JPS5887221A - Production of high tensile steel having excellent resistance to sulfide corrosion cracking - Google Patents

Abstract

PURPOSE:To produce titled high tensile steel by forming molten steel contg. specific contents of C, Si, Mn, P, S, Nb, B, solAl and contg. P, C, Mn in specific relations to an ingot by continuous casting or large sized steel ingot-blooming stages and hot rolling the ingot. CONSTITUTION:Molten steel is composed, by wt% of 0.002-0.15 C, 0.1-1 Si, 0.5-2.5 Mn, <0.03 P, <0.005 S, 0.01-0.12 Nb, 0.0002-0.005 B, 0.01-0.005 sol Al, and the balance Fe and unavoidable impurities wherein the contents of P, C, Mn are so controlled as to satisfy the formula. After such molten steel is formed into an ingot by continuous casting or large sized steel ingot-blooming stages, the ingot is hot rolled. If necessary, the steel is subjected to accelerated cooling or heat treatments.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-strength steel with excellent sulfide corrosion cracking resistance, which is used for oil and natural gas transmission pipes and LPG storage tanks exposed to wet hydrogen sulfide corrosion environments. In order to manufacture the above-mentioned steel by continuous casting or large-sized steel ingot-blooming process, molten steel is hot-rolled and further heat-treated as necessary, the composition of the molten steel is determined by The main idea is to take into account the segregation at the center of a continuously cast slab or large steel ingot, and to make adjustments to a specific range where the cracking does not become a problem even in the central segregation area where sulfide corrosion cracking is most susceptible. In this way, it is possible to
I C (Hydrogen Induc@d Crac)
king = hydrogen induced cracking) resistance and SSC (5ulfi
d@5tressCrackinir = produces round steel with excellent sulfide stress corrosion cracking resistance1. Conventionally, HIC occurs when hydrogen generated on the steel surface penetrates into the steel and forms inclusions/crackinir through a diffusion process. It is said that it gathers at the matrix interface, resulting in hydrogen embrittlement of the matrix and the pressure caused by hydrogen. Therefore, HI resistance
For C steel, (a) prevent hydrogen from entering, (b)
Reducing or controlling the morphology of inclusions that are the starting point of cracks, (C
) Measures have been taken to reduce the sensitivity of the matrix to hydrogen embrittlement. Regarding (a), a method is known in which, for example, an anticorrosive film is formed on the steel surface by adding Cu. However, in a corrosive environment where the pH of the solution is low, the formation of a film is insufficient and hydrogen penetration cannot be sufficiently prevented. Regarding (b) K, REM-treated steel (e.g., JP-A No. 54-31020) and Ca-treated steel (e.g., JP-A-54-38214) have been proposed, and nine of them are highly effective in low-strength materials. Regarding (e) above, where high-strength materials may not provide sufficient improvement effects, segregation is particularly problematic. That is, the segregated part is Mn, Cr,
Since the amount of alloying elements such as Mo is high, low-temperature transformation products that are hard and highly sensitive to hydrogen embrittlement are likely to be produced. Therefore, in order to prevent the formation of such structures, the steel plate is slowly cooled after rolling, or quenched and tempered after rolling, and these structures are replaced with tempered martensite or tempered behnite (K Methods such as changing the temperature have been adopted (for example, Japanese Patent Application Laid-Open No. 138916/1983). However, these conventional methods have many problems in terms of productivity and economy.

In addition, SSC is a constant value for hydrogen embrittled steel - (limit stress σ
In this case, the critical stress for cracking, which is a crack that occurs when a stress greater than or equal to th is applied, is significantly lowered when a hard structure that is easily susceptible to hydrogen embrittlement is present. In addition, in the case of line pipes, the amount and shape of inclusions, and the amount of hydrogen in steel are SS
It causes the occurrence of C.

In other words, the difference between HIC and SSC is only in the presence or absence of stress, and there is essentially no difference, and the measures against HICK described above are also effective against SSCK, and it can be considered that the problems are the same.

Additionally, in the production of high-strength steel, it is common practice to increase the content of alloying elements such as Mn, Cr, and Mo in order to increase the strength. However, when molten steel containing these alloying elements is solidified into large pieces, especially when it is made into slabs, billets, blooms, etc. by continuous casting, or when it is made into large steel ingots (weighing 20 tons or more), these castings The alloying elements are concentrated in the center of the piece or steel ingot,
It is known that so-called central segregation is likely to be formed.

