JP3296591B2 - Low yield ratio high strength hot rolled steel sheet and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention, the inner plate of a motor vehicle, suitable for use in applications such as underbody parts and strength members, tensile strength 70~100kgf / mm ² class low yield ratio high-strength hot-rolled steel sheet It is about.

[0002]

2. Description of the Related Art Conventionally, high-strength steel sheets have been widely used as steel sheets used for inner plates, undercarriage parts, strength members and the like for automobiles in order to reduce vehicle body weight. Such a high-strength steel sheet for automobiles, in addition to having the strength required to ensure the safety of the automobile, good formability represented by press working, and also after forming Good fatigue resistance and the like are also required. Furthermore, in addition to the above characteristics, in order to reduce the cost of the steel plate used, the use ratio of the hot rolled steel plate to the inner plate and the strength member, which has conventionally been mostly cold rolled steel plate, is increasing. In addition, in recent years, under the situation where the regulation on the total amount of exhaust gas from automobiles has been significantly strengthened, it is urgently required to further increase the strength.

[0003] Under the present circumstances as described above, the intensity level of the high-strength hot-rolled steel sheet, the conventional TS: from 50 to 60 kgf / mm ² class entities, tends to rise to 70 to 100 kgf / mm ² class is there. Moreover, the following are the main characteristics required for such a high-strength hot-rolled steel sheet. (1) Stable and high strength characteristics (material variation is small). (2) The yield ratio is low. (3) No severe hot rolling conditions are required. (4) The spot weldability is good. (5) Good fatigue characteristics. (6) The winding shape is good.

By the way, in the case of a conventional TS: 50 to 60 kgf / mm ² grade steel sheet, there are many options for the strengthening method, such as solid solution strengthening, structure strengthening, precipitation strengthening and grain refinement strengthening. According to the application to be used, it was possible to make different products utilizing their material and economic characteristics. However, the choices will be much less if 70~100 kgf / mm ² class is the target. First, it is not possible to cope with means mainly comprising solid solution strengthening or grain refinement strengthening. In the case of favorable precipitation strengthened in terms of weldability and stability manufacturability is, 80 kgf / mm ² or higher grade of will range the strength quite difficult, in practice, deposited on the tissue strengthening by pearlite or bainite In many cases, manufacturing methods that take into account reinforcement are used. However, the disadvantage of these precipitation-strengthened high tensile steels is, of course, a high yield ratio (generally 0.80 or more). In particular, TS: 80 kgf / mm High-strength class ² or higher has a problem of springback after press forming.

On the other hand, the structure strengthened steel has the advantage that it is easy to achieve both high strength and low yield ratio, and the elongation characteristics of the ferrite-martensite two-phase mixed steel, also called Dual Phase steel, Microstructure steel (for example, Japanese Patent Publication No. 61-151)
No. 28), it is possible to obtain characteristics superior to those of other methods, and the fatigue resistance is better than that of precipitation-strengthened steel. However, TS, such as more than 80 kgf / mm ²
In order to obtain, the production conditions are strict, and there is a problem that a shape defect is caused in the production process and the material is apt to vary.

Recently, TS: 80 to 100 kgf / mm ² grade (α + γ) structured steel which exhibits higher ductility than the above-mentioned Dual Phase steel, so-called TRIP steel (for example,
0049) has attracted attention. This TRIP steel is TS: 80
kgf / mm class ² or higher, which satisfies the necessary characteristics as a high tensile steel that emphasizes workability. However,
In the case of this TRIP steel, since the tensile properties are greatly affected by the phase fraction in the steel, mainly the amount of retained austenite, it is extremely difficult to obtain a uniform material in the width and longitudinal directions of the steel strip, and it contains C. Because of the high volume,
Deterioration of spot weldability is inevitable. As described above, to date, there is no steel that sufficiently satisfies the characteristics required as a high-strength hot-rolled steel sheet, and its development has been desired.

