JP7768362B2 - Hot-rolled steel and its manufacturing method - Google Patents

Description

The present invention relates to hot-rolled steel and a method for manufacturing the same.

Parts used in automobiles and other applications are made by first applying preliminary processing such as wire drawing to the hot-rolled steel material, then hot or cold working it to form the part shape, followed by cutting or heat treatment as needed to create the final product. In recent years, the use of cold working (e.g., cold forging) to manufacture parts has become widespread, as it offers advantages in terms of manufacturing costs, such as dimensional accuracy and yield, as well as reducing the energy used to heat the material. Parts manufactured using cold working are either left as they are, or undergo heat treatment for strength adjustment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, etc.), depending on the required strength, before becoming the final product.

The surface of hot-rolled steel subjected to the above-mentioned cold working process contains a decarburized layer formed during hot rolling. If the decarburized layer of hot-rolled steel is excessively thick, a softened area with a low carbon content remains near the surface of the steel in parts after cold working or parts that have undergone cold working and induction hardening and tempering heat treatment, resulting in poor strength or fatigue properties as parts. Furthermore, if the decarburized layer of hot-rolled steel is excessively thick, in parts that have undergone cold working and quenching and tempering heat treatment, carbon diffuses from the core of the steel to the decarburized layer during heating and quenching, preventing the softened area from remaining in the surface of the steel. However, because the carbon content differs between the decarburized layer and the core, the austenite reverse transformation temperatures differ. In this case, the timing of austenite reverse transformation differs between the surface and core of the steel, resulting in uneven reverse-transformed austenite grain size between the surface and core of the steel. Such uneven grain size induces coarsening of the crystal grains, resulting in a deterioration of the fatigue properties and toughness of the parts.

As mentioned above, if the decarburized layer on the surface of hot-rolled steel is excessively thick, various properties of the final product will deteriorate, so it is necessary to prevent the decarburized layer from becoming excessively thick during the hot-rolling stage. Furthermore, if cracks occur during cold working, the steel cannot be used as a final product, so hot-rolled steel is also required to have excellent cold workability.

Patent document 1 describes how adding specific amounts of Te, Se, or S as trace elements and appropriately controlling the thermal history after hot rolling makes it possible to both soften hot-rolled steel and suppress decarburization reactions.

Japanese Patent Application Laid-Open No. 2004-250768

However, Patent Document 1 uses Te and Se, which are rare and toxic elements that require special care in handling, making it difficult to say that this technology is generally applicable. Furthermore, both Patent Document 1 and the technologies prior to it require precise control of the temperature history after hot rolling, which also poses challenges from the perspective of productivity. As such, no technology has been reported that combines suppression of the decarburized layer thickness with excellent cold workability.

In consideration of the above problems, the present invention aims to provide a hot-rolled steel material in which the thickness of the decarburized layer is sufficiently suppressed and which has excellent cold workability, and a manufacturing method thereof.

To solve the above-mentioned problems, the inventors focused on suppressing decarburization reactions by adding Cu and Ni, elements that are less easily oxidized than Fe, and discovered the following: When scale forms on the surface of steel due to heating during hot rolling, Fe is preferentially oxidized to form scale. Meanwhile, Cu and Ni are not oxidized and are left behind in the surface layer of the steel, forming concentrated regions where at least one of Cu and Ni is concentrated. By forming these concentrated regions, it is possible to effectively suppress decarburization reactions that occur in the surface layer of the steel.

However, it is well known that Cu concentrated in the surface layer of steel penetrates into grain boundaries during hot rolling, embrittling the grain boundaries and inducing cracking during cold working. Therefore, as a result of further intensive research, the inventors discovered that by optimizing the balance of Ni and Cu contents in hot-rolled steel, it is possible to limit the depth of the enriched region in which at least one of Cu and Ni is concentrated in the surface layer of the steel within a certain range, thereby suppressing the occurrence of cracking during cold working.

In addition, the inventors have discovered that in order to make the enriched regions of hot-rolled steel, where at least one of Cu and Ni is enriched, suitable for suppressing decarburization reactions, it is necessary to appropriately control the maximum heating temperature during hot rolling. They have also discovered that in order to effectively suppress decarburization reactions, it is necessary to appropriately control the residence time of the steel material in the heating furnace during hot rolling.

Furthermore, as a result of extensive research, the inventors have come to the following findings: In hot-rolled steel to which Sn is added in addition to Cu and Ni, Sn is simultaneously concentrated in enriched regions in the steel surface where at least one of Cu and Ni is enriched. Because Sn has the effect of lowering the melting point of these enriched regions, enriched regions are more likely to form in the steel surface compared to when Sn is not added, and the enriched regions are also more likely to suppress decarburization reactions.

On the other hand, if the Sn concentration in an enriched region where at least one of Cu and Ni is enriched is excessively high, the enriched region will penetrate deeply into the grain boundaries, embrittling them and causing cracking during cold working. Therefore, when manufacturing hot-rolled steel containing Sn in addition to Cu and Ni, it is necessary to limit the Sn concentration in the enriched region to a specified range to prevent excessive grain boundary penetration. Effective specific methods include adjusting the amount of Sn added depending on the amount of Cu and Ni, and shortening the residence time in the heating furnace during hot rolling depending on the amount of Sn.

