EP4596742A1

EP4596742A1 - Non-tempered steel for hot forging, and hot forging material and method for producing same

Info

Publication number: EP4596742A1
Application number: EP23911855.7A
Authority: EP
Inventors: Katsuyuki Ichimiya; Kazuaki Fukuoka
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2022-12-26
Filing date: 2023-12-19
Publication date: 2025-08-06
Also published as: EP4596742A4; WO2024143075A1; MX2025007520A; CN120418465A; JP7552959B1; JPWO2024143075A1; KR20250108687A

Abstract

Non-heat-treated steel for hot forging that has a chemical composition, in mass%, of C: 0.25 % to 0.37 %, Si: 0.50 % to 1.00 %, Mn: 1.45 % to 2.20 %, P: 0.005 % to 0.030 %, S: 0.030 % to 0.080 %, Al: 0.015 % to 0.050 %, Ni: 0.03 % or less, Cr: 0.01 % to 0.20 %, V: 0.10 % to 0.25 %, B: 0.0003% or less, and N: 0.0030 % to 0.0200 %, with the balance being Fe and inevitable impurities, wherein Ceq defined by the Expression (1) is 0.62 to 0.72, α defined by the Expression (2) is 4.4 to 6.2, and β defined by the Expression (3) is 20.0 to 60.0. Expression (1): Ceq = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14, Expression (2): α = 2.7Mn + 4.6Cr + V, Expression (3): β = Mn/S

Description

TECHNICAL FIELD

The present disclosure relates to non-heat-treated steel for hot forging, and hot forged material and a method of producing same.

BACKGROUND

Components for machine structure used in automobiles, construction machinery, and industrial machinery are becoming more energy efficient and compact. However, even when size is decreased, the same fatigue strength is still required, and therefore steel material that has a higher yield stress is typically used for components for machine structures.
In particular, steel material that has a higher yield stress is required for the production of components such as shafts that are subject to repeated bending stress or repeated compressive stress in the axial direction. For this reason, quenching and tempering are carried out after hot forging to secure strength and toughness, and yield stress is restored to 70 % to 90 % of tensile strength.
In recent years, from the viewpoint of energy and cost saving, there has been a demand for steel material that obtains the required properties without quenching and tempering after hot forging, so-called non-heat-treated steel. Therefore, technologies have been developed to improve ductility and toughness by adding precipitation hardening elements such as V and Nb to JIS carbon steel for machine structural use and alloy steel for machine structural use, as material for hot forging, in order to improve strength and decrease carbon content. However, most conventional non-heat-treated steel that has a tensile strength from 650 MPa to 850 MPa is ferritic/pearlitic non-heat-treated steel, making further strength improvement difficult.
Regarding such strength improvement, for example, in Patent Literature (PTL) 1, a so-called bainitic non-heat-treated steel is proposed that has extremely low carbon content and a bainite single phase microstructure to improve toughness as well as strength. However, bainitic microstructure has the problem of yielding at low strain, resulting in inferior fatigue resistance.
In PTL 2, a high proof stress ratio non-heat-treated steel suitable for components such as connecting rods is described, in which large amounts of P and S are added to increase the proof stress ratio, which is the ratio of 0.2 % proof stress to tensile strength, to produce components that have high proof stress without loss of machinability by cutting. However, there is concern that grain boundary embrittlement due to grain boundary segregation of P may occur, resulting in decreased fatigue strength.
In PTL 3, a rolled steel bar for hot forging is proposed in which S content is limited to a range from 0.008 % to less than 0.030 % in order to increase fatigue strength in the direction perpendicular to the forging direction. However, a large amount of V is added together with S, in a range from 0.20 % to 0.32 %, and low-temperature toughness is a problem.
Further, in PTL 4, a non-heat-treated steel for hot forging that has excellent bending fatigue strength is proposed. However, assuming relatively large components, the heating time before hot forging is long and there is concern about surface roughness of the hot forged material.

