JPS61204327A - Production of as-rolled thin steel sheet for working having excellent ridging resistance and deep drawability - Google Patents

Abstract

PURPOSE:To obtain a titled thin steel sheet having excellent ridging resistance and deep drawability with a new process without including cold rolling and recrystallization annealing by specifying rolling conditions in a stage for rolling a low carbon steel to a prescribed sheet thickness. CONSTITUTION:The low-carbon steel is rolled in the temp. range from the recrystallization temp. of ferrite to 300 deg.C and at >=35% draft and >=300 s<-1> strain rate in at least one pass in finish rolling in the stage of rolling the low- carbon steel to the prescribed sheet thickness. The thin steel sheet having the good deep drawability and excellent ridging resistance is thus obtd. as-rolled without executing not only the conventional cold rolling but also recrystallization annealing by the high draft and high strain rate rolling in the above-mentioned temp. range.

Description

[Detailed Description of the Invention] (Industrial Application Field) The technical content described in this specification regarding the production of thin steel sheets with excellent ridging resistance and deep drawability includes cold rolling and recrystallization annealing by regulating rolling conditions. The purpose is to disclose the development results of new processes that can omit steps.

Processing thin steel sheets with a thickness of approximately 2+++- or less and used for applications such as building materials, automobile body materials, can stock, and various surface-treated original sheets are required to have the following properties.

(1) Mechanical properties In order to obtain good bending workability, stretchability and drawing workability, high ductility and high Lankford value (r value) are mainly required.

In particular, parts such as automobile panels, oil pans, and gasoline tanks are particularly difficult to form by deep drawing, and are required to have an r value of 1.7 or more, although it depends on the shape of the part.

(2) Surface properties Since these materials are mainly used on the outermost side of the final product, not only the shape and surface beauty of the material but also various surface treatments are important.

The general manufacturing method for these 'a steel plates is as follows.

First, low carbon steel is mainly used as the steel cable material, and after being made into a piece of rope with a thickness of about 200 mm by ingot-forming and blooming rolling, it is heated and soaked in a heating furnace, and then a rough hot rolling process is performed to thicken the plate. Approximately 30
After forming a sheet bar into a hot-rolled steel strip with a predetermined thickness in a final hot rolling process at a finishing temperature of Ar3 transformation point or higher, the strip is then pickled and cold-rolled to a predetermined thickness (2 mm). , 0 mm or less), and then subjected to recrystallization annealing to produce the final product.

The biggest drawback of this practice is the extremely long process required to reach the final product. As a result, not only does the amount of energy, personnel and time required to produce the product increase, but additional disadvantages are added during these long steps which can cause problems with the quality of the product, particularly its surface properties. For example, unavoidable troubles include the occurrence of surface defects in the cold rolling process, deterioration in surface beauty due to surface concentration of impurity elements and surface oxidation in the recrystallization annealing process, and deterioration in surface treatability.

By the way, as a method of manufacturing thin steel sheets for processing, a method of producing the final product through a hot rolling process is also considered. According to this method, cold rolling and recrystallization annealing steps can be omitted, which is a great advantage.

However, the mechanical properties of a hot-rolled thin steel sheet are far inferior to those obtained through a cold rolling-annealing process. In particular, press-formed materials used for automobile bodies require excellent deep drawability, but hot-rolled steel sheets have a low r value of around 1.0, so their processing applications are extremely limited. Become something. This is because in the conventional hot rolling method, the finishing temperature is Ar, which is higher than the transformation temperature, so that the texture becomes random during the γ-α transformation.

In addition, it is extremely difficult to manufacture thin steel sheets with a thickness of 2.0 mm or less using only a hot rolling process. Moreover, in addition to the problem of dimensional clarity, the decrease in steel sheet temperature due to thinning forces the rolling of low carbon steel below the Ar = transformation point (and the material quality (
ductility, drawability), and even A
Even if the quality of the material can be ensured by rolling below r=transformation point, a new problem arises in that steel sheets rolled in the ferrite region are more likely to suffer from cringing.

