JPS60248868A - P-added ferritic stainless steel having excellent formability and fabrication property - Google Patents

Abstract

PURPOSE:To provide the titled ferritic stainless steel which has excellent corrosion resistance, high-temp. characteristic, etc. and can be inexpensively produced by specifying the compsn. consisting of C, Cr, Si, Mn, P, S, Ni, Sol.Al, B and Fe and the compsn. relation. CONSTITUTION:Said stainless steel contains 0.0050-0.0500wt% C, 10.00-18.00% Cr, <=0.50% Si, <=0.50% Mn, 0.040% or over - 0.200% P, <=0.030% S, <=0.60% Ni, 0.005-0.200% Sol.Al, tr-0.0050, more preferably 0.0005-0.0050% B, satisfies the relation to remain within the region of the quadrilateral formed by connecting the coordinate points, A(12.0, 0.30), B(12.0, 0.005), C(22.0, 0.020), D(22.0, 0.30) by straight lines in the figure taking Cr+50P at the axis of abscissa and C+ 10B+Sol.Al at the axis of ordinate and is constituted of the balance Fe with inevitable impurities. The ferritic stainless steel having excellent formability and fabrication property is thus inexpensively obtd.

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, but 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 particular, Al5I409 series and 5
Low chromium ferritic stainless steel of the US410L series is used in large quantities for applications such as automobile exhaust gas-related parts due to its superior high-temperature strength and high-temperature oxidation resistance when compared with ordinary steel. but.

These low chromium ferritic stainless steels too.

Compared to ordinary cold-rolled steel sheets and surface-treated steel sheets, although they are superior in terms of material properties such as corrosion resistance and high-temperature properties, they are economically constrained by their high price. The development of this is strongly desired.

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 is as described in the claims.

In weight%.

C, 0.0050-0.0500%.

Cr; 10.00-18.00%.

Si; 0.50% or less.

Mn: 0.50% or less.

P; more than 0.040% ~ 0.200%, S; 0
．． 030% or less.

Ni: 0.60% or less.

Sol, Al; 0.005 to 0.200%.

B; tr (trace level) ~0.200%.

And.

The horizontal axis represents the content according to the formula (Cr+50XP).

When the vertical axis is the content according to the formula (C+lQx B +So1.AI), there are four coordinate points, A (12,0,0,30), B (12
,0,0,005).

C (22,0,0,020) and D (22, sail 0.
30) satisfies the relationship within the quadrilateral area formed by connecting them with straight lines.

The present invention provides a P-added ferritic stainless steel with excellent formability and secondary workability, the remainder being re and unavoidable impurities.

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 steel, 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 JISG
For example, let us take the ferritic stainless steel specified as 4305 cold rolled stainless steel plate.

5US441J 1 (Cr; 28.50-32%)
and SUSXM27 (Cr; 25.00-27.
50%).

P is specified to be 0.030% or less, and for other ferritic stainless steels, P is 0.0% or less.
It is specified as 40% or less. This is because ferritic stainless steel has a body-centered cubic crystal structure, and this crystal structure also has poor toughness, and the Cr content is 11%.
Because it contains the above elements, it has the inherent disadvantage that its toughness tends to deteriorate further, so it is necessary to reduce elements such as P that are known to have a negative effect on toughness as much as possible. With consideration, P is 0.0
It is defined as 40% 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 the results of melting of ferritic stainless steel with varying P content in the basic composition system of 13%Cr-0.03%C. This figure shows the relationship between the Y value and the P content of a steel plate with a thickness of 0.7 mm obtained through annealing. The change in Y value versus P content is shown. As is well known, the Y value is a typical 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 shown in Figure 2, approximately 0.025% of P is contained in normal ferritic stainless steel.
At concentration, the Y value is lower than 1.0, but as the P content increases, the Y value increases, and when the P content exceeds 0.075%, the Y value reaches 1.4 or higher.

In addition, the addition of P improves the pickling properties of ferritic stainless steel, and as a result, it is possible to replace the commonly used pickling method for hot-rolled ferritic stainless steel strips with nitric-hydrofluoric acid. 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 two elements, T
Expensive alloying elements such as i, Nb, and AI are used,
This inevitably led to an increase in product prices. P can be added in two ways: by adding a P source such as Fe-P alloy, and by using P-containing hot metal; however, even in the former case, the impact on product prices is extremely small; In this case, there is no impact on the product price because what was previously removed as impurities is effectively used.

