JP7448874B1 - steel bar - Google Patents

Abstract

A steel bar according to one aspect of the present invention includes a parent phase and a layer containing Si, Cr, Fe, and O that covers the surface of the parent phase, and the parent phase has C: 0.10 to 0.30%, Si: 0.51 to 2.40%, Mn: 0.50 to 1.50%, P: 0.050% or less, S: 0.050% or less, Cr: 0 .05 to 2.00%, Al: 0.010 to 0.100%, N: 0.003 to 0.030%, and O: 0.0050% or less, with the remainder consisting of iron and impurities, The Cr/Si mass concentration ratio of the layer is 0.10 or more.

Description

The present invention relates to a steel bar.
This application claims priority based on Japanese Patent Application No. 2023-001176 filed in Japan on January 6, 2023, the contents of which are incorporated herein.

When manufacturing steel materials such as steel bars, scale is generated on their surfaces. Scale deteriorates the surface properties of steel materials and causes surface flaws during rolling of steel materials. On the other hand, scale also has the function of protecting steel materials from oxygen in the atmosphere and improving the corrosion resistance of steel materials.

Scale is removed from steel materials when the steel materials are processed into mechanical structural parts. However, it may take up to several months from the completion of steel production to the start of steel processing. During this period, steel products are often stored outdoors where moisture is high. At this time, the scale exhibits the effect of improving the corrosion resistance of the steel material.

Various studies have been conducted regarding scale adhering to the surface of steel materials.

Patent Document 1 describes a method for producing a Cr-containing long steel material containing 0.10 to 2.0% Cr (meaning mass %; the same applies hereinafter in terms of the chemical composition of steel), in which a steel billet is taken from a heating furnace. After taking out, the steel billet is subjected to descaling. After step a, the steel billet is subjected to hot rolling in an atmospheric atmosphere. After step b, the steel billet is subjected to hot rolling. A method for manufacturing a Cr-containing long steel material is disclosed, which includes a step c of performing descaling, and the hot rolling in the step b is performed under predetermined conditions.

Patent Document 2 discloses that a steel slab containing 0.10 to 2.00% Cr (meaning mass %; the same applies hereinafter for the chemical composition of steel) is taken out of a heating furnace, descaled, and then hot rolled. A method for producing a Cr-containing long steel material, the method comprising heating the steel piece in the heating furnace in a temperature range of 1000°C or more and 1150°C or less for 15 minutes or more, and then the surface temperature (extraction temperature) The steel slab in the temperature range is taken out from the heating furnace, and then in an atmosphere where the O ₂ concentration is 10 volume % or more and the H ₂ O concentration is 5 volume % or more and 35 volume % or less, the surface temperature of the steel slab is Excellent scale removability characterized by rapidly heating the steel billet at a heating rate of 20°C/min or more from the extraction temperature to a temperature range of 1200°C or more and 1350°C or less, and then descaling. A method for manufacturing a Cr-containing long steel material is disclosed.

Patent Document 3 describes a steel piece made of Cr-containing steel containing Si: 0.05 to 0.4% by mass, Cr: 0.1 to 2.0% by mass, and the balance being Fe and unavoidable impurities. , When performing heat treatment immediately before descaling treatment, the heat treatment temperature is 1000 to 1150 ° C., the holding time at the heat treatment temperature is 1 to 30 minutes, and the heat treatment atmosphere is H ₂ O [steam]: 25 A method for manufacturing a Cr-containing steel with excellent scale removability is disclosed, which is characterized by containing 1 to 10 volume % of O ₂ and 1 to 10 volume % of O 2 and the balance being N ₂ and inevitable gas components.

Patent Document 4 describes a method in which a steel billet containing Cr: 0.2 to 4.0% by mass is heated in a combustion atmosphere heating furnace, kept uniformly heated, and then extracted outside the heating furnace and hot rolled. , assuming that the Cr concentration in the steel is X% by mass, the water vapor concentration in the heating furnace is (4.47X+17.1) vol. % or more, and in an atmosphere where the ratio of water vapor concentration to oxygen concentration (vol.%) in the heating furnace is adjusted to 10 or more, soaking is maintained at a temperature of less than 1100°C for 10 minutes or more. A method for producing a high Cr-containing cold-pressing steel material with good descaling properties is disclosed.

Patent Document 5 describes C: 0.4 to 0.8 mass%, Si: 1.0 to 2.5 mass%, Mn: 0.2 to 1 mass%, P: more than 0 mass%, 0.05 % by mass or less, S: more than 0 mass % and 0.05 mass % or less, Cr: 0.6 to 2 mass %, and the balance is iron and unavoidable impurities, the steel wire surface The proportion of iron oxide in the iron oxide scale satisfies the following conditions: FeO: 10 to 60 volume %, Fe ₂ O ₃ : more than 0 volume %, 15 volume % or less, balance: Fe ₃ O ₄ and Fe ₂ SiO ₄ , and the above-mentioned A steel wire with excellent coiling properties, characterized in that the average thickness of the iron oxide scale is 0.3 to 2.0 μm, and the average grain size of the iron oxide scale is 0.2 μm or less. is disclosed.

Patent Document 6 describes C: 0.05 to 1.2 mass%, Si: 0.01 to 0.50 mass%, Mn: 0.1 to 1.5 mass%, P: 0.02 mass% or less. (including 0%), S: 0.02% by mass or less (including 0%), N: 0.005% by mass or less (including 0%), and the balance consists of iron and inevitable impurities. An FeO layer consisting of fine crystal grains with random orientation is formed as an inner scale on the surface of the steel, and a _4- layer Fe ₂ SiO layer with a thickness of 0.01 to 1.0 μm is formed at the interface between the inner scale FeO layer and the steel. Disclosed is a steel wire rod characterized in that the thickness of the inner layer scale is 1 to 40% of the total thickness of the scale.

