JPS582261B2 - Touhou Teki Dekatsukou Engine Seino SI - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a high-strength hot-rolled steel that has a large n value (uniform elongation), high ductility, and low anisotropy in mechanical properties despite having high strength. .

Recent automobiles, especially passenger cars, have increased in weight due to the installation of pollution prevention devices such as exhaust gas, and in order to ensure driver safety, there has been a shift toward safer and less polluting vehicles, and high-strength steel plates are inevitably required as material countermeasures. There is a need to reduce weight by using

On the other hand, high-strength steel plates with excellent bendability and weldability and a strength of 60 to 100 kg/class 1 are beginning to be used for the purpose of reducing the weight of trucks for transporting heavy goods and other industrial machinery. .

These high-strength steel plates not only come in a wide range of thicknesses, from thin to thick, depending on the purpose of use, but the required level of performance is also gradually increasing.

In particular, there is a demand for high ductility steel that maintains ductility as high as possible, which normally inevitably deteriorates with increasing strength, and also the deterioration of ductility, which becomes even more noticeable as the steel becomes thinner, and isotropy (especially L, C). In addition to the important issues that need to be improved in terms of materials, such as the need for thin steel with excellent isotropic ductility, which has improved the increase in direction difference), special elements such as B, Mo, and Ni are required for manufacturing. There are also manufacturing issues, such as the need for a low-cost steel that does not require additives and exhibits excellent performance, and solutions to maintain a high yield and stable supply of high-strength steel.

In solving these difficult problems, the inventors made the following three points.
Based on these basic concepts, we have discovered extremely effective countermeasures to realize them, and have constructed the present invention based on these.

The basic concept will be explained below.

Regarding improving ductility and toughness (1) Improved ductility by increasing n value.

(2) Refinement of structure, second phase (pearlite phase, inclusions)
Improved ductility due to uniform dispersion and apparent reduction.

There is.

The ductility scale in uniaxial tensile deformation is total elongation (T.M), uniform elongation (ε*) and waist elongation (εL
) is expressed as the sum of

By the way, the conventional n value and uniform elongation (ε
*: The intersection of the true stress direct strain curve and the da/change curve, that is, the ε value at 0-σ) has a proportional relationship, although there is some difference in absolute value.

FIG. 1 shows the relationship including the steel of the present invention.

As is clear from this figure, ε* is approximately 0.015 smaller than n.
It's just too high.

Therefore, improving the n value is completely synonymous with improving uniform elongation (ε*), and is an effective solution to improving ductility unless waist elongation is reduced.

It is important to find a way to maintain the n value, which generally decreases rapidly with increasing strength, at a high level.

On the other hand, from the perspective of plastic deformation and ductile fracture, the reduction, refinement, and uniform dispersion of apparent pearlite and inclusions not only improve uniform elongation (ε*) but also improve waist elongation (ε
It is also effective in improving L).

Figure 7 a s b + C + d. , e t f show the microstructures of various samples starting from the same slab and controlling the grain size and pearlite fraction, and Figure 2 shows the changes in the total elongation (T.M) and n value of the ductility scale. It is shown in relation to the pearlite fraction.

As shown in the photograph in FIG. 7, fine-grained steel has a small percentage of pearlite, but the ferrite grains are well-sized and the pearlite colony size is small and uniformly dispersed.

When the grain becomes coarse, the opposite phenomenon occurs.

Figure 2 clearly shows that the n value (
That is, both ε*) and εL are improved, and a significant improvement in ductility is recognized.

These facts suggest that microstructure improvement plays a large role in improving ductility, and show that the above-mentioned microstructure refinement is an effective solution for improving ductility.

Regarding the elimination of anisotropy in mechanical properties, there is (3) elimination of anisotropy by smoothing of the effect due to isotropic organization and micro-segregation of main additive elements.

Practical alloy steels inevitably involve segregation of added or accompanying elements.

