JPH0699779B2 - Hot-rolled steel sheet for ultra deep drawing with good resistance to secondary processing brittleness - Google Patents

Description

Description: TECHNICAL FIELD The technology disclosed in the present specification relates to a hot rolled steel sheet used in a portion such as a compressor cover for an air conditioner that requires ultra-deep drawability, and particularly to a drawing process. It is a proposal for a material exhibiting excellent resistance to embrittlement during low-temperature impact after undergoing primary working, such as secondary working brittleness resistance.

(Prior Art) Conventionally, as a hot-rolled steel sheet for deep drawing, for example, low carbon (C:
0.02 to 0.07 wt%; hereinafter simply expressed as%) Al killed steel or rimmed steel is taken up at high temperature during hot rolling, or extremely low C steel (C:
0.01%) is used as a material, and a type of steel sheet to which B or Nb is added is known.

Further, recently, as shown in Japanese Patent Publication No. 60-7690, low carbon rimmed steel with C: 0.10% or less is used as a material, and the remaining effective Mn consumed as oxides and sulfides is 0.10%.
After limiting to the above, low temperature heating of the slab (1050 ~ 1200
C.) and low temperature rolling (700 to 800.degree. C.), a hot rolled steel sheet manufactured through a special treatment is also known.

However, the literature relating to such a conventional hot-rolled steel sheet for ultra-deep drawing does not consider brittleness that appears in a portion that undergoes ultra-deep drawing such as a compressor cover.

(Problems to be Solved by the Invention) Generally, in the case of a hot-rolled steel sheet, unlike a cold-rolled steel sheet, a {111} texture effective for drawability is hard to develop, and a value of 0.5-1.0 is a measure of drawability. The degree is low (by the way, the value of cold-rolled steel sheet is about 1.3 to 2.2), which is low.

However, the hot-rolled steel sheet has a large thickness, and even if the value is small, it is somewhat advantageous over the cold-rolled steel sheet for drawing, because it is thick. That is, in order to improve the drawability of the hot rolled steel sheet, it is necessary to improve the ductility of the steel sheet as much as possible.

Further, this type of hot-rolled steel sheet has a problem that it shows brittleness after primary working such as drawing work, and cracks do not occur due to impact after primary working, that is, it is excellent in secondary work brittleness resistance. In addition, since parts such as the compressor cover are thermally affected by welding, it is important that the secondary processing brittleness resistance does not deteriorate even in this heat-affected zone.

That is, the necessary properties that the hot-rolled steel sheet for ultra-deep drawing should have are summarized as follows.

High ductility.

Must have low yield stress.

No cracking due to impact during or after drawing, that is, excellent secondary work embrittlement resistance.

Secondary processing brittleness resistance shall not deteriorate even in areas that are affected by heat such as welding.

(Means for Solving the Problems) The inventors of the present invention have conducted research by paying attention to the chemical composition of steel in order to provide the above-mentioned various properties, and as a result, in order to solve the above-mentioned conditions to be provided. Have an extremely low C composition,
In order to meet the requirements for ~, the C, N, S, Ti content of ultra-low C steel, especially S, should be set to an extremely lower level than the normal level (S = 0.005 to 0.015%), and the C, N, S content It was found that the Ti content can be advantageously adjusted by limiting the Ti amount in accordance with the above, and by containing an extremely small amount of B.

In short, the present invention, C: 0.0040% or less, B: 0.0002 to 0.0015%, Al: 0.005 to 0.10%, P: 0.015% or less N: 0.0040% or less, S: 0.0035% or less, with the balance being inevitable impurities and Fe Good secondary work embrittlement resistance It is a hot rolled steel sheet for ultra deep drawing.

(Operation) The reason why the composition of the steel sheet of the present invention is limited as described above will be described below.

[C]: In order to improve the secondary work embrittlement resistance of C, it is necessary to leave an appropriate amount (2 to 9 ppm) in a solid solution state in the steel sheet. The C content will be described later in [Ti],
As will be described in more detail in relation to the [S] content, when the C content increases, the solid solution C tends to remain at 10 ppm or more when the Ti content is small, which deteriorates the aging resistance and the ductility, that is, the deep drawability. To do. Further, when the amount of Ti is large, the amount of carbide (TiC) that is formed increases, precipitation hardening occurs, and ductility also deteriorates. Therefore, the smaller the amount of C, the better, and the upper limit is 0.0040% (40ppm), but more preferably 35ppm.
Should be:

[Mn]: If the Mn content is high, the workability deteriorates, so the upper limit is
0.20%

[Ti]: Ti is the most important element in the steel composition in the present invention, and is at least the minimum in order to fix a part of S, N and C in the steel and to improve the workability. Needs.

here, Corresponds to the Ti amount for fixing N and S. The value obtained by adding 0.003% to the sum of these is set as the lower limit of Ti, because the aging resistance is deteriorated by fixing a part of C in the steel as TiC and leaving the remaining amount as solid solution C. This is because the secondary processing brittleness resistance is imparted without causing the deterioration.

