the other hand, if the Sn content exceeds 0.30%, a scale containing Sn is easily formed on the steel surface in a high temperature environment. Further, in this case, Sn contained in the scale is excessively concentrated at the interface between the scale and the base material surface of the steel material, and Fn1 described later becomes 0.35 or more, so that the adhesion between the scale and the base material surface is improved. Reduce. For this reason, Sn content shall be 0.01 to 0.30%. The Sn content is preferably 0.05% or more and preferably 0.20% or less.

Al: 0.005 to 0.05%
Al is an element that improves the corrosion resistance of steel. To obtain the effect, Al content needs to be 0.005% or more. On the other hand, when the Al content exceeds 0.05%, the above effect is saturated. For this reason, Al content shall be 0.005-0.05%. In addition, since it will become easy to embrittle steel materials when Al is contained in a large amount, the Al content is preferably 0.03% or less. The Al content is preferably 0.01% or more. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

N: 0.0005 to 0.01%
N contained in the steel material becomes ammonia and dissolves in moisture in contact with the steel material, and suppresses the pH decrease due to the hydrolysis of Fe ^{3+ in} an environment with a large amount of incoming salt. Therefore, N has an effect of improving the corrosion resistance of the steel material in a salt environment. In order to obtain this effect, the N content needs to be 0.0005% or more. On the other hand, when the N content exceeds 0.01%, not only the effect is saturated, but also the toughness of the steel material is deteriorated. For this reason, N content shall be 0.0005 to 0.01%.

Mo: 0 to 0.5%
W: 0 to 0.5%
Ti: 0 to 0.5%
Any of these elements may be contained as necessary in order to improve the corrosion resistance of the steel material. However, when the content of each element exceeds 0.5%, not only the effect is saturated but also the cost of the steel material increases. For this reason, the content of each element is set to 0.5% or less. On the other hand, in order to obtain the effect of improving the corrosion resistance, the content of each element is preferably set to 0.01% or more. In addition, when combining 2 or more types of these elements, it is preferable that the total content shall be 1.0% or less.

  The balance of the chemical composition of the steel material of the present invention consists of Fe and impurities. “Impurity” is a component that is mixed due to various factors in raw materials such as ore and scrap and manufacturing processes when industrially producing steel materials, and is allowed within a range that does not adversely affect the present invention. Means something.

(B) Scale The steel material of the present invention has a scale formed on the surface, and the scale has a structure in which magnetite, firelite, wustite, and hematite are laminated or partially mixed. As for the composition of each oxide in the scale, Fn1 represented by the following formula (i) satisfies 0.15 <Fn1 <0.35.
Fn1 = (Mm + Mf) / (Mw + Mh) (i)
However, the meaning of each symbol in the above formula (i) is as follows.
Mm: Magnetite content in the scale (% by mass)
Mf: Firelight content in the scale (% by mass)
Mw: Wustite content in the scale (% by mass)
Mh: hematite content in the scale (% by mass)

  Magnetite, firelite and wustite improve the adhesion between steel and scale. In addition, wustite decreases as the Cr content increases. On the other hand, hematite is likely to occur on the surface layer of the scale and has little influence on the adhesion between the steel material and the scale. When Fn1 satisfies 0.15 <Fn1 <0.35, the scale has a structure in which magnetite, firelite, wustite, and hematite are laminated in order from the steel material side, or a partially mixed structure, and this scale is high with the steel material. Adhesion.

  Examples of the case where Fn1 is 0.15 or less include a case where the content of hematite is large. In this case, since the adhesion between the scale and the steel material is reduced, scale peeling is likely to occur during the manufacturing process of the steel material, and the peeled scale causes a scale indentation flaw. On the other hand, as a case where Fn1 is 0.35 or more, for example, there is a case where the content of magnetite is large. In this case, the corrosion potential difference is increased and local corrosion is promoted. For this reason, Fn1 is defined as 0.15 <Fn1 <0.35. Fn1 is preferably 0.18 or more, and preferably 0.32 or less.

