JP2011084784A - Steel to be worm-worked - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel to be worm-worked which is supplied to worm working for various structures, automotive components and the like; a method for worm-working the steel; and a steel material and steel components obtained by this worm working method. <P>SOLUTION: The steel to be worm-worked is a steel which forms a particle-dispersed type fiber structure in its matrix by the worm working; and forms 7×10<SP>-3</SP>or more by a volume rate in total amount of dispersed particles of the second phase at a room temperature, and shows hardness by Vickers hardness (HV) shown by expression (2) HH=(5.2-1.2×10<SP>-4</SP>λ)×10<SP>2</SP>or more, when having been subjected to any one of annealing treatment, tempering treatment and aging treatment without being worked beforehand, in a predetermined temperature range between 350°C and the Ac1 point, on such a condition that a parameter λ expressed by expression (1) λ=T(logt+20) (wherein T is a temperature (K); and t is a time (hr)) is 1.4×10<SP>4</SP>or more. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a steel for warm working in which a particle-dispersed fiber structure is generated by warm working in a predetermined temperature range of 350 ° C. or higher and Ac1 point or lower.

As shown in Patent Document 1 (PCT / JP2006 / 323248), this seed steel has a tensile strength of 1.2 GPa or more, excellent ductility, delayed fracture resistance, and toughness. It was developed as a high-strength steel for warm working that has been dramatically improved.
According to the present invention, it is desirable that the content of phosphorus (P) be 0.03 wt or less, following the general practice of high-strength steel.

  The addition of P is desired in terms of improving the hardenability of the steel and further expanding the range of use of the high-strength steel. However, the present invention is supposed to reduce as much as possible in order to embrittle the high-strength steel. It was common technical knowledge at the time of filing.

  In view of such circumstances, the present invention uses a steel for warm-working that effectively uses P, which has been removed as much as possible by refining as an impurity element, as an alloying element that enhances hardenability and provides good toughness. It was an issue to provide.

  The present invention is a high-strength steel having a particle-dispersed fibrous structure shown in Patent Document 1 and details how the content of P influences, and as a result, is based on the knowledge obtained.

  The warm-working steel of the invention 1 is a warm-working steel in which a particle-dispersed fiber structure is produced by warm working in a predetermined temperature range of 350 ° C. or higher and Ac1 point or lower, and its phosphorus content is 0. 0.03 wt% and less than 0.1 wt%.

Invention 2 is the steel for warm-working of Invention 1, wherein 80% by volume or more of the base structure is a single structure of martensite or bainite, or a mixed structure thereof. In the temperature range, the following formula (1 Tempering parameter λ
λ = T (logt + 20) (T; temperature (K), t; time (hr)) (1)
Has a temper softening resistance in which the Vickers hardness (HV) is equal to or higher than the hardness H of the following formula (2) by performing a tempering process without processing under a condition that becomes 1.4 × 10 ⁴ or more. By applying the tempering treatment, the second phase dispersed particles are precipitated and dispersed so that the total amount of the second phase dispersed particles at room temperature is 7 × 10 ⁻³ or more and 12 × 10 ⁻² or less as a volume ratio. It is characterized by containing an alloy element.
H = (5.2-1.2 × 10 ⁻⁴ λ) × 10 ² (2)

As described above, the hardenability of high-strength steel could be remarkably improved by intentionally containing an appropriate amount of P, which was conventionally avoided. Furthermore, by adopting Invention 2, by controlling the softening resistance when the steel is heated, that is, by controlling the thermal stability and the total amount of the base structure and the second phase dispersed particles, when subjected to warm working A particle-dispersed fibrous structure can be generated, and the Vickers hardness after warm working can be increased to 3.7 × 10 ² or more. As a result, it was possible to provide a steel for warm working that can dramatically improve its toughness while maintaining and improving a tensile strength of 1.2 GPa or more at room temperature.
In addition, high phosphorus content enhances hardenability, and the use of scraps with high phosphorus content has been considered impossible for this type of high-strength steel, but it can be used. In any case, it has the effect of improving the productivity of high-strength steel.

FIG. 1 is a diagram illustrating the relationship between tempering hardness and T (logt + 20) = λ, where T is a tempering temperature (K) and t is a tempering time (hr). FIG. 2 is a diagram illustrating an ultrafine fiber structure of a high P material (developed steel). FIG. 3 is a diagram illustrating the relationship between the absorbed energy and the P amount.