Here, ridging is a defect in surface irregularities that occurs during the processing of a product, and is a fatal defect for this type of steel plate, which is mainly used on the outermost side of processed products.

In terms of metallurgy, ridging is caused by crystal orientation groups (for example, (100) oriented grain groups) that are not easily divided even after undergoing the annealing recrystallization process and remain stretched in the rolling direction. , generally tends to occur when processing is performed at a relatively high temperature in the ferrite (α) region, and this tendency is particularly strong when the reduction rate in the ferrite region is high, that is, when manufacturing thin steel sheets.

Recently, these thin steel sheets for processing have become more complex,
As materials become more sophisticated, they are often subject to more severe processing, and excellent ridging resistance is now required.

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

In other words, in recent years, the introduction of the continuous casting process has made it possible to omit the blooming process, and the heating temperature of the steel strip has tended to be lowered from the conventional 1200°C to around 1100°C or lower in order to improve material quality and save energy. It is in. Furthermore, a process is being put into practical use in which a steel strip having a thickness of 5 mm or less is produced immediately from molten steel, thereby omitting the heat treatment of hot rolling and the rough rolling process.

However, these new manufacturing processes are disadvantageous in that they destroy the group Va (casting structure) formed when molten steel solidifies. Especially formed during solidification (100
) It is extremely difficult to destroy the strong casting texture whose main orientation is <uVW>.

As a result, the final thin steel sheet was prone to the aforementioned jinging.

(Prior art) Several methods have been proposed for manufacturing thin steel sheets for processing, which are formed into a thin steel sheet of a predetermined thickness in a relatively low temperature range below the Ar transformation point, and then are not subjected to cold rolling or recrystallization annealing steps. There is. For example, in JP-A-48-4329, low carbon rimmed steel is rolled to 411I by 90% rolling at a temperature below the Ar3 transformation point.
Yield point: 26.1 kg due to steel strip having thickness of I11
/mm”, tensile strength 37.3kg/mm!, elongation 49.
7%.

r=l. A fabrication example with 29 properties is shown.

Furthermore, in JP-A-52-44718, a low carbon rimmed steel is hot-rolled to a thickness of 2.0 mm at a finishing temperature of 800 to 860°C (below the Ars transformation point), and a coiling temperature of 600 to 730°C is disclosed.
℃ has been shown to produce a low yield point steel plate with a yield point of 20 kg/mm or less. However, the conical cup value, which is an index of drawability, is 60.
．． It is about 60 to 62.18 mm, which is different from the conventional example 6.
Compared to 0.58 to 60.61, the drawability is the same or lower. Furthermore, in JP-A-53-22850, a low carbon rimmed steel is hot rolled to a thickness of 1.8 to 2.3 mm at a finishing temperature of 710 to 750°C, and a coiling temperature of 530 to 60°C is disclosed.
A method of manufacturing a low carbon hot rolled steel sheet by setting the temperature to 0°C is shown. However, the conical cup value of the product obtained by this method is also higher than that of the conventional example, as in the case of JP-A-52-44718, and the drawability is inferior. Furthermore, JP-A-54-109022 discloses that low carbon aluminum killed steel is hot-rolled to a thickness of 1.6 mm at a finishing temperature of 760 to 820°C and has a yield point of 14.6 mm at a coiling temperature of 650 to 690°C. 9～18.8kg/mm”
.

Tensile strength 27.7-29.8kg/nun”, elongation 39
．． An example of manufacturing a low strength mild steel plate having properties of 0 to 44.8% is disclosed. In addition, JP-A-59-226149 has C10,002°5i10.02. Mn0.
23. Plo, 009. S10.008. Al
10.025°N10.0021. Ti10.10
An example of manufacturing a thin steel plate having a property of r = 1.21 by rolling 76% of low carbon Afi killed steel at 500 to 900°C with lubricating oil to a plate thickness of 1.6 mm. It is shown.