Japanese Patent Application Publication No. 2010-264469 Japanese Patent Application Publication No. 2010-23087 Japanese Patent Application Publication No. 2010-524 JP2009-275285A JP2017-115228A Japanese Patent Application Publication No. 2009-263750

During the period from the completion of manufacturing the steel bar to the start of processing the steel bar, red rust may occur on the surface of the steel bar. Customers of steel bars are demanding that red rust be suppressed. Upon investigation by the present inventors, it became clear that scale exfoliation occurs at locations where red rust occurs.

In the prior art, sufficient consideration has not been given to means for suppressing scale peeling. For example, in the techniques disclosed in Patent Documents 1 to 5, it is an issue to improve scale removability. These patent documents do not disclose any means for suppressing scale peeling or improving the corrosion resistance of scale peeled portions.

The technology of Patent Document 6 aims to provide a steel wire rod that is difficult to peel off scale during cooling after hot rolling or during storage and transportation, has good scale peeling properties during mechanical descaling, and has excellent mechanical descaling properties. This is an issue. However, Patent Document 6 relates to steel wire rods. The manufacturing conditions for suppressing scale peeling of steel wire rods described in Patent Document 6 cannot be applied to steel bars. Moreover, in the example of the technique of Patent Document 6, the scale peeling rate is only evaluated at the time when manufacturing of the steel wire rod is completed. It cannot be determined from the examples of Patent Document 6 whether the technology of Patent Document 6 can suppress scale peeling during storage and transportation of steel wire rods.

In view of the above circumstances, an object of the present invention is to provide a steel bar that can suppress the occurrence of red rust during the period from the completion of manufacturing to the start of processing.

The gist of the invention is as follows.

(1) A steel bar according to one aspect of the present invention includes a parent phase and a layer containing Si, Cr, Fe, and O that covers the surface of the parent phase, wherein the parent phase is , C: 0.10 to 0.30%, Si: 0.51 to 2.40%, Mn: 0.50 to 1.50%, P: 0.050% or less, S: 0.050% or less, Contains Cr: 0.05-2.00%, Al: 0.010-0.100%, N: 0.003-0.030%, and O: 0.0050% or less, with the balance being iron and impurities. The layer has a Cr/Si mass concentration ratio of 0.10 or more.
(2) In the steel bar according to (1) above, preferably, the parent phase further contains, in unit mass %, Mo: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50%. Below, W: 0.500% or less, V: 0.50% or less, Bi: 0.10% or less, Co: 0.500% or less, Nb: 0.10% or less, Ti: 0.20% or less, Contains one or more of Ca: 0.0015% or less, Pb: 0.09% or less, Sn: 0.10% or less, and B: 0.007% or less.
(3) In the steel bar according to (2) above, preferably, the parent phase, in unit mass %, Bi: 0.10% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Ca: 0.0015% or less.
(4) In the steel bar according to (2) above, preferably, the parent phase, in unit mass %, Mo: 0.50% or less, W: 0.500% or less, B: 0.007% or less, Contains one or more of Cu: 0.50% or less, Ni: 0.50% or less, and Co: 0.500% or less.
(5) In the steel bar according to (2) above, preferably, the parent phase contains, in unit mass %, Ti: 0.20% or less, Nb: 0.10% or less, and V: 0.50% or less. Contains one or more of the following.
(6) The steel bar according to any one of (1) to (5) above preferably further includes a scale layer covering the layer.

According to the present invention, it is possible to provide a steel bar that can suppress the occurrence of red rust during the period from the completion of manufacturing to the start of processing.

It is a cross-sectional schematic diagram of the surface of the steel bar concerning this embodiment. It is a flowchart of the manufacturing method of the steel bar concerning this embodiment. It is a perspective view explaining the analysis position of the iron-based oxide layer of a steel bar. FIG. 3 is a schematic cross-sectional view showing a location where a line analysis region for measuring a Cr/Si mass concentration ratio is arranged. FIG. 3 is a schematic cross-sectional view showing the interface between the parent phase of the steel bar and the iron-based carbide layer, and a line analysis region for measuring the Cr/Si mass concentration ratio. FIG. 3 is a schematic cross-sectional view showing the interface between the parent phase of the steel bar and the iron-based carbide layer, and a line analysis region for measuring the Cr/Si mass concentration ratio.

A steel bar 1 according to one embodiment of the present invention includes a matrix 11 and a layer containing Si, Cr, Fe, and O (iron-based oxide layer 12) that covers the matrix 11, and the matrix 11 is It has a predetermined chemical composition, and the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is 0.10 or more. First, technical knowledge for arriving at the steel bar according to this embodiment will be explained.

The present inventors conducted microscopic observation and compositional analysis of the interface between the scale layer 13 and the matrix 11. As a result, it was found that a layer containing Si, Cr, Fe, and O (iron-based oxide layer 12) was formed at the interface between the scale layer 13 and the matrix 11. Further, the present inventors have found that Cr may be concentrated in the iron-based oxide layer 12. In addition, the present inventors have found that the steel bar 1 in which the iron-based oxide layer 12 enriched with Cr is formed has excellent corrosion resistance.

While the scale layer 13 is easily peeled off from the steel bar 1, the iron-based oxide layer 12 is difficult to peel off from the steel bar 1. The present inventors confirmed that the iron-based oxide layer 12 remained at the peeled portion of the scale layer 13. The present inventors estimate that the iron-based oxide layer 12 enriched with Cr improves the corrosion resistance at the location where the scale layer 13 has peeled off.

Furthermore, the inventors have further investigated the method of enriching Cr in the iron-based oxide layer 12. As a result, by keeping the Cr content of the steel slab within a predetermined range before hot rolling, and optimizing the rolling conditions in the rough rolling stage of hot rolling and the cooling conditions after hot rolling, it is possible to It has been found that the oxide layer 12 can be enriched with Cr.

Next, a specific configuration of the steel bar 1 according to this embodiment will be explained. Hereinafter, the unit of element content "mass %" will be simply written as "%".

A steel bar 1 according to one embodiment of the present invention includes at least a matrix 11 and an iron-based oxide layer 12 that covers the matrix 11. The matrix 11 is also called a base iron. The matrix 11 is steel having the chemical composition described below. Si and Cr contained in the matrix 11 greatly influence the structure of the iron-based oxide layer 12. Other elements do not directly affect the structure of the iron-based oxide layer 12 or the corrosion resistance of the steel bar, but are used to improve the mechanical properties of the steel bar 1.

