JPS6152317A - Manufacture of hot rolled steel plate having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To improve the toughness at low temp. by subjecting a steel contg. prescribed percentages of C, Si, Mn and Al to different peripheral speed rolling or skew rolling under prescribed conditions so as to produce a sufficient working strain in the steel and to form a fine ferrite structure. CONSTITUTION:A steel billet or ingot consisting of, by weight, 0.03-0.35% C, <=0.5% Si, 0.5-2% Mn, 0.003-0.06% sol. Al and the balance Fe is cast. The billet or ingot is subjected to different peripheral speed rolling in one or more passes with upper and lower rolls whose peripheral speeds are different from each other by >=10%, or it is subjected to skew rolling in one or more passes with upper and lower rolls whose axes intersect each other at >=0.5 deg. angle. The resulting hot rolled steel plate is finish-rolled at the Ar3 point or above and coiled at 500-600 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot rolled steel sheet having poor low temperature toughness.

In recent years, line pipes, oil country tubular goods, etc. are often used in cold regions, so there is a trend toward thicker walls and higher tensile strength, and at the same time, excellent low-temperature toughness is required. However, in general, steel plates have the problem of deterioration of toughness due to thickening and high tensile strength. Therefore, conventionally, steel plates used in low-temperature ranges are steel slabs containing precipitation-strengthening elements such as MOINb, ■, and Ti. Alternatively, it is manufactured by so-called controlled rolling of a steel ingot. This controlled rolling is a method in which various conditions such as steel heating temperature, rough rolling, finish rolling, runout cooling, and coiling temperature are optimized according to the required characteristics. It is already known that lower temperatures are preferred.

Although such a low temperature heating method is a preferable method from the point of view of energy saving, on the other hand, it has various problems. Therefore, low-temperature heating reduces the strength of, for example, aluminum-killed steel intended for cold rolling, requiring the rods to be clustered together to avoid cross-mixing, resulting in an inflexible rolling schedule. . Further, steel sheets obtained by controlled rolling methods have problems such as deterioration of surface properties due to low-temperature heating, or occurrence of warping or bending.

In order to solve the above-mentioned problems, the present inventors conducted intensive research from a viewpoint fundamentally different from that of the conventional controlled rolling as described above, and as a result, the present inventors developed a method that increases the amount of effective strain in the austenite region of a steel billet or steel ingot. The inventors have discovered that the ferrite grains can be refined by hot-rolling the steel sheet while introducing it into a steel sheet, finishing rolling it at a predetermined temperature, and coiling it at a predetermined temperature range, thereby obtaining a hot-rolled steel sheet with excellent low-temperature toughness. This led to the present invention.

That is, the method for producing a hot-rolled steel sheet with excellent low-temperature toughness according to the present invention includes, in weight percent, co, O'3 to 0.35%, Si 0.50% or less, Mn' 0.50 to 2.0%, and sol. A7! A method for producing a hot-rolled steel plate, in which a steel billet or steel ingot consisting of 0.003 to 0.060%, balance iron and unavoidable impurities is finish rolled at a temperature of Ar 3 or higher, and then wound at a temperature of 500 to 650°C. At least one pass of different circumferential speed rolling is performed in which the circumferential speeds between the upper and lower rolls differ by at least 10%, or the intersection angle of the roll axes between the upper and lower rolls is at least 5.5. The method is characterized in that at least one pass of skew rolling is performed.

First, the reasons for limiting the chemical composition of the steel used in the method of the present invention will be explained.

C combines with Nb, V, etc. to precipitate fine carbides and make the crystal grains finer, but on the other hand, it reduces the toughness of the weld heat affected zone.
Alternatively, a bainite structure containing a large amount of cementite is generated during low-temperature winding of 600° C. or lower, resulting in deterioration of low-temperature toughness. Therefore, in consideration of the ease of melting and in order to effectively develop precipitation hardening, in the present invention, the lower limit of the amount of C added is set to 0.03%. On the other hand, when adding too much C, it deteriorates weldability and low-temperature toughness, so the upper limit is set to 0.35%.

Although Si is effective for deoxidizing steel and increasing its strength, adding too much deteriorates weldability, so the amount added is 0.
50% or less.

Mn has the effect of improving the strength and toughness of steel, and in order to effectively express this effect, it is necessary to add at least 0.50% of IW. However, excessive addition deteriorates weldability.
(Therefore, the upper limit is set at 2.0%.

