JPS5940203B2 - Manufacturing method of high cleanliness, low yield ratio hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing hot rolled steel sheets with high cleanliness and low yield ratio.

In recent years, as line pipes have become more sophisticated, strict regulations have been put in place to prevent defects caused by inclusions near so-called welds.

- As pressures become higher, as seen in gas line pipes, there is a tendency for materials to have material properties that have greater work hardening after yielding as a measure to prevent damage due to increased internal pressure.

It is generally accepted that materials with a small ratio of yield point to tensile strength (hereinafter referred to as yield ratio) are advantageous, and this is the main purpose of the present invention for gas line pipes. It is preferable that the hot-rolled steel sheet used as a raw material has a coefficient of about 0.80 or less.

In other words, in order to meet the demands for line pipe safety, it is absolutely necessary to reduce the inclusions and yield ratio of the material.

The present invention shows a method for manufacturing a steel pipe material for such purpose using a hot rolled steel plate.

The term hot-rolled steel sheet referred to here mainly refers to hot-rolled coils manufactured by the so-called rapid heating (abbreviation for continuous hot rolling) process, but it also includes thick plates manufactured by the so-called thick plate process (abbreviation for thick plate rolling process). In principle, it is applicable.

Well, first of all, the characteristic feature of the chemical composition of the present invention is that it is so-called low C-high Mn as seen in conventional line bump materials.
It is not a component with Nb or Ti added to the yarn base,
It is composed of a unique component yarn in which one or both of Nb and Ti are added to the base of the so-called high C-low Mn yarn.

In terms of metallurgical principles, increasing the C content increases the amount of pearlite produced, which increases the so-called work hardening ability, which lowers the yield ratio, which can significantly improve ductility such as elongation. It can be understood that

Defects caused by inclusions near pipe welds are generally detected using ultrasonic testing (Ultra 5onicWavesTe).
It is often determined by the defective defect rate (hereinafter abbreviated as UST defective rate), etc., and the larger and more inclusions there are, the higher the UST defective rate is, which is not preferable.

Especially electric resistance welded steel pipe (Electric Re5i)
Inclusions in 5tance welding pipes (hereinafter referred to as ERW steel pipes) are the main cause of UST defects as hook cracks along the metal flow, so they are undesirable and are usually 5tance welding pipes per charge.
If it is less than %, there will be less process trouble.
UST defects caused by inclusions are also a big problem in Arc Welding Pipe (hereinafter referred to as SAW steel pipe) (including spiral welded steel pipes).

The term "inclusions" as used herein mainly means alumina class inclusions mainly composed of Al2O3, but other inclusions are also included.

Now, in the present invention, unlike the conventional method, when steel is tapped in a converter, A
The purpose is to perform a sufficient combination of Al and O in the steel by adding 7, and then perform a degassing treatment to sufficiently float a compound of Al and O, that is, Al2O3.

As a result, we succeeded in reducing at least the amount of coarse alumina clusters remaining in the steel.
As a result, the number of UST defects in the welded parts and base metal parts of steel pipes has been significantly reduced.

The method for producing high-cleanliness, low-yield-ratio hot-rolled steel sheets of the present invention involves adding Al when tapping steel melted in a converter, sufficiently combining with O in the steel, and then degassing the steel. and C
: 0.15~0.25%, Si: 0.05~0
．． 25%, Mn: 0.60”1.00%, Al: 0
．． 005-0.030%, S: 0.015% or more, 0:
0.0030% or less, and either Nb or Ti
containing 0.005 to 0.025% of one or two species, and the ratio of (Mn%)/(C%) is 3.0 to 6.0, (Mn
%), / (Si%) ratio of 3.0 to 9.0, leaving the remainder Fe and unavoidable impurities.
Characterized by hot rolling.

Further, in the above manufacturing method, the present invention provides R
EM, Ca, Mg, Zr.

0.1% or less of one or more of B and/or N
The steel is characterized by containing 1.5% or less of one or more of i, Cr, Cu + V9Mo.

