JP2022122483A - Hot rolled steel sheet and method for producing the same - Google Patents

Abstract

To provide a high-carbon hot rolled steel sheet that is excellent in cold workability and wear resistance after heat treatment such as induction hardening and normal hardening, and a method for producing the same.SOLUTION: A hot rolled steel sheet has a predetermined composition. A microstructure has ferrite and cementite. In the cementite, the ratio of the number of cementite particles with a circle-equivalent diameter of 0.1 μm or less to the total number of cementite particles is 20% or less, the average cementite diameter is 0.15 μm or more, and the proportion of the cementite in the total microstructure is 4.5% or more and less than 20.0% in area ratio. In the range from the steel sheet surface to 100 μm, the mass ratio of X contained in XC (X=Ti, Mo, Nb) of 1 μm or more to the amount of X (X=Ti, Mo, Nb) in the steel is 5% or less, and the total elongation (EI) is 20% or more.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet having excellent wear resistance suitable for automotive drive system parts, and a method for producing the same. More specifically, the present invention relates to a steel sheet subjected to induction hardening and dip quenching after obtaining a component shape by cold working, and to a hot rolled steel sheet having excellent wear resistance after heat treatment and a method for producing the same. The term "excellent wear resistance" means that the wear scar cross-sectional area after 30,000 cycles is 300 μm ² or less in a wear test using a ball-on-disk type wear tester.

At present, automobile parts such as transmissions, gears, and seat recliners are produced by cold-working hot-rolled steel sheets, which are carbon steel for machine structural use and alloy steel for machine structural use, as specified in JIS G4051, into desired shapes. After that, induction hardening and dip hardening are often applied to ensure the desired hardness. In general, the wear resistance of a steel sheet is improved by increasing its hardness. For this reason, for parts where wear resistance is important, steel materials that have been quenched and then tempered at a low temperature to achieve higher hardness, and steel materials with a high content of alloying elements that generate high-hardness carbides are used. ing.

In Patent Document 1, in mass %, C: 0.32 to 0.70%, Si: 0.5% or less, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.02% or less, Nb: 0.1 to 0.5%, the balance being Fe and unavoidable impurities, having a chemical composition of 200 to 1000/mm ² carbides with a particle diameter of 1 μm or more containing Nb A material steel sheet for machine parts is described which has an annealed structure present in a matrix with density.

In Patent Document 2, in mass%, C: 0.30 to 0.90%, Si: 0.05 to 1.00%, Mn: 0.10 to 1.50%, P: 0.003 to 0 0.030%, S: 0.001 to 0.020%, Nb: 0.10 to 0.70%, the balance having a chemical composition consisting of Fe and unavoidable impurities, and after refining heat treatment in which Nb-containing carbides are dispersed and the number of Nb-containing carbide particles having a particle size of 1.0 μm or more is 200, where the square root of the area of each Nb-containing carbide particle observed by observing the cross-sectional structure is the particle size of the particle. A wear-resistant steel material with excellent fatigue properties is proposed, in which the maximum particle size Dmax of Nb ^- containing carbide particles in 10 ³ mm ³ estimated by the extreme value statistics method is adjusted to 18.0 μm or less. It is

In Patent Document 3, in mass%, C: 0.3 to 0.6%, Si: 0.005 to 0.5%, Mn: 0.2 to 1.5%, P: 0.03% or less , S: 0.03% or less, Al: 0.01 to 0.1%, and N: 0.015% or less, and further as other elements, Cr: 0.5% or less, Cu: 0.25% or less , Ni: 0.25% or less, Mo: 0.25% or less, and B: 0.01% or less, and the metal structure of the steel before spheroidizing annealing is It has pearlite and pro-eutectoid ferrite, the total area ratio of pearlite and pro-eutectoid ferrite to the entire structure is 90% or more, and the area ratio A of pro-eutectoid ferrite is Ae = (0.8-Ceq ₁ ) × 96 Average circle of bcc-Fe crystal grains surrounded by large-angle grain boundaries that satisfy A>Ae in relation to the Ae value represented by .75 and have an orientation difference of more than 15° between two adjacent crystal grains A metal having an equivalent diameter of 15 to 35 μm, an average value of the largest grain size and the second largest grain size of the circle equivalent diameter of the bcc-Fe crystal grains of 50 μm or less, and further, after spheroidizing annealing. The structure has an average equivalent circle diameter of bcc-Fe crystal grains of 15 to 35 μm, and a cementite major diameter of 3 μm or more in the bcc-Fe crystal grains has an aspect ratio of 2.5 or less. Machine structural steels for cold working have been proposed.

Japanese Unexamined Patent Application Publication No. 2010-216008 JP 2013-136820 A JP 2013-147728 A

The technique described in Patent Document 1 controls the amount of Nb and Ti carbides to improve wear resistance, but focuses on carbides with a particle size of 1 μm or more, and does not describe carbides with a particle size of 1 μm or less. Control of cementite is important from the viewpoint of cold workability, but the control method is not described.

The technology described in Patent Document 2 defines the density and maximum particle size of NbC with a particle size of 1 μm or more, but does not describe carbides with a particle size of 1 μm or less. Control of cementite is important from the viewpoint of cold workability, but the control method is not described.

The technique described in Patent Document 3 mentions cementite of 3 μm or more in a spheroidized material, but does not describe cementite with an average diameter of less than 3 μm. Although the ratio of fine cementite is important from the viewpoint of cold workability, the control method is not described.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet excellent in cold workability and wear resistance after heat treatment such as induction hardening and dip hardening, and a method for producing the same. do.

