JPH07300654A - Stainless steel strip having high strength and high toughness and its production - Google Patents

Abstract

PURPOSE:To produce a high toughness stainless steel strip increased in Erichsen fracture stress, as an index of toughness of steel sheet, while maintaining high strength. CONSTITUTION:This stainless steel strip has a composition which consists of, by mass, <=0.10% (not including 0%) C, 1.0-4.0% Si, <=5.0% (not including 0%) Mn, 4.0-10.0% Ni, 12.0-18.0% Cr, <=3.5% (not including 0%) Cu, 1.0-5.0% Mo, <=0.15% (not including 0%) N, and the balance Fe with industrially inevitable impurities and in which C+N>=0.10% is satisfied and the value of Md(N) defined by Md(N)=580-520X[%C]--2X[%Si]--16X[%Mn]--16X[%Cr]-23X[% Ni]--300X[%N]--26X[%Cu]--10X[%Mo] is regulated to 20-100. The stainless steel strip has a metallic structure in which 30-80vol.% of working induced martensitic phases exist as a mixture in retained austenitic phases.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to members and parts which are required to have high strength and toughness as well as corrosion resistance, such as leaf springs, coil springs, and blade plates for producing Si single crystal wafers (I
The present invention relates to a stainless steel plate or a steel strip (in the present specification, simply referred to as a steel strip for the sake of including the steel plate) most suitable for a material such as a metal gasket constituting an engine of a D-blade) or an automobile, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, when manufacturing the above-mentioned members and parts using stainless steel, martensitic stainless steel, work hardening stainless steel or precipitation hardening stainless steel has been used.

[0003] Martensitic stainless steel is hardened by rapidly cooling it from a high temperature austenitic state and transforming it into martensite, and steel types such as SUS420J1 and SUS420J2 correspond to this. High strength and toughness can be obtained from these steels by tempering-tempering heat treatment. However, if the product is extremely thin, it is difficult to create the desired shape by deforming due to thermal strain during quenching.

Therefore, work-hardening austenitic stainless steel is usually used for such applications.
These are typified by SUS301 and SUS304, which exhibit an austenite phase in the solution heat treated state, and in the subsequent cold working, work-induced martensite is generated to obtain high strength. Its strength depends on the amount of cold working and martensite, but it is very difficult to adjust the strength only by cold working, and if the cold rolling rate is increased significantly, the anisotropy of the material increases, Toughness decreases.

Precipitation hardening type stainless steel is one in which an element having a high precipitation hardening ability is added to be hardened by aging treatment, and SUS630 containing Cu and SUS631 containing Al are typical. The former SUS630 has a tensile strength of at most about 1400 N / mm ² when it is hardened by aging after solution treatment. On the other hand, the latter SUS631 has considerably higher strength. That is, in the latter case, the metastable austenite phase after solution treatment was changed to a martensite phase by a pretreatment such as cold working, and then the intermetallic compound Ni ₃ Al Precipitates and hardens. By positively generating a work-induced martensite phase, for example, tensile strength 18
Can be raised to 00N / mm ² .

Further, by utilizing the strength increase due to such aging treatment, stainless steel having a higher strength than the above-mentioned steel types has been developed. For example, as described in JP-A-62-256949 and JP-A-4-202643, Cu and Si
A metastable austenite system with multiple additions of cold processing is cold-worked to form a two-phase structure of work-induced martensite and austenite phase, and subsequent aging treatment gives a tensile strength of 2000 N / mm ² and Vickers hardness of 580. . However, in order to obtain this level of strength, considerable cold working and a large amount of martensite are required, so the toughness deterioration associated with this is undeniable.

[0007]

When attempting to increase the strength of austenitic stainless steel by utilizing age hardening and work hardening, there is a problem that the toughness is significantly impaired because the cold rolling rate is increased. Furthermore, if the product is an extremely thin material, its shape will also be impaired.

Therefore, if the problem of deterioration of toughness due to heavy cold rolling can be solved, an optimum material can be provided as a material for spring materials, ID blade plates, and metal gaskets that require high strength and high toughness. The present invention is intended to solve this problem.

