JP2008127595A - High strength hot forged non-heat treated steel having excellent fatigue limit ratio and toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot forged non-heat treated steel having excellent fatigue limit ratio and toughness even as-hot forged. <P>SOLUTION: (1) The high strength hot forged non-heat treated steel has a composition comprising 0.10 to 0.50% C, 0.05 to 2% Si, 0.3 to 3% Mn, 0.005 to 0.1% Al, ≤0.05% P, ≤0.5% S, ≤0.003% O, ≤0.02% N, at least Nb and/or Ti among Nb, Ti and V, and in which V may be comprised, where, in the case Nb is comprised, ≤0.2% Nb is satisfied, in the case Ti is comprised, ≤0.20% Ti, <0.010% N and Ti/N≥3.4 are all satisfied, and, in the case V is comprised, ≤0.6% V is satisfied, and the balance Fe with inevitable impurities. (2) Prescribed MP1 and MP2 obtained in accordance with prescribed procedures satisfy MP1≥5×10<SP>-4</SP>and MP2/MP1≥0.05. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel for hot forging used as a raw material for parts such as transportation equipment such as automobiles, construction machines, and other industrial machines, and in particular, without performing heat treatment after hot forging (non-tempered). The present invention relates to a high-strength non-tempered hot forging steel having a high fatigue limit ratio and high toughness. The high-strength non-tempered hot forging steel 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 machines, and other various industrial machines, especially those that require high strength, are usually hot-rolled, hot forged and then quenched and tempered. It is manufactured by applying heat treatment (so-called tempering treatment) to give necessary mechanical properties. However, from the viewpoints of cost reduction and manufacturing efficiency, even with hot forging (non-tempering), the desired mechanical properties, especially fatigue limit ratio and strength, and excellent toughness are also obtained. There is an urgent need to provide hot forging steel (hot rolled material).

  From the viewpoint of improving the strength, for example, it is conceivable to increase the C content, but there is a problem that the machinability is lowered.

Therefore, a technique for improving fatigue strength [particularly fatigue limit ratio (= fatigue strength / tensile strength)] by adding precipitation strengthening elements such as Ti, Nb, and V to steel has been proposed (for example, patents). Literature 1 and Patent literature 2). However, in the method of Patent Document 1, a high fatigue limit ratio can be achieved, but the tensile strength is as low as about 850 MPa. On the other hand, in the method of Patent Document 2, a high tensile strength can be secured, but the fatigue limit ratio decreases. is there.
Japanese Patent Laid-Open No. 62-167855 Japanese Patent No. 3485805

  The present invention has been made in view of the above circumstances, and its purpose is high strength that is excellent in both tensile strength and fatigue limit ratio and is improved in toughness even in hot forging (non-tempered). It is to provide non-tempered hot forging steel.

The high strength non-tempered hot forging steel excellent in fatigue limit ratio and toughness 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% (Meaning of mass%, the same shall apply hereinafter), Si: 0.05-2%, Mn: 0.3-3%, Al: 0.005-0.1%, P: 0.05% or less (0% S: 0.5% or less (not including 0%), O: 0.003% or less (not including 0%), N: 0.02% or less (not including 0%), Among Nb, Ti, and V, at least Nb and / or Ti are included, and V may be included. When Nb is included, Nb is 0.2% or less (not including 0%), and Ti If included, Ti: 0.20% or less (not including 0%), N: less than 0.010% (not including 0%), and all satisfying Ti / N ≧ 3.4 And V: 0.6% or less (excluding 0%), the balance: satisfying Fe and inevitable impurities, and (2) following the procedures (A) to (D) below The obtained MP1 and MP2 have a gist where MP1 ≧ 5 × 10 ⁻⁴ and MP2 / MP1 ≧ 0.05 are satisfied.
(A) Using a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride as an electrolyte, a sample after hot rolling is extracted under a current of 200 A / m ² or less.
(B) The sample after extraction is filtered using a 0.2 μm filter, and the amount of each element of Nb and / or Ti (which may further contain V) in the extraction residue remaining on the filter is inductively coupled. Quantification is performed by plasma (Inductively Coupled Plasma, ICP) emission spectrometry.
(C) From the amount of each element of Nb and / or Ti (which may further contain V) contained in the steel, Nb and / or Ti (V is further contained in the extraction residue obtained by (B) above. Each element amount of Nb and / or Ti (which may further contain V) in the extract that has passed through a 0.2 μm mesh filter. [Nb], [Ti], and [V].
(D) MP1 and MP2 are calculated based on the following equation.
MP1 = {[V] / 51} + {[Nb] / 93} + {[Ti] / 48}
MP2 = {[Nb] / 93} + {[Ti] / 48}

