JP5344329B2 - Winding method of hot rolled maraging steel strip - Google Patents

Description

  The present invention relates to a method for winding a hot rolled maraging steel strip.

Since maraging steel has a very high tensile strength of around 2000 MPa, it is used for members that require high strength. Among them, for example, maraging steel used for a continuously variable transmission part of an automobile engine is processed into a steel strip by hot rolling and then processed into a thin plate of about 0.5 mm by cold rolling.
Examples of the hot rolling method for the maraging steel described above include, for example, Japanese Patent Application Laid-Open No. 60-234920 (Patent Document 1), C ≦ 0.02%, Si ≦ 0.1%, Mn ≦ 0.2. %, P ≦ 0.01%, S ≦ 0.01%, N ≦ 0.01%, Ni 15-25%, Co ≦ 10.0%, Mo ≦ 7.0%, Al ≦ 0 .2%, Ti ≦ 1.5% containing any two or more of steels, the balance being Fe and unavoidable impurities, the cumulative reduction ratio of 1000 ° C. or less to 60% or less In addition, the hot rolling is finished at 900 ° C. or more with a cumulative reduction ratio of 950 ° C. or less being 20% or less, winding at 300 to 600 ° C., and then recrystallization annealing and solution treatment after cold rolling. A manufacturing method is disclosed.

JP 60-234920 A

The hot rolling conditions disclosed in Patent Document 1 described above are such that hot rolling is finished at 900 ° C. or higher, winding is performed at 300 to 600 ° C., preventing non-uniform transformation at the run-out table, and martensite after winding. It is intended to be transformed.
According to the study of the present inventor, even if the temperature is controlled so as to cause martensitic transformation after winding, the hardness unevenness that locally hardens in the wound maraging steel strip (hereinafter referred to as a coil). Was confirmed to occur. In this case, for example, when trying to obtain a belt material for a continuously variable transmission part (CVT) for an automobile engine that requires high thickness accuracy by cold rolling, the hardness unevenness is a deformability at the time of rolling. This leads to a big problem that the plate thickness accuracy is not obtained.
The objective of this invention is providing the winding method of the hot rolling maraging steel strip which can suppress the raise of the local hardness of the coil after hot rolling.

The present invention has been made in view of the above-described problems.
That is, in the present invention, after the maraging steel material is heated to 1050 to 1300 ° C., hot rolling is performed to obtain a maraging steel strip, and then the maraging steel strip is wound to form a coil. In the method of winding a maraging steel strip, the temperature of the maraging steel coil wound with the maraging steel strip is maintained at a temperature exceeding the Ms point over the entire length of the maraging steel strip. After cooling to a temperature below the Ms point, this is a method of winding a hot-rolled maraging steel strip that does not reheat to a temperature exceeding the Ms point again.
Preferably, after the end temperature of the hot rolling is set to 700 ° C. or higher and lower than 900 ° C. to obtain a maraging steel strip, winding of the maraging steel strip into a coil shape is started within 30 seconds from the end of hot rolling. In the coil after the end of winding + 2 minutes, the temperature of the coil outer peripheral surface width direction central portion, the coil inner peripheral surface width direction central portion temperature, and the coil central portion side surface temperature are all Ms points. This is a method of winding a hot-rolled maraging steel strip that has a temperature difference of 700 ° C. or less and a temperature difference of 300 ° C. or less.

  According to the method for winding a hot-rolled maraging steel strip of the present invention, it is possible to suppress an increase in the hardness of the maraging steel strip after hot rolling, so that steps such as cutting by hardening can be excluded. It is also excellent in economic efficiency and productivity.

It is a schematic diagram which shows the coil wound up.

The present inventor has investigated the reason why the hot-rolled maraging steel strip is locally hardened, and has obtained the following knowledge.
First, the local increase in hardness is thought to be due to the aging effect unique to maraging steel, which occurs in the structure after martensitic transformation by cooling, and the local increase in hardness due to this aging effect. This is because the portion that has been cooled coincides with the portion where the temperature has increased, and the increase in the temperature of the portion once cooled is due to heat transfer from the inside of the coil.
In other words, the portion that is simply cooled does not generate unevenness of hardness, but the region where the temperature rises clearly after the martensitic transformation has a local aging effect and causes unevenness of hardness. That's what it means.