  The present invention has the features as described above. The requirements of the present invention will be described in detail below.

The present invention is a steel for warm working in which a particle-dispersed fiber structure is generated by warm working in a predetermined temperature range of 350 ° C. or higher and Ac1 point or lower.
Preferably, the parameter λ represented by the following formula (1)
λ = T (logt + 20) (T; temperature (K), t; time (hr)) (1)
Is a second phase at room temperature when any one of annealing, tempering, and aging treatment is performed without processing under the condition that is 1.4 × 10 ⁴ or more, more preferably 1.5 × 10 ⁴ or more. It contains an alloy component or / and second phase dispersed particles in which the total amount of dispersed particles is 7 × 10 ⁻³ or more as a volume ratio, and the Vickers hardness (HV) is a hardness H of the following formula (2)
H = (5.2-1.2 × 10 ⁻⁴ λ) × 10 ² (2)
It is desirable to show the above.
The steel for warm working of the present invention is a non-processed material obtained by heat treatment simulating the heat history of warm working because the dispersion state of the second phase dispersed particles and the base structure change during warm working applied thereto. It is comprised by setting the minimum of Formula (2) with respect to hardness (structure | tissue). That is, as described below, the tissue state is represented by hardness.
(A) Structure of steel for warm working In order to achieve high strength and improvement of toughness of duplex structure steel simultaneously by warm working, strengthening by dispersing the second phase dispersed particles as small as possible and as small as possible, It is desired that the base structure can be refined and the fiber structure can be formed simultaneously. In order to achieve this ultrafine multiphase structure, the fine dispersion or fine dispersion ability of the second phase dispersed particles in the steel for warm working as the material is important.

In the present invention, for the fine dispersion or fine dispersion ability of the second phase dispersed particles,
(I) The second phase dispersed particles are already dispersed in the warm working steel. (Ii) The second phase dispersed particles are not dispersed in the warm working steel, but the second phase dispersed during the warm working. One or more kinds of particles are precipitated, and a particle-dispersed fiber structure is formed after the processing (iii) The second phase dispersed particles are already dispersed in the steel for warm working. Three ways of precipitation of other particles can be considered.

  The dispersion (precipitation) strengthening by the second phase dispersed particles depends on the dispersion state such as the volume ratio of the second phase dispersed particles, the size of the particles, the hardness and the shape. When dispersion strengthening is based on the Orowan mechanism, the particle diameter (d) is smaller than the following formula (A) (“Steel Precipitation Control Metallurgy Frontline (Japan Iron and Steel Institute) (2001) P.69”), and the volume fraction ( The dispersion strengthening amount increases as f) increases. That is, the dispersion state (and dispersibility) of the second phase dispersed particles has a close relationship with the hardness.

Δσ = (3.2 Gb) / [(0.9f ^−1/2 −0.8) d] (A)
Here, G is a steel rigidity of 80 GPa, and b is a Burger spectrum of 0.25 nm.
However, if the particle becomes too smaller than a certain critical particle size, the dislocation is not pinned by the particle, and the particle is sheared by the dislocation, so that the Orowan mechanism is not established. In the so-called Cutting mechanism in which particles are sheared by dislocation, the amount of dispersion strengthening increases as the particle diameter increases. That is, the maximum dispersion strengthening amount can be obtained with the minimum particle diameter at which the Orowan mechanism is established. The minimum particle size at which the maximum dispersion strengthening can be achieved depends on the hardness of the particles and decreases in inverse proportion to the hardness of the particles (frontier of precipitation control metallurgy of steel (Japan Iron and Steel Institute) (2001) p. 69) . Therefore, when compared at the same volume ratio, the harder particles have a smaller minimum particle diameter at which the Owanan mechanism is established, and therefore the maximum amount of particle dispersion strengthening is increased.

