JP4699342B2 - High strength non-tempered steel for cold forging with excellent fatigue limit ratio - Google Patents

Description

  The present invention relates to a high-strength non-heat treated steel for cold forging used as a material for transportation equipment such as automobiles, construction machinery, and other industrial machines, and in particular, even if heat treatment is not performed after cold forging (non- Tempering), and relates to non-tempered steel for cold forging having a high fatigue limit ratio and high strength. The non-tempered steel for high-strength cold forging of the present invention is particularly suitably used as a material for gears, shafts, gears with shafts, and the like.

  Machine parts used in automobiles, construction machinery, and other various industrial machines that require particularly high strength are usually quenched after hot rolling and cold forging / forming. It is manufactured by applying heat treatment such as tempering (so-called tempering treatment) to give necessary mechanical properties. However, from the viewpoints of cost reduction and production efficiency, high strength cooling with excellent mechanical properties, particularly fatigue limit ratio and strength, is maintained without compromising cold forgeability even in cold forging (non-tempering). There is an urgent need to provide non-tempered steel for hot forging (hot rolled material).

For example, Patent Document 1 discloses a technique for improving fatigue characteristics and cold forgeability by controlling the ferrite fraction and the hardness of the ferrite part. Here, precipitation strengthening elements such as Ti, Nb, and V are added to improve fatigue characteristics and strength, but further improvement is desired.
Japanese Patent Laid-Open No. 11-140583

  The present invention has been made in view of the above circumstances, and its purpose is to provide a high-strength cold steel excellent in fatigue limit ratio without reducing the cold forgeability even in cold forging (non-tempering). It is to provide non-tempered steel for forging.

The high-strength non-refined steel for cold forging with excellent fatigue limit ratio according to the present invention that has solved the above problems is as follows: (1) The component in the steel is C: 0.10 to 0.50% (mass) %: Same as below), Si: 0.05-2%, Mn: 0.3-3%, Al: 0.005-0.1%, P: 0.05% or less (including 0%) No), S: 0.5% or less (not including 0%), O: 0.003% or less (not including 0%), N: 0.02% or less (not including 0%) Furthermore, among Nb, Ti, and V, at least Nb and / or Ti may be included, and V may be included. When Nb is included, Nb is 0.2% or less (not including 0%). When Ti is contained, Ti: 0.20% or less (excluding 0%), N: less than 0.010% (excluding 0%), and Ti / N ≧ 3.4 In addition, when V is included, V: 0.6% or less (excluding 0%), the balance: satisfying Fe and inevitable impurities, and (2) Nb and / or Ti contained in ferrite precipitates containing (may further comprise V), and, the precipitate particle size of less than 15nm the MP value of the following formula (1) satisfies the MP ≧ 0.05, according to the following procedure When measured, it has a gist in that it contains 50 precipitates / μm ² or more.
MP = [{[Nb] / 93} + {[Ti] / 48}] / [{[V] / 51} + {[Nb] / 93} + {[Ti] / 48}] (1 )
In the formula, [] means the content (% by mass) of the element contained in the precipitate.
(A) An extraction replica for TEM observation is prepared using an electrolytic solution containing 10% acetylacetone, 80% methanol, and 10% tetramethylammonium chloride.
(A) Next, the sample treated by the extraction replica method is observed with a TEM at a magnification of 100,000 times, and optionally 20 precipitates having a particle size of 15 nm or less are measured, and then Ti contained in the precipitates is measured. , Nb and V are analyzed by EDX.
(C) Next, based on the above-described formula (1), the MP value of each precipitate is calculated. As a result, the number of precipitates satisfying MP ≧ 0.05 in the above-mentioned precipitates (total of 20) identified by TEM observation (100,000 times) can be obtained, so that MP ≧ 0. The ratio of precipitates satisfying 05 is calculated (this ratio is “X”).
(D) Next, for a given area, take a picture of 20 fields of view at a magnification of 150,000 with a TEM (total area of 0.75 μm ^{2 of} 20 fields of view ) and calculate the number of precipitates with a particle size of 15 nm or less. To do. The number of precipitates thus obtained (total number for 20 fields of view) is converted to the number per 1 μm ² (this number is referred to as “Y”).
(E) Finally, Y (the number of precipitates having a particle size of 15 nm or less / μm ² ) thus obtained is added to the above-described X (all precipitates satisfying MP ≧ 0.05). By multiplying the ratio, the number of ultrafine precipitates satisfying MP ≧ 0.05 is obtained.

  In a preferred embodiment, the high-strength cold forged non-heat-treated steel further contains Mo: 1% or less and / or B: 0.015% or less.

  In a preferred embodiment, the non-tempered steel for high-strength cold forging further contains at least one selected from the group consisting of Ni: 2% or less, Cu: 2% or less, and Cr: 3% or less. To do.

  In a preferred embodiment, the high-strength cold forged non-heat treated steel is further selected from the group consisting of Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.02% or less. Containing at least one of the above.

  In a preferred embodiment, the high-strength cold forged non-heat-treated steel is further selected from the group consisting of Zr: 0.1% or less, Ta: 0.1% or less, and Hf: 0.1% or less. Containing at least one of the above.

  According to the present invention, cold forged non-heat treated steel excellent in both strength and fatigue limit ratio can be provided without reducing cold forgeability even in cold forging (non-heat treated). It was.