The present inventors have made intensive studies in order to achieve the above objects. As a result, TiC, NbC, and MoC of 1 μm or more are reduced as much as possible, and TiC, NbC, and MoC of less than 1 μm greatly contribute to the improvement of wear resistance, and the amount of Ti used for their formation, The ratios of the Nb content and the Mo content to the total Ti content, the total Nb content and the total Mo content, and the manufacturing conditions for obtaining a predetermined steel sheet structure were studied and the following findings were obtained.
i) The mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 µm or more to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 µm from the steel plate surface is 5% or less. Excellent wear resistance is obtained by this.
ii) The mass ratio of the amount of X contained in XC (X=Ti, Mo, Nb) of 100 nm or more to the amount of X contained in XC (X=Ti, Mo, Nb) in the range of 100 μm from the steel plate surface is 65% A more excellent abrasion resistance is obtained by setting it as above.
iii) To obtain a predetermined amount of Ti, Nb and Mo contained in TiC, NbC and MoC, start rough rolling within 70 seconds after holding the slab in a temperature range of 1250 ° C. or less for 1.5 hours or more, It is important to carry out the rough rolling under the conditions of a temperature range of 1070° C., 5 passes or more, a rolling reduction of 50% or more, and an interpass time of 1 s or more and 15 s or less. By rolling while applying a predetermined strain in these temperature ranges, TiC, NbC and MoC of less than 1 μm are likely to be produced.
iv) Furthermore, by performing rough rolling so that the rolling reduction rate is 30% or less in the temperature range of 1000 to 1070 ° C. in the first pass of rough rolling, TiC, NbC and MoC of less than 1 μm are more likely to be generated. .
v) Cementite having an equivalent circle diameter of 0.1 μm or less greatly affects cold workability, hardness, and total elongation (hereinafter sometimes simply referred to as elongation) of hot-rolled steel sheets before quenching. is doing. By setting the number of cementites having an equivalent circle diameter of 0.1 μm or less to 20% or less of the total number of cementites, a total elongation (El) of 20% or more can be obtained.
vi) Start rough rolling within 70 seconds after holding the slab in a temperature range of 1250 ° C. or less for 1.5 hours or more, temperature range of 900 to 1070 ° C., 5 passes or more, rolling reduction of 50% or more, time between passes of 1 s or more Rough rolling is performed under conditions of 15 seconds or less, finish rolling end temperature: finish rolling is performed at the Ar ₃ transformation point or higher, then average cooling rate: 20 to 100 ° C./sec to 700 to 750 ° C. After coiling at a temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet, the hot-rolled steel sheet is annealed at an annealing temperature of less than the Ac ₁ transformation point for 0.5 h or more, thereby cold working. It can impart properties and improve wear resistance after heat treatment.
vii) Alternatively, the sheet is held in a temperature range of 1250°C or lower for 1.5 hours or more, and rough rolling is started within 70 seconds after removal from the heating furnace, and the temperature range is 900 to 1070°C, 5 passes or more, and the rolling reduction is 50% or more. , Perform rough rolling under the condition that the time between passes is 1 s or more and 15 s or less, finish rolling finish temperature: Ar ₃ transformation point or higher, and then average cooling rate: 700 to 750 at 20 to 100 ° C./sec. After cooling to ° C. and coiling at a coiling temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet, the hot-rolled steel sheet is held at the Ac ₁ transformation point or more and Ac ₃ transformation point or less for 0.5 hours or more, Then, it is cooled to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20° C./h, and is annealed at less than the Ar ₁ transformation point for 20 hours or more to impart cold workability and wear resistance after heat treatment. can improve sexuality.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] % by mass,
C: 0.30% or more and less than 1.0%,
Si: 0.80% or less,
Mn: 0.10% or more and 1.0% or less,
P: 0.03% or less,
S: 0.010% or less,
sol. Al: 0.001% or more and 0.10% or less,
N: 0.01% or less,
Cr: 0.05% or more and 0.50% or less,
B: containing 0.0005% or more and 0.005% or less,
Furthermore, Ti: more than 0.06% and 0.2% or less, Mo: more than 0.10% and 0.4% or less, Nb: more than 0.10% and 0.2% or less ,
Having a component composition in which the balance is Fe and unavoidable impurities,
The microstructure is
having ferrite and cementite,
The cementite has a ratio of cementite with a circle equivalent diameter of 0.1 μm or less to the total number of cementite of 20% or less, an average cementite diameter of 0.15 μm or more, and a ratio of the cementite to the total microstructure in terms of area ratio of 4.5. % or more and less than 20.0%,
The mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 µm or more with respect to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 µm from the steel plate surface is 5% or less,
A hot-rolled steel sheet having a total elongation (El) of 20% or more.
[2] The mass ratio of the amount of X contained in XC (X = Ti, Mo, Nb) of 100 nm or more to the amount of X contained in XC (X = Ti, Mo, Nb) in the range of 100 µm from the steel plate surface is 65. % or more of the hot-rolled steel sheet according to [1].
[3] The hot-rolled steel sheet according to [1] or [2], containing 0.002% or more and 0.1% or less in total of one or two selected from Sb and Sn.
[4] In addition to the above component composition, one or more selected from Ta, Ni, Cu, V, and W, each containing 0.0005% or more and 0.1% or less in mass%. The hot-rolled steel sheet according to any one of [1] to [3].
[5] The hot-rolled steel sheet according to any one of [1] to [4], wherein the ferrite has an average grain size of 4 to 25 μm.
[6] A method for manufacturing a hot-rolled steel sheet according to any one of [1] to [5],
Rough rolling is started within 70 seconds after holding the steel material having the above chemical composition in a temperature range of 1250 ° C. or less for 1.5 hours or more, and the temperature range is 900 to 1070 ° C., 5 passes or more, and the total rolling reduction is 50% or more. , Perform rough rolling under the condition that the time between passes is 1 s or more and 15 s or less, finish rolling finish temperature: perform finish rolling at the Ar ₃ transformation point or higher,
Then, cool to 700-750°C at an average cooling rate of 20-100°C/sec,
Coiling temperature: After coiling at a temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet,
A method for producing a hot-rolled steel sheet, wherein the hot-rolled steel sheet is annealed at an annealing temperature of less than the Ac ₁ transformation point for 0.5 hours or more.
[7] The method for producing a hot-rolled steel sheet according to [6], wherein the first pass of the rough rolling performs rough rolling at a temperature range of 1000 to 1070° C. so that the rolling reduction is 30% or less.
[8] The method for producing a hot-rolled steel sheet according to any one of [1] to [5], wherein the steel material having the chemical composition is held in a temperature range of 1250 ° C. or lower for 1.5 hours or longer, and then held for 70 seconds. Rough rolling is started within 900 to 1070 ° C., 5 passes or more, the total rolling reduction is 50% or more, and the time between passes is 1 s or more and 15 s or less. Perform finish rolling at ₃ transformation points or more,
Then, cool to 700-750°C at an average cooling rate of 20-100°C/sec,
Coiling temperature: After coiling at a temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet,
The hot-rolled steel sheet is held at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower for 0.5 h or more, and then cooled to less than the Ar ₁ transformation point at an average cooling rate of ₁ to 20 ° C./h. A method for manufacturing a hot-rolled steel sheet that is annealed at a temperature of 20 h or more.
[9] The method for producing a hot-rolled steel sheet according to [8], wherein the first pass of the rough rolling performs rough rolling in a temperature range of 1000 to 1070°C so that the rolling reduction is 30% or less.

According to the present invention, a hot-rolled steel sheet having excellent cold workability and wear resistance after heat treatment can be obtained. By applying the hot-rolled steel sheet produced by the present invention to parts such as drive system parts that require wear characteristics, it can greatly contribute to the production of automotive drive system parts that require stable quality. It has a remarkable effect on industry.

The hot-rolled steel sheet and the method for producing the same according to the present invention are described in detail below. In addition, this invention is not limited to the following embodiment.