A7! is an essential element for killed steel in terms of deoxidation of steel, but sol △l is 0. When the amount of OO is less than 3%, deoxidation is insufficient and the strength of the steel base metal decreases.

On the other hand, if 5olAp exceeds 0.060%, HAZ
? Deteriorates PJJ properties. Therefore, in the present invention,
5ol AI! , is in the range of 0.003 to 0.060%.

In the present invention, the steel used is allowed to contain unavoidable impurities such as P and S.

For example, when P undergoes a slow cooling process such as coiling after controlled rolling, separation becomes apparent in the Charpy test of the steel plate and lowers the Charpy absorbed energy in the ductile fracture surface temperature range, so the lower the P content, the lower the P content. It is preferable for low-temperature toughness, but in consideration of economic efficiency during operation, the upper limit is preferably set to 0.02%.

In addition, S combines with Mn etc. to form sulfides and lowers the Charpy absorbed energy in the ductile fracture surface temperature range.
As with P, the lower the content, the better for low-temperature toughness, but in consideration of economic efficiency during operation, the upper limit is preferably 0.02%.

In the present invention, in addition to the above-mentioned elements, Cu, Cr, Mo, N
It is also possible to further add one or more elements selected from the group consisting of b, V and 'I' i.

Cu is effective in increasing the strength of steel, but when added in excess, it tends to deteriorate the toughness and may also cause hot cracking, so the upper limit is set at 1.0%.

Cr is also effective in increasing the strength of steel, but when added in excess, it increases the hardenability of HAZ and reduces toughness and weld cracking resistance, so the upper limit is set to 0.5%.

Mo is effective in increasing strength, but 0. - If added in large quantities exceeding 5%, the toughness of the steel will deteriorate, so the upper limit should be set at 0.
．． 5%.

Both Nb and V can significantly improve the strength and toughness of the base metal by adding a small amount of 0.01% or more, but when added in excess, the base metal and high heat input welding pound In the present invention, the upper limit is set at 0.1% for Nb and 0.1% for Nb.
Regarding (2), it is set at 0.15%.

Ti is effective in increasing the strength of steel, and also has the effect of improving the shape of sulfides in steel and suppressing coarsening of crystal grains. In order to effectively express such an effect, it is necessary to add at least 0.1%, but on the other hand, when adding too much, the HAZ toughness is deteriorated.
The upper limit is set to 0.1%.

Moreover, in the present invention, Ca can also be added. Ca helps improve toughness by eliminating the anisotropy of mechanical properties of steel, but when added in excess, it increases nonmetallic inclusions and actually deteriorates toughness, so the upper limit should be set at 0.005. %.

In the method of the present invention, when producing a hot rolled steel plate by hot rolling a steel billet or steel ingot having the above-mentioned chemical composition, different circumferential speed rolling is performed in which the circumferential speeds between the upper and lower rolls are different. or at least one pass of skew rolling in which the roll axes between the upper and lower rolls have an intersection angle.

As shown in FIG. 1, different circumferential speed rolling refers to rolling when the circumferential speed of the upper roll 1 and the circumferential speed of the lower roll 2 are different during hot rolling.
By this method, shearing horns act within the thickness section of the steel billet or steel ingot 3, so that large deformation can be applied to the center of the thickness. On the other hand, according to the conventional hot rolling using constant circumferential speed lower rolls, as shown in FIG. 2, the deformation at the center of the plate thickness section is small.

In the method of the present invention, when performing hot rolling by different circumferential speed rolling, as shown in FIG. 1, different circumferential speed ratio R= (-- --1) X l 00 (%)
■.

It is necessary to carry out at least one rolling pass in which the When the different circumferential speed ratio is less than 10%, it is difficult to apply sufficient processing strain to the center of the sheet thickness, and therefore the ferrite structure in the resulting hot rolled steel sheet becomes coarse, resulting in insufficient low temperature toughness. does not have.

On the other hand, skew rolling is, as shown in FIG. This refers to a rolling method in which a shearing force acts on a steel billet or steel ingot in the width direction, so that deformation can also be imparted in the direction perpendicular to rolling.

When performing hot rolling by such skew rolling, the shearing force acting in the width direction of the sheet increases approximately in proportion to the skew angle. In the method of the present invention, in order to impart sufficient deformation to the steel billet or steel ingot in the direction orthogonal to rolling, it is necessary to set the skew angle to at least 0.56 and perform at least one pass of such skew rolling. be.

