JP2000096181A - Steel sheet free from uts defect in weld heat-affected zone, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet hardly causing UTS defect in the heat affected zone under severe welding conditions at large heat input welding, welding at high cooling velocity, etc., and its manufacturing method. SOLUTION: The steel sheet has a composition containing, by weight, 0.02-0.47% C, 0.3-2.5% Mn, <=0.05% P, and <=0.03% S. In this steel sheet C concentration in the central part of sheet thickness is equal to or lower than that in the part at a position, one - fourth the sheet thickness. This steel sheet can be manufactured by carrying out, in the continuous casting process where a cast slab leaving an unsolidified region in the course of casting is subjected to bulging in a thickness direction and then to reduction, bulging of 5 to 30% with respect to the size of the shorter side of a mold and then performing, just before the solidification completion point, reduction having 0.5-2.0 reduction ratio in unsolidified part and >=20 mm/m rolling reduction in a drawing direction to obtain a cast slab to be a rolling stock.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate used for welding structures such as buildings, ships, marine structures, tanks, construction machines, line pipes and the like.

[0002]

2. Description of the Related Art In various steel structures such as buildings, ships, bridges and tanks, submerged arc welding (SAW), electro-gas welding (E) is used to reduce the number of welding steps.
G), high efficiency and high heat input welding such as electroslag welding (ES) is performed, and a steel sheet excellent in welding crack resistance at that time is applied. As a typical example of such a steel sheet,
There is an accelerated cooling steel sheet that reduces carbon equivalent, performs hot rolling, and then performs forced cooling instead of cooling to obtain a required strength.

When large heat input welding is performed, ultrasonic flaws (UST defects) may occur in the heat affected zone. The ultrasonic flaw detection method is based on JIS-G-0901,
If a defect is detected, it will need to be repaired by welding or remanufactured. U of such heat affected zone
In the suppression of ST defects, further reduction of P and S correlated with the occurrence of defects can be achieved, and furthermore, for example, a heat input of 20 KJ / mm
For the purpose of changing the form of MnS-based inclusions that are likely to be the starting point of defects, addition of Ca or REM is performed as in the invention of Japanese Patent Application Laid-Open No. 62-56554, which presupposes a large heat input welding. ing. However, the reduction of P and S and the addition of Ca, REM, etc. all greatly increase the production cost, so that a more inexpensive and effective measure is desired.

[0004] Further, it is expected that the welding heat input will increase to about 100 KJ / mm in the future, and the strength will increase and the C equivalent will tend to increase. Thus, hand welding with a very high cooling rate is still unavoidable. Therefore, the risk of occurrence of UST defects is increasing, and more effective countermeasures are required.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet which is unlikely to cause UTS defects in a heat-affected zone under severe welding conditions such as large heat input welding and high cooling rate welding. On offer.

[0006]

The present inventors have conducted a detailed investigation on UTS defects in a heat affected zone of a weld generated under particularly severe welding conditions. As a result, the defect is located at the center of the thickness of the steel sheet. It was confirmed that the heat was generated or propagated starting from the hardened portion at the center. The position corresponds to a part called so-called center segregation.

[0007] In the central segregation part, not only C but also Mn and P are localized and their concentration is high, and MnS-based inclusions are present more than other parts. C, Mn, and P are all elements that greatly improve the hardenability of steel. Since all three elements are localized at a high concentration, the hardenability of the central segregation part is significantly improved, and the heating during welding is performed. Upon subsequent cooling, it is easily quenched into a brittle hardened structure. In addition, since the Mn concentration is high, MnS is easily formed, and these inclusions are the starting points of cracking.

Therefore, if this center segregation is reduced,
UTS defects in the weld heat affected zone should be eliminated. Conventionally, in order to suppress extreme center segregation in continuous casting, a method of electromagnetically stirring molten steel, a method of suppressing bulging as much as possible, and applying a reduction to the slab slightly larger than the solidification shrinkage when changing from a liquid phase to a solid phase. Methods, etc. have been implemented. On the other hand, recently, in order to further greatly reduce the center segregation, a means of reducing bulge pressure in continuous casting has been developed. For example, the method of the invention disclosed in JP-A-9-57410, the thickness of the slab during casting,
At the stage where the central part is not solidified, the thickness is positively increased to 20 to 100 mm, and reduced to the original mold size immediately before the end of solidification. When a steel plate was manufactured using a slab with significantly reduced center segregation cast by such a method, and the heat affected zone of welding was examined, the UTS defect was considerably reduced, but it can be said that it was not necessarily completely absent. Did not.

[0009] Then, as a result of further study of the casting conditions for continuous casting, the segregation at the center was eliminated by increasing the reduction ratio of the unsolidified portion and performing the reduction rapidly. It was found that reduction to the same value or less was possible.

