JP2001288530A - High strength and high toughness martensitic steel and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide martensitic steel having sufficient strength and also excellent toughness even when used as automotive springs and bolts, and to provide its production method. SOLUTION: This high strength and high toughness martensitic steel contains >=0.1 mass % C, and >=80% or its metallic structure is composed of martensite. The average value of the old austenite crystal grain size numbers is >=7, and also, the area ratio occupied by the old austenite grains with the grain size numbers different by >=3.0 from the grain size number having the maximum frequency is <=10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-strength and high-toughness martensitic steel and a method for producing the same, and to a high-strength spring and a high-strength bolt using the high-strength and high-toughness martensitic steel.

[0002]

2. Description of the Related Art In recent years, there has been an increasing need to reduce the weight of steel materials used for improving fuel efficiency of automobiles, and higher strength steels such as spring steel and bolt steel are required to have higher strength. ing.

[0003] Martensitic steel is used as the above high-strength steel. However, toughness is degraded as a detrimental effect of high strength, and it is important to improve delayed fracture susceptibility and corrosion fatigue characteristics while increasing strength. It has been taken up as an issue and various technologies have been proposed.

For example, Japanese Patent Publication No. 60-30736 discloses a method of generating fine martensite by induction hardening for the purpose of improving the toughness of a cold-formed coil spring. However, since this is a technique for refining austenite grains and indirectly refining martensite by ordinary high-frequency heat treatment, the degree of improvement in toughness is not sufficiently satisfactory. Development is desired.

Japanese Patent Application Laid-Open No. Hei 6-116637 discloses that
As a component composition, while containing a large amount of Ni (8 to 11%), a shear-type reverse transformed austenite phase is generated during temperature rise, and the toughness of the martensitic steel is increased by quenching from untransformed austenite having a high dislocation density. A method for improving is disclosed. However, there is a problem that Ni is an expensive element for actively utilizing.

Further, Japanese Patent Application Laid-Open No. 11-229075 discloses a method of increasing the delayed fracture resistance of a high-strength steel by limiting the component composition and limiting the rate of temperature rise and cooling. However, this technology has a limited use range for thick plates, and the ultimate strength is up to 1551MP in tensile strength.
a, and the breaking stress indicating toughness is as low as 945 MPa,
High strength steel used for automobiles lacks strength and toughness.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has sufficient strength and excellent toughness even when used as a spring or bolt for automobiles. An object of the present invention is to provide steel and a method for manufacturing the same.

[0008]

The high-strength and high-toughness martensitic steel of the present invention which has solved the above-mentioned problems contains 0.1% by mass or more of C and 80% or more of the metal structure is martensite. A martensitic steel having an average value of prior austenite crystal grain size numbers of 7 or more and an area ratio occupied by 10% or less of old austenite grains having a grain size number different from the most frequently used grain size number by 3.0 or more. The gist is that. The above martensitic steel has V ≦
1 selected from the group consisting of 0.2% by mass, Nb ≦ 0.2% by mass, Ti ≦ 0.2% by mass and Hf ≦ 0.2% by mass
It is desirable to contain at least one species, and it is preferable to contain 1.0% by mass or less of Cr and / or 1.0% by mass or less of Mo. Further, the hydrogen diffusion coefficient D _H is 1.
It is recommended that it be 0 × 10 ⁻⁵ cm ² / s or less.

In producing such a high-strength and high-toughness martensitic steel, a step of performing cold working at a temperature of 500 ° C. or less and at least a true strain of 0.20 or more, at a heating rate of 50 ° C./sec or more, Ac ₃ points + 150 ° C or higher 120
The step of heating to less than 0 ° C., the total heating time from the start of heating to the start of cooling (for example, the time during which heating is actively performed by high-frequency heating or the like) is set to 20 seconds or more and less than 40 seconds, and is maintained at a predetermined heating temperature. After that, it is desirable to adopt a method having a quenching step of cooling at least 1.5 times the critical cooling rate or more, and the cooling rate in the quenching step is 2.0 times or more the critical cooling rate. Is preferred.