Therefore, steel products rolled from such slabs, steel products manufactured from such face steel ingots through the blooming-rolling process, or steel products further heat-treated as necessary, are Also, the central segregation part) (IC, SSC
It is said to increase sensitivity.

On the other hand, the composition of conventional steel materials is assumed to be uniform throughout the steel material (or, it is known that alloying elements segregate in the center - but without considering the original fate), at the molten steel stage. The composition is adjusted to match that of steel. However, in the case of fractures such as HIC and SSC, where cracks first occur in the crack-sensitive parts of the steel material and then propagate and spread, this macroscopic approach alone is insufficient. .

From this point of view, the present invention particularly focuses on the part where the susceptibility to sulfide cracking is highest, that is, the central segregation part of steel products manufactured through a continuous casting process where center segregation is likely to develop or an ingot forming process using a large steel ingot weighing 20 tons or more. This is an attempt to adjust the composition of molten steel to prevent cracks from occurring.

The inventors first prepared molten steel in which the chemical components such as C, Mn, Si, and P were varied over a wide range, determined the composition of the molten steel using samples sampled from the molten steel, and then transformed the molten steel into large steel ingots. A cross-section of the steel material obtained by rolling (thermogeography if necessary) or continuous casting-rolling (original treatment if necessary) is macro-etched to reveal the central segregation part of the steel ingot or slab, and it is analyzed using a microanalyzer. The content of each element at that location was determined. In this manner, the content of each element in the segregated area is known, and the segregation coefficient of the central segregated area can be determined as the ratio of the content in the molten steel. Table 1 shows the segregation coefficients of each element determined by the Kakaru method.

Table 1 From this Table 1, the component axes of each element in large steel ingots or continuously cast slabs of 20 tons or more are not uniform, and in the central segregation area, P is 5 times that of molten steel, Mn and Mo are 2・5 times, C,
It can be seen that Cr is concentrated to 1.5 times the amount.・Also, not all elements segregate in the central segregation area (Cu
It was confirmed that the elements other than those mentioned above, such as , Ni, and Nb, were almost unchanged from the composition of molten steel and were at the same level [6].

Next, the inventors conducted detailed studies on a large number of steels corresponding to segregation areas. The steel corresponding to the segregated area here refers to the central segregated area where sulfide corrosion cracking is most susceptible to sulfide corrosion cracking in large steel ingots or steel products manufactured through the continuous casting process as explained earlier. Based on the knowledge that cracks occur at these locations, in order to prevent or reduce cracking at the relevant locations,
First of all, this is steel in the sense that the entire steel material has the same composition as the part corresponding to the central segregation part.

In other words, the steel used was melted in a test melting furnace and made into small steel ingots of about 50 noodles, then rolled and heat treated as necessary.)
It is considered that there is no segregation throughout the steel material (i.e., the steel ingot). We determined the conditions for making a steel with excellent sulfide corrosion cracking resistance for such a segregated phase steel, and
Next, using this and the segregation coefficients in Table 1, we determined the molten steel composition for improving the sulfide corrosion cracking resistance of steel products (having a central segregation part) manufactured by continuous casting and large steel ingot-blooming processes. This is what I wanted.

That is, in the present invention, C: α002 to 0.15chi, Sl:
111-1.0%, Mn: 0.5-2.5chi,
P: 0.03% or less, S: 0. O05chi or less, Nb: [
101~0.12chi, B:0.002~α0050chi,
5otAt: Contains 0.01 to 0.05 To, or in addition, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 1.0% or more), Mo: 1.0% or less , 720.01~015%, Ti, and 115%
200 A method for manufacturing steel, in which after the hot rolling,
It is characterized by performing accelerated cooling or heat treatment as necessary.

Therefore, in the present invention, the lower limit of C in the composition of molten steel is set to 0.
The reason why it is set as 0021 is that it is impossible to impart the necessary strength to this type of steel with less than that, and the upper limit is set to 0.
．． 1! The reason for choosing M is that exceeding this value significantly increases weld hardenability and cracking susceptibility. Lower limit of Sl is 0.1
The reason why it is set as s is because it is extremely disadvantageous if it is less than this in terms of steel manufacturing, and the reason why the upper limit is set at 1.0 is because if it exceeds this, toughness will be significantly impaired. Mn is an essential element to give strength and ductility to steel, but its lower limit is 0.
．． The reason for setting the upper limit to 5 is because the effect is reduced below this value, and the reason that the upper limit is set to 2.5 is that the weld hardenability and HIC susceptibility increase significantly. H is contained in steel as an impurity, but it is extremely easy to segregate, especially when it exceeds 0.03%.
This was set as the upper limit because the sensitivity of IC increases. It is known that sulfide-based nonmetallic inclusions become the starting point of HIC and Sac, and it is not desirable for S to exceed 0.005% from the shelf energy standpoint, so this is set as the upper limit.

