JP5419820B2 - Rolled steel bar or wire rod for hot forging - Google Patents

Description

  The present invention relates to a rolled steel bar or wire for hot forging. More specifically, the present invention relates to a steel bar or wire rod for hot forging that is weldable and has excellent strength and toughness (particularly toughness at low temperatures).

  Automotive undercarriage parts such as spindles require high strength and good toughness.

  Generally, “Carbon steels and alloy steels for mechanical structures” such as S48C defined in JIS G 4051 (2009) and SCM435H defined in JIS G 4051 (2009) are used as the material for the above parts. ing.

  Usually, the above steel is hot forged and then subjected to quenching-tempering heat treatment, and then processed into a predetermined shape by cutting. Furthermore, in order to join with other parts, processing such as bolt hole processing and spline shape processing is performed to finish a desired part.

  However, the cutting cost of high-strength parts is extremely high. For this reason, joining with other components by welding is desired.

  If the C content is reduced, welding cracks can be prevented, so that welding becomes possible.

  However, if the C content is reduced, it becomes difficult to provide the parts with high strength.

  Furthermore, in the case of welding, crystal grains are coarsened in a heat-affected zone (hereinafter referred to as “HAZ”), making it difficult to ensure good toughness, particularly at low temperatures. In addition, it is necessary not only to suppress a decrease in the toughness of the HAZ but also to improve the toughness of the base material of the component itself for an automobile undercarriage component that is expected to receive a strong impact such as a spindle.

  Therefore, it is assumed that the material for the above parts is used in a cold region, and the HAZ and the base material are required to have excellent toughness particularly at a low temperature.

  Therefore, Patent Document 1 discloses “steel that can be welded with high strength and high toughness and a method for producing a member using the steel”.

JP 2007-84909 A

  In the case of the technique disclosed in Patent Document 1, it has not been sufficient to consider the toughness of the HAZ and the base material and weld cracking. Furthermore, in order to increase the toughness of the base metal, it is necessary to lower the heating temperature during hot forging to 1100 ° C. or lower and perform controlled forging. When the heating temperature during hot forging is lowered, Since deformation resistance becomes high, it was not necessarily preferable when manufacturing parts.

  Moreover, there exists a thick board as what requires weldability. However, unlike steel plates, the steel bars and wires used for parts such as spindles are further hardened and tempered after the rolled material is further heated to a high temperature of about 1200 ° C. and formed into a part shape by hot forging. The heat treatment is performed. Therefore, it is also necessary to have stable base material toughness even when the whole material is heated again to a high temperature of about 1200 ° C. after rolling.

  The present invention has been made in view of the above situation, and its purpose is that welding is possible, and the strength and toughness of the base material are excellent, as well as the toughness of HAZ. It is to provide a rolled steel bar or wire rod for hot forging that is suitable as a material for parts.

  Among the above-mentioned problems, the present inventor first made various investigations and studies on preventing weld cracking and ensuring excellent strength of the base material. As a result, the following knowledge was obtained.

(A) About weld crack:
(A-1) The presence or absence of weld cracking depends on the highest hardness of HAZ (hereinafter referred to as “Hmax”). When Hmax exceeds 400 in terms of Vickers hardness (hereinafter referred to as “HV hardness”), weld cracks are likely to occur.

(A-2) Carbon equivalent (Ceq) is often used as an index for predicting Hmax. For example, the following formula is described in JIS G 3106 (2008).
Ceq = C + (Si / 24) + (Mn / 6) + (Ni / 40) + (Cr / 5) + (Mo / 4) + (V / 14)
C, Si, Mn, Ni, Cr, Mo, and V in the above formulas represent the content of each element in mass%.

Although many relational expressions between Ceq and Hmax have been proposed, the present inventors based on the data shown in the examples,
Hmax = 583 × Ceq + 65
Was used to predict Hmax.

  (A-3) From the above (a-1) and (a-2), in order to prevent weld cracking, only the contents of C, Si, Mn and Cr as elements intentionally contained in the base material are included. Instead, it is effective to limit the contents of Ni, Mo and V in the impurities.

(B) Strength of base material:
(B-1) For Hmax correlated with weld cracks, in other words, the C content has the largest influence on Ceq. For this reason, it is preferable to lower the C content to prevent weld cracking and to contain an element that enhances hardenability instead of C to ensure strength.

(B-2) Ensuring sufficient strength (tensile strength of 800 MPa or more) after tempering at 400 to 500 ° C. (especially after tempering at 475 ° C.) after performing quenching-tempering heat treatment. In order to do so, for example, the chemical composition of the base material may be adjusted so that the ideal critical diameter (DI) represented by the following formula is 70 or more.
^{DI = 25.4 × 0.311 × C 0.5} × (1 + 0.64 × Si) × (1 + 4.10 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2. 33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) × {1 + 1.50 × (0.90−C)}
C, Si, Mn, P, S, Cr, Ni, Mo, and Cu in the above formulas represent the content of each element in mass%.

  Even if welding cracks can be prevented as described above and high strength can be secured in the base material, the toughness of the HAZ and the base material is not necessarily excellent.

  Then, next, the present inventor repeated various investigations and examinations about ensuring excellent toughness in the HAZ and the base material, which were left out of the above-mentioned problems. As a result, the following knowledge was obtained.