(C: 0.10-0.30%)
C increases the strength of steel. If C is insufficient, the hardness, tensile strength, etc. of the steel bar will be impaired. Therefore, the C content is set to 0.10% or more. The C content may be 0.12% or more, 0.14% or more, or 0.16% or more.

On the other hand, if the C content is excessive, the hardness of the steel bar will be excessive, and the machinability of the steel bar will be impaired. Therefore, the C content is set to 0.30% or less. The C content may be 0.28% or less, 0.26% or less, or 0.25% or less.

(Si: 0.51-2.40%)
Si forms an iron-based oxide layer 12 that covers the matrix 11 and improves the corrosion resistance of the steel bar 1. Moreover, Si increases the strength of steel. Therefore, the Si content is set to 0.51% or more. The Si content may be 0.60% or more, 0.80% or more, or 1.00% or more.

On the other hand, if the Si content is excessive, the hardness of the steel bar will be excessive, and the cuttability, machinability, and manufacturability of the steel bar will be impaired. Therefore, the Si content is set to 2.40% or less. The Si content may be 2.00% or less, 1.80% or less, or 1.60% or less.

(Mn: 0.50-1.50%)
Mn increases the strength of steel. If Mn is insufficient, the hardness, tensile strength, etc. of the steel bar will be impaired. Therefore, the Mn content is set to 0.50% or more. The Mn content may be 0.55% or more, 0.60% or more, or 0.70% or more.

On the other hand, an excessive amount of Mn may interfere with carburizing treatment of the steel bar. When a steel bar is used as a material for carburized parts, an excessive amount of Mn increases the amount of retained austenite on the surface of the part after carburizing, reduces the surface hardness of the part, and reduces the fatigue strength of the part. Sometimes I let it happen. Therefore, the Mn content is set to 1.50% or less. The Mn content may be 1.45% or less, 1.40% or less, or 1.35% or less.

(P: 0.050% or less)
P is a component mixed into the matrix as an impurity. Excessive P content impairs the mechanical properties of the steel material. Therefore, the P content is set to 0.050% or less. The P content may be 0.040% or less, 0.030% or less, or 0.020% or less.

P is not required to improve the corrosion resistance of the steel bar. Therefore, the P content is preferably as low as possible, and may be 0%. However, the lower the P content, the higher the refining cost. In order to reduce refining costs, the P content may be greater than 0%, 0.001% or more, or 0.005% or more.

(S: 0.050% or less)
S is a component mixed into the matrix as an impurity. If the S content is excessive, coarse inclusions will occur in the matrix, impairing the mechanical properties of the steel material. Therefore, the S content is set to 0.050% or less. The S content may be 0.040% or less, 0.030% or less, or 0.020% or less.

S is not required to improve the corrosion resistance of the steel bar. Therefore, the S content is preferably as low as possible, and may be 0%. However, the lower the S content, the higher the refining cost. In order to reduce refining costs, the S content may be greater than 0%, 0.001% or more, or 0.005% or more.

(Cr: 0.05-2.00%)
Cr concentrates in the iron-based oxide layer 12 covering the matrix 11 and improves the corrosion resistance of the iron-based oxide layer 12. Thereby, Cr dramatically improves the corrosion resistance of the steel bar 1. Furthermore, Cr increases the temper softening resistance of the steel and improves the surface fatigue strength of the steel. Therefore, the Cr content is set to 0.05% or more. The Cr content may be 0.10% or more, 0.20% or more, 0.50% or more, or 1.00% or more.

On the other hand, if the Cr content is excessive, the above-mentioned effects are saturated, but the hardness of the steel material is increased, leading to an increase in manufacturing cost. Therefore, the Cr content is set to 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 1.20% or less, 0.50% or less, or 0.30% or less.

(Al: 0.010-0.100%)
Al forms fine inclusions and refines the γ grains. Therefore, the Al content is set to 0.010% or more. The Al content may be 0.015% or more, or 0.020% or more.

On the other hand, if the Al content is excessive, coarse inclusions are generated in the matrix, impairing the mechanical properties of the steel bar. Therefore, the Al content is set to 0.100% or less. The Al content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(N: 0.003-0.030%)
N refines the crystal grains of the parent phase. Therefore, the N content is set to 0.003% or more. The N content may be 0.005% or more, 0.010% or more, or 0.015% or more.

On the other hand, if the N content is excessive, the high-temperature ductility of the matrix is impaired, and the manufacturing yield of the steel bar is reduced. Therefore, the N content is set to 0.030% or less. The N content may be 0.028% or less, 0.025% or less, or 0.020% or less.

(O: 0.0050% or less)
O is a component mixed into the parent phase as an impurity. If the O content is excessive, coarse oxides are generated in the matrix, impairing the mechanical properties of the steel material. Therefore, the O content is set to 0.0050% or less. The O content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

O is not required to improve the corrosion resistance of the steel bar. Therefore, the O content is preferably as low as possible, and may be 0%. However, the lower the O content, the higher the refining cost. In order to reduce refining costs, the O content may be greater than 0%, 0.0001% or more, or 0.0005% or more.

(Remainder: iron and impurities)
The remainder of the chemical components of the matrix are iron and impurities. Impurities are components that are mixed in by raw materials such as ore or scrap, or various factors in the manufacturing process, for example, when steel materials are manufactured industrially, and do not have an adverse effect on the steel bar according to the present embodiment. means permissible within range.

The parent phase may further contain one or more of the arbitrary elements listed below. This makes the properties of the steel bar even better. However, the arbitrary elements listed below are not essential for solving the problems of the steel bar according to this embodiment. Therefore, the lower limit of the arbitrary elements listed below may be 0%.

(Mo: preferably more than 0% and 0.50% or less)
Mo improves the hardenability of steel. Therefore, the Mo content may be set to more than 0%, 0.10% or more, or 0.15% or more.

