JP2984884B2 - Non-aging steel sheet for deep drawing and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aging steel sheet for deep drawing and a method for producing the same. The thin steel sheet according to the present invention is intended for press working of automobile parts, household electric appliances, buildings, etc., and is subjected to cold-rolled steel sheet and, for example, Zn plating or alloyed Zn plating for rust prevention. Includes both surface treated steel sheets.

[0002]

2. Description of the Related Art With the recent progress in vacuum degassing of molten steel, it has become easier to produce ultra-low carbon steel. At present, the demand for ultra-low carbon steel sheets having good workability is increasing.
A conventional ultra-low carbon steel sheet contains at least one of Ti and Nb which have a strong attractive interaction with interstitial solid solution elements (C, N) in the steel and easily form carbonitrides. It is well known. Steel free of interstitial solid solution elements (IF steel: Interstitial Fr
ee Steel) does not include interstitial solid-solution elements that cause strain aging and deterioration of workability, and thus has the characteristic of non-aging and extremely good workability. Furthermore, Ti and N
The addition of b also plays an important role in reducing the crystal grain size of the hot-rolled sheet of ultra-low carbon steel, which tends to become coarse, and improving the deep drawability of the cold-rolled annealed sheet. However, the ultra-low carbon steel to which Ti or Nb is added has the following problems. First, in addition to the vacuum processing cost for ultra-low carbon, expensive Ti and Nb
This is because the production cost is high because of the necessity of the addition of. Secondly, the addition of Ti or Nb increases the recrystallization temperature, so that high-temperature annealing is indispensable, and there are problems such as the occurrence of heat buckles and sheet breakage during sheet passing and high energy consumption. Third, there is a problem that the strength of crystal grain boundaries is reduced due to the absence of solid solution C, and brittle cracking occurs during secondary working. In particular, this problem becomes apparent in a high-strength steel sheet containing a large amount of P. Fourth, in ultra-low carbon steel to which Ti or Nb having a strong tendency to form oxides is added, there is a problem that scratches due to oxide-based inclusions are likely to occur.

[0003] In order to solve such a problem of IF steel, many researches and developments have been conventionally performed. For example,
JP-A-59-80727, JP-A-60-1031
No. 29, Japanese Unexamined Patent Publication No. 1-184251, etc.
A cold-rolled steel sheet including a region where the amount of carbon is 0.0015% or less without adding an element such as Ti or Nb which causes the above-described problem and a method for manufacturing the same are disclosed. However, when the C content is in the range of 0.0015% or less, the crystal grain size of the hot-rolled sheet becomes coarse, and sometimes it becomes an extremely coarse columnar crystal elongated in the thickness direction. When a hot-rolled sheet having such coarse crystal grains is used as a material for cold rolling and annealing, the r-value of the product sheet is rather deteriorated despite the extremely low carbon steel. In particular, the r value at 45 degrees from the rolling direction decreases. Then, since the anisotropy increases, embrittlement of the secondary processing also becomes apparent. To deal with this new problem, Japanese Patent Application Laid-Open No. 59-93834.
In the gazette, the addition of (Ni + Cu + Cr) and optimization of the rolling schedule of the hot rolling are performed for the ultra-low carbon steel having a C content of 0.0020% or less to increase the crystal grain size of the hot-rolled sheet. It discloses a method for preventing and improving the in-plane anisotropy of the r value. In addition, JP-A-1-188628 and JP-A-1-188629 disclose that the C content is 0.0
A method of improving the r-value of a product sheet by controlling cooling after hot rolling for an ultra-low carbon steel of from 0 to 0.0030% is disclosed. The effect of preventing the grain size of the hot-rolled sheet from being coarsened by these improved techniques is exerted as such.
However, even if these techniques are applied, there is a slight difficulty in always obtaining a crystal grain size of a hot-rolled plate stably. The idea of adding Cr to ultra-low carbon steel containing no Ti or Nb is described in Japanese Unexamined Patent Application Publication Nos. 60-50152 and 60-1
No. 84669, JP-A-60-197846,
JP-A-62-1845, JP-A-63-72830
No. 6,086,045. However, these are not necessarily added to prevent the grain size of the hot-rolled sheet from becoming coarse, and only under the disclosed conditions,
Grain refinement is not effectively achieved.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is to solve the problem of Ti
A simple ultra-low carbon steel sheet that does not add expensive carbonitride forming elements such as Nb and Nb, achieves stable hot-rolled sheet crystal grain refinement, non-aging deep drawing thin steel sheet and its It is to provide a manufacturing method.

