JPS6338520A - Production of steel plate having excellent hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To prevent the formation of martensite and hardness increase of bainite and to obtain a titled steel plate by specifying the cooling conditions after hot rolling of a slab which is confined in the compsn. CONSTITUTION:The compsn. consisting of 0.01-0.20wt% C, 0.03-0.80% Si, 0.40-1.80% Mn, <=0.025% P, <=0.002% S, 0.008-0.032% Ti, 0.01-0.10% solAl and the balance unavoidable impurities and iron is applied. The compsn. is confined to 3.5<(Ca/S) <10.0 and 3.4<(Ti/N)<8.0. The CC slab made of such compsn. is subjected to the hot rolling at >=50% draft in the temp. region of <=(Ar3+150 deg.C) and the Ar3 or above and the hot rolling is ended at the Ar3 point or above. The steel is then subjected to accelerated water cooling at 5-20 deg.C/sec cooling rate from the temp. region of >=(Ar3-30 deg.C) down to <=600 deg.C and the temp. in excess of the Ms point and is allowed to cool naturally after the suspension of water cooling, by which the steel plate is produced.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to the production of steel sheets with excellent resistance to hydrogen-induced cracking, which are useful for line pipes used to transport crude oil and natural gas containing H and S. In particular, the present invention relates to a method for manufacturing a steel sheet having a fine and uniform bainite + ferrite structure and excellent resistance to hydrogen-induced cracking.

(Prior Art) Hydrogen-induced cracking (HIC) is a hydrogen embrittlement phenomenon that occurs when hydrogen generated when steel corrodes in a wet ToS environment invades the steel.

The region with the highest HI cQ receptivity is the Vi,N-center, which is due to central segregation of the slab.

Conventionally, the following methods have been used to prevent HIC.

■ Reducing segregation by slab soaking ■ Reducing segregation by U due to low P range ■ Controlling the morphology of nonmetallic inclusions using Ca and REM to reduce inclusions that serve as starting points for H[C.

However, among these methods, (2) and (3) lead to a significant increase in costs. Moreover, the effect is ambiguous when it comes to (■).

Therefore, recently, attempts have been made to produce steel that prevents the concentration of alloying elements by performing accelerated water cooling after rolling, creates a uniform and fine structure, and suppresses the formation of low-temperature a-structures in segregated areas. For example, JP-A-54-118325, JP-A-57-8
No. 5928, No. 58-77530, and No. 60-33
See No. 310.

The method for producing hydrogen-induced cracking resistance (hereinafter referred to as HrCfi) using accelerated water cooling is roughly based on Ar
Finish rolling is completed at the + point or higher, and water cooling is performed from -30°C or higher to change the structure to a fine ferrite↓pearlite structure or a ferrite + bainite in combination structure.
(This is to improve IC performance.

However, if the water cooling conditions are not appropriate, a low-temperature transformed structure such as martensite or bainite with high hardness may be formed, resulting in a decrease in HIC resistance.

Furthermore, even if the rolling-water cooling conditions are appropriate, some component systems may result in n having poor HIC resistance.

For example, JP-A-54-118325 and JP-A-57-8
In the case of No. 5928, after hot rolling, water cooling is started at Ar1 point or higher, and the temperature range is 3 to 550°C.
It is cooled at a cooling rate of S, and then allowed to cool.

Perlite remains unavoidable. Also, JP-A-58-
In the case of No. 77530 and No. 60-33310, A
Start water cooling from r=-30°C or higher, and reduce to 550-350°C
The temperature range is 10~b, and then it is allowed to cool as in the case described above.

Pearlite does not exist, but martensite and hard bainite do exist.

In addition, these conventional products are made of steel, and T
There is no mention of N content in any of those containing i, those not containing i, and even those containing Ca.

(8 points to be solved by the invention) Therefore, an object of the present invention is to provide a method for manufacturing a steel plate with excellent HIC resistance.

A further object of the present invention is to provide a method for manufacturing a steel sheet with excellent HIC resistance using a water cooling method that prevents the formation of martensite and the increase in hardness of bainite.

Still another object of the present invention is to provide an economical method for manufacturing a steel plate for line pipes that has excellent abrasiveness against HIC, which is a problem in a humid 11□S environment.

