JPS63166931A - Manufacture of high tension hot rolled steel sheet having high magnetic flux density - Google Patents

Abstract

PURPOSE:To accurately manufacture a high tension hot rolled steel sheet having high magnetic flux density by subjecting a steel slab having a specified compsn. consisting of C, Si, Mn, P, S, Al, N, Ti, B and Fe to specified hot rolling, cooling and coiling. CONSTITUTION:A steel slab consisting of 0.05-0.15% C, <=0.50% Si, 0.70-2.00% Mn, <=0.020% P, <=0.010% S, 0.010-0.10% sol.Al, <=0.0050% N, 0.10-0.30% Ti, 0.0015-0.0050% B and the balance Fe with inevitable impurities is heated to >=1,200 deg.C, hot rolled at a hot rolling finish temp. between the Ar3 transformation point and 950 deg.C, cooled at 30-<70 deg.C/sec cooling rate and coiled at <=500 deg.C. Thus, a high strength hot rolled steel sheet having >=about 80kg/mm<2> yield point, >=about 100kg/mm<2> tensile strength and >=about 1.7 T magnetic flux density B100 is obtd.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet having a high magnetic flux density, and particularly to a method for producing a high-strength hot-rolled steel sheet having a yield point of 8 Q kg/mm, which is particularly suitable for the recent high performance of power generators. ” or more, tensile strength 100kg/w
The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet suitable for use as a high-strength hot-rolled steel sheet for two or more magnetic poles.

(Prior Art) The recent oil crisis has led to a worldwide trend toward a shift away from petroleum energy, and there is also a trend toward hydroelectric power generation as a source of electricity. Along with this, there has been a demand for improved power generation capacity, and generators are being made to have higher rotation speeds and higher outputs. Steel plates with higher strength are now required for use in automobiles.

Conventionally, high-strength hot-rolled steel sheets with a tensile strength of 80 kg/1 m'' or more have been considered that have good cold workability and are mainly used in automobiles and construction machinery, and the ones shown in the following patent documents are There is.

Special Publication No. 53-43884, Special Publication No. 54-1404
Publication No. 6.

Japanese Patent Publication No. 58-21010, Japanese Patent Publication No. 53-8862
Publication No. 0.

JP-A-59-1632, JP-A-59-15001
Publication No. 8 However, all of these are aimed at having good cold workability, and are not aimed at having both magnetic flux density and strength like the present invention.

On the other hand, there is a high-strength hot-rolled steel sheet with a high magnetic flux density, such as that shown in Japanese Patent Application Laid-Open No. 58-91121, but its strength level is below the yield point of 80 kg/1 m'',
A material with a tensile strength of 100 kg/n+" or less, which satisfies both the yield point and tensile strength to be higher so that the magnetic pole can be maintained even at high speed rotation as the performance of generators increases as mentioned above. isn't it.

In addition, in Japanese Patent Publication No. 61-11288, the yield point is 70~
One with a tensile strength of 110 kg/Tsurutake and a tensile strength of 80 to 120 kg/Tsuru2 has been proposed. However, this method has been studied only from the viewpoint of the shape and toughness of the hot rolled steel sheet, and no consideration has been given to the required magnetic flux density.

(Problems to be Solved by the Invention) The present invention has a yield point of 80 kg/Tsuru” or more and a tensile strength of 100
kg/w” or more, magnetic flux density B1+o1.7Tesl
It is an object of the present invention to provide a new method that can accurately produce a high-strength hot-rolled steel sheet having a strength of at least a.

(Means for Solving the Problems) The gist of the present invention is that the weight ratio of C; 0.05
~0.15%, Si: 0.50% or less, Mn: 0.
70-2.00%, P: 0.020% or less, S:
0.010% or less, s'ol Aj; 0.010~
0.10%, N: 0.0050% or less, Ti: 0.
10-0630%, B: 0.0015-0.0050%
A steel slab containing Fe and the remainder consisting of Fe and unavoidable impurities is heated to a heating temperature of 1200°C or higher, hot-rolled at a hot-rolling finishing temperature Ar, above the transformation point and below 950°C, and at a cooling rate of 30°C/sec or above. Cool at less than 70°C/sec,
The present invention provides a method for producing a high-tensile hot-rolled steel sheet having a high magnetic flux density, which is characterized by winding at a temperature of 00°C or lower.

The details of the present invention will be explained below.

It is well known that it is effective to reduce impurities in steel to make it closer to pure iron in order to obtain a high magnetic flux density, which is the objective of the present invention, but this does not ensure high strength. Yield point 801qr/n+" or higher, tensile strength 10
In order to obtain high-strength hot-rolled steel sheets with a strength of 0 kg/va" or higher, solid solution strengthening alone is not possible, and it is necessary to combine precipitation strengthening or transformation structure strengthening. These strengthening mechanisms must be used without deteriorating the magnetic flux density. This is the gist of the present invention.

Among the impurities, C has the most negative effect on magnetic flux density.
These are interstitial atoms such as N. These penetrate into the crystal lattice of Fe, increase lattice strain, and impede movement of magnetic domains*
Substituted atoms of St+ Mn also distort the crystal lattice, but C
The effect is less than that of interstitial atoms such as , N, etc. On the other hand, Tt, Nb+V, Zr, which are known as precipitation strengthening elements
It is thought that they precipitate in steel as fine carbides or nitrides and strengthen the steel by hindering the movement of dislocations, but do not impede the movement of magnetic domains. These elements are effective for the purpose of the present invention because they fix C as carbide and strengthen the steel. Especially T
i is an extremely effective element because it is inexpensive, can improve strength with a small amount, and fixes N to increase magnetic flux density, and B, which is added to improve hardenability, works effectively. It is. That is, as shown in Figures 1 and 2, when B is added and the cooling rate on the runout table after hot rolling is fast, it is effectively strengthened without adding large amounts of alloying elements that reduce magnetic flux density. It is possible to do so.

The reasons for limiting the above-mentioned steel components in the present invention are as follows.

C: C is the most effective element for obtaining high tensile strength and requires at least 0.05% for this purpose. However, as the magnetization characteristics deteriorate as C increases,
The upper limit was set to 0.15%, and the range was limited to 0.05 to 0.15%.

Si: Si is useful as a reinforcing element, but it degrades the magnetization properties, so it is desirable to have a small amount. However, in order to economically produce steel with minimal addition of reinforcing elements, 0.
It was decided to add up to 50%.

Mn: Mn is also an effective element for improving strength, but it deteriorates magnetization characteristics. However, compared to Si, the degree of deterioration is small. Therefore, at least 0.70% of the reinforcing element is required. However, if it exceeds 2.00%, the deterioration of the magnetization characteristics will be significant, so the upper limit is set at 2.00%, and 0.70 to 2.
．． 00% range.

5oZAf: 5olkl is required for deoxidation at 0.010% or more, but 0
．． If it exceeds 10%, the crystal grains become coarse and the strength deteriorates, and the magnetization characteristics also deteriorate significantly, so it is limited to 0.10% or less.

P, S: Both P and S are impurity elements, and since they impair the ductility and toughness of steel, it is preferable to have as little as possible, and P is 0.020%.
Hereinafter, S needs to be 0.010% or less.

N: N acts as an interstitial atom, distorting the crystal lattice and deteriorating the magnetization properties, and also combines with B, which is especially added in the present invention, to form BN.
is formed, and the hardenability is no longer effectively improved. To prevent this, Ti is added, but if a large amount of N is present, BN will be formed and the effect of improving hardenability will be reduced, so the upper limit was limited to 0.0050%.

Ti: Ti is inexpensive, and when added in small amounts, it combines with C to form TiC and strengthens steel, so it should be at least 0.10%.
Requires. If Ti increases, it causes surface flaws, so the upper limit was set at 0.30%.

B: B improves hardenability under rapid cooling at a cooling rate of 30°C/sec or more after hot rolling, and has the effect of making it easier to obtain a haynite structure, but if it is less than 0.0015%, the above effect is impaired. The desired effect was not obtained, while 0.0050
Since the effect will be saturated even if the content exceeds 0.0015 to 0.0015 o, o o s
o%.

Hot rolling conditions The varnish slab is heated to 1200° C. or higher once it has been turned into a cold piece, or even if it is still a hot piece. Normally 1200℃
It is heated to a temperature range of ~1400°C and then rolled.

The reason why the heating temperature is set to 1200° C. or higher is to promote solid solution of Ti, and this is because Ti is not sufficiently dissolved at a heating temperature of less than 1200° C. The upper limit is not particularly limited, but from the perspective of preventing scale loss and saving energy, it is 140
Heating above 0°C is unnecessary.

