JP2001107136A - Method for producing high strength steel excellent in weldability and low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing high strength steel economically and efficiently satisfying strength TS >=600 N and EV-80>=100 J in a welded joint without requiring the addition of large quantities of expensive alloy elements and also without applying loads of equipment and operation. SOLUTION: As for the method for producing Cu precipitation hardening type high strength steel, by controlling the content of Al to <=0.015%, a steel composition preventing the formation of AlN at the time of welding is made, a slab is heated to the temperature range of 950 to 1,200 deg.C, is subjected to hot rolling in which finishing temperature is controlled to 700 to 850 deg.C, is thereafter let to cool or is cooled from >=(680 to 830 deg.C) at a cooling rate of 1 to 50 deg.C/sec to the temperature range of <=580 deg.C, is then reheated at 450 to 650 deg.C and is then subjected to air cooling treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a steel material having excellent weldability and low-temperature toughness.

[0002]

2. Description of the Related Art In recent years, from the viewpoints of economy, safety, and the like, the strength of welded structures such as large industrial machines, welded steel pipes, offshore structures, bridges, and the like has been increasingly increased, and they have been used in these fields. The required properties for steel materials are increasing. A common requirement for these steel materials is further improvement in weldability and low-temperature toughness.

Conventionally, to improve the weldability of steel,
It is known that lowering the C content is effective. In order to compensate for the reduction in strength due to the lowering of the C content, various alloying elements have been added and the strength has been increased from the process aspect. For example, ASTMA710 and US Pat. No. 3,692,514 disclose steel materials utilizing Cu precipitation strengthening. These steels are distinguished by their excellent weldability, but cannot be said to have sufficient low-temperature toughness.

On the other hand, techniques for improving low-temperature toughness include:
Japanese Patent Nos. 2611565 and 2690578 disclose a method of applying cold rolling or cold rolling after ordinary hot rolling. However, these techniques usually apply a reduction in cold or warm conditions in a hot plate mill that performs hot rolling, and have a large facility load and cannot be said to be generally applicable techniques. For example, Japanese Patent No. 2690578 discloses that from the temperature range of 700 to 850 ° C. after hot rolling, air cooling or cooling at a cooling rate of 30 ° C./sec or less, and 400 ° C.
A method for producing a Cu-precipitated high-strength steel which is excellent in low-temperature toughness and resistance to local corrosion of a weld heat-affected zone by rolling at 3 to 5% in a temperature range of ~ 450 ° C is disclosed.

Also, Japanese Patent No. 2710846 discloses a method for producing a thick steel plate having a high thickness of Cu precipitation type. However, it is necessary to add more than 2.0% or less of Ni, which is an expensive element, to 6.0% or less. There is a problem in the economics as.

Japanese Patent Application Laid-Open No. 7-292416 discloses a low P (≦ 0.
015%), low S (≦ 0.0010%)) with Ca addition, strictly controlled rolling in austenite unrecrystallized area and two-phase area, and then accelerated cooling, low temperature toughness, on-site weldability, sour resistance A method for manufacturing a steel plate for an ultra-high-strength line pipe of API × 100 class or more capable of simultaneously achieving the above-mentioned requirements is disclosed.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high-strength steel material excellent in weldability and low-temperature toughness, particularly in low-temperature toughness in a welded joint, by simpler and less expensive means.

[0008] Specifically, an object of the present invention is to provide a weldability and low-temperature toughness economically and efficiently without requiring a large amount of expensive alloying elements and without imposing load on equipment and operation. It is an object of the present invention to provide a method for producing a high-strength steel material.

[0009] More specifically, the object of the present invention is to provide TS ≧
An object of the present invention is to provide a method for manufacturing a high-strength steel material satisfying _E V _-80 ≧ 100 J at 600 N and a welded joint by simpler and less expensive means.

[0010]

Means for Solving the Problems The present inventor has made various studies in order to solve the above-mentioned problems, and as a result, obtained the following findings and completed the present invention.

That is, in the conventional Cu precipitation hardening type steel material, strength and weldability are simultaneously improved by utilizing the precipitation hardening effect of Cu, the precipitation hardening effect of Nb and the refining effect of crystal grains. I am satisfied. In this case, when Al is positively added, the austenite grains are prevented from coarsening by the formation of AlN, but it is usually considered only as a kind of deoxidizing agent. However, if Si cannot be used as a deoxidizing agent,
It has been considered to use as a deoxidizing agent and to prevent austenite grain coarsening due to generation of AlN. In any case, the addition of Al was conventionally considered to be essential.

