JP3474661B2 - Sour-resistant steel plate with excellent crack arrestability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sour-resistant steel sheet having excellent crack propagation arresting properties, a tensile strength of 500 MPa or more and a sheet thickness of 25 mm or more. The steel sheet according to the present invention can be used for line pipes, pressure vessels and the like used in high pressure, wet hydrogen sulfide environment and low temperature.

[0002]

2. Description of the Related Art Conventionally, the excellent hydrogen-induced cracking resistance (HIC resistance) of thick steel sheets has been described in Japanese Patent Publication No. 63-001369 and Japanese Patent Publication No. 62-112722. Technology such as high cleanliness, morphology control of sulfide inclusions by adding Ca, reduction of center segregation by light reduction during continuous casting, improvement of microstructure of center segregation part by accelerated cooling after hot rolling, etc. It has been achieved by making full use of it.
In particular, the application of accelerated cooling significantly improves the HIC resistance by controlling the microstructure including the center segregated portion to bainite or acicular ferrite. Such structure control is important near the center segregation portion, and accelerated cooling from Ar ₃ (transformation start temperature) or higher is essential. Therefore, in the conventional rolling of the sour-resistant steel sheet, the rolling end temperature is limited to a high temperature of Ar ₃ or higher, so that the refinement of the structure is insufficient and it is difficult to achieve a good crack propagation stopping property.

Further, as the plate thickness increases, the effect of rolling on the central part of the plate thickness becomes smaller and the structure becomes coarser, so that it is difficult to obtain good crack propagation stopping characteristics in thick materials. The method of refining the thickness center structure of the thick material is to use the difference in deformation resistance in the thickness direction to strongly reduce the thickness center, that is, to roll the surface with a hard surface and a soft center. There is. The means for achieving such a deformation resistance difference is to increase the temperature difference in the plate thickness direction, or as disclosed in JP-A-63-307216, the surface layer has a hard ferrite structure and the central portion has a soft structure. For example, it has an austenitic structure. Further, as disclosed in Japanese Patent Publication No. 6-929, there is also a method of strongly reducing the central portion of the plate thickness by high shape ratio rolling having a large roll diameter and a large reduction ratio. However, even with these conventional techniques, there is a limit to the miniaturization of the plate thickness center structure of the thick material, and it has been difficult to achieve good crack propagation stopping characteristics. Therefore, Japanese Patent Application Laid-Open No. 61-235534 discloses that a steel sheet having excellent crack propagation stopping properties can be manufactured by forming a thick ultrafine grain layer on the surface layer portion of the sheet thickness. However, this technique does not consider sour resistance at all.

As described above, there has been no steel sheet capable of simultaneously satisfying an excellent crack propagation stopping property and a good sour resistance in a thick material.

[0005]

The present invention has a plate thickness of 25 m.
It is an object of the present invention to provide a sour-resistant steel sheet that satisfies both excellent brittle crack propagation arresting characteristics and hydrogen-induced cracking resistance (HIC resistance) in a steel sheet having a tensile strength of 500 MPa or more at m or more.

[0006]

SUMMARY OF THE INVENTION The gist of the present invention is C: 0.02 to 0.12% by weight, Si: 0.6% or less, Mn: 0.6 to 1.5%, P: 0. .01
5% or less, S: 0.001% or less, Al: 0.06%
Below, Ti: 0.005-0.03%, Nb: 0.01
~ 0.10%, Ca: 0.001-0.005%, O: 0.00
3% or less, N: 0.001 to 0.005%, and 0.5 ≦ [Ca] (1-124 [O]) / 1.25
[S] ≤ 7.0 is satisfied, and if necessary, by weight% Ni: 0.1-0.5% Cr: 0.1-
0.5% Mo: 0.1-0.5% Cu: 0.1
0.5% V: 0.01 to 0.1% of one or more kinds, with the balance having a chemical composition consisting of iron and unavoidable impurities, and the thickness of the steel sheet from both surfaces toward the center of the thickness. The area ratio of ferrite having a crystal grain size of 5 μm or less over 70% and the Vickers hardness of 140% or more over the distance of 10% to 25 %, and the remaining thickness direction inside is bainite or acicular ferrite and the Vickers hardness is Is less than 248, and the plate thickness is 2
It is a sour-resistant steel plate having a crack propagation stopping property of 5 mm or more and excellent.

