JP7444096B2 - Hot rolled steel sheet and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet with excellent cold workability and wear resistance, which is suitable as an automobile drive system component, and a method for manufacturing the same. Specifically, it relates to a steel plate that is subjected to quenching treatment such as carburizing and induction hardening after obtaining a part shape by cold working using a steel plate, and relates to a hot-rolled steel plate that has excellent wear resistance after heat treatment and a method for manufacturing the same. . Excellent cold workability means that the total elongation of the steel plate, which is one indicator of workability, is 30% or more. Excellent wear resistance means that it is determined by a wear test using a ball-on-disc abrasion tester. , the cross-sectional area of wear scars after 30,000 cycles is 300 μm ² or less.

Currently, drive train parts for automobiles are produced by cold-working hot-rolled steel sheets, which are carbon steel for machine structures and alloy steel for machine structures specified in JIS G4051, into a desired shape and then hardened to the desired hardness. In some cases, it is manufactured by applying hardening treatment such as carburizing hardening or induction hardening to ensure the hardness. Generally, the wear resistance of a steel plate improves by increasing its hardness. Therefore, for parts where wear resistance is important, steel materials that have been tempered to a high degree of hardness through quenching treatment or steel materials that have a high content of alloying elements that produce highly hard carbides are used.

Various steel plates using carbides such as Nb and Ti have been proposed as techniques for improving wear resistance. For example, in Patent Document 1, in mass %, C: 0.10 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.10 to 0.50%, P: 0.030 % or less, S: 0.030% or less, Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0.10%, N: 0.0300% Contains the following, and optionally contains Ti: less than 0.050% and B: 0.0010 to 0.0050%, has a structure consisting of ferrite and pearlite, and has an average ferrite grain size. A mechanical structural steel for carburized parts is described which is characterized by having a grain coarsening resistance of 15 μm or more and excellent toughness.

Patent Document 2 describes, 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: It has a chemical composition of 0.02% or less, Nb: 0.1-0.5%, the balance Fe and unavoidable impurities, and has 200-1000 carbides/ ^mm2 containing Nb with a particle size of 1 μm or more. A material steel sheet for mechanical parts is described which has an annealed structure that is present in a dense matrix.

Patent Document 3 describes, 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. .030%, S: 0.001 to 0.020%, Nb: 0.10 to 0.70%, balance Fe and inevitable impurities, after tempering heat treatment in which Nb-containing carbides are dispersed. When the square root of the area of each Nb-containing carbide particle observed by cross-sectional structure observation is taken as the particle size, the number of Nb-containing carbide particles with a particle size of 1.0 μm or more is 200. We propose a wear-resistant steel material with excellent fatigue properties in which the maximum grain size Dmax of Nb-containing carbide particles in 10 ³ mm ³ estimated by the extreme value statistical method is adjusted to 18.0 μm or less ^. has been done.

Japanese Patent Application Publication No. 2013-040376 Japanese Patent Application Publication No. 2010-216008 Japanese Patent Application Publication No. 2013-136820

The technology described in Patent Document 1 contributes to increasing hardness by suppressing grain growth in the carburizing process, but 1.8% or more of Cr is added to improve hardenability after carburizing. , there is no description regarding steel with a small amount of Cr.

In addition, the technology described in Patent Document 2 improves wear resistance by controlling the amount of Nb and Ti carbides, 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 more. do not have. Furthermore, no technology regarding improvement of cold workability through cementite control is described.

The technique described in Patent Document 3 specifies the density and maximum particle size of NbC with a particle size of 1 μm or more, but does not describe carbide with a particle size of 1 μm or less. Furthermore, no technology regarding improvement of cold workability through cementite control is 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 with excellent cold workability and wear resistance after heat treatment, and a method for manufacturing the same.

The present inventors conducted extensive studies to achieve the above object. As a result, TiC, NbC, and MoC with a diameter of 100 nm or more and less than 1 μm greatly contribute to improving wear resistance, and the amount of Ti, Nb, and Mo used in their formation, including the total Ti amount and total Nb The amount of Mo, the ratio to the total amount of Mo, 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 up to 100 μm from the steel plate surface is 5% or less By doing so, excellent wear resistance can be obtained.
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 up to 100 μm from the steel plate surface is 65 % or more, even better wear resistance can be obtained.
iii) To obtain the predetermined amounts 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 1.5h or more, and then It is important to carry out rough rolling under the conditions of a temperature range of 1070° C., 5 passes or more, a reduction rate 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 those temperature ranges, TiC, NbC, and MoC with a size of 100 nm or more and less than 1 μm are easily generated.
iv) Furthermore, by performing rough rolling so that the first pass in rough rolling is at a temperature range of 1000 to 1070 ° C. and a rolling reduction of 30% or less, more TiC, NbC, and MoC of 100 nm or more and less than 1 μm are generated. It becomes easier.
v) Cementite with an equivalent circle diameter of 0.1 μm or less has a large influence on cold workability, hardness (hardness), and total elongation (hereinafter sometimes simply referred to as elongation) in hot-rolled steel sheets before quenching. are doing. By setting the number of cementites with a circular equivalent diameter of 0.1 μm or less to 20% or less of the total cementite number, a tensile strength of 500 MPa or less and a total elongation (El) of 30% 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 rate of 50% or more, interpass time of 1 second or more. Rough rolling is carried out under the conditions of 15 seconds or less, finishing rolling is carried out at a finish rolling temperature of Ar ₃ transformation point or higher, and then cooling is performed at an average cooling rate of 20 to 100 degrees Celsius to 700 to 750 degrees Celsius, and rolling is performed. After coiling at a temperature of more than 580°C and less than or equal to 700°C to form a hot-rolled steel plate, the hot-rolled steel plate is annealed at an annealing temperature of less than Ac ₁ transformation point for 0.5 hours or more to perform cold working. properties and can improve wear resistance after heat treatment.
vii) Alternatively, hold the product in a temperature range of 1250°C or less for 1.5 hours or more, start rough rolling within 70 seconds after taking it out from the heating furnace, and start rough rolling within 70 seconds at a temperature range of 900 to 1070°C, 5 passes or more, and a reduction rate of 50% or more. , Rough rolling is carried out under the conditions that the interpass time is 1 s or more and 15 s or less, finish rolling is carried out at a finish rolling temperature of Ar ₃ or more transformation point, and then the average cooling rate is 700 - 750 at 20 - 100 ° C / sec. After cooling to °C and coiling at a coiling temperature of more than 580 °C and less than 700 °C to form a hot-rolled steel plate, the hot-rolled steel plate is held at an Ac ₁ transformation point or more and an Ac ₃ transformation point or less for 0.5 h or more, Next, annealing is performed by cooling to below the Ar ₁ transformation point at an average cooling rate of 1 to 20°C/h and maintaining it below the Ar ₁ transformation point for 20 hours or more to impart cold workability and improve wear resistance after heat treatment. can improve sex.

