JPH0350827B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a P-added ferritic stainless steel that has excellent formability and secondary workability. Ferritic stainless steel is relatively inexpensive compared to austenitic stainless steel, and has appropriate workability and corrosion resistance.
It is used commercially in large quantities, mainly in durable consumer goods such as kitchen equipment and building materials. In addition, low chromium ferritic stainless steels, especially AISI409 series and SUS410L series, are often used in applications such as automobile exhaust gas related parts due to their superior high temperature strength and high temperature oxidation resistance when compared to ordinary steel. . However, although these low-chromium ferritic stainless steels have superior material properties such as corrosion resistance and high-temperature properties compared to ordinary cold-rolled steel sheets and surface-treated steel sheets, they are expensive and economical. There are limitations in terms of performance, and there is a strong desire to develop cheaper materials. The present invention is aimed at meeting such demands, and provides a ferritic stainless steel that can be manufactured at low cost and has excellent formability and secondary workability. That is, the present invention, as described in the claims, contains, in weight percent, C: 0.0050 to 0.0500%, Cr: 10.00 to 18.00%, Si: 0.50% or less, Mn: 0.50% or less, P: 0.040%. Exceeding ~0.200%, S: 0.030% or less, Ni: 0.60% or less, Sol.Al: 0.005~0.200%, B: tr (trace level) ~ 0.0050%, and the formula (Cr+50×P) The horizontal axis is the content according to
When the content according to the formula (C+10×B+Sol.Al) is taken as the vertical axis, there are four coordinate points, A (12.0, 0.30), B (12.0, 0.005), and C, as shown in Figure 1.
(22.0, 0.020) and D (22.0, 0.30) with straight lines, and satisfies the relationship of being within the quadrilateral region formed by connecting straight lines, and has excellent formability and secondary workability, with the remainder being Fe and unavoidable impurities. The present invention provides a P-added ferritic stainless steel. The content of the present invention will be explained in detail below. The basic feature of the ferritic stainless steel of the present invention is that P, which was required to be kept low in conventional ferritic stainless steels, is actively contained in an appropriate amount in relation to other components. There is. Therefore, first, this P will be explained. JISG4304 hot rolled stainless steel plate and
For example, ferritic stainless steels specified for JISG4305 cold-rolled stainless steel sheets include SUS447J ₁ (Cr; 28.50-32%) and
For two types of SUSXM27 (Cr; 25.00 to 27.50%), P is specified to be 0.030% or less, and for other ferritic stainless steels, P is specified to be 0.040% or less. This is because ferritic stainless steel has a body-centered cubic crystal structure and has poor toughness due to this crystal structure, and it also tends to have poorer toughness because it contains more than 11% Cr. Considering that it is necessary to reduce elements such as P that are said to have a negative effect on toughness as much as possible, P is 0.040.
% or less or 0.030% or less. On the other hand, according to research conducted by the present inventors, it has become clear that when an appropriate amount of P is added to ferritic stainless steel, deep drawability and pickling properties can be improved. FIG. 2 shows an example of improvement in deep drawability by adding P. This figure 2 shows 13%Cr
A steel plate with a thickness of 0.7 mm obtained by melting ferritic stainless steel with a basic composition of −0.03% and varying the P content, and then undergoing conventional hot tube rolling, hot rolled sheet annealing, cold rolling, and annealing. This shows the relationship between the value and P content, and taking into account some variation, the value of P is calculated within the range of the width of the hatched part in the figure.
