JPS63230825A - Improvement of yielding ratio, ductility and delayed aging characteristic at room temperature of steel stock for working - Google Patents

Abstract

PURPOSE:To advantageously improve yielding ratio, ductility and delayed aging characteristic at room temp., by subjecting a steel stock after hot rolling or cold rolling-annealing to cold working under specific conditions determined by rate of strain and temp. and at a specific draft. CONSTITUTION:The steel stock for working passed through the course of hot rolling or cold rolling-annealing and comparatively reduced in initial dislocation density is subjected to cold working under the conditions that the value of A determined by equations, A=epsiloneXp(500/T) and epsilon=vR[2/D(H-h)]<1/2>.ln(H/h) [where T represents absolute temp.(K), D represents roll diameter(m), H and (h) mean entry thickness(m) and exit thickness(m), respectively, vR is the peripheral speed of a roll(m/S), and (epsilon) stands for average rate of strain(S<-1>)], is regulated to >=100 and a draft is also regulated to <=10%. By this method, yielding ratio can be reduced and ductility and delayed aging characteristic at room temp. can also be improved in the steel stock for various workings independently of the component system, structure, etc., of the steel.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for improving the yield ratio, ductility, and room temperature slow aging properties of steel materials for processing, particularly those that require excellent workability such as steel sheets for automobiles. It is particularly suitable for application to steel materials.

(Prior Art) As a method of manufacturing a low yield ratio steel plate, there is a method of subjecting a hot rolled steel plate or a cold rolled annealed plate to skin pass rolling. Such skin pass rolling is particularly effective for steel plates exhibiting a remarkable yield point phenomenon, and is carried out as a process to prevent stretcher strain. At that time, the appropriate skin pass rolling reduction rate is determined by microstructural factors such as the ferrite grain size of the steel sheet, but generally steel sheets with a fine structure require a large rolling reduction rate, which in turn leads to deterioration of elongation. Ta.

As a solution to the above problems, a so-called two-phase system [
% (Dual Phase) steel was developed. For example, hot-rolled steel sheets are disclosed in JP-A-51-12317 and JP-A-54414426.
Both are two-phase composite fibers consisting of ferrite and martensite, and have excellent properties such as a low yield ratio, high ductility, and hardenability when baked. There are also many similar techniques for cold-rolled steel sheets.

Detailed reports on these can be found, for example, in Tetsu to Hagane νo168N
Collected in the special issue of high-strength thin steel sheets in α9 (1982).

(Problems to be Solved by the Invention) However, such dual-phase steel also has the following problems.

+ ) Since relatively large amounts of Mn, Cr+ Sil P, etc. are added, weldability and surface treatment properties deteriorate.

ii) When manufactured by a rapid cooling process such as water quenching or a cryogenic winding process, the shape is greatly disturbed and a leveler, skin pass, etc. are required, but an increase in yield ratio is unavoidable.

This invention advantageously solves the above-mentioned problems, and applies not only to two-phase fiber steel, but also to all steel materials for processing, regardless of the composition or structure of the steel.
The purpose is to propose a method that can advantageously improve ductility and room temperature slow aging properties.

(Means for Solving the Problems) Based on the knowledge of conventional skin pass rolling, the inventors have conducted repeated studies focusing mainly on the rolling temperature and strain rate, which are not conventional. In other words, conventional skin pass rolling is performed at room temperature or higher at a relatively low speed (about several hundred mpm).
On the other hand, we investigated the effect of rolling at a lower temperature and higher speed.

As a result, it was found that high-speed rolling at low temperature and low reduction rate not only significantly lowers the yield ratio but also improves ductility and room temperature slow aging properties.

This invention is based on the above knowledge.

In other words, the present invention provides processing steel material that has undergone hot rolling or cold rolling and annealing, and has an A value determined by the following formula of 1.
Yield ratio of steel material for processing, characterized by cold working under conditions of 00 or more and rolling reduction: 10% or less,
This is a method for improving ductility and slow aging at room temperature.

Note A = a exp (500/T) a -Vl i I j! n (H/h) where T: Absolute temperature (K), D: Roll diameter (m), H: Inlet side plate thickness (n+), h: Outlet side plate thickness (m), vR: Roll circumferential speed (O+/ S), ``e: Average strain rate (s-').

It is.

The scope of application of the present invention is applicable to general steel materials, and can also be applied to thick plates, wire rods, etc., as long as they require a low yield ratio and high ductility. A particularly typical example is a thin plate such as an automobile steel plate.

Typical compositions of steel materials for processing are listed below.

Ci 0.0020-0.20% Si; 1.5% or less Mn; 0.2-3.0% P; 0.01-0.10% 81; 0.005-0.060% Now, this invention There are three requirements: That is, i) processing temperature ii) processing speed (strain rate) ■i) rolling reduction rate.

Regarding i) and ii), they are expressed by the following three equations based on their equal characteristics. The formula below was obtained based on data obtained through numerous experiments.