(Means for Solving the Problems) As a result of the inventors' detailed study of the HIC resistance of accelerated water-cooled steel in relation to the rolling-water-cooling conditions, the following facts were found.

■Steel plates with hunt-like I, 11 weaves of ferrite + pearlite, or structures in which pearlite remains have high resistance to HI.
Poor C properties.

■A mixed structure consisting of bainite and ferrite with a hardness of 250 or less has excellent HIC resistance.

(2) Steels having a structure containing bainite or martensite with a hardness of 250 or more have poor I″1tHIC properties.

In other words, if the cooling rate after rolling is selected excessively to suppress the formation of pearlite while causing haynite transformation, and preventing the formation of martensite, it is possible to
This improves carbon properties.

Therefore, the present inventors further investigated in detail the relationship between the IIC resistance of accelerated water-cooled steel and its component system, and found that
The following facts were discovered.

■Steel with high N content has poor HIC resistance.

■Even if the Nl content is as low as 40 ppm or less, steel without T1 addition still has poor HrC resistance.

■Steel with Ca/S of 3.5 or less or 10.0 or more has a N content, Ti! Poor HIC resistance regardless of IL.

Therefore, the gist of the present invention is, in weight %, C: 0.01-0.20%, Si: 0.03-0.
80%, Mn: 0.40-1.80%, P: C1
, (125% or less, S: 0.002% or less, Ti
:0.0080-0.032%, sol, AQ: 0
．． 01-0.10%N: 0.0040% or less, Ca
:0.0020~0.0050%, further as necessary, Cu: 0.05~0.50%, Ni: 0.05~
0.5050%, Cr: 0.05-0.50%, ha
: 0°05~0.50%, Nb: 0.01~0.1
5%, V: 0.01-0.15%, and REM:
Contains one or more of O,0QO5~0.01%, and 3.5 < Ca/S < 10.0 and 3.4
<Ti/N<B, Ol A CC slab with a composition satisfying the relational expression and the remainder consisting of unavoidable impurities and iron is heated to form Ar3 + 15
Hot rolling is performed at a reduction rate of at least 50% in a temperature range of 0°C or lower and Ar3 points or higher, and the hot rolling is finished at Ar1 or higher, and from a temperature range of Arc-30°C or higher, 600
Cooling rate of 5 to 20℃ to temperatures below ℃ and above Ms point
This is a method for producing a steel sheet with excellent hydrogen-induced cracking resistance, which is characterized by performing accelerated cooling in the range of /S, and cooling after stopping water cooling.

In this way, the steel plate obtained by cooling after stopping the water cooling was heated to 500°C.
As mentioned above, tempering may be performed by reheating to a temperature range below the Ac point.

As described above, according to the present invention, the following structural features are achieved.
You can see it.

■Use a CC slab with reduced N to 40 ppm or less.■3.4 Add Ti so that <Ti/N<8.0, fix N as TiN, and actively precipitate TiC. let

■Ca in the range of 20 to 50 ppm and 3.5<
Add so that Ca/S<10.0.

In addition, conventionally, there are no regulations regarding N for this type of steel (
, the normal level of N content is considered to be greater than 4 (1 ppm).

(Function) The reason why the steel composition is limited as described above in the present invention is as follows.

C: 0.01% or more is required to ensure the strength of the steel, and 0.2% to ensure the toughness of the steel and prevent welding cold cracking.
0% or less.

Si: 0.03% or more is required to ensure the strength of the steel and deoxidize, and it is 0.80% or less to ensure the toughness of the steel and prevent temper embrittlement.

0.40% or more is required to ensure the strength and toughness of the steel, and an increase in Mn increases the concentration of alloying elements in the segregated portion, but a content of 1.80% or less is permissible.

P: The lower the concentration of alloying elements in the folded part, the higher the resistance to HIC.
However, reducing the P content increases manufacturing costs, so the upper limit of the content is set at 0.025%, which is as high as possible within a range that does not have any negative effects on the present invention. However, it goes without saying that the smaller the amount, the better.

When S·S exceeds 0.002%, MnS, whose morphology cannot be controlled by Ca, is generated and becomes the starting point of HrC.

Therefore, in the present invention, S is limited to 0.002% or less. Preferably it is 0.001% or less.