Changes to content are shown. As is well known, the value is a representative index of deep drawability. It can be said that the larger this value is, especially the larger it exceeds 1, the better the deep drawability is. As seen in Figure 2, the value is lower than 1.0 at a P concentration of about 0.025%, which is contained in ordinary ferritic stainless steel, but as the P content increases, the value increases, and at P concentrations of 0.075% or more, the value increases. When it comes to content,
The value is even greater than 1.4. In addition, the addition of P improves the pickling properties of ferritic stainless steel, and as a result, it is possible to replace the pickling method with nitric-hydrofluoric acid that is normally used as a pickling method for hot-rolled ferritic stainless steel strips. It was also found that it can be pickled with hydrochloric acid in the same way as ordinary steel. The effect of improving the workability and pickling property of ferritic stainless steel by adding P has two important meanings in providing inexpensive stainless steel. First, P itself is a very inexpensive additive element. Conventionally, to improve workability by adding elements, expensive alloying elements such as Ti, Nb, and Al were used, which inevitably led to an increase in product prices. For addition of P, P such as Fe-P alloy is used.
There are cases where P-containing hot metal is added and cases where P-containing hot metal is used, but even in the former case, the impact on the product price is extremely small, and in the latter case, conventionally it was removed as an impurity. Since resources are used effectively, there is no rebound in product prices. The second reason is manufacturing significance. When blast furnace hot metal is used as the main source of iron, the removal treatment can be omitted, which brings great benefits to the refining operation, and iron ore and Cr ore, which have low economic value due to their P content, It is also possible to target raw materials for use. Further, in pickling hot rolled steel sheets, hydrochloric acid pickling can be performed which is advantageous in terms of cost and operation. In this way, P-added ferritic stainless steel is meaningful in providing inexpensive steel from both the alloy composition and manufacturing standpoints, and in addition,
It can be said that it has an excellent effect in terms of material quality, such as improved workability. However, on the other hand, it cannot be denied that P in steel usually has an adverse effect on material properties. One of them, as mentioned above, is the negative effect on toughness. Regarding this point, it is possible to suppress the decrease in toughness due to P by regulating the amount of C and the amount of Cr, and adding a small amount of Sol.Al. The second is an adverse effect on secondary processability. The term "secondary workability" as used herein refers to the workability after deep drawing with a press is performed as the primary work. The improvement in formability due to the addition of P makes it possible to perform more severe deep drawing, so if this steel is applied to fields that require more severe forming in order to fully enjoy this effect, new ( It was found that problems (of secondary processability) occurred. For example, when performing a secondary drawing (restriking) following the primary drawing (first draw), or after pressing the flange.
It was found that when an impact is applied from a cut, brittle cracks (cracks/vertical cracks) may occur parallel to the drawing direction. This cracking is caused by a decrease in toughness due to primary processing, and the more severe the primary processing and the lower the temperature, the more likely it is to occur. Therefore, it can be said that this secondary workability is a different type of material property that is different from the toughness and formability of the material. Due to this problem of secondary workability, even if the value of the material is high, that is, even if the material has excellent deep drawability, it may not be possible to process it into a final press product because of the poor secondary workability. be. The detailed mechanism of the effect of P on this secondary workability is not necessarily clear at present, but since P is an element with a strong tendency to segregate at grain boundaries, it has the effect of weakening the bonding strength of grain boundaries. The present inventors conjecture that secondary workability deteriorates as a result of the primary processing becoming more pronounced and making vertical cracks more likely to occur due to intergranular fracture. In order to eliminate the above-mentioned adverse effects of P on the material quality of ferritic stainless steel, the present inventors have conducted various tests and studies. As a result, by strictly controlling the alloy composition, the first problem, the decrease in toughness, is suppressed as much as possible, and the second problem, secondary workability, is also solved at the same time. We were able to invent a P-added ferritic stainless steel that has excellent workability and secondary workability. The content of the present invention will be explained in more detail below using experimental data. FIG. 3 shows an example of the toughness improving effect of Sol.Al. The sample material is basically 13% Cr,
Based on ferritic stainless steel containing 0.03%C and 0.10%P, steel with varying Sol.Al content is melted into 30Kg steel ingots, and after forging at 1100℃. A test piece was cut from the sample which was further subjected to soaking treatment at 760°C for 4 hours. A Charpy impact test was conducted at each temperature of 20°C and 0°C. The impact value that causes no manufacturing problems when manufacturing products such as steel plates is 5Kgf.