(C) HAZ toughness:
(C-1) During welding, the HAZ is heated to a high temperature exceeding 1200 ° C. As “pinning particles” that suppress coarsening of HAZ crystal grains at such a high temperature, it is preferable to use TiN having a high solid solution temperature because carbides and / or carbonitrides form a solid solution.

  (C-2) TiN does not dissolve in the matrix during heating for hot forging or during heating for quenching. For this reason, if the TiN is deposited in an appropriate size at the stage of the steel bar or wire after hot rolling, the HAZ crystal grain coarsening can be suppressed during welding. That is, the toughness of the HAZ can be increased by controlling the precipitation form of TiN that precipitates at the stage of the steel bar or wire after hot rolling.

(D) About the toughness of the base material:
(D-1) Even if the chemical composition of the base material is adjusted so that the DI is 70 or more, if the contents of C, Si, Mn and Cr are excessively increased, the hardness of the base material is increased. Thus, the toughness of the base material decreases.

  (D-2) Not only the contents of C, Si, Mn and Cr but also the contents of P, S, Cu, Ni and Mo in the impurities affect the toughness of the base material.

  (D-3) In order to ensure good toughness of the base material, it is preferable to adjust the chemical composition of the base material so that DI does not exceed 170.

  (D-4) In order to increase the toughness of the base material, in addition to the adjustment of the DI, it is necessary to prevent the austenite grains from becoming coarse during heating for quenching.

  (D-5) Since the above-mentioned TiN has a large size as pinning particles at the time of heating for quenching, the effect is small. On the other hand, TiC and TiC in which N is dissolved, that is, Ti (C, N), is finer than TiN, and thus acts as “pinning particles” during heating for quenching to a temperature around 900 ° C. It has the effect of suppressing the coarsening of austenite grains.

  (D-6) If the size of TiC and Ti (C, N) is too small, TiC and Ti (C, N) are dissolved in the matrix by heating during hot forging to a temperature of around 1200 ° C. On the other hand, when the sizes of TiC and Ti (C, N) are coarse, the number is reduced.

  (D-7) In order for TiC and Ti (C, N) to sufficiently act as “pinning particles” during heating for quenching, before heating for quenching, that is, after hot forging. In this state, it is important that a large number of fine TiC and Ti (C, N) are distributed.

  (D-8) If TiC and Ti (C, N) are deposited in an appropriate size at the stage of the steel bar or wire after hot rolling, the condition (d-7) can be satisfied. Therefore, austenite grain coarsening during heating for quenching can be suppressed. That is, the toughness of the base material can be increased by controlling the precipitation form of TiC and Ti (C, N) precipitated at the stage of the steel bar or wire after hot rolling. For this reason, it is possible to use the parts in a low temperature environment.

  (D-9) Ti binds preferentially to N rather than C. For this reason, in order to use both [TiN] and [TiC and Ti (C, N)] as “pinning particles”, it is necessary to contain an amount of Ti that binds to both N and C.

  The present invention has been completed based on the above findings, and the gist of the present invention is that of the rolled steel bar or wire for hot forging shown in (1) below and the rolled steel bar or wire for hot forging shown in (2). In the manufacturing method.

(1) In mass%,
C: 0.10 to 0.20%,
Si: 0.01-0.30%,
Mn: 1.00-2.30%,
S: 0.040% or less,
Cr: 0.10 to 0.80%,
Al: 0.010-0.080%,
B: 0.0002 to 0.0050%,
Ti: 0.010 to 0.080%, and N: 0.0020 to 0.0080%
The balance consists of Fe and impurities,
P, Cu, Ni, Mo and V in the impurity are
P: 0.040% or less,
Cu: less than 0.10%,
Ni: less than 0.10%,
Mo: limited to less than 0.05%, and V: 0.01% or less,
Furthermore, fn1 represented by the following formula (1) is 0.001 or more, Ceq represented by the formula (2) is 0.57 or less, and DI represented by the formula (3) is 70 to 170. Having a composition,
Furthermore, in an area of 100 μm ² , 10 or more Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm and 10 or more Ti deposits of 0.01 to 0.05 μm were deposited. A rolled steel bar or wire rod for hot forging characterized by
fn1 = Ti-3.4N (1)
Ceq = C + (Si / 24) + (Mn / 6) + (Ni / 40) + (Cr / 5) + (Mo / 4) + (V / 14) (2)
^{DI = 25.4 × 0.311 × C 0.5} × (1 + 0.64 × Si) × (1 + 4.10 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2. 33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) × {1 + 1.50 × (0.90−C)} (3)
Ti, N, C, Si, Mn, Ni, Cr, Mo, V, P, S, and Cu in the above formulas represent the content of each element in mass%.

  Said Ti deposit refers to TiN, TiC, and Ti (C, N). Hereinafter, TiC in which N is dissolved, that is, Ti (C, N) may also be referred to as TiC.

  Further, the “impurities” in the “Fe and impurities” as the balance refers to those mixed from the ore, scrap, or production environment as raw materials when the steel material is industrially produced.

(2) Continuously casting the molten steel having the chemical composition described in (1) above under the condition that the cooling rate is 2.0 ° C./min or more,
The slab is heated to a temperature range of 1100 ° C. or higher and held for 30 minutes or more, and is subjected to split rolling, bar rolling, or wire rod rolling under the conditions satisfying the following formula (4), and further, bar rolling or wire rod A method for producing a rolled steel bar or wire rod for hot forging characterized by cooling a temperature range of 800 to 600 ° C after rolling at a cooling rate of 5 to 100 ° C / min.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 7.5 × 10 ³ (4)
In the above formula (4), T is the heating temperature in units of “° C.”, t is the holding time in units of “s” at the heating temperature T, subscript 1 is the block rolling process, and subscript 2 is the steel bar rolling Or it represents a wire rod rolling process and means T (2) = (T ₂ +273) / (T ₁ +273).