[0009]

According to the present invention, the mass% is
In, C: 0.10% or less (not including 0%), Si
1.0% to 4.0%, Mn: 5.0% or less (not including 0%), Ni: 4.0% to 10.0%, Cr: 12.0% to
18.0%, Cu: 3.5% or less (including no addition), Mo
Including 1.0% to 5.0%, N: 0.15% or less (not including 0%), C + N ≧ 0.10% and Md (N) = 580−520 × [% C] -2 x [% Si] -16 x [% Mn]
−16 × [% Cr] −23 × [% Ni] −300 × [% N] −26 × [% Cu]
A stainless steel containing these elements so that the value of Md (N) defined as −10 × [% Mo] is 20 to 100, the balance being Fe and impurities unavoidably mixed in during manufacturing. A high-strength and high-toughness stainless steel strip having a metal structure in which a work-induced martensite phase of 30 to 80% by volume is mixed in a retained austenite phase.

Further, according to the present invention as a method for producing this high-strength and high-toughness stainless steel strip, the stainless steel strip adjusted to the above-mentioned composition is first prepared in the temperature range of 980 to 1150 ° C. in the average grain size. A stainless steel strip having a metallographic structure, which is solution-treated under the condition that austenite single-phase crystal grains with a diameter of 5 to 50 μm are formed and the number of undissolved precipitates larger than 100 nm in each austenite grain is 5 or less Is cold-rolled at a cold-rolling rate sufficient to produce 30-80% by volume of work-induced martensite, and then 3
There is provided a manufacturing method characterized by performing an aging treatment at 00 to 650 ° C for 0.5 to 5 minutes.

[0011]

Stainless steel strip of the effects of the present invention is described below in exemplary tensile strength as shown in the example of simultaneously including a high-strength and high toughness say and Erichsen breaking stress at 1800 N / mm ² or 1000 N / mm ² or more . The Erichsen rupture stress is a value obtained by dividing the load when cracking occurs in the specimen in the known Erichsen test by "plate thickness of the specimen x punch diameter", and is an index suitable for evaluating the toughness of thin plate materials. is there.

The steel strip of the present invention has such high strength as well as high toughness, the details of which will be demonstrated in Examples. Mainly, (1) C, N, Si, Mn in steel , Ni, Cr, Cu, Mo are contained within the ranges specified in the present invention, and the amount of each component is strictly adjusted so that the Md (N) value becomes 20 to 100, thereby inducing processing. When the steel of the present invention having a mixed structure of martensite and retained austenite phase is subjected to aging treatment and strained, the retained austenite phase has a property of being capable of being transformed into martensite to an appropriate degree (for example, as described below). Examples 1), (2) A relatively large amount of Mo is solid-dissolved in steel to suppress coarsening of crystal grains during solution treatment and to perform aging treatment at a relatively high temperature after cold rolling. Even if it does, it has an advantageous effect on the improvement of strength and toughness, (3)
By austenite grain size finer than 50μm and minimizing the number of precipitates in each grain, and then cold rolling and aging treatment, the precipitates are finely precipitated during aging and the strain in the aging treated material is It is considered that the characteristic action that the retained austenite phase transforms into fine martensite and relaxes the concentrated stress leading to cracking is also interrelated when added.

The features of the steel of the present invention will be specifically described below. First, the outline of the action of each component and the reason for limiting the content in the steel of the present invention will be explained individually. "%" Of each component amount in the steel of the present invention represents "mass%" unless otherwise specified.

C is an austenite-forming element, which acts very effectively in suppressing δ-ferrite produced at high temperature and strengthening the martensite phase induced by cold working. However, since the steel of the present invention has a high Si content, C The solid solubility limit of has decreased.
Therefore, when C is increased, Cr carbide is precipitated in the grain boundaries, which causes intergranular corrosion resistance and a decrease in toughness. For these reasons, the C content is 0.10% or less (not including 0%), and preferably 0.05 to 0.10%.

Si is usually added for deoxidation,
When added for this purpose, it is generally 1.0% or less, as seen in work-hardening stainless steels SUS301 and 304. However, in the present invention, the content of Si is made higher than this, and the content of Si accelerates the formation of the martensite phase during cold rolling and hardens the martensite phase, and the retained austenite phase is formed. The strength after cold rolling is increased by utilizing the action of forming a solid solution and hardening it. Further, in the aging treatment, the inclusion of Si promotes the age hardening ability of the steel due to the interaction with Cu. As described above, in the steel of the present invention, Si has a characteristic effect, but the effect is 1.0 as in the conventional steel.
%, It is small, and when it exceeds 4.0%, hot cracking is likely to occur during hot deformation, which causes various problems in manufacturing.
Therefore, the Si content is set to more than 1.0% and 4.0% or less. The preferable Si content is more than 1.0% and 3.5% or less.