When the skew angle is smaller than 0.5°, it is difficult to apply a sufficiently large processing strain in the direction perpendicular to rolling.
Therefore, the obtained hot-rolled steel sheet does not have sufficient low-temperature toughness as described above. Therefore, in order to impart a large deformation, the larger the skew angle, the more advantageous it is, but in view of the rollability of the plate crown, etc., the upper limit is set at 46 in practical terms.

Then, in the method of the present invention, the rolling temperature is k
It is preferable that the temperature be at least the rs point, particularly at least the Ar3 point, and as low as possible. When the finishing temperature is lower than the Ar3 point, the crystal grains become mixed and coarsened, deteriorating the low-temperature toughness. Also, the winding temperature is 5
It is in the range of 00 to 650°C. When the coiling temperature is outside this range, low temperature toughness deteriorates.

As described above, according to the method of the present invention, when hot rolling a steel billet or steel ingot, performing different circumferential speed rolling at a predetermined different circumferential speed rate or skew rolling at a predetermined skew angle,
Since sufficient working strain is imparted to the steel billet or steel ingot to make the steel structure a fine ferrite structure, a hot-rolled steel sheet with excellent low-temperature toughness can be obtained.

The present invention will be explained below with reference to Examples.

Example 1 Thick 35-long rope pieces made of the invention steels A to ■ having the chemical compositions shown in Table 1 were skew-rolled at different circumferential speeds under the conditions according to the method of the invention shown in Table 2, and a finished plate was obtained. After finishing rolling to a thickness of 9°31 mm, the material was wound at a temperature of 600°C. Regarding the hot-rolled steel sheet obtained in this way, JIS
A No. A 2 mm V sub-size test piece was prepared and subjected to an impact test. Charpy fracture transition temperature (νTrs) and −
The absorbed energy (vELz.) at 20°C is shown in Table 2.

For comparison, comparative steel J-L having the chemical composition shown in Table 1 was rolled at a constant peripheral speed according to a conventional method and then wound. The finished plate thickness is the same as above. Table 2 shows the results of the same impact test as above for the obtained hot rolled steel sheets.

Apparently the hot rolled steel sheet obtained by the method of the invention has vT
rs and vE-z. Both are excellent.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining different circumferential speed rolling, FIG. 2 is a diagram for explaining conventional uniform circumferential speed rolling, and FIG. 3 is a diagram for explaining skew rolling. 1... Upper roll, 2... Lower roll, 3... Steel billet or steel ingot. Patent applicant: Kobe Steel, Ltd. Representative: Patent attorney: Ittsu Makino Department 1 Figure 2. (Mi hit

Claims (1)

[Claims] (1) C 0.03-0.35%, Si 0.50% or less, Mn 0.50-2.0%, solAl 0.003-0.060%, balance iron A method for producing a hot-rolled steel plate in which a steel billet or steel ingot containing unavoidable impurities is finish rolled at a temperature of Ar_3 or higher and rolled at a temperature of 500 to 650°C, wherein the circumferential speed between the upper and lower rolls is at least 10 % different circumferential speed rolling or at least one pass of skew rolling in which the cross angle of the roll axes between the upper and lower rolls is at least 0.5°. A method for producing hot-rolled steel sheets with excellent toughness. (2) Contains (a) C 0.03 to 0.35%, Si 0.50% or less, Mn 0.50 to 2.0%, solAl 0.003 to 0.060%, and further, (b) Cu 1.0% or less, Cr 0.5% or less, Mo 0.5% or less, Nb 0.010 to 0.10%, V 0.010 to 0.15%, and Ti 0.01 to 0.10%, at least one element selected from the group consisting of, the balance iron and unavoidable impurities, finish rolling a steel billet or steel ingot at a temperature of Ar_3 points or higher and coiling at a temperature of 500 to 650°C. In the method for producing a steel plate, at least one pass of different circumferential speed rolling is performed between the upper and lower rolls, in which the circumferential speed differs by at least 10%, or the crossing angle of the roll axes between the upper and lower rolls is at least 0. A method for producing a hot-rolled steel sheet with excellent low-temperature toughness, the method comprising performing at least one pass of skew rolling with a skew rolling angle of .5°.