However, in the case of ferrite-topperlite, the final transformed phase, inclusions, etc. other than the ferrite matrix are reduced, finely and uniformly dispersed, and therefore Mn
Even if there is segregation, inclusions, and precipitates of reinforcing elements such as , Si, and Or, and accompanying impurities (C, O, S, etc.) that interact with them, the harmful effects of these elements are reduced due to the refinement of their structures. It was concluded that if the localized action is dispersed and diluted as a whole, an effect close to a non-segregation state can be produced, anisotropy can be eliminated, and an isotropic material can be obtained.

Therefore, the present inventors have developed the following solutions to obtain isotropic high-strength steel with excellent ductility: (1) improving the n value, (2) reducing the fineness of the transformation-generated phases (ferrite grains, pearlite), and As a result of conducting various experiments on the following three items: homogeneous dispersion, reduction of pearlite and harmful inclusions, and (3) smoothing of effects through micro-segregation of major additive elements, we have come up with an original method to effectively materialize this. Invented it.

That is, the present invention [(1) Carbon (C) 0.03 to 0.15% silicon]
Element (Si) 0.7-243% Manganese (Mn
) 0.7-2.0% Sulfur (S) 0
．． In the range of 0.01% or less, the Si/Mn component ratio is 0.6 or more and 1.5
Si-added high-strength steel that is isotropic and has high ductility and toughness, with the remainder consisting of iron and unavoidable accompanying impurities.

(2) Carbon (C) 0.03-0.15% silicon
Element (Si) 0.7-2.3% Manganese (Mn)
．． 0.7-2.0% chromium (Cr)
Less than 0.3% Sulfur (S) In the range of 0.01% or less, the Si/Mn component ratio is 0.6 or more and 1.5
Si-added high-strength steel that is isotropic and has high ductility and toughness, with the remainder consisting of iron and unavoidable accompanying impurities.

(3) Carbon (C) 0.03-0.15% silicon
Element (Si) 0.7-2.3% Manganese (Mn)
The Si/Mn component ratio is 0.6 to 1.5 in the range of 0.7 to 2.0%, and Ce/S;
1.5 ~ 2.0 42>Ti/S>4,5>Zr
/S>2 Ce, Ti, or Zr is added to the steel, and the remainder is iron and unavoidable accompanying impurities, which is an isotropic Si-added high-strength steel with high ductility and toughness.

(4) Carbon (C) 0.03-0.15% silicon
Element (Si) 0.7-2.3% Manganese (Mn
) 0. 7 to 2.0% chromium (Cr)
Less than 0.3% Sulfur (S) In the range of 0.01% or less, the Si/Mn component ratio is 0.6 or more and 1.5
In addition, one or two of Nb, V, and Ti are added in a combined amount of 0.4% or less, and the remainder is iron and unavoidable accompanying impurities. strength steel.

(5) Carbon (C) 0.03-0.15% silicon
Element (Si) 0.7-2.3% Manganese (Mn
) 0. 7 ~2.0% chromium (Cr)
In the range of less than 0.3%, the Si/Mn component ratio is 0.6 or more and 1.5
Up to and Ce/S: 1. 5 ~2. 0
．． 42>Ti/S>4. 5>Zr/S>2 Ce, Ti, or Zr is added, and the remainder is iron and unavoidable accompanying impurities, which is an isotropic Si-added high-strength steel with high ductility and toughness.

”.

Below, solutions to each of the above-mentioned measures in the present invention and reasons for limiting each of the above-mentioned components will be explained.

(1) Measures to improve n value Figure 3 shows the true stress-true strain curves of various high-strength steels, including the steel of the present invention, and the n value determined by the two-point method of 10%-15%.

A and B are steels of the present invention, both of which are 0. 7% or more of Si is added.

E is a 3% SiFe alloy.

A characteristic of the deformation curves is that for both A and B, the rate of increase in deformation stress, that is, the work hardening rate, changes rapidly and greatly from an elongation strain of 4% or more.