On the other hand, by setting the Mo content to 0.50% or less, excessive hardening of the steel can be avoided and the workability of the steel bar can be improved. Therefore, the Mo content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Cu: preferably more than 0% and 0.50% or less)
Cu improves the hardenability of steel. Therefore, the Cu content may be more than 0%, 0.10% or more, or 0.15% or more.

On the other hand, by setting the Cu content to 0.50% or less, excessive hardening of the steel can be avoided and the workability of the steel bar can be improved. Therefore, the Cu content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Ni: preferably more than 0% and 0.50% or less)
Ni improves the hardenability of steel. Therefore, the Ni content may be more than 0%, 0.10% or more, or 0.15% or more.

On the other hand, by setting the Ni content to 0.50% or less, excessive hardening of the steel can be avoided and the workability of the steel bar can be improved. Therefore, the Ni content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(W: Preferably more than 0% and 0.500% or less)
W increases the hardenability of steel. Therefore, the W content may be set to more than 0%, 0.100% or more, or 0.150% or more.

On the other hand, by setting the W content to 0.500% or less, excessive hardening of the steel can be avoided and the workability of the steel bar can be improved. Therefore, the W content may be set to 0.500% or less, 0.400% or less, or 0.300% or less.

(V: preferably more than 0% and 0.50% or less)
V forms fine carbides, nitrides, and/or carbonitrides. As a result, V suppresses grain growth, contributes to fine homogenization of the structure, and improves the fatigue properties of the steel bar. Therefore, the V content may be more than 0%, 0.10% or more, or 0.15% or more.

On the other hand, by controlling the V content to 0.50% or less, the formation of hard coarse carbides can be suppressed and the machinability of the steel bar can be ensured. Therefore, the V content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Bi: preferably more than 0% and 0.10% or less)
Bi improves the machinability of steel. Therefore, the B content may be greater than 0%, 0.01% or more, or 0.03% or more.

On the other hand, by setting the Bi content to 0.10% or less, it is possible to suppress the occurrence of cracks during rolling of the steel bar. Therefore, the B content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(Co: preferably more than 0% and 0.500% or less)
Co improves the hardenability of steel. Therefore, the Co content may be set to more than 0%, 0.100% or more, or 0.150% or more.

On the other hand, by setting the Co content to 0.500% or less, excessive hardening of the steel can be avoided and the workability of the steel bar can be improved. Therefore, the Co content may be set to 0.500% or less, 0.400% or less, or 0.300% or less.

(Nb: preferably more than 0% and 0.10% or less)
Nb forms fine carbides, nitrides, and/or carbonitrides. As a result, Nb suppresses grain growth, contributes to fine homogenization of the structure, and improves the fatigue properties of the steel bar. Therefore, the Nb content may be greater than 0%, 0.01% or more, or 0.03% or more.

On the other hand, by controlling the Nb content to 0.10% or less, the formation of hard coarse carbides can be suppressed and the machinability of the steel bar can be ensured. Therefore, the Nb content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(Ti: preferably more than 0% and 0.20% or less)
Ti forms fine carbides, nitrides, and/or carbonitrides. As a result, Ti suppresses grain growth, contributes to fine homogenization of the structure, and improves the fatigue properties of the steel bar. Therefore, the Ti content may be more than 0%, 0.01% or more, or 0.03% or more.

On the other hand, by setting the Ti content to 0.20% or less, the formation of hard coarse carbides can be suppressed and the machinability of the steel bar can be ensured. Therefore, the Ti content may be set to 0.20% or less, 0.18% or less, or 0.15% or less.

(Ca: preferably more than 0% and 0.0015% or less)
Ca improves the machinability of steel. Therefore, the Ca content may be greater than 0%, 0.0002% or more, or 0.0005% or more.

On the other hand, by setting the Ca content to 0.0015% or less, the fatigue strength of steel can be improved. Therefore, the Ca content may be set to 0.0015% or less, 0.0012% or less, or 0.0010% or less.

(Pb: preferably more than 0% and 0.09% or less)
Pb improves the machinability of steel. Therefore, the Pb content may be greater than 0%, 0.01% or more, or 0.03% or more.

On the other hand, by setting the Pb content to 0.09% or less, the forgeability of steel can be improved. Therefore, the Pb content may be set to 0.09% or less, 0.06% or less, or 0.05% or less.

(Sn: preferably more than 0% and 0.10% or less)
Sn improves the machinability of steel. Therefore, the Sn content may be more than 0%, 0.01% or more, or 0.03% or more.

On the other hand, when the Sn content is 0.10% or more, hot forging cracks occur. Therefore, the Sn content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(B: Preferably more than 0% and 0.007% or less)
B increases the hardenability of steel. Therefore, the B content may be set to more than 0%, 0.001% or more, or 0.002% or more.

On the other hand, by setting the B content to 0.007% or less, saturation of the above-mentioned effects can be avoided and the cost of raw materials for steel can be reduced. Therefore, the B content may be set to 0.007% or less, 0.006% or less, or 0.005% or less.

The method for measuring the chemical components of the matrix is not particularly limited. For example, an analysis method defined in JIS G 1201:2014 can be used to identify the chemical components of the matrix.

(Iron-based oxide layer 12)
A layer containing Si, Cr, Fe, and O (iron-based oxide layer 12) is formed to cover the surface of the matrix 11. Fe and Si contained in the iron-based oxide layer 12 originate from Fe and Si contained in the matrix 11. The steel bar 1 is manufactured by hot rolling a slab. During cooling after this hot rolling, the surface of the steel is oxidized to form an iron-based oxide layer 12.
In addition, Takeda et al., "Effect of Si concentration on the structure and adhesion of the primary scale of steel" (Kobe Steel Technical Report Vol. 55 No. 1 (Apr. 2005) p. 31) states, "In high-temperature oxidation of steel, It is known that Si affects the growth rate of scale and its properties.Si in steel forms SiO ₂ on the alloy surface, and then reacts with FeO to form Fe ₂ SiO ₄ (fayalite). "to form." Since the iron-based oxide layer 12 of the steel bar 1 according to the present embodiment is formed on the surface of the matrix 11 containing Si, it is presumed to be a layer containing fayalite. However, since the iron-based oxide layer 12 of the steel bar 1 according to this embodiment contains Cr, it is not the same as the scale described in the document.