That is, T ₁ is the heating temperature (° C.) in the batch rolling process, T ₂ is the heating temperature (° C.) in the bar rolling or wire rolling process, and t ₁ is the holding time (s) at T ₁ ° C. in the batch rolling process. , T ₂ is the holding time (s) at T ₂ ° C in the steel bar rolling or wire rod rolling process.

  The temperature and cooling rate in each of the above treatments refer to the temperature and cooling rate based on the surface.

  The rolled steel bar or wire rod for hot forging of the present invention can be welded, and is excellent in the strength and toughness of the base material and also in the toughness of HAZ. It can be used suitably.

It is a figure explaining the positional relationship of the R / 2 part in the round bar of 36 mm in diameter which extract | collected the test piece for 25 mm in width lap joint welding used in the Example. In this figure, the longer dimension of the test piece is referred to as the “width direction” of the test piece. It is a figure which shows the dimension shape of the test piece for the lap joint welding used in the Example. The unit of the dimension in the figure is “mm”. In an Example, it is a figure which illustrates typically the condition which used the two test pieces for lap joint welding, and performed the lap joint welding. In the examples, the test material subjected to the lap joint welding was vertically cut at the center in the width direction and divided into two equal parts. Further, the test piece was cut into 30 mm so that the weld metal became the center in the length direction in the longitudinal section. It is a figure explaining the position which measured the crystal grain size of HAZ using FIG. The unit of the dimension in the figure is “mm”. In the examples, the test material subjected to the lap joint welding was vertically cut at the center in the width direction and divided into two equal parts. Further, the test piece was cut into 30 mm so that the weld metal became the center in the length direction in the longitudinal section. It is a figure explaining the condition which measured HV hardness using. The unit of the dimension in the figure is “mm”.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition of steel bar or wire:
C: 0.10 to 0.20%
C has an effect of increasing the martensite hardness after quenching of the steel, and also has an effect of forming TiC together with Ti and suppressing grain coarsening during heating for quenching. In order to sufficiently secure the effect, it is necessary to contain 0.10% or more of C. On the other hand, C increases the hardness of the HAZ and as a result causes weld cracking. For this reason, the upper limit was provided and the C content was set to 0.10 to 0.20%. The C content is preferably 0.12% or more and 0.18% or less.

Si: 0.01-0.30%
Si is an element effective for deoxidation of steel and contributes to improvement of hardenability. In order to ensure these effects, it is necessary to contain Si by 0.01% or more. On the other hand, when the content of Si exceeds 0.30%, the above effect is saturated and hot ductility is reduced. Therefore, the Si content is set to 0.01 to 0.30%. The Si content is preferably 0.25% or less.

Mn: 1.00-2.30%
Mn has the effect of increasing tensile strength by improving hardenability. In order to acquire this effect, it is necessary to make Mn content 1.00% or more. On the other hand, if the Mn content is excessive, the hardness of the HAZ is increased, and as a result, it causes welding cracks. For this reason, the upper limit was provided and the content of Mn was set to 1.00 to 2.30%. The Mn content is preferably 1.50% or more and 2.00% or less.

S: 0.040% or less S is an element contained as an impurity in steel. Moreover, if it contains positively, it will combine with Mn and form MnS and has the effect of improving machinability. However, if the S content increases and exceeds 0.040% in particular, it segregates at the crystal grain boundaries and lowers the toughness of the base material. Therefore, the content of S is set to 0.040% or less. When importance is attached to the toughness of the base material, the S content is preferably 0.020% or less, and the lower the content, the better. On the other hand, when emphasizing machinability, it is desirable to positively contain S in an amount exceeding 0.020%.

Cr: 0.10 to 0.80%
Cr is an element effective for improving the hardenability. In order to acquire this effect, it is necessary to contain Cr 0.10% or more. On the other hand, if the Cr content is excessive, the hardness of the HAZ is increased, and as a result, it causes welding cracks. Therefore, an upper limit is set and the Cr content is set to 0.10 to 0.80%. The Cr content is preferably 0.20% or more and 0.60% or less.

Al: 0.010-0.080%
Al is added as a deoxidizer. In order to acquire this effect, it is necessary to contain Al 0.010% or more. However, even if Al is contained in an amount exceeding 0.080%, the effect is saturated and the alloy cost is increased. Therefore, the content of Al is set to 0.010 to 0.080%.

B: 0.0002 to 0.0050%
B is a very important element that enhances hardenability. In order to acquire this effect, it is necessary to contain B 0.0002% or more. On the other hand, if the content of B exceeds 0.0050%, not only the effect of improving hardenability is saturated but also the cost increases. For this reason, an upper limit is provided, and the B content is set to 0.0002 to 0.0050%. The B content is preferably 0.0030% or less.

Ti: 0.010 to 0.080%
Ti is an important element in the present invention. That is, Ti combines with free N to form TiN preferentially, and thus has the effect of preventing B, which is effective in hardenability, from combining with N. Moreover, said TiN acts as a pinning particle | grain and has the effect of suppressing the crystal grain coarsening of HAZ at the time of welding, and improving toughness. Furthermore, Ti combines with C to form TiC. The above TiC also acts as pinning particles, and has the effect of suppressing toughening of the austenite grains of the base material and increasing toughness during heating for quenching to a temperature of around 900 ° C. Therefore, the content of Ti is set to 0.010 to 0.080%.