The present invention has been made based on the above findings, and its gist is as follows.
[1] In mass%,
C: 0.05% or more and less than 0.30%,
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: Contains 0.0005% or more and 0.005% or less,
Furthermore, it contains one or more selected from Ti: more than 0.06% and less than 0.2%, Mo: more than 0.10% and less than 0.4%, and Nb: more than 0.10% and less than 0.2%. ,
The remainder has a component composition consisting of Fe and unavoidable impurities,
The microstructure is
Contains ferrite and cementite,
In the cementite, the ratio of the number of cementites with a circle equivalent diameter of 0.1 μm or less to the total number of cementites is 20% or less, the average cementite diameter is 0.15 μm or more, and the ratio of the cementite to the total microstructure is 1.0 in terms of area ratio. % or more and less than 5.5%,
The mass proportion 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 up to 100 μm from the steel plate surface is 5% or less,
A hot rolled steel sheet having a tensile strength of 500 MPa or less and a total elongation of 30% or more.
[2] 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 up to 100 μm from the steel plate surface The hot rolled steel sheet according to [1], wherein is 65% or more.
[3] The hot rolled steel sheet according to [1] or [2], containing one or two selected from Sb and Sn in a total amount of 0.002% or more and 0.1% or less.
[4] In addition to the above component composition, it further contains one or more selected from Ta, Ni, Cu, V, and W in an amount of 0.0005% or more and 0.1% or less, respectively, 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] The method for producing a hot rolled steel sheet according to any one of [1] to [5],
After holding the steel material having the above-mentioned composition in a temperature range of 1250°C or less for 1.5 hours or more, start rough rolling within 70s, in a temperature range of 900 to 1070°C, 5 passes or more, total rolling reduction of 50% or more. , Rough rolling is performed under conditions such that the inter-pass time is 1 s or more and 15 s or less, finish rolling is performed at a finish rolling temperature of Ar ₃ or more transformation point,
After that, it was cooled to 700 to 750 °C at an average cooling rate of 20 to 100 °C/sec,
Coiling temperature: After coiling at more than 580°C and less than 700°C to form a hot rolled steel plate,
A method for producing a hot-rolled steel sheet, which comprises annealing the hot-rolled steel sheet by holding the hot-rolled steel sheet at an annealing temperature of less than 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 rough rolling is performed such that the first pass has a rolling reduction of 30% or less in a temperature range of 1000 to 1070°C.
[8] The method for producing a hot rolled steel sheet according to any one of [1] to [5], comprising:
After holding the steel material having the above-mentioned composition in a temperature range of 1250°C or less for 1.5 hours or more, start rough rolling within 70s, in a temperature range of 900 to 1070°C, 5 passes or more, total rolling reduction of 50% or more. , Rough rolling is performed under conditions such that the inter-pass time is 1 s or more and 15 s or less, finish rolling is performed at a finish rolling temperature of Ar ₃ or more transformation point,
After that, it was cooled to 700 to 750 °C at an average cooling rate of 20 to 100 °C/sec,
Coiling temperature: After coiling at more than 580°C and less than 700°C to form a hot rolled steel plate,
The hot rolled steel sheet is maintained at a temperature of Ac ₁ transformation point or more and Ac ₃ transformation point or less for 0.5 hours or more, and then cooled to less than Ar ₁ transformation point at an average cooling rate of 1 to 20°C/h, and then cooled to less than Ar ₁ transformation point. A method for manufacturing a hot-rolled steel sheet, which is annealed for 20 hours or more.
[9] The method for producing a hot rolled steel sheet according to [8], wherein in the rough rolling, the first pass is performed in a temperature range of 1000 to 1070° C. with a reduction ratio of 30% or less.

According to the present invention, a hot rolled steel sheet with excellent cold workability and wear resistance after heat treatment can be obtained. By applying the hot-rolled steel sheet manufactured according to the present invention to parts such as drive train parts that require good wear characteristics, it will be possible to greatly improve the production of drive train parts for automobiles that require stable quality. It can contribute greatly to the industry, and has a significant industrial effect.

Below, the hot rolled steel sheet of the present invention and its manufacturing method will be explained in detail. Note that the present invention is not limited to the following embodiments.

1) Composition The composition of the hot rolled steel sheet of the present invention and the reason for its limitation will be explained. Note that "%", which is the unit of content in the following component compositions, means "% by mass" unless otherwise specified.

