JP2002212665A - High strength and high toughness steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength and high toughness steel which is suitable as a bar steel stock for a bolt, a spring or the like without requiring the addition of large quantities of Ni and Cr and special thermomechanical treatment. SOLUTION: The steel contains, by mass, 0.2 to 2.0% Mn and <=3.0% Cr also so as to satisfy Cr/Mn<=2.5%, (0.77-0.12Cr-0.02Mn)% to 0.8% C, <=3.0% Si, <=0.1% Al, <=0.01% P and <=0.03% S, and the balance Fe as essential components. The steel has a structure essentially consisting of tempered martensite, and the balance retained austenite. The average particle size of the old austenite particles is <=15 μm, and, in the structure, undissolved carbides whose aspect ratio expressed by a minor axis/a major axis is >=0.8, and having a particle size of 0.3 to 0.6 μm are contained by >=50 pieces in the field of 2,300 μm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength high-strength steel suitable as a steel strip for springs and bolts.

[0002]

2. Description of the Related Art In recent years, there has been an increasing need to reduce the weight of steel materials used in automobiles in order to improve fuel efficiency of automobiles, and it is desired to increase the strength of strip steel materials such as spring steel materials and bolt steel materials. For example, in the case of steel for bolts, the tensile strength is 1200 to 20
00MPa (HRC39-5 in Rockwell C hardness conversion)
4) For spring steel materials, 1600-2200 MPa (HRC)
The strength of HRC 47 to 58) in conversion is desired.

[0003] Martensitic steel is used as a high-strength steel. However, the adverse effects of the high strength are delayed fracture susceptibility and corrosion fatigue properties. Therefore, it is necessary to improve these properties. Enhancing is a fundamental technical issue. Although there is no standardized evaluation standard for the toughness of the steel for bolts and steel for springs,
Summary of the Iron and Steel Institute of Japan Lecture Meeting CAMP-ISIJ Vo
l. 11 (1988) -p495, the method of substituting and evaluating the fracture toughness value K _IC by the cathode charge life test, it is desired to secure a life of 1000 seconds or more.

[0004]

As a measure for increasing the toughness of martensitic steel, for example, Japanese Patent Publication No. 60-3073 is disclosed.
No. 6 discloses a method for manufacturing a cold-formed coil spring that forms fine martensite by induction hardening. This high-frequency heating treatment is a technique for indirectly refining martensite by refining austenite grains. However, there is a limit to improvement in toughness due to refinement of austenite grains, and development of a technique for further increasing toughness is desired.

As another technique for improving the toughness of martensite steel, a technique of quenching untransformed austenite having a high dislocation density has been proposed. For example, JP-A-6-1166
No. 37 discloses a technique using a component steel to which a large amount of Ni is added, generating a shear-type reverse transformed austenite phase during heating, and utilizing this. But N
There is a problem that i is an expensive element to use positively. Japanese Patent Application Laid-Open No. 11-229075 discloses a method for solving the above-mentioned problem by manufacturing a thick steel plate in which the steel composition is limited and the rate of temperature rise and the rate of cooling are further limited. However, this technology has a limited use range for thick plates, and the ultimate strength is 1551MP in TS (tensile strength).
a) Since the breaking stress indicating toughness stops at 945 MPa, the strength and toughness of the high-strength high-toughness steel used for automobile parts are insufficient.

[0006] Japanese Patent Application Laid-Open No. 8-67950 discloses that
As a means of improving the toughness of martensitic stainless steel, Cr carbide (M
₂₃ C ₆ ) to reduce the prior austenite grain size from 15 to 3
A technique for improving toughness by reducing the size to 0 μm has been disclosed. However, this technology is
Since the addition is indispensable, the tempered carbide of iron which contributes to the strengthening of the steel material for springs and bolts does not precipitate and grows.
Since the base carbide is mainly used, it is difficult to effectively increase the strength. The toughness improved by this technique is low-temperature toughness, and has different technical characteristics from toughness that contributes to delayed fracture susceptibility required for springs and bolts.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and can be applied to materials such as springs and bolts without requiring a large amount of alloying elements such as Ni and Cr and special working heat treatment. It is an object of the present invention to provide a high-strength high-toughness steel excellent in environmental embrittlement characteristics, which is suitable as a strip steel material.