1) Component composition The component composition of the hot-rolled steel sheet of the present invention and the reasons for its limitation will be described. In addition, "%", which is a unit of content in the following component compositions, means "% by mass" unless otherwise specified.

C: 0.30% or more and less than 1.0% C is an important element for obtaining strength after quenching. If the amount of C is less than 0.30%, the desired hardness cannot be obtained when heat treatment such as induction hardening and dip hardening is used after molding into a part. There is a need to. However, if the amount of C is 1.0% or more, the base metal becomes hard and the toughness and cold workability deteriorate. Therefore, the amount of C should be 0.30% or more and less than 1.0%. Preferably, it is 0.75% or less. More preferably, it is 0.65% or less. It is preferably 0.32% or more.

Si: 0.80% or less Si is an element that increases strength by solid solution strengthening. As the Si content increases, the steel hardens and the cold workability deteriorates, so the Si content is made 0.80% or less. It is preferably 0.60% or less, more preferably 0.50% or less. From the viewpoint of ensuring a predetermined softening resistance in the tempering process after quenching, the Si content is preferably 0.10% or more, more preferably 0.20% or more, and still more preferably 0.30% or more. do.

Mn: 0.10% to 1.0% Mn is an element that improves hardenability and increases strength by solid solution strengthening. If the Mn content is less than 0.10%, the hardenability in induction hardening and dip hardening begins to deteriorate, so the Mn content is made 0.10% or more. When a thick material is to be quenched to the inside, the content is preferably 0.25% or more, more preferably 0.30% or more. On the other hand, if the Mn content exceeds 1.0%, a band structure due to Mn segregation develops, the structure becomes non-uniform, and solid solution strengthening hardens the steel and reduces cold workability. Therefore, the Mn content is set to 1.0% or less. As a material for parts requiring formability, a predetermined cold workability is required, so the Mn content is preferably 0.7% or less. More preferably, it is 0.55% or less.

P: 0.03% or less P is an element that increases the strength by solid solution strengthening. When the P content exceeds 0.03%, intergranular embrittlement is caused and the toughness after quenching deteriorates. Moreover, cold workability is also deteriorated. Therefore, the amount of P is set to 0.03% or less. In order to obtain excellent toughness after quenching, the P content is preferably 0.02% or less. Since P deteriorates cold workability and toughness after quenching, the smaller the amount of P, the better. However, excessive reduction of P increases the refining cost, so the P content is preferably 0.005% or more. More preferably, it is 0.007% or more.

S: 0.010% or less S is an element that must be reduced because it forms sulfides and lowers the cold workability of hot-rolled steel sheets and the toughness after quenching. If the S content exceeds 0.010%, the cold workability of the 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 S content is preferably 0.005% or less. Since S degrades cold workability and toughness after quenching, the smaller the amount of S, the better. However, excessive reduction of S increases the refining cost, so the S content is preferably 0.0005% or more.

sol. Al: 0.001% or more and 0.10% or less sol. If the amount of Al is less than 0.001%, the amount of AlN in the steel sheet is too small, grain growth of austenite occurs during heating in the austenite region, and a predetermined fatigue strength cannot be obtained. On the other hand, if the content exceeds 0.10%, a large number of Al-derived inclusions are formed, which may cause cracks during molding of parts. In addition, sol. Al has a deoxidizing effect, and is preferably 0.005% or more for sufficient deoxidizing.

N: 0.01% or less When the amount of N exceeds 0.01%, the amount of dissolved N in the steel sheet increases and the steel sheet hardens, so that the desired tensile strength and elongation cannot be obtained, and the cold workability deteriorates. descend. Therefore, it should be 0.01% or less. Preferably, it is 0.0065% or less. More preferably, it is 0.0050% or less. Note that N forms AlN, Cr-based nitrides and B-nitrides. As a result, it is an element that moderately suppresses the growth of austenite grains during heating in the quenching process and improves the toughness after quenching. Therefore, the N content is preferably 0.0005% or more. More preferably, it is 0.0010% or more.

Cr: 0.05% or more and 0.50% or less In the present invention, Cr is an important element for enhancing hardenability. If the Cr content is less than 0.05%, no sufficient effect is observed, so the Cr content must be 0.05% or more. Also, if the Cr content in the steel is 0%, ferrite tends to occur in the surface layer, especially in induction hardening and dip hardening, and a completely hardened structure cannot be obtained, and hardness tends to decrease. Therefore, from the viewpoint of emphasizing hardenability, the Cr content is set to 0.05% or more, preferably 0.10% or more. On the other hand, when the amount of Cr exceeds 0.50%, the steel sheet before quenching hardens and the cold workability is impaired. Therefore, the Cr content is set to 0.50% or less. In addition, when processing parts that require high workability that are difficult to press-form, even better cold workability is required, so the Cr content is preferably 0.45% or less. .30% or less is more preferable.

B: 0.0005% or more and 0.005% or less In the present invention, B is an important element for enhancing hardenability. If the amount of B is less than 0.0005%, a sufficient effect is not recognized, so the amount of B must be 0.0005% or more. Preferably, it is 0.0010% or more. On the other hand, when the amount of B exceeds 0.005%, the recrystallization of austenite after finish rolling is delayed, and as a result, the texture of the hot-rolled steel sheet develops, the anisotropy after annealing increases, and in drawing forming Ears are more likely to occur. Therefore, the amount of B is set to 0.005% or less. Preferably, it is 0.004% or less.

Furthermore, in the present invention, one selected from Ti: more than 0.06% and 0.2% or less, Mo: more than 0.10% and 0.4% or less, and Nb: more than 0.10% and 0.2% or less contain the above.

Ti: more than 0.06% and 0.2% or less Ti forms TiC in the steel sheet, and TiC has a high effect of improving wear resistance after heat treatment. If the amount of Ti is 0.06% or less, the effect is not recognized, so when Ti is contained, the amount of Ti should be more than 0.06%. More preferably, it is 0.1% or more. On the other hand, when the amount of Ti exceeds 0.2%, the steel sheet before quenching hardens and the cold workability is impaired. Therefore, when Ti is contained, the amount of Ti should be 0.2% or less. Preferably, it is 0.15% or less.

Mo: More than 0.10% and 0.4% or less Mo is an element highly effective in improving wear resistance after heat treatment. If it is 0.10% or less, the addition effect is small, so when Mo is contained, the lower limit is made more than 0.10%. More preferably, it should be 0.11% or more. When Mo exceeds 0.4%, the effect of addition is saturated and the cost increases, so when Mo is contained, the upper limit is made 0.4%. It is more preferably 0.35% or less, more preferably 0.3% or less.

