JPS5819465A - Manufacture of galvanized steel plate with superior press formability - Google Patents

Abstract

PURPOSE:To efficiently manufacture the titled steel plate at a low cost by adding a very small amount of B to a low carbon steel with a restricted N content and hot rolling the steel at a specified finish temp. CONSTITUTION:A steel consisting of <=0.010% C, <=0.30% Si, 0.05-0.30% Mn, <=0.070% sol. Al, <=0.0050% N, 0.0010-0.0050% B and the balance essentially Fe or further contg. 0.001-0.100% one or more among Ti, Nb, Zr as required and V is hot rolled at 850-900 deg.C finish temp. and coiled. It is then cold rolled at >=60% draft, annealed at the recrystallization temp. -<= the A3 transformation point, and galvanized.

Description

[Detailed description of the invention] This invention relates to a method for producing plated steel sheets.

There are various methods for producing hot-dip galvanized steel sheets, but a method using a continuous galvanizing line having an in-line annealing furnace, such as the Sendzimir method, is considered to be advantageous and has become popular. However, since such in-line annealing methods involve rapid heating, short-time soaking, and rapid cooling, the resulting products generally have poor press formability, and for this reason, conventional hot-dip galvanized steel sheets have been used for was limited to types of books that did not require much press formability. However, recently hot-dip galvanized steel sheets have been recognized for their excellent anti-corrosion ability, and there is a growing demand for their use in large quantities in applications that require high press formability, such as automobile parts. In response to such demands, various proposals have been made to impart high press formability to hot-dip galvanized steel sheets. That is, the following methods are currently available for producing hot-dip galvanized steel sheets with excellent press formability using an in-line annealing continuous hot-dip galvanizing line.

b) Perform overage treatment.

口) Use pre-annealed material as the material.

c) Degassing and ultra-low EC) - Used as a so-called 11m pipe material such as [Ti]-added steel.

The main purpose of method A) above is to improve workability (mainly ductility) and aging property through supersaturated co-cut carbon precipitation, but it has the problem that it has no effect on improving deep drawability. be.

In addition, the method described above involves box annealing after cold rolling and before processing in a continuous melt plating line.
It is possible to produce a steel plate with excellent deep drawing properties. However, this method has problems in terms of efficiency in that box annealing is performed, and since an extra step is required for this, there is a problem in that it is extremely disadvantageous in terms of manufacturing costs. Furthermore, method c) only needs to ensure an annealing temperature higher than the recrystallization temperature, and therefore has the advantage that the annealing process is very simple. ), it is necessary to add a large amount of (N), several times the amount required to completely fix it, and there is a problem that the manufacturing cost of the material becomes extremely high. As described above, the conventional method has some drawbacks in terms of the material of the steel plate obtained and the manufacturing cost.

The present invention has been devised to solve the above-mentioned conventional problems, and aims to provide a method for efficiently producing hot-dip galvanized steel sheets with excellent press formability at low cost. Its characteristics are: [C]: 0.010% or less, [Si]: 0.30% or less, Low] = 0.05 to 0.30%, (SotAt'l:
0.070X or less, [N]: 0.0050% or less,
[B]: 0.0010~0゜0050%, remainder [F.
] and unavoidable impurities, the steel is rolled at a finishing temperature of 850 to 900°C in the hot rolling stage, rolled before and after, and then cold rolled at a rolling reduction of 60% or more, and then rolled at a temperature of A above the recrystallization temperature.
The method involves annealing at a temperature below the s transformation point, followed by hot-dip galvanizing. In addition to the above component system, other features include [Ti
) (Corner) ["Zr] The purpose is to contain one or more of (V) in a total of 0.001 to 0.100%. As a result of the above, a hot-dip galvanized steel sheet with excellent press formability can be obtained. It becomes possible to manufacture containers efficiently and at low cost.

The chemical components according to the present invention are adjusted within the following ranges.

[C): 0.010% or less [81]: 0.30% or less [nu>]: 00.05-0.30%5otAt): 0.070% or less [:N]: 0.
0050% or less [B]: 0.0010 to 0.0050% In addition, the present invention further includes [Ti], [Nb] in addition to the above basic components.
, [Zr:], and [V) in a total amount of o, oot to 0.100%.

In the present invention, the reason why the component range is limited as described above is as follows.

[C] is reduced to 0.010% or less by degassing treatment.

The lower [C] is, the more preferable it is, the better the deep drawability and aging resistance will be. (C:] is preferably less than 0.00!S%, but it is defined as 0.010% or less based on the current degassing equipment capacity.

[Other] is an element that impairs the adhesion of zinc plating, and therefore 0.30. %' is the maximum.

When [Mn) exceeds 0.30%, deep drawability deteriorates rapidly. [Seed] From the viewpoint of deep drawing housing improvement, the lower the 5- is, the better, but considering the problems of steel properties and hot embrittlement, the lower limit is set to 0.
．． 05%.

(8oAAj) is used for deoxidation, and [”B]/[N
)〈In the case of 1, the solid solution [N] that cannot be fixed in (B) is
It is added to fix it as tN, 0.070
The upper limit is %. Adding more than this is not preferable because it increases the cost.

[N] is inevitably mixed, but the lower the better, the upper limit is o, ooso%, and the amount of 0 [N] will be described later [
B] Although it is closely related to the amount added, [N] is 0.005
If it exceeds 0%, a large amount of nitrides will be produced, which will inhibit the growth of ferrite grains during annealing and deteriorate workability. Moreover, if the amount of [N) is large, the amount of added elements for fixing [N] in solid solution increases, which is disadvantageous in terms of costs.

CB) is the most important additive element in the present invention.0 [B] is an element that has a strong affinity with [N], and combines with [N] to form BN, which has the effect of improving aging resistance. Some things are already known. The present inventors have discovered that by combining this steel with a small amount of CB) and a specific hot rolling finishing temperature, a steel plate with excellent deep drawability can be obtained even in rapid heating annealing such as continuous annealing. This is what we discovered. [”B
] The above effect obtained by the combination of the two conditions of addition and specific hot rolling finishing temperature will be described in detail later, but it is thought to be based on the effect of adjusting the austenite grain size of hot rolling. The amount of CB specified in the present invention is 0.0010 to o
, ooso%. When CB) is added alone for the purpose of fixing [N], it is necessary to satisfy the condition (BE/[N]≧10; however, if the reverse pressure ratio becomes too large, solid solution [
B) remains and has an adverse effect on the press formability of the product.

In the present invention, nine (B) is added to obtain the effect of grain adjustment during hot rolling and the effect of improving deep drawing properties after cold rolling annealing.
[:Bl/[:N) need not be equivalent. Solid solution (N
), the purpose can be sufficiently achieved by adding (SojAt) (Tl) or the like. However, it is necessary to avoid deterioration of press formability due to solid solution CB, and in this sense, C
The upper limit of B) was set to o, ooio% in relation to the upper limit of [N]. Further, if [B] exceeds 0°oosoXt-, there is a problem that edge cracking of the slab occurs and the slab becomes rough. The lower limit of CB) is 0.0010%, and below this, the effect of adding CB) cannot be obtained.

[Ti) (Scary) Regarding [Zr'l (V)], these elements are vertical compound forming elements, and these can be added singly or in combination to completely fix solid solution [N] and solid solution [C]. ] If these elements are added sufficiently, so-called 1.F in which the solid solution (C) (N) is completely fixed.
, the product becomes steel, and the product after annealing becomes non-aging. but,
In past examples aimed at achieving this non-aging property, the amount of added elements was required to be several times the amount required to completely fix the solid solution [C][N], which was extremely disadvantageous in terms of cost. As mentioned above, the main focus of the present invention is on cost, and the solid solution (N) is completely fixed, but the solid solution [C] is limited to the minimum amount necessary to partially or completely fix the solid solution [C]. The upper limit shall be 0.100% for the total of individual or combined additions.
And so. Further, if the total amount is less than 0.001%, the effect of the addition cannot be obtained, and therefore the lower limit was defined as above.

In addition to the above components, CP) is added as necessary for the purpose of adjusting the strength level of the product. This (Pl) is an effective element for increasing strength without impairing press formability, and if necessary o, o a O ~ 0.1 s
o X is added. If it is less than 0.030%, no strength-increasing effect can be expected, and if it exceeds 0.150%, it impairs plating adhesion and causes deterioration in weldability and workability, which is not preferable.