The present inventor, in the fatigue strength improvement technology by adding precipitation strengthening elements such as Ti, Nb, V, etc., without impairing the cold forgeability even without non-tempering, and without increasing the tensile strength more than necessary. We have intensively studied for the purpose of further increasing the fatigue limit ratio. As a result, (a) in the ferrite, (specifically, particle size 15nm or less) nano-level and those containing a large number of Nb and / or Ti-containing precipitates, the Nb and / or Ti-containing further including V A precipitate containing a certain amount or more of Nb and / or Ti (specifically, MP ≧ 0.05 described later) achieves desired characteristics, and (b) In order to obtain such a non-tempered steel, in particular, the heating conditions of the block rolling process and the hot rolling process before cold forging are heated and maintained at a high temperature for a long time (particularly, the heating time is 1 in any process). It has been found that it is extremely important to set the time or longer) and that it is necessary to appropriately control the cooling conditions (average cooling rate) after hot rolling, and the present invention has been completed.

Hereinafter, the background to the present invention will be described. In the following, for convenience of explanation, there are cases less of Nb and / or Ti-containing precipitates particle size 15nm referred to as "ultra-fine precipitates of nano-sized", or simply "ultra-fine precipitates". Said Nb and / or Ti containing deposit may contain V further, and does not need to contain it. In order to distinguish the two, the precipitate containing V is particularly referred to as “V-containing ultrafine precipitate” or the like, and the precipitate not containing V is particularly referred to as “V-free ultrafine precipitate” or the like. is there.

  In general, precipitation strengthening is obtained by precipitation of a large number of precipitation strengthening elements such as V, which are solid-dissolved during heating, in the ferrite during the ferrite transformation during cooling after cold forging. Precipitates that contribute to precipitation strengthening are mainly MX type compounds (M = Metal elements, X = carbon or nitrogen) having a NaCl-type crystal structure. For example, precipitation strengthening of Ti, Nb, and V in steel When elements are included, (Ti, Nb, V) (C, N) precipitates are formed. When Ti and Nb precipitation strengthening elements are included in the steel, (Ti, Nb) (C, N) precipitates are generated. Things are generated. However, when the precipitation strengthening element is added in a certain amount or more, the above action is saturated, and it is difficult to further improve the characteristics (particularly increase the strength).

  Therefore, the present inventors have investigated the cause of saturation of the precipitation strengthening action. As a result, even if the amount of precipitation strengthening element added is increased in order to further enhance the precipitation strengthening action, MX type precipitates precipitate in austenite rather than ferrite during cooling after cold forging. In addition, even when precipitated in ferrite, the grain growth after precipitation is remarkably fast, so MX type precipitates grow coarsely, reducing the precipitation strengthening action and increasing the strength. I was unable to plan. Based on the above findings, the present inventors have further studied. As a result, in order to suppress precipitation in the austenite region and precipitate a large number of the above precipitates in the ferrite, heating conditions for split rolling and hot rolling before forging, as well as cooling after hot rolling. It has been found that it is effective to appropriately control the conditions, and in particular, it is necessary to maintain the above heating conditions at a higher temperature and for a longer time than before. When the precipitates thus obtained are observed in detail, (a) a number of nano-sized ultrafine precipitates that are finer than before are precipitated, and (b) in particular, V is contained. In the ultrafine precipitate, it was found that a certain amount or more of Nb and / or Ti was contained in the V-containing ultrafine precipitate. That is, it is considered that these ultrafine precipitates greatly contribute to the improvement of strength and fatigue limit ratio. In the V-containing ultrafine precipitate, the reason why the characteristics are improved by being controlled as in the above (b) is unknown in detail, but a predetermined amount of Nb and / or in the V ultrafine precipitate is not known. When Ti is present, (i) Nb and / or Ti has a slower diffusion rate than V, and therefore grain growth is suppressed, and a large number of desired nano-sized ultrafine precipitates can be secured, resulting in higher strength. This is presumably because (ii) the compatibility with ferrite deteriorates and the fatigue limit ratio can be improved.

  In this specification, “non-tempered steel for cold forging” is a non-tempered hot-rolled material before cold forging after hot rolling, and further after the cold forging. This means that no quality treatment is required.

  In the present specification, “high strength” means that the tensile strength after cold working (specifically, wire drawing) is about 1000 MPa or more and less than 1300 MPa. The cold working includes drawing, cold rolling, forward extrusion, upsetting and the like in addition to the wire drawing described above.

  Further, in this specification, “the fatigue limit ratio is high” means that the fatigue limit ratio represented by the ratio of fatigue strength / tensile strength after cold working is generally 0.30 or more (preferably 0.00. 33 or more).

  Hereinafter, the cold forged non-heat treated steel of the present invention will be described in detail.

  First, the nano-sized ultrafine precipitate that most characterizes the present invention will be described.

  The non-tempered steel of the present invention contains a precipitate containing at least one of Nb and Ti in ferrite. The above precipitate may further contain V. V is a selected component. As described above, Nb, Ti, and V are all useful as precipitation strengthening elements, and desired characteristics are ensured by existing as an MX type compound in ferrite.

  Hereinafter, the V-containing ultrafine precipitate and the V-free ultrafine precipitate will be described, respectively.

(V-containing ultrafine precipitate)
First, the V-containing ultrafine precipitate will be described. V-containing ultrafine precipitates can be represented as (Ti, Nb, V) (C, N) precipitates, specifically, for example, Nb carbide, Nb nitride, Nb carbonitride, Ti carbide, Ti nitride, Ti carbonitride, Nb-Ti composite carbide, Nb-Ti composite nitride, Nb-Ti composite carbonitride, Nb-V carbide, Nb-V nitride, Nb-V carbonitride, Ti- Examples thereof include V carbide, Ti-V nitride, Ti-V carbonitride, Nb-Ti-V composite carbide, Nb-Ti-V composite nitride, and Nb-Ti-V composite carbonitride. All of these containing at least one kind are included in the “V-containing ultrafine precipitate” in the present specification. In addition, the presence form of the precipitate in the present invention is not particularly limited. For example, the Nb carbide may be present alone, or other precipitate (for example, Al nitride) may be present in the Nb carbide. Or the like) may be present in a combined state. Moreover, when it further contains Cr or Mo, it may exist as a carbide or carbonitride containing Cr or Mo.

For V-containing ultra-fine precipitates, in the ferrite, the following equation (1) above the value of MP represented the following particle size 15nm satisfying MP ≧ 0.05 ultrafine precipitates 50 / [mu] m ² or more Contains. By a measuring method described later in detail, when measuring the number of ultra-fine precipitates and grain size ≦ 15 nm in MP ≧ 0.05 present in the ferrite, the number of ultra-fine precipitates that satisfies all of these requirements It was found that not only high strength but also a high fatigue limit ratio can be obtained when the number is 50 / μm ² or more (see Examples described later). According to the present invention, the strength can be improved by depositing a large number of nano-sized ultrafine precipitates as described above, and the fatigue limit ratio can be further increased by controlling the MP value as described above. Can be enhanced. In addition to increasing the strength, in order to ensure a high fatigue limit ratio, it is not sufficient to precipitate a large number of nano-sized ultrafine precipitates, and the MP value needs to satisfy the above range, Experiments have confirmed that when a large number of nano-sized ultrafine precipitates whose MP values do not satisfy the above range are precipitated, a high fatigue limit ratio cannot be obtained even if high strength can be ensured.
MP = [{[Nb] / 93} + {[Ti] / 48}] / [{[V] / 51} + {[Nb] / 93} + {[Ti] / 48}] (1 )
In the formula, [] means the content (% by mass) of the element contained in the precipitate.

  MP represented by the above formula (1) is an abbreviation for Metallic elements (Note: M of MX type compound) Precipitate. As shown in the above formula, all elements of Ti, Nb, and V are expressed in atomic ratios, and in the formula, the denominator, that is, [{[V] / 51} + {[Nb] / 93} + {[Ti] / 48}] means the sum of atomic ratios of M (here, Ti, Nb, V) in the ultrafine precipitate (MX type compound) present in the ferrite, and is a molecule, [{[Nb] / 93} + {[Ti] / 48}] means the total atomic ratio of Ti and Nb. That is, the MP value is an atomic ratio in which Ti and Nb effective for realizing high strength and a high fatigue limit ratio are contained in the V-containing ultrafine precipitate containing V as an essential component. As a requirement for effectively exhibiting the above-described effects of Ti and Nb, MP ≧ 0.05 was defined. This is because, in the V-containing ultrafine precipitate, if the atomic ratio of Ti and Nb in the total ultrafine precipitate is 5% or more (MP ≧ 0.05), the growth (diffusion) of crystal grains is inhibited, This is based on empirical knowledge that desired characteristics can be secured.

  The larger the MP value, that is, the greater the atomic ratio of Nb and / or Ti in the V-containing ultrafine precipitate, the more the above-described effect is exhibited. Although the preferable value of MP varies depending on the amount of the above-mentioned components contained in the steel, it is generally preferably 0.10 or more, and more preferably 0.15 or more.

  Next, a method for measuring the number of ultrafine precipitates satisfying MP ≧ 0.05 will be described. In the present invention, a precipitate is identified by observation with a transmission electron microscope (TEM), and elemental analysis in the precipitate is performed by an energy dispersive X-ray analyzer (Energy Dispersive X-ray, EDX). The value is being measured. As will be described in detail below, in the present invention, precipitates are observed and analyzed by changing the magnification of TEM observation (initially 100,000 times → next 150,000 times). This is done for the purpose of improving the measurement efficiency in consideration of the time required for the measurement.

(Preparation of extraction replica for TEM observation)
First, using a sample after cold forging, an extraction replica for TEM observation is prepared from a D / 4 (D is a diameter or thickness) position. Specifically, when the sample is cylindrical, an extraction replica is produced from the D / 4 position (D: diameter) at the center in the height direction. When the sample is square or plate-shaped, the longitudinal direction and the width direction From the D / 4 position (D: thickness) in the center of the sample, an extraction replica is produced. The extraction replica was performed according to the following procedures (a) to (e).
(A) Electrolytic corrosion of a sample is performed using an electrolytic solution containing 10% acetylacetone, 80% methanol, and 10% tetramethylammonium chloride.
(B) Deposit carbon on the surface of the sample.
(C) A grid-like cut of 2 to 3 mm square is made on the sample plane.
(D) Electrolytic corrosion is performed with the above electrolytic solution, and carbon is floated.
(E) Store in alcohol and use for observation.

(TEM observation and EDX analysis at a magnification of 100,000 times)
Then, the treated sample in the extraction replica method, and TEM observation in magnification of 100,000, optionally, after the particle size was measured more than 20 precipitates 15 nm, Ti contained in the precipitate, Nb, EDX analysis of V. In the examples described later, a transmission electron microscope “H-800” manufactured by Hitachi, Ltd. is used as the TEM. The particle size is based on the following equation, it is obtained by converting the equivalent circle diameter.

  Next, the MP value of each precipitate is calculated based on the above-described formula (1). As a result, the number of precipitates satisfying MP ≧ 0.05 in the above-mentioned precipitates (total of 20) identified by TEM observation (100,000 times) can be obtained, so that MP ≧ 0. The ratio of precipitates satisfying 05 is calculated (this ratio is “X”).

