JPS6338518A - Production of steel plate having excellent hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To obtain a high tensile steel plate having excellent hydrogen induced cracking resistance and low yield ratio type by subjecting a specifically composed slab to two-stage cooling after hot rolling. CONSTITUTION:The CC slab consisting of 0.01-0.20wt% C, 0.03-0.80% Si, 0.40-1.80% Mn, <=0.025% P, <=0.002% S, 0.008-0.15% Ti, 0.01-0.10% solAl and >=1 kinds among 0.05-0.50% Cu, 0.0005-0.0050% Ca, and 0.0005-0.01% REM and the balance unavoidable impurities and iron is heated. The slab is subjected to >=50% hot rolling 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 rolled steel is then subjected to a 1st stage of the quick water cooling from >=(Ar3-30 deg.C) down to (650-550 deg.C) at 15-30 deg.C/sec cooling rate. The steel is in succession subjected to a 2nd stage of the slow water cooling down to (650-550 deg.C) (550-400 deg.C) at 3-15 deg.C/sec cooling rate. The steel is allowed to cool after the stop of the water cooling. The formation of martensite and the hardness increase of bainite are thereby prevented and the title steel plate is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention provides H! A method for producing a steel plate, which is useful for line pipes used for transporting S-containing crude oil and natural gas, and is characterized by excellent resistance to hydrogen-induced cracking and a low yield ratio, in particular,
The present invention relates to a method for manufacturing a steel sheet that employs a two-stage cooling method to create a fine and uniform bainite + ferrite structure, which has excellent resistance to hydrogen-induced cracking and is characterized by a low yield ratio.

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

The part with the highest HICIC property is the central part of the plate thickness, which is the part caused by central segregation of the slab.

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

■Reducing segregation by slab soaking ■Reducing segregation by reducing P ■Controlling the morphology of nonmetallic inclusions using Ca and REM to reduce inclusions that are the starting point of HIC.

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

Therefore, attempts have recently been made to produce steel that prevents the quaternization of alloying elements by performing accelerated water cooling after rolling, creates a uniform and fine structure, and suppresses the formation of low-temperature Liu weave in the segregated areas. For example, JP-A-54-118325, JP-A-57
-85928, 58-77530, and 60-
See No. 33310.

The manufacturing method of hydrogen-induced cracking resistant steel (hereinafter referred to as HICM resistant) using such accelerated water cooling is roughly based on Arz
Finish rolling is finished at Ars -30°C or above, and the structure is changed to fine ferrite + pearlite [1'
Or use a ferrite + bainite mixed structure to resist HI.
The purpose is to improve C properties.

In other words, the water cooling condition is to uniformly cool a certain temperature range at a certain cooling rate. Also, its metallurgical structure is known as ferrite/pearlite structure or ferrite+bainite structure.

However, if the water-cooling conditions are not appropriate, a low-temperature transformed structure such as martensite or haynite 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 a τ4 that is not very good in HIC resistance.

On the other hand, low yield ratio (low YR) high-strength steels are now being sought after to prevent serious accidents such as explosions from occurring even if HrC occurs. In other words, YR(Y
If the S/TS ratio is low, it is thought that even if HIC occurs and stress is concentrated at the cracked part and a partial yield phenomenon occurs, it will not lead to rupture. In that case, YR is thought to be about 85% or less.

By the way, in order to manufacture such low YR type steel, it is desirable to have as high a carbon content as possible, but since carbon promotes segregation at the center of the slab, it becomes difficult to obtain sufficient HIC resistance with simple water cooling alone. . On the other hand, when the amount of C is low (0.05
~0.10%), relatively fast water cooling rate to obtain low YR by water cooling (10~b), since it results in the formation of martensite, 13
It is inevitable that the tHIc property will deteriorate.

Here, Fig. 1 and Fig. 2 show typical water cooling patterns of the conventional method, and as shown in each of them, it is possible to uniformly cool a certain temperature range in any case. It has become a feature.

In other words, in the case of Figure 1, after hot rolling, water cooling is started above the Arz point and the temperature is increased to 650 to 550°C.
~b It performs cooling. In the case of FIG. 2, water cooling is started from Arc-30° C. or higher, and the cooling is continued until the temperature ranges from 550 to 350° C. in the same way as in the case of FIG.

