JPS5825419A - Preventing method for low-temperature cracking of martensitic stainless steel - Google Patents

Abstract

PURPOSE:To prevent the low-temp. cracking of martensitic stainless steel quickly and surely by cooling the martensitic stainless steel after hot working once and holding and the same for a prescribed time then reheating the steel up to a pearlite transformation temp. CONSTITUTION:The martensitic stainless steel after hot working is cooled from its high temp. state down to a temp. T in the temp. region wherein the upper limit is Ms point +150 deg.C and the lower limit is the higher of Mf point of 100 deg.C at the cooling rate that does not enter the nose part of the pearlite transformation in a continuous cooling transformation curve; thereafter, the steel is held at said temp. region for at least the time (t) minute specified by the formula: t= 1/10 (T-Ms) when T>=Ms and t=0 when T<Ms (where t; minute, T: deg.C, Ms; deg.C), then the steel is reheated up to the prescribed temp. below the Ac1 point and is held at said temp. for at least one minute to undergo tempering of the transformed martensite and/or pearlite convertion of untransformed austenite, after which the steel is cooled down to an ordinary temp.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing low-temperature cracking such as delayed fracture and quench cracking after hot working of martensitic stainless steel, medium carbon or high carbon martensitic stainless steel. be.

Conventionally, martensitic stainless steels, such as martensitic chromium stainless steels, exhibit self-hardening properties due to martensitic transformation, and have therefore been widely used for the purpose of utilizing this property. In addition, recently, Fi, its gas corrosion resistance has attracted attention, and medium carbon 18Cr stainless steel has become widely used in resource and energy related fields such as oil country tubular goods.

However, since this type of rutinsite stainless steel has high hardenability, it does not form a sufficiently hardened structure, or even a martensitic structure, when left to cool after hot working at high temperatures. Cool to 9 s Ip#
If the K M m point is 800° C. or lower, quench cracking may occur. Furthermore, depending on the environment in which the cooling machine is placed, it may be delayed and destroyed before moving on to the next process. Such cold cracking is an extremely serious problem in the production of martensitic stainless steel products, as it makes post-hot processing impossible or requires a large amount of man-hours to clean. .

Conventionally, in order to prevent such cold cracking of martensitic stainless steel copper, after hot working at high temperatures, treatment is performed to prevent martensitic transformation, or a large amount of per2ite transformation is caused. Measures have been taken to lower the hardness after cooling, such as through treatment.

For example, in the method 1) ti, martensitic stainless steel processed by heating to a high temperature is slowly cooled at a cooling rate of approximately 60°C/h or less from the Ar1 point to the Ms point. However, this treatment method requires an extremely long annealing time, requiring a long storage space and a long process period, making it extremely unsuitable for short-term mass production of KFi.

On the other hand, as a method to prevent cold cracking during the cooling process after hot working of martensitic stainless steel and the subsequent process,
There is a method of softening using constant temperature transformation. That is, martensitic stainless steel processed by high temperature heating is processed in the temperature range from its high-temperature forest state to pearlite transformation, preferably at the temperature near the nose of the isothermal transformation curve (hereinafter referred to as T, T, T, curve). This is a method in which the temperature is lowered and kept at that temperature for a long time to cause isothermal pearlite transformation.

However, this method also involves directly cooling a high-temperature object to an appropriate temperature, and setting a specific temperature during cooling of the high-temperature region as the optimum temperature, converging the temperature of the object during cooling to this temperature and performing isothermal transformation. Not only does it require advanced technology to control the temperature, but it also requires practical use, especially in medium and high carbon temperatures! Rutinsite chromium stainless steel @
T, T, T, even if the temperature is maintained in the vicinity of the nose of the curve, at least 26 to
It took more than 8 hours to soften, and it took a long time for softening, so we had no choice but to conclude that KFi is not suitable for mass production in a short period of time.