(TEM observation and photography at a magnification of 150,000)
Then, for any area, taking the 20 fields of view at a magnification 150,000 × photograph (total area 0.75 .mu.m ² 20 fields of) at TEM, calculates the number of the following precipitate particle size 15 nm. The number of precipitates thus obtained (total number for 20 fields of view) is converted to the number per 1 μm ² (this number is referred to as “Y”).

(Calculation of the number of ultrafine precipitates satisfying MP ≧ 0.05)
Finally, the thus obtained Y (number / [mu] m ² of particle size 15nm following deposit), (the total deposit, the ratio of the precipitates which satisfies the MP ≧ 0.05) X as described above the By multiplying, the number of ultrafine precipitates satisfying MP ≧ 0.05 was obtained.

In the present invention, further, in the ferrite, a V-containing ultrafine precipitates satisfies the MP value ≧ 0.05, that following the ultra-fine precipitates particle size 15nm is present 50 / [mu] m ² or more is necessary. As described above, since a large number of nano-sized ultrafine precipitates are present in ferrite, it is possible to ensure a high fatigue limit ratio while effectively exerting the effect of improving the strength by the precipitation strengthening element (Examples described later). See).

The size of the V-containing ultrafine precipitates, the particle size may be at 15nm or less. The particle size may smaller, generally, it is preferably 10 nm.

The number of the V-containing ultrafine precipitates may be 50 / μm ² or more, and the larger the number, the more effective the desired characteristics. In general, the number of ultrafine precipitates is preferably 100 / μm ² or more, and more preferably 200 / μm ² or more.

  In the present invention, V-containing ultrafine precipitates observed in the ferrite structure are measured. The non-heat treated steel of the present invention is composed of a ferrite-pearlite structure mainly composed of a ferrite structure. However, the non-refined steel is limited to the ferrite structure if the V-containing ultrafine precipitate is not present in the ferrite structure. This is because the above characteristics are not effectively exhibited.

(V-free ultrafine precipitate)
Next, the V-free ultrafine precipitate will be described. V-free ultrafine precipitates can be represented as (Ti, Nb) (C, N) precipitates, specifically, for example, Nb carbide, Nb nitride, Nb carbonitride, Ti carbide, Ti Examples thereof include nitride, Ti carbonitride, Nb—Ti composite carbide, Nb—Ti composite nitride, and Nb—Ti composite carbonitride. All of these containing at least one kind are included in the “V-free ultrafine precipitate” in the present specification. In addition, the presence form of the precipitate in the present invention is not particularly limited. For example, the Nb carbide may be present alone, or other precipitate (for example, Al nitride) may be present in the Nb carbide. Or the like) may be present in a combined state. Moreover, when it further contains Cr or Mo, it may exist as a carbide or carbonitride containing Cr or Mo.

  The details of the V-free ultrafine precipitate are the same as those of the V-containing ultrafine precipitate described above except for the requirement of MP ≧ 0.05. For the size, number, etc., the above contents may be referred to. As described above, the MP value is a useful requirement in the V-containing ultrafine precipitate, and when V is not contained, the MP value is 1 by calculation, and therefore MP ≧ 0.05 is necessarily satisfied. It is.

  The deposits characterizing the present invention have been described above.

  Next, the chemical composition of steel will be described.

C: 0.10 to 0.50%
C is an element that forms pearlite and combines with Ti, Nb, and Vi to form an MX type compound to strengthen ferrite and contribute to high strength. In order to ensure a predetermined strength, the C amount is 0.10% or more. However, if the amount of C becomes excessive, the pearlite fraction increases too much and the precipitation strengthening amount due to ferrite decreases, and the fatigue limit ratio decreases, so the upper limit is made 0.50%. The C content is preferably 0.20% or more and 0.45% or less, and more preferably 0.26% or more and 0.40% or less.

Si: 0.05-2%
In addition to acting as a deoxidizer, Si is an element that strengthens ferrite and pearlite by solid solution strengthening and contributes to improvement of the fatigue limit ratio. In order to effectively exhibit such an action, the Si amount is set to 0.05% or more. However, if the amount of Si exceeds 2%, an overcooled structure such as bainite is generated at the time of cooling, and instead the fatigue limit ratio is lowered, so the upper limit is made 2%. The amount of Si is preferably 0.1% or more and 1.5% or less, and more preferably 0.4% or more and 1.0% or less.

Mn: 0.3 to 3%
Mn is an element that contributes to improvement of strength, fatigue limit ratio, and toughness by refining ferrite by lowering the transformation temperature. If the amount of Mn is less than 0.3%, the hardenability improving effect is small and the above effect is not effectively exhibited, so the lower limit is made 0.3%. However, if the amount of Mn becomes excessive, an overcooled structure such as bainite is generated during cooling, and the fatigue limit ratio is lowered. Therefore, the upper limit is made 3%. The lower limit of the amount of Mn is preferably 0.5%, and more preferably 0.75% or more. Further, the upper limit of the amount of Mn is preferably 2.5% or less, more preferably 2.0% or less, and further preferably 1.8% or less.

Al: 0.005 to 0.1%
Al is useful as a deoxidizer, and for that purpose, 0.005% or more is added. However, when the amount of Al becomes excessive, a lot of inclusions are generated, and the fatigue characteristics and further the toughness are lowered, so the upper limit was made 0.1%. The amount of Al is preferably 0.01% or more and 0.07% or less, and more preferably 0.015% or more and 0.05% or less.