Figure 3 shows the water cooling stop temperature and YR and Hi
It is a graph showing the relationship with C-official acceptability, that is, CLR (%). From this graph, it can be seen that HIC resistance must be at the water cooling stop temperature of \375℃ or higher, but on the other hand, low Y
It can be seen that in order to obtain R, the water cooling stop temperature must be 375°C or lower.

Therefore, in the conventional water cooling method, especially low C materials (0,05
It can be seen that it is difficult to obtain a steel plate with low YR (YR < 85%) (approximately 50%) and excellent IC properties.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to provide a method for manufacturing a steel plate with low YR1 and excellent HIC resistance.

A further object of the present invention is to provide a method for producing a steel sheet with low YR (85% or less) and excellent 54+11C properties using a water cooling method that produces martensite and increases the hardness of bainite in a low C material. It is to be.

Still another object of the present invention is to provide an economical method for producing a low YR type steel plate for line pipes, which has excellent abrasiveness against HIC, which is a problem in a warm and humid 1hS environment. It is to be.

(Means for Solving the Problems) The present invention aims at producing a low YR type high tensile strength steel for line pipes without impairing HIC resistance.

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

■A batten having a band-like structure of ferrite + pearlite or a structure in which pearlite remains has poor HIC resistance.

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

■ Steels with a structure containing bainite or martensite with a hardness of 250 or more have poor HIC resistance.

In other words, if the cooling rate after rolling is appropriately selected to suppress the formation of pearlite while causing bainite transformation, and preventing the formation of martensite, it is possible to achieve high resistance to HI.
C properties are improved.

Further, in order to obtain a low YRg, a polygonal ferrite structure, an acicular acicular ferrite structure, or a mixed structure of ferrite and bainite is desirable. Steel with such a structure shows no elongation at yield point in a tensile test.S-3IJ! I-line, low yield ratio (approximately 85%
(below).

The importance of the present invention is not only the low yield ratio phenomenon, but also the discovery of conditions that combine two important elements (for L/P J11 steel): this low yield ratio and improved 111C resistance. be.

In other words, uniform and fine ferrite-vainaite is obtained by water cooling, resulting in a low yield ratio, and low-temperature transformation of the central segregated part that impairs HIC resistance. HI resistance is achieved by eliminating the weave and pearlite band MiPa.
Aim to improve C properties.

At this time, the alloy composition is also adjusted so that the quaternization of alloying elements is simultaneously suppressed as much as possible and center segregation is also reduced.

Therefore, the gist of the present invention is, in weight %, C: 0.01 to 0.20%, Si: 0.03
~0.80%, Mn: 0.40~1.80%, P
: 0.025% or less, s: 0.002% or less, T
i:0.008-0.15%, sol. Al: 0.
01-0.10% and Cu: 0.05-0.50%, Ca: 0.0005
~0.0050%, and R Kano: Q, GO05~0.
One or more of the following: 0.01%, Ni: 0.05-0.50%, Cr: 0.05-0
．． 50%, M.

: O, OS~0.50%, Nb: 0.01~0.1
5%, and one or more of V: 0.01 to 0.15%, the remainder being unavoidable impurities and iron is heated, and Ar3 + 15
At least 50% in the temperature range of 0℃ or below and Ar3 point or above
The above hot rolling is carried out, and the hot rolling is finished at the Ars point or higher, and the cooling rate is 15 to 30°C/S from the temperature range of Ar3 30°C or higher to the temperature range of 650°C or lower to 550°C or higher.
Accelerated water cooling is performed in the range of
This is a method for producing a steel sheet with excellent hydrogen-induced cracking resistance, which is characterized by accelerated water cooling in the range of ~15° C./S, and cooling after stopping the water cooling.

The steel plate thus obtained was heated to Ac at 500°C or higher.

Tempering may be performed by heating to a temperature range below the dot plate.

The present invention can be referred to as a two-stage water cooling method, in which the formation and growth of pro-eutectoid ferrite is suppressed so that the first stage rapid water cooling (>Ar, -30℃-650~550℃, 15~b state occurs). , second stage slow water cooling (650~550t-55
The bainite transformation is continued to proceed so as to prevent the formation of 3-b at 0 to 400°C and to prevent the hardness from exceeding the upper limit, and then allowed to cool to prevent the formation of martensite.

(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% or more is required 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 mouth and deoxidize, and it is 0.80% or less to ensure the toughness of the steel and prevent temper embrittlement.

