JPS6156245A - Manufacture of molten galvanized steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To obtain the titled steel sheet having extremely superior secondary workability and powdering resistance, by using a steel in which trace B is added to ultralow carbon, Nb bearing steel, and combining a specified heat treatment and molten Zn plating condition, etc. with said steel. CONSTITUTION:The steel consisting of, by weight <=0.0025% C, <=0.05% Si, <=0.30% Mn, <=0.030% P, <=0.020% S, 0.015-0.080% Sol Al, <=0.0050% N, <=0.050% Nb, 0.0005-0.0015% B and the balance Fe while satisfying 5<=Nb/C<= 20 is melted. This is hot rolled at >=Ar3 finishing temp., and 650-800 deg.C winding temp., then said plate is cold rolled by >=65% draft after pickling to obtain cold rolled steel sheet. This is recrystallized at >= recrystallization temp. -850 deg.C, then molten Zn plated, held at 400-600 deg.C for >=5sec after the plating to perform alloying treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a hot-dip galvanized steel sheet for deep drawing, particularly a relatively soft hot-dip galvanized steel sheet.

[Conventional technology and its problems]

In recent years, alloyed hot-dip galvanized steel sheets have been widely used as rust-preventing steel sheets for automobile bodies.In particular, for parts that require deep drawing processing, cold-rolled steel sheets made of low-carbon At-killed steel, which are the base plate for plating, are used in advance. After decarburizing annealing in a box furnace, passing it through a continuous molten galvanizing line (hereinafter referred to as CGL), or performing vacuum degassing treatment during melting of steel to remove as much carbon as possible from the molten steel and remove residual carbon. A method is adopted in which a cold-rolled steel sheet fixed as a stable carbide by addition of t-Ti+Nb or the like is passed through a CGL.

However, in the former case, it is an inefficient method to pass pre-annealed cold-rolled steel sheets through a CGL that has annealing equipment in the line, which increases the burden on the box-type annealing process. The result is not good. In the latter case, solid solution C, which is a powerful grain boundary strengthening element, is fixed as a stable carbide, so in molded parts that require severe deep drawing, it is difficult to use during deep drawing or subsequent secondary processing. There is a problem in that brittle structural fractures called vertical cracks are likely to occur.

To solve this problem, in JP-A No. 59-74232, a steel containing trace amounts of Ti, Nb, and B was used, and the N in the steel was fixed as TiN, so that solid solution B remained.
Nb with Nb in it which does not substantially inhibit the non-aging property of solid solution C
By fixing it as C, some solid solution Ct- remains,
It has been proposed that solid solution B and solid solution C are used to strengthen grain boundaries, improve secondary workability, and further impart BH properties. However, steel containing T1 has the disadvantage that surface flaws are likely to occur due to nonmetallic inclusions such as Ti0z and At203 generated in the steel. For this reason, scratchiness due to surface flaws and plating peeling during severe deep drawing are likely to occur. JP-A-50-31531 and JP-A-54-10
No. 4417 proposes a method for improving surface properties, but it does not necessarily yield favorable results.In particular, since Ti is an element that easily combines with N, S, and O in addition to C, which is an element in steel, Nb, etc. It has the disadvantage that it requires a large amount of addition compared to nft, which inevitably increases manufacturing costs. Furthermore, due to the rapid alloying of steel with Ti mirrors during the galvanizing process, the hard and brittle alloy layer of the Fe IJ latch grows thickly, which significantly reduces the workability of the plated layer and requires severe deep coating. It is easy to cause plating peeling and powdering during drawing process. The w1 diagram is an indicator of deep drawing because the alloy layer of steel containing Ti is harder and more brittle than that of steel containing Nb; it shows an example where the value deteriorates significantly. In this way, with conventional methods, even if secondary workability is improved, problems such as deterioration of deep drawing properties, peeling of plating, or powdering become apparent, and it is difficult to obtain sufficiently satisfactory alloyed galvanized steel sheets for deep drawing. I couldn't get it.