CITATION LIST

Patent Literature

PTL 1: JP 2000-265245 A
PTL 2: JP 2006-206934 A
PTL 3: JP 2013-108130 A
PTL 4: JP 2010-270346 A

SUMMARY

(Technical Problem)

As mentioned above, increasing strength in a microstructure composed of ferrite and pearlite is difficult. In contrast, bainitic non-heat-treated steel secures higher tensile strength and toughness. However, in bainitic non-heat-treated steel, permanent deformation begins from a range of low load in tensile tests, and therefore proof stress in a low strain range even smaller than 0.2 % is lower than that of ferritic/pearlitic non-heat-treated steel, which has lower tensile strength. Accordingly, when repeatedly subjected to stresses such as bending in the low strain range, deformation is likely to occur. Therefore, in this case, bainitic non-heat-treated steel is not applicable to the production of components for which durability is required.
Further, in both ferritic/pearlitic and bainitic microstructures, when strengthened as non-heat-treated steel, the steel becomes too hard during machining after forging, shortening tool life, and therefore the addition of free-cutting elements becomes necessary. Alternatively, annealing is required after forging, which increases costs and does not make sense for a non-heat-treated product.
In view of the above problems, it would be helpful to provide a non-heat-treated steel for hot forging suitable as material to obtain hot forged material that has no surface roughness and has excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in a low strain range. Further, it would be helpful to provide hot forged material that has no surface roughness and has excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in the low strain range. Further, it would be helpful to provide a method of producing hot forged material that can produce hot forged material that has the excellent properties described above using a commonly used hot forging apparatus.
Here, the term "excellent tensile properties" means that 0.05 % proof stress is 650 N/mm² or more, 0.2 % proof stress is 680 N/mm² or more, and tensile strength is 900 N/mm² or more, as measured by the tensile test described later. The term "excellent impact properties" means that the impact value at -50 °C measured by the Charpy impact test described later is 25 J/cm² or more. The term "excellent rotating bending fatigue resistance" means that the fatigue limit is 500 N/mm² or more at 10⁷ cycles of loading by the Ono-type rotating bending fatigue test described later. The term "excellent deformation properties under repeated stress loading in the low strain range" means that the number of repetitions until a deformation amount reaches 0.5 mm in the repeated stress loading test described below is 80 or more.

(Solution to Problem)

The inventors engaged in extensive studies regarding the above problems and made the following discoveries.

(A) To suppress deformation due to repeated stress loading, it is important to increase strength while maintaining ferritic/pearlitic microstructure, which has a long completely elastic range in the low strain range. Further, by increasing the yield point, strength in the low strain range can be increased.
(B) By strengthening crystal grain boundaries of microstructure, fatigue strength can be further improved and deformation due to repeated stress can be decreased.
(C) Strengthening by precipitation of ferrite is the most effective way to increase the yield point, and yield stress improvement can be maximized by controlling the area fraction of ferrite and the distribution state of precipitates in the ferrite. This is accomplished through chemical composition and microstructure optimization.
(D) Surface roughness due to heating prior to hot forging is due to nonuniform oxidation of Ni, and it is important to avoid substantial Ni addition.
(E) In comparison with existing ferritic/pearlitic microstructure non-heat-treated steel, steel produced based on the above discoveries (A) to (D) suppresses surface roughness caused by heating before hot forging, and can suppress hardness of as-forged steel by increasing the yield point without increasing the tensile strength substantially. The steel can also obtain the same workability and machinability without increasing the amount of free-cutting elements added.

The present disclosure was completed based on these discoveries, and primary features of the present disclosure are described below.

[1] Non-heat-treated steel for hot forging comprising a chemical composition containing (consisting of), in mass%,
- C: 0.25 % or more and 0.37 % or less,
- Si: 0.50 % or more and 1.00 % or less,
- Mn: 1.45 % or more and 2.20 % or less,
- P: 0.005 % or more and 0.030 % or less,
- S: 0.030 % or more and 0.080 % or less,
- Al: 0.015 % or more and 0.050 % or less,
- Ni: 0.03 % or less,
- Cr: 0.01 % or more and 0.20 % or less,
- V: 0.10 % or more and 0.25 % or less,
- B: 0.0003 % or less, and
- N: 0.0030 % or more and 0.0200 % or less,
- with the balance being Fe and inevitable impurities, wherein Ceq defined by the following Expression (1) is 0.62 or more and 0.72 or less, α defined by the following Expression (2) is 4.4 or more and 6.2 or less, and β defined by the following Expression (3) is 20.0 or more and 60.0 or less, $Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14$ $α = 2.7 Mn + 4.6 Cr + V$ $β = Mn / S$
- where the element reference signs in the Expressions (1) to (3) indicate the content in mass% of the element.
[2] The non-heat-treated steel for hot forging according to [1], wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of:
- Cu: 0.05 % or less,
- Mo: 0.08 % or less,
- Nb: 0.040 % or less,
- Ti: 0.025 % or less,
- Sb: 0.0100 % or less,
- Pb: 0.30 % or less,
- Ca: 0.0050 % or less,
- Mg: 0.0050 % or less, and
- Bi: 0.30 % or less.
[3] The non-heat-treated steel for hot forging according to [1] or [2], wherein the chemical composition further contains, in mass%,
Sn: 0.030 % or less.
[4] Hot forged material comprising:
- the chemical composition according to any one of [1] to [3], and
- a microstructure in which the sum of area fractions of ferrite and pearlite is 90 % or more, an area fraction of intergranular ferrite is 10 % or more and 35 % or less, an average grain size of intergranular ferrite is 2 µm or more and 20 µm or less, an average size of intergranular ferrite and pearlite blocks is 5.0 µm or less, and an average interlamellar spacing of pearlite is 0.10 µm or more and 0.25 µm or less.
[5] A method of producing hot forged material, the method comprising:
- a step of heating the non-heat-treated steel for hot forging according to any one of [1] to [3] to a temperature of 1200 °C or more and 1300 °C or less;
- a subsequent step of hot forging the non-heat-treated steel for hot forging at a finishing temperature of 1050 °C or more to obtain hot forged material; and
- a subsequent step of cooling the hot forged material at a cooling rate of 0.80 °C/s or less to produce the hot forged material according to [4].