The second reason is manufacturing significance. When blast furnace hot metal is used as the main iron source, the dephosphorization process can be omitted, which brings great benefits to refining operations, and iron ore and Cr ore, which have low economic value due to their P content, can be eliminated. 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 terms of the two alloy components and in terms of manufacturing, as well as providing an inexpensive steel.

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 in the normal case, P in steel has an adverse effect on the material properties.

One of them is the negative effect on toughness, as mentioned above. Regarding this point, it is possible to suppress the decrease in toughness due to P by regulating the amount of C and Cr1l, respectively, and adding trace amounts of Sol and AI.

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 a second 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, brittle cracks (cracks, cracks, It was found that vertical cracking may occur. This cracking is caused by a decrease in toughness due to secondary processing, and the more severe the secondary processing is 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 material has a high Y value, that is, even if the material has excellent deep drawability, it cannot be processed into a final pressed product due to poor secondary workability. There is.

Regarding the influence of P on this secondary workability, the detailed mechanism is not necessarily clear at present, but since P is an element that originally has a strong tendency to segregate at two grain boundaries, it has the effect of weakening the bonding force between two grain boundaries. The inventors speculate that this becomes more noticeable during primary processing and tends to cause vertical cracking due to intergranular fracture (as a result, secondary workability deteriorates).

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 properties and secondary workability.

The content of the present invention will be explained in more detail below with reference to experimental data.

FIG. 3 shows an example of the toughness improving effect of Sol and AI. The sample material is basically 13% Cr.

Based on ferritic stainless steel containing 0.03%C and 0.10%P, steels with varying contents of Sol and AI were melted into steel ingots weighing 30kg each, and heated to 1100℃. After forging at 760 °C for 4 hours, a test piece was cut from the forged specimen. The Charpy impact test was conducted at 20°C and 0°C. 5 kgf-m/d is a standard impact value that causes no manufacturing problems when manufacturing products such as steel plates. As is clear from Figure 3.

For steel containing 0.002% Sol and AI, the impact value is close to zero at each temperature, but it rapidly increases from around 0.010% Sol and AI.
Even if more than 0.020% of Sol, 8■ is contained, the impact value does not change much. in this way,
Although the addition of small amounts of Sol and AI has a remarkable effect on improving the toughness of this type of P-added ferritic stainless steel network,
In order to obtain toughness that does not cause problems in manufacturing, it is necessary to add 0.005% or more of Sol and AI.

FIG. 4 shows the effects of alloying elements on the secondary workability of P-added ferritic stainless steel, and the results are summarized. The sample material is

By varying the alloy components, 1. Normal hot rolling, cold rolling,
It is a cold-rolled annealed plate with a thickness of 0.7 mm created through annealing,
In the test to evaluate secondary workability, each sample material was used at a drawing ratio of 2.0. A deep drawn cup with an outer diameter of 27.0 mm was made,
This cup was expanded using a conical punch at 0° C., and it was determined whether the crack was ductile or brittle.

In addition, 5 to 10 cups were made from one sample steel, each of which was expanded, and the ratio of the cups that were brittlely cracked was determined to be the brittle fracture rate. Hereinafter, this test will simply be referred to as the tube expansion test. In this tube expansion test, if no brittle cracks occur, ductile fracture occurs in all 9 cases, and the brittle fracture rate is 0.
%, it can be said that the secondary workability is practically sufficient unless particularly severe deep drawing is performed.

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 steel medium (7) (Cr+5
0X P )% is plotted on the vertical axis as (C + 10X B +
So1. Al)%, above the straight line BC connecting two points B (12,0°0.005) and point C (22,0,0,020>, and at point C (22,0,0 ,02
0) and point D (22, 0, 0, 30) in the area to the left of the straight line, sample steel P1 and sample steel P2
If the exceptional steel (described below) is omitted, no brittle cracks occur at all, and there is a clear correlation.

In other words, from Fig. 4, it can be seen that Cr and P deteriorate secondary workability, and C, B, Sol, and AI are elements that improve secondary workability, but each component cannot be regulated or controlled individually. It is not enough just to add C, B,
It is clear that by appropriately regulating Sol, AI, Cr, and P in relation to each other, precise and good secondary processability can be obtained.