In other words, the gist of the present invention is as follows:

[1] In mass%,
C: 0.03 to 0.80%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
Cu: 0.010-0.500%,
Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%
a base steel having a composition in which the ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], satisfies 0.10 or more and 3.00 or less, with the balance consisting of Fe and unavoidable impurities; and a decarburized layer formed on the surface of the base steel,
The decarburized layer has a concentrated region in which at least one of Cu and Ni is concentrated,
a coverage of the enriched region on the surface of the decarburized layer is 50% or more,
the maximum depth of the concentrated region in the decarburized layer is 1 μm or more and 150 μm or less,
The total decarburization depth (DM-T) of the decarburized layer as defined in JIS G 0558 is 0.80 mm or less,
The total area ratio of ferrite and pearlite in the base steel is 90.0% or more,
The hot-rolled steel material has an average Vickers hardness of 250 HV or less in the base steel.

[2] The component composition further comprises, in mass %,
Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less;
In the enriched region, Sn is enriched in addition to at least one of Cu and Ni,
The hot-rolled steel material according to the above [1], wherein the ratio [Sn]/([Cu]+[Ni]) of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region is 0.50 or less in atomic ratio.

[3] The component composition further comprises, in mass %,
Cr: 0.01-1.50%,
Mo: 0.01-0.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
V: 0.001 to 0.300%,
Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
The hot-rolled steel material according to the above [1] or [2], containing at least one element selected from the group consisting of:

[4] The component composition further comprises, in mass %,
P: 0.001-0.100%,
S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%
The hot-rolled steel material according to any one of [1] to [3] above, containing at least one element selected from the group consisting of:

[5] The component composition further comprises, in mass %,
Pb: 0.01 to 0.50%,
Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
The hot-rolled steel material according to any one of [1] to [4] above, containing at least one element selected from the group consisting of:

[6] In mass%,
C: 0.03 to 0.80%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
Cu: 0.010-0.500%,
Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%
and a ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], of 0.10 or more and 3.00 or less, with the balance being Fe and unavoidable impurities, under conditions where a maximum heating temperature T in a heating furnace is 1000°C or more and 1200°C or less, and the residence time of the steel material in the heating furnace is equal to or less than a time _t1 (minute) determined by the following formula (1), to obtain a hot-rolled steel material.
t ₁ =1150-0.8T-3([Ni]/[Cu])-10[Sn]...(1)
Here, T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass%), [Cu] is the amount of Cu in the steel material (mass%), and [Sn] is the amount of Sn in the steel material (mass%).

[7] The component composition further comprises, in mass %,
The method for producing a hot-rolled steel material according to the above [6], containing Sn: 0.001% or more ([Ni] + [Cu])/2 or less.

[8] The component composition further comprises, in mass %,
Cr: 0.01-1.50%,
Mo: 0.01-0.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
V: 0.001 to 0.300%,
Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
The method for producing a hot-rolled steel material according to the above [6] or [7], containing at least one element selected from the group consisting of:

[9] The component composition further comprises, in mass %,
P: 0.001-0.100%,
S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%
The method for producing a hot-rolled steel material according to any one of [6] to [8] above, containing at least one element selected from the group consisting of:

[10] The component composition further comprises, in mass%,
Pb: 0.01 to 0.50%,
Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
The method for producing a hot-rolled steel material according to any one of [6] to [9] above, containing at least one element selected from the group consisting of:

The present invention provides a hot-rolled steel material and a manufacturing method thereof in which the thickness of the decarburized layer is sufficiently suppressed and which has excellent cold workability.

1 is a schematic diagram showing a cross section of a hot-rolled steel material according to an embodiment of the present invention.

The following describes an embodiment of the hot-rolled steel material and its manufacturing method according to the present invention. Note that the embodiment described below is an example of how the present invention is embodied, and the specific example does not limit the configuration of the present invention.

(hot rolled steel)
FIG. 1 shows a cross section of a hot-rolled steel material according to one embodiment of the present invention. The hot-rolled steel material 100 includes a base steel 10 and a decarburized layer 20 formed on the surface of the base steel 10. The decarburized layer 20 may also include a scale 30 on the decarburized layer 20. The decarburized layer 20 is further characterized by having enriched regions 22 in which at least one of Cu and Ni is enriched, a coverage of the enriched regions 22 on the surface 24 of the decarburized layer 20 of 50% or more, a maximum depth A of the enriched regions 22 in the decarburized layer 20 of 1 μm or more and 150 μm or less, a total decarburization depth B (DM-T) of the decarburized layer 20 as defined in JIS G 0558 of 0.80 mm or less, a total area fraction of ferrite and pearlite in the base steel 10 of 90.0% or more, and an average Vickers hardness of the base steel 10 of 250 HV or less.

<Subway>
First, we will explain the base steel of a hot-rolled steel material according to one embodiment of the present invention. The base steel contains, by mass%, 0.03 to 0.80% C, 0.01 to 1.00% Si, 0.01 to 1.50% Mn, 0.010 to 0.500% Cu, 0.010 to 1.000% Ni, and 0.0020 to 0.0250% N, with the ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], being 0.10 to 3.00, and the balance being Fe and unavoidable impurities. In the following description of the composition, "%" representing the content means "% by mass" unless otherwise specified. Furthermore, [Ni] and [Cu] represent the amount of Ni and the amount of Cu contained in the base steel, respectively.

[C:0.03-0.80%]
C is an element added to ensure the strength of hot-rolled steel. If the C content of the base steel is less than 0.03%, the necessary strength cannot be ensured. Therefore, the C content of the base steel is set to 0.03% or more, and preferably 0.05% or more. On the other hand, if the C content of the base steel exceeds 0.80%, the hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the C content of the base steel is set to 0.80% or less, and preferably 0.65% or less, and more preferably 0.50% or less.