(Cr/Si mass concentration ratio of iron-based oxide layer 12: 0.10 or more)
Moreover, the iron-based oxide layer 12 of the steel bar 1 according to this embodiment contains Cr. Cr improves the corrosion resistance of the iron-based oxide layer 12, prevents corrosion of the matrix 11 covered with the iron-based oxide layer 12, and suppresses the occurrence of red rust. Like Fe and Si, Cr contained in the iron-based oxide layer 12 also originates from Cr contained in the matrix 11.

In the iron-based oxide layer 12 of the steel bar 1 according to this embodiment, the ratio of the Cr concentration and the Si concentration is defined. Specifically, the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is 0.10 or more. The Cr/Si mass concentration ratio of the iron-based oxide layer 12 is an index of Cr concentration in the iron-based oxide layer 12. By setting the Cr/Si mass concentration ratio of the iron-based oxide layer 12 to 0.10 or more, the corrosion resistance of the steel bar 1 can be ensured. The Cr/Si mass concentration ratio of the iron-based oxide layer 12 may be 0.50 or more, 1.00 or more, or 5.00 or more.

The higher the Cr/Si mass concentration ratio of the iron-based oxide layer 12, the more preferable it is. Therefore, the upper limit of the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is not particularly limited. According to the investigation results of the present inventors, the Cr/Si mass concentration ratio in the iron-based oxide layer did not necessarily match the Cr/Si mass concentration ratio of the parent phase. Considering the chemical components of the matrix described above, the Cr/Si mass concentration ratio of the matrix is at most 3.92, while the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is greater than 24. There is also. However, considering the upper limit of Cr in the matrix, the Cr/Si mass concentration ratio of the iron-based oxide layer 12 may be set to 40 or less, 35 or less, or 30 or less.

The Cr/Si mass concentration ratio of the iron-based oxide layer is determined by EPMA component analysis according to the following procedure.

First, a steel bar is embedded in cross-sectional observation resin 5 and cut. The cut plane is preferably perpendicular to the longitudinal direction of the steel bar 1. However, when the iron-based oxide layer 12 is thin, in order to increase the apparent thickness of the iron-based oxide layer 12 at the cut surface, the angle between the cut surface and the longitudinal direction of the steel bar 1 is set to 90 degrees. It may be less than Next, the cut surface is polished. Then, the cut surface is observed using EPMA to identify the position of the iron-based oxide layer 12.

Next, the Si concentration, Fe concentration, Cr concentration, and Mn concentration of the iron-based oxide layer 12 are subjected to line analysis. The procedure for line analysis will be described with reference to FIGS. 3, 4, 5A, and 5B. FIG. 3 is a perspective view of the steel bar 1. The region 1C shown in FIG. 3 includes the surface of the steel bar 1 and its vicinity in the cross section of the steel bar 1. This area 1C is observed under magnification to set an area for line analysis. FIG. 4 is an enlarged view of region 1C. Line analysis is performed in each region 2 shown in FIG. 5A and 5B are enlarged views of region 2. FIG. 5A is an enlarged view of a portion to which the scale layer 13 is not attached, and FIG. 5B is an enlarged view of a portion to which the scale layer 13 is attached. Reference numeral 3 shown in FIGS. 5A and 5B is a starting point of line analysis, and reference numeral 4 is a line analysis area. Reference numeral 5 indicates resin for cross-sectional observation.
The extending direction of the line analysis region is perpendicular to the interface between the iron-based oxide layer 12 and the matrix 11, as shown in FIGS. 5A and 5B. At the interface between the iron-based oxide layer 12 and the matrix 11, fine irregularities are formed that are visible by microscopic observation. Therefore, the direction perpendicular to the interface between the iron-based oxide layer 12 and the matrix 11 does not necessarily match the direction perpendicular to the surface of the steel bar 1, which is identified by observing the steel bar 1 with the naked eye.
Further, the line analysis region is made to include at least the iron-based oxide layer 12 within 10 μm from the interface between the matrix 11 and the iron-based oxide layer 12. By this line analysis, the position in the iron-based oxide layer 12 within 10 μm from the interface between the matrix 11 and the iron-based oxide layer 12 where the Si concentration is maximum is specified. The Si concentration and Cr concentration at this position are measured, and the value obtained by dividing the Cr concentration by the Si concentration is calculated. Line analysis conditions for the iron-based oxide layer 12 are as follows.
・Measurement magnification 1000 times ・Acceleration voltage 15.0kV
・Irradiation current 4.993×10 ^-8 A
・Measurement interval 0.20μm
・Number of measurement points: 500 points (line analysis length 100 μm)

Measurement of the Cr/Si mass concentration ratio by the above-mentioned means is carried out at five locations. As shown in FIG. 4, the interval between regions 2 where line analysis is performed is 50 μm. The average value of the Cr/Si mass concentration ratio specified in each region 2 is regarded as the Cr/Si mass concentration ratio of the iron-based oxide layer 12 of the steel bar 1.

The configuration of the iron-based oxide layer 12 other than the Cr/Si mass concentration ratio, such as the thickness, is not particularly limited. The iron-based oxide layer 12 has many irregularities and its thickness is non-uniform. Although FIG. 1 shows the iron-based oxide layer 12 having a uniform thickness, the irregularities are omitted here for convenience of explanation. It is extremely difficult to quantitatively evaluate the thickness of the iron-based oxide layer 12. Further, when the present inventors observed cross sections of various iron-based oxide layers 12, it was qualitatively confirmed that the thickness of the iron-based oxide layer 12 tends to be proportional to the Si content of the matrix 11. In addition, if the iron-based oxide layer 12 is extremely thin, Cr concentrated in the iron-based oxide layer 12 will not be detected, and the measured value of the Cr/Si mass concentration ratio will not exceed 0.10. In other words, the iron-based oxide layer 12 whose measured value of the Cr/Si mass concentration ratio is 0.10 or more naturally has a sufficient thickness to ensure the corrosion resistance of the steel bar 1. As long as the iron-based oxide layer 12 is not removed and the Si content of the matrix 11 is within the above range, the steel bar 1 will have a sufficient thickness of the iron-based oxide layer 12 to ensure corrosion resistance. necessarily be prepared.
The components of the iron-based oxide layer 12 are also not particularly limited. The components of the iron-based oxide layer 12 usually have values that correspond to the components of the matrix 11. This is because the iron-based oxide layer 12 is an oxide layer generated by oxidizing the surface of the matrix 11. Therefore, as long as the components of the matrix 11 are within the above-mentioned range, the components of the iron-based oxide layer 12 will naturally be mainly composed of Fe and Si.