The aging effect peculiar to this maraging steel will be described in detail.
The present inventors represent 18% Ni-9% Co-5% Mo-0.45% Ti-0.1% Al-bal. A test specimen was collected from the Fe maraging steel material to investigate the increase in hardness due to the increase in temperature (recuperation) of the part once cooled. In the investigation of the effect of recuperation, the test piece once martensitic transformed was reheated, and the relationship between temperature and time was investigated.
The aging effect due to recuperation appears when a part of the maraging steel strip in the coil falls below the Ms point and the temperature rises again. Therefore, after holding the test piece at 850 ° C. for 1 hour, a solution treatment was performed under conditions of water cooling and air cooling. The water cooling assumes the case where shower cooling is performed on a run-out table. The air cooling is assumed to be a coil as it is after hot rolling. All the test pieces exhibited a martensite structure, and the Vickers hardness was 310 HV.
The test piece was reheated at 200 ° C., 300 ° C., 400 ° C. and 500 ° C. for 5 minutes, 10 minutes, 30 minutes and 60 minutes, and the increase in hardness due to the aging effect was measured. The hardness was measured using a Vickers hardness meter. The results are shown in Table 1. The Ms point of the maraging steel shown in Table 1 is 220 ° C.

As shown in Table 1, the test piece reheated at 200 ° C. below the Ms point showed no increase in hardness. However, in the reheating at 300 ° C., the hardness started to increase slightly by holding for 5 minutes, and this tendency became more prominent as the reheating temperature was higher, and when the reheating temperature was 400 ° C. or higher, the hardness exceeded 350 HV. Further, the water-cooled specimen and the air-cooled specimen were slightly different in hardness, but showed almost the same tendency.
For this reason, once cooled to below Ms and the metal structure becomes martensite, if it is reheated again by recuperation after that, an aging effect appears in a short time. It can be seen that the higher the temperature and the longer the time, the more prominent.
Therefore, in the hot rolling process of maraging steel, after cooling to a temperature below the Ms point, the temperature must not exceed the Ms point again so that the aging effect does not appear. If the aging effect appears, the hardness will increase as shown in Table 1 above, and a local aging effect will occur at that location, causing unevenness of the hardness. Directly linked to deterioration. That is, the expression of the aging effect of maraging steel is sensitive to temperature.

From the above knowledge, in the present invention, the following means are applied in order to eliminate unevenness in hardness. The configuration defined by the present invention will be described in detail below.
In this invention, after heating the raw material of maraging steel to 1050-1300 degreeC, it hot-rolls and obtains a maraging steel strip, Then, this maraging steel strip is wound up into a coil, The said maraging steel The temperature of the maraging steel coil wound with the strip is maintained at a temperature exceeding the Ms point over the entire length of the maraging steel strip, and after the maraging steel coil is cooled to a temperature below the Ms point, the temperature exceeding the Ms point again. Shall not be reheated.
The temperature of the maraging steel coil wound in the present invention is maintained at a temperature exceeding Ms over the entire length of the maraging steel strip. The maraging steel strip wound in the coil has an aging effect due to martensitic transformation and recuperation. This is to prevent the expression of. The Ms point temperature is preferably maintained at a temperature of 30 ° C. or higher, more preferably the Ms point temperature of + 70 ° C. or higher.
In addition, the hot workability of a maraging steel raw material falls that the temperature of the heating in hot rolling is less than 1050 degreeC. Therefore, the lower limit of the heating temperature of the hot rolling material is 1050 ° C. On the other hand, if the heating temperature exceeds 1300 ° C., the ductility of the maraging steel material may be lowered. Therefore, the upper limit of the heating temperature of the material for hot rolling is set to 1300 ° C.

In addition, after the maraging steel coil is cooled to a temperature below the Ms point, it is important not to reheat to a temperature exceeding the Ms point again. As described above, maraging steel that has undergone martensitic transformation is a material in which an aging effect appears even when reheated to a temperature exceeding the Ms point for several minutes. Once the aging effect is exhibited, the hardness of the portion increases and unevenness of hardness occurs. Then, in the cold rolling performed after hot rolling, it becomes difficult to control the thickness of the cold rolling due to unevenness in hardness. Therefore, in the present invention, after the maraging steel coil is cooled to a temperature below the Ms point, it is not reheated to a temperature exceeding the Ms point again.
If the manufacturing method prescribed | regulated by said this invention is applied, the difference in the hardness of the maraging steel strip in a coil can be made into 20 HV or less by Vickers hardness.