At least 0.01% is necessary for deoxidizing steel with 5000 Attachment, and since this effect is saturated if it exceeds 0.05%, this is set as the upper limit.

As for Nb K, the effect on strength and toughness cannot be seen unless 0.01% or more is dissolved in solid solution, so this was set as the lower limit2.
If the amount added is large, the toughness of the base material and HAZ will deteriorate, so the upper limit was set at 0.12%.

B is an element effective in improving the hardenability of steel in a small amount, but the effect is small at 0.0002% or less, so this was set as the lower limit. Moreover, if it exceeds 0.0050S, the toughness of the weld zone will be significantly impaired, so this is set as the upper limit.Also, as an optional addition element, Cu contained in the molten steel in the present invention is mainly added from the viewpoint of corrosion resistance, but 1. If the temperature exceeds .0, KCu defects will occur frequently on the steel plate surface, and the welding heat affected zone (H
This is set as the upper limit because it also has an adverse effect on the toughness and hardenability of AZ). Ni, Cr, and Mo are the toughness of HAZ,
It improves the strength and toughness of the base material without affecting the hardenability, but exceeding 1.01 is unfavorable for the toughness and hardenability of the HAZ mentioned above, and from an economic point of view. With this as the upper limit, 5, N'b, V, and TI were set as the lower limit since no effect on strength and toughness would be observed unless any one of them was dissolved in a solid solution of 0.01% or more. In addition, when the addition amount of these is large, the base material and HAZ
The upper limit was set at 0.15% because it deteriorates the toughness of the steel. Ca
If the ratio is less than 0.0005, no inclusion control effect is observed, and therefore the sensitivity to HIC is not reduced, so this was set as the lower limit. Moreover, if it exceeds 0.01%, large inclusions will be formed and the HIC sensitivity will be increased, so this was set as the upper limit.

Next, in the present invention, the conditions for regulating the composition of molten steel are set to 115-
200 × pH-s OCeq"270 CHowever, P"
The reason for setting =5P will be explained.

Figure 1 shows the ductile fracture ratio (hereinafter referred to as SAl'lT) at room temperature in the Charpy impact test determined for the steel corresponding to the segregation zone.
2 is a graph showing the relationship between crack length ratio (hereinafter referred to as CLR) in the HIC test. This first
As is clear from the figure, as the 5ART value increases, the CLR
It can be seen that the value decreases, and when the SARτ value reaches 70 degrees or more, no cracks occur at all. This tendency is observed not only in hot-rolled materials but also in heat-treated materials. The inventors further studied various metallurgical factors that affect this 5ART, and found that reinforcing elements such as C and Mn, and especially the amount of P, have a large influence.Based on such experimental data, the 5ART value and P value were determined. , the regression equation with the Ceq value is 5A.
RT (19= 115 200-P -50・Ceq
--=(1) was obtained. Here, P is P amount (%), Ce
q is the carbon equivalent of Lloyd, Ceq(@=C+Mn/S+(Cu+Ni)/15+(
It is expressed by the formula (2) (each component is expressed in weight %).

Therefore, the composition of the segregated part of steel raw eel, which is considered to be highly susceptible to HIC and SSC, is 115-200.
"-50・Ceq"270・・・・・・・・・・・・・
・・・・・・・・・・・・(3) is full of HI
C. Preventing the occurrence of SSC (1) In other words, the molten steel composition of the steel obtained by continuous casting of high-strength steel with excellent sulfide corrosion cracking resistance and the large steel ingot-blooming process is as described above. This means that equation (3) needs to be satisfied. on the other hand,
As already shown in Table 1, the steel material (3) obtained through continuous casting or large ingot-blooming process in which central segregation tends to develop (
According to the present invention, it is possible to produce a steel material with excellent sulfide corrosion cracking resistance.