[Means to solve the problem]

The inventors of the present invention have repeatedly conducted various experiments and studies to solve these surface defects, and have found that ultra-low carbon, cutting-added steel with a trace amount of B'f
: An alloy for deep drawing that has extremely excellent secondary workability and powdering resistance by adopting a component system containing the following ingredients and combining special heat treatment and hot-dip galvanizing conditions. It has been discovered that a hot-dip galvanized steel sheet can be obtained.

The feature of the present invention is that C: 0.0
025wt% or less, 81: 0.05wt% or less, M
n: 0.30 wt% or less, P: 0.030w
t% or less, S: 0.020wt% or less, 5oZAZ
: O-015~0.080wt%, N: 0.0
050 wt% or less, Nb: 0.05 wt% or less, B: 0.0005 to 0.0015 wt%, the balance consisting of iron and inevitable impurities, and satisfying 5 < Nb/C20, After hot rolling at a finishing temperature of 55°C or higher and a coiling temperature of 650 to 800°C, the cold rolled steel sheet is pickled and then cold rolled at a rolling reduction of 55cm or higher, and the resulting cold rolled steel sheet is heated to a recrystallization temperature. After recrystallizing at a temperature of above 850°C or below, hot-dip galvanizing is applied, and after the plating 40°C
The purpose is to perform alloying treatment by holding the temperature at 0 to 600°C for 5 seconds or more.

The reasons for limiting the component composition and manufacturing conditions of the present invention will be explained below.

C is an index of deep drawing properties. In order to increase iI, it is preferable to reduce # as much as possible, and if there is less C, the amount of Nl required for fixation as carbide (NbC) can be reduced. .

From this point of view, C is restricted to 0.0025 wt% or less. Such extremely low C can be easily obtained industrially by vacuum degassing treatment.

In the present invention, Sl is preferably #lily, with a small amount of 0.05
If it exceeds wt %, there is a problem that the plating adhesion deteriorates, so it is set to be less than o, oswt*.

Mn does not contribute to deep drawability, and is set at 0.30 wt% or less due to the relationship with S described below. Further, if an attempt is made to significantly reduce Mnji, the cost will increase, so 0.04 Wt is actually the lower limit.

P is an element that increases the strength of steel sheets, significantly deteriorates secondary workability, and also deteriorates plating adhesion.
For this reason, 0. (+30 wtl or less.

Since S combines with Mn to form MnS'i and deteriorates the cleanliness of the steel, it is preferable to have a small amount of S, and it is necessary to set Mn/S≧5 to prevent hot brittleness during hot rolling. (Mnl is normally 0.10% or less when not actively added), so it is regulated to 0.020 wt% or less.

S o tkl is required as a deoxidizing agent during melting, and at least 0.015 wt% or more is required to fix it as solid solution N''i AtN, which causes strain aging.
If it exceeds 0.080 wt%, the steel will be hardened and its ductility will be reduced. Therefore, sou'-t is 0.015 to 0.080w
It is assumed to be t%.

Since N causes a decrease in ductility, it is preferable to have less N.
It is regulated to 0.0050wt% or less.

B is one of the main elements of the present invention, and when added in an extremely small amount, the secondary processability is significantly improved.

In other words, and B are o and o in order to obtain sufficient secondary workability.
005wt% or more is required, but 0.0015w
If it exceeds t%, the ductility and deep drawability will be significantly reduced, so the limit is set at 0.0015%. Furthermore, the effect of adding B is to improve the powdering resistance along with the improvement in plating adhesion. The reason for this is not clear enough, but B is an element that tends to concentrate on the surface during annealing before plating, and has the effect of suppressing alloying in Fe'J. It is presumed that this is to suppress the development of an alloy layer that would damage the material. From this point of view, the amount of B added should be 0.0005 wt% or more, and there is no need to particularly regulate the upper limit, but as mentioned above, from the material standpoint, Olo 015 wt% is the upper limit. to snatch

Nb is also one of the main elements in the present invention, and C in steel
By fixing it as l carbide (NbC), deep drawability and ductility are significantly improved.

