JP2854313B2 - Method for producing low phosphorus rolled steel sheet for processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention provides a high ductility of 50% or more, a value of about 0.9 to 1.0,
The present invention relates to a method for producing a low-phosphorous hot-rolled steel sheet for processing, which has each | Δr | value of 0.1 or less and does not cause vertical cracks.

Longitudinal cracking refers to brittle cracking that occurs in the wall of a processed product when a secondary load is applied to a single-stage or multi-stage drawing process on a disc-shaped blank obtained from a steel plate. And a steel sheet free of such cracks is considered to be a steel sheet having excellent secondary workability.

<Prior art> In a steel sheet for the above-mentioned applications, a steel sheet having no vertical cracks during pressing and having a small wave height of deformation generated at an upper end portion when a blank is pressed, that is, earring e (shown in FIG. 3) is used. Is desired.

In recent years, the conditions for drawing have become increasingly severe, and various studies have been made on the development of steel materials that meet these requirements.

A proposal for a steel material that meets this demand and a method for manufacturing the same is disclosed in Kawasaki Steel Technology Vol.19 No.3 1987 P41-46.

This proposal is a very low C material of 0.0015-0.0025%, Mn: 0.09-0.11% P: 0.01-0.12% S: ≦ 0.003% Al: 0.028-0.031% N: 0.0022-0.0028% Nb: 0.007-0.019% Ti: 0.024 to 0.028% is added, and rolling is performed at 910 ° C or more at _three or more points of Ar and wound at 540 ° C to 680 ° C. YP: 16.8 to 19.5 El: 53.4 to 59.6: 0.8 to 0.92 Δr: −0.15 It provides steel with a material of 0.37.

There is also a proposal by Japanese Patent Application Laid-Open No. 52-125411.

This proposal includes: C: ≦ 0.010% Mn: 0.03-0.15% Si: ≦ 0.015% S: ≦ 0.05% Al: 0.04-0.10% B: 0.001-0.005% Rem: 0.002-0.05% Rem% / S% : 0.5 ~ 20.0 B% / N%: 0.2 ~ 1.0, and normal hot rolling at an Ar ₃ point temperature or higher is performed.
Wound at 710 ℃, YP: 18.2~22.6kgf / mm 2 El: 54 ~56% value: from 0.9 to 0.98 grain size: a method of manufacturing a steel sheet having a material of 8.0 to 8.8 th.

In general, it is known that there is a correlation between the | Δr | value (r value anisotropic difference) and the earring as shown in FIG. 2. In order to obtain a good earring having a diameter of 4 mm or less, it was necessary to produce a steel sheet having good workability having a | Δr | value of 0.2 or less.

It is known that the molding height (a) increases as the elongation increases, as shown by the dotted line in FIG. 4, and becomes saturated when the elongation reaches 50%.

As shown in FIG. 3, the molding height (a) is represented by the dimension (b) of the shallowest part at the time of press molding-the required product height (c) = (a). The material yield is improved by reducing the blank diameter based on this. Therefore, in the steel sheet in which (e) is small and (b) is large and the forming height (a) is high, the material yield is remarkably improved.

However, when the elongation reaches about 50%, the improvement of the forming height (a), that is, the reduction of the earring (e) is saturated, so conventionally, if the elongation is about 50%, the height of the forming is reduced. It was good.

However, the drawing conditions in recent years have become increasingly severe, and there has been a demand for further improvement in the molding height. However, the improvement due to elongation is limited, and in order to further reduce the earrings, the | Δr | value is shaded in FIG. It is necessary to set the area to 0.1 or less.

<Problem to be Solved by the Invention> The present invention satisfies this demand and has the above-mentioned problem that the proposal has been waiting for. That is, since is based on extremely low C, the C in steel is reduced to this level. Requires a great deal of processing cost, and further requires the addition of expensive Nb and Ti, so that the production cost inevitably increases.
Is fixed as AlN or BN to improve the non-aging property and elongation of the steel sheet. For this reason, Al required for each precipitation and B for forming nitride at a high temperature are added in required amounts.
When the amount of addition becomes 0.001% or more, the current demand described above, 0.1%
The following | Δr | value cannot be obtained stably.