P: 0.05% or less, O: 0.003% or less Since P and O are elements that deteriorate toughness, it is preferable to reduce them as much as possible. Here, P is set to 0.05% and O is set to 0.003% as the upper limit of the amount that does not significantly deteriorate the toughness without reduction by special refining treatment. The smaller the number of these elements, the better. P is preferably 0.03% or less, more preferably 0.02% or less, and further preferably 0.015% or less. Further, O is preferably 0.002% or less, and more preferably 0.0015% or less.

S: 0.5% or less S is an element that forms MnS and contributes to improvement of machinability. Therefore, when used for applications requiring machinability, the amount of S is preferably 0.1% or more. However, if the amount of S becomes excessive, the toughness deteriorates, so the upper limit is made 0.5%. The amount of S is preferably 0.2% or less. When toughness is required, the S amount is more preferably 0.1% or less.

N: 0.02% or less (excluding 0%)
N is an element that combines with Ti, Nb, and V (and Zr, Ta, and Hf added as necessary) to form an MX type compound and contributes to an improvement in tensile strength and fatigue limit ratio. . In order to effectively exhibit such an action, the N content is preferably 0.0030% or more. However, if added excessively, a coarse MX-type compound is generated and the fatigue characteristics deteriorate, so the upper limit is made 0.02%. In general, the N amount is preferably 0.01% or less, more preferably 0.007% or less, and still more preferably 0.0055% or less. In particular, when adding Ti as a precipitation strengthening element, it is preferable to appropriately control the amount of N according to the amount of Ti so that a desired ultrafine precipitate can be obtained, as will be described later.

For Nb, Ti, and V, these elements are elements that combine with C and N to generate MX type compounds and contribute to high strength. Of these, Nb and Ti, unlike V, have a very slow precipitation rate, so the addition of Nb and Ti significantly suppresses the growth of MX-type compounds in ferrite and produces many desired ultrafine precipitates. To come. Therefore, in the present invention, at least Nb and / or Ti are contained, and V is a selective element. From the viewpoint of further improving the strength and fatigue limit ratio, V is preferably contained, and most preferably Nb, Ti, and V are all contained. Hereinafter, each component will be described.

In the case where Nb is contained, Nb: 0.2% or less In order to effectively exhibit the above-described action due to the addition of Nb, the amount of Nb is preferably 0.022% or more, and preferably 0.04% or more. More preferred. However, if added excessively, the amount of undissolved substances increases without heating during heating, and coarse compounds are likely to precipitate, and the coarse compounds become the starting point of fatigue, reducing the fatigue limit ratio. Is 0.2%. The upper limit of the amount of Nb is preferably 0.1%, and more preferably 0.08% or less.

When Ti is contained, Ti: 0.20% or less, N: less than 0.010%, and Ti / N ≧ 3.4
In order to effectively exhibit the above-described action by addition of Ti, it is necessary to appropriately control not only the amount of Ti but also the amount of N and the atomic ratio of Ti and N as described above. Ti has a higher reactivity with N than C. When N is 0.01% or more, coarse TiN is formed during solidification, and the amount of Ti contained in the ultrafine precipitate is reduced. It is. Preferably, Ti: 0.02% or more and 0.1% or less, N: 0.0070% or less, Ti / N ≧ 4.0, and more preferably, Ti: 0.04% or more and 0.08% or less. , N: 0.0055% or less, and Ti / N ≧ 5.0.

  Note that Nb, unlike Ti, hardly exhibits the above-described reactivity difference, and therefore when Nb is added, it is not particularly necessary to appropriately control the amount of N and the atomic ratio of Nb to N.

When V is included, V: 0.6% or less In order to effectively exhibit the above-described action due to the addition of V, the amount of V is preferably 0.15% or more, and 0.2% or more. Is more preferable. However, if added excessively, the amount of undissolved substances increases without heating during heating, and coarse compounds are likely to precipitate, and the coarse compounds become the starting point of fatigue, reducing the fatigue limit ratio. Is 0.6%. The upper limit of the amount of V is preferably 0.5%, and more preferably 0.4% or less.

  The non-heat treated steel for cold forging of the present invention contains the above components, and the balance is Fe and inevitable impurities.

  Furthermore, the non-heat treated steel of the present invention may contain the following components for the purpose of improving other properties.

Mo: 1% or less, and / or B: 0.015% or less Both Mo and B contribute to the improvement of strength and fatigue limit ratio by reducing the transformation temperature to refine the ferrite and to improve the toughness ratio. It is an element that contributes to improvement. In order to effectively exert such an action, it is preferable to add Mo 0.1% or more and B 0.0003% or more, and to add Mo 0.2% or more and B 0.0006% or more. Is more preferable. However, if added excessively, a supercooled structure such as bainite is generated at the time of cooling, and the fatigue limit ratio is lowered. Therefore, the upper limit is preferably set to Mo: 1% and B: 0.015%. More preferable upper limits are Mo: 0.75% and B: 0.005%, and still more preferable upper limits are Mo: 0.5% and B: 0.0035%. These elements may be added alone or in combination of two or more.

At least one of Ni, Cu, and Cr selected from the group consisting of Ni: 2% or less, Cu: 2% or less, and Cr: 3% or less all have a strength-improving action, and further improve toughness. It is an element that also contributes. In order to exhibit such an action effectively, it is preferable to set the lower limit to Ni: 0.2%, Cu: 0.2%, and Cr: 0.3%. More preferable lower limits are Ni: 0.5%, Cu: 0.5%, Cr: 0.5%. However, since the said effect | action will fall if it adds excessively, it is preferable to make an upper limit into Ni: 2%, Cu: 2%, Cr: 3%. More preferable upper limit is Ni: 1.5%, Cu: 1.5%, Cr: 2%, and further preferable upper limit is Ni: 1.2%, Cu: 1.2%, Cr: 1.5. %. These elements may be added alone or in combination of two or more.

