JP2783809B2 - High tensile hot-rolled steel strip with excellent cold workability and weldability and a tensile strength of 55 kg / f / mm 2 or more - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention is excellent in weldability in direct current butt welding and has good cold workability such as brim overhanging (burring) processing, and is particularly suitable for automobile wheels. It relates to a hot-rolled high-tensile steel strip having a tensile strength of 55 kgf / mm ² or more suitable for use.

For example, it is effective to reduce the body weight as one of the measures to improve the fuel efficiency of automobiles, and the ratio of using high-strength hot-rolled steel sheets for structural members tends to increase more and more from the viewpoint of thinning steel sheets and safety. It is in.

By the way, the properties required for high-strength hot-rolled steel sheets as structural members for automobiles include not only high tensile strength of 55 kgf / mm class ² or higher and excellent workability, but also these structural members. Since the assembling is performed by direct current butt welding, flash butt welding, spot welding or the like, it is basically important that the weldability is excellent. DC butt welding, which is superior to conventional flasher butt welding in terms of productivity and working environment, is more efficient than conventional flasher butt welding in the high-efficiency production line applied to wheel rims, which are particularly oriented to high-tensile steel among automotive parts. Has been transferred to. The heat history at the welded part in this DC butt welding method is more severe than the conventional welding method, that is, a heat cycle of rapid heating and quenching. With the weldability in the range provided by the steel sheet, it is difficult to ensure sufficient reliability against fatigue fracture or the like of the welded portion. In general, the higher the strength, the higher the C equivalent, so the demand for improvement in weldability becomes a more serious problem as the material used increases in strength. The situation is the same not only in the wheel rim but also in the wheel disc on which spot welding is performed.

Generally, as an index indicating the weldability of steel material, for example, C + Mn
/ 6 + Si / 24 + Cr / 5 + Mo / 6 + V / 8 + Ni / 12 C equivalent is used. Actually, if a steel material with a low C equivalent is used, hardening at the weld is suppressed and crack susceptibility and fatigue characteristics are reduced. It is known to improve. Therefore, in the case of a high-strength hot-rolled steel strip for automobiles used for applications in which weldability is particularly important, such as wheel rims, it is important to thoroughly reduce the C equivalent.

(Prior art) Conventionally, the range of the C content of a high-strength steel sheet for such an application is usually 0.05 to 0.15 wt% in consideration of weldability (hereinafter simply referred to as%).
However, in order to obtain high strength with a tensile strength of 55 kgf / mm ² or more in such a range of the C content, the Mn content is generally set to 1.3% or more, or if the Mn content is small, Si 1.0
% Or more is included as a steel component, or the amount of Mn and
The amount of Si is reduced to the range of 0.80 to 1.3%, and the winding is performed in a low temperature region of, for example, less than 500 ° C., so that the microstructure includes a bainite phase or a martensite phase in a large amount. Thus, means for increasing the strength are adopted.

However, when the amount of Mn or the amount of Si is increased, an increase in the C equivalent is unavoidable in any case, and there is a limit to the level of weldability that can be obtained. First, the supercooling down to the low temperature range during production deteriorates the shape at the time of steel plate production and increases the inhomogeneity of the material. If this is selected, the low-temperature transformation phase, which is the parent material of the reinforcement, will be tempered due to the heat effect at the time of welding and will be softened conversely, resulting in inconvenience in use such as not being able to maintain the required high strength at the welded part. Become.
By the way, in order to avoid the above-mentioned problem caused by the softening at the welded portion, the lower limit of the C equivalent of the conventional manufacturing method is almost 0.
This is 20%, so that the reduction of the C equivalent is naturally limited.

Thus, for example, it relates to high-tensile steel sheet for suitable tensile strength of 55 kgf / mm ² grade wheel rim for DC butt welding in JP 61-264159, the C eq in consideration of the DC butt weldability conventional Although it is disclosed that the range of components is lower than that of the high-tensile steel sheet, the achievable decrease in C equivalent is still insufficient, and the tensile strength is 55 kgf / mm.
^In the case of a material having a higher strength, which is attainable only in the ^second class, and higher than that, it is naturally difficult to manufacture in the range of this C equivalent.