  The content of each oxide in the scale can be measured, for example, by the following procedure. First, the scale on the surface of the steel material is collected with a hammer and a cutter knife to a depth where the base material can be confirmed, and the collected scale is pulverized to obtain a powder sample. By applying a powder X-ray diffraction method (internal standard method) to this powder sample, the contents of magnetite, firelite, wustite and hematite can be measured.

In the steel material of the present invention, Fn2 represented by the following formula (ii) satisfies 9.0 <Fn2 <23.0 as the contents of Cr, Sn, and Si in the scale formed on the surface.
Fn2 = ([Cr] + [Sn]) / [Si] (ii)
However, the meaning of each symbol in the above formula (ii) is as follows.
[Cr]: Cr content (% by mass) in the scale
[Sn]: Sn content (% by mass) in the scale
[Si]: Si content (% by mass) in the scale

  As an example of a method for measuring the contents of Cr, Sn and Si in the scale, there is a method in which the scale on the surface of the steel material separated using a hammer or the like is chemically dissolved in aqua regia and analyzed by ICP.

  When Fn2 is 9.0 or less and 23.0 or more, the corrosion potential difference is large, for example, exceeding 40 mV, and local corrosion is promoted. On the other hand, when Fn2 satisfies 9.0 <Fn2 <23.0, the corrosion potential difference is small, and the corrosion resistance of the steel material is improved. In this case, since Cr and Sn are moderately concentrated in the scale, this scale has high adhesion. Therefore, Fn2 is defined as 9.0 <Fn2 <23.0. Fn2 is preferably 10.0 or more, and more preferably 20.0 or less.

  In the steel material of the present invention, the thickness of the scale is 5 to 50 μm. When the scale thickness is less than 5 μm, when the scale is physically peeled off by shot blasting or the like, the scale may be pushed into the surface of the steel material, and as a result, excessive unevenness is imparted to the surface of the steel material. It will be. On the other hand, when the thickness of the scale exceeds 50 μm, when the steel material is hot-rolled, partial scale peeling occurs, which may cause scale wrinkles generated in the rolling roll and the steel material. For this reason, the thickness of a scale shall be 5-50 micrometers.

(C) Manufacturing method Although there is no restriction | limiting in particular about the manufacturing method of the steel material of this invention, it heats so that the steel which has the chemical composition demonstrated above may be included, for example, process (P1)-(P6) shown below. , Hot rolling, and winding. This manufacturing method will be described in detail below.

  Molten steel having the above chemical composition is produced in a converter, electric furnace or the like. If necessary, the molten steel may be further subjected to a treatment such as vacuum degassing. Thereafter, the molten steel is made into a steel piece (slab) by a known method, for example, a slab by a continuous casting method, or after casting into a mold to form a steel ingot, and then rolling it into pieces. Moreover, you may manufacture a slab directly from molten steel by methods, such as what is called strip casting. When component segregation occurs in slabs, steel ingots, or slabs, the variation in the particle size of carbides produced in the steel increases, so methods such as electromagnetic agitation and unsolidified zone reduction are adopted to reduce component segregation during solidification. It is preferable to suppress the occurrence. This slab is processed by a method including the following steps (P1) to (P6).

<Process (P1)>
The manufactured slab is heated. Heating temperature shall be 1100-1300 degreeC. If the heating temperature is less than 1100 ° C, the slab may not be heated sufficiently, and the surface of the slab is cooled by descaling in the step (P2), and the rough rolling start temperature in the step (P3) is reduced. Sometimes it cannot be secured. Further, when the heating temperature exceeds 1300 ° C., a highly adhesive scale containing Cr and Si is formed, but this scale causes roll wrinkles during rolling and deteriorates the surface properties of the steel material. The soaking is preferably performed until the temperature difference between the steel sheet surface temperature and the steel sheet inside is 40 ° C. or less. The soaking time may be sufficient to satisfy the above conditions according to the equipment specifications. Preferably it is 1 hour or more.

<Process (P2)>
The primary scale formed on the heated slab surface is removed. This descaling is not limited to one time, and may be performed a plurality of times as necessary. In addition, the water pressure and the injection flow rate may be in a range that is normally employed, and the conditions under which the primary scale can be removed may be appropriately selected depending on conditions such as the material of the steel material. For descaling, for example, a scale removing device using high-pressure water can be used as a scale breaker.

