JPS61270341A - Manufacture of cold rolled steel sheet for deep drawing superior in ridging resistance and chemical conversion treatability - Google Patents

Abstract

PURPOSE:To manufacture the titled steel sheet, by finishing rolling low carbon steel at a specified low temp. range, further finishing the sheet using lubricant at least in one pass, when the steel is hot rolled to a prescribed thickness. CONSTITUTION:Continuously cast slab or bloomed slab of low carbon steel contg. <0.10% C, <0.01% N is heated to 950-1,150 deg.C and hot rolled, finishing rolling is at 600-800 deg.C temp. range, and at least one pass is rolled by using lubricant. Next, the plate is cold rolled by 50-95wt%s draft to finish it to the final sheet thickness, successively the sheet is recrystallization annealed at 650 deg.C being recrystallization temp.-950 deg.C by continuous annealing furnace. Further if necessary, it is cooled slowly at <=10 deg.C/sec rate, or over aging treated at about 350 deg.C. Steel sheet superior in ridging resistance, deep drawability and chemical conversion treatability can be manufactured.

Description

Detailed Description of the Invention (Industrial Field of Application) This specification relates to a method for producing cold-rolled steel sheets for deep drawing that has excellent ridging resistance and chemical conversion properties. The following describes the results of the development research that made low-temperature hot rolling possible.

Cold-rolled steel sheets, which are mainly used for automobile panels, are required to have excellent deep drawability. In order to improve deep drawability, the mechanical properties of the steel sheet need to be high ductility and high Lankford value (r value).

In addition, since deep-drawn products such as automobile exterior panels often serve as the outer surface, excellent surface quality is an important property, and chemical conversion treatment, which is a pretreatment for baking painting, is also important. It is.

Incidentally, the manufacturing process of steel materials has changed significantly in recent years, and the case of thin steel sheets for deep drawing is no exception.

In other words, conventionally, steel is made into slabs with a thickness of about 250 mm through ingot-making and blooming rolling, then heated and soaked in a heating furnace, processed into a sheet bar with a thickness of about 30 mm through a rough rolling process, and then heated. Hot-rolled steel strips were produced with a predetermined thickness in the finishing hot-rolling process so that the finishing temperature was higher than the Ar3 transformation point, but in recent years, the introduction of the continuous casting process has made it possible to omit the blooming process, and A new process that can eliminate the heat treatment and rough rolling steps of hot rolling is becoming a reality, by means of which sheet bars or copper strips with a thickness of 50 mm or less can be obtained immediately. However, all of these new manufacturing processes are disadvantageous in that they destroy the structure (casting group!ia) formed when molten steel solidifies. Especially formed during solidification (
110) It is extremely difficult to destroy a strong casting texture with <uvw> as the main orientation.

As a result, the final thin steel sheet tends to be prone to creasing, and when subjected to deformation such as press working, obvious irregularities extending in the rolling direction occur, impairing product value.

On the other hand, in the conventional production of cold-rolled steel sheets for deep drawing, it is necessary to raise the finishing temperature of hot rolling to a high temperature, which necessitates high-temperature slab heating and high-temperature hot rolling, but high-temperature slab heating requires less heating energy. There are disadvantages such as increased cost, reduced yield due to slab surface oxidation, quality problems due to increased internal oxidation products, and troubles during cold rolling. In addition, hot rolling at high temperatures tends to cause damage to the rolling rolls and causes deterioration of surface quality.

On the other hand, if a cold-rolled steel sheet with good deep drawability can be produced by low-temperature slab heating and low-temperature hot rolling, the above-mentioned problems can be eliminated and there are great manufacturing advantages.

(Prior Art) In fact, several methods for manufacturing deep-drawn steel plates by low-temperature hot rolling have been disclosed, such as Japanese Patent Application Laid-open No. 57-13123.

Examples include Japanese Patent Laid-Open No. 59-67322.

Both are characterized by low-temperature heating and low-temperature hot rolling, and are advantageous in terms of energy savings and improved material quality. However, these known techniques do not provide any suggestions for improving the above-mentioned ridging resistance and chemical conversion treatment properties, and more generally, regarding the ridging resistance of thin steel sheets, low temperature heating - low temperature hot rolling This is more disadvantageous than the case of high temperature heating-high temperature hot rolling.

On the other hand, in hot and cold rolling, lubrication rolling is performed in which lubricating oil is sprinkled on the roll surface, and this lubrication rolling significantly reduces the rolling load, resulting in energy savings, improvement in the surface properties of the steel strip, and This is advantageous in reducing roll wear, and various lubricant supply methods and lubricants have been considered for this purpose. In fact, regarding hot rolling, Japanese Patent Publication No. 52-462 and Japanese Unexamined Patent Application Publication No. 51-62167 are cited as known techniques.