Nb/C〉s is sufficient to obtain the properties necessary for deep drawing properties, but to obtain more stable properties, Nb/C≧8
is desirable. However, increasing Nb/C more than necessary will not only increase manufacturing costs but also reduce ductility, so the upper limit is set at 0.050wt% and Nl)/
It is regulated that C≦20. Furthermore, when secondary workability is important, 5<Nb/C (8 is the most desirable) where solid solution C remains.

In this range of 5 < Nb/C (8), extremely excellent secondary workability can be obtained by adjusting the B addition amount in the range of 5 to 15 ppm. The extremely excellent properties are due to the synergistic effect of solid solution C and solid solution B.

Figure 2 shows the grain size m (A
This figure shows the relationship between STMN [l] and vertically divided limit temperature (secondary workability). The vertical cracking limit temperature here is a drawing ratio of 2.0 at which all of the test materials can be easily formed. 50 in l
This cup was immersed in a low-temperature bath for 3 minutes, and a conical cone with an opening angle of 60 mm was pressed to reach the limit temperature at which brittle vertical cracks occurred on the side wall of the cup. As shown in this figure, the larger the grain size, that is, the smaller the crystal grain size, the lower the longitudinal cracking limit temperature shifts. In this case, the influence of grain size on the longitudinal cracking limit temperature is as follows:
Experimental results have shown that as the number increases, the vertical splitting limit temperature shifts to the lower side by about -10°C. In other words, when evaluating the secondary workability of gold, all grain sizes must be considered. The composite addition steel of Nb and B, which corresponds to the present invention, not only has well-organized grains, but also has superior secondary workability than Nl) addition steel without B addition when compared in the same grain size order. It has extremely superior secondary workability compared to T1-added steel.

In the present invention, a trace amount of B is added to allow solid solution B to remain, and the mechanism thereof is as follows.

That is, according to what the present inventors confirmed through experiments, BN
The precipitation initiation temperature of ib is in the temperature range of about 900 to 1100°C.
, its precipitation peak is in the temperature range of about 600-700°C. Furthermore, the smaller the amount of B added, the lower the BN precipitation start temperature and the precipitation peak shift to the lower temperature side. Since the precipitation initiation temperature of BN is high, the temperature dependence during hot rolling coiling is small, and precipitation occurs over a wide temperature range. Furthermore, since BC-based compounds are also precipitated preferentially over BN, most of the total B amount is precipitated as B(C,N). The precipitation start temperature and precipitation peak of BN in the present invention are as follows at a B addition amount of 14 pprrl.
It has been confirmed that these temperatures are around 900°C and 600°C, respectively, which are in a lower temperature range than the generally known precipitation peak of AtN.

In the hot rolling coiling temperature range of 650 to 800°C in the present invention, most of the solid solution N remaining in the ladle preferentially precipitates as AtN, so some B cannot be precipitated as BN and is dissolved in solid solution. It remains as B. Furthermore, in the present invention, since the steel is made of ultra-low carbon steel, the amount of precipitated BC compounds is small, which also contributes to the increase in solid solution B.

In short, in the present invention, most of the solid solution C is fixed as NbC by adding Nb to ultra-low carbon steel, suppressing the precipitation of BC-based compounds, and adding a small amount of B to the steel.
By making the precipitation peak of AtH lower than that of AtH, the precipitation of B(C,N) -41Tr is suppressed and a large amount of solid solution B remains. Solid solution BJi in this case
is 3 to lOppmg degree, and a sufficient improvement effect on secondary processability has been confirmed. As described above, the mechanism for allowing the solid solution B to remain in the present invention is based on the above-mentioned Japanese Patent Application Laid-open No. 5
This is fundamentally different from that of No. 9-7423, and furthermore, with the combined addition of trace amounts of B and Nb in the present invention, sufficient deep drawing properties can be obtained with minimal material deterioration.