[0005]

Means for Solving the Problems and Actions The present inventors have studied a method for reducing the crystal grain size of a hot-rolled sheet in a simple ultra-low carbon steel.
The addition of r, P and B is effective, and in particular, Cr + 20
The effect is remarkable when P ≧ 0.2%. 2) The C content is 0.1%.
It has been found that in such a steel of 0015% or less, if the steel is cooled at a cooling rate of 50 ° C./sec or more within 0.5 seconds after the end of the hot rolling, the grains are further refined.

The present invention has been made based on such new findings, and the gist thereof is as follows. (1) In weight%, C: 0.0001 to 0.0015
%, Si: 1.2% or less, Mn: 0.01 to 3%, P:
0.01 to 0.15%, Cr: 0.1 to 3%, and Cr
+ 20P ≧ 0.2%, S: 0.0010 to 0.020
%, Al: 0.005 to 0.15%, N: 0.0001
-0.0080%, B: 0.0001-0.0020%
A non-aging steel sheet for deep drawing having a ferrite single-phase sizing structure composed of Fe and unavoidable impurities.

(2) A slab comprising the chemical components described in the above item 1 is subjected to hot rolling (Ar ₃ transformation temperature −100).
° C) or more, and immediately after that, within 0.5 seconds at 50 ° C / s
Rapid cooling to 750 ° C or less at a cooling rate of ec or more,
Non-aging deep-drawing thin film having a ferrite single-phase sizing structure characterized in that it is rolled at a temperature of 750 ° C., cold-rolled at a rolling ratio of 70% or more, and then continuously annealed at 600 to 900 ° C. Steel plate manufacturing method.

Hereinafter, the present invention will be described in detail. C: C is a very important element that determines the material properties of a product. If the C content exceeds the upper limit of 0.0015%, non-aging properties can no longer be ensured at room temperature.
0015%. On the other hand, when the C content is less than 0.0001%, secondary working embrittlement occurs. In addition, this is an area that is extremely difficult to reach in terms of steelmaking technology, and the cost is also significantly increased.
Therefore, the lower limit is made 0.0001%. Preferably, the range is 0.0005 to 0.0010%.

Si: Si is an element which increases the strength at low cost, and when used for strengthening, it is preferable to add 0.05% or more. However, when the content exceeds 1.2%, problems such as a decrease in chemical conversion property and a decrease in plating property occur. Therefore, the upper limit of Si is set to 1.2%. Usually, it may be 0.05% or less. Mn: Mn is an element effective for increasing the strength,
It also has the role of preventing cracking during hot rolling. To prevent cracking, 0.01% or more is required. On the other hand, Mn
If the amount exceeds 3%, the r value decreases.

[0010] Cr, P, B: Cr, P, B are the most important components in the present invention. That is, in order to attain the object by reducing the crystal grain size of the hot-rolled sheet of the ultra-low carbon steel having a C content of 0.0015% or less as in the present invention, it is necessary to reduce
r ≧ 0.1%, P ≧ 0.010%, and Cr + 20P ≧
0.2%, B: It is essential to add B in the range of 0.0001 to 0.0020%. Cr and P are Si,
Like Mn, it is an element effective for increasing the strength.
If the added amount of Cr exceeds 3%, the r value decreases, and further, the chemical conversion property and the plating property deteriorate, so the upper limit is set to 3%. Further, if the added amount of P exceeds 0.15%, the cold rolling property and the secondary workability are deteriorated.
15%. B has an effect of grain refinement when added in an amount of 0.0001% or more, but when it exceeds 0.0020%, on the contrary, the r value decreases or slab cracking is caused.
The upper limit is 0.0020%. Usually, Cr: 0.
2 to 1.0%, P: 0.10% or less, B: 0.0002
It may be 0.0010%. The effects of these elements are still unknown, but the addition of these elements suppresses recrystallization in the γ region and increases the nucleation frequency of the transformation by lowering the γ → α transformation temperature, It is presumed that the grain growth is suppressed to achieve finer grains.

