JPS5924183B2 - Heat-resistant ferritic stainless steel with excellent stretch flangeability and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a heat-resistant ferritic stainless steel with excellent press formability, and particularly with high oxidation resistance and corrosion resistance at high temperatures such as automobile exhaust gas purification devices and various combustion devices.
This invention relates to a stainless steel suitable for parts requiring excellent press formability, especially stretch flangeability.

JIS? This specified SUS430 is the most widely used ferritic stainless steel, but 8500C
Oxidation resistance is not sufficient at high temperatures such as ~1,000°C, and even at temperatures of 8,000°C to 850°C, scaling often peels off when subjected to repeated heating and cooling. It is unsuitable for applications where clogging is a problem.

Furthermore, since press moldability is poor, it is difficult to manufacture parts with complicated shapes. In addition, austenitic stainless steels such as SUS304 have good heat resistance and press formability, but are not only expensive but also have large thermal expansion. There is a risk that the difference may cause deformation or destruction of the device.

For the reasons mentioned above, the present inventors first added an appropriate amount of Z.
A heat-resistant ferritic stainless steel containing r was developed (Japanese Patent Application Laid-Open No. 146512/1983).

This Zr-added ferritic stainless steel has improved not only oxidation resistance but also weldability and cold workability to some extent. However, the value is not necessarily sufficient for press molding.
By the way, especially for parts that have complex shapes and are manufactured by press forming, such as automobile exhaust gas purification devices, an extremely high degree of press formability is required, and the previously proposed Zr-added steel does not necessarily have sufficient performance. cannot demonstrate.

Press formability is classified into deep drawability, stretchability, stretch flangeability, etc. depending on the forming method, and it is known that these properties are correlated with various properties measured by tensile tests. In addition, it is also necessary to have so-called roughness resistance, which means that the surface does not become rough due to press molding. By the way, stretch flange forming called louver processing has become an important forming element in automobile exhaust gas purification devices and the like in order to create gas passages. For this reason,
Materials that have a certain level of moldability and particularly excellent stretch flangeability have high utility value. As a result of conducting research focusing on this point, the present inventor discovered that Zr
By appropriately adjusting the component balance of % and C%+N%, we succeeded in particularly improving stretch flangeability.

The steel of the present invention is Crll. O~20.0%, Sil. Less than 0%, Mnl. It is characterized in that the balance between the stress Zr and the C+N content is adjusted as described later, so that the content is 5% or less.

The reason why the component contents in the steel of the present invention are determined as described above is as follows. Cr must have a coefficient of 11 or higher to ensure basic oxidation resistance and corrosion resistance. However, 2
When the ratio exceeds 0, press formability deteriorates. It is effective to include Si as a deoxidizing agent during steel manufacturing, but at 1.0
% or more, press formability deteriorates.

Mn promotes the deoxidizing effect of Si, and changes the form of nonmetallic inclusions in combination with Si.

However, when the modulus exceeds 15, the hardening of the steel becomes significant and cold working becomes difficult. Next, Zr%, which is a feature of the present invention
The balance between C% and N% will be described.

According to the research results of the present inventor, in order to improve stretch flangeability, C%+N'% must be less than 0.02% and Zr
% = 4 (C% 10N%) ~ [4 (C% 10N%) + 0.20
It is necessary to adjust it within the range of If (C%-N%) is less than 0.02%, there are few Zr carbonitrides, so the crystal grains become coarse and roughness tends to occur, and the texture is also affected, resulting in poor deep drawability and stretchability. Although it deteriorates slightly, the stretch flangeability improves considerably. However, even if deep drawability, stretchability, etc. are deteriorated, it is not inferior to SUS43O, etc., and there are many cases in which there is no problem in practical use unless particularly high formability is required.

If Zr% is greater than the above amount, Zr and Cr. An intermetallic compound with Fe is formed, inhibiting ductility and deteriorating stretch flangeability.

In addition, if Zr% is too small, carbonitrides as well as Zr carbonitrides
Carbonitrides of r are formed, and this has a large effect of reducing ductility, resulting in deterioration of stretch flangeability. Therefore, Z
The above ranges are appropriate for r% and C%10N%.

In addition, to make C%10N% less than 0.02%, C
% and N% both need to be at extremely low levels (for example, 0.01% or less), which can be achieved industrially by vacuum melting, argon-oxygen decarburization (AOD), and the like.
Next, a method for manufacturing steel having the above-mentioned components will be described.

From melting to hot rolling, there are no factors that particularly affect formability, so normal methods are sufficient. The annealing process has the greatest effect on press formability.