[Si: 0.01 to 1.00%]
Si is a deoxidizing element during refining and also an element that improves the strength and hardenability of hot-rolled steel. If the Si content of the base steel is less than 0.01%, the above effects cannot be obtained. Therefore, the Si content of the base steel is set to 0.01% or more. On the other hand, if the Si content of the base steel exceeds 1.00%, the hardenability becomes too high, resulting in a structure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the Si content of the base steel is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[Mn: 0.01 to 1.50%]
Mn is an element that improves the strength and hardenability of hot-rolled steel. If the Mn content of the base steel is less than 0.01%, the above effects cannot be obtained. Therefore, the Mn content of the base steel is set to 0.01% or more. On the other hand, if the Mn content of the base steel exceeds 1.50%, the hardenability becomes too high, resulting in a structure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the Mn content of the base steel is set to 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less.

[Cu:0.010-0.500%]
Cu is an element that is less oxidized than Fe and that forms an enriched region in the surface layer of the steel material due to scale formation during hot rolling, thereby suppressing the decarburization reaction. If the Cu content of the base steel is less than 0.010%, an enriched region is not sufficiently formed in the surface layer of the steel material, and the decarburization suppression effect is not sufficiently achieved. Therefore, the Cu content of the base steel is set to 0.010% or more. On the other hand, if the Cu content of the base steel exceeds 0.500%, the enriched region in the surface layer of the steel material becomes excessively deep, making cracks more likely to occur during cold working. Therefore, the Cu content of the base steel is set to 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less.

[Ni:0.010-1.000%]
Like Cu, Ni is an element that is less susceptible to oxidation than Fe. Ni is an element that concentrates in the steel surface layer as scale is generated during hot rolling, forming an enriched region and suppressing decarburization reactions. If the Ni content of the base steel is less than 0.010%, an enriched region is not sufficiently formed in the steel surface layer, and the decarburization suppression effect is not sufficiently achieved. Therefore, the Ni content of the base steel is set to 0.010% or more. On the other hand, if the Ni content of the base steel exceeds 1.000%, the enriched region in the steel surface layer becomes excessively deep, making cracks more likely to occur during cold working. Therefore, the Ni content of the base steel is set to 1.000% or less, preferably 0.800% or less, and more preferably 0.600% or less.

[N:0.0020-0.0250%]
N is an element that bonds with nitride-forming elements in the steel to form nitrides and acts as grain boundary pinning particles, thereby preventing grain coarsening. If the N content of the base steel is less than 0.0020%, the above effect cannot be obtained. Therefore, the N content of the base steel is set to 0.0020% or more. On the other hand, if the N content of the base steel exceeds 0.0250%, not only will blowholes form in the steel, but the solute N in the steel will undergo dynamic strain aging, making it more susceptible to cracking during cold working. Therefore, the N content of the base steel is set to 0.0250% or less, preferably 0.0200% or less, and more preferably 0.0180% or less.

[The ratio (mass ratio) of the amount of Ni to the amount of Cu, [Ni]/[Cu], is 0.10 or more and 3.00 or less]
In the present invention, since the decarburization reaction is suppressed by the enriched region in which at least one of Cu and Ni is enriched in the steel surface layer, it is not sufficient to focus on the Cu content and the Ni content independently; it is necessary to consider the balance of the contents of these elements. When the ratio (mass ratio) of the Ni content to the Cu content ([Ni]/[Cu]) is less than 0.10, i.e., when the Cu content is excessive relative to the Ni content, the Cu enriched in the steel surface layer penetrates into the grain boundaries, making the enriched region excessively deep and prone to cracking during cold working. Therefore, [Ni]/[Cu] is set to 0.10 or more, preferably 0.15 or more, and more preferably 0.20 or more. On the other hand, when [Ni]/[Cu] exceeds 3.00, i.e., when the Ni content is excessive relative to the Cu content, the enriched region in the steel surface layer also becomes excessively deep and prone to cracking during cold working. Therefore, the [Ni]/[Cu] ratio is set to 3.00 or less, preferably 2.50 or less, and more preferably 2.00 or less.

The base steel of the hot-rolled steel material according to one embodiment of the present invention may further contain the following elements as necessary.

[Sn: 0.001% or more ([Ni] + [Cu])/2 or less]
Since Sn has the effect of lowering the melting point of enriched regions where at least one of Cu and Ni is enriched, adding Sn makes it easier for enriched regions to form in the surface layer of the steel material, making it easier to obtain the decarburization suppression effect. When Sn is added to the base steel to obtain the above effect, the Sn content is set to 0.001% or more. On the other hand, if excessive Sn is added, the grain boundary penetration depth of the enriched regions increases, embrittling the grain boundaries and causing cold work cracks. Therefore, when Sn is added to the base steel, the Sn content is set to ([Ni] + [Cu])/2 or less, and preferably ([Ni] + [Cu])/3 or less.

[Cr: 0.01 to 1.50%]
Cr is an element that improves the hardenability of steel. If the Cr content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, when Cr is added to the base steel, the Cr content should be 0.01% or more. On the other hand, if the Cr content of the base steel exceeds 1.50%, the effect of the addition saturates, and the hardenability of the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Cr is added to the base steel, the Cr content should be 1.50% or less, preferably 1.30% or less, and more preferably 1.15% or less.