  In a preferred embodiment, the high-strength non-tempered hot forging steel further contains Mo: 1% or less and / or B: 0.015% or less.

  In a preferred embodiment, the high-strength non-tempered hot forging steel 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 non-tempered hot forging 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 non-tempered hot forging 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, it is possible to provide a steel for non-tempered hot forging that is excellent in both tensile strength and fatigue limit ratio and is further improved in toughness even in hot forging (non-tempered). It was.

  The inventor of the present invention is a technique for improving fatigue strength by adding precipitation strengthening elements such as Ti, Nb, V, etc., wherein the fatigue limit ratio is further increased without increasing the tensile strength more than necessary, and the toughness is further improved. We have intensively studied for the purpose of provision. As a result, first, in order to increase the strength and fatigue limit ratio, in (a) a steel material (hot steel for forging) after hot rolling and before hot forging, Nb and / or Or it is effective to set Ti (MP1 which may contain V, which will be described later) to a predetermined value or more, (b) In particular, steel contains V in addition to Nb and / or Ti. In some cases, it is effective to increase the ratio (MP2 / MP1) of the amount of Nb and / or Ti (MP2 to be described later) (MP2 / MP1) contained in MP1 in addition to the requirement (a). And (b), during hot forging heating, V and Nb and / or Ti, which are harder to dissolve than V, are easily re-dissolved, so that the precipitation strengthening action by these elements is effectively exhibited, Ascertaining high fatigue limit ratio and strength . And in order to achieve these requirements, the high temperature and long time heat holding process which heat-holds at 1250 degreeC or more for 1 hour or more in at least any process at the time of a lump rolling process or hot rolling before forging. It was found that it is extremely important to apply Furthermore, in order to obtain high toughness in addition to improvement in strength and fatigue limit ratio, the heating conditions during hot forging may be controlled so that the austenite grains do not become coarse (generally, 1150 to 1250 ° C. at 1 The present invention has been completed.

  Hereinafter, the background to the present invention will be described. In the following, for convenience of explanation, an electrolytic extraction method described later → filtering with a 0.2 μm mesh filter → passing through the filter may be referred to as “filtered material”. As will be described in detail later, the “filter-passed product” is a nano-sized ultrafine precipitate (specifically, generally, an ultrafine precipitate having an average particle size of 30 nm or less), or a solid solution in the matrix. Solid solutions are also included. The “filter-passed product” contains Nb and / or Ti and may further contain V or may not contain V. In order to distinguish between the two, a substance containing V is particularly referred to as a “V-containing filter-passed product” or the like, and a substance not containing V is particularly referred to as a “V-free filter-passed product” or the like. Moreover, the ultrafine precipitate contained in the V-containing filter passage is called “V-containing ultrafine precipitate”, and the ultrafine precipitate contained in the V-free filter passage is “V-free ultrafine precipitate”. May be called.

  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 ferrite transformation during cooling after 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 addition amount of the precipitation strengthening element is increased in order to further enhance the precipitation strengthening action, MX type precipitates are precipitated in austenite, not in ferrite, during cooling after 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 desired strength. I was unable to achieve that. Based on the above findings, the present inventors have further studied. As a result, in order to suppress the precipitation in the austenite region and precipitate a large number of the precipitates in the ferrite, it is possible to appropriately control the heating conditions of the partial rolling and / or hot rolling before forging. It has been found that it is effective, and in particular, it is necessary to maintain the heating conditions at a higher temperature than before and for a long time.