In the present invention, steel having the above-mentioned composition is melted by vacuum degassing treatment, etc., and the obtained slab is hot-rolled at a finishing temperature of Ar1 or higher and a coiling temperature of 650 to SOO℃, and then After pickling, the copper strip is cold-rolled at a reduction rate of 65 inches or more, and the cold-rolled copper strip is recrystallized at a temperature above the crystallization temperature and below 850 degrees Celsius, and then hot-dip galvanized, and after the plating is
Alloying treatment is performed by holding the temperature at ~600°C for 5 seconds or more.

To explain the reason for limiting the manufacturing conditions, first, setting the finishing temperature to Ar3 or higher in hot rolling is because
This is because, at temperatures below 1000 yen, the r value, which is an index of deep drawability, tends to decrease, and ductility and secondary workability deteriorate due to coarse grains caused by low-temperature finishing.

The reason why the coiling temperature is set in the range of 650 to 800° C. is to fix solid solution N, which causes strain aging, as AtN, and to allow a large amount of solid solution B to remain as described above.

In cold rolling, the reduction ratio is set to 65 inches or more in order to obtain a sufficient rolled shape and improve the r value.

The annealing temperature must be higher than the recrystallization temperature in order to have sufficient deep drawability, and this improves both the r value and the total elongation. Particularly at temperatures above 750°C, extremely excellent properties are obtained. However, in high temperature ranges exceeding 850℃, N1
) A significant increase in solid solution C is likely to occur due to redissolution of C (, B
Although it contributes to H properties, it causes deterioration in aging properties and deep drawability. For this reason, the annealing temperature was set to be at least the recrystallization temperature, preferably at least 750°C and at most 850°C.

The reason why the alloying treatment temperature is limited to 400 to 600°C is 4.
At temperatures below 00°C, sufficient alloying is not achieved, and at temperatures below 600°C,
If the temperature is exceeded, the powdering resistance will be significantly impaired due to overalloying, and in order to achieve sufficient alloying, it is necessary to hold the temperature in the above temperature range for 5 seconds or more.

The n-h steel plate plated as described above was heated to 200°C.
After being rapidly cooled to below, pressure regulation or leveling is performed. The purpose of rapid cooling to 200° C. in this way is to prevent grain boundary concentration of P, which is harmful to secondary workability, and in this case, it is sufficient that the cooling rate is 10° C./sec or higher.

The steel sheets manufactured in this way are less prone to brittle fractures called vertical cracks under the severe conditions of deep drawing, and have extremely excellent secondary workability, as well as powdering resistance. It also has excellent t'LfC characteristics.

〔Example〕

Alloyed hot-dip galvanized steel sheets were manufactured according to the composition and manufacturing conditions shown in Table 1, and their mechanical properties, secondary workability, and powdering resistance were investigated. As a result t-#i
It is shown in Table 2.

However, all mechanical properties were determined using test specimens specified in JIS No. 5. All of the material can be formed by cup drawing at drawing ratios of 2.1 and 3.0.
This cup was immersed in a low-temperature bath and pushed into a conical cone with an opening angle of 60 mm, and the temperature was set at the critical temperature at which brittle longitudinal cracking occurred on the side wall of the cup. Powderability is determined by a 90 bending test and a drawing speed of 200 wV'min.

Pressing with a 0.5R punch tip was performed using a drawbead test with a load of 500 kg in duplicate, and the damaged plated surface was peeled off with tape and evaluated by the amount of zinc attached to the tape.

Figure 3 shows the relationship between the drawing ratio and the vertical split n limit temperature for representative examples of the invention material and comparative materials.
Steel No. 7 is a comparative material of the present invention with a composite addition of B and Nb, steel No. 7 is a comparative material with a single addition of Nb, and steel N112 is a comparative material with a single addition of Ti. For deep drawing, it is generally necessary to have workability with a drawing ratio of 3.0 or more. However, the higher the drawing ratio, that is, the more severe the deep drawing process, the higher the vertical division n limit temperature shifts, and the secondary workability deteriorates significantly. This point n
The same is true for the non-additive material with the addition of Nby6, but it has excellent secondary workability even in ryohoku with a high drawing ratio when compared to the comparative material with only Nb added and even the material with only Ti added. .