[0008]

Means for Solving the Problems As a result of intensive studies by the present inventor on measures to improve the toughness of martensitic steel, the fundamental principle that achieves both high strength and high toughness is not austenitic crystal grain refinement. It is effective to refine the underlying structure of the martensitic structure, which is the underlying structure, and further suppress the growth of the martensitic underlying structure to refine the martensitic underlying structure without requiring special heat treatment. The present inventors have found that it is important to disperse obstacles in steel, and have completed the present invention. The feature of the present invention completed on the basis of the above findings is that the amount of C is suppressed to 0.8% or less, which is called a hypoeutectoid composition, to prevent the steel from being embrittled. , Mn,
By controlling the Cr content, fine carbides are dispersed and precipitated in austenite as in the hypereutectoid composition, whereby the substructure of martensite is refined and the toughness is significantly improved. Hypereutectoid means a component system in which, when cooled from the austenite phase at the time of high temperature heating, the phase which is thermodynamically equilibrium precipitated is not ferrite but carbide.

That is, the high-strength and high-toughness steel of the present invention comprises:
Despite being 0.8% C or less, it has a component system that can relatively easily obtain a state in which carbides are dispersed in the austenite phase. In mass%, Mn: 0.2% ≦ Mn ≦ 2.0%, Cr: Cr ≦ 3.0% and Cr / Mn ≦ 2.5, C: (0.77-0.12Cr-0.02Mn)% ≦ C
≦ 0.8%, Si: Si ≦ 3.0%, Al: Al ≦ 0.1%, P: P ≦ 0.01%, S: S ≦ 0.03%, and the balance Fe as an essential component Containing, having a structure mainly composed of tempered martensite and a balance of retained austenite, an average particle size of prior austenite grains of 15 μm or less, and an aspect expressed by a minor axis / major axis in the organization When the ratio is 0.8 or more, the particle size is 0.3 ~
Undissolved carbides of 0.6μm to in the field of view 2300μm ² 50
Have more than one.

[0010] As steel components, Ni ≦ 2.0%, M
at least one of Ni, Mo, Cu and W satisfying o ≦ 1.0%, Cu ≦ 1.0% and W ≦ 1.0, or further V ≦ 0.01%, Ti ≦ 0.01 %, Nb ≦ 0.0
1%, Hf ≦ 0.01%, Zr ≦ 0.01%, Ta ≦
At least one of V, Ti, Nb, Hf, Zr and Ta satisfying 0.01%, or (Ca + Mg +
REM) At least one of Ca, Mg, and REM satisfying 0.01% can be contained. Since the steel of the present invention has not only high strength but also high toughness, it is suitably used as a steel material for bolts and a steel material for springs.

[0011]

First, the reasons for limiting the components of the steel of the present invention will be described. All units are mass%. Mn: 0.2% ≦ Mn ≦ 2.0% Mn is an element that enhances the hardenability and, when added, moves the eutectoid composition to the low carbon side. 0.
If it is less than 2%, hardenability required for high strength cannot be secured. On the other hand, the addition of more than 2.0% reduces lath martensite, which is a martensite structure having high toughness, increases the fraction of twin martensite which is not preferable for improving toughness, and lowers toughness. Therefore, the lower limit of the amount of Mn is 0.2%,
It is preferably 0.3%, and the upper limit is 2.0%, preferably 1.5%.

Cr: Cr ≦ 3.0%, and Cr / Mn ≦ 2.5 Cr is an element that largely shifts the eutectoid composition toward the low carbon side, and is preferably added in an amount of 0.1% or more. However, if the amount of Cr exceeds 3.0%, a remarkable adverse effect is exerted on the cold workability. Therefore, the upper limit of Cr is set to 3.0%, preferably 2.0%. On the other hand, if the Cr / Mn ratio exceeds 2.5, the effect of improving toughness is lost even if the distribution of undissolved carbides is controlled as in the present invention, so the Cr / Mn ratio is specified to be 2.5 or less. To ensure the toughness of the parent martensite.