[0007] In response to the above demands, the present inventors have proposed TiC and NbC as the ones having the characteristics of the conventional precipitation-strengthened steel and the structure-strengthened steel while advantageously solving the drawbacks. Developed a hot-rolled high-strength steel sheet having a structure consisting of ferrite and martensite precipitation-strengthened with, or ferrite precipitation-strengthened with TiC or NbC, martensite and retained austenite (Japanese Patent Application No. 3-35).
No. 8007). However, the above steel sheets include C, Ti, Nb
There was a drawback that strict balance adjustment was necessary. In addition, when carbides of Ti or Nb are used as precipitation strengthening, solution treatment at a high temperature is necessary, and from the viewpoint of cost reduction and energy saving, it will become mainstream in the near future, at 1100 ° C.
Considering the low-temperature heating operation, it is pointed out that Ti and Nb-based alloys have insufficient solution solution and are hardly useful for strengthening.

[0008]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-mentioned situation, and has the characteristics of conventional precipitation-strengthened steel and structure-strengthened steel in low-temperature heating operation of a slab. An object of the present invention is to propose a low-yield-ratio high-strength hot-rolled steel sheet that advantageously overcomes these disadvantages, together with a method for producing the same.

[0009]

Means for Solving the Problems Now, the inventors have conducted numerous experiments and studies to achieve the above object, and as a result,
Add appropriate amounts of Si and Cu to the components of conventional precipitation strengthened steel,
Further, by performing hot rolling under optimum conditions, C discharged from the ferrite grains is concentrated in the untransformed austenite grains, and finally, the precipitation-strengthened ferrite phase is mainly contained, and the martensitic phase is formed as the second phase. Or a composite structure containing a small amount of a retained austenite phase, and the finding that the above object can be advantageously achieved by using such a structure. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.05-0.18%, Si: 0.5-2.5%, Mn: 0.5-2.5
%, P: 0.05% or less, Al: 0.01 to 0.1%, and Cu: 0.5 to
3.0%, the balance is composed of Fe and unavoidable impurities, the yield ratio is 0.58 or less, and the structure is ε-Cu
And high strength hot rolled steel sheets with martensite or martensite and retained austenite. 2. The low-yield-ratio high-strength hot-rolled steel sheet further containing Cr: 0.3 to 1.5%. 3. The low-yield-ratio high-strength hot-rolled steel sheet according to 1 or 2, further containing 0.5 to 2.0% of Ni. 4. In the above 1, 2, or 3, Nb: 0.01 to 0.
10%, Ti: One or two selected from 0.01 to 0.10%
Low yield ratio high strength hot rolled steel sheet containing seeds. 5. C: 0.05-0.18%, Si: 0.5-2.5%, Mn: 0.5-2.5
%, P: 0.05% or less, Al: 0.01 to 0.1%, and Cu: 0.5 to
Continuous cast slab with a composition containing 3.0%, directly or 130
After heating to a temperature of 0 ° C. or less, it is subjected to hot rolling. After the hot rolling is completed at a temperature of 800 ° C. or more, the material is retained in a temperature range of 800 to 720 ° C. for 6 seconds or more, and then 10 ° C./s or more. cooling at a cooling rate, epsilon-Cu by winding at 600 ° C. below the temperature
Ferrite and martensite or
A method for producing a low-yield-ratio high-strength hot-rolled steel sheet having a structure consisting of rutensite and retained austenite .

In the present invention, the ratio of the ferrite phase in the steel structure is preferably about 50 to 90%. The amount of ε-Cu precipitated in the ferrite phase is preferably about 20 to 100%.