The reasons for limiting the range of chemical components, etc. of the present invention will be described below.

C is not only effective for the strength of steel, but in the present invention, 0.15% is currently required to obtain particularly low yield ratio properties.

On the other hand, if it exceeds 0.25%, it significantly increases the hardness near the welding part, so it is not preferable.

In addition to strengthening steel, Si is also effective in suppressing the formation of Al2O3, similar to Mn, and a minimum content of 0.05% is required.

- If the force exceeds 0.25%, inclusions of silicate threads will be generated and UST defects will increase.

In addition to strengthening steel, Mn is also effective in suppressing the formation of Al2O3, and a minimum content of 0.60% is required.

- If the force exceeds 1.00%, inclusions of manganese oxide yarns will be generated and UST defects will increase.

Furthermore, if the ratio of (Mn%)/(C%) exceeds 6.0, pearlite formation is insufficient and it is difficult to obtain low yield ratio characteristics.
．． If it is less than 0, the toughness of the steel for a line pipe cannot be obtained.

On the other hand, if the ratio of (Mn%)/(C%) exceeds 9.0, penetrators will be generated during welding, especially in ERW steel pipes, which is not preferable for UST failure.

If this ratio is less than 3.0, the formation of silicate yarn inclusions is promoted, which is not preferable for UST defects.

If Al is less than 0.005%, the deoxidizing ability of the steel will be insufficient and the cleanliness will deteriorate; however, if it is more than 0.030%, the amount of Al2O3 produced will increase, which is undesirable.

If more than 0.015% of S is contained, MnS will be generated, which is unfavorable for the toughness and UST defects of the steel.

O combines not only with Al but also with Si and Mn [JST
Since it increases the defective rate, the content must be particularly low in the component yarns of the present invention, and must be 0.0030% or less.

Nb and T1 are necessary for improving the strength of steel and toughness through grain refinement, and in the present invention, the total amount of one or both of them is required to be at least 0.005%.

- If more than 0.025% of force is included, some of it will combine with C and N
A large amount of bC or Tic yarn precipitates are generated, and the yield ratio becomes high.

Furthermore, if it becomes higher than 0.025%, Nb or Ti
[JST defect rate also increases due to the formation of oxides.

The main constituent elements of the present invention are as described above, but in addition to these, REM (rare earth element), Ca, and Mg.

Oxide-, nitride-, and sulfide-forming elements such as Zr and B are effective in improving impact properties, but if one or more of these exceeds 0.1% [JST defective rate increases].

- Strong elements such as Ni, Cr, Cu, V, and Mo are effective in strengthening the steel, but if one or more of these elements exceeds 1.5%, the toughness deteriorates.

Next, the structural conditions regarding the manufacturing process and material characteristics of the present invention will be explained.

In the present invention, steel is smelted in a converter, and when the molten steel is transferred to a ladle after smelting blow-off, or before or after, Al is preferably added at 1.0 to 2.0% per ton of molten steel. The steel pot is characterized in that the amount of O and Al in the steel are sufficiently combined, and then the pot is degassed.

Other alloying additive elements are mainly added during the degassing process, but some are added in the steps before and after the degassing process.

Note that some amount of Al may be additionally added during degassing treatment for the purpose of increasing the accuracy of the Al component value of the final product.

Through the above series of treatments, O in the steel becomes Al before degassing treatment.
After being sufficiently combined with the alumina, it floats up as Al2O3 during the degassing process, and finally the residual alumina class in the steel is significantly reduced.

At the same time, O in the hot-rolled steel sheet becomes 0.0030% or less, which improves the cleanliness.

It should be noted that a high blow-stop C amount in the converter is good for reducing the UST defective rate, and is preferably 0.04% or more.

The degassed molten steel is turned into steel slabs by the usual two-blanking method or continuous casting method.

A steel billet is continuously hot rolled or plate rolled into a hot rolled steel plate, such as a hot rolled coil or a plate.