C: (0.77-0.12Cr-0.02)
Mn)% ≦ C ≦ 0.8% In order to secure toughness, the C content needs to be 0.8% or less, and preferably 0.77% or less. On the other hand, in order to increase the toughness of martensite by precipitating fine carbides in austenite, it is necessary to increase the eutectoid C content.
Parameter f (Mn, Cr) = (0.77−0.12C)
(r-0.02Mn) is important.
For this reason, the C content is defined in the above range.

Si: Si ≦ 3.0% Si is an effective element for deoxidation, but excessive addition has an adverse effect such as decarburization on the surface layer, so that it is limited to 3.0% or less.

Al: Al ≦ 0.1% Al is also an effective element for deoxidation like Si,
Excessive addition has an adverse effect such as decarburization on the surface layer.
Keep it below 1%.

P: P ≦ 0.01%, S: S ≦ 0.03% These elements are impurities, and the smaller, the better. If contained excessively, it causes grain boundary embrittlement and the like.
01% or less, S: 0.03% or less. Since S has an effect of improving machinability, S can be added within the above range when machinability is required.

The steel of the present invention contains the above components and the balance of Fe as essential components. In addition to the unavoidable impurities, other elements can be added as long as the characteristics of the steel of the present invention are not impaired. For example, N tends to cause embrittlement, but may be up to about 0.015%. further,
In order to further improve the mechanical properties of the steel of the present invention, for example,
One or more of Ni, Mo, Cu, and W in the following range, or one or more of V, Ti, Nb, Hf, Zr, and Ta, or one or more of Ca, Mg, and REM (rare earth element). More than one species can be added. REM
As examples, La, Ce, Pr, Nd, and Y can be given.

Ni ≦ 2.0%, Mo ≦ 1.0%, Cu ≦
1.0%, W ≦ 1.0% These elements are elements that affect the hardenability of steel. It can be added for the purpose of adjusting the strength. However, excessive addition has an adverse effect on hot workability and the like, so the upper limit of each element is determined as described above.

V ≦ 0.01%, Ti ≦ 0.01%, Nb
≦ 0.01%, Hf ≦ 0.01%, Zr ≦ 0.01%,
Ta ≦ 0.01% These elements form fine carbonitrides and act as hydrogen trap sites, and therefore can be added for the purpose of improving hydrogen embrittlement resistance. However, for each element,
Since the addition of more than 0.01% causes excessive precipitation strengthening and lowers the toughness, each is limited to 0.01% or less.

0.01% or less in total of at least one of Ca, Mg, and REM Ca, Mg, and REM are deoxidizing elements that can control the form of inclusions and have an effect of improving machinability. , 0.0
It can be added up to 1%.

Next, the structure of the steel of the present invention will be described.
The steel of the present invention has a structure mainly composed of tempered martensite. The tempered martensite desirably occupies the entire structure, but there is no practical problem even if it contains residual austenite of 10% by volume or less, preferably 5% by volume or less. On the other hand, since ferrite and pearlite deteriorate the strength, they are not contained in the structure.

As described above, the steel of the present invention is mainly made of tempered martensite and occupies at least 90% by volume, preferably 95% by volume in the structure, thereby ensuring high strength. In view of the above, the average particle size of the prior austenite is 15 μm or less, preferably 10 μm
The following is assumed. If the prior austenite average particle size exceeds 15 μm, so-called intergranular cracks will act as a fracture mechanism, and the toughness will be significantly reduced.

Further, in order to further improve the toughness, the steel of the present invention has a structure in which undissolved carbides are dispersed in a structure mainly composed of martensite. The average particle size of undissolved carbide is
0.3 μm or more and 0.6 μm or less. In order to be an obstacle that suppresses the growth of the martensite substructure, the carbide must be wider than the width of the “las”, which is the minimum unit of the martensite substructure. Since the carbide smaller than 0.3 μm cannot suppress the growth of “lath”, the lower limit of the average particle size is set to 0.3 μm. on the other hand,
Coarse carbides of 0.6 μm or more serve as starting points for cracks and reduce toughness. For this reason, the lower limit of the average particle size of the undissolved carbide is set to 0.3 μm, preferably 0.35 μm,
The upper limit is set to 0.6 μm, preferably 0.55 μm.