<Process (P3)>
When the slab subjected to descaling after heating is hot-rolled at a rough rolling start temperature of 1000 ° C. or higher and a finish rolling completion temperature of 950 to 800 ° C. by a rolling mill group including at least a rough rolling mill and a finish rolling mill, rough rolling is performed. A scale formed on the surface of the steel material during rolling by injecting water with a water pressure of 10 MPa or more and an injection flow rate of 20 to 300 L / min in the exit stage of the mill and the initial stage of rolling in the finishing mill. Remove. By this descaling, the scale other than the scale having high adhesion is removed from the secondary scale formed on the surface of the slab during rough rolling.

  When the starting temperature of rough rolling is below 1000 ° C., the roll load during rolling increases. In rough rolling, the starting temperature is preferably 1050 ° C. or higher and 1200 ° C. or lower. The rough rolling method includes a lever rolling method in which a steel material in a case where the rough rolling mill is composed of a single stage stand is reciprocated and rolled a plurality of times, and a steel material in a case where the rough rolling mill is a tandem type composed of a multi-level stand. Any of the methods for conveying the film in only one direction may be used.

  Since finish rolling is desirably performed in a low temperature range of austenite, the finish rolling completion temperature is set to a temperature range of 950 to 800 ° C. The conditions for finish rolling are not particularly specified except for the finish rolling completion temperature. When the finish rolling completion temperature is less than 800 ° C., the austenite grains are flattened, and the mechanical properties of the steel material vary in the rolling direction and the width direction, so that the workability of the steel material may be deteriorated. Moreover, there exists a possibility that a deposit may become coarse. On the other hand, when the finish rolling completion temperature exceeds 950 ° C., there are problems that the crystal grains become coarse, the scale grows too thick by the start of cooling of the steel material, and scale wrinkles are likely to occur.

  In the hot rolling, it is preferable that the cumulative rolling reduction in the temperature range from the rough rolling start temperature to the finish rolling completion temperature is 60% or more. If the cumulative rolling reduction is less than 60%, the austenite grains are not sufficiently refined, and the toughness of the steel material may be deteriorated.

  The exit side of the roughing mill means that when the roughing mill consists of a single-stage stand, it means the exit side from the stand after finishing the levers rolling, and the rough rolling mill is a tandem type consisting of a multi-stage stand. In this case, it means the exit of the last stand. The initial stage of rolling in the finish rolling mill means at least one pass from the first pass to the third pass of finish rolling performed in the finish rolling mill. Finish rolling is generally performed for about 5 to 7 passes, although it depends on the dimensions of the product (steel material). The provisions of the present invention do not exclude descaling to be performed at a time other than “the exit side of the roughing mill and the initial stage of rolling in the finishing mill”. For example, when the rough rolling mill is composed of a single stage stand, descaling may be performed in the middle of the levers rolling. In the case of a tandem type composed of a plurality of stage stands, the descaling is performed each time rolling is performed at each stand. Also good.

At the time of descaling, the water pressure of the scale breaker is 10 MPa or more. When the water pressure is less than 10 MPa, it is difficult to remove firelite (Fe ₂ SiO ₄ ), and thus descaling cannot be performed sufficiently. However, if the water pressure exceeds 60 MPa, strict operation regulations may be required for the heating conditions of the slab, and the equipment may be increased in size and cost may increase. Therefore, the water pressure in descaling is preferably 60 MPa or less. The lower limit of water pressure in descaling is preferably 5 MPa, and the upper limit is more preferably 30 MPa.

  The water injection flow rate per nozzle is set to 20 to 300 L / min. If it is less than 20 L / min, the thermal shock force is small, so that it cannot be sufficiently scaled. On the other hand, when the injection flow rate per nozzle exceeds 300 L / min, the temperature of the steel material surface decreases and temperature unevenness occurs on the steel material surface, making uniform rolling difficult. In order to achieve both stable descaling and rolling, the lower limit of the water injection flow rate per nozzle is preferably 50 L / min, and the upper limit is preferably 200 L / min. The number and arrangement of the nozzles of the scale breaker may be set according to the shape and size of the slab. For example, the number and arrangement of the scale breaker that can be descaled for the entire circumference of the slab may be used.