N: 0.0020 to 0.0080%
N is an important element in the present invention. That is, as described above, TiN formed by combining N with Ti acts as pinning particles, suppresses coarsening of HAZ crystal grains during welding, and increases toughness. In order to obtain this effect, the N content needs to be 0.0020% or more. On the other hand, when the content of N increases, BN is formed and the effect of improving the hardenability of B is reduced. Furthermore, an excessive amount of N forms coarse TiN and reduces the toughness of the base material. Therefore, the upper limit is set and the N content is set to 0.0020 to 0.0080%.

  The rolled steel bar or wire rod for hot forging of the present invention, in addition to the above elements, the balance consists of Fe and impurities, the content of P, Cu, Ni, Mo and V in the impurities is limited as follows, Fn1 represented by the formula (1) is 0.001 or more, Ceq represented by the formula (2) is 0.57 or less, and DI represented by the formula (3) is 70 to 170. Is.

P: 0.040% or less P is an element contained as an impurity in steel, and its content increases. In particular, when it exceeds 0.040%, it segregates at the crystal grain boundary and increases the toughness of the base material. Reduce. Therefore, the content of P in the impurities is set to 0.040% or less. The P content in the impurities is preferably 0.030% or less.

Cu: Less than 0.10% Cu may be contained as an impurity in the steel although it is in a trace amount, and affects the hardenability similarly to C, Si, Mn and Cr, and increases the hardenability. If the hardenability is excessive, the toughness of the base material is lowered, so the Cu content needs to be reduced as much as possible. Therefore, the content of Cu in the impurities is set to less than 0.10%.

Ni: Less than 0.10% Ni may be contained as an impurity in the steel in a small amount, but affects the hardness and hardenability of HAZ as well as C, Si, Mn, and Cr, and hardness and quenching of HAZ Increase sex. If the hardness of the HAZ increases, the risk of weld cracking increases, and if the hardenability becomes excessive, the toughness of the base material decreases. Therefore, the content of Ni in the impurities needs to be reduced as much as possible, and is set to less than 0.10%.

Mo: less than 0.05% Mo may be contained as an impurity in the steel in a small amount, but affects the hardness and hardenability of HAZ as well as C, Si, Mn and Cr, and hardness and quenching of HAZ Increase sex. If the hardness of the HAZ increases, the risk of weld cracking increases, and if the hardenability becomes excessive, the toughness of the base material decreases. Therefore, the Mo content in the impurities needs to be reduced as much as possible, and is set to less than 0.05%.

V: 0.01% or less V may be contained as an impurity in the steel even though it is in a small amount, and affects the hardness of HAZ as in the case of C, Si, Mn, and Cr, and increases the hardness of HAZ. As the hardness of the HAZ increases, the risk of weld cracking increases. Therefore, the content of V in the impurities needs to be reduced as much as possible, and is set to 0.01% or less.

fn1: 0.001 or more Ti binds preferentially to N rather than C. For this reason, in the present invention using both TiN and TiC as pinning particles, it is necessary to contain an amount of Ti that binds to both N and C.

fn1, that is,
fn1 = Ti-3.4N (1)
When the value represented by the formula is less than 0.001, a sufficient amount of Ti to bond with C cannot be secured, and therefore the pinning effect of TiC becomes insufficient. Therefore, fn1 represented by the above formula (1) is determined to be 0.001 or more. fn1 may be a calculated value of 0.073 when Ti is 0.080% of the upper limit and N is 0.0020% of the lower limit.

Ceq: 0.57 or less The presence or absence of occurrence of weld cracks depends on Hmax (the highest hardness of HAZ). If Hmax exceeds 400 in terms of HV hardness, weld cracks are likely to occur. And Hmax has a correlation with Ceq represented by the following formula (2).
Hmax = 583 × Ceq + 65
Ceq = C + (Si / 24) + (Mn / 6) + (Ni / 40) + (Cr / 5) + (Mo / 4) + (V / 14) (2).

Therefore, in order to suppress the occurrence of weld cracking,
Hmax = 583 × Ceq + 65 ≦ 400
If it is.

Therefore, Ceq ≦ 335/583 = 0.57
Therefore, Ceq represented by the above formula (2) is 0.57 or less. Ceq may be 0.29 calculated from the lower limit value of each element.

DI: 70-170
After the quench-temper heat treatment, especially after tempering at 400-500 ° C. (especially after tempering at 475 ° C.), DI,
^{DI = 25.4 × 0.311 × C 0.5} × (1 + 0.64 × Si) × (1 + 4.10 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2. 33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) × {1 + 1.50 × (0.90−C)} (3)
If the value represented by the formula is less than 70, sufficient strength (tensile strength of 800 MPa or more) cannot be ensured. On the other hand, if the DI increases and exceeds 170, the hardness of the base material becomes too high and the toughness of the base material decreases. Therefore, the DI represented by the above formula (3) is 70 to 170.