Mn: 0.40% or more is required to ensure the strength and toughness of Mn, and an increase in Mn increases the temperature of the alloying element in the segregated part, but it is permissible up to 1.80%.

The smaller P=, the lower the alloy element concentration in the 02 fold, and the higher the HIC resistance.
However, since lowering the P content will increase the manufacturing cost, the upper limit is set at 0.025%, which is as high as possible without adversely affecting the present invention. However, it goes without saying that the smaller the amount, the better.

S: When S exceeds 0.002%, MnS whose morphology cannot be controlled by Ca is generated, which becomes the starting point of HIC.

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

Ti: TiN aims to refine the rolling structure and promotes extremely fine and uniform heinite transformation during the first stage water cooling. 0.008% or more is required to improve HIC resistance due to the hydrogen trapping effect of TiN and TiC. On the other hand, if it exceeds 0.15%, toughness is significantly impaired, so the upper limit is set at 0.15%. .

sol. Al: Requires 0.01% or more for deoxidizing steel, and 0.10% or less to ensure cleanliness.

In the present invention, in addition, 6 (as an edibility-imparting element,
At least one of Cu, Ca and REM is added.

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

However, if it exceeds 0.50%, weldability deteriorates.

Ca: O,oos% or more is added to control the morphology of MnS, which is an inclusion in steel, and to improve HIC resistance. However, when the content exceeds 0.0050%, Ca-based inclusions conversely deteriorate HIC resistance and 5scc resistance.

RE Gate: As in the case of Ca, is it possible to control the morphology of MnS? For il, more than 0°0005% is added, but if it exceeds 0.01%,
Cleanliness deteriorates, HIC resistance and 5scc resistance decreaseN
i: 0.05% or more is required to ensure the strength and toughness of steel,
If it exceeds 0.50%, the 5scc resistance deteriorates.

Cr + Mo: Both are 0 to ensure the strength or toughness of the steel.
．． Add 0.05% or more and 0.50% or less.

! Jt+-V+ Both are steel strength or toughness 61! For protection, 0.01% or more and 0.15% or less are required, respectively.

According to the present invention, the CC slab having such a composition is then subjected to hot rolling and two-stage accelerated water cooling, and FIG. 4 shows the water cooling pattern at this time.

That is, after heating to an appropriate temperature for hot rolling, the finishing temperature is controlled at Ar 3 points or higher and Ar++150°C or lower. 1
Rolling was performed for 17JA, and then (Ar, point ~ Ar)
The first stage accelerated water cooling is performed from a temperature range of 30° C. or higher to 15° C. to 15° C. This first stage rapid water cooling is 650~
Stop at temperature range of 550℃, then 550-400℃
3 to b, which is lower than the cooling rate described above, to a temperature in the range of 3 to b, which is then allowed to cool.

The reason why the hot rolled strip 1 and the water cooling conditions for water cooling and slow water cooling in the present invention are limited as described above is as follows.

First, during hot rolling, the material is heated to a temperature that allows hot rolling of 1 or less.

Hot rolling finishing temperature 5: The upper limit is Arz"L50°C, but if rolling is finished at a temperature exceeding this, the grains will not be sufficiently fine and high strength and toughness will not be obtained.

Furthermore, if the process ends at less than 3 Ar points, a predetermined water cooling start temperature cannot be obtained. Rolling between heating and finishing temperature is optional.

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

When the water cooling start temperature is lower than 30°C, the alloying elements change into polarized portions due to the growth of pro-eutectoid ferrite, and a low-temperature transformed Il texture is generated during water cooling, resulting in a decrease in HIC resistance. More preferably, the score is 1 point or more.

The lower limit of the cooling rate for the first stage water cooling is 15°C/S. If the cooling rate is lower than this, a large amount of pro-eutectoid ferrite will be generated, so uniform and fine bainite transformation will not occur, and low YR and HIC resistance will be satisfied. do not. Further, at 30° C./Sl, it becomes difficult to control the water cooling stop temperature. Preferably 18
~b The water cooling stop temperature in the first stage water cooling has an upper limit of 650°C. At a temperature higher than this, water cooling has no effect and pro-eutectoid ferrite grows, while a temperature lower than 550°C causes an increase in the hardness of heinite.

The second-stage water-cooling cooling rate is lower than the first-stage water-cooling cooling rate, with a lower limit of 3° C./S, and below this, some barlite is generated. Moreover, if the temperature exceeds 15° C./S, the hardness of bainite will increase, and in some component systems, martensite will be formed. Preferably, the upper limit of the water cooling stop temperature in the second stage water cooling is 550°C, and if this temperature is exceeded, pearlite will be generated during cooling. Further, if the temperature is lower than 400°C, martensite will be generated during water cooling.

