JP3304191B2 - Method for producing high-strength galvannealed steel sheet with excellent secondary work brittleness resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a high-strength alloy which has high strength and excellent hot pressability and surface treatment properties, and is subjected to hot-dip galvanizing suitable for high-strength cold-rolled steel sheets and has excellent secondary work brittleness resistance. The present invention proposes a method for producing a galvannealed steel sheet. The high-strength alloyed hot-dip galvanized surface-treated steel sheet according to the present invention is used, for example, for an automobile body after being subjected to press working, and particularly simultaneously imparts the required formability and strength to them. Therefore, there is an effect that the thickness of the steel sheet can be reduced, and the weight of those products can be reduced.

[0002]

2. Description of the Related Art Conventionally, decarburization has been sufficiently performed in a steel making stage to obtain extremely low carbon, and solute C and solute N in steel are reduced to Ti, Nb.
By containing carbonitride forming components, such as carbonitride, these are precipitated as carbonitrides, and a steel to which C and N are fixed is used as a base steel. Many proposals have already been made for the raised high-tensile cold-rolled steel sheets. For example, Japanese Patent Publication No. 63-190141 discloses a cold-rolled steel sheet obtained by adding a large amount of Mn and P to the ultra-low carbon Ti-added steel. In this case, by adding an appropriate amount of Mn and P, a small amount of solid solution C remains after annealing, which significantly improves the average r value and further increases the amount of solid solution C existing at the grain boundary. It is stated that secondary working brittleness is effectively prevented.

[0003] In addition, Si, Mn,
There are several disclosed examples of cold rolled sheets to which a solid solution strengthening component of P is added in combination. However, these components are known to affect the plating properties, and it has been very difficult to hot-dip galvanize a steel sheet containing these components.

[0004]

From the above, the present inventors, of course, have excellent galvanizing properties, a tensile strength of 38 kgf / mm ² or more, and an average r-value of
There is a demand for a hot-dip galvanized steel sheet of 1.5 or more, and a production method that can obtain the desired properties by adding a low-cost alloy and ordinary continuous annealing.

[0005] Accordingly, an object of the present invention is to solve the above-described problems advantageously, and to achieve a very low carbon Ti, Nb,
The B-complex-containing steel as a base steel, a tensile strength of 38kgf / mm ² or more, excellent resistance to secondary work embrittlement and hot-dip galvanizing properties,
Another object of the present invention is to propose a method for producing a high-strength alloyed hot-dip galvanized steel sheet having a high average r-value with an average r-value of 1.5 or more.

[0006]

Means for Solving the Problems In order to achieve the above object, the inventors have paid close attention to the ultra-low carbon Ti, Nb, and B composite added steels and have conducted intensive research. As a result, extremely low carbon Ti,
It has been found that a high average r value can be obtained by maintaining the content of Mn in the Nb, B composite added steel in an appropriate balance with respect to the Si, P content. There is a problem that iron is concentrated on the surface and non-plating tends to occur during hot-dip galvanizing. To overcome this problem, NOF (non-oxidizing furnace) treatment and pickling after brushing have been considered.

[0007] As a result of further studies, the primary and secondary annealing in the annealing, brushing and pickling / plating processes, and the secondary annealing brittleness must be optimized unless the secondary annealing temperature immediately before plating is optimized. The inventor has newly found that the present invention is greatly deteriorated, and has achieved the present invention. This is
Segregation by P is considered to be the main cause, but the detailed reason is unknown. That is, the gist of the present invention is as follows. C: 0.001 mass% or more, 0.007 mass% or less, Si: 0.05 mass% or more, 0.5 mass% or less, Mn: 0.5 mass% or more, 2.0 mass% or less, Ni: 0.8 mass% or less, Ti: 0.01 mass% or more, 0.2 mass% or less, Nb: 0.001 mass% or more, 0.1 mass% or less, B: 0.0002 mass% or more, 0.005 mass% or less, Cu: 1.0 mass% or less, P: 0.03 mass% or more, 0.12 mass% or less, sol Al : 0.1 mass% or less, S: 0.02 mass% or less and N: comprises 0.005 mass% or less, the balance as material a steel slab comprising the composition of Fe and unavoidable impurities, Ar ₃ point (℃) above, Ar ₃ point (℃)
After hot rolling is completed at a rolling finishing temperature range of + 100 ° C or less, it is wound around a coil at a temperature range of 650 ° C or less, scale-removed, cold-rolled, and subjected to primary continuous annealing at a temperature of 750 ° C or more. Temperature T after pickling or brushing
(° C) is 500 ° C or more, and the content of P
A high-strength alloy excellent in secondary work brittleness resistance, characterized by subjecting to secondary annealing in a temperature range satisfying the following formula (1) or (2) expressed as P), followed by galvanizing and alloying treatment. This is a method for producing a galvannealed steel sheet. [Note] T (° C) ≤ 700 (° C)-[(% P)-0.03] x 1000 (° C) --- (1) T (° C) ≥ 700 (° C) + [(% P)-0.03] × 1000 (℃) --- (2)

[0008]

The reasons for limiting the steel composition and the production conditions in the present invention will be described.