(Scale layer 13)
The steel bar 1 may further include a scale layer 13 covering the iron-based oxide layer 12. Since the scale layer 13 suppresses corrosion of the matrix 11, the corrosion resistance of the steel bar 1 is further improved. However, the scale layer 13 easily falls off from the steel bar 1. Furthermore, as shown in FIG. 1, the iron-based oxide layer 12 remains even at the locations where the scale layer 13 has fallen off. In the steel bar 1 according to the present embodiment, the corrosion resistance of the iron-based oxide layer 12 is enhanced, so that corrosion of the matrix 11 is suppressed even at locations where the scale layer 13 has fallen off. Therefore, the steel bar 1 does not need to have the scale layer 13.

(Other configurations of steel bars)
Other configurations of the steel bar according to this embodiment are not particularly limited. For example, the cross-sectional shape of the steel bar is not particularly limited. Examples of the cross-sectional shape of the steel bar include a circle, square, rectangle, hexagon, or octagon. The size of the steel bar is also not particularly limited. For example, the diameter or equivalent circle diameter of the steel bar may be 18 mm or more, 20 mm or more, or 30 mm or more. The diameter or equivalent circle diameter of the steel bar may be 180 mm or less, 150 mm or less, or 120 mm or less. The steel bar may be a bar-in-coil, that is, a steel material obtained by winding a long hot-rolled steel bar into a coil shape.

(Method for producing steel bars)
Next, a method for manufacturing a steel bar according to another aspect of the present invention will be described. According to the manufacturing method described below, a steel bar having a Cr-enriched iron-based oxide layer can be obtained. However, it should be noted that a steel bar that satisfies the above-mentioned requirements is considered to be a steel bar according to the present embodiment, regardless of its manufacturing method. That is, the scope of the steel bar according to the present embodiment described above is not limited by the method of manufacturing the steel bar described below.

As illustrated in FIG. 2, the method for manufacturing a steel bar according to the present embodiment includes (S1) a step of first descaling a steel piece;
(S2) Rough rolling a steel billet;
(S3) a step of performing second descaling on the steel billet;
(S4) Finish rolling a steel billet to obtain a steel bar;
(S5) A step of cooling the steel bar;
and a steel piece, in unit mass %, C: 0.10 to 0.30%, Si: 0.51 to 2.40%, Mn: 0.50 to 1.50%, P: 0.050 % or less, S: 0.050% or less, Cr: 0.05 to 2.00%, Al: 0.010 to 0.100%, N: 0.003 to 0.030%, and O: 0.0050 % or less, with the balance consisting of iron and impurities, the number of rough rolling passes is 5 or more, the rolling reduction ratio in all passes of rough rolling is 10% or more and 30% or less, and all passes of rough rolling are The rolling temperature is set to 950°C or higher and 1100°C or lower, and the average cooling rate in the range from the finish rolling end temperature to 500°C is set to 1.0°C/second or lower.

First, the technical concept of the manufacturing conditions of the steel bar according to this embodiment will be explained. In order to form an iron-based oxide layer sufficiently enriched with Cr, a manufacturing method that satisfies the following two requirements must be applied to the steel piece.
(A) The steel billet immediately before finish rolling is sufficiently descaled.
(B) After finish rolling, the steel bar is slowly cooled in a temperature range where an iron-based oxide layer is formed.

(A) If the steel billet immediately before finish rolling is not sufficiently descaled, the Cr/Si mass concentration ratio of the iron-based oxide layer of the steel bar will not be sufficiently increased. This is presumed to be because the oxide layer existing on the surface of the steel slab before finish rolling prevents Cr from concentrating when an iron-based oxide layer is formed after finish rolling.

Note that the important conditions for sufficiently descaling a steel billet immediately before finish rolling are not the descaling conditions but the rough rolling conditions before finish rolling. The temperature and rolling reduction during rough rolling affect the density of scale formed on the surface of the steel billet after rough rolling. If the oxide layer formed after rough rolling is dense, the oxide layer cannot be sufficiently removed even by descaling after rough rolling (second descaling).

(B) If the cooling rate after finish rolling is too high, the Cr/Si mass concentration ratio of the iron-based oxide layer of the steel bar cannot be sufficiently increased. This is because the growth of the iron-based oxide layer stops before Cr is sufficiently concentrated in the iron-based oxide layer.

Next, specific details of the manufacturing conditions of the steel bar according to this embodiment will be explained in detail.

(S1 first descaling)
In the method for manufacturing a steel bar according to this embodiment, first, a steel slab is heated, and then the steel slab is descaled. Heating of the steel billet is performed to bring the steel billet to a temperature suitable for rough rolling. Descaling refers to removing an oxide layer (for example, scale, etc.) on the surface of a steel piece caused by heating by jetting high-pressure water. To distinguish from the descaling of the steel billet after rough rolling, the descaling of the steel billet before rough rolling is referred to as first descaling.

