JP5413350B2 - Rolled steel for hot forging and method for producing the same - Google Patents

Description

  The present invention relates to a rolled steel material for hot forging and a manufacturing method thereof. Specifically, the present invention relates to a rolled steel material for hot forging that can be suitably used as a material for automobile parts, construction machine parts, and industrial machine parts, and a method for producing the same. More specifically, it is excellent in tensile strength and toughness, has a high yield ratio, and is inexpensive, for example, a rolled steel material for hot forging that can be suitably used as a material for automotive parts such as connecting rods and hubs, and the like. It relates to a manufacturing method.

  For example, automotive parts such as connecting rods and hubs are required to have high tensile strength, toughness, and buckling resistance.

  In order to increase the buckling resistance among the above characteristics, it is necessary to increase the yield strength. However, if the tensile strength increases simultaneously with the yield strength, the machinability is sacrificed. Therefore, it is also necessary to increase the yield strength without excessively increasing the tensile strength, that is, to increase the yield ratio represented by “yield strength / tensile strength”.

  Stable carbon steel materials such as S48C as defined in JIS G 4051 (2009) are formed by hot forging into a rough shape of the parts, and this is subjected to a so-called tempering treatment of “quenching-tempering” to stabilize Parts with high tensile strength, yield ratio and toughness can be secured. For this reason, the conventional hot forged parts have been manufactured by tempering a carbon steel material for mechanical structure such as S48C.

However, reflecting the recent severe economic situation and the environmental situation of CO ₂ reduction, the movement of reducing the manufacturing cost of various parts and reducing the weight of parts has been activated. Parts are no longer an exception.

  For this reason, non-tempered steel that can obtain the same tensile strength, yield ratio, and toughness as when tempering carbon steel for mechanical structure without performing tempering of “quenching-tempering”, which increases the manufacturing cost. The demand for is growing and it is beginning to be adopted.

  On the other hand, in non-heat treated steel, it is common to use V as an alloying element. However, due to the recent rise in alloying elements such as V, there is an increasing need for material cost reduction in non-heat treated steel. Yes.

  Therefore, for example, Patent Document 1 can provide non-heat treated steel for hot working that is excellent in fatigue resistance and suitable for materials such as machine structural parts at low cost without using expensive alloy elements. Non-heat treated steel for hot working excellent in fatigue resistance, specifically, in mass%, C: 0.2 to 0.6%, Si: 0.01 to 0.1%, Mn: 0.00. 7-2.0%, S: 0.01-0.1%, Cr: 0.1-2.0%, Nb: 0.005-0.05%, Al: 0.002-0.02% And N: 0.01 to 0.05%, the balance being made of Fe and inevitable impurities, and having a ferrite and pearlite structure, “non-heat treated steel for hot working with excellent fatigue resistance” is disclosed. Yes.

Patent Document 2 discloses a high-toughness, high-strength non-refined steel having mechanical properties equal to or higher than that of tempered steel in a state where tempering is not performed, and a method for producing the same, specifically, by mass%. , C: 0.35 to 0.45%, Si: 0.15 to 0.35%, Mn: 0.80 to 1.50%, S: 0.005 to 0.050%, Cr: 0.30 %: Al: 0.01 to 0.05%, V + Nb: 0.05 to 0.15%, Ti: 0.03% or less, N: 0.006 to 0.020%, P: 0.005 as impurities. 03% or less, O ₂ : 0.0050% or less, and “high toughness and high strength non-heat treated steel” containing Fe and impurities inevitably contained in the steel making process, and in mass%, C: 0.40 ˜0.50%, Si: 0.25 to 0.65%, Mn: 1.00 to 1.60%, S: 0.005 to 0.050%, r: 0.30% or less, Al: 0.01 to 0.05%, V + Nb: 0.05 to 0.20%, Ti: 0.03% or less, N: 0.006 to 0.020%, and further if necessary, B: 0.0030% or less, P as an impurity: 0.03% or less, O _2: 0.0050% or less, and "high toughness comprising impurities contained on necessarily Fe and steelmaking High-strength non-tempered steel "and methods for their production are disclosed.

JP-A-8-67944 Japanese Patent Laid-Open No. 7-90485

  At present, there is a demand for an advanced technique for improving tensile strength, toughness, and yield ratio after reducing the amount of V contained in non-tempered steel.

  In addition, the heating temperature at the time of hot forging for forming a rough part shape often exceeds 1200 ° C. Therefore, even in the above case, it is necessary to suppress the coarsening of the austenite grains and to prevent the yield ratio and toughness of the parts from decreasing.

  The non-heat treated steel disclosed in Patent Document 1 described above is a steel that can ensure excellent fatigue resistance without using V. However, this non-tempered steel does not contain Ti, and therefore it cannot be said that a sufficiently high yield ratio is obtained.

  The non-heat treated steel described in Patent Document 2 cannot be said to have a sufficiently high yield ratio and excellent toughness when hot forging is performed by heating to a high temperature exceeding 1200 ° C. . In addition, the non-tempered steel must contain a large amount of V or B, and an expensive alloy element is used, or cracking during casting is suppressed, so that the material cost can be reduced. could not.

  Thus, conventionally, there has been no technique that can achieve both a reduction in the V content and an improvement in tensile strength, toughness, and yield ratio.

  The present invention has been made in view of the above situation, and the V content can be reduced, and even when the heating temperature during hot forging exceeds 1200 ° C., the tensile strength, toughness, and yield ratio. It is an object to provide an inexpensive rolled steel for hot forging that can be used as a material for automobile parts, construction machine parts, and industrial machine parts, and a method for producing the same.

  The present inventors first conducted various studies on securing tensile strength without containing a large amount of V.

  As a result, in order to ensure the tensile strength without containing a large amount of V, it is effective to contain appropriate amounts of C, Mn, and Cr. It has been found that there is a possibility that the machinability may be deteriorated, and therefore it is necessary to secure a certain degree of tensile strength and further increase the yield ratio.

  And in order to achieve the above-mentioned content, it combines with C and N, and precipitates as a carbonitride in ferrite, thereby increasing the yield strength, and further increasing the yield strength without excessively increasing the tensile strength. It was concluded that the action, that is, the inclusion of Nb having the effect of increasing the yield ratio is effective.

  The present inventors have further studied various ways to further increase the strength and toughness.

  As a result, in order to further improve the yield strength and toughness, it has been found that the pinning effect of TiN during hot forging may be used.

  However, if Ti is simply contained, N is combined with Ti to form TiN, so the amount of N combined with Nb is reduced, the Nb carbonitride is reduced, and the yield strength of the steel is reduced. End up.

  Therefore, as a result of further studies by the present inventors, the following knowledge has been obtained.

  If the content of Ti is adjusted to an appropriate range and N is also actively contained, precipitation strengthening by Nb carbonitride is not hindered, and TiN is finely dispersed, and after hot forging. The crystal grains can be further refined, and as a result, the tensile strength, toughness and yield ratio of the steel are improved.

  The present invention has been completed based on the above findings, and the gist of the present invention is the rolled steel for hot forging shown in (1) and (2) below, and the rolled steel for hot forging shown in (3). It is in the manufacturing method.

(1) In mass%,
C: 0.20 to 0.45%,
Si: 0.90% or less,
Mn: more than 1.00% and 1.90% or less,
P: 0.050% or less,
S: 0.010 to 0.10%,
Cr: 0.10 to 0.60%,
Ti: 0.0016 to 0.0080%,
Nb: 0.005 to 0.050%,
N: 0.0050 to 0.0250%,
Al: 0.010-0.080%, and O: 0.0050% or less, and fn represented by the following formula (1) is in the range of 0.50-1.00, The balance has a chemical composition composed of Fe and impurities, and more than 10 TiNs having a length of 0.005 to 0.100 μm are deposited in an area of 100 μm ² . Rolled steel.
fn = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V + Nb− (5/7) S (1)
C, Si, Mn, Cr, V, Nb and S in the above formula (1) mean the content of the element in mass%.

  The “impurities” in the remaining “Fe and impurities” refer to those mixed from ore as a raw material, scrap, or the manufacturing environment when the steel material is industrially produced.

  TiN having a length of 0.005 to 0.100 μm means TiN having a long side length of 0.005 to 0.100 μm as a precipitate.

  (2) The rolled steel for hot forging as described in (1) above, which contains V: less than 0.05% by mass% instead of part of Fe.

(3) A slab having the chemical composition described in the above (1) or (2) is heated to a steel bar under the conditions satisfying the following formula (2) while heating at a temperature range of 1100 ° C. or more for 30 minutes or more. A method for producing rolled steel for hot forging characterized by the following.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 6.7 × 10 ³ (2)
In the formula (2), T is the heating temperature (° C.), t is the heating holding time (s), the subscript 1 is the block rolling process, the subscript 2 is the bar rolling process, and T (2) = (T ₂ +273 ) / (T ₁ +273).

That is, T ₁ is the heating temperature (° C.) of the block rolling process, T ₂ is the heating temperature (° C.) of the bar rolling process, t ₁ is the heating and holding time (s) of the block rolling process, and t ₂ is the bar rolling process. Is the heating and holding time (s).

  Moreover, heating temperature means the average value of the furnace temperature of a heating furnace.

  The rolled steel material for hot forging of the present invention is inexpensive because it can reduce the V content, has excellent tensile strength and toughness, and has a high yield ratio. Therefore, it is used for automobile parts, construction machine parts, and industrial machinery. It is suitable for use as a material for parts, for example, a material for automobile parts such as connecting rods and hubs.

  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 material C: 0.20 to 0.45%
C has an effect of improving the tensile strength. In order to acquire the effect, it is necessary to contain 0.20% or more of C. On the other hand, when the content of C exceeds 0.45%, the ratio of pearlite in the ferrite / pearlite structure increases, so that a high yield ratio cannot be obtained and the toughness is also lowered. Furthermore, the tensile strength becomes too high for the yield strength, resulting in a decrease in hot workability and machinability. Therefore, the content of C is set to 0.20 to 0.45%. The content of C is preferably 0.25% or more, and preferably 0.43% or less.

Si: 0.90 or less%
Si is an impurity that enters molten steel during the refining process. On the other hand, Si is an element effective for deoxidation of steel, and when added, it has the effect of increasing the yield strength by solid-solution strengthening of ferrite and consequently improving the yield ratio. However, if the Si content increases as a result of addition and exceeds 0.90%, not only the above effects are saturated, but also hot workability is reduced. Therefore, the Si content is set to 0.90% or less. The Si content is preferably 0.70% or less. On the other hand, in order to obtain the deoxidation effect and the yield ratio improvement effect, the Si content is preferably 0.05% or more.

Mn: More than 1.00% and 1.90% or less Mn has a deoxidizing action and contributes to solid solution strengthening of ferrite to increase the yield strength, and as a result, to improve the yield ratio. In order to obtain these effects, Mn must be contained in an amount exceeding 1.00%. However, if the Mn content exceeds 1.90%, the hot workability decreases, and the hardenability becomes too high, resulting in a bainite structure, resulting in a decrease in yield ratio and toughness. Therefore, the Mn content is more than 1.00% and 1.90% or less. The Mn content is preferably 1.70% or less.

P: 0.050% or less P is an element contained as an impurity in steel, and its content increases, and particularly when it exceeds 0.050%, the hot workability and toughness are greatly reduced. Therefore, the content of P is set to 0.050% or less. The P content is preferably 0.040% or less.

S: 0.010 to 0.10%
S combines with Mn to form MnS and has an effect of improving machinability. In order to acquire the effect, it is necessary to contain 0.010% or more of S. On the other hand, when the content of S exceeds 0.10%, hot workability is lowered and toughness is also lowered. Therefore, the content of S is set to 0.010 to 0.10%. The S content is preferably 0.08% or less.

Cr: 0.10 to 0.60%
Cr contributes to strengthening of cementite in pearlite and has an effect of increasing yield strength. In order to obtain this effect, the Cr content needs to be 0.10% or more. However, when the Cr content exceeds 0.60%, a bainite structure is formed, and the yield ratio and toughness are lowered. Therefore, the content of Cr is set to 0.10 to 0.60%. The content of Cr is preferably 0.15% or more, and preferably 0.45% or less.

Ti: 0.0016 to 0.0080%
Ti is an important element in the present invention. That is, TiN formed by combining Ti with free N acts as pinning particles, and austenite grains become coarse even when heated to a high temperature range exceeding 1200 ° C. during hot forging. Is effective in increasing the yield ratio and toughness. In order to obtain the function and effect as the pinning particles of TiN, Ti must be contained in an amount of 0.0016% or more. On the other hand, when the Ti content increases, TiN coarsens, and when it exceeds 0.0080%, the amount of N bonded to Nb decreases, and the Nb carbonitride decreases, and instead yield. The ratio will be lowered. Therefore, the content of Ti is set to 0.0016 to 0.0080%. The Ti content is preferably 0.0018% or more, and preferably 0.0060% or less.

Nb: 0.005 to 0.050
Nb is an important element in the present invention. That is, Nb combines with C and N and precipitates as a carbonitride in ferrite, and has the effect of improving toughness and yield ratio. In order to obtain such an effect, the Nb content needs to be 0.005% or more. However, even if Nb is contained in excess of 0.050%, it cannot be completely dissolved in the matrix at the heating temperature at the time of hot forging. The cost of things that don't exist will increase. Therefore, the Nb content is set to 0.005 to 0.050%. The Nb content is preferably 0.010% or more, and preferably 0.045% or less.

N: 0.0050 to 0.0250%
N is also an important element in the present invention. That is, N is an important element that forms a carbonitride with Nb together with C to increase the yield ratio. Furthermore, TiN formed by combining N with Ti acts as pinning particles even when heated to a high temperature range exceeding 1200 ° C. during hot forging, and refines austenite grains. Has the effect of improving yield strength and toughness. In order to obtain these effects, an N content of 0.0050% or more is required. However, the above effect is saturated even if N is contained in excess of 0.0250%. Therefore, the N content is set to 0.0050 to 0.0250%. The N content is preferably 0.0150% 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%. The Al content is preferably 0.015% or more, and preferably 0.070% or less.

O: 0.0050% or less O (oxygen) is an element that exists as an oxide inclusion in steel and impairs fatigue strength. In particular, when the O content exceeds 0.0050%, the fatigue strength decreases greatly. Therefore, the O content is set to 0.0050% or less. The O content is preferably 0.0030% or less.

fn: 0.50 to 1.00
The rolled steel for hot forging of the present invention is
fn = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V + Nb− (5/7) S (1)
Fn represented by the formula (1) must be in the range of 0.50 to 1.00. However, C, Si, Mn, Cr, V, Nb and S in the formula (1) mean the content of the element in mass%.

  The above fn is a parameter serving as an index of tensile strength. If fn is 0.50 or more, the tensile strength required for the component can be stably obtained. On the other hand, if fn exceeds 1.00, the tensile strength becomes too high and the machinability is lowered. fn is preferably 0.65 or more, and preferably 0.90 or less.

  One of the rolled steel materials for hot forging of the present invention has a chemical composition in which the balance is composed of Fe and impurities in addition to the above elements. As already described, “impurities” in “Fe and impurities” refers to those mixed from ore, scrap, or production environment as raw materials when industrially producing steel materials.

  Another one of the rolled steel materials for hot forging of the present invention contains, as its chemical composition, V in the following amount in place of a part of the Fe.

V: Less than 0.05% The rolled steel for hot forging of the present invention achieves high yield strength and yield ratio, and excellent toughness with an inexpensive chemical composition that limits the content of expensive V as much as possible. It is aimed. Therefore, V may not be intentionally contained.

  However, ferrite precipitation strengthening and grain refinement by V carbonitride are effective in improving the yield ratio. For this reason, V may be intentionally contained. However, when the amount is 0.05% or more, the cost increase becomes significant, and deviates from the original purpose of the present invention of providing inexpensive steel materials. End up. Therefore, even when V is contained, the amount is made less than 0.05%.

(B) TiN size and precipitation density Even when heated to a high temperature range exceeding 1200 ° C. during hot forging, TiN acts as pinning particles to suppress austenite grain growth, yield ratio and toughness In order to ensure the effect of increasing the thickness, 10 or more TiN having a length of 0.005 to 0.100 μm must be deposited in an area of 100 μm ² .

First, when the length of TiN is less than 0.005 μm, if heated to a high temperature range exceeding 1200 ° C. during hot forging, it does not act as pinning particles because it dissolves in the matrix. On the other hand, if the length of TiN exceeds 0.100 μm, the pinning effect cannot be obtained. Even if the length of TiN is 0.005 to 0.100, coarsening of austenite grains is inevitable unless ten or more precipitates are deposited in an area of 100 μm ² . Therefore, the yield ratio and toughness of the parts after hot forging are reduced. The effect is saturated even when more than 100 TiNs having a length of 0.005 to 0.100 μm are deposited in an area of 100 μm ² .

(C) Manufacturing method of rolled steel for hot forging The rolled steel for hot forging according to the present invention shown in the above (1) and (2) is, for example, the rolled steel for hot forging shown in (3) above. And, specifically, a slab having the chemical composition described in the above section (A) is heated in a temperature range of 1100 ° C. or higher for 30 minutes or more, and
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 6.7 × 10 ³ (2)
It can manufacture by rolling to a bar steel etc. on the conditions which satisfy | fill Formula (2) of this.

In the formula (2), T represents the heating temperature (° C.), t represents the heating and holding time (s), the subscript 1 represents the split rolling process, the subscript 2 represents the bar rolling process, and T (2) = (T ₂ +273) / (T ₁ +273).

  In order to reduce the segregation of the slab, it is generally desirable to increase the heating temperature when rolling the slab and to increase the heating and holding time.

  However, when the heating and holding time is increased, TiN becomes agglomerated and coarsened, the TiN precipitation density is lowered, and the effect of suppressing coarsening during hot forging heating is lost. Therefore, the yield ratio and toughness of the parts after hot forging are reduced.

  Therefore, when rolling the slab, the lower limit of the heating temperature is set to 1100 ° C. in order to reduce the load on the rolling mill, and the heating holding time is set to 30 min or more in order to equalize the temperature to the center. If the rolling is performed under the condition satisfying the formula (2), TiN aggregation and coarsening can be suppressed, and thereby austenite grain coarsening during hot forging heating can be easily suppressed. As a result, a hot forged part having a desired yield ratio and toughness can be obtained.

  Hereinafter, the expression (2) 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 is manufactured by a two-stage rolling process including a block rolling and a steel bar rolling. Therefore, when the heating temperatures in the block rolling process and the bar rolling process are T ₁ (° C.) and T ₂ (° C.), respectively, and the heating holding times are t ₁ (s) and t ₂ (s), respectively. The tempering parameters in the rolling process 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),
And 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 by the heating temperature T ₁ (° C.) and the heating holding time x ( s), the degree Y of Ostwald growth of TiN for two rolling processes of the batch rolling process and the steel bar rolling process can be expressed as a formula (c) as a parameter for one batch rolling process. Furthermore, the expression (d) can be obtained by substituting the expression (b) into the expression (c).
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 of the formula (d) thus determined, if the value of Y is 6.7 × 10 ³ or less, (1) and The rolled steel for hot forging related to the present invention shown in (2) can be obtained.

From the above, equation (2) was defined as a parameter representing the degree of Ostwald growth of TiN.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 6.7 × 10 ³ (2).

  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. Moreover, even if the value of Y is within a specified range, the heating and holding time 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 set to 14400 s (240 min) or less. Is more preferable.

  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 which case the value of Y ′ is 6.7 × 10 ^3. If it is below, the rolled steel material for hot forging related to the present invention shown in the above (1) and (2) 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.

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

  Steels 1 to 14 having the chemical composition shown in Table 1 were melted by a 70-ton converter and made into a slab by continuous casting, and then rolled into 180 mm × 180 mm steel pieces under the conditions shown in Table 2, and thereafter Further, the steel bar was rolled to produce a steel bar having a diameter of 55 mm.

  Steels 1 to 6, Steel 13 and Steel 14 in Table 1 are steels of the present invention examples having chemical compositions within the range defined by the present invention, while Steels 7 to 12 have chemical compositions of the present invention. It is a steel of a comparative example that deviates from the conditions specified in.

  Using the steel bar having a diameter of 55 mm produced as described above, the precipitation density of TiN was investigated, and further, a test simulating hot forging was conducted to investigate the mechanical properties.

That is, a sample was taken from the longitudinal section of R / 2 part (“R” represents the radius of the steel bar) of the steel bar having a diameter of 55 mm by the extraction replica method, and observed with a transmission electron microscope to determine the precipitation density of TiN. investigated. Specifically, 20 fields of view were observed at a magnification of 30000, the number of TiN having a length of 0.005 to 0.100 μm was counted, and this was converted to the number per 100 μm ² area.

  Further, a test piece having a diameter of 38.5 mm and a length of 55 mm was collected from the central portion of the steel bar having a diameter of 55 mm, and first subjected to a hot extrusion test. That is, the above test piece was heated to 1220 ° C. for 10 minutes, and then extruded at 1100 ° C. with a reduction in area of 60% to produce a hot extruded product having a diameter of 23.1 mm and a length of 85 mm. After hot extrusion, it was allowed to cool in the atmosphere.

  Next, a 14A tensile test piece having a parallel part diameter of 5 mm described in JIS Z 2201 (1998) was cut out from R / 2 part of the above hot-extruded product, and a tensile test was performed at room temperature by a usual method. . The tensile strength and the 0.2% yield strength were measured, and the yield ratio was calculated using the 0.2% yield strength as the yield strength. The targets for tensile strength and yield ratio were 500 to 1000 MPa and 0.60 or more, respectively.

Further, a U-notch test piece having a width of 10 mm and a notch depth of 2 mm was cut out from the R / 2 part of the above hot-extruded product, and subjected to a Charpy impact test at room temperature by an ordinary method, and an impact value was measured. The target of toughness was that the Charpy impact value at room temperature using the U-notch test piece having a width of 10 mm and a notch depth of 2 mm was 40 J / cm ² or more.

  Table 3 summarizes the above test results.

  From Table 3, it is clear that all of the test numbers of the inventive examples having the chemical composition and microstructure defined in the present invention, that is, test numbers 1 to 6, have the target tensile strength, yield ratio, and toughness. It is.

  On the other hand, at least one of the chemical composition and the microstructure is out of the conditions specified in the present invention, that is, the test numbers 7 to 14 are those of tensile strength, yield ratio and toughness. At least one has not reached the goal.

  The rolled steel material for hot forging of the present invention is inexpensive because it can reduce the V content, has excellent tensile strength and toughness, and has a high yield ratio. Therefore, it is used for automobile parts, construction machine parts, and industrial machinery. It is suitable for use as a material for parts, for example, a material for automobile parts such as connecting rods and hubs. This rolled steel material for hot forging can be obtained by the method of the present invention, for example.

Claims (3)

% By mass
C: 0.20 to 0.45%,
Si: 0.90% or less,
Mn: more than 1.00% and 1.90% or less,
P: 0.050% or less,
S: 0.010 to 0.10%,
Cr: 0.10 to 0.60%,
Ti: 0.0016 to 0.0080%,
Nb: 0.005 to 0.050%,
N: 0.0050 to 0.0250%,
Al: 0.010-0.080%, and O: 0.0050% or less, and fn represented by the following formula (1) is in the range of 0.50-1.00, The balance has a chemical composition composed of Fe and impurities, and more than 10 TiNs having a length of 0.005 to 0.100 μm are deposited in an area of 100 μm ² . Rolled steel.
fn = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V + Nb− (5/7) S (1)
C, Si, Mn, Cr, V, Nb and S in the above formula (1) mean the content of the element in mass%.   The rolled steel material for hot forging according to claim 1, wherein, instead of a part of Fe, the material contains V: less than 0.05% by mass%. A hot slab characterized by heating a slab having the chemical composition according to claim 1 or 2 in a temperature range of 1100 ° C. or higher for 30 minutes or more and rolling it into a steel bar under a condition satisfying the following formula (2): A method for producing rolled steel for forging.
Y = (T ₁ +273) × log (t ₁ + t ₂ ^{T (2)} ) ≦ 6.7 × 10 ³ (2)
In the formula (2), T is the heating temperature (° C.), t is the heating holding time (s), the subscript 1 is the block rolling process, the subscript 2 is the bar rolling process, and T (2) = (T ₂ +273 ) / (T ₁ +273).