However, all of these methods are for use in a hot region (usually finishing temperature Ar+ point or higher), which is different from use in a low-temperature hot rolling region (finishing temperature Ars point or lower) in this invention. Moreover, it did not provide any suggestions on methods for improving ridging resistance and chemical conversion treatment properties.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing a thin steel sheet having excellent ridging resistance and chemical conversion treatment properties by low-temperature heating and low-temperature hot rolling.

(Means for Solving the Problems) This invention, when hot rolling low carbon steel to a predetermined thickness, sets the finishing temperature to 600 to 800°C, uses lubricating oil in at least one bath, and continues rolling. Rolling reduction rate 50-95%
This is a method for producing a cold-rolled steel sheet for deep drawing, which has excellent ridging resistance and chemical conversion treatment properties, and is characterized by cold-rolling the steel sheet, followed by recrystallization annealing.

The research results that formed the basis of this invention are as follows.

The sample materials are shown in Table 1. Two types of low carbon aluminum killed steel slabs shown in Figure 1 were heated and soaked to 1100°C, rolled for 3 baths at a rolling reduction of 90%, and then cold rolled at a rolling reduction of 70%.

The relationship between the hot rolling finishing temperature (PDT) at this time, the lower value after annealing (soaking temperature 800° C.), and the ridging index is shown in FIG.

It was found that the lower value and ridging resistance deteriorate significantly as the finishing temperature decreases in non-lubricated hot rolling (marked with a *), but do not depend so much on the finishing temperature in lubricated hot rolling.

In addition, it was newly discovered by comparing cold rolled steel sheets that have undergone subsequent processes that lubricated hot rolling has better chemical conversion properties than non-lubricated hot rolling. As a result of repeated research based on this basic data, the inventors discovered that by regulating the manufacturing conditions as shown below, it is possible to manufacture thin steel sheets with excellent deep drawability, ridging resistance, and chemical conversion treatment properties.

The effect of lubricated rolling is related to the phenomenon that crystal orientation (for example, (100)), which is unfavorable for drawability and ridging resistance, is reduced by the use of lubrication, but the mechanism is not clear.

(Manufacturing conditions and effects) +1) Steel composition The effects of low temperature lubrication hot rolling essentially do not depend on the steel composition. However, in order to ensure deep drawability above a certain level, the interstitial solid solution elements C and N should each be 0.10%,
It is preferable that it is less than o, oi%.

In addition, reducing O in steel by adding Al is advantageous for improving material quality and ductility.Furthermore, in order to obtain better deep drawability, C and N are precipitated as stable carbonitrides. Special elements that can be fixed, such as Ti+ Nb+ Zr,
Addition of B and the like is also effective.

(2) Rolled material As for the rolled material, steel slabs obtained by conventional methods such as ingot-blowing rolling or continuous casting can of course be used. In this case, the appropriate heating temperature for the steel slab is 800 to 1250°C, especially 9
A temperature of 50 to 1150°C is suitable. Of course, a continuous casting-direct rolling method (so-called CG-DR method), in which rolling is started without reheating the steel billet after continuous casting, is also applicable.

On the other hand, methods of directly casting rolled material of 50 mm or less from molten steel (sheet bar caster method and strip caster method) also have great economic merits from the viewpoint of energy saving and process saving, and are particularly advantageous for manufacturing the rolled material of this invention. It is.

(3) Hot rolling This step is the most important in this invention.

It is essential to carry out finish rolling at a temperature range of 600° C. to 800° C., and to finish using lubrication in a small amount (at least one bath).

When the finish rolling temperature is in a high temperature range of 800° C. or higher, various problems occur in high temperature hot rolling, and there are few merits based on low temperature hot rolling in this invention. In addition, this finish rolling temperature is
At temperatures below 600°C, there is a rapid deterioration of the r value and a sudden increase in the rolling load, and there is little merit from the viewpoint of energy saving.

The coiling temperature after hot rolling is not particularly limited, but is preferably 600° C. or lower in order to improve pickling efficiency.

The number of rolling passes and the distribution of the rolling reduction ratio may be arbitrary as long as the condition is satisfied that the product is finished with lubrication in at least one pass, and of course the arrangement structure of the rolling mill, roll diameter, tension, etc. have no essential influence.

The type of lubricating oil and the method of dispersing it are also not limited in this invention. Preference is given to applying suspension oils based on mineral oil or the like in the usual manner.