At least one of Ca, Mg, and REM selected from the group consisting of Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.02% or less forms a sulfide, and MnS It is an element that contributes to improvement of toughness by preventing the elongation of. In order to effectively exhibit such an action, it is preferable that the lower limit of the element is Ca: 0.0005%, Mg: 0.0002%, REM: 0.0005%. However, if added in excess, the toughness is rather lowered, so the upper limit is preferably made Ca: 0.005%, Mg: 0.005%, and REM: 0.02%. More preferable upper limits are Ca: 0.003%, Mg: 0.003%, and REM: 0.01%. These elements may be added alone or in combination of two or more.

  In this specification, REM means an element group obtained by adding Sc (scandium) and Y (yttrium) to a lanthanoid element (a total of 15 elements from La to Ln in the periodic table). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable to contain La and / or Ce. Moreover, the form of REM added to molten steel is not specifically limited, For example, as REM, pure La, pure Ce, pure Y, etc., pure Ca, pure Zr, pure Ti, and also Fe-Si-La alloy, Fe -Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Ni-Ca alloy, etc. may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, the amount of Ca preferably satisfies the above range. In the examples described later, misch metal is added.

At least one of Zr, Ta, and Hf selected from the group consisting of Zr: 0.1% or less, Ta: 0.1% or less, and Hf: 0.1% or less is bonded to N and stable. It is an element that forms a simple nitride, promotes the formation of ultrafine precipitates by suppressing the growth of the austenite grain size during heating, and particularly contributes to toughness improvement. In order to effectively exhibit such an action, it is preferable that the lower limit of the element is Zr: 0.005%, Ta: 0.005%, and Hf: 0.005%. However, if added excessively, coarse nitrides are formed and fatigue characteristics are lowered, so the upper limit of each element is preferably 0.1%. A more preferable upper limit is 0.05% for any element, and a further preferable upper limit is 0.025% for any element.

  In the above, the component in steel of this invention was demonstrated.

  Next, referring to FIG. 1, an embodiment of a method for producing a high-strength cold forging non-heat treated steel according to the present invention and a method for producing a forged part using the cold forging steel will be described. To do. FIG. 1 schematically shows an outline of a casting process, a block rolling process, a hot rolling process, and a cold forging process in the order of the manufacturing process. Among these, the processes characterizing the present invention are a block rolling process and a hot rolling process. In FIG. 1, a conventional representative heat pattern is indicated by dotted lines in the block rolling process and the hot rolling process for reference.

  The production method of the present invention, as will be described in detail later, is heated and held at 1250 ° C. for 1 hour or more in the block rolling process before cold forging, and is heated and held at 1150 ° C. or more for 1 hour or more in the subsequent hot rolling process. This includes high-temperature and long-time heat-holding treatment, whereby desired characteristics are effectively exhibited. Here, the heating temperature at the time of hot rolling is lower than the heating temperature at the time of ingot rolling, in the present invention, because cold forging is performed after hot rolling, from the viewpoint of suppressing wrinkles, etc. This is based on the reason that the hot heating temperature should be as low as possible.

  First, steel satisfying the above-described component composition is melted and cast. Casting conditions are not particularly limited, and a generally used method may be employed. This is because, as will be described in detail later, in the present invention, desired ultrafine precipitates and solid solutions are ensured by appropriately controlling the split rolling conditions and / or hot rolling conditions. However, it is also effective to appropriately control the casting conditions for the purpose of producing finer and more precipitates. For example, the average cooling rate during casting (the average cooling rate at the center of the cast product) is as fast as possible. (Generally, 200 ° C./hr or more) is preferable.

  After casting, ingot rolling is performed. Partial rolling may include soaking before the partial rolling. In the case of performing soaking, it is preferable to carry out under the same conditions as in the bulk rolling process.

  The block rolling process is the most important process for securing desired ultrafine precipitates and increasing the strength and fatigue limit ratio. Specifically, as described in detail below, the heating conditions at the time of the ingot rolling are controlled so as to be heated and maintained for as long a time as possible.

  First, regarding the heating conditions at the time of ingot rolling, in the present invention, a temperature of 1250 ° C. or higher (T1 in FIG. 1) is maintained for 1 hour or longer (t1 in FIG. 1). The heating time (t1) means a holding time when the heating temperature (T1) is reached. The heating pattern of the present invention has a higher temperature and a longer holding time than a conventional representative heating pattern (dotted line portion in FIG. 1). Thus, by heating and holding at a high temperature for a long time, the solid solution of precipitation strengthening elements Nb, Ti, and V is promoted. In particular, the solid solution of Nb and Ti that is difficult to dissolve in comparison with V is further promoted. Therefore, the penetration of Nb and Ti into the precipitate that precipitates in the subsequent cooling process becomes easy, and as a result, the MP value increases. In the above process, it is particularly important to appropriately control the heating time (t1). Even if the heating temperature (T1) is increased to 1250 ° C. or higher, the heating time (t1) is less than 1 hour. The desired properties cannot be obtained (see FIG. 1 described later). The higher the heating temperature (T1), the better the holding time (t1). For example, the heating temperature (T1) is preferably 1275 ° C. or higher, and more preferably 1300 ° C. or higher. The heating time (t1) is preferably 2 hours or longer. Although the upper limit of the heating temperature is not particularly limited, it is preferably about 1325 ° C. because of the relationship with the equipment.