Ti: The rolling structure is made finer with TiN, and further TiN
0.0080% or more is required to improve HTc resistance due to the hydrogen trapping effect of TiC.
If it exceeds 0.032%, toughness will be significantly impaired.
The upper limit is set to 0.032%.

sol, A(1,: 0.01% or more is required for deoxidizing steel, and ?f
To ensure f purity, it should be 0.10% or less.

If N2N exceeds 0.0040%, the HIC resistance rate will decrease, so it is set to 0.0040% or less.

Ca: 0.0020% or more is added to control the morphology of MnS, which is an inclusion in steel, and to improve IIC resistance. However, when the content exceeds o, ooso%, Ca-based inclusions conversely deteriorate HIC resistance and 5scc resistance.

Ca/S: The lower limit of Ca/S is 3 to ensure sufficient spheroidization of Mn5.
．． Over 5. Ca/S is set to less than 10.0 in order to prevent precipitation of Ca-based inclusions (oxides).

Ti/N: In order to improve HIC resistance through complete fixation of N and precipitation of TiC, the Ti/H ratio needs to be more than 3.4, and on the other hand, to ensure toughness, the ratio is less than 8.0.

In the present invention, carbon is used as the corrosion resistance imparting element.
At least one of u and REM is added.

Cu: Add 0.05% or more to impart corrosion resistance.

However, if the amount of Cu added exceeds 0.50%, weldability will be impaired.

REM: As in the case of Ca, 0°0005% or more is added to control the morphology of MnS 'JII, but if it exceeds 0.01%, cleanliness is impaired and HIC resistance and 54sscc properties are reduced. .

In the present invention, Ni, Ni,
At least one of Cr, Mo, Nb, and (2) is added.

Ni: Ni needs to be added in an amount of 0.05% or more to ensure strength and toughness, and if it exceeds 0.50%, the 5scc resistance will deteriorate.

Cr+'Io: 0.05% each to ensure the strength and toughness of the steel
Above, 0.50% or less is added.

Nb, V: 0.01% each to ensure the strength and toughness of the steel
Above, 0.15% or less is required.

According to the present invention, the CC slab having such a composition is then hot-rolled and accelerated water-cooled, and FIG. 1 shows the water-cooling pattern at this time.

That is, the material is heated to an appropriate temperature for hot rolling, then controlled hot rolling is performed at a finishing temperature of 3 points or more, Ar) + 150°C or less and a reduction rate of 50% or more, and then Ar3-
Accelerated water cooling is performed from a temperature in the range of 30 DEG C. or higher to a temperature range of 600 DEG C. or lower at a cooling rate of 5 to 20' C/S, exceeding the Ms point, and then allowed to cool. If necessary, the steel plate obtained by cooling is directly tempered at a temperature of 500° C. or higher at 3 AC points.

The reason why the hot rolling conditions and the accelerated water cooling conditions are limited as described above in the present invention is as follows.

First, in hot rolling, the material is heated to a temperature that allows the following hot rolling.

The upper limit of the hot rolling temperature is Ar3” 150°C,
If rolling is carried out at a temperature exceeding this range, the grains will not be sufficiently fine and strong deformation will not be achieved. Furthermore, if the temperature is lower than Ar, a predetermined water cooling start temperature cannot be obtained.

Rolling between heating and finishing temperature is optional.

If the rolling reduction rate is less than 50%, the austenite grains will not become sufficiently fine, and even if accelerated cooling is performed, a uniform composition will not be obtained.

The starting temperature for water cooling is Art. At temperatures lower than 30°C, alloying elements are converted into the polarized part due to the growth of pro-eutectoid ferrite, and a low-temperature transformation structure is formed when water is cooled.
Sexuality decreases. More preferably, it is at least the Arz point.

The upper limit of the water cooling stop temperature in continuous water cooling is 600°C, and the heinite transformation does not proceed sufficiently at temperatures higher than this. There is no effect of rapid cooling because the perlite tone progresses. Below the water-cooling stop temperature range ■, there is a Ms point pressure. In addition, 4
Since HIC resistance does not improve even if the temperature is lower than 00°C, rapid cooling is preferably performed to a temperature range of 400°C or higher.

The lower limit of the water-cooling rate is 5° C., and when the temperature is less than 7, Paral 1-1- is partially formed. At temperatures above 20'C/S, the water cooling stop temperature cannot be controlled.
.