The standard is m/cm ² . As is clear from Figure 3, for steel containing 0.002% Sol.Al, the impact value is close to zero at each temperature, but it increases rapidly from around 0.010% Sol.Al (for steel containing 0.005% or more Sol. .Al, the impact value exceeds 5Kgf・m/ ^cm2 ),
Even if Sol.Al is contained in excess of 0.020%, the impact value does not change much. As described above, the addition of a small amount of Sol.Al has a remarkable effect on improving the toughness of this type of P-added ferritic stainless steel.
It is necessary to add FIG. 4 shows the effects of alloying elements on the secondary workability of P-added ferritic stainless steel, and the results are summarized and shown. The sample material is
This is a 0.7mm thick cold-rolled annealed plate created by varying the alloy composition through normal hot rolling, cold rolling, and annealing. A deep-drawn cup with a drawing ratio of 2.0 and an outer diameter of 27.0 mm was prepared, and this cup was expanded using a conical punch at 0°C, and it was determined whether the cracks were ductile or brittle. In addition, 5 to 10 cups were made from one sample steel, each of which was expanded, and the percentage of cups that were brittlely cracked was determined, which was defined as the brittle fracture rate. Hereinafter, this test will simply be referred to as the tube expansion test. If no brittle cracks occur in this tube expansion test, that is, if all ductile fractures occur and the brittle fracture rate is 0%, the secondary workability is not practical unless particularly severe deep drawing is performed. It can be said that this is sufficient. Figure 4 shows the relationship between the alloy components of the test materials and their brittle fracture rates after conducting a number of tube expansion tests. As seen in the results in Figure 4, the horizontal axis shows (Cr+50×P)% in steel, and the vertical axis shows (C+50×P)% in steel.
If 10×B+Sol.Al)% is taken, point B (12.0,
0.005) and point C (22.0, 0.020), and above the straight line BC connecting point C (22.0, 0.020) and point D (22.0,
0.30), no brittle cracks occur at all if the exceptional steels of test steel P ₁ and test steel P ₂ (this will be discussed later) are omitted. There is a clear correlation. That is,
From this Figure 4, it can be seen that Cr and P deteriorate secondary workability, and C, B, and Sol.Al are elements that improve secondary workability, but each component is regulated or added individually. alone is not enough, C.
It is clear that good secondary processability can only be obtained by appropriately regulating B, Sol.Al, Cr, and P in relation to each other. Regarding the relationship shown in Figure 4, when the analysis value of B (boron) is so small that it is appropriate to display it as tr (trace, trace) (especially when B is not added), the vertical axis of (C+10×B+
Sol.Al) is calculated by setting B to 0, that is, (C+Sol.
Calculated as Al). In addition, in Figure 4, we can see the exceptional behavior, that is, the straight line.
P ₁ and P ₂ , which were located above BC and to the left of the straight line CD, were brittle cracks, but the cause is thought to be the content of a specific component. _P1 and _P2
Table 1 shows the chemical composition values of the sample materials.

[Table] As shown in Table 1, P ₁ has a very low C content of 0.0035%, so even if it contains a certain amount of Sol.Al, it does not show sufficient secondary workability, and P ₂ has a very low Cr content.