The high-strength and high-toughness martensitic steel according to the present invention is suitable as a high-strength spring or a high-strength bolt.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The average grain size of prior austenite crystals greatly affects the strength-toughness balance. The larger the average grain size, that is, the finer the average grain size, the better the strength-toughness balance. It is known to However, the effect of the prior austenite grain size on the toughness has been often discussed so far based on the average grain size, and the average grain size was the average value of the whole regardless of the mixed grain structure or the sized grain structure. . JIS
In G 0551, as a definition of a mixed grain structure, "a grain having a grain size number different from that of a grain number having the maximum frequency of about 3 or more is unevenly distributed in one visual field, and these grains have about 20% or more. That occupies an area, or that there are visual fields having three or more different granularity numbers between visual fields. " Speaking of this mixed grain structure,
When the difference between the grain numbers of adjacent crystal grains is large, extreme stress concentration is likely to occur at the interface, and the toughness is deteriorated.
In particular, when grain boundary cracking such as corrosion fatigue or delayed fracture becomes a problem, the characteristics are significantly deteriorated as the mixed grain structure becomes more pronounced. That is, to further improve the strength-toughness balance, it has been found that it is very important not only to reduce the average crystal grain size but also to control the mixed grain structure. Specifically, the average value of the prior austenite grain size number was 7
As described above, by setting the area occupied by the prior austenite grains having a grain size number different from the grain size number having the maximum frequency by 3.0 or more to 10% or less, dispersion and relaxation of the stress-concentrated portion are achieved, which has never been seen before. The strength-toughness balance can be obtained.

As described above, the average prior austenite grain size greatly affects the strength-toughness balance. As the average grain size is larger, that is, as the average grain size is finer, the strength-toughness balance is larger. Therefore, the lower limit of the average prior austenite grain size was set to 7. In addition, according to the current industrial technology, the production of steel having an average prior austenite average grain size of more than 12 is limited in terms of equipment capacity.
Have difficulty.

Hereinafter, other reasons for limitation of the present invention will be described.

C ≧ 0.1% In order to secure the strength of the martensitic steel, 0.1% or more of C is required. High strength is obtained with an increase in the C content of the steel material. However, as the C content increases, quenching cracks are likely to occur.

80% or more of the metal structure: When martensite is quenched, the structure as-quenched contains martensite, bainite, retained austenite, etc., but if the ratio of structures other than martensite increases, the strength decreases. When applied as a high-strength spring or high-strength bolt steel, if at least 80% or more of the metal structure is martensite, the required strength (for example, 1 for bolt steel)
200 N / mm ² class strength).

V ≦ 0.2% by mass, Nb ≦ 0.2% by mass,
Group consisting of Ti ≦ 0.2% by mass and Hf ≦ 0.2% by mass
One or more of V, Nb, Ti and Hf selected from the following are arbitrary additive elements, and form a precipitate by adding a trace amount thereof to bring about precipitation strengthening. The precipitate also acts as a hydrogen trap site, has the effect of further lowering D _H and improving delayed fracture resistance. However, if added excessively, the number of precipitates increases and the toughness is impaired. Therefore, the upper limit of each element is set to 0.2% by mass.

Cr: 1.0% by mass or less and / or Mo:
1.0 mass% or less Cr and / or Mo are also optional elements, and when added, improve hardenability and form carbides or nitrides to bring about precipitation strengthening. However, since the excessive addition lowers the toughness, the upper limit is set to 1.0% by mass.

As other elements, Si, Mn,
Elements such as Ni, Cu, P, S, Al, N, and B may be added in an appropriate amount according to the required characteristics. For example, in spring steel, it is common to add 1% by mass or more of Si for the purpose of improving sag resistance.

Hydrogen diffusion coefficient D _H : 1.0 × 10 ⁻⁵ cm ² /
In order to put into practical use a high-strength steel having a tensile strength of 1400 N / mm ² or less, it is important to control the hydrogen diffusion coefficient in addition to the uniformity of the grain size. The slower the diffusion of hydrogen in the steel, the better the resistance to stress corrosion cracking and delayed fracture. The diffusion of hydrogen is delayed by the introduction of hydrogen trap sites, and effective trap sites include carbides, dislocations, and grain boundaries. In order to trap hydrogen without using additional elements such as V and Nb, it is very effective to increase the dislocation density. By adjusting the critical cooling rate to be described later, a structure with a dislocation density higher than before can be obtained, and as a result, _DH becomes 1.0 × 10 ⁻⁵ cm ² / s or less, and a remarkable effect of improving toughness is obtained. be able to.

Next, the reasons for limiting the manufacturing method will be described in detail.