C: 0.05% or more and less than 0.30% C is an important element for obtaining strength after quenching. When the amount of C is less than 0.05%, the desired hardness cannot be obtained by heat treatment after molding, so the amount of C needs to be 0.05% or more. However, if the amount of C is 0.30% or more, the steel becomes hard and the toughness and cold workability deteriorate. Therefore, the amount of C is set to 0.05% or more and less than 0.30%. When used for cold working of parts with complicated shapes and difficult to press, the amount of C is preferably 0.25% or less. Preferably it is 0.1% or more, more preferably 0.12% or more.

Si: 0.80% or less Si is an element that increases strength through solid solution strengthening. As the amount of Si increases, it becomes harder and the cold workability deteriorates, so the amount of Si is set to 0.80% or less. Preferably it is 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 amount of Si is preferably 0.10% or more, more preferably 0.20% or more, and still more preferably 0.30% or more. do.

Mn: 0.10% or more and 1.0% or less Mn is an element that improves hardenability and increases strength through solid solution strengthening. If the Mn content is less than 0.10%, the hardenability begins to deteriorate, so the Mn content is set to 0.10% or more. When reliably hardening the inside of a thick material, etc., the content is preferably 0.25% or more, and more preferably 0.30% or more. On the other hand, when the amount of Mn exceeds 1.0%, a band structure due to segregation of Mn develops, the structure becomes non-uniform, and the steel becomes hard due to solid solution strengthening, resulting in a decrease in cold workability. Therefore, the amount of Mn is set to 1.0% or less. As a material for parts requiring moldability, 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 strength by solid solution strengthening. If the amount of P increases beyond 0.03%, grain boundary embrittlement will occur, and the toughness after quenching will deteriorate. It also reduces cold workability. Therefore, the amount of P is set to 0.03% or less. In order to obtain excellent toughness after quenching, the amount of P is preferably 0.02% or less. Since P reduces cold workability and toughness after quenching, it is preferable that the amount of P be as small as possible. However, if P is reduced too much, refining costs will increase, so the amount of P 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 reduces the cold workability and toughness after quenching of hot rolled steel sheets. When the amount of S exceeds 0.010%, the cold workability and toughness after quenching of the hot rolled steel sheet deteriorate significantly. 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 reduces cold workability and toughness after quenching, it is preferable that the amount of S be as small as possible. However, if S is reduced too much, refining costs will increase, so the amount of S is preferably 0.0005% or more.

sol. Al: 0.001% or more and 0.10% or less sol. If the Al content is less than 0.001%, the AlN in the steel sheet is too small, and austenite grain growth occurs during heating in the austenite region, making it impossible to obtain the desired fatigue strength. Therefore, the Al content is set to 0.001% or more. On the other hand, if it exceeds 0.10%, a large number of inclusions caused by Al will be generated, causing cracks to occur during component molding. Therefore, from the viewpoint of suppressing cracks during component molding, the content is set to 0.10% 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 If the N amount exceeds 0.01%, the amount of solid solute N in the steel sheet will increase and it will become hard, making it impossible to obtain the desired tensile strength and elongation, and resulting in poor cold workability. descend. Therefore, the content 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 nitride, and B nitride. This is an element that moderately suppresses the growth of austenite grains during heating during quenching treatment and improves toughness after quenching. For this reason, the amount of N 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 that improves hardenability. When it is less than 0.05%, sufficient effects are not observed, so the amount of Cr needs to be 0.05% or more. Further, if the Cr content in the steel is 0%, ferrite is likely to be generated in the surface layer especially during carburizing and quenching, and a completely quenched structure cannot be obtained, which may easily cause a decrease in hardness. Therefore, from the viewpoint of placing emphasis on hardenability, the Cr content is set to 0.05% or more, preferably 0.10% or more. On the other hand, if the Cr content exceeds 0.50%, the steel plate before quenching becomes hard, impairing cold workability. 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. It is more preferable to set it to .30% or less.

B: 0.0005% or more and 0.005% or less In the present invention, B is an important element that improves hardenability. When the amount of B is less than 0.0005%, sufficient effects are not observed, 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 austenite after finish rolling is delayed, resulting in the development of the texture of the hot rolled steel sheet, the anisotropy after annealing becomes large, and the drawing process 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 type selected from Ti: more than 0.06% and not more than 0.2%, Mo: more than 0.10% and not more than 0.4%, and Nb: more than 0.10% and not more than 0.2%. Contain the above.

Ti: More than 0.06% and 0.2% or less Ti generates TiC in the steel plate, and TiC is highly effective in 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 is set to be more than 0.06%. More preferably, it is 0.1% or more. On the other hand, if the Ti content exceeds 0.2%, the steel sheet before quenching becomes hard and cold workability is impaired. Therefore, when containing Ti, the amount of Ti should be 0.2% or less. Preferably it is 0.15% or less.

Mo: more than 0.10% and not more than 0.4% Mo is an element that is highly effective in improving wear resistance after heat treatment. If it is less than 0.10%, the effect of addition is small, so when Mo is contained, the lower limit is set to more than 0.10%. More preferably, it is 0.11% or more. If Mo exceeds 0.4%, the effect of addition will be saturated and the cost will increase, so when Mo is contained, the upper limit is set to 0.4%. More preferably, it is 0.35% or less, and even more preferably 0.3% or less.

Nb: More than 0.10% and 0.2% or less Nb is an element that forms NbC and NbN and is highly effective in improving wear resistance after heat treatment. If Nb is less than 0.10%, the effect is weak, so if Nb is contained, it should be more than 0.10%. On the other hand, if Nb exceeds 0.2%, not only will the additive effect be saturated, but the Nb carbide will reduce the elongation as the tensile strength of the base material increases. is set to 0.2%. More preferably, it is 0.18% or less, even more preferably 0.15% or less.