Mn is an element that controls the stability of the austenite phase, and its content in the steel of the present invention is determined by the balance with other elements. If the Mn content is too high, the martensite phase is less likely to be induced when the steel is cold rolled, so the upper limit of the Mn content is set to 5.0%. The preferred Mn content is 4.5% or less (not including 0%).

Ni is an essential element for obtaining an austenite phase at high temperature and room temperature, but in the case of the steel of the present invention, it must be a metastable austenite phase at room temperature and induce a martensite phase by cold rolling. Ni content 4.0
If it is lower than%, a large amount of δ-ferrite phase is formed at high temperature, and a martensite phase is formed during the cooling process up to room temperature, and it cannot exist as an austenite single phase.
If it exceeds 10.0%, the martensite phase is less likely to be induced by cold working, so it was set to 4.0 to 10.0%. Preferred Ni
The content is 5.0 to 9.5%.

Cr is an essential component for ensuring the corrosion resistance of steel. At least to give the intended corrosion resistance
Requires 12.0% Cr. However, Cr is also a ferrite-forming element, so if it is made too high, a large amount of δ-ferrite phase will be formed at high temperatures. Therefore, in order to suppress the δ-ferrite phase, austenite forming elements such as C, N, N
Although i, Mn, Cu, etc. must be added in excess, excessive addition of these elements results in stabilization of austenite at room temperature, and no work-induced martensite phase is formed during cold rolling. It becomes impossible to obtain high strength. Therefore, the upper limit of Cr is set to 18.0%. The preferable Cr content is 12.0-16.5%.

As described above, Cu hardens the steel by the interaction with Si as described above, but excessive addition deteriorates the hot workability and causes cracking. Below. The preferable Cu content is 1.0 to 3.5%
Is.

Mo has the effect of improving corrosion resistance and suppressing grain growth of austenite during solution treatment. Also, Mo acts extremely effectively in suppressing the rapid release of strain during high temperature aging. In manufacturing the steel of the present invention,
The aging temperature is relatively high to reduce excessive rolling strain,
It is advantageous to prevent the deterioration of toughness due to excessive rolling strain, but the too rapid release of strain by this high temperature aging is also disadvantageous in terms of strength. When Mo is contained, the excessive disappearance of dislocations can be suppressed by the drag effect even when the aging temperature is set high. Further, since Mo forms a precipitate that contributes to the strength by the aging treatment, from this point as well, it is effective in preventing the decrease in the strength when the aging treatment is performed in the high temperature region. The content of 1.0% or more is required to obtain the characteristic effects of Mo. However, if Mo is added too much, δ
Ferrite is formed, and when the amount of Mo increases, the deformation resistance at high temperature also increases and the hot workability decreases, so Mo is 5.0% or less, preferably 4.5%.
Below.

N is an element which forms austenite and is an element which is extremely effective in hardening the austenite phase and martensite phase. However, addition of a large amount causes blowholes during casting, so 0.15% or less (0 % Is not included). The preferable N content is 0.04 to 0.10%.

C and N show the same hardening action as each other, and in order to fully exert their effects, the total amount of C + N is 0.
Must be 10% or higher. That is, the total amount of C + N
If it is less than 0.10%, it will be difficult to adjust other components within the range specified in the present invention in order to simultaneously impart sufficient strength and toughness intended in the present invention to the steel.

[Regarding Md (N) Value] In the steel of the present invention, the Md (N) value defined by the above-mentioned formula has a very important meaning. High strength and high toughness can be simultaneously provided for the first time by adjusting the amount of each component so that the Md (N) value falls within the range of 20 to 100. That is, even if the Md (N) value is less than 20 or exceeds 100, as shown in Example 1, the tensile strength is 1800 N / mm ² or more and the Erichsen breaking stress is high.
Not able to satisfy 1000 N / mm ² or more at the same time.

The meaning of this Md (N) value is not related to the method for producing the stainless steel strip of the present invention. That is,
The stainless steel strip of the present invention basically has a work-induced martensite phase of 30 to 80 after undergoing a solution treatment step for solid solution of precipitates in a matrix phase (austenite phase).
Manufactured by a series of processes in which cold rolling is performed at a cold rolling rate such that it is produced in an amount of volume% (the balance is substantially a retained austenite phase), and finally this cold rolled material is aged. However, the stainless steel strip of the present invention that has undergone the final aging treatment is given a strain that would otherwise cause cracking due to proper adjustment of the Md (N) value. In addition, the property that the austenite phase remaining in the steel transforms into the martensite phase in a form that contributes to the improvement of toughness is imparted.