At least 500 ° C. and true strain of 0.2
Prior to the austenitizing treatment of 0 or more , at least 500 ° C. and true strain of 0.20 or more, preferably 0.35 or more are applied. Conventionally, if a wire is used, wire drawing with a low degree of processing for the purpose of adjusting the wire diameter (sizing) is common (slightly as disclosed in JP-A-3-698).
No. 1 only discloses that "after being drawn, rapid heating to a predetermined quenching temperature of 900 to 1050 ° C. within a time not exceeding 10 sec....") Absent. In the present invention, the austenite nucleation is made uniform by actively performing strong working before the austenitizing treatment and introducing a large amount of defects serving as austenite nucleation sites during the austenitizing treatment into the structure. It is intended to be finely dispersed. That is, since the crystal nuclei are uniformly finely dispersed and generated, there is an effect of promoting the reduction of the variation in the final crystal grain size. When the processing temperature exceeds 500 ° C., the defect density in the structure decreases due to recovery, the number of nucleation sites during austenitization decreases, and uniform nucleation cannot be obtained. Therefore, the upper limit of the processing temperature was set to 500 ° C. . The processing method may be rolling, drawing, or any other method.

At least Ac ₃ points + 150 ° C. or higher, 12
When heating austenite at a heating rate of 50 ° C./sec or more to a temperature lower than 00 ° C., the temperature is raised to maintain high-density defects introduced by processing for the purpose of uniformly and finely dispersing nucleation of austenite to the heating temperature. Speed is 5
It is necessary to be 0 ° C./sec or more. If the heating rate is less than 50 ° C./sec, the recovery proceeds during the heating, the defect density decreases before the heating temperature is reached, and uniform nucleation during austenitization cannot be obtained. Thus, in order to maintain a high defect density up to the heating temperature, the rate of temperature rise is preferably higher, and is preferably 100 ° C./sec or more.

In order to completely austenite the entire steel material, the heating temperature is at least Ac ₃ points + 150 ° C.
It is necessary to be above (desirably, more than Ac ₃ points + 200 ° C.). In addition, during nucleation during austenitization,
The nucleation rate increases as the temperature rises rapidly to higher heating temperatures. In order to equalize the austenite grain size, it is desirable to increase the nucleation rate and disperse the nucleation, and it is better to increase the heating temperature and raise the temperature rapidly. But,
Overheating causes the austenite grains formed to become coarser,
Eventually, coarsening occurs such that the austenite particle size number becomes less than 7, and as a result, toughness is reduced. Therefore, the upper limit of the heating temperature is desirably less than 1200 ° C. Incidentally, provision of the heating temperature of the present invention is a skeleton to be a single-phase austenite temperature, the over-eutectoid formation of more than 0.8%, it is desirable to define with the Acm temperature changes in Ac _3, heating The temperature may be set to Acm point + 150 ° C. or higher, preferably Acm point + 200 ° C. or higher.

Total heating time: 20 seconds or more and less than 40 seconds The finer the austenite average particle size is, the more the toughness is improved, so that the austenite average particle size is not coarsened.
Generally, the total heating time is set to the minimum necessary for completing the austenitization of the entire steel material. However, if the total heating time is such that the austenitization is completed over the entire steel material, the heating time inside the structure varies, and austenite crystal grains that have been sized cannot be obtained. In order to reduce the area ratio of austenite grains having grain sizes that differ from the mode of the austenite grain size by 3.0 or more to 10% or less, the total heating time must be 20 seconds or more. The grain size is leveled. In addition, the crystal grains grow after 40 seconds or more,
Since the average austenite grain size number is less than 7, and the toughness is reduced, the upper limit of the holding time is set to less than 40 sec.

[0025]Cooling more than 1.5 times the critical cooling rate CRcri
Cool at reject speed Cooling is performed at the critical cooling rate CRcri
Quench on the top to cause martensitic transformation
Has a tensile strength of 1200 N / mm^TwoOdor level
In order to secure the expected toughness, the critical cooling rate C
It is necessary to cool at a cooling rate of 1.5 times or more of Rcri
It is. Furthermore, a tensile strength of 1400 N / mm ^TwoGrade strength
Expected toughness in steel materials with high strength up to the bell
In order to secure the hydrogen diffusion coefficient D_HTo 1.0 × 10
^-Fivecm^Two/ S or less. D_HTo 1.0 × 10
^-Fivecm^Two/ S or less, the conversion of martensite
It is necessary to secure the phase density, but the cooling rate is CR
If less than 2.0 times cri, self-tempering occurs during cooling.
And the dislocation density decreases, and carbides precipitated by tempering
Not finely dispersed, D_HIs 1.0 × 10^-Fivecm^TwoGreater than / s
It will be good. Cooling rate should be more than 2.0 times of CRcri
If high dislocation density is secured, D_HTo 1.0 × 10^-Fivecm ^Two
/ S or less, and a tensile strength of 1400 N / mm^TwoGrade
Expected toughness can be obtained even in high strength steel. More excellent
3.0 times higher than CRcri
Good. However, considering the prevention of burning cracking,
Less than 4.0 times is desirable.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples do not limit the present invention. Are included within the technical scope of

[0027]

EXAMPLE A steel material having the composition shown as steel material a in Table 1 was experimentally melted and shaped to obtain a bar having a diameter of 18 mm.

[0028]

[Table 1]

Next, the bar was drawn at room temperature and quenched under various conditions shown in Table 2 using a high-frequency heating furnace.

[0030]

[Table 2]

At the time of quenching, a thermocouple is welded to the surface of the sample to measure the surface temperature. When the measured temperature is raised to the set heating temperature, the total heating time from the start of the temperature rise is 15 to 45 seconds.
After holding at the heating temperature so as to be ec, quenching was performed. The rate of temperature rise and the rate of cooling were calculated from the results of measurement of the sample surface temperature using a thermocouple installed on the sample. First, the critical cooling rate was determined by changing the cooling rate, and then quenching was performed at a desired cooling rate. After quenching, the sample was prepared, and the martensite ratio and the prior austenite grain size were measured using picral as an etching solution. The martensite ratio is determined by observing the cross section of the as-quenched sample at a D / 4 position (a position at a depth of 1/4 from the surface in the thickness direction) at a magnification of 400 using an optical microscope in four visual fields, and analyzing the image. The average value was evaluated. In the measurement of the prior austenite grain size, the D / 4 position of the cross section of the as-quenched sample was observed with an optical microscope at a magnification of 200 times or 400 times, and the area of 100 or more old austenite crystal grains was measured using an optical microscope photograph. . At this time, the number of prior austenite crystal grains measured in one visual field was set to 40 or more.
Only steel materials having a visual field of less than 40 prior austenite crystal grains in the visual field were observed and measured at a magnification of 200 times. In addition, one of the crystal grains adjacent to the crystal grain to be measured is always a measurement crystal grain. Thereafter, the measurement result of the prior austenite crystal grain area was converted into a particle size number using the following equation (1). The average grain size is the average value of all measured prior austenite grain sizes. In addition, the grain size number was arranged by rounding off the second decimal place of the grain size number, and the crystal grain size number indicating the maximum frequency was obtained. Further, for crystal grains having a grain size number different from the grain size number showing the maximum frequency by 3.0 or more, their areas were totaled, and the ratio to the total area was calculated.

[0032]

(Equation 1)

The quenched sample is subjected to a tempering treatment using a lead bath to adjust the strength to a tensile strength of about 1100 to 1800 N / mm ² , and then, as a strength evaluation characteristic value, JIS Z 22
According to No. 41, a room temperature tensile test was performed using a No. 2 test piece in JIS Z 2201 as a test piece, and a tensile strength was measured.

The characteristic value of the toughness evaluation is four-point bending.
The rupture life in the cathode charge test was adopted. The details of the four-point bending-cathode charge test are described below. First, from the tempered sample, the length was 60 m by electric discharge machining.
A plate-shaped test piece having a size of m, a width of 15 mm and a thickness of 1.5 mm was cut out and restrained at four points with a bending stress of 1400 MPa using a jig shown in FIG. The jig to which this test piece is attached is 0.5 mo
1 / liter sulfuric acid and 0.01mo1 / liter K
The test piece was electrochemically supplied with hydrogen by immersing in a mixed solution of SCN, using a platinum electrode as an anode, and applying a cathode potential of -700 mV. After the potential was applied, the time until the test piece subjected to the bending stress was broken was measured. Life 1000sec
Since those exceeding c have toughness suitable for practical use, a life of 1000 sec was used as a pass / fail criterion in this experiment.

The hydrogen diffusion coefficient was measured using the sample after tempering. The hydrogen diffusion coefficient was determined using a measurement method described in "New Development of Delayed Fracture Elucidation" (The Iron and Steel Institute of Japan and others). A specific measuring method will be described below. First, a sample was treated with 0.5 mol / liter sulfuric acid,
In a 0.01 mol / liter KSCN mixture, the sample was cathode-charged at a current density of 20 mA / cm ² to absorb hydrogen into the sample. Thereafter, thermal analysis was performed using an atmospheric pressure ionization mass spectrometer (APIMS) equipped with an infrared image furnace. The temperature of the infrared furnace was continuously raised at 12 ° C./sec, and the amount of hydrogen gas released with the temperature rise was measured. From the hydrogen gas release curve measured in the low temperature range, the measurement result was fitted to Fick's second law to determine the hydrogen diffusion coefficient in steel. The above results are shown in Table 3 and FIG.

