TW202031912A - High-carbon hot-rolled steel sheet and method for manufacturing same - Google Patents

Abstract

Provided are a high-carbon hot-rolled steel sheet and a method for manufacturing the same. The present invention is a high-carbon hot-rolled steel sheet having a specific component composition, wherein: the microstructure comprises ferrite, cementite, and pearlite accounting for 6.5% or less, in area percentage, of the whole microstructure; as regards the cementite, the proportion of the quantity of cementite having a circle-equivalent diameter of 0.1 [mu]m or less is 20% or less relative to the total quantity of cementite, the average cementite diameter is 2.5 [mu]m or less, and the proportion of cementite relative to the whole microstructure is from 3.5% to 10.0% in area percentage; the average concentration of the amount of solid-soluted B in a region up to a depth of 100 [mu]m from the surface layer is 10 ppm by mass or greater; and the average concentration of the amount of N existing as AlN in a region up to a depth of 100 [mu]m from the surface layer is 70 ppm by mass or less.

Description

High-carbon hot-rolled steel plate and manufacturing method thereof

The present invention relates to a high-carbon hot-rolled steel sheet with excellent cold workability and hardenability (immersion hardenability and carburizing hardenability) and a manufacturing method thereof.

At present, automotive parts such as transmission and seat recliner are used as carbon steel materials for mechanical structures and alloys for mechanical structures in accordance with Japanese Industrial Standards (JIS) G4051 for cold working. After the hot-rolled steel sheet (high-carbon hot-rolled steel sheet) of the steel material is processed into a desired shape, the desired hardness is ensured, and in many cases, it is manufactured by quenching. Therefore, the hot-rolled steel sheet used as a raw material requires excellent cold workability and hardenability, and various steel sheets have been proposed so far.

For example, in Patent Document 1, it is described that a component composition containing C: 0.15% to 0.9%, Si: 0.4% or less, Mn: 0.3% to 1.0%, and P: 0.03 in weight% % Or less, T.Al: 0.10% or less, more Cr: 1.2% or less, Mo: 0.3% or less, Cu: 0.3% or less, Ni: 2.0% or less, or Ti: 0.01%～0.05%, B : 0.0005% to 0.005%, N: 0.01% or less, a high carbon steel sheet for fine blanking with a spheroidization rate of 80% or more and a structure in which carbides with an average particle size of 0.4 μm to 1.0 μm are dispersed in ferrous iron.

Patent Document 2 describes a high-carbon steel sheet with improved workability, which contains C: 0.2% or more, Ti: 0.01% to 0.05%, and B: 0.0003% to 0.005% by mass. The average particle size of carbides is 1.0 μm or less, and the ratio of carbides of 0.3 μm or less is 20% or less.

Patent Document 3 describes a B-added steel having C: 0.20% or more and 0.45% or less, Si: 0.05% or more and 0.8% or less, Mn: 0.5% or more and 2.0% or less in mass %, P: 0.001% or more and 0.04% or less, S: 0.0001% or more and 0.006% or less, Al: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.2% or less, B: 0.001% or more and 0.01% or less, And N: 0.0001% or more and 0.01% or less, and Cr: 0.05% or more and 0.35% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0.05% or more and 0.5% or less, Mo: 0.01% or more and 1.0% Or less, Nb: 0.01% or more and 0.5% or less, V: 0.01% or more and 0.5% or less, Ta: 0.01% or more and 0.5% or less, W: 0.01% or more and 0.5% or less, Sn: 0.003% or more and 0.03% Below, Sb: 0.003% or more and 0.03% or less, As: 0.003% or more and 0.03% or less of one or two or more components.

In Patent Document 4, it is described that a component composition containing C: 0.10% to 1.2%, Si: 0.01% to 2.5%, Mn: 0.1% to 1.5%, and P: 0.04 in mass% % Or less, S: 0.0005% to 0.05%, Al: 0.2% or less, Te: 0.0005% to 0.05%, N: 0.0005% to 0.03%, Sb: 0.001% to 0.05%, and Cr: 0.2% to 2.0%, Mo: 0.1% to 1.0%, Ni: 0.3% to 1.5%, Cu: 1.0% or less, B: 0.005% or less, and it contains a structure mainly composed of ferrite and porlite, Fertilizer iron has a crystal grain size of No. 11 or higher, which is a mechanical structural steel with improved cold workability and low decarburization.

In Patent Document 5, it is described that in terms of mass %, C: 0.20% to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al ：0.10% or less, N: 0.005% or less, B: 0.0005%～0.0050%, and moreover contains one or more of Sb, Sn, Bi, Ge, Te, Se in total 0.002%～0.03%, and contains ferrite iron and Xueming carbon iron, the microstructure of Xueming carbon iron in the fertilized iron grains with a density of 0.10 pcs/μm ² or less, the hardness is 75 or less in Rockwell hardness (HRB), and the total elongation is more than 38% The high-carbon hot-rolled steel sheet with improved hardenability and workability.

In Patent Document 6, it is described that, in terms of mass %, C: 0.20% to 0.48%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al ：0.10% or less, N: 0.005% or less, B: 0.0005%～0.0050%, and moreover, it contains more than one of Sb, Sn, Bi, Ge, Te, Se, which contains 0.002%～0.03% in total. And Xueming carbon iron, and the Xueming carbon iron density in the fertilizer grains is 0.10 pieces/μm ² or less, the hardness is 65 or less in HRB, and the total elongation is more than 40%, which improves the quenching High carbon hot-rolled steel sheet with high performance and processability.

In Patent Document 7, it is described that C: 0.20% to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al : 0.10% or less, N: 0.005% or less, B: 0.0005% to 0.0050%, moreover, it contains more than one of Sb, Sn, Bi, Ge, Te, Se in a total of 0.002% to 0.03%, and the amount of solid solution B is in The proportion of B content is more than 70%, and it has a microstructure that contains fat iron and snow carbon iron, and the snow carbon iron density in the fat iron grain is 0.08 pieces/μm ² or less, and the hardness is HRB Counted as 73 or less, high carbon hot-rolled steel sheet with a total elongation of 39% or more.

Patent Document 8 describes a composition that contains C: 0.15% to 0.37%, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, and S: 0.03% by mass%. Or less, sol.Al: 0.10% or less, N: 0.0005%～0.0050%, B: 0.0010%～0.0050%, and at least one of Sb and Sn: 0.003%～0.10% in total, and 0.50≦(14[B ])/(10.8[N]), the remainder contains Fe and unavoidable impurities, and contains the fat iron phase and snow carbon iron, and the average particle size of the fat iron phase is 10 μm or less. It is a high-carbon hot-rolled steel sheet with a microstructure with a spheroidization rate of 90% or more and a total elongation rate of 37% or more. [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2009-299189 [Patent Document 2] Japanese Patent Laid-Open No. 2005-344194 [Patent Document 3] Japanese Patent No. 4012475 [Patent Document 4] Japanese Patent No. 4782243 [Patent Document 5] Japanese Patent Laid-Open No. 2015-017283 [Patent Document 6] Japanese Patent Laid-Open No. 2015-017284 [Patent Document 7] International Publication No. 2015/146173 [Patent Document 8] Japanese Patent No. 5458649

[The problem to be solved by the invention] The technique described in Patent Document 1 is related to fine blanking properties, and describes the influence of the dispersion form of carbides on fine blanking properties and hardenability. Specifically, Patent Document 1 describes that by controlling the average carbide particle size to 0.4 μm to 1.0 μm, the spheroidization rate is 80% or more, and a steel sheet with improved fine blanking properties and hardenability can be obtained. However, Patent Document 1 does not discuss cold workability, nor does it describe carburizing and hardenability.

The technology described in Patent Document 2 pays attention not only to the average particle size of carbides, but also to the influence of fine carbides of 0.3 μm or less on workability. The average particle size of carbides is controlled to 1.0 μm or less and 0.3 μm or less The proportion of carbides is controlled below 20%. Thereby, it is described that a steel sheet with improved workability can be obtained, and a steel sheet with excellent hardenability added with Ti and B is described. However, in Patent Document 2, there is no description of solid solution B or the like that affects hardenability, and no description of which position of the steel sheet corresponds to the quenching hardness.

The technology described in Patent Document 3 describes that by adjusting the component composition, it is possible to obtain steel with improved cold workability and decarburization resistance. However, Patent Document 3 does not describe immersion hardenability and carburizing hardenability.

The technical description described in Patent Document 4: Contains B, and moreover contains one or two or more of Cr, Ni, Cu, Mo, Nb, V, Ta, W, Sn, Sb, and As by ensuring a predetermined amount The solid solution B in the surface layer of the steel can achieve high hardenability steel. However, Patent Document 4 stipulates that the hydrogen concentration in the environment in the annealing step is 95% or more, and there is no description about whether it is possible to suppress nitrogen absorption and ensure solid solution B in the annealing step in a nitrogen atmosphere.

The techniques described in Patent Documents 5 to 7 describe that by containing B and further containing one or more of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002% to 0.03%, the effect of preventing nitriding is high. In the case of annealing in a nitrogen atmosphere, nitriding is also prevented, and the hardenability is improved by maintaining a predetermined amount of solid solution B. However, none of Patent Documents 5 to 7 describes the quenching hardness in the surface layer.

The technique described in Patent Document 8 proposes a steel with high hardenability by containing C: 0.15% to 0.37%, B, and one or more of Sb and Sn. However, in Patent Document 8, higher hardenability such as carburizing and hardenability has not been studied.

