JPH0472017A - Production of baking hardening type hot-dip galvanized steel sheet having high workability - Google Patents

Abstract

PURPOSE:To obtain a plated steel sheet excellent in workability and ageing characteristics add having moderate baking hardenability by hot-rolling a slab of a steel with a specific composition and successively subjecting the resulting plate to cold rolling, decarburizing annealing, hot-dip galvanizing, and temper rolling under respectively specified conditions. CONSTITUTION:A slab of a steel having a composition which consists of, by weight, <=0.0050% C, <=0.04% Si, 0.10-0.60% Mn, <=0.080% P, 0.002-0.010% S, 0.030-0.100% Al <=0.0030% N, 48/14N(%)-{48/14N(%)+48/32S(%)} Ti, and the balance Fe with inevitable impurities and in which Mn(%)+10P(%) <=1.20 is satisfied is hot-rolled and then cold-rolled at >=50% cold rolling rate, and the resulting sheet is subjected to decarburizing annealing at a temp. between the recrystallization temp. and 900 deg.C for >=40sec under an atmosphere having a dew point between -30 and +15 deg.C to undergo the regulation of C after this decarburizing annealing to 0.0010-<0.0030%. Subsequently, the above sheet is subjected to hot-dip galvanizing and further to temper rolling at 1.0-2.0% elongation percentage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention has excellent processability and aging properties, and has a property of 2.5 to 3.
The present invention relates to a method for producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having an appropriate bake hardenability of 9 kg/*m''.

(Prior art) In recent years, galvanized steel sheets, especially alloyed hot-dip galvanized steel sheets,
Because of its good corrosion resistance and paintability, its demand as an automotive material is rapidly expanding, and along with this, there is a need for further improvements in properties, particularly in terms of processability.

By the way, materials for automobiles are required to have excellent workability so that they can be press-formed into complex shapes, and also to have high strength in consideration of reducing the weight of the vehicle body for energy saving.

That is, in order to have excellent workability, it is necessary to have a low yield point and a high f value, and on the other hand, in order to have high strength, it is necessary to have a high yield point.

(Intelligence B to be Solved by the Invention) By the way, materials with high strength generally have low elongation and t value, and tend to have poor workability.

Therefore, as a method to achieve both of these contradictory required properties, a so-called bake-hardenable steel sheet has been developed (Japanese Patent Application Laid-Open No. 62-112731, Japanese Patent Application Laid-open No. 6115763).
9, JP-A-61-26757, JP-A-59
-31827, Japanese Patent Application Laid-open No. 57-70258)
.

This bake hardenability utilizes the strain aging of the steel plate, and is a property in which the steel plate ages and hardens due to the heat during baking of the paint and its yield point increases.When forming, the yield point is low and it is easy to process. Since the baking coating after molding increases the yield point and increases the strength, it is possible to simultaneously satisfy the above-mentioned contradictory required characteristics.

However, care must be taken when imparting bake hardenability. In other words, increasing the bake hardenability is to age the steel plate and make it stiffer, which deteriorates the age hardenability at room temperature and causes room temperature aging. Therefore, between manufacturing and molding, age hardening increases the yield point and reduces workability, and yield point elongation (YPE) occurs during molding, which is called stretcher strain. This results in fatal wrinkle-like defects that cannot be repaired. Generally, it is said that clear room temperature aging occurs when the amount of bake hardening, that is, the increase in yield point after baking exceeds 5 kg/ms. However, according to detailed research by the inventor, the amount of bake hardening It is considered that when the temperature exceeds 4 kg/mm, the room temperature aging property cannot be ignored.

Until now, bake-hardenable steel sheets have mainly been cold-rolled steel sheets, so in order to control the amount of bake-hardening, medium-low carbon steel is box-annealed to fix N as AQN, and excess C is precipitated and hardened. The conventional method was to control the amount of dissolved C. However, when manufacturing a plated steel sheet, there is a drawback that the C precipitated by the heat during the plating process re-dissolves into solid solution, and it is difficult to control the amount of solute C even if an overaging treatment is performed after plating.