The structure of the stainless steel strip of the present invention which has undergone the final aging treatment has a two-phase structure in which the residual austenite phase and the martensite phase are finely mixed. To be precise, two types of retained austenite phase and tempered work-induced martensite
It is a phase-mixed structure.

When the retained austenite of the aged material is represented by γ and the work-induced martensite is represented by α ′, the soft γ phase and the hard α ′ phase (actually, the α ′ phase tempered by the aging treatment) are When a deformation stress is applied to the mixed aging material, the soft γ phase is deformed and the stress is concentrated, resulting in the occurrence of minute cracks. The strain is concentrated and a new α'phase is generated. Therefore, the generated α'phase blocks the propagation of cracks, and if external stress is applied, stress concentrates on another γ phase and new cracks start to appear. Again, α'is formed at the crack tip. Phases form.

At that time, the ease of forming the α'phase is Md.
There is a correlation with the value of (N), and if Md (N) is small, it will not be generated,
Therefore, the crack propagates in γ and progresses to a large crack, which eventually leads to fracture. That is, the toughness is low. On the other hand, if Md (N) is too large, the α'phase is immediately generated and it becomes difficult to deform.
If this is forcibly deformed, it will develop into cracks, and in this case as well, the toughness will decrease. That is, even if Md (N) is too large, the crack propagation due to the transformation of γ → α 'cannot be prevented. In the steel of the present invention, Md (N) represented by the above formula
One of the reasons for defining the range of 20 to 100 is to improve the toughness of the aged material by advantageously utilizing the transformation behavior of the γ phase, which is softer than the α'phase.

The Md (N) value is also an index showing the ease with which the α'phase is generated during cold rolling. Although the amount of α'phase generated in cold rolling is related to the rolling reduction and rolling temperature, Fig. 1 shows the amount of α'phase generated by cold rolling, Md (N) value and cold rolling temperature. It shows a correlation. The amount of α'phase after cold rolling is intended in the present invention when the cold rolling ratio is set to a normal operable range of 30 to 70%.
Depending on the Md (N) value, A of FIG.
Cold rolling may be carried out at a rolling temperature in the range surrounded by B, C and D.

In FIG. 1, in the region below the straight line AB, the γ phase may be insufficient and the toughness of the aged material may be impaired.
Further, in the region above the straight line DC, the α'phase may be insufficient and the strength of the aged material may be impaired.

Next, a method for manufacturing the steel strip of the present invention will be described.

According to the present invention, after the stainless steel whose composition has been adjusted as described above is melted, it is hot-rolled and, if necessary, further cold-rolled to produce the raw material steel strip first. Then, in this steel strip, austenite single-phase crystal grains having an average grain size of 5 to 50 μm are formed in a temperature range of 980 to 1150 ° C., and the number of undissolved precipitates larger than 100 nm in each austenite grain is 5 or less. Solution treatment under the following conditions, stainless steel strip with this metallographic structure
It is cold-rolled at a cold-rolling rate sufficient to generate 80% by volume of work-induced martensite, and then subjected to an aging treatment at 300 to 650 ° C for 0.5 to 5 minutes.

First, in the solution treatment step, in order to improve the toughness of the final aging material, the solution treatment temperature is set to 980 to 115.
It is necessary to set the temperature in the range of 0 ° C., and after holding the temperature in this temperature for a required time, the cooling is performed at a cooling rate such that precipitates do not appear in the cooling process from this temperature to normal temperature. If the heating temperature is lower than 980 ° C, the precipitates existing in the raw steel strip cannot be completely dissolved in the austenite of the matrix,
On the contrary, relatively large precipitates are deposited and the purpose of solution treatment cannot be achieved. When precipitates are formed during the solution heat treatment, the amount of fine precipitates that contribute to the strength is reduced during the final aging treatment, the strength improving effect is diminished, and coarse precipitates remain. Has a great effect on the toughness because it gives the final aged material a large notch sensitivity. On the other hand, if the solutionizing temperature exceeds 1150 ° C, the austenite grains become coarse, and in this case also, the toughness of the final aged material decreases.

