JP2008240130A - Non-heat treated steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-heat treated steel material securing a 0.2% proof stress of ≥900 MPa as hot-forged. <P>SOLUTION: The non-heat treated steel material has a composition comprising >0.3 to 0.55% C, 0.01 to 1.0% Si, 0.6 to 2.0% Mn, ≤0.15% P, 0.005 to 0.20% S, 0.02 to 0.4% Cr, 0.35 to 1.0% V, ≤0.05% Al, <0.008% N and ≤0.0035% O, and one or more kinds selected from >0.05 to 0.4% Ti, 0.02 to 0.4% Nb and 0.02 to 0.4% Zr, and the balance Fe with impurities, and satisfying 0<(0.293Ti+0.151Nb+0.154Zr)-N≤0.l2. One or more kinds selected from ≤0.05% Ca, ≤0.4% Pb, ≤0.3% Bi, ≤0.1% Te and ≤0.5% Se may be incorporated therein. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a non-heat treated steel material, and more particularly, to a non-heat treated steel material having good yield characteristics and suitable as a material for a connecting rod such as an automobile engine or a knuckle that is an undercarriage part of an automobile.

  A connecting rod (hereinafter referred to as a “connecting rod”) such as an automobile engine is an engine component that couples a piston and a crankshaft, and plays a role of transmitting explosive force to a drive shaft. For this reason, a high yield stress (hereinafter referred to as “0.2% yield strength”) is required for the connecting rod. In particular, with the recent increase in engine output, the yield stress required for connecting rods is increasing.

  In addition, there is a similar trend of increasing the strength of knuckle, which is an undercarriage part of an automobile, and the required 0.2% proof stress is increasing.

  Of the “carbon steels for machine structural use” defined in JIS G 4051 (2005), so-called “medium carbon steels” such as S48C are subjected to a so-called “tempering treatment” of “quenching-tempering”. The 0.2% yield strength of 600 MPa or more can be secured stably.

  For this reason, conventional connecting rods and knuckles have been manufactured by tempering a carbon steel material for machine structure of medium carbon such as S48C.

  However, reflecting recent severe economic conditions and competition in the automobile industry, movements to reduce manufacturing costs and improve performance of various automobile parts have become active, and this movement is the knuckle of connecting rods and undercarriage parts that are engine parts. It is no longer an exception.

  For this reason, 0.2% which is equivalent to the case of tempering the carbon steel material for mechanical structure of medium carbon without performing the tempering treatment of “quenching-tempering”, which increases the manufacturing cost, that is, non-tempering. The demand for securing proof strength has increased, and it has begun to be adopted in some models.

  In addition, automobile parts have been made lighter for higher performance, and therefore, it is necessary to ensure sufficient 0.2% proof stress in a light state. Conventional carbon steel for machine structures such as S48C. There is an increasing demand for a steel having a 0.2% proof stress higher than that of a “quenched-tempered” steel material. For example, a part having a high strength of 900 MPa or more with a 0.2% proof stress may be specifically required.

  Therefore, especially for automobile parts, a large 0.2% proof stress of 900 MPa or more is ensured with hot forging without applying special treatments such as tempering and tempering for quenching and tempering. There is a growing demand for non-heat treated steel.

  For this reason, for example, Patent Documents 1 to 3 disclose a non-tempered steel material and a manufacturing method thereof for controlling the chemical composition and manufacturing method of steel to obtain a high 0.2% proof stress. Patent Document 4 discloses a non-heat treated steel excellent in wear resistance, and Patent Document 5 discloses a non-heat treated steel excellent in anisotropy of impact characteristics.

Specifically, in Patent Document 1, as a weight ratio, C: 0.15 to 0.50%, Si: 0.005 to 2.00%, Mn: 0.40 to 2.00%, S: 0.01-0.10%, Al: 0.0005-0.05%, Ti: 0.003-0.05%, N: 0.0020-0.0200%, V: 0.20-0. 70% is contained, and if necessary, (a) Cr: 0.02 to 1.50%, Mo: one or two of 0.02 to 1.00%, (b) Nb: One selected from the group of 0.001 to 0.20%, (c) Pb: 0.05 to 0.30%, Ca: 0.0005 to 0.010%, or containing two or more elements, the balance a steel material having a composition consisting of Fe and impurity elements, subjected to hot forging is heated to a temperature of more than ₃ points Ac, allowed to cool “A method for producing a non-tempered steel with excellent fatigue characteristics” in which 90% or more of the metal structure after completion of the state is a ferrite + pearlite structure and further subjected to aging treatment at a temperature of 200 to 700 ° C. It is disclosed.