Furthermore, in recent years, continuous annealing has become mainstream in order to improve productivity, and it is said that it is difficult to sufficiently control the amount of solid solute C when medium-low carbon materials are used.

Therefore, techniques have been developed to use extremely low C materials as bake-hardenable steel sheets for continuous annealing.

During steel manufacturing, C, N, etc. are sufficiently reduced through vacuum degassing treatment, N is precipitated as AQN during coiling after hot rolling, and all C in the steel is solid solution C. Alternatively, the amount of solid solute C is controlled by adding a small amount of carbonitride such as Ti or Nb to precipitate some C as TiC or NbC. However, in order to set the amount of bake hardening within a specific range (for example, 2.5 to 3.9 kg/++v+"), it is necessary to keep the amount of solid solute C within a somewhat narrow range, but these methods The solid solution C amount is C, N, Ti
, Nb, S, etc., so the solid solution C
It is difficult to control the amount sufficiently, and solid solution C
Since the amount is almost determined at the time of casting, yields are poor and costs are high. Only when Nb is precipitated with some C as NbC, it is possible to slightly adjust the amount of solid solute C by taking advantage of the fact that NbC re-dissolves depending on the annealing temperature, but this requires a considerable amount of High-temperature annealing is required, which inevitably increases costs and shortens the life of the furnace. Furthermore, the adjustment range for the amount of solid solute C is small, so the benefits are small.

An object of the present invention is to solve these problems and provide a method for producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet with excellent workability.

(Means for Solving the Problems) Therefore, the present inventor improves the above-mentioned problems and produces a (alloyed) hot-dip galvanized steel sheet that has excellent workability and aging resistance, and has appropriate bake hardenability. For this purpose, we conducted many experiments and prototypes.

As a result, during continuous annealing, we added a small amount of Ti to completely fix N and improve deep drawability to ultra-low carbon steel whose C content had been reduced to some extent through vacuum degassing. By adjusting the dew point and decarburizing the solute C in the steel sheet, the amount of solute C can be controlled quite accurately, which allows for appropriate bake hardenability and good wettability during plating. By suppressing the amounts of Si and Mn added and limiting the range of addition of Mn, and P in consideration of processability, it is possible to achieve the desired characteristics (
The present invention was completed based on the discovery that hot-dip galvanized steel sheets (alloyed) can be manufactured reliably at a relatively low cost.

As a conventional technique similar to the present invention, there is a technique in which a small amount of Ti or Nb is added to an ultra-low carbon material such as the one mentioned above, and the optimum range of addition thereof has been widely published (
JP-A-53-114717, JP-A No. 57-70258
Publication No. 57-79161, Publication No. 59-3182
Publication No. 7, Publication No. 61 26757, Publication No. 61-157
(No. 639 and No. 62-112731) In most of them, the hard 78C is determined at the time of casting, and nothing is said about readjustment after casting, or they just use remelting of NbC. Therefore, with these conventional techniques, it is difficult to control solid solution C, and it is thought that the yield will be poor.

In addition, many technologies have been published to decarburize ultra-low carbon materials by continuous annealing, such as decarburizing electrical steel sheets, but these decarburize and remove as much C as possible to increase grain growth. It is intended to improve electromagnetic properties, and the less C is, the better.There is no concept of adjusting C, and decarburization causes grains to grow significantly, resulting in a decrease in strength.The continuous annealing of the present invention It differs from decarburization in terms of purpose and degree.

That is, in the present invention, the composition ratio at the time of casting is (1,: 0.0050% or less, Si: 0.04% by weight)
% or less, Mn: 0.10-0.60%, P: 0.
080% or less, but Mn(%i + 10 P m
≦1.20S: 0.002~O, O1, O%, AQ
: 0.030-0.100%, N: 0.0030
% or less, Ti: -N (g) ~ (transfer N ~ 10-3 ~), remainder: Fe
After hot rolling, a steel billet containing unavoidable impurities is cold rolled at a cold rolling rate of 50% or more, and then 6 points -30°C or more +15'
Decarburization annealing is performed for 40 seconds or more at a temperature higher than the recrystallization temperature and lower than 900°C in an atmosphere of C or lower, and the C after the decarburization annealing is reduced to 0.
．． 0.0010% or more and less than 0.0030%, hot-dip galvanizing is applied, and the elongation is 1.0% or more and 2.0% or less. This is a method for producing a hot-dip galvanized steel sheet that has good workability and aging properties and has appropriate bake hardenability, which is characterized by performing semi-rolling.

