JPS6112828A - Manufacture of ferrite stainless steel sheet surperior in surface property and workability - Google Patents

Abstract

PURPOSE:To manufacture economially a steel sheet superior in surface property and workability by making a ferrite stainless steel billet contg. a specified Al quantity to hot rolled plate under a specified condiction, winding it, and applying descaling, cold rolling and final annealing thereto. CONSTITUTION:The ferrite stainless fillet contg. 0.08-0.5% Al is rolled at 1,100-1,180 deg.C temp. range by >=30% cumulative draft. Next, said plate is held at 1,100-1,180 deg.C temp. for at least >=30sec, then heated to 1,150-1,300 deg.C temp. range and made hot rolled plate at >=850 deg.C temp. by rolling mill consisting or rough rolling mill and plural continuous rolling mills. Next, the hot rolled plate is wound, descaled, then cold rolled and subjected to final annealing. By this way, annealing process of hot rolled plate and surface grinding process after pickling can be omitted.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for economically producing a ferritic stainless thin steel sheet having excellent surface properties and workability.

(Prior art) A technology for manufacturing At-added ferritic stainless steel sheets without hot-rolled sheet annealing has already been disclosed in JP-A No. 5.
Although these technologies are introduced in Japanese Patent Application Laid-open No. 7-3564 and Japanese Patent Application Laid-Open No. 57-20234, it is not clear that the mechanical properties, T value, and ridging properties required for ferritic stainless steel sheets are necessarily satisfied. I can't say that.

(Problems to be Solved by the Invention) The present invention provides an economical method for producing an At-containing ferritic stainless steel sheet with no surface flaws and excellent workability by omitting the hot-rolled sheet annealing process and the surface polishing process after pickling. The present invention provides a method for manufacturing.

(Means for Solving the Problems) The object of the present invention is to heat a ferritic stainless steel slab containing ~0.5% of p, to, o s to 1100°C or higher.
Rolling at a cumulative reduction rate of 30% or more in a temperature range of 180°C or lower, then holding at a temperature range of 1100°C or higher and 1180°C or lower for at least 30 seconds, then rolling at a temperature of 1150°C or higher
After heating in a temperature range of 300°C or less, a rolling mill consisting of a rough rolling mill and a plurality of continuous rolling mills is used to form a hot rolled sheet at a temperature of 850°C or more, and the sheet is rolled in a temperature range of 600 to 800°C. Next, after main descaling was performed by pickling mainly with acids other than nitric-fluoric acid, the work roll diameter was 300 waφ.
At least 60 inches of the total amount of cold rolling to be cold rolled is rolled by a tandem cold rolling mill consisting of a plurality of cold rolling mills as described above, and the remaining part is rolled by a cold rolling mill having a work roll diameter of 100tIILφ or less. and then 850-10
This is achieved by a method for producing ferritic stainless thin steel sheets with excellent surface texture and workability, which is characterized by final annealing within 60 seconds at a temperature of 0.000C.

In the present invention, a ferritic stainless steel slab containing 0.08 to 0.5% by weight of At is first rolled in a temperature range of 1100°C or more and 1180°C or less at a cumulative reduction rate of 30°C multiple times before normal hot rolling. After performing the above rolling, 11
Do not lower the temperature to below 00°C (hold at a temperature of 1100°C to 1180°C for at least 30 seconds or more, because river crystals can destroy the casting structure and reduce the gluing properties of the final product. The present inventor found that static recrystallization progresses and the cast structure is destroyed by rolling at a cumulative reduction rate of 30% or more in the α+γ2 phase region. When static recrystallization progresses, when the material temperature drops to a temperature range where the already existing γ phase begins to transform into the α phase, the strain energy accumulated under cumulative pressure drives static recrystallization. It has been found that static recrystallization does not proceed because it is not used as a force and is consumed to promote the r→α transformation.In the present invention, 1
The temperature range was limited to 100℃ to 1180℃ because α+
This is to perform rolling in the γ2 phase region, and the reason why the cumulative reduction rate is set to t30% or more is because static recrystallization is insufficient with a reduction amount less than this. After such rolling, the material temperature is 11
3 in the temperature range of 1100 to 1180℃ without exceeding 00℃
The reason for holding for 0 seconds or more is to promote static recrystallization without causing γ+α transformation, and since γ→α transformation occurs at temperatures below 1100°C, the temperature is set at 1100°C or higher.