[0012]

[Function] Unlike the conventional Dual Phase steel and TRIP steel, the steel of the present invention improves the strength by precipitating and strengthening the soft ferrite phase, thereby increasing the amount of martensite or the amount of martensite required to obtain the same strength. The ratio of the retained austenite phase can be made smaller than that of the conventional one, and therefore, an increase in the C equivalent can be suppressed. Further, compared with the conventional precipitation strengthened steel, high strength can be achieved by the presence of the hard second phase, and a high-density movable dislocation steel is formed around the second phase in the same manner as the conventional structure strengthened steel. Therefore, low yield ratio characteristics are exhibited. Further, since the consistency between the second phase and the ferrite grains is maintained, the strength-ductility balance is improved, and the propagation of fatigue cracks is prevented by the second phase, so that the fatigue resistance is improved. . Furthermore, conventional Dual
Compared with the Phase steel, the difference in strength between the ferrite grains and the second phase is smaller than that of the conventional steel, so local stress concentration on the ferrite grains is eased, which has been considered a disadvantage of this type of reinforced steel. Local deformability is also improved. Moreover, the present inventors previously proposed, compared with low-yield high-strength hot-rolled steel sheets having a structure composed of ferrite and martensite precipitation-strengthened by carbides of Ti and Nb, or precipitation-strengthened ferrite and martensite and retained austenite. In order to obtain the same level of strength, the steel of the present invention requires a lower C equivalent component system.

Hereinafter, the results of an experiment on which the present invention is based will be described. C: 0.07%, Si: 1.5%, Mn: 1.5
%, P: 0.01%, S: 0.001%, Al: 0.04%, and various steel slabs added with varying Cu in the range of ~ 1.0%, slab heating temperature: 1080 ° C, heat Rolling finish temperature: 850 ° C, residence time at 800 to 720 ° C: 10 seconds, cooling rate until winding: 20 ° C / s, winding temperature: 550
Hot rolling at ℃ (slab dimensions and finish dimensions are the same)
And a hot rolled sheet having a sheet thickness of 2.00 mm. Fig. 1 shows the results of an investigation on the side bend elongation and the hole expansion rate, which are indicators of stretch flangeability, for each of the obtained hot rolled sheets.
It is shown in relation to the content. In addition, each investigation method is as showing in the Example mentioned later.

As apparent from FIG. 1, the Cu content is 0.5%.
%, Both the side bend elongation and the hole expansion rate are improved. It is considered that this is because the difference in strength between the ferrite grains in which ε-Cu precipitated and the second phase was reduced, and local stress concentration on the ferrite grains was suppressed.

Next, in the present invention, the reason why the composition of steel is limited to the above range will be described. C: 0.05 to 0.18% If C is less than 0.05%, martensite and retained austenite will not be obtained as a second phase, whereas if it exceeds 0.18%, spot weldability will be significantly deteriorated. , C content is limited to the range of 0.05 to 0.18%.

Si: 0.5-2.5% Si is an important element in the present invention. That is, Si
Is an element effective for transforming the second phase into martensite and retained austenite, and its effect is exhibited by adding 0.5% or more. However, if the content exceeds 2.5%, not only does the effect reach saturation, but also the descaling property after hot rolling deteriorates, and the cost increases. Therefore, the Si content was limited to the range of 0.5 to 2.5%.

Mn: 0.5-2.5% If the Mn content is less than 0.5%, a desired composite structure cannot be obtained, while if the Mn content exceeds 2.5%, the Ar ₃ transformation point is excessively lowered. Mn is contained in the range of 0.5 to 2.5% because α grains are hard to appear during cooling after hot rolling.

P: 0.05% or less P is limited to 0.05% or less in order to secure workability and weldability.

Al: 0.01 to 0.1% Al must be added at least 0.010% for cleaning steel, and improvement of cleanliness is indispensable especially for high strength. However, addition exceeding 0.10% is not preferable because it causes surface defects due to alumina clusters. Accordingly, Al is contained in the range of 0.010 to 0.10%.

Cu: 0.5-3.0% Cu is an important element in the present invention in that it precipitates as ε-Cu in α grains after hot rolling and effectively contributes to strengthening. It also has the effect of substituting Mn. However, if the content is less than 0.5%, the precipitation strengthening ability of ε-Cu is not exhibited, resulting in a bias toward microstructure-enhanced high-tense, and the improvement of local deformability cannot be achieved. , Hot brittleness is likely to occur during hot rolling, so Cu
Was contained in the range of 0.5 to 3.0%.