Nb: More than 0.10% and 0.2% or less Nb is an element highly effective in forming NbC and NbN and improving wear resistance after heat treatment. If the Nb content is 0.10% or less, the effect is weak. On the other hand, when Nb exceeds 0.2%, not only does the effect of addition saturate, but the Nb carbide increases the tensile strength of the base material and reduces the elongation. is 0.2%. It is more preferably 0.18% or less, and still more preferably 0.15% or less.

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

With the above essential elements, the hot-rolled steel sheet of the present invention can obtain the desired properties. In addition, the hot-rolled steel sheet of the present invention may contain the following elements, if necessary, for the purpose of further improving hardenability, for example.

Sum of one or two selected from Sb and Sn: 0.002% or more and 0.1% or less Sb and Sn are elements effective in suppressing nitriding from the steel sheet surface layer. If the total content of one or more of these elements is less than 0.002%, a sufficient effect cannot be obtained. 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 anti-nitriding effect is saturated. In addition, since these elements tend to segregate at grain boundaries, if the total content exceeds 0.1%, the content becomes too high and may cause grain boundary embrittlement. Therefore, when one or two selected from Sb and Sn are contained, the total content shall be 0.1% or less. It is preferably 0.03% or less, more preferably 0.02% or less.

In order to stabilize the mechanical properties and hardenability of the present invention, one or two or more selected from Ta, Ni, Cu, V, and W are added in an amount of 0.0005% or more and 0.1% or less, respectively. may

Ta: 0.0005% or more and 0.1% or less Ta, like Nb, forms carbonitrides, prevents abnormal grain growth and coarsening of crystal grains during heating before quenching, and improves temper softening resistance. is an effective element for If it is less than 0.0005%, the addition effect is small, so when Ta is contained, the lower limit is preferably 0.0005%. More preferably, it should be 0.0010% or more. When Ta exceeds 0.1%, the effect of addition is saturated, the quenching hardness is reduced due to excessive carbide formation, and the cost increases, so when Ta is contained, the upper limit is made 0.1%. is preferred. It is more preferably 0.05% or less, and still more preferably less than 0.03%.

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

Cu: 0.0005% or more and 0.1% or less Cu is an element effective for ensuring hardenability. If Cu is less than 0.0005%, the effect of addition is not sufficiently confirmed, so when Cu is contained, the lower limit is preferably 0.0005%. More preferably, it should be 0.0010% or more. If Cu exceeds 0.1%, flaws are likely to occur during hot rolling, resulting in deterioration in manufacturability such as a drop in yield. More preferably, it is 0.05% or less.

V: 0.0005% or more and 0.1% or less V, like Nb and Ta, forms carbonitrides, prevents abnormal grain growth of crystal grains during heating before quenching, improves toughness, and improves temper softening resistance. It is an effective element. If V is less than 0.0005%, the effect of addition is not sufficiently exhibited, so when V is contained, the lower limit is preferably set to 0.0005%. More preferably, it should be 0.0010% or more. When V exceeds 0.1%, not only does the effect of addition saturate, but also the elongation decreases as the tensile strength of the base material increases due to the formation of carbides. 1% is preferable. It is more preferably 0.05% or less, and still more preferably less than 0.03%.

W: 0.0005% or more and 0.1% or less W, like Nb and V, forms carbonitrides and is effective in preventing abnormal grain growth of austenite grains during heating before quenching and improving temper softening resistance. element. If it is less than 0.0005%, the addition effect is small, so when W is contained, the lower limit is preferably 0.0005%. More preferably, it should be 0.0010% or more. When W exceeds 0.1%, the effect of addition is saturated, the quenching hardness is reduced due to excessive carbide formation, and the cost increases, so when W is included, the upper limit is made 0.1%. is preferred. It is more preferably 0.05% or less, and still more preferably less than 0.03%.

In the present invention, when two or more selected from Ta, Ni, Cu, V, and W are contained, the total amount is preferably 0.0010% or more and 0.1% or less.

2) Microstructure The reasons for limiting the microstructure of the hot-rolled steel sheet of the present invention will be explained.

2-1) Ferrite and Cementite The microstructure of the hot-rolled steel sheet of the present invention contains ferrite and cementite. In the present invention, the area ratio of ferrite is preferably 92% or more. When the area ratio of ferrite is less than 92%, formability deteriorates, and it may become difficult to cold work parts with a high degree of working. Therefore, the area ratio of ferrite is preferably 92% or more. More preferably, it is 94% or more.

In addition to the ferrite and cementite described above, the microstructure of the hot-rolled steel sheet of the present invention may contain pearlite. As long as the area ratio of pearlite to the entire microstructure is 6.5% or less, it does not impair the effect of the present invention, so it may be contained. Since it may be 0%, the lower limit is set to 0%.

2-2) Percentage of cementite with an equivalent circle diameter of 0.1 μm or less to the total number of cementite: 20% or less If there is a large amount of cementite with an equivalent circle diameter of 0.1 μm or less, the cementite becomes hard due to dispersion strengthening, and the elongation decreases. From the viewpoint of obtaining cold workability, in the present invention, the number of cementites having an equivalent circle diameter of 0.1 μm or less is made 20% or less of the total number of cementites. As a result, a total elongation (El) of 20% or more can be achieved.

Furthermore, when the hot-rolled steel sheet of the present invention is used for difficult-to-form parts, high cold workability is required. In this case, the number of cementites having an equivalent circle diameter of 0.1 μm or less is preferably 10% or less of the total number of cementites. A total elongation (El) of 25% or more can be achieved by setting the number of cementites having an equivalent circle diameter of 0.1 μm or less to 10% or less of the total number of cementites. The reason why the ratio of cementite having an equivalent circle diameter of 0.1 μm or less is defined is that cementite having a diameter of 0.1 μm or less produces dispersion strengthening ability, and if cementite having that size increases, cold workability is hindered. is.

From the viewpoint of suppressing abnormal grain growth of ferrite grains during annealing, it is preferable that the number of cementites having an equivalent circle diameter of 0.1 μm or less is 3% or more of the total number of cementites. The diameter of cementite that exists before quenching is about 0.07 to 3.0 μm in equivalent circle diameter. The dispersed state of cementite having an equivalent circle diameter of more than 0.1 μm before quenching is not particularly defined in the present invention because the size does not significantly affect precipitation strengthening.

2-3) Average Cementite Diameter: 0.15 μm or More A large amount of excessively small cementite decreases cold workability. From the viewpoint of cold workability, it is necessary to disperse cementite having a predetermined size, and the average cementite diameter is set to 0.15 μm or more. More preferably, the thickness is 0.2 μm or more. On the other hand, it is necessary to melt all cementite during quenching to ensure a predetermined solid solution amount of C in ferrite. If the average cementite diameter exceeds 2.5 μm, the cementite cannot be completely dissolved during holding in the austenite region, and especially in short-time heating such as induction hardening, cementite remains undissolved. .5 μm or less. More preferably, it is 2.0 μm or less.

In the present invention, the term "cementite diameter" refers to the equivalent circle diameter of cementite. The "average cementite diameter" refers to a value obtained by dividing the sum of equivalent circle diameters of all cementites converted to equivalent circle diameters by the total number of cementites.

2-4) The ratio (area ratio) of cementite to the entire microstructure is 4.5% or more and less than 20.0% When the ratio of the area ratio of cementite to the entire microstructure is less than 4.5%, the base material strength decreases. Since the strength of a member used without heat treatment may be insufficient, the content is made 4.5% or more. Preferably it is 5.0% or more. On the other hand, if the strength of the base material increases and the elongation is particularly small, the risk of cracking in the difficult-to-form parts increases, so it is necessary to ensure a predetermined elongation. In order to obtain the desired elongation, the above proportion should be less than 20.0%. More preferably, it is 15.0% or less.

2-5) Average grain size of ferrite: 4 to 25 μm (preferred conditions)
If the average grain size of the ferrite is less than 4 µm, the strength before cold working increases, and there is a risk that the press formability may deteriorate. On the other hand, if the average grain size of ferrite exceeds 25 μm, the strength of the base material may decrease. In addition, the base material must have a certain degree of strength in areas where it is used without being quenched after being molded into the intended product shape. Therefore, the average grain size of ferrite is preferably 25 μm or less. It is more preferably 5 μm or more, and still more preferably 6 μm or more. It is more preferably 20 μm or less, still more preferably 18 μm or less.

In the present invention, the equivalent circle diameter of cementite, the average cementite diameter, the ratio of cementite to the total microstructure, the area ratio of ferrite, the average grain size of ferrite, etc. are measured by the method described in the examples described later. can do.

2-6) The mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 μm or more to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 μm from the steel plate surface is 5% or less. In the present invention, the mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 μm or more with respect to the amount of X (X = Ti, Mo, Nb) in the steel at a surface layer of 100 μm is 5% or less. , X required for coarse XC can be suppressed, a large amount of XC of less than 1 μm can be precipitated, and a predetermined wear resistance can be obtained. Preferably, it is 3% or less. Note that the mass ratio of X contained in XC (X=Ti, Mo, Nb) of 1 μm or more is preferably as low as possible, so it may be 0%.

2-7) Mass ratio of the amount of X contained in XC (X = Ti, Mo, Nb) of 100 nm or more to the amount of X contained in XC (X = Ti, Mo, Nb) in the range of 100 μm from the steel plate surface is 65% or more (preferred conditions)
Since TiC, MoC, and NbC with a particle size of 100 nm or more greatly contribute to the improvement of wear resistance, the mass ratio thereof is preferably 65% or more.

In the present invention, the ratio of Ti, Nb, and Mo used for precipitation of TiC, NbC, and MoC of 100 nm or more is closely related to the holding time from slab heating to rough rolling, the temperature in rough rolling, the reduction ratio, and the time between passes. , it has been found necessary to optimize these sets of manufacturing conditions. The reasons for obtaining the proportions of Ti, Nb, and Mo used for depositing TiC, NbC, and MoC of 100 nm or more will be described later.

In the range of 100 μm from the steel plate surface, the mass ratio of the amount of X contained in XC (X=Ti, Mo, Nb) of 100 nm or more to the amount of X in the steel (X=Ti, Mo, Nb) is the wear resistance , Ti is preferably 65% or more, Mo is 30% or more, and Nb is 80% or more. More preferably, Ti is 70% or more, Mo is 35% or more, and Nb is 85% or more.

3) Mechanical Properties The hot-rolled steel sheet of the present invention is required to have excellent cold-workability because it is cold-pressed to form automotive drive train parts. In addition, it is necessary to increase the hardness by quenching treatment to impart wear resistance. Therefore, the hot-rolled steel sheet of the present invention has excellent cold workability by increasing the total elongation and making the total elongation (El) 20% or more, and at the time of heat treatment such as induction hardening and dip hardening, austenite grains It is possible to suppress growth and obtain excellent wear resistance after heat treatment. More preferably, El is 25% or more.

The above-mentioned total elongation (El) can be measured by the method described in Examples below.

4) Manufacturing method Hereinafter, reasons for limitation in the manufacturing method of the hot-rolled steel sheet of the present invention will be described. In the description, "°C" regarding temperature indicates the temperature on the surface of the steel plate or the surface of the steel material.

In the present invention, the method for manufacturing the steel material is not particularly limited. For example, both a converter and an electric furnace can be used to smelt the steel of the present invention. Steel melted by a known method such as a converter is made into a slab or the like (steel material) by ingot making-slabbing rolling or continuous casting. A slab is usually hot rolled (rough hot rolling, finish rolling) after being heated.

For example, in the case of a slab manufactured by continuous casting, direct rolling may be applied in which the slab is rolled as it is or while being heat-retained for the purpose of suppressing a temperature drop. In the hot rolling, the material to be rolled may be heated by a heating means such as a bar heater in order to secure the finishing temperature of finish rolling.

Heating temperature of the steel material (slab heating temperature): Hold at a temperature of 1250°C or less for 1.5 hours or more If the slab heating temperature is too high, a large number of coarse TiN precipitates, which is effective for improving wear resistance. Less Ti is used for the more TiC. Therefore, the slab heating temperature is set to 1250° C. or less. It is preferably 1200° C. or less. The holding time is set to 1.5 hours or longer in order to uniformly disperse the alloying elements in the steel.

Rough rolling starts within 70 seconds The mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 μm or more to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 μm from the steel plate surface is 5 % or less, it is necessary to shorten the holding time at high temperatures. Therefore, after the temperature range of 1250° C. or less is maintained for 1.5 hours or more, the time until the start of rough rolling is set to 70 seconds or less. Preferably, it is 65s or less.

Rough rolling is performed under the conditions of a temperature range of 900 to 1070 ° C, 5 passes or more, a total rolling reduction of 50% or more, and an interpass time of 1 s to 15 s. In order to make the mass ratio of X contained in XC (X=Ti, Mo, Nb) of 1 μm or more to the amount of Mo, Nb) 5% or less, 5 passes or more in the temperature range of 900 to 1070 ° C. , it is necessary to perform rough rolling so that the total rolling reduction is 50% or more. Furthermore, if an interpass time of 1 s or more cannot be secured, XC (X = Ti, Mo, Nb) of 1 μm or more with respect to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 μm from the steel plate surface 1 s or more because the mass ratio of X contained in the film cannot be ensured to be 5% or less. Considering the workability, the time between passes is preferably 15 seconds or less. More preferably, it is 10 s or less.