[Mo:0.01~0.50%]
Mo is an element that significantly improves the hardenability of steel with a small amount of addition. If the Mo content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, when Mo is added to the base steel, the Mo content should be 0.01% or more. On the other hand, if the Mo content of the base steel exceeds 0.50%, the effect of the addition saturates, and the hardenability of the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Mo is added to the base steel, the Mo content should be 0.50% or less, and preferably 0.30% or less.

[Al: 0.001-0.100%]
Al is a deoxidizing element and also bonds with N in the steel to form nitrides, contributing to grain refinement. If the Al content of the base steel is less than 0.001%, the above-mentioned effects cannot be obtained. Therefore, when Al is added to the base steel, the Al content should be 0.001% or more. On the other hand, if the Al content of the base steel exceeds 0.100%, the amount of Al oxide in the steel increases, making the steel more susceptible to cracking during cold working and also deteriorating the fatigue fracture properties of the component. Therefore, when Al is added to the base steel, the Al content should be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[Ti: 0.001 to 0.100%]
Like Al, Ti is an element that combines with N in steel to form nitrides and contributes to grain refinement. If the Ti content of the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Ti is added to the base steel, the Ti content should be 0.001% or more. On the other hand, if the Ti content of the base steel exceeds 0.100%, the amount of Ti-based precipitates in the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Ti is added to the base steel, the Ti content should be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[V:0.001-0.300%]
Like Al and Ti, V is an element that combines with N in steel to form nitrides and contributes to grain refinement. Furthermore, V contributes to increasing the strength of the steel. If the V content of the base steel is less than 0.001%, the above-mentioned effects cannot be obtained. Therefore, when V is added to the base steel, the V content should be 0.001% or more. On the other hand, if the V content of the base steel exceeds 0.300%, the amount of V-based precipitates in the steel becomes excessive, increasing the hardness of the steel, reducing cold workability, and also reducing the toughness of the steel. Therefore, when V is added to the base steel, the V content should be 0.300% or less, preferably 0.200% or less, and more preferably 0.150% or less.

[Nb: 0.001 to 0.100%]
Nb is an element that combines with carbon in steel to form carbides and contributes to grain refinement. If the Nb content of the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Nb is added to the base steel, the Nb content should be 0.001% or more. On the other hand, if the Nb content of the base steel exceeds 0.100%, the amount of Nb-based carbides in the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Nb is added to the base steel, the Nb content should be 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[B:0.0005-0.0050%]
B is an element that greatly improves the hardenability of steel with a small amount of addition. If the B content of the base steel is less than 0.0005%, the above effect cannot be obtained. Therefore, when B is added to the base steel, the B content should be 0.0005% or more. On the other hand, if the B content of the base steel exceeds 0.0050%, the effect of improving hardenability saturates. Therefore, when B is added to the base steel, the B content should be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[P:0.001-0.100%]
P is an element that increases the strength of steel. If the P content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when P is added to the base steel, the P content should be 0.001% or more. On the other hand, if the P content of the base steel exceeds 0.100%, P segregates at grain boundaries and reduces the toughness of the steel. Therefore, when P is added to the base steel, the P content should be 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[S:0.001-0.100%]
S is an element that combines with Mn in steel to form MnS inclusions, improving the machinability of the steel. If the S content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when S is added to the base steel, the S content should be 0.001% or more. On the other hand, if the S content of the base steel exceeds 0.100%, the large amount of MnS inclusions present acts as crack initiation sites during cold working, making cracks more likely to occur during cold working. Therefore, when S is added to the base steel, the S content should be 0.100% or less, preferably 0.070% or less, and more preferably 0.050% or less.

[Sb: 0.0010-0.0300%]
Like Sn, Sb is an element that easily segregates in the surface layer of a steel material, and has the effect of suppressing decarburization reactions, similar to the enriched regions where at least one of Cu and Ni is enriched. If the Sb content of the base steel is less than 0.0010%, the above-mentioned effect cannot be obtained. Therefore, when Sb is added to the base steel, the Sb content is set to 0.0010% or more. On the other hand, if the Sb content of the base steel exceeds 0.0300%, the amount of Sb segregating in the surface layer becomes excessive, deteriorating the surface properties of the steel material. Therefore, when Sb is added to the base steel, the Sb content is set to 0.0300% or less, preferably 0.0200% or less, and more preferably 0.0150% or less.

[Pb: 0.01 to 0.50%]
Pb is an element that improves the machinability of steel. If the Pb content of the base steel is less than 0.01%, the above-mentioned effect cannot be obtained. Therefore, when Pb is added to the base steel, the Pb content should be 0.01% or more. On the other hand, if the Pb content of the base steel exceeds 0.50%, the effect of improving machinability saturates, and the amount of inclusions in the steel increases, resulting in a decrease in toughness. Therefore, when Pb is added to the base steel, the Pb content should be 0.50% or less, and preferably 0.35% or less.

[Bi:0.001-0.100%]
Like Pb, Bi is an element that improves the machinability of steel. If the Bi content in the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Bi is added to the base steel, the Bi content should be 0.001% or more. On the other hand, if the Bi content in the base steel exceeds 0.100%, the machinability improvement effect saturates, and the amount of inclusions in the steel increases, resulting in a decrease in toughness. Therefore, when Bi is added to the base steel, the Bi content should be 0.100% or less, and preferably 0.050% or less.

