JPH0699760B2 - Method for producing steel plate with hot dip zinc for ultra deep drawing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot dip galvanized steel sheet for ultra-deep drawing which has bake hardenability and is slow aging.

<Background Art> In recent years, the idea of resource saving and energy saving has permeated every corner of the industrial world. For example, in the automobile industry as well, various vehicles such as passenger cars have been used to reduce weight and fuel consumption. The use of high-strength cold-rolled steel sheets has become common in automobiles. Along with this, the demand for cold-rolled steel sheets having higher strength and improved deep drawability has become more and more severe.

On the other hand, in automobiles, countermeasures against anti-freezing agents such as rock salt sprinkled on roads in cold regions are also problematic, and from these viewpoints, even more stringent demands are made on the corrosion resistance of steel sheets used. As a result, there has been a demand for a high strength cold rolled steel sheet for deep drawing having an anticorrosion plating layer.

Of course, much effort has been made to develop a high-strength cold-rolled steel sheet for ultra-deep drawing, which has high strength and is further improved in deep drawability. Al-killed steel plate added, Nb-killed steel plate added with P to ultra-low C steel, steel plate added with Ti and Nb to ultra-low C steel, and further suppressing P content (JP-A-59-31827), However, since it is difficult to secure a sufficiently satisfactory Rankford value (r value) with the steel sheets mentioned above and a low yield ratio, a complicated press forming process is required in terms of styling. It is unsatisfactory as a cold-rolled steel sheet for ultra-deep drawing required for automobiles, etc. Moreover, the above steel sheet cannot obtain bake hardenability, and the problem of secondary work brittleness has not been completely solved. It was Here, the secondary working brittleness appears in the form of causing brittle cracking when a cylinder drawn by a press or the like is expanded from the mouth end, for example, and an extremely low C system having a low grain boundary strength. It is known to exist as a material defect.

<Means for Solving Problems> The inventors of the present invention, in view of the above-mentioned problems found in the conventional cold-rolled steel sheet for deep drawing, have the purpose of ensuring excellent deep drawability of the cold-rolled steel sheet. Is essential to prevent aging at room temperature, and it is necessary to secure a bake hardening amount of at least 3 kg / mm ^{2 in} order to impart sufficient dent resistance to high tensile strength steel sheets for automobiles. In addition, from the viewpoint that it is essential to efficiently perform anticorrosion plating under suitable plating conditions in order to provide the corrosion resistance required for steel sheets for automobiles, etc., it has sufficient delayed aging and is extremely excellent. As a result of conducting research to efficiently and stably provide anti-corrosion plated steel sheets for ultra-deep drawing that have deep bakeability, have sufficient bake hardenability, and have eliminated problems such as secondary work brittleness, As a raw material steel, reduce the amount of C and N, and While using a steel with Ti added to suppress normal temperature aging, the P content is secured to compensate for the strength decrease due to C reduction, and the addition of B further results in an extremely low C-high P steel. The secondary working brittleness is suppressed, the amount of Ti is adjusted more appropriately and carefully, and an appropriate amount of Nb is added for adjusting the amount of solid solution C. The cold-rolled steel sheet obtained by cold rolling is subjected to appropriate "annealing" and subsequent "retention treatment in the low temperature range" in a "Sendimia type continuous galvanizing line" in which heat treatment and hot dip galvanizing are combined. As a result, delicate solid solution C control can be performed stably and reliably, and an appropriate bake hardening amount can be secured. Moreover, if this "low temperature range retention treatment" is followed by hot dip galvanizing treatment, adhesion will be improved. Excellent galvanized layer is formed and excellent deep drawing Sex is the ultra-deep drawing plated steel sheet excellent in corrosion resistance with sufficient bake hardenability and good slow aging property came to obtain a knowledge that "obtained.

