JPS6256519A - Production of acceleratedly cooled steel sheet without generating ust defect at part affected by high heat input welding - Google Patents

Abstract

PURPOSE:To obtain the titled acceleratedly cooled steel sheet without generating UST defects and having excellent toughness by controlling the conditions in the heat treatment of steel having a specified composition and controlling the total length of the MnS type inclusion. CONSTITUTION:Steel consisting of, by weight, <=0.20% C, <=0.50% Si, <=1.60% Mn, <=0.020% P, <=0.008% S, the balance iron and inevitable impurities is heated at a temp. above the Ac3 point, hot-rolled at 730-820 deg.C finishing temp. and then acceleratedly cooled. Consequantly, the total length of the MnS type inclusion is regulated to <=0.1mm/mm<2> and a steel sheet without generating UST defects at the part affected by the welding with a high heat input of <=80KJ/cm is obtained. Besides, the steel is preferably incorporated with <=0.010% Ca and <=0.010% of <=1 kind among REM of contg., independently of the elements, <=1 kind of element selected from a group of <=0.50% Cu, <=0.50% Ni, <=0.30% Cr and <=0.30% Mo.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides a heat-affected zone with a large heat input of 80 KJ/cm or more, with UST sensitivity V + s-z, a: U at 80%.
The present invention relates to a method for producing an accelerated cooling steel sheet without ST defects.

(Prior art) In the construction of various steel structures such as ships, offshore structures, and bridges, suffrage arcs are used to reduce welding man-hours. Acceptance welding (SAW) and electrogas welding (
High-strength steel plates, which can be subjected to high-efficiency, high-heat-input welding such as EC) and have excellent weld cracking resistance, have been used conventionally. A typical example of such a steel plate is an accelerated cooling steel plate in which a hot-rolled steel plate is water-cooled to have a lower carbon equivalent than normalized steel or as-rolled steel having the same strength level.

However, when such an accelerated cooling copper plate is subjected to high efficiency large heat input welding such as SAW or EC, a UST defect (UST sensitivity V+s-z, s: 80%) may occur in the weld heat affected zone. Generally, when ultrasonic waves of 2 to 5 MHz are applied from the surface into a steel plate through an appropriate couplant, cracks,
When defects such as incomplete adhesion exist within the steel plate, sound waves are reflected from within the steel plate. In this way, defects revealed by ultrasonic waves are tested using ultrasonic
estjng) defects, i.e., UST defects, but when such UST defects occur, welding rework or remanufacturing is required.
There is a strong demand for accelerated cooling steel sheets that do not produce defects.

(Object of the Invention) The present inventors have determined that the UST sensitivity V's-z,s: 80% in the heat-affected zone of large heat input welding of 80 KJ/cm or more
In order to obtain an accelerated cooling steel sheet without UST defects, U
As a result of detailed research into the causes of ST defects, it was found that MnS-based inclusions in the segregation zone of steel sheets are the main cause of UST defects, and that there is a good correlation between UST defects and the total length of MnS-based inclusions. Based on this knowledge, we reduced the segregated elements C, Mn, and P as much as possible to reduce S, and further controlled the morphology of inclusions with Ca and/or REM. By controlling the conditions when hot rolling steel with a chemical composition of
The present invention was achieved by discovering that it is possible to obtain a high-toughness accelerated cooling steel sheet that does not generate UST defects.

(Structure of the Invention) The method for producing an accelerated cooling steel sheet in which UST defects do not occur in the heat affected zone of large heat input welding of 80 KJ/CII+ or more according to the present invention is as follows: C0.20% or less, Si 0.50% or less, Mn in weight% After heating a steel consisting of 1.60% or less, P 0.020% or less, S 0.008% or less, and the balance iron and unavoidable impurities to a temperature of Ac 3 or higher and hot rolling to a finishing temperature of 730 to 820°C,
Accelerated cooling to reduce the total length of MnS inclusions to 0.1 ryu/m
m'' or less.

First, the present inventors investigated the U generated in the heat affected zone of large heat input welding.
ST defect (UST sensitivity V+s-z, a: UST defect at 80%, defect definition EH > 20%, the same applies hereinafter)
The effects of manufacturing conditions on steel sheets were investigated.

In addition, UST sensitivity VIS-2, 11: UST at 80%
A defect is defined as U, which is set so that the echo height from the standard flaw in the standard test piece (STB-G V + , -2, 11) specified in JIS Z 2345 is 80%.
It refers to ST defects, and the defect definition EH > 20% is:
A defect inside a steel plate that causes an echo height exceeding 20% at a specified UST sensitivity.

As the results are shown in FIG. 1, it was found that UST defects are more likely to occur in accelerated cooled steel sheets than in as-rolled steel sheets, and that a decrease in finishing temperature leads to an increase in UST defects. In addition, the symbols added in each figure are shown in Table 1 or Table 3.
Corresponds to the steel chemical composition of the steel symbol shown in the table. Furthermore, as shown in FIG. 2 showing the influence of a large amount of heat on the occurrence of UST defects, it has been found that as the amount of large heat increases, the number of UST defects increases.