  The rolling temperature after heating (T2 in FIG. 1) is not particularly limited, and is preferably 950 ° C. or higher, for example.

  Hot rolling is performed after the block rolling. In order to ensure the desired characteristics, it is preferable to promote the solid solution of Nb, Ti, V during hot rolling as well as during partial rolling. In this embodiment, heating during hot rolling is performed. It is preferable to maintain the conditions at 1125 ° C. or higher (T3 in FIG. 1) for 1 hour or longer (t3 in FIG. 1). The heating pattern of the present invention has a higher temperature and a longer holding time than a conventional representative heating pattern (dotted line portion in FIG. 1). Thus, by heating and holding at a high temperature for a long time, the solid solution of precipitation strengthening elements Nb, Ti, and V is promoted. In particular, the solid solution of Nb and Ti that is difficult to dissolve in comparison with V is further promoted. Therefore, the penetration of Nb and Ti into the precipitate that precipitates in the subsequent cooling process becomes easy, and as a result, the MP value increases. The higher the heating temperature (T3) and the longer the holding time (t3), the better, and this further promotes solid solution of the above elements. For example, the heating temperature (T3) is preferably 1150 ° C. or higher, and more preferably 1175 ° C. or higher. The heating time (t1) is preferably 2 hours or longer. As described above, the upper limit of the heating temperature is preferably lower than the heating temperature (T1) at the time of the block rolling, and is preferably about 1250 ° C.

  Moreover, the final temperature (finish rolling temperature, T4) at the time of hot rolling is controlled to 950 ° C. or higher. This suppresses the formation of precipitates in austenite that does not contribute to precipitation strengthening. The final temperature (T4) is preferably 1000 ° C. or higher.

  The cooling conditions after the hot rolling are as follows: the range up to 650 ° C. after hot rolling is cooled (rapidly cooled) at an average cooling rate of about 120 ° C./min or more (CR1 in FIG. 1), and then is 650 to 500 ° C. The range is cooled (slowly cooled) at an average cooling rate of about 60 ° C./min or less (CR2 in FIG. 1). In this way, precipitation in the austenite region is suppressed by rapidly cooling the temperature range until the ferrite transformation, and then the ultrafine precipitates in the ferrite are gradually cooled until the ferrite transformation is completed. Since the generation increases, finally, desired characteristics are effectively exhibited. The larger CR1 and the smaller CR2 are better, but considering the balance with production efficiency and the like, it is generally preferable that CR1: 150 to 600 ° C / min, CR2: 10 to 40 ° C / min.

  After hot rolling, cold forging is performed, and a forged part is formed by machining such as cutting into a desired part shape.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and is appropriately within a range that can meet the purpose described above and below. It is also possible to carry out with modification, and they are all included in the technical scope of the present invention.

(Production method)
A to V steels shown in Table 1 (remainder: iron and inevitable impurities) were melted and cast using a small vacuum melting furnace. Next, partial rolling was performed under the conditions shown in Table 2 to obtain a steel ingot having a cross section of 155 mm × 155 mm. Subsequently, it hot-rolled on the conditions shown in Table 2, and obtained the steel piece of (phi) 12mm.

  Using the above steel pieces, wire drawing (total 6 passes) was performed at a wire drawing area reduction rate of 20% to obtain a wire drawn material having a diameter of 10.7 mm.

(Characteristic evaluation)
Using the thus obtained drawing material (Nanba1～29 in Table 2), based on the method described above, the number of particle size 15nm following ultrafine precipitates satisfying MP ≧ 0.05 Calculated. Furthermore, the tensile strength and fatigue limit were measured as follows.

  The tensile strength was measured according to JIS Z 2241 using a JIS No. 4 test piece. Here, those having a tensile strength of 1000 MPa or more and less than 1300 MPa were evaluated as ◯ (passed), and those having a tensile strength of less than 900 MPa were evaluated as x (failed). Those having a tensile strength of less than 1300 MPa are considered indirectly excellent in cold forgeability.

  The fatigue strength was measured according to JIS Z 2274 after electrolytic polishing was performed to remove the influence of the surface processed layer using a notched Ono-type rotary bending fatigue test piece with a stress concentration factor α = 1.9. The fatigue test was performed by the method and measured. The fatigue limit ratio was calculated by the ratio of fatigue strength / tensile strength. Here, those having a fatigue limit ratio of less than 0.30 were evaluated as x (failed), those having 0.30 or more were evaluated as ◯, and those having a fatigue limit ratio of 0.33 or more were evaluated as ◎ (◯ and ◎ passed).

These results are summarized in Table 2. Table 2 also shows the steel types used (the steel types in Table 1). Moreover, the column of comprehensive evaluation was provided in Table 2, and it evaluated comprehensively with the following reference | standard.
×: At least one of strength and fatigue limit ratio is ×
○: Both tensile strength and fatigue limit ratio are ○
◎: Tensile strength ○, fatigue limit ratio ◎

  Among the steel types listed in Table 1, steel types A, C, and F to S are steels whose chemical components satisfy the scope of the present invention, and steel types B, D, E, and T to V in Table 1 are described in detail later. As noted, any chemical component is a steel that does not satisfy the scope of the present invention. Further, among the above steel types that satisfy the scope of the present invention, when focusing on the precipitation strengthening elements, the steel types A, G to I, and K to S contain Nb and V (without Ti), the steel type C is An example containing Ti and V (no Nb), steel type F is an example containing all of Ti, Nb and V, and steel type J is an example containing only Ti (Nb, no V).

  From Table 2, it can be considered as follows.