For example, it is known that TiC has a high hardness among alloy carbides and has a small density, so that effective dispersion particle strengthening can be performed. Now, assuming that 7 nm is obtained as the minimum particle diameter applicable to the Owanan mechanism in TiC, about 0.9 GPa (TS (GPa) ≈0.0032 HV, HV 2.8 ×) with a volume fraction dispersion of 7 × 10 ^−3. A particle dispersion strengthening amount of 10 ² ) can be expected. Incidentally, when the density of TiC is 4.94 Mg / m ³ , the atomic weight of Ti is 47.9, and the atomic weight of C is 12, the Ti necessary for precipitating TiC with a volume ratio of 7 × 10 ⁻³ is 0.35 wt%, C Is 0.087 wt%. In addition, since the strength of the base of the practical ferritic steel is about 0.3 GPa (about HV 0.9 × 10 ² ), the room temperature strength of the steel in which the TiC is dispersed in the ferrite base is 1.2 GPa or more (HV 3.7). X10 ² or more). Therefore, considering the ideal dispersion state for TiC, if the dispersed particles to which the Orowan mechanism can be applied have a size of 7 nm, HV3.7 × 10 ² can be obtained only by dispersion strengthening at a small volume ratio of 7 × 10 ^−3. You will be fully satisfied. The same effect can be expected for the second phase dispersed particles composed of carbonitride, intermetallic compound, oxide, Cu particles and the like.

Metal carbide particles such as Mo and Ti are generally about 10 nm in size, and it is known that high strength can be effectively achieved even with a small amount of dispersion having a volume ratio of less than 10 × 10 ⁻³ . However, the size of the second phase dispersed particles and the distribution in the matrix structure also vary due to segregation of alloy elements and the like. Therefore, in the present invention, even if there is variation in the distribution of the second phase dispersed particles, the volume fraction of the second phase dispersed particles at room temperature is considered so that a fine crystal structure can be stably obtained by warm working. ^Is defined as 7 × 10 ⁻³ or more. For low alloy martensitic steels and baitnite steels, the second phase dispersed particles are taken into account when the average particle size of general cementite (Fe ₃ C) before warm working is several tens of nanometers or more. The volume ratio is preferably 20 × 10 ⁻³ or more.

In addition, the upper limit of the volume fraction of the second phase dispersed particles is not particularly limited for increasing the strength, but is preferably 12 × 10 ⁻² or less in consideration of toughness. Further, the particle dispersion strengthening by the Orowan mechanism is expected to become remarkable in the region of several tens of nm or less from the formula (A), and 1 in the dispersed state of the second phase dispersed particles having an average particle diameter larger than 0.5 μm. It is difficult to obtain a strength of 2 GPa or more. Therefore, it is desired for the steel for warm working that the average particle size of the second phase dispersed particles is 0.5 μm or less, more preferably 0.1 μm or less.

However, the above conditions are based on the premise that the second phase dispersed particles do not grow even in a temperature range of 350 ° C. or higher in the third stage or higher. In other words, in order to have a strength of 1.2 GPa or more even after warm processing, it is necessary that the second phase dispersed particles notably decrease in strength due to remarkable Ostwald growth in addition to the matrix structure during and after heating, processing. It becomes a condition. Therefore, when the thermal stability of the tissue is evaluated using λ represented by the following equation (1), which is generally known as a tempering parameter, as an index, in a predetermined temperature range of 350 ° C. or more and Ac1 point or less, λ Hardness at which Vickers hardness (HV) at room temperature is given by the following formula (2) when at least one of annealing, tempering, and aging heat treatment is performed without processing under the condition of ≧ 1.4 × 10 ⁴ It can be considered that a softening resistance that is greater than or equal to H is a necessary and sufficient condition for the pre-worked structure, that is, the steel for warm working of the present invention.

λ = T (logt + 20) (1)
Here, T is temperature (K), and t is time (h).

H = (5.2-1.2 × 10 ⁻⁴ λ) × 10 ² (2)
The term “in the predetermined temperature range” means that the above condition should be satisfied at any temperature from 350 ° C. to Ac1 point, and means that the above condition need not be satisfied over the entire temperature range. That is, in the case of aging or tempering treatment, when the material undergoes remarkable age hardening or secondary hardening and becomes a hardness H or more only in a certain temperature range within the above range, Can be steel.

