JPH06192786A - High strength steel excellent in brittle fracture occurrence characteristic - Google Patents

Abstract

PURPOSE:To obtain a high strength steel plate excellent in brittle fracture occurrence characteristic by hot-rolling the continuously cast slab of a low carbon steel with a specific composition and controlling the concentrations of P and Mn in a microsegregation zone to specific values or below, respectively. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.03-0.15% C, 0.02-0.50% Si, 0.5-1.9% Mn, 0.005-0.050% Al, <0.015% P, and the balance Fe or further containing one or >=2 kinds among <0.04% Nb, <0.1% V, <1.5% Ni, and <1.5% Cu or <0.02% Ti and/or <0.02% REM independently or in combination is continuously forged at about 30% draft in the course of a continuous casting, and the resulting rolling stock is heated up to 960-1150 deg.C, hot-rolled, and cooled rapidly. By this method, the high strength steel material where the concentrations of P and Mn in the microsegregation zone in the hot rolled steel plate are regulated to 0.020wt.% and >=2.0wt.%, respectively, and which has superior brittle fracture occurrence characteristic in a weld zone can be stably produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel excellent in brittle fracture occurrence characteristics suitable for use as a structural material for marine structures.

[0002]

2. Description of the Related Art The most important characteristic required for a steel sheet for a structural material of an offshore structure is a brittle fracture occurrence characteristic of a welded portion.
This is because the welded portion is liable to have a brittle structure due to the heat history of the welding, and because the structure is apt to receive stress concentration, brittle fracture is likely to occur. Since brittle fracture of a structural member causes a serious accident, a steel material having excellent toughness at a welded portion is required from the viewpoint of preventing occurrence of the accident.

The cause of brittle fracture is conventionally believed to be that island-like martensite generated by welding heat remains without being decomposed. JP-A-63-183152 discloses that the P content should be 0.0 to improve brittle fracture initiation characteristics.
Suppresses below 10wt% (hereinafter simply indicated as%) and 0.5mm
The maximum value of P concentration in the thick plate direction per area of × 0.5 mm is 0.
Steel with a content of 08% or less is disclosed.

[0004]

However, according to the study by the inventors, the maximum value of the local P concentration is suppressed to 0.08% or less as in the above-mentioned Japanese Patent Laid-Open No. 183152/1988. However, when the Mn content is high, the local segregation of Mn is 2.0% or more frequently and the island martensite is not decomposed. As a result, good brittle fracture initiation characteristics are obtained. Turned out not. The purpose of this invention is
This is to propose a high-strength steel in which the above problems are advantageously solved and brittle fracture initiation characteristics are effectively improved.

[0005]

The above object can be achieved by the following gist structure. (1) C: 0.03 to 0.15%, Si: 0.02 to 0.50%, Mn: 0.
5 to 1.9%, Al: 0.005 to 0.050%, P: 0.015%
The following are contained, and the balance is composed of Fe and unavoidable impurities, and the maximum values of P concentration and Mn concentration in the microsegregated portion are 0.020% or less and 2.0% or less, respectively. Excellent high-strength steel (first invention).

(2) C: 0.03 to 0.15%, Si: 0.02 to 0.
50%, Mn: 0.5-1.9%, Al: 0.005-0.050%,
P: 0.015% or less, and Nb: 0.04% or less,
V: 0.1% or less, Ni: 1.5% or less, Cu: 1.5% or less, and one or more kinds selected, with the balance being Fe and inevitable impurities, and the P concentration in the microsegregated portion. A high-strength steel excellent in brittle fracture initiation characteristics characterized by having a maximum value of 0.020% or less and an Mn concentration of 2.0% or less (second invention).

(3) C: 0.03 to 0.15%, Si: 0.02 to 0.
50%, Mn: 0.5-1.9%, Al: 0.005-0.050%,
P: 0.015% or less is included, and Ti: 0.02% or less, R
EM: Contains one or two selected from 0.02% or less, the balance is Fe and inevitable impurities, and the maximum values of P concentration and Mn concentration in the microsegregated portion are 0.0
A high-strength steel excellent in brittle fracture initiation characteristics characterized by 20% or less and 2.0% or less (third invention).