If the amount is less than this, C and N are dissolved in the steel, and the resistance to 2
Subsequent workability becomes very good, but the strain aging resistance deteriorates rapidly and workability, especially ductility, deteriorates. On the other hand, Ti has a very small amount of S in the present invention even if it is added in a considerably large amount, so an appropriate amount of solid solution C remains,
Secondary processing brittleness resistance is good.

Also, the upper limit of Ti is Is. If the content of Ti exceeds this amount, all C
Is fixed as TiC and solid solution C does not remain so that the secondary work embrittlement resistance is deteriorated, and even the workability is deteriorated by solid solution strengthening of Ti, so the amount is limited to this amount.

[B]: The steel sheet for ultra-deep drawing of the present invention is rarely used alone after pressing, and is often used by being joined to other members by welding or the like. In such a case, the heat-affected zone due to welding is heated to 500 to 600 ° C, so that part of the solid solution C precipitates as fine carbides in the form of dislocation lines due to strain aging, and
The solute C, which is advantageous for the subsequent processing brittleness, decreases. B does not undergo strain aging even by such heat treatment and is stably present in a solid solution state, and segregates at grain boundaries to enhance grain boundary fracture resistance.

Here, in order to confirm the effect of B content, a secondary work embrittlement resistance test was performed using hot-rolled sheets of vacuum-melted steel having different B contents.

That is, C: 0.0028%, Si: 0.01%, Mn: 0.11%, Ti: 0.026
%, Al: 0.035%, N: 0.0030%, P: 0.009% and S: 0.002
The laboratory made vacuum melted steel slabs with different basic B content and different B contents. After slab rolling, hot rolling was performed at a heating temperature of 1250 ° C, a finishing temperature of 900 ° C and a winding temperature of 520 ° C. 3.0 mm
A thick hot rolled steel plate was used. Then, after pickling, a secondary working brittleness test was performed.

That is, a hot-rolled steel sheet is punched out to 100 mmφ, deep-drawn by a 50 mmφ cylindrical punch (drawing ratio 2.0), and then 10 ° C at 500 ° C.
After holding the heat-treated test material held for a minute at a predetermined temperature, a weight of 5 kg was dropped from a height of 1.0 m to measure whether or not brittle cracking occurred.

This result, as shown in FIG. 1, shows that the brittle crack initiation temperature shifts significantly to the low temperature side as the B content increases. When B is contained in an amount of 2 ppm, the effect becomes remarkable, and when it exceeds 4 ppm, the embrittlement temperature becomes stable in the low temperature range. If it exceeds 8 ppm, the effect tends to be saturated.

On the other hand, it is known that when B is contained in a large amount, the secondary work embrittlement resistance is improved even without the presence of solid solution C. However, B suppresses the recrystallization of austenite during hot rolling and is Since the texture tends to develop and the in-plane anisotropy increases, the upper limit is 15 ppm. Most preferred is B: 0.0004
The range is from 0.0010% to 0.0010%, and by leaving an appropriate amount of solute C as in the present invention, the secondary work embrittlement resistance of the hot rolled steel sheet can be maintained in the heat affected zone without increasing the in-plane anisotropy. It can be improved more than steel.

[Al]: At least 0.005% is required to fix 0 in steel and increase the yield of Ti. If the content is 0.10 or more, the cost will increase and the effect will be saturated.

[P]: P is also limited to 0.015% or less for the same reason as Mn.

[N]: N is Ti, which is preferentially fixed as TiN in a high temperature region (during slab heating at ≧ 1000 ° C. or rough rolling) as in S described below, so that the adverse effect of solute N can be almost ignored. However, when the N content increases, the precipitation strengthening of TiN causes hardening and the workability deteriorates. Therefore, the upper limit of N is N ≦ 0.0040%, and more preferably N ≦ 0.0035%.

