JP3538981B2 - Automotive steel sheet with excellent impact resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly used as a part for automobiles after being subjected to a process such as press molding, and is particularly required for a part where excellent impact resistance is required in the event of a collision while the automobile is running. The present invention relates to a steel sheet for automobiles which is suitably used as a material for the automobile.

[0002]

2. Description of the Related Art Recently, momentum for global environmental protection has been increasing.
Against the background, CO from automobiles _TwoTo reduce emissions
As a means, weight reduction of an automobile body is required. like this
One of the methods of weight reduction is to increase the thickness of the steel sheet by increasing its thickness.
Is effective. Furthermore, in the design concept of the car body
Based on this, it is possible to not only increase the strength of steel sheets, but also
In the event of a collision, steel sheets with excellent impact resistance
In other words, steel with high deformation resistance when deformed at a high strain rate
The development of boards will increase the safety of cars
In addition, it effectively contributes to the realization of a lighter body
You.

[0003] Conventionally, in the ferrite single-phase structure, solid-solution strengthening by adding a substitution element such as Si, Mn, or P, or adding a carbonitride forming element such as Nb or Ti is conventionally performed in a ferrite single phase structure. In general, a method based on precipitation strengthening was used. For example, JP-A-56-139654
In the gazette, the workability of ultra-low carbon steel, in order to improve the aging, contain Ti, Nb, and further strengthening components such as P contained within the range that does not impair the workability, to achieve high strength. A steel plate is proposed. Also, for example, Japanese Patent Application Laid-Open No. 59-193221 proposes a method for increasing the strength of ultra-low carbon steel by adding Si. Furthermore, Japanese Patent Application Laid-Open No. 60-52528 proposes a method for producing a high-strength steel sheet in which low-carbon steel is annealed at a high temperature, and after cooling, a martensite phase is precipitated to improve ductility.

[0004]

However, when the strength of a steel sheet is increased by such a method, the above-described impact resistance is essentially improved even though the thickness of the automobile body can be reduced to some extent. Not something. Because, in these proposals, the yield strength or tensile strength, which is an index of the strength of the steel sheet, is obtained based on only the so-called static evaluation method, in which the strain rate is extremely slow, 10 ⁻³ to 10 ⁻² (s ⁻¹ ). However, in actual vehicle body design, rather than such static strength, the strain rate is 10 to 10 in consideration of safety at the time of collision.
This is because the strength based on a so-called dynamic evaluation method accompanied by a shock deformation of ⁴ (s ^-1 ) becomes more important. Therefore,
Each of the above-mentioned conventional proposals, which have been developed focusing only on the static strength, has a problem that it cannot be used as a fundamental index for reducing the weight of an automobile body.

An object of the present invention is to develop a steel sheet excellent in impact strength under a high strain rate, which has not been studied at all. Specifically, static-dynamic ratio = dynamic yield stress (yield stress at a strain rate of 10 ³ (s ⁻¹ )) / static yield stress (strain rate of 10 3
^-3 (s ^-1 ) yield of a steel plate with a static-dynamic ratio of 1.6 or more. That is, the present invention has a static-dynamic ratio of 1.6 or more, and thus has a strain rate dependence of strength equal to or higher than that of a conventional mild steel sheet, and achieves safety of an automobile body and weight reduction of an automobile body. The purpose of the present invention is to provide a steel plate that realizes the above.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies for realizing the above-mentioned objects, and as a result, have found that the above-mentioned problems can be solved by appropriately adjusting the composition and structure of steel. . That is, the present invention has the following content as a gist configuration.

(1) C: 0.0001 to 0.0050 wt%, Si: 1.50 wt%
%, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S:
0.010 wt% or less, Al: containing 0.001 to 0.08 wt%, the balance being
A static-dynamic ratio characterized by having a composition of Fe and unavoidable impurities, and having a mixed structure of a transformed phase mainly composed of 5-40% by volume of massive ferrite and a balance of equiaxed ferrite phase. Has excellent impact resistance of 1.6 or more.

(2) C: 0.0001 to 0.0050 wt%, Si: 1.50 wt%
%, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S:
0.010 wt% or less, Al: 0.001 to 0.08 wt% , B: 0.01 wt% or less, the balance has a component composition consisting of Fe and unavoidable impurities, and is mainly composed of 5 to 40% by volume of massive ferrite. A steel sheet for automobiles having a static-dynamic ratio of 1.6 or more and excellent in impact resistance, characterized by having a mixed structure of a transformed phase and an equiaxed ferrite phase.