Further, as an criterion of the undissolved carbide, an aspect ratio of the carbide is set to 0.8 or more. The carbide effectively acting on the refinement of the substructure of martensite is a so-called undissolved carbide existing before quenching, and is not a carbide that precipitates during tempering performed for strength adjustment and ensuring toughness. Since the two are basically carbides having the same composition, it has been difficult to discriminate them. However, the present inventors have found that the granular carbide can be determined to be undissolved carbides from the characteristics of the precipitation form. As a standard of the carbide form that can be determined as undissolved carbide, in the present invention, the aspect ratio of the carbide is set to 0.8 or more. The aspect ratio is a value determined by the ratio of the minor axis / major axis of the carbide. As the aspect ratio approaches 1, the carbide takes a form closer to a perfect circle.

Further, as the number of the undissolved carbides,
In the present invention, the number of carbides in the visual field of 2300 μm ² needs to be 50 or more. The undissolved carbides exhibit their effects only when present in the austenite grains. 23
The presence of 50 or more, preferably 100 or more carbides in 00 μm ² divides the substructure in austenite grains, that is, forms martensite having various orientations in one austenite grain. Work effectively. If it is less than 50, the effect of refinement of the lower structure is small, and a further improvement in toughness cannot be expected.

Next, the recommended manufacturing conditions of the steel of the present invention will be described. The steel of the present invention is manufactured by quenching and tempering a steel material having the above composition under the conditions described below. The structure of the steel material (raw material) before the quenching treatment is not particularly limited. For example, a strip steel material obtained by hot rolling a billet having a predetermined composition according to a conventional method can be used as a material as appropriate.

However, in order to more easily control the distribution state of undissolved carbide, it is desirable that the structure of the material is tempered martensite or spheroidized carbide structure. The tempered martensite is obtained by heating the material after quenching at a temperature of (Acm point + 100) ° C. or higher for about 30 minutes to 1 hour, then cooling at a critical cooling rate or higher, and further reducing the tempering temperature to 300 ° C. or higher, desirably. 450 ° C or higher,
It can be obtained by setting the temperature to 600 ° C. or lower. By making the material have a tempered structure under such conditions, it is possible to shorten the heating time in the subsequent quenching process to about 30 minutes or less. On the other hand, as a method of obtaining a spheroidized structure, a method of gradually heating and maintaining the temperature to just below the A1 point temperature, and then slowly cooling, which is known as an ordinary method, can be mentioned.

In order to obtain a martensite-based structure containing the undissolved carbide having the specified particle size, aspect ratio, and number as defined above, at least a part of the austenite phase in which the undissolved carbide remains remains at a critical cooling rate or higher. It is necessary to cool to below the Ms point. The upper limit of the heating temperature is not particularly defined, but if the heating is carried out for a long time at a temperature higher than the temperature at which the carbide forms a solid solution, the undissolved carbide, which is an important factor of the present invention, will completely disappear after a holding time. For this reason,
When heating above the Acm point, it is necessary to heat at a temperature and within a time at which undissolved carbide is not completely dissolved. Specifically, when heating above the Acm point, (Ac
m point +80) ° C or less, desirably (Acm point +40) ° C
By heating in the following range, variation in the number of undissolved carbides due to the length of the holding time can be reduced.
When the heating is performed to the Acm point or less, even if the heating is performed for a long time, the carbide does not completely form a solid solution, so that the structure intended by the present invention can be stably obtained. Since the heating time is related to the control of the prior austenite grain size, Ac
1h regardless of whether it is above or below the m point
r, preferably within 30 minutes.