(4) C: 0.03 to 0.15%, Si: 0.02 to 0.
50%, Mn: 0.5-1.9%, Al: 0.005-0.050%,
P: 0.015% or less, and Nb: 0.04% or less,
V: 0.1% or less, Ni: 1.5% or less, Cu: 1.5% or less, one or more selected, and Ti: 0.02%
Below, REM: Contains one or two selected from 0.02% or less, the balance is Fe and unavoidable impurities composition, and the maximum values of P concentration and Mn concentration of the microsegregated portion are 0.020%, respectively. Or less and 2.0% or less, a high-strength steel excellent in brittle fracture initiation characteristics (fourth invention).

The experimental results that led to the invention will be described below. Now, the inventors first of all, C: O.
08%, Si: 0.38%, Mn: 0.5 to 1.8%, P: 0.002 to 0.
The composition is 005%, S: 0.009%, Al: 0.033%, Ti: 0.006% and N: 0.0030%, and the shape is 50 mm × 150 mm ×
A 150 mm sample steel was prepared. This test piece was cut at the central portion, subjected to K-shaped groove processing, and this portion was multi-layer welded. Then, a crack tip opening displacement (CTOD) test piece with a notch in the fusion line was taken from this welded joint, and CT
It was subjected to the OD test. The test temperature was -30 ° C. After the CTOD test, the test piece is cut along the plane that passes through the fracture starting point as shown in Fig. 1, and this plane is cut by an X-ray microanalyzer (beam diameter: 1
μm) and the concentrations at the starting points of P and Mn were examined. The obtained results are shown in FIGS. 2 (a) and 2 (b), respectively.

As is clear from the figure, Mn at the starting point
Concentration and P concentration are Mn ≦ 2.0% and P ≦ 0.020, respectively
%, It was found that the CTOD value stably exhibited a high value, and good toughness fracture initiation characteristics were obtained.

[0011]

The reason why the component composition is limited to the above range in the present invention will be described below. C: 0.03 to 0.15% C needs to be contained at least 0.03% in order to obtain the strength required as a steel plate for offshore structure materials, but 0.
If the content exceeds 15%, the weld hardenability and the weld cracking susceptibility increase, so the content was limited to 0.03 to 0.15%.

Si: 0.02 to 0.50% Si needs to be 0.02% or more for the sake of deoxidation. The addition of Si also contributes to the increase in strength, but if it exceeds 0.50%, the toughness of the base material is deteriorated, so the upper limit is made 0.50%.

Mn: 0.5-1.9% Mn must be contained in an amount of 0.5% or more to secure the strength of the base material. However, if the content exceeds 1.9%, the coarse-grain heat-affected zone (ICCGHA
The island-like martensite formed in (Z part) is not decomposed even by the thermal effect of the next pass, and as a result, the brittle fracture initiation characteristics are significantly deteriorated, so the range was set to 0.5-1.9%.

Al: 0.005 to 0.050% Al requires 0.005% or more for deoxidation, but if the content exceeds 0.050%, the toughness of the base material deteriorates significantly, so the range of 0.005 to 0.050% Limited to.

P: 0.015% or less P not only prevents the island-like martensite formed in the ICCGHAZ part from decomposing in the next pass, but also segregates at the grain boundaries to cause grain boundary fracture, resulting in toughness. However, the content of 0.015% or less is included.

The basic components have been described above. However, in the present invention, Nb, V, Ni and
It is also possible to add one or more selected from Cu and one or two selected from Ti and REM as grain growth suppressing components in the following composition ranges.

Nb: 0.04% or less Nb not only improves the toughness by expanding the unrecrystallized region in hot rolling to reduce ferrite grains after transformation in austenite, but also accelerates after hot rolling. During cooling, it is a useful element that increases the amount of low-temperature transformation products such as bainite and martensite to significantly increase the strength. However, if the content exceeds 0.04%, the weld cracking property deteriorates and the toughness of the welded part after stress relief annealing deteriorates, so the content was made 0.04% or less.

V: 0.1% V, like Nb, is an element useful for improving strength and toughness, but if the content exceeds 0.1%, the toughness of the welded portion after stress relief annealing is deteriorated, so V is 0.1%. % Or less.