Next, as shown in Figure 3, the location of UST defects in the thickness direction is most common at the center of the plate thickness (3 (In)), but is distributed over 1/4 to 3/4 of the plate thickness. As a result of detailed observation of this UST defect, the present inventors found that Mn and S-based inclusions are present in the segregation zone at the center of the sheet thickness, and MnS-based inclusions are also present at other locations. There are voids between the sub-rails, and from these observation results,
It is assumed that these are the causes of UST defects, and in particular, there are many MnS-based inclusions (furthermore, hardened segregation bands are likely to exist) in the central part of the sheet thickness, so it seems that there are many UST defects.

Therefore, in order to obtain an accelerated cooling steel sheet in which no UST defects occur, it is ideal to completely eliminate MnS-based inclusions and segregation, but this is impossible due to the current state of technology. Therefore, in order to reduce the segregation elements C, Mn, and P as much as possible, as well as to reduce harmful MnS-based inclusions, the present inventors aimed to reduce S as shown in Figure 4, and further , Ca
It has been found that controlling the morphology of inclusions by using the following method is extremely effective in preventing the occurrence of UST defects.

Furthermore, as shown in FIG. 5, the present inventors investigated the critical amount of M n S-based inclusions for UST defects.
There is a good correlation between the total length of MnS-based inclusions and the number of UST defects, and in particular, by regulating the total length of MnS-based inclusions to 0.1 mm/mm'' or less, the finishing temperature can be reduced to 730°C. We have found that even at temperatures as low as ˜750° C., no U ST defects occur.

Next, the reason for limiting the chemical composition of steel in the method of the present invention will be explained.

C is an element useful for ensuring strength, but on the other hand, since C is a segregated element, in the present invention, it is preferable to reduce its addition amount as much as possible. In particular, 0.20%
When adding in excess of
Because it deteriorates weld cracking resistance and high heat input weld joint toughness,
The upper limit of the amount of C added is 0.20%.

Si is an effective element for deoxidizing and ensuring strength, but when added in large amounts exceeding 0.50%, the ferrite hardens and the toughness of the base metal deteriorates, so the upper limit of the amount added must be set. It shall be 0.50%.

Like C, Mn is effective for ensuring strength, but
It tends to segregate, and in particular, when the amount added exceeds 1.60%, coarse bainite is generated, which deteriorates the toughness of the adult weld joint as well as the base metal toughness. Therefore, the upper limit is set at 1.60%.

P is an element that concentrates in segregated areas and embrittles the isthmus.
In particular, when the content exceeds 0.020%, the above harmful effects become significant, so the upper limit is set at 0.020%.

As mentioned above, S causes the generation of M n S,
Since UST defects are likely to occur, in the present invention, the S content is set to 0,00 s% or less.

In addition to the above-mentioned elements, the steel used in the present invention contains C
a and/or REM.

Ca and RtoM are MnS, which is easily stretched by hot rolling.
It is an element that has the effect of spheroidizing.

This effect can be sufficiently achieved by adding a small amount of 0.010% or less. On the other hand, if the amount added exceeds 0.010%, internal quality deteriorates, so the upper limit is set to 0.010%.

The steel used in the present invention includes one or more of the above-mentioned Ca and REM, or independently from these,
It can contain at least one element selected from the group consisting of Cu -, Ni, Cr, and MO, and/or at least one element selected from the group consisting of Nb, V, Ti, and B.

Cu, Ni, Cr, and Mo are effective as elements for adjusting steel strength, and as elements for improving weldability and toughness of high heat input welded joints by replacing them with C and Mn.

However, when adding large amounts, the amount of coarse bainite increases and the toughness of the steel deteriorates.
Regarding MO, it is set at 0.30%.

Nb, ■, Ti, and B form carbonitrides and are useful for improving the strength of steel and toughness by refining grains due to their precipitation hardening effect, but when added in excess, Bainite forms and deteriorates toughness. Therefore, the upper limit of the amount added is 0.10% for each of Nb, V, and Ti, and 0.10% or less for B.

In the method of the present invention, steel having the above-mentioned chemical composition is
The material is heated to a temperature of point c3 or higher and hot rolled. At this time, finish rolling is performed at a temperature of 730 to 820° C., and then accelerated cooling is performed. The above finishing temperature is an important factor governing the toughness of the obtained steel sheet, and in order to obtain fine austenite grains, a lower finishing temperature is desirable;
When the temperature is lower than 30°C, MnS stretches and tends to cause UST defects, while when the temperature is higher than 820°C, the toughness is significantly deteriorated. Therefore, in the method of the present invention, the finishing temperature is limited to a temperature in the range of 730-820°C.

In this way, by finishing rolling at a predetermined temperature and then performing accelerated cooling, the total length of M n S inclusions can be reduced to 0.
．． It is possible to obtain an accelerated cooling steel plate having a cooling rate of 1+u/mm2.