It is thought that because the P content was as high as 18.60%, the secondary workability deteriorated even though the P content was slightly low. Therefore, in the steel of the present invention, it is necessary to regulate the lower limit of the content of C and the upper limit of the content of Cr. For this reason, in the steel of the present invention, after each component mutually satisfies the relationship shown in Figure 4 (the same applies to Figure 1), C is regulated at 0.005% or more and Cr is regulated at 18.00% or less. are doing. Thus, in P-added ferritic stainless steel, it is necessary to strictly control the contents of C, B, Sol.Al, Cr, and P in relation to each other in order to improve secondary workability. However, the reason for the beneficial effects of C, B and Sol.Al on secondary workability is not necessarily clear at present, but the inventors speculate as follows. . In other words, C and B either suppress the grain boundary segregation of P, which is harmful to secondary workability, by segregating to the grain boundaries, or they can suppress the grain boundary segregation by segregating to the grain boundaries. I think this is because it will be strengthened. In addition, regarding Sol.Al, since Al suppresses the precipitation of Cr carbides,
As a result of reducing C consumed as Cr carbide,
This substantially increases the amount of solid solution C or grain boundary segregated C, and it is presumed that this C improves secondary workability through the above-mentioned mechanism. The results shown in Figure 4 are important results that form the basis of the present invention, and show that it is important not only to regulate each component individually, but also to regulate them in relation to each other. , the reasons for regulating the upper and lower limits for each component will be explained. As mentioned above, C is undesirable from the viewpoint of secondary workability if it is too low (in the sample steel of P ₁ above).
It needs to be 0.0050% or more. However, if it is too high, the material becomes hard and formability deteriorates, and weldability is also adversely affected. To avoid these,
The upper limit needs to be 0.0500%. The lower limit of 10.00% of Cr is the minimum amount necessary to maintain corrosion resistance of steel. The straight line AC in Figure 1 is
This value was determined from the lower limit value of Cr and the lower limit value of P. On the other hand, if the Cr content is high, the toughness is impaired and, as mentioned above, the secondary workability is also deteriorated (as in the _P2 sample steel), so the upper limit is set at 18.00%. Si improves high-temperature oxidation resistance, but if the content is too high, the material will harden, so the upper limit is set at 0.50%. Mn is an element that improves hot workability and the toughness of welded parts, but if it is contained in an amount exceeding 0.50%, the effect will be saturated and the cost will increase, so 0.50% is set as the upper limit. S is an element harmful to corrosion resistance and hot workability, and a lower content is preferable, so it is set to 0.030% or less. Ni is effective in improving the toughness of ferritic metal materials, but if it is too high, the product becomes expensive and does not meet the purpose of the present invention, and the permissible upper limit is 0.60% or less, which is the upper limit specified for ordinary ferritic stainless steel. shall be. As mentioned above, P is a basic alloying element in the alloy of the present invention, and in order to obtain an inexpensive ferrite stainless steel, it is necessary to contain it in an amount exceeding 0.040%. Although the present invention suppresses the adverse effects of P on toughness, hot workability, and furthermore secondary workability, the P content is not limited to any limit and is limited to 0.20%. . During steel manufacturing, Al acts as a deoxidizer, reducing oxygen in the steel and cleaning the steel. It is also a useful element that suppresses and improves the adverse effects of P on toughness and secondary workability (Figure 3). To ensure this effect, it is necessary to add 0.005% or more of Sol.Al. However, even if Sol.Al is contained in an amount exceeding 0.200%, this effect will be saturated and may cause nozzle clogging during casting, which may cause manufacturing problems. Therefore, the upper limit of Sol.Al is set at 0.200%. B effectively acts on improving secondary processability even in a very small amount. Even if B is tr (trace, to the extent that it is shown as a trace in analysis), C as defined in the present invention
Secondary workability can be ensured depending on the amount of addition of Sol.AL and Cr and P content, but in order to obtain good secondary workability, it is necessary to contain B at 0.0005% or more. preferable. However, if it is contained too much, the moldability will be deteriorated, so the content is preferably 0.0050% or less. Although not particularly specified in the present invention, N is an element that is inevitably mixed in during the steel manufacturing stage, and can be contained in an amount of 0.0050 to 0.05%, which is the same as that contained in ordinary ferritic stainless steel. . Merely limiting the upper and lower limits of each element above is insufficient for secondary processability, and as already mentioned, it is necessary to It is necessary to satisfy the relationship shown in FIG. In the relationship shown in Figure 1, if B is not contained within the analytical accuracy, the index on the vertical axis in Figure 1 (C+10×B+Sol.Al) is
= 0, that is, (C+Sol.Al). As described above, an inexpensive ferritic stainless steel that has excellent formability and secondary workability and can be applied to applications involving strong forming processing is provided. The characteristics of the steel of the present invention will be explained in more detail with reference to Examples below. Each steel having the chemical composition shown in Table 2 was melted, and cold rolled annealed sheets with a thickness of 0.7 mm were manufactured under normal hot rolling, cold rolling and annealing conditions. Table 3 shows the values and secondary workability test results for each steel plate. The value is relative to the rolling direction of the plate,
It was calculated from the values of γ ₀ , γ ₄₅ , and γ ₉₀ obtained in the 0°, 45°, and 90° directions, as follows: =(γ ₀ +2γ ₄₅ +γ ₉₀ )/4. Regarding secondary workability, the tube expansion test described in FIG. 4 above was conducted at 0°, and the form of fracture was evaluated in the same manner as described in FIG. 4.