<Process (P4)>
The obtained hot-rolled steel material starts cooling within 5 s after finishing the finish rolling, and cools to a temperature T1 in the temperature range of 650 to 500 ° C. at an average cooling rate of 25 ° C./s or more in the water injection zone. If the start of cooling exceeds 5 s after the finish rolling is completed, recovery of dislocations introduced by hot rolling occurs, and the core of ferrite transformation is insufficient, so that the crystal grains become coarse and the toughness of the steel material may be reduced. Also, the scale becomes too thick. When the cooling rate after finish rolling is less than 25 ° C./s, a pearlite structure and a ferrite structure are generated, and a hard structure and a soft structure are mixed, so that there is a possibility that the material of the steel material may vary. In addition, when the scale becomes too thick and part of the scale is peeled off, the moisture that has entered the peeled portion evaporates, causing the swelling of the scale. On the other hand, when the cooling rate after finish rolling exceeds 100 ° C./s, a martensite structure tends to be formed, and the toughness of the steel material may be deteriorated. Therefore, the cooling rate is preferably 100 ° C./s or less. When T1 exceeds 650 ° C., the transformation of the scale proceeds, the value of Fn1 becomes 0.15 or less, and the adhesion of the scale becomes low. Further, as the scale transformation proceeds, the value of Fn2 becomes 23.0 or more, the corrosion potential difference exceeds 40 mV, and local corrosion is promoted. On the other hand, when T1 is less than 500 ° C., it is difficult to wind the steel material in a coil shape, it becomes difficult to control the temperature of the steel material in the transformation temperature range of the scale, and further, the surface of the steel material is taken up when the steel material is wound. There arises a problem that the scale formed on the surface peels off and the surface properties become non-uniform.

<Process (P5)>
The cooled hot-rolled steel material is wound into a coil at a temperature T2 in the temperature range of 650 to 500 ° C. When T2 exceeds 650 ° C., the low temperature transformation layer is not sufficiently formed, and the strength of the steel material cannot be sufficiently increased. On the other hand, when T2 is less than 500 ° C., shape failure may occur in the steel material. Moreover, when T2 is less than 500 ° C., the transformation of the scale proceeds before the steel material is wound, and the value of Fn1 becomes 0.35 or more and the value of Fn2 becomes 23.0 or more, and the adhesion of the scale. Decreases.

<Process (P6)>
The hot rolled steel material wound up in a coil shape is heated for 10 hours at an average cooling rate of 0.005 to 0.1 ° C / s from a temperature T3 in the temperature range of 650 to 500 ° C to a temperature T4 in the temperature range of 300 to 150 ° C. Allow to cool over. By allowing to cool from T3 to T4 over 10 hours at the above average cooling rate, Sn can be sufficiently diffused in the scale. When T3 exceeds 650 ° C., the scale grows in the coil, and the surface properties of the steel material become non-uniform. On the other hand, when T3 is less than 500 ° C., it becomes difficult to control the temperature of the steel material in the transformation temperature range, the surface property of the steel material becomes non-uniform, and the adhesion of the scale becomes low.

  Even if the temperature conditions of T3 and T4 are satisfied, if the average cooling rate is less than 0.005 ° C./s, the transformation does not proceed, the scale composition ratio changes, the value of Fn1 is 0.35 or more, and Fn2 The value becomes 9.0 or less, and the adhesion of the scale decreases. Further, the value of Fn1 is 0.35 or more and the value of Fn2 is 23.0 or more, and the adhesion of the scale is lowered. Furthermore, even if the temperature condition and the cooling rate condition are satisfied, if the cooling time is less than 10 h, the transformation does not proceed, the scale composition ratio changes, the values of Fn1 and Fn2 are not satisfied, and the adhesiveness decreases. .

  The steel material of this invention is completed by the process containing the above process (P1)-(P6). In addition, it is preferable that it is T1> = T2> = T3 from a viewpoint of productivity improvement, energy saving, etc. which does not need to heat steel materials in the middle of a process. Moreover, the cooling from T4 may be an arbitrary method.