Rolling reduction of the first pass in rough rolling in a temperature range of 1000 to 1070 ° C.: 30% or less (preferred conditions)
When a high strain is applied at a high temperature, a large number of TiC, NbC, and MoC of less than 100 nm are precipitated, and XC (X = Ti, Mo, Nb) of 100 nm or more with respect to the amount of X contained in XC (X = Ti, Mo, Nb). It becomes impossible to ensure that the mass ratio of the amount of X contained is 65% or more. Therefore, it is preferable to set the rolling reduction to 30% or less. The temperature range is preferably from 1000 to 1070° C. in order to obtain a predetermined XC.

Finish rolling temperature: Finish rolling above the Ar ₃ transformation point If the finish rolling temperature is lower than the Ar ₃ transformation point, coarse ferrite grains are formed after hot rolling and after annealing, resulting in a marked drop in elongation. For this reason, the finish rolling end temperature is set to the Ar ₃ transformation point or higher. It is preferably (Ar ₃ transformation point + 20°C) or higher. Although the upper limit of the finishing temperature of finish rolling need not be specified, it is preferably 1000° C. or less in order to perform cooling smoothly after finish rolling.

Average cooling rate after finish rolling: cooling to 700-750°C at 20-100°C/sec After finish rolling, the average cooling rate to 700-750°C greatly affects the size of spheroidized cementite after annealing. If the average cooling rate after finish rolling is less than 20° C./sec, the structure before annealing becomes ferrite and pearlite structures with too many ferrite structures, so that the desired cementite dispersion state and size cannot be obtained after annealing. Therefore, it is necessary to cool at 20° C./sec or more. Preferably, it is 25° C./sec or more. On the other hand, if the average cooling rate exceeds 100°C/sec, it becomes difficult to obtain cementite having a predetermined size after annealing, so the cooling rate is set to 100°C/sec or less. Preferably, it is 75°C/sec or less.

Coiling temperature: more than 580°C and 700°C or less The hot-rolled steel sheet after finish rolling is coiled into a coil shape. If the coiling temperature is too high, the strength of the hot-rolled steel sheet becomes too low, and when the steel sheet is wound into a coil shape, it may be deformed by its own weight. Therefore, it is not preferable from an operational point of view. Therefore, the upper limit of the winding temperature is set at 700°C. It is preferably 690° C. or less. On the other hand, if the coiling temperature is too low, the hot-rolled steel sheet is hardened, which is not preferable. Therefore, the winding temperature should be above 580°C. It is preferably 600° C. or higher.

After winding into a coil shape, it may be cooled to room temperature and pickled. Annealing is performed after the pickling treatment. A known method can be applied to the pickling treatment. After that, the obtained hot-rolled steel sheet is subjected to the following annealing.

Annealing temperature: less than the Ac ₁ transformation point and held for 0.5 h or longer When the annealing temperature is equal to or higher than the Ac ₁ transformation point, austenite precipitates, and a coarse pearlite structure is formed in the cooling process after annealing, resulting in a non-uniform structure. Become. Therefore, the annealing temperature should be less than the Ac ₁ transformation point. It is preferably (Ac ₁ transformation point -10°C) or less. Although the lower limit of the annealing temperature is not specified, the annealing temperature is preferably 650° C. or higher, more preferably 700° C. or higher, in order to obtain a predetermined state of cementite dispersion. Any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used as the atmospheric gas. Further, the holding time at the above annealing temperature is set to 0.5 hours or more. If the holding time at the annealing temperature is less than 0.5 hours, the effect of annealing is poor and the target structure of the present invention cannot be obtained. As a result, the target hardness and elongation of the steel sheet cannot be obtained. sometimes not. Therefore, the holding time at the above annealing temperature should be 0.5 hours or more. It is more preferably 5 hours or more, and still more preferably over 20 hours. On the other hand, when the holding time at the above annealing temperature exceeds 40 hours, the productivity decreases and the manufacturing cost becomes excessive. Therefore, the holding time at the above annealing temperature is preferably 40 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 annealing described above. Specifically, after winding, it is held at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower for 0.5 h or longer (first stage annealing), and then the average cooling rate: 1 to 20 ° C./h for Ar ₁ transformation. It is also possible to manufacture by performing two-stage annealing in which the steel is cooled below the Ar ₁ transformation point and held for 20 hours or longer (second-stage annealing).

In the present invention, the hot-rolled steel sheet is held at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower for 0.5 hours or longer, and relatively fine carbides precipitated in the hot-rolled steel sheet are dissolved to form a solid solution in the γ phase. and then cooled to below the Ar ₁ transformation point at an average cooling rate of 1 to 20° C./h and held below the Ar ₁ transformation point for 20 hours or more. As a result, solid solution C is precipitated using relatively coarse undissolved carbides or the like as nuclei, and the ratio of the number of cementites having an equivalent circle diameter of 0.1 μm or less to the total number of cementites is 20% or less. (cementite) distribution can be controlled. That is, in the present invention, the steel sheet is softened by controlling the dispersion form of carbides by performing two-stage annealing under predetermined conditions. In the hot-rolled steel sheet targeted by the present invention, it is important to control the dispersion form of carbides after annealing in order to soften the steel. In the present invention, by holding the hot-rolled steel sheet at the Ac ₁ transformation point or more and the Ac ₃ transformation point or less (first stage annealing), fine carbides are dissolved and C is dissolved in γ (austenite). . In the subsequent cooling stage below the Ar ₁ transformation point and the holding stage (second annealing), the α / γ interface and undissolved carbide existing in the temperature range above the Ac ₁ transformation point become nucleation sites, and relatively coarse carbide precipitates. The conditions for such two-step annealing will be described below. Any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used as the atmosphere gas during annealing.

Hold for 0.5 h or more at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower (first stage annealing)
By heating the hot-rolled steel sheet to an annealing temperature equal to or higher than the Ac ₁ transformation point, part of the ferrite in the steel sheet structure is transformed into austenite, fine carbides precipitated in the ferrite are dissolved, and C is added to the austenite. Dissolve. On the other hand, the remaining ferrite that has not been transformed into austenite is annealed at a high temperature, so that the dislocation density is reduced and the ferrite is softened. In addition, relatively coarse carbides (undissolved carbides) that have not been dissolved remain in the ferrite, but become coarser due to Ostwald growth. When the annealing temperature is lower than the Ac ₁ transformation point, austenite transformation does not occur, so carbides cannot be dissolved in austenite. On the other hand, if the annealing temperature in the first stage exceeds the Ac ₃ transformation point _, a large amount of rod-shaped cementite is obtained after annealing, and the predetermined elongation cannot be obtained. Further, in the present invention, fine carbides cannot be sufficiently dissolved if the holding time at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower is less than 0.5 hours. Therefore, as the first stage annealing, the steel is held at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower for 0.5 hours or longer. The retention time is preferably 1.0 h or more. Also, the holding time is preferably 10 hours or less.