  In addition, although a method of performing heating and holding for a long time in the forging process after hot rolling was also attempted, in this case, austenite grains become coarse, and as a result, the structure after transformation becomes coarse and toughness decreases. Was found by experiment. That is, in order to ensure excellent toughness, it is not preferable to set the heating conditions at the time of hot forging at high temperature and for a long time, and it is necessary to heat and maintain the conditions so that the austenite grains do not become coarse. I understood. Therefore, in the present invention, even if the heating temperature at the time of hot forging is set so low that the austenite grains do not become coarse, precipitation strengthening elements Nb, Ti, and V (particularly, Nb and Ti that are difficult to dissolve) are precipitated. In order to re-dissolve, the bulk rolling and / or the hot rolling before forging are kept at a high temperature for a long time. According to the present invention, the strength and fatigue limit ratio are increased by high temperature heating and holding in the partial rolling and / or hot rolling before forging, and the low temperature heat treatment during forging (“low temperature” means that austenite grains are coarse. The toughness is increased by the low temperature).

And about the hot rolled material (steel for forging) obtained in this way, when the composition of a filter passage thing is quantitatively analyzed by the method of postscript, (a) In this filter passage thing, many Nb and / or Ti (further V) is contained (ie, MP1 ≧ 5 × 10 ⁻⁴ ), (b) In particular, a filter-passing material containing V is contained in the V-containing filter-passing material (strictly speaking, In MP1, it was found that Nb and / or Ti contained in a certain amount or more (that is, MP2 / MP1 ≧ 0.05). That is, it is thought that the filter passing material in which MP1 and MP2 satisfy the above requirements greatly contributes to the improvement in strength and fatigue limit ratio. The reason why the above characteristics are improved by controlling the ratio of (MP2 / MP1) particularly in the V-containing filter passage material is unknown in detail, but a predetermined amount in the V-containing filter passage material (MP1) In the presence of Nb and / or Ti (MP2), (i) Nb and / or Ti have a slower diffusion rate than V, and therefore coarsening of the precipitate is suppressed, and nano-sized ultrafine precipitates and solid It is presumed that a large number of solutes can be secured and high strength can be achieved, and (ii) the compatibility with ferrite is deteriorated and the fatigue limit ratio can be improved.

  As described above, the present invention goes one step further from the conventional basic idea of “precipitating a large amount of fine precipitates to increase the strength and fatigue limit ratio”. For Nb and Ti, hot rolling is performed so that these elements are re-dissolved even at a low heating temperature during hot forging (“low heating temperature” means a temperature that is low enough that the austenite grains do not become coarse). In a later state, the strength and fatigue limit ratio were improved by using nano-sized ultrafine precipitates or solid solutions, which has a technical idea. The technical idea of the present invention is expressed by the ratio of MP1 and (MP2 / MP1) described above. Hereinafter, the ultrafine precipitate or solid solution represented by MP1 may be referred to as “ultrafine precipitate / solid solution”. Further, the ratio of (MP2 / MP1) may be referred to as a Q value.

  In the present specification, “non-tempered hot forging steel” is a hot-rolled material after hot rolling and before hot forging, and does not require tempering after hot forging. Means things.

  In the present specification, “high strength” means that the tensile strength after hot forging is about 900 MPa or more and less than 1300 MPa.

  Further, in this specification, “high fatigue limit ratio” means that the fatigue limit ratio expressed by the ratio of fatigue strength / tensile strength after hot forging is generally 0.30 or more (preferably 0 .33 or more).

In the present specification, “excellent toughness” means that the absorbed energy (vE ₂₀ ) at 20 ° C. is 50 J or more.

  Hereinafter, the high strength non-tempered hot forging steel of the present invention will be described in detail.

  First, MP1 and Q value (MP2 / MP1) that characterize the present invention will be described.

  “MP” in MP1 and MP2 is an abbreviation for Metallic elements (Note: M of MX type compound) Precipitate. In MP1 and MP2, all elements of Ti, Nb, and V are expressed in atomic ratios.