C: 0.001 to 0.007 mass%, C greatly deteriorates the average r value when a large amount of solid solution C remains during recrystallization. In addition, since Ti and Nb for fixing solid solution C must also be contained in accordance with the C content, the C content is preferably as low as possible, but the allowable upper limit is 0.007 mass%.
And On the other hand, the lower limit of the C content is preferably as low as possible, but is set to 0.001 mass% from the current steelmaking technology.

Si: 0.05 to 0.5 mass%, Si has a large solid solution strengthening ability and does not significantly deteriorate the average r value, and is therefore an optimal solid solution strengthening component. 0.05mass% for this
It is necessary to contain the above. However, when the content increases, the surface treatment property deteriorates, so the upper limit is 0.5 mas.
s%.

Mn: 0.5 to 2.0 mass%, unlike Si and P, Mn is a component that lowers the transformation point, and by making effective use of this, the grain size of the hot-rolled sheet is made extremely fine. be able to. The grain refinement of the hot-rolled sheet is very effective in improving the average r-value by developing the (111) texture of the annealed sheet from the grain boundaries of the hot-rolled sheet. The effect is obtained when the content is 0.5 mass% or more. On the other hand, since Mn itself is a component that deteriorates the average r value, it is not effective to include a large amount of Mn. If the content exceeds 2 mass%, the plating property is reduced, and a low-temperature transformation phase tends to appear. As a result, the material does not have a ferrite structure, and the average r value is greatly deteriorated. Therefore, the content is 0.5 mass% or more and 2.0 mass% or less. In addition, the balance with the contents of Si, Ni, Cu and P described below is 0.2 ≦ (Si (mass%) + P (mass%)) / (Mn (mass%) + Cu
(mass%) + Ni (mass%)) ≦ 1.0. This is because when the above-mentioned balance is smaller than 0.2, the average r-value is deteriorated, and when the above-mentioned balance is more than 1.0, the transformation point is increased, and it is not possible to reduce the grain size of the hot-rolled sheet.

Ni: 0.8 mass% or less, Ni is mainly contained with the addition of Cu. When Cu is contained, a low-melting phase is formed during hot rolling, which becomes a sesame shedding defect. In order to avoid this, it is effective to add Ni to form a solid solution with Cu.
Moreover, Ni is a component that lowers the transformation point similarly to Cu and Mn, and therefore, the same effect as these components can be expected. However, when the content exceeds 0.8 mass%, a low-temperature transformation phase is likely to appear and the ferrite structure disappears, and the average r value is greatly deteriorated. Therefore, the upper limit of the content
0.8 mass%.

Ti: 0.01 to 0.2 mass%, Ti is a solid solution C,
N and S are contained for fixing as TiC, TiN and TiS. If the content is less than 0.01 mass%, the effect is not sufficient. If the content exceeds 0.2 mass%, phosphide is generated, and the elongation and the average r value are deteriorated. Therefore, its content is 0.01ma
ss% or more and 0.2 mass% or less.

Nb: 0.001 to 0.1 mass%, Nb is used to fix solid solution C by using C as NbC as in Ti. Although solid solution C can be fixed by Ti alone, solid solution C can be fixed more effectively by adding Nb in combination, and the average r
The value can be improved. The effect is not sufficient if the content is less than 0.001 mass%, and if the content is more than 0.1 mass%, hot rolling is performed in an austenite non-recrystallized state, adversely affecting the formability of the annealed material. Therefore, the content is set to 0.001 mass% or more and 0.1 mass% or less.

B: 0.0002 to 0.005 mass%, B is contained to prevent secondary working embrittlement. In particular, when a solid solution strengthening component is contained in an ultra-low carbon steel sheet, secondary work embrittlement resistance is deteriorated, so that B must be contained. The effect is manifested when the content is 0.0002 mass% or more.
If it is contained in excess of ss%, recrystallization of austenite is delayed during hot rolling, the load during rolling is increased, and the material of the annealed sheet is deteriorated. Therefore,
Its content should be 0.0002 mass% or more and 0.005 mass% or less.

Cu: 1.0 mass% or less, Cu is a component that lowers the transformation point like Mn and Ni. By effectively utilizing this, it is possible to make the grain size of the hot-rolled sheet extremely fine. it can. Further, Cu is a component that does not deteriorate the plating property, and therefore is very effective in terms of surface treatment properties. However, if the content exceeds 1.0 mass%, a low-temperature transformation phase is likely to appear, and the ferrite structure is not obtained, and the average r value is greatly deteriorated. Therefore, the upper limit of the content is set to 1.0 mass%.

P: 0.03 to 0.12 mass% P is important as a solid solution strengthening component, and has a solid solution strengthening ability.
It is higher than Si and Mn and also increases the average r value. In order to obtain these effects, the content needs to be 0.03 mass% or more. However, if the content exceeds 0.12 mass%, segregation occurs at the grain boundaries and the grain boundaries are embrittled. Further, it causes center segregation at the time of solidification, and further slows down the alloying speed during galvannealing. Therefore,
Its content should be 0.03 mass% or more and 0.12 mass% or less.

Sol Al: 0.1 mass% or less, Al is a component necessary for deoxidation. When the sol Al content exceeds 0.1 mass%, not only the deoxidation effect is saturated, but also inclusions increase. Adversely affects moldability. Therefore, its content is 0.1
mass% or less.

S: not more than 0.02 mass%, S does not affect the average r value, but if the content exceeds 0.02 mass%, inclusions such as MnS increase, and local ductility represented by stretch flangeability is reduced. May cause a decrease. Therefore, its content is
0.02 mass% or less.

N: 0.005 mass% or less, N is an impurity component unavoidably mixed into steel.
To improve moldability. However, when a large amount of TiN is formed, workability is deteriorated. The upper limit of the allowable content is set to 0.005 mass%.

Next, the reasons for limiting the manufacturing conditions will be described.
You. Hot rolling end temperature: Ar_ThreeDot to Ar_ThreePoint + 100 ° C The hot rolling end temperature must be changed according to the transformation point
is there. Ar_ThreeBelow the transformation point, it becomes two-phase rolling and annealed sheet
Texture that adversely affects the average r value of
U. On the other hand, Ar_ThreeHigh temperature relative to transformation point,
That is, when the temperature exceeds 100 ° C, the grain size of the hot-rolled sheet structure becomes coarse.
And the texture effective for deep drawability does not develop during annealing.
It is not suitable for becoming. Therefore, the hot rolling end temperature
The degree is Ar _ThreeFrom point (° C) to Ar_ThreePoint (° C) + 100 ° C temperature range
Enclose. Winding temperature: 650 ° C or less The winding temperature is determined by adding Ti and P as in the present invention.
If the temperature exceeds 650 ° C, the average r
It is limited by finding things. This
This is because at winding temperatures above 650 ° C, the phosphorus
Chloride precipitates in large quantities on the hot-rolled sheet, the details of which are unknown.
It is presumed that this adversely affects the average r value of the annealed material.
You.

The cold rolling may be performed according to a conventional method.
The rolling reduction may be in the range of common sense. The primary annealing is preferably continuous annealing. In this case, the annealing temperature is set to 750 ° C. or higher so that recrystallization is completed, and the upper limit is preferably set to ₁ point of Ac + 50 ° C. or lower.

Secondary annealing temperature T (° C.): 500 ° C. or higher and T
(° C) ≤ 700 (° C)-[(% P)-0.03] x 1000 (° C)
Or T (° C) ≧ 700 (° C) + [(% P) -0.03] × 1000
(° C.) FIG. 1 shows a graph of the limited range of the present invention in relation to (% P) and the secondary annealing temperature. Regarding the secondary annealing temperature, in this component system, the brittleness is severe near 700 ° C., and the temperature range is affected by the P content. Although the detailed reason for this is unknown, it is considered that the major cause is that the grain boundary segregation of phosphorus in the temperature range is greatly promoted. Therefore, the secondary annealing temperature T (° C.) is T (° C.) ≦ 700 (° C.) − [(% P) −0.
03] × 1000 (℃) or T (℃) ≧ 700 (℃) + [(% P)
−0.03] × 1000 (℃). On the other hand, if the temperature is not higher than 500 ° C., the reduction of the surface of the steel sheet becomes insufficient, and the plating property deteriorates.

[0024]

EXAMPLE A steel slab having the chemical composition shown in Table 1 was hot-rolled at a rolling start temperature of 1250 ° C., wound around a coil, pickled, and then cold-rolled at a rolling reduction of 80%. The primary annealing was performed by performing an alundum bath treatment at the annealing temperature shown in Table 2 for 40 seconds. After the acid washing, the secondary annealing was performed at the annealing temperature shown in Table 2, followed by galvanizing and alloying.

[0025]

[Table 1]

[0026]

[Table 2] About the alloyed hot-dip galvanized steel sheet thus obtained,
Tensile properties, secondary work brittleness resistance (brittle transition temperature), and the like were investigated, and zinc plating was evaluated for plating properties, plating adhesion, alloying speed, and the like. The results of these surveys are also shown in Table 2.

Here, the tensile test was performed using a JIS No. 5 test piece (parallel width 25 mm, gauge length 50 mm) sampled in the rolling direction after temper rolling of 0.8%. The r value was measured using a JIS No. 5 test piece taken from the rolling direction (L), the 45 ° direction (D) and the direction perpendicular to the rolling direction (T), with a gauge length of 25 mm and a 15% tensile strain. Was. And the average r
Values were determined from the average r value _{= (r L + r T +} 2r D) / 4. The secondary working brittleness was determined as the transition temperature at which a cup was formed into a cup with a drawing ratio of 1.8 and then subjected to an impact of 80 cm-5 kg at a predetermined temperature and cracking started.

In addition, the evaluation and judgment criteria for zinc plating are as follows:
It went according to the following. -Plating property Evaluation was made by visual inspection after hot-dip galvanizing. ○ No plating × No plating ・ Plating adhesion Dupont impact test (hemispherical diameter: 6.35 mm (1/4 inch)
Weight: 1 kg, height: 500 mm). ○ No plating peeling × Plating peeling ・ Alloying speed The alloying speed was evaluated based on whether or not the η phase of zinc remained on the surface of the galvannealed steel sheet. ○ Without η phase × With η phase

As is clear from Tables 1 and 2, all of the conforming examples conforming to the present invention in terms of the composition of the components and the manufacturing conditions are all tensile strength: 40 kgf / mm ² or more, average r value: 1.5 or more, brittle An alloyed hot-dip galvanized steel sheet having a transition temperature of −50 ° C. or less and good evaluation of galvanization is obtained.

Next, regarding steel No. 1 shown in Table 1, 2
The brittle transition temperatures were measured at different annealing temperatures.
The primary annealing temperature was fixed at 850 ° C. The results are shown in FIG. FIG. 2 is a graph showing the relationship between the secondary annealing temperature and the brittle transition temperature. When the secondary annealing temperature is within the limited range of the present invention, the brittle transition temperature is equal to or higher than -45 ° C., which is the criterion. It shows that it is excellent in secondary working brittleness.

[0031]

According to the present invention, a high-strength steel with excellent secondary work brittleness resistance is provided by limiting the composition of the ultra-low carbon Ti, Nb, B composite added steel and specifying the secondary annealing temperature range immediately before plating. An alloyed hot-dip galvanized steel sheet is obtained. The steel sheet according to the present invention can be advantageously applied to automobiles, home electric appliances, and the like, and can contribute to their weight reduction.

[Brief description of the drawings]

FIG. 1 is a graph showing a limited range of the present invention in relation to (% P) and a secondary annealing temperature.

FIG. 2 is a graph showing a relationship between a secondary annealing temperature and a brittle transition temperature.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C23C 2/28 C23C 2/28 2/40 2/40 (72) Inventor Toshiyuki Kato 1 Kawasakicho, Chuo-ku, Chiba-shi, Chiba Kawasaki (56) References JP-A-6-179922 (JP, A) JP-A-7-70723 (JP, A) JP-A-7-70724 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60 C23C 2/00-2/40

Claims (1)

(57) [Claims] [Claim 1] C: 0.001 mass% or more, 0.007 mass%
Below, Si: 0.05 mass% or more, 0.5 mass% or less, Mn: 0.5 mass% or more, 2.0 mass% or less, Ni: 0.8 mass% or less, Ti: 0.01 mass% or more, 0.2 mass% or less, Nb: 0.001 mass% 0.1 mass% or less, B: 0.0002 mass% or more, 0.005 mass% or less, Cu: 1.0 mass% or less, P: 0.03 mass% or more, 0.12 mass% or less, sol Al: 0.1 mass% or less, S: 0.02 mass % And N: 0.005 mass% or less, with the balance being a steel slab having a composition of Fe and unavoidable impurities, and a rolling finish temperature of at least ₃ points (° C) and not more than ₃ points (° C) + 100 ° C After completing the hot rolling in the range, it is wound around a coil in a temperature range of 650 ° C or less, and after the scale is removed, it is cold rolled.
After performing primary continuous annealing at a temperature of 750 ° C. or more, pickling or brushing and then pickling, the temperature T (° C.) is 500 ° C. or more, and the value of the above P content is expressed as (% P) below. formula
High-strength alloyed galvanized steel sheet with excellent secondary work brittleness characterized by subjecting it to hot dip galvanizing followed by alloying treatment after secondary annealing in a temperature range that satisfies (1) or (2) Production method. [Note] T (° C) ≤ 700 (° C)-[(% P)-0.03] x 1000 (° C) --- (1) T (° C) ≥ 700 (° C) + [(% P)-0.03] × 1000 (℃) --- (2)