As described above, the basic components of the present invention have been described. In the present invention, the following elements can be appropriately added. Cr: 0.3-1.5% Cr is an element that effectively contributes as a substitute for Mn, and its proper addition range is 0.3-1.5%.

Ni: 0.5-2.0% Ni is an element having an effect of suppressing the adverse effect of Cu.
The proper addition range is 0.5 to 2.0%.

Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10% Each of Nb and Ti is an element which effectively contributes to improvement of stretch flangeability by grain refinement of the structure. However, if the content deviates from the above range, the effect of the addition is lost. Therefore, in either case of single addition or composite addition, the content is set in the above range.

It is inevitable to mix S and N as unavoidable impurities, but each of them can be tolerated in a range of 0.02% or less.

Next, the reasons for limiting the manufacturing conditions of the steel according to the present invention will be described. Hot rolling can be performed immediately after slab casting (so-called CC-direct rolling), or when heating, 13
The temperature is not higher than 00 ° C. CC-When performing direct rolling,
Heat retention or some heating to the edges is acceptable. Further, when heating, if the heating temperature exceeds 1300 ° C., undesirably, austenite is rapidly coarsened and good characteristics cannot be obtained, and in addition, the bad influence of Cu scabble becomes remarkable. Note that the lower limit of the heating temperature is at least the temperature range of the austenite single phase, and there is no problem as long as the target finish rolling temperature can be secured.
There is no particular limitation.

If the finish rolling temperature in hot rolling is less than 800 ° C., the ductility after hot rolling is remarkably deteriorated. Therefore, the hot rolling finish temperature is set to 800 ° C. or higher.

After the completion of the hot rolling, the steel is kept in a temperature range of 800 to 720 ° C. to cause a γ → α transformation. If the residence time is less than 6 seconds, the transformation from γ to α is insufficient, so that the residence time in the temperature range of 800 to 720 ° C. is 6 seconds or more (preferably 30 seconds or less).

Thereafter, the cooling rate until winding is 10 ° C. /
s or more. Because the cooling rate is 10 ℃
This is because pearlite appears when the ratio is less than / s.

The winding temperature needs to be 600 ° C. or less. This is because pearlite appears when the winding temperature exceeds 600 ° C. The lower limit of the winding temperature does not need to be particularly limited, and may be any temperature at which the shape after winding does not deteriorate.

[0030]

EXAMPLE A steel slab adjusted to various component compositions shown in Table 1 (9 kinds of applicable steels of the present invention, 6 kinds of comparative steels, a total of 15 kinds) was subjected to hot rolling under various conditions shown in Table 2. And a hot rolled sheet having a sheet thickness of 2.00 mm. Table 3 shows the results obtained by examining the tensile properties, side bend elongation (C direction), hole expansion rate, fatigue strength, and microstructure of each hot-rolled sheet thus obtained.
Shown in

The test procedure for each characteristic is as follows. (1) The tensile test was performed in the L direction using a JIS No. 5 test piece in a usual manner. (2) For side bend elongation, measure the test piece size, length: 200 m
m, width: 40 mm, according to the side bend test method shown in FIG. 2, distance between fulcrums: 150 mm, distance between gauge points l ₀ : 50 mm
The distance l between the gauges at the time when the crack is generated by bending
₁ was measured and calculated by the following equation. Side bend elongation _{(%) = (1 1 -1} 0) / 1 0 × 100 (3) holes spread rate, the specimen dimensions 150mm Shun Sumi, in accordance with the hole expansion test method shown in FIG. 3, the diameter 36mmφ (D the center of the test piece opened the perforations of _0), the tip portion is pushed up by the spherical head punch having a radius of 50 mm, a diameter D ₁ of the when the micro-cracks occur was measured and calculated from the following equation. Hole expansion rate (%) = (D ₁ −D ₀ ) / D ₀ × 100 (4) The fatigue strength was determined by a complete swing plane bending fatigue test using a sample having the dimensions shown in FIG.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

As is clear from Table 3, all the conforming examples obtained according to the present invention have a tensile strength of 70 kgf / mm ^2.
More than that, it has a low yield ratio, good strength-ductility balance, side bend elongation and hole expansion rate, and high fatigue strength. The organization of the conforming example
Ferrite and martensite were strengthened mainly by precipitation of ε-Cu, but retained austenite was also observed in Samples 11 and 14, in particular. Further, in all of these conforming examples, the strength of the spot welds separately investigated was also good.

On the other hand, among the comparative examples, sample No. 16
Since the C content was less than the lower limit of the appropriate range, although the side bend elongation and the hole expansion ratio were good, the steel exhibited characteristics close to what is called precipitation-strengthened steel, in which the yield ratio was high and the fatigue strength was low. Sample No. 17 had good strength-ductility balance and fatigue strength because the C content exceeded the upper limit of the appropriate range, but was poor in side-bend elongation and hole expansion rate. And the strength of the spot welds was significantly deteriorated. Sample No. 21 was Cu
Was not added, the strength-ductility balance and the fatigue strength were good, but the ferrite was not precipitation-strengthened, so the side-bend elongation and the hole expansion rate were poor, showing characteristics close to what is called structurally strengthened steel. . Sample No.2
Is because the slab heating temperature has exceeded the upper limit of the appropriate range,
Sesame hedging occurred.

[0037]

As described above, according to the present invention, while having the characteristics of the conventional precipitation strengthened steel and the structure strengthened steel,
In addition, overcoming those disadvantages, the tensile strength is 720 k
A high-strength hot-rolled steel sheet of gf / mm ² or more can be obtained. The hot-rolled sheet obtained by the present invention has high strength, low yield ratio, and good strength-ductility balance, and stretch flangeability represented by side bend elongation and hole expansion rate. Since it is also excellent in fatigue characteristics and spot weldability, it is very effective when used for automobile inner plates, suspension parts, strength members and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between Cu content, side bend elongation, and hole expansion ratio.

FIG. 2 is a schematic diagram showing a side bend test procedure.

FIG. 3 is a schematic diagram illustrating a procedure of a hole expanding test.

FIG. 4 is a view showing the shape of a fatigue test piece.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-140652 (JP, A) JP-A-4-329848 (JP, A) JP-A-4-337026 (JP, A) JP-A-2- 38523 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10 C21D 9/46-9/48

Claims (5)

(57) [Claims] 1. C: 0.05 to 0.18%, Si: 0.5 to 2.5%, Mn: 0.5 to 2.5%, P: 0.05% or less, Al: 0.01 to 0.1%, and Cu: 0.5 to 3.0%, with the balance being the balance Is the composition of Fe and unavoidable impurities, the yield ratio is 0.58 or less, and its structure is ferrite and ε-Cu precipitated, low yield ratio high strength hot rolled steel sheet consisting of martensite or martensite and retained austenite . 2. The method according to claim 1, further comprising Cr: 0.3 to 1.5%.
Low yield ratio high strength hot rolled steel sheet containing. 3. The method according to claim 1, further comprising:
Low yield ratio high strength hot rolled steel sheet containing 5 to 2.0%. 4. The method according to claim 1, 2 or 3, further comprising:
b: A low-yield-ratio high-strength hot-rolled steel sheet containing one or two selected from 0.01 to 0.10% and Ti: 0.01 to 0.10%. 5. A composition containing C: 0.05 to 0.18%, Si: 0.5 to 2.5%, Mn: 0.5 to 2.5%, P: 0.05% or less, Al: 0.01 to 0.1%, and Cu: 0.5 to 3.0%. The continuous cast slab is directly or after heating to a temperature of 1300 ° C. or less, subjected to hot rolling, and after hot rolling at a temperature of 800 ° C. or more, stays in a temperature range of 800 to 720 ° C. for 6 seconds or more. Then, cool at a cooling rate of 10 ° C / s or more and wind up at a temperature of 600 ° C or less to produce 0.58 or less.
With a yield ratio of
Ferrite and martensite or martensite
And a low-yield-ratio high-strength hot-rolled steel sheet comprising: