JPH04198421A - Manufacture of austenitic stainless stell strip - Google Patents

Abstract

PURPOSE:To effectively execute cold rolling to a hot rolled steel strip even if its annealing stage is obviated and to make better the formability of the steel strip by specifying the hot rolling temp., cooling conditions and coiling temp. of a steel strip and heating the material immediately before its cold rolling. CONSTITUTION:The slab of an austenitic stainless steel is heated, and its hot rolling is finished at >=850 deg.C. Immediately, the rolled stock is rapidly cooled at >=10 deg.C/s cooling rate and is coiled in the temp. range of <=500 deg.C. Next, the material is pickled without executing heat treatment, is thereafter heated to 50 to 200 deg.C immediately before its cold rolling and is rolled. In this way, an austenitic stainless steel strip having good surface properties and workability can be manufactured at low cost by a relatively simple cold rolling stage.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for stably producing an austenitic stainless steel strip having good formability without a solution heat treatment step.

<Conventional technology and its issues> Normal hot rolling (hot rolling end temperature = 900°C or higher).

Cooling rate after hot rolling: less than 10°C/s 5 Coiling temperature = 8
The austenitic stainless steel hot-rolled steel strip obtained at a temperature of 00 to 650℃) is in a sensitized state with carbides precipitated at the austenite grain boundaries. Therefore, if it is pickled as it is during cold rolling, intergranular corrosion will occur and the quality will deteriorate. Failure to do so may result in defects. Therefore, conventionally, in manufacturing austenitic stainless steel strip (cold rolled steel strip), 1. [The hot-rolled steel strip obtained by hot rolling is heated and held at around 1100°C prior to pickling, and then rapidly cooled by gas cooling or water spray cooling to dissolve carbides into solid solution. This was indispensable and was a huge burden in terms of equipment, workability, manufacturing costs, etc.

Therefore, the present inventors first proposed a technique for omitting zero-hour annealing (solution heat treatment) of an austenitic stainless steel strip. Austenitic stainless steel that suppresses carbide precipitation and prevents sensitization after hot rolling by rapidly cooling at a cooling rate of
We would like to propose a manufacturing method for hot-rolled steel strips.
No. 921).

However, as a result of continued studies from various viewpoints, the following problems were pointed out in the above-mentioned method for producing hot-rolled austenitic stainless steel strips. In other words, although the method for producing austenitic stainless steel hot-rolled steel strip proposed by Pasaki is certainly very effective in preventing sensitization, when such treatment is carried out, Residual strain recovers and recrystallizes during low-temperature coiling, resulting in significant hardening of the hot-rolled sheet, making cold rolling difficult and requiring many passes, so the cost increase from this aspect could not be ignored. .

In other words, it is generally known that austenitic stainless steel has higher deformation resistance than ordinary steel, but as mentioned above, hot-rolled sheets that do not undergo hot-rolled sheet annealing have a higher deformation resistance due to the residual strain of hot-rolling. Cold rolling was more difficult due to higher deformation resistance.

Therefore, the purpose of the present invention is to enable sufficiently efficient cold rolling even if the annealing process of hot rolled steel strip is omitted, and to produce a stable product with good formability. The objective was to establish a means of manufacturing austenitic stainless steel strips that could be obtained by

(Means for Solving the Problems) As a result of further research to achieve the above object, the inventors have obtained the following new knowledge.

That is, 8) suppression of carbide precipitation at the hot-rolled sheet stage is a technical issue that requires omitting hot-rolled sheet annealing in the process of manufacturing austenitic stainless steel cold-rolled steel strip.

B) Reduction of rolling load in the cold rolling process.

However, regarding "suppression of carbide precipitation in the hot-rolled sheet stage," as clarified in the previous proposal by the present inventors, "the precipitation of carbides is suppressed during hot-rolling." Since there are "end-of-rolling temperatures, cooling rates, and winding temperature ranges" in which sensitization does not occur, it is effective to implement these controls.

On the other hand, regarding the above-mentioned "reduction of rolling load in the cold rolling process", it is necessary to solve the problem of "hardening of the material in cold rolling". Therefore, the present inventor investigated the hardening mechanism of austenitic stainless steel by cold rolling and found that a) SUS304 steel and 5US301@
The increase in cold rolling deformation resistance in austenitic stainless steels, such as typified by , is due to the extremely large influence of work hardening due to the introduction of strain during the cold rolling process, as well as work-induced martensite that precipitates due to strain.

b) However, "precipitation of deformation-induced martensite", which is the main cause of this hardening, has a strong temperature dependence, and rolling in a warm region minimizes the precipitation of deformation-induced martensite.

