JPS62156250A - High strength and high toughness maraging steel and its production - Google Patents

Abstract

PURPOSE:To obtain a titled maraging steel by specifying the contents of Mo and Ti in the maraging steel and controlling the content of N thereby specifying the component ratios of the maraging steel essentially consisting of Mo. CONSTITUTION:This maraging steel consists, by weight %, of <=0.005 C, <=0.06 total of Si, Mn and Cr, 16.5-18.0 Ni, 12.0-15.0 Co, 0.03-0.15 Al, 5.3-6.0 Mo, 1.30-1.50 Ti, <=0.0015 N and the balance Fe and satisfies the formula. The steel is subjected to a soln. heat treatment for heating for 30min-3hr at 800-860 deg.C and aging treatment for 1-10hr at 470-520 deg.C. The steel is subjected to cold working at >=30% reduction of area prior to the above-mentioned soln. heat treatment if necessary. Or the steel is further subjected successively to cold working at >=30% reduction of area and the soln. heat treatment for heating up within 10min up to the solutionizing temp. of 830-950 deg.C and holding for 1-2min then cooling down to an air temp. at a cooling rate above the air cooling rate prior to the cold working. The steel having >=250kg/mm<2> tensile strength and good toughness is thus obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a marage steel that exhibits a tensile strength of 250 kgf 7112 or more and also has excellent toughness.
and its manufacturing method.

Ultra-high-speed rotating bodies such as rocket motor cases, various tools (e.g. M hot extrusion dies), various springs, and power storage flywheels usually require the highest possible strength. In metal materials, there is generally a tendency for toughness to decrease as strength increases, so at present there are very few materials that fully satisfy the above requirements.

<Prior art and its problems> Currently, "marage steel" can be cited as one of the few metal materials used for the above-mentioned purposes, and it is one of the ultra-high tensile steels with a tensile strength of 200 kgf/m2 or more. This is because it shows the most excellent toughness.

By the way, the toughness of materials used for the above-mentioned purposes is evaluated by the strength of a "notched tensile test piece", which is a tensile test piece with a notch. The strength is 240 kg when using a test piece of
Up to f / 1 tat2, the "strength of the notched tensile test piece (N, T, S, )" exceeds the "strength of the smooth tensile test piece (T, S, )", so this level of strength is It has been put to practical use as a tough material.

However, when the tensile strength exceeds 250 kgf/B2, the “notch strength ratio (N, T, S, / T, S, )
If " is less than 1.0, the toughness is insufficient and it has not yet been put to practical use as a tough material.

That is, conventionally, tensile strength: 250 kgf/rtan
13%Ni-15%Co as a maraging steel of 2 or more
-10%Mo system” and '18%Ni-15%Co-6
, 5 Ti Mo - 1,1% Ti system (hereinafter, the component ratio is expressed as weight %), but its toughness is 200 to 220 kgf. It showed only an inferior value.

However, the above ''' 13%Ni -15%Co -1
Although it is known that there is a method for rapidly heating 0% Mo-based maraging steel to the solution treatment temperature to improve its toughness,
This method requires a solid solution temperature of 1000°C or higher, so there is a problem that a sufficient heating rate cannot be obtained for thick-walled members.
, 51Mo-1,1%Ti" marage steel, no effect of rapid heating was clearly observed.

Means for Solving the Problems〉 From the above-mentioned viewpoint, the present inventors have determined that the tensile strength is 25.
0 kgf/1tai2 or more, and the "notch strength ratio (N, T, S, / T, S, )" is 1.0
As a result of the above-mentioned research to provide a marage steel that can be used as a tough material, we found that: (a) MO and T, which are strengthening elements in marage steel;
By adjusting the content of i to a specific value and reducing the amount of N, which is an impurity in the steel, to o, oolss (x 5 ppm), a maraging steel with significantly improved toughness while maintaining sufficient strength can be created. (b) Furthermore, if the composition ratio of the maraging steel is adjusted to a specific value, especially with MO as the main component, the solution treatment temperature can be lowered to the conventional "13"Ni-15"Co without deteriorating its performance. -1
The strength of the 0% MO system can be significantly lowered, and this makes it possible to achieve a remarkable increase in strength that cannot be obtained by ordinary solution treatment and aging treatment. In addition, toughness can also be improved; (C) If such marage steel is subjected to cold working before solution treatment, the grains after solution treatment become significantly finer, and toughness is further improved; It was found that when grain refinement treatment is performed twice by combining cold working and solution treatment, the effect of improving toughness becomes even more remarkable.