The cooling rate at the time of quenching after heating must be higher than the critical cooling rate in order to prevent ferrite transformation or pearlite transformation. If the cooling rate is lower than the critical cooling rate, the structure becomes mainly composed of ferrite or pearlite, and the required strength cannot be secured. Although the upper limit of the cooling rate is not particularly defined, there is no particular problem as long as it is in a cooling rate range such as water cooling which can be obtained industrially. Further, in order to obtain a martensite structure, it is important to cool to a temperature lower than the temperature at which martensitic transformation defined as the Ms point starts, at a critical cooling rate or higher. However, depending on the steel composition and cooling conditions, retained austenite may remain in the steel, but as described above, 10% by volume is used in the present invention.
The following retained austenite is acceptable. Although the Ms point can be determined by laboratory measurement, there is usually no practical problem even if the approximate calculation is performed by the following equation. Ms (℃) = 550-361 × C-39Mn-35V-20Cr-17Ni-10Cu
-5 (Mo + W)

After the quenching treatment, 1200 to 2300MP
A tempering process is performed for the purpose of strength adjustment so as to obtain a strength of about a. Maximum temperature T during tempering
(Absolute temperature K) and heating time t (hr) indicating the time from the start of heating to the start of cooling, as in a conventional method, T =
In the range of 573 to 873, λ = T × (20 + log t) may be set so as to substantially satisfy the following expression. 1000
0 ≦ λ ≦ 18000

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not construed as being limited to such examples.

[0032]

EXAMPLES Example 1 Steels having the components shown in Tables 1 and 2 were experimentally melted, and the cast pieces were subjected to hot working to obtain a diameter of 16 mm and a length of 150 m.
m round bar material was obtained. These test materials were heat-treated under the conditions shown in Table 3. Heating was performed in an air furnace, and quenching was oil-cooled (70 ° C.). A salt bath was used for tempering.
The tempering processing time (the time from when the test material was put into the salt bath until it was taken out) was all set to 1 hr. Tables 1 and 2 also show the Ms point, A1 point, and Acm point of each steel, and Table 2 also shows the critical cooling rate at which ferrite and pearlite are not formed.

A part of the obtained sample was processed into a test piece for structure observation, and a part was processed into a test piece for evaluating toughness. The grain size of the prior austenite and the number of undissolved carbides were measured using the test piece for structure observation, and the entire structure was observed to measure the amount of retained austenite. On the other hand, using a test piece for evaluating toughness, a Rockwell C scale hardness (HRC) as an index of strength and a cathode charge life (cathode CH life) as a toughness evaluation characteristic value were measured. Cathode CH life means the rupture life in a 4-point bending-cathode charge test.

The measurement of the prior austenite grain size is performed according to JI
With reference to “Austenitic grain size test method of steel” specified in SG0551, the surface of the sample after tempering was mirror-polished, and a picric acid alcohol solution was applied to make the structure appear, and Magnification 400 under microscope
Four tissue photographs were taken from an arbitrary position at a magnification of 2, and the average section length of each photograph was determined by the linear intersection line segmentation method shown in Appendix 4 of JIS G0551, and this was taken as the prior austenite grain size.

The number of undissolved carbides was measured by subjecting the surface of the tempered sample to mirror polishing by wet polishing, and then immersing the sample in 5% nital for 2 seconds without drying so as not to form an oxide film. The obtained tissue surface to be exposed is SEM (model used: Hitachi FE-SEM, model name S-
4500), a Polaroid (registered trademark) photograph was taken by an automatic exposure at an acceleration voltage of 20 kV and a magnification of 5,000 times, and the obtained photograph (field of view: 2300 μm ² ) was taken in with a scanner (model used: EP2000, manufactured by EPSON, Inc.) Capture software: EP Scan! II32, capture conditions: 256 color gray, no halftone, no dropout, high quality, resolution 120 dpi), and captured image analysis software (The Proven Solution, I
With mage Pro Plus ver. 4.0.0.11), the carbides recognized as white particles on the SEM photograph were counted, and the reciprocal of the ratio of the major axis to the minor axis was 0.8 or more as the aspect ratio, and the diameter (average) was 0. Those having a size of 3 to 0.6 μm were counted. In addition, the structure photograph (sample No.
FIG. 3 shows an example of 5).