However, none of the above-mentioned known techniques gives any consideration to improving the above-mentioned ridging resistance.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing thin steel sheets with excellent ridging resistance and deep drawability by a new process that does not include not only cold rolling but also recrystallization annealing. It is.

(Means for Solving the Problems) This invention provides at least one pass in the process of rolling low carbon steel to a predetermined thickness in a temperature range from the ferrite recrystallization temperature to 300°C, and rolling reduction = 35. % or more, strain rate: 300 (s”1
) This is a method for manufacturing an azu-rolled thin steel sheet with excellent ridging resistance and deep drawability, which is characterized by completing the process as described above.

First, the research results that formed the basis of this invention will be explained.

A hot-rolled low-carbon aluminum killed steel sheet shown in Table 1 was used, heated and soaked to 450°C, and heated to 20% in one bath.

Rolling was performed at a rolling reduction of 40% and 60%, respectively.

Figure 1 shows the results of an investigation into the relationship between the strain rate (ε) and the rolling index of the steel sheet after rolling.

The ridding index strongly depends on the strain rate and reduction ratio, and is 40
When high strain and high reduction rate rolling of 300 s-' or more was performed at a rolling reduction of 60% and 60%, the ridging resistance was significantly improved.

The strain rate (ε) is calculated according to the following formula: n: Number of rotations of rolling rolls (rpm) r: Reduction rate (χ)/100 R: Radius of rolling rolls (mm) Ho: Thickness of plate before rolling ( mm) Next, Fig. 2 shows the results of investigating the r-value of the rolled plates of A and B steel slabs shown in Table 1 and varying the rolling temperature. At this time, ; =852s-', the rolling reduction rate was 65χ. In addition, the recrystallization temperature of A and B111 is as shown in Table 1, and this recrystallization temperature is based on the hardness and structure when cold rolling is performed at room temperature by 75% and heated at a heating rate of 20°C/h. This is what we seek from change.

It can be seen that both the image quality and the r value sharply increase due to rolling below the recrystallization temperature. However, when rolled at a temperature of about 300° C. or lower, recrystallization does not occur in Azurold, and therefore the r value deteriorates rapidly.

As a result of repeated research based on these basic data, the inventors confirmed that a thin steel sheet with excellent ridging resistance and deep drawability can be manufactured by regulating the manufacturing conditions as described below.

(1) Steel composition The effects of high strain rate rolling do not essentially depend on the steel composition. However, in order to ensure deep drawability above a certain level, the interstitial solid solution elements C and N must each be 0.10
% or less, preferably 0.01% or less. Also, AI for Hagane Medium 0! Reduction by addition of is advantageous for improving material quality, especially ductility. Furthermore, in order to obtain even better deep drawability, it is also effective to add special elements such as Tt+Nb+Zr and B, which can precipitate and fix C and N as stable carbonitrides.

Further, in order to obtain high strength, P, Si, Mn, etc. can be added depending on the strength.

(2) Manufacturing method of rolled material Steel slabs obtained by conventional methods, ie, ingot-blowing rolling or continuous casting methods, can of course be applied.

The heating temperature of the steel piece is suitably 800 to 1250°C, and preferably less than 1100°C from the viewpoint of energy saving. Of course, the so-called CC-DR (continuous casting-direct rolling) method, in which rolling is started without reheating the steel billet after continuous casting, is also applicable.

On the other hand, the method of immediately casting a rolled material of 50 mm or less from molten steel (sheet bar caster method and trip caster method) also has great economic merit from the viewpoint of energy saving and process saving, so it is particularly advantageous as a method for manufacturing rolled material.

(3) Rolling process This process is the most important.When rolling low carbon steel to a predetermined thickness, at least one bath is performed in the finish rolling at a temperature ranging from the ferrite recrystallization temperature to 300°C. It is essential to roll under conditions of a rolling reduction of 35% or more and a strain rate of 300 s-' or more.