Further, at temperatures exceeding 1180°C, γ→α transformation occurs and static recrystallization is inhibited, so the upper limit was set at 1180°C. The reason why we set the retention time to 30 seconds or more is because the retention time is shorter than this.
This is due to insufficient progress of static recrystallization.

It is then pre-rolled in this way and has a 1100-1180
A stainless steel slab kept at a temperature of 30 seconds or more is heated, and the heating temperature is 1150°C or more and 1300°C or less. The reason for this is that when heating the slab at a temperature lower than this, the temperature of the material decreases during hot rolling and the rolling load increases, resulting in defects occurring during hot rolling and defects after hot rolling. This is because a grinding process to remove these flaws is essential. In order to prevent the occurrence of defects during hot rolling, it is preferable to heat the slab at a high temperature, but excessive temperatures exceeding 1,300°C will cause abnormal growth of crystal grains, deteriorate the gluing properties of the final sheet, and Since heating requires energy and is uneconomical, the upper limit was set at 1300°C.

The reason why the finish rolling finishing temperature is limited to 850° C. or higher is that if the finishing temperature is lower than this, the T value decreases. The finish rolling finishing temperature is preferably a high temperature, but in consideration of the upper limit of the slab heating temperature of the present invention, it is preferably about 1000° C. or less. The reason why the T value deteriorates as the final rolling finish temperature becomes lower than 850°C is that shear deformation bands occur inside the steel sheet and a (111) texture, which is advantageous for deep drawing properties, develops in the final annealing. This is because it becomes difficult.

Next, the reason why the winding temperature is limited to 600°C or higher is that if the winding temperature is lower than this, edge cracks are likely to occur during cold rolling, and in some cases, breakage may occur during cold rolling. This is because the cold rollability deteriorates significantly and the lower value becomes low, making it unsuitable for deep drawing and the like. The reason why the cold rollability deteriorates due to low-temperature rolling and the 7 value of the final product decreases is that the α' phase remains in the as-hot-rolled state. In other words, since the α' phase is hard and brittle, cold-rollability decreases, and the presence of such a hard phase is advantageous for deep drawability during final annealing (111
) The development of texture is suppressed. The reason why the winding temperature was set to be 800° C. or lower is that if the winding temperature is higher than this, the final product will deteriorate in gluing. The reason why the sillizing properties deteriorate due to high-temperature winding 9 exceeding 800° C. is that in the case of such high-temperature winding, the α' phase transforms into α decacarbide.

In the annealing process, the crystal orientation is not randomized (Zoo) and (
This is because the texture of 111) develops preferentially.Next, the reason for limiting the descaling conditions will be described. The ferritic stainless steel hot-rolled sheet of the present invention is descaled in the hot-rolled state, so compared to normal hot-rolled sheet descaling that involves hot-rolled sheet annealing and descaling, the properties of the scale are different, and the scale is descaled. However, in order to effectively descale, light reduction rolling of 10 inches or less, mechanical descaling by shot blasting or spraying iron sand powder with high pressure water, and descaling by acid solution are recommended. It is effective to use them in combination.When hot-rolled sheets are annealed, the scale itself is difficult to pickle compared to the as-hot-rolled state; , the ferrite phase is completely dispersed in the decacarbide phase, the Cr concentration in the ferrite phase of the matrix is uniform, and it dissolves uniformly in any pickling process.
There is no need to particularly limit the pickling liquid to be used.