  In the production of the steel material according to the present invention, a so-called “high pressure water scale breaker” may be used as a scale breaker, and a so-called “bar heater” may be used for heating the steel material after rough rolling (hereinafter referred to as “bar”). There is no problem even if hot rolling continuation for rolling the bar joint material after rough rolling is utilized. “High-pressure water scale breaker” refers to a scale breaker in which the water pressure of water to be sprayed is 5 MPa or more. By using these facilities, it is possible to improve the temperature process capability, prevent the generation of scale soot and improve the yield.

  The steel material according to the present invention is not particularly limited in shape. For example, when the steel material is a steel plate, the thickness is preferably 16 mm or less, and when the steel plate is used as a steel sheet pile, or after the spiral steel pipe, When used for a pile, the thickness is more preferably 10 mm or less.

  Hereinafter, the present invention will be described more specifically with reference to examples in which a steel plate is taken as an example of a steel material, but the present invention is not limited to these examples.

Using a slab having the chemical composition of steels 1 to 11 shown in Table 1 as a raw material and using a rolling mill equipped with a roughing mill and a finishing mill, hot rolling is performed under the conditions shown in Table 2 to obtain a thickness of 3 A hot-rolled steel sheet having a width of 2 mm and a width of 1000 mm was produced. Although not shown in Table 2, the step (P2) was carried out under any conditions, and the primary scale was removed. Descaling at the initial stage of rolling in the finishing mill in the process (P3) is performed in the first to third passes, and the injection conditions from the scale breaker on the exit side of the roughing mill and the initial stage of the finishing mill are shown in Table 2. Also written. The number of nozzles of the scale breaker was set so that it could be descaled over the entire circumference of the slab, and was arranged at equal intervals around the slab. In the step (P3), all of the steels 1 to 11 could be sufficiently descaled for the entire circumference of the slab. Moreover, the cooling from the temperature range below T4 to room temperature was also performed by the same cooling method as the cooling to T4.

Among the production methods A to O shown in Table 2, A to C satisfied the provisions of the method for producing a steel material of the present invention. About production method DH, manufacture of the hot-rolled steel plate was stopped for the following reasons.
Production method D: The heating temperature of the slab in the step (P1) exceeded 1300 ° C., and a highly adhesive scale containing Cr and Si grew in the heating furnace. However, since this scale was found to cause roll wrinkling during rolling, the subsequent steps were stopped.
Production method E: Since the heating temperature of the slab in the step (P1) was less than 1100 ° C., the slab could not be heated sufficiently within a predetermined time (2.5 h). Moreover, the surface of the slab was cooled by descaling after heating, and the rough rolling start temperature became less than 1000 ° C.
Production methods F to H: A scale with high adhesion containing Cr and Si grew by heating the slab in the step (P1). This scale could not be removed sufficiently unless descaling was performed at least 3 times in the step (P2) with a water pressure of 10 MPa or more and a jet water amount of 40 L / min or more. In the production methods F to H, any one of the water pressure, the amount of jet water, and the number of descaling does not satisfy these conditions, and the scale remains on the slab surface, and this scale causes roll wrinkles in the subsequent rolling. It was.

The production methods I to O did not satisfy the provisions of the steel material production method of the present invention in the following points.
Production method I: The finish rolling completion temperature in step (P3) exceeded 950 ° C.
Production method J: The time until the cooling of the hot-rolled steel sheet in the step (P4) was started exceeded 5 s after completion of finish rolling.
Manufacturing method K: The cooling rate to the cooling stop temperature T1 in the step (P4) was less than 25 ° C./s.
Production Method L: The cooling stop temperature T1 in the step (P4) was higher than 650 ° C.
Production method M: The winding temperature T2 of the hot-rolled steel sheet in the step (P5) was less than 500 ° C, and the temperature T4 in the step (P6) was less than 150 ° C.
Production method N: The average cooling rate from the temperature T3 to T4 in the step (P6) exceeded 0.1 ° C./s, and the cooling time was less than 10 h.
Production Method O: The average cooling rate from T3 to T4 in the step (P6) was less than 0.005 ° C./s.