On the other hand, Japanese Patent Publication No. 62-35453 discloses that the Mn content is 0.40 to 0.70%.
Using a steel having a C equivalent in the range of 0.19 to 0.28%, containing 0.05 to 0.90% Si and 0.0005 to 0.006% B,
The microstructure by the following low-temperature winding is a composite structure,
Although there is a disclosure regarding a method for producing a high-strength steel sheet having a tensile strength of 50 to 60 kgf / mm ^{2 and} a low C equivalent, the tensile strength is 55 kgf / mm2.
To obtain a strength of mm ² class or higher, the C equivalent is low.

On the other hand, high-strength steel sheets applied to wheel discs are required to be materials that can withstand severe cold working such as burring, and the factors affecting the burring workability are factors such as MnS. The presence of non-metallic inclusions and anisotropy of microstructure such as fibrous structure, these effects appear as deterioration of ductility in the direction perpendicular to the rolling direction. Of these, MnS is relatively easy to improve by reducing the amount of S, but it is extremely difficult to improve the microstructure anisotropy in high-strength steel. This is because when the conventional tensile strength of 55 kgf / mm ² or higher grade high tensile steel, generally above containing Mn content of 1.0% or more, since the use of reinforcing elements Nb, such as B and Ti, hot by these elements Recrystallization of γ grains in the rolling process is suppressed, and γ grains at the end of hot rolling undergo γ → α transformation in an unrecrystallized state expanded in the rolling direction, and the final microstructure obtained is also obtained. This is because a structure having anisotropy depending on the expanded γ grains is exhibited. In addition, factors on the structure that affect the cold workability other than the anisotropy include the ratio between the second phase such as pearlite, bainite, and martensite and the ferrite phase. The workability is good, and the workability deteriorates as the ratio of the second phase increases. However, when the Mn content is high and Nb and B are contained as in the conventional high-tensile steel, the Ar3 point decreases, the ferrite fraction in the microstructure decreases, and the ratio of the second phase increases. It is disadvantageous for gender. This tendency becomes more remarkable as the Mn content is increased for higher strength.

However, no effective remedy for the above problem has been proposed.

(Problems to be Solved by the Invention) The present invention provides a high tensile strength of 55 kgf / mm ² or more, which has excellent weldability required for a material for an automobile wheel rim and excellent burring workability required for a material for a wheel disc. The purpose is to provide a hot rolled steel strip.

(Means for Solving the Problems) In order to secure the weldability necessary for a material for a wheel rim, it is advantageous to reduce the C equivalent, and as a means for strengthening the C amount, After reducing the Mn amount as much as possible to the minimum range, the strength reduction caused by the reduction of Mn should be supplemented with other strengthening components that do not increase the C equivalent, that is, Ti having solid solution hardening ability and precipitation hardening ability. However, in order to improve the burring workability required for wheel disc materials, since the burring workability is governed by the material ductility in the direction perpendicular to the rolling direction, the microstructure should be mainly made of fine polygonal ferrite. And to eliminate the anisotropy by making the microstructure uniform, and completed the present invention.

That is, in the present invention, C: 0.04 to 0.18 wt%, Si: 0.05 to 1.00 wt%, Mn: 0.10 to 0.1.
47wt%, Ti: 0.05 ~ 0.30wt%, Al: 0.001 ~ 0.100wt%, N:
0.0100 wt% or less, P: 0.030 wt% or less and S: 0.015 wt% or less, the balance being unavoidable impurities and Fe, and Cr in Ti, C, N, S, Mn, Si and unavoidable impurities Of 0.3 ≦ Ti / (C + S + N) <5 and C + Mn / 6 + Si / 2
55kgf / mm with excellent cold workability and weldability characterized by satisfying 4 + Cr / 5 ≦ 0.20wt% and having a polygonal ferrite fraction of 70% or more in the final microstructure.
^Two or more high-strength hot-rolled steel strips and, furthermore, Cr: 0.10 to 0.50 wt% containing Mn + Cr ≤ 0.50 wt%, with excellent tensile strength of 55 and excellent in cold workability and weldability
It is a high-strength hot-rolled steel strip of kgf / mm ² or more.