[S]: S is one of the most important elements together with Ti in the present invention.
Is one. Most of S is fixed as TiS in a high temperature range of 1000 ° C. or higher, for example, during cooling after slab casting, heating of the slab, or rough rolling stage during hot rolling. What is important here is that the generated TiS converts C in the steel into TiC.
Is a point serving as a precipitation nucleus for fixing. That is, when S is in the extremely low S region of 0.0035% or less as in the present invention, the total amount of C in the steel does not precipitate, and 2 to 10 ppm of solute C remains in the steel. In the ultra low C steel as in the present invention, the secondary work crack usually occurs at the grain boundary.
When the amount of solute C remains as much as 2 to 10 ppm, most of it is segregated at the grain boundaries to increase the grain boundary strength and improve the secondary work embrittlement resistance. In this respect, the amount of S of steel manufactured conventionally is usually 0.005% or more, and such amount of S does not have the effect of improving the secondary work embrittlement resistance. That is, the effect aimed at by the present invention is realized only by reducing this S amount to an extremely low S region of 0.0035% or less.

The steel having the above-described composition is made into a hot-rolled steel sheet by a treatment according to a conventional method. That is, it is a general method that a slab is produced by degassing the steel after converter tapping and continuously casting, but any method of this melting process does not affect the effects of the present invention. Therefore, for example, the same effect can be expected when cast as a sheet bar having a plate thickness of about 30 mm. Also, for hot rolling conditions, after slab reheating, a process of rough rolling-finish rolling and winding the coil is generally used, but CC-DR, that is, the same effect can be obtained by performing slab direct rolling. Can be expected. Next, the hot-rolled steel sheet obtained is subjected to leveling processing or descaling as required to obtain a product. Further, in the present invention, similar effects can be obtained even if surface treatment such as hot dip Zn plating is performed.

(Examples) Steels having the chemical compositions shown in Table 1 were taken out from the converter, degassed from RH, and continuously cast into slabs. This slab was heated to 1250 ° C., finished by hot rolling at 920 ° C. ± 5 ° C., and wound at 570 ° C. (thickness 3.2 mm).

After descaling, mechanical properties and resistance to secondary work embrittlement were examined. The aging index was adopted as a measure of strain aging resistance. If the aging index is 3 kg / mm ² or less, the progress of strain aging at room temperature is extremely slow, and it is substantially non-aging. In addition, the secondary processing brittleness resistance test is performed by punching a sample to 100 mmφ, deep drawing using a flat bottom punch of 50 mmφ (drawing ratio 2.0), and then rapidly heating it to 600 ° C (heating rate 5 ° C / s). A heat treatment of holding for seconds and air cooling was performed. Add a 5 kg weight to the sample after heat treatment at -50 ° C.
It was judged by the presence or absence of cracks when dropped from a height of m.
The results are shown in Table 2.

Here, ▲ ▼, ▲ ▼, and ▲ ▼ are defined by the rolling direction, the direction perpendicular to the rolling direction, the rolling direction, and the average value of the values of 45 °. For example, the ductility ▲ ▼ is [(El ₀ + El ₉₀ + 2El ₄₅ ) /
4]. (Note subscripts angle between the rolling direction and the test piece) The plane anisotropy was evaluated in _{ΔEl (= El 0 + El 90} -2El 45/2).

Specimen Nos. 1, 2, and 3 are examples in which C, Mn, and P are out of the range of the present invention, and the materials are inferior.

Further, the sample material No. 7 is an example in which S was removed, and the secondary work embrittlement resistance was poor.

Specimen No. 9 is inferior in material because N is removed.

Specimen Nos. 10 and 11 are examples that deviate from the upper and lower limits of Ti. Specimen No. 10 has poor strain aging resistance, and Specimen No. 11 has poor secondary work embrittlement resistance. .

Specimen Nos. 12, 13, and 16 are examples in which the upper and lower limits of B are not satisfied. Specimen No. 12 has poor secondary work embrittlement resistance, and Specimen No. 13 has in-plane anisotropy. Remarkably large.

Specimen Nos. 4,5,6,8,14,15,17,18 are steels of the present invention, which have good material and secondary work embrittlement resistance, and have small in-plane anisotropy.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain a hot-rolled steel sheet suitable for use in a compressor cover, a structural member of an automobile, or the like, particularly in a portion where ultra-deep drawability is required, and It is possible to significantly improve the secondary work brittleness resistance of a steel sheet without deterioration in the heat-affected zone.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of B on the secondary work embrittlement resistance.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Keno Higashino 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no address) Inside Mizushima Works, Kawasaki Steel Co., Ltd. (56) Reference JP-A-62-37341 (JP, A) ) JP-A-61-73836 (JP, A)

Claims (1)

[Claims] 1. C: 0.0040 wt% or less, Mn: 0.20 wt% or less, B: 0.0002 to 0.0015 wt%, Al: 0.005 to 0.10 wt%, P: 0.015 wt% or less, N: 0.0040 wt% or less, S: 0.0035 wt% or less, and the balance containing inevitable impurities and Fe Two
Hot-rolled steel sheet for ultra-deep drawing with good subsequent work brittleness.