(This refers to the change from the arrow in the figure.) In other words, the increase in deformation stress in the uniform elongation deformation range has been significantly improved compared to the conventional material, and the uniform elongation has become larger.

On the other hand, the n values listed in Table 1 indicate the clear superiority of the steel of the present invention over the conventional steel.

The main reason for this is thought to be the high Si addition.

Incidentally, E of 3% Si Fe shows a similar tendency.

The present inventors have reached the following conclusion from these new findings.

Among steel types with approximately the same strength, high Si-added steel maintains the n value at a higher level than steel with lower Si content.

The lower limit of its addition amount is 0.7%. Also, S of A and B
The amount of i and the amount of Mn are almost the same, and the component ratio Si/Mn is 0.
．． 89 to 1.07, the n value decreases as Si/Mn decreases, and the lower limit of Si/'Mn showing a high n value is 0.
．． It is about 6.

(2) Refinement of structure, uniform dispersion and reduction of second phase
It has already been explained in FIGS. 7 and 2 that the structural modification described above improves ductility (both ε* and εL).

Therefore, in order to compare the characteristics of the present invention and steels other than the present invention,
Produced by the experimental method of composition table 2 shown in Table 1, in which 1% or more of each of effective reinforcing elements M'n and 'Cr other than C, including Si, which is an n-value improving element, is added. The microscopic structure and segregation state of the sample were investigated.

Experimental method Vacuum induction heating melting (20kg) 1200°G-1/2hr heating 13 passes continuous rolling 1000-950°C Finished plate thickness 12 mm t-{"Microscopic observation air cooling EPM
,'#Analysis As seen in the photographs in Figure 8, A, C, and C, the addition of Si alone not only makes the ferrite grains finer, but also improves the ductility of pearlite, which has a small colony size and is almost uniformly dispersed. Close to the ideal organization necessary for improvement.

However, when Mn or Cr is added alone, the pearlite colony size is large and a so-called pearlite band is formed.

In addition, ferrite matrix grains also exhibit a mixed grain state, which is disadvantageous for ductility and exhibits a shape that seems to promote anisotropy.

By the way, the two types of composite addition S shown in the photos in Figure 8 2, E and
In the case of i and Mn, a very remarkable change appears.The combined addition of Mn and Cr results in a remarkable band structure in which the negative effects of each addition alone are superimposed.

On the other hand, with the combined addition of Si and Cr, the so-called band structure almost completely disappears.

However, among the three, including the combined addition of Si and Mn described below, the crystal grain size is coarser and the pearlite colony size is extremely large.

Although this is advantageous in eliminating anisotropy from a microstructural perspective, it is insufficient for improving ductility, and is questionable in light of the results of Si and Mn composite addition steels.

The Si and Mn composite addition steel reproduces a structure close to the ideal structure that was observed only in the Si-only addition steel.

The structure of high-Si steel, which is close to the ideal structure, hardly collapses even when Mn is added.

The conditions for obtaining such an ideal structure are determined by the Si/Mn component ratio, and Si/Mn can range from 0.6 (results obtained from Figures 3 and 4) to infinity (Si alone without Mn). be.

As a result of this experiment, Si/Mn~1.5 is included therein.

Apart from other constraints, the bottom line as far as the organization is concerned is that
If S i /Mn is in the range of 0.6 to gloss, the pearlite fraction is small, the pearlite colony size is small, a uniformly dispersed structure is obtained, and the n value is improved.

For this purpose, the lower limit amount of Si required is 0. 7%, so S i/Mno. The lower limit addition amount of Mn regulated by 6 is 0.45%.

On the other hand, regarding inclusions, an increase in S i + Mn brings about a change in the composition of the inclusions, which are finely divided during rolling and are finely and uniformly dispersed in the matrix.

This effect has a direct impact on improving ductility.

(3) Elimination of anisotropy Increase in Si and Mn, especially when Si/Mn > 0.6, changes in inclusion composition promote finer inclusions and uniformity of distribution, improving not only ductility but also ductility and toughness. It has become clear that this contributes to the improvement of the anisotropy of the material, resulting in an almost isotropic material.

In addition to the form of these inclusions, the strengthening element S
i, which is thought to depend on the characteristics of the micro-segregation of Mn.
As shown in the results of EPMA line analysis, the segregation of Si and Mn in steel with Mn composite addition is contradictory.

That is, it can be imagined that areas with high Si correspond to areas with low Mn, as if the segregation were smoothed and equalized as an effect.

Such a unique distribution of micro-segregation of Si and Mn is effective for eliminating anisotropy, and the present invention steel (Si>0.
7, S i/Mn > 0. It is recognized that the isotropic property of 6) originates from this area, and from this point of view, the heights of the distribution of both elements in Si and Cr-added steel completely overlap, and the concentrated segregation of Si and Cr does not occur. However, in order to eliminate the anisotropy, similar to Mn and Cr composite steel, which has the same segregation tendency as Si and Cr steel, it still remains and therefore promotes the localization of the above elements and harmful elements that interact with them. Questions remain.

As described in the examples, the steel of the present invention supports not only the rolling direction (cross direction (C)) but also the rolling direction (cross direction (C)),
There is no difference in ductility even in the 45° direction (up to), and it is extremely isotropic.

From the various results mentioned above, in order to eliminate anisotropy, a wide range of controls are required, including not only modification of the structure, but also the problem of inclusions and micro-segregation.

There is no example in which it has been possible to manufacture an isotropic material, which has been difficult to achieve, by simply regulating the amount of Si and the component ratio Si/Mn.

From the above various experiments, the appropriate value of Si/Mn is 0.6 to 1.
It was confirmed that within the range of 5, the anisotropy elimination, which is one of the objectives of the present invention, was achieved.

It is clear that the smoothing of the effects due to the very characteristic isotropic structure, inclusion morphology, and micro-segregation eliminates the anisotropy in strength and ductility.

This means the appearance of a material that is isotropic in mechanical properties.

This will be explained in the examples.

As mentioned above, improving ductility and eliminating anisotropy are:
■Improve the n value by adding Si, ■Improve the structure and inclusions by adding Si and Mn within the appropriate value of Si/Mn, and smooth the effect by effectively distributing the micro-segregation of elements. Since this is possible, we investigated the Si and Si-Mn balances that should be clarified in order to achieve this, and the effects of the addition of reinforcing elements C and Cr on them.

We selected ε* as the most appropriate measure.

Figure 4 shows the results of this study.

All of these results were obtained using a sample in which a 20 kg vacuum melted steel ingot was soaked at 1250°C for 2 hours, hot rolled to a thickness of 6 mm with a finishing temperature of 930°C, and then cooled in the air.

However, 0.0 shown in b in the figure. I C-I Mn -0.2
Only 5cr - [¥] (meaning that only the amount of s i added is a variable, the same applies hereinafter) has a plate thickness of 4.5 mm.

In the figure, a indicates the most standard Si addition amount of 1%, and S i /
These are the results of investigating the relationship between the C addition amount up to 0.15% and the ε* and n values in Mn~1 base steel.

n of 0.2 or more in the strength range of 50 to 60 kg/1
You can see that the value is obtained.

In b, 0.25% Cr was added to the upper and lower component amounts (0.3% and 1% s s %, so Si/Mn: O, 3 and 1) across the lower limit Si amount, and the effect was observed. .

Although Cr lowers ε*, a new finding has been obtained that if the amount of Si added and Si/Mn are appropriate, the amount of decrease can be suppressed considerably.

This result may be explained by the structural difference between the low-Si steel and the high-Si steel that satisfies Si/Mn~1, and the difference in the amount and distribution of the variable components Si and Cr.

In C, Si was added to a low carbon-Nb steel in an amount close to the lower limit of 0.7%, and the optimum Si/Mn range was investigated.

In the figure (ε* decreases with the addition of Mn, but the degree of deterioration is small, and Si/Mn
ranges from 1.1 to 0.61.

A lower limit value of 0.6, which is exactly the same as the previous result, was obtained.

d is 0. It shows the influence of Cr on 50-60 kg/m4 grade steel whose base component is I C-I Si-I Mn.

As shown in the figure, the reduction in ε* due to the addition of Cr is 1/10 of that of C per unit addition amount, but the amount of Cr required to obtain the same strength is extremely large (see Figure 5).
, is not a very effective element for strengthening and improving ε*.

However, as shown in Figure 5, C up to 0.3%
Although the addition of r causes a decrease in the n value (ε*),
There is no change in total elongation (T.E7).

This fact is extremely important.

After all, the less Cr, the higher the n value (ε*).

However, the aperture value (≠ (good correlation with bendability) is 0.
In the range up to 5%, it increases as the Cr content increases.

In other words, the waist elongation (εL) is increased.

This range is a unique region in which the improvements and decreases in ε* and εL are complementary.

When placing emphasis on the n value, it is desirable to reduce Cr to the level of an unavoidable impurity as much as possible, and for bending formability, Cr should be reduced to 0. It is recommended to add up to 3%.

Furthermore, although the reason is not clear, Si≧0.7, S
i/Mn 〉0. When Cr is added to component 6 in a range of less than 0.3%, the occurrence of abnormal scale and rust is suppressed.

As explained above, the composition of the steel of the present invention is a high-strength steel with a high n value, excellent ductility, and no anisotropy, which enables improvement of the structure and inclusions, and proper distribution and smooth distribution of segregation of reinforcing elements. , Si/Mn is from 0.6 to 1.5, preferably S
Under the addition conditions of i/Mn~1, (1) carbon (C)
:0.03~0.15% (2) Silicon (Si)
:0.7-2.3% (3) Manganese (Mn)
: 0.7 to 2.0%, and chromium (Cr): less than 0.3%.

Furthermore, in addition to isotropy, which is the objective of the present invention, o. o1o
% or less, or when S is less than 0.025%, which is the upper limit of S obtained by normal steelmaking methods.
e/S 1. 5 ~2. 0,4 2 >T i/S
>4. 5>. Ce, Ti so that Zr/S>2
, Zr, by controlling the shape of elongated A-based inclusions (such as MnS) that inhibit the C-direction properties into grains, the isotropic high It further promotes ductility and eliminates the LC anisotropy that is inevitable with high strength.

Of course S is o. It goes without saying that the effects of the steel of the present invention can be achieved even if at least one of Ce, Ti, and Zr is added so as to satisfy the above relationship when the amount is less than o+o%.

(For example, Examples I, J, L') On the other hand, the present invention's paste steel (Cr<0.3% addition) was prepared by adding one or two of the precipitation strengthening elements Nb, V, and Ti in an amount up to 0.4%.
Not only can the addition of ferrite effectively increase strength, but it can also eliminate L and C anisotropy, achieve high n-values, and high ductility toughness, which conventional precipitation-strengthened high-strength steels could not achieve. It is.

The reasons for limiting each component in the steel of the present invention are summarized as follows.

(1) Carbon (C) C is most effective as an inexpensive and powerful reinforcing element.

However, strengthening with C depends on the pearlite fraction, and an increase in the fraction significantly impedes ductility, so it is not preferable to add a large amount of C.

Since no problem occurs up to about 0.15% regarding the balance between strength and ductility, the upper limit was set at 0.15%.

On the other hand, reducing C to the extent permitted by strength is extremely advantageous in terms of ductility, but it is necessary to guarantee a strength of 50 kg/1 or more, and the lower limit is set to 0.000 kg/1 in consideration of on-site manufacturing limits.
03%.

(2) Silicon (Si) Compared to C, Si has a less reinforcing ability, but C
It is essential for increasing strength when the amount is kept low,
Effective as a supplementary strengthening element.

Even more important effects are, as explained earlier, the effect of dramatically improving the n value and the effect of improving the structure due to the combined addition with Mn.

The n-value improvement effect does not appear below 0.42%, and 0.7%
The above becomes evident.

Therefore, the lower limit was set at 0.7%.

On the other hand, even at a Si content of 3%, the effect of Si still remains.

Therefore, it seems to be effective to add up to about 3%, but it is important to avoid the (α+γ) region during manufacturing, and to
In order to make it possible to soak the steel ingot or slab in the single-phase γ range, which is necessary to obtain a uniform structure and smooth micro-segregation, it is necessary to increase the Si content to 2.3% or more based on the phase diagram. Unacceptable.

(Goyumi and Abe: Silicon Steel Plate, p. 7) In addition, the addition of a large amount of Si is disadvantageous in terms of quality and operation, such as poor surface scale properties, so the upper limit was set at 2.3%.

(3) Manganese (Mr1) Although solid solution hardening itself is lower than that of C and Si, Mn is useful as an element that does not significantly inhibit ductility.

Particularly, as described above, when both elements are added in a Si/Mn ratio of 0.6 to 1.5 to ensure proper distribution and smoothness of the distribution, anisotropy can be eliminated.

Mn is important as an essential element to obtain an isotropic material.

The lower limit amount that satisfies this effect is 0.7% Si, so 0.
42% is sufficient.

However, in order to maintain strength and appropriate distribution of Si and Mn, the lower limit is o. 7%, and the upper limit is 2.0%.

(4) Chromium (Cr) Cr acts as a reinforcing element when added in an amount of 1% or more.

Coexistence with Mn increases the anisotropy, but ε* remains at a high level, so in that sense it is meaningful to add Cr, but it has low reinforcing ability and requires a large amount of addition, so it is costly. This is disadvantageous.

Conversely, if Cr is 0. When decreasing from 3% to CrO%, the uniform elongation increases rapidly even though the strength hardly changes.

That is, it is advantageous for the n value to avoid adding Cr as much as possible.

However, the addition of less than 0.31% Cr reduces the aperture value (
This has the effect of improving ≠ (Fig. 5), and is advantageous in improving bending formability.

Furthermore, the addition of 0.5% or less of Cr is the same as that of Si, M of the present invention.
Since the occurrence of abnormal scale and rust is suppressed within the n addition amount range, the upper limit of Cr is set to less than 0.3%.

(5) Sulfur (S) As mentioned above, the amount of sulfur was determined in order to improve the processing method, impact properties, and achieve the purpose of the present invention (elimination of anisotropy).

In other words, when adding one or more of Ce + Ti + Zr, it should be added within a range that does not exceed the upper limit of sulfur (0.025%) normally obtained in normal steelmaking, and Ce + Ti + Zr. + If one or more of Zr is not added, o. The objective is achieved by desulfurizing to below 1% o.

In addition, when S is 0.010% or less, Ce. Ti,Z
It goes without saying that the effects of the present invention can be achieved even if one or more of r is added within the scope of the claims.

The present invention steel within these limited component ranges and conventional steel will be compared below. Clarify the characteristics of 4.

Example 1 In the steel of the present invention, 50 kg/first class steel was targeted and 0.
I C I. Based on O Si-1.0Mn, 0
．． 2% Cr and Cr-free steels; comparative materials include Si-Cr-Mn steels with the same amount of Si and Cr at about 1%, and 2% of each of Si-Cr-Mn steels;
A 0 kgfia vacuum melted steel ingot was soaked at 1250° C. for 2 hours, hot rolled to a thickness of 6 mm with a finishing temperature of 930° C., and the mechanical properties of the sample were measured after cooling in the air.

As shown in Table 3 (2), the comparative material seems to have the highest level of properties of conventional steel, but the steel of the present invention has no anisotropy and exhibits a dramatically high n value and ductility, giving it outstanding performance.

It is also seen that the reduction of area, which corresponds well to bending formability, is improved by adding 0.2% Cr.

Furthermore, the reduction of area is equal to or higher than that of the comparative material (steel with 1.02% Cr added), and both exceed 75%.

It has been confirmed that close bending is possible even in the C direction.

In the above sample, the S content was 0.006%, and while the steel of the present invention was isotropic, the comparative material showed anisotropy in L and C.

It is presumed that this is due to differences in micro-segregation or inclusions.

Example 2. As shown in the table above, both were selected and compared, with almost no major differences in composition except for Si and ■.

Both are in-situ materials melted in a converter.

Although the steel F of the present invention is a dog steel ingot material, as shown in Fig. 6, there are variations depending on the steel ingot parts from the coil top to the bottom, L (rolling direction), C (perpendicular direction to the rolling direction),
There is no anisotropy in D (direction at 45° with respect to the rolling direction), and it is extremely isotropic, showing a good yield.

The mechanical test results are for the middle part of the steel ingot.

As in the case of 50 kg/mm class inventive steels A and B, the strength -
The balance of ductility and isotropy are amazing, and compared to conventional steels, the n-value and ductility are significantly higher, and the bending properties indicate that close bending is possible in both the C direction, indicating that the steel of the present invention is extremely Recognized as having a high level of performance.

In particular, the comparison material is 6mm material, whereas the thinner 2.3m material
It is noteworthy that even the M material shows excellent ductility.

In addition, although there is a slight portion at the edge that looks like a band structure as shown in FIGS. 9 and 10, there is almost no anisotropy and the film is isotropic as shown in FIG.

Example 3 This example relates to claims 3 and 4 of the present application, and I, J, M, and N are the base steels of the present invention (
Ti is added to S i /Mn (each from 1.1 to 0.96), and M and N are added to 0.01 to the base steel of the present invention.
% or more and less than 0.025% of S, to which Ce and Zr within the composition range of the present invention are added.

On the other hand, as a comparative steel, a base steel of the present invention (comparative steel 2) in which S was added to the same extent as M and N was prepared.

The manufacturing conditions were the same as in Example 1, and the thickness was 6 mm.

As shown in Table 5 (2), the conventional 50. 60. 65 to 7
Compared to 0kg/mm class steel, its strength-ductility balance is at the highest level, and it has an extremely large n value, and L
, C is an extremely excellent isotropic high ductility steel with almost no recognized anisotropy.

L and C that are common in conventional Nb steel, Nb-V steel, and Ti steel
The problems of anisotropy and decrease in n value were solved at once by the steel of the present invention.

This result confirms that the base steel of the present invention has extremely unique performance in that the original performance can be inherited even with the addition of Ti.

On the other hand, steel containing S of 0.01% or more and less than 0.025%, such as M and N, represents an example of claim 4 of the present application, and as can be seen by comparing it with comparative steel 2, The C-direction ductility, reduction of area, and bending formability of Zr-added M steel (5>Zr/S>2) and Ce-added N steel (Ce/S~1.8) are all the same as those of S. The deleterious influence of S on the mechanical properties can be eliminated by the addition of these elements.

Example 4. In this example, Cr is about 0. Ti, Nb or Nb-V, Nb-Ti are added to the base steel of the present invention with a 2% addition, and finished to a thickness of 6 mm under the same manufacturing conditions as in Example 1. This relates to item 5.

The characteristics of the reduction of area depending on the presence or absence of Cr observed in the present invention steels A and B in Example 1 are similar to those observed in the present invention steels A and B.
It does not change even if S is added alone or in combination (including V), and when S is 0.01% or more and less than 0.025%, Ce and Zr are within the scope of the present patent claims as shown in Example 3. Even by adding Ti, even in Nb-added steel with S content of 0.010% or more, the strength, ductility, and n-value anisotropy in the L and C directions can almost completely be eliminated, which is a feature of the present invention. High isotropic ductility is not lost.

Above, we have described the characteristics of the samples melted within the composition range of the present invention.As is well known, phosphorus P is homologous to Si and has a small atomic radius in iron, as can be seen from the periodic table. , they have strong similarities in their effects as solid solution strengthening elements, and have common properties in various respects.

Therefore, there is a possibility that P can be added to replace Si.

Moreover, since the steel of the present invention is based on solid solution strengthening, it goes without saying that even if it is subsequently cold rolled and annealed, the excellent properties obtained during hot rolling will hardly be lost.

[Brief explanation of the drawing]

Figure 1 shows the uniform elongation (ε*) n of the inventive steel and conventional steel.
Figure 2 shows the relationship between pearlite fraction and ductility, that is, total elongation (T.E7), and n value of samples using the same slab and controlling the structure (grain size, pearlite fraction, etc.). Figure 3 is a diagram showing the true stress-true strain curves of the inventive steel and conventional steel and the n value determined by the two-point method (10-15%). Figure 4 is a diagram showing the uniform elongation (ε*). and C, Si,
Figure 5 is a diagram showing the relationship between Mn and the amount of Cr added; Figure 6 is a chart showing changes in mechanical properties in the longitudinal direction of the coil of the steel of the present invention in Example 2, Figure 7 a, b, c, d, e, and f show crystal grains and pearlite fractions starting from the same slab. Fig. 8 Micrographs of various samples with controlled conditions.
Photographs showing the cross-sectional microstructure and EPMA line analysis results when 1 or more of each of the two types of Cr and Mn are added. Figure 9 is an L cross-sectional micrograph of the M section (center of the longitudinal coil) of the steel of the present invention. The structure photograph, FIG. 10, is a high-magnification microscopic structure photograph taken at the same position as FIG. 9.

Claims (1)

[Claims] 1 Carbon (C) 0.03-0.15% Silicon (Si) 0.7-2.3% Manganese (Mn)
0.7-2.0% Sulfur (S) 0.
In the range of 0.01% or less, the Si/Mn component ratio is 0.6 or more and 1.5
Si-added high-strength hot-rolled steel that is isotropic and has high elongation toughness, with the remainder consisting of iron and unavoidable accompanying impurities. 2 Carbon (C) 0.03-0.15% Silicon (Si) 0.7-2.3% Manganese (Mn)
0.7-2.0% chromium (Cr) 0.
Less than 3% Sulfur (S) In the range of 0.01% or less, the Si/Mn component ratio is 0.6 or more and 1.5
Si-added high-strength hot-rolled steel that is isotropic and has high elongation toughness, with the remainder consisting of iron and unavoidable accompanying impurities. 3 Carbon (C) 0.03-0.15% Silicon (Si) 0.7-2.3% Manganese (Mn)
o. '7 ~2.0% Sulfur (S) 0
．． The Si/Mn component ratio is 0 in the range of less than 0.025%.
．． 6 or more and up to 1.5, and Ce/S 1.5 ~
Isotropic and Si-added high strength hot rolled steel with high elongation toughness. 4 Carbon (C) 0.03-0.15% Silicon (Si) 0.7-2.3% Manganese (Mn)
0.7 to 2.0% chromium (Cr) 0
．． Less than 3% Sulfur (S) In the range of less than 0.01%, the Si/Mn component ratio is 0.6 or more and 1.5
In addition, one or two types of Nb, V, and Ti are added in a combined amount of 0.4% or less, and the remainder is iron and unavoidable accompanying impurities. Inter-rolled steel. 5 Carbon (C) 0.03-0.15% Silicon (S i ) 0. 7-2.3% manganese (
Mn) 0.7 to 2.0% chromium (Cr)
Less than 0.3% Sulfur (3) In the range of less than 0.025%, the Si/Mn component ratio is 0.6 to 1.5, and Ce/S 1.5 to 2.0.42>Ti /S>4
, Medical≧Zr/S>2, Ce, T i or Zr
A Si-added high-strength hot-rolled steel which is isotropic and has high elongation toughness, with the remainder being iron and unavoidable accompanying impurities.