(Advantageous Effect)

By using the non-heat-treated steel for hot forging according to the present disclosure, hot forged material is obtainable that has no surface roughness and has excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in the low strain range. Further, the hot forged material according to the present disclosure has no surface roughness and has excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in the low strain range. Further, the method of producing a hot forged material according to the present disclosure can produce hot forged material that has the excellent properties described above using a commonly used hot forging apparatus.

DETAILED DESCRIPTION

(Non-heat-treated steel for hot forging)

It is essential that the non-heat-treated steel for hot forging according to an embodiment of the present disclosure has a defined chemical composition. Reasons for limiting the content of each element in the chemical composition are explained below. In the following description, "%" means "mass%" unless otherwise specified.

C: 0.25 % or more and 0.37 % or less

To secure desired strength, C content is 0.25 % or more. The C content is preferably 0.27 % or more. On the other hand, when the C content exceeds 0.37 %, bainitic phase in the microstructure increases and the proof stress in the low strain range decreases. Alternatively, toughness is decreased and impact properties degrade. The C content is therefore 0.37 % or less. The C content is preferably 0.35 % or less.

Si: 0.50 % or more and 1.00 % or less

To obtain the defined microstructure and toughness, Si content is 0.50 % or more. The Si content is preferably 0.55 % or more. On the other hand, when the Si content exceeds 1.00 %, toughness decreases and impact properties degrade. Alternatively, deformation resistance during hot forging increases, decreasing the life of the press die. The Si content is therefore 1.00 % or less. The Si content is preferably 0.90 % or less.

Mn: 1.45 % or more and 2.20 % or less

Mn is an element that improves strength through solid solution strengthening. To secure the desired strength and toughness, Mn content is 1.45 % or more. The Mn content is preferably 1.48 % or more. On the other hand, when the Mn content exceeds 2.20 %, bainite is included in the microstructure, resulting in lower proof stress, larger deformation under repeated stress loading, and lower fatigue strength. Further, toughness is decreased and impact properties degrade. The Mn content is therefore 2.20 % or less. The Mn content is preferably 2.00 % or less.

P: 0.005 % or more and 0.030 % or less

P contributes to ferrite strengthening by solid solution strengthening, but also segregates to grain boundaries, embrittling grain boundaries and decreasing toughness. To suppress this decreases in toughness, P content is 0.030 % or less. The P content is preferably 0.027 % or less. On the other hand, an excessive decrease in P content significantly increases production costs, and therefore the P content is 0.005 % or more.

S: 0.030 % or more and 0.080 % or less

S is an element that improves machinability by cutting and toughness, and to obtain these effects, S content is 0.030 % or more. The S content is more preferably 0.035 % or more. On the other hand, when the S content exceeds 0.080 %, a large amount of MnS inclusions may reduce toughness and accelerate the progression of fatigue fracture, resulting in decreased fatigue strength. The S content is therefore 0.080 % or less. The S content is preferably 0.070 % or less.

Al: 0.015 % or more and 0.050 % or less

Al is necessary as a deoxidation element, and to achieve this effect, the Al content is 0.015 % or more. On the other hand, excessive addition of Al increases alumina inclusions and decreases toughness and fatigue strength. The Al content is therefore 0.050 % or less.

Ni: 0.03 % or less

Ni causes the surface of the steel to be unevenly oxidized by heating before hot forging, resulting in a surface defect called surface roughness. Ni content is therefore limited to 0.03 % or less. The Ni content is preferably 0.02 % or less. A lower limit of the Ni content is not particularly limited, and the N content may be 0.00 % or more.

Cr: 0.01 % or more and 0.20 % or less

Cr has an effect of increasing strength, and to obtain this effect, Cr content is 0.01 % or more. The Cr content is preferably 0.02 % or more. On the other hand, when the Cr content exceeds 0.20 %, toughness decreases and impact properties degrade. The Cr content is therefore 0.20 % or less. The Cr content is preferably 0.15 % or less.

V: 0.10 % or more and 0.25 % or less

V strengthens ferrite by precipitating carbides and carbonitrides in the ferrite. To achieve this effect, V content is 0.10 % or more. The V content is preferably 0.11 % or more. On the other hand, when the V content exceeds 0.25 %, the amount of precipitates becomes large and the toughness of ferrite decreases, resulting in degradation of impact properties. The V content is therefore 0.25 % or less. The V content is preferably 0.19 % or less.

B: 0.0003 % or less

B is an element that enhances hardenability by segregating to grain boundaries and suppressing ferrite transformation. To obtain this effect, B content is preferably 0.0001 % or more. On the other hand, when the B content exceeds 0.0003 %, formation of bainitic microstructure is promoted and toughness and fatigue strength decrease. The B content is therefore 0.0003 % or less.

N: 0.0030 % or more and 0.0200 % or less

N combines with V to form nitrides or carbonitrides to strengthen ferrite by precipitation. When N content is less than 0.0030 %, the strengthening by precipitation is insufficient, resulting in low proof stress in the low strain range, low rotating bending fatigue strength, and large deformation due to repeated stress. The N content is therefore 0.0030 % or more. The N content is preferably 0.0040 % or more. On the other hand, when the N content exceeds 0.0200 %, this effect is saturated while toughness is adversely affected. The N content is therefore 0.0200 % or less. The N content is preferably 0.0150 % or less.

Ceq: 0.62 or more and 0.72 or less

$Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14$
The element reference signs in Expression (1) indicate the content in mass% of the element.
Ceq is an index that correlates with hardenability of steel. To obtain the defined strength, Ceq is 0.62 or more. Ceq is preferably 0.63 or more. On the other hand, when Ceq exceeds 0.72, bainitic microstructure is formed and toughness and fatigue strength decrease. For this reason, Ceq is 0.72 or less. Ceq is preferably 0.71 or less.

α: 4.4 or more and 6.2 or less

$α = 2.7 Mn + 4.6 Cr + V$
The element reference signs in Expression (2) indicate the content in mass% of the element.
α is an index that correlates with the proof stress at 0.05 % strain. When α is less than 4.4, the proof stress in the low strain range decreases, resulting in low fatigue strength. For this reason, α is 4.4 or more. On the other hand, when α exceeds 6.2, bainitic microstructure is more likely, and similarly, proof stress decreases and deformation due to repeated stress occurs earlier. α is therefore 6.2 or less.

β: 20.0 or more and 60.0 or less

$β = Mn / S$
The element reference signs in Expression (3) indicate the content in mass% of the element.
Mn and S form MnS in the steel, causing γ grains to be refined, and MnS becomes the core of ferrite transformation, which contributes to toughness improvement. To achieve this effect, β, the ratio of Mn content to S content, needs to be controlled to be 20.0 or more and 60.0 or less. β is preferably 20.0 or more and 55.0 or less. β is more preferably 20.0 or more and 50.0 or less.
The above is a description of the basic components of the chemical composition of the non-heat-treated steel for hot forging according to the present embodiment, but the chemical composition may contain one or more elements selected from the group consisting of Cu, Mo, Nb, Ti, Sb, Pb, Ca, Mg, Bi, and Sn. These elements are optional elements, and therefore a lower limit of content of each element is not particularly limited. That is, the content of each element may of course be 0 %, or more than 0 %.

Cu: 0.05 % or less

Cu may be included to increase strength. To obtain this effect, Cu content is preferably 0.01 % or more. However, Cu affects scale separability of the steel surface after hot forging. When the Cu content exceeds 0.05 %, scale is more likely to remain adhered, which may affect tool life and component precision in subsequent machining. Therefore, when Cu is added, the Cu content is 0.05 % or less.

Mo: 0.08 % or less

Mo may be included to increase strength. To obtain this effect, Mo content is preferably 0.01 % or more. However, when the Mo content exceeds 0.08 %, a large amount of bainitic microstructure is formed, resulting in lower proof stress and larger deformation due to repeated stress. Therefore, when Mo is added, the Mo content is 0.08 % or less.

Nb: 0.040 % or less

Nb may be included to increase the hardenability of the steel. To obtain this effect, Nb content is preferably 0.001 % or more. However, when the Nb content exceeds 0.040 %, bainitic microstructure is formed and fatigue strength is reduced. Therefore, when Nb is added, the Nb content is 0.040 % or less.

Ti: 0.025 % or less

Ti has an effect of improving impact properties by refining microstructure. To obtain this effect, Ti content is preferably 0.001 % or more. On the other hand, when the Ti content exceeds 0.025 %, toughness decreases and impact properties degrade, and therefore, when Ti is included, the Ti content is 0.025 % or less.

Sb: 0.0100 % or less

Sb has the effects of suppressing surface decarburization and increasing fatigue strength. To obtain these effects, Sb content is preferably 0.0010 % or more. The Sb content is more preferably 0.0015 % or more. On the other hand, when the Sb content exceeds 0.0100 %, toughness decreases and impact properties degrade, and therefore, when Sb is included, the Sb content is 0.0100 % or less.

Pb: 0.30 % or less

Pb has an effect of improving machinability by cutting. To obtain this effect, Pb content is preferably 0.01 % or more. The Pb content is more preferably 0.05 % or more. On the other hand, when the Pb content exceeds 0.30 %, toughness decreases and impact properties degrade, and therefore, when Pb is included, the Pb content is 0.30 % or less.

Ca: 0.0050 % or less

Ca has an effect of improving machinability by cutting. To obtain this effect, Ca content is preferably 0.0001 % or more. The Ca content is more preferably 0.0005 % or more. On the other hand, when the Ca content exceeds 0.0050 %, toughness decreases and impact properties degrade, and therefore, when Ca is included, the Ca content is 0.0050 % or less.

Mg: 0.0050 % or less

Mg has an effect of improving machinability by cutting. To obtain this effect, Mg content is preferably 0.0001 % or more. The Mg content is more preferably 0.0005 % or more. On the other hand, when the Mg content exceeds 0.0050 %, toughness decreases and impact properties degrade, and therefore, when Mg is included, the Mg content is 0.0050 % or less.

Bi: 0.30 % or less

Bi has an effect of improving machinability by cutting. To obtain this effect, Bi content is preferably 0.01 % or more. The Bi content is more preferably 0.05 % or more. On the other hand, when the Bi content exceeds 0.30 %, toughness decreases and impact properties degrade, and therefore, when Bi is included, the Bi content is 0.30 % or less.

Sn: 0.030 % or less

Sn has the effects of suppressing surface decarburization and increasing fatigue strength. To obtain these effects, Sn content is preferably 0.0010 % or more. On the other hand, when the Sn content exceeds 0.030 %, toughness decreases and impact properties degrade, and therefore, when Sn is included, the Sn content is 0.030 % or less.
The balance of the chemical composition of the non-heat-treated steel for hot forging according to the present embodiment consists of Fe and inevitable impurities. The content of each element may be measured by spark discharge atomic emission spectrometry, X-ray fluorescence analysis, inductively coupled plasma (ICP) optical emission spectrometry, ICP mass spectrometry, a combustion method, or the like.

(Hot forged material)

The hot forged material according to an embodiment of the present disclosure is produced by hot forging non-heat-treated steel for hot forging that has the chemical composition described above. That is, the chemical composition of the hot forged material is the same as the chemical composition described above. The hot forged material according to the present embodiment has the following microstructure. That is, the microstructure of the hot forged material according to the present embodiment is mainly ferrite and pearlite with a suppressed bainitic microstructure proportion. The microstructure indicated below is achievable by a combination of chemical composition (mainly controlled to satisfy Expressions (1) and (2)) of the non-heat-treated steel for hot forging and hot forging conditions.

Total area fraction of ferrite and pearlite: 90 % or more

When the total area fraction of ferrite and pearlite is less than 90 %, the bainite fraction increases and strength and fatigue strength decrease. The total area fraction of ferrite and pearlite is therefore 90 % or more. The total area fraction is preferably 92 % or more. The total area fraction may of course be 100 %. The balance, other than ferrite and pearlite, consists of bainite. The method of measuring the total area fraction of ferrite and pearlite is described later in the EXAMPLES section of the present disclosure.

Area fraction of intergranular ferrite: 10 % or more and 35 % or less

Intergranular ferrite is polygonal ferrite crystal grains that form from prior austenite grain boundaries during the early stages of transformation during cooling after hot forging. When the area fraction of intergranular ferrite is less than 10 %, the toughness of the hot forged material is decreased, and therefore the area fraction of intergranular ferrite is 10 % or more. On the other hand, when the area fraction of intergranular ferrite exceeds 35 %, the strength of the hot forged material decreases, and therefore the area fraction of intergranular ferrite is 35 % or less. The method for measuring the area fraction of intergranular ferrite is described later in the EXAMPLES section of the present disclosure.

Average grain size of intergranular ferrite: 2 µm or more and 20 µm or less

When the average grain size of intergranular ferrite is less than 2 µm, strength becomes too high and toughness is adversely affected. The average grain size of intergranular ferrite is therefore 2 µm or more. On the other hand, when the average grain size of intergranular ferrite exceeds 20 µm, toughness is decreased, and therefore the average grain size of intergranular ferrite is 20 µm or more. The method of measuring the average grain size of intergranular ferrite is described later in the EXAMPLES section of the present disclosure.

Average size of intergranular ferrite and pearlite blocks: 5.0 µm or less

A pearlite block is a region where the crystal orientation of layered ferrite that makes up pearlite is identical. Intergranular ferrite and pearlite blocks exhibit the same effect with respect to resistance to crack propagation in a toughness evaluation test, and therefore the average size of intergranular ferrite and pearlite blocks is used as an index for toughness evaluation. When the average size is 5.0 µm or less, toughness is excellent. The smaller the average size, the better the toughness, and therefore a lower limit of the average size is not particularly limited. According to the present embodiment, the average size may be 2.0 µm or more. The method of measuring the average size of intergranular ferrite and pearlite blocks is described later in the EXAMPLES section of the present disclosure.

Average interlamellar spacing of pearlite: 0.10 µm or more and 0.25 µm or less

When the average interlamellar spacing of pearlite is less than 0.10 µm, strength becomes too high and toughness is decreased. The average interlamellar spacing of pearlite is therefore 0.10 µm or more. Further, to achieve the defined strength, the average interlamellar spacing of pearlite is 0.25 µm or less. The method of measuring the average interlamellar spacing of pearlite is described later in the EXAMPLES section of the present disclosure.

(Method of producing non-heat-treated steel for hot forging)

The following describes a suitable method of producing non-heat-treated steel for hot forging according to an embodiment of the present disclosure. Molten steel that has the chemical composition described above is smelted using an ordinary converter, electric furnace, or the like, and made into steel material by ordinary continuous casting or blooming methods. The steel material is heated as required and made into a steel bar (non-heat-treated steel for hot forging) by hot rolling such as bloom rolling, bar rolling, or the like. Conditions for the heating and rolling described above are not particularly limited, and may be determined according to material properties. For example, the microstructure may be controlled to be advantageous for subsequent forging, machining, and the like.

(Method of producing hot forged material)

The method of producing hot forged material according to an embodiment of the present disclosure includes: a step of heating the non-heat-treated steel for hot forging according to an embodiment of the present disclosure to a defined temperature; a subsequent step of hot forging the non-heat-treated steel for hot forging at a defined finishing temperature to obtain hot forged material; and a subsequent step of cooling the hot forged material at a defined cooling rate.

Heating temperature before hot forging: 1200 °C or more and 1300 °C or less

When the heating temperature before hot forging is lower than 1200 °C, Ti precipitates cannot be redissolved, and coarse Ti precipitates remain, leading to reduced forging press die life. The heating temperature before hot forging is therefore 1200 °C or more. On the other hand, when the heating temperature before hot forging exceeds 1300 °C, the microstructure of the hot forged material becomes coarse and hardenability increases, resulting in a larger bainitic microstructure proportion and lower yield stress. Further, it becomes difficult to satisfy the toughness required for the hot forged material. The heating temperature before hot forging is therefore 1300 °C or less.

Finishing temperature of hot forging: 1050 °C or more

When the finishing temperature of hot forging is less than 1050 °C, deformation resistance during forging becomes higher, and the life of the forging press die decreases rapidly. The finishing temperature of hot forging is therefore 1050 °C or more. An upper limit of the finishing temperature of hot forging is not particularly limited. From the viewpoint of improving toughness, the finishing temperature of hot forging is preferably 1200 °C or less.

Cooling rate after hot forging: 0.80 °C/s or less

When the cooling rate after hot forging exceeds 0.80 °C/s, bainite becomes mixed in the microstructure and toughness and proof stress in the low strain range are significantly decreased. The cooling rate after hot forging is therefore 0.80 °C/s or less. A lower limit of the cooling rate after hot forging is not particularly limited. From the viewpoint of improving toughness, the cooling rate after hot forging is preferably 0.05 °C/s or more.
According to the method of producing hot forged material according to an embodiment of the present disclosure, it is possible to obtain the hot forged material having the chemical composition and microstructure described above. That is, hot forged material can be produced that has no surface roughness and has excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in the low strain range.
When the hot forged material according to the present embodiment is machined, hot-forged components that have high bending fatigue strength and low deformation with respect to repeated stress are obtainable. After machining into a component shape, one or more of cold working, induction hardening/tempering, shot peening, nitrocarburizing treatment, or carburizing-quenching/tempering may be further carried out.

EXAMPLES

Ingots that had chemical compositions including the elements listed in Table 1, with the balance consisting of Fe and inevitable impurities, were made into round bars that each had a diameter of 35 mm by hot rolling, and after heating according to the conditions listed in Table 2, hot forged to a diameter of 25 mm and cooled to obtain hot forged material. Table 2 lists the heating temperature before hot forging, the finishing temperature of hot forging, and the cooling rate after hot forging. The obtained hot forging test material was subjected to the following microstructure observation and various property tests.

[Microstructure observation]

For each test material, an arbitrary cross-section was cut out and collected, and after nital etching, the total area fraction of ferrite and pearlite and the area fraction of intergranular ferrite were determined in four fields of view observed under an optical microscope at a magnification of 200×. The average values of the four fields of view were used as the "total area fraction of ferrite and pearlite" and the "intergranular ferrite area fraction" of the present disclosure. Further, the circle equivalent diameter of intergranular ferrite in all four fields of view was determined, and the average value was used as the "average grain size of intergranular ferrite" of the present disclosure.
Further, for each test material, an arbitrary cross-section was cut out and collected, and after mirror polishing, the cross-section was subjected to electron back scattering pattern (EBSP) measurement. An electron beam was scanned at a pitch of 0.2 µm over a region 200 µm square in the field of view, and intergranular ferrite and pearlite blocks were specified using accompanying software. The average of the circle equivalent diameters of the specified intergranular ferrite and pearlite blocks was defined as the "average size of intergranular ferrite and pearlite blocks" of the present disclosure.
Further, for each test material, an arbitrary cross-section was cut out and collected, and after nital etching, the area having the narrowest pearlite lamella was observed using a scanning electron microscope (SEM) at a magnification of 5000×, and the average interlamellar spacing of 10 lamellae was defined as the "average interlamellar spacing of pearlite" of the present disclosure.

[Surface roughness presence]

The presence or absence of surface roughness of the test material was determined from the visual appearance of the test material surface. No surface roughness was considered a pass and surface roughness was considered a fail. Results are listed in Table 2.

[Tensile properties]

A JIS No. 4 tensile test piece was taken from the center of the test material, and proof stress and tensile strength were determined in accordance with JIS Z 2241:2022. Here, the proof stress was measured at 0.05 % strain and 0.2 % strain. Tensile properties were judged to pass testing when 0.05 % proof stress was 650 N/mm² or more, 0.2 % proof stress was 680 N/mm² or more, and tensile strength was 900 N/mm² or more; otherwise, tensile properties were judged to fail testing. Results are listed in Table 2.

[Charpy impact properties]

Three U-notch Charpy test pieces were taken from each test material, and Charpy impact tests according to JIS Z 2242:2018 were performed at -50 °C, and the average impact value was determined. When the average impact value at -50 °C was 25 J/cm² or more, the impact property was judged to pass testing, and when less than 25 J/cm², the impact property was judged to fail testing. Results are listed in Table 2.

[Rotating bending fatigue resistance]

Rotating bending fatigue tests were carried out on examples where all of the above tests for surface roughness, tensile properties, and impact properties were passed. Rotating bending fatigue tests were omitted for all but a few of the examples that failed the tests for surface roughness, tensile properties, or impact properties.
Ono-type rotating bending fatigue test pieces that had a parallel portion diameter of 8 mm were taken from each test material and subjected to rotating bending fatigue tests. The stress value at 10⁷ load cycles was used as the fatigue limit. The bending fatigue resistance was judged to pass testing when the fatigue limit at 10⁷ load cycles was 500 N/mm² or more, and fail testing when the fatigue limit was less than 500 N/mm². Results are listed in Table 2.