【table】

【table】

[Table] The following facts are clear from the results in Tables 2 and 3. Inventive steels No. 1 to No. 7 all have high values and are excellent in deep drawability. At the same time, none of them showed any brittle fracture in the tube expansion test and had good secondary workability. Comparative steel No. 8 has P exceeding the range of the present invention,
Furthermore, (Cr+50P) is 23.84, which is outside the scope of the present invention shown in FIG. As a result, although it is relatively good, it is inferior in secondary workability. Comparative steel No. 9 has low P. This is why the value is low. Thus, when P is low, deep drawability is poor. In other words, P improves deep drawability. Comparative steel No. 10 has a C content of 0.0028%, which is lower than the range defined by the present invention. Therefore, secondary workability is not sufficient. Comparative steel No. 11 has (Cr+50P), although the content of each component is within the range of the present invention when viewed individually.
is outside the scope of the present invention, and therefore the secondary processability is poor. Comparative steel No. 12 has Sol.Al of 0.003%, which is lower than the range of the present invention, and (C+10B+Sol.Al) for the value of (Cr+50P)=20.59 is outside the present invention. As a result, the secondary processability is poor. Comparative steel No. 13 has Cr and (Cr+50P) exceeding the range of the present invention, and is also inferior in secondary workability.

[Brief explanation of drawings]

Figure 1 shows C, Cr, P, Sol.Al, and B of the steel of the present invention.
Figure 2 shows the relationship between the contents of 13% and 13%.
Relationship diagram between P content and value when P is added to Cr-0.03%C ferritic stainless steel, Part 3
The figure shows the relationship between Sol.Al content and Charpy impact value in P-added ferritic stainless steel of 13%Cr-0.03%C-0.10%P. Figure 4 shows the relationship between C, Cr, P, Sol. .Secondary workability test (tube expansion test) that served as the basis for deriving the mutual relationship between Al and B contents
FIG.

Claims (1)

[Claims] In 1% by weight, C: 0.0050 to 0.0500%, Cr: 10.00 to 18.00%, Si: 0.50% or less, Mn: 0.50% or less, P: more than 0.040% to 0.200%, S: 0.030 % or less, Ni: 0.60% or less, Sol.Al: 0.005 to 0.200%, B: tr (trace level) to 0.0050%, and the content according to the formula (Cr + 50 × P) is taken as the horizontal axis,
When the content according to the formula (C+10×B+Sol.Al) is taken as the vertical axis, there are four coordinate points, A (12.0, 0.30), B (12.0, 0.005), and C, as shown in Figure 1.
(22.0, 0.020) and D (22.0, 0.30) with straight lines, and satisfies the relationship of being within the quadrilateral region formed by connecting straight lines, and has excellent formability and secondary workability, with the remainder being Fe and unavoidable impurities. P-added ferritic stainless steel. 2. The P-added ferritic stainless steel having excellent formability and secondary workability according to claim 1, wherein B is 0.0005 to 0.0050%.