In Patent Document 2, in terms of weight ratio, C: 0.15-0.50%, Si: 0.005-2.00%, Mn: 0.40-2.00%, S: 0.01-0 .10%, Al: 0.0005 to 0.050%, Ti: 0.003 to 0.050%, N: 0.0020 to 0.0200%, V: 0.20 to 0.70% If necessary, (a) one or two of Cr: 0.02 to 1.50%, Mo: 0.02 to 1.00%, (b) Nb: 0.001 to 0 20%, (c) one or more elements selected from the group consisting of Pb: 0.05 to 0.30% and Ca: 0.0005 to 0.010% containing the balance being a steel having a composition consisting of Fe and impurity elements, is heated to a temperature of more than ₃ points Ac, forging finishing temperature of 750 to 900 ° C. The sub-hot forging is performed, and after cooling and the transformation is completed, 90% or more of the metal structure is a ferrite + pearlite structure, and an aging treatment is further performed at a temperature of 200 to 700 ° C. Further, a method for producing a sub-hot forged non-heat treated steel material having excellent toughness and fatigue properties is disclosed.

  In Patent Document 3, in mass%, C: 0.15-0.40%, Si: 0.4-1.5%, Mn: 0.5-2.0%, P: 0.10-0. 15%, S: 0.01 to 0.15%, V: 0.15 to 0.40%, Al: 0.001 to 0.1%, if necessary, (a) Cr: 0.05-0.2%, (b) N: 0.002-0.03%, (c) Ti: 0.05-0.30%, Nb: 0.01-0.10% A material steel containing one or more elements selected from the group consisting of one or two kinds, the balance being composed of Fe and inevitable impurities, is heated to 1000 ° C. or more and subjected to hot forging, “Method for manufacturing non-tempered steel hot-forged member” has been developed, which is cooled to room temperature to make the microstructure of ferrite and pearlite and cold work with a workability of 2 to 10%. It is.

Patent Document 4 contains, in mass%, C: 0.3 to 0.8%, Mn: 0.3 to 2.0% and Si: 0.5 to 2.5%, F = [Si] F value specified by + [Mn] /3.5 is 1.0 or more, or, if necessary, (a) V: 0.4% or less, Nb: 0.15% or less, Ti: 0 One or more of 15% or less, (b) Cr: 1.5% or less, (c) Al: 0.04% or less, (d) S: 0.12% or less, Pb: 0.3% Hereinafter, one or two selected from the group consisting of Zr: 0.2% or less, Ca: 0.01% or less, Te: 0.1% or less, Bi: 0.1% or less Containing the above elements, F ′ value defined by F ′ = [Si] + [Mn] /3.5+3 [V] +2.5 [Nb] +2.5 [Ti] is 1.0 or more, And in any longitudinal section Oxide inclusions above average particle size 20μm of per area 300 mm ² is 10 or less "wear resistance excellent hot forging microalloyed steels" is disclosed.

  In Patent Document 5, in mass%, C: 0.01-0.70%, Si: 0.05-1.80%, Mn: 0.20-3.50%, S: 0.03-0. 20%, Al: 0.003-0.10%, N: 0.003-0.025%, if necessary, (a) O: 0.01% or less, Cr: 1.50 %, Mo: 1.00% or less, Ni: 1.50% or less, B: 0.015% or less, (b) V: 0.50% or less, Nb: 0 10% or less, Ti: 0.5% or less of 1 type or 2 types, (c) Bi: 0.30% or less, Pb: 1 type or 2 types of 0.30% or less, An element that contains one or more elements selected from the group, and more easily forms a sulfide than Mn as a chemical component that controls the sulfide form; and M And the balance is Fe and inevitable impurities, and the impact value perpendicular to the rolling or forging direction of steel and the impact value parallel to the rolling or forging direction are the stretch ratio of sulfide inclusions. “Non-tempered steel excellent in impact properties in a direction perpendicular to the rolling or forging direction” satisfying a specific relational expression with (aspect ratio) is disclosed.

JP-A-7-102340 JP-A-7-157824 JP 2004-137542 A JP 2000-328193 A JP 2002-180194 A

The 0.2% yield strength (yield strength) of the non-heat treated steel disclosed in Patent Document 1 is only as low as 834 MPa (85.0 kgf / mm ² ) at most.

Similarly, the 0.2% yield strength (yield strength) of the non-heat treated steel disclosed in Patent Document 2 is only as low as 829 MPa (84.5 kgf / mm ² ).

  On the other hand, in the case of the non-tempered steel material disclosed in Patent Document 3, there is one that has achieved a large 0.2% proof stress of 955.8 MPa exceeding 900 MPa, which is the paragraph [0043] of Patent Document 3 described above. As described in, the initial yield stress (0.2% proof stress) increases with an increase in the amount of plastic deformation during cold working, and is due to cold working with a working degree of 5%. It is. However, when cold working is performed, the parts manufacturing process becomes complicated and the manufacturing cost becomes excessive.

  One of the main points of the technology disclosed in Patent Document 4 is that, as described in the paragraph [0011], the additive element is optimized in order to strengthen the ferrite by solid solution of Si or Mn in the ferrite. It is to be. That is, it aims at strengthening of non-tempered steel by solid solution strengthening of ferrite with Si or Mn instead of conventional precipitation strengthening with carbonitride such as V. Therefore, since V and Nb are positioned as so-called “arbitrary additive elements”, for example, the V content in the steel specifically disclosed in the examples of Patent Document 4 is as low as 0.195% at most. Is. For this reason, the hardness obtained in the said Example is also a value about 292HV (Vickers hardness 292).

  According to the “approximate conversion value for Vickers hardness of steel” in the hardness conversion table of SAE J 417 (1983), the above-mentioned Vickers hardness of 292 corresponds to a tensile strength of approximately 923 MPa. In the case of non-tempered steel, its yield ratio (“0.2% yield strength / tensile strength”) is low, and a large value exceeding 0.975 is never obtained. For this reason, the non-heat treated steel material disclosed in Patent Document 4 cannot achieve a large 0.2% proof stress of 900 MPa or more.

  The non-heat treated steel material disclosed in Patent Document 5 focuses only on the improvement of toughness anisotropy caused by sulfides, and has a high 0.2% proof stress as required for automobile parts of 900 MPa or more. Not trying to get. Therefore, as in the case of Patent Document 4, V and Nb are positioned as so-called “arbitrary additive elements”. For example, the V content in the steel specifically disclosed in the Example of Patent Document 5 is It is as low as 0.25% at most. Further, the structure of the non-tempered steel material may be a bainite structure, but the yield ratio of the bainite structure is lower than that of the ferrite-pearlite structure. Therefore, in the non-heat treated steel material disclosed in Patent Document 5, it is difficult to secure a high 0.2% proof stress.

  Therefore, an object of the present invention is to secure a large 0.2% proof stress of 900 MPa or more as it is with hot forging without performing special treatment such as tempering treatment of “quenching-tempering” or cold working. It is to provide non-tempered steel.

  The present inventors have made various studies in order to solve the above-described problems. As a result, it was found that the following <1> to <6> are effective in increasing the 0.2% proof stress.

  <1> When the ratio of ferrite + pearlite in the structure is 80% or more, the yield ratio when the bainite structure is generated hardly decreases, and a high 0.2% proof stress can be obtained.

  <2> In order to increase the 0.2% proof stress, a large amount of carbide may be precipitated in the ferrite.

  <3> In order to precipitate carbide, it is necessary to prevent so-called “carbide-forming elements” from being consumed as nitrides. For this purpose, the content of N (nitrogen) mixed in the steelmaking process is reduced. There is a need to reduce it.

