JPS5924182B2 - Heat-resistant ferritic stainless steel with excellent press formability 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, particularly oxidation resistance and corrosion resistance at high temperatures such as automobile exhaust gas purification devices and various combustion devices, as well as excellent press formability. Concerning stainless steel suitable for parts that require high performance.

SUS430 specified by JIS is the most widely used ferritic stainless steel, but it is
Oxidation resistance is not sufficient at high temperatures such as ooooC, and even at temperatures of 8000C to 8500C, when heating and cooling are repeated, there is a lot of peeling of scale and clogging, such as in automobile exhaust gas purification equipment. Unsuitable for the application in question. Furthermore, since press moldability is poor, it is difficult to manufacture parts with complicated shapes. Also, SUS3
Austenitic stainless steel such as 04 has good heat resistance and press formability, but it is not only expensive but also has large thermal expansion, so when used in combination with ordinary steel and low alloy steel, there is a difference in thermal expansion. There is a risk of deformation or destruction of the device.

In view of the above-mentioned circumstances, the present inventors have developed a heat-resistant ferritic stainless steel to which an appropriate amount of Zr is added (Japanese Patent Laid-Open No. 146512/1983).

This Zr-containing ferritic stainless steel not only has excellent heat resistance, but also has improved weldability and cold workability to some extent, but cold workability is evaluated only by the total elongation in a tensile test. Moreover, the value is not necessarily sufficient for press molding. By the way, parts that have complex shapes and are manufactured by press forming, such as special carbon automobile exhaust gas purification devices, require extremely high press formability, and conventionally known Zr-added steels do not necessarily meet this requirement. cannot demonstrate sufficient performance.

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. Generally speaking, actual press parts include various forming methods such as deep drawing and stretch forming at the same time, and therefore have a suitable balance for all of the above formability and Materials are required to have excellent moldability.

The present inventor has conducted detailed research with a view to developing a material with excellent overall moldability as described above.

As a result, the component balance of Zr%, C% + N% is important, and by strictly adjusting them, the conventional Zr
We succeeded in achieving a high degree of press formability that cannot be stably achieved with additive ferrite stainless steel. The steel of the present invention is Crll. O~20.0%, Sil. 5% or less, Mn
l. It is characterized by having a ratio of 5 or less and adjusting the balance of the Zr and C0N contents as described later.

The reason for determining the component content as described above in the steel of the present invention is as follows.

Cr is 11 to ensure basic oxidation resistance and corrosion resistance.
% or more is required.

However, when the ratio exceeds 20, press formability deteriorates. S
It is effective to include i as a deoxidizing agent during steel manufacturing, but if it exceeds 1.5%, press formability deteriorates.

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

However, if it exceeds 1.5%, the steel will become significantly hardened, making cold working difficult. Next, the balance between Zr% and C%+N%, which is a feature of the present invention, will be described.

According to the results of research conducted by the present inventors, when Zr is not contained, C and N combine with Cr to form carbonitrides of Cr, which decreases the ductility of steel and deteriorates its formability. . Z
When a suitable amount of r is contained, C and N are fixed as carbonitrides of Zr, and their influence on formability ζ is considerably reduced. Note that the effects of C and N are almost the same. Good moldability cannot be maintained unless Zr% satisfies the following relational expression.
10(C%10N%)-0.15f0≦Zr%≦10(C
If Zr is too small, some Cr carbonitrides will remain, affecting ductility and texture, resulting in noticeable decreases in total elongation and r value in the tensile test, and deep drawing. sex,
It is harmful to both stretchability and stretch flangeability.

Furthermore, since the effect of crystal refinement by carbonitride of Zr is reduced, surface roughness occurs during molding, and not only the appearance but also the deterioration of extrusion properties due to the surface roughness becomes significant. If there is too much Zr, the excess Zr forms an intermetallic compound with Cr and Fe, which reduces ductility and causes elongation, leading to deterioration in stretchability.
The content of Zr% and C% and N% is also a problem, and Zr%
Alternatively, if C%+N% is too large, no matter how appropriate the balance is, the amount of Zr carbonitride itself increases and ductility deteriorates, resulting in deterioration of stretchability and stretch flangeability.

Therefore, it is appropriate that Zr be 0.6% or less and C+N be 0.06% or less. If C + N is too low, the amount of Zr carbonitride is too low, and during hot rolling or annealing after cold rolling, the crystal grains become coarse and roughness tends to occur, and the texture is also affected. Therefore, the r value, elongation, etc. decrease, and deep drawability and stretchability tend to deteriorate. Therefore, C
%+N% is suitably 0.02 or more. that's all,
With the above-mentioned component balance, a steel having good properties in all of stretchability, stretch flangeability, and deep drawability can be obtained. 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 the usual 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 depending on whether the hot rolled sheet is in a coiled state or 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 of maintaining the temperature in the 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 by holding it at 770° C. to 1000° C. in a batch furnace, 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.
Further, even in steels in which austenite does not precipitate, the crystal grains become coarse and the cold-opening formability deteriorates. However, Z of the steel of the present invention
Since the r-containing ferritic stainless steel remains in the ferrite phase even at high temperatures and coarsening of crystal grains is less likely to occur, it has the characteristic that steel with excellent formability can be obtained by high-temperature annealing. However, when it exceeds 1000C, Z
This is not preferable because decomposition and agglomeration of carbonitrides of r occur and crystal grains become coarse. Further, if the temperature is lower than 770°C, the dispersion form of Zr carbonitride during hot rolling is not sufficiently improved, and recrystallization is inhibited during annealing after cold rolling, resulting in deterioration of formability. Next, annealing after cold rolling will be described.

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

Typical formability of steel made into a thin plate with a thickness of about 1.2 mm using the above-mentioned ingredients and manufacturing method is as shown in Table 1.

The figure is a diagram summarizing the research results that form the basis of the present invention.

Four typical types of formability indexes are illustrated, but in order to obtain the characteristics shown in Table 1, Zr%c! =(C%10N
It is understood that the component balance (%) is extremely important. In addition, the scope of claims includes moldability indicators other than those shown in the figures. Needless to say, it was determined by taking into consideration heat resistance and other properties. Example 1 A steel having the components shown in Table 2 t was melted, hot-rolled to make a plate with a thickness of 3.2 mm, annealed at 830°C for 15 minutes, and then cold-rolled to a thickness of 3.2 mm. A thin plate of 1.2 mm was made and annealed at 850 cold CX 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. It is clear that any steel that falls outside the composition range of the present invention has some kind of deterioration in formability, and that well-balanced and overall good formability cannot be obtained.

Example 2 Steel having components within the range of the steel of the present invention as shown in Table 4 is melted, a plate with a thickness of 3.2 mm is made by hot rolling, and subsequent annealing is not performed. , when annealing was performed at 830°C for 15 minutes, rough cold rolling was carried out at 1.
A thin plate with a thickness of 2 mm was made, and after annealing at 850°C for 5 minutes, a plate-shaped test piece was prepared.

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

Example 3 Steel having the components of Example 2 was hot rolled to a plate thickness of 3.2
After annealing at 830°C for 15 minutes, a thin plate with a thickness of 121n1n was made by cold rolling.
Annealing was performed at 30° C. for 5 minutes and at 850° C. for 15 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 formability generally deteriorates significantly.

[Brief explanation of the drawing]

The figure shows representative values from the formability test values for Zr, Elixir value Er (stretchability), tensile test r value (deep drawability), hole expansion ratio λ (stretch flangeability), and surface roughness resistance. This is a diagram showing the relationship between C + N(!:), and the shaded area indicates an area where moldability is generally good.

Claims (1)

[Claims] 1 Cr 11.0-20.0%, Si 1.5% or less, M
n1.5% or less, C%+N%0.02-0.06%, Z
r0.2~0.6%, and Zr%=10(C%+N
%) A heat-resistant ferritic stainless steel with excellent press formability, containing Zr within a range of ±0.15%, and the remainder being substantially Fe. 2 Cr11.0-20.0%, Si1.5% or less, M
n1.5% or less, C%+N%0.02-0.06%, Z
r0.2~0.6%, and Zr%=10(C%+N
%) ±0.15% of Zr, with the remainder essentially consisting of Fe, after hot rolling at 770°C to 100°C.
770 after annealing in the temperature range of 0℃ and then cold rolling
A method for producing heat-resistant ferritic stainless steel with excellent press formability, characterized by annealing at a temperature range of 1000°C to 1000°C.