The present invention was completed in view of the above-mentioned problems, and its object is to provide a high-carbon hot-rolled steel sheet having excellent cold workability and excellent hardenability (immersion hardenability, carburizing hardenability) and a method of manufacturing the same. [Means to solve the problem]

In order to achieve the above-mentioned problems, the inventors of the present inventors studied the manufacturing conditions, cold workability and hardenability (immersion) of high-carbon hot-rolled steel sheets containing B and one or two selected from Sn and Sb as the component composition of steel. The relationship between hardenability and carburizing and hardenability) has been studied hard. As a result, the following findings were obtained.

i) When annealing is carried out in a nitrogen environment, the nitrogen in the environment penetrates and concentrates into the steel sheet, combines with B and Al in the steel sheet, and generates B nitride and Al nitride on the surface layer. As a result, the amount of solid solution B in the steel sheet is reduced, or Al nitrides are present, so that the austenitic iron grain size becomes smaller during the heating of the austenitic iron (austenite) region before quenching, and the quenching may be insufficient. Therefore, in the present invention, when annealing is performed in a nitrogen atmosphere, a predetermined amount of at least one of Sb and Sn is added to the steel for steel sheets requiring higher hardenability (high carburizing and hardenability). In addition, by heating in a temperature range of 450°C to 600°C at a predetermined heating rate during annealing, it is possible to suppress nitriding from the environment into the steel to a predetermined amount. This prevents the aforementioned nitriding, suppresses a decrease in the amount of solid solution B and an increase in Al nitrides, and can ensure higher hardenability (high carburizing hardenability).

ii) Xueming carbon iron with an equivalent circle diameter of 0.1 μm or less has a great influence on the cold workability, the hardness (hardness) of the high carbon hot-rolled steel sheet before quenching, and the total elongation (hereinafter, sometimes referred to as elongation) . By setting the number of snow carbon and iron having an equivalent circle diameter of 0.1 μm or less to 20% or less of the total snow carbon and iron number, a tensile strength of 480 MPa or less and a total elongation (El) of 33% or more can be obtained.

iii) Xueming carbon iron with an equivalent circle diameter of 0.1 μm or less has a great influence on the hardness (hardness) and total elongation of the high-carbon hot-rolled steel sheet before quenching. By setting the number of snow carbon iron with an equivalent circle diameter of 0.1 μm or less to 10% or less of the total snow carbon iron number, a tensile strength of 440 MPa or less and a total elongation (El) of 36% or more can be obtained.

iv) After hot rough rolling, perform final rolling at the final rolling end temperature: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec to coil Winding temperature: 500 ℃ ~ 700 ℃ for winding, cooling to normal temperature, after the hot-rolled steel plate is made, the hot-rolled steel plate is heated at an average heating rate: 15 ℃/h or more at 450 ℃ ~ 600 ℃, by Annealing temperature: below Ac ₁ transformation point for more than 1.0 h of annealing to ensure the specified microstructure.

v) Or, after hot rough rolling, perform final rolling end temperature: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec to Coiling temperature: 500 ℃ ~ 700 ℃ for winding, cooling to normal temperature, after the hot-rolled steel sheet is made, the hot-rolled steel sheet is heated at an average heating rate: 15 ℃/h or more at 450 ℃ ~ 600 ℃, by Keep it above the Ac ₁ transformation point and below the Ac ₃ transformation point for more than 0.5h, and then cool to less than the Ar ₁ transformation point at an average cooling rate: 1℃/h～20℃/h, and keep it for 20 h below the Ar ₁ transformation point The above two-stage annealing can ensure a predetermined microstructure.

The present invention was completed based on the above knowledge, and its gist is as follows. [1] A high-carbon hot-rolled steel sheet comprising the following composition: in mass %, containing C: 0.20% or more and 0.50% or less, Si: 0.8% or less, Mn: 0.10% or more and 0.80% or less, P: 0.03 % Or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.01% or less, Cr: 1.0% or less, B: 0.0005% or more and 0.005% or less, and the total content is 0.002% or more and 0.1% The following are selected from one or two of Sb and Sn, the remaining part contains Fe and unavoidable impurities. The microstructure includes: ferrite, snow carbon iron, and the area ratio of the total microstructure accounts for less than 6.5% In the Xueming carbon iron, the ratio of the Xueming carbon and iron number with an equivalent circle diameter of less than 0.1 μm to the total Xueming carbon and iron number is less than 20%, and the average Xueming carbon and iron diameter is 2.5 μm or less, the ratio of the Xueming carbon iron to the total microstructure is 3.5% or more and 10.0% or less in terms of area ratio, and the average concentration of the solid solution B amount in the region from the surface layer to the depth of 100 μm is 10 Mass ppm or more, and the average concentration of N amount existing as AlN in the region from the surface layer to a depth of 100 μm is 70 mass ppm or less. [2] The high carbon hot-rolled steel sheet according to [1], wherein the tensile strength is 480 MPa or less, and the total elongation is 33% or more. [3] The high-carbon hot-rolled steel sheet according to [1] or [2], wherein the average grain size of the ferrous iron is 4 μm to 25 μm. [4] The high-carbon hot-rolled steel sheet according to any one of [1] to [3], wherein, in addition to the component composition, it contains in mass% selected from the following A group and B group One or two groups in group A: Ti: 0.06% or less Group B: Make one or more of Nb, Mo, Ta, Ni, Cu, V, and W be 0.0005 % Above and below 0.1%. [5] A method for manufacturing a high-carbon hot-rolled steel sheet, which is the method for manufacturing a high-carbon hot-rolled steel sheet according to any one of [1] to [4], wherein the steel having the composition After rolling, perform final rolling at the final rolling end temperature: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, and the winding temperature: 500 After coiling at ℃～700℃ to make a hot-rolled steel sheet, the hot-rolled steel sheet is heated to a temperature range of 450℃～600℃ at an average heating rate: 15℃/h or more, with annealing temperature: lower than Ac _{1 The} transformation point remains annealed for 1.0 h or more. [6] A method for manufacturing a high-carbon hot-rolled steel sheet, which is the method for manufacturing a high-carbon hot-rolled steel sheet according to any one of [1] to [4], wherein the steel having the composition After rolling, perform final rolling at the final rolling end temperature: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, and to coil temperature: 500 After coiling at ℃～700℃ to make a hot-rolled steel sheet, the hot-rolled steel sheet is heated at an average heating rate: 15℃/h or more to a temperature range of 450℃-600℃, with Ac ₁ transformation point or more Ac ₃ Keep below the transformation point for more than 0.5h, and then cool down to below the Ar ₁ transformation point at an average cooling rate of 1°C/h to 20°C/h, and keep the annealing below the Ar ₁ transformation point for more than 20 hours. [Effects of the invention]

According to the present invention, a high-carbon hot-rolled steel sheet excellent in cold workability and hardenability (immersion hardenability, carburization hardenability) can be obtained. In addition, by applying the high-carbon hot-rolled steel sheet manufactured according to the present invention as a raw material steel sheet to automotive parts requiring cold workability, such as backrests, door latches, and transmission systems, it can greatly contribute to automotive parts requiring stable quality The manufacturing of this product has a significant effect in the industry.

Hereinafter, the high carbon hot rolled steel sheet of the present invention and its manufacturing method will be described in detail. In addition, the present invention is not limited to the following embodiments.

1) Composition The composition of the high-carbon hot-rolled steel sheet of the present invention and the reason for its limitation will be described. In addition, "%" as a content unit of the following component composition means "mass %" unless otherwise specified.

C: 0.20% or more and 0.50% or less C is an important element to obtain the strength after quenching. When the C content is less than 0.20%, the desired hardness cannot be obtained by the heat treatment after forming, so the C content needs to be 0.20% or more. However, if the amount of C exceeds 0.50%, the hardness will be hardened, and toughness and cold workability will deteriorate. Therefore, the amount of C is set to 0.20% or more and 0.50% or less. When used for cold working of parts that are complicated in shape and difficult to press, the amount of C is preferably 0.45% or less, and more preferably 0.40% or less.

Si: 0.8% or less Si is an element that increases the strength by solid solution strengthening. As the amount of Si increases, it hardens and deteriorates cold workability. Therefore, the amount of Si is 0.8% or less. It is preferably 0.65% or less, and more preferably 0.50% or less. When cold workability is further required in the use of difficult-to-form parts, it is preferably 0.30% or less. From the viewpoint of ensuring a predetermined softening resistance in the tempering step after quenching, the amount of Si is preferably 0.1% or more, and more preferably 0.2% or more.

Mn: 0.10% or more and 0.80% or less Mn is an element that increases the strength by solid solution strengthening while improving hardenability. When the Mn content is less than 0.10%, both the immersion hardenability and carburizing and hardenability begin to decrease, so the Mn content is 0.10% or more. When it is reliably hardened to the inside in thick materials, etc., it is preferably 0.25% or more, and more preferably 0.30% or more. On the other hand, if the amount of Mn exceeds 0.80%, the banded structure due to segregation of Mn spreads, the structure becomes non-uniform, and the steel is hardened due to solid solution strengthening, and cold workability is reduced. Therefore, the amount of Mn is set to 0.80% or less. As a material for parts requiring formability, a predetermined cold workability is required, so the amount of Mn is preferably 0.65% or less. More preferably, it is 0.55% or less.

P: Below 0.03% P is an element that increases the strength by solid solution strengthening. If the amount of P is increased by more than 0.03%, it will cause grain boundary embrittlement and deterioration of toughness after quenching. In addition, cold workability is also reduced. Therefore, the amount of P is 0.03% or less. In order to obtain excellent toughness after quenching, the amount of P is preferably 0.02% or less. Since P decreases cold workability and toughness after quenching, the smaller the amount of P, the better. However, if P is excessively reduced, refining costs increase, so the amount of P is preferably 0.005% or more. More preferably, it is 0.007% or more.

S: Below 0.010% S is an element that has to be reduced due to the formation of sulfides, which reduces the cold workability and toughness of the high-carbon hot-rolled steel sheet after quenching. If the S content exceeds 0.010%, the cold workability of the high-carbon hot-rolled steel sheet and the toughness after quenching are significantly deteriorated. Therefore, the amount of S is set to 0.010% or less. In order to obtain excellent cold workability and toughness after quenching, the amount of S is preferably 0.005% or less. Since S reduces cold workability and toughness after quenching, the smaller the amount of S, the better. However, if S is excessively reduced, refining costs increase, so the amount of S is preferably 0.0005% or more.

sol.Al: 0.10% or less When the amount of sol.Al exceeds 0.10%, AlN is generated during the heating of the quenching treatment and the austenite grains are excessively refined. As a result, the formation of the fat iron phase is promoted during cooling, the microstructure becomes fat iron and martensite, and the hardness after quenching decreases. Therefore, the amount of sol.Al is set to 0.10% or less. It is preferably set to 0.06% or less. In addition, sol.Al has a deoxidizing effect, and in order to sufficiently deoxidize, it is preferably 0.005% or more.

N: 0.01% or less When the amount of N exceeds 0.01%, due to the formation of AlN, the austenitic iron grains are excessively refined during the heating of the quenching treatment, and the formation of the fat grain iron phase is promoted during cooling, so the hardness after quenching will decrease. Therefore, the amount of N is set to 0.01% or less. Preferably it is 0.0065% or less. More preferably, it is 0.0050% or less. Furthermore, N forms AlN, Cr-based nitrides, and B nitrides. This is an element that moderately suppresses the growth of austenitic iron grains during quenching heating and improves toughness after quenching. Therefore, the amount of N is preferably 0.0005% or more. More preferably, it is 0.0010% or more.

Cr: 1.0% or less In the present invention, Cr is an important element for improving hardenability. When the amount of Cr in the steel is 0%, especially during carburizing and quenching, the surface layer is likely to produce fat iron, and a complete quenched structure cannot be obtained, which may easily cause a decrease in hardness. Therefore, when it is used for applications where hardenability is important, it is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. On the other hand, when the amount of Cr exceeds 1.0%, the steel sheet before quenching hardens, and cold workability is impaired. Therefore, the amount of Cr is 1.0% or less. Furthermore, when processing parts requiring high processing that are difficult to press and form, since more excellent cold workability is required, the amount of Cr is preferably 0.7% or less, and more preferably 0.5% or less.

B: 0.0005% or more and 0.005% or less In the present invention, B is an important element for improving hardenability. When the amount of B is less than 0.0005%, no sufficient effect is seen, so the amount of B needs to be 0.0005% or more. Preferably it is 0.0010% or more. On the other hand, when the B content exceeds 0.005%, the recrystallization of the austenitic iron after final rolling is delayed. As a result, the texture of the hot-rolled steel sheet develops, and the anisotropy after annealing becomes large, which tends to produce edges during coil forming. . Therefore, the amount of B is set to 0.005% or less. Preferably it is 0.004% or less.

One or a total of two selected from Sb and Sn: 0.002% or more and 0.1% or less. Sb and Sn are effective elements for suppressing nitriding from the surface layer of the steel sheet. When the total of one or more of these elements is less than 0.002%, sufficient effects are not seen, so the total of one or more of these elements is 0.002% or more. More preferably, it is 0.005% or more. On the other hand, even if the total content of one or more of these elements exceeds 0.1%, the effect of preventing nitriding is saturated. In addition, these elements tend to segregate to grain boundaries, so if the total exceeds 0.1%, the content is too high, which may cause grain boundary embrittlement. Therefore, the total content of one or two selected from Sb and Sn is set to 0.1% or less. It is preferably 0.03% or less, and more preferably 0.02% or less.

In the present invention, by making the total of one or two selected from Sb and Sn 0.002% or more and 0.1% or less, even in the case of annealing in a nitrogen atmosphere, nitriding from the surface of the steel sheet is suppressed, thereby suppressing An increase in the concentration of nitrogen in the surface of the steel sheet. Thus, according to the present invention, since nitriding from the surface of the steel sheet can be suppressed, even in the case of annealing in a nitrogen atmosphere, it is possible to appropriately ensure the amount of solid solution B from the surface of the annealed steel sheet to a depth of 100 μm. In addition, the formation of Al nitride (AlN) in the region from the surface of the steel sheet to a depth of 100 μm is suppressed, thereby enabling the growth of ferrous iron grains during heating before quenching. As a result, it is possible to delay the production of ferrous iron and pulverized iron during cooling, thereby obtaining high hardenability.

In the present invention, the remainder other than the above is Fe and unavoidable impurities.

With the above essential elements, the high-carbon hot-rolled steel sheet of the present invention can obtain target characteristics. Furthermore, the high-carbon hot-rolled steel sheet of the present invention is for the purpose of further improving hardenability, and may contain the following elements as necessary.

Ti: Below 0.06% Ti is an effective element for improving hardenability. When only B is contained and the hardenability is insufficient, the hardenability can be improved by containing Ti. When the amount of Ti is less than 0.005%, the effect is not seen, so when Ti is contained, the amount of Ti is preferably 0.005% or more. More preferably, it is 0.007% or more. On the other hand, if the Ti content exceeds 0.06%, the steel sheet before quenching hardens and the cold workability is impaired. Therefore, when Ti is contained, the Ti content is set to 0.06% or less. Preferably it is 0.04% or less.

Furthermore, in order to stabilize the mechanical properties and hardenability of the present invention, one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W may be added in required amounts.

Nb: 0.0005% or more and 0.1% or less Nb is an effective element for forming carbonitrides, preventing abnormal grain growth of crystal grains during heating before quenching, improving toughness, and improving temper softening resistance. If it is less than 0.0005%, the effect of the addition cannot be sufficiently observed, so when Nb is contained, the lower limit is preferably 0.0005%. More preferably, it is 0.0010% or more. When Nb exceeds 0.1%, not only the effect of addition is saturated, but also the tensile strength of the base material increases due to the Nb carbides, and the elongation decreases with this. Therefore, when Nb is contained, the upper limit is preferably set to 0.1%. It is more preferably 0.05% or less, and more preferably less than 0.03%.

Mo: 0.0005% or more and 0.1% or less Mo is an effective element for improving hardenability and tempering softening resistance. If it is less than 0.0005%, the effect of addition is small. Therefore, when Mo is contained, the lower limit is preferably 0.0005%. More preferably, it is 0.0010% or more. When Mo exceeds 0.1%, the effect of addition is saturated and the cost increases. Therefore, when Mo is contained, the upper limit is preferably set to 0.1%. It is more preferably 0.05% or less, and still more preferably less than 0.03%.

Ta: 0.0005% or more and 0.1% or less Ta is an element that forms carbonitrides like Nb, and is effective for preventing abnormal grain growth of crystal grains during heating before quenching, preventing coarsening of crystal grains, and improving temper softening resistance. If it is less than 0.0005%, the effect of addition is small. Therefore, when Ta is contained, the lower limit is preferably set to 0.0005%. More preferably, it is 0.0010% or more. If Ta exceeds 0.1%, the effect of the addition is saturated, the quenching hardness due to the formation of excess carbides is reduced, and the cost increases, so when Ta is contained, the upper limit is preferably set to 0.1%. More preferably, it is 0.05% or less, and still more preferably less than 0.03%.

Ni: 0.0005% or more and 0.1% or less Ni is an element that is highly effective in improving toughness and improving hardenability. Since there is no additive effect if it is less than 0.0005%, when Ni is contained, the lower limit is preferably 0.0005%. More preferably, it is 0.0010% or more. When Ni exceeds 0.1%, the effect of addition is saturated and the cost increases. Therefore, when Ni is contained, the upper limit is preferably set to 0.1%. More preferably, it is 0.05% or less.

Cu: 0.0005% or more and 0.1% or less Cu is an effective element for ensuring hardenability. If it is less than 0.0005%, the effect of the addition cannot be fully confirmed. Therefore, when Cu is contained, the lower limit is preferably made 0.0005%. More preferably, it is 0.0010% or more. When Cu exceeds 0.1%, defects during hot rolling are likely to occur, and manufacturability deteriorates such as a decrease in yield. Therefore, when Cu is contained, the upper limit is preferably set to 0.1%. More preferably, it is 0.05% or less.

V: 0.0005% or more and 0.1% or less V, like Nb and Ta, is an effective element that forms carbonitrides, prevents abnormal grain growth of crystal grains during heating before quenching, improves toughness, and improves temper softening resistance. If it is less than 0.0005%, the effect of the addition cannot be sufficiently observed. Therefore, when V is contained, the lower limit is preferably 0.0005%. More preferably, it is 0.0010% or more. When V exceeds 0.1%, not only the effect of addition is saturated, but also the tensile strength of the base material increases due to the Nb carbide, and the elongation decreases with this. Therefore, when V is contained, the upper limit is preferably set to 0.1%. More preferably, it is 0.05% or less, and still more preferably less than 0.03%.

W: 0.0005% or more and 0.1% or less Like Nb and V, W is an effective element that forms carbonitrides, prevents abnormal grain growth of austenitic iron crystal grains during heating before quenching, and improves temper softening resistance. If it is less than 0.0005%, the effect of addition is small. Therefore, when W is contained, the lower limit is preferably made 0.0005%. More preferably, it is 0.0010% or more. When W exceeds 0.1%, the effect of the addition is saturated, the quenching hardness due to the formation of excess carbides is reduced, and the cost increases. Therefore, when W is contained, the upper limit is preferably set to 0.1%. It is more preferably 0.05% or less, and still more preferably less than 0.03%.

Furthermore, in the present invention, when two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W are contained, the total amount is preferably 0.001% or more and 0.1% or less.

2) Micro organization The reason for the limitation of the microstructure of the high carbon hot rolled steel sheet of the present invention will be described.

In the present invention, the microstructure has fat-grained iron and snow carbon iron. With respect to the snow carbon iron, the snow carbon iron number with an equivalent circle diameter of 0.1 μm or less relative to the total snow carbon iron number is 20% or less, The average snow carbon iron diameter is 2.5 μm or less, and the ratio of the above snow carbon iron to the total microstructure is 3.5% or more and 10.0% or less in terms of area ratio, solid solution from the surface layer to a depth of 100 μm The average concentration of the B content is 10 mass ppm or more, and the average concentration of the N content existing as AlN in the region from the surface layer to the depth of 100 μm is 70 mass ppm or less. In addition, in the present invention, the average particle size of the ferrite grains is preferably 4 μm to 25 μm. More preferably, it is 5 μm or more.

2-1) Fertilizer iron and Xueming carbon iron The microstructure of the high-carbon hot-rolled steel sheet of the present invention has fat grain iron and snow carbon iron. Furthermore, in the present invention, the fertilizer grain iron is preferably 90% or more in terms of area ratio. If the area ratio of ferrous iron is less than 90%, the formability will deteriorate, and it may be difficult to cold work with parts with high workability. Therefore, the area ratio of fertilizer grains is preferably above 90%. More preferably, it is 92% or more.

In addition, the microstructure of the high-carbon hot-rolled steel sheet of the present invention can also produce polished iron in addition to the above-mentioned fat iron and snow carbon iron. When the area ratio of pleite relative to the total microstructure is 6.5% or less, the effect of the present invention will not be impaired, and therefore pleite may be contained.

2-2) The ratio of the number of carbon and iron of the equivalent circle diameter of 0.1 μm or less to the total number of carbon and iron of the snow: 20% or less When there is a large amount of snow carbon iron with a circle diameter of 0.1 μm or less, it will be hardened by dispersion strengthening and the elongation will decrease. From the viewpoint of obtaining cold workability, in the present invention, the number of snow carbon and iron having an equivalent circle diameter of 0.1 μm or less is set to 20% or less of the total snow carbon and iron number. As a result, the tensile strength can further reach 480 MPa or less, and the total elongation (El) can reach 33% or more. When used for difficult-to-form parts, high cold workability is required. In this case, the number of snow carbon iron with an equivalent circle diameter of 0.1 μm or less is preferably 10% or less relative to the total snow carbon iron number. By making the number of snow carbon iron with an equivalent circle diameter of 0.1 μm or less relative to the total snow carbon iron number 10% or less, the tensile strength can reach 440 MPa or less, and the total elongation (El) can reach 36% or more. Furthermore, the reason for defining the ratio of snow carbon iron with an equivalent circle diameter of 0.1 μm or less is that: snow carbon iron of 0.1 μm or less produces dispersion strengthening energy, and the increase in the size of snow carbon iron will affect the cold workability. Come obstacles. From the viewpoint of suppressing abnormal grain growth of fertilizer grains during annealing, the number of snow-carbon iron with an equivalent circle diameter of 0.1 μm or less is preferably 3% or more with respect to the total snow-carbon iron number.

In addition, the diameter of the snow carbon iron existing before quenching is about 0.07 μm to 3.0 μm in terms of the equivalent circle diameter. Therefore, there is no particular regulation in the present invention regarding the dispersion state of the snow carbon iron having a size that does not greatly affect the precipitation strengthening and the equivalent circle diameter before quenching exceeds 0.1 μm.

2-3) Average Xueming Carbon Iron Diameter: 2.5 μm or less It is necessary to melt all the Xueming carbon iron during quenching to ensure the specified amount of solid solution C in the fertilizer grain iron. When the average Xueming carbon iron diameter exceeds 2.5 μm, the Xueming carbon iron cannot be completely dissolved in the austenitic iron area, so the average Xueming carbon iron diameter is set to 2.5 μm or less. More preferably, it is 2.0 μm or less. Furthermore, when the Xueming Iron Carbon is too fine, the precipitation strengthening of the Xueming Carbon Iron leads to a decrease in cold workability. Therefore, the average Xueming Carbon Iron diameter is preferably 0.1 μm or more. More preferably, it is 0.15 μm or more. Furthermore, in the present invention, the "Xueming Carbon Iron Diameter" refers to the equivalent circle diameter of Xueming Carbon Iron. The Equivalent Circle Diameter of Xueming Carbon Iron is to measure the long and short diameters of Xueming Carbon Iron and convert them into equivalent circles. The value obtained from the diameter. In addition, the "average snow carbon iron diameter" refers to a value obtained by dividing the total of the equivalent circle diameters of all snow carbon iron converted into the equivalent circle diameter by the total number of snow carbon iron.

2-4) The ratio of Xueming carbon iron to the total microstructure (area ratio): 3.5% or more and 10.0% or less When the ratio of snow carbon iron to the total microstructure exceeds 10.0%, the number of snow carbon iron of 0.1 μm or less that contributes to precipitation strengthening increases with this, and the steel is hardened, so it is set to 10.0% or less. Preferably it is 9.5% or less. On the other hand, when the above ratio is less than 3.5%, the actual C content does not reach 0.20%, and the specified hardness cannot be obtained after heat treatment, so it is set to 3.5% or more. More preferably, it is 4.0% or more.

2-5) Average particle size of fertilizer grains: 4 μm～25 μm (best condition) When the average particle size of the fat iron is less than 4 μm, the strength before cold working increases, which may deteriorate the press formability, so it is preferably 4 μm or more. On the other hand, when the average particle size of the ferrous iron exceeds 25 μm, the strength of the base metal may decrease. In addition, after forming into the target product shape, the strength of the base material is required to a certain extent in the area that is not used for quenching. Therefore, the average particle size of the ferrous iron is preferably 25 μm or less. It is more preferably 5 μm or more, and still more preferably 6 μm or more. More preferably, it is 20 μm or less. More preferably, it is 18 μm or less.

Furthermore, in the present invention, the equivalent circle diameter of the above-mentioned Xueming carbon iron, the average Xueming carbon iron diameter, the ratio of the Xueming carbon iron to the total microstructure, the area ratio of the fat iron, and the proportion of the fat iron The average particle diameter and the like can be measured by the method described in the below-mentioned Examples.

2-6) The average concentration of solid solution B in the region from the surface layer to the depth of 100 μm: 10 mass ppm or more In the high-carbon hot-rolled steel sheet of the present invention, in order to prevent the quenched structure called polished iron or matte iron that is easily formed on the surface during quenching of the steel sheet, a region from the surface of the steel sheet to a position of 100 μm in the thickness direction is required (Location) (100 μm surface layer) The amount of B is 10 mass ppm or more in average concentration as unnitrided or unoxidized solid solution B. For automotive parts that require abrasion resistance for use by quenching, surface hardness is required. In order to obtain a predetermined surface hardness, it is necessary to obtain a completely quenched structure at 100 μm of the surface layer after quenching. It is preferable that the average concentration of the solid solution B amount is 12 mass ppm or more. More preferably, it is 15 mass ppm or more. Furthermore, too high solid solution B hinders the development of the texture of the hot-rolled structure, so it is set to 40 mass ppm or less. More preferably, it is 35 mass ppm or less.

2-7) The average concentration of the amount of N existing as AlN from the surface layer to the depth of 100 μm: 70 mass ppm or less By setting the average concentration of the amount of N present as AlN in the region 100 μm in the thickness direction from the steel sheet surface layer to 70 mass ppm or less, the austenitic iron region heated before quenching promotes the growth of crystal grains. By this, it is difficult to obtain a structure such as polished iron and matte iron in the cooling stage, and the specified surface hardness is obtained without causing insufficient quenching. The average concentration of the amount of N present as AlN in the region from the surface layer to a depth of 100 μm is preferably 50 mass ppm or less. In addition, from the viewpoint of suppressing abnormal grain growth during the heating of the austenitic iron region, the average concentration of the amount of N is preferably 10 mass ppm or more, and more preferably 20 mass ppm or more.

In the present invention, it is found that the amount of solute B in the surface layer of the steel sheet and the amount of N present as AlN are closely related to the manufacturing conditions in each step of heating conditions, winding conditions, and annealing conditions, and it is necessary to optimize this series Manufacturing conditions. In addition, the reason required to obtain the amount of solid solution B and the amount of N as AlN in each step will be described later.

3) Mechanical characteristics The high-carbon hot-rolled steel sheet of the present invention needs to have excellent workability because it is cold-pressed to form automotive parts such as gears, transmissions, and backrests. In addition, it is necessary to increase the hardness by quenching treatment to impart wear resistance. Therefore, the high-carbon hot-rolled steel sheet of the present invention reduces the tensile strength of the steel sheet so that the tensile strength (TS) is 480 MPa or less, and the elongation is increased to make the total elongation (El) 33% or more, thereby having Excellent cold workability, and can have excellent hardenability (immersion hardenability, carburizing hardenability). More preferably, TS is 460 MPa or less, and El is 35% or more.

In addition, imagine forming difficult-to-form parts requiring cold pressability, further reduce the tensile strength of the steel sheet, make TS less than 440 MPa, and increase the total elongation to make El 36% or more, thereby having excellent cold workability, and Both excellent hardenability (immersion hardenability, carburizing hardenability). More preferably, TS is 410 MPa or less, and El is 38% or more.

In addition, the above-mentioned tensile strength (TS) and total elongation (El) can be measured by the method described in the below-mentioned Examples.

4) Manufacturing method The high-carbon hot-rolled steel sheet of the present invention is manufactured by using steel with the above-mentioned composition as a raw material, and after hot rough rolling the raw material (steel raw material), the final rolling end temperature: Ar _The final rolling is performed above the transformation point, and then cooled to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, coiled at a winding temperature: 500°C to 700°C, and cooled to room temperature After the hot-rolled steel sheet is made, the hot-rolled steel sheet is heated to a temperature range of 450°C to 600°C at an average heating rate: 15°C/h or more, and annealing is performed at an annealing temperature: less than the Ac ₁ transformation point for more than 1.0 h.

In addition, it is manufactured by using steel having the above-mentioned composition as a raw material, after hot rough rolling the raw material (steel raw material), and then performing final rolling at a final rolling end temperature: Ar ₃ transformation point or higher, Then, cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, wind at a winding temperature: 500°C to 700°C, and cool to room temperature to make a hot rolled steel sheet. Heat the hot-rolled steel sheet at an average heating rate: 15°C/h or more to a temperature range of 450°C to 600°C, and keep it at the Ac ₁ transformation point or more and Ac ₃ transformation point for 0.5 h or more, and then the average cooling rate: 1°C /h～20℃/h to cool down to less than Ar ₁ transformation point, and keep it less than Ar ₁ transformation point for more than 20 hours of two-stage annealing.

Hereinafter, the reason for the limitation in the method of manufacturing the high-carbon hot-rolled steel sheet of the present invention will be explained. Furthermore, in the description, the temperature-related "°C" indicates the temperature of the surface of the steel sheet or the surface of the steel material.

In the present invention, the manufacturing method of the steel raw material does not need to be particularly limited. For example, in order to melt the high carbon steel of the present invention, both a converter and an electric furnace can be used. The high-carbon steel melted by a known method such as a converter is converted into a slab or the like (steel raw material) by ingot-dividing rolling or continuous casting. Slabs are usually hot rolled (hot rough rolling, final rolling) after heating.

For example, in the case of a slab manufactured by continuous casting, in the case of a steel slab manufactured by continuous casting, straight rolling may also be applied. The straight rolling is directly rolled, or in order to suppress temperature drop and heat preservation After rolling. In addition, in the case of heating a slab for hot rolling, in order to avoid deterioration of the surface condition due to rust, it is preferable to set the heating temperature of the slab to 1280°C or lower. Furthermore, the lower limit of the heating temperature of the slab is preferably 1100°C, more preferably 1150°C, and still more preferably 1200°C or higher. Furthermore, in hot rolling, in order to ensure the final rolling end temperature, the material to be rolled may be heated by a heating mechanism such as a sheet bar heater during hot rolling.

The final rolling is performed at the final rolling end temperature: Ar ₃ transformation point or higher. When the final rolling end temperature is less than the Ar ₃ transformation point, coarse iron grains are formed after hot rolling and annealing, and the elongation is significantly reduced. Therefore, the final rolling end temperature is set to be equal to or higher than the Ar ₃ transformation point. It is preferably (Ar ₃ transformation point + 20°C) or higher. In addition, the upper limit of the final rolling end temperature does not need to be particularly specified, but in order to smoothly perform cooling after the final rolling, it is preferably 1000° C. or lower.