The present invention will be described in detail below. The technical idea of the present invention is to achieve excellent crack propagation stopping properties by forming a superfine grain ferrite layer thickly in the plate thickness surface layer portion, particularly in a steel sheet with a chemical composition adjusted from the viewpoint of sour resistance. A steel sheet excellent in crack propagation stopping characteristics and HIC resistance is obtained by securing the HIC resistance by controlling the microstructure inside the thickness to bainite or acicular ferrite and at the same time controlling the hardness of the surface layer part and the inside of the thickness. Is to get.

The basic idea for improving the crack propagation arresting property is to reduce the structure. Generally, the outermost surface portion of a steel sheet has a fine structure and thus has excellent ductility, and even when the inside of the plate thickness is brittle, the extremely thin region of the outermost surface portion forms a ductile fracture surface called shear lip. JP 61-23
In the invention described in Japanese Patent No. 5534, the thickness of the shear lip is increased by increasing the thickness of the ultrafine grain layer in the surface layer portion of the plate thickness, and the crack propagation stopping property is enhanced. However, no consideration is given to the sour resistance of such an ultrafine grain surface layer.

Most of the HIC is generated in the center portion of the plate thickness due to the hardened structure of the center segregation portion, but cracks called blister may also occur in the surface layer portion of the plate thickness. The method for preventing blisters in the superfine grain surface layer having excellent crack propagation stopping properties is not clear, and the present inventors have made extensive studies on this point. As a result, as shown in FIG. 1, a correlation was recognized between the Vickers hardness (load 25 g: hereinafter referred to as Hv25 g) of the superfine grain surface layer and the number of surface blister generation, and the minimum value of Hv25 g was 140 or more. It has been found that in some cases surface blister can be prevented. When a soft ferrite having an Hv25g of less than 140 is present in the superfine grain surface layer, such soft ferrite cannot be strength-strengthened against the pressure of hydrogen gasified at a certain place, and the surface is not It is believed that blister is generated. Therefore, a Vickers hardness of 140 or more is required for the superfine grain surface layer having excellent crack propagation stopping properties from the viewpoint of sour resistance.

Such an ultrafine grain surface layer has an area ratio of ferrite of 70% or more with a crystal grain size of 5 μm or less over a distance of 10% to 25% of the plate thickness from both surfaces of the steel plate toward the center of the plate thickness. There must be. The thickness of one side of the ultrafine grain surface layer is less than 10% of the plate thickness, the area ratio of ferrite having a crystal grain size of 5 μm or less is less than 70%, or the ferrite crystal occupying 70% or more in area ratio. If the particle size exceeds 5 μm, sufficient crack propagation stopping properties cannot be obtained. Also, the thickness of one side of the ultrafine grain surface layer is 2
If it exceeds 5%, the strength tends to decrease, and it becomes difficult to control the structure in the vicinity of the center segregation portion.