In the high temperature range where the finish rolling temperature exceeds the recrystallization temperature of ferrite and the low temperature range below 300°C, even if the rolling reduction is 35
% or more and at a strain rate of 300 s-' or more, only poor deep drawability can be obtained as shown in Figure 2 above, so the finish rolling temperature is between the ferrite recrystallization temperature and 300°C. limited to the range of

Regarding the strain rate, if the strain rate is less than 300 s-', the target material quality cannot be secured, so a strain rate of 300 s-' or more, particularly 500 to 2500 s-', is suitable.

The number of rolling passes and the distribution of the rolling reduction ratio may be arbitrary as long as the above conditions are satisfied.

The arrangement, structure, tension, presence or absence of lubrication of the rolling mill, etc. have no essential influence.

In principle, recrystallization annealing treatment is not necessary, but due to material requirements, heat retention and soaking treatment must be performed on the runout table after rolling and during the winding process, and if necessary, some heat treatment must be performed after rolling. Heat treatment is not prohibited.

(4) Pickling and temper rolling The copper strip obtained by the above procedure is rolled at a lower temperature than conventional methods, so the oxidation layer is thinner and the pickling property is extremely good, so no pickling is required. It can also be used for a wide range of purposes. In addition to conventional acid removal, mechanical removal can also be used for descaling. Furthermore, for the purpose of shape correction, surface roughness adjustment, etc.
Temper rolling of 10% or less can be added.

(5) Surface treatment The copper strip thus obtained is galvanized (including alloy-based)
It has excellent surface treatment properties such as tin plating and enameling, so it can be used as various surface-treated base plates.

(Function) The reason why the ridging resistance and T value are significantly improved by rolling at a high reduction rate and high strain rate according to the present invention has not yet been clearly elucidated.
This is thought to be closely related to changes in the texture and processing strain of the rolled material.

(Example) Each of the composition steels shown in Table 2 was processed to a thickness of 2 by the method shown in Table 3.
After forming the sheet bar into a sheet bar having a thickness of 5 to 40 a+m, a thin steel plate having a thickness of 0.8 to 1.0 cm was formed using a rolling mill consisting of 6 rows.

At this time, high strain and high reduction rolling was performed using six stands. Note that the recrystallization temperature of the test steel was determined by the method described above.

Table 3 shows the material properties of the thus obtained thin steel sheet after pickling and greenhouse rolling (reduction ratio of 0.5 to 1.2x). The tensile properties were determined as a JISS No. test piece. In addition, the ridging property was evaluated using a JIS No. 5 test piece cut out from the rolling direction and subjected to 15% tensile strain by visually observing the surface unevenness from 1 (good) to 5 (poor). . No evaluation criteria have been established for this evaluation because virtually no shearing occurred using the conventional manufacturing method for low-carbon cold-rolled steel sheets. Therefore, in the present invention, the index evaluation criteria based on the visual method for conventional stainless steels are applied as they are.

An evaluation of 1.2 indicates ridging properties with no practical problems.

Steel sheets manufactured according to the invention exhibit far superior lower values and ridging resistance than the comparative examples.

(Effects of the Invention) Thus, according to the present invention, by rolling at a high reduction rate and high strain rate in the temperature range from the ferrite recrystallization temperature to 300°C, the azurolled steel sheet that not only omits conventional cold rolling but also recrystallization annealing can be achieved. It is possible to obtain a thin steel plate with good deep drawability and excellent ridging resistance without changing the process.
Moreover, the rolled material is compatible with the sheet bar caster method, strip caster method, etc., and the manufacturing process of thin steel sheets for processing can be greatly simplified.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effect of strain rate on the lower value and ridging index using the rolling reduction as a parameter. FIG. 2 is a graph showing the relationship between rolling temperature and lower value. Figure 1 100 200,! 100 600 strain ts-a11 (S-re

Claims (1)

[Claims] 1. In the process of rolling low carbon steel to a predetermined thickness, at least one pass is performed at a temperature range from the recrystallization temperature of ferrite to 300°C, rolling reduction: 35% or more, strain rate: 300(s^-^
1) A method for manufacturing an as-rolled thin steel sheet having excellent ridging resistance and deep drawability, which comprises rolling at the above steps.