[Ca: 0.0005-0.1000%]
Ca is an element that dissolves in sulfides in steel to spheroidize the sulfides, thereby suppressing deterioration of the cold workability and toughness of the steel. If the Ca content of the base steel is less than 0.0005%, the above effect cannot be obtained. Therefore, when Ca is added to the base steel, the Ca content should be 0.0005% or more. On the other hand, if the Ca content of the base steel exceeds 0.1000%, the amount of Ca-based inclusions in the steel increases, adversely affecting the toughness and fatigue properties of the steel. Therefore, when Ca is added to the base steel, the Ca content should be 0.1000% or less, preferably 0.0500% or less, and more preferably 0.0300% or less.

[Total area ratio of ferrite and pearlite is 90.0% or more]
To ensure cold workability, the structure of the base steel needs to be a mixed structure of relatively soft ferrite and pearlite. Therefore, the total area ratio of ferrite and pearlite in the base steel is set to 90.0% or more, and preferably 95.0% or more. On the other hand, there is no particular upper limit to the total area ratio of ferrite and pearlite in the base steel, and the total area ratio may be 100.0%.

The total area ratio of ferrite and pearlite in the present invention can be determined by the following procedure. Three samples containing the steel surface layer are prepared by cutting the steel material, and the structure of each sample is observed on a cross section perpendicular to the rolling direction. The microstructure of the region of the sample's surface structure, excluding the decarburized layer described below, is observed using an optical microscope at a magnification of 100x. The area of one field of view is set to 600 μm x 800 μm, and the total area ratio of ferrite and pearlite in the observed area of each of five randomly selected fields of view is calculated. The average value of the total area ratios obtained in each field of view for each sample is calculated, and this is used as the total area ratio of ferrite and pearlite for that steel material. The image analysis software ImageJ can be used to calculate the area ratio.

[Average Vickers hardness is 250 HV or less]
If the average Vickers hardness of the base steel exceeds 250 HV, cold workability deteriorates. Therefore, the average Vickers hardness of the base steel is set to 250 HV or less, preferably 230 HV or less, and more preferably 220 HV or less. On the other hand, from a practical standpoint, the average Vickers hardness of the base steel is preferably 80 HV or more.

The average Vickers hardness in this invention can be determined by the following procedure. Three samples are prepared by cutting the steel material, and a Vickers hardness tester is used to measure the hardness of the cross section perpendicular to the rolling direction of the sample. If the sample is round steel, the measurement points are five: one point at a position equivalent to 1/2 of the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D/2 position"), and four points at positions equivalent to 1/4 of the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D/4 positions"). Furthermore, when the sample is a plate, the measurement points are a total of five points: one point at a position corresponding to 1/2 of the plate width from one end toward the center and corresponding to 1/2 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W/2-t/2 position"), and four points at positions corresponding to 1/4 of the plate width from one end toward the center and corresponding to 1/4 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W/4-t/4 positions"). The average value of the hardness obtained at each measurement point on each sample is calculated and used as the average Vickers hardness of the steel. Note that hardness measurements can be performed under a load of 10 kgf.

<Decarburized layer>
Next, the decarburized layer of the hot-rolled steel material will be described.

[The total decarburization depth (DM-T) of the decarburized layer as defined in JIS G 0558 is 0.80 mm or less]
If the decarburized layer is excessively thick, it will lead to a decrease in the fatigue properties and toughness of the component, so it is necessary to prevent the decarburized layer from becoming excessively thick. In the present invention, the thickness of the decarburized layer is evaluated using the total decarburization depth (DM-T) (hereinafter simply referred to as "total decarburization depth") specified in JIS G 0558. If the total decarburization depth of the decarburized layer exceeds 0.80 mm, the deterioration of properties becomes significant. Therefore, the total decarburization depth of the decarburized layer is set to 0.80 mm or less, preferably 0.50 mm or less, and more preferably 0.30 mm or less. Meanwhile, there is no particular lower limit to the total decarburization depth of the decarburized layer.

The total decarburization depth of the decarburized layer in this invention can be determined by the following procedure. Three samples containing the steel surface layer are prepared by cutting the steel material, and a structural observation is performed on the cross section perpendicular to the rolling direction of the sample. The surface structure of the sample is observed using an optical microscope at 100x magnification, with an area of one field of view being 600 μm x 800 μm. The total decarburization depth of the surface layer of the sample is measured in three randomly selected fields of view using the method described in JIS G 0558, and the total decarburization depth at the deepest point of the decarburized layer is defined as the total decarburization depth in that field of view. The average value of the total decarburization depth obtained in each field of view for each sample is calculated and defined as the total decarburization depth of the decarburized layer of that steel material.

[The decarburized layer contains a concentrated region where at least one of Cu and Ni is concentrated]
In one embodiment of the present invention, a decarburized layer of a hot-rolled steel material has an enriched region where at least one of Cu and Ni is enriched. Furthermore, in the enriched region, Sn is preferably enriched in addition to at least one of Cu and Ni. In the present invention, the enriched region is defined as a region in the surface layer of the hot-rolled steel material where the concentration of at least one of Cu and Ni is three times or more the reference concentration. The reference concentration refers to the concentrations of Cu and Ni at the D/4 position of the hot-rolled steel material when the hot-rolled steel material is a round steel material, or at the W/4-t/4 position of the hot-rolled steel material when the hot-rolled steel material is a plate material.

[Coverage rate of concentrated area on the surface of the decarburized layer is 50% or more]
The formation of an enriched region in which at least one of Cu and Ni is enriched on the surface of a hot-rolled steel material contributes to the suppression of decarburization reactions. If the coverage of the enriched region on the surface of the decarburized layer is less than 50%, the decarburization suppression effect becomes insufficient. Therefore, the coverage of the enriched region on the surface of the decarburized layer is set to 50% or more, preferably 60% or more, and more preferably 70% or more. On the other hand, there is no particular upper limit to the coverage, and the coverage of the enriched region may be 100%.