This invention was made based on the above knowledge, and the following steps were taken to prepare marage steel: C: 0.005% or less, Si, Mn and Cr: 0.06% or less in total, Ni: 1
6.5-18.0%, Co: 12.0-15.0%, All: 0.03-0.15%, Mo: 5.3-6.0%, Ti: 1.3 0-1 .50%, N:0.0.
0.0015% or less, Fe and other unavoidable impurities: the remainder, and constituted so that the amount of Mo, the amount of Ti, and the amount of Ti satisfy the formula Mo (%), 250 kgf /wx2
It is characterized by having both the above tensile strength and excellent toughness.
Solid solution treatment by heating and holding at 470 °C for 30 minutes to 3 hours
Aging treatment at ~520°C for 1 to 10 hours, or cold working to a reduction in area of 30 inches or more before the solution treatment, and then subjecting it to a similar heat treatment, or Before the solid solution treatment, cold working with a cross-section reduction rate of 30 inches or more is performed, and the temperature is raised to a solid solution temperature of 830 to 950°C within 10 minutes, held for 1 to 20 minutes, and then cooled at a cooling rate higher than air cooling. A solution treatment of cooling to room temperature is sequentially applied, followed by cold working with an area reduction rate of 30% or more, followed by a similar heat treatment, resulting in a tensile strength of 250kgf/11uIL2 or more and excellent toughness. It is also characterized by the fact that it manufactures marage steel that has the following characteristics.

Next, the reason why the composition of the marage steel or its manufacturing conditions are numerically limited as described above in this invention will be explained.

A. Composition of steel a) C C is an element that reduces the toughness of steel by becoming TiC and increasing nonmetallic inclusions, and also consumes Ti and interferes with precipitation strengthening by Ti. It is desirable to reduce the C content as much as possible, but if the C content is reduced to 0.0 O5% or less, the adverse effects on toughness and strength can be suppressed to a practically acceptable level. OO is set at 5% or less.

b) Si, Mn, and cr In maraging steel, all of these elements cause an increase in inclusions as oxides.
If the total content exceeds 0.06%, the JIS-specified 6 steel cleanliness exceeds 0.03%, which is particularly noticeable for materials with a tensile strength of 250 kgf/mm2 or higher. The total content of Si, Mn, and Cr was determined to be 0.06% or less since it would cause a decrease in toughness.
Preferably, the total content of these elements is suppressed to 0.03% or less.

c) Ni The N1 component has the effect of actively improving the toughness of marage steel, and after aging treatment, the amount of N1 in the matrix decreases to 10
% or less, the toughness of the steel will drop significantly.

By the way, T, which is a constituent component of the maraging steel of this invention,
All of i becomes an intermetallic compound N i3T:L, which contributes to precipitation strengthening and consumes Ni, and MO is also an element contributing to precipitation strengthening, which partially forms Ni 5MO and consumes Ni. Therefore, the Ni content is 16.
If it is less than 5'%, it will not be possible to secure 10 N1 views in the base after the aging process.

On the other hand, if the Ni content exceeds 18.0%, martensitic transformation will not be completed sufficiently even if the steel is cooled to room temperature after the solution treatment, and austenite will remain, making it impossible to ensure sufficient strength. This is because austenite does not have aging strengthening ability.

For this reason, the Ni content is 16.5 to 18.0.
It was decided that

d) C.

The CO acid component has a solid solution strengthening effect, an effect of promoting the precipitation of intermetallic compounds including MO, and an effect of suppressing austenite that precipitates during aging.Since it is an expensive element, it is desirable to suppress its content as much as possible. However, it is not possible to increase the content of MO and Ti, which contribute to precipitation strengthening, beyond a certain value, so in order to secure a tensile strength of 250 k17f / ML2, it is necessary to contain more than 12.0 Ti. be. On the other hand, if its content exceeds 15.0%, it will not only cause a decrease in toughness but also be uneconomical.
The o content was determined to be 12.0 to 15.0%.

e) M.

The Mo component produces two types of precipitates, FeMo and Ni5Mo, and contributes to strengthening without impairing toughness, but if the content is less than 5.3%, the desired strength cannot be secured. On the other hand, if the content exceeds 6.096, a large amount of intermetallic compound of MO will precipitate during heating during solution treatment, and this will not dissolve at the solution temperature and will significantly increase the toughness. Therefore, the MO content was set at 5.3 to 6.0%.