The chemical composition of the steel slab is the same as the chemical composition of the matrix of the steel bar. Therefore, the chemical composition of the steel slab is within the range of the chemical composition of the matrix mentioned above. The method of manufacturing the steel piece is not particularly limited. A steel billet manufactured by normal continuous casting can be used in the method for manufacturing a steel bar according to this embodiment.
In the first descaling, the surface pressure of the water injected onto the steel billet is within the range of 5 to 30 MPa. If the surface pressure is too high, the temperature of the steel material will drop excessively. In this case, a large number of flaws occur on the surface of the steel material during rough rolling performed subsequent to the first descaling. As a result, the quality of the steel bar deteriorates or it becomes impossible to manufacture the steel bar. On the other hand, if the surface pressure is too low, removal of coarse scale generated during heating before descaling will be insufficient.

(S2 rough rolling)
The descaled steel billet is hot rolled. Hot rolling includes rough rolling and finish rolling. Rough rolling is rolling for bringing the shape of a steel piece closer to the intended steel bar shape. Finish rolling is rolling for making the shape of a roughly rolled steel piece match the shape of a target steel bar. A typical hot rolling facility is separately equipped with a rolling facility for rough rolling and a rolling facility for finish rolling.

In rough rolling, the number of passes must be 5 or more. Further, in all passes, the rolling reduction ratio must be 10% or more and 30% or less, and the rolling temperature must be 950°C or more and 1100°C or less. If these requirements are not met, a dense oxide layer will form on the surface of the rough-rolled steel billet that cannot be completely removed by descaling, and this oxide layer will cause Cr concentration to fayalite. This is thought to hinder the development of In the manufacturing method according to the present embodiment, the formation of this dense oxide layer is suppressed and the Cr concentration of the iron-based oxide layer is increased.

(S3 second descaling)
After rough rolling, the billet is again descaled. To distinguish from the descaling of the steel billet before rough rolling, the descaling of the steel billet after rough rolling is referred to as second descaling.
In the second descaling, the surface pressure of the water injected onto the steel billet is within the range of 2.0 to 5.0 MPa. If the surface pressure is too high, the temperature of the billet will drop excessively. As a result, there is insufficient time for Cr to concentrate in the iron-based carbide, and the Cr concentration in the iron-based carbide becomes insufficient. On the other hand, if the surface pressure is too low, the desired iron-based carbide cannot be obtained.
Note that even if the surface pressure and injection time in the second descaling are appropriate, if the rough rolling conditions are inappropriate, the oxide layer that adversely affects the iron-based carbide will not be sufficiently removed.

(S4 finish rolling)
After the second descaling, the billet is finish rolled. The rolling reduction rate and finish rolling temperature in finish rolling are not particularly limited, and can be appropriately set depending on the chemical composition of the steel billet and the shape of the steel bar. A steel bar having a predetermined shape is obtained by finish rolling. For example, the finishing temperature of finish rolling may be within the range of 850 to 1000°C.

(S5 cooling)
After finishing rolling, the bar is cooled. Here, the average cooling rate in the range from the finish rolling end temperature to 500°C is 1.0°C/second or less. The average cooling rate in the range from the finish rolling end temperature to 500°C is the time required for the surface temperature of the steel bar to decrease from the finish rolling end temperature to 500°C, which is the difference between the finish rolling end temperature and 500°C. This is the divided value.

In a temperature range from the finish rolling end temperature to 500° C., an iron-based oxide layer is formed on the surface of the steel bar. By slowly cooling the steel bar at an average cooling rate of 1.0°C/sec or less in this temperature range, Cr contained in the matrix of the steel bar migrates to the iron-based oxide layer, and Cr in the iron-based oxide layer /Si mass concentration ratio is increased.

EXAMPLES The effects of one embodiment of the present invention will be explained in more detail with reference to Examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example condition. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

Steel pieces having chemical components shown in Tables 1A and 1B were prepared. Steel bars were manufactured by applying any of the manufacturing methods of Patterns 1 to 5 below to these steel pieces.

(Pattern 1)
The billet was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was 1020°C, and the finish rolling temperature was 980°C. In the rough rolling, rolling with a rolling reduction of 10% or more and 30% or less was performed for 5 or more passes. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.

(Pattern 2)
The billet was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was 1080°C, and the finish rolling temperature was 870°C. In the rough rolling, the rolling reduction per pass was 5%. Instead, the number of passes was increased to 20% compared to pattern 1. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.

(Pattern 3)
The billet was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was 1020°C, and the finish rolling temperature was 1000°C. In the rough rolling, the reduction rate was set at 30 to 40% per pass. Instead, the number of passes was reduced to 3 compared to pattern 1. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.

(Pattern 4)
The billets were first descaled, rough rolled, finish rolled, and cooled. The rough rolling temperature was 1020°C, and the finish rolling temperature was 1000°C. In the rough rolling, the reduction rate was 20% per pass, and the number of passes was 5. On the other hand, the second descaling after rough rolling was omitted. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.

(Pattern 5)
The billet was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was 1020°C, and the finish rolling temperature was 980°C. In the rough rolling, rolling with a reduction ratio of 10% or more was performed for 5 or more passes. However, after finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was set to 10°C/sec or more.
(Pattern 6)
The steel billet was subjected to rough rolling, second descaling, finish rolling, and cooling without performing the first descaling. The rough rolling temperature was 1020°C, and the finish rolling temperature was 980°C. In the rough rolling, rolling with a reduction ratio of 10% or more was performed for 5 or more passes. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.
(Pattern 7)
The steel billet was subjected to first descaling and rough rolling, and finish rolling and cooling without performing second descaling. The rough rolling temperature was 1020°C, and the finish rolling temperature was 980°C. In the rough rolling, rolling with a reduction ratio of 10% or more was performed for 5 or more passes. After finish rolling, the average cooling rate in the range from finish rolling end temperature to 500°C was 1.0°C/sec or less.

The Cr/Si mass concentration ratio of the iron-based oxide layer of the steel bar obtained by the above procedure was determined by the above procedure, and is listed in Table 2. If the Cr/Si mass concentration ratio of the iron-based oxide layer cannot be measured because the iron-based oxide layer is not formed or the iron-based oxide layer is very thin, the symbol " -” was written.
Further, for reference, the thickness of the iron-based oxide layer of a steel bar with a Cr/Si mass concentration ratio of 0.10 or more was measured. In the line analysis for measuring the Cr/Si mass concentration ratio, the region where a Si concentration more than twice that of the parent phase was detected was estimated to be an iron-based oxide layer, and the thickness of the region was measured. . In steel bars in which the Cr/Si mass concentration ratio was 0.10 or more, the thickness of this region was generally within the range of 0.4 μm to 5.0 μm.

