JPH10176239A - High strength and low yield ratio hot rolled steel sheet for pipe and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet low in a yield ratio and furthermore small in the amt. of YS to be reduced after tube making and to provide a method for producing the same. SOLUTION: This steel sheet has a compsn. contg., by weight, 0.02 to 0.12% C, 0.1 to 1.5% Si, <=2.0% Mn, <=0.05% P, <=0.01% S and 0.01 to 0.10% Al, furthermore contg. 0.1 to 1.5% Mo+Cr or moreover contd. prescribed amounts of one or more kinds among Cu, Ni, Ti, Nb, V, Ca and rare earth metals, and the balance Fe with inevitable impurities and has a structure essentially consisting of martensite and ferrite by 1 to 20 area %. As for its producing method, the steel having the above components is heated at 1000 to 1300 deg.C, after that, hot rolling is finished in the temp. range of 750 to 950 deg.C, and it is cooled to a coiling temp. at a cooling rate of 10 to 50 deg.C/s and is coiled in the temp. range of 480 to 600 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a high-strength, low-yield-ratio hot-rolled steel sheet suitable as a material steel sheet for line pipes and oil country tubular goods manufactured by OE pipe making and the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Pipes used for line pipes and oil well pipes are required to have low-temperature toughness and weldability. Therefore, a low C type is used as a component system of the material steel plate.
Many precipitation strengthening elements such as i and Nb are used. Although this precipitation strengthening is an effective means for increasing the strength of a steel sheet, it has a disadvantage that the yield ratio (YR) tends to increase with the increase in strength. For example, in a class of API standard X60 or more, the yield ratio often reaches 95 to 98%.

[0003] However, a steel sheet having a high yield ratio has poor pipe formability, and a steel sheet having a low yield ratio is desired from the viewpoint of burst resistance and the like. On the other hand, in the case of a steel sheet whose yield strength (YS) is simply lowered in order to lower the yield ratio, YS may be greatly reduced due to the Bauschinger effect when formed into a pipe, and may not satisfy the API standard.

[0004]

In the prior art, it is difficult to reduce the yield ratio, particularly when high strength is achieved by adding a precipitation strengthening element, and furthermore, the yield ratio and the YS reduction after pipe forming are reduced. Obtaining a small amount of steel plate was particularly difficult.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a steel sheet having a reduced yield ratio and a reduced YS reduction after pipe making, and a method for producing the same.

[0006]

In order to provide a steel sheet having a low yield ratio, it is effective to form an appropriate amount of martensite in ferrite. Martensite produced by more rapid cooling has an excessive amount and a large size, and the Bauschinger effect is too large, so that the amount of YS reduction after pipe formation is large, and the API standard may not be satisfied.

Further, when martensite is generated by rapid cooling from the austenite region, there are various problems in production. In other words, it is difficult to control the quenching speed with a thick plate such as a line pipe, etc.In addition, when winding a hot rolled steel sheet, it will be wound at a low temperature, and the coil shape etc. will be unstable This makes the winding process difficult.

On the other hand, the present inventors suppress the pearlite transformation and bainite transformation which usually occur immediately after the end of hot rolling, leave untransformed austenite even in the temperature region where the winding treatment is performed, and It has been found that by transforming the austenite into martensite by cooling, martensite having a relatively sparse dispersion in ferrite and a smaller size can be obtained. Based on such knowledge, a hot-rolled steel sheet having a low yield ratio and a suitable martensite structure with a small YS reduction amount after pipe forming was developed, and a production method which was easy to implement in a practical production process was completed. Things.

That is, the hot-rolled steel sheet for a high-strength low-yield-ratio pipe of the present invention has a C content of 0.02 to 0.12% by weight,
Si: 0.1 to 1.5%, Mn: 2.0% or less, P:
0.05% or less, S: 0.01% or less, Al :: 0.0
1 to 0.10%, further Mo + Cr: 0.1 to 1.5%
Or further contains Cu: 1.0% or less, Ni:
1.0% or less, Ti: 0.08% or less, Nb: 0.08
% Or less, V: 0.08% or less, Ca: 0.005% or less, REM: at least one of 0.005% or less, the balance being Fe and unavoidable impurities.
It has a structure mainly composed of up to 20% of martensite and ferrite. Preferably, the average size of martensite is not more than 10 μm.

[0010] In the production method of the present invention, the steel of the above-mentioned composition is heated to 1000 to 1300 ° C, and then heated to 750 to 950.
The hot rolling is completed in a temperature range of 10 ° C., and the cooling rate is 10 to 5
It is characterized by cooling to a winding temperature at 0 ° C / s and winding in a temperature range of 480 to 600 ° C.

First, the steel composition of the hot-rolled steel sheet of the present invention will be described.

C: 0.02 to 0.12% C is an important element for improving the strength.
It contributