JP2008240076A - Cold forged non-tempered high-strength steel component having excellent impact characteristic in direction orthogonal to axial direction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve both strength and the impact characteristics in a direction orthogonal to an axial direction in the cold forged non-tempered high-strength steel components having shaft-like parts. <P>SOLUTION: The cold forged non-tempered high-strength steel components having the shaft-like parts have components containing 0.20 to 0.35% C, ≤0.1% Si, 1.0 to 2.0% Mn, ≤0.02% P, ≤0.02% S, 0.05 to 0.5% Cr, ≤0.07% Al, ≤0.006% N, and ≤0.010% C, and the balance iron and inevitable impurities, in which the ratio of Mn to C (Mn/C) is ≥4, the ratio Al to N (AI/N) is ≥8, the shaft-like part is a ferrite-pearlite structure and the Gp grain size number at its cross section is ≥9.0 and the Fgc grain size number is ≥10.0 and the aspect ratio of an Mns-based inclusion at the longitudinal direction section is 6 to 7.5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cold forged non-tempered high-strength steel part (for example, a tie rod, a ball stud, etc.) that has a shaft-like portion and requires impact characteristics in a direction orthogonal to the axial direction.

Tie rods, ball studs, etc. are parts manufactured by, for example, cold forging one end of a shaft steel (bar steel, wire, etc.) obtained by wire drawing into a spherical shape and drawing the other end. Tensile strength (for example, about 800 to 1000 N / mm ² ) is required. Therefore, normally, after performing cold forging and drawing, a predetermined strength is ensured by performing quenching and tempering treatment.

In recent years, there has been a demand for non-tempered steel parts in which quenching and tempering processes after cold forging and drawing are omitted for the purpose of reducing part manufacturing costs and reducing CO ₂ emissions (for example, Patent Document 1). ). Non-tempered steel parts are also superior in that they can prevent thermal strain deformation due to quenching and tempering treatments and do not require straightening.

However, if the quenching / tempering treatment is omitted, it is difficult to ensure the strength. In order to secure a predetermined tensile strength with the non-heat treated parts, for example, it is conceivable to increase the alloy components or improve the processing rate. However, toughness (impact characteristics) deteriorates when the alloy components are increased or the processing rate is improved. In recent years, automobile underbody parts such as tie rods and ball studs have been required to have excellent impact characteristics in a direction orthogonal to the axial direction (hereinafter sometimes simply referred to as an orthogonal direction). Even a tempered part is required to have both strength and impact characteristics in the orthogonal direction of the shaft portion.
JP-A-6-55237

  The present invention has been made paying attention to the above-described circumstances, and its purpose is to achieve both strength and impact characteristics in the orthogonal direction in a cold forged non-heat treated steel part having a shaft-like portion. is there.

  As a result of intensive studies in order to solve the above problems, the present inventors have adjusted the steel component to an appropriate range while making the structure of the shaft-like portion a fine fibrous structure composed of ferrite-pearlite. It was found that both strength and impact characteristics in the orthogonal direction could be achieved without quenching and tempering, and the present invention was completed.

  That is, the cold forged non-tempered high-strength steel part having excellent impact characteristics in the orthogonal direction according to the present invention has a shaft-shaped portion of a ferrite-pearlite structure, and a cross section of the shaft-shaped portion was observed. When the old austenite has a Gp grain size number (2005 JIS G 0551) of 9.0 or more and a ferrite FGc grain size number (1998 JIS G 0552) of 10.0 or more, the longitudinal section of the shaft portion is observed. In this case, the aspect ratio of the MnS inclusion is 6 to 7.5. Steel components are also important, and the steel parts are: C: 0.20 to 0.35% (meaning mass%, the same shall apply hereinafter), Si: 0.1% or less (not including 0%), Mn: 1.0-2.0%, P: 0.02% or less (not including 0%), S: 0.02% or less (not including 0%), Cr: 0.05-0.5% ( 0% not included), Al: 0.07% or less (not including 0%), N: 0.006% or less (not including 0%), O: 0.010% or less (not including 0%) The balance is iron and inevitable impurities, and the ratio of Mn to C (Mn / C) is 4 or more, and the ratio of Al to N (Al / N) is 8 or more. Further, the steel part may further contain Cu and Ni. In that case, Cu: 0.1% or less (not including 0%), Ni: 0.1% or less (not including 0%) ).

When the longitudinal section of the shaft-like portion is observed, the average particle diameter of Al ₂ O ₃ inclusions precipitated in the MnS inclusions is, for example, 5 μm or less. The cold forged non-tempered high strength steel part of the present invention may be baked at a temperature of 250 to 600 ° C. The cold forged non-tempered high-strength steel part of the present invention has a tensile strength of the shaft-shaped portion of, for example, 850 N / mm ² or more, and is based on a subsize U-notch test piece having a width of 5 mm taken from the shaft-shaped portion. The impact absorption energy KU ₂ is, for example, 100 J / cm ² or more at both test temperatures −50 ° C. and 20 ° C.

  In this specification, the heating temperature of the steel material refers to the temperature of the furnace unless otherwise specified.

  According to the cold forged non-tempered high-strength steel part of the present invention, the steel component is adjusted to an appropriate range, and the structure of the shaft portion is made into a fine fibrous structure made of ferrite-pearlite. -Both strength and impact characteristics in the orthogonal direction can be achieved without tempering.

  A high-strength steel part (cold forged part) of the present invention is obtained from a shaft steel (bar steel, wire, etc.) obtained by wire drawing, and a cold forged part obtained by cold forging part of the shaft steel. It is a non-tempered part that has a shaft-shaped portion obtained by cold-reducing (drawing, rolling, etc.) a part of shaft-shaped steel and does not require quenching / tempering after cold working. In the present invention, the steel component of the high-strength steel part is adjusted to an appropriate range, and the shaft portion has a fine fibrous structure made of ferrite-pearlite. Therefore, both strength and orthogonal impact characteristics can be achieved without quenching and tempering.

  The components of the steel part of the present invention are as follows.

C: 0.20 to 0.35% (meaning mass%, the same shall apply hereinafter)
C is a basic element that governs the balance between strength and ductility of the steel material, and is essential for increasing the strength. Therefore, C is 0.20% or more, preferably 0.23% or more, and more preferably 0.25% or more. However, when C is increased, toughness and ductility are lowered, and impact characteristics are lowered. Moreover, the cold forgeability (especially crack generation limit compression rate) is reduced. Accordingly, the C content is 0.35% or less, particularly 0.33% or less.

Si: 0.1% or less (excluding 0%)
Si acts as a deoxidizer during melting and is a useful element in the production of steel materials. However, if added in a large amount, it causes hardening and embrittlement of ferrite and inhibits cold forgeability and impact characteristics. Therefore, in the present invention, Si is made 0.1% or less, preferably 0.07% or less, more preferably 0.05% or less.

Mn: 1.0-2.0%
Mn acts as a deoxidizer and combines with S in steel to suppress embrittlement due to S. It is also an effective element from the viewpoint of ensuring the strength of non-heat treated steel parts. Accordingly, Mn is set to 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more. Mn is 4 times or more of C (that is, Mn / C is 4 or more), preferably 4.2 times or more, more preferably 4.5 times or more. Mn is excellent in improving the low temperature impact characteristics such as lowering the transition temperature of the impact value, and it is necessary to add a sufficient amount to C that lowers the low temperature impact characteristics. If Mn is excessive, ferrite becomes brittle and cold forgeability deteriorates. Therefore, Mn is 2.0% or less, preferably 1.8% or less, more preferably 1.6% or less.

P: 0.02% or less (excluding 0%)
P is more preferably as small as possible because it causes grain boundary segregation and causes a drop in impact value and cold forgeability. The amount of P is 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less.

S: 0.02% or less (excluding 0%)
S forms MnS inclusions in the steel. This MnS inclusion becomes a stress concentration source and lowers impact characteristics and cold forgeability. The amount of S is 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less.

Cr: 0.05-0.5%
Cr is an element effective in producing carbonitrides in steel and suppressing strain aging due to solute C and solute N. Further, it is an important element because it contributes to an increase in strength in the same manner as C and Si and has a smaller adverse effect on cold forgeability than C and Si. Therefore, the Cr content is 0.05% or more, preferably 0.08% or more. However, if added in a large amount, coarse carbonitrides are formed, and cold forgeability and impact properties are deteriorated. Therefore, Cr is 0.5% or less, preferably 0.3% or less, more preferably 0.15% or less.

Al: 0.07% or less (excluding 0%)
Al is effective in preventing strain aging by fixing solute N as AlN. Further, AlN is effective in suppressing the coarsening of crystal grains. Therefore, Al is added so as to be 8 times or more (that is, Al / N is 8 or more), preferably 9 times or more, and more preferably 10 times or more with respect to N. A specific amount of Al is, for example, 0.03% or more, preferably 0.035% or more, and more preferably 0.040% or more. However, if Al is added in a large amount, the hardness of ferrite increases, Al ₂ O ₃ inclusions become coarse, toughness and ductility decrease, and cold forgeability deteriorates. Accordingly, the Al content is 0.07% or less, preferably 0.06% or less, and more preferably 0.055% or less.

N: 0.006% or less (excluding 0%)
N combines with Al to form nitride (AlN). N that cannot be combined with Al remains as a solid solution N, which causes a decrease in impact characteristics due to strain aging and a decrease in cold forgeability. Therefore, the total N amount in the steel is 0.006% or less, preferably 0.005% or less. From the viewpoint of effectively using AlN, the N amount is, for example, 0.001% or more, preferably 0.002% or more, and more preferably 0.003% or more.

O: 0.010% or less (excluding 0%)
O hardly dissolves in steel at room temperature and exists as a hard oxide. Oxide inclusions act as a stress concentration source during cold forging and when impact stress is applied, greatly reducing both properties. Therefore, the O content should be reduced as much as possible. In the present invention, the O content is 0.010% or less, preferably 0.007% or less, and more preferably 0.005% or less.

  The balance is usually iron and inevitable impurities.

  The steel part of the present invention may also contain impurities mixed in from steelmaking raw materials, manufacturing processes, and the like. Examples of this impurity include Cu and Ni. Each allowable amount and the reason for its suppression are as follows.

Cu: 0.1% or less (excluding 0%)
When Cu is contained in a small amount, there is little adverse effect on the impact value and cold forgeability, but if it exceeds 0.1%, the transition temperature rises and the low temperature impact value is lowered. Therefore, Cu is 0.1% or less, preferably 0.07% or less, more preferably 0.05% or less.

Ni: 0.1% or less (excluding 0%)
When Ni is contained in a small amount like Cu, there is little adverse effect on the impact value and cold forgeability, but if it exceeds 0.1%, the transition temperature rises and the low temperature impact value is lowered. Therefore, Ni is 0.1% or less, preferably 0.07% or less, more preferably 0.05% or less.

  And in the steel part of this invention, the structure | tissue of the shaft-shaped part is made into the fine fibrous structure which consists of ferrite-pearlite. In the cross section (longitudinal section) along the axial direction, this fibrous structure has a layered structure as shown in the optical micrographs of FIGS. 1 (a) and 1 (b), for example. Moreover, in the cross section (transverse cross section) orthogonal to the axial direction, for example, it is a fine grain as shown in the optical micrograph of FIG.

  As is clear from FIG. 1, it is difficult to directly and quantitatively show the layered tissue state of the longitudinal section. Since the degree of ferrite-pearlite fiberization is correlated with the aspect ratio of the MnS inclusions in the longitudinal section, in the present invention, the structure state is indirectly determined by the aspect ratio (major axis / minor axis) of the MnS inclusions. It was decided to prescribe. The aspect ratio of the MnS-based inclusion is 6 or more, preferably 6.2 or more, and more preferably 6.5 or more. If the fiberization becomes excessive (that is, if the aspect ratio of the MnS inclusions becomes too large), the toughness and ductility are lowered, and the cold forgeability is deteriorated. Therefore, the aspect ratio of the MnS inclusions is 7.5 or less, preferably 7.4 or less.

  On the other hand, the fine grain structure state of the cross section can be quantitatively shown by the crystal grain size numbers of prior austenite and ferrite. The Gp particle size number (2005 JIS G 0551) of the austenite in the cross section is 9.0 or more, preferably 9.5 or more. The upper limit of the Gp particle size number is not particularly limited, but it is included in the present invention even if it is about 14 or less. Moreover, the FGc particle size (1998 JIS G 0552) of the ferrite of a cross section is 10.0 or more, Preferably it is 10.5 or more, More preferably, it is 11.0 or more. The upper limit of the FGc particle size number is not particularly limited, but it is included in the present invention even if it is about 14 or less.

In addition, as above-mentioned, the steel component of this invention refine | miniaturizes the oxide type inclusion by reducing Al and oxygen. Specifically, the size of the oxide inclusions can be further clarified by examining the Al ₂ O ₃ inclusions present in the MnS inclusions in the longitudinal section. The average value (average particle size) of the particle size (= (major axis + minor axis) / 2) of the Al ₂ O ₃ inclusion is, for example, 5 μm or less, preferably 4 μm or less, and more preferably 3 μm or less. Although the minimum of an average particle diameter is not specifically limited, Even if it is about 1 micrometer or more, it is included in this invention.

  The steel parts of the present invention are roughly outlined by hot-rolled shaft steel (bar steel, wire rods, etc.), as appropriate, pickled, coated, etc., then drawn and then cold forged and reduced in diameter into the part shape. Can be obtained. Most importantly, in order to make the shaft-like portion into a fine fibrous structure made of ferrite-pearlite as described above, the wire is strongly processed in wire drawing and diameter reduction processing. More specifically, the area reduction rate of wire drawing is set to 20% or more, and the total area reduction after diameter reduction (area reduction based on before wire drawing) exceeds 40% (preferably 42%). Or more, more preferably 45% or more) and 80% or less (preferably 70% or less, more preferably 65% or less). The degree of fibrosis of the ferrite-pearlite structure can be adjusted by adjusting the degree of strong processing.

  Other manufacturing processes are recommended as follows.

  The heating temperature in the hot rolling is selected from a range of about 1000 to 1200 ° C. When the heating temperature is low, a mixed phase of an austenite phase and a ferrite phase is locally generated, and there is a risk of causing cracks during rolling. On the low temperature side, the roll load during rolling increases, and the productivity decreases. On the other hand, when the heating temperature is high, ferrite crystal grains are coarsened, the cold forgeability is lowered, and the impact value is not improved sufficiently by the fibrous organization.

  The finishing temperature of hot rolling is selected from the range of about 850 to 975 ° C. (preferably about 875 to 950 ° C.). If the finishing temperature is low, the load on the rolling roll increases and the steel material productivity is significantly reduced. Conversely, even if the finishing temperature is too high, the ferrite grains become coarse.

  The winding temperature after rolling is about 800 to 950 ° C. When the coiling temperature is too high, the precipitation of nitride is delayed, and the solid solution N in the steel material increases. Since solute N causes an increase in deformation resistance due to strain aging, the cold forging die life is reduced.

  The steel part obtained as described above may be baked as necessary. If baking is performed at an appropriate temperature, the tensile strength can be increased without deteriorating the impact characteristics. Baking temperature is about 250-600 degreeC, for example, Preferably it is about 280-500 degreeC, More preferably, it is about 300-450 degreeC. The baking time is usually about 5 to 30 minutes, preferably about 10 to 20 minutes. Allow to cool after baking.

The steel part of the present invention has a shaft-like portion and is excellent in strength and impact characteristics in the orthogonal direction of the shaft-like portion regardless of the presence or absence of baking treatment. Tensile strength of the shaft-like portion is, for example, 850N / mm ² or more, preferably at 900 N / mm ² or more, when the most excellent 950 N / mm ² or more. The upper limit of the tensile strength is not particularly limited, but even if it is 1100 N / mm ² or less, it is included in the present invention.

Impact absorption energy KU ₂ by the sub-size U-notch test piece of width 5mm taken from the shaft-like part, in the case of test temperature -50 ° C., for example, 100 J / cm ² or more, preferably 105 J / cm ² or more, more preferably Is 110 J / cm ² or more. The upper limit of KU _{2 at} a temperature of −50 ° C. is not particularly limited, but even if it is 150 J / cm ² or less, it is included in the present invention. Impact absorption energy KU ₂ at a test temperature of 20 ° C. Further, for example, 100 J / cm ² or more, preferably at 120 J / cm ² or more, when the most excellent 150 J / cm ² or more. The upper limit of KU ₂ at a temperature of 20 ° C. is not particularly limited, but 200 K / cm ² or less is included in the present invention.

  The HRC hardness of the shaft-like portion is, for example, about 10-40 (preferably about 25-40). It can be said that the higher the HRC hardness, the better the tensile strength.

  Since the steel part of the present invention is excellent in the tensile strength and impact characteristics of the shaft part, it can be used for a shaft part for connecting to a tie rod outer socket, such as a tie rod and a ball stud. . This shaft-shaped part is a cold forged part. More specifically, a part of shaft steel (bar, wire, etc.) obtained by wire drawing is cold-forged while the other part is reduced in diameter. It is common in that it can be obtained by (drawing, etc.), and is characterized in that excellent impact characteristics in the orthogonal direction are required in this reduced diameter shaft-shaped portion.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experiment No. 1-24
150 kg of test materials having the components shown in Table 1 were manufactured by vacuum melting. After forging the melted material into a 155 mm × 155 mm square and welding it to a dummy billet material, it was hot-rolled under the conditions of a heating temperature of 1050 ° C., a finishing temperature of 925 ° C., and a winding temperature of 825 ° C. A rolled material was obtained. This rolled material was treated in the order of washing with sulfuric acid, washing with water, washing with hydrochloric acid and washing with water, followed by coating treatment (zinc phosphate coating), and then drawing to the diameter shown in Table 2.

The obtained wire was cold forged by the method described later and examined for cold forgeability. Further, the obtained wire was cold-reduced (skin pass rolling) to produce a non-tempered shaft-like part, and in some examples (Nos. 8 to 11), baking was further performed. Details of the obtained shaft-like part equivalent (structure, particle size, aspect ratio of MnS inclusions, average particle diameter of Al ₂ O ₃ inclusions, HRC hardness, tensile strength, impact value) are described later. Investigated by.

Experiment No. 25-27
In the experiment No. In the same manner as in 1 to 24, wire rods having final diameters shown in Table 2 were obtained.

  This wire was cold forged by the method described later and examined for cold forgeability. In addition, this wire is quenched (heating temperature 840 ° C., heating time 30 minutes, cooling condition: oil cooling), and tempered (heating temperature 420 to 530 ° C., heating time 120 minutes, cooling condition: air cooling). An equivalent product was produced. The details of the obtained shaft-like part were examined by the method described later.

Evaluation methods of cold forgeability, particle size, aspect ratio of MnS inclusions, average particle diameter of Al ₂ O ₃ inclusions, Rockwell hardness, tensile strength, and impact value are as follows.

(1) Cold forgeability A sample having a diameter of 10 mm and a height of 15 mm was taken in parallel with the axial direction of the wire, and subjected to end face constrained compression (strain rate = 10 / second) in the axial direction. The relationship between the presence or absence of cracking after compression and the compression ratio was examined, and the crack compression limit compression ratio was determined.

(2) Structure After embedding in a supporting substrate with the cross-section of the shaft-like portion equivalent product exposed, polishing, and immersing in a 5% picric acid alcohol solution for 15 to 30 seconds to corrode, then an optical microscope The tissue of D / 4 (D is a diameter) region was photographed at 10 × with 10 fields. Further, the longitudinal section (longitudinal section) of the shaft-like part was corroded in the same manner as the transverse section, and then 10 fields of view of the structure of the D / 4 part were photographed at 400 × with an optical microscope. From these micrographs, the structure state (whether it is a fine fibrous structure made of ferrite-pearlite) was confirmed. The degree of refinement is quantified by the crystal grain size number. The degree of fiberization is quantified by the aspect ratio of the MnS inclusions.

(3) Grain size Based on the optical micrograph (10 fields of view) of the cross section taken by the above structure observation, the former austenite grain size number Gp (6.3.3.3 “Proeutectoid ferrite method” of JIS G 0551, 2005) And ferrite grain size number FGc (1998 JIS G 0552).

(4) Aspect ratio of MnS inclusions The aspect ratio of MnS inclusions was determined based on a scanning electron micrograph of the longitudinal section taken in the above structure observation (2 cross sections x 5 observation points = 10 fields of view). It was. The five observation locations are 2 locations from the D / 8 (D is diameter) location, 2 locations from the D / 4 (D is diameter) location, and 1 location from the D / 2 (D is diameter) location. is there. More specifically, a reflection electron image of an electron microscope was taken in each field of view (magnification: 400 times). The reflected electron image changes in luminance depending on the chemical component (see FIG. 3A). Next, a MnS portion (including MnTiS and the like) was identified by an EDS spectrum (see FIG. 3B), and a portion having a luminance corresponding to the MnS portion was extracted from the reflected electron image using image analysis software. By this operation, MnS inclusions are extracted as black portions (see FIG. 3C). The aspect ratio was calculated for MnS inclusions having a thickness of 0.5 μm or more, and the average (arithmetic average) was calculated as the aspect ratio of the MnS inclusions.

(5) Average particle diameter of Al ₂ O ₃ -based inclusions In addition, the longitudinal section (longitudinal section) of the shaft-shaped portion equivalent product is viewed from D / 8 (D is the diameter) from D / 8 (D is a diameter) by a scanning electron microscope (SEM). Two visual fields were observed from the D / 4 (D is diameter) site, and one visual field was observed from the D / 2 (D is diameter) site. For each field of view, a reflected electron image was first taken at a magnification of 400 times to confirm that there was no significant segregation (see FIG. 3A). The reflected electron image changes in luminance depending on the chemical component. The portion where the luminance change was recognized was magnified (magnification 1600 times), the component was examined by the EDS spectrum, and the Al ₂ O ₃ inclusion was searched (see FIG. 3B). It was also confirmed whether MnS inclusions exist around the found Al ₂ O ₃ inclusions. Obtain the particle size (= (major axis + minor axis) / 2) for Al ₂ O ₃ inclusions with MnS inclusions in the surroundings and a minor axis of 0.5 μm or more. The arithmetic average of the particle size of the product was defined as the average particle size.

(6) Rockwell hardness Based on JIS Z 2245, the Rockwell hardness (scale C) of the cross section of a shaft-like part equivalent product was measured.

(7) Tensile strength No. 14 test piece specified in JIS Z 2201 was sampled from the shaft-like portion equivalent product, and the tensile strength was determined according to JIS Z 2241.

(8) Impact value A 5 mm wide sub-size U-notch test piece (the length direction of the test piece is the same as the axial direction of the shaft-like part) is taken from the shaft-like part and the room temperature is measured according to JIS Z 2242. The impact absorption energy KU ₂ at (20 ° C.) or −50 ° C. was determined.

  The results are shown in the table below. An example of an optical micrograph of the longitudinal section (example of Experiment No. 3) is shown in FIG. 1 (a) (magnification: 100 times) and FIG. 1 (b) (magnification: 400 times). An example of the photograph (examples of Experiment Nos. 3 and 5) is shown in FIG. 2 (magnification: 400 times).

  Experiment No. Nos. 1 and 2 have low tensile strength, impact value, and the like because the processing by wire drawing and diameter reduction is insufficient and the fine fiber formation of the ferrite-pearlite structure is insufficient. No. No. 6, the ferrite-pearlite structure is excessively fiberized, and the cold forgeability is poor. No. In Nos. 8 and 11, the baking temperature is too low or too high, and the tensile strength or impact value is deteriorated. No. No. 12 is a point with low Mn / C. No. 13 is a low Al / N point. No. 14 is a point where the amount of C is insufficient. No. 15 is that C is excessive, No. 15; No. 16 is a point where Si is insufficient. No. 17 is a point where Mn is insufficient. No. 18 is that Mn is excessive. No. 19 is that P is excessive. No. 20 is that S is excessive, No. 20; No. 21 is that Cr is excessive. No. 22 is that Al is excessive. No. 23 is that N is excessive. No. 24 is inappropriate in that O is excessive, and any of cold forgeability, tensile strength, and impact value deteriorates.

  Experiment No. Nos. 25 to 27 simulate simulated tempered cold forged products (equivalent to S45C-W) and have low impact values.

  No. In 3-5, 7, and 9-10, the steel component is adjusted to an appropriate range, and the structure of the shaft-like portion is made into a fine fibrous structure composed of ferrite-pearlite, so it is not quenched and tempered. Can achieve both strength and impact characteristics in the orthogonal direction.

FIG. 1 is an optical micrograph showing the structure of the cross section in the longitudinal direction of the axial part of the cold forged non-tempered high strength steel part of the present invention. FIG. 2 is an optical micrograph showing the cross-sectional structure of the shaft-shaped portion of the cold forged non-tempered high strength steel part of the present invention. FIG. 3A is an electron micrograph for explaining a method for measuring inclusions. FIG. 3B is a diagram combining an electron micrograph and an EDS spectrum for explaining a method for measuring inclusions. FIG.3 (c) is the electron micrograph after the image analysis process for demonstrating the measuring method of an inclusion.

Claims (5)

In cold forged non-tempered high-strength steel parts having a shaft-shaped part,
The steel part has components of C: 0.20 to 0.35% (meaning mass%, the same shall apply hereinafter), Si: 0.1% or less (not including 0%), Mn: 1.0 to 2. 0%, P: 0.02% or less (not including 0%), S: 0.02% or less (not including 0%), Cr: 0.05 to 0.5% (not including 0%) Al: 0.07% or less (excluding 0%), N: 0.006% or less (not including 0%), O: 0.010% or less (not including 0%), the balance Is iron and inevitable impurities, the ratio of Mn to C (Mn / C) is 4 or more, the ratio of Al to N (Al / N) is 8 or more,
The shaft part is a ferrite-pearlite structure,
When the cross section of this shaft-like part was observed, the Gp grain size number of old austenite (2005 JIS G 0551) was 9.0 or more, and the FGc grain size number of ferrite (1998 JIS G 0552) was 10.0 or more. ,
When the longitudinal section of the axial part is observed, the aspect ratio of the MnS inclusion is 6 to 7.5,
Cold forged non-tempered high strength steel parts with excellent impact characteristics in the direction perpendicular to the axial direction.   Furthermore, Cu and Ni are contained, and the amount is suppressed to Cu: 0.1% or less (not including 0%), Ni: 0.1% or less (not including 0%). 1. Cold forged non-tempered high-strength steel part according to 1. The cold forging non-adjustment according to claim 1 or 2, wherein an average particle diameter of Al ₂ O ₃ inclusions precipitated in the MnS inclusions is 5 µm or less when a longitudinal section of the shaft-like portion is observed. High quality steel parts.   A cold forged non-tempered high strength steel part obtained by baking the cold forged non-tempered high strength steel part according to claim 1 at a temperature of 250 to 600 ° C. The tensile strength of the shaft-shaped portion is 850 N / mm ² or more, and the impact absorption energy KU ₂ by the sub-size U-notch test piece with a width of 5 mm collected from the shaft-shaped portion is 100 J / at both the test temperature −50 ° C. and 20 ° C. cm ² or more in cold forging microalloyed high-strength steel part according to any one of claims 1 to 4.