We were able to clarify the fact that

When manufacturing austenitic stainless steel strip, hot rolling is performed by selecting a final temperature, cooling rate, and coiling temperature that suppress carbide precipitation and do not cause sensitization, and cold rolling is performed to harden the strip. If the process is carried out in a temperature range (warm) where deformation-induced martensite, which is the main cause, does not precipitate, "cold-rolled steel with good surface properties can be obtained with good cold-rolling workability even if hot-rolled sheet annealing is omitted.""Obi can be obtained" and "
When hot-rolled sheet annealing is omitted, the ductility after cold-rolling annealing is usually slightly inferior to that of hot-rolled sheet annealing; however, when cold rolling is performed warmly, the ductility decreases. It was also found that there was a significant improvement in

The present invention was completed based on the above-mentioned findings, etc. [In manufacturing an austenitic stainless steel strip,
The hot rolling is finished at 850°C or higher, and immediately after the hot rolling is finished, it is rapidly cooled at a cooling rate of 10°C/s or higher to 500°C.
After being coiled at a temperature below ℃, the material immediately before rolling is heated to 50 to 200℃ before rolling, making the austenitic stainless steel strip with good formability easy to work with and stable. It has a major feature in that it can be manufactured by

Here, the type of austenitic stainless steel to which the method of the present invention is applied is not particularly limited, and the present invention applies to any type of austenitic stainless steel including those specified in the JTS standard. It goes without saying that the desired effect can be obtained even if the method is applied.

As described above, the present invention suppresses the precipitation of carbides after hot rolling by performing hot rolling and winding under specific conditions when producing an austenitic stainless steel strip, thereby making it possible to perform the solution heat treatment process. If omitted, cold rolling becomes difficult due to “hardening due to processing strain in the latter half of the hot rolling process (below 1000°C)” and “hardening due to precipitation of deformation-induced martensite in the cold rolling process”. The main idea is to avoid this through warm rolling and to be able to efficiently produce austenitic stainless steel strip with good formability.
The proposal regarding only "warm rolling of stainless steel" is published in Japanese Patent Application Laid-open No. 1983. -62115, JP-A-63-10009, JP-A-64-21013, JP-A-1210
Several attempts have been made so far, as seen in Publication No. 103 and the like.

However, it goes without saying that these proposals are on a completely different level from the technology related to the present invention, which is ``introducing warm rolling as a means to industrially realize the omission of hot-rolled sheet annealing.''

Next, the hot rolling end temperature in the method of the present invention.

The reason why the cooling rate after hot rolling, the one-winding temperature, and the heating temperature before cold rolling are limited as described above will be explained.

(al) Hot rolling final temperature If the hot rolling final temperature is below 850°C, precipitation of carbides is promoted due to strain-induced precipitation during rolling, so the hot rolling final temperature is set to be 850°C or higher. .

(b) Cooling rate after completion of hot rolling Cooling rate from completion of hot rolling to coiling is 10'C/c
If the cooling rate is slower than 1, carbide precipitation will occur during cooling, resulting in deterioration of product quality, so the cooling rate after hot rolling is 1.
It was set as 0°C/c or higher.

(C) Winding temperature If the winding temperature exceeds 500°C, carbide precipitation occurs during slow cooling after winding, so the winding temperature was set at 500°C or lower. However, if the temperature is lower than 200°C, the deformation strength of the copper strip increases and winding becomes difficult, so the winding temperature is preferably not lower than 200°C.

(d) Heating temperature before cold rolling If the cold (warm) rolling temperature is less than 50°C, the amount of martensite will be large and the deformation resistance will be large, making rolling difficult and forcing multi-bus rolling. However, the rolling temperature is 50
In the temperature range above ℃, the rolling load is noticeably reduced and smooth rolling becomes possible. However, heating to a temperature exceeding 200°C is not preferable because temper color occurs.

For this reason, it was decided that the steel should be heated to a temperature range of 50 to 200°C immediately before cold rolling. Here, the amount of martensite precipitated by warm rolling is not affected by the presence or absence of hot-rolled sheet annealing or the hot-rolling conditions.

In addition, Fig. 1 shows a hot rolled sheet of ordinary 5US304 steel (hot rolling end temperature = 890°C 1 cooling rate after hot rolling: Q-15°C/c, coiling temperature: 450°C 1 hot rolled sheet without annealing) ) using J = 1.3xlO-'/s.

It is a graph showing the "relationship between test temperature, amount of martensite, and tensile strength" obtained by conducting a tensile test at a test temperature of 50 to 400°C. Note that the "amount of martensite" here refers to the amount of martensite at the fractured part after the tensile test expressed as "%" in terms of the amount of pufferite. From the results shown in Figure 1, it can be seen that the test temperature is 50%. It can be confirmed that when the temperature exceeds .degree. C., the precipitation of martensite after tensile processing (deformation) is suppressed, and the tensile strength decreases.

Further, as mentioned above, when cold rolling is carried out at a warm temperature within the above temperature range, the ductility of the resulting product is also improved, and this is thought to be due to the following reasons.

When warm rolling is performed at a temperature of 50°C or higher, the residual strain caused by low-temperature coiling uniformly precipitates deformation-induced martencide that precipitates in a controlled manner during warm rolling. It is presumed that the ductility was improved by the effect of equalizing and grain-regulating the recrystallized structure by =10.

Next, the present invention will be explained in more detail with reference to Examples.

<Example> 5US304 equivalent steel obtained by continuous casting method and 5
After heating a slab of US301 equivalent steel to 1250℃,
Thickness under the hot rolling two winding conditions shown in Table 1: 4 +u+
produced hot-rolled steel strips.

Subsequently, after pickling this without heat treatment, the first
After heating to the cold rolling temperature (biting temperature) shown in the table, it was immediately cold rolled to a thickness of 0.41 m.

For comparison, the hot-rolled steel strip was heat-treated at 1100°C for 30 seconds, pickled, and then heated to a thickness of 0.
A test of cold rolling to 4 mm was also conducted.

The pickling conditions at this time were as follows.

Pickling liquid composition - HF: 3wt%, HNO,: 15wt
%.

Hz O: 82wt%, Liquid temperature...50℃1 Immersion time: 30 seconds.

The surface gloss of the cold rolled steel strip thus obtained was visually observed, and the results are shown in Table 1 along with the number of cold rolling passes up to 4t-0,4'.

As is clear from the results shown in Table 1, the cold-rolled steel strip obtained according to the method of the present invention has no gloss defects, whereas the manufacturing conditions deviate from the conditions specified by the present invention. It can be confirmed that gloss defects occur in the comparative method.

In addition, if the cold rolling temperature is 50 to 200°C, the number of cold rolling passes is 8 or less, which is good (hot rolled plate annealed material has 8 passes or less).
(Since cold rolling is possible with 8 passes or less, it is judged as good); if the cold rolling temperature is less than 50°C, the number of cold rolling passes:
It is also clear that 10 or more baths are required and the cold rolling is defective.

It can also be seen that temper color adheres when the cold rolling temperature is 250°C.

Furthermore, the ``elongation'' measured by conducting a tensile test on material J that was annealed at 1100°C for 30 seconds after cold rolling is also listed in Table 1, and these results also show that the elongation was obtained according to the method of the present invention. All of the cold-rolled steel strips showed an elongation of over 60%,
This is not only a significant improvement over test numbers 9 to 13, in which cold rolling was performed without heating to a predetermined temperature immediately before (warm rolling was not performed), but it was also significantly improved compared to test numbers 9 to 13, in which cold rolling was performed without heating to a predetermined temperature immediately before (warm rolling was not performed). It is confirmed that the value is better than that of Test No. 17 which was rolled (rolling temperature: 25°C).

These results also show that even if hot-rolled sheet annealing is omitted in the manufacturing process of austenitic stainless steel strip, sufficient elongation (formability) can be achieved with good rolling workability by combining the specified warm rolling. It is clear that it is possible to obtain a product with

In addition, in this example, 5LJS304 series and 5US301
Although only examples of stainless steels are shown, other austenitic stainless steels (SUS310 series, 5US316 series, 5US3 series) are shown.
It goes without saying that similarly good results were obtained with respect to 17 series, etc.).

Summary of Effects> As explained above, according to the present invention, austenite with good surface texture and workability can be produced by a relatively simple cold rolling process without requiring a hot-rolled sheet heat treatment process after hot rolling. Industrially useful effects are brought about, such as making it possible to manufacture stainless steel strips at low cost.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the tensile test temperature, the tensile strength, and the amount of martensite for a hot-rolled sheet of 5US304 steel that has been omitted from annealing.

Claims (1)

[Claims] In producing austenitic stainless steel strip, hot rolling is completed at 850°C or higher, and immediately after the hot rolling is completed, it is rapidly cooled to 500°C at a cooling rate of 10°C/s or higher.
A method for producing an austenitic stainless steel strip with good formability, which comprises rolling in the following temperature range and then heating the material just before rolling to 50 to 200°C before rolling.