However, the present inventor has found that Al has a bad effect on the toughness of a welded joint, and that low-temperature toughness is significantly improved by minimizing it.

Therefore, according to the present invention, Al is not used as a deoxidizing agent, and the content of Al is made as small as possible. In the present invention, deoxidation is performed using Si and Mn.

[0014] Here, the present invention relates to the following:
0.08%, Mn: 0.60 to 1.6%, P: 0.025% or less, S: 0.
01% or less, Cu: 0.50 to 1.20, Al: 0.015% or less, Nb: 0.
It has a steel composition containing 005 to 0.05%, and Ni: 0 to 0.60%, and further contains an element generally added to steel for adjusting Ceq, Pcm strength and the like, if desired, and the balance of Fe and A slab with a steel composition that is an inevitable impurity
Weldability characterized by heating to a temperature range of 1200 ° C, performing hot rolling at a finishing temperature of 700 to 850 ° C, allowing to cool, then reheating to 450 to 650 ° C, and then air cooling. And a high-strength steel excellent in low-temperature toughness.

From another aspect, the present invention relates to a method for heating a steel slab having the above-described steel composition to a temperature range of 950 to 1200 ° C. and performing hot rolling at a finishing temperature of 700 to 850 ° C. , 68
Cooling from 0 to 830 ° C or higher to 580 ° C or lower at a cooling rate of 1 to 50 ° C / sec, then reheating to 450 to 650 ° C, and then air-cooling. This is a method for producing high-strength steel with excellent toughness.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method for producing steel according to the present invention will be described below. Hereinafter, in this specification, unless otherwise specified, “%” means “% by mass”. In the following description of the embodiments, a case where the “steel material” is a “steel plate” will be taken as an example. First, the reason for limiting the composition of the molten steel used in the present invention will be described.

C: 0.03 to 0.08% C is an element that contributes to an increase in strength. However, it is difficult to secure the strength when the content is less than 0.03%. Deteriorates the weldability and toughness of the resulting product. Therefore, the C content is limited to 0.03% or more and 0.08% or less.

Mn: 0.60 to 1.6% Mn is an element necessary for ensuring the strength and toughness of steel, but if it is less than 0.60%, such effects are small.
On the other hand, when added in a large amount exceeding 1.6%, the weldability is deteriorated. Therefore, the Mn content is limited to 0.60 to 1.6%. Preferably, the lower limit is 0.80% and the upper limit is 1.3%.

P: 0.025% or less, S: 0.01% or less Both P and S are elements that are liable to cause segregation during solidification of steel, and this segregation causes the welded portion to become brittle and lower the toughness. For this reason, it is desirable to reduce the content of both P and S, but a remarkable reduction requires a corresponding processing cost.

Therefore, in the present invention, the P content is set to 0.025%
Limited to the following. On the other hand, S precipitates in steel as MnS, which is an A-based inclusion, and is elongated during rolling to reduce toughness. The S content is set to 0.01% or less.

Cu: 0.50% to 1.20% Cu is an element that exerts a strengthening effect by precipitation strengthening treatment after rolling and cooling, realizes a low C, and improves weldability and low-temperature toughness. The lower limit is 0.50 for effective precipitation strengthening
%. Excessive addition also causes a decrease in toughness, as well as the occurrence of Cu cracks and HAZ during hot rolling of steel.
The adverse effect of promoting grain boundary cracking becomes significant. Therefore, the upper limit is set to 1.20%. Preferably, it is limited to 0.80 to 1.0%.

Ni: 0 to 0.60% Ni may be added as needed, and does not adversely affect the weldability and the HAZ toughness almost similarly to Cu and improves the strength and toughness of the base material. If added excessively, it becomes extremely expensive as a structural steel material and loses economic efficiency. Therefore, when added, the Ni content is limited to 0.60% or less.

Nb: 0.005 to 0.05% Nb is an element effective for improving the toughness by precipitation hardening and grain refinement. However, if it is less than 0.005%, such an effect cannot be obtained. On the other hand, if the content exceeds 0.05%, the toughness of the welded portion deteriorates.
Therefore, the amount of Nb added is limited to 0.005% or more and 0.05% or less.

Al: 0.015% or less Al is HA due to not producing AlN during large heat input welding.
It is characterized by improved Z toughness. That is, when the amount of Al is high, it is dispersed and precipitated as AlN in the steel, but in the vicinity of a FL (fusion line) which is heated to a high temperature during welding, AlN forms a solid solution when the temperature rises, and solute N increases. This solid solution N does not re-precipitate as AlN because of its high cooling rate during cooling after welding, and as solid solution N lowers the toughness near FL. In order to avoid such effects, it is desirable that Al is as low as 0.015% or less, but considering stability in the steel making process, 0.01%
It is preferable to set the following. Therefore, the Al content is 0.015
%, Preferably 0.010% or less.

In the present invention, Si may be added as a deoxidizing agent, if necessary. However, since addition of a large amount causes deterioration of weldability and toughness, it is preferred that the amount be as small as possible. Si is limited to 0.50% or less, preferably 0.30% or less.

Other Elements Other than the elements defined above in the present invention, elements generally added in the production of steel, for example, Cr, Mo,
The addition of V, Ti, B, etc. is not limited as long as the effects of the present invention are not impaired. In other words, as long as the addition amount is economically acceptable, the effect of the present invention is not promoted or hindered, so that the addition amount is not limited.

That is, in the present invention, Ceq, Pcm
At least one or more of the above-mentioned elements generally added to steel for adjustment of strength and the like may be further contained, and the balance is iron and inevitable impurities.

(Hot Rolling) The material for hot rolling may be a slab obtained by the slab rolling method or a slab obtained by continuous casting. And

In the present invention, the steel slab thus manufactured is heated to a temperature range of 950 to 1200 ° C. to perform hot rolling. The steel slab that has been once cooled to room temperature may be reheated, or may be maintained or heated at the above-mentioned temperature through a soaking furnace without cooling to room temperature after continuous casting by a so-called direct rolling process.

When the heating temperature is lower than 950 ° C., since Nb does not sufficiently dissolve in the matrix, recrystallization of austenite cannot be suppressed in the subsequent hot rolling, and the microstructure cannot be refined. Will be enough. On the other hand, if the heating temperature exceeds 1200 ° C., the austenite crystal grains become coarse when the steel slab is heated, and the toughness of not only the central part of the sheet thickness but also of the entire base material decreases.

In the present invention, the heating temperature of a steel slab as a hot-rolled material such as a continuous cast slab is 950 to 1200 ° C.

The conditions for the hot rolling are not particularly limited, as long as the rolling end temperature of the hot rolling is set at 700 to 850 ° C. This is so-called controlled rolling, and the crystal grains can be further refined by rolling in a two-phase region or an austenite recrystallization region as in the past, but in the case of the present invention, As long as the rolling end temperature is within the above range, the rolling may be performed according to, for example, a conventional method, and is not particularly limited.

When the rolling end temperature is lower than 700 ° C., the deformation resistance of the steel increases, so that it is difficult to finish the shape of the steel sheet after hot rolling to a target shape. On the other hand, if the rolling end temperature is high, the effect of grain refinement by controlled rolling cannot be obtained, and the toughness of the base material cannot be secured. Therefore, the upper limit of the rolling end temperature is limited to 850 ° C.

(Cooling) After the completion of the hot rolling, the steel sheet is left to cool or, if necessary, cooled at a cooling rate of from 680 to 830 ° C. to 1 to 50 ° C./sec to a temperature range of 580 ° C. or less.

The term "as needed" is determined from both the target thickness of the steel material and securing the low-temperature toughness. For example, when high toughness at a lower temperature is required,
Alternatively, when the cooling rate at the time of cooling is slow with a thick steel material, it means that it is effective to perform accelerated cooling. In this case, time elapses from the end of rolling to the start of cooling, and depending on the steel, hardenability decreases during cooling, and strength cannot be secured. Therefore, in the present invention, the cooling start temperature is set to 680 ° C. or higher. Usually, cooling is started immediately after hot rolling is completed.

The average cooling rate in this cooling is 1 ° C./s
If it is less than ec, a bainite structure or the like with coarse carbides is likely to be generated, so that it is not possible to secure a sufficient yield strength especially at the center of the steel sheet. On the other hand, the cooling rate is 50 ℃ / sec
If it exceeds, the surface layer of the steel sheet may be easily quenched in the vicinity thereof, and the toughness of the surface layer may be reduced. Therefore, in the present invention,
The average cooling rate from the cooling start temperature range of 680 ° C. or higher to the temperature range of 580 ° C. or lower is limited to 1 ° C./sec to 50 ° C./sec.

If the cooling stop temperature in this cooling exceeds 580 ° C., not only in the central part but also in the surface part of the steel sheet,
Since the formation of martensite or lower bainite becomes insufficient, the strength cannot be secured. Therefore,
In the present invention, the cooling stop temperature is 580 ° C. or lower.

By such a heat treatment, a martensite or bainite structure is obtained, and in the present invention, it is mainly a bainite structure.

(Heat treatment) 45 ° C
After reheating to 0 to 650 ° C, an air cooling process is performed. This is for the purpose of efficiently and stably exerting the precipitation strengthening of Cu. At this time, the reheating is performed in a temperature range of 450 to 650 ° C. depending on the desired strength and low-temperature toughness.

Here, the reason why the lower limit is limited to 450 ° C. is that if the temperature is lower than this, the precipitation of Cu is not sufficiently completed during the heat treatment.
This is because there is a concern that the properties of the base material may be significantly changed in the vicinity of the joint due to welding heat when being processed into a structure or the like thereafter. Further, the upper limit is limited to 650 ° C. because when heated to a high temperature, sufficient strength cannot be secured due to overaging.

The steel material thus produced according to the present invention can be used, for example, in a case where a structure is formed by using welding, for example, in the case of -80 in a welded joint, especially a welded base material.
Toughness at low temperature of ℃, -60 ℃ in heat affected zone
The toughness at a low temperature is greatly improved. This is particularly advantageous as a steel material used for shipbuilding and marine structures whose structures are formed by welding.

Therefore, when the steel material according to the present invention is used for shipbuilding or offshore structures, its excellent properties are exhibited.

[0043]

Next, the operation and effect of the present invention will be described more specifically with reference to the embodiments. Note that the target performance in this embodiment is Gr.80 class for marine structures (YS ≧ 80
Ksi) steel, and the target value of joint toughness is vE-80 ≧ 100J at the center of the plate thickness.

From the steel compositions shown in Table 1, 43 types of steel sheets were manufactured by hot rolling under the same manufacturing conditions as shown in Table 1.

A test piece was cut out from the obtained steel sheet, and YS,
TS and Charpy impact value were measured. In addition, as an evaluation of the toughness of the weld, a weld joint was created by applying a SAW heat input of 10 kJ / mm to the groove on each sample, and the vE-
A Charpy impact value of 60 was measured. The results are shown in Tables 1 and 2.
Are shown together.

In Table 1, Examples Nos. 1 to 22 are Examples of the present invention, and Examples Nos. 23 to 43 in Table 2 are Comparative Examples. Examples No. 1 to 22 which are examples of the present invention are all YS ≧ 552 N / mm ² , TS ≧
^{621N / mm 2, vE-80} ≧ 295J, HAZ of vE-60 ≧ 108 J (target ≧
47J), and is excellent in weldability and low-temperature toughness.

On the other hand, Comparative Examples Nos. 23 to 43 are examples in which the production method is outside the scope of the present invention. In Examples Nos. 23 to 24, the heating temperature was out of the range of the invention, and the strength or the joint toughness did not achieve the target.

In Examples Nos. 25 to 26, the rolling finishing temperature was out of the range of the invention, and the strength or the joint toughness did not achieve the target. In Examples Nos. 27 to 30, cooling was out of the range of the invention, and strength or joint toughness did not achieve the target.

Examples Nos. 31 to 41 are examples in which the chemical components are outside the range of the present invention, and the strength or the joint toughness did not achieve the target. In Examples Nos. 42 to 43, the heat treatment temperature was out of the range of the present invention, and the strength or the joint toughness did not achieve the target.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

As described in detail above, according to the present invention, a steel-strength steel material excellent in weldability and low-temperature toughness can be effectively produced by inexpensive means. The practical significance of the present invention having such an effect is extremely large.

Claims (2)

[Claims] C .: 0.03 to 0.08%, Mn: 0.60 to 1.6%, P: 0.025% or less, S: 0.01% or less, Cu: 0.50 to 1.20, Nb: 0.005 to 0.
A slab having a steel composition containing 05%, Al: 0.015% or less, and Ni: 0 to 0.60% was heated to a temperature range of 950 to 1200 ° C, and hot-rolled at a finishing temperature of 700 to 850 ° C. rear,
A method for producing a high-strength steel material excellent in weldability and low-temperature toughness, characterized by allowing to cool, then reheat to 450 to 650 ° C, and then air-cooling. 2. A steel slab having the steel composition according to claim 1 is heated to a temperature range of 950 to 1200 ° C., and a finishing temperature of 700 to 1200 ° C.
After hot rolling at ~ 850 ° C, it is cooled from 680 ~ 830 ° C to 580 ° C at a cooling rate of 1 ~ 50 ° C / sec, then reheated to 450 ~ 650 ° C, A method for producing a high-strength steel material having excellent weldability and low-temperature toughness characterized by air-cooling.