Table 2, which further supplements the explanation of the present invention based on the examples below, shows the chemical composition of the sample molten steel, Ceq (Lloyd), Ceq
and 5ART, and Table 3 shows rolling conditions, heat treatment conditions, and their accuracy results.

Molten steel B and F in Table 2 are large steel ingots of 34 tons, and the other molten steels are slabs continuously cast in a mold with dimensions of 220 x 1 <550.

Next, aBP is generally used as a test method to evaluate HIC performance.
However, in the present invention, the HIC
Sensitivity was evaluated - CLR and C8R were used here as major meters to indicate HIC susceptibility. These values are determined by the following formula.

However, the total area of Σai/bi mini-step crack A: specimen width B: specimen thickness However, currently CLR is
, Since there is no established theory regarding the upper limit of C8R, the present invention uses ARAMCO8PECAM, which is currently considered to have a strict standard.
Assess HIC susceptibility using the following criteria adopted in SS-1 (S9).

CLR<15% C3R=0 (no step cracking) - Molten steels A-D in Table 2 above are comparison steels, and each component including P is relatively high, so 5ART = 11
5-200 P"-50ceq'-・---------・-
- Mountain - <4> Based on the above formula (4), <5ART or both are low values of less than 70. Steels E-H are steels of the present invention, and all 5ART values are 70 or more. The rolling conditions and accuracy results for each of the sample steels are as shown in Table 6.
CLR in the NACE environment for comparative steels, regardless of whether they are plate materials), heat-treated materials, controlled rolling materials, or accelerated cooling materials.
, C8H values are all standard values (CLR<:15ch, C
3R=O1), whereas in the steel of the present invention, CL
Both R and C8H exhibit excellent HIC resistance below the standard value. Figure 2 shows SART and CLR in the example.
This is a graph showing the relationship between

Comparing the comparative steel and the steel of the present invention, the effect of lowering P is particularly large; for example, steel C has a CLR of 60% up to CR4;
Heat treated (quenched and tempered) material improved slightly by 20%.
However, the value will eventually reach 1, which is higher than this reference value, whereas in G, the CR remains as is, and the CLR is 8%, which is within the reference value.

From the above, the effects of the present invention can be expected regardless of whether the material is hot-plate material, as-rolled material, accelerated cooling material, or heat-treated material. It is an excellent manufacturing method as it can achieve good performance as rolled, and has extremely large economical effects.Although the above embodiments of the present invention have only mentioned examples of steel plates, the present invention is not limited to this. Needless to say, the present invention is not limited to the production of steel pipes, shaped steel, and other steel materials.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between ductile fracture ratio and CLR at room temperature, and Figure 2 is a graph showing the relationship between 5ART and CLR. Osamu

Claims (2)

[Claims] (1) C: 0.002-0.15%, Si:
0.1-1.0%%Mn: 0.5-2.5chi, P
: 0.03 or less, S2o, oos or less, Nb: 0
．． 01~0.12%, B:Q, 0002~o, oos
s, 5otAt: 0.01 to 0.05%, the remainder consisting of iron and inevitable impurities, and 115-200X
P”-30Ceq270 (However, P”-5P, Ceq
” = 1.5C + 2.5Mn/6) by continuous casting or large steel ingot-blooming process.
A method for producing high-strength steel with excellent sulfide corrosion cracking resistance, characterized by hot rolling after forming into a steel billet, and further subjecting it to accelerated cooling or heat treatment as necessary. (2) C: 0.002~o, i 5%, St: 0.
1-1.0%, Mn: 0.5-2.5S, P:
0.01 or less, s: 0.005% or less, ・Nb:
0.01-0.12%, B: 0.0002-0.0
059G%5oAAA: 0.01 to 0.05%, further Ni: 1.0% or less, Cu: 1.01 or less, Cr:
1. O1 or less, Mo: 1.0% or less, V2 0.01 to 0
．． 11, Ti: 0.01-G, 11, Ca: 0.00
It contains 0.05 to 0.01% of one or two types of base, and the remainder consists of iron and inevitable impurities, and is 115 to 200×?
-30Ceq”270C However, r=sp, Ceq*
= 1.5C+ is made into steel slabs by continuous casting or large steel ingot-blooming process, then hot rolled, and further subjected to accelerated cooling or heat treatment as necessary. A method for manufacturing high-strength steel with excellent sulfide corrosion cracking resistance.