In the present invention, as a manufacturing condition for steel made of such components, finish rolling is performed at a finishing temperature in the range of 850 to 900°C after the hot rolling stage, and this is one of the major characteristics of the present invention.
By combining this heat treatment with the above component system, especially [B], suitable surface press formability can be obtained. This seems to be based on the austenite grain size of hot rolling as mentioned above. Figure 1 shows CB) additives (B: o, ooss,
%, coiling temperature: 660°C) and the relationship between the hot-rolled sheet ferrite grain size and the hot-rolling finishing temperature. As shown in No. 111, when the hot-rolling finishing temperature exceeds 50oct, the grain size of the hot-rolled sheet suddenly increases. The cause of this is not clear, but it is believed that the solid solution (B) reduces the frequency of occurrence during transformation, and that the ferrite grain size itself is not large due to the extremely low (C) steel. Conceivable. At a finishing temperature of 850 to 900°C (shaded area in the figure), the appropriate 7-item grain size is exhibited, but this is because BN precipitates due to the temperature drop and the solid solution (B) decreases, resulting in the above-mentioned transformation. Elimination of the effect of reducing the frequency of occurrence and precipitation of 9BN
It is presumed that this is caused by the presence of precipitates that inhibit grain growth. Furthermore, when the finishing temperature is lower than 850°C, a low temperature finishing layer begins to appear, so the average ferrite grain size becomes large. Figure 2 is the same as Figure 1, with CB) additives (
[B): o, o o s s%, coiling temperature 26
60℃, annealing temperature: yso℃), and the relationship between the hot-rolling finishing temperature (deep drawability value) and YP value of the plated steel sheet.As can be seen from this, the ferrite grain size of the hot-rolled sheet When the temperature exceeds 9, the deep drawing properties of the product are considerably inferior to those of the appropriate grain size (the shaded area in the figure). This is thought to be due to the extremely large grain size and the appearance of a low-temperature finish structure at a finishing temperature of less than 850°C.

The steel plate finish-rolled at the finishing temperature as described above is wound into a coil, but the winding temperature KII is not limited to 41. That is, low-temperature to high-temperature deposition of about sso to 720° C. is performed.

Next, the steel plate is descaled, then subjected to normal cold rolling at a rolling rate of 60 to 90X, and then sent to an in-line annealing continuous hot-dip galvanizing line where it is annealed and hot-dip galvanized. The annealing temperature is above the recrystallization temperature and below the transformation point. Within this temperature range, ferrite grain growth progresses as the temperature increases and a product with excellent deep drawability can be obtained.
Even with annealing at c@ degree, a material with good quality comparable to that of a cold-rolled steel sheet obtained by box-annealing At-killed steel can be obtained.
C] The effect of rounding, which is targeted at steel, does not depend on the cooling conditions after annealing, and therefore the cooling conditions are not particularly specified.In addition, the above composition range and heat treatment are sufficient for the present invention to obtain the desired properties. In order to obtain this steel, it is necessary to undergo an over-aging treatment, which is normally required for this low carbon steel. This is the same as a normal plating method.Although zinc plating is sometimes subjected to alloying treatment for the purpose of improving its corrosion resistance, the effects of the present invention do not change in any way.

Examples Chunin Steel to E steel in Table 0 are steels of the present invention, which are made by melting steel having the chemical composition shown in Table 1 and making slabs by continuous casting.
A steel, B steel are extremely low [C)-CB) system, C steel is extremely low [C]
-CB) -(Ti) system, D steel is extremely low (C) -CB
) -(P) series, E steel has extremely low (C) -CB) -[T
i) -CP) system. Moreover, G steel to L steel are comparative steels.

The above slag was hot rolled into 18 plate needles under various hot rolling conditions shown in Table 2. Then, after descaling by pickling, the plate thickness was reduced to 0.
8■ (rolling ratio 71.4%) K cold rolling. Thereafter, the sheet was passed through a continuous hot-dip galvanizing line having an in-line annealing furnace for annealing, and hot-dip galvanizing was applied.

The annealing temperature was 760°C or 850°C.