(3) C: 0.0001 to 0.0050 wt%, Si: 1.50 wt%
%, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S:
0.010 wt% or less, Al: 0.001 to 0.08 wt% , B: 0.01 wt% or less, and one or two kinds selected from Ti: 2.0 wt% or less and Nb: 1.0 wt% or less, The balance is Fe
And a static-dynamic ratio characterized by having a composition composed of an inevitable impurity, a transformation phase mainly composed of 5-40% by volume of massive ferrite, and a balance consisting of an equiaxed ferrite phase. Automotive steel sheet with excellent impact resistance of 1.6 or more.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above-mentioned target value of the static-dynamic ratio of a steel sheet, the inventors of the present invention use steel containing Si, Mn and P and vary the manufacturing conditions of the steel in various ways. The effect of steel structure on the mechanical yield stress was studied in detail.

As a result, if a mixed structure in which a transformation phase mainly composed of hard massive ferrite is dispersed in a soft equiaxed ferrite phase is formed and the amount of each phase is appropriately controlled, the dynamic yield stress can be increased. And a static-dynamic ratio of 1.6 or more. The volume ratio of each phase for obtaining a static-dynamic ratio of 1.6 or more is calculated as follows: volume ratio of the transformed phase mainly composed of massive ferrite = (volume of the transformed phase mainly composed of massive ferrite) / ｛(mass ferrite It is necessary to set 5% to 40% of the volume of the transformation phase as the main component) + (the volume of the equiaxed ferrite phase), and to make the remainder the equiaxed ferrite phase. That is, the former phase is less than 5% or 4
In any case exceeding 0% and in any case where the remaining structure is not an equiaxed ferrite phase, a static-dynamic ratio of 1.6 cannot be obtained.

Next, the reason why the chemical composition of steel is limited as described above in the present invention will be described. C: 0.0001 to 0.0050 wt% C is desirably small from the viewpoint of elongation and r-value, which are indicators of press formability. Bring. on the other hand,
If the content exceeds 0.0050 wt%, it is necessary to add excessive amounts of Ti and Nb to stabilize C. Therefore, the C content is 0.0001 to 0.0050 wt%, preferably 0.0001%.
To 0.003 wt%.

Si: 1.50 wt% or less Si is an element to be added in a necessary amount depending on the desired strength. In addition, the surface treatment properties are significantly reduced. For this reason, the Si content is 1.50 wt% or less, preferably 1.
0 wt% or less.

Mn: 2.50 wt% or less Mn is an element effective as a steel strengthening component.
%, The steel sheet is extremely hardened and the cold rollability is reduced. Therefore, the content is added in the range of 2.50 wt% or less. In addition, the elongation as an index of cold-rolling press formability, from the viewpoint of the r value, a smaller one is desirable, but if it is less than 0.05, a sufficient strengthening effect as an automotive material cannot be obtained, so that it is preferably 0.05 to 2.50. %, more preferably 0.1 to 2.0% by weight.

P: 0.01 to 0.15 wt% P is an effective element as a strengthening component of steel. The effect can be obtained by adding 0.01% by weight or more, but if it exceeds 0.15% by weight, the hot rolled base sheet is hardened remarkably, the cold rolling property is deteriorated, and the surface treatment property is also significantly reduced. Therefore, P
The content is in the range of 0.01 to 0.15 wt%, preferably 0.05 to 0.1 wt%.

S: not more than 0.010 wt% By reducing the content of S, precipitates in the steel are reduced and workability is improved. Such an effect can be obtained by setting the amount of S to 0.010 wt% or less.

Al: 0.001 to 0.05 wt% Al is an element necessary for deoxidizing steel and improving workability, and it is necessary to add 0.001 wt% or more.
The effect saturates even if it exceeds 0.05 wt%. Therefore, the Al content is in the range of 0.001 to 0.05 wt%.

B: 0.01 wt% or less B is an element effective for improving the resistance to secondary working embrittlement. However, even if added over 0.01 wt%, the effect is saturated, so that 0.01 wt% or less. Add in the range.

Ti: 2.0 wt% or less Ti is an element useful for increasing the r value and improving the formability. However, even if added in excess of 2.0 wt%, the effect is saturated and the production cost is reduced. Since it causes an increase, it is added in an amount of 2.0 wt% or less, preferably in the range of 0.5 to 1.5 wt%.

Nb: 1.0 wt% or less Nb is an element useful for increasing the r value and improving the formability, but its effect is saturated even if it is added in excess of 1.0 wt%. % Or less, preferably 0.005 to 0.5 wt%
Add in the range.

A method for producing a steel sheet for automobiles according to the present invention, that is, a steel sheet having a mixed structure of the above-mentioned transformation phase mainly composed of 5-40% by volume of massive ferrite and the rest being an equiaxed ferrite phase will be described. I do. The steel sheet of the present invention is manufactured by hot rolling a steel slab having the above composition, followed by cold rolling and annealing to obtain a cold rolled steel sheet. Hot rolling and cold rolling may be performed according to a conventional method.
The conditions are, for example, heating a steel slab to 1200 ° C and a thickness of 3 mm
Hot rolling, and then cold rolling to a thickness of 0.7 mm. Annealing after cold rolling (normally, continuous annealing) is a particularly important step, annealing at a temperature range which is two-phase region of ferrite and austenite, followed by 1000 ° C. / sec (water cooled or equivalent) or faster It is manufactured by cooling to 300 ° C. Needless to say, the effect of improving the static-dynamic ratio of the present invention is exerted not only on the cold-rolled steel sheet but also on a surface-treated steel sheet manufactured using the material.

FIG. 1 shows that the cold-rolled steel sheet is annealed in a single-phase region of γ phase or in a two-phase region of α-γ, and then immediately at 1000 ° C./s.
It is the result of investigating the static-dynamic ratio of the steel sheet cooled by ec. From this figure, it can be seen that the target static-dynamic ratio can be obtained when the alloy is annealed in the α-γ two phase region and then rapidly cooled, and the volume ratio of the massive ferrite is 5 to 40%.

[0023]

【Example】

Example 1 Steel having the composition shown in Table 1 was melted in a converter. These steel materials were heated to 1200 ° C., hot-rolled to a thickness of 3 mm, and then cold-rolled to a thickness of 0.7 mm. This cold rolled sheet was annealed for 10 seconds to 2 minutes using a continuous annealing facility, and subsequently cooled in a cooling zone. At this time, each annealing temperature, cooling rate, and the like were set to the conditions shown in Table 1. For the obtained cold-rolled steel sheets, tensile tests were performed at strain rates of 10 ³ (s ^-1 ) and 10 ^-3 (s ^-1 ), and the static-dynamic ratio was determined from the yield stress of each.
Further, the structure volume ratio was determined by observing the structure of a sample collected from each steel sheet. The results are shown in Tables 1 to 4.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

As is clear from the results shown in Tables 1 to 4, all the steel sheets conforming to the present invention exhibited characteristics in which the static-dynamic ratio was 1.6 or more, which is the target value. On the other hand, in the comparative example, a static-dynamic ratio of 1.6 or more was not obtained.

[0029]

As described above, according to the present invention,
By optimizing the chemical composition and structure of the steel sheet,
Characteristics with a static-dynamic ratio of 1.6 or more can be satisfied. Therefore, by applying the steel sheet according to the present invention to automobile parts, it becomes possible to reduce the weight of the automobile body and improve safety.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a static-dynamic ratio, a structure, and an annealing temperature.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-34186 (JP, A) JP-A-6-122939 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. C: 0.0001 to 0.0050 wt%, Si: 1.50 wt% or less, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S: 0.01
0 wt% or less, Al: 0.001 to 0.08 wt%, the balance has a component composition consisting of Fe and unavoidable impurities, and the transformation phase mainly composed of 5 to 40% by volume of massive ferrite and the balance are the same. A steel sheet for automobiles having a static-dynamic ratio of 1.6 or more and excellent in impact resistance, comprising a mixed structure with a shaft ferrite phase. 2. C: 0.0001 to 0.0050 wt%, Si: 1.50 wt% or less, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S: 0.01
0% by weight or less, Al: 0.001 to 0.08 % by weight , B: 0.01% by weight or less, the balance having a component composition of Fe and unavoidable impurities, and mainly containing 5 to 40% by volume of massive ferrite. A steel sheet for automobiles having a static-dynamic ratio of 1.6 or more and having excellent impact resistance, characterized by having a mixed structure of the transformed phase and the equiaxed ferrite phase. 3. C: 0.0001 to 0.0050 wt%, Si: 1.50 wt% or less, Mn: 2.50 wt% or less, P: 0.01 to 0.15 wt%, S: 0.01
0 wt% or less, Al: 0.001 to 0.08 wt% , B: 0.01 wt% or less, and one or two selected from Ti: 2.0 wt% or less and Nb: 1.0 wt% or less, and the balance Has a component composition of Fe and unavoidable impurities, and has a volume fraction of 5
A steel sheet for automobiles excellent in impact resistance with a static-dynamic ratio of 1.6 or more, characterized in that a transformed phase mainly composed of up to 40% of a massive ferrite and a balance of an equiaxed ferrite phase are formed.