(MP1)
MP1 is represented by the following formula. MP1 is a precipitation strengthening element (MX type compound) contained in an electrolytic extraction method to be described later → filtered with a 0.2 μm mesh filter → filter passing material (including both ultrafine precipitates and solid solution) passing through the filter. In this case, the atomic ratio of Nb, Ti, V) is defined.
MP1 = {[V] / 51} + {[Nb] / 93} + {[Ti] / 48}

As the amount of MP1 increases, the amounts of Nb, Ti, and V that are easily re-dissolved during hot forging heating increase, and not only the strength and fatigue limit ratio but also the toughness can be increased. In order to effectively exhibit the above action, MP1 ≧ 5 × 10 ⁻⁴ is set in the present invention. The larger MP1 is, the better. For example, it is preferably 7.0 × 10 ⁻⁴ or more, and more preferably 10.0 × 10 ⁻⁴ or more.

  The filter passing material contains Nb and / or Ti, and may further contain V. V is a selected component. Nb, Ti, and V contained in the filter passing material may exist as precipitates as described in detail below, or may exist as a solid solution. Precipitates that can pass through a 0.2 μm mesh filter are ultrafine precipitates having an average particle size of nano-size, and generally have an average particle size of 30 nm or less.

  Hereinafter, the ultrafine precipitate contained in the filter passage will be described.

  In the V-containing filter passing material in which the filter passing material contains V, the ultrafine precipitate can be expressed as a (Ti, Nb, V) (C, N) precipitate, and 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-V carbide, Ti-V nitride, Ti-V carbonitride, Nb-Ti-V composite carbide, Nb-Ti-V composite nitride, Nb- Ti-V composite carbonitride is mentioned. 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.

  On the other hand, in the V-free filter passage in which the filter passage does not contain V, the ultrafine precipitate can be expressed as (Ti, Nb) (C, N) precipitate, 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. 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.

{Q value [= (MP2 / MP1)]}
MP2 is represented by the following formula. MP2 is an electrolytic extraction method to be described later → filtered with a 0.2 μm mesh filter → of M (where M = Ti, Nb, V) that constitutes an MX type compound contained in the filter passage through the filter , Nb and Ti mean atomic total amount.
MP2 = {[Nb] / 93} + {[Ti] / 48}

  The Q value expressed by the ratio of (MP2 / MP1) is a V-containing filter containing V as an essential component. Ti and Nb, which are effective for realizing a high strength and high fatigue limit ratio, are V-containing ultrafine precipitates. This defines the atomic ratio contained in the product / solid solution. In the present invention, Q value ≧ 0.05 is defined as a requirement for effectively exhibiting the above-described effects of Ti and Nb. This is because when the atomic ratio of Ti and Nb in the V-containing ultrafine precipitate / solid solution is 5% or more in the V-containing filter-passed product, the growth of the precipitate is inhibited and the desired characteristics are secured. Based on empirical knowledge that it can. According to the present invention, the amount of precipitation after hot forging can be ensured by defining MP1 as described above, and the strength and fatigue limit ratio can be further increased by controlling the Q value. Become. In order to ensure these characteristics, it is not sufficient to generate a large number of nano-sized ultrafine precipitates or solid solutions, and a large number of ultrafine precipitates or solid solutions having a Q value of less than 0.05 are insufficient. When generated, it has been confirmed by experiments that a high fatigue limit ratio cannot be obtained even if high strength can be ensured.

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

  The Q value is a useful requirement when the filtered material contains V. This is because when the filter-passed product does not contain V, the Q value is 1 by calculation, and the Q value ≧ 0.05 is inevitably satisfied.

  Next, a method for measuring the element amounts ([Nb], [Ti], [V]) of Nb, Ti, and V in the filter passage will be described. These are obtained according to the following procedures (A) to (C). As will be described below, the amount of the above-mentioned element contained in the V-containing filter passage is determined by quantitatively analyzing the amount of the above-mentioned element contained in the extraction residue remaining on the filter (extraction residue amount). It is calculated indirectly by subtracting the amount of extraction residue from the amount of the element contained. This is because it is difficult to directly quantitatively analyze the amount of the element contained in the V-containing filter passage.