In addition, the rust resistance of the steel bar obtained by the above procedure was evaluated. The test for evaluating rust resistance was a wet test described in JIS K 2246:2018. The conditions for the wet test were as follows.
・Test temperature: 50℃
・Relative humidity: 95%
・Test time: 240 hours ・Test piece shape: Cylindrical shape with a diameter of 20 to 80 mm and a length of 50 mm

The pass/fail of rust resistance was determined based on the state of rust on the surface of the test piece. Samples with a rust area ratio of 10% or less were judged to have excellent rust resistance and were evaluated as "GOOD" in Table 2. The rust resistance evaluation results are shown in Table 2.

Inventive Examples 1 to 14, in which the chemical composition of the slab and manufacturing conditions were appropriate, had excellent corrosion resistance. In the iron-based oxide layers of these invention examples, the Cr/Si mass concentration ratio was increased.

In Comparative Example 15, the slab lacked Cr. It is therefore presumed that in Comparative Example 15, Cr was not sufficiently supplied to the iron-based oxide layer during the cooling period after finish rolling. In Comparative Example 15, the Cr/Si mass concentration ratio of the iron-based oxide layer was insufficient. In Comparative Example 15, corrosion resistance could not be ensured.

In Comparative Example 16, the slab lacked Si. Therefore, in Comparative Example 16, no iron-based oxide layer was formed. Therefore, in Comparative Example 16, corrosion resistance could not be ensured.

In Comparative Example 17, while the chemical composition of the slab was appropriate, the rolling reduction in rough rolling was insufficient. As a result, it is presumed that in Comparative Example 17, the oxide phase became dense before finish rolling and was not completely removed by the second descaling. In Comparative Example 17, the iron-based oxide layer was not appropriately formed on the surface of the matrix. In Comparative Example 17, corrosion resistance could not be ensured.

In Comparative Example 18, while the chemical composition of the slab was appropriate, the rolling reduction in rough rolling was excessive. It is therefore presumed that in Comparative Example 18, the oxide phase bit into the parent phase before finish rolling and was not completely removed by the second descaling. In Comparative Example 18, a large amount of scale was involved in the matrix, and an iron-based oxide layer was not properly formed on the surface of the matrix. In Comparative Example 18, corrosion resistance could not be ensured.

In Comparative Example 19, while the chemical composition of the slab was appropriate, the second descaling was not performed. As a result, in Comparative Example 19, a large amount of scale was attached to the surface of the slab before finish rolling. In Comparative Example 19, the iron-based oxide layer was not properly formed on the surface of the matrix. In Comparative Example 19, corrosion resistance could not be ensured.

In Comparative Example 20, while the chemical composition of the slab was appropriate, the cooling rate before finish rolling was excessive. Therefore, in Comparative Example 20, it is estimated that the formation reaction of the iron-based oxide layer (that is, oxidation of the surface of the steel bar) was completed before Cr was sufficiently supplied to the iron-based oxide layer. In Comparative Example 20, the Cr/Si mass concentration ratio of the iron-based oxide layer was insufficient. In Comparative Example 20, corrosion resistance could not be ensured.
In Comparative Example 21, while the chemical composition of the slab was appropriate, the first descaling after heating was not performed. As a result, in Comparative Example 21, the scale generated during heating could not be removed, and the desired iron-based oxide layer could not be formed during subsequent cooling after rolling. In Comparative Example 21, corrosion resistance could not be ensured.
In Comparative Example 22, while the chemical composition of the slab was appropriate, the second descaling before finish rolling was not performed. As a result, in Comparative Example 22, the scale generated during rolling could not be removed, and the desired iron-based oxide layer could not be formed during subsequent cooling after rolling. In Comparative Example 22, corrosion resistance could not be ensured.

1 Steel bar 11 Mother phase 12 Iron-based oxide layer 13 Scale layer 3 Starting point of line analysis 4 Line analysis area 5 Resin for cross-sectional observation

Claims (6)

Mother phase and
a layer containing Si, Cr, Fe, and O that covers the surface of the matrix;
Equipped with
The parent phase is in unit mass %,
C: 0.10-0.30%,
Si: 0.51 to 2.40%,
Mn: 0.50 to 1.50%,
P: 0.050% or less,
S: 0.050% or less,
Cr: 0.05-2.00%,
Al: 0.010-0.100%,
Contains N: 0.003 to 0.030%, and O: 0.0050% or less, with the remainder consisting of iron and impurities,
A steel bar in which the layer has a Cr/Si mass concentration ratio of 0.10 or more. The parent phase further comprises, in unit mass %,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
W: 0.500% or less,
V: 0.50% or less,
Bi: 0.10% or less,
Co: 0.500% or less,
Nb: 0.10% or less,
Ti: 0.20% or less,
Ca: 0.0015% or less,
Pb: 0.09% or less,
Sn: 0.10% or less, and B: 0.007% or less,
The steel bar according to claim 1, characterized in that it contains one or more of the following. The parent phase is in unit mass %,
Bi: 0.10% or less,
Pb: 0.09% or less,
The steel bar according to claim 2, containing one or more of Sn: 0.10% or less and Ca: 0.0015% or less. The parent phase is in unit mass %,
Mo: 0.50% or less,
W: 0.500% or less,
B: 0.007% or less,
Cu: 0.50% or less,
Ni: 0.50% or less, Co: 0.500% or less,
The steel bar according to claim 2, characterized in that it contains one or more of the following. The parent phase is in unit mass %,
Ti: 0.20% or less,
Nb: 0.10% or less, and V: 0.50% or less,
The steel bar according to claim 2, characterized in that it contains one or more of the following. The steel bar according to any one of claims 1 to 5, further comprising a scale layer covering the layer.

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