  A plate-shaped test piece having a thickness of 3.2 mm, a width of 60 mm, and a length of 100 mm was cut out from the hot-rolled steel sheet obtained by the above production method, and various tests were performed. In the soil embedding test, a test piece having another dimension described later was used.

<Oxide content in scale>
The scale on the surface of the test piece was collected with a hammer and a cutter knife to a depth where the base material could be confirmed, and the collected scale was pulverized to obtain a powder sample. By applying the powder X-ray diffraction method (internal standard method) to this powder sample, the contents (mass%) of magnetite, firelite, wustite and hematite, that is, Mm, Mf, Mw and Mh were measured. Using this measurement result, the value of Fn1 represented by the following formula (i) was calculated.
Fn1 = (Mm + Mf) / (Mw + Mh) (i)

<Cr, Sn and Si contents in scale>
The scale on the surface of the test piece was peeled off using a hammer, and the peeled scale was chemically dissolved in aqua regia and analyzed by ICP, whereby the Cr, Sn and Si contents (% by mass) in the scale, ie, [Cr ], [Sn] and [Si] were measured. Using this measurement result, the value of Fn2 represented by the following formula (ii) was calculated.
Fn2 = ([Cr] + [Sn]) / [Si] (ii)

<Scale adhesion test>
The adhesion of the scale was investigated by a DuPont impact test specified in JIS K 5600: 1999. Specifically, a 500 g weight was dropped from a height of 200 mm for each test piece, and the scale breakage was visually judged. If the scale was not cracked or peeled off, it was evaluated as “◯” (scale adhesion was good), and if the scale was cracked or peeled off, it was evaluated as “x” (scale adhesion was poor).

<Local corrosion test>
A hole having a depth of 1 mm was provided on the surface of the test piece using a drill having a diameter of 3 mm. With respect to this test piece, the potential difference (corrosion potential difference) between the scale and the base material was measured using a scanning reference electrode in a 0.003% NaCl aqueous solution adjusted to pH 2.0 with hydrochloric acid. This simulates local corrosion at a part where the scale on the steel material surface is partially peeled off.

<Acid immersion test>
The test piece was immersed in a 0.5 M Na ₂ SO ₄ aqueous solution adjusted to pH 1.0 with sulfuric acid for 24 hours. The mass of the test piece was measured before and after immersion. The amount of mass decrease due to acid immersion calculated using these mass measurement results was converted to the amount of corrosion (unit: mm / y) per year (365 days).

<Soil embedding test>
A test clay in which iron (II) sulfate (FeSO ₄ ) was contained as an acidic sulfate in kaolin clay (kaolin 70 mass%, water 30 mass%) was prepared. The iron (II) sulfate contained was 5% of the mass of kaolin clay. The test clay was packed in a test tank having a depth of 30 cm and compacted by applying a load from the upper part for one day to obtain test soil.

  As the test piece, a plate-shaped test piece having a thickness of 3 mm, a width of 30 mm, and a length of 150 mm, which was collected from the center portion in the thickness direction of a hot-rolled steel sheet having a thickness of 3.2 mm, was used. Both the surface and the end face of the test piece were wet-polished with No. 600 emery abrasive paper and then dried. An epoxy resin paint was applied to a portion 30 mm from one end of the test piece for the purpose of preventing corrosion.

  The test piece was embedded from the top of the test soil so as to be perpendicular to the bottom surface of the test tank. The portion of the test piece where the paint was applied was exposed from the test soil. In this state, the atmosphere around the test tank was set to 30 ° C. and maintained for 3 months. During the test period, the moisture content of the test soil is periodically measured by inserting an underground conductivity meter from the surface of the test soil to a depth of 1.5 cm, and when the moisture content is less than 30%, Mist water was supplied to the surface of the test soil to maintain the moisture content of the test soil.

  At the end of the test period, the test piece was removed from the test soil and the maximum corrosion depth was measured. The corrosion depth was measured by setting the corrosion depth of the portion where the paint was applied to 0.