Next, the preferable structure prescribed | regulated by this invention is demonstrated.
In this invention, after heating the raw material of maraging steel to 1050-1300 degreeC, hot rolling was performed and the hot rolling completion temperature was 700 degreeC or more and less than 900 degreeC. This is intended to reduce the basic amount of heat that causes the aging effect by making the hot rolling end temperature as low as possible. Further, when the hot rolling is finished at a temperature lower than the temperature exceeding 900 ° C., the temperature difference between the coil inner circumference, outer circumference, and central portion can be reduced.
Here, the reason why the hot rolling end temperature is set to 700 ° C. or more is that when the temperature is lower than this temperature, the workability is low and the rate of occurrence of defects such as cracks increases. On the other hand, the reason why the hot rolling end temperature is set to less than 900 ° C. is that, if it is higher than this, the retained heat is large, and the local aging effect in the coil tends to occur. For example, the inner surface of the coil and the outer surface of the coil are exposed to the outside air, so that they are cooled quickly and are likely to fall below the Ms point. On the other hand, the retained heat is large near the center of the coil. Therefore, even when the coil inner surface and coil outer surface that are easy to cool become Ms point or less, the temperature of the inner surface and outer surface of the coil may be reheated by the retained heat near the center of the coil. Therefore, it is desirable to reduce the retained heat of the coil, and the hot rolling end temperature is defined as less than 900 ° C.

As the thickness of the maraging steel strip obtained by hot rolling becomes thinner, cooling in the strip state is promoted, and the maraging steel strip is uniformly cooled between the end of hot rolling and coil winding. . As a result, by reducing the thickness of the coil, it is possible to reduce the retained heat near the center of the coil when it is used as a coil. For example, the local area exposed to the outside air such as the coil inner surface and the coil outer surface It becomes possible to prevent recuperation more reliably, and the occurrence of an aging effect can be suppressed. Therefore, the thickness of the maraging steel strip obtained by hot rolling is preferably 2.5 mm or less. The lower limit of the thickness of the maraging steel strip after hot rolling is preferably about 1.5 mm for cold rolling.
Moreover, in this invention, it is preferable to start winding the above-mentioned maraging steel strip to a coil within 30 seconds after completion | finish of hot rolling. This is because if the coil is immediately wound from the end of the hot rolling, the entire coil is cooled, so that the temperature of the part transformed into martensite can be prevented from rising locally. The preferred time until the start of winding is within 25 seconds. In the present invention, the end of rolling refers to the moment when the rear end of the steel material is discharged from the roll, and the start of winding refers to the moment when the leading end of the maraging steel strip is bitten by the winding device. say.
Moreover, since the one where the time to complete | finishing winding to a coil is quicker can advance cooling as the whole coil, it is good to complete | finish winding up within 5 minutes. Preferably it is 4 minutes or less.

In addition to the above-described conditions, in the present invention, in the coil after 2 minutes from the end of winding, the temperature of the coil outer peripheral surface width direction central portion, the coil inner peripheral surface width direction central portion, and the coil It is preferable that the temperature of the side surface of the central portion is 700 ° C. or less exceeding the Ms point, and the temperature difference is within 300 ° C.
As described above, the local aging effect due to the temperature rise after cooling occurs after the martensitic transformation. Therefore, this can be prevented by cooling the martensite as an integrated coil and performing martensitic transformation. For that purpose, it is necessary that the whole exceeds the Ms point immediately after the winding is completed, and this is defined from the above three points as a representative part of the whole.
Further, if the coil exceeds 700 ° C. or the temperature difference exceeds 300 ° C., the temperature difference between the coil surface and the inside in the cooling process becomes large, and unevenness in hardness occurs between the inside and the surface of the coil. There is a fear.
In addition, in order to set it as said temperature, in addition to adjustment of said hot rolling completion | finish temperature and adjustment of winding start time, for example, the method of not performing forced water cooling on the runout table currently performed normally Can be realized. If there is a possibility of exceeding 700 ° C., an appropriate amount of water may be discharged while the maraging steel strip is being wound, and the temperature may be adjusted by shower cooling.

In addition, the coil inner surface said by this invention mentioned above is the inner side of the coil in which the maraging steel strip front-end | tip part 3 where the maraging steel strip after a hot rolling is wound up first among the coils 1 shown in FIG. 1 is located. The surface is referred to as a coil inner surface 5, and the coil outer surface 4 refers to the outer surface of the coil where the maraging steel strip rear end portion 2 where the maraging steel strip after hot rolling is wound last is located. Moreover, the coil center part 6 says the area | region of the thickness of about half of the thickness direction of a coil.
In the above three measurements, the inner and outer surfaces of the coil are measured at the center in the width direction of the maraging steel strip. This is because recuperation (temperature increase) is caused by the retained heat in the coil central portion because the vicinity of the central portion in the width direction of the maraging steel strip is the most severe. Moreover, the temperature measurement of the coil center part is performed from the side surface of the coil.
In addition, it is easy to perform temperature measurement of these three places by infrared thermography.