Conventionally, although similar attempts have been made to reduce the C equivalent by using reinforcing components such as Nb, Ti, V and B, a significant reduction in the amount of Mn as in the present invention has not been achieved, The reason why the amount of Mn can be significantly reduced in the present invention is that a method for maximizing the solid solution hardening ability and precipitation hardening ability of Ti has been found. In the steel strip according to the present invention in which Mn is reduced and the reinforcing component is Ti, the center segregation in the slab is smaller than that of the conventional steel. That is, conventionally
To obtain a 55 kgf / mm ² or more tensile strength, it usually has to be contained at least 1.0% or more Mn amount, when the composition of such a high Mn, the greater the center segregation of the slab, This effect appears as a weak point that the center segregation layer tends to cause hub cracking during burring,
In the present invention, since the composition of the composition is low Mn, the degree of center segregation is remarkably reduced as compared with the conventional steel, and the burring workability does not deteriorate due to the center segregation.

Further, to obtain a structure mainly composed of fine and polygonal ferrite, (1) the γ → α transformation proceeds easily, that is, Ar3
High transformation point (2) It is necessary that γ → α transformation occurs uniformly, in other words, γ grains before transformation must be finely sized. It is necessary to apply metallurgical methods such as easy crystal refinement. Means of using Ti as the main component of the strengthening component and using a component system in which the amount of Mn is reduced as much as possible works extremely effectively.That is, Ti does not greatly affect the Ar3 point, so the Ar3 point is reduced by a significant reduction of Mn. Can be raised. Also, unlike Nb, which has been widely used in conventional steels, Ti has a small effect of suppressing the recrystallization of γ grains, and TiN acts as an inhibitory action on the growth of recrystallized γ grains. It is done.
Further, the reduction of the amount of Mn also contributes to the promotion of recrystallization of γ grains.

By optimizing the action of the components as described above, a high-strength steel strip for wheels having both direct current butt weldability and burring workability can be obtained.

(Operation) Next, the reasons for limiting the component composition ranges will be described.

C: 0.04 to 0.18% C is an indispensable element for securing the strength of steel, and in order to achieve high strength with a tensile strength of 55 kgf / mm ² or more
0.04% or more is necessary, while if it exceeds 0.18%, the C equivalent increases and the weldability deteriorates remarkably, and the amount of polygonal ferrite decreases and the ratio of the second phase such as pearlite and nbainite Increases, and the cold formability such as burring deteriorates.
The range was about 0.18%.

Si: 0.05-1.00% Si is a useful element that has a solid solution hardening action and a deoxidizing action.
In order to utilize the deoxidizing action, by including 0.05% or more of Si together with Al, a stable deoxidizing effect can be expected and the cleanliness of the steel can be enhanced. When the content is 0.30% or more, an increase in strength due to the solid solution effect can be expected. However, when the content exceeds 1.0%, weldability deteriorates, and descalability during hot rolling deteriorates, and scale defects remain in the product. Therefore, the content was set to 0.05 to 1.0%.

Mn: 0.10 to 0.47% Mn has the effect of increasing the strength of steel and fixing harmful S that causes hot embrittlement as MnS.
Although it is preferable to contain 0.10% or more, 0.47%
When the content exceeds 0.1%, the object of the present invention cannot be achieved by the use described below, so the content is set to 0.10 to 0.47%.

In other words, when the Mn content increases, the C equivalent also increases and the DC butt weldability during wheel rim forming deteriorates, and the Ar3 point during hot rolling decreases, so that the γ → α transformation is suppressed and polygonal ferrite is reduced. Since it is difficult to obtain the main microstructure and the rolling recrystallization of γ grains during hot rolling is suppressed, γ grains in the expanded state of γ grains before transformation are
The α transformation is caused, and the anisotropy of the microstructure after the transformation is increased to cause deterioration of the microstructure, thereby deteriorating the burring processability.