[Deformation properties under repeated stress loading]

Repeated stress loading tests were carried out on only the examples that passed bending fatigue resistance testing. The process of applying a 5 tonne tensile load to the tensile test piece and then removing the load was repeated, and the number of times until deformation became 0.5 mm was investigated. This test is a repeated stress loading test in the low strain range of less than 0.2 %. The number of repetitions was limited to 100. The deformation property under repeated stress loading in the low strain range was judged to pass testing when the number of times until the deformation amount reached 0.5 mm was 80 times or more, and judged to fail testing when the number of times was less than 80. Results are listed in Table 2.
When the non-heat-treated steel for hot forging having the chemical composition according to the present disclosure was used and the hot forging conditions according to the present disclosure were adopted, the hot forged material having the microstructure according to the present disclosure could be obtained. The obtained hot forged material had no surface roughness and had excellent tensile properties, impact properties, rotating bending fatigue resistance, and deformation properties under repeated stress loading in the low strain range.
In contrast, when the non-heat-treated steel for hot forging of a Comparative Example was used, or when the hot forging conditions were outside the scope of the present disclosure, the hot forged material according to the present disclosure could not be obtained. The obtained hot forged material had surface roughness or was inferior in at least one of the following properties: tensile properties, impact properties, rotating bending fatigue resistance, or deformation properties under repeated stress in the low strain range.

INDUSTRIAL APPLICABILITY

The non-heat-treated steel for hot forging and the hot forged material according to the present disclosure are suitable for use as material of components for machine structures used in automobiles, construction machinery, and industrial machinery.

Claims

Non-heat-treated steel for hot forging comprising a chemical composition containing, in mass%,
C: 0.25 % or more and 0.37 % or less,

Si: 0.50 % or more and 1.00 % or less,

Mn: 1.45 % or more and 2.20 % or less,

P: 0.005 % or more and 0.030 % or less,

S: 0.030 % or more and 0.080 % or less,

Al: 0.015 % or more and 0.050 % or less,

Ni: 0.03 % or less,

Cr: 0.01 % or more and 0.20 % or less,

V: 0.10 % or more and 0.25 % or less,

B: 0.0003 % or less, and

N: 0.0030 % or more and 0.0200 % or less,

with the balance being Fe and inevitable impurities, wherein Ceq defined by the following Expression (1) is 0.62 or more and 0.72 or less, α defined by the following Expression (2) is 4.4 or more and 6.2 or less, and β defined by the following Expression (3) is 20.0 or more and 60.0 or less, $Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14$ $α = 2.7 Mn + 4.6 Cr + V$ $β = Mn / S$

where the element reference signs in the Expressions (1) to (3) indicate the content in mass% of the element.
The non-heat-treated steel for hot forging according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of:
Cu: 0.05 % or less,

Mo: 0.08 % or less,

Nb: 0.040 % or less,

Ti: 0.025 % or less,

Sb: 0.0100 % or less,

Pb: 0.30 % or less,

Ca: 0.0050 % or less,

Mg: 0.0050 % or less, and

Bi: 0.30 % or less.
The non-heat-treated steel for hot forging according to claim 1 or 2, wherein the chemical composition further contains, in mass%,
Sn: 0.030 % or less.
Hot forged material comprising:
the chemical composition according to any one of claims 1 to 3; and

a microstructure in which the sum of area fractions of ferrite and pearlite is 90 % or more, an area fraction of intergranular ferrite is 10 % or more and 35 % or less, an average grain size of intergranular ferrite is 2 µm or more and 20 µm or less, an average size of intergranular ferrite and pearlite blocks is 5.0 µm or less, and an average interlamellar spacing of pearlite is 0.10 µm or more and 0.25 µm or less.
A method of producing hot forged material, the method comprising:
a step of heating the non-heat-treated steel for hot forging according to any one of claims 1 to 3 to a temperature of 1200 °C or more and 1300 °C or less;

a subsequent step of hot forging the non-heat-treated steel for hot forging at a finishing temperature of 1050 °C or more to obtain hot forged material; and

a subsequent step of cooling the hot forged material at a cooling rate of 0.80 °C/s or less to produce the hot forged material according to claim 4.