However, in the as-hot-rolled state, the matrix
It consists of ferrite phase + α′ phase + carbide, especially α′
A chromium-deficient layer exists at the boundary between the ferrite phase and the ferrite phase. The γ phase, which is generally referred to as the α' phase, is thought to be formed by transformation during hot rolling. When observed closely, fine ferrite grains with a grain size of about 1 μm are found on the surface of the steel sheet. This is an aggregate of fine ferrite grains, and finer carbides are precipitated in a dot array surrounding specific grain boundaries of these fine ferrite grains. A chromium-deficient layer exists at the boundary between these carbides. The Cr concentration of the matrix and matrix of these fine ferrite grain aggregates is about 1 to 2 times lower than that of the coarse ferrite coarse phase with a grain size of about 100 μm, which was the ferrite phase during hot rolling. In this way, the chromium concentration varies significantly locally.) IJ socks, pickling mainly using nitric and hydrofluoric acid, e.g. 60g/AHNO3+ 2097tHF
, When pickling is carried out under normal conditions such as 50℃ and 40 seconds, these parts are preferentially dissolved, resulting in significant intergranular corrosion after pickling, and the surface irregularities are easily absorbed by other pickling liquids. (acids mainly composed of sulfuric acid and hydrochloric acid) are used.

If a plate with such a large uneven surface is cold-rolled as it is, the metal in the curved parts will fall down onto the metal in the concave parts, and some parts will overlap, and the parts that have fallen down will break off. Such defects may occur. Such overlapping and torn parts remain even after final annealing and are considered surface defects. In order to prevent such surface defects, it is possible to smooth out the unevenness by polishing with a fine-grained belt abrasive paper after pickling and before cold rolling, but this is not economical. However, when descaling hot-rolled material is pickled with an acid other than nitric or hydrofluoric acid, for example, an acid solution mainly composed of sulfuric acid or hydrochloric acid, intergranular corrosion does not occur after pickling, and the degree of unevenness is reduced. In the present invention, the pickling used for descaling is limited because the surface defects are less likely to occur even if polishing is not performed after pickling.

Next, the conditions for cold rolling will be described. The purpose of cold rolling is to perform cold rolling using a large diameter roll in the first stage and a small diameter roll in the second stage to improve the bottom value, reduce ridging, and prevent surface defects from occurring. The lower value is at the final annealing step (111)
According to the research of the inventor (111), the plastic deformation in the cold rolling process develops when the deformation due to shear deformation is as small as possible. I found it. Cold rolling of stainless steel is
It is generally rolled in a Sendzimir rolling mill with a roll diameter of about 50 mm, compared to ordinary steel rolling in a tandem cold rolling mill with a roll diameter of 300 mm or more.
The drawback is that the production volume is extremely low. Therefore, if a tandem cold rolling mill that rolls ordinary steel can be used to roll stainless steel fields, production will increase and the economic effect will be significant. By the way, rolling with such large diameter rolls causes less shear deformation than rolling with small diameter rolls, so the (Ill) interlocking structure develops better in the final annealing step. In the case of a material that contains about 0.12 of A7 in normal 430@ and is hot-rolled by the method of the present invention, it can be rolled with a roll of 300 mm in diameter or rolled with a roll of 50 mm in diameter. The T value is improved by approximately 10% to 30% compared to the case where the According to the inventor's research, this T value improvement effect is
If 60% or more of the total rolling amount to be cold rolled is rolled with large diameter rolls, the effect will not change even if the remaining portion is rolled with small diameter rolls, so the large diameter roll cold rolling rate is set to 60%. -This is the above. The reason for this is that in processing after 60 strips, the strain becomes loose due to rolling with large diameter rolls, and the manner in which strain accumulates during deformation is not affected by slight changes in the roll diameter.