(A) Using a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride as the electrolyte, the sample after hot rolling is extracted (electrolytically decomposed) under a current of 200 A / m ² or less. Thereby, a matrix melt | dissolves and a deposit is exposed to the surface. In collecting the sample after hot rolling, it is necessary to pay particular attention to the sampling position. For example, when the hot rolled material is cylindrical, the D / 4 position (D is the diameter), the hot rolled material is In the case of a square shape or a plate shape, the sample is taken from the D / 4 position (D is the thickness). In addition, the size of the sample after hot rolling is not particularly limited, and may be appropriately adjusted depending on the shape of the hot rolled material. In the examples described later, a sample having a length of 15 mm, a width of 15 mm, and a length of 5 mm was taken from the D / 4 (D is thickness) position of the bar after hot rolling.

  (B) Next, the sample after extraction is filtered using a filter having a mesh size of 0.2 μm, and the amount of each element of Nb and / or Ti (which may further contain V) in the extraction residue remaining on the filter Is quantified by inductively coupled plasma (ICP) emission spectrometry. In Examples described later, a membrane filter (material polycarbonate) manufactured by Advantech Co., Ltd. was used as a 0.2 μm filter.

  (C) From the amount of each element of Nb and / or Ti (which may further contain V) contained in the steel, Nb and / or Ti (V is further contained in the extraction residue obtained by (B) above. Each element amount of Nb and / or Ti (which may further contain V) in the extract that has passed through a 0.2 μm mesh filter. [Nb], [Ti], and [V].

  The MP1 and Q value that characterize 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 is more reactive with N than C, and if N is contained in an amount of 0.01% or more, coarse TiN is formed during solidification, and the amount of Ti contained in the ultrafine precipitate is reduced. Because. 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 high-strength non-tempered hot forging steel of the present invention contains the above components, and the balance is Fe and inevitable impurities.

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

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, an embodiment of a method for producing high-strength non-tempered hot forging steel according to the present invention and a method for producing forged parts using the forging steel will be described with reference to FIG. FIG. 1 schematically shows an outline of a casting process, a block rolling process, a hot rolling process, and a hot 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.

  In the present invention, as will be described in detail later, high temperature and long time heating at 1250 ° C. or higher for 1 hour or longer in at least one of the rolling step and / or the hot rolling step before hot forging. Including the holding treatment, the desired characteristics are effectively exhibited. Such a high-temperature and long-time heat holding treatment may be performed in any of the block rolling process and the hot rolling process, and may be performed in both processes. In consideration of the fact that it is a block rolling process that is easy to heat to a high temperature, it is preferable to control the heating conditions during the block rolling as described above.

  Below, as a typical embodiment of the production method of the present invention, the heating conditions at the time of the ingot rolling will be described in the order of steps, taking as an example a pattern in which the heating and holding treatment is performed at a high temperature for a long time. The production method of the present invention is not limited to this. For example, the heating conditions during hot rolling may be maintained by heating at a high temperature for a long time.

  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. When soaking is performed before the block rolling, the soaking is preferably performed under substantially the same conditions as the block rolling described in detail below. Specifically, it is generally preferable to carry out at 1200 ° C. for 1 hour or longer.

  The block rolling process is the most important process in order to control the MP1 and Q value within the range of the present invention and to increase all the strength, fatigue limit ratio, and toughness. Specifically, as described in detail below, it is necessary to appropriately control the heating conditions (heating temperature and holding time) during the batch rolling, and further the cooling conditions after the batch rolling. Only those manufactured under conditions that satisfy all of these requirements have the desired characteristics (see Examples below).

  First, regarding the heating conditions at the time of ingot rolling, in the present invention, the temperature is maintained at a temperature of 1250 ° C. or higher (T1 in FIG. 1) 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, it becomes easy for Nb and Ti to penetrate into the precipitate that precipitates in the subsequent cooling process, and as a result, the MP1 and Q values increase. 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 more preferably 2 hours or longer. Although the upper limit of heating temperature is not specifically limited, It is preferable to set it as 1325 degreeC in general in relation to an installation etc.