(4) Cold rolling This step is also essential in this invention, and in order to reduce the high r value and in-plane anisotropy, the cold rolling reduction rate is 50
~95% is essential.

If the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained.

(5) Annealing A cold rolled steel strip that has undergone an annealing cold rolling process requires recrystallization annealing. The annealing method may be either a box annealing method or a continuous annealing method, but the latter is advantageous from the viewpoint of homogeneity and productivity.

The heating temperature ranges from the recrystallization temperature (approximately 650°C) to 950°C. Regarding the heat cycle in the case of continuous annealing, the conditions such as the cooling rate after soaking, the presence or absence of overaging treatment, and the heat cycle do not have any essential influence on this invention, but The following slow cooling or overaging treatment at around 350°C is effective in improving the material quality and ductility.

After annealing, temper rolling of 10% or less can be applied to the belt for shape correction, adjustment of surface roughness, etc.

The cold-rolled steel sheet obtained by the present invention can also be used as an original sheet for surface steel sheets for processing, and surface treatments include galvanizing (including alloy-based), tin plating, and enameling.

(Example) Steel slabs having the chemical compositions shown in Table 2 were manufactured by a converter steel manufacturing process and a continuous casting method.

Table 2 (Kat%) After heating and soaking this steel slab to 950°C to 1150°C, it was roughly rolled into a sheet bar with a thickness of 20-30mm, and was continuously rolled using a finishing rolling machine consisting of 7 rows. A hot-rolled plate with a thickness of 2 am was used. At this time, lubricated rolling was performed using all rows of rolling mills.

Subsequently, after pickling, a cold rolled plate having a thickness of 0.8 mm (cold rolling reduction 75%) or 1.7 mm (cold rolling reduction 47%) was prepared, and then recrystallization annealing was performed.

Table 3 shows the material properties after continuous annealing under different hot rolling conditions.

The tensile properties were determined using a JISS No. test piece.

The ridging property was evaluated using a No. J'ISS test piece cut out from the rolling direction, subjected to 15% tensile strain, and visually evaluated for surface irregularities on a scale of 1 (good) to 5 (poor). Rating 1
．． No. 2 indicates ridging properties that pose no problem in practical use.

For chemical conversion treatment, the steel plate is degreased, washed with water, and treated with phosphate.
Evaluation was made based on the pinhole area ratio and the number of crystal precipitates when performing the pinhole test described below.

In addition, the phosphoric acid treatment is performed using Nippon Barcalizing BT31.
12, total acidity 14.3, free acidity 0.5 at 55℃
and sprayed for 120 seconds.

In the pinhole test, a filter paper soaked with a reagent (ferrooxyl solution) that reacts with iron ions to form a color is placed in close contact with the test surface to detect areas where phosphoric acid crystals remain on the steel plate surface and perform image analysis. It was quantified as the pinhole area ratio. The number of crystal precipitation was determined by SEM observation. The evaluation criteria for chemical conversion treatment properties were determined as follows. First, the pinhole area ratio is less than 2%, 0.2 to 9% is △,
For exceeding 9%, X or the number of crystal precipitation (X 10'
/mm2) was evaluated as 5 or more and less than 0.5 as 3, and as less than 3 as X.

On the other hand, the sheet bars shown in Table 4 were manufactured by the converter steelmaking-sheet bar caster method.

Table 4 (Sail %) The sheet bar was made into a hot-rolled sheet with a thickness of 3.2 mm without being reheated using equipment in which a finishing rolling mill consisting of four rows was connected to the sheet bar caster.

At this time as well, lubricated rolling was performed using all rows of rolling mills.

After subsequent pickling, a cold-rolled plate with a thickness of 0.8 mm was obtained.

Regarding the annealing conditions and material testing methods, as already described and compared in Table 5, the steel sheets obtained according to the present invention exhibit a high T value, as well as excellent ridging resistance and chemical conversion treatment properties.

(Effects of the Invention) According to the present invention, it is possible to improve deep drawability and chemical conversion treatment properties without causing ridging defects, regardless of the working degree of hot rolling applied to cold rolled sheets. Therefore, it is also useful for reducing energy costs.

[Brief explanation of the drawing]

FIG. 1 is a graph comparing the influence of hot rolling temperature on T-value and rolling index with and without lubrication.

Claims (1)

[Claims] 1. When hot rolling low carbon steel to a predetermined thickness, the finishing temperature is 600 to 800°C, and at least one pass is finished using lubricating oil, followed by cold rolling at a reduction rate of 50 to 95%. , followed by recrystallization annealing. A method for producing a cold-rolled steel sheet for deep drawing, which has excellent ridging resistance and chemical conversion treatment properties.