<4> However, the N content mixed in the steelmaking process cannot be reduced to zero. For this reason, if N is fixed as a nitride by containing elements having high affinity with N, such as Ti, Nb, and Zr, this makes it possible to precipitate a carbide-forming element as a carbide. Then, in order to fix N as a nitride by Ti, Nb and Zr, the value of fn1 represented by the following formula (1) is expressed by using the element symbol in the formula as the content in mass% of the element. It is necessary to control to exceed zero.
fn1 = (0.293Ti + 0.151Nb + 0.154Zr) −N (1).

  <5> Ti, Nb, and Zr combine with O (oxygen) to form a hard oxide, resulting in a decrease in machinability. For this reason, in order to ensure good machinability, it is necessary to reduce the O content in the steel to suppress the formation of hard oxides.

  <6> The remaining Ti, Nb, and Zr that form nitrides have the effect of forming carbides in ferrite to increase 0.2% proof stress, but the degree of precipitation strengthening is smaller than that of V carbides. Therefore, in order to ensure a large 0.2% proof stress, it is preferable to effectively use V carbides without forming excessive carbides of Ti, Nb and Zr. In order not to excessively form Ti, Nb and Zr carbides, it is necessary to control so that the value of fn1 represented by the above formula (1) is 0.12 or less.

  This invention is completed based on said knowledge, The summary exists in the non-heat-treated steel materials shown to following (1) and (2).

(1) By mass%, C: more than 0.3% and 0.55% or less, Si: 0.01 to 1.0%, Mn: 0.6 to 2.0%, P: 0.15% Hereinafter, S: 0.005 to 0.20%, Cr: 0.02 to 0.4%, V: 0.35 to 1.0%, Al: 0.05% or less, N: less than 0.008% And O: 0.0035% or less, Ti: more than 0.05% and 0.4% or less, Nb: 0.02 to 0.4% and Zr: 0.02 to 0.4% And the balance is composed of Fe and impurities, the chemical composition satisfying the condition that the value of fn1 represented by the following formula (1) exceeds 0 and is 0.12 or less, and A non-tempered steel material characterized in that the ratio of ferrite + pearlite in the structure is 80% or more.
fn1 = (0.293Ti + 0.151Nb + 0.154Zr) −N (1).
Here, the element symbol in the formula (1) represents the content in mass% of the element.

  (2) Instead of a part of Fe, in mass%, Ca: 0.05% or less, Pb: 0.4% or less, Bi: 0.3% or less, Te: 0.1% or less, and Se: 0 The non-tempered steel material according to (1) above, containing one or more of 5% or less.

  Hereinafter, the inventions relating to the non-heat treated steel materials (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The non-tempered steel material of the present invention ensures a large 0.2% proof stress of 900 MPa or more in hot forging without performing special treatment such as tempering and quenching of “quenching-tempering”. Therefore, it is suitable as a material for connecting rods such as automobile engines and knuckle that are undercarriage parts of automobiles that are required to have high strength in recent years, and greatly contributes to reduction of manufacturing costs.

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

(A) Chemical composition C: more than 0.3% and 0.55% or less C forms carbides with V, Ti, Nb and Zr and precipitates in ferrite, and has the effect of increasing steel strength by precipitation strengthening. Have. In order to acquire this effect, it is necessary to contain C exceeding 0.3%. However, if the content of C exceeds 0.55%, the yield ratio does not become high for the content, and the ratio of pearlite to ferrite becomes too large, leading to a decrease in machinability. Therefore, the content of C is set to more than 0.3% and not more than 0.55%. A preferable range of the C content is 0.32 to 0.50%.

Si: 0.01 to 1.0%
Si is effective for deoxidation and has an effect of increasing the strength of the steel material by solid solution strengthening. Therefore, in order to obtain these effects, Si is contained in an amount of 0.01% or more. However, when the Si content increases and exceeds 1.0%, the solid solution strengthening action is saturated, and the productivity is deteriorated due to a decrease in hot ductility. Therefore, the Si content is set to 0.01 to 1.0%. A preferable range of the Si content is 0.1% or more and less than 0.5%.

Mn: 0.6 to 2.0%
Mn has a deoxidizing action and an action of improving hardenability and improving steel strength. Further, Mn combines with S to form MnS and has an effect of improving machinability. In order to obtain these effects, the Mn content needs to be 0.6% or more. However, if the Mn content exceeds 2.0%, the hot workability decreases, and the hardenability becomes too high and a bainite structure is likely to be formed. 2% yield strength cannot be obtained. Therefore, the Mn content is set to 0.6 to 2.0%. A preferable range of the Mn content is 0.9 to 1.4%.

P: 0.15% or less P is an element contained as an impurity and reduces toughness and hot workability. In particular, when the content exceeds 0.15%, the hot workability is reduced. It becomes remarkable. Therefore, the content of P is set to 0.15% or less. If very good toughness is required, the lower the P content, the better.

  In addition, P has the effect | action which raises steel material intensity | strength as a solid solution strengthening element, and since it embrittles ferrite, it has the effect | action which improves the surface roughness as a machinability, and chip disposal property. When it is desired to obtain the effects as described above, the P content is preferably 0.03 to 0.15%.

S: 0.005-0.20%
S forms MnS together with Mn and has an effect of improving machinability. In order to obtain this effect, the S content needs to be 0.005% or more. However, if the content of S is too large, not only the effect is saturated, but also the hot workability is lowered. In particular, when it exceeds 0.20%, the hot workability is significantly lowered. Therefore, the content of S is set to 0.005 to 0.20%. A preferable range of the S content is 0.03 to 0.13%.

Cr: 0.02-0.4%
Cr has the effect | action which improves the hardenability of steel and raises steel material intensity | strength. In order to obtain this effect, the Cr content needs to be 0.02% or more. However, if the Cr content exceeds 0.4%, not only the alloy cost increases, but also the hardenability becomes too high and a bainite structure is likely to be formed, so the yield ratio decreases and the desired high 0.2% The yield strength cannot be obtained. Therefore, the Cr content is set to 0.02 to 0.4%. A preferable range of the Cr content is 0.05 to 0.2%.

V: 0.35-1.0%
V is the most important element in the present invention, and has the effect of forming V carbide to increase the 0.2% proof stress. That is, V has the effect of increasing the strength of the steel by precipitating as a carbide in the ferrite in the ferrite-pearlite structure, thereby obtaining a high 0.2% proof stress. In order to obtain this effect, the V content needs to be 0.35% or more. However, if the content of V exceeds 1.0%, not only the effect is saturated, but also hot workability is reduced. Therefore, the content of V is set to 0.35 to 1.0%. A preferable range of the V content is 0.4 to 0.9%.

Al: 0.05% or less Al is an element having a deoxidizing action. However, even if Al is contained in excess of 0.05%, not only the above effects are saturated, but also the cost increases, and the machinability also decreases due to the hard oxide. Therefore, the Al content is set to 0.05% or less. A preferable range of the Al content is 0.008 to 0.05%.

N: Less than 0.008% N is an undesirable element in the present invention because it inhibits the strengthening action due to carbide precipitation. Therefore, the N content is less than 0.008%. The lower the N content, the better.

O: 0.0035% or less O is an element contained as an impurity, and in the case of the present invention containing Al, a hard Al oxide is formed and machinability is lowered. When the amount exceeds 0.0035%, the machinability is significantly lowered. Therefore, the content of O is set to 0.0035% or less. The lower the O content, the better.

Ti: more than 0.05% to 0.4% or less, Nb: 0.02 to 0.4% and Zr: one or more of 0.02 to 0.4% Ti, Nb and Zr Is an important element for fixing N. In the case of Ti, it exceeds 0.05%, in the case of Nb it is 0.02% or more, and in the case of Zr, it is 0.02% or more alone or in combination, and the above formula (1) In the case where the value of fn1 represented by the formula is included so as to exceed 0, a nitride is formed to fix nitrogen, and Ti carbide in the case of Ti, Nb carbide in the case of Nb, and Zr carbide in the case of Zr. As it precipitates in ferrite and contributes to precipitation strengthening, the 0.2% proof stress can be improved.

  However, the degree of precipitation strengthening of the Ti carbide, Nb carbide and Zr carbide is smaller than that of the V carbide described above, and the content of any element of Ti, Nb and Zr exceeds 0.4%. Then, the precipitation amount of carbides of these elements becomes excessive and the formation of V carbides is hindered, so that V carbides having a large precipitation strengthening effect cannot be used effectively.

  Therefore, the contents of Ti, Nb, and Zr are set to more than 0.05% and 0.4% or less, 0.02 to 0.4%, and 0.02 to 0.4%, respectively.

  Ti, Nb and Zr may be contained alone or in combination of two or more, but not only to fix N, but also to prevent excessive formation of carbides of Ti, Nb and Zr, As described, it is necessary that the value of fn1 represented by the above formula (1) exceeds 0 and becomes 0.12 or less.

fn1 value: greater than 0 and 0.12 or less fn1 represented by the above formula (1) is obtained by subtracting the total content of N from the sum of N amounts consumed as TiN, NbN, and ZrN. .

  When the value of fn1 exceeds 0, N is fixed by Ti, Nb and Zr, and part of Ti, Nb and Zr forms carbides, and precipitation strengthening action by these carbides. Is obtained. That is, when the value of fn1 exceeds 0, precipitation strengthening is achieved not only by V carbides but also by carbides of Ti, Nb and Zr, or composite carbides thereof, so that a large 0.2% proof stress is ensured. be able to. On the other hand, when the value of fn1 becomes large, especially when it exceeds 0.12, the total amount of carbides of Ti, Nb and Zr becomes too large, so that the precipitation amount of V carbide most effective for precipitation strengthening is sufficiently secured. Can not do. Therefore, the value of fn1 represented by the above formula (1) is set to be more than 0 and 0.12 or less.

  For the above reasons, the chemical composition of the non-tempered steel material according to the present invention (1) contains C, Si, Mn, P, S, Cr, V, Al, N, O in the above-described range, and Ti. , Nb and Zr are contained in the above-mentioned range, the balance is made of Fe and impurities, and the value of fn1 represented by the above formula (1) satisfies the above-mentioned definition. .

  In addition, the chemical composition of the non-tempered steel material according to the present invention (1) is replaced with a part of Fe, and further, if necessary, Ca: 0.05% or less, Pb: 0.4% or less, Bi : 0.3% or less, Te: 0.1% or less, and Se: 0.5% or less can be selectively contained.

  That is, in order to improve machinability, one or more of the aforementioned Ca, Pb, Bi, Te and Se may be added and contained as optional elements.

  Hereinafter, the above optional elements will be described.

Ca: 0.05% or less Ca is an element effective for improving machinability. In order to reliably obtain this effect, the Ca content is preferably 0.0005% or more. However, when the content exceeds 0.05%, the hot workability is deteriorated. Therefore, when Ca is added, the content of Ca is set to 0.05% or less. When Ca is added, the content of Ca is preferably 0.0005 to 0.05%, and more preferably 0.0005 to 0.01%.

Pb: 0.4% or less Pb has an effect of improving machinability. In order to reliably obtain this effect, the Pb content is preferably 0.02% or more. However, when the content exceeds 0.4%, the hot workability is reduced. Therefore, the content of Pb when added is set to 0.4% or less. In addition, it is preferable to make content of Pb in the case of adding 0.02-0.4%, and it is more preferable if it is 0.09-0.35%.

Bi: 0.3% or less Bi has an effect of improving machinability. In order to reliably obtain this effect, it is preferable that Bi is contained in an amount of 0.03% or more. However, when the content exceeds 0.3%, the hot workability is deteriorated. Therefore, the Bi content when added is set to 0.3% or less. In addition, when Bi is added, the content of Bi is preferably 0.03 to 0.3%, and more preferably 0.05 to 0.25%.

Te: 0.1% or less Te is an element effective for improving machinability. In order to reliably obtain this effect, the Te content is preferably 0.002% or more. However, when the content exceeds 0.1%, hot workability is deteriorated. Therefore, when Te is added, the content of Te is set to 0.1% or less. In addition, when Te is added, the content of Te is preferably 0.002 to 0.1%, and more preferably 0.005 to 0.06%.

Se: 0.5% or less Se has an effect of improving machinability. In order to ensure this effect, Se is preferably contained in a content of 0.0005% or more. However, when the content exceeds 0.5%, the hot workability is lowered. Therefore, the content of Se when added is set to 0.5% or less. In addition, when adding, it is preferable that content of Se shall be 0.0005 to 0.5%, and if it is 0.0005 to 0.1%, it is more preferable.

  In addition, said Ca, Pb, Bi, Te, and Se can contain only any 1 type in them, or 2 or more types of composites.

  For the above reasons, the chemical composition of the non-tempered steel material according to the present invention (2) is, in mass%, Ca: 0.05 instead of a part of Fe of the non-tempered steel material according to the present invention (1). %, Pb: 0.4% or less, Bi: 0.3% or less, Te: 0.1% or less, and Se: 0.5% or less.

  In the non-tempered steel material according to the present invention, elements other than those described above are essentially impurities and are not intentionally added.

  Here, from the viewpoint of avoiding mischievous cost increase in the steelmaking process for removing impurities and preventing hot cracking due to excessive inclusion, the contents of Cu, Ni and Mo in the impurities are respectively , 0.3% or less, 0.25% or less, and 0.3% or less.

(B) Structure Even if it has the chemical composition described in the above section (A), the yield ratio decreases when the proportion of bainite in the structure increases and the ratio of ferrite + pearlite falls below 80%. Therefore, a large 0.2% proof stress of 900 MPa or more cannot be obtained.

  Therefore, in the present invention, the ratio of ferrite + pearlite in the structure is 80% or more.

  In addition, the non-tempered steel material which concerns on this invention can be manufactured by passing through the process of [1]-[3] shown next in order, for example.

  [1] After the steel having the chemical composition described in the item (A) is melted by a blast furnace-converter process or an electric furnace melting process, a slab or a steel ingot is manufactured by a continuous casting method or an ingot casting method. .

  [2] The above slab or steel ingot is formed into, for example, a 180 mm square steel slab as an intermediate material, and then hot rolled into a round bar having a diameter of about 20 to 200 mm to produce a forging material. .

  [3] After heating the forging material obtained as described above to 1200 to 1250 ° C. and hot forging into a desired part shape at 1200 to 900 ° C., the carbides of V, Ti, Nb and Zr The precipitation temperature range of 800 to 500 ° C. is cooled at a cooling rate of 0.1 to 3 ° C./second, and the temperature range lower than 500 ° C. is cooled to room temperature by appropriate cooling means such as air cooling or mist cooling. As a result, the ratio of ferrite + pearlite in the structure of the non-heat treated steel material can be 80% or more.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels 1 to 28 having chemical compositions shown in Table 1 were melted in a vacuum melting furnace to produce an ingot.

  Steels 1 to 16 and Steel 28 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels 17 to 27 are steels whose chemical compositions deviate from the conditions defined in the present invention.

  After making said ingot into a steel piece by a normal method, a round bar having a diameter of 20 mm was produced by hot rolling at 1250 ° C. and finishing at 1000 ° C.

  Next, characteristics assuming hot forging were evaluated.

  That is, for test numbers 1 to 27 using steels 1 to 27, a round bar having a diameter of 20 mm obtained as described above was heated to 1250 ° C. assuming heating during hot forging, and in the atmosphere. It was allowed to cool and cooled to room temperature. In addition, the cooling rate in 800-500 degreeC at this time was 1 degree-C / sec.

  A round bar having a diameter of 20 mm made of steel 28 is divided into two equal parts, both of which are heated to 1250 ° C. assuming heating during hot forging. The bainite was intentionally formed easily by cooling at a cooling rate. On the other hand, as for test number 29, as in the case of the test numbers 1 to 27, the sample was allowed to cool in the air and cooled to room temperature. In addition, the cooling rate in 800-500 degreeC of this test number 29 was also 1 degree-C / sec.

  Various test pieces were collected from the thus obtained heated and cooled round bar with a diameter of 20 mm, and the microstructure, Vickers hardness (hereinafter referred to as “Hv hardness”) and tensile properties were investigated.

  The microstructure was cut out from each of the above-mentioned round bars, a specimen having an observation plane perpendicular to the forging axis, mirror-polished and corroded with nital, and then observed in four fields with an optical microscope at a magnification of 400 times. The phase ”(structure) was determined, and image analysis was performed by a normal method to investigate the ratio of ferrite + pearlite in each field of view.

  For Hv hardness, after cutting a test piece having a surface perpendicular to the forging axis as a test surface from each round bar and mirror polishing, R / 2 site (where “R” represents the radius of the round bar). A total of 5 points including 4 points and one central part were measured with a test force of 98.07 N, and arithmetically averaged to obtain Hv hardness (hereinafter referred to as “THv”) as total hardness.

  As for the tensile properties, a 14A test piece (however, the diameter of the parallel part: 7 mm) described in JIS Z 2201 (1998) was cut out from each round bar, and a 0.2% proof stress was measured by performing a tensile test at room temperature.

  Table 2 summarizes the results for tissue, THv and 0.2% yield strength. In the structure column of Table 2, “O” when the ratio of ferrite + pearlite is 80% or more, “X” when the amount of bainite is large and the ratio of ferrite + pearlite is less than 80%. It was described.

  As is clear from Table 2, in the case of Test Nos. 1 to 16 and Test No. 29 whose chemical composition and structure are within the range defined by the present invention, tempering treatment and cold working of “quenching-tempering” A high 0.2% proof stress of 900 MPa or more, specifically, a 0.2% proof stress of 905 to 1250 MPa is obtained without special treatment.

  On the other hand, in the case of test numbers 17 to 28 whose chemical composition or / and structure deviate from the conditions specified in the present invention, the 0.2% proof stress is less than 900 MPa.

  That is, in the case of the test numbers 17 to 21 and the test numbers 24 to 27, although the structure satisfies the conditions specified in the present invention, the chemical compositions deviate from the conditions specified in the present invention. Since 27 was used, the 0.2% yield strength is below 900 MPa.

  In the case of test number 22 and test number 23, the steel 22 and steel 23 whose chemical compositions deviate from the conditions defined in the present invention were used, and the structure deviated from the conditions defined in the present invention. The 0.2% proof stress is below 900 MPa.

  Furthermore, in the case of the test number 28, the steel 28 having a chemical composition within the range defined by the present invention is used. However, since the structure deviates from the conditions defined by the present invention, the 0.2% proof stress is It is 880 MPa lower than 900 MPa.

  The non-tempered steel material of the present invention ensures a large 0.2% proof stress of 900 MPa or more in hot forging without performing special treatment such as tempering and quenching of “quenching-tempering”. Therefore, it is suitable as a material for connecting rods such as automobile engines and knuckle that are undercarriage parts of automobiles that are required to have high strength in recent years, and greatly contributes to reduction of manufacturing costs.

Claims (2)

C: more than 0.3% and 0.55% or less, Si: 0.01 to 1.0%, Mn: 0.6 to 2.0%, P: 0.15% or less, S : 0.005 to 0.20%, Cr: 0.02 to 0.4%, V: 0.35 to 1.0%, Al: 0.05% or less, N: less than 0.008% and O: Including 0.0035% or less, Ti: more than 0.05% and 0.4% or less, Nb: 0.02 to 0.4%, and Zr: 0.02 to 0.4% Or it contains 2 or more types, and the balance consists of Fe and impurities, the chemical composition satisfying the condition that the value of fn1 represented by the following formula (1) exceeds 0 and is 0.12 or less, and in the structure The ratio of ferrite + pearlite is 80% or more.
fn1 = (0.293Ti + 0.151Nb + 0.154Zr) −N (1).
Here, the element symbol in the formula (1) represents the content in mass% of the element.   Instead of a part of Fe, in mass%, Ca: 0.05% or less, Pb: 0.4% or less, Bi: 0.3% or less, Te: 0.1% or less, and Se: 0.5% The non-heat treated steel material according to claim 1, comprising one or more of the following.