Next, let's consider ridging. When hot-rolled material is rolled with small-diameter rolls, the deformation in the central region of the sheet thickness is relatively small compared to when rolled with large-diameter rolls, so the collection of (100) systems that exist in the hot-rolled state is reduced. Even after cold rolling and annealing, a large proportion of the structure remains in that form without recrystallization, resulting in deterioration of the ridging properties. The (ioo) type texture is the final stable orientation of cold rolling and recrystallization, and conversely, when the roll diameter is larger and the cold rolling rate is higher, when cold rolling is performed with small diameter rolls. The stable orientation will be reached earlier than (1
00) The degree of accumulation of the system texture is higher than that in small diameter roll rolling9, resulting in deterioration of the ridging properties. In other words, the degree of accumulation of the (ioo) type texture that deteriorates the ridging properties decreases first as the correlation between the cold rolling rate and the roll diameter increases, and as the cold rolling rate and roll diameter each increase, Then, the phenomenon shows an increase again. After all, there is a cold rolling rate and a roll diameter that result in the minimum value of the (ioo)-based texture accumulation degree. On the other hand, the cold rolling rate and roll diameter at which the degree of accumulation of the (,100) type texture becomes the minimum value also differ depending on the condition of the material to be rolled. As in the present invention, materials that have not been annealed in hot rolled sheets have a higher degree of accumulation of (100) texture than materials that have been annealed, so the degree of accumulation of (100) texture after finish annealing is higher. When the cold rolling rate shows the lowest value, the roll diameter shifts to the larger density side. Furthermore, as in the present invention, when a hard phase called the so-called α' phase is present in the material, various sliding deformations occur around these phases, making it difficult for the degree of accumulation of the (Zoo)-based texture to increase. become a state.

In this way, in the present invention, the roll diameter is determined by confirming the range in which the ridging characteristics do not deteriorate even if the roll diameter is moved to the larger diameter side, but by using a roll with a maximum diameter of about 700 mφ,
Even when rolled at a high reduction rate of about 90%, the ridging properties do not deteriorate.

The reason why the present invention stipulates that the first stage of cold rolling be rolled by a rolling mill with a roll diameter of 300 mφ or more at a cold rolling rate of 60% or more is based on the above reasons, but the workability (
From the viewpoints of production efficiency (value, linong properties) and production efficiency, it is sufficient to roll the entire rolling amount in one circuit with a tandem cold rolling mill. However, in consideration of the surface properties, it is preferable that the first stage be cold rolled using large diameter rolls, and the second stage be cold rolled using small diameter rolls. The reason is as follows. First, if the first stage is cold rolled with a large diameter roll of 300fflφ or more, even if the surface of the steel plate is uneven during the pickling process, as mentioned above, if the unevenness is not extremely large, it will be more effective than cold rolling with a small diameter roll. Since the plastic deformation of the surface layer is reduced, surface defects due to convex parts collapsing into concave parts and overlapping υ parts do not occur. This is because the polishing process becomes unnecessary. If the purpose is only to prevent surface defects caused by such unevenness, the entire cold rolling process can be carried out in a tandem cold rolling mill with large diameter rolls, but the tandem cold rolling mill used for rolling ordinary steel When rolling the entire process using a cold rolling mill, there is a drawback that the surface gloss required for the stainless steel sheet cannot be obtained. The reason for this is that when high-speed cold rolling is performed using large-diameter rolls, the lubricating oil film thickness at the roll bite increases, depending on the level of lubricating oil, and the oil present in the recesses on the surface of the copper plate causes so-called oil This is because there is a tendency for the surface to become dull and the surface gloss to deteriorate. Furthermore, when using tandem cold rolling '+f&, which is normally used for through-rolling, to cold-roll stainless steel, the rolling oil, surface roughness of the rolls, crown, etc. should be adjusted to suit rolling of ordinary steel. By paddling it to a state suitable for all-stainless steel rolling, it is possible to obtain almost the shape 1 surface texture of stainless steel.
When rolling stainless steel, the conditions change.
Since it is not economical, it is not preferable from an economic point of view to roll the entire process using a tandem cold rolling mill.