Figure 4 shows the influence of the vertical cracking limit temperature due to the addition of B at a drawing ratio of 3.0 for each sample material in the example. The range of B of the present invention is higher than the rLi comparison material, the mark e is a comparison material with Nb added without B, and the mark △ is a TI addition ratio nail material. As is clear from this example, as the amount of B added increases, the longitudinal cracking limit temperature shifts to the lower temperature side, and excellent secondary workability is obtained when B is 5 pprn or more.

Figure 5 shows the relationship between Nb/C, r value, total elongation, and longitudinal cracking limit temperature for each sample material of the example, and is illustrated with the same symbols as in Figure 4. In order to obtain excellent deep drawing properties, it is desirable that the 7 value, which is an index of deep drawing properties, be high and that the total elongation be high. When Nb/C is around 8.0, both the i value and the total elongation are the highest, and excellent deep drawing properties are obtained. When Nb/C<8.0, both the i value and the total elongation begin to decrease and the deep drawing properties deteriorate.

However, the vertical cracking limit temperature rapidly shifts to the lower temperature side, and the secondary workability improves markedly. From this, the lower limit of the Nb/c ratio that is allowable for deep drawing p is Nb/C≧5.0, and a stable high i value can be obtained when N′b/C≧8.0. On the other hand, it is clear that Nl)/C<20 is preferable because the total weight gradually decreases and the manufacturing cost also increases.

As mentioned above, the effect of B addition is that the vertical cracking limit temperature is Nb
Even when added alone, the temperature shifts to the low temperature side and secondary workability is significantly improved, but the i value and total elongation deteriorate.

In particular, when B is 20 ppm or more, material deterioration is significant, so even if secondary workability can be improved, it is difficult to obtain sufficient deep drawing properties.

When B is 15 ppm or less, the degree of material deterioration is slight, and a material that is sufficiently acceptable for deep drawing cannot be obtained.
Moreover, when B is 5 PI)m or more, excellent secondary workability can be obtained, which is advantageous for severe deep drawing processing.

Furthermore, in the 900 bend test and the drawbeat test for evaluating powdering properties, it can be seen that the material of the present invention exhibits better powdering resistance than the comparative material.

[Brief explanation of drawings]

FIG. 1 shows the relationship between the i values before and after peeling of each plating alloy layer in deep drawing of ultra-low carbon Ti-added steel and ultra-low carbon Nb-added steel. FIG. 2 shows the relationship between grain size and vertical division n limit temperature for an alloyed hot-dip galvanized steel sheet. FIG. 3 shows the relationship between the drawing ratio and the longitudinal cracking limit temperature for representative examples of the present invention materials and comparative materials in Examples. FIG. 4 shows the influence of the addition of B on the longitudinal cracking limit temperature at a drawing ratio of 3.0 for each sample material of the example. Figure 15 shows the relationship between Nb/C, i value, total elongation, and vertical cracking limit temperature for each sample material of the example.

Claims (1)

[Claims] C: 0.0025wt% or less, Si: 0.05wt% or less, Mn: 0.30wt% or less, P: 0.030wt%
Below, S: 0.020wt% or less, SolAl: 0.0
15-0.080wt%, N: 0.0050wt% or less, Nb: 0.050wt% or less, B: 0.0005-0
．． 0015wt%, the balance being iron and unavoidable impurities, and satisfying 5≦Nb/C≦20 is melted, and then Ar
After hot rolling at a finishing temperature of 3 or more and a coiling temperature of 650 to 800℃, the cold rolled steel sheet is cold rolled at a rolling reduction of 65% or more after pickling. A method for producing a hot-dip galvanized steel sheet for deep drawing, which comprises recrystallizing it at a high temperature, then hot-dip galvanizing it, and after the plating, holding it at a temperature of 400 to 600°C for 5 seconds or more to perform an alloying treatment. .