Average cooling rate: 1 to 20 ° C./h to less than the Ar ₁ transformation point After the above-described first stage annealing, the temperature range for the second stage annealing, which is the Ar ₁ transformation point, average cooling rate: 1 Cool at ~20°C/h. During cooling, C discharged from austenite accompanying transformation from austenite to ferrite precipitates as relatively coarse spherical carbides using α/γ interfaces and undissolved carbides as nucleation sites. In this cooling, it is necessary to adjust the cooling rate so as not to generate pearlite. If 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 made 1° C./h or more. It is preferably 5° C./h or more. On the other hand, if the average cooling rate exceeds 20° C./h, pearlite precipitates and the hardness increases, so the cooling rate is set to 20° C./h or less. It is preferably 15° C./h or less.

Hold for 20 hours or more below the Ar ₁ transformation point (second stage annealing)
After the above-described first-stage annealing, the steel is cooled at a predetermined average cooling rate and held below the Ar ₁ transformation point to further grow coarse spherical carbides and eliminate fine carbides by Ostwald growth. If the holding time below the Ar ₁ transformation point is less than 20 hours, the carbide cannot be sufficiently grown, and the hardness after annealing becomes too large. For this reason, the second stage annealing is performed at a temperature lower than the Ar ₁ transformation point and maintained for 20 hours or longer. Although not particularly limited, the second annealing temperature is preferably 660° C. or higher in order to sufficiently grow carbides, and the holding time is preferably 30 hours or less from the viewpoint of production efficiency. preferable.

The above-mentioned Ac ₃ transformation point, Ac ₁ transformation point, Ar ₃ transformation point, and Ar ₁ transformation point can be determined by actual measurement by thermal expansion measurement and electrical resistance measurement during heating and cooling by a Formaster test. can.

The average heating rate and average cooling rate described above are obtained by measuring the temperature with a thermocouple installed in the furnace.

Steels having chemical compositions of Steel Nos. A to Q shown in Table 1 were melted and then hot-rolled according to the production conditions shown in Tables 2 and 3. Next, it was pickled and annealed (spheroidized annealing) in a nitrogen atmosphere (atmosphere gas: nitrogen) at the annealing temperature and annealing time (h) shown in Tables 2 and 3 to obtain a plate with a thickness of 3.0 mm. A hot-rolled and annealed sheet was produced.

A test piece was taken from the hot rolled and annealed sheet thus obtained, and the microstructure, the amount of Ti contained in TiC of 100 nm or more, the amount of Mo contained in MoC of 100 nm or more, and the amount of Mo contained in NbC of 100 nm or more The Nb content, tensile strength, total elongation, steel plate hardness after sub-quenching, and wear resistance after sub-quenching were determined. The Ac ₃ transformation point, Ac ₁ transformation point, Ar ₁ transformation point and Ar ₃ transformation point shown in Table 1 were determined by the Formaster test.

(1) Microstructure The microstructure of the steel sheet after annealing is obtained by cutting and polishing a test piece (size: 3 mmt × 10 mm × 10 mm) taken from the center of the width of the plate, and then applying nital corrosion. was used to photograph at 3000 times magnification at 5 locations from the surface layer to 1/4 of the plate thickness. Each phase (ferrite, cementite, pearlite, etc.) was identified by image processing of the photographed structure photograph. Tables 2 and 3 show the "perlite area ratio" as the microstructure, and the steels in which the pearlite area ratio exceeds 6.5% are given as comparative examples. Steels containing pearlite, ferrite, and cementite with an area ratio of 6.5% or less are examples of the present invention.

Further, the area ratio (%) of ferrite was obtained by binarizing ferrite and regions other than ferrite from the SEM image using image analysis software such as a GIMP file. Cementite was similarly binarized for areas other than cementite and cementite to determine the area ratio (%) of cementite. For pearlite, a value obtained by subtracting each area ratio (%) of ferrite and cementite from 100(%) was defined as the area ratio (%) of pearlite.

In addition, individual cementite diameters were evaluated for the photographed tissue photographs. The cementite diameter was obtained by measuring the major axis and the minor axis and converting them into circle-equivalent diameters. The average cementite diameter was obtained by dividing the sum of equivalent circle diameters of all cementites converted into equivalent circle diameters by the total number of cementites. The number of cementite particles having an equivalent circle diameter of 0.1 μm or less was measured and defined as the number of cementite particles having an equivalent circle diameter of 0.1 μm or less. In addition, the number of all cementites was obtained and used as the number of all cementites. Then, the ratio of the number of cementite having an equivalent circle diameter of 0.1 μm or less to the total number of cementite ((number of cementite having an equivalent circle diameter of 0.1 μm or less/total number of cementite)×100(%)) was obtained. In addition, this "proportion of cementite with an equivalent circle diameter of 0.1 μm or less" may be simply referred to as cementite with an equivalent circle diameter of 0.1 μm or less.

In addition, the average grain size of ferrite was obtained from the photographs of the structure by using the grain size evaluation method (cutting method) specified in JIS G 0551.

(2) Measurement of the amount of each X (X = Ti, Mo, Nb) It was determined by the same method as described in Reference Document 1 and Patent Document 4 below. That is, in a region from the surface of the steel sheet to 100 μm, electrolysis is performed in a 10% AA (10 vol% acetylacetone-1% by mass tetramethylammonium chloride-methanol) electrolytic solution, and solid solution X (X = Ti, Mo, Nb ) was measured. Subsequently, it was immersed in an aqueous solution of sodium hexametaphosphate whose concentration was changed by 7 levels in the range of 0 to 2000 mg/l, which is a solution that disperses the steel remaining after electrolysis, to separate the deposits. Next, using a filter with straight pores, a porosity of 47% determined by electron microscope observation, and a filter pore diameter of 100 nm, precipitates of 100 nm or more and precipitates of 100 nm or less are separated, and TiC and NbC of 100 nm or more are separated. and MoC were determined respectively. Precipitates of less than 100 nm were determined by subtracting the solid-solution X amount from each X (X=Ti, Mo, Nb) obtained from the liquid from which the precipitates of 100 nm or more were separated. Amount of X in solid solution + Amount of X contained in precipitates = Amount of X in steel.
TiC, NbC and MoC of 1 μm or more were determined as follows. Only Nb carbides, Ti carbides, and Mo carbides were extracted by a two-stage replica method as shown in Patent Document 5. Using a transmission electron microscope (TEM), 30 fields of view were photographed at a magnification of 160,000 times, and the size was measured. The length and breadth of the observed 30 particles were measured, converted to circle equivalent diameter, and the ratio of X contained in XC of 1 μm or more was determined.
[Reference 1] Satoshi Kishiro, Tomoharu Ishida, Kunio Inose, Kyoko Fujimoto, Tetsu to Hagane, vol. 99 (2013) No. 5, p. 362-365
[Patent Document 4] JP-A-2010-127789 [Patent Document 5] JP-A-2018-194539 (3) Elongation of steel plate From the steel plate (original plate) after annealing, the direction (L A tensile test was performed at 10 mm/min using a JIS No. 5 tensile test piece cut out in the direction), and the total elongation was determined by butting the fractured samples. The result was defined as elongation (El).

