JP2598357B2 - Manufacturing method of high strength steel sheet with excellent low temperature toughness - Google Patents

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 high-strength steel sheet having excellent low-temperature toughness. Steel produced by this method has severe strength and toughness (DWTT: Drop Weig).
HT Tear Test) (-)
(60 ° C.).

[0002]

2. Description of the Related Art With natural gas being attracting attention as an energy source, development of new gas fields is being promoted in extremely cold regions in the Arctic Circle. Along with this, the demand for large-diameter, thick-wall, high-tensile gas line pipes has been increasing in order to efficiently and economically transport gas to consumption areas.

[0003] From the viewpoint of safety, in order to prevent large-scale destruction of the pipeline, the DWTT has an excellent ductile fracture ratio and a Charpy impact value, which are indices of the characteristic of stopping propagation of unavoidable cracks. Steel sheet is required.

In order to satisfy such severe material properties, a so-called controlled rolling method disclosed in, for example, JP-A-52-128821 and JP-A-58-77528.
g, hereinafter referred to as CR) and online processing heat treatment method (Th
thermo-mechanical Control P
process (hereinafter referred to as TMCP) is well known.

[0005]

In recent years, with the depletion of natural resources, the laying area of line pipes has been further advanced to an extremely low temperature region of -60 ° C. Higher tensile strength of 590 N / mm ² or more is required. However, there is a problem that this requirement cannot be sufficiently satisfied only by manufacturing a steel sheet by the conventional CR or TMCP.

[0006] An object of the present invention is to solve the problems of the conventional method and to clarify the optimum components and production conditions, thereby providing a high-strength steel plate (590N) having excellent low-temperature toughness.
/ Mm ² or more).

[0007]

Means for Solving the Problems The gist of the present invention is that the weight%
And C: 0.01 to 0.08%, Si: 0.6% or less,
Mn: 1.2 to 2.0%, Mo: 0.05 to 0.35
%, Nb: 0.01 to 0.10%, Ti: 0.004 to
0.03%, N: 0.001 to 0.006%, Al:
0.10% or less, or further Ni: 0.05 to 1.
Less than 0% , Cu: 0.05 to 1.50%, Cr: 0.0
5 to 1.00%, V: 0.005 to 0.080%, C
a: A steel slab containing 0.0005 to 0.005% of one or more kinds and the balance consisting of Fe and unavoidable impurities is heated to a temperature of 900 to 1000 ° C., and is rolled up to 900 ° C. or less in subsequent rolling. after the reduction ratio and the in and rolling end temperature of 50% or more 830 ° C. or less, the cooling rate 5-4
This is a method for producing a high-strength steel sheet excellent in low-temperature toughness, characterized by accelerated cooling at a rate of 0 ° C./sec to a temperature of 550 ° C. or lower, and then allowing it to cool.

[0008]

It is well known that Nb has an ability to refine ferrite grains and precipitate harden, and is an important element indispensable for CR and TMCP. The good low-temperature toughness of Nb-added CR and TMCP steels is due to the refinement of ferrite grains, which is considered to largely depend on the non-recrystallization of austenite during rolling by solid-solution Nb. . Nb dissolved as a solid solution at the time of slab reheating is strain-induced precipitated as Nb (CN) in lattice defects introduced by rolling, and remarkably suppresses austenite recrystallization. Furthermore, a number of deformation zones are introduced into the grains of unrecrystallized austenite, and these deformation zones serve as ferrite nucleation sites,
This is because ferrite grains are refined.

Further, in order to improve the toughness by reducing the size of the ferrite grains, it is necessary to reduce the reheating temperature of the slab to reduce the size of the initial austenite grains. However, when the slab reheating temperature is lowered, N
Since the amount of solid solution b decreases, the strength decreases, and it is extremely difficult to achieve both high strength and high toughness. Accordingly, the present inventors have studied the optimum components and rolling conditions for producing a steel having a strength of 590 N / mm ² or more, which is extremely excellent in low-temperature toughness, and have reached the present invention.