FIG. 5 shows the 12!2 AA described below within the scope of the present invention.
A 0.05% C material corresponding to the above was hot rolled at a finishing temperature of 830°C, water cooling was started from 770°C at a cooling rate of 25°C/S, and then the second stage water cooling was performed after cooling to 600°C. It is a graph showing the relationship between the second stage water cooling stop point degree and YR (%) and CI-R (%) when cooling was performed at a cooling rate of 8° C./S and left to cool. 2nd stage water cooling stop point degree 400~5
It can be seen that by setting the temperature to 50°C, both YR and CLR are within a satisfactory range.

Furthermore, in a preferred embodiment of the present invention, the steel plate manufactured as described above is tempered, and the tempering temperature is Ac1.
Recrystallization occurs when the point exceeds the point. On the other hand, if the temperature is less than 500°C, the effect of tempering cannot be obtained.

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

EXAMPLES CC slabs were manufactured by conventional methods using test steels having the compositions shown in Table 1, and were hot rolled and accelerated cooled under the conditions shown in Tables 2 and 3.

The mechanical properties and HIC resistance of the obtained and human-rolled materials are also summarized in Tables 2 and 3.

Table 2 shows YR and 'iti+ of 0.05% C steel manufactured by the conventional water cooling method and the method of the present invention, respectively.
A comparison of Ic properties is shown.

As is clear from the results shown in Table 2, the conventional water cooling method satisfies HIC resistance depending on the water cooling conditions, but YR
is not necessarily less than 85%.

Also, Yl'? If it is desired to reduce the temperature to 85% or less, the water cooling stop temperature must be lowered, which will reduce the HIC resistance.

In this regard, the method of the present invention yields a steel plate that satisfies both HIC resistance and YR of 85% or less.

Table 3 shows the manufacturing conditions, mechanical properties and YR
and HIC resistance are collectively shown.

As is clear from the results shown in Table 3, the products according to the present invention satisfy the required conditions for both YR and HIC resistance. However, since F5m does not contain Ti,
Does not satisfy either HIC resistance or low YR.
- Figures 6 [11) to f) show microscopic Mi weave photographs of various steel plates manufactured according to this example.

Figure 6fa) is a micrograph of A9 in Table 2.
゜O) in Figure 6 (bl is the same enlarged version (x 5
00). Uniform, fine ferrite BJ
Ii! You can see that it is.

Figure 6 tel shows the steel structure (X1
00), and it can be seen that martensite is generated. FIG. 6 fd+ is the same microscopic structure photograph taken at X500.

The tel in Figure 6 is the microscopic structure photograph of 81 in Table 2 (X10
0). This is due to conventional one-stage cooling, and since the cooling rate is slow at 12° C./S, it can be seen that pearlite is generated. Figure 6 (fl is the same X5
This shows a photograph of the microscopic structure of No. 00.

The HIC resistance test was conducted as shown in FIGS. 7 and 8.

In other words, for the HIC test, as shown in Figure 7, a steel plate with a thickness of 2111111 mm cut from the front and back surfaces and a width of 100 mm was used.
A plate-shaped test piece with a length of 100 mm was taken over the entire width, and also at several locations along the length. These test pieces were polished with 600 mensier emery and then degreased with acetone. The test solution used in the HTC test was called NACEN, and it was 0.5% acetic acid (C113CO□I+)-
During the test, H2S was bubbled through the test using a 5% aqueous solution of common salt (Na(Q)) to achieve a saturated state.The test was conducted at a temperature of 25° C. for 100 hours.

Figure 8 shows the end face of the specimen after the HIC test, where I (IC) is the length of the crack in the width direction ai
The ratio of the total sum of alj to the cross-sectional width was defined as the crack length ratio (%) (C, L, R,). and,
The HIG resistance accuracy rate is expressed by the following formula.

Number of specimens

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing typical water cooling patterns in the conventional method; Figure 3 is a graph showing the relationship between water cooling stop temperature and YR (%) and CLR (%) in the conventional method; Figure 4 is a diagram showing the water cooling pattern of the method of the present invention; Figure 5 is a diagram showing the second stage water cooling stop temperature and Y in the method of the present invention.
A graph showing the relationship between R (%) and CLR (%); and Figures 7 and 8 show the relationship between the II I C test and the
FIG. 2 is a schematic explanatory diagram showing a 1C property evaluation procedure.