  The rolling temperature after heating (T2 in FIG. 1) is not particularly limited, and is preferably 900 ° 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 not only during the batch rolling but also during the hot rolling. In this embodiment, a temperature of 1150 ° C. or higher ( In FIG. 1, it is preferable to hold at T3) 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, it becomes easy for Nb and Ti to penetrate into the precipitate that precipitates in the subsequent cooling process, and as a result, the MP1 and Q values increase. The higher the heating temperature (T1) and the longer the holding time (t1), the better, and this further promotes solid solution of the above elements. For example, the heating temperature (T3) is preferably 1175 ° C. or higher, and more preferably 1200 ° C. or higher. The heating time (t1) is more preferably 2 hours or longer. The upper limit of the heating temperature is not particularly limited, but is preferably about 1250 ° C. because of the relationship with the equipment.

  Hot forging is performed after hot rolling. According to the present invention, a steel for hot forgings excellent in strength, fatigue limit ratio, and toughness can be obtained by the above-described series of steps, but as described below, by further appropriately controlling the forging step, Forged parts excellent in the above characteristics can be obtained with certainty.

  First, regarding the heating conditions at the time of hot forging, in this embodiment, the temperature is maintained at 1150 ° C. or more and 1250 ° C. or less (T5 in FIG. 1) for 0.5 hour or more and 1 hour or less (t5 in FIG. 1). Is preferred. The heating time (t5) means a holding time when the heating temperature (T5) is reached. In the present embodiment, the heating temperature is set to be equal or slightly higher and the heating time is set to be equal or slightly longer than a typical representative pattern (dotted line portion in FIG. 1). By controlling to such heating conditions, it is possible to promote the solid solution of the precipitation strengthening elements Nb, Ti, and V, and to suppress the coarsening of the austenite grains. If the heating temperature (T5) becomes too high or the heating time (t5) becomes too long, the austenite grains become coarse and the toughness decreases. The heating temperature (T5) is more preferably about 1175 ° C. or higher.

  The forging temperature after heating (T6 in FIG. 1) is preferably 1100 ° C. or higher. By setting the forging temperature higher than in the prior art, precipitate formation in austenite that does not contribute to precipitation strengthening can be suppressed. The higher the forging temperature, the better. In general, the forging temperature is more preferably 1125 ° C or higher, and further preferably 1150 ° C or higher. The upper limit of forging temperature is not specifically limited, Usually, it becomes below heating temperature (T5).

  After forging as described above, cooling is performed, but in the present embodiment, the range up to 650 ° C. after hot forging is cooled (rapidly cooled) at an average cooling rate (CR1) of about 60 ° C./min or more. It is preferable to cool (slowly cool) the range from 650 to 500 ° C. at an average cooling rate (CR2) of about 10 ° C./min or less. In this way, grain growth in the austenite region is suppressed by rapidly cooling the temperature region until ferrite transformation, and then ultrafine precipitates in ferrite are obtained by gradually cooling the temperature region until ferrite transformation is completed. As a result, the desired characteristics are effectively exhibited. The larger CR1 and the smaller CR2, the better. However, considering the balance with production efficiency and the like, it is generally preferable that CR1: 80 to 120 ° C / min, CR2: 3 to 8 ° C / min.

  After forging as described above, it is formed into a desired part shape by machining such as cutting to obtain a forged part.

  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, hot rolling and hot forging were performed under the conditions shown in Table 2 to obtain a forged part having a diameter of 30 mm × 500 mm. The average cooling rate after hot rolling was about 150 ° C./min.

(Characteristic evaluation)
Using the forged parts thus obtained (Nos. 1 to 28 in Table 2), MP1 and MP2 were measured based on the method described above, and the ratio (Q value) of (MP2 / MP1) was calculated. . Furthermore, the tensile strength and fatigue limit were measured as follows.

  The tensile strength was measured according to JIS Z 2241. Here, those having a tensile strength of 900 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).

  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).