The addition of B has an effect on longitudinal cracking resistance, and | Δr |
Due to the decrease in the value, when B is not added, the frequency of occurrence of vertical cracks increases.

 An object of the present invention is to eliminate these problems.

<Means for Solving the Problems> In order to solve the above problems, the present invention contains, by weight%, C: more than 0.025 to 0.07% Mn: 0.01 to 0.4% P: less than 0.005% S: 0.01% or less , With the balance being Fe and unavoidable impurities
550 ℃ ～ 65 after rolling in the recrystallization area of Ar ₃ points + 20 ℃ or more
Winding in the range of 0 ° C. is a solution.

The reasons for limiting the upper limit or lower limit of each component described above are as follows.

C is desirably at an extremely low carbon level from the viewpoint of the material of the steel sheet of the present invention.

However, at the present time, processing to an extremely low carbon level requires high processing costs, and an increase in manufacturing costs is inevitable. Therefore, the present invention greatly reduces the necessity of the above-mentioned extremely low carbon level by a characteristic manufacturing method described below, so that the lower limit of C is set to 0.025% in consideration of the manufacturing cost.

 The upper limit of C is set for ensuring workability.

The lower limit of Mn is for maintaining strength, the upper limit is for ensuring workability, the upper limit of P is for preventing vertical cracking, and the upper limit of S is for preventing deterioration of workability due to sulfide-based inclusions.

For the components other than the above, the present invention is not particularly limited, but letting the components usually used in the art contain usual amounts for usual reasons can be used as long as the basic effects of the present invention are not impaired. .

Further, steel having a low content of C as in the present invention has a high transformation point, and even if rolling is performed at a temperature higher than the rolling temperature of a normal material, the steel tends to be rolled in the unrecrystallized region just above the Ar ₃ point temperature. The anisotropy of the product increases due to the development of the texture.

Therefore, in the present invention, the rolling just above the Ar ₃ point temperature is avoided, and A
r Rolling should be performed in the complete recrystallization area at the _three- point temperature + 20 ° C or higher.

The winding temperature is in the range of 550 ° C. to 650 ° C. in order to secure an appropriate crystal grain size of about 7 to 9 for the purpose of improving the elongation and the press breaking load.

<Effects> The present inventors are effective in establishing a means for solving the above-mentioned problems, but are effective for longitudinal cracking resistance, but do not use B which causes an adverse effect on | Δr | In order to develop a steel sheet having cracks and low | Δr | values, various experiments and examinations were repeated using the test steels shown in Table 1.

The rolling at that time used the temperature range of Ar ₃ point temperature + 20 ° C. or more, and the winding was performed in the temperature range of 500 to 700 ° C.

As a result, a steel sheet having a winding temperature of 550 ° C. to 650 ° C. has a grain size of 7 to 9, which is subjected to press working to obtain a 50
A number of press tests were performed.

(A) shows that the | Δr | value is about 0 to 0.1, the earring height is 2.0 mm or less, the elongation is about 50% to 60%, and the molding height is 6 mm.
As described above, no vertical crack was observed.

However, in (b), the | Δr | value was 0.1 or less, the earring height was as low as 2 mm or less, the elongation was about 50% to 60%, and the molding height was 6 mm or more. Occurred.

Therefore, the present inventors searched for the cause of the difference in the occurrence of vertical cracks due to the difference in the P content, and obtained the relationship shown in FIG.

This summarizes the relationship between the P content, the | Δr | value, and the state of occurrence of vertical cracks.

Conventionally, P and S are generally provided with an upper limit in order to improve the cleanliness of steel, and the amount normally contained is the above-mentioned Kawasaki Steel Technology Vol. 19 No. 3 1987 P41-46, and JP-A-52- No. 125411, JP-A-49-134509, JP-B-57-2101
No. 1 and Japanese Patent Publication No. 58-14858 each show a range of P = 0.007 to 0.01% and S = 0.002 to 0.01%.