Thus, in order to obtain an ultrafine multiphase structure having a tensile strength of 1.2 GPa by warm working on the basis of dispersion strengthening by the ideal dispersion state of TiC carbide, the volume fraction of the second phase dispersed particles is The lower limit is 7 × 10 ⁻³ and the hardness of the steel after any of annealing, tempering, and aging is HV ≧ (5.2-1) under the condition of T (logt + 20) ≧ 1.4 × 10 ^4. .2 × 10 ⁻⁴ λ) × 10 ² is a necessary and sufficient condition for the pre-processed structure. That is, as the steel for warm working, the second phase dispersed particles are finely dispersed or precipitated as the particle dispersion strengthened particles in the matrix structure, and the structure control for improving the thermal stability of the second phase dispersed particles It is a feature of the invention.

  About the structure | tissue of the steel for warm processing of this invention as mentioned above, since the dispersion state and base structure | tissue of a 2nd phase dispersion particle are variously changed during the process of warm processing, it is limited by the structure | tissue form of room temperature. However, in reality, all steels having a strength of 1.2 GPa or more, excluding steels having a pearlite structure as the main structure, can be considered as warm work steels. As such a material, specifically, for martensite steel (tempered martensite structure), a low alloy steel of JIS-G4053, a spring steel of JIS-G-4801, and a strength level of 2 or higher. Secondary hardened steel, maraging steel, TRIP steel, ausformed steel and the like.

In the second warm working steel of the present invention, 80% by volume or more of the base structure is made to be either a single structure of martensite or bainite or a mixed structure thereof. This is because, in a medium carbon low alloy steel, the width of a block that is considered to be an effective crystal grain of martensite is 1 μm or less (Script Mater., 49 (2003), P. 1157). In addition to being able to efficiently form a fiber structure by performing warm working on a tempered martensite structure in which carbides are finely dispersed, the bainite structure is also a needle-like or plate-like structure form in which carbides are finely dispersed. This is because a fibrous structure can be obtained in the same manner when this is used as a pre-processed structure. In the steel for warm working of the present invention, it is more preferable that the single structure of such martensite and bainite or the mixed structure thereof is 90% by volume or more of the base structure.
In particular, in order to stably maintain a strength of 1.2 GPa or more after warm working, a martensite or bainite structure having a temper softening resistance equal to or higher than that of tempered martensite steel of JIS-SCM430 steel is 80. % Or more is desirable. In addition, 20 volume% or less other than martensite or bainite and mixed structure thereof may be any structure such as ferrite, pearlite, and austenite structure. This is because such a ferrite, pearlite, austenite structure, etc. decomposes / disappears during the warm working heat treatment or changes to a fine structure, so it is judged that there is no problem if it is 20 volume% or less. is there.

(B) Chemical composition The steel for warm working of the present invention is an alloy designed based on the above knowledge, and the gist thereof is as follows: P: more than 0.03 wt% and 0.1 wt% Less than, C: 0.70 wt% or less, Si: 0.05 wt% or more, Mn: 0.05 wt% or more, Cr: 0.01 wt% or more, Al: 0.5 wt% or less, O: 0.3 wt% or less, N: It is steel for warm work characterized by containing 0.3 wt% or less, and the remainder is substantially Fe and inevitable impurities. Moreover, this steel for warm working is further Mo: 5.0 wt% or less, W: 5.0 wt% or less, V: 5.0 wt% or less, Ti: 3.0 wt% or less, Nb: 1.0 wt% Hereinafter, it contains 1 type or 2 or more types selected from the group consisting of Ta: 1.0 wt% or less, or Ni: 9.0 wt% or less, Cu: 2.0 wt% or less Can be considered. The reasons for limiting the component structure of steel in the present invention will be described below.

C: C forms carbide particles and is the most effective component for increasing the strength. However, if it exceeds 0.70 wt%, the toughness is deteriorated, so the content is set to 0.70 wt% or less. In order to sufficiently expect an increase in strength, it is preferable to contain 0.08 wt% or more, more preferably 0.15 wt% or more.

Si: Si is an element effective for deoxidizing and dissolving in ferrite to increase the strength of steel and finely disperse cementite. Therefore, the content of 0.05 wt% or more including those added as a deoxidizer and remaining in the steel is used. The upper limit is not particularly limited for increasing the strength, but it is preferably 2.5 wt% or less in consideration of the workability of the steel material.