Claims (4)

[Claims] (1) 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: 0.008-0.1
5%, sol. Al: 0.01-0.10%, Cu: 0.05-0.50%, Ca: 0.0005-0
．． 0050%, and REM: 0.0005-0.01
%, the balance is unavoidable impurities, and iron is heated to form an Ar_3+150
At least 50% in the temperature range below ℃ and above Ar_3 points
The above hot rolling is carried out, and the hot rolling is finished at the Ar_3 point or more, and the temperature range is 650°C from the temperature range of Ar_3-30°C or more.
Below, the cooling rate is 15-30℃ up to a temperature range of 550℃ or higher.
Accelerated water cooling in the range of /S, followed by 400℃ below 550℃
Excellent hydrogen-induced cracking resistance and low yield, characterized by accelerated water cooling at a cooling rate of 3 to 15°C/S, which is lower than the above cooling rate, to a temperature range of 3°C or higher, and cooling after stopping the water cooling. A method for manufacturing a steel plate characterized by the ratio. (2) 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: 0.008-0.1
5%, sol. Al: 0.01-0.10%, Cu: 0.05-0.50%, Ca: 0.0005-0
．． 0050%, and REM: 0.0005-0.01
%, the balance is unavoidable impurities, and iron is heated to form an Ar_3+150
At least 50% in the temperature range below ℃ and above Ar_3 points
The above hot rolling is carried out, and the hot rolling is finished at the Ar_3 point or more, and the temperature range is 650°C from the temperature range of Ar_3-30°C or more.
Below, the cooling rate is 15-30℃ up to a temperature range of 550℃ or higher.
Accelerated water cooling in the range of /S, followed by 400℃ below 550℃
A steel plate obtained by accelerated water cooling at a cooling rate of 3 to 15 °C/S lower than the above cooling rate to a temperature range of 500 °C or higher and below Ac_1 point after stopping the water cooling. A method for manufacturing a steel plate that is characterized by heating and tempering, and is characterized by excellent hydrogen-induced cracking resistance and a low yield ratio. (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: 0.008-0.1
5%, sol. Al: 0.01-0.10%, Cu: 0.05-0.50%, Ca: 0.0005-0
．． 0050%, and REM: 0.0005-0.01
%, and Ni: 0.05-0.50%, Cr: 0.05-0.5
0%, Mo: 0.05~0.50%, Nb: 0.01~
Ar_3+150
At least 50% in the temperature range below ℃ and above Ar_3 points
The above hot rolling is carried out, and the hot rolling is finished at the Ar_3 point or more, and the temperature range is 650°C from the temperature range of Ar_3-30°C or more.
Below, the cooling rate is 15-30℃ up to a temperature range of 550℃ or higher.
Accelerated water cooling in the range of /S, followed by 400℃ below 550℃
Excellent hydrogen-induced cracking resistance and low yield, characterized by accelerated water cooling at a cooling rate of 3 to 15°C/S, which is lower than the above cooling rate, to a temperature range of 3°C or higher, and cooling after stopping the water cooling. A method for manufacturing a steel plate characterized by the ratio. (4) 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: 0.008-0.1
5%, sol. Al: 0.01-0.10%, Cu: 0.05-0.50%, Ca: 0.0005-0
．． 0050%, and REM: 0.0005-0.01
%, and Ni: 0.05-0.50%, Cr: 0.05-0.5
0%, Mo: 0.05~0.50%, Nb: 0.01~
Ar_3+150
At least 50% in the temperature range below ℃ and above Ar_3 points
The above hot rolling is carried out, and the hot rolling is finished at the Ar_3 point or more, and the temperature range is 650°C from the temperature range of Ar_3-30°C or more.
Below, the cooling rate is 15-30℃ up to a temperature range of 550℃ or higher.
Accelerated water cooling in the range of /S, followed by 400℃ below 550℃
A steel plate obtained by accelerated water cooling at a cooling rate of 3 to 15 °C/S lower than the above cooling rate to a temperature range of 500 °C or higher and below Ac_1 point after stopping the water cooling. A method for manufacturing a steel plate that is characterized by heating and tempering, and is characterized by excellent hydrogen-induced cracking resistance and a low yield ratio.