JPH0924448A - Method for manufacturing steel sheet with excellent toughness in heat-affected zone - Google Patents

Abstract

(57) Abstract: [PROBLEMS] To provide a method for manufacturing a steel material having good toughness in a heat-affected zone of high heat input welding. SOLUTION: When continuously casting molten steel in which components such as Ti and Al are specified, the solidification rate of the slab is 65% or more and 90% or more.
In the following unsolidified region, the surface member is used to reduce the rolling speed 2
mm / sec or more and less than 15 mm / sec, reduction rate (δ /
d) (however, δ = amount of reduction (mm), d = unsolidified thickness (mm) at the side of entering the rolled zone) is 0.1 or more and less than 0.5 until the completion of solidification, The toughness is improved by increasing the number of Ti oxides and refining the structure of the high heat input welding heat affected zone.

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 a welded structural steel material having excellent toughness in a weld heat affected zone (hereinafter referred to as HAZ) used in marine structures, ships, storage tanks, middle- and high-rise buildings and the like. It is a thing.

[0002]

2. Description of the Related Art In recent years, offshore structures, ships, storage tanks,
The demand for the material properties of welded structural steel used for large-scale structures such as middle- and high-rise buildings is increasing, and due to the damage caused by destruction and the degree of social unrest, the toughness of the steel material itself, The demands on the toughness of HAZ are increasing.

Further, when constructing such a structure,
In order to promote the efficiency of welding, application of a large heat input welding method such as a single-sided one-pass welding method represented by flux-copper backing welding is desired. In response to this, there have been many proposals that have focused on the HAZ toughness of steel materials at the time of high heat input welding.

For example, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 55-26164, the fine austenite grains in the HAZ are made small by ensuring fine Ti nitride in the steel,
There is a method of improving toughness. In addition, Japanese Patent Laid-Open No. 3-264
Japanese Patent No. 614 proposes a method of utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite to improve the toughness of HAZ.

However, since Ti nitride almost forms a solid solution in the vicinity of the boundary (referred to as a weld bond portion) with the weld metal of which the maximum attainable temperature exceeds 1400 ° C. in the HAZ, the effect of suppressing deterioration of toughness decreases. However, it is difficult to meet the recent demands for strict steel material properties. As a method for improving the toughness in the vicinity of the weld bond portion, there is a method of utilizing Ti oxide as a transformation nucleus of intragranular ferrite, as disclosed in JP-A-61-79745. The oxide is stable even in a high temperature range of 1400 ° C. or higher, and there is no problem that it forms a solid solution like a nitride. However, it is very difficult to uniformly disperse Ti oxide of several μm or less, which is effective as a transformation nucleus of the intragranular ferrite, in the steel. Due to the existence of the center segregation, the toughness deteriorates more and more, and it is difficult to satisfy the required toughness.

[0006]

In view of these circumstances, the present invention increases the number of Ti oxides in the central portion of the plate thickness, improves the structure near the weld bond portion, and improves toughness. An object of the present invention is to provide a method for producing a steel material for a welded structure, which stably satisfies the toughness required for HAZ of a large welded structure.

[0007]

The present invention has been made to solve the above problems, and the gist thereof is as follows. (1) C: 0.03 to 0.15% by weight, Si: ≤
0.50%, Mn: 0.50 to 1.80%, P: ≤0.
020%, S: 0.001 to 0.005%, N: 0.0
020-0.0060%, Ti: 0.005-0.02
0%, Al: ≦ 0.005% is contained, the remainder is iron and inevitable impurities, and substantially no Al-containing molten steel is continuously cast, and a slab having an unsolidified portion is pressed using a face member. In doing so, until the solidification is completed in the region where the solidification rate is 65% or more and 90% or less, the welding heat effect is characterized in that the material is cast by successively applying reduction so as to satisfy the relationship of the following equation, and then rolled. Method for producing a steel sheet having excellent toughness in the welded part.