 The present inventors have arranged these effects of P.

As a result, as shown in FIG. 1, no vertical cracks were observed when the P content was less than 0.005%, and the molding height was reduced to 4 mm.
As shown by the solid line in the figure, it was found that the elongation was higher for the same elongation as the conventional steel sheet.

It is conventionally known that the limitation of the P content simply contributes to the purification of steel to improve workability.

The present invention is different from this conventional finding and is based on the above-mentioned new finding that vertical cracks are completely eliminated from a specific range of the P content limit amount.

<Examples> Examples of the present invention are shown below together with comparative examples.

(1) Test steel composition (shown in Table 2) (2) Rolling conditions (shown in Table 2) (3) Material (shown in Table 2) Steel numbers 1-2 of the present invention examples are shown in Table 2. As you can see, El is 50
%, Low | Δr | value of 0 to 0.1, earrings at the time of final pressing are 2 mm or less, and high molding heights of 6 mm or more are obtained. Good products are obtained with no vertical cracks. Further, since the steel sheet has the characteristics shown by the solid line in FIG. 4, even a steel sheet having a low elongation exhibited a forming height equivalent to that of the conventional steel sheet.

On the other hand, among the comparative examples, steel No. 3 in which the P content exceeded the upper limit was used.
Has a | Δr | value of 0.13 and an earring of 2.7 mm,
A vertical crack occurred.

Although the composition is within the range of the conditions of the present invention, steel No. 4 whose finish rolling temperature deviated on the low temperature side below the Ar ₃ point had a | Δr | value of 0.41, a high earring of 7.5 mm, and an elongation of 43.9%. Was low.

Further, steel number 5 in which the winding temperature was deviated to a high value and the crystal grains were coarsened frequently caused vertical cracks during pressing.

In addition, steel No. 6 in which C exceeded the upper limit, steel No. 7 in which Mn exceeded the upper limit, and steel No. 8 in which the winding temperature was lower than the lower limit all had an El of 44.
%, The molding height is 3.7mm, 4.3mm and 3.5mm
And a low value.

<Effect of the Invention> The present invention regulates the content of P based on new findings and rolls a steel obtained by regulating the contents of C, Mn, and S in a recrystallization region of _three or more Ar points. By rolling at a predetermined temperature to produce a low-phosphorous hot-rolled steel sheet for processing, it is equal to or greater than a steel sheet conventionally produced using Nb, Ti or B,
It is necessary to produce a steel sheet for press working that has excellent elongation, value, earring value, | Δr | value, exhibits excellent forming height, and does not generate vertical cracks without using the Nb, Ti, and B. It is possible to significantly reduce both the processing cost and the manufacturing cost of a steel sheet exhibiting excellent press workability, and it is possible to manufacture with high productivity.

[Brief description of the drawings]

FIG. 1 shows the relationship between the P content, the B content, the | Δr | value, and the state of occurrence of vertical cracks. FIG. 2 shows the relationship between the | Δr | value of the steel material and the earring height. FIG. 3 shows the molding height, the shallowest part size, the required product height, earrings and the like at the time of press molding. FIG. 4 shows the relationship between the elongation of the steel material and the forming height.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-143021 (JP, A) JP-A-60-204827 (JP, A) JP-A-62-161919 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C21D 9/46-9/48 C21D 8/02-8/04

Claims (1)

(57) [Claims] 1. A steel containing, by weight, C: more than 0.025 to 0.07% Mn: 0.01 to 0.4% P: less than 0.005% S: 0.01% or less, with the balance being Fe and unavoidable impurities
A method for producing a low-phosphorus hot-rolled steel sheet for processing, comprising rolling at a temperature of 550 ° C. to 650 ° C. after rolling in a recrystallization region of Ar ₃ points + 20 ° C. or more.