Mn: Mn is an element effective in lowering the austenitizing temperature and effective in refining austenite, and also in hardenability and solid solution in cementite to suppress cementite coarsening. If the amount is less than 0.05 wt%, the desired effect cannot be obtained. More preferably, 0.2 wt% or more is contained. The upper limit is not particularly limited for increasing the strength, but considering the toughness of the obtained steel material, it is preferably 3.0 wt% or less.

Cr: Cr is an element effective for improving hardenability and is an element having a strong effect of delaying the growth of cementite by being dissolved in cementite. In addition, it is also one of the important elements in the present invention that, by adding a relatively large amount, forms a high Cr carbide that is more thermally stable than cementite and improves corrosion resistance. Therefore, it is necessary to contain at least 0.01 wt% or more. Preferably it is 0.1 wt% or more, More preferably, 0.8 wt% or more is contained.

Al: Al is an element effective for deoxidizing and forming an intermetallic compound with an element such as Ni to increase the strength of the steel. However, excessive addition reduces toughness, so it was made 0.5 wt% or less. In the case where an intermetallic compound of Al and another element, Al nitride, oxide, or the like is not used as the second phase dispersed particles, it is 0.02 wt% or less, and more specifically 0.01 wt% or less. It is preferable.

If O: O (oxygen) can be finely and uniformly dispersed as an oxide, it effectively acts not as inclusions but as grain growth suppression and dispersion strengthening particles. However, if excessively contained, the toughness is lowered, so the content was made 0.3 wt% or less. When the oxide is not used as the second phase dispersed particles, the content is preferably 0.01 wt% or less.

If N: N (nitrogen) can be finely and uniformly dispersed as a nitride, it effectively acts as a grain growth inhibiting particle or dispersion strengthening particle. However, if excessively contained, the toughness is lowered, so the content was made 0.3 wt% or less. When the nitride is not used as the second phase dispersed particles, the content is preferably 0.01 wt% or less.

Mo: Mo is an element effective for increasing the strength of steel in the present invention, and not only improves the hardenability of the steel, but also solidifies a small amount in cementite to make the cementite thermally stable. In particular, the steel is strengthened by causing secondary hardening by separate nucleation of alloy carbides on dislocations in the matrix phase completely separate from cementite. Moreover, the formed alloy carbide is effective for atomization and also for hydrogen replacement. Therefore, preferably 0.1 wt% or more, more preferably 0.5 wt% or more is contained, but it is an expensive element and excessive addition forms coarse undissolved carbides or intermetallic compounds to deteriorate toughness. Therefore, the upper limit of the addition amount is set to 5 wt%. From the viewpoint of economy, it is preferable that it is 2 wt% or less or not contained.

  W, V, Ti, Nb and Ta also showed the same effect as Mo, and the upper limit addition amount was determined for each. Furthermore, the combined addition of these elements is effective in finely dispersing the dispersion strengthening particles.

Ni: Ni is an element that is effective for improving hardenability and is effective for reducing the austenitizing temperature and reducing the austenite, improving toughness, and improving corrosion resistance. Moreover, if it contains an appropriate amount, it is an element that is effective in forming an intermetallic compound with Ti or Al to strengthen precipitation of steel. If less than 0.01 wt%, the desired effect cannot be obtained. More preferably, 0.2 wt% or more is contained. Although there is no restriction | limiting in particular about an upper limit, Since it is an expensive element, it is preferable that it is 9 wt% or less or does not contain.

Cu: Cu is a harmful element that causes hot brittleness, but if an appropriate amount is added, it causes precipitation of fine Cu particles at 500 ° C. to 600 ° C. and strengthens the steel. When added in a large amount, hot brittleness is caused, so the amount was set to 2 wt% or less, which is the almost maximum solid solution amount in ferrite.

  In addition, when increasing strength by precipitation of fine intermetallic compounds, it is effective to contain Co: 15 wt% or less.

When P: P (phosphorus) is 0.03 wt% or less, the effect of improving the quenching characteristics is not recognized. However, when the amount exceeds this as in the following examples, the hardenability is greatly increased. Could be improved.
However, it is less than 0.1 wt%, preferably 0.08 wt% or less, more preferably 0.07 wt% or less, and further preferably 0.06 or less.
When this upper limit is exceeded, the grain boundary strength is lowered, and embrittlement is caused during the formation of the material.

S: S (sulfur) is not particularly defined, but S is an element to be removed as much as possible in order to reduce the grain boundary strength, and is preferably 0.03 wt% or less.

