JP2000256738A - Production of high tensile strength steel for welded structure excellent in strip cuttability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel for welded structures small in the generating level of strains after strip cutting. SOLUTION: Steel added with a trace amt. of B and contg., by weight, 0.03 to 0.20% C, 0.01 to 0.80% Si, 0.40 to 2.00% Mn, 0.0001 to 0.0010% B, and consisting of the balance Fe with inevitable impurities is hot-rolled in such a manner that the rolling finishing temp. is controlled to Tf>Ar3+50 deg.C. The steel desirably contains one or >=two kinds among 0.03 to 1.0% Cu, 0.03 to 2.0% Ni, 0.03 to 1.0% Cr, 0.01 to 0.50% Mo, 0.003 to 0.05% Nb, 0.003 to 0.10% V and 0.003 to 0.05% Ti.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-strength steel for welded structures which is used in sections of bridges, shipbuilding, construction, civil engineering, industrial construction machines and the like.

[0002]

2. Description of the Related Art With the enlargement of structures, the required characteristics of thick steel plates to be used have been increased, and in order to respond to them, thermal processing control technology characterized by precise temperature control during rolling has been developed. Methods for reducing the finishing temperature and controlling cooling after rolling have been put to practical use as toughening techniques for steel sheets. For example, JP
As described in JP-A-9-031537 and JP-A-06-093332, in hot rolling of a thick steel plate, the rolling in a specific temperature range is strengthened, and further, the finishing temperature is limited to a low temperature range. A method is disclosed in which rolling in the recrystallization region is strengthened to achieve toughness. JP-A-09-
In No. 031537, the cumulative rolling reduction under 900 ° C. is 40.
% Or more, a method for the finishing temperature is _{Ar 3 + 50 ℃ ~Ar 3 -50} ℃, and after the rolling, cooled at 2 ° C. / S or higher to 550 ° C. or less, describes a method to achieve toughening to produce a low-temperature transformation structure Have been. In JP-A-06-093332,
The cumulative draft at 900 ° C or less is 50% or more, and the finishing temperature is in the range of 3 to 800 ° C.
A method of water cooling at 10 ° C./s or more to 0 ° C. is shown. On the other hand, a thick steel plate used for a welded structure is often subjected to sectioning to be cut into a shape of a member before welding, and often causes a problem of sectioning distortion. Stripping strain is a phenomenon in which a steel sheet is deformed when it is cut in a state where residual stress is generated in the steel sheet, and hinders subsequent welding work. Therefore, correction by a press machine or correction of deformation by local heating is necessary. This has caused an increase in the load of the work process and an increase in cost.

[0003]

The shearing strain is a phenomenon that occurs when a residual stress is generated in a steel sheet, and the utilization of a low-temperature transformation structure by low-temperature finish rolling or controlled cooling in toughening of the steel sheet reduces the residual stress. It is easy to produce, and is not preferable from the viewpoint of preventing shearing distortion. An object of the present invention is to provide a method for producing a high-strength steel sheet for a welded structure which is excellent in strippability and prevents the occurrence of stripping strain and achieves toughening of a steel sheet.

[0004]

Means for Solving the Problems In order to achieve the above object, the present inventors have investigated the relationship between rolling conditions and strain generation during stripping using steel for welded structures. As a result, the generation of the shearing strain is most affected by the rolling finish temperature, and when the finish rolling is performed in a range where the austenite recrystallization fraction is higher than Ar ₃ + 50 ° C., the generation of the shearing strain is suppressed to a low level. However, it has been found that the strength of welded structural steel, in which the amount of C and the amount of alloying elements added are kept low in order to ensure weldability, decreases, and a new component design guideline is needed.

Accordingly, the present inventors have focused on the use of trace elements and the effect of the composite addition of each element, utilizing B, Nb, and V in an appropriate range, and also using Cu, an alloying element,
The composite effect with Cr, Ni, and Mo was examined.

As a result, by adding a very small amount of B, 20
N / mm ² , a small amount of Nb and Mo combined with 3
It has been found that a strength increase of 0 to 50 N / mm ² can be obtained. In particular, it has been newly found that B is particularly effective for strengthening under cooling conditions in which ferrite is formed, in addition to the conventional effect of increasing hardenability. It is thought that the transformation of the second phase is shifted to the low temperature side to form a second phase morphology that works more effectively by strengthening.

The present invention has been made based on the above findings, and the gist thereof is as follows.

[0008] 1. By weight%, C: 0.03 to 0.20
%, Si: 0.01-0.80%, Mn: 0.40-
A strip comprising 2.00%, B: 0.0001 to 0.0010%, and the balance consisting of Fe and unavoidable impurities is hot-rolled at a rolling finish temperature: Tf> Ar ₃ + 50 ° C. A method for producing high-strength steel for welded structures with excellent cutability.

[0009] 2. The steel further contains Cu: 0.03% by weight.
1.0%, Ni: 0.03 to 2.0%, Cr: 0.0
3 to 1.0%, Mo: 0.01 to 0.50%, Nb:
0.003-0.05%, V: 0.003-0.10
%, One or two or more of Ti: 0.003 to 0.05%.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the present invention will be described in detail.

C: 0.03 to 0.20% C is an element for increasing the strength, and is added in an amount of 0.03% or more in order to secure the necessary strength as a welded structural steel.
Since addition of a large amount exceeding 0.20% may impair weldability and toughness, it is set to 0.03% to 0.20%.

Si: 0.01% to 0.80% Si is an element for increasing the strength. To be used as a deoxidizing element, 0.01% or more is added.
If a large amount exceeds the above range, the weldability and the toughness may be impaired.

Mn: 0.40 to 2.00% Mn is an element for increasing the strength, and is added in an amount of 0.40% or more to secure the necessary strength as a welded steel, but exceeds 2.00%. Since addition of a large amount may impair weldability and toughness, the content is set to 0.40 to 2.00%.

B: 0.0001 to 0.0010% B is an element effective for strengthening even with a very small amount of addition. Even under cooling conditions in which ferrite is formed, the transformation of the second phase is shifted to a lower temperature side, and B is strengthened. 0.
0001% or more is added, but if it exceeds 0.0010%, there is a concern that the welding heat affected zone may be adversely affected.
001 to 0.0010%.

The basic component composition of the present invention is as described above.
Although the object can be sufficiently achieved, more preferable effects for improving the strength and toughness can be obtained by selectively adding one or more of the following elements.

Cu: 0.03 to 1.0% Cu is an element for increasing the strength, and when it is used as a strengthening element, 0.03% or more is required.
%, The weldability and toughness are impaired.

Ni: 0.03 to 2.0% Ni is an element for improving strength and toughness, and when its effect is utilized, 0.03% or more is required.
% Is too economically disadvantageous.
3 to 2.0%.

Cr: 0.03 to 1.0% Cr is an element for improving the strength, and when it is used as a strengthening element, 0.03% or more is required. A large amount exceeding 1.0% may impair the weldability and toughness.

Mo: 0.01 to 0.50% Mo is an element which increases the strength, and when its effect is utilized, 0.01% or more is required. A large amount of addition exceeding 0.50% may impair weldability and toughness, so the content is made 0.01 to 0.50%.

Nb: 0.003-0.05% Nb is an element that increases the strength by precipitation strengthening, and when its effect is used, 0.003% or more is required. A large amount exceeding 0.05% may impair weldability and toughness, so the content is made 0.003 to 0.05%.

V: 0.003 to 0.10% V is an element which increases the strength by precipitation strengthening, and when its effect is used, 0.003% or more is required.
A large amount exceeding 0.10% may impair weldability and toughness, so the content is made 0.003 to 0.10%.

Ti: 0.003 to 0.05% Ti is an element for improving the effect of the heat affected zone and increasing the strength of the welded heat affected zone.
Need more. Since a large amount of addition exceeding 0.05% may impair the base metal toughness, 0.003 to 0.05%
And

Rolling finish temperature: Tf> Ar ₃ + 50 ° C. The rolling finish temperature strongly affects the generation of strain at the time of stripping, the rolling finish temperature decreases, and rolling mainly in the austenite unrecrystallized region is performed. Stripping distortion increases. Since the recrystallized fraction of austenite increases when the temperature exceeds Ar ₃ + 50 ° C., the finish rolling temperature is set to exceed Ar ₃ + 50 ° C.

[0024]

EXAMPLES A steel sheet was manufactured by using a steel having the composition shown in Table 1 while changing the rolling finish temperature, and the mechanical properties and the state of occurrence of strain after stripping were investigated. The state of occurrence of distortion was visually evaluated in five stages (the larger the number, the larger the amount of generation). Table 2
FIG. 1 shows the test results. Addition of a very small amount of B
The strength can be increased by 0 to 40 N / mm ² . B alone has the effect of increasing the strength, but Nb, Mo, Cu, N
When combined with i, Cr, etc., the effect is even greater. As shown in FIG. 1, in the comparative steel to which B was not added, the finish temperature was set to Ar in order to achieve the target strength.
_It is necessary to carry out rolling that places a burden on a rolling mill near _three points, and a lot of distortion occurs after stripping. On the other hand, in the steel of the present invention to which a small amount of B was added, the desired strength was obtained at any finishing temperature exceeding Ar ₃ + 50 ° C., and good results were obtained with almost no occurrence of strain after stripping. .

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

As described above, since the present invention has the above-described structure, it is possible to manufacture steel for welded structures with less occurrence of strain after stripping, and it is possible to obtain industrially The effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of finishing temperature on tensile strength and onset level of strain after cutting.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA02 AA04 AA05 AA11 AA14 AA16 AA19 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CC03 CC04

Claims (2)

[Claims] (1) C: 0.03 to 0.20% by weight
Si: 0.01 to 0.80%, Mn: 0.40 to 2.0
0%, B: 0.0001% to 0.0010%, the balance being hot rolled at a rolling finish temperature: Tf> Ar ₃ + 50 ° C., the balance being Fe and unavoidable impurities. Method for manufacturing high-strength steel sheets for welded structures with excellent quality. 2. The steel further comprises Cu: 0.03% by weight.
1.0%, Ni: 0.03 to 2.0%, Cr: 0.03
~ 1.0%, Mo: 0.01 ~ 0.50%, Nb: 0.
003 to 0.05%, V: 0.003 to 0.10%, T
The method for producing a high-strength steel for welded structures having excellent strippability according to claim 1, characterized in that one or more kinds of i: 0.003 to 0.05% are contained.