In view of the above-mentioned situation K11ii, the present inventors quickly, stably, and reliably prevent low-temperature cracking after hot working of martensitic stainless steel, and mass-produce martensitic stainless steel products with high yield. We have considered ways to find a way to do so. As a result, martensitic stainless steel is produced by appropriately cooling the austenite that is in a high temperature state after hot working, once cooling it to a low temperature #iIL below the Ms point + 160°C, and holding it for a predetermined period of time, until it reaches the pearlite transformation temperature range. When the temperature is raised again and held, the carbide generated and grown during proper cooling, holding in a low temperature range, and subsequent heating is used as a core to form pearlite more quickly than in normal constant temperature transformation treatment, that is, directly from the high temperature state. Pearlite transformation starts and ends more quickly than the method of lowering the temperature to the transformation temperature range and holding it at that temperature to cause pearlite transformation, and partially formed martensite is tempered, so softening ends quickly. death,
Furthermore, if a certain degree of pearlite transformation is caused by such appropriate cooling, holding, and reheating, the untransformed austenite becomes lower in carbon, the Ms point increases, and the martensite generated in the subsequent cooling process to room temperature becomes They came to the knowledge that the occurrence of low-temperature cracking can be suppressed due to self-tempering and softening.

Therefore, the present invention was made based on this knowledge, and the present invention is based on the above findings.
Continuous cooling from the high temperature state, cooling rate that does not affect the nose of pearlite transformation in the transformation curve, and the upper limit is Ms
After cooling to a temperature T in a temperature range where the m point is 150°C and the lower limit is the higher of the Mf point or Fi 100°C, the time t (minutes) specified by the following formula at least is maintained in that temperature range. : (1) T '2Mm and I on 1/10 (T-M
s) (1) When T<Ms, 1-0 (however, t: minutes T: "CM: °C)" is held, and then the stainless steel is heated again to a predetermined temperature below Ae1 point, and then heated to that temperature. By holding the stainless steel for at least 41 minutes, tempering the martensite that has been transformed and/or pearlitizing the untransformed austenite, and then cooling the stainless steel to room temperature, the untransformed austenite that has been reduced in carbon can be heated. In order to soften martensite with a high Ma point that is generated through transformation, it is possible to cool and soften martensitic stainless steel in a short time after hot working, thereby preventing delayed cracking or quenching cracking at low temperatures. It has a certain characteristic in preventing low temperature cracking such as.

That is, in the method of the present invention, in order to rapidly mass-produce martensitic stainless steel products, the high temperature state after one hot working is continuously cooled at a cooling rate that does not affect the nose part K of pearlite transformation on the transformation surface IsK. Since it has a cooling step, it goes without saying that martensitic transformation may occur during the cooling. This cooling rate is generally 20° C./min or more. Even if martensitic transformation occurs, if cooling is stopped at the Mf point or 100°C, whichever is higher, and the temperature is raised again, martensite in the high temperature state has a plastic performance deformation line and will crack due to quenching. The temperature was raised again without causing A.
By maintaining the temperature at the c1 point or lower for at least 1 minute, the martensite produced through transformation undergoes a tempering treatment and is softened.

On the other hand, the remaining nine portions of untransformed austenite that did not undergo martensitic transformation during the cooling process contain carbides generated and grown during cooling, holding in a low temperature range, and reheating as nuclei below the S AJ point. Pearlite transformation occurs more quickly when the temperature is maintained at K. Although it is not a necessary condition to complete pearlite transformation, it is necessary to complete pearlite transformation to some extent)! It is necessary to lower the carbon content of untransformed austenite by inducing the 5-state. This is because during the next cooling process to room temperature, the untransformed austenite that remains without pearlite change becomes martensite, but the starting point of this martensitic transformation, that is, the Ma point, is raised to a sufficiently high pressure and high temperature side.
This is because carbon reduction is necessary in order for the nine-martensite produced at a sufficiently high temperature to be softened by self-tempering during the further cooling process.

As described above, the re-heating in the method of the present invention is preferably carried out to Fi600° C. or higher, ideally to #iT, T, T, a temperature near the nose of the curve. In this case, since the stainless steel piece has been cooled once, it is extremely easy to raise the temperature again and maintain it at a temperature near the nose of the curve. Ms.
To be sure that the pearlite transformation is greatly promoted by re-heating after cooling to a temperature below +160°C and holding it for a predetermined time, it is sufficient to hold the temperature at the re-heating set temperature for at least 1 minute, preferably Fi 5 minutes or more. Through this treatment, martensite generated during cooling from a high temperature state after hot working is tempered, or untransformed austenite is turned into pearlite, or both of these occur simultaneously to advance softening. On the other hand, the remaining untransformed orstenite is reduced in carbon.