  No. in Table 2 Nos. 1 to 8 are examples in which the steel type A satisfying the requirements of the present invention is used and the forging conditions and the cooling rate after forging are changed.

  Of these, No. 3 to 5 are all examples of the present invention produced under the conditions specified in the present invention, and many desired ultrafine precipitates satisfying MP ≧ 0.05 are generated. Both are excellent.

  In contrast, no. No. 1 is an example where the heating time (t1) at the time of ingot rolling and the heating time (t3) at the time of hot rolling are short. No. 2 is an example in which the heating time (T3) during hot rolling is low. No. 6 is an example in which the final temperature (T4) during hot rolling is low. No. 7 is an example in which the average cooling rate (CR1) after hot rolling is slow. No. 8 is an example in which the average cooling rate (CR2) after hot rolling is fast. In either case, a desired ultrafine precipitate satisfying MP ≧ 0.05 was not obtained, and the fatigue limit ratio was lowered.

  Next, no. Consider 9-29.

  No. Nos. 10 and 13 to 26 are examples of the present invention produced under the conditions specified in the present invention, and many desired ultrafine precipitates satisfying MP ≧ 0.05 are generated. Both ratios are excellent.

  In contrast, no. No. 9 is an example using steel type B that does not contain both Nb and Ti. 11 is an example using a steel type D which is a Ti-added steel and has a low Ti / N ratio. In either case, ultrafine precipitates satisfying MP ≧ 0.05 were not obtained at all, and the fatigue limit ratio was lowered. .

  No. No. 12 is an example using a steel type E which is a Ti-added steel and has a large amount of N. Since the amount of N is large, coarse TiN is generated and becomes the starting point of fracture, and the fatigue limit ratio becomes low.

  No. No. 27 is an example using steel type T with a large amount of Ti. 28 is an example using steel type U with a large amount of C, and the fatigue limit ratio was lowered.

  No. 29 is an example using the steel type V with a small amount of C, and the strength decreased. In addition, No. Since 29 had low strength, fatigue strength and fatigue limit ratio were not measured ("-" in Table 2).

FIG. 1 is a schematic process diagram schematically showing one embodiment of the production method of the present invention.

Claims (5)

(1) Components in steel are
C: 0.10 to 0.50% (meaning mass%, the same shall apply hereinafter),
Si: 0.05-2%
Mn: 0.3-3%,
Al: 0.005 to 0.1%,
P: 0.05% or less (excluding 0%),
S: 0.5% or less (excluding 0%),
O: 0.003% or less (excluding 0%),
N: 0.02% or less (excluding 0%),
Among Nb, Ti, and V, at least Nb and / or Ti may be included, and V may be included.
When Nb is included, Nb is 0.2% or less (not including 0%),
When Ti is contained, Ti: 0.20% or less (excluding 0%), N: less than 0.010% (excluding 0%), and Ti / N ≧ 3.4 are all satisfied,
When V is included, V: 0.6% or less (not including 0%),
Balance: Fe and inevitable impurities are satisfied, and
(2) The ferrite contains Nb and / or Ti-containing precipitates (which may further contain V), and the MP value represented by the following formula (1) satisfies MP ≧ 0.05. the particle size 15nm or less of the precipitates, when measured according to the following procedure, the precipitates 50 / [mu] m ² or more, characterized in that it contains fatigue ratio excellent high strength cold forging microalloyed steel .
MP = [{[Nb] / 93} + {[Ti] / 48}] / [{[V] / 51} + {[Nb] / 93} + {[Ti] / 48}] (1 )
In the formula, [] means the content (% by mass) of the element contained in the precipitate.
(A) An extraction replica for TEM observation is prepared using an electrolytic solution containing 10% acetylacetone, 80% methanol, and 10% tetramethylammonium chloride.
(A) Next, the sample treated by the extraction replica method is observed with a TEM at a magnification of 100,000 times, and optionally 20 precipitates having a particle size of 15 nm or less are measured, and then Ti contained in the precipitates is measured. , Nb and V are analyzed by EDX.
(C) Next, based on the above-described formula (1), the MP value of each precipitate is calculated. As a result, the number of precipitates satisfying MP ≧ 0.05 in the above-mentioned precipitates (total of 20) identified by TEM observation (100,000 times) can be obtained, so that MP ≧ 0. The ratio of precipitates satisfying 05 is calculated (this ratio is “X”).
(D) Next, for a given area, take a picture of 20 fields of view at a magnification of 150,000 with a TEM (total area of 0.75 μm ^{2 of} 20 fields of view ) and calculate the number of precipitates with a particle size of 15 nm or less. To do. The number of precipitates thus obtained (total number for 20 fields of view) is converted to the number per 1 μm ² (this number is referred to as “Y”).
(E) Finally, Y (the number of precipitates having a particle size of 15 nm or less / μm ² ) thus obtained is added to the above-described X (all precipitates satisfying MP ≧ 0.05). By multiplying the ratio, the number of ultrafine precipitates satisfying MP ≧ 0.05 is obtained.   Furthermore, Mo: 1% or less, and / or B: 0.015% or less, The high-strength cold forging non-heat-treated steel for cold forging.   The non-tempered steel for high-strength cold forging according to claim 1 or 2, further comprising at least one selected from the group consisting of Ni: 2% or less, Cu: 2% or less, and Cr: 3% or less. .   The high content according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.02% or less. Non-tempered steel for cold forging.   The high content according to any one of claims 1 to 4, further comprising at least one selected from the group consisting of Zr: 0.1% or less, Ta: 0.1% or less, and Hf: 0.1% or less. Non-tempered steel for cold forging.

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