(B) Ti precipitate size and precipitation density:
The Ti precipitates in the present invention are TiN, TiC, and Ti (C, N). TiN is a precipitate that precipitates at high temperatures, while TiC and Ti (C, N) are precipitates that precipitate at low temperatures. Since TiN precipitated at high temperature and TiC and Ti (C, N) precipitated at low temperature have the same crystal structure, it is difficult to obtain the respective precipitation densities. However, it is known that TiN among these Ti precipitates takes the form of a hexahedron. Therefore, as a result of investigating the shape and size of the Ti precipitates by observation with a transmission electron microscope, the present inventors have found that most of the Ti precipitates have a circle equivalent diameter of 0.07 μm or more. In the case of angularity, and when the equivalent circle diameter is 0.05 μm or less, there was almost no angularity in the Ti precipitates. Therefore, TiN that precipitates Ti precipitates having an equivalent circle diameter of 0.07 μm or more at high temperatures, Ti precipitates having an equivalent diameter of 0.05 μm or less were considered as TiC or Ti (C, N) that precipitates at a low temperature, and the precipitation density of both was defined.

(B-1) Size and precipitation density of Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm:
TiN does not dissolve in the matrix during heating for hot forging or during heating for quenching. For this reason, at the stage of the steel bar or wire rod after hot rolling, the size and precipitation density of TiN are adjusted, and the grain coarsening of HAZ is suppressed by the pinning action of TiN during welding.

If 10 or more Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm are deposited in the area of 100 μm ² at the stage of the steel bar or wire after hot rolling, the HAZ crystal grains become coarse Is suppressed, and it becomes easy to ensure good toughness in the HAZ.

  That is, the HAZ is heated to a high temperature exceeding 1200 ° C. during welding. However, even at such a high temperature, Ti precipitates having an equivalent circle diameter of 0.07 μm or more do not dissolve in the matrix. For this reason, it acts as pinning particles. On the other hand, when the equivalent circle diameter exceeds 1.0 μm, it is too large to have a pinning action.

However, at the stage of the steel bar or wire rod after hot rolling, even if the size of the Ti precipitate is 0.07 to 1.0 μm in equivalent circle diameter, 10 or more precipitates are deposited in the area of 100 μm ^2. Otherwise, coarsening of crystal grains is unavoidable in the HAZ during welding.

Therefore, in the rolled steel bar or wire rod for hot forging of the present invention, 10 or more Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm are precipitated in an area of 100 μm ² . The Ti precipitate having an equivalent circle diameter of 0.07 to 1.0 μm may be 1000 or less because the pinning effect is saturated even if the precipitation density is too high.

(B-2) Size and precipitation density of Ti precipitate having an equivalent circle diameter of 0.01 to 0.05 μm:
TiC acts as pinning particles during heating for quenching to a temperature of around 900 ° C., and suppresses coarsening of austenite grains. For this reason, it contributes to the toughness improvement of a base material.

  In order to fully exhibit the above effects, it is important that many fine TiCs are distributed before heating for quenching, that is, after hot forging.

That is, many fine TiCs should just precipitate after the hot forging which is a pre-quenching process. And to that end, at the stage of the raw material before hot forging, that is, the steel bar or wire rod after hot rolling, a Ti precipitate having an equivalent circle diameter of 0.01 to 0.05 μm is present in an area of 100 μm ^2. Ten or more must be deposited.

  Ti precipitates at the stage of steel bars or wire rods after hot rolling have a size that is too small and has an equivalent circle diameter of less than 0.01 μm, by heating during hot forging to a temperature of around 1200 ° C. It dissolves in the matrix. For this reason, the pinning effect cannot be ensured during heating for quenching. On the other hand, when the size of the Ti precipitate exceeds 0.05 μm in equivalent circle diameter in the above stage, coarse Ti precipitates remain during heating for quenching, and the number of fine Ti precipitates is reduced, resulting in a pinning effect. Run short. Therefore, in order for the Ti precipitates to exhibit the effect as pinning particles during heating for quenching, the size is 0.01 to 0.05 μm in terms of the equivalent circle diameter at the stage of the steel bar or wire after hot rolling. Must.

However, at the stage of the steel bar or wire after hot rolling, even if the size of the Ti precipitate is 0.01 to 0.05 μm in equivalent circle diameter, 10 or more precipitates in the area of 100 μm ^2. If not, coarsening of austenite grains during heating for quenching cannot be suppressed.

Therefore, in the rolled steel bar or wire rod for hot forging of the present invention, 10 or more Ti precipitates having an equivalent circle diameter of 0.01 to 0.05 μm are deposited in an area of 100 μm ² . The Ti precipitate having an equivalent circle diameter of 0.01 to 0.05 μm may be 1000 or less because the pinning effect is saturated even if the precipitation density is too high.

(C) Method for producing rolled steel bar or wire rod for hot forging:
The rolled steel bar or wire rod for hot forging according to the present invention shown in the above (1) can be produced, for example, by the method for producing a rolled steel bar or wire rod for hot forging shown in the above (2).

Specifically, the molten steel having the chemical composition described in the section (A) is continuously cast under the condition that the cooling rate is 2.0 ° C./min or more,
The slab is heated to a temperature range of 1100 ° C. or higher and held for 30 minutes or more, and is subjected to split rolling, bar rolling, or wire rod rolling under the conditions satisfying the following formula (4), and further, bar rolling or wire rod It can manufacture by cooling a 800-600 degreeC temperature range after a rolling with the cooling rate of 5-100 degrees C / min.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 7.5 × 10 ³ (4).

In the above formula (4), T is the heating temperature (° C.), t is the holding time (s) at T ° C., the subscript 1 is the split rolling process, the subscript 2 is the bar rolling or wire rolling process, (2) = (T ₂ +273) / (T ₁ +273).