The inside of the plate thickness excluding the superfine grain surface layer must be controlled to have a structure mainly composed of bainite or acicular ferrite so that the Vickers hardness is 248 or less. Specifically, it is effective to control the structure by accelerated cooling after hot rolling. In particular, even in a steel sheet whose chemical composition is adjusted from the viewpoint of sour resistance, if the inside of the sheet thickness is mainly composed of polygonal ferrite, the center segregation part has a hardened structure (island martensite) with a Vickers hardness of more than 248. Etc.) is formed in a large amount, and the HIC resistance is significantly deteriorated. This is because the central segregated portion has a lower Ar ₃ point than the surroundings due to the concentration of the alloying element, so C discharged by the surrounding ferrite transformation is concentrated in the untransformed central segregated portion. This is because many hardened tissues are formed. Such a hardened structure is also disadvantageous for toughness and crack propagation arrest properties. When the structure is controlled mainly by bainite or acicular ferrite, such C concentration in the central segregated portion is reduced, so that the formation of a hardened structure is suppressed. When the inside of the plate thickness becomes martensite due to quenching, etc., a large amount of a hard region with a Vickers hardness of more than 248 is formed.
The C property deteriorates and the crack propagation stopping property also deteriorates. However, the area ratio of acicular ferrite is 20
% Of polygonal ferrite is allowed, and bainite is allowed to contain island martensite of 3% or less in area ratio. Further, from the viewpoint of toughness and crack propagation stopping characteristics, it is desirable that the structure inside the plate thickness be fine.

The object of the present invention is a thick sour resistant steel plate having a plate thickness of 25 mm or more. In the comparatively thin sour-resistant steel plate having a plate thickness of less than 25 mm, the TMCP technique can make the entire plate a fine structure and achieve good crack propagation arresting properties. Board thickness is 25m
In the case of a thick sour-resistant material having a thickness of m or more, excellent crack propagation stopping characteristics can be achieved only by the present invention.

The present invention is achieved, for example, by the following manufacturing method. That is, 10
After heating to 00 to 1250 ° C. and rolling to a thickness of 2 t or more, the water amount density becomes 0.5 to 2.0 from a temperature of 950 ° C. or more.
Cooling with water cooling time of 10 to 50 s at m ³ / m ² · min, then finishing rolling to the thickness of t in the process where the surface layer of the plate thickness recovers due to sensible heat inside, and the temperature of Ar ₃ or more points To 300 to 550 ° C. at a cooling rate of 5 to 40 ° C./s, and then air-cooling. Here, t is the final plate thickness after rolling, and 25 mm ≦ t. At this time, rolling should not be performed in the temperature decreasing process after the plate thickness surface layer part has recovered heat. When such rolling is performed, part of the strongly worked ferrite in the surface layer of the plate thickness is recrystallized, and soft ferrite having a Vickers hardness of less than 140 is generated in the superfine grain surface layer. Because I will do it. From the viewpoint of productivity and cost, it is desirable to make full use of such TMCP technology to achieve the present invention.

The reasons for limiting the chemical components will be described below. An increase in the amount of C strengthens the segregation of Mn and P in the central segregation of the slab and significantly deteriorates the hydrogen induced cracking resistance, so the upper limit was made 0.12%. The lower limit was made 0.02% in order to secure strength and low temperature toughness. The upper limits of the amounts of Mn and P were set to 1.5% and 0.015%, respectively, in order to reduce center segregation and secure hydrogen-induced cracking resistance. The lower limit of the amount of Mn is set to 0 in order to secure the strength and low temperature toughness of the base material and the weld.
It was 6%. On the other hand, the smaller the amount of P, the higher the resistance to hydrogen-induced cracking.

Nb and Ti are essential elements for the steel of the present invention. Nb contributes to refinement of the austenite structure and precipitation strengthening in controlled rolling to strengthen the steel. Ti forms fine TiN, suppresses coarsening of heated austenite grains during slab heating and welding, and improves base material toughness and HAZ.
Improves toughness. The lower limit of the Nb amount of 0.01% and the lower limit of the Ti amount of 0.005% are the minimum amounts for these elements to exert their effects, and the upper limit of the Nb amount is 0.10% and the upper limit of the Ti amount is 0.03. % Is the limit of the amount of addition that does not deteriorate the HAZ toughness and field weldability.

N forms TiN and improves the base metal and HAZ toughness by suppressing coarsening of γ grains during slab reheating and welding. The minimum amount required for this is 0.001%. However, if it is too much, HA will be caused by slab surface defects and solid solution N.
Since it causes deterioration of Z toughness, its upper limit is 0.005%.
Need to be kept to. Addition of a large amount of Si deteriorates the field weldability and HAZ toughness, so the upper limit is 0.6%.
And Only Al and Ti are sufficient for deoxidizing steel.
It is not always necessary to add i.

In the steel of the present invention, S which is an impurity is 0.
001% or less and adding Ca, 0.5 ≦ [C
a] (1-124 [O]) / 1.25 [S] ≤ 7.0 is satisfied. S forms MnS inclusions, and MnS is elongated by rolling and becomes a starting point of HIC generation. In order to prevent this, the absolute amount of inclusions must be reduced, and the form of sulfide must be controlled to obtain CaS (-O) that is difficult to be stretched by rolling.

Therefore, the amount of S is set to 0.001% or less and Ca
0.001 to 0.005% is added to sufficiently control the morphology of sulfides by Ca. Therefore, [Ca] (1-124
[O]) / 1.25 [S] ≧ 0.5. However, when this value is too large, it has been an increase in the Ca-type inclusions, since the starting points of HIC, was the upper limit of 7.0.

In relation to the above, the amount of O is limited to 0.003% or less. This is because oxide-based inclusions, which are the starting point of HIC, are reduced and the sulfide morphology is controlled by the amount of Ca.
Al is an element contained in steel as a deoxidizing element, but deoxidizing is also possible with Ti or Si, and it is not always necessary to add it. If the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%.

Next, the selective elements Ni, Mo, Cr, C
The reason for adding u and V will be described. The main purpose of adding these elements to the basic composition is to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself.

When Ni is 0.1% or more, weldability and HAZ
The strength and toughness of the base material are improved without adversely affecting the toughness, but excessive addition is not preferable for weldability, so the upper limit was made 0.5%. Mo is 0.1% or more, the strength of the base material,
Improves both toughness, but excessive addition of base metal and H
The upper limit is 0.5% because it deteriorates the toughness and weldability of AZ.
And

Cr is not easily segregated in the center of the CC slab, and if it is 0.1% or more, it improves the strength of the base metal, but excessive addition deteriorates the toughness and weldability of the base metal and HAZ, so the upper limit is 0. It was set to 0.5%. Cu has an effect similar to that of Ni at 0.1% or more, but excessive addition causes Cu-cracks during hot rolling, which makes production difficult.
The upper limit was 0.5%.

When V is 0.01% or more, it has substantially the same effect as Nb, and it makes it possible to improve the toughness by micronizing the microstructure, increase the hardenability, and improve the strength by precipitation hardening. However, excessive addition causes deterioration of HAZ toughness and weldability, so the upper limit was made 0.1%.

[0024]

EXAMPLES Table 1 shows the chemical composition of steel. Table 2 shows the structural characteristics and hardness of the steel sheet, as well as the mechanical properties and sour resistance. Steels 1 to 6 in Tables 1 and 2 are steels of the present invention, and steels 7 to 19 are comparative steels. The steel of the present invention has a high strength of TS of 500 MPa or more, and has excellent crack propagation stopping characteristics and excellent sour resistance (NACE environment). On the other hand, the comparative steel is inferior in mechanical properties or sour resistance because the chemical composition, structure and hardness are not appropriate. Steel 7, 8 and 9 are C
Content, Mn content, and P content are too large, and even if the inside of the plate thickness is controlled to a structure mainly composed of bainite or acicular ferrite, a large amount of hardened structure is formed in the central segregation portion, and the HIC resistance is poor. . Steel 10 has too much S, so ESSP (= [Ca] (1-124 [O]) / 1.25
[S]) is less than 0.5, and morphology control of sulfide inclusions is insufficient, resulting in poor HIC resistance. Steel 11 is N
Since the amount of b is too small, refinement of the austenite structure by rolling and precipitation hardening become insufficient, the toughness and strength are poor, and good crack propagation stopping characteristics (for example, Kc (a
t-100 ℃) ≧ 400kgf · mm -3/2) not be obtained. Steel 12 has an insufficient amount of Ca, resulting in inadequate morphology control of sulfide inclusions, resulting in poor HIC resistance. Steel 13 has an excessively small amount of Ti, and therefore TiN does not sufficiently suppress the growth of heated austenite grains.
Due to the coarsening of grains, the structure after transformation also becomes coarse, the toughness is inferior, and good crack propagation stopping properties are not obtained. Steel 14 has a thin ultrafine grain surface layer, and Steel 16 has a small ferrite area ratio of 5 μm or less in the ultrafine grain surface layer, so that the crack propagation arresting property is significantly deteriorated. Steel 15 has a superfine grain surface layer that is too thick, and therefore has low strength, resulting in poor HIC resistance due to insufficient structure control of the center segregation portion by accelerated cooling. Steels 17 and 18 have a Vickers hardness of less than 140 in the superfine grain surface layer, so that many surface blisters occur. In steel 19, since polygonal ferrite having an area ratio of more than 20% is formed inside the plate thickness, many hardened structures are formed in the central segregation portion, and the HIC resistance and toughness are poor.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

Industrial Applicability The sour-resistant thick high-strength steel sheet provided by the present invention has very excellent crack propagation arresting property as compared with the conventional steel, and can be used under high pressure, wet hydrogen sulfide environment and low temperature. such safety line pipes and pressure vessels used to upper remarkably improvement.

[Brief description of drawings]

FIG. 1 shows the relationship between the number of surface blisters and the minimum Vickers hardness of the superfine grain surface layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-236420 (JP, A) JP-A-6-256842 (JP, A) JP-A-61-235534 (JP, A) JP-A-5- 271860 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.02 to 0.12% by weight,
Si: 0.6% or less, Mn: 0.6 to 1.5%, P: 0.01
5% or less, S: 0.001% or less, Al: 0.06%
Below, Ti: 0.005-0.03%, Nb: 0.01
~ 0.10%, Ca: 0.001-0.005%, O: 0.00
3% or less, N: 0.001 to 0.005% is contained, and 0.5 ≦ [Ca] (1-124 [O]) / 1.25
[S] ≦ 7.0 is satisfied, the balance has a chemical composition of iron and unavoidable impurities, and is 5 μm or less over a distance of 10% to 25% of the plate thickness from both surfaces of the steel plate toward the center of the plate thickness. A plate having an area ratio of 70% or more and a Vickers hardness of 140 or more, and bainite or acicular ferrite having a Vickers hardness of 248 or less in the remaining thickness direction. A sour-resistant steel plate with a thickness of 25 mm or more and excellent in crack propagation stopping properties. 2. C: 0.02 to 0.12% by weight, Si: 0.6% or less, Mn: 0.6 to 1.5%, P: 0.01
5% or less, S: 0.001% or less, Al: 0.06%
Below, Ti: 0.005-0.03%, Nb: 0.01
~ 0.10%, Ca: 0.001-0.005%, O: 0.00
3% or less, N: 0.001 to 0.005% is contained, and 0.5 ≦ [Ca] (1-124 [O]) / 1.25
[S] ≦ 7.0 is satisfied, and Ni: 0.1 to 0.5% Cr: 0.1
0.5% Mo: 0.1-0.5% Cu: 0.1
0.5% V: 0.01 to 0.1% of one or more kinds, with the balance having a chemical composition consisting of iron and unavoidable impurities, and the thickness of the steel sheet from both surfaces toward the center of the thickness. The area ratio of ferrite having a crystal grain size of 5 μm or less over 70% and the Vickers hardness of 140% or more over the distance of 10% to 25 %, and the remaining thickness direction inside is bainite or acicular ferrite and the Vickers hardness is Is less than 248, and the plate thickness is 2
A sour-resistant steel plate with excellent crack propagation stopping characteristics of 5 mm or more.