The accuracy results of the product after pressure adjustment (test piece: JISs number, rolling direction) are shown in Table 2. Note that A (1) steel to mu (s) steel in Table 2 are A steel processed under various hot rolling conditions and annealing conditions, and among these, A (2) steel and A (4) steel , Mu(s) Table 2, F steel has a high content of [C], no addition of G steel wire [B], and H#1 company (Ti) has a high content of [C].
．． Although 0.017% is added, there is no addition of [B], and the content of [C] is high, both of which are outside the scope of the present invention, and this value is low. Saraki steel is CB)
The content of J steel is high, and the content of [seed] is high, both of which are outside the scope of the present invention, and the i values of these are also low. Steel X has a high content of [Si], which is outside the scope of the present invention, and therefore has a low r value and poor plating adhesion. The L steel has a high content of CP (CP) which is outside the above range, and as a result, both the i value and the Et value are low and the plating adhesion is poor.

On the other hand, steel B and steel E are steels of the present invention, and both have good mechanical properties.

Among M & A steels, the invention steels A(2) steel, A(4) steel and A(5) steel have good properties, but A(1) steel has good properties.
Steel and A(3) steel have finishing temperatures outside the range of the present invention (910° C., 830° C.) in the hot rolling stage, and therefore have low i values.

C steel and E steel are examples according to the second invention of the present application,
and 0.03ON of [Tl), which is a carbonitride-forming element, in 4.
, 0.0!13N is added, and according to this, i
In addition to showing good properties in terms of mechanical properties such as value and plating adhesion, other steels of the present invention show values of 3.0 to 3.5 #/- especially regarding aging index. 1.0~i, s by/w? It is a relatively low value, and it can be seen that moderate aging resistance has been obtained compared to other steels of the present invention. However, when looking at the steel of the present invention as a whole, the aging index for evaluating aging resistance is at a level of about 1 to 3.5ψ, and this state is not completely non-aging due to the presence of a small amount of solid solution (C). However, the recovery of yield point elongation due to aging and the amount of aging deterioration of the material hardly poses a problem in practice, so it can be said that it has slow aging properties. Conversely, the presence of such a small amount of solid solution [C] means that it has bake hardenability, and high yield strength can be expected after it is manufactured into products such as automobile parts.

In addition, in the present invention, since CB) is added, it is possible to make the material of the product uniform in the longitudinal direction or width direction of the coil. This is an invention with extremely high practical value, as hot-dip galvanized steel sheets with excellent formability can be manufactured efficiently and with low production costs without adding large amounts of special elements or special processes. It can be said that there is.

[Brief explanation of the drawing]

FIG. 1 shows the relationship between hot-rolling finishing temperature and hot-rolled sheet ferrite grain size of steel sheets according to the composition range of the present invention. Second
The figure also shows the relationship between the hot rolling finishing temperature of a steel sheet and the i value and YP value of the pressure bale according to the composition range of the present invention. Figure 1: Heat 1 can Temperature (0C) Figure 2: Mugien (0C)

Claims (2)

[Claims] (1) C: 0.010% or less, B1: o, ao% or less, Mn: 0.0! i ~0.30%, 5oth
: o, 0.070% or less, N: 0.0050% or less,
B: 0.0010 to 0.0050%, balance Fe and unavoidable impurities, the steel is heated to a finishing temperature of 85% during hot rolling.
After rolling at 0-900℃, winding, then rolling rate 60%
Method 0 for producing a hot-dip galvanized steel sheet with excellent press formability, characterized in that after cold rolling as described above, annealing is performed at a temperature above the recrystallization temperature and below the As transformation point, followed by hot-dip galvanizing. (2) c: 0.010% or less, Si: 0.30
% or less, Mn: 0.05-0.30%, 5oLA1
: 0.070% or less, N: 0.0050% or less, B
:0.0010~o, ooso%, Ti*Nb@
One or more of Zr + V has a total of 0.
001~0.100%, the balance is Fe and unavoidable impurities, and the finishing temperature is SSO~900℃ during the hot rolling stage.
A molten metal with excellent press formability, which is characterized by being rolled, then coiled, then cold rolled at a rolling rate of 60% or more, annealed at a temperature above the recrystallization temperature and below the transformation point, and then hot-dip galvanized. Method of manufacturing galvanized steel sheet.