(4) Steel plate hardness after quenching (hardenability)
The steel plate hardness after quenching was evaluated by a method assuming submerged quenching. The steel sheets after annealing were heated to 900°C at a heating rate of 20°C/s, held at 900°C for 30 minutes, and then oil-cooled (oil-cooling temperature: 70°C) to prepare samples. The hardness was measured at a depth of 0.1 mm from the steel plate surface and at a position of 1/4 of the plate thickness under the condition of a load of 1 kgf, and the hardness of the surface layer 0.1 mm (HV) and the hardness inside the steel plate ( HV) was obtained. Table 4 shows acceptance criteria for the dip hardenability according to the C content, which can be evaluated as sufficient dip hardenability. If the hardness (HV) of the surface layer 0.1 mm and the hardness (HV) inside the steel sheet satisfy the criteria in Table 4, it is judged to be acceptable (symbol: ○), and the submerged hardenability is excellent. evaluated.

(5) Abrasion Resistance after Heat Treatment A test piece of 30 mm×30 mm was sampled from the annealed material, submerged quenching was performed, and the abrasion resistance after heat treatment was evaluated. A hardening treatment was performed under the conditions shown in (4) above, and an abrasion test was performed using a ball-on-disk tester. In the abrasion test, a 1N load is applied with a carbide ball (sphere diameter 6 mm) in contact with the surface of the test piece, and a concentric circle with a radius of 8 mm is rotated at a rotation speed of 20 cm / s, and rotated 30,000 times. The cross-sectional area of the wear scar after the wear was determined. Those with a wear scar cross-sectional area of 300 μm ² or less after 30,000 rotations were judged to be acceptable (symbol: ○), and judged to be excellent in wear characteristics.

For those who got a judgment of 〇 in the result of (4) and those who got a judgment of 〇 in the result of (5), the overall evaluation is 〇, and if any value is not satisfied, it is disqualified. (Symbol: X).

Tables 2 and 3 show the results.

From the results in Tables 2 and 3, the invention examples are all excellent in cold workability and wear resistance after heat treatment.

Claims (9)

in % by mass,
C: 0.30% or more and less than 1.0%,
Si: 0.80% or less,
Mn: 0.10% or more and 1.0% or less,
P: 0.03% or less,
S: 0.010% or less,
sol. Al: 0.001% or more and 0.10% or less,
N: 0.01% or less,
Cr: 0.05% or more and 0.50% or less,
B: containing 0.0005% or more and 0.005% or less,
Furthermore, Ti: more than 0.06% and 0.2% or less, Mo: more than 0.10% and 0.4% or less, Nb: more than 0.10% and 0.2% or less ,
Having a component composition in which the balance is Fe and unavoidable impurities,
The microstructure is
having ferrite and cementite,
The cementite has a ratio of cementite with a circle equivalent diameter of 0.1 μm or less to the total number of cementite of 20% or less, an average cementite diameter of 0.15 μm or more, and a ratio of the cementite to the total microstructure in terms of area ratio of 4.5. % or more and less than 20.0%,
The mass ratio of X contained in XC (X = Ti, Mo, Nb) of 1 µm or more with respect to the amount of X (X = Ti, Mo, Nb) in the steel in the range of 100 µm from the steel plate surface is 5% or less,
A hot-rolled steel sheet having a total elongation (El) of 20% or more. The mass ratio of the amount of X contained in XC (X=Ti, Mo, Nb) of 100 nm or more to the amount of X contained in XC (X=Ti, Mo, Nb) in the range of 100 μm from the steel plate surface is 65% or more. The hot-rolled steel sheet according to claim 1. The hot-rolled steel sheet according to claim 1 or 2, containing 0.002% or more and 0.1% or less in total of one or two selected from Sb and Sn. 0.0005% or more and 0.1% or less of one or more selected from Ta, Ni, Cu, V, and W in mass % in addition to the above component composition. 4. The hot-rolled steel sheet according to any one of 1 to 3. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the ferrite has an average grain size of 4 to 25 µm. A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5,
Rough rolling is started within 70 seconds after holding the steel material having the above chemical composition in a temperature range of 1250 ° C. or less for 1.5 hours or more, and the temperature range is 900 to 1070 ° C., 5 passes or more, and the total rolling reduction is 50% or more. , Perform rough rolling under the condition that the time between passes is 1 s or more and 15 s or less, finish rolling finish temperature: perform finish rolling at the Ar ₃ transformation point or higher,
Then, cool to 700-750°C at an average cooling rate of 20-100°C/sec,
Coiling temperature: After coiling at a temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet,
A method for producing a hot-rolled steel sheet, wherein the hot-rolled steel sheet is annealed at an annealing temperature of less than the Ac ₁ transformation point for 0.5 hours or more. 7. The method for producing a hot-rolled steel sheet according to claim 6, wherein the first pass of the rough rolling is performed in a temperature range of 1000 to 1070° C. so that the rolling reduction is 30% or less. The method for producing a hot-rolled steel sheet according to any one of claims 1 to 5, wherein the steel material having the chemical composition is held in a temperature range of 1250 ° C. or less for 1.5 hours or more and then rough-rolled within 70 seconds. Starting, rough rolling is performed under the conditions of a temperature range of 900 to 1070 ° C., 5 passes or more, a total rolling reduction of 50% or more, and an _interpass time of 1 s to 15 s. Perform finish rolling,
Then, cool to 700-750°C at an average cooling rate of 20-100°C/sec,
Coiling temperature: After coiling at a temperature of more than 580 ° C. and 700 ° C. or less to form a hot-rolled steel sheet,
The hot-rolled steel sheet is held at the Ac ₁ transformation point or higher and the Ac ₃ transformation point or lower for 0.5 h or more, and then cooled to less than the Ar ₁ transformation point at an average cooling rate of ₁ to 20 ° C./h. A method for manufacturing a hot-rolled steel sheet that is annealed at a temperature of 20 h or more. The method for producing a hot-rolled steel sheet according to claim 8, wherein the first pass of the rough rolling is performed at a temperature range of 1000 to 1070°C so that the rolling reduction is 30% or less.