In addition, in the relationship shown in Figure 4, 1 for B (boron)
If the analysis value is so small that it is appropriate to display it as tr (L race, trace) (especially when B is not added), (C + 10 × B + So1.AI) on the dark blue axis is B
is O, i.e. (C+Sol.

AI).

In addition, in Figure 4, there is an exceptional behavior, that is, P that has developed a brittle crack even though it is above the line BC and to the left of the line CD.
1 and P2 are considered to be caused by the content of specific components. Table 1 shows the chemical composition values of the pi and P2 test materials.

As shown in Table 1, PI has a very low C content of 0.0035%, so it does not show sufficient secondary workability even if Sol and AI are contained to some extent, and P2 has a Cr content of 18.0%.
It is thought that because P was as high as 60%, secondary workability was deteriorated even though P was somewhat low. Therefore, in the steel of the present invention, it is necessary to regulate the lower limit of the content of C, and also to regulate the lower limit of the content of C.
It is necessary to regulate the amount of these substances contained. For this reason, in the steel of the present invention, each component mutually satisfies the relationship shown in Figure 4 (the same applies to Figure 1), and 0.005% or more for C and 1 for Cr.
It is regulated at 8.00% or less.

In this way, in P-added ferritic stainless steel, it is necessary to strictly control the contents of C, B, Sol, AI, Cr, and P in relation to each other in order to improve secondary workability. Although it has become clear, the reason for the beneficial effects of C, B, Sol, and AI on secondary processability is not necessarily clear at present, but the present inventors speculate as follows. That is, for C and B.

This is thought to be due to either ■ suppressing the grain boundary segregation of P, which is harmful to secondary workability, by segregating to the grain boundaries, or ■ strengthening the grain boundaries by segregating to the grain boundaries. ing. Also, regarding Sol and AI, AI is Cr
Since it suppresses carbide ρ precipitation, it reduces the amount of C consumed as Cr carbide, resulting in less solid solution C or grain boundary segregation C.
This results in a substantial increase in the amount of 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 already mentioned (in the sample steel P1), if C is too low, it is not preferable from the viewpoint of secondary workability.

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. How to avoid these.

Set the upper limit to 0. It is necessary to set it to Q500%.

The lower limit of 10.00% of Cr is the minimum amount necessary to maintain corrosion resistance of steel. The straight line AC in Fig. 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 will be impaired and, as mentioned above, the secondary workability will also deteriorate (in the P2 sample steel), so 18.
The upper limit is 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 joints, but if it is contained in excess of 0.50%, the effect will be saturated and the cost will increase, so 0.50% is the upper limit. .

S is an element harmful to corrosion resistance and hot workability, and a lower content is preferable, so the content 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. 2 The upper limit specified for ordinary ferritic stainless steel is 0.60. % or less.

As mentioned above, P is a basic alloying element in the alloy of the present invention, and in order to obtain an inexpensive ferritic stainless steel, it is necessary to contain it in an amount exceeding 0.040%. Although the present invention suppresses the negative effects of P on toughness, hot workability, and even secondary workability, the P content is not limited to any limit, and is limited to 0.20%. .

AI reduces oxygen in steel as a deoxidizer during steel manufacturing, reducing the amount of oxygen in the steel to 9M.
cleanse. It is also a useful element that suppresses and improves the adverse effects of P on toughness and secondary workability (Figure 2). To ensure this effect, it is necessary to add 0.005% or more of Sol and AI. However, S
Even if the content exceeds 0.200% as ol, AI, this effect will be saturated and may cause nozzle clogging during casting, which may cause manufacturing problems.
The upper limit of A1 is 0.200%.

Even if B is extremely small (even a very small amount acts effectively on improving secondary workability; B is tr() race, and is shown as traces in analysis), it can be added to C, Sol, and AI defined in the present invention. Although secondary workability can be ensured depending on the addition amount and the content of Cr and P, it is preferable to contain B in an amount of 0.0005% or more in order to obtain good secondary workability. However, if too much is contained, the moldability deteriorates, so 0.
It is preferable to set it to 0.050% or less.