Note that the undissolved precipitates include Si%Mn and Cr.

It has the same toughness deterioration effect as nonmetallic inclusions formed by N1, etc., and for this reason, in solution treatment at a normal heating rate (for example, 30 minutes to 2 hours to the solution temperature), it is difficult to maintain the temperature above 860℃. Temperature is required. However, when the temperature exceeds 860° C., the dislocation density of the marage steel after cooling decreases, the strength after aging decreases, and the strengthening ability by Mo significantly decreases. Therefore, from this point of view, MOO
The amount must be kept below 6.0 inches.

f) Ti The Ti component has the effect of forming Ni3Ti and improving the strength of steel, but if its content exceeds 1.5% or
Or the ratio of (MO(96)/Ti(chi)) is 3.6
If it is less than 100%, acicular Ni 3 Ti will also precipitate at grain boundaries, resulting in a sharp decrease in toughness. On the other hand, if the Ti content is less than 1.3096, not only the desired strength cannot be obtained, but also the effect of suppressing austenite precipitated during aging becomes small, and softening during aging (overaging) is promoted. For this reason, the Ti content is 1.30 to 1.5
0%, and the MOO content and Ti content are (MO(%)
) / Ti (%)≧3.6].

g) htM is a deoxidizing agent that is added before adding Ti during melting in order to remove oxygen from molten steel, prevent oxidation of Ti, and improve its yield. When the amount of oxygen is less than 0.03 ppm, the amount of oxygen in the steel ingot exceeds 15 ppm, resulting in large oxide inclusions and a decrease in the toughness of the steel.
If the N content exceeds 15%, the toughness will be significantly reduced.
i. Since Al comes to be precipitated, the AL content is determined to be 0.03 to 0.15%.

h) N Marage steel is usually produced by vacuum melting, so its N
The amount is very low, about 25 to 35 ppm, and therefore the effect of this small amount of N has not received any attention in the past. However, research conducted by the present inventors has revealed that the toughness of marage steel is greatly affected by even a very small amount of N, and is a significant cause of variations in toughness of ultra-high tensile steel.

In other words, almost all N in steel is in the form of non-metallic inclusions of TiN, and it has been known that this is the main cause of poor cleanliness.
does not simply turn into TiN and deteriorate toughness, but the generated TiN is arranged in a dot array, which plays the same role as defects (cracks) in steel and deteriorates toughness. This made it clear.

Moreover, when the N content is higher than 15% (15 ppm) of N content: o, oo, the above-mentioned dot-like inclusions increase rapidly, and conversely, when the N content is below the above value, the dot-like inclusions increase rapidly. It was also confirmed that the inclusions arranged in rows almost disappeared and became dispersed.

Figure 1 shows C: 0.003% or less, Si+Mn+C
r: 0.03cm or less, P: 0.003% or less, S: 0
．． 003 or less, Ni: 17.0-17.3%, C
o: 14.0-14.4%, Mo: 5.6-5.7%
, Ti: 1.32-1.35%, Al2 0.06-0
．． 08%, balance: steel of the present invention consisting of Fe and impurities;
C: 0.006%, Si+kJn-1-Cr: 0
．． 05%, P: 0.003% or less, S: 0.003% or less, Ni: 18.0-18.4%, Co: 12.0-
12.3 To%Mo: 4.2-4.4%,'ri:
1.55-1.60%, Al: 0.04-0.0
Regarding comparative steel consisting of 6%, balance: Fe and impurities (
Both were subjected to solid solution treatment at 840°C for 1 hour and aging treatment at 500°C for 4 hours), ``N content in steel'', ``tensile strength with notches'', and ``TiN dot arrangement''. This is a graph showing the relationship between the maximum length of inclusions and the maximum length of a row of inclusions (measured according to the D method in the ASTM inclusion measurement method). is 0.001
5% or less (15 ppm or less), preferably 0,0 OL
It can be seen that when the content is below the OS (10 ppm or below), the notched tensile strength and toughness are rapidly improved. In addition, the tensile strength with notches in this case is 240 to 246 k for the comparative steel.
gf/sm2, whereas in the steel of the present invention it is about 26 gf/sm2.
It can be confirmed that the steel has a high value of 5 to 269 kgf/IIjE2, and that the toughness is higher than that of the comparative steel with low strength.

For the above reasons, the N content was determined to be 0.0015 inches or less.

In addition, the content of P and S as impurities should be 0.01% or less, preferably 0.01%, to prevent deterioration of the toughness of the steel.
．． It is best to adjust it to OO5S or less.