  An example of the composition of the maraging steel that can be used in the present invention is C: 0.15% or less, Ni: 8-22%, Co: 3-20%, Mo: 2-9 in mass%. %, Ti: 0 to 2%, Al: 2.5% or less, Cr: 0 to 4%, Oxygen: less than 10 ppm, nitrogen: less than 15 ppm, and the balance composed of Fe and impurities may be used. Furthermore, you may contain any 1 type or 2 types or more of Mg: 0.01% or less, Ca: 0.01% or less, and B: 0.01% or less as a selection element.

The following examples further illustrate the present invention.
A consumable electrode was manufactured by vacuum melting, and vacuum arc remelting was performed using the consumable electrode to obtain a maraging steel ingot. The obtained maraging steel ingot was hot forged to obtain a maraging steel material having a thickness of 60 mm for hot rolling.
Table 2 shows the composition of the maraging steel ingot. The Ms point is 220 ° C.

Hot rolling was performed using a maraging steel material for hot rolling having a thickness of 60 mm. In addition, forced water cooling on the run-out table was not performed. In addition, shower cooling was performed during coiling.
The coil temperature was measured by infrared thermography. The temperature of the wound maraging steel coil was 260 to 410 ° C., and the temperature exceeded the Ms point over the entire length of the maraging steel strip. Further, after the maraging steel coil was cooled to a temperature below the Ms point, it did not reheat to a temperature exceeding the Ms point again.
End temperature of hot rolling, thickness of maraging steel strip after hot rolling, time from the end of hot rolling to the start of winding into a coil shape, temperature of coil immediately after end of winding and temperature change with time Tables 3 and 4 show. Of the three temperature measurements shown in Table 3, the “inner surface” and “outer surface” are the coil inner surface 5 and coil outer surface 4 shown in FIG. 1 and near the center in the width direction of the maraging steel strip. Measurement temperature. Further, “center” is the result of measuring the coil central portion 6 on the side surface of the coil shown in FIG.
For the measurement of hardness, the coil was unwound after completion of cooling, and specimens for hardness measurement were collected from maraging steel strips at locations corresponding to the aforementioned “inner surface”, “outer surface”, and “center”. The hardness scale was measured from the surface side by removing the oxidized scale of the collected test piece for hardness measurement and further polishing.

As shown above, since the hot rolling end temperature is within the range defined by the present invention, the temperature difference in the coil can be reduced.
Further, the maraging steel strip obtained by the winding method defined in the present invention showed no unevenness in hardness due to an increase in hardness due to a local aging effect, and no defective shape such as deformation was observed. .
In addition, the maraging steel strip obtained by the winding method defined in the present invention could have a hardness of 350 HV or less over the entire length. In particular, the variation in hardness was within 20 HV over the entire length of the coil, and the hardness could be made uniform.

  The production method of the present invention can be applied to a material having a problem of hardness change at the time of hot rolling and winding, with respect to a material having a transformation point of the metal structure.

DESCRIPTION OF SYMBOLS 1 Coil 2 Maraging steel strip rear end part 3 Maraging steel strip tip part 4 Coil outer surface 5 Coil inner surface 6 Coil center part

Claims (2)

After the maraging steel material is heated to 1050 to 1300 ° C., hot rolling is performed to obtain a maraging steel strip, and then the maraging steel strip is wound to form a coil. In the winding method,
The end temperature of the hot rolling is 700 ° C. or more and less than 900 ° C.,
In the coil after 2 minutes from the end of winding, the temperature at the central portion in the width direction of the coil outer peripheral surface, the temperature at the central portion in the width direction of the inner peripheral surface of the coil, and the temperature at the side surface in the central portion of the coil all exceed Ms point. 700 ° C. Ri Do below, the coil outer circumferential surface widthwise central portion of the temperature, as the temperature difference between the temperature of the temperature and the side of the coil center in the width direction central portion of the said coil circumference is within 300 ° C. Winding on a coil
Thereby, the temperature of the maraging steel coil wound with the maraging steel strip is maintained at a temperature exceeding Ms over the entire length of the maraging steel strip, and then the maraging steel coil is cooled to a temperature below the Ms point. After that, a method of winding a hot-rolled maraging steel strip, which is not reheated to a temperature exceeding the Ms point again.   After obtaining the maraging steel strip, the coiling of the maraging steel strip is started within 30 seconds from the end of hot rolling, and the winding is finished within 5 minutes. The hot rolling maraging steel strip winding method according to claim 1.

Images