The coverage of the enriched area on the surface of the decarburized layer can be determined using the following procedure. First, the steel is cut to prepare three samples containing the steel surface layer, and the reference concentrations of Cu and Ni are measured for each sample. To eliminate the effects of enriched areas and central segregation in the steel surface layer, three randomly selected points at the above-mentioned positions (D/4 position on hot-rolled steel for round steel, W/4-t/4 position on hot-rolled steel for plate steel) are analyzed using an EPMA (electron probe microanalyzer), and the average values of the Cu and Ni concentrations at these three points are taken as the reference concentrations of Cu and Ni, respectively. The EPMA measurement conditions can be a magnification of 100x and an area per field of view of 600 μm x 800 μm.

Next, quantitative analysis (mapping) is performed using EPMA on three randomly selected fields on the steel surface of each sample to determine the concentrations of Cu and Ni. EPMA measurement conditions can be a magnification of 100x and an area of 600 μm x 800 μm per field. Regions where at least one of Cu and Ni is present at a concentration three times or more the reference concentrations of Cu and Ni determined above are considered to be enriched regions, and the coverage of the enriched regions in each field is determined. The average of the coverage obtained in each field of view for each sample is calculated and used as the coverage of the enriched regions for that steel.

[Maximum depth of concentrated region in decarburized layer is 1 μm or more and 150 μm or less]
If the maximum depth of the enriched region in the decarburized layer is less than 1 μm, the effect of suppressing the decarburization reaction cannot be obtained. Therefore, the maximum depth of the enriched region in the decarburized layer is set to 1 μm or more. On the other hand, if the maximum depth of the enriched region in the decarburized layer exceeds 150 μm, although this is preferable from the viewpoint of suppressing the decarburization reaction, the enriched region is too deep and is therefore prone to cracking during cold working. Therefore, the maximum depth of the enriched region in the decarburized layer is set to 150 μm or less, preferably 120 μm or less, and more preferably 100 μm or less.

The maximum depth of the enriched region in the decarburized layer can be determined using the following method. Three samples containing the steel surface layer are prepared by cutting the steel material, and the enriched region is observed in three fields of view (each field having an area of approximately 600 μm x 800 μm) on a cross section perpendicular to the rolling direction of the sample using the method described above, and the maximum depth of each enriched region is measured. The average of the maximum depths obtained in each field of view for each sample is calculated, and this is regarded as the maximum depth of the enriched region in the decarburized layer of that steel material. The minimum depth of the enriched region can also be determined by observing using the same method.

Furthermore, if the hot-rolled steel contains Sn, Sn may be further concentrated in the enriched region where at least one of Cu and Ni is enriched. Regarding Sn, Sn is considered to be enriched when its concentration is three times or more the reference concentration.

[In the enriched region, the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration, [Sn]/([Cu]+[Ni]), is 0.50 or less in atomic ratio (preferred range)]
As described above, Sn has the effect of lowering the melting point of the enriched region where at least one of Cu and Ni is enriched. However, if the Sn concentration in the enriched region is excessively high, the grain boundary penetration depth of the enriched region increases, causing cold work cracks. Therefore, the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region, [Sn]/([Cu] + [Ni]), is set to an atomic ratio of 0.50 or less, preferably 0.40 or less, and more preferably 0.30 or less. On the other hand, the lower limit of the atomic ratio [Sn]/([Cu] + [Ni]) is not particularly limited, and [Sn]/([Cu] + [Ni]) may be 0.00.

The ratio of the Sn concentration to the sum of the Cu and Ni concentrations in the enriched region, [Sn]/([Cu] + [Ni]), can be determined using the following procedure. Three samples containing the surface layer of the steel are prepared by cutting the steel. Quantitative analysis (mapping) is performed using EPMA on three fields of view on the cross section perpendicular to the rolling direction of the sample to determine the concentrations of Cu, Ni, and Sn. The EPMA measurement conditions can be a magnification of 100x and an area of 600 μm x 800 μm per field. The enriched region is determined using the method described above, and the atomic ratio of [Sn]/([Cu] + [Ni]) is calculated from the measured concentration at the point with the highest Sn concentration in the enriched region. The average atomic ratio obtained in each field of view of each sample is calculated, and this is defined as the [Sn]/([Cu] + [Ni]) in the enriched region of that steel.

(Method for manufacturing hot-rolled steel)
Next, a method for producing hot-rolled steel material will be described. The method for producing hot-rolled steel material according to one embodiment of the present invention includes a step of hot-rolling a steel material having the above-described chemical composition under conditions in which the maximum heating temperature T in a heating furnace is 1000°C or higher and 1200°C or lower, and the residence time of the steel material in the heating furnace is within a predetermined range, to obtain a hot-rolled steel material.

[Steel material]
The steel material used in the method for producing a hot-rolled steel material has a chemical composition containing C, Si, Mn, Cu, Ni, and N, with the balance being Fe and unavoidable impurities. In addition to these, the steel material may contain the above-mentioned elements as necessary. The content of each element is as described above.