Ni: 1.5% or less Ni is a useful element that improves the strength and toughness of steel without adversely affecting the hardenability and toughness of the heat-affected zone of welding, but its upper limit is 1.5% from the viewpoint of cost. did.

Cu: 1.5% or less Cu has the same function and effect as Ni and also contributes effectively to the improvement of corrosion resistance, but if it is contained in excess of 1.5%, hot brittleness tends to occur, The upper limit was 1.5%.

Ti: 0.02% or less Ti exists as TiN in the steel and is a useful element that effectively suppresses the growth of austenite grains in the heat-affected zone of welding. However, if the content exceeds 0.02%, if TiN decomposes into solid solution Ti at the bond part that is rapidly heated to near the melting point in the next pass, the hardness of the weld heat affected zone increases.
CTOD value decreases. Therefore, the Ti content is limited to 0.02% or less.

Rare earth metal (REM): 0.02% or less REM exists as REM (O, S) in steel, and the sulfides and oxides of this REM are stable even in the bond part of the welded part. In the same manner as above, suppressing the growth of austenite grains,
It has the effect of improving toughness. However, the REM content is 0.
If it exceeds 02%, the cleanliness of the steel will deteriorate and the toughness of the steel will deteriorate, so the content was made 0.02% or less.

Although the preferable component composition range in the present invention has been described above, in order to achieve the intended object of the present invention, it is not sufficient to limit the component composition to the above range, and the microsegregation portion of It is important to control the maximum P concentration and Mn concentration to 0.020% or less and 2.0% or less, respectively. The brittle fracture initiation characteristics depend largely on the existence of island martensite in the ICCGHAZ part, which is said to have the worst brittle fracture characteristics. In particular, when such island martensite is formed at a location that coincides with the P and Mn enrichment in the ICCGHAZ part, the island martensite does not decompose even in the subsequent heat cycle of the welding pass, and As a result, the brittle fracture initiation characteristics deteriorate. However, as shown in FIG. 2 above, if the maximum values of the P concentration and the Mn concentration are 0.020% or less and 2.0% or less, respectively, the above-mentioned adverse effects are eliminated. So, in this invention,
The maximum values of P concentration and Mn concentration in the microsegregated portion were limited to 0.020% or less and 2.0% or less, respectively.

The maximum values of P concentration and Mn concentration can be measured in the following manner. The P and Mn concentrations can be measured by cutting and polishing the measurement part and mapping or line analysis by EPMA.

Further, in the production of the steel of the present invention, during continuous casting, a continuous forging method is applied in which the crater end portion is continuously forged from above and below or left and right and the liquid phase enriched in P and Mn is discharged. It is preferable to manufacture.

[0026]

EXAMPLES Steel sheets having the compositions shown in Table 1 were manufactured under the following conditions. Nos. 1 to 16 were made into slabs by continuous casting. Among them, Nos. 1, 2 and 3 are slabs that are continuously cast according to the usual method, and are used as rolling materials, while Nos. 4 to
For No. 16, a slab that was continuously cast while applying continuous forging pressure at the crater end during continuous casting was used as the rolling material.

That is, the slabs of Nos. 4 to 16 were subjected to continuous forging at a reduction rate of 30% during continuous casting, and the steady portion was used as a material for rolling. These rolling materials were
After heating to a temperature of ℃, finishing the hot rolling up to 740 ℃ and making a steel plate with a plate thickness of 50 mm, immediately cool it at a cooling rate of 5 ℃ / s.
Accelerated cooling to below 00 ° C.

From the test materials thus obtained, 50
A plate of mm × 150 mm × 500 mm was cut out, subjected to K-shaped groove processing, and subjected to multi-layer welding by submerged arc welding with a heat input of 5 kJ / mm. The test piece had a shape of 50 mm × 100 mm × 450 mm so that the Fusion-line was located at the center of the test piece. Fatigue notch introduction and test method was in accordance with BS5762; 1979. CTOD
Test pieces are 3 from each of the No. 1, 2, 3, 5 to 16 test materials.
9 pieces from the No. 4 test material
The CTOD value, Mn amount and P amount at the fracture starting point were measured. Further, a tensile test piece was sampled from a position sufficiently distant from the welded portion and a tensile test was performed. Strength of base metal of each steel sheet, local
Table 2 shows the results of examining the maximum Mn and P concentrations, the CTOD value, the tensile properties and the toughness of the base material.