Here, "good workability" in the present invention refers to mechanical properties before aging of the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet manufactured by the manufacturing method of the present invention, yp≦24 kgf/Mm" , preferably YP≦
22kgf/ms”, E42≧37.5% (0.75
t), preferably LfEl≧40% (0,75t), T≧
1.55 (0,75t) preferably f≧1.70 (0,
75t), satisfies -')YPE=O, and "good aging properties" means that the mechanical properties after aging due to accelerated aging heat treatment at 100°C for 60 minutes show that YP does not deteriorate. ΔYP≦+”kg/am”, and YPE
= O (yield point elongation does not occur).

(Function) The reason why the composition and manufacturing conditions of the plated steel sheet are limited as described above in the present invention is as follows.

C: C exists as solid solution C or TtC in ultra-low carbon-T1 addition steel, but solid solution C has the effect of providing bake hardenability and improving secondary processing resistance. Therefore, solid solution C must be present in an amount of at least 0.0010%.

On the other hand, if C is present in an excessive amount, the age hardenability will deteriorate and aging will occur easily, resulting in poor workability and easy to cause stretcher strain during processing, so 0.0030%
It needs to be less than 0.002, preferably 0.002
It is 0% or less.

With the current vacuum degassing technology, it is extremely difficult to constantly keep C within the above range, but in the present invention, by controlling the dew point and annealing temperature, decarburization is performed simultaneously during recrystallization annealing. The amount is 0 at the time of casting.
It is sufficient if it is 0.0050% or less. However, the higher the C content during casting, the more difficult it becomes to perform sufficient decarburization in a short period of time, so it is desirable that the C content be 0.0040% or less.

Si: Si is an element added to increase the strength of high-strength steel sheets, etc., but in the case of manufacturing plated steel sheets as in the present invention, Si causes surface concentration during continuous annealing. The adhesion tension with molten zinc is significantly reduced, plating adhesion deteriorates, and non-plating is likely to occur.

In order to produce a good plated steel sheet as in the present invention, Si needs to be 0.04% or less.

Preferably it is 0.02% or less.

Mn: Mn is an effective element for fixing S, which is generally mixed as an impurity element in molten steel, reducing its harmful effects, and, like Si, for increasing the strength of steel sheets. In ordinary ultra-low C, Ti-added steel, a considerable amount of S combines with Tj, so Mn addition for S fixation is not very necessary, but when the amount of Ti added is small as in the present invention, a certain amount of addition is necessary. It is. Also, from the viewpoint of strength, it is necessary to increase the strength with Mn and P to compensate for the small amount of Si added. However, as the amount of Mn added increases, the strength increases and the elongation decreases, with a particularly strong tendency for the f value to decrease.Furthermore, like Si, surface concentration has a slight adverse effect on plating properties.

For the above reasons, -n is set to 0.10 to 0.60%.

P: Like Si and Mn, P is an element that has a strong effect on increasing the strength of steel sheets, and has a smaller effect on lowering elongation and f-value than Mn. However, its influence is Although the amount is small, it still tends to lower the elongation and Y' value, and especially when P is added in an amount of 0.1% or more, the elongation deteriorates significantly, and it also segregates at the grain boundaries, causing grain boundary embrittlement. This causes deterioration of the embrittlement resistance. Moreover, if there is a large amount of P, streak-like patterns are likely to occur due to differences in alloying processing speed in partially concentrated areas on the surface, and although this tendency depends on casting conditions and slab heating conditions, P If it exceeds 0.08%, it becomes more likely to occur. For the above reasons, P is set to 0.080% or less, preferably 0.070% or less.