Further, from the viewpoint of weldability and burring workability, it is advantageous to reduce Mn, but on the other hand, a decrease in the Mn content results in a decrease in strength. In the present invention, Ti is used as a strengthening element to compensate for the decrease in strength. This is because Ti has a strengthening function of 0.47% or less of the high Mn content region of 1.0% or more as compared with the conventional steel. This is derived from the new finding that the content is increased by setting the content range, and thus the strength reduction due to the reduction of Mn can be sufficiently compensated for by Ti.

In addition, the increase of the strengthening function of Ti in the low Mn content region as described above (the increase of the strengthening function as referred to herein is the amount of increase in strength per Ti addition amount) is due to (1) polygonal ferrite in the microstructure. In the case of 70% or more, (2) the reheating temperature of the slab before hot rolling is a temperature condition under which all of the Ti content can be dissolved in the austenite phase under the condition indicated by the solubility product of TiC of the following formula. If hot rolling directly without reheating from a state where the temperature of the billet after casting is not cooled to a temperature range of less than 1000 ° C,
(3) In the case where the winding temperature after hot rolling is a temperature condition of 500 ° C. or higher, it becomes remarkable when conditions such as are satisfied.

Log (% Ti) (% C) =-10475 / T + 5.33 where T is the reheating temperature (K) Although the details of these mechanisms are not always clear, the above phenomenon (1) is based on the precipitation of TiC. Is precipitated in the ferrite phase as fine precipitates effective for strengthening, whereas in the second phase such as pearlite and bentonite, the dislocation density is high, so that a relatively coarse precipitate form with a small strengthening mechanism is taken. Needless to say, the above (2) relates to conditions necessary for sufficiently dissolving Ti in the initial state, and the above (3) relates to a region where the winding temperature is low.
This corresponds to the fact that the precipitation effect does not appear because the precipitation of TiC does not occur.

In any case, if the base component is low Mn,
The strengthening ability of Ti became remarkably higher than that of Mn system, indicating that these are phenomena related to the change of precipitation hardening mechanism through the precipitation behavior of TiC, and completed this invention based on this finding That is.

Cr: 0.10 to 0.50%, Mn + Cr0.50% Cr has the same effect as Mn, but has a smaller effect of inhibiting hardening of Ar3 transformation than Mn, and a smaller effect of suppressing the recrystallization of γ grains. More than 0.1% C
If r is used in place of Mn as long as r satisfies the condition of Mn + Cr 0.50%, a microstructure can be easily obtained. However, if the Cr content or Mn + Cr exceeds 0.50%, the direct current butt weldability deteriorates. Therefore, the upper limit of Cr and the upper limit of Mn + Cr are set to 0.50%.

Ti: 0.05 to 0.30% Ti is the main element of strengthening in the present invention. For this reason, 0.05% or more is necessary. However, if added in excess of 0.30%, a penetrator is likely to be generated in the welded portion, so that The range was 0.05-0.30%. Ti is more S than Mn.
It has a strong affinity with Ti and fixes harmful S as TiS, and thus has an effect of improving burring workability and fatigue characteristics.

Al: 0.001 ~ 0.100% Al is added as a deoxidizer at the time of welding steel, at least
0.001% is required, while using more than 0.100% saturates the effect.

N: 0.0100% or less N combines with Ti as TiN to reduce the amount of Ti effective for strengthening and to deteriorate the cleanliness of steel as TiN.

P: 0.030% or less If P exceeds 0.030%, the secondary working brittleness resistance is likely to deteriorate, so the range is set to 0.030% or less.

S: 0.015% or less When S exceeds 0.015%, the amount of non-metallic inclusions in the A system increases, and the burring processability deteriorates. Range.

The upper limit of C equivalent (C + Mn / 6 + Si / 24 + Cr / 5) is 0.20.
The reason for setting the C% is that it is extremely effective to set the C equivalent to 0.20% or less, particularly for improving the weldability during DC butt welding.

The reason for setting the range of Ti / (C + N + S) to the range of 0.3 to 5 is that if the ratio is less than 0.3, the tensile strength required for the present invention of 55 kgf / mm ² or more cannot be obtained, while the ratio is 5%.
If it exceeds the limit, the amount of Ti effective for strengthening becomes excessive, so that when subjected to the self-annealing effect after coil winding, the precipitation behavior of TiC tends to fluctuate greatly in the longitudinal direction of the coil, so the mechanical properties in the material This is because the variation increases, which is not preferable.

In the present invention, in addition to the component ranges specified above, Ca
It does not prevent the addition of an element having the effect of controlling the sulfide form, such as sulfide, and the addition of less than 50 ppm of Ca can particularly improve the burring workability.

The reason why the fraction of polygonal ferrite in the microstructure is set to 70% or more in the present invention will be described with reference to FIG. The figure for high-tensile hot-rolled steel sheet produced by various production conditions strength ranging tensile 60~65kgf / mm ^2, and the polygonal ferrite fraction in the microstructure,
It is a plot of the relationship between the elongation in the direction perpendicular to the rolling direction (hereinafter referred to as C direction) and the elongation in the direction parallel to the rolling direction (hereinafter referred to as L direction). This indicates that this ratio is small in the region where the polygonal ferrite fraction is less than 70%, and the material anisotropy in the L direction and the C direction is large. Hub cracking during burring is affected by characteristics in the C direction, which is inferior in workability. Therefore, to prevent hub cracking, it is necessary to eliminate anisotropy and improve elongation characteristics in the C direction. The reason that the anisotropy deteriorates in the region where the polygonal ferrite fraction is less than 70% is that the second phase in the microstructure tends to have anisotropy, and the polygonal ferrite fraction should be 70% or more. This adverse effect can be avoided.

By the way, in the present invention, it is advantageous to adopt the following range of hot rolling conditions after satisfying the above requirements on the components in order to obtain the desired microstructure and mechanical properties.

The heating temperature is preferably in the range of 1100 ° C to 1450 ° C.
The reason for this is that TiC is insufficiently dissolved at a temperature lower than 1100 ° C., so that the precipitation hardening ability of Ti is not sufficiently exhibited and a desired strength cannot be obtained.On the other hand, as the heating temperature increases, TiC becomes Dissolution proceeds, and the amount of strength increase due to Ti per added amount increases. However, if it exceeds 1450 ° C., the amount of oxidation during heating and during rolling increases, resulting in economic disadvantage.

In addition, in the present invention, if a step of immediately starting hot rolling without casting and then a reheating step after casting into a slab, that is, a direct rolling method, the strengthening function of Ti is exhibited to the maximum. It will be more convenient. In this case, when the temperature of the cast slab after casting is less than 1000 ° C, precipitation of TiC occurs before hot rolling, and the precipitation hardening ability of Ti decreases, so the hot rolling start temperature may be 1000 ° C or more. desirable.

The finishing temperature of hot rolling is preferably in the range of 800 to 950 ° C. The reason for this is that if the temperature is lower than 800 ° C., a fibrous microstructure is easily formed, and the anisotropy of mechanical properties increases.
If the temperature exceeds 950 ° C., the γ grains become coarse and the γ → α transformation is delayed, so that it is difficult to obtain a microstructure having a polygonal ferrite fraction of 70% or more.

The cooling rate after hot rolling is preferably in the range of 5 to 100 ° C / s. Because, when the cooling rate exceeds 100 ° C / s, the γ → α transformation in the cooling process is suppressed, so that it becomes difficult to obtain a microstructure with a polygonal ferrite fraction of 70%. In such a case, the grain size of the polygonal ferrite becomes coarse, which adversely affects the burring workability.

The winding temperature is desirably in the range of 500 to 700 ° C. The reason for this is that if the temperature is lower than 500 ° C., the precipitation hardening of TiC does not occur, so that the desired strength cannot be obtained, and the steel sheet shape also deteriorates.
On the other hand, when the temperature exceeds 700 ° C., the deposited TiC becomes coarse and precipitation hardening is reduced, so that it is difficult to obtain strength.

(Examples) Steels having the chemical components shown in Table 1 were hot-rolled to a thickness of 3.0 mm by the production steps shown in Table 2 to obtain hot-rolled steel strips. Table 1 shows the results of examining the microstructure, mechanical properties, and direct current butt weldability of these steel strips.

In Table 1, the microstructure of the optical microscopy sample taken from the cross section parallel to the rolling direction of the hot-rolled steel sheet was exposed with a corrosive liquid, and then a 10-field photograph was taken at 400 times magnification. The ratio of polygonal ferrite phase, acicular ferrite phase, pearlite phase, and bainite phase in the total was measured, and the average value of each phase ratio was determined.The hole expansion rate was determined using a tool with the shape and dimensions shown in Fig. 2. Using a blank with a diameter D ₀ = 70 mm and a hole diameter D ₁ = 12 mm, perform deep drawing and stop it at the moment when a crack is formed around the hole, measure the hole diameter d ₁ at that time, and calculate the formula Collision determined, weld hardness welding current density at the weld hardness DC welding machine [150A / mm ^2], welding time 50 [cycle], the test piece in the conditions of pressure 10 kg / mm ² at After butt welding, measure the hardness distribution with a Vickers hardness tester on the cross section of the test piece including the welded part and the base material, and determine the maximum hardness (Hv m
ax) and the hardness difference (ΔHv) between the maximum hardness and the base metal part.

As is apparent from Table 1, the steel strip having the components and microstructure according to the present invention has excellent direct current butt weldability, a tensile strength of 55 kgf / mm ² or more, and good stretch flange properties.

FIG. 3 shows a comparison between the steel strip shown in Table 1 and the steel plates described in the above-mentioned JP-A-61-264159 and JP-B-62-35453 for the relationship between the C equivalent and the tensile strength. Shown. From the figure, it can be seen that the steel strip according to the present invention has a significantly lower C equivalent required to obtain the same tensile strength as compared with the conventional steel sheet.

(Effects of the Invention) According to the present invention, a high-long-strength hot-rolled steel strip excellent in direct current butt weldability and burring workability and having a tensile strength of 55 kgf / mm ² or more, that is, an optimum material is particularly provided as an automobile wheel. obtain.

It also has many advantages such as low component cost, low center segregation, and suitability for the direct rolling method, which has recently been implemented as a manufacturing process for pursuing energy saving.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the polygonal ferrite fraction in the microstructure and the ratio of elongation in the C direction / elongation in the C direction. FIG. 2 is an explanatory view of a tool for a hole expansion test. FIG. 3 is in accordance with the present invention. 4 is a graph showing the relationship between carbon equivalent and tensile strength of a steel strip and a conventional steel sheet.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-58-42725 (JP, A) JP-A-61-264160 (JP, A) JP-A-57-155348 (JP, A) JP-A 52- 17319 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301

Claims (2)

(57) [Claims] 1. C: 0.04 to 0.18 wt%, Si: 0.05 to 1.00 wt%, M
n: 0.10 ~ 0.47wt%, Ti: 0.05 ~ 0.30wt%, Al: 0.001 ~ 0.1
00wt%, N: 0.0100wt% or less, P: 0.030wt% or less and S:
0.015wt% or less, the balance being unavoidable impurities and Fe
And the content of Cr in Ti, C, N, S, Mn, Si and inevitable impurities is 0.3 ≦ Ti / (C + S + N) <5 and C
+ Mn / 6 + Si / 24 + Cr / 5 ≦ 0.20wt%, and the final microstructure has a polygonal ferrite fraction of 70% or more. High tension hot rolled steel strip of mm ² or more. 2. C: 0.04 to 0.18 wt%, Si: 0.05 to 1.00 wt%, M
n: 0.10 ~ 0.47wt%, Cr: 0.10 ~ 0.50wt%, Ti: 0.05 ~ 0.30
wt%, Al: 0.001 to 0.100 wt%, N: 0.0100 wt% or less, P: 0.0
30% by weight or less and S: 0.015% by weight or less, the balance being unavoidable impurities and Fe, and the content of Ti, C, N, S, Mn, Si and Cr is 0.3 ≦ Ti / (C + S + N) < 5, C + Mn
/6+Si/24+Cr/5≦0.20wt% and Mn + Cr ≦ 0.50wt%, polygonal ferrite fraction of final microstructure
A high-strength hot-rolled steel strip with a cold workability and weldability of 70% or more and a tensile strength of 55 kgf / mm ² or more.