In this solution treatment, M in the steel of the present invention
o plays the role of enabling the solution temperature range to a relatively high temperature range of 1150 ° C. Grain growth is suppressed by the drag effect that the diffusion rate of Mo atoms themselves is slow. The effect of adding Mo in improving the toughness of the final aged material in relation to the solution treatment temperature is shown in FIG.

In this solution treatment, in addition to the regulation of the heating temperature, the holding time at that temperature is appropriately adjusted to form austenite single-phase crystal grains with an average grain size of 5 to 50 μm. In addition, the number of undissolved precipitates larger than 100 nm in each austenite grain must be 5 or less. If the solution-treated steel strip has austenite grains larger than 50 μm, the toughness of the aged material cannot be improved as intended even if the precipitates are completely solid-solved, and the individual austenite grains If the number of undissolved precipitates larger than 100 nm is more than 5, even if the crystal grains are within 50 μm,
Again, the toughness of the aged material cannot be improved as intended. This fact is shown in Figure 3 below.

Then, the solution-treated steel strip is cold-rolled to generate a work-induced martensite phase of 30 to 80% by volume. As described in FIG. 1, the amount of the martensite phase generated has a correlation with the Md (N) value, the cold rolling rate and the cold rolling temperature. %. If the amount of martensite is less than 30% by volume, the strength of the final aging material will not be sufficient. That is, the amount of work-induced martensite is required to be 30% or more in order to increase the strength of the material itself and increase the nuclear sites of precipitates that contribute to the increase in strength due to precipitation aging during aging treatment. However, when the amount of martensite is more than 80% by volume, the amount of retained austenite is decreased, and the amount of austenite involved in the toughness of the aged material is insufficient, so that sufficient toughness cannot be obtained.

Assuming that a normal cold rolling rate of 30 to 70%, which is advantageous in operation, is adopted as the rolling temperature, as shown in FIG.
As described above, the temperature may be set in the range of -20 ° C to 100 ° C depending on the Md (N) value.

The aging treatment performed after cold rolling is from 300 to
It is desirable to carry out the treatment in the temperature range of 650 ° C. for 0.5 to 5 minutes. Such a short time heat treatment makes it possible to continuously pass the cold-rolled steel strip through a continuous heat treatment furnace for treatment. Even if the component composition specified in the present invention and other manufacturing conditions are satisfied, a sufficient strength increase cannot be expected at an aging temperature of less than 300 ° C., and at a temperature of more than 650 ° C. It reversely transforms into an austenite phase and causes a decrease in strength. If the soaking time is 0.5 minutes or less, a sufficient aging effect cannot be expected, and it is inconvenient that soaking takes 5 minutes or more in consideration of the continuous heat treatment.

In the steel of the present invention, the elements that exert the main hardening action by aging are Si, Cu, C, N and Mo,
Si and Cu contribute to hardening by fixing the strain introduced during rolling, and C, N and Mo contribute to hardening as precipitates. In particular, Mo acts effectively to suppress the excessive disappearance of dislocations due to the drag effect when the aging temperature is set to be high in addition to the formation of precipitates.

[0039]

【Example】

[Example 1] Table 1 shows the chemical component values (% by mass) of the test materials and the Md (N) values described in the text. T1 to T12 in the table
Is a steel of the present invention having a composition and Md (N) value within the ranges specified in the present invention, and a to i are the amounts of any component or Md.
It is a comparative steel whose (N) value is out of the range specified in the present invention.
All of the steels were melted in a vacuum melting furnace and forged, hot rolled, intermediate annealed, and cold rolled into a cold rolled steel strip with a thickness of 2.0 mm.

Each steel strip (excluding comparative steel i) was subjected to solution treatment by holding it at 1050 ° C. for 1 minute and then water-cooled. After that, each material was cold-rolled at the cold rolling rate shown in Table 2, the amount of martensite of each material induced by this cold rolling was measured, and the measured value (volume%) is shown in Table 2. .

Next, each cold rolled material was subjected to an aging treatment at 570 ° C. for 1 minute. A test piece was taken from this aged material,
Tensile strength and Erichsen breaking stress were measured. The results are also shown in Table 2. The Erichsen rupture stress is the value obtained by dividing the load at the time of crack occurrence by "plate thickness x punch diameter" in the well-known Erichsen test. This Erichsen rupture stress is suitable for evaluating the toughness of thin plate materials, and the higher this value, the better the toughness.

[0042]

[Table 1]

[0043]

[Table 2]

As can be seen from the results shown in Table 2, all the steels of the present invention have a tensile strength of 1800 N / mm ² or more and an Erichsen rupture stress.
It is clear that both strength and toughness are excellent, satisfying 1000 N / mm ² or more at the same time.

On the other hand, the comparative steels a to h cannot satisfy both characteristics at the same time. This result can be analyzed as follows. Comparative steels a and b are C, Si, Mn, Ni, Cr, Cu, Mo,
The amount of each component of N is within the range specified by the present invention, but the Md (N) value is lower than the range 20-100 specified by the present invention. Therefore, for example, in Comparative Steel a having an Md (N) value of −21.6, the amount of martensite is as low as 25% even at a cold rolling ratio of 70%, and therefore the tensile strength of the aged material is also low. Further, in the comparative steel b having Md (N) value of 9.2, the amount of martensite was 42% at the cold rolling reduction rate of 70%, but the austenite phase remaining in the aged material was Does not transform into the martensitic phase during transformation. For this reason, the comparative steel b does not exhibit sufficient toughness because the Erichsen rupture stress is low even though the tensile strength is high.

Comparative steels c and d are C, Si, Mn, Ni,
The amounts of Cr, Cu, Mo and N are all within the range specified by the present invention, but the Md (N) value is higher than the range specified by the present invention. For this reason, a sufficient amount of martensite is produced even at a low cold rolling ratio, but the austenite phase remaining in the aged material transforms into the martensite phase during the initial deformation during deformation in the Erichsen test. Therefore, the Erichsen rupture stress is low and it does not exhibit sufficient toughness.

As described above, even if the amount of each component is within the range specified in the present invention, if the Md (N) value is lower than the value specified in the present invention, the martensite phase is generated during the deformation of the aging material. On the contrary, when the Md (N) value is higher than the value specified in the present invention, the martensite phase is apt to be generated during the deformation of the aged material. Absent. On the other hand, in the steel of the present invention in which the amounts of the respective components are mutually adjusted so that the Md (N) value is in the range of 20 to 100, the austenite → martensite transformation is likely to occur during the deformation of the aged material. Therefore, it can be considered that the alloy has a high strength and a high Erichsen rupture stress. That is, one reason why the steel of the present invention has high strength and toughness at the same time is that the amount of each component is adjusted so that the Md (N) value falls within an appropriate range.

On the other hand, the comparative steels e to h have Md (N) values in the range specified by the present invention, but have low tensile strength. This is because the comparative steels e and f had a Mo content lower than that specified in the present invention, so that softening started quickly during the aging treatment, and in the comparative steels g and h, the Si content was specified in the present invention. It is considered that the contribution of Si to the strengthening due to strain aging is small because it is lower than the amount.

[Example 2] A cold rolled steel strip of T12 in Table 1 was used as a test material, which was solution-treated at 1050 ° C for 1 minute, water-cooled, and then cold-rolled at a cold rolling rate of 55%. . The amount of work-induced martensite of this cold rolled material was 58% by volume.

A test piece was taken from this cold rolled material and subjected to an aging treatment by changing the aging conditions (temperature and holding time). For each of the obtained aged materials, the tensile strength and the Erichsen breaking stress were measured in the same manner as in Example 1. Table 3 shows the aging conditions and the measurement results.

[0051]

[Table 3]

From the results shown in Table 3, the aging temperature is 300 to 65.
0 aging to the tensile strength at conditions ℃ at 0.5 to 5 minutes 1800 N / mm ² or more, satisfying Erichsen break stress 1000 N / mm ² or more at the same time, the material strength and toughness is excellent in both is obtained I understand. On the other hand, when the aging temperature is as low as 250 ° C., it is considered that the strain aging and the precipitation hardening are insufficient, but the tensile strength is low. On the other hand, even when the aging temperature is as high as 660 ° C and 700 ° C, the tensile strength is low. This is probably because part of the work-induced martensite formed during cold rolling undergoes reverse transformation to the austenite phase during this high-temperature aging. Further, in the case of aging treatment at 700 ° C., the Erichsen rupture stress decreases with the strength. The cause of this decrease in toughness is that this temperature is in the sensitization temperature range.
It is considered that this is due to the formation of coarse grain boundary precipitates.