If it is too much, it will lead to the growth of martensite and cause the hardness of haynite to increase. Next, C: 0.09%, Si: 0.25%, Mn: I
, 0%, P+Q015%, S:O,0O11%: (A
Ti-N% Ca-
1' and 21 with various cold rolls of S were heated to 1100°C, then subjected to color rolling 2 at a reduction rate of 709%, and then rolled to 910°C.
The sample was then finished at 840° C. and then acceleratedly cooled to 500° C. at 18° C./S, and then allowed to cool. (The damaged ′, -1 board was subjected to HIC cutting and λ in the same manner as in the example described later.
I calculated the IC rate. The experimental results are summarized in graphs in FIGS. 2171 and 3. Figure 2 Snake 1! I
This figure summarizes the effects of Ti and Nl on carbon properties and mechanical properties, and in both cases Ca/S was within the scope of the present invention. Furthermore, FIG. 3 shows the influence of Ni and Ca/s tg on the HIC resistance rate.

In addition, the numerical value inside the circle in the figure indicates the HIC resistance accuracy rate.

As is clear from the illustrated results, NF440ppm or less,
Ti/N 3.4-8. O, Ca/33.5-10.0
At this time, the HIC resistance and mid-range resistance will be 100%.

The reason why a reduction rate of 50% or more and Ca/S of 3.5 to 10.0 is required is the shape of MnS! This seems to be due to its relationship with the song. Conventionally, extended MnS has been known to be the starting point of HIC, and an amount of C,1 has been added such that Ca/S=2 to 3 or more.

However, the reason why a large amount of Ca of 3.5 or more is required as shown in LJ in the present invention is not clear in detail, but water-cooled materials are better around coarse inclusions such as MnS than air-cooled materials. It is thought that this is because distortion tends to remain in the plane, so more f and Y shapes are required.

In addition, the reason why HIC resistance decreases even if Ca/S is 3.5 or more when N140 ppm1 is unknown is that N impairs the action of Ca and that a large amount of TiN 11 The story of IC! ! It is conceivable to form a route.

Furthermore, the advantage of reducing N and adding T1 with Ti/N > 3.4 is that a uniform ferrite layer is formed using TiN as a nucleus.
It can be considered that a haynite structure is obtained and that i'1tHIc properties are improved because TiC and TiN act as hydrogen trap sites.

Further, in a preferred embodiment of the present invention, the eight plates manufactured as described above are tempered, and the tempering temperature is Ac+
Above this point, recrystallization will occur.

Note that if the temperature is less than 500'C, the sufficient effect of tempering cannot be obtained, so it is preferably 500°C or more, AC,...
Tempering is carried out at less than t'-1°.

Next, the present invention will be further explained by examples.

Example Using the test steel with the composition shown in Table 1, CC was obtained by using 44 methods.
Slabs were produced, hot rolled under the conditions shown in Tables 2 to 5, and subjected to I]D rapid cooling.

Mechanical 4′r properties and +11c resistance of the obtained hot rolled material
The gender is also summarized in Tables 2 to 5.

Table 2 examines the optimum rolling and water cooling conditions for A steel. A-7 and A-8 are rolled and air-cooled materials and have very poor HIC resistance. A-5 has a water cooling start temperature of 73
As low as 0°C (56°C below the Ar* point). A-6 has a finishing temperature of Ar 3 or lower, and the water cooling start temperature is accordingly low, so it is inferior in HIC resistance. In A-9, the cooling rate is too fast.

A-1 to A-4 are within the scope of the present invention, and are
It can be seen that the carbon properties are good.

Table 3 shows the effects of Ti and N using the optimum rolling-water cooling conditions determined in Table 2.

No Ti was added in the 8-piece, and the N amount in the CI4 was 40 ppm or less, but Ti/N<3.4. Steel D has Ti/N of 4.0, but NN of over 140 pp. B-02
'A's HIC resistance rate is poor. E-G steel is Ti/N,
Both NWi are within the scope of the present invention. Moreover, Ca/S of all B-G@ is within the scope of the present invention.

In Table 4, J steel has too much Ca added (54 ppm).
and Ca/S is large. The HIC-resistant guys are going downhill.

K steel has a small amount of Ca added (, Ca/S is small, and has a high T resistance.
(Decrease in Ic properties.

Both L and M steels have Ca/S and Ti/N within the range of the present invention, but have Ni exceeding 40 ppm and are inferior in HIC resistance.

Table 5 summarizes the results of Examples conducted to confirm the effects of the present invention.

The 111 C resistance test was conducted as shown in FIGS. 4 and 5.

That is, for the H■C test, as shown in Figure 4, a piece with a thickness of 2 mm cut from the surface z-plane from the batten, a width of 100 mm, and a length of 1 mm is used.
A plate-shaped test piece of 00 mm was sampled over the entire width, and also at several locations in the length direction. These specimens were 60
After polishing with 0% emery, it was degreased with acetone. H
The test solution used in the IC test is called NACE solution, which contains 0.5% (C1hCOzll) and 5% common salt (
During the test with NaCQ) aqueous solution, H and S were bubbled through to form a saturated 1
I made it into a song. The temperature is 25°C and the soaking time is 100 hours.

Figure 5 shows the end face of the test piece after the HI C test.
, =), and the ratio of the sum of t and θ of this aij to the cross-sectional width was defined as the crack length ratio (%) (C, L, R,). The i4 HI C rate is expressed by the following formula.

Number of specimens

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the water cooling pattern of the method of the present invention; FIGS. 2 and 3 are graphs showing the influence of Tie, Nl, and Ca/S ratio on IT IC resistance in the present invention; and Figures 4 and 5 show the I (IC test and HIc resistance).
FIG. 2 is a schematic explanatory diagram illustrating the procedure for evaluating gender. Applicant: Sumitomo Metal Industries (Sumitomo Metal Industries, Ltd., company agent, patent attorney, Hiroshi dn Chapter - Figure 1 Sekikai Figure 2 Figure 3 B5) 0.0 IL15

Claims (1)

[Claims] (1) In weight%, C: 0.01 to 0.20%, Si: 0.03 to 0.80
%, Mn: 0.40-1.80%, P: 0.025% or less, S: 0.002% or less, Ti: 0.0080-0.
032%, sol. Al: 0.01-0.10%, N: 0.0040% or less, Ca: 0.0020-0.0
050%, and 3.5<Ca/S<10.0 and 3.4<Ti/N<8.0, and the remainder consists of unavoidable impurities and iron. Ar_3+150
℃ or below, hot rolling is carried out at a reduction rate of at least 50% in a temperature range of Ar_3 or above, and the hot rolling is finished at Ar_3 or above, and from a temperature range of Ar_3-30℃ or above, 6
Below 00℃, cooling rate 5-2 to temperature range exceeding Ms point
A method for producing a steel sheet with excellent hydrogen-induced cracking resistance, characterized by performing accelerated water cooling in the range of 0° C./S, and cooling after stopping the water cooling. (2) The steel plate obtained by cooling after stopping the water cooling was
The method according to claim 1, characterized in that the tempering is carried out by reheating to a temperature range of not less than 0.degree. C. and less than Ac_1 point. (3) In weight%, C: 0.01-0.20%, Si: 0.03-0.80
%, Mn: 0.40-1.80%, P: 0.025% or less, S: 0.002% or less, Ti: 0.0080-0.
032%, sol. Al: 0.01-0.10%, N: 0.0040% or less, Ca: 0.0020-0.0
050%, further Cu: 0.05-0.50%, Ni
:0.05~0.50%, Cr:0.05~0.50%
, Mo: 0.05-0.50%, Nb: 0.01-0.
15%, V: 0.01-0.15%, and REM: 0
．． 0005 to 0.01%, and satisfies the following relational expressions: 3.5<Ca/S<10.0 and 3.4<Ti/N<8.0, and the remainder is unavoidable. A CC slab with a composition consisting of impurities and iron is heated to Ar_3+150
℃ or below, hot rolling is carried out at a reduction rate of at least 50% in a temperature range of Ar_3 or above, and the hot rolling is finished at Ar_3 or above, and from a temperature range of Ar_3-30℃ or above, 6
Below 00℃, cooling rate 5-2 to temperature range exceeding Ms point
A method for producing a steel sheet with excellent hydrogen-induced cracking resistance, characterized by performing accelerated water cooling in the range of 0° C./S, and cooling after stopping the water cooling. (4) The steel plate obtained by cooling after stopping the water cooling was
4. The method according to claim 3, wherein the method is tempered by reheating to a temperature range of .degree. C. or above and below Ac_1 point.