In addition, the aforementioned Ar ₃ transformation point can be determined by thermal expansion measurement during cooling using a fully automatic phase changer (formastor) or the like or actual measurement by electrical resistance measurement.

After the final rolling, cool down to 650℃～750℃ at an average cooling rate: 20℃/sec～100℃/sec After the final rolling, the average cooling rate to 650°C to 750°C has a great influence on the size of the annealed spheroidized snow carbon iron. After the final rolling, when the average cooling rate is less than 20°C/sec, the structure before annealing becomes the ferrous iron and the poriferous iron structure with too much ferrous iron structure. Therefore, the specified Xueming carbon iron dispersion state and size cannot be obtained after annealing. . Therefore, cooling must be performed at 20°C/sec or more. Preferably it is 25°C/sec or more. On the other hand, when the average cooling rate exceeds 100°C/sec, it is difficult to obtain snow carbon iron having a predetermined size after annealing, so it is set to 100°C/sec or less. Preferably it is 75°C/sec or less.

Winding temperature: 500℃～700℃ The hot-rolled steel sheet after final rolling is wound into a coil shape. When the winding temperature is too high, the strength of the hot-rolled steel sheet is too low, and when wound into a coil shape, the coil may deform due to its own weight. Therefore, it is not good from the operational point of view. Therefore, the upper limit of the winding temperature is 700°C. Preferably it is 690°C or less. On the other hand, when the winding temperature is too low, the hot-rolled steel sheet becomes hardened, which is not preferable. Therefore, the winding temperature is set to 500°C. It is preferably 530°C or higher.

After being wound into a coil shape, it can be cooled to room temperature and pickled. After the pickling treatment, annealing is performed. In addition, a known method can be applied to the pickling treatment. After that, the following annealing was performed on the obtained hot-rolled steel sheet.

Average heating rate in the temperature range of 450℃～600℃: 15℃/h or more The hot-rolled steel sheet obtained as described above is annealed (spheroidizing annealing of Xueming carbon iron). During annealing in a nitrogen environment, ammonia gas is easily generated in the temperature range of 450°C to 600°C. The nitrogen decomposed from the ammonia gas enters the surface steel plate and combines with B and Al in the steel to form nitrides. Therefore, the heating time in the temperature range of 450°C to 600°C should be shortened as much as possible. The average heating rate in this temperature range is 15°C/h or more. Preferably it is 20°C/h or more. From the viewpoint of suppressing variations in the furnace for the purpose of improving productivity, it is preferably 100°C/h or less, and more preferably 70°C/h or less.

Annealing temperature: below the Ac ₁ transformation point for more than 1.0 h. When the annealing temperature is above the Ac ₁ transformation point, austenitic iron is precipitated, and a coarse iron structure is formed during the cooling process after annealing, forming an uneven structure. Therefore, the annealing temperature is set to be less than the Ac ₁ transformation point. It is preferably (Ac ₁ transformation point -10°C) or lower. In addition, the lower limit of the annealing temperature is not particularly specified, but in order to obtain a predetermined dispersion of snow carbon iron, the annealing temperature is preferably 600°C or higher, and more preferably 700°C or higher. Furthermore, the ambient gas can use any of nitrogen, hydrogen, a mixed gas of nitrogen and hydrogen. In addition, the holding time at the above-mentioned annealing temperature is 1.0 hour (h) or more. If the holding time at the annealing temperature is less than 1.0 hour, the effect of annealing is insufficient and the structure targeted by the present invention cannot be obtained. As a result, the hardness and elongation of the steel sheet targeted by the present invention cannot be obtained. Therefore, the holding time at the annealing temperature is 1.0 hour or more. It is preferably 5 hours or more, and more preferably more than 20 hours. On the other hand, if the holding time at the above-mentioned annealing temperature exceeds 40.0 hours, the productivity decreases and the manufacturing cost becomes excessive. Therefore, the holding time at the above annealing temperature is preferably 40.0 hours or less. More preferably, it is 35 hours or less.

In the present invention, the following two-stage annealing can be performed instead of the above-mentioned annealing. Specifically, it can also be manufactured by the following method: after winding and cooling to room temperature to form a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to an average heating rate: 15°C/h or more at a temperature of 450°C to 600°C Range heating, keep at least Ac ₁ transformation point and below Ac ₃ transformation point for 0.5 h or more (the first stage of annealing), and then cool down to less than Ar ₁ transformation point at an average cooling rate: 1℃/h～20℃/h , Keep the transformation point less than Ar _{1 for} more than 20 h (second annealing) two-stage annealing.

In the present invention, the hot-rolled steel sheet is heated at an average heating rate: 15°C/h or more in the temperature range of 450°C to 600°C, and maintained at the Ac ₁ transformation point or more for 0.5h or more, and the precipitates in the hot-rolled steel sheet are relatively finely dissolved. The carbides are dissolved in the γ phase, and then cooled to less than the Ar ₁ transformation point at an average cooling rate of 1°C/h to 20°C/h, and kept for more than 20 h if the Ar ₁ transformation point is less than. As a result, solid solution C is precipitated with relatively coarse undissolved carbides as the nucleus, and the ratio of the number of snow carbon and iron with an equivalent circle diameter of 0.1 μm or less to the total number of snow carbon and iron is 20% or less. , Control the dispersion state of carbides (Xue Ming Carbon Iron). That is, in the present invention, by performing two-stage annealing under predetermined conditions, the dispersion form of carbides is controlled and the steel sheet is softened. In the high-carbon steel sheet targeted in the present invention, it is important to control the dispersion form of carbides after annealing in addition to softening. In the present invention, by maintaining the high carbon hot-rolled steel sheet above the Ac ₁ transformation point and below the Ac ₃ transformation point (first stage annealing), fine carbides are dissolved, and C is dissolved in γ (austenitic iron). )in. In the subsequent cooling stage and holding stage below the Ar ₁ transformation point (second stage annealing), the α/γ interface and undissolved carbides existing in the temperature range above the Ac ₁ transformation point become nucleation sites and precipitate Larger carbides. Hereinafter, the conditions of this two-stage annealing will be described. Furthermore, the ambient gas during annealing can be any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen.

Average heating rate in the temperature range of 450℃～600℃: 15℃/h or more For the same reason as above, ammonia gas is easily generated in the temperature range of 450°C to 600°C. Nitrogen decomposed from ammonia gas enters the surface steel plate and combines with B and Al in the steel to form nitrides. Therefore, shorten 450°C as much as possible Heating time in the temperature range of ~600°C. The average heating rate in this temperature range is set to 15°C/h or more. Preferably it is 20°C/h or more. The upper limit of the average heating rate is preferably 100°C/h, and more preferably 90°C/h or less.

Keep at the Ac ₁ transformation point or more and Ac ₃ transformation point for 0.5h or more (first stage annealing). By keeping the hot-rolled steel sheet at the Ac ₁ transformation point or higher, part of the ferrous iron in the steel sheet structure is transformed into Voss Tien iron dissolves the fine carbides precipitated in the fertilizer grain iron, so that C is dissolved in the austenitic iron. On the other hand, the ferrous iron remaining without being transformed into austenitic iron is annealed at a high temperature, so the dislocation density is reduced and softened. In addition, there are relatively coarse undissolved carbides (undissolved carbides) remaining in the fertilized iron, but they become coarser by Oswald coarsening. When the annealing temperature is lower than the Ac ₁ transformation point, the austenitic iron phase transformation does not occur, and therefore the carbide cannot be dissolved in the austenitic iron. On the other hand, when the annealing temperature in the first stage exceeds the Ac ₃ transformation point, many rod-shaped snow carbon irons are obtained after annealing, and the predetermined elongation cannot be obtained, so the Ac ₃ transformation point is set to be below the Ac ₃ transformation point. In addition, in the present invention, when the retention time of Ac ₁ transformation point or more and Ac ₃ transformation point or less is less than 0.5 h, fine carbides cannot be sufficiently dissolved. Therefore, as the first stage of annealing, it is assumed to be maintained at the Ac ₁ transformation point or more and Ac ₃ transformation point or less for 0.5 h or more. The retention time is preferably 1.0 h or more. In addition, the holding time is preferably 10 h or less. In addition, even in the case of annealing by holding the Ac ₁ transformation point or more and Ac ₃ transformation point or less, the heating rate is preferably set to an average heating rate of 15°C/h or more in the temperature range of 450°C to 600°C , Set the upper limit to 100°C/h or less.

Cooling at an average cooling rate: 1℃/h～20℃/h to less than Ar ₁ transformation point After annealing in the first stage above, cooling at an average cooling rate: 1℃/h～20℃/h to less than the second stage Ar ₁ transformation point in the annealing temperature range. During the cooling process, with the phase transition from austenitic iron to fat iron, the C discharged from austenitic iron takes the α/γ interface or undissolved carbides as nucleation sites, as relatively coarse spherical carbonization Matter precipitated out. In this cooling, the cooling rate must be adjusted so as not to generate wavelet iron. When the average cooling rate from the first stage annealing to the second stage annealing is less than 1°C/h, the production efficiency is poor, so the average cooling rate is 1°C/h or more. Preferably it is 5°C/h or more. On the other hand, when the average cooling rate exceeds 20° C./h and becomes larger, poriferous iron is precipitated and the hardness becomes higher, so it is made 20° C./h or less. Preferably it is 15 degrees C/h or less.

Keep at less than Ar ₁ transformation point for more than 20 hours (second stage annealing) After the above-mentioned first stage annealing, cool at a predetermined average cooling rate, keep it at a lower transformation point than Ar ₁ , and coarsen it by Oswald Spherical carbides grow further, and fine carbides disappear. If the retention time shorter than the Ar ₁ transformation point is less than 20 h, the carbide cannot be grown sufficiently, and the hardness after annealing becomes too large. Therefore, the annealing in the second stage is kept below the transformation point of Ar _{1 for} more than 20 h. In addition, it is not particularly limited, but the annealing temperature in the second stage is preferably 660° C. or higher in order to fully grow carbides. In addition, from the viewpoint of production efficiency, the holding time is preferably 30 h or less.

In addition, the above-mentioned Ac ₃ transformation point, Ac ₁ transformation point, Ar ₃ transformation point, and Ar ₁ transformation point can be measured by thermal expansion during heating and cooling such as the transformation point measurement (automatic phase changer (formastor)) test. Or it can be determined by the actual measurement of resistance measurement.