[Example 3] Of the steels in Table 1, cold rolled steel strips of comparative steel i having different Mo amounts and steels T6, T7 and T8 of the present invention were selected, and the solution heat treatment temperature was changed to carry out the solution heat treatment. Then, after that, each was cold-rolled at a cold-rolling rate of 50% and then subjected to an aging treatment at 500 ° C. for 1 minute. For each aged material, the tensile strength and Erichsen rupture stress were measured as in Example 1. The tensile strength of each steel is 1950 N / mm ^{2 to} 2000
The strength level was almost the same as N / mm ² .

However, the measured values of the Erichsen rupture stress of each steel showed various values, and the results shown in FIG. 2 were obtained by arranging the measured values by the solution treatment temperature. The following can be seen from the results shown in FIG. First, in all steels, the Erichsen rupture stress value peaks when the solution treatment temperature is around 1000 ° C. Secondly, the Erichsen rupture stress value tends to decrease as the temperature rises above about 1000 ° C, but the low Mo amount of comparative steel i shows a sharp decrease, whereas the Mo amount decreases. The degree of decrease is low for all steels with 1.0% or more.
Thirdly, only the low Mo amount of the comparative steel i has a low Erichsen rupture stress value, and among the steels of the present invention, the higher the Mo amount, the higher the overall value.

From the above, Mo contributes greatly to the improvement of the toughness of the steel of the present invention in the present invention, and the inclusion of this Mo causes the Erichsen rupture stress value of 1000 N / m.
It can be seen that the solution treatment temperature at which m ² or more is obtained is 980 to 1150 ° C, which is a relatively wide range. The fact that the solution treatment temperature range may be wide is very convenient in terms of manufacturing the steel strip of the present invention.

On the other hand, according to the results of FIG. 2, when the solution heat treatment temperature is lower than 980 ° C., the Erichsen rupture stress value of all the steels sharply decreases. It can be considered that this is because undissolved precipitates remained, and if there are undissolved precipitates, sufficient toughness cannot be secured even if the subsequent cold rolling and aging treatment are appropriately performed. On the other hand, the present inventors have considered that the coarsening of the austenite grain size may be the reason why the toughness decreases as the solution treatment temperature increases. Therefore, the following tests were conducted.

Example 4 From Table 1 from the invention steels T6 and T
Eleven cold-rolled steel strips were selected, a large number of test pieces were cut out from these strips, and the solution treatment conditions were changed as shown in Table 4. Then, the average austenite grain size and the number of precipitates (including grain boundary precipitates) existing in each austenite grain were examined for each solution treated test piece.

[0058]

[Table 4]

The grain size was measured by a comparative method using an optical microscope, and the number of precipitates was arbitrarily selected from 10 to 15 austenite grains by electron microscope observation. The value was calculated. In addition, the number of precipitates having a size of 100 nm or more was counted. After this observation, all solution heat treated materials had a cold rolling rate of 50%.
Then, a cold reduction corresponding to the above was applied and aging treatment was performed at 500 ° C for 1 minute. Then, the Erichsen rupture stress of these aged materials was measured in the same manner as in Example 1.

FIG. 3 shows that the Erichsen rupture stress value is 1000 N / mm.
Those with ² or more are marked with ●, those with less than 1000 N / mm ² are marked with ○, the average austenite grain size (μm) after solution treatment is plotted on the horizontal axis and the intra-grain precipitation of 100 nm or more on the vertical axis. It is plotted in the coordinates of the number of objects. The number attached to each plot represents the sample number in Table 4.

From the results shown in FIG. 3, the average austenite grain size is 50 μm or less (5 μm or more) and the number of intragranular precipitates is 5.
It can be seen that the number of pieces or less shows an Erichsen breaking stress of 1000 N / mm ² or more. On the other hand, sample No. 10
Even if the number of intragranular precipitates of 100 nm or more like No. 11 is 0, those with a large austenite grain size do not show good toughness, and the austenite grain size like Sample No. 1 and No. 2 Even if the value is small, a material having a large number of precipitates does not show good toughness.

Therefore, in order to improve the toughness of the final aged material, as shown in Examples 1 to 3, proper adjustment of the Md (N) value, proper aging treatment conditions, proper Mo amount and solution treatment. In addition to consideration of temperature, etc., good results can be obtained by making the austenite grain size as small as possible and completely precipitating the precipitates in the matrix phase or minimizing undissolved precipitates in the solution treatment stage. Obtainable.