The present invention was made based on the above findings, and C: 0.001 to 0.006% (hereinafter,% representing a component ratio is% by weight).
, Mn: 0.06-0.40%, P: 0.100% or less, S: 0.030% or less, Ti: 0.003-0.020%, and the formula Satisfying the condition, Nb: 0.060% or less, and the formula Satisfying the following conditions: N: 0.0040% or less, sol.Al: 0.010 to 0.090%, B: 0.0002 to 0.0010%, Fe and unavoidable impurities: Steel of the composition consisting of the remainder is heated in the temperature range of Ar ₃ points or more. Rolling after hot rolling at 600 ℃ or less, then pickling, cold rolling, and then at a recrystallization temperature above 900 ° C on a Sendzimir type continuous galvanizing line, for example, as in the heat pattern shown in Fig. 1.
Anneal at a temperature of ℃ or less and cool the subsequent 700 to 500 ℃ section at a cooling rate of 1 ℃ / sec or more, and further 48
By keeping it in the temperature range of 0 to 430 ° C for 20 to 120 seconds and then performing hot dip galvanizing, it has excellent deep drawability, bake hardenability, and delayed aging. It is characterized in that it enables stable and highly efficient mass production of plated steel sheets.

Next, in the present invention, the reason why the composition of the raw material steel, the hot rolling condition and the heat treatment condition are limited as described above will be explained.

A) Component composition of raw steel (a) CC is a component necessary for imparting bake hardenability to a steel sheet, and if the content thereof is less than 0.001%, a desired bake hardenability cannot be secured. On the other hand, if the C content exceeds 0.006, the room temperature aging becomes large and the performance as a steel plate for ultra deep drawing deteriorates. Therefore, the C content is 0.001 to 0.006%
I decided.

(B) Mn Mn is a component necessary to secure hot workability of steel,
If the content is less than 0.06% it is not possible to completely prevent the occurrence of surface defects due to red heat embrittlement, while if it exceeds 0.40% causes deterioration of drawability due to solid solution hardening,
Since it is difficult to control the bake hardening amount, the Mn content is 0.
It was set at 06 to 0.40%.

(C) P P has the effect of improving the strength of the steel sheet, and is a component that is positively included to secure the desired strength, but if it exceeds 0.100%, spot welding performance for users The P content is 0.100.
Defined to be less than or equal to%.

The P content is adjusted according to the desired strength, but at least 0.03 is required to secure the strength improving effect.
It is preferable to contain 0% or more.

(D) S S is an impurity element that is inevitably accompanied in steel, but if its content exceeds 0.030%, surface defects due to red heat embrittlement tend to occur, so the S content is 0.030% or less. Specified.

(E) Ti Ti component has a function of fixing N in steel to prevent normal temperature aging, but if its content is less than 0.003%, the desired effect cannot be obtained on the other hand, while 0.020% is added. If it is contained in excess, N and S as well as C become precipitates and the bake hardening amount decreases, so the Ti content was set to 0.003 to 0.020%.

Further, in order to surely precipitate N in the steel as TiN, the Ti content of [48/14 × N (%)] or more is required in relation to the N content.
It is necessary to secure the amount (%).

On the other hand, Ti tends to precipitate TiS and TiC in association with S and C in steel, but the strength of the bond with Ti is N → S → C.
In that order. Therefore, if Ti is present in excess of the amount associated with N and S, the remaining Ti associated with N and S is Ti.
It precipitates as C and reduces the solid solution C necessary for ensuring the bake hardenability. Therefore, in order to prevent such a decrease, the Ti content should be reduced. It is necessary to keep below.

Therefore, the Ti content is in the range of 0.003 to 0.020%, and Was determined as a value that satisfies.

(F) The hot-dip galvanized steel sheet produced by the method of the present invention is designed to secure a desired bake hardenability by an appropriate amount of solute C, but the amount of C and the amount of Ti are adjusted. It is not possible to reliably control the amount of dissolved C alone.
Therefore, solid solution C is formed by the low temperature retention treatment subsequent to the annealing.
It is necessary to precipitate a part of NbC as NbC to optimize the amount of dissolved C.