  In addition, it is permissible for elements other than those described above to be contained in various elements as long as the effects of the present invention are not reduced.

(C) Preparation of steel for warm working Note that the method for producing the steel for warm working as described above is based on, for example, a variety of JIS standard martensite structure and bainite structure manufacturing method. Can be considered.

(D) Warm working The warm working method of the present invention is a temperature that gives a strain of 0.7 or more to any one of the above-mentioned steels for warm working in a temperature range of 350 ° C or higher and Ac1 point -20 ° C or lower. It is characterized by inter-processing. It is also considered to perform aging and annealing treatment in a temperature range of 350 ° C. or higher and Ac1 point or lower after the warm working.

  More specifically, with respect to such a processing temperature, for example, in the case of a medium carbon low alloy steel used as a general machine structural steel and based on a martensite structure, a tempering third stage in which cementite precipitates. It is possible to set the temperature to 350 ° C. or higher that substantially corresponds to In particular, in order to effectively use alloy carbides, intermetallic compounds, Cu, and the like as the second phase dispersed particles, it is necessary to process in a temperature range of 500 ° C. to 650 ° C., which is the precipitation temperature of these second phase dispersed particles. desirable.

On the other hand, in the portion that has undergone austenite transformation during processing, phase transformation such as pearlite transformation or martensite transformation occurs in the cooling process, and as a result, there is a high possibility that a non-uniform structure that causes cracking is formed. Moreover, the upper limit temperature of processing was made into Ac1 point-20 degreeC also considering the temperature rise by process heat_generation | fever. However, as a combination of the processing temperature and time of the material, when the hardness is arranged by the tempering parameter λ, Vickers hardness at room temperature when the material is annealed, tempered, or aging treated without processing. A combination that does not become HV 3.7 × 10 ² or less is preferable in order to obtain a strength of 1.2 GPa or more after warm working. Particularly in processing at high temperatures, it is necessary to shorten the time required for processing in consideration of the softening resistance of the material and the heating time.

  The degree of tissue development depends on the pre-processed structure, processing temperature and strain. In other words, the amount of strain required varies depending on the pre-processed structure and processing temperature, so the amount of strain cannot be strictly defined here, but when trying to form a fibrous structure inside the material, 0.7 or more, more It is preferable to apply one or more strains. Less than 1 strain amount for warm work steels with martensite and bainite structure in which prior austenite grains are elongated into fine fibers by processing in advance in the non-recrystallization temperature range of austenite A fine fiber structure can be uniformly generated by the application of. However, in an approximate case, the amount of strain is preferably 1 or more, more preferably 1.5 or more.

  At this time, the strain to be applied is not limited to a single process, and may be introduced in a plurality of processes. Further, the processing direction is not always limited to the same direction. Furthermore, the time between passes is not particularly limited. Furthermore, it includes applying a predetermined strain to a specific region (for example, a surface layer that requires high strength, an R portion of a part, or the like) instead of the entire area of the workpiece. However, the actual amount of strain depends on the material properties of the workpiece, the conditions of the roll (mold if it is forged) and the workpiece (such as the type and presence of lubricant), and the roll (the mold if forged). ), The rolling (forging) speed, the rolling (forging) temperature, etc., can only be understood. In particular, when parts are formed by forging, it is essential that non-uniform strain is introduced. Therefore, it is desirable to predict the amount of strain by a highly accurate numerical analysis technique, but generally the rolling reduction is 45% or more in the case of sheet rolling on the premise of a plane strain state, and the cumulative surface reduction in the case of bar rolling. If the rate is 45% or more, it is considered that a strain of 0.7 or more is introduced in the entire area of the workpiece. If the cumulative rolling reduction or cumulative area reduction is 58% or more, it is considered that a strain of 1 or more is introduced throughout the workpiece. However, for example, even if the rolling reduction (area reduction) is less than 45%, a strain of 0.7 or more may be introduced into the entire area of the workpiece or a specific region due to the influence of friction or the like. It is necessary to quantitatively study the amount of strain introduced by numerical analysis.

(E) Steel material The steel material of the present invention is a steel obtained by warm-working the steel for warm working as described above, and has a base structure composed of fibrous crystals having a minor axis average particle size of 3 μm or less. And the second phase dispersed particles are finely dispersed in the matrix structure at a volume ratio of 7 × 10 ⁻³ or more at room temperature, and the Vickers hardness at room temperature is HV3.7 × 10 ² or more. . The base structure in the steel material of the present invention has a degree of extension (aspect ratio) exceeding 2 and is typically composed of fibrous ferrite crystals having an aspect ratio of 5 or more, and the second phase dispersed particles are finely dispersed therein. Can be understood.

  The effect of grain refinement on the mechanical properties of steel is known to be noticeable in the grain region of several μm or less. In the present invention, the average interval between the base structures composed of fibrous crystals (that is, the minor axis) The upper limit of (average particle diameter) is 3 μm. Here, the crystal grain is a base crystal grain surrounded by a grain boundary having a crystal orientation difference of 15 ° or more. On the other hand, when the average particle size of the long axis of the dispersed particles is larger than 0.3 μm, the particle dispersion strengthening can hardly be expected, and the steel having 1.2 GPa or more is highly likely to significantly deteriorate the toughness. Therefore, it is desirable that the average particle diameter of the major axis is 0.3 μm or less.

  In particular, the effect of crystal grain refinement is particularly remarkable in the region where the average grain size is 1 μm or less and the particle dispersion strengthening by the Orowan mechanism is in the region where the average grain size is 0.1 μm or less. Therefore, in order to effectively utilize the strengthening by crystal fiber formation and the particle dispersion strengthening, it is effective to further reduce the minor axis average particle diameter of the fibrous crystal to 1 μm or less, further 0.5 μm or less. . The average particle diameter of the major axis of the second phase dispersed particles is also preferably 0.1 μm or less, more preferably 0.05 μm or less, depending on the refinement of the matrix structure.

  In such warm-worked steel materials, strengthening mechanisms such as solid solution strengthening and dislocation strengthening can be added in addition to the strengthening mechanism described above, and a simple addition of the strengthening mechanism due to the effect of superimposing these strengthening mechanisms. In this way, highly functional materials that cannot be predicted have been obtained.

  Such a fine fiber structure can be formed by warm forming of a plate material, a rod wire, a bolt thread portion, and the like. In particular, even when the amount of accumulated strain is small, a fiber structure can be formed in a surface layer portion or the like that has undergone local strong deformation, and the characteristics of various parts and desired portions can be greatly improved.

  Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Of course, the present invention is not limited to the following examples, and it goes without saying that various aspects are possible in detail.

Table 1 shows the steel component (A) within the scope of the present invention and the steel component (B) outside the range. In addition, the basic component of steel is based on the component of JIS-SCM440 steel. The volume fraction of cementite that can be dispersed as second phase dispersed particles in A and B steels is as follows: carbon content 0.4 wt%, cementite density 7.68 Mg / m ³ , base ferrite iron density 7.86 Mg / m ³ to 60 It can be determined as ^x10-3 . Therefore, the only major difference between steel A and steel B is the amount of P.

First, a 4 cm square × 12 cm long square bar cut out from a hot-rolled steel plate or a forged material was hot-rolled into a square bar material having a cross-sectional area of 10 cm ² and a length of 19 cm, and then air-cooled. The square bar material was quenched after being austenitized at 920 ° C. for 0.5 hour to obtain a martensite single structure close to approximately 100% by volume. Here, what is important is that the A material can obtain a nearly 100% martensite structure even with air cooling, whereas the B material cannot obtain a martensitic structure unless it is cooled with ice brine. That is, the hardenability of the steel material could be improved by adding P. Next, the square was heated to 500 ° C. for 0.5 hour and tempered for 1 hour, and then subjected to warm rolling using a grooved roll to a reduction in area of 77% to give strain and air-cooled. The warm processed material was further annealed at 550 ° C. for 1 hour (TF material). For comparison, a part of the warm processed material is subjected to a normalizing treatment from 880 ° C., and then quenched after annealing at 920 ° C. for 0.5 hour and tempered at 550 ° C. for 1 hour. A material with the same hardness was prepared (QT material).

  The structure of the obtained steel material was observed by polishing a cross section parallel to the rolling process (RD) direction using an FE-SEM and an EBSP analyzer. For the minor axis of the elongated grains in the fiber structure, the average section length of the minor axes of the elongated grains having a crystal orientation difference of 15 ° or more was measured by a cutting method by EBSP analysis.