[0036]

[Table 3]

Further, steel materials having various compositions shown in Table 1 were subjected to a true strain (ε) of 0.21, a heating rate of 200 ° C./sec.
c, heating temperature: 1070 ° C., total heating time: 22 sec,
Quenching was performed under the condition of a cooling rate ratio (CR / CRcri): 2.5. The results are shown in Table 4 and FIG.

[0038]

[Table 4]

The steel materials a to g in Table 4 and the examples of the present invention shown in FIG. 3 satisfy the conditions of the present invention in the composition and the production method. Comparative examples in which the composition is out of the specified range of the present invention (steel materials h to n)
It is apparent that the balance between strength and toughness after quenching and tempering is significantly improved as compared with the case of FIG.

No. 1 produced by the method according to the present invention.
Nos. 1 to 10 have excellent toughness in a wide range of strength levels after tempering in a lead bath. No. whose manufacturing method is outside the range specified in the present invention. In 11 to 20, the average grain size N
₁ is less than 7.0 or the crystal grain size N ₂ showing the maximum frequency
, The area ratio of prior austenite grains having a grain size different from No. 3.0 or more exceeds 10%, and deterioration in toughness is observed.

[0041]

Since the present invention is constructed as described above, the present invention provides a martensitic steel having sufficient strength as a spring steel and a bolt steel for automobiles and excellent in toughness, and a method for producing the same. It can be done.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a method of measuring a cathode charge life.

FIG. 2 is a graph showing the relationship between the tensile strength of various martensitic steels in Table 3 and the cathode charge life.

FIG. 3 is a graph showing the relationship between the tensile strength of various martensitic steels in Table 4 and the cathode charge life.

[Explanation of symbols]

 1 jig 2 test piece

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/22 C22C 38/22 (72) Inventor Hiroshi Ieguchi 1-5-5 Takatsukadai, Nishi-ku, Kobe Stock Company Kobe Steel, Kobe Research Institute F-term (reference) 4K032 AA11 AA19 BA02 CG00 CH01 CH06 4K042 AA02 BA01 BA02 CA06 CA08 CA09 CA12 CA13 DA01 DB01 DC01 DC02 DC03

Claims (8)

[Claims] 1. A martensitic steel containing 0.1% by mass or more of C and 80% or more of the metal structure being martensite, wherein the average value of prior austenite grain size numbers is 7 or more, and the maximum frequency A high-strength, high-toughness martensitic steel characterized in that the area ratio occupied by prior austenite grains having a grain size number different from the grain size number by at least 3.0 is 10% or less. 2. The composition according to claim 1, wherein the composition contains at least one member selected from the group consisting of V ≦ 0.2% by mass, Nb ≦ 0.2% by mass, Ti ≦ 0.2% by mass and Hf ≦ 0.2% by mass. Item 4. A high-strength, high-toughness martensitic steel according to item 1. 3. Cr content of not more than 1.0% by mass and / or Mo
1 or 2 in a range of 1.0% by mass or less.
2. A high-strength and high-toughness martensitic steel according to item 1. 4. A hydrogen diffusion coefficient D _H of 1.0 × 10 ⁻⁵ cm ²
The high-strength and high-toughness martensitic steel according to any one of claims 1 to 3, which is not higher than / s. 5. The method for producing a high-strength and high-toughness martensitic steel according to claim 1, wherein cold working at a temperature of 500 ° C. or less and at least a true strain of 0.20 or more is performed. Application process, at a heating rate of 50 ° C / sec or more, Ac ₃ points + 150 ° C or more 1
A step of heating to less than 200 ° C., the total heating time from the start of heating to the start of cooling is reduced to 20 seconds or more and less than 40 seconds, and after maintaining at the heating temperature, cooling is performed at a cooling rate of at least 1.5 times the critical cooling rate A method for producing a high-strength and high-toughness martensitic steel, comprising a quenching step. 6. The method according to claim 5, wherein a cooling rate in the quenching step is 2.0 times or more a critical cooling rate. 7. A high-strength spring made of the high-strength and high-toughness martensitic steel according to claim 1. 8. A high-strength bolt comprising the high-strength and high-toughness martensitic steel according to any one of claims 1 to 4.