Note that the hot-rolled coil may be flattened by a cold straightening process and then sheared to be used as a cut plate.

In this case, the maximum surface strain rate during cold straightening is preferably set in the range of 0.6 to 3.0% using a leveler or the like.

By adopting such a technical configuration, the method of the present invention can reduce the yield ratio of a hot rolled steel sheet to 0.80 or less.

The hot rolled steel sheets of the present invention are ER'W steel pipes, SAM steel pipes ((JO
Used as steel pipes (including spiral welded steel pipes in addition to bending type steel pipes).

It goes without saying that the hot-rolled steel sheet of the present invention is most suitable as a material for steel pipes, but since it has a low yield ratio and good elongation properties, it can be used for general press parts, industrial machinery, automobiles, shipbuilding, and other applications. It is also suitable for uses such as tanks.

Examples of the present invention will be described below.

Steel having the chemical composition shown in Table 1 was tapped and subjected to a hot rolling process to obtain a hot rolled steel plate.

The intermediate steps are shown in Table 2.

In addition, the amount of residual metal after pouring the last steel ingot of the present invention in the ingot making method is 6T or more.

Table 3 shows the mechanical test values of the hot-rolled steel sheets produced in this manner, the yield ratio of the mechanical tests of the pipes after pipe production, and the EJSTJST defective rate (number of defective pipes per jersey).

The pipe made by the method of the present invention (marked with ○) had a low yield ratio in all 7 mechanical tests of hot-rolled steel sheets, and the
The defective rate is low and good.

As can be seen from Tables 1 to 3, steels other than the invention are inferior to the invention steel in the following respects.

In other words, in A2, 6+18, the timing of Al injection is inappropriate, and the JST failure rate (UST failure rate of pipe) is high.

A3 without degassing treatment also has a high UST defect rate.

A13 has high C and Mn and has a high UST failure rate.

71g14.24 has a high (Mn%)/(Si%) and a high UST defect rate.

Al5,21°27 has a high A7 and a high UST failure rate.

522 has a high Nb + Ti) and a high yield ratio, UST
The defective rate is also high.

A27 does not contain Nb or Ti and has low strength.

Also &2,3,6,14,15゜18.21.23,2
4, 25, and 26 have high O and a high UST defect rate.

Claims (1)

[Claims] 1. When tapping steel melted in a converter, A7 is added to sufficiently combine with O in the steel, and then degassing treatment is performed, C: 0.15 to 0. .25%, Sl: 0.05-0.
25%, Mn: 0.60-1. O0%, Al:0
．． 005-0.030%, S: 0.015% or less, O:
0.0030% or less, and Nb is also any one of Ti
0 seeds or 2 seeds. (Contains JO5~0.025%,
and the ratio of (Mn%)/(C%) is 3.0 to 6.0, (M
n%) / (Si%) ratio satisfies 3.0 to 9.0 and is made into a slab consisting of female part Fe and unavoidable impurities,
A method for producing a hot-rolled steel sheet with high cleanliness and low yield ratio, characterized by hot rolling. 2. When tapping steel melted in a converter, Al is added to fully combine with O in the steel, and then degassed, C: 0.15-0.25%, Si: 0.05~0.
25%, Mn: 0.60-1.00%, 11? :0
．． 005-0.030%, S: 0.015% or less, 0:
0.0030% or less, and either Nb or TI
containing 0.005 to 0.025% of one or two species, and the ratio of (Mn%)/(C%) is 3.0 to 6.0, (Mn
%)/(Si%) satisfies 3.0 to '9.0, and furthermore, 1 of REM r Ca 2 Mg r Zr + B
species or two or more species with 0.1% or less and/or Ni
, Cr, one or more of Cu+V+Mo
High cleanliness characterized by hot rolling after forming a slab containing 5% or less and the balance Fe and unavoidable impurities;
A method for producing low yield ratio hot rolled steel sheets.