The amount of retained austenite was measured by X-ray diffraction. RAD-RU30 as X-ray diffractometer
0 (manufactured by Rigaku Denki), CoKα was used as the X-ray source, and the intensity was measured at 40 kV-200 mA. For the quantification, α-Fe (2
00) and each peak of (200) was selected for γ-Fe.
In addition, in order to remove the orientation, the measurement was performed while rotating and rocking the sample. The integrated intensity was calculated using each data measured for quantification, and the integrated intensity of the peak was determined. Using the obtained peak intensity, the residual γ amount was calculated by the following equation. _Vγ = 1 / (( _Iα / _Iγ ) × K + 1) × 100 (vol
%) Where _Vγ is the volume concentration of γ-Fe (vol%), _Iα
Is the peak intensity of α-Fe (200), and I _γ is γ-Fe
(200) is the peak intensity. K is a constant for the combined peak, and K = 2.3616.

The hardness HRC was measured at four points after wet mechanical polishing of the sample surface, and the average was determined.

The four-point bending-cathode charge test was performed as follows. First, a plate-shaped test piece having a length of 60 mm, a width of 15 mm, and a thickness of 1.5 mm was cut out from the tempered sample by electric discharge machining, and as shown in FIG.
The test piece 2 is restrained at four points by a jig 1 at a bending stress of 1400 MPa. The jig 1 to which the test piece 2 is attached is 0.5 m
ol / liter sulfuric acid and 0.01 mol / liter K
The test piece is electrochemically supplied with hydrogen by immersing it in a mixed solution with SCN, using a platinum electrode as the anode, and applying a cathode potential of -700 mV. After the potential is applied, the time until the test piece subjected to bending stress is broken is measured. Life 1000s
Since those exceeding ec are suitable for practical use as high-strength and high-toughness steel, a life of 1000 sec was determined as a pass / fail judgment criterion. In the case of steel for springs and steel for bolts, the evaluation of toughness is not the dynamic toughness such as the Charpy impact test, nor the toughness shown by TS (tensile strength) × EL (elongation) found in thin plates. , Fracture toughness values K _IC and J _IC are suitable for evaluation. However, the exact K _IC because there is a size effect, since it is difficult to measure the J _IC, also in the present embodiment, as the evaluation of the toughness, the Japan Iron and Steel Institute lecture tournament Overview Collection C
AMP-ISIJ Vol. 11 (1988) -p. 4
As described in No. 95, toughness was determined by four-point bending-cathode CH life.

Table 3 also shows the results of these measurements. FIG. 2 is a graph showing the relationship between the hardness HRC (a substitute for strength) and the cathode CH life. Regarding the structure of the sample by the whole observation of the structure, with respect to the samples Nos. 1 to 14 of the present invention, in Nos. 1 to 12, the whole structure was almost entirely occupied by tempered martensite, while in No. 13 was the tempered martensite. In addition, retained austenite is about 8% by volume,
In No. 14, about 3% of retained austenite was observed. No. 15 is a comparative example in which about 14% of retained austenite was observed.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

As shown in Table 3 and FIG. 2, the samples of No. 1 to No. 14 using the steel of the component system of the present invention and having a prior austenite grain size and the number of undissolved carbides satisfying the conditions of the present invention have the toughness. It can be seen that the cathode CH life as an index has good toughness exceeding 1000 seconds. However, the organization of the present invention,
In the case of steel using steel materials (steel Nos. 15 to 41) deviating from the components, the maximum is less than 540 seconds, and high toughness is not obtained.

Example 2 A steel satisfying the following components of the present invention was melted in the same manner as in Example 1, and the cast piece was hot-worked into a round bar. Quenching and tempering (treatment time: 1 hr) were performed under various conditions shown in Table 4, and in the same manner as in Example 1, the prior austenite particle size, the number of undissolved carbides, H
The RC and cathode CH lifetimes were examined. Table 4 also shows the obtained results. FIG. 4 shows the relationship between HRC and cathode CH life.
Shown in The structure of each sample was almost 100% of tempered martensite. -Steel component (mass%, balance Fe) C: 0.55%, Si: 0.19%, Mn: 1.61
%, P: 0.008%, S: 0.007%, Cr: 3.%
0%, Al: 0.031%, N: 0.006%. Ms = 2
28.7 ° C., A1 = 762 ° C., Acm = 850 ° C., critical cooling rate = 3 ° C./s.