B. Manufacturing conditions a) Solution treatment conditions before aging treatment In this solution treatment, the treatment temperature was less than 80°C, and if the treatment time was less than 30 minutes, the strength of the steel would increase, but the Intermetallic compounds containing solid solution Mo are formed, resulting in extremely low toughness.On the other hand, when the solution treatment temperature exceeds 860°C, toughness decreases due to coarsening of crystal grains, and when heated for more than 3 hours, strength decreases. began to invite Therefore, in the solution treatment before the aging treatment, the holding temperature was set at 800 to 860°C, and the holding time was set at 30 minutes to 3 hours. The optimum solution treatment temperature for obtaining excellent values for both strength and toughness varies depending on the MO, and is 5.3
% is 800-820°C, and 6-chi is 840-860°C.

b) Aging treatment conditions When the aging temperature was less than 470°C and the aging time was less than 1 hour, precipitation strengthening by Mo and Ti was insufficient (
, it is not possible to secure a tensile strength of 250 kgf/mm2 or more. On the other hand, if the aging temperature exceeds 520℃,
If the aging time exceeds 10 hours, austenite will precipitate in the steel resulting in over-aging, making it impossible to secure the desired strength. For this reason, the aging treatment temperature is 470~5
The aging treatment time was set at 20° C. for 1 to 10 hours.

C) Area reduction rate during cold working Cold working before solution treatment significantly refines the grains of the steel after solution treatment and improves toughness, but the area reduction rate during cold working If it is less than 30%, grain refinement is not sufficient and no particular effect can be obtained by cold working. Furthermore, it is even more effective to combine two grain refinement treatments by combining one cold working and one solid solution treatment.

d) First treatment conditions when performing solution treatment twice When performing solution treatment twice, simply cold working and 800
You may repeat the solution treatment for 0.5 to 3 hours at ~86CIC, but if the first solution treatment is performed with rapid heating and short time treatment, the grains of the steel will become finer and the strength and strength will increase. Toughness is further improved. This effect may occur if the time required to raise the temperature during heating exceeds 10 minutes, or if the soaking time is 2.
It will disappear if the heating time exceeds 0 minutes or the heating temperature exceeds 950℃ (However, the toughness will not decrease)
On the other hand, if the soaking time is less than 1 minute or the soaking temperature is less than 830° C., the strength of the steel increases, but the toughness significantly decreases. In addition, cooling after solution treatment is
If it is air cooled or higher, sufficient strength and toughness can be ensured.

Therefore, when performing solution treatment twice, the first treatment condition is to raise the temperature to a solution treatment temperature of 830 to 950°C within 10 minutes, hold it for 1 to 20 minutes, and then air cool at a cooling rate higher than that of air cooling. It was decided to cool it down to room temperature.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

<Example> First, a steel ingot obtained by vacuum melting of 5ooky was used as an electrode for remelting with a consumable electrode type high vacuum arc.
In addition to manufacturing the A steel shown in the table, in addition to this, 1
B-L steel shown in Table 1 was manufactured by vacuum melting 50 kg.

Each steel ingot is homogenized at 1200-1250℃ during the forging process necessary to create the hot-rolling material required in the next process (total holding time before, during, and after forging). (for 15 to 30 hours) to completely eliminate microsegregation caused by Mo and Ti.

Next, the hot rolling material was heated to 1200 to 1250°C and rolling was started to obtain a hot rolled steel plate having a thickness of 7 to loB.

Subsequently, each of the above-mentioned hot-rolled steel sheets subjected to external cutting was subjected to the treatments shown in Table 2 to produce plate products having a thickness of 0.2 to 2.5N.

For each of these products, its mechanical properties and TiN
Measure the maximum length of the dot-like inclusions and use the obtained results as a second
It is also shown in the table.

The thickness and mechanical properties of products exceeding 1.0 mm are measured by reducing the thickness to 1.0 mx.
For elongation of undropped product, go to 4 (however, A is the cross-sectional area)
Comparisons were made by adjusting the gauge length.

Furthermore, the tensile test piece has a parallel part width of 5. The tensile test piece with a notch is a plate-shaped test piece with a parallel part width of 7u and a distance between the notch bottoms of 5R1 (the notch depth at both ends is 1.5mm).
0 van and a stress concentration factor of 3.5).

The maximum length of the TiN dot array-like inclusion is determined by ASTM
It was measured according to method D in the inclusion measurement method.