Therefore, if additional cold rolling is performed using small diameter rolls of t-100mφ or less up to the final gauge in the latter stage of cold rolling with lubricating oil and roll roughness suitable for cystine-less steel, the conditions for rolling ordinary steel can be used as is. At the same time, the oil pit is repaired, the surface roughness is reduced, and a stainless steel plate with excellent gloss can be obtained. By rolling 60% or more of the total cold rolling amount with large diameter rolls in the previous stage of cold rolling,
The unevenness during pickling becomes shallower, and furthermore, work hardening of the surface layer progresses, so even if subsequent rolling is carried out using small diameter rolls, the aforementioned heavy scratches will no longer occur, and surface defects due to heavy scratches will not occur. I can't see it. Also, by using small diameter rolls,
Since the contact area between the roll and the rolled material becomes smaller, it is possible to prevent the occurrence of oil film breakage and oil pits, so if the surface roughness of the roll is made fine, it is possible to produce a thin steel sheet with a good surface gloss. . In this case, the smaller the roll diameter, the better, but if it is 100mφ or less, it will be effective.
Loom the small diameter roll in cold rolling with small diameter rolls that is performed subsequent to cold rolling with large diameter rolls.
It is limited to φ or less. The larger the amount to be cold-rolled with rolls of 100mφ or less, the more it becomes possible to improve oil pits caused by large-diameter roll rolling, surface roughness (in case of large-diameter roll rolling with large roll roughness), etc. However, the amount of reduction by the small diameter rolls can be improved by rolling at least 1 inch or more of the sheet thickness before cold rolling.

Cold rolling according to the method of the present invention is carried out at a factory that produces both ordinary steel thin sheets and stainless thin steel sheets, that is, ordinary steel rolling is carried out in a tandem mill, and rolling is carried out in a Sendzimir rolling mill exclusively for stainless steel thin steel sheets. In this process, the stainless steel is rolled as it is in the tandem cold rolling mill used for -M through-rolling, and then rolled in the Sendzimir mill, so that the entire cooling step is rolled in the Sendzimir mill as in the conventional process. Not only is the productivity of the cold rolling process significantly improved, but the workability ('F value.

The quality of
This cold rolling technology is considered to be extremely superior in terms of both cost and cost. As a cold rolling machine that achieves the object of the present invention,
As mentioned above, cold rolling may be carried out using a combination of all existing tandem cold rolling mills and Sendzimir cold rolling mills, or cold rolling mills may be used in which the roll diameter of the stand at the rear stage of the tandem cold rolling machine is rolled to a smaller diameter. Needless to say, it is good to roll it.

Next, the final annealing is specified to be performed within 60 seconds at a temperature range of 850 to 1000° C., in particular, with the aim of lowering the yield point and improving workability. In the case of the target steel of the present invention, the slab heating temperature is preferably 1150°C or higher and 1300°C from the viewpoint of preventing the occurrence of hot rolling defects.
Even in the case of At-added steel, the effect of reducing solid solution N due to precipitation of AtH during the hot rolling process cannot be expected. Moreover, under the usual annealing conditions for ferritic stainless steel sheets, annealing at 800 to 820° C. for about 20 seconds or less can hardly lower the yield point because almost no precipitation of AtN can be expected in this annealing step.

However, according to the present invention, the final annealing is carried out at 850°C or higher.
By heating the slab at a temperature of 000°C or less and keeping the annealing temperature at a low temperature for a long time, AtN precipitates and it becomes possible to reduce the solid solution N that increases the yield point, so high-temperature slab heating is performed. , and even if hot-rolled sheet annealing is omitted, it is possible to lower the yield point.

In addition, as a basic component of the target steel of the present invention, At is 0.08
The reason why it is contained in the range of 0.5% is that if A, ao, 08% or less, 1) cold rollability decreases and edge cracking occurs in the cold rolling process, making stable cold rolling impossible; 11) The unevenness of the surface becomes large during pickling, and the uneven parts become thicker during cold rolling, and the thickened and thinner parts break off, resulting in surface defects on the final product: tf) r value 9) At 0.08
By adding % or more, preferably 0.1% or more,
These defects can be prevented. The higher the amount of At added, the better; however, even if it is added in excess of 0.5%, the effect is slight, but it becomes almost saturated, so the upper limit of the amount added is 0.5%.
%.