2.0 ≦ v <15.0 ... 0.10 ≦ δ / d <0.50, where v is the reduction speed (mm / sec), and δ is the reduction amount (m
m) and d are the thickness (m
m). (2) C: 0.03 to 0.15% by weight, Si: ≤
0.50%, Mn: 0.50 to 1.80%, P: ≤0.
020%, S: 0.001 to 0.005%, N: 0.0
020-0.0060%, Ti: 0.005-0.02
0%, Al: ≦ 0.005% as a basic component, and further C
u: ≤1.0%, Ni: ≤2.0%, Nb: ≤0.03
0%, V: ≦ 0.1%, Cr: ≦ 0.6%, Mo: ≦
Molten steel containing 0.6% of one kind or two kinds or more, the balance of which is iron and unavoidable impurities and substantially not containing Al is continuously cast, and a slab having an unsolidified portion is used as a face member. In the reduction, the welding heat is characterized in that after the solidification rate is in the range of 65% or more and 90% or less, solidification is completed until the solidification is completed, the material is cast by successively applying reduction so as to satisfy the relationship of the following equation, and then rolled. A method for manufacturing a steel sheet having excellent toughness in the affected zone.

2.0 ≦ v <15.0 ... 0.10 ≦ δ / d <0.50, where v is the reduction speed (mm / sec), and δ is the reduction amount (m
m) and d are the thickness (m
m). In the present invention, C, Si, Mn, P, S, N,
The limited amounts of Ti, Al, Cu, Ni, Nb, V, Cr and Mo and the reasons therefor will be described below.

The lower limit of C is 0.03% because it is important for obtaining strength, and 0.15% is the upper limit because it is desirable that C be low for weldability of steel. Although Si is added for preliminary deoxidation of molten steel, 0.50% is made the upper limit in order to prevent deterioration of toughness due to hardening of HAZ. Mn needs to be added in an amount of 0.50% or more as a component for improving the strength of the steel material and forming transformation nuclei of intragranular ferrite. If the content is excessive, hardenability increases and weldability and HAZ toughness decrease. Therefore, the upper limit was set to 1.80%.

The smaller the content of P is, the more preferable it is, but 0.020% is set as the upper limit because it takes a great cost to industrially reduce the amount. The lower limit of S is 0.001% in order to form MnS which becomes a nucleus of ferrite transformation, and the upper limit is 0.005% because an increase in the content causes an increase in anisotropy of the steel material and a decrease in toughness.

Since N is necessary for the formation of TiN precipitates, the lower limit is 0.0020%.
The upper limit was made 0.0060% because it causes a decrease in Z toughness. Since Ti is added for the purpose of forming Ti oxide and Ti nitride, the lower limit is 0.005%, and if the addition amount is too large, carbide (TiC) is generated and the toughness is lowered, so 0.020% is the upper limit. And

[0013] Al is desirable to be small as it reduces Ti oxide, but 0.005% is the upper limit because Al is inevitably mixed from refractory materials. Cu is effective for improving the strength of the steel material, but if it is too much, it lowers the HAZ toughness, so 1.0% was made the upper limit. Ni is effective for improving the strength and toughness of steel, but if too much Ni reduces the HAZ toughness, 2.0
% As the upper limit.

Nb is a very effective element for improving the strength and toughness of the base material by improving the hardenability. However, in the HAZ part, excessive addition significantly lowers the toughness, so 0.030%. Was set as the upper limit. V, C
Since r and Mo also have the same effect as Nb, the upper limits were 0.10%, 0.60%, and 0.60%, respectively.

Further, in the method of rolling steel, the method of accelerated cooling and the method of quenching and tempering, even if a method conventionally applied in the field is used, the HAZ toughness is not affected at all. Can be applied.

[0016]

The present inventors have studied a method of increasing the number of Ti oxides in the central portion of the cast slab in order to solve the problems of the above-mentioned prior art. There are many reports on the phenomenon that the number of Ti oxides decreases in the center of the slab. For example, in “Materials and Processes” 4 (1991) 284, the Ti oxide observed in the slab crystallizes during solidification. It is an oxide, and it is shown that the number of oxides formed in the central portion is slow because the solidification rate is slow, and the oxides formed by the solidification are aggregated and coalesced until the solidification is completed.

Conventionally, as a method for increasing the number of oxides in the central part, as disclosed in Japanese Patent Laid-Open No. 202422/1993, a compound oxide is formed by adding Zr after deoxidizing Ti. Therefore, a method of increasing the number of oxides in the central portion has been proposed. However, as a result of studies by the present inventors, it has been found that when the proportion of Ti oxide in the oxide decreases, the intragranular ferrite forming ability deteriorates and the HAZ toughness does not improve so much.

Therefore, the present inventors have focused on the unsolidified reduction as a method for increasing the number of Ti oxides in the central portion of the slab. It has been found that when the uncoagulated reduction is performed, the coagulated structure becomes finer, and the cause thereof is a phenomenon in which the coagulation rate becomes faster than that in the case without the reduction. It is considered that when the solidification rate is increased, the above-described aggregated coalescence of oxides is unlikely to occur. The inventors of the present invention have No. 1 in Table 1. As a result of investigating the number of Ti oxides in the center of the slab obtained by casting molten steel having the composition shown in FIG. 1 and applying unsolidified reduction, as shown in FIG.
It was found that the number of i-oxides increased.

In order to clarify the relationship between the unsolidified rolling condition and the number of Ti oxides, No. 1 in Table 1 was used. 2 molten steel with 1 component
80 mm thick cast, unsolidified thickness d (mm), reduction amount δ
(Mm) and rolling speed v (mm / sec) were variously changed to carry out unsolidified rolling, and the oxide of the slab was investigated. Further, a steel plate having a plate thickness of 35 mm was manufactured by hot rolling, and the weld HAZ toughness was investigated.

First, the reduction rate (δ) of the unsolidified lower part represented by the ratio of the amount of reduction δ affecting the formation of Ti oxide and the unsolidified thickness d.
In order to see the effect of / d), the results of arranging the relationship between the reduction rate and the number of Ti oxides under the condition that the reduction rate is almost constant are shown in FIG. As the rolling reduction increases,
It was confirmed that the number of Ti oxides increased. However, it was found that internal cracking occurs when the rolling reduction is 0.5 or more. Further, as a result of a welded HAZ toughness test, it was found that a toughness satisfying the standard was obtained when the rolling reduction was 0.1 or more.

Next, the relationship between the number of Ti oxides and the rolling speed is shown in FIG. When the reduction rate exceeds 2.0, the number of Ti oxides tends to increase. This is presumably because a large reduction rate is not sufficient to increase the solidification rate in the unsolidified rolling section, and a reduction rate for stirring molten steel is necessary. However, the rolling speed is 15 mm / se
It has been found that internal cracking may occur if it is more than c. Further, as a result of a welding HAZ toughness test, it was found that a material satisfying the standard can be obtained when the rolling speed is 2.0 mm / sec or more.

Further, in the range of the solidification rate at the start of rolling, if the solidification rate is less than 65%, the thickness of the solidified shell becomes thin, so that internal cracking may occur even under the conditions where the rolling rate and the rolling speed are at the lower limits. Since it became clear, the lower limit of the solidification rate is 65%
And On the other hand, if the solidification rate exceeds 90%, the area in which the number of oxides increases due to the unsolidification pressure becomes narrower, so HAZ
Since it was found that the improvement in toughness was insufficient, 90% was made the upper limit. Further, it has been found that unless the reduction is continued until the solidification is completed, the unsolidified portion causes center segregation when solidifying, which causes deterioration of the HAZ toughness. Therefore, it was found that it is necessary to continue applying the reduction from the state where the solidification rate is 65% to 90% to the completion of solidification.

The molten steel components are as described above, and as shown in FIG. 4, it was found that Ti oxide is generated under the condition that Al is 0.005% or less, and the weld HAZ toughness is improved. The present invention has been made by applying a new action based on the above findings, and thereby achieved the object of the present invention.

[0024]

EXAMPLES Examples of the present invention and comparative examples will be specifically described below. 280 mm molten steel with the chemical composition shown in Table 1
Continuous casting was performed to a thickness of 1900 mm width, and unsolidified rolling was carried out by using a rolling-down equipment composed of surface members shown in FIG. In FIG. 5, 1 is an inner bar, 2 is an outer bar, 3 is a slab, and 4 is an unsolidified portion.

The specifications of the reduction equipment used in this example are as follows. Model Walking bar system Configuration Inner bar 3 Outer bar 4 Shift amount 100 mm Bar length (rolling strip length) 2500 mm Rolling strip width 1600 mm Under the conditions shown in Table 2, the unsolidified strip is reheated and hot rolled. After that, the large heat input welding HAZ toughness of the steel plates with plate thicknesses of 25 mm and 35 mm produced by air cooling are shown in Table 3 separately for the present invention example and the comparative example.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

No. 1 of the present invention example. 1 to No. No. 8 exhibited excellent characteristics of 50 J or more at -40 ° C. On the other hand, in No. 9 is an example with a large amount of Al,
Since the i oxide was reduced to Al, no intragranular ferrite was formed in the welded HAZ portion, and only a low toughness of less than 50 J was shown at -40 ° C.

No. No. 10 is an example in which the non-coagulation reduction was not performed, No. 14 is an example in which the rolling speed during rolling is too slow, N
o. No. 15 is an example in which the amount of reduction is small with respect to the unsolidified thickness.
It exhibited a low toughness of less than 50 J at -40 ° C. N
o. No. 11 is an example in which solidification was not completed in the rolling zone and an unsolidified portion remained, and central segregation occurred, and only a low toughness of less than 50 J at -40 ° C was exhibited.

No. No. 12 is an example in which the solidification rate at the time of rolling is small and the solidified shell is thin, No. No. 13 is an example in which the rolling speed is too fast, No. 13 No. 16 is an example in which the amount of reduction is too large, the number of oxides is large, and the HAZ toughness shows a high value, but internal cracking occurs, which is a poor steel sheet product. In this example, after rolling, an example of a steel sheet manufactured by air cooling was shown, but as described above, in the method of rolling steel material, the method of accelerated cooling and the method of quenching and tempering, it is conventionally applied in the relevant field. Since the HAZ toughness is not affected even if the method described above is used, the conventional method can be applied.

[0032]

INDUSTRIAL APPLICABILITY The present invention is to provide a steel sheet to which a large heat input welding method is applied and which satisfies the severe toughness requirement against the destruction of large welded structures such as marine structures, ships and storage tanks. The effect of seeds on the industrial field is extremely large, and the contribution to society is also very large in terms of the safety of structures.

[Brief description of drawings]

FIG. 1 is a diagram comparing the number distribution of Ti oxides in the thickness direction of a slab with and without unsolidified rolling.

FIG. 2 is a diagram showing the relationship between the reduction rate of unsolidified reduction and the number of Ti oxides.

FIG. 3 is a graph showing the relationship between the rolling speed and the number of Ti oxides during unsolidified rolling.

FIG. 4 is a diagram showing the relationship between the amount of Al and the number of Ti oxides.

5A and 5B show a reduction equipment used in an example of the present invention, in which FIG. 5A is a front view and FIG. 5B is a side view.

[Explanation of symbols]

 1 inner bar 2 outer bar 3 slab 4 unsolidified part

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical display location C22C 38/14 C22C 38/14 (72) Inventor Masaki Minagawa Oita-shi 1st Nishinosu Nippon Steel Stock Company Oita Works (72) Inventor Ken Seki Oita City 1st Nishinosu Nippon Steel Works Oita Works Stock Company

Claims (2)

[Claims] 1. By weight%, C: 0.03 to 0.15%, Si: ≤ 0.50%, Mn: 0.50 to 1.80%, P: ≤ 0.020%, S: 0. 0.001 to 0.005%, N: 0.0020 to 0.0060%, Ti: 0.005 to 0.020%, Al: ≤ 0.005%, with the balance being iron and inevitable impurities. When continuously casting molten steel containing substantially no Al and rolling down a slab having an unsolidified portion using a surface member, the solidification rate is from 65% to 90% until solidification is completed,
A method for producing a steel sheet having excellent toughness in a heat-affected zone of a welding heat-treated part, which comprises rolling by sequentially applying a reduction so as to satisfy the relationship of the following equation: 2.0 ≦ v <15.0 ... 0.10 ≦ δ / d <0.50, where v is the reduction speed (mm / sec) and δ is the reduction amount (m
m) and d are the thickness (m
m). 2. By weight%, C: 0.03 to 0.15%, Si: ≤ 0.50%, Mn: 0.50 to 1.80%, P: ≤ 0.020%, S: 0. 0.001 to 0.005%, N: 0.0020 to 0.0060%, Ti: 0.005 to 0.020%, Al: ≤ 0.005% as a basic component, and further Cu: ≤ 1.0% , Ni: ≦ 2.0%, Nb: ≦ 0.030%, V: ≦ 0.1%, Cr: ≦ 0.6%, Mo: ≦ 0.6% In the continuous casting of molten steel containing the balance of iron and unavoidable impurities and containing substantially no Al, and when a cast piece having an unsolidified portion is rolled down using a face member, the solidification rate is 65% or more and 90% or less. Until solidification is completed from the region, casting is performed by successively applying reduction so as to satisfy the relationship of the following formula, and then rolling. Method for manufacturing a steel sheet excellent in toughness of the weld heat affected zone to symptoms. 2.0 ≦ v <15.0 ... 0.10 ≦ δ / d <0.50, where v is the reduction speed (mm / sec) and δ is the reduction amount (m
m) and d are the thickness (m
m).