In addition, the above-mentioned average heating rate and average cooling rate are obtained by measuring the temperature of a thermocouple installed in the furnace. Example

Steels having the component compositions of the steel numbers A to T shown in Table 1 were melted, and then hot rolled under the production conditions shown in Tables 2-1 and 3-1. Then, pickling is performed, and annealing (spheroidizing annealing) is performed in a nitrogen environment (ambient gas: nitrogen) at the annealing temperature and annealing time (h) shown in Table 2-1 and Table 3-1 to produce a plate 3.0 mm thick hot rolled annealed sheet.

In the examples of the present invention, test pieces were collected from the hot-rolled and annealed sheets thus obtained, and the microstructure, the amount of solid solution B, the amount of N in AlN, tensile strength, total elongation, and Quenching hardness (hardness of steel plate after quenching, steel plate hardness after carburizing and quenching). In addition, the Ac ₃ transformation point, Ac ₁ transformation point, Ar ₁ transformation point, and Ar ₃ transformation point shown in Table 1 were obtained by a transformation point measurement test.

(1) Micro organization The microstructure of the annealed steel sheet is a test piece (size: 3 mmt×10 mm×10 mm) collected from the center of the width of the plate, cut and polished, and then corroded by a nitric acid etching solution. The scanning electron microscope is used. , SEM), shooting at 3000 times magnification at 5 locations at 1/4 of the thickness from the surface. The photographs of the tissues obtained were used to determine the phases (ferrous iron, snow carbon iron, pleite, etc.) through image processing. In Table 2-2 and Table 3-2, the "Polalite area ratio" is described as the microstructure, and the steel whose area ratio exceeds 6.5% was confirmed as a comparative example. Steels having polished iron, ferrite iron, and snow carbon iron in an area ratio of 6.5% or less are examples of the present invention.

In addition, using image analysis software, binarize the areas other than the fat iron and the fat iron based on the SEM image to obtain the area ratio (%) of the fat iron. The same applies to Xueming Carbon Iron. Using image analysis software, the area other than Xueming Carbon Iron and Xueming Carbon Iron is binarized based on the SEM image to obtain the area ratio (%) of Xueming Carbon Iron. In addition, the area ratio (%) of plethyrite is the value obtained by subtracting the area ratios (%) of the ferrite iron and the snow-carbon iron from 100 (%).

In addition, the diameter of each Xueming carbon iron was evaluated with respect to the photograph of the structure. Measure the long and short diameters, and convert the Xueming carbon iron diameter into an equivalent circle diameter. The average Xueming carbon iron diameter is calculated by dividing the sum of the equivalent circle diameters of all Xueming carbon irons converted to the equivalent circle diameter by the total number of Xueming carbon irons. The value of the equivalent circle diameter is measured as the number of snow carbon irons with an equivalent circle diameter of 0.1 μm or less, and is taken as the number of snow carbon irons with an equivalent circle diameter of 0.1 μm or less. In addition, the number of total Xueming carbon and iron was calculated as the total Xueming carbon and iron number. In addition, calculate the ratio of the number of snow carbon and iron with an equivalent circle diameter of 0.1 μm or less to the total number of snow carbon and iron ((the number of snow carbon and iron with an equivalent circle diameter of 0.1 μm or less/total snow carbon and iron number)×100 (%)). In addition, the "ratio of the iron with an equivalent circle diameter of 0.1 μm or less" may also be simply referred to as an iron with an equivalent circle diameter of 0.1 μm or less.

In addition, with regard to the photographic structure photograph, the average particle size of the fat iron was determined by using the crystal grain size evaluation method (cutting method) specified in JIS G 0551.

(2) Determination of the average concentration of solid solution B It is determined by the same method as described in the following references. That is, the grinding powder from the surface layer to a depth of 100 μm is collected and measured three times, and the average value is determined as the average concentration of the solid solution B amount. [References] Tetsushi Castle, Tomoharu Ishida, Kunio Inose, Kyoko Fujimoto, ≪Iron and Steel≫, vol.99 (2013) No.5, p.362-365 (3) Measurement of the average concentration of the amount of N present as AlN In the same manner as described above, the average concentration of the amount of N present as AlN was determined by the same method as that described in the following references. [References] Tetsushi Castle, Tomoharu Ishida, Kunio Inose, Kyoko Fujimoto, "Iron and Steel", vol.99 (2013) No.5, p.362-365 (4) Tensile strength and elongation of steel plate Use a JIS No. 5 tensile test piece cut from the annealed steel sheet (original plate) in the direction (L direction) at 0° relative to the rolling direction, and perform a tensile test at 10 mm/min to obtain the nominal stress The nominal strain curve uses the maximum stress as the tensile strength. In addition, the broken samples were butted, and the total elongation was determined. The result is defined as elongation (El).

(5) Hardness of steel plate after quenching (immersion hardenability) A flat test piece (width 15 mm × length 40 mm × plate thickness 3 mm) was collected from the center of the width of the annealed steel sheet, and quenched with oil cooling at 70°C as follows to determine the quenching hardness (immersion hardenability). The quenching treatment is performed by a method (70°C oil cooling) in which the above flat test piece is kept at 900°C for 600s and immediately cooled with 70°C oil. Regarding the quenching hardness, the cut surface of the test piece after the quenching treatment was measured at 5 places in the area 70 μm from the surface layer and 1/4 of the plate thickness using a Vickers hardness tester under a load of 0.2 kgf The hardness is calculated and the average hardness is determined, and this is defined as the quenching hardness (HV). In addition, the area within the thickness of 70 μm from the above-mentioned surface layer is indicated as "surface layer" in Table 2-2 and Table 3-2.

(6) Hardness of steel plate after carburizing and quenching (carburizing and hardenability) For the annealed steel sheet, carburizing and quenching treatments such as soaking, carburizing, and diffusion treatment are carried out at 930°C for a total of 4 hours. After keeping at 850°C for 30 minutes, it is oil-cooled (the temperature of oil cooling). : 60℃). The hardness is measured at a depth of 0.1 mm from the surface of the steel plate and a position of 1.2 mm in depth at intervals of 0.1 mm under a load of 1 kgf, and the hardness (HV) and effective hardened layer of the surface layer at the time of carburizing and quenching are determined by 0.1 mm Depth (mm). The effective hardened layer depth is defined as the depth at which the hardness of the surface is measured after heat treatment and becomes 550 HV or more.

Then, based on the results obtained in (5) and (6) above, the hardenability evaluation was performed under the conditions shown in Table 4. Table 4 shows the acceptance criteria of the hardenability corresponding to the C content that can be evaluated as sufficient hardenability. If the hardness (HV) after oil cooling at 70°C, the hardness (HV) of the surface layer depth of 0.1 mm during carburizing and quenching, and the effective hardened layer depth during carburizing and quenching all meet the criteria in Table 4, it is judged as qualified (marked by ○) , It is evaluated as excellent hardenability. On the other hand, when any of the values did not satisfy the criteria shown in Table 4, it was judged to be unacceptable (indicated by the symbol: ×), and the hardenability was evaluated as poor.

[Table 1] Steel number Composition (mass%) Ac ₁ transformation point (℃) Ar ₁ transformation point (℃) Ac ₃ transformation point (℃) Ar ₃ transformation point (℃) Remarks C Si Mn P S sol.Al N Cr B Sn, Sb Ti Nb Mo Ta Ni Cu V W A 0.20 0.01 0.55 0.02 0.004 0.030 0.0044 0.50 0.0030 Sb+Sn: 0.010 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 726 716 836 824 Steel of the invention B 0.22 0.03 0.45 0.01 0.003 0.050 0.0041 0.40 0.0030 Sb: 0.012 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 726 716 835 825 Steel of the invention C 0.28 0.79 0.50 0.02 0.004 0.010 0.0044 0.12 0.0020 Sb: 0.025 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 743 737 852 840 Steel of the invention D 0.24 0.64 0.60 0.02 0.004 0.010 0.0044 0.65 0.0015 Sb+Sn: 0.020 ｰ 0.001 ｰ ｰ ｰ ｰ ｰ ｰ 746 736 842 832 Steel of the invention E 0.23 0.85 0.40 0.02 0.004 0.010 0.0044 0.50 0.0025 Sb: 0.010 ｰ 0.001 ｰ ｰ ｰ ｰ ｰ ｰ 752 742 861 851 Comparative steel F 0.22 0.15 0.85 0.02 0.004 0.050 0.0050 0.30 0.0035 Sb+Sn: 0.010 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 723 713 829 827 Comparative steel G 0.22 0.30 0.40 0.01 0.003 0.006 0.0045 0.02 0.0030 Sb: 0.012 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 728 718 835 825 Steel of the invention H 0.23 0.35 0.45 0.01 0.003 0.030 0.0050 0.40 0.0032 Sb: 0.008 0.02 ｰ ｰ ｰ ｰ ｰ ｰ ｰ 735 725 839 829 Steel of the invention I 0.22 0.01 0.35 0.01 0.003 0.050 0.0050 1.05 0.0020 Sb+Sn: 0.010 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 737 727 832 820 Comparative steel J 0.23 0.01 0.45 0.01 0.003 0.030 0.0050 0.40 0.0032 0.000 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 725 727 824 814 Comparative steel K 0.35 0.01 0.35 0.02 0.004 0.010 0.0044 0.01 0.0020 Sb: 0.009 ｰ ｰ 0.0015 ｰ 0.001 ｰ ｰ ｰ 720 716 810 798 Steel of the invention L 0.33 0.15 0.31 0.01 0.003 0.120 0.0110 0.10 0.0015 Sb: 0.025 0.01 ｰ ｰ ｰ ｰ ｰ ｰ ｰ 726 716 855 845 Comparative steel M 0.35 0.05 0.35 0.02 0.004 0.020 0.0044 0.20 0.0001 Sb+Sn: 0.012 ｰ ｰ ｰ ｰ ｰ ｰ ｰ ｰ 724 714 811 801 Comparative steel N 0.36 0.01 0.04 0.02 0.003 0.050 0.0047 0.20 0.0030 Sb+Sn: 0.011 ｰ ｰ ｰ ｰ ｰ 0.0150 ｰ ｰ 726 717 830 819 Comparative steel O 0.45 0.03 0.35 0.02 0.004 0.030 0.0050 0.02 0.0026 Sn+Sb: 0.100 ｰ ｰ ｰ 0.0020 ｰ 0.0015 ｰ ｰ 720 723 801 790 Steel of the invention P 0.48 0.25 0.31 0.01 0.004 0.010 0.0044 0.06 0.0025 Sb: 0.009 ｰ ｰ ｰ ｰ 0.025 ｰ 0.0015 ｰ 728 718 794 782 Steel of the invention Q 0.48 0.03 0.28 0.01 0.002 0.035 0.0052 0.45 0.0010 Sb+Sn: 0.012 ｰ ｰ ｰ ｰ ｰ ｰ ｰ 0.0015 728 716 788 778 Steel of the invention R 0.44 0.10 0.32 0.01 0.003 0.040 0.0047 0.05 0.0048 Sb: 0.011 0.04 ｰ ｰ 0.0015 ｰ ｰ ｰ ｰ 723 713 803 793 Steel of the invention S 0.18 0.05 0.45 0.01 0.004 0.035 0.0050 0.40 0.0030 Sb: 0.010 0.04 ｰ ｰ ｰ ｰ ｰ ｰ ｰ 726 715 840 829 Comparative steel T 0.55 0.25 0.45 0.01 0.003 0.040 0.0050 0.45 0.0020 Sn+Sb: 0.008 ｰ ｰ 0.0012 ｰ ｰ ｰ ｰ ｰ 733 722 785 775 Comparative steel