The reason why the slab heated above 1200°C was hot-rolled and the finishing temperature was set at above the Ar3 transformation point was to ensure strength due to TiC precipitation and to prevent magnetic permeability deterioration due to residual strain. This is because this objective cannot be achieved if the finish rolling temperature is below this point.

Further, if the finish rolling temperature exceeds 950°C, the temperature is too high and the crystal grains of the steel sheet become coarse, making it impossible to obtain the necessary strength.

Cooling and coiling conditions: The cooling rate after hot rolling is an essential condition for controlling the structure of the steel sheet, as well as the composition. Tensile strength 100kg/
In order to obtain high-strength hot-rolled steel sheets with f12 or higher, solid solution strengthening alone is not possible; it is necessary to combine precipitation strengthening or transformation structure strengthening, and a fine ferrite structure is aimed at through cooling control. If the cooling rate is less than 30° C./see, the ferrite (Jl) weave becomes coarse and the necessary strength cannot be obtained. In addition, the tensile strength is 100 kg/mm2 (7) high strength t! iJ
[j Te also requires elongation huffing % or more, but if the cooling rate is 7
If it exceeds 0° C./sec, it becomes a martensitic structure and elongation decreases, which is not suitable for the purpose of use.

If the winding temperature is high, it will be rolled up before the transformation is completed,
The temperature was limited to 500° C. or lower because the crystal grains would become coarse due to transformation during heat retention after winding and the desired strength could not be obtained.

A high tensile strength hot rolled steel sheet with a high magnetic flux density can be obtained by such a manufacturing method.

(Example) Steel having the chemical composition shown in Table 1 was melted in a converter and made into slabs by continuous casting. Looking at the chemical components, A
, B, C, and D steels satisfy the gist of the present invention.

E, F, and G steels are for comparison. Table 2 shows the hot rolling conditions and the mechanical properties and magnetic flux density of the steel sheets obtained. According to this, steel plate 11m1 ~
No. 5 satisfies the gist of the present invention. However, 6th
Although the chemical composition is satisfactory, the cooling rate after hot rolling is slow and the crystal grains are large, making it impossible to obtain the desired strength.

In addition, although the chemical composition of sample No. 7 is satisfactory, the coiling temperature is high and the crystal grains are large, making it impossible to obtain the desired strength. NIIL
No. 8 has satisfactory strength, but 0% is high and the desired magnetic flux density cannot be obtained. Na9. In Na1O, the transformation structure was not strengthened because the amount of B added was small, and in No. 11, precipitation strengthening was not performed because the amount of Ti added was small, and the desired strength was not obtained in either case.

Figure 1 shows the relationship between cooling rate and tensile test value for A steel and F steel, and it shows that adding B and increasing the cooling rate are effective in increasing strength. . In this way, by adding an appropriate amount of h+B and increasing the cooling rate of steel whose chemical composition is as low as possible with 0%, a high-tensile hot-rolled steel sheet with a high magnetic flux density can be obtained.

(Effect of the invention) As explained above, according to the present invention, the yield point is 80k.
It is possible to accurately produce high-strength hot-rolled steel sheets having a tensile strength of 100 k+r/mu" or more, a magnetic flux density of B1°.1.7 Tesla or more, and therefore has great industrial effects.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the presence or absence of B addition in Ti-added steel, the cooling rate, and the tensile test value, and Figure 2 shows the relationship between the presence or absence of B addition, C content, cooling rate, and magnetic flux density in Ti-added steel. This shows the relationship. Figure 1 Figure 2 Cooling efficiency EC

Claims (1)

[Claims] C: 0.05-0.15% Si: 0.50% or less Mn: 0.70-2.00% P: 0.020% or less S: 0.010% or less solAl: 0 A steel slab containing .010 to 0.10% N: 0.0050% or less Ti: 0.10 to 0.30% B: 0.0015 to 0.0050%, with the remainder consisting of Fe and unavoidable impurities, Heating to a heating temperature of 1200° C. or higher, hot rolling at a hot rolling finishing temperature Ar_3 transformation point or higher and 950° C. or lower, cooling at a cooling rate of 30° C./sec or higher and lower than 70° C./sec,
A method for producing a high-tensile hot-rolled steel sheet having a high magnetic flux density, which comprises winding at a temperature of 500°C or less.