  As described above, the temperature and cooling rate in each of the above treatments refer to the temperature and cooling rate based on the surface.

(C-1) Cooling rate during continuous casting of molten steel:
TiN precipitates during cooling of continuous casting. And, at the stage of the steel bar or wire after hot rolling, in order to obtain the precipitation density of 0.07 to 1.0 μm Ti precipitate having a circle equivalent diameter having the pinning action described in the above section (B-1). The molten steel is preferably continuously cast under the condition that the cooling rate is 2.0 ° C./min or more. Since there is a limit to increase the cooling rate, 50 ° C./min or less is a preferable range.

When the cooling rate is slow, TiN deposited during the cooling of continuous casting grows in the cooling process, and 10 or more Ti precipitates having a circle equivalent diameter of 0.07 to 1.0 μm are deposited in an area of 100 μm ² . It becomes difficult.

  The cooling rate at the time of continuous casting is a temperature range where TiN precipitates, that is, a cooling rate up to 1000 ° C. after the steel solidifies.

(C-2) About ingot rolling, hot bar rolling, and wire rod rolling:
In order to reduce segregation of the slab, it is generally desirable to increase the heating temperature when rolling the slab and to increase the holding time at the heating temperature.

However, when the holding time is increased, TiN becomes agglomerated and coarsened, making it difficult to make 10 or more Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm in an area of 100 μm ^2. Become. For this reason, the effect which suppresses the coarsening of the HAZ crystal grain by the pinning effect | action of TiN will lose | disappear in the case of welding.

  The slab heating temperature is set to 1100 ° C. or higher, and the holding time at the heating temperature is set to 30 min or higher in order to equalize the temperature to the center portion, and then the rolling is performed under the condition satisfying the above-described formula (4). If steel bar rolling or wire rod rolling is performed, TiN aggregation and coarsening can be suppressed, and thereby HAZ crystal grain coarsening during welding can be easily suppressed.

  Hereinafter, Formula (4) will be described.

  The degree of Ostwald growth of TiN is affected by the heating temperature T (° C.) and the heating time t (s). Therefore, it was considered to arrange the degree of Ostwald growth of TiN by the tempering parameter “(T + 273) × log (t)”.

  In general, a steel bar or a wire rod is produced by a two-stage rolling process, in which a slab is rolled into pieces and a steel bar or wire rod is rolled.

  Hereinafter, the production of the steel bar will be described in detail as an example.

In the block rolling process and the bar rolling process, the heating temperatures are T ₁ (° C.) and T ₂ (° C.), respectively, and the holding times at the heating temperatures are t ₁ (s) and t ₂ (s), respectively. The tempering parameters in each rolling step are “(T ₁ +273) × logt ₁ ” and “(T ₂ +273) × logt ₂ ”.

Here, the time x (s) required for causing the Ostwald growth of TiN, which occurs at the time of heating in the bar rolling process, to occur at the heating temperature T ₁ (° C.) in the block rolling process will be obtained.

The tempering parameter (T ₂ +273) × logt _{2 for} the Ostwald growth of TiN in the steel bar rolling process can be expressed by the formula (a) using the heating temperature T ₁ (° C.) in the block rolling process.
(T ₂ +273) logt ₂ = (T ₁ +273) logx (a).

Here, assuming T (2) = (T ₂ +273) / (T ₁ +273), the Ostwald growth of TiN, which occurs at the time of heating in the steel bar rolling process, is the heating temperature T ₁ (° C.) in the block rolling process. The time x (s) required for waking up can be expressed as shown in equation (b).
x = t ₂ ^{T (2)} (b).

Then, the degree of Ostwald growth of TiN that occurs in the steel bar rolling process at the heating temperature T ₂ (° C.) and the heating holding time t ₂ (s) is determined based on the heating temperature T ₁ (° C.) and the temperature T ₁ (° C. in the block rolling process. ), The degree Y of Ostwald growth of TiN for the two rolling steps of the split rolling process and the steel bar rolling process is expressed as a parameter for one batch rolling process by the formula (c ) And by substituting equation (b) into equation (c), equation (d) can be obtained.
Y = (T ₁ +273) × log (t ₁ + x) (c),
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) (d).

As a result of investigating in detail the relationship between the parameter Y, the size of TiN and the number of precipitates obtained in this way, the value of Y is 7.5 × 10 ³ or less. The rolled steel bar for hot forging related to the present invention shown can be obtained.

From the above, equation (4) is defined as a parameter representing the degree of Ostwald growth of TiN.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 7.5 × 10 ³ (4).

In addition, even if the value of Y is in a regulation range, it is preferable that the heating temperature of a lump rolling process and a steel bar rolling process shall be 1300 degrees C or less from a viewpoint of energy saving, and it is more preferable to set it as 1270 degrees C or less. Even if the value of Y is within the specified range, the holding time at the heating temperature in the block rolling process and the bar rolling process is preferably 18000 s (300 min) or less from the viewpoint of energy saving, and is 14400 s (240 min). It is more preferable to set the following. From the viewpoint of productivity, the value of Y is preferably 4.0 × 10 ³ or more.

  In addition, the parameter Y showing the degree of Ostwald growth of TiN shown by said Formula (d) is a general case where a bar steel is manufactured by the two-stage rolling process of a block rolling and a bar rolling.