[0045]

[Table 4]

As shown in Nos. 1 to 5 in Table 4, by appropriately controlling the heat treatment conditions at the time of quenching, a predetermined structure specified in the present invention can be obtained. Good toughness of 1000 seconds or more was obtained. On the other hand, even if the steel composition of the present invention was satisfied, in No. 56, the heating temperature at the time of quenching was too low to make austenite. When the heating temperature is too high as in No. 57, the undissolved carbide is dissolved, the number of carbides becomes insufficient, and the toughness is deteriorated up to about 260 seconds. Also, as shown in No. 58, when the holding time for austenitization is too long, the prior austenite grains are coarsened and the toughness is also deteriorated.

[0047]

According to the high-strength and high-toughness steel of the present invention, it is possible to obtain a steel having a high strength and a high toughness.
Under a predetermined component of 8% C or less, not only the prior austenite particle size but also a structure having a predetermined particle size and the number of undissolved carbides dispersed and precipitated in the structure, so that the tempered martensite can be refined. It has excellent toughness as well as high strength. For this reason, it is suitably used as a strip steel material for various uses requiring high strength and high toughness, particularly a steel material for bolts and a steel material for springs.

[Brief description of the drawings]

FIG. 1 is a cross-sectional explanatory view of a jig for applying a bending stress used in a four-point bending-cathode charge test.

FIG. 2 shows hardness HRC as a strength index in Example 1.
4 is a graph showing the relationship between the lifetime of a cathode and the lifetime of a cathode CH (rupture lifetime obtained by a four-point bending-cathode charge test).

FIG. 3 is a drawing substitute photograph showing an example of a metallographic photograph used for the measurement of the number of undissolved carbides in Example 1.

FIG. 4 is a graph showing the relationship between hardness HRC and cathode CH life in Example 2.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Ieguchi 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo F-term in Kobe Steel Research Institute, Kobe Research Institute, Ltd. 3J059 AB20 AD06 BC02

Claims (6)

[Claims] 1. Mass%, Mn: 0.2% ≦ Mn ≦ 2.0%, Cr: Cr ≦ 3.0% and Cr / Mn ≦ 2.5, C: (0.77-0.12Cr) −0.02 Mn)% ≦ C
≦ 0.8%, Si: Si ≦ 3.0%, Al: Al ≦ 0.1%, P: P ≦ 0.01%, S: S ≦ 0.03%, and the balance Fe as an essential component Containing, having a structure mainly composed of tempered martensite and a balance composed of retained austenite, an average particle diameter of prior austenite grains of 15 μm or less, and an aspect expressed by a minor axis / major axis in the organization field undissolved carbides particle size 0.3~0.6μm ratio is 0.8 or more 2300Myuemu ²
High-strength, high-toughness steel with 50 or more in it. 2. Ni ≦ 2.0%, Mo ≦ 1.0
%, Cu ≦ 1.0%, Ni, M satisfying W ≦ 1.0
The high-strength and high-toughness steel according to claim 1, containing at least one of o, Cu and W. 3. V ≦ 0.01%, Ti ≦ 0.0
1%, Nb ≦ 0.01%, Hf ≦ 0.01%, Zr ≦
V, Ti, N satisfying 0.01% and Ta ≦ 0.01%
The high-strength and high-toughness steel according to claim 1 or 2, containing at least one of b, Hf, Zr, and Ta. 4. The method according to claim 1, wherein (Ca + Mg + REM) ≦ 0.
The high-strength and high-toughness steel according to any one of claims 1 to 3, comprising at least one of Ca, Mg, and REM satisfying 01%. 5. A bolt steel material formed of the high-strength and high-toughness steel according to any one of claims 1 to 4. 6. A spring steel material formed of the high-strength and high-toughness steel according to any one of claims 1 to 4.