Combined addition of Mn and P: Mn and P are elements that increase strength, and when added in large amounts, elongation occurs as the strength increases and the f value decreases. Therefore, if the amounts of both Mn and P added are increased at the same time, a steel plate with good workability as in the present invention cannot be obtained.

Therefore, - for boat fishing, Mn (g) + l0P (g) ≦1.2
Set to 0.

However, to ensure a strength of 35.5 kgf/m + ~+IOP~<0.95 is sufficient.

If it is important to keep the E2 and P values within a preferable range, it is necessary to keep them within this range.

S: S is contained as an unavoidable impurity in steel, reduces the ductility and t-value of steel sheets, and also causes embrittlement during hot working. In addition, in ordinary ultra-low carbon-Ti added steel, it combines with Ti before hot rolling, for example during slab heating, which increases alloy cost and causes heave defects due to coarse precipitates due to TiS. In this case, Ti is not added in such a large amount as to fix all the S, so the alloy cost does not increase, but as Si increases, the required amount of Mn increases.
．． It is necessary that it be 0.01% or less. However, when Si becomes extremely low, Ti is reduced to N/14≦Ti/48≦N/
Since it becomes difficult to add in the range of 14+S/32, the lower limit is set to 0.002%. As a result, S becomes Ti
acts like a cushion, fixing N but not C.

AQ: AQ is added to deoxidize the steel, and prevents the formation of nonmetallic inclusions (oxides) in the steel that reduce the addition yield due to the loss of Ti through oxidation or deteriorate the ductility. It is an element that has the function of suppressing N, and also has the function of fixing N as AQN, which causes aging at room temperature and has an adverse effect on ductility.

However, in the case of ultra-low carbon-Ti-added steel like the present invention, the amount of N is lowered, and most of the N is fixed by Ti, so it is necessary to specifically add it to fix N. There's no need. Moreover, when the amount added increases, the alloy cost increases, and inclusions such as AQzOs increase, which causes deterioration of ductility and surface defects called bald spots and blisters.

For these reasons, in the present invention, AQ is 0.03 to 0.
．． 10%.

Like S, N is an element that is contained as an unavoidable impurity in steel, and if it is dissolved in solid solution, it reduces the ductility and t-value of the steel sheet, and causes aging at room temperature, causing stretcher strain. It is.

In addition, in the case of ultra-low carbon-Ti added steel like the present invention, most of it is fixed as TiN before hot rolling, similar to and preferentially than S, so its content is If the amount of Ti increases, the amount of Ti required to fix it will increase, leading to an increase in alloy cost, and the precipitated TiN will have a negative effect on the total elongation and F value, and if the precipitates are coarse, it will cause sagging. It may also cause defects.

For these reasons, N is set to 0.0030% or less, preferably 0.0020% or less.

Ti: Ti is ductile and f in ordinary ultra-low carbon-Ti added steel.
(!!, solid solution N and S in steel that have a negative effect on aging properties, etc.)
, C is removed by fixing them as precipitates, and is added for the purpose of improving formability.

But hard? If all 8C is fixed, the bake hardenability will be lost, so Ti should not be added in such a large amount that all C is fixed. Fortunately, for Ti, N and S are fixed before hot rolling before A (! = Nb, Mn, etc.), and then C is fixed.
It preferentially combines with N and S before combining with C to form TiN and T.
Consumed as iS. Therefore, Ti/48≦N/14
+ If S/32, there is almost no Ti bonded to C. In other words, if the total amount of C is large, it is okay for some C to bond with Ti, but if this happens, it becomes difficult to control the solid solution C, and stable productivity decreases.

On the other hand, if N is not fixed even slightly, these N
As mentioned above, N/
It is necessary that 14≦Ti/48.

For the above reasons, the amount of Ti added should be N/14≦Ti.
The range satisfies /48≦N/14 + S/32.

Other incidental impurities such as Cu, Cr, 0, etc. are not particularly limited as long as they are in an impurity amount, but E! ,? In order to secure the value, preferably Cu: less than 0.1%, Cr
: less than 0.1%, o: limited to less than o, oos%.