S: Since the manufacturing cost increases when the S content is less than 0.0010%, the lower limit is 0.0010%. On the other hand, if it exceeds 0.020%, a large amount of MnS precipitates and the workability deteriorates, so 0.020% is made the upper limit. However, S also has a role of preventing the coarsening of the crystal grains of the hot-rolled sheet, so that the content is preferably 0.007% or more and 0.015% or less.

Al: Al is used for adjusting deoxidation and fixing N, but if it is less than 0.005%, it is difficult to stably obtain these effects. On the other hand, 0.15%
Beyond that leads to increased costs. Therefore, 0.005
To 0.15%. In order to effectively fix N, 0.04 to 0.12% is preferable. N: N is preferably lower. However, if the content is less than 0.0001%, a remarkable cost increase is caused.
1% is the lower limit. On the other hand, if it exceeds 0.0080%, it becomes difficult to fix N with Al anymore, so that dissolved N which causes strain aging remains or the fraction of AlN increases, resulting in deterioration of workability. I do. Therefore, 0.00
80% is set as the upper limit of the amount of N. Usually 0.0030%
The following is preferred.

Next, the reasons for limiting the manufacturing conditions will be described. Hot rolling: The conditions of hot rolling are extremely important components in the present invention. First, the finish temperature may be (Ar ₃ transformation temperature −100 ° C.) or more in the case of ultra-low carbon steel in order to secure the deep drawability of the product sheet. Next, in order to reduce the crystal grain size of the hot-rolled sheet, after the hot-rolling is finished, the hot-rolled sheet has a diameter of 0.1 mm.
Within 5 seconds, it is rapidly cooled to 750 ° C. or less at a cooling rate of 50 ° C./sec or more. When the time of the quenching start is more than 0.5 seconds after the hot rolling finish, the austenite grain size becomes extremely large, and the ferrite grain size after transformation becomes coarse,
0.5 seconds or less. Also, the cooling rate is 50 ° C./sec.
If less than c, the growth of austenite grains, the reduction of the grain refinement effect during γ → α transformation, the growth of ferrite grains after transformation,
For this reason, it is impossible to reduce the ferrite grain size, so the lower limit of the cooling rate is set to 50 ° C./sec. Further, the hot-rolled coil is heated from 600 ° C to 75 ° C.
Wind at a temperature below 0 ° C. If the winding temperature exceeds 750 ° C., ferrite grains grow coarsely, and the pickling properties deteriorate, and the material becomes non-uniform in the longitudinal direction of the coil.
50 ° C is the upper limit. On the other hand, when the temperature is lower than 600 ° C., the precipitation of AlN on the hot-rolled sheet becomes insufficient, and the workability of the product sheet deteriorates.

Cold rolling: Cold rolling may be performed under ordinary conditions.
For the purpose of securing the r value of the product plate, the rolling reduction is set to 70% or more. From the viewpoint of reducing the deep drawability, particularly the in-plane anisotropy, the content is preferably 80 to 95%. Continuous annealing: annealing temperature for recrystallization annealing is 600 to 90
0 ° C. If the annealing temperature is lower than 600 ° C., recrystallization is insufficient, and the workability of the product plate becomes a problem. Although the workability improves with an increase in the annealing temperature, if the temperature exceeds 900 ° C., the temperature is too high and the sheet breaks and the flatness of the sheet deteriorates, and the r-value remarkably decreases. Since the present invention relates to a cold-rolled steel sheet and a continuous hot-dip galvanized steel sheet, continuous annealing includes annealing by a continuous hot-dip galvanizing line.