This is considered to be due to the following reasons. When the central portion of the slab starts to solidify, primary dendrites having a composition lower in concentration than the molten steel are initially formed, and the components are concentrated in the molten steel. If the solidification proceeds as it is, as the solidification shrinks, the concentrated molten steel between the dendrites gathers at the center of the slab, resulting in central segregation. On the other hand, if a reduction of more than solidification shrinkage is applied at a stage where solidification has progressed to a certain extent, the concentrated molten steel is discharged to the low solid phase fraction side, leaving primary dendrites in the center, and slabs A low-concentration portion such as C or Mn is formed at the center. However, in this case, it is inevitable that a portion having a higher concentration than the ladle composition remains in the final portion of the casting.

However, as a result of rolling a thick steel plate using this slab and performing a welding test, it was possible to obtain a steel plate that would be almost free of UTS defects. In this steel sheet, generation of a hardened structure was not recognized in the center part of the sheet thickness after welding. Based on such results, the present invention was completed by further clarifying the limit conditions. The gist of the present invention is as follows.

(1) By weight%, C: 0.02-0.47%, Mn:
Including 0.3-2.5%, P: 0.05% or less, S: 0.03% or less,
A steel sheet having no UTS defect in a weld heat affected zone, characterized in that the C concentration at the center of the thickness is equal to or lower than the C concentration at a quarter of the thickness.

(2) In a continuous casting method in which a slab that leaves an unsolidified region during casting is bulged in the thickness direction and then reduced, 5 to 30% bulging is performed on the short side dimension of the mold to complete solidification. Just before the point, the reduction ratio of the unsolidified part is 0.5 to 2.0
The method for producing a steel sheet according to the above (1), characterized in that a reduction in an amount of reduction in a drawing direction is 20 mm / m or more, thereby producing a slab as a rolled material.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition, segregation,
The reason for limiting the manufacturing conditions will be described. Here, all chemical compositions are% by weight.

The content of C is 0.02 to 0.47%. C is an important element that determines the strength of steel, and if it is less than 0.02%, the strength as steel cannot be obtained. On the other hand, when the content exceeds 0.47%, the toughness of the welded portion deteriorates, and the steel sheet becomes unsuitable for the welded structure.

As the C content distribution in the steel sheet, it is assumed that the C concentration in the center part of the sheet thickness is equal to or lower than the C concentration in a part at a quarter position of the sheet thickness. This means that the C concentration in the central portion of the sheet thickness is equal to or less than the average C content of the steel sheet represented by the C concentration in a portion at a quarter position of the sheet thickness.
Thereby, the occurrence of UTS defects in the weld heat affected zone can be eliminated. The part with a low C concentration at the center of the thickness is 4% or more of the thickness and 80% of the width of the width.
It is desirable to have the above.

The content of Mn is set to 0.3 to 2.5%. Mn has an effect of preventing hot brittleness of steel due to S and an effect of improving the strength of steel. Content below 0.3% is especially S
It is not enough to deter the harm of steel and has almost no effect on the strength improvement. If the content exceeds 2.5%, the toughness is deteriorated, and the workability is also deteriorated.

The content of P is set to 0.05% or less. P is one of the unavoidable impurities of steel and degrades the toughness of steel, so the smaller the content, the better. It is an element that easily segregates in the center together with C and Mn. However, when C in the central portion is reduced as in the present invention, the central portion concentration is reduced at the same time, and the upper limit of the content as steel can be relaxed. Desirable is 0.02% or less.

The content of S is set to 0.03% or less. S is also one of the unavoidable impurities of steel, and the smaller the better, the better. Usually, sufficient Mn is simultaneously contained to prevent hot embrittlement due to its presence, so that it is present in the steel in the form of MnS inclusions. MnS serves as a starting point of cracking when stress is applied to steel and deteriorates toughness, and serves as a site for hydrogen precipitation to lower resistance to hydrogen cracking. If the segregation of Mn at the center of the plate thickness is eliminated, the uneven distribution of MnS-based inclusions at the center is also reduced, and the adverse effect is alleviated. Therefore,
The upper limit of the content is 0.03%, but preferably 0.01% or less.

To prevent the occurrence of defects during casting of the slab, a deoxidizing agent is naturally added to the molten steel. As a result, the steel sheet contains 0.1% or less of Si or Al.
Further, in order to improve the strength, toughness, corrosion resistance, etc. of the steel sheet product, for example, each of Cr, Cu, Mo, etc. of 1.0% or less, Ti, V, N, etc. of 0.1% or less, or 0.0% or less, respectively.
B of up to 05% may be simultaneously contained. The inclusion of these elements, like other unavoidable impurities of steel, does not reduce the effect of the present invention that UTS defects do not occur in the weld heat affected zone.

The C concentration at the center of the thickness of the steel sheet is 1% of the thickness.
The same or lower than the C concentration at the / 4 position is set during continuous casting of a slab that is a rolled material of a steel sheet.
This is because the C concentration in the center of the slab is positively reduced.
In such a slab, the interval in the slab thickness direction of the guide rolls arranged immediately below the continuous casting mold increases in the drawing direction, thereby increasing the maximum thickness of the slab bulging. An apparatus with rolling rolls that can perform up to 30% of the short side dimension of the mold, and then can roll down to the short side dimension of the original mold at a rolling reduction of 20 mm / m or more in the drawing direction. Use and cast.

Using this apparatus, 5 to 30
% Bulging, and the thickness of the unsolidified portion at the center of the slab is 0.5 to 2.0 with respect to the reduction amount, that is, the bulging amount.
Start rolling when the ratio is The thickness of the unsolidified portion is determined by the casting speed and the amount of bulging. The thickness and the solid fraction of the unsolidified shell may be determined by a one-dimensional transient heat transfer analysis calculation from the casting speed, the surface cooling of the slab, and the chemical composition of the cast steel type.

If the bulging amount is less than 5%, it is not possible to obtain a slab in which the C concentration at the center of the slab is sufficiently reduced. In addition, bulging exceeding 30% is not preferable because there is no further decrease and increase in the concentration of C at the center, the amount of reduction is large, and the load on the equipment is too large. If the rolling ratio of the unsolidified portion is less than 0.5, fluctuations due to the temperature of the molten steel, the surface temperature of the slab, etc. cannot be absorbed, and the C concentration in the center of the slab may not be able to be reduced. The reduction ratio may be large, but if the reduction ratio exceeds 2.0, the stress required for reduction becomes too large, and troubles such as roll breakage are likely to occur. In addition, the reduction rate in the pull-out direction is 20mm /
The reason for setting it to m or more is that if it is less than 20 mm / m, coagulation proceeds too much and the central concentration cannot be sufficiently reduced. The rolling reduction per length may be large, but is limited to about 100 mm / m due to limitations of facilities and the like.

[0024]

EXAMPLE A steel plate having a strength of 400 MPa or more and a toughness (vE ₀ ) of 47 J or more was used as a target of steel properties, and bulging and slab reduction were performed on steel having a C content of 0.02 to 0.47 shown in Table 1 under the conditions shown in Table 2. A slab having a thickness of 235 mm and a width of 2300 mm was cast, and these slabs were hot-rolled to obtain a steel plate of 30 to 70 mm. With respect to the obtained steel sheet, the C content of the center part of the sheet thickness and the part at the position of 1/4 thickness was analyzed, and the strength and toughness were measured. Each of these steel sheets was subjected to various weldings of 5 m length, and then subjected to ultrasonic flaw detection for the heat-affected zone to count the number of defects. Table 1 also shows these results.
Shown in

[0025]

[Table 1]

[0026]

[Table 2]

As is apparent from these results, in the steel sheet according to the present invention, in which the C content at the center of the steel sheet thickness is lower than the C content at the 1/4 thickness position, UTS defects are present in the heat-affected zone due to welding. Not at all. Further, it is clear that such a steel sheet can be produced by performing bulging by 5% or more by continuous casting, increasing the reduction ratio of an unsolidified portion, and rolling the obtained slab.

[0028]

By applying the present invention, the conventional S
The generation of UTS defects in the heat affected zone under severe welding conditions, which was suppressed by the extreme reduction of P and P and the morphological change of MnS-based inclusions, can be more efficiently and more easily suppressed at lower cost. be able to. As a result, it is possible to provide and manufacture a steel sheet that can withstand welding with higher heat input and welding under more severe conditions.

Claims (2)

[Claims] C .: 0.02 to 0.47% by weight, Mn: 0.3% by weight.
-2.5%, P: 0.05% or less, S: 0.03% or less, characterized in that the C concentration at the center of the plate thickness is equal to or lower than the C concentration at the 1/4 position of the plate thickness. A steel sheet that does not generate UTS defects in the heat affected zone. 2. A continuous casting method in which a slab that leaves an unsolidified region during casting is bulged in a thickness direction and then reduced.
5-30% bulging is performed on the short side dimension of the mold, and the reduction ratio of the unsolidified portion is 0.5-2.0 immediately before the solidification completion point.
2. The method for producing a steel sheet according to claim 1, wherein the reduction is performed such that the reduction amount in the drawing direction is 20 mm / m or more to obtain a slab as a rolled material.