[0063]

As described above, according to the method of the present invention, stainless steel not only achieves high strength but also high toughness as compared with the conventional work hardening type stainless steel or precipitation hardening type stainless steel. was gotten. Therefore, the high-strength and toughness stainless steel strip of the present invention is a leaf spring, a coil spring, or Si which is required to have high strength and toughness simultaneously with corrosion resistance.
As a material for blade plates for making single-crystal wafers and metal gaskets that compose engines for automobiles, etc., it exerts an effect not seen in conventional materials and expands the use of stainless steel.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an Md (NT) value and a cold rolling temperature on a generation amount of a work-induced martensite phase.

FIG. 2 is a diagram showing the effect of solution treatment temperature on the Erichsen rupture stress (index of toughness) of the final aged material for each steel having a different Mo content.

FIG. 3 is a diagram showing the influence of the average austenite grain size at the stage of solution treatment and the number of precipitates in each grain on the Erichsen rupture stress (toughness index) of the final aged material.

Claims (4)

[Claims] 1. In mass%, C: 0.10% or less (not including 0%), Si: 1.0% to 4.0%, Mn:
5.0% or less (not including 0%), Ni: 4.0% to 10.
0%, Cr: 12.0% to 18.0%, Mo: 1.0% to
5.0%, N: 0.15% or less (not including 0%) is included, C + N ≧ 0.10% is satisfied, and Md (N) = 580−520 × [% C] −2 × [% Si] -16 × [% Mn]
-16 × [% Cr] -23 × [% Ni] −300 × [% N] −10 × [% Mo] These elements are defined so that the value of Md (N) is 20 to 100. It is a stainless steel strip that contains Fe, and the balance is Fe and impurities that are unavoidably mixed in the manufacturing process, and has a metallographic structure in which a work-induced martensite phase of 30 to 80% by volume is mixed in the retained austenite phase. High strength and toughness stainless steel strip. 2. In mass%, C: 0.10% or less (not including 0%), Si: 1.0% to 4.0%, Mn:
5.0% or less (not including 0%), Ni: 4.0% to 10.
0%, Cr: 12.0% to 18.0%, Cu: 3.5% or less (not including 0%), Mo: 1.0% to 5.0%, N:
Including 0.15% or less (not including 0%), satisfying C + N ≧ 0.10%, and Md (N) = 580−520 × [% C] −2 × [% Si] −16 × [% Mn]
−16 × [% Cr] −23 × [% Ni] −300 × [% N] −26 × [% Cu]
A stainless steel containing these elements so that the value of Md (N) defined as −10 × [% Mo] is 20 to 100, the balance being Fe and impurities unavoidably mixed in during manufacturing. A high-strength and high-toughness stainless steel strip having a metal structure in which a work-induced martensite phase of 30 to 80% by volume is mixed in a retained austenite phase. Wherein a tensile strength of 1800 N / mm ² or more and Eriksen breaking stress is 1000 N / mm ² or more according to claim 1 or 2
High strength and high toughness stainless steel strip described in. 4. In mass%, C: 0.10% or less (not including 0%), Si: 1.0% to 4.0%, Mn:
5.0% or less (not including 0%), Ni: 4.0% to 10.
0%, Cr: 12.0% to 18.0%, Cu: 3.5% or less (including no addition), Mo: 1.0% to 5.0%, N:
Including 0.15% or less (not including 0%), satisfying C + N ≧ 0.10%, and Md (N) = 580−520 × [% C] −2 × [% Si] −16 × [% Mn]
−16 × [% Cr] −23 × [% Ni] −300 × [% N] −26 × [% Cu]
A stainless steel containing these elements so that the value of Md (N) defined as −10 × [% Mo] is 20 to 100, the balance being Fe and impurities unavoidably mixed in during manufacturing. Steel strip of
Temperature range of 980 to 1150 ° C and average particle size of 5 to 5
The solution treatment is performed under the condition that 0 μm austenite single-phase crystal grains are formed and the number of undissolved precipitates larger than 100 nm in each austenite grain is 5 or less, and the stainless steel strip having this metallographic structure High-strength, high-toughness stainless steel consisting of cold-rolling at a cold-rolling rate sufficient to form 80% by volume of work-induced martensite, and then aging treatment at 300-650 ° C for 0.5-5 minutes. Method of manufacturing obi.