Thus, the present invention has been constructed based on new ideas and new findings, and according to the present invention, Ti or Nb
A thin steel sheet which is non-aging and excellent in deep drawability can be obtained without adding an expensive element such as.

[0016]

【Example】

Example 1 Steel having the composition shown in Table 1 was vacuum-melted in a laboratory.
In steel A, the amount of C was changed from 0.0004% to 0.0030%. On the other hand, in steel B, the Cr content was changed from 0.01% to 1.50%, and the P content was changed from 0.005% to 0.120%. The obtained steel slab was hot-rolled under the following conditions. That is, after finish rolling at a slab heating temperature of 1150 ° C and a finishing temperature of 910 ° C, 80 ° C / sec within 0.2 seconds.
It cooled at the cooling rate of c, and wound up at 710 degreeC. The board thickness is
4.0 mm. After pickling, cold rolling was performed at a rolling reduction of 80% to form a 0.8 mm cold-rolled sheet.
° C / sec, soaking 800 ° C x 50 seconds, cooling rate 20 ° C /
Continuous annealing was performed for sec. Furthermore, temper rolling at a rolling reduction of 0.8% was performed and subjected to a tensile test. The tensile test method is J
The method described in IS2241 was followed. The strain aging characteristic is 10
Yield point elongation after artificial aging at 0 ° C for 1 hour (YP-El)
When it was 0.2% or less, it was regarded as non-ageing. Also,
Paint bake hardening characteristics (BH property) after 2% tensile prestrain
This is the amount of rise in the yield point when a treatment equivalent to paint baking at 170 ° C. for 20 minutes is performed and a tensile test is performed again. The secondary workability is to punch a blank with a diameter of 110 mm from a tempered rolled steel sheet, and then form a cup with a punch having a diameter of 50 mm,
Up to 20 with a conical punch with a vertical angle of 53 degrees at various temperatures
It was evaluated by the transition temperature between ductility and brittleness when mm was pressed and fractured, and a value of -50 ° C or less was regarded as good.

As is clear from Table 2, when the total C content becomes 0.0015% or less without adding Ti or Nb, the yield point elongation (YP-El) after 100 ° C. for one hour is 0.1%.
It is less than 2%, achieving the non-aging goal at room temperature. In addition, as is clear from the table, the C content is 0.0006 to 0.0006.
0.0013% ultra-low carbon steel, Cr ≧ 0.1%, P
By defining ≧ 0.01% and Cr + 20P ≧ 0.2% and performing cooling control after hot rolling, the r value, especially r _45, is remarkably improved, and it is a sufficient level as a steel sheet for deep drawing. Therefore, according to the present invention, a cold-rolled steel sheet excellent in normal-temperature non-aging properties and deep drawability can be obtained without adding expensive elements such as Ti and Nb. Further, as is apparent from Table 2, the steel of the present invention exhibits paint bake hardening characteristics and good secondary workability.

Example 2 Based on the findings of Example 1, steel having the chemical composition shown in Table 3 was melted and cast on an actual machine scale, followed by hot rolling (heating temperature: 1200 ° C., finishing temperature: 930). ° C, cooling after finishing: 7 seconds at 100 ° C / sec 0.3 seconds after hot rolling finishing
Cooling to 40 ° C, winding temperature: 710 ° C), cold rolling (rolling reduction: 84%), continuous galvanizing (maximum heating temperature:
820 ° C., hot-dip galvanizing: 460 ° C. (Al concentration in the bath: 0.11%), alloying treatment: 520 ° C. × 20 seconds, and temper rolling (0.8%). The tensile test method is the same as in Example 1. Further, as plating characteristics, evaluation of plating adhesion and Fe concentration in the plating film were measured. Here, the plating adhesion, after performing 180 ° close contact bending, the adhesion state of the zinc film after bonding the adhesive tape to the bent part,
This was peeled off and judged from the amount of peel plating adhered to the tape. The evaluation was based on the following five levels.

1 ... large peeling, 2 ... during peeling, 3 ... small peeling, 4
... Peeling slight, 5 ... No peeling Further, the Fe concentration in the plating layer was determined by X-ray diffraction. Further, the method of evaluating the secondary workability is exactly the same as that of the first embodiment. As is evident from Table 4, the steel of the present invention is a non-aging cold-drawn galvannealed steel sheet having excellent hot-dip galvanizing properties, and has a coating bake hardening property and a good secondary work brittle resistance. It also shows chemical properties.

Example 3 In Example 2, continuous hot-dip galvanizing without alloying treatment was performed. The sample is steel 3 of Example 2, and the continuous hot-dip galvanizing condition is that the maximum heating temperature is 780 ° C and the hot-dip galvanizing temperature is 460 ° C. Temper rolling (0.8%)
After that, the same evaluation as in Example 2 was performed. The characteristic values are as shown in Table 5, and according to the present invention, a room-temperature non-aging hot-dip galvanized steel sheet for deep drawing can be manufactured.

Example 4 Using steels 2 and 3 in Table 3, the cooling conditions after the completion of hot rolling were studied using actual equipment. Table 6 shows the hot rolling conditions, the relationship between r and r ₄₅ of the product sheet. Here, as the hot rolling conditions, the cooling conditions after finishing, particularly the time until the start of rapid cooling and the cooling rate were studied. In the cold rolling, the rolling reduction is 84% and the sheet thickness is 0.8 mm. The steel sheet was subjected to continuous annealing at 780 ° C. for 40 seconds and temper rolling at a rolling reduction of 0.8%. As is evident from Table 6, in the steel component of the present invention, after the completion of the hot rolling, cooling to a temperature of 750 ° C or less at a cooling rate of 50 ° C / sec or more within 0.5 seconds requires an r value, especially r. Important for ₄₅ improvement.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

[Table 6]

[0028]

As described in detail above, according to the present invention, T
Even without adding an expensive element such as i or Nb, a thin steel sheet which is non-aging and has excellent bake hardening properties and excellent deep drawability can be obtained, and the secondary work embrittlement resistance is also satisfied. Further, the present invention can be applied to a surface-treated steel sheet to be subjected to electroplating and the like, and its manufacture. As described above, the present invention not only enables production of a steel sheet having excellent performance stably at a low cost as compared with the prior art, but also secures the earth resources of expensive elements, or It is thought that the use of high-strength steel sheets contributes to global environmental protection, and the effect is remarkable.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-346625 (JP, A) JP-A-58-84928 (JP, A) JP-A-58-48636 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C22C 38/00 C21D 8/00-9/48

Claims (2)

(57) [Claims] C: 0.0001 to 0.00% by weight
15%, Si: 1.2% or less, Mn: 0.01 to 3%,
P: 0.01 to 0.15%, Cr: 0.1 to 3%, and Cr + 20P ≧ 0.2%, S: 0.0010 to 0.02
0%, Al: 0.005 to 0.15%, N: 0.000
1-0.0080%, B: 0.0001-0.0020
% Non-ageing deep-drawing thin steel sheet having a ferrite single-phase sizing structure composed of Fe and unavoidable impurities. 2. A slab comprising the chemical component according to claim 1 is finished by hot rolling at (Ar ₃ transformation temperature −100 ° C.) or more, and immediately thereafter, a cooling rate of 50 ° C./sec or more within 0.5 seconds. And rapidly cooled to 750 ° C or less
Winding at 0 ° C, cold rolling at a rolling rate of 70% or more,
A method for producing a non-ageing deep-drawing thin steel sheet having a ferrite single-phase sizing structure, characterized by continuously annealing at 600 to 900 ° C.