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

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

Total 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 surface layer of the steel sheet. If the total amount of one or more of these elements is less than 0.002%, sufficient effects will not be observed, so it is preferable that the total amount 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 nitriding prevention effect is saturated. Furthermore, 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 should be 0.1% or less. Preferably it is 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 more selected from Ta, Ni, Cu, V, and W are added in an amount of 0.0005% to 0.1%, respectively. It's okay.

Ta: 0.0005% or more and 0.1% or less Ta, like Nb, forms carbonitrides, prevents abnormal growth of crystal grains during heating before quenching, prevents coarsening of crystal grains, and improves resistance to temper softening. It is an effective element for If it is less than 0.0005%, the effect of addition is small, so when Ta is contained, it is preferable to set the lower limit to 0.0005%. More preferably, it is 0.0010% or more. If Ta exceeds 0.1%, the effect of adding it will be saturated, the quenching hardness will decrease due to excessive carbide formation, and the cost will increase, so when Ta is contained, the upper limit is set to 0.1%. It is preferable. More preferably, it is 0.05% or less, and even 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 hardenability. If less than 0.0005%, there is no effect of addition, so when Ni is contained, it is preferable to set the lower limit to 0.0005%. More preferably, it is 0.0010% or more. If Ni exceeds 0.1%, the effect of addition is saturated and costs increase, so when Ni is contained, it is preferable to set the upper limit to 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 in ensuring hardenability. If the content is less than 0.0005%, the effect of addition cannot be sufficiently confirmed, so when Cu is contained, it is preferable to set the lower limit to 0.0005%. More preferably, it is 0.0010% or more. If Cu exceeds 0.1%, flaws are likely to occur during hot rolling, resulting in deterioration of manufacturability such as a drop in yield, so when Cu is contained, the upper limit is preferably 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, forms carbonitrides and prevents abnormal grain growth of crystal grains during heating before quenching, improves toughness, and improves tempering softening resistance. It is a valid element. If it is less than 0.0005%, the effect of addition will not be sufficiently expressed, so when V is contained, it is preferable to set the lower limit to 0.0005%. More preferably, it is 0.0010% or more. When V exceeds 0.1%, not only does the effect of adding it become saturated, but also the elongation decreases as the tensile strength of the base material increases due to the formation of carbides, so when V is contained, the upper limit should be set at 0.1%. It is preferably 1%. More preferably, it is 0.05% or less, and even 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 crystal grains during pre-quenching heating and improving temper softening resistance. It is an element. If it is less than 0.0005%, the effect of addition is small, so when W is contained, it is preferable to set the lower limit to 0.0005%. More preferably, it is 0.0010% or more. If W exceeds 0.1%, the effect of adding it will be saturated, the quenching hardness will decrease due to excessive carbide formation, and the cost will increase, so when W is contained, the upper limit is set to 0.1%. It is preferable. More preferably, it is 0.05% or less, and even 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 thereof 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 includes 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 cold working may become difficult in highly workable parts. Therefore, the area ratio of ferrite is preferably 92% or more. More preferably, it is 94% or more.

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

2-2) Ratio of the number of cementites with an equivalent circle diameter of 0.1 μm or less to the total number of cementites: 20% or less If there is a large amount of cementite with an equivalent circle diameter of 0.1 μm or less, the material becomes hard due to dispersion strengthening and elongation decreases. From the viewpoint of obtaining cold workability, in the present invention, the number of cementites with an equivalent circle diameter of 0.1 μm or less is set to 20% or less of the total number of cementites. As a result, a tensile strength of 500 MPa or less and a total elongation (El) of 30% 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, it is preferable that the number of cementites having an equivalent circle diameter of 0.1 μm or less is 10% or less of the total number of cementites. By setting the number of cementites with a circular equivalent diameter of 0.1 μm or less to 10% or less of the total number of cementites, it is possible to achieve a tensile strength of 450 MPa or less and a total elongation (El) of 35% or more. The reason why we defined the percentage of cementite with a circular equivalent diameter of 0.1 μm or less is that cementite with a diameter of 0.1 μm or less produces dispersion strengthening ability, and an increase in cementite of that size will impede cold workability. It is.

Further, 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 with respect to the total number of cementites. Note that the diameter of cementite existing before quenching is approximately 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 If there is too much small cementite, cold workability will deteriorate. From the viewpoint of cold workability, it is necessary to disperse cementite of 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, during quenching, it is necessary to melt all the cementite to ensure a predetermined amount of solid solution C in the ferrite. If the average cementite diameter exceeds 2.5 μm, the cementite cannot be completely dissolved during retention in the austenite region, so the average cementite diameter is set to 2.5 μm or less. More preferably, it is 2.0 μm or less.

In the present invention, the term "cementite diameter" refers to the equivalent circular diameter of cementite, and the equivalent circular diameter of cementite is a value obtained by measuring the major axis and minor axis of cementite and converting it into the equivalent circular diameter. Moreover, the "average cementite diameter" refers to a value obtained by dividing the sum of the equivalent circle diameters of all cementites converted into equivalent circle diameters by the total number of cementites.

2-4) The ratio of cementite to the total microstructure (area ratio) is 1.0% or more and less than 5.5% When the ratio of the area ratio of cementite to the total microstructure is less than 1.0%, the strength of the base material decreases. Since this may lead to insufficient strength in parts used without heat treatment, it is set to 1.0% or more. Preferably it is 1.5% 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 difficult-to-form parts increases, so it is necessary to ensure a predetermined elongation. In order to obtain the desired elongation, this proportion should be less than 5.5%. More preferably, it is 5.0% or less.

2-5) Average grain size of ferrite: 4 to 25 μm (preferred conditions)
The average particle size of ferrite is preferably 4 μm or more, since if it is less than 4 μm, the strength before cold working may increase and press formability may deteriorate. On the other hand, if the average particle size of ferrite exceeds 25 μm, the strength of the base material may decrease. Furthermore, in areas where the product is used without being quenched after being molded into the desired product shape, the base material must have a certain level of strength. Therefore, the average grain size of ferrite is preferably 25 μm or less. More preferably, it is 5 μm or more, and still more preferably 6 μm or more. More preferably, it is 20 μm or less, and still more preferably 18 μm or less.

In addition, in the present invention, the above-mentioned equivalent circle diameter of cementite, average cementite diameter, ratio of cementite to the total microstructure, area ratio of ferrite, 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 up to 100 μm from the steel plate surface is 5% Below In the present invention, 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 up to 100 μm from the steel plate surface 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 in order to improve wear resistance, it is effective to make the mass ratio of X contained in XC (X=Ti, Mo, Nb) of 1 μm or less to 5% or less. Note that the mass proportion of X contained in XC of 1 μm or more (X=Ti, Mo, Nb) may be 0% because it is better as it is smaller.

2-7) Mass of the amount of X contained in XC (X = Ti, Mo, Nb) of 100 nm or more with respect to the amount of X contained in XC (X = Ti, Mo, Nb) in the range up to 100 μm from the steel plate surface Ratio is 65% or more (suitable conditions)
Since TiC, MoC, and NbC having a size of 100 nm or more greatly contribute to improving wear resistance, it is preferable that the mass proportion thereof is 65% or more.

In the present invention, the proportions of Ti, Nb, and Mo used for the precipitation of TiC, NbC, and MoC of 100 nm or more are closely related to the holding time from heating the slab to rough rolling, the temperature during rough rolling, the rolling reduction rate, and the time between passes. It has been found that it is necessary to optimize a series of these manufacturing conditions. Note that the reason necessary 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 addition, 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 (X = Ti, Mo, Nb) in the steel in the range up to 100 μm from the steel plate surface is the In terms of obtaining properties, Ti is preferably 65% or more, Mo is 30% or more, and Nb is preferably 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 needs to have excellent cold workability because drive-train automobile parts are formed by cold pressing. In addition, it is necessary to increase the hardness through quenching treatment to impart wear resistance. Therefore, the hot rolled steel sheet of the present invention has excellent properties by reducing the tensile strength of the steel sheet to make the tensile strength (TS) 500 MPa or less, and increasing the total elongation to make the total elongation (El) 30% or more. In addition to having cold workability, it is possible to suppress austenite grain growth during carburizing and obtain excellent wear resistance after carburizing. More preferably, TS is 450 MPa or less and El is 35% or more.

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

4) Manufacturing method The reasons for limitations in the method for manufacturing a hot rolled steel sheet of the present invention will be explained below. In addition, in the description, the expression "°C" related to temperature represents the temperature at the surface of the steel plate or the surface of the steel material.

In the present invention, the method for manufacturing the steel material does not need to be particularly limited. For example, both a converter furnace and an electric furnace can be used to melt 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-blowing rolling or continuous casting. The slab is usually heated and then hot rolled (hot rough rolling, finish rolling).

For example, in the case of a slab manufactured by continuous casting, direct rolling may be applied, in which the slab is rolled as is or with heat retention for the purpose of suppressing temperature drop. In addition, in hot rolling, the material to be rolled may be heated by heating means such as a bar heater in order to ensure the finish rolling end temperature.

Heating temperature of steel material (slab heating temperature): Hold for 1.5 hours or more in a temperature range of 1250℃ or less If the slab heating temperature is too high, a large number of coarse TiN will precipitate, which is effective in improving wear resistance. Therefore, less Ti is used for TiC. Therefore, the slab heating temperature is set to 1250°C or less. Preferably it is 1200°C or less. The holding time is set to 1.5 h or more in order to ensure that the alloying elements are uniformly dispersed 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 up to 100 μm from the steel plate surface In order to keep it below 5%, it is necessary to shorten the holding time at high temperature. Therefore, after maintaining the temperature in a temperature range of 1250° C. or less for 1.5 hours or more, the time until the start of rough rolling is set to within 70 seconds. Preferably it is 65 seconds or less.

Rough rolling is carried out in the temperature range of 900 to 1070°C, 5 passes or more, total reduction rate of 50% or more, and interpass time of 1 s or more and 15 s or less. In order to make the mass ratio of X contained in XC of 1 μm or more (X = Ti, Mo, Nb) to the amount of As mentioned above, it is necessary to carry out rough rolling so that the total rolling reduction ratio is 50% or more. Furthermore, if the inter-pass time cannot be ensured at 1 s or more, XC (X = Ti, Mo, Nb) of 1 μm or more for the amount of X (X = Ti, Mo, Nb) in the steel in the range up to 100 μm from the steel plate surface Since it becomes impossible to ensure that the mass proportion of X contained is 5% or less, the time is set to 1 s or more. In consideration of operability, the interpass time is preferably 15 seconds or less. More preferably it is 10 seconds or less.

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

Finish rolling finish temperature: Finish rolling at Ar ₃ transformation point or higher If the finish rolling finish temperature is less than Ar ₃ transformation point, coarse ferrite grains are formed after hot rolling and annealing, and elongation is significantly reduced. For this reason, the finish rolling end temperature is set to be equal to or higher than the Ar ₃ transformation point. Preferably, the temperature is (Ar ₃ transformation point +20°C) or higher. The upper limit of finish rolling finishing temperature does not need to be particularly specified, but is preferably 1000° C. or less in order to smoothly cool down after finish rolling.

After finish rolling, cool down to 700 to 750°C at an average cooling rate of 20 to 100°C/sec After finish rolling, the average cooling rate to 700 to 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 pre-annealing structure will be a ferrite and pearlite structure with too many ferrite structures, making it impossible to obtain a predetermined cementite dispersion state and size 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 average 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 less than 700°C The hot rolled steel sheet after finish rolling is wound into a coil shape. If the winding temperature is too high, the strength of the hot-rolled steel sheet will be too low, and when it is wound into a coil shape, it may deform due to its own weight. Therefore, this is not preferable from an operational standpoint. Therefore, the upper limit of the winding temperature is set to 700°C. Preferably it is 690°C or lower. On the other hand, if the coiling temperature is too low, the hot rolled steel sheet becomes hard, which is not preferable. Therefore, the winding temperature is set to exceed 580°C. Preferably it is 600°C or higher.

After winding it up into a coil, it may be cooled to room temperature and subjected to pickling treatment. After pickling treatment, annealing is performed. Note that a known method can be applied to the pickling treatment. Thereafter, the obtained hot rolled steel sheet is subjected to the following annealing.

Annealing temperature: less than Ac ₁ transformation point and maintained for 0.5 hours or more If the annealing temperature is above Ac ₁ transformation point, austenite will precipitate and a coarse pearlite structure will be formed in the cooling process after annealing, resulting in a non-uniform structure. Become. Therefore, the annealing temperature is set below the Ac ₁ transformation point. Preferably it is below (Ac ₁ transformation point -10°C). Although there is no particular lower limit to the annealing temperature, in order to obtain a predetermined cementite dispersion state, the annealing temperature is preferably 650°C or higher, more preferably 700°C or higher. Note that as the atmospheric gas, any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used. Further, the holding time at the above annealing temperature is 0.5 h or more. If the holding time at the annealing temperature is less than 0.5 h, the annealing effect will be poor and the target structure of the present invention will not be obtained, and as a result, the hardness and elongation of the steel plate that are the target of the present invention will not be obtained. There may be no. Therefore, the holding time at the above annealing temperature is set to 0.5 h or more. More preferably it is 5 hours or more, and even more preferably more than 20 hours. On the other hand, when the holding time at the annealing temperature exceeds 40 hours, productivity decreases and manufacturing costs become 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 above-described annealing. Specifically, after winding, the temperature is maintained at Ac ₁ transformation point or more and Ac ₃ transformation point or less for 0.5 h or more (first stage annealing), and then Ar ₁ transformation is carried out at an average cooling rate of 1 to 20°C/h. It is also possible to produce the alloy by performing two-stage annealing in which the alloy is cooled to a temperature below the Ar ₁ transformation point and maintained at a temperature below the Ar 1 transformation point for 20 hours or more (second-stage annealing).

In the present invention, a hot-rolled steel sheet is heated and held at a temperature above the Ac ₁ transformation point and below the Ac ₃ transformation point for 0.5 hours or more, thereby dissolving relatively fine carbides precipitated in the hot-rolled steel sheet into the γ phase. After that, it is cooled to below the Ar ₁ transformation point at an average cooling rate of 1 to 20° C./h, and maintained below the Ar ₁ transformation point for 20 hours or more. As a result, solid solution C is precipitated using relatively coarse undissolved carbides as nuclei, and carbides are formed such that the ratio of the number of cementites with an equivalent circle diameter of 0.1 μm or less to the total number of cementites is 20% or less. The dispersion of (cementite) can be controlled. 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 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 sheet. In the present invention, by holding the hot-rolled steel sheet at a temperature 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 (austenite). . In the subsequent cooling stage and holding stage (second annealing) below the Ar ₁ transformation point, the α/γ interface and undissolved carbides that exist in the temperature range above the Ac ₁ transformation point become nucleation sites, resulting in relatively coarse particles. carbide precipitates. The conditions for such two-stage annealing will be explained below. Note that as the atmospheric gas during annealing, any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used.

Hold for 0.5 hours or more at Ac ₁ transformation point or more and Ac ₃ transformation point or less (1st stage annealing)
By heating the hot-rolled steel sheet to an annealing temperature higher than the Ac ₁ transformation point, part of the ferrite in the steel sheet structure is transformed into austenite, the fine carbides precipitated in the ferrite are dissolved, and C is converted into austenite. Make a solid solution. On the other hand, the ferrite that remains without being transformed into austenite is annealed at a high temperature, so its dislocation density decreases and it becomes soft. In addition, relatively coarse carbides (undissolved carbides) that are not dissolved remain in the ferrite, but they become coarser due to Ostwald growth. When the annealing temperature is less than the Ac ₁ transformation point, austenite transformation does not occur, and thus carbides cannot be dissolved in austenite. On the other hand, if the first stage annealing temperature exceeds the Ac ₃ transformation point, a large number of rod-shaped cementites will be obtained after annealing, making it impossible to obtain the desired elongation. Therefore, the temperature is set below the Ac ₃ transformation point. Further, in the present invention, if the holding time at the Ac ₁ transformation point or more and the Ac ₃ transformation point or less is less than 0.5 h, fine carbides cannot be sufficiently dissolved. Therefore, as the first stage annealing, the temperature is maintained at the Ac ₁ transformation point or more and the Ac ₃ transformation point or less for 0.5 hours or more. The holding time is preferably 1.0 h or more. Further, the holding time is preferably 10 hours or less.

Average cooling rate: 1 to 20°C/h to below Ar ₁ transformation point After the first stage annealing described above, average cooling rate: 1 to below Ar ₁ transformation point, which is the temperature range of second stage annealing. Cool at ~20°C/h. During cooling, C discharged from austenite as austenite transforms into ferrite precipitates as relatively coarse spherical carbides using the α/γ interface and undissolved carbides as nucleation sites. In this cooling, it is necessary to adjust the cooling rate so that pearlite is not generated. 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 set to 1°C/h or more. Preferably it is 5°C/h or more. On the other hand, if the average cooling rate exceeds 20°C/h, pearlite will precipitate and the hardness will increase, so the cooling rate is set to 20°C/h or less. Preferably it is 15°C/h or less.

Ar: Maintained at less than ₁ transformation point for 20 hours or more (2nd stage annealing)
After the first stage annealing described above, by cooling at a predetermined average cooling rate and maintaining it below the Ar ₁ transformation point, coarse spherical carbides are further grown by Ostwald growth and fine carbides are disappeared. If the holding time below the Ar ₁ transformation point is less than 20 hours, carbide cannot be grown sufficiently and the hardness after annealing becomes too large. Therefore, the second stage annealing is maintained at less than the Ar ₁ transformation point for 20 hours or more. Although not particularly limited, the second stage 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.

In addition, the Ac ₃ transformation point, Ac ₁ transformation point, Ar ₃ transformation point, and Ar ₁ transformation point mentioned above can be determined by actual measurements such as thermal expansion measurement during heating and cooling or electrical resistance measurement by Formaster test etc. can.

Moreover, the above-mentioned average heating rate and average cooling rate are determined by measuring the temperature with a thermocouple installed in the furnace.

Steels having the compositions of steel numbers A to R shown in Table 1 were melted and then hot rolled according to the manufacturing conditions shown in Tables 2 and 3. Next, it was pickled and annealed (spheroidizing 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 annealed plate was manufactured.

A test piece was taken from the hot-rolled annealed plate 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 NbC contained in NbC of 100 nm or more were collected as shown below. The amount of Nb, tensile strength, total elongation, steel plate hardness after gas carburizing and oil quenching, and wear resistance after gas carburizing and oil quenching were determined. Note that 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 plate after annealing was determined by cutting and polishing a test piece (size: 3 mm x 10 mm x 10 mm) taken from the center of the plate width, subjecting it to nital corrosion, and using a scanning electron microscope (SEM). Photographs were taken at 3000x magnification at five locations from the surface layer to 1/4 of the board thickness. Each phase (ferrite, cementite, pearlite, etc.) was identified by image processing of the photographed structure. In Tables 2 and 3, "pearlite area ratio" is listed as a microstructure, and steels in which pearlite was observed in an area ratio exceeding 6.5% are treated as comparative examples. Steels containing pearlite, ferrite, and cementite with an area ratio of 6.5% or less are considered as examples of the present invention.

Furthermore, using image analysis software such as a GIMP file from the SEM image, ferrite and areas other than ferrite were binarized to determine the area ratio (%) of ferrite. Similarly, for cementite, the cementite and non-cementite areas were binarized to determine the area ratio (%) of cementite. For pearlite, the area ratio (%) of pearlite was calculated by subtracting the area ratios (%) of ferrite and cementite from 100 (%).

In addition, the diameter of each cementite was evaluated for the microstructure photographs taken. The cementite diameter was determined by measuring the major axis and minor axis and converting it into a circular equivalent diameter. The average cementite diameter was determined by dividing the sum of the equivalent circle diameters of all cementites converted into equivalent circle diameters by the total number of cementites. The number of cementites with an equivalent circle diameter of 0.1 μm or less was measured, and was defined as the number of cementites with an equivalent circle diameter of 0.1 μm or less. In addition, the total number of cementites was determined and used as the total number of cementites. Then, the ratio of the number of cementites with an equivalent circle diameter of 0.1 μm or less to the total number of cementites ((number of cementites with a circle equivalent diameter of 0.1 μm or less/total number of cementites)×100(%)) was determined. Note that 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.

Furthermore, the average grain size of ferrite was determined from the photographed structure using the crystal 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 up to 100 μm from the steel plate surface, electrolysis is performed in a 10% AA (10 vol% acetylacetone-1 mass % tetramethylammonium chloride-methanol) electrolyte, and solid solution X (X = Ti, Mo, Nb ) amount was measured. Subsequently, the steel remaining after electrolysis was immersed in an aqueous solution of sodium hexametaphosphate, which is a dispersible solution whose concentration was varied in seven levels from 0 to 2000 mg/l, to separate the precipitates. Next, a filter consisting of straight pores with a porosity of 47% determined from electron microscopy and a filter pore diameter of 100 nm was used to separate precipitates of 100 nm or more and precipitates of smaller size. and MoC were determined. Precipitates with a diameter of less than 100 nm were determined by subtracting the amount of solid solution X from each X (X = Ti, Mo, Nb) determined from a liquid from which precipitates with a diameter of 100 nm or more were separated. The amount of X in solid solution + the amount of X contained in the precipitate = the amount of X in the steel.
TiC, NbC, and MoC with a diameter of 1 μm or more were determined as follows. From the Nb carbide, Ti carbide, and Mo carbide extracted by the two-stage replica method as shown in Patent Document 5, 30 fields of view were photographed at a magnification of 160,000 times using a transmission electron microscope (TEM) to determine the size. was measured. The major axis and minor axis of the 30 observed specimens were measured and converted into a circular equivalent diameter to determine the proportion of X contained in XC of 1 μm or more.
[Reference 1] Satoshi Joshiro, 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) Tensile strength and elongation of steel plate From the steel plate (original plate) after annealing, the angle of 0° with respect to the rolling direction is A tensile test was performed at 10 mm/min using a JIS No. 5 tensile test piece cut in the direction (L direction), a nominal stress nominal strain curve was determined, and the maximum stress was taken as the tensile strength. In addition, the total elongation was determined by comparing the broken samples. The result was defined as elongation (El).

(4) Steel plate hardness after carburizing oil quenching (carburizing hardenability)
After annealing, the steel plate was subjected to carburizing and quenching treatments such as soaking, carburizing, and diffusion treatment at 930°C for a total of 4 hours, held at 850°C for 30 minutes, and then oil-cooled (oil cooling temperature: 70°C). ℃). The hardness was measured at 0.1 mm intervals from the steel plate surface to the 0.1 mm depth position and the 1.2 mm depth position under a load of 1 kgf, and the hardness of the surface layer 0.1 mm during carburizing and quenching ( HV) and the hardness (HV) at 1.0 mm from the surface layer. Table 4 shows the acceptance criteria for hardenability according to the C content, which can be evaluated as having sufficient hardenability. If the hardness (HV) at a depth of 0.1 mm from the surface layer during carburizing and quenching is 700 HV or more, and the hardness at a depth of 1.0 mm from the surface layer satisfies the criteria in Table 4, the test is passed. (Symbol: Indicated by ○), and the hardenability was evaluated to be excellent.

(5) Wear resistance properties after carburizing A 30 mm x 30 mm test piece was taken from the annealed material, carburized and quenched, and the wear resistance of the carburized material was evaluated. Carburizing and quenching were performed under the conditions shown in (4) above, and a wear test was conducted using a ball-on-disc tester. In the wear test, a cemented carbide ball (ball diameter 6 mm) was placed in contact with the surface of the specimen, a load of 1 N was applied, and concentric circles with a radius of 8 mm were rotated at a rotational speed of 20 cm/s for 30,000 revolutions. The cross-sectional area of the wear scar after this 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 (indicated by a symbol ◯) and were judged to have excellent wear resistance.

The overall evaluation will be 〇 for those for which a 〇 judgment was obtained in the results of (4) and 〇 judgment was obtained in the results of (5), and if any value is not satisfied, the result will be rejected. (Symbol: Indicated by ×).

The results are shown in Tables 2 and 3.

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

Claims (9)

In mass%,
C: 0.05% or more and less than 0.30%,
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: Contains 0.0005% or more and 0.005% or less,
Furthermore, it contains one or more selected from Ti: more than 0.06% and less than 0.2%, Mo: more than 0.10% and less than 0.4%, and Nb: more than 0.10% and less than 0.2%. ,
The remainder has a component composition consisting of Fe and unavoidable impurities,
The microstructure is
Contains ferrite and cementite,
In the cementite, the ratio of the number of cementites with a circle equivalent diameter of 0.1 μm or less to the total number of cementites is 20% or less, the average cementite diameter is 0.15 μm or more, and the ratio of the cementite to the total microstructure is 1.0 in terms of area ratio. % or more and less than 5.5%,
The mass proportion 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 up to 100 μm from the steel plate surface is 5% or less,
A hot rolled steel sheet having a tensile strength of 500 MPa or less and a total elongation of 30% 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 up to 100 μm from the steel plate surface is 65% The hot-rolled steel sheet according to claim 1, which is the above. The hot rolled steel sheet according to claim 1 or 2, containing one or two selected from Sb and Sn in a total amount of 0.002% or more and 0.1% or less. In addition to the above component composition, the composition further contains one or more selected from Ta, Ni, Cu, V, and W in an amount of 0.0005% or more and 0.1% or less, respectively, in mass %. 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 producing a hot rolled steel sheet according to any one of claims 1 to 5, comprising:
After holding the steel material having the above-mentioned composition in a temperature range of 1150°C or more and 1250°C or less for 1.5 hours or more, start rough rolling within 70 seconds, and apply a total reduction rate of 5 passes or more in a temperature range of 900 to 1070°C. 50% or more and the interpass time is 1 s or more and 15 s or less, and finish rolling is performed at a finish rolling temperature of Ar ₃ or more transformation point,
After that, it was cooled to 700 to 750 °C at an average cooling rate of 20 to 100 °C/sec,
Coiling temperature: After coiling at more than 580°C and less than 700°C to form a hot rolled steel plate,
A method for producing a hot-rolled steel sheet, which comprises annealing the hot-rolled steel sheet by holding the hot-rolled steel sheet at an annealing temperature of less than 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 rough rolling is performed such that the first pass is in a temperature range of 1000 to 1070° C. with a reduction ratio of 30% or less. A method for producing a hot rolled steel sheet according to any one of claims 1 to 5, comprising:
After holding the steel material having the above-mentioned composition in a temperature range of 1150°C to 1250°C for 1.5 hours or more, start rough rolling within 70 seconds, and apply a total rolling reduction in a temperature range of 900 to 1070°C, 5 passes or more, and a total rolling reduction. 50% or more and the interpass time is 1 s or more and 15 s or less, and finish rolling is performed at a finish rolling temperature of Ar ₃ or more transformation point,
After that, it was cooled to 700 to 750 °C at an average cooling rate of 20 to 100 °C/sec,
Coiling temperature: After coiling at more than 580°C and less than 700°C to form a hot rolled steel plate,
The hot rolled steel sheet is maintained at a temperature of Ac ₁ transformation point or more and Ac ₃ transformation point or less for 0.5 h or more, and then cooled to less than Ar ₁ transformation point at an average cooling rate of 1 to 20°C/h, and then cooled to less than Ar ₁ transformation point. A method for manufacturing a hot-rolled steel sheet, which is annealed for 20 hours or more. 9. The method for producing a hot-rolled steel sheet according to claim 8, wherein the rough rolling is performed such that the first pass is in a temperature range of 1000 to 1070° C. and the rolling reduction is 30% or less.