(Effects of the Invention) As described above, according to the method of the present invention, the segregation elements C, Mn, and P can be extremely reduced and S can be reduced, and MnS
By regulating the total length of inclusions in the system, and more preferably by controlling the morphology of inclusions using Ca, the UST sensitivity V15-2.8 can be maintained even in the heat affected zone of high heat input welding of 80 KJ/cm or more.
At 80%, an accelerated cooling steel plate with no UST defects and excellent toughness can be obtained.

(Examples) The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

Example Steel having the chemical composition and Caq shown in Table 1 was
After heating and finish rolling according to the conditions shown in the table, the material was cooled to produce an accelerated cooling steel sheet having a predetermined thickness. The tensile properties and impact properties of these steel plates are shown in Table 2. Also,
The number of UST defects (pieces/m) at the joints of these steel plates is shown.

All accelerated cooling steel sheets obtained by the method of the present invention have a total MnS length of 0.1 + u/mm2 or less,
No UST defects occur and the toughness is excellent. A typical example is shown in FIG.

On the other hand, the amount of C in comparative steel J, the amount of Mn in comparative steel, the amount of P in comparative steel, and the amount of S in comparative steel M exceed the range specified in the present invention, and all Also, the total length of MnS exceeds 0.1 mm/mmz.

A typical example is shown in FIG. Comparative steel N has a chemical composition within the range specified by the present invention, but because the finishing temperature is too low, MnS is elongated and the total length of M n 3 is 0.16.
w/n+n+”, a UST defect occurs.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between finish rolling temperature and the number of UST defects in the heat-affected zone of a large heat input weld with a large heat amount of 200 KJ/cm, and Figure 2 is a graph showing the relationship between the large heat amount and the number of UST defects. The graph shown in Figure 3 shows the position in the plate thickness direction and the UST
Figure 4 is a graph showing the relationship between the number of defects and the number of defects.
FIG. 5 is a graph showing the influence of the amount of Ca and the amount of Ca on the number of UST defects. FIG. 5 is a graph showing the relationship between the total length of M n S-based inclusions and the number of UST defects. Fig. 1 Fig. 2 0 100 :200 30θ entering) (to k≠) Fig. 3 Fig. 4 Fig. 5 0 0.1 0.2 θ3 0.1 ψ
Part 5 S total mosaku 1 wheel 2)

Claims (4)

[Claims] (1) Steel consisting of C 0.20% or less, Si 0.50% or less, Mn 1.60% or less, P 0.020% or less, S 0.008% or less, the balance being iron and unavoidable impurities. was heated to Ac_3 point or higher, hot-rolled to a finishing temperature of 730 to 820°C, and then acceleratedly cooled to reduce the total length of MnS-based inclusions to 0.1 mm.
/mm^2 or less 80KJ/cm
A method for producing an accelerated cooling steel sheet in which UST defects do not occur in the heat-affected zone of high heat input welding as described above. (2) In terms of weight %, (a) C 0.20% or less, Si 0.50% or less, Mn 1.60% or less, P 0.020% or less, S 0.008% or less, and (b) Ca0. A steel containing at least one element selected from the group consisting of 0.010% or less and REM 0.010% or less, with the balance consisting of iron and unavoidable impurities is heated to a temperature of Ac_3 or higher to a finishing temperature of 730 to 820°C. After rolling, accelerated cooling was performed to reduce the total length of MnS inclusions to 0.1 mm.
/mm^2 or less 80KJ/cm
A method for manufacturing an accelerated cooling steel plate in which UST defects do not occur in the heat-affected zone of high heat input welding as described above. (3) In weight%, (a) C 0.20% or less, Si 0.50% or less, Mn 1.60% or less, P 0.020% or less, S 0.008% or less, and (b) Ca 0 at least one element selected from the group consisting of 0.010% or less and REM 0.010% or less, and (c) Cu 0.50% or less, Ni 0.50% or less, Cr 0.30% or less, and Mo 0 After heating a steel containing at least one element selected from the group consisting of .30% or less and the balance consisting of iron and unavoidable impurities to a temperature of Ac_3 or higher and hot rolling to a finishing temperature of 730 to 820°C, Accelerated cooling to reduce the total length of MnS inclusions to 0.1 mm
/mm^2 or less 80KJ/cm
A method for producing an accelerated cooling steel sheet in which UST defects do not occur in the heat-affected zone of high heat input welding as described above. (4) In weight%, (a) C 0.20% or less, Si 0.50% or less, Mn 1.60% or less, P 0.020% or less, S 0.008% or less, and (b) Ca 0 at least one element selected from the group consisting of 0.010% or less and REM 0.010% or less, and (c) Nb 0.10% or less, V 0.10% or less, Ti 0.10% or less, and B 0. A steel containing at least one element selected from the group consisting of: Cool and reduce the total length of MnS inclusions to 0.1 mm.
/mm^2 or less 80KJ/cm
A method for producing an accelerated cooling steel sheet in which UST defects do not occur in the heat-affected zone of high heat input welding as described above.