[Hot rolling]
Hot-rolled steel is obtained by hot-rolling a steel material. If the maximum heating temperature T in the heating furnace during hot rolling is less than 1000°C, scale is unlikely to form and grow on the surface of the steel material, and the formation of enriched regions is unlikely to progress. As a result, the coverage of the enriched regions on the surface of the decarburized layer is reduced, and the decarburization reaction cannot be suppressed. Therefore, the maximum heating temperature T in the heating furnace is set to 1000°C or higher, and preferably 1030°C or higher. On the other hand, if the maximum heating temperature T exceeds 1200°C, the scale growth rate on the surface of the steel material is too fast, and the enriched regions formed on the surface of the steel material are expelled to the scale side (scale off). As a result, the coverage of the enriched regions on the surface of the decarburized layer is reduced, and the decarburization reaction cannot be sufficiently suppressed. Therefore, the maximum heating temperature T in the heating furnace is set to 1200°C or lower, and preferably 1150°C or lower.

During hot rolling, the longer the residence time of the steel material in the heating furnace, the longer the time for which the decarburization reaction occurs in the heating furnace. The inventors discovered that by setting the upper limit of the residence time of the steel material in the heating furnace to time t ₁ (minutes) determined by the following formula (1), the decarburized layer and concentrated region can be made into a state suitable for suppressing the decarburization reaction. On the other hand, since a shorter residence time is more advantageous for suppressing the decarburization reaction, there is no need to particularly set a lower limit for the residence time. However, an excessively short residence time may have adverse effects such as the temperature of the material varying depending on the position. Therefore, the residence time is preferably 30 minutes or more. In the present invention, the residence time refers to the time the material stays in the heating furnace from the time the material is charged into the heating furnace until the heated material leaves the heating furnace.

t ₁ =1150-0.8T-3([Ni]/[Cu])-10[Sn]...(1)

where T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass%), [Cu] is the amount of Cu in the steel material (mass%), and [Sn] is the amount of Sn in the steel material (mass%). If the steel material does not contain Sn, [Sn] = 0.

In addition, conventional methods can be used for steps and conditions not described in this invention.

The following examples are provided to specifically explain the configuration and effects of the present invention. The present invention is not limited to the examples below, and modifications may be made as appropriate within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

A 160 mm square billet material having the chemical composition shown in Tables 1 and 2 was heated in a heating furnace under the hot rolling conditions shown in Tables 3 and 4, and then hot rolled to obtain a wire rod with a diameter of 15 mm. Samples were taken from the resulting hot-rolled wire rod for structural observation and hardness measurement. Using the methods described above, the coverage of the enriched regions where at least one of Cu and Ni was enriched, the maximum and minimum depths of the enriched regions, the total decarburization depth (DM-T) of the decarburized layer, the total area ratio of ferrite and pearlite, and the average Vickers hardness were measured for each example. The results are shown in Tables 3 and 4. Furthermore, for the steel types listed in Table 2, the atomic ratio of [Sn]/([Cu] + [Ni]) was measured using the method described above, and the results are shown in Table 4.

The cold workability of the resulting hot-rolled wire rod was evaluated as follows. After complete removal of scale from the surface of the hot-rolled wire rod by pickling, the wire rod was drawn to a diameter of 14 mm and then cut to a height of 21 mm to obtain cylindrical test specimens. The test specimen shape conformed to the No. 1 test specimen described in the literature "Cold Upsetting Test Method" (Plasticity and Processing, 22 (1981), 139.). The resulting test specimens were subjected to a cold compression test (end-constrained condition) in which they were compressed 60% in the height direction at a strain rate of 10/s. N=3 cold compression tests were conducted for each example, and the side surfaces of the specimens were observed after the compression test to confirm the presence or absence of cracks associated with cold compression. In Tables 3 and 4, steel types that showed no cracks in all N=3 samples were designated as passing products and indicated with "absent," while steel types that showed cracks in at least one of the N=3 samples were designated as failing products and indicated with "present."

Below, each example of the invention and comparative examples will be explained.

Nos. 1, 2, and 3 are comparative examples in which the amounts of C, Si, and Mn exceeded the ranges set forth in the present invention. These steels contained excessive amounts of either C, Si, or Mn, resulting in excessively high hardenability, resulting in a microstructure containing bainite or martensite, and the total area ratio of ferrite and pearlite in the base steel was below 90.0%. Furthermore, the average Vickers hardness of the base steel exceeded 250 HV, resulting in poor cold workability and the occurrence of cracks after cold compression testing.

Nos. 4 and 6 are comparative examples in which the Cu and Ni contents were below the ranges specified in the present invention. Because the Cu and Ni contents were low in these steels, the coverage of the enriched region was below the range specified in the present invention, and the total decarburization depth exceeded the range specified in the present invention.

Nos. 5 and 7 are comparative examples in which the Cu and Ni contents exceeded the ranges set forth in the present invention. These steels contained excessive amounts of Cu or Ni, and the maximum depth of the enriched region exceeded the ranges set forth in the present invention, resulting in cracks during the cold compression test.

No. 8 is a comparative example in which the N content exceeded the range specified in the present invention. This steel type had a high amount of dissolved N in the steel, and the cold workability was poor due to the effects of dynamic strain aging, resulting in cracks occurring during cold compression testing.

Nos. 9 and 10 are comparative examples in which the ratio of Ni to Cu, [Ni]/[Cu], was outside the range of the present invention. Because the [Ni]/[Cu] ratio was inappropriate for these steels, the maximum depth of the enriched region exceeded the range of the present invention, causing cracks to occur during cold compression testing.