Hereinafter, the present invention will be described.

A feature of the present invention is that a steel containing a certain amount of Mo and Nb is heated to a low temperature region, and then appropriately rolled, and the structure is made into fine ferrite-martensite, thereby providing high strength and low temperature. An object is to obtain a steel sheet having excellent toughness.

Mo also raises the austenite non-recrystallization temperature similarly to Nb and is effective for refining the ferrite structure. However, the inclusion of Mo and Nb significantly promotes the austenite non-recrystallization effect. Even when the slab reheating temperature is lowered in order to refine the initial austenite grains, it is extremely effective for the non-recrystallization of austenite and the refinement of ferrite based thereon. further,
Mo is effective in forming fine martensite subsequent to the formation of fine ferrite at the time of transformation from stretched unrecrystallized austenite, thereby forming a two-phase structure of ferrite-martensite. The dislocation density inside the fine martensite is very high, and high strength can be easily achieved.

In order to produce these effects, Mo must be used.
The amount should be 0.05-0.35%. 0.05
%, The effect is weak, and the addition of 0.35% or more is not preferable for weldability, so the upper limit is set to 0.35%. Also Nb
The amount should be 0.01-0.10%. 0.01
%, The effect is weak, and the addition of 0.10% or more is not preferable for weldability, so the upper limit is set to 0.10%.

Not only the amounts of Mo and Nb but also the heating and rolling conditions are important.

The reheating temperature of the slab is 900-1000 ° C.
Need to be This is because the initial austenite grains during heating are kept small and the rolling structure is refined. Further, as the initial austenite grains are smaller, the two-phase structure of fine ferrite-martensite is more likely to occur. 1000 ° C. is the upper limit temperature at which the austenite grains during heating do not become coarse. On the other hand, if the heating temperature is too low, the added alloying element is not sufficiently solution-solutioned, the internal quality of the steel is deteriorated, and the temperature at the end of rolling is too low, so that a sufficient material improvement effect by controlled cooling or the like cannot be expected. Therefore, the lower limit is set to 900 ° C.

However, even if the heating temperature is limited to a low value as described above, it is not possible to obtain a good material if the rolling conditions are inappropriate, so that the rolling reduction in the non-recrystallization temperature region of 900 ° C. or less is required. It needs to be 50% or more. This is because by applying sufficient rolling in the non-recrystallization temperature range to low-temperature heating, the austenite grains are thoroughly refined and elongated, and furthermore, a two-phase structure of ferrite-martensite is achieved.

Further, the rolling end temperature must be 830 ° C. or lower. This is because if the rolling is completed at a temperature exceeding 830 ° C., the structure is not sufficiently refined and the two-phase structure is not sufficiently formed, and good strength and toughness cannot be obtained.

Next, there are two methods of cooling after rolling: a method of air cooling after rolling, and a method of accelerated cooling to a temperature of 550 ° C. or less after rolling, at a cooling rate of 5 to 40 ° C./sec, and then allowing to cool.
Either one can be selected in terms of required sheet thickness, strength level and cost. In particular, in the case where it is thick and requires high strength and high toughness, it is preferable that after rolling, accelerated cooling is performed at a cooling rate of 5 to 40 ° C./sec to a temperature of 550 ° C. or less, and then the material is allowed to cool.

The reason for setting the cooling rate to 5 to 40 ° C./sec is as follows.
If the rate is less than 5 ° C./sec, a fine martensite structure is hardly generated, and improvement in strength cannot be expected. If the rate is more than 40 ° C./sec, coarse and large amounts of martensite are generated, which deteriorates ductility. . The reason why the cooling stop temperature is set to 550 ° C. or lower is that if the cooling is performed at too low a temperature, a dehydrogenation effect or sufficient precipitation hardening cannot be obtained. In this case, it is desirable that the cooling be stopped at 350 to 550 ° C. and then left to cool. However, if the cooling stop temperature exceeds 550 ° C., a sufficient increase in strength cannot be expected. Note that, as a cooling medium, spray water or water is generally suitable.

Further, when the steel produced according to the present invention is reheated for the purpose of dehydrogenation or the like, if the temperature exceeds 600 ° C., the strength is undesirably deteriorated. However, reheating to a temperature of about 600 ° C. or less does not impair the effects of the present invention, although the strength is slightly reduced.

Next, the reasons for limiting other components will be described.

C is 0.01 to obtain the required tensile strength.
% Or more is required. However, excessive addition of C causes deterioration of weldability, so the upper limit is made 0.08%.

[0023] Si is an element contained in the deoxidized upper steel,
Excessive addition impairs weldability and weld heat affected zone (HAZ) toughness. Therefore, the upper limit is set to 0.6%.

Mn is a useful element for securing strength, toughness and hardenability, and it is necessary to add 1.2% or more. However, if the amount of Mn is too large, the weldability and the HAZ toughness are deteriorated, so the upper limit is made 2.0%.

Ti is useful for suppressing austenite grain coarsening during welding and ensuring HAZ toughness. However, the addition of less than 0.004% has no effect.
On the other hand, if the addition is more than 03%, the precipitation hardening of TiC causes HA.
In order to cause deterioration of Z toughness, the amount of addition is 0.004 to
Limited to 0.03%.

Although N is generally contained in steel as an inevitable impurity, the presence of TiN is useful for suppressing the austenite grains from coarsening and ensuring the HAZ toughness. However, addition of less than 0.001% has no effect, and excessive addition of N causes deterioration of HAZ toughness. Therefore, the upper limit is made 0.006%.

Al is generally an element contained in the deoxidized upper steel. However, since deoxidation is also performed by Si, Mn or Ti, the lower limit of Al is not limited in the present invention. However, when the amount of Al increases, the cleanliness of the steel deteriorates and the HAZ toughness deteriorates. Therefore, the upper limit is set to 0.1%.

P and S are contained in steel as unavoidable impurities. In the present invention, the amount is not particularly limited,
Because these deteriorate the toughness of the base metal and the weld,
The amount is preferably as small as possible.
%, 0.01% or less.

In the present invention, if necessary, N
i: 0.05 to 1.0% , Cu: 0.05 to 1.5
0%, Cr: 0.05-1.00%, V: 0.005-
One or two or more of 0.080% and Ca: 0.0005 to 0.005% can be contained. The main purpose of containing these elements is to enable improvement of strength, toughness and expansion of the production plate thickness without impairing the effect of the method of the present invention, and the added amount thereof is such as weldability and HAZ toughness. It is of a nature that should be naturally restricted in terms of aspects.

Ni improves the strength and toughness of the base material without adversely affecting weldability and HAZ toughness.
If the content is less than 5%, the effect is weak, and the addition of more than 1.0% is not preferable for weldability, so the upper limit is made less than 1.0% .

Cu has almost the same effect as Ni, and also has an effect on corrosion resistance and resistance to hydrogen-induced cracking.
%, Cu-cracks occur during hot rolling, making production difficult. Therefore, the upper limit is set to 1.50%.

Cr is an element that increases the strength of the base material,
Add 0.05% or more. However, the Cr content is 1.00%
If it exceeds, the weldability and the HAZ toughness are degraded, so the upper limit is made 1.00%.

V is an element contained for the purpose of grain refinement and precipitation strengthening of the rolled structure, and is an element which improves both strength and toughness. However, if it is less than 0.005%, the effect cannot be sufficiently obtained. If it exceeds 0.080%, it is harmful to the weldability and the toughness of the welded portion.
Limited to 80%.

Ca controls the form of sulfide, increases Charpy absorbed energy and improves low temperature toughness.
It is also effective in improving the resistance to hydrogen-induced cracking. But,
If the Ca content is 0.0005% or less, there is no practical effect, and if it exceeds 0.005%, CaO and CaS are generated in large amounts to form large inclusions, impairing not only the toughness of the steel but also the cleanliness. Since the weldability is also adversely affected, the range of the amount of Ca added is set to 0.0005 to 0.005%.

[0035]

Next, an embodiment of the present invention will be described.

Tables 1 to 3 show the chemical components, production conditions and mechanical properties of the test steels. Steel plates of various thicknesses were manufactured and their mechanical properties were investigated. Tensile properties were investigated using an API tensile test piece, Charpy properties were examined using a JIS No. 4 test piece taken from a 1/4 t portion, and DWTT properties were measured using a DWTT test piece reduced in thickness to 19.05 mm from the surface. Regarding weldability, a HAZ toughness at −60 ° C. was evaluated by applying a reproducible heat cycle at a peak temperature of 1400 ° C. In Tables 1 to 3, steels 1 to 5 show inventive examples and 6 to 16 show comparative examples.
Inventive Examples 1 to 5 have a tensile strength of 590 N / mm ² or more, and show extremely good low-temperature toughness. In contrast, Comparative Example 6 had sufficient strength because the slab reheating temperature was too low.
No toughness is obtained, and internal defects are also observed. In Comparative Example 7, since the slab reheating temperature was too high, the initial austenite grains were large, and good strength and toughness could not be obtained. Comparative Example 8
Since the rolling reduction at 900 ° C. or less is small, good strength and toughness cannot be obtained. In Comparative Example 9, good rolling strength and toughness were not obtained because the rolling end temperature was too high. In Comparative Example 10, since the cooling rate during the accelerated cooling is low, sufficient strength cannot be obtained. In Comparative Example 11, sufficient toughness was not obtained because the cooling rate during accelerated cooling was too high. In Comparative Example 12, sufficient strength was not obtained because the cooling stop temperature was too high. Since Comparative Example 13 does not contain Mo, good strength and toughness cannot be obtained. In Comparative Example 14, good HAZ toughness was not obtained because the amount of Mo was too large. Since Comparative Example 15 does not contain Nb, good strength and toughness cannot be obtained. In Comparative Example 16, good HAZ toughness was not obtained because the amount of Nb was too large.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

According to the present invention, it is possible to manufacture an epoch-making steel sheet having both high strength of 590 N / mm ² or more and good low-temperature toughness, and the safety of a line pipe manufactured using this steel sheet. Can be achieved.

Claims (2)

(57) [Claims] C: 0.01 to 0.08%, Si: 0.6% or less, Mn: 1.2 to 2.0%, Mo: 0.05 to 0.35% by weight% Nb: 0.01 to 0.10%, Ti: 0.004 to 0.03%, N: 0.001 to 0.006%, Al: 0.10% or less, with the balance being Fe and inevitable impurities the billet consisting heated to a temperature of 900 to 1000 ° C., after which the subsequent cumulative reduction rate of 900 ° C. or less when rolling was less and 830 ° C. the rolling end temperature of 50% or more, the cooling rate 5-4
Accelerated cooling to a temperature of 550 ° C or less at 0 ° C / second,
A method for producing a high-strength steel sheet having excellent low-temperature toughness, characterized by being left to cool . 2. In% by weight, C: 0.01 to 0.08%, Si: 0.6% or less, Mn: 1.2 to 2.0%, Mo: 0.05 to 0.35%, Nb: 0.01 to 0.10%, Ti: 0.004 to 0.03%, N: 0.001 to 0.006%, Al: 0.10% or less, Ni: 0.05 to 1 Less than 0.0% , Cu: 0.05 to 1.50%, Cr: 0.05 to 1.00%, V: 0.005 to 0.080%, Ca: 0.0005 to 0.005% Alternatively, a steel slab containing two or more kinds and the balance consisting of Fe and unavoidable impurities is heated to a temperature of 900 to 1000 ° C., and in subsequent rolling, the cumulative draft of 900 ° C. or less is 50% or more, and the rolling end temperature is and 830 ℃ or less
After cooling, the cooling rate is 5 to 40 ° C / sec.
A method for producing a high-strength steel sheet with excellent low-temperature toughness, characterized in that the steel sheet is accelerated and cooled after cooling .