Therefore, rutinsite, which transforms during the cooling process to room temperature after re-heating and holding, has low hardness in quenching 11 because of its low carbon content, and also has a high Ma point, so it self-transforms during the cooling process of modified m*. It is tempered and becomes sufficiently soft.

Now, it is clear that it is extremely easy to maintain a stainless steel piece at a low temperature below the Ms point +150°C because the slope of the cooling curve is gentle in this temperature range, especially when the %Ms Time t at temperature T ('C) above point
Holding for -1/10 (T-Ma) (minutes) is an effective treatment for the generation and growth of carbide, which becomes the core of pearlite transformation by holding after the next reheating, and this holding is carried out. This means that pearlite transformation is greatly promoted by holding the temperature at the next re-heating set temperature.

Next, the reason for limiting the temperature range in which stainless steel is rapidly cooled from a high temperature state in the method of the present invention, and the reason for limiting the holding time in the appropriate cooling low temperature range and reheating temperature will be summarized again.

First, the reason why we set the upper limit of the target temperature range for rapid cooling to M1 point + 150°C is because above this temperature, the holding time there, which is effective for the generation of nuclei for pearlite transformation, becomes extremely long υ
In addition, the bar-2ite transformation time in the next temperature increase and hold becomes longer, making it impossible to perform a rapid process.On the other hand, the lower limit was set to the higher of the Mf point or 100°C. This is because when the temperature is lower than this, the plastic deformability of the martensite produced is significantly reduced and there is a possibility that dawn cracking may occur. f, the temperature T('
The retention time in C); When T>Ms, t-1/10 (T-Ms)T<
The reason for setting the time to be longer than t (minutes) such that 9t=0 for Me is that holding the temperature in the temperature range above the Ms point for the above period promotes pearlite transformation during the holding at the next temperature increase. There it is,
If the time is less than t (minutes), the effect is small. On the other hand, the holding time in the reheating temperature range was limited to 1 minute or more because
If the time is less than 1 minute, the amount of pearlite produced to reduce the carbon content of untransformed austenite will not reach the desired level, and the martensite produced will not be softened sufficiently by tempering. be.

left Next, examples of the present invention, comparative examples, and K will be explained.

Example L 024%C-042%5i-058X Mn-1a28
A piece of i-rutinsite stainless steel with %Cr (@%″ is weight%, the same applies hereinafter) is heated to 1180°C to produce 1016
It was rolled into a steel pipe of 2 (diameter) x 20 (wall thickness), and after the rolling was completed, it was cooled from 960°C to 880°C at a cooling rate of 80°C/min. flkJI kept at this temperature for 20 minutes
C. The product was placed in a reheating furnace and heated to 700'CK, held at this temperature for 80 minutes, and then extracted and air cooled. After cooling to room temperature,
It was left outdoors for KIO days, and ultrasonic testing revealed no cracks. The hardness was HRc=2a8. Incidentally, the M1 point of this stainless steel was about 210°C, and the Mf point was about 70°C.

Example 2 A martensitic stainless steel pipe with the same composition and size as in Example 1 was heated from about 960°C to 280°C after rolling.
The sample was cooled at a cooling rate of 80°C/min, held at this temperature for 6 minutes, then charged into a reheating furnace, heated to 700°C, held at that temperature for 16 minutes, and then extracted and air-cooled. .

After cooling to room temperature, it was left in water for 10 days and subjected to ultrasonic flaw detection, but no cracks were found. Hardness FiHBc=M2 ton 8.

Example 8 A martensitic stainless steel pipe with the same composition and size as in Example 1 was heated from about 950°C to 226°C after rolling.
The sample was cooled at a cooling rate of 80°C/min, held at this temperature for 1 minute, charged into a reheating furnace, heated to 700°C, held at this temperature for 5 minutes, and then extracted and air-cooled. . After cooling to room temperature, it was left for 10 days and subjected to ultrasonic flaw detection, but no cracks were found. The hardness was 2a2.

Comparative Example 1 A martensitic stainless steel pipe of the same composition and size as in Example 1 was heated from 960°C to 80°C/mi after rolling.
Cool to 450'C at a cooling rate of n and hold for 20 minutes,
Thereafter, it was placed in a reheating furnace and heated to 700°C, held at this temperature for 80 minutes, and then extracted and air-cooled. Cool to room temperature vk
It was left outside at Nikkan and underwent ultrasonic flaw detection before moving on to the next heat treatment process.