From the results shown in Table 2, the steel according to the present invention has a tensile strength T, S, ): 250 kgf/mm2 or more, and a good notch tensile strength N, which exceeds the tensile strength. T, S, ). However, even if the steel has a composition that satisfies the conditions of the present invention,
It can be confirmed from the comparative examples that sufficient characteristics cannot be obtained unless the manufacturing conditions are appropriate.

Furthermore, it is clear that conventional steels whose compositions do not meet the conditions specified in the present invention (particularly those in which Mo, Ti, and N exceed the upper limit values) do not exhibit sufficient toughness values.

Figure 2 is a graph comparing the properties of the steel of the present invention and the conventional steel measured in this way.
The excellent performance of the steel of the present invention is clearly apparent from the figure.

Overall effect> As mentioned above, according to the present invention, 250 kgf/m
This brings about industrially useful effects, such as the ability to obtain marage steel that exhibits a tensile strength of 2 or more and also has extremely excellent toughness.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the amount of N in steel, the tensile strength with notches, and the maximum length of TiN dot array inclusions, and Figure 2 is a graph comparing the properties of the steel of the present invention and conventional steel. It is. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 2 others autopsy θN+
(pr:)m)

Claims (4)

[Claims] (1) In terms of weight percentage, C: 0.005% or less, Si, Mn and Cr: 0.06% or less in total, Ni: 1
6.5-18.0%, Co: 12.0-15.0%, Al: 0.03-0.15%, Mo: 5.3-6.0%, Ti: 1.30-1. 50%, N: 0.0015% or less, Fe and other unavoidable impurities: the remainder, and the amount of Mo and the amount of Ti are expressed by the formula Mo (%)
/Ti (%)≧3.6, and has a tensile strength of 250k.
Marage steel with good toughness of gf/mm^2 or more. (2) In terms of weight percentage, C: 0.005% or less, Si, Mn and Cr: 0.06% or less in total, Ni: 1
6.5-18.0%, Co: 12.0-15.0%, Al: 0.03-0.15%, Mo: 5.3-6.0%, Ti: 1.30-1. 50%, N: 0.0015% or less, Fe and other unavoidable impurities: the remainder, and the amount of Mo and the amount of Ti satisfy the formula Mo (%) / Ti (%) ≧ 3.6 Steel is subjected to solution treatment by heating and holding at 800 to 860°C for 30 minutes to 3 hours, and by heating and holding at 470 to 520°C.
A method for producing marage steel having a tensile strength of 250 kgf/mm^2 or more and good toughness, which comprises subjecting it to aging treatment for ~10 hours. (3) In terms of weight percentage, C: 0.005% or less, Si, Mn and Cr: 0.06% or less in total, Ni: 1
6.5-18.0%, Co: 12.0-15.0%, Al: 0.03-0.15%, Mo: 5.3-6.0%, Ti: 1.30-1. 50%, N: 0.0015% or less, Fe and other unavoidable impurities: the remainder, and the amount of Mo and the amount of Ti satisfy the formula Mo (%) / Ti (%) ≧ 3.6 Steel is subjected to cold working with an area reduction rate of 30% or more, followed by solution treatment in which the steel is heated and held at 800 to 860°C for 30 minutes to 3 hours, and then heated at 470 to 520°C for 1 to 3 hours.
A method for producing marage steel having a tensile strength of 250 kgf/mm^2 or more and good toughness, the method comprising subjecting it to an aging treatment for 10 hours. (4) In terms of weight percentage, C: 0.005% or less, Si, Mn and Cr: 0.06% or less in total, Ni: 1
6.5-18.0%, Co: 12.0-15.0%, Al: 0.03-0.15%, Mo: 5.3-6.0%, Ti: 1.30-1. 50%, N: 0.0015% or less, Fe and other unavoidable impurities: the remainder, and the amount of Mo and the amount of Ti satisfy the formula Mo (%) / Ti (%) ≧ 3.6 Steel is subjected to cold working with an area reduction rate of 30% or more, heated to a solution temperature of 830 to 950°C within 10 minutes, held for 1 to 20 minutes, and then cooled to room temperature at a cooling rate higher than air cooling. After applying solid solution treatment in sequence, cold working with an area reduction rate of 30% or more and solution treatment of heating and holding at 800 to 860°C for 30 minutes to 3 hours are performed again, and then 1 time at 470 to 520°C. A method for producing marage steel having a tensile strength of 250 kgf/mm^2 or more and good toughness, the method comprising aging treatment for ~10 hours.