[0029]

[Table 1]

[0030]

[Table 2]

No. 1 is a comparative example in which the Mn content is out of the proper range of the present invention, No. 2 is a comparative example in which the P content is also out, and No. 3 is in the Mn content and P content. Although both are comparative examples, the CTOD characteristics at -30 ° C are both inferior. On the other hand, not only Mn content and P content but also Mn concentration and P concentration at the starting point of fracture satisfy the appropriate range of the present invention, Nos. 15 and 16 have excellent CTOD characteristics. Shows. In addition, No. 4 to 7 are Ti
No. 8 for V and Nb, No. 9 for Ni, No. 10 for RE
M, No. 11 was added with Ti, Nb, Ni and REM, No. 12 was added with V and Nb, No. 13 was added with V, and No. 14 was added with Cu within the ranges specified in the present invention. However, all the steel sheets have excellent CTOD characteristics, and Cu, N
The strength is improved by adding i, Nb and V.

[0032]

As described above, according to the present invention, not only the Mn and P contents but also the maximum values of the P concentration and the Mn concentration of the microsegregated portion are limited to an appropriate range, so that
It is possible to stably obtain a high-strength steel having excellent CTOD characteristics, in other words, excellent characteristics of occurrence of brittle fracture in the welded portion.

[Brief description of drawings]

FIG. 1 is a diagram showing suitable positions for measuring P concentration and Mn concentration in a microsegregated portion.

FIG. 2 is a graph showing the relationship between the P concentration and Mn concentration at the fracture starting point and the CTOD value.

Claims (4)

[Claims] 1. C: 0.03 to 0.15 wt%, Si: 0.02 to 0.50
wt%, Mn: 0.5 to 1.9 wt%, Al: 0.005 to 0.050 wt%, P: 0.015 wt% or less, the balance being Fe and inevitable impurities, and the P concentration and Mn concentration in the microsegregation Of maximum strength of 0.020 wt% or less and 2.0 wt% or less, respectively, of high strength steel with excellent brittle fracture initiation characteristics. 2. C: 0.03-0.15 wt%, Si: 0.02-0.50 wt
%, Mn: 0.5 to 1.9 wt%, Al: 0.005 to 0.050 wt%, P: 0.015 wt% or less, and Nb: 0.04 wt% or less, V: 0.1 wt% or less, Ni: 1.5 wt% or less, Cu : Contains 1 or 2 or more selected from 1.5 wt% or less, the balance is
High-strength steel with excellent brittle fracture initiation characteristics, characterized by the composition of Fe and unavoidable impurities, and the maximum P and Mn concentrations in the microsegregated portion being 0.020 wt% or less and 2.0 wt% or less, respectively. . 3. C: 0.03-0.15 wt%, Si: 0.02-0.50 wt
%, Mn: 0.5-1.9 wt%, Al: 0.005-0.050 wt%, P: 0.015 wt% or less, and one or two selected from Ti: 0.02 wt% or less and REM: 0.02 wt% or less. Occurrence of brittle fracture characterized by containing seeds, the balance being Fe and inevitable impurities, and the maximum P concentration and Mn concentration in the microsegregated portion being 0.020 wt% or less and 2.0 wt% or less, respectively. High strength steel with excellent properties. 4. C: 0.03-0.15 wt%, Si: 0.02-0.50 wt
%, Mn: 0.5 to 1.9 wt%, Al: 0.005 to 0.050 wt%, P: 0.015 wt% or less, and Nb: 0.04 wt% or less, V: 0.1 wt% or less, Ni: 1.5 wt% or less, Cu : 1 or 2 or more selected from 1.5 wt% or less and 1 or 2 selected from Ti: 0.02 wt% or less and REM: 0.02 wt% or less, with the balance being Fe
And the composition of unavoidable impurities, P of the microsegregation part
A high-strength steel with excellent brittle fracture initiation characteristics characterized by maximum concentrations of 0.020 wt% or less and 2.0 wt% or less, respectively.