Next, according to the present invention, cold rolling is performed with a reduction rate of 50% or more after hot rolling, but if the cold rolling rate, that is, the reduction rate is less than 50%, it is difficult to obtain a preferable texture after recrystallization annealing.
It becomes difficult to obtain a high Y value.

The subsequent decarburization annealing, usually continuous decarburization annealing and skin pass, is performed under the following conditions.

In the present invention, during this recrystallization annealing, water vapor is blown into the atmospheric gas to raise the dew point and decarburize at the same time. The decarburizing atmosphere at this time was one in which steam was blown into a mixed gas of AX gas (ammonia decomposition gas) and N2.

In addition, in the process of manufacturing a plated steel sheet by normal continuous annealing, recrystallization annealing after cold rolling is usually performed in a reducing atmosphere (n point is -50''C or less).

The steel sheet whose casting composition has been adjusted to the composition range of the present invention and which has been cold rolled as described above recrystallizes during this continuous annealing and decarburization, and at the same time, the solid solution C of the C in the steel is recrystallized.
81 (the amount decreases and aging properties are improved, but the amount of decarburized increases as the amount of solid solute C in the steel increases, and conversely decreases when the amount is small. Solid solution C in steel
It is easy to converge the quantity within a certain range.

In addition, this decarburization is accelerated as the annealing temperature or dew point increases, so if the conditions are set according to the C value of the cold rolled steel sheet before annealing, it is possible to Molten C
It becomes easier to converge the quantity within a certain range.

Note that if the annealing temperature is increased, grain growth will proceed at the same time as decarburization, resulting in a decrease in strength, but decarburization by only adjusting the dew point will not change the grain size much, and mechanical properties such as strength and elongation will also hardly change. Only the amount of bake hardening decreases.

Therefore, regarding the decarburization annealing conditions in the present invention, firstly, if the dew point is less than 130°C, the decarburization power is poor, and if the C content is high, it is difficult to perform sufficient decarburization.
If the temperature exceeds 5°C, oxidation of the surface of the steel sheet becomes intense, which deteriorates plating adhesion and tends to cause poor powdering properties.

For this reason, the dew point is -30℃ or more +15℃
The temperature is below, preferably -20°C to -10°C.

Next, regarding the annealing temperature and time, the annealing temperature may be equal to or higher than the recrystallization temperature, as in the production of ordinary cold rolled steel sheets.

However, if the annealing temperature at this time is too high, significant coarsening of crystal grains will occur, so the upper limit is set at 900°C. Preferably it is 750-850°C.

Regarding the annealing time, a minimum of about 40 seconds is required in order for appropriate grain growth to occur after recrystallization and for decarburization to proceed to a certain extent, and preferably 60 seconds or more.

It goes without saying that these decarburization annealing conditions are preferably set in accordance with the C value of the coil before annealing.

As described above, according to the present invention, the solid solution C can be controlled with high accuracy through this decarburization annealing treatment, and the amount of bake hardening thereby can also be accurately controlled.

The subsequent hot-dip galvanizing treatment and alloying treatment method may be carried out by a conventional method.Finally, the skin pass elongation condition is not particularly limited.According to the present invention, the steel sheet is coated with YPE after annealing. There are some remains,
B) The amount tends to be larger as the l amount is higher. For this reason, the higher the BH content, the greater the elongation rate required to eliminate YPE, but in order to completely eliminate YPE, at least 1.0% or more is required. However, if the growth rate in the skin pass is too high, YP will increase, El,
Since this causes a decrease in the r value, the upper limit is set at 2.0%.

Regarding the mechanical properties obtained for the hot-dip galvanized steel sheet thus manufactured according to the present invention, the elongation (El) of the alloyed hot-dip galvanized steel sheet is due to the hardness of the alloyed coating film. As a result, the strength tends to be about 2 to 3% lower than that of cold-rolled steel sheets of the same level, and for the same reason, the r value also decreases by 0.2 to 3%.
It becomes lower by about 0.3.

Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 Two types of molten steel, whose compositions are shown in Table 1, were melted in a converter and subjected to vacuum degassing treatment. After continuous casting according to a conventional method, the steel was heated to a temperature of 1200°C and hot-casted. Rolled, about 600
After winding at °C and pickling, cold rolling was performed at a rolling reduction of 75% to produce a 0.75 m cold rolled steel plate.

Next, these cold-rolled steel sheets were subjected to decarburization annealing for about 60 seconds at varying temperatures in an N2 + steam atmosphere with various dew points, and then mechanical properties and C were analyzed.

In the test of this example, no plating was applied and the tensile test was conducted without applying a skin pass in order to minimize the factors that would cause errors in the properties. However, BH
The strength test was conducted after temper rolling of about 1.5% was performed to completely eliminate the elongation at the depression point. At this time, YP was also measured. The results of these investigations are also summarized in Table 1.

As is clear from Table 1, when decarburization is performed by increasing the dew point during annealing, C in the steel decreases and the BH amount decreases. The amount of decarburization increases as the C content in the steel increases, and as the temperature and dew point increase.

It should be noted that when the decarburization annealing temperature is increased, the TS also decreases, which is thought to be due to grain growth.

In addition, when decarburizing only by increasing the dew point, the crystal grain size does not change much and TS and Ei! , B) only the amount of I decreases, with almost no change.

Example 2 Test steels whose compositions are shown in Table 2 were melted in a converter, subjected to vacuum degassing treatment, and then continuously cast according to a conventional method. Rolled for about 690°
After winding with C and pickling, cold rolling was performed at a reduction ratio of 75% to produce a cold rolled steel sheet of 0.75 mm.

Next, these cold-rolled steel sheets were subjected to 8
Continuous annealing for about 60 seconds at around 00°C to a dew point of 60 to +
After applying it in an atmosphere where steam is blown into AX gas and N2 gas adjusted to 20°C, it is cooled to about 460°C to perform hot-dip galvanizing, and then in an alloying furnace for 500°C.
~600'CXl0 seconds alloying treatment, then elongation rate 1
．． A 0-2.0% skin bath was applied to produce an alloyed hot-dip galvanized steel sheet.

The composition of these steel sheets and the mechanical properties investigated are summarized in Table 3 together with the recrystallization annealing conditions and skin bath conditions. In addition, a material whose components or manufacturing conditions differ from those of the present invention is shown as a comparison material.

As is clear from the results shown in Table 3, the plated steel sheet according to the present invention has superior workability compared to the comparative plated steel sheet.
In addition to being hard to cause age hardening, it is 2.5 to 3.9 kg/ms.
Because of its bake hardenability, it is very suitable for automobile materials and special exterior materials.

(Effects of the Invention) Since the present invention is constructed as described above, it is possible to produce a hot-dip galvanized steel sheet that has excellent formability and improves strength through bake hardening after forming, and is extremely useful industrially. .

Claims (3)

[Claims] (1) Composition ratio at the time of casting is C: 0.0 in weight%
050% or less, Si: 0.04% or less, Mn: 0.10
~0.60%, P: 0.080% or less, however, Mn (%
)+10P(%)≦1.20S: 0.002-0.01
0%, Al: 0.030-0.100%, N: 0.00
30% or less, Ti: 48/14N (%) ~ {48/14N (%) + 4
8/32S (%)}, balance: Fe and unavoidable impurities A steel billet is cold rolled at a cold rolling rate of 50% or more after hot rolling, and then the dew point is -30°C or more +15°C
Decarburization annealing is performed for 40 seconds or more at a temperature higher than the recrystallization temperature and lower than 900°C in the following atmosphere, and the C after the decarburization annealing is 0.
0.0010% or more and less than 0.0030%, hot-dip galvanizing, and temper rolling with an elongation of 1.0% or more and 2.0% or less. A method for producing a hot-dip galvanized steel sheet having good and appropriate bake hardenability. (2) The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein Mn (%) + 10P (%)≦0.95. (3) 0.95<Mn(%)+10P(%)≦1.20
The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet has a tensile strength of 35.5 Kgf/mm^2 or more.