2N is an element that is unavoidably mixed in during the steel manufacturing stage, although it is not particularly specified in the present invention.
It can contain 0 to 0.05%.

Merely limiting the upper and lower limits of each element above is insufficient for secondary processability, and as already mentioned, it is necessary to connection of,
Furthermore, 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 precision, the index on the vertical axis in Figure 1 is (C + 10X B + S
o1. AI) with B=0, i.e. (C+So1
．． AI). As described above, an inexpensive ferritic stainless steel that has excellent formability and secondary workability and can be applied to applications involving dual-strength forming is provided.

The features of the steel of the present invention will be explained in more detail with reference to Examples below.

After melting each steel with the chemical composition shown in Table 2,
A cold-rolled annealed plate having a thickness of 0.7 mm was produced under normal hot rolling, cold rolling and annealing conditions.

Table 3 shows the Y value and secondary workability test results for each steel plate. The Y value is relative to the rolling direction of the plate.

γ measured in the 0°, 45°, and 90° directions. , γ45. From the value of γ9o.

Y i direct - (γ. +2 γ45 + γ90) /4,
Calculated.

Regarding secondary workability, the tube expansion test described in FIG. 4 above was conducted at 0° C., and the form of fracture was evaluated in the same manner as described in FIG. 4.

The following facts are clear from the results in Tables 2 and 3 of Table 3.

Steel fishes 1 to 7 of the present invention all have a high Y value 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 floor 8 has P exceeding the range of the present invention.

Further, (Cr+50P) 23' and 84 are outside the scope of the present invention shown in FIG. As a result, although the Y value is relatively good, the secondary workability is poor.

Comparative steel plate 9 has low P. For this reason, the Y value is low. Thus, when P is low, deep drawability is poor. In other words, P improves deep drawability.

Comparison WIN[Llo has a C content of 0.0028%, which is lower than the range specified in the present invention. Therefore, secondary workability is not sufficient.

Comparative steel N11ll has a content within the range of the present invention when looking at each component individually, but (Cr+50P) is outside the range of the present invention, and therefore has poor secondary workability.

Comparative mNa12, Sol, AI Kao, ooa% are lower than the range of the present invention, and (Cr+50P) -20,
(C+10B+So1.8l) for the value of 59 is outside the scope of the present invention. As a result, the secondary processability is poor.

Comparative steel 13 is made of Cr and (Cr + 50P
) is beyond the scope of the present invention, and is also inferior in secondary processability.

[Brief explanation of drawings]

Figure 1 shows C, Cr, P, Sol, AI,
The figure which shows the mutual relationship of content of B. FIG. 2 is a diagram showing the relationship between P content and Y value when P is added to 13%Cr-0.03%C ferritic stainless steel. Figure 3 shows P of 13%Cr-0.03%c-o, io%P.
FIG. 2 is a diagram showing the relationship between Sol and AI contents and Charpy impact value in additive ferritic stainless steel. Figure 4 shows the alloy of the present invention, C+ Cr+ P + Sol,
FIG. 2 is a diagram showing the results of a secondary processability test (tube expansion test) that served as the basis for deriving the mutual relationship between the contents of AI and B. Figure 1 Cr +50),' (person) Figure 2 0 0.025 0.050 0.075 0.10[
] 0. i25 0.150 0.175 0.200
P (%) Fig. 3 SOko, A] (%) Fig. 4 10 20 5θ Cr -l-50P (%)

Claims (1)

[Claims] (1), iii amount%. C; 0.0050-0.0500%. Cr i 10.00-1B, OO%. Si; 0.50% or less. Mn; 0.50% or less. PiO, exceeding 040% ~ 0.200%. S: 0.030% or less. Nf; 0.60% or less. Sol, Al; 0.005-0.200%. B; tr (trace level) ~0.0050%. And. The horizontal axis represents the content according to the formula (Cr+50XP). When the vertical axis is the content according to the formula (C+IOx B +So1.AI), there are four coordinate points, A (12,0,0,30), B(12,
0,0,005). C (22,0,0,020) and D (22,0,0
, 30) within the quadrilateral area formed by connecting them with straight lines. A P-added ferritic stainless steel with excellent formability and secondary workability, the balance being Fe and unavoidable impurities. +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%.