  Table 3 shows the results of the above tests.

  Examples 1 to 6 are examples of the present invention, and all satisfied the provisions of the present application. In any case, the evaluation of the adhesion of the scale was “◯”, and the corrosion rate in the acid immersion test was 0.01 to 0.08 mm / y. The difference in corrosion potential measured in the local corrosion test was 40 mV or less, and the maximum corrosion depth in the soil burying test was 40 to 200 μm.

  In all of Comparative Examples 1 to 12, the value of Fn2 did not satisfy the definition of the present invention. Therefore, in all cases, the corrosion potential difference was a large value of 45 mV or more. Furthermore, at least one characteristic among the adhesion of the scale, the acid immersion corrosion rate, and the maximum soil burial depth was inferior.

  The steel material of the present invention has excellent corrosion resistance in a soil corrosive environment (particularly in an acidic soil environment) because the scale formed on the surface has adhesion. Even if the scale is peeled off and the steel base material is exposed, the potential difference generated between the scale and the base material is small, so that excellent corrosion resistance can be maintained. The steel material of the present invention can be suitably used for steel structures for underground use such as steel sheet piles and steel pipe piles. Moreover, according to the manufacturing method of the steel material of this invention, the steel material of this invention can be manufactured efficiently.

Claims (3)

A steel material with a scale formed on its surface,
The chemical composition of the steel material is mass%,
C: 0.01-0.20%
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.5%
P: 0.01% or less,
S: 0.01% or less,
Cr: 3.0-5.0%,
Sn: 0.01-0.30%,
Al: 0.005 to 0.05%,
N: 0.0005 to 0.01%,
Mo: 0 to 0.5%,
W: 0 to 0.5%
Ti: 0 to 0.5%,
Balance: Fe and impurities,
The scale is a structure in which magnetite, firelite, wustite and hematite are laminated or partially mixed,
The scale has a thickness of 5 to 50 μm,
Steel materials in which Fn1 and Fn2 represented by the following formulas (i) and (ii) are 0.15 <Fn1 <0.35 and 9.0 <Fn2 <23.0, respectively.
Fn1 = (Mm + Mf) / (Mw + Mh) (i)
Fn2 = ([Cr] + [Sn]) / [Si] (ii)
However, the meaning of each symbol in the above formulas (i) and (ii) is as follows.
Mm: Magnetite content (% by mass) in the scale
Mf: Firelight content (% by mass) in the scale
Mw: Wustite content (% by mass) in the scale
Mh: hematite content (% by mass) in the scale
[Cr]: Cr content (% by mass) in the scale
[Sn]: Sn content (% by mass) in the scale
[Si]: Si content (% by mass) in the scale   A steel structure for embedding in soil using the steel material according to claim 1. The manufacturing method of the steel materials of Claim 1 including the following process (P1)-(P6).
(P1) a step of heating the slab having the chemical composition according to claim 1 to 1100 to 1300 ° C.,
(P2) removing the scale formed on the surface of the heated slab;
(P3) When the slab after scale removal is hot-rolled at a rough rolling start temperature of 1000 ° C. or higher and a finish rolling completion temperature of 950 to 800 ° C. by a rolling mill group including at least a rough rolling mill and a finish rolling mill, Formed on the surface of the steel material during rolling by injecting water having a water pressure of 10 MPa or more and an injection flow rate of 20 to 300 L / min at the exit side of the roughing mill and the initial stage of rolling in the finishing mill. Removing the scales,
(P4) The process of starting cooling of the obtained hot-rolled steel material within 5 s after completion of finish rolling and cooling to a temperature T1 in the temperature range of 650 to 500 ° C. at an average cooling rate of 25 ° C./s or more in the water injection zone ,
(P5) a step of winding the cooled hot rolled steel material in a coil shape at a temperature T2 in a temperature range of 650 to 500 ° C;
(P6) The coiled hot-rolled steel material is subjected to an average cooling rate of 0.005 to 0.1 ° C / s from a temperature T3 in a temperature range of 650 to 500 ° C to a temperature T4 in a temperature range of 300 to 150 ° C. The process which cools over 10 hours or more.