For this reason, it is essential to add an appropriate amount of Nb, but if the Nb content is less than the value of [93/12 × C (%)], the amount of dissolved C is insufficiently adjusted and the room temperature aging property is increased. On the other hand, if 0.060% is also contained, it is sufficient for the solid solution C to be fixed, and if it is contained in excess of this amount, the curing will be saturated.
Content is 0.060% or less, and formula Is limited to satisfy the value.

(G) NN N is also an impurity element that is inevitably accompanied in steel and causes aging at room temperature. Therefore, the smaller the content, the better. Particularly, if the content exceeds 0.0040%, the amount of Ti for fixing N is large. However, the N content is
The limit was 0.0040% or less.

(H) sol.Al sol.Al is an important component as a deoxidizer for steel, and it is necessary to secure a content of at least 0.010% in order to perform sufficient deoxidation, but exceeding 0.090%. Since the effect is saturated even if it is contained, the sol.Al content is 0.010 ~
It was set at 0.090%.

(I) BB has the effect of suppressing the adverse effect of P segregating at the crystal grain boundaries earlier than segregating at the crystal grain boundaries and preventing secondary work embrittlement of the steel sheet. When the content is less than 0.0002%, the desired effect cannot be obtained, and when the content is more than 0.0010%, the effect is saturated, so the B content is defined as 0.0002 to 0.0010%.

The components of the material steel targeted by this invention are those containing the above components, but in addition, since Si present in the steel also adversely affects the deep drawability due to the solid solution effect, it is 0.05%.
It is preferable to suppress it to the following.

B) Hot rolling conditions (a) Finishing temperature When the hot rolling finishing temperature is lower than the temperature at the Ar ₃ point, the crystal orientation during hot rolling becomes unfavorable for drawability and the deep drawability of the galvanized steel sheet is descend. Therefore, hot rolling requires Ar ₃
It was decided to finish in the temperature range above the point.

(B) In order to sufficiently fix N in the steel sheet to TiN at the winding temperature Ti, the effect does not change even if the winding temperature is sufficiently low. On the other hand, if the coiling temperature exceeds 600 ° C., the formation of NbC will be excessive, and the separation and solid solution of Nb and C will be suppressed in the continuous hot-dip galvanizing line, making it difficult to control the solid solution C.

Therefore, the hot rolling coiling temperature is set to 600 ° C or lower.

C) Annealing condition (a) Annealing temperature The bake hardenability of the hot-dip galvanized steel sheet produced by the method of the present invention is obtained by an appropriate amount (about 10 ppm) of solid solution C. Of the NbC formed is adjusted by adjusting the recrystallization annealing to obtain solid solution C
Need to be generated. However, since abnormal grain growth occurs when the annealing temperature is higher than 900 ° C, the annealing temperature is limited to the recrystallization temperature or more and 900 ° C or less.

(B) Cooling rate after annealing The cooling rate at the time of cooling after the annealing was slowed down to 1 ° C.
If it is less than / sec, NbC will precipitate again and the desired amount of solid solution C cannot be secured. Although there is no particular upper limit of the cooling rate, it is not actually necessary to set the cooling rate to more than 100 ° C./sec.

D) Retention condition in low temperature region Recrystallization, NbC dissolution and solid solution C were secured by the annealing, but C was 10 pp when the annealing / quenching was performed.
At least m, at least 30 ppm or more will be dissolved. Therefore, if it is left as it is, the amount of solid solution C is too large and the bake hardening amount is large, but the room temperature aging is also large and it is not suitable for "for ultra deep drawing".

For this reason, during cooling after annealing, a certain amount of solid solution C must be precipitated as NbC by allowing it to stay (hold or slowly cool) in the temperature range of 480 to 430 ° C. for 20 to 120 seconds, and form an appropriate amount of solid solution C. I have to.

If this low-temperature holding or slow cooling is performed in a temperature range exceeding 480 ° C, precipitation of NbC will occur too much and bake hardenability will be reduced, while if the low-temperature holding or slow cooling temperature range falls below 430 ° C, hot dip galvanizing will occur. Will cause poor adhesion.

Further, if the residence time in the above temperature range is less than 20 seconds, the aging resistance due to excessive solid solution C deteriorates, while if it exceeds 120 seconds, precipitation of NbC also occurs too much and the bake hardenability increases. Get smaller.

Therefore, it was decided that the low temperature retention treatment should be carried out in the temperature range of 480 to 430 ° C for 20 to 120 seconds.

Cold-rolled steel sheets treated as described above are usually temper-rolled at an elongation of 1.5% or less and hot-dip galvanized, but there is no particular limitation on the hot-dip galvanizing type, only hot-dip galvanizing is performed. However, the alloying treatment may be performed subsequently.

Next, the present invention will be described with reference to examples.

<Example> First, a continuous cast slab having a composition as shown in Table 1 was manufactured by a conventional method, and subsequently hot-rolled under the conditions shown in Table 2 to obtain a hot rolled sheet having a thickness of 3.0 mm. After forming the coil, it was pickled and cold-rolled to obtain a cold-rolled steel sheet having a plate thickness of 0.8 mm.

Next, the obtained cold rolled steel sheet was annealed in a Sendzimir type continuous hot dip galvanizing line, subjected to low temperature retention treatment, temper rolling (elongation ratio: 1%) and hot dip galvanized to obtain a galvanized steel sheet. The processing conditions were as shown in Table 2.

"Tensile properties", "transition temperature" and "bake hardening amount" of the hot-dip galvanized steel sheet produced in this way were investigated,
The results are also shown in Table 2.

Here, the "transition temperature" means the boundary temperature at which brittle cracking occurs, and a method of investigating brittle cracks by drawing a cylinder with a drawing ratio of 1.6, covering it with a truncated cone, and then pushing it in by impacting It was measured.

The "bake hardening amount" was measured as the amount of increase in the yield point when a prestrain of 2% was applied by tension and then tension was applied again.

As is clear from the results shown in Table 2, the steel sheets manufactured according to the conditions of the present invention have excellent deep drawing. On the other hand, those exhibiting a good bake-hardening property and an appropriate bake-hardening amount, while those whose manufacturing conditions deviate from the conditions specified in the present invention, have sufficiently satisfactory properties as a galvanized steel sheet for ultra-deep drawing which is expected to have a bake-hardening property. It turns out that they don't have it.

<Summary of Effects> As described above, according to the present invention, a hot-dip galvanized steel sheet for super deep drawing having excellent deep drawability, bake hardenability, and slow aging and excellent corrosion resistance can be stably and efficiently provided. It can be mass-produced and can sufficiently meet the demands as a steel sheet for automobiles with strict contents, resulting in an extremely useful effect in industry.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat pattern of "a processing example of a cold rolled steel sheet in a Sendzimir type continuous hot dip galvanizing line" according to the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C23C 2/02

Claims (1)

[Claims] 1. A weight ratio of C: 0.001 to 0.006%, Mn: 0.06 to 0.40%, P: 0.100% or less, S:
0.030% or less, Ti: 0.003 to 0.020%, and formula Satisfying the condition, Nb: 0.060% or less, and the formula Satisfying the following conditions: N: 0.0040% or less, sol.Al: 0.010 to 0.090%, B: 0.0002 to 0.0010%, Fe and unavoidable impurities: Steel of the composition consisting of the remainder is heated in the temperature range of Ar ₃ points or more. Rolling after hot rolling at 600 ℃ or less, then pickling, cold rolling, then annealing at a temperature of recrystallization temperature or more and 900 ℃ or less in the Sendzimir type continuous galvanizing line, then 700 ~ 500 ℃
While cooling the section at a cooling rate of 1 ° C / sec or more, let it stay in the temperature range of 480 to 430 ° C for 20 to 120 seconds,
A method for producing a galvanized steel sheet for ultra-deep drawing having bake hardenability, which is characterized by performing hot dip galvanization subsequently.