The parameter Y ′ representing the degree of Ostwald growth of TiN when the number of rolling processes is performed in i stages can be expressed as in equation (e). In this case, the value of Y ′ is 7.5 × 10 ^3. If it is below, the rolled steel bar for hot forging related to the present invention shown in the above (1) can be obtained.

Y ′ = (T ₁ +273) × log {Σ (t _i ^{T (i)} )} (e).
Here, T (i) = (T _i +273) / (T ₁ +273) is meant.

  The same applies to the case where the slab is produced by two stages of rolling and wire rolling, and the number of rolling processes is i.

(C-3) About cooling after steel bar rolling or wire rolling:
In order to obtain the precipitation density of 0.01 to 0.05 μm Ti precipitate having a circle equivalent diameter having the pinning action described in the above (B-2) at the stage of the steel bar or wire after hot rolling, After rolling or wire rod rolling, it is preferable to cool a temperature range of 800 to 600 ° C. at a cooling rate of 5 to 100 ° C./min.

If the cooling rate is too high, TiC cannot be precipitated. On the other hand, if the cooling rate is too slow, the precipitated TiC aggregates and grows. Therefore, in any case, the equivalent circle diameter is 0.01 to 0 in an area of 100 μm ^2. It is difficult to deposit 10 or more 0.05 μm Ti precipitates.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels 1 to 14 shown in Table 1 were melted by a 70-ton converter, made into slabs by continuous casting under the conditions shown in Table 2, and then rolled into 180 mm x 180 mm steel pieces, and then further rolled into bar steel. Thus, a steel bar having a diameter of 54 mm was produced.

  Steels 1 to 7 and Steels 12 to 14 in Table 1 are steels of the present invention examples having chemical compositions within the range defined by the present invention. On the other hand, steels 8 to 11 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  With respect to the steel bar having a diameter of 54 mm prepared as described above, the precipitation density of TiN and TiC was investigated.

  That is, a sample is taken from a longitudinal section of R / 2 part (“R” represents a radius) of a steel bar having a diameter of 54 mm by an extraction replica method, and a transmission electron microscope equipped with an energy dispersive X-ray analyzer (EDX). Was observed to investigate the precipitation density of Ti precipitates.

  Specifically, 20 fields of view were observed at a magnification of 40000 times, the area of Ti precipitates was calculated by image analysis, converted to the area of a circle, and the equivalent circle diameter was determined.

Subsequently, the number of Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm was counted and converted into the number per 100 μm ² area. Similarly, the number of Ti precipitates having an equivalent circle diameter of 0.01 to 0.05 μm was counted and converted into the number per 100 μm ² area.

  Further, the steel bar having a diameter of 54 mm was heated at 1200 ° C. for 30 minutes and hot forged to produce a round bar having a diameter of 36 mm. Next, this round bar with a diameter of 36 mm was heated at 900 ° C. for 1 h for water quenching, and then tempered at 475 ° C. for 1 h.

  Tensile tests, impact tests, and HRC hardness measurements were carried out using a round bar with a diameter of 36 mm subjected to the above quenching-tempering heat treatment.

  In the tensile test, a specimen No. 14A specified in JIS Z 2201 (1998) (however, the diameter of the parallel part: 5 mm) was collected from R / 2 part of a round bar having a diameter of 36 mm, and at 25 ° C. by a usual method. A tensile test was performed and the tensile strength (TS) was measured. The target TS was 800 MPa or more.

For the impact test, a U-notch standard impact test piece (however, a notch depth of 2 mm and a notch bottom radius of 1 mm) defined in JIS Z 2242 (2005) was taken from R / 2 part of a round bar having a diameter of 36 mm. Charpy impact tests were conducted at -40 ° C and -40 ° C, and impact values were measured. Impact value as a target is, 25 ° C. at 160 J / cm ² or more, it was at -40 ℃ 135J / cm ² or more.

  The HRC hardness is measured by crossing the round bar having a diameter of 36 mm, polishing so that the cut surface becomes the test surface, and performing R / 2 part 4 points and central part 1 point. The value was HRC hardness.

  Further, from the R / 2 part of the round bar having a diameter of 36 mm subjected to the above quenching and tempering heat treatment, the test piece for lap joint welding shown in FIG. 2 was collected in the positional relationship shown in FIG. Lap joint welding was performed as shown.

  The welding conditions were MAG welding with a voltage of 20 V, a current of 180 A, a welding speed of 40 cm / min, and an 800 MPa class welding wire (AWS A5.28 (2005) ER110S-G) was used. First, the presence or absence of weld cracks was investigated, and then the HAZ grain size was examined. Furthermore, HV hardness of HAZ was measured and Hmax (maximum hardness of HAZ) was evaluated.

  The presence or absence of weld cracks was determined by penetration inspection.

  The HAZ crystal grain size evaluation method is as follows. First, a test material subjected to lap joint welding is longitudinally cut at a center position in the width direction to divide it into 12.5 mm, and further, the weld metal is longitudinal in the longitudinal section. It cut | disconnected to 30 mm so that it might become a center part, embedded in resin, and mirror-polished. Next, the weld metal was corroded with a saturated aqueous solution of picric acid to which a surfactant was added. As shown in FIG. The austenite crystal grain size at a location 0.3 mm away from the steel was measured. For the prior austenite crystal grains, the grain size number was set to 3.0 or more.