Here, T: Absolute temperature (K) D: Roll diameter (m) H: Inlet side plate thickness (a+) h; Outlet side plate thickness C++) ν; Roll circumferential speed (+++/s) λ: Average strain rate (s- ') In Figure 1, the rolling speed is set to 60 pII + constant, and D-3
The results are shown in which the tensile properties were investigated at room temperature after rolling at a wide range of rolling temperatures while applying a 2% reduction with a 00 cylinder roll.

As is clear from the figure, YP is low on the low temperature side where A calculated by Dowaage (1) and (2) equations is >100.

A decrease in Y, R, and an increase in El are observed.

In Figure 2, the rolling temperature is constant at -20°C, and D = 300.
The results are shown in which the tensile properties were investigated at room temperature after rolling at a wide range of rolling speeds while applying a reduction of 5% with a roll.

In this figure, as in Figure 1, on the high rolling speed side where A calculated by formulas (1) and (2) is ≧100,
Low y, p, , low Y, R, and further increase in Bit were observed.

Next, in Fig. 3, the rolling temperature is 40°C and the rolling speed is 100f.
The results are shown in which the tensile properties at room temperature were investigated after rolling was performed with a roll of D=300 mm and a wide range of rolling reductions with a constant fipII+.

According to the same figure, A! It has been found that lower Y and R can be achieved by maintaining a better relationship between strength and ductility by rolling at low temperatures and high speeds with a rolling reduction of up to 10%.

From the above, for cold working conditions, A = e e
xp (-) e=vJ Low resistance in (H/h) where T: Absolute temperature (K) D: Roll diameter (m) H: Inlet side plate thickness (m) h; Outlet side plate thickness (m) Ga; The range is limited to a range in which A, which is determined by roll circumferential speed (m/s) and average strain rate (s-'), is 100 or more and the rolling reduction is 10% or less.

(Function) Although the reason why material quality is improved by cold working according to the present invention has not necessarily been clearly elucidated,
The inventors speculate as follows.

When rolling at low temperatures and/or high speeds, it becomes difficult for screw dislocations to move, and cross-slip is also difficult, so the dislocation structure after such processing will have dislocations distributed very uniformly. , when the fibers are rolled at room temperature or higher and/or at low speeds, they have a non-uniform so-called cell structure.

When a material with a structure in which such dislocations are distributed very uniformly is processed at room temperature, these dislocations contribute to deformation as mobile dislocations, resulting in a low yield ratio, which also affects the high strain side. Therefore, total elongation is also improved. In addition, regarding room temperature aging based on the fixation of dislocations by interstitial solid solution atoms, aging becomes slower when dislocations are uniformly distributed. This was estimated based on the observation results of the sample using a transmission electron microscope.

In this invention, the target material is not particularly limited, but
If it is manufactured in an annealed state or by a normal hot rolled steel sheet manufacturing process, in other words, if the initial dislocation density is not very high (or not), a sufficient effect will be recognized.

(Example) Hot-rolled sheets and cold-rolled sheets having the component compositions shown in Table 1 were added to Table 2.
Table 2 also shows the results of investigating the tensile properties and aging properties of each steel plate obtained by cold rolling under the various conditions shown in Table 2.

The aging characteristics were evaluated by the difference JY and Effi between Y and El after rolling and YJf after aging treatment for 3 months at room temperature.

As is clear from Table 2, according to the present invention, A (direction is 10
When rolling is performed under conditions of 0 or more, y, p, and eventually Y, R, decrease, and workability improves, regardless of the steel type.

In addition, regarding aging properties, steel types A and B, which are originally slow aging properties,
However, there was no difference in steel types with aging properties (C-F).
By setting the A value to 100 or more, lJY
, Eβ is decreasing.

(Effects of the Invention) Thus, according to the present invention, it is possible not only to lower the yield ratio of various steel materials for processing, but also to improve the ductility and slow aging properties at room temperature.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the A value and tensile properties when rolling is performed with the rolling reduction rate and rolling speed constant and the rolling temperature varied. Figure 3 is a graph showing the relationship between the A value and the tensile properties when rolling was performed at a constant temperature and at various rolling speeds.
It is a graph showing the relationship between the A value and the rolling reduction ratio and the tensile properties when the rolling reduction ratio is varied.

Claims (1)

[Scope of Claims] 1. For steel materials for processing that have undergone hot rolling or cold rolling and annealing, cold working is performed under conditions where the A value determined by the following formula is 100 or more and the rolling reduction is 10% or less. A method for improving the yield ratio, ductility, and room temperature slow aging property of a working steel material, the method comprising: Note A=■exp(500/T) ■=V_R√{2/D(H-h)}・ln(H/h) where T: Absolute temperature (K), D: Roll diameter (m), H : Inlet side plate thickness (m), h: Outlet side plate thickness (m), V_R: Roll circumferential speed (m/s), ■: Average strain rate (s^-^1).