Annealing includes annealing performed after hot rolling and before cold rolling, and annealing performed after cold rolling. First, annealing after hot rolling will be described. This annealing step is indispensable when high formability is desired, such as in the steel of the present invention. In the actual manufacturing process, there are differences between when the hot-rolled sheet is in a coil state and when it is in a sheet state. In the case of a coil state, there are two types of annealing methods. One is 770 by batch annealing as a coil. This is a method in which the temperature is maintained in a temperature range of C to 1000 C for several hours or more. The other method is to unwind the coil and pass it through a continuous annealing furnace, annealing it at a temperature in the range of 770°C to 1000°C. In this case, since the material is in a so-called sheet form in the annealing furnace, the temperature of the material increases quickly. Therefore, the heating time may be much shorter than in the batch annealing method. In the case of a sheet state, it is annealed in a batch furnace while being maintained at 770° C. to 1000° C., but the holding time may be short as in the continuous furnace method described above. The reason for limiting the temperature range as described above is as follows.

For normal ferritic stainless steel, the annealing temperature is 83
At temperatures higher than 0°C, austenite precipitates and martensite precipitates during cooling, making cold rolling difficult.
Furthermore, even in steels in which austenite does not precipitate, the crystal grains become coarse and the cold-opening formability deteriorates. However, Zr of the steel of the present invention
Ferrite-based stainless steel containing ferrite remains in a ferrite phase even at high temperatures, and coarsening of crystal grains is less likely to occur, so it has the characteristic that a steel with excellent formability can be obtained by high-temperature annealing. However, when the temperature exceeds 1000°C, Zr
This is not preferable because decomposition and aggregation of carbonitrides occur and crystal grains become coarse. In addition, at temperatures lower than 770°C, Zr during hot rolling
Since the dispersion form of carbonitrides is not sufficiently improved, recrystallization is inhibited during annealing after cold rolling, resulting in deterioration of formability. Next, annealing after cold rolling will be described.

This annealing needs to be carried out at a temperature range of 770°C to 1000°C. If the temperature exceeds 1000°C, crystal grains will become coarse, which is not preferable. If the temperature is lower than 770°C, the rolled structure cannot be sufficiently recrystallized, resulting in marked deterioration of formability.

With the above ingredients and manufacturing method, itasara 1.2mm
Typical formability of steel made into thin plates is shown in Table 1.

The figure shows the test results that form the basis of the present invention. Example 1 shows that stretch flangeability can be improved by keeping (C + N)% extremely low and maintaining a balance with Zr%. Steel having the components shown in Table 2 is melted, hot-rolled to make a plate with a thickness of 3.2mm, annealed with 830'CXl for 5 minutes, and then cold-rolled into a thin plate with a thickness of 1.2mm. year,
Annealing was performed at 850°C for 5 minutes.

Next, surface scale was removed by pickling, a plate-shaped test piece was prepared, and the various formability shown in Table 1 was examined. The results are shown in Table 3. From this result, it is clear that stretch flangeability deteriorates when the composition of the steel of the present invention is outside the range.

Example 2 Table 4 ζ A case in which a steel having a composition within the range of the steel of the present invention as shown in the table is melted, a plate with a thickness of 32 mm is produced by hot rolling, and subsequent annealing is not performed. ,830℃
× For the case where annealing was performed for 15 minutes, a thin plate with a thickness of 1.2 mm was further cold-rolled at ゜2850℃ ×
After annealing for 5 minutes, a plate-shaped test piece was created.

The formability of this steel is as shown in Table 5, and it is clear that the stretch flangeability deteriorates significantly without annealing after hot rolling.

Example 3 A steel having the composition of Example 2 was hot rolled into a plate with a thickness of 32mm, annealed at 830°C for 15 minutes, and then cold rolled into a plate with a thickness of 1.2mm. Make thin plates and heat them to 730℃
Annealing was performed for 5 minutes and then for 850 minutes to prepare a plate-shaped test piece.

The formability of this steel is shown in Table 6, and it is clear that if the annealing temperature after cold rolling is too low, the stretch flangeability will be significantly degraded.

[Brief explanation of the drawing]

The figure is a chart showing the relationship between stretch flangeability and the amount of Zr and CfN in ferritic stainless steel.

Claims (1)

[Claims] 1 Cr11.0-20.0%, Si less than 1.0, Mn
1.5% or less, C%+N% less than 0.02%, and Zr
%=4(C%+N%) ~ [4(C%+N%)+0.20
A heat-resistant ferritic stainless steel with excellent stretch flangeability, containing Zr within a range of %] and the remainder being substantially Fe. 2 Cr1.10-20.0%, Si less than 1.0%, C
%+N% less than 0.02%, and Zr=4(C%+N%
) ~ 4 (C% + N%) + 0.20% of Zr after hot rolling steel with the balance essentially consisting of Fe77
A method for producing heat-resistant ferritic stainless steel with excellent stretch flangeability, characterized by annealing at a temperature range of 0°C to 1000°C, then cold rolling, and then annealing at a temperature range of 770°C to 1000°C.