As for toughness, a JIS No. 3 test piece was sampled at a position of depth D / 4 (D = diameter), and the absorbed energy (vE ₂₀ ) at room temperature (20 ° C.) was measured. Here, a value of vE ₂₀ of 50 J or more was evaluated as ◯ (passed), and a value of less than 50 J was evaluated as × (failed).

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 tensile strength, fatigue limit ratio, and toughness is ×
○: Tensile strength, fatigue limit ratio, and toughness are all ○
◎: Tensile strength and toughness ○, 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 7 are examples in which the steel type A satisfying the requirements of the present invention is used and the rolling temperature, the hot rolling conditions, and the heating temperature during hot forging are changed.

  Of these, No. 4 to 5 and 7 are examples of the present invention produced under the conditions specified in the present invention, and the MP1 and Q values satisfy the requirements of the present invention, so both the strength and the fatigue limit ratio are excellent. And toughness is high.

  In contrast, no. 1 is an example in which the heating time (t1) at the time of block rolling and the heating time (t3) at the time of hot rolling are short, and the heating temperature (T5) at the time of hot forging is high, the Q value becomes small, and the toughness Decreased.

  No. No. 2 is an example in which the heating time (t1) at the time of ingot rolling and the heating time (t3) at the time of hot rolling are short, the Q value was reduced, and the fatigue limit ratio was reduced.

  No. No. 3 is an example in which the heating temperature (T5) at the time of hot forging is high, austenite grains become coarse and toughness decreases.

  No. 6 is an example in which the heating temperature (T3) at the time of hot rolling is low, and MP1 is small, so that the fatigue limit ratio is lowered.

  Next, no. Consider 8-28.

  No. 9, 12 to 25 are all examples of the present invention produced under the conditions specified in the present invention, and MP1 and Q value satisfy the requirements of the present invention, so that the strength, fatigue limit ratio, and toughness of Excellent for everything.

  In contrast, no. 8 is an example using a steel type B that does not contain both Nb and Ti. Since MP2 = 0, the Q value becomes 0 and the action of Nb and / or Ti is not effectively exhibited. Declined.

  No. No. 10 is an example of using a steel type D which is a Ti-added steel and has a low Ti / N ratio, and the fatigue limit ratio has decreased.

  No. 11 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 a starting point of fracture, and the fatigue limit ratio becomes low.

  No. No. 26 is an example using the steel type T with a large amount of Ti, and the fatigue limit ratio was lowered.

  No. 27 is an example using steel type U with a large amount of C, and the fatigue limit ratio and toughness were lowered.

  No. 28 is an example using the steel type V with a small amount of C, and the strength decreased. In addition, No. Since No. 28 was low in strength, the 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) Fatigue limit ratio, wherein MP1 and MP2 obtained according to the following procedures (A) to (D) satisfy MP1 ≧ 5 × 10 ⁻⁴ and MP2 / MP1 ≧ 0.05, and High strength non-tempered hot forging steel with excellent toughness.
(A) Using a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride as an electrolyte, a sample after hot rolling is extracted under a current of 200 A / m ² or less.
(B) The sample after extraction is filtered using a 0.2 μm filter, and the amount of each element of Nb and / or Ti (which may further contain V) in the extraction residue remaining on the filter is inductively coupled. Quantification is performed by plasma (Inductively Coupled Plasma, ICP) emission spectrometry.
(C) From the amount of each element of Nb and / or Ti (which may further contain V) contained in the steel, Nb and / or Ti (V is further contained in the extraction residue obtained by (B) above. Each element amount of Nb and / or Ti (which may further contain V) in the extract that has passed through a 0.2 μm mesh filter. [Nb], [Ti], and [V].
(D) MP1 and MP2 are calculated based on the following equation.
MP1 = {[V] / 51} + {[Nb] / 93} + {[Ti] / 48}
MP2 = {[Nb] / 93} + {[Ti] / 48}   The steel for high-strength non-tempered hot forging according to claim 1, further comprising Mo: 1% or less and / or B: 0.015% or less.   The steel for high-strength non-tempered hot 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. High strength non-tempered steel for hot 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. High strength non-tempered steel for hot forging.

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