No. 11 is a comparative example in which the maximum heating temperature T during hot rolling exceeded the range of the present invention. In this example, the heating temperature of the steel material was too high, resulting in a rapid growth rate of scale, a low coverage rate of the enriched region, and a total decarburization depth that exceeded the range of the present invention.

No. 12 is a comparative example in which the maximum heating temperature T during hot rolling was below the range specified in the present invention. In this example, the heating temperature was too low, so enriched regions were not sufficiently formed in the steel surface layer, and the coverage rate of the enriched regions was significantly below the range specified in the present invention. As a result, the total decarburization depth exceeded the range specified in the present invention.

No. 13 is a comparative example in which the residence time in the heating furnace during hot rolling exceeded the range of the present invention. In No. 13, the coverage and maximum depth of the enriched region were within the range of the present invention, but the total decarburization depth exceeded the range of the present invention because the decarburization reaction took place over a long period of time.

No. 35 is a comparative example of a Sn-containing steel in which the Sn content exceeded the upper limit of ([Cu] + [Ni])/2. Because of the excessive Sn content in this steel, the atomic ratio of [Sn]/([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range specified in the present invention, resulting in cracks during cold compression testing.

No. 36 is a comparative example in which the residence time in the heating furnace during hot rolling of a Sn-containing steel exceeded the range of the present invention. In this example, the atomic ratio of [Sn]/([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range of the present invention, resulting in cracks during the cold compression test.

In contrast to the above comparative examples, Nos. 14 to 34 and Nos. 37 to 57 had steel composition and hot rolling conditions within the ranges of the present invention, as shown in Tables 1 to 4. In these hot-rolled steels, the coverage and maximum depth of the enriched region, the total decarburization depth, and the total area ratio of ferrite and pearlite were all within the ranges of the present invention. Furthermore, in the examples shown in Tables 2 and 4, the [Sn]/([Cu] + [Ni]) ratio in the enriched region was within the range of the present invention. Furthermore, these examples had excellent average Vickers hardness and cracking properties during cold compression. In other words, Nos. 14 to 34 and Nos. 37 to 57 exhibited excellent decarburization reaction suppression and cold workability.

The present invention provides a hot-rolled steel material and a manufacturing method thereof in which the thickness of the decarburized layer is sufficiently suppressed and which has excellent cold workability.

100 Hot-rolled steel material 10 Base steel 20 Decarburized layer 22 Enriched region 24 Decarburized layer surface 30 Scale A Maximum depth of enriched region B Total decarburization depth of decarburized layer

Claims (9)

In mass%,
C: 0.03 to 0.80%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
Cu: 0.010-0.500%,
Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%
a base steel having a composition in which the ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], satisfies 0.10 or more and 3.00 or less, with the balance consisting of Fe and unavoidable impurities; and a decarburized layer formed on the surface of the base steel,
The decarburized layer has a concentrated region in which at least one of Cu and Ni is concentrated,
a coverage of the enriched region on the surface of the decarburized layer is 50% or more,
the maximum depth of the concentrated region in the decarburized layer is 1 μm or more and 150 μm or less,
The total decarburization depth (DM-T) of the decarburized layer as defined in JIS G 0558 is 0.80 mm or less,
The total area ratio of ferrite and pearlite in the base steel is 90.0% or more,
The hot-rolled steel material has an average Vickers hardness of 250 HV or less in the base steel. The component composition further comprises, in mass %,
Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less;
In the enriched region, Sn is enriched in addition to at least one of Cu and Ni,
2. The hot-rolled steel material according to claim 1, wherein the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region, [Sn]/([Cu]+[Ni]), is 0.50 or less in atomic ratio. The component composition further comprises, in mass %,
Cr: 0.01-1.50%,
Mo: 0.01-0.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
V: 0.001 to 0.300%,
Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
The hot-rolled steel material according to claim 1, further comprising at least one element selected from the group consisting of: The component composition further comprises, in mass %,
Cr: 0.01-1.50%,
Mo: 0.01-0.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
V: 0.001 to 0.300%,
Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
The hot-rolled steel material according to claim 2, containing at least one element selected from the group consisting of: The component composition further comprises, in mass %,
P: 0.001-0.100%,
S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%
The hot-rolled steel material according to any one of claims 1 to 4, containing at least one element selected from the group consisting of: The component composition further comprises, in mass %,
Pb: 0.01 to 0.50%,
Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
The hot-rolled steel material according to any one of claims 1 to 4, containing at least one element selected from the group consisting of: The component composition further comprises, in mass %,
Pb: 0.01 to 0.50%,
Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
The hot-rolled steel material according to claim 5, containing at least one element selected from the group consisting of: In mass%,
C: 0.03 to 0.80%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
Cu: 0.010-0.500%,
Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%
and a ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], of 0.10 or more and 3.00 or less, with the balance being Fe and unavoidable impurities, under conditions where a maximum heating temperature T in a heating furnace is 1000°C or more and 1200°C or less, and the residence time of the steel material in the heating furnace is equal to or less than a time _t1 (minute) determined by the following formula (1), to obtain a hot-rolled steel material.
t ₁ =1150-0.8T-3([Ni]/[Cu])-10[Sn]...(1)
where T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass%), [Cu] is the amount of Cu in the steel material (mass%), and [Sn] is the amount of Sn in the steel material (mass%). If the steel material does not contain Sn, [Sn] = 0. The component composition further comprises, in mass %,
The method for producing a hot-rolled steel material according to claim 8, wherein Sn is contained in an amount of 0.001% or more and ([Ni] + [Cu])/2 or less.