  The hardness of the obtained steel material was measured at a load of 20 kg and a holding time of 15 s using a Vickers hardness tester in accordance with a test method defined in JIS Z 2244.

  The tensile test was performed at room temperature on a JIS No. 14A proportional test piece having a parallel part diameter of 6 mm, a length of 42 mm, and a distance between scores of 30 mm in accordance with the test method defined in JIS Z 2241. The crosshead speed was 0.5 mm / min, and the elongation was measured with a Shimadzu video non-contact extensometer (DVE-201).

  The impact test was carried out on a V-notch test piece having a length of 55 mm and a height and width of 10 mm produced from a steel material by cutting in accordance with a test method specified in JIS Z 2242.

  FIG. 1 shows the relationship between T (logt + 20) = λ and the hardness of tempered martensitic steel as it is.

In both steel A and steel B, the volume fraction of cementite is 60 × 10 ⁻³ , and in the tempering treatment of λ = 1.4 × 10 ⁴ or more, the hardness is H = (5 becomes .2-1.2 × ^{10 -4} λ) above, can be achieved HV3.7 × 10 ² by warm working above 350 ° C..

  FIG. 2 shows an example in which the structure of the A material obtained by warm groove rolling is analyzed. As can be seen from the EBSP analysis diagram of the Bcc phase, an ultrafine fiber structure elongated in the rolling direction is obtained. As a result of measuring the average grain size of the minor axis of the crystal grains having a crystal orientation difference of 15 ° or more by the cutting method, the average grain size of the minor axis of the elongated crystal grains is either A-TF material or B-TF material. Was 0.4 μm.

  From the inverse pole figure regarding the rolling direction (RD), it can be seen that the <011> // RD texture is a developed fiber structure. Similar textures were formed for other developed steels. Since the cleavage plane of Bcc iron is {100}, the formation of such <011> fiber structure is extremely effective for fracture due to tensile deformation in the fiber axis direction or bending deformation that receives a bending moment along the fiber direction. I think there is.

  Table 2 summarizes the results of the tensile test.

  FIG. 3 shows the relationship between absorbed energy and P content. In the QT material, the toughness is remarkably lowered by the addition of P. On the other hand, it was confirmed that the toughness of the warm processed material (particle-dispersed fiber structure steel (TF)) does not deteriorate so much even when P is added.

As described above in detail, the present invention
High-strength steels that have been made use of P, which has been removed by refining as impurities, effectively as an element that enhances hardenability and are made into a multiphase by fine dispersion of a small amount of second-phase dispersed particles, especially softening is difficult. Even for difficult-to-form ultra-high-strength steels, given deformation (thin plate, thick plate, bar wire, parts) in a temperature range where deformation resistance decreases and cracks do not occur in the material By molding, conventional spheroidizing annealing and quenching and tempering after molding of the parts are omitted, and at the same time, the ultrafine multiphase structure is developed into a fiber shape, greatly improving the toughness that has a relationship between high strength and trade-off balance. Provided are high strength steel and members.

  These are useful as steels or members that are processed into various structures and parts of automobiles.

Claims (2)

  A steel for warm working in which a particle-dispersed fiber structure is produced by warm working in a predetermined temperature range of 350 ° C. or more and Ac1 point or less, and its P content is more than 0.03 wt% and less than 0.1 wt% It is a steel for warm working characterized by being. In the warm-working steel according to claim 1, 80% by volume or more of the base structure is a single structure of martensite and bainite, or a mixed structure thereof. In the temperature range, the following formula (1) Tempering parameter λ expressed by
λ = T (logt + 20) (T; temperature (K), t; time (hr)) (1)
Has a temper softening resistance in which the Vickers hardness (HV) is equal to or higher than the hardness H of the following formula (2) by performing a tempering process without processing under a condition that becomes 1.4 × 10 ⁴ or more. By applying the tempering treatment, the second phase dispersed particles are precipitated and dispersed so that the total amount of the second phase dispersed particles at room temperature is 7 × 10 ⁻³ or more and 12 × 10 ⁻² or less as a volume ratio. A steel for warm working characterized by containing an alloying element.
H = (5.2-1.2 × 10 ⁻⁴ λ) × 10 ² (2)