[table 2-1] Sample No Steel number Hot rolling conditions Annealing conditions Final rolling end temperature (℃) After final rolling, the average cooling rate to 650℃～750℃ (℃/sec) Winding temperature (℃) Average heating rate in the temperature range of 450℃～600℃ (℃/h) Annealing (annealing temperature-holding time) 1 A 850 40 550 45 715℃-30h 2 A 850 55 480 60 715℃-30h 3 A 850 55 680 15 715℃-30h 4 A 850 55 620 15 710℃-15h 5 B 845 30 550 30 715℃-30h 6 B 845 30 550 30 770℃-25h 7 B 845 30 510 15 715℃-30h 8 B 845 30 620 5 715℃-30h 9 C 860 40 620 35 710℃-22h 10 D 860 60 680 90 710℃-25h 11 E 880 50 580 20 715℃-30h 12 F 840 50 620 30 710℃-25h 13 G 850 50 600 30 680℃-25h 14 H 850 40 580 50 715℃-30h 15 H 850 40 510 40 715℃-0.25h 16 I 840 50 600 40 715℃-30h 17 J 830 80 700 20 715℃-30h 18 K 830 60 700 40 715℃-30h 19 L 860 40 700 50 715℃-30h 20 M 880 50 680 60 715℃-30h twenty one N 840 50 660 40 715℃-30h twenty two O 830 50 590 40 715℃-30h twenty three P 830 25 550 40 715℃-30h twenty four Q 830 25 560 30 715℃-30h 25 R 820 40 650 45 715℃-30h 26 S 860 40 650 40 715℃-30h 27 T 810 40 650 40 710℃-25h

[Table 2-2] Sample No Steel number Microstructure {(Snow-ming carbon iron with an equivalent circle diameter of 0.1 μm or less)/(total snow-ming carbon iron)}×100 (%) Average Xueming Carbon Iron Diameter (μm) Fertilizer iron average particle size (μm) Fertilizer iron area rate (%) The proportion of Xueming carbon iron in the total microstructure (area%) Rail area rate (%) The average concentration of dissolved B in the surface layer at 100 μm (mass ppm) The average concentration of N as AlN in the surface layer at 100 μm (mass ppm) TS (MPa) Total elongation (%) Immersion hardenability (HV) Carburizing and hardenability Hardenability evaluation Remarks 70℃ oil cooling (surface layer) 70°C oil cooling (1/4 plate thickness) 0.1 mm hardness of the surface layer during carburizing and quenching (HV) Effective hardened layer depth during carburizing and quenching (mm) 1 A Fertilizer iron + Xueming carbon iron 15 0.40 7 95 3.5 1.5 15 35 460 37 360 390 670 0.70 ○ Example of the invention 2 A Fertilizer iron + Xueming carbon iron twenty two 0.18 6 91 3.9 5.1 14 40 490 32 355 385 665 0.68 ○ Comparative example 3 A Fertilizer iron + Xueming carbon iron 12 0.45 8 95 3.5 1.5 16 60 450 38 360 395 675 0.65 ○ Example of the invention 4 B Fertilizer iron + Xueming carbon iron 18 0.35 4 94 3.6 2.4 15 60 470 34 360 395 675 0.65 ○ Example of the invention 5 B Fertilizer iron + Xueming carbon iron 11 0.55 8 94 3.6 2.4 15 35 450 38 380 420 655 0.65 ○ Example of the invention 6 B Fertilizer Iron + Xueming Carbon Iron + Polly Iron 4 0.65 11 80 0.5 19.5 15 40 460 31 370 420 655 0.64 ○ Comparative example 7 B Fertilizer iron + Xueming carbon iron 1 0.50 7 94 3.6 2.4 10 70 460 36 360 425 600 0.62 ○ Example of the invention 8 C Fertilizer iron + Xueming carbon iron 13 0.52 7 93 3.7 3.3 9 80 395 41 340 415 580 0.60 × Comparative example 9 D Fertilizer iron + Xueming carbon iron 14 0.45 7 93 4.7 2.3 17 40 460 35 390 470 680 0.62 ○ Example of the invention 10 E Fertilizer iron + Xueming carbon iron 12 0.50 8 94 4.0 2.0 15 30 480 33 380 430 690 0.70 ○ Example of the invention 11 F Fertilizer iron + Xueming carbon iron 14 0.38 7 93 3.8 3.2 14 40 490 32 370 425 670 0.70 ○ Comparative example 12 G Fertilizer iron + Xueming carbon iron 14 0.40 6 92 3.7 4.3 14 40 490 32 390 425 700 0.80 ○ Comparative example 13 H Fertilizer iron + Xueming carbon iron 17 0.30 5 94 3.7 2.3 15 40 430 40 370 425 650 0.65 ○ Example of the invention 14 H Fertilizer iron + Xueming carbon iron 12 0.50 8 92 3.8 4.2 12 35 430 40 370 430 700 0.70 ○ Example of the invention 15 H Fertilizer Iron + Xueming Carbon Iron + Polly Iron 30 0.20 4 90 3.3 6.7 15 40 480 32 370 430 700 0.65 ○ Comparative example 16 I Fertilizer iron + Xueming carbon iron 10 0.30 8 94 3.7 2.3 15 35 490 30 400 425 710 0.80 ○ Comparative example 17 J Fertilizer iron + Xueming carbon iron 15 0.50 7 93 3.7 3.3 5 50 430 39 300 430 580 0.65 × Comparative example 18 K Fertilizer iron + Xueming carbon iron 12 0.40 8 94 5.9 0.1 12 60 420 40 510 570 680 0.65 ○ Example of the invention 19 L Fertilizer iron + Xueming carbon iron 12 0.41 8 94 5.5 0.5 12 200 420 38 380 480 580 0.65 × Comparative steel 20 M Fertilizer iron + Xueming carbon iron 12 0.35 8 93 5.9 1.1 0 70 430 41 470 530 570 0.55 × Comparative steel twenty one N Fertilizer iron + Xueming carbon iron 11 0.40 7 94 6.0 0.0 15 50 410 42 470 490 590 0.50 × Comparative example twenty two O Fertilizer iron + Xueming carbon iron 7 0.40 9 92 7.5 0.5 17 40 460 35 670 680 700 0.70 ○ Example of the invention twenty three P Fertilizer iron + Xueming carbon iron 13 0.45 10 92 8.0 0.0 15 35 460 36 680 700 675 0.70 ○ Example of the invention twenty four Q Fertilizer iron + Xueming carbon iron 12 0.48 9 91 8.1 0.9 14 34 450 37 680 710 675 0.70 ○ Example of the invention 25 R Fertilizer iron + Xueming carbon iron 9 0.50 9 92 7.4 0.6 14 38 440 37 580 620 695 0.70 ○ Example of the invention 26 S Fertilizer iron + Xueming carbon iron 12 0.40 7 95 2.7 2.3 15 40 400 42 310 380 700 0.60 × Comparative example 27 T Fertilizer iron + Xueming carbon iron 25 0.35 6 90 9.2 0.8 15 40 520 28 680 720 700 0.68 ○ Comparative example

[Table 3-1] Sample No Steel number Hot rolling conditions Annealing conditions Final rolling end temperature (℃) After final rolling, the average cooling rate to 650℃～750℃ (℃/sec) Winding temperature (℃) Average heating rate in the temperature range of 450℃～600℃ (℃/h) The first stage of annealing (annealing temperature-holding time) The average cooling rate from the 1st stage to the 2nd stage (℃/h) The second stage of annealing (annealing temperature-holding time) 28 A 850 40 680 50 790℃-8h 10 710℃-30h 29 A 850 55 680 50 790℃-8h 10 710℃-15h 30 A 850 55 680 10 790℃-8h 10 710℃-30h 31 A 850 40 680 50 820℃-0.1h 10 710℃-30h 32 B 845 30 620 40 780℃-10h 12 710℃-20h 33 B 845 30 620 40 860℃-8h 10 710℃-30h 34 B 845 30 670 15 800℃-6h 50 710℃-30h 35 C 860 40 680 35 790℃-7h 12 710℃-25h 36 D 860 60 620 90 750℃-8h 10 715℃-20h 37 E 880 50 480 30 755℃-4h 10 690℃-30h 38 F 840 50 510 30 750℃-4h 10 690℃-20h 39 G 850 50 550 60 790℃-8h 10 710℃-30h 40 H 850 40 510 50 760℃-8h 10 710℃-25h 41 I 840 50 600 40 770℃-6h 10 710℃-30h 42 J 830 80 700 20 800℃-6h 10 710℃-25h 43 K 830 60 700 15 800℃-6h 10 710℃-25h 44 L 860 40 700 50 800℃-6h 10 710℃-25h 45 M 880 50 680 50 800℃-6h 10 710℃-20h 46 N 840 50 660 40 790℃-8h 15 705℃-30h 47 O 830 50 590 40 790℃-4h 8 710℃-25h 48 Q 830 25 610 30 770℃-8h 10 710℃-20h 49 R 820 40 700 45 800℃-8h 10 710℃-30h 50 S 860 40 850 40 810℃-4h 10 710℃-21h 51 T 810 40 650 40 800℃-6h 10 710℃-25h