A crack was discovered. So we measured the hardness and found that it was H,C.
It was w5α4. This is 460℃ in the appropriate cooling temperature range.
It is considered that the cooling temperature was high, and therefore, the isothermal transformation at 700° C. for 80 minutes did not sufficiently cause bar2ite transformation, and cracks occurred due to martensitic transformation.

Comparative Example 2 After rolling, a martensitic stainless steel pipe having the same composition and size as in Example 1 was water-cooled from about 950°C to room temperature, and left in water for 5 hours. Thereafter, it was placed in a reheating furnace and heated to 700°C, held at this temperature for 80 minutes, and then extracted and air-cooled. After cooling to room temperature, the hardness was measured to be Hmc-2a2, but cracks were discovered by ultrasonic testing. Since the structure was tempered martensite, it is thought that cracking was caused by martensitic transformation during water cooling, or delayed cracking occurred.

Comparative Example 8 A martensitic stainless steel pipe with the same composition and size as in Example 1 was rolled and then charged into a furnace from about 900°C and heated to 46°C.
Controlled cooling was performed at a cooling rate of /hr, and the mixture was cooled to room temperature over approximately 20 hours and left for 910 days. This was subjected to ultrasonic testing, but no cracks were found.

Cirrhosis was Hmcz2α8, and there was sufficient pearlite transformation.

Example 4 α41%C-(L28%St-α25%Mn-142
12 pieces of 2% Cr martensitic stainless steel
It was heated to 00'CK and rolled into a steel pipe with a diameter of 60 mm and a wall thickness of XaOm, and after the rolling was completed, it was cooled in the atmosphere from about 950 °C to 22 (It) and held for 10 minutes, and then charged into a reheating furnace. 72
The sample was heated to 0°C, held at that temperature for 80 minutes, extracted, cooled in the air, left in water for 5 days, and then subjected to ultrasonic testing. No cracks were found. Further, the hardness in this case was Hmctxx22.9, and the structure was pearlite. By the way, the M1 point of this stainless steel is approximately 160℃.
The Mf point is below room temperature.

Example 6 A martensitic stainless steel pipe with the same composition and size as in Example 4 was heated from about 900°C to 150°C after rolling.
The mixture was left to cool in the air until the temperature reached 710° C., held for 1 minute, charged into a reheating furnace, heated to 710° C., held at that temperature for 15 minutes, and then extracted and cooled in air. Ultrasonic flaw detection was performed immediately after cooling, but no cracks were found. In this case, the hardness was ijHRcm2&8, and the structure was a mixed structure of tempered martensite and pearlite.

Comparative Example 4 A martensitic stainless steel pipe with the same composition and size as in Example 4 was heated from about 920°C to 880°C after rolling.
Leave to cool in the air until the fIk is 20 minutes, then
Raise the temperature to 00℃ and hold for 46 minutes, then extract and air cool.9. After cooling VK in water for 5 days, ultrasonic testing revealed cracks.

The hardness was Hacm4117. This has a high cooling temperature of 880℃ in the suitable cooling temperature range, so 700℃
It is considered that the isothermal transformation for 46 minutes did not sufficiently cause the per2ite transformation, and therefore cracking due to martensitic transformation or delayed cracking occurred.

From the above Examples and Comparative Examples, the method of the present invention can reduce cold cracking after hot working of martensitic stainless steel.
It turns out that it can be prevented quickly and reliably.

As described above, according to the present invention, cold cracking after hot working of martensitic stainless steel can be prevented by simple operations, and martensitic stainless steel products can be produced stably, reliably, and quickly with good yield. It can be mass-produced and provides industrially useful effects.

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Claims (1)

[Claims] For martensitic stainless steel after hot working, the cooling rate does not affect the nose part of pearlite transformation in the continuous cooling transformation curve from its high temperature state, and the upper limit is Ma point +16
After cooling to a temperature T in a temperature range where the lower limit is the higher of the Mf point or 100°C, the time t (minutes) specified by the following formula: 1) When 'r>Mj, t=1/10(T-Ml
) (-) When T<Ms 1.0 (however, t: minutes, T!'C, Ms!'C) is held, and then the stainless steel is heated again to a predetermined temperature below 1 AC point, and the temperature is 4) Z-? 1. A method for preventing cold cracking of martensitic stainless steel after hot working, the method comprising converting the stainless steel into a metal and then cooling the stainless steel to room temperature.