(Example) The present invention will be explained in detail below according to examples.

Example A ferritic stainless steel slab with a thickness of 300 wm and the composition shown in Table 1 was heated at 1150°C and then heated with 3 nogs to form a slab with a thickness of 200 TJan (cumulative rolling reduction rate of 33.
3%). The material surface temperature after 3-pass rolling was 1130°C. This slab t-1 was immediately placed in a heating furnace maintained at a temperature of 1,100°C, held for 5 minutes, and then reheated to a temperature of 1,200°C to form a rough rolled piece with a thickness of 20+m+ in 7 passes. It was rolled into a hot-rolled plate with a thickness of 3.0 mm. The hot rolling end temperature was 870°C, and the sheet was rolled up at a temperature of 650°C. After the hot-rolled sheet thus produced was shot blasted, it was descaled at a temperature of 90°C with a H2SO4 concentration of 300 g/l for 40 seconds, followed by a HNO concentration of 1509/l for 40 seconds at a temperature of 50°C. I did it. Next, it was cold rolled to a thickness of 1 in a 5-stand tandem cold rolling mill with a work roll diameter of 500+mφ, and then rolled to a thickness of 0.4 in 4 passes in a Sendzimir cold rolling mill with a roll diameter of 55 sIφ.
Cold rolling was performed up to the throat. Then at a temperature of 875℃ 3
Annealing was performed for 0 seconds. For comparison, 430 steel that does not contain At was subjected to the same treatment as in the present invention to form a thin steel plate, and 84 steel was subjected to the same treatment as in the present invention, but
A comparative product was a thin steel plate obtained by annealing the hot-rolled plate at 0CX for 4 hours. The r value, ridging property, yield point 2 surface texture, cold rollability, etc. of the thin steel sheet thus produced are summarized in Table 2.As is clear from Table 2, the product of the present invention has the following properties:
It can be seen that even though the hot-rolled plate annealing process is omitted, the surface quality, r value, linong properties, and yield point are all equivalent to or higher than the conventional material.

Ash 2. Product properties and cold rollability (effect) As detailed above, according to the present invention, the ingredients.

By combining hot rolling conditions, pickling conditions, cold rolling conditions, and annealing conditions, we are able to eliminate the hot rolled sheet annealing process and the surface polishing process after pickling, which were indispensable in the production of conventional ferritic stainless thin steel sheets. The present invention provides a ferritic stainless thin steel sheet with no surface defects and excellent workability using an extremely economical manufacturing method in which the main cold rolling is performed using a tandem cold rolling mill that can be omitted and has high productivity. Therefore, its industrial effects are extremely large.

Procedural amendment (voluntary) August 27, 1982

Claims (1)

[Claims] A ferritic stainless steel slab containing 0.08 to 0.5% Al is rolled at a cumulative reduction rate of 30% or more in a temperature range of 1100°C or higher and 1180°C or lower, and then rolled in a temperature range of 1100°C or higher and 1180°C or lower. Holding for at least 30 seconds or more, then heating at a temperature range of 1150°C or more and 1300°C or less, and then forming a hot rolled sheet at a temperature of 850°C or more in a rolling mill consisting of a rough rolling mill and a plurality of continuous rolling mills, 6
After rolling in a temperature range of 00 to 800°C, and then main descaling by pickling mainly with acids other than nitric-fluoric acid, tandem cold rolling consisting of multiple cold rolling mills with work roll diameters of 300 mm or more is performed. At least 60% or more of the total cold rolling amount to be cold rolled is rolled by a rolling mill, the remaining rolling is performed by a cold rolling mill with a work roll diameter of 100 mmφ or less, and then at a temperature range of 850 to 1000 ° C. A method for producing a ferritic stainless thin steel sheet with excellent surface texture and workability, characterized by performing final annealing within 60 seconds.