[Table 3-2] Sample No Steel number Microstructure {(Snow-ming carbon iron with an equivalent circle diameter of 0.1 μm or less)/(total snow-ming carbon iron)}×100 (%) Average Xueming Carbon Iron Diameter (μm) Fertilizer iron average particle size (μm) Fertilizer iron area rate (%) The proportion of Xueming carbon iron in the total microstructure (area%) Rail area rate (%) The average concentration of dissolved B in the surface layer at 100 μm (mass ppm) The average concentration of N as AlN in the surface layer at 100 μm (mass ppm) TS (MPa) Total elongation (%) Immersion hardenability (HV) Carburizing and hardenability Hardenability evaluation Remarks 70℃ oil cooling (surface layer) 70°C oil cooling (1/4 plate thickness) 0.1 mm hardness of the surface layer during carburizing and quenching (HV) Effective hardened layer depth during carburizing and quenching (mm) 28 A Fertilizer iron + Xueming carbon iron 1 1.5 15 95 3.8 1.2 15 30 400 42 360 395 670 0.72 ○ Example of the invention 29 A Fertilizer Iron + Xueming Carbon Iron + Polly Iron 5 1.3 13 83 3.9 13.1 15 30 400 32 360 400 670 0.73 ○ Comparative example 30 A Fertilizer iron + Xueming carbon iron 1 1.5 16 94 3.7 2.3 10 80 390 43 340 395 590 0.60 × Comparative example 31 A Fertilizer iron + Xueming carbon iron 25 0.4 10 92 3.5 4.5 15 30 480 32 365 398 671 0.72 ○ Comparative example 32 B Fertilizer iron + Xueming carbon iron 1 1.5 17 94 4.2 1.8 17 35 370 44 375 410 655 0.65 ○ Example of the invention 33 B Fertilizer Iron + Xueming Carbon Iron + Polly Iron 5 1.1 17 80 0.5 19.5 13 35 370 32 375 410 655 0.67 ○ Comparative example 34 B Fertilizer Iron + Xueming Carbon Iron + Polly Iron 3 1.2 15 83 4.2 12.8 14 36 371 32 375 405 655 0.65 ○ Comparative example 35 C Fertilizer iron + Xueming carbon iron 1 1.3 14 94 5.2 0.8 15 70 410 40 390 465 680 0.63 ○ Example of the invention 36 D Fertilizer iron + Xueming carbon iron 1 2.0 17 94 4.5 1.5 10 50 385 41 380 415 675 0.70 ○ Example of the invention 37 E Fertilizer iron + Xueming carbon iron 1 1.1 12 93 4.3 2.7 16 30 485 32 370 425 671 0.70 ○ Comparative example 38 F Fertilizer iron + Xueming carbon iron 1 1.1 12 92 4.1 3.9 16 30 485 32 370 430 698 0.68 ○ Comparative example 39 G Fertilizer iron + Xueming carbon iron 1 1.5 15 93 4.1 2.9 14 30 370 45 370 430 690 0.65 ○ Example of the invention 40 H Fertilizer iron + Xueming carbon iron 1 1.5 14 92 4.4 3.6 15 35 380 44 370 420 702 0.85 ○ Example of the invention 41 I Fertilizer iron + Xueming carbon iron 1 1.1 10 91 4.1 4.9 13 45 490 32 370 420 710 0.80 ○ Comparative example 42 J Fertilizer iron + Xueming carbon iron 1 1.3 14 93 4.3 2.7 16 35 365 45 340 420 590 0.55 × Comparative example 43 K Fertilizer iron + Xueming carbon iron 1 1.8 15 93 6.5 0.5 14 60 370 42 510 570 680 0.65 ○ Example of the invention 44 L Fertilizer iron + Xueming carbon iron 1 1.5 13 93 6.2 0.8 12 200 370 43 380 480 580 0.65 × Comparative example 45 M Fertilizer iron + Xueming carbon iron 1 1.3 15 93 6.7 0.3 0 70 390 43 470 530 570 0.55 × Comparative example 46 N Fertilizer iron + Xueming carbon iron 1 2.5 20 93 6.8 0.2 15 35 365 44 470 490 590 0.50 × Comparative example 47 O Fertilizer iron + Xueming carbon iron 1 1.9 18 91 8.4 0.6 16 30 410 38 670 680 705 0.68 ○ Example of the invention 48 Q Fertilizer iron + Xueming carbon iron 1 1.3 13 91 8.9 0.1 14 35 400 40 680 710 675 0.70 ○ Example of the invention 49 R Fertilizer iron + Xueming carbon iron 1 1.4 13 91 8.2 0.8 15 40 400 39 685 705 670 0.67 ○ Example of the invention 50 S Fertilizer iron + Xueming carbon iron 1 1.4 15 94 3.0 3.0 14 35 360 45 310 380 700 0.60 × Comparative example 51 T Fertilizer Iron + Xueming Carbon Iron + Polly Iron 25 1.5 12 50 10.5 39.5 14 35 490 32 680 720 700 0.68 ○ Comparative example

[Table 4] C amount Hardness after oil cooling at 70°C (HV) Hardness (HV) at a depth of 0.1 mm of the surface layer during carburizing and quenching Effective hardened layer depth during carburizing and quenching (mm) C>0.20% ≧340 ≧600 ≧0.60 0.20%≦C>0.30% ≧350 ≧600 ≧0.60 0.30%≦C>0.35% ≧400 ≧600 ≧0.60 0.35%≦C>0.40% ≧490 ≧600 ≧0.60 0.40%≦C>0.45% ≧580 ≧600 ≧0.60 0.45%≦C>0.50% ≧670 ≧600 ≧0.60 0.50%≦C ≧700 ≧650 ≧0.60

According to the results of Table 2-2 and Table 3-2, it can be seen that in the high-carbon hot-rolled steel sheet of the example of the present invention, the ratio of the number of snow carbon iron with an equivalent circle diameter of 0.1 μm or less to the total snow carbon iron number is 20% Hereinafter, the average Xueming carbon iron diameter is 2.5 μm or less, and the proportion of the Xueming carbon iron relative to the total microstructure is 3.5% or more and 10.0% or less, and includes those with fertilizer grain iron and snow carbon iron Microstructure, excellent cold workability, and excellent hardenability. In addition, excellent mechanical properties with a tensile strength of 480 MPa or less and a total elongation (El) of 33% or more were also obtained.

On the other hand, it can be seen that in the comparative examples outside the scope of the present invention, any one or more of the composition, microstructure, the amount of solid solution B, and the amount of N in AlN does not satisfy the scope of the present invention, and the result is cold workability and hardenability. Any one or more cannot meet the above target performance. In addition, there are cases where one or more of the tensile strength (TS) and total elongation (El) cannot meet the target characteristics. For example, in Table 2-2 and Table 3-2, the amount of C in steel S is lower than the range of the present invention, and therefore does not satisfy the immersion hardenability. In addition, the C content of the steel T is higher than the range of the present invention, and therefore does not satisfy the characteristics of TS and total elongation of the steel sheet.

no

Claims (6)

A high-carbon hot-rolled steel sheet comprising the following ingredients: In terms of mass%, containing C: 0.20% or more and 0.50% or less, Si: 0.8% or less, Mn: 0.10% or more and 0.80% or less, P: 0.03% or less, S: less than 0.010%, sol.Al: 0.10% or less, N: 0.01% or less, Cr: 1.0% or less, B: 0.0005% or more and 0.005% or less, Furthermore, it contains one or two selected from Sb and Sn in a total of 0.002% or more and 0.1% or less, The remainder contains Fe and unavoidable impurities, The microstructure includes: ferrite grains, snow carbon iron, and perlite with an area ratio of 6.5% or less relative to the total microstructure, In the Xueming carbon iron, the ratio of the Xueming carbon iron with an equivalent circle diameter of 0.1 μm or less to the total Xueming carbon iron number is 20% or less, and the average Xueming carbon iron diameter is 2.5 μm or less. The ratio of carbon iron to the total microstructure is 3.5% or more and 10.0% or less in terms of area ratio, The average concentration of the amount of solid solution B in the region from the surface layer to a depth of 100 μm is 10 mass ppm or more, The average concentration of the amount of N present as AlN in the region from the surface layer to a depth of 100 μm is 70 mass ppm or less. The high-carbon hot-rolled steel sheet according to claim 1, wherein the tensile strength is 480 MPa or less, and the total elongation is 33% or more. The high-carbon hot-rolled steel sheet according to claim 1 or claim 2, wherein the average grain size of the ferrous iron is 4 μm to 25 μm. The high-carbon hot-rolled steel sheet according to any one of claims 1 to 3, which, in addition to the component composition, further contains a group selected from the following A group and B group in terms of mass% Group or two groups, Group A: Ti: Below 0.06% Group B: Let one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W be 0.0005% or more and 0.1% or less. A method for manufacturing a high-carbon hot-rolled steel sheet is the method for manufacturing a high-carbon hot-rolled steel sheet as described in any one of Claims 1 to 4, wherein after hot rough rolling of the steel having the composition, Perform final rolling at the end temperature of the final rolling: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, and at the winding temperature: 500°C to 700 After coiling at ℃ to make a hot-rolled steel sheet, heat the hot-rolled steel sheet at an average heating rate: 15°C/h or more to a temperature range of 450°C to 600°C, and an annealing temperature: lower than Ac ₁ transformation point Keep annealing for more than 1.0 h. A method for manufacturing a high-carbon hot-rolled steel sheet is the method for manufacturing a high-carbon hot-rolled steel sheet as described in any one of Claims 1 to 4, wherein after hot rough rolling of the steel having the composition, Perform final rolling at the end temperature of the final rolling: Ar ₃ transformation point or higher, and then cool to 650°C to 750°C at an average cooling rate: 20°C/sec to 100°C/sec, and at the winding temperature: 500°C to 700 After coiling at ℃ to prepare a hot-rolled steel sheet, the hot-rolled steel sheet is heated at an average heating rate of 15°C/h or more to a temperature range of 450°C to 600°C, which is above the Ac ₁ transformation point and Ac ₃ transformation Keep the temperature below the temperature for more than 0.5 h, and then cool down to below the Ar ₁ transformation point at an average cooling rate of 1°C/h to 20°C/h, and keep annealing for more than 20 h at a temperature lower than the Ar ₁ transformation point.