  The evaluation method of the HV hardness of the HAZ is that the resin-embedded sample used for the crystal grain size measurement is polished again, and as shown in FIG. The HV hardness at a position of 1.5 mm and the HV hardness at a position of 1.0 mm from the upper surface of the test piece were measured at a pitch of 0.5 mm from end to end of a 30 mm cut product. The target of Hmax was set to 400 or less in HV hardness.

  Table 3 summarizes the results of the above investigations.

From Table 3, using the steel 1-7 whose chemical composition is in the range prescribed by the present invention, and in the area of 100 μm ² , two kinds of Ti precipitates having equivalent circle diameters defined by the present invention are respectively obtained. In the case of Test Nos. 1 to 7 where 10 or more have been deposited, since Hmax is less than 400 in terms of HV hardness, no weld cracking occurs, and the strength and toughness of the base material are excellent. it is obvious. Furthermore, in the case of the above test numbers, the HAZ former austenite grain size number is as large as 3.6 to 4.1 (in other words, the former austenite crystal grains are small), and therefore the HAZ toughness is also excellent. It is estimated that

  On the other hand, in the case of test numbers 8 to 14 of “Comparative Examples” that do not satisfy all the conditions specified in the present invention at the same time, at least the weld crack, the strength of the base material, the toughness of the base material, the HAZ There is a problem with either toughness.

  That is, in the case of test number 8, fn1 of the used steel 8 is “−0.015”, which is outside the conditions defined in the present invention. For this reason, the precipitation density of the Ti precipitate having an equivalent circle diameter of 0.01 to 0.05 μm deviates from the conditions defined in the present invention, and the toughness of the base material is inferior.

  In the case of test number 9, the Ceq of the steel 9 used is as large as 0.59, which is outside the conditions defined in the present invention. For this reason, Hmax is HV hardness of 419, which greatly exceeds 400. Therefore, a weld crack occurred.

  In the case of test number 10, DI of steel 10 used is as large as 172, which is out of the conditions defined in the present invention. For this reason, the toughness of the base material is inferior.

  In the case of the test number 11, the DI of the steel 11 used is as small as 66, which is outside the conditions defined in the present invention. For this reason, the tensile strength of the base material is low.

  In the case of Test No. 12 and Test No. 13, the chemical compositions of Steel 12 and Steel 13 used are both within the range defined by the present invention, but precipitation of Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm. The density deviates from the conditions specified in the present invention. For this reason, the prior austenite grain size number of HAZ is as small as 2.3 and 2.2 (in other words, the prior austenite crystal grains are large), and it is estimated that the HAZ is inferior in toughness.

  Also in the case of test number 14, the chemical composition of the steel 12 used is within the range defined by the present invention, but the precipitation density of Ti precipitates having an equivalent circle diameter of 0.01 to 0.05 μm is defined by the present invention. Out of condition. For this reason, the toughness of the base material is inferior.

  The rolled steel bar or wire rod for hot forging of the present invention can be welded, and is excellent in the strength and toughness of the base material and also in the toughness of HAZ. It can be used suitably.

Claims (2)

% By mass
C: 0.10 to 0.20%,
Si: 0.01-0.30%,
Mn: 1.00-2.30%,
S: 0.040% or less,
Cr: 0.10 to 0.80%,
Al: 0.010-0.080%,
B: 0.0002 to 0.0050%,
Ti: 0.010 to 0.080%, and N: 0.0020 to 0.0080%
The balance consists of Fe and impurities,
P, Cu, Ni, Mo and V in the impurity are
P: 0.040% or less,
Cu: less than 0.10%,
Ni: less than 0.10%,
Mo: limited to less than 0.05%, and V: 0.01% or less,
Furthermore, fn1 represented by the following formula (1) is 0.001 or more, Ceq represented by the formula (2) is 0.57 or less, and DI represented by the formula (3) is 70 to 170. Having a composition,
Furthermore, in an area of 100 μm ² , 10 or more Ti precipitates having an equivalent circle diameter of 0.07 to 1.0 μm and 10 or more Ti deposits of 0.01 to 0.05 μm were deposited. A rolled steel bar or wire rod for hot forging characterized by
fn1 = Ti-3.4N (1)
Ceq = C + (Si / 24) + (Mn / 6) + (Ni / 40) + (Cr / 5) + (Mo / 4) + (V / 14) (2)
^{DI = 25.4 × 0.311 × C 0.5} × (1 + 0.64 × Si) × (1 + 4.10 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2. 33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) × {1 + 1.50 × (0.90−C)} (3)
Ti, N, C, Si, Mn, Ni, Cr, Mo, V, P, S, and Cu in the above formulas represent the content of each element in mass%.
Said Ti deposit refers to TiN, TiC, and Ti (C, N). Continuously casting the molten steel having the chemical composition according to claim 1 under a condition where the cooling rate is 2.0 ° C./min or more,
The slab is heated to a temperature range of 1100 ° C. or higher and held for 30 minutes or more, and is subjected to split rolling, bar rolling, or wire rod rolling under the conditions satisfying the following formula (4), and further, bar rolling or wire rod A method for producing a rolled steel bar or wire rod for hot forging characterized by cooling a temperature range of 800 to 600 ° C after rolling at a cooling rate of 5 to 100 ° C / min.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 7.5 × 10 ³ (4)
In the above formula (4), T is the heating temperature in units of “° C.”, t is the holding time in units of “s” at the heating temperature T, subscript 1 is the block rolling process, and subscript 2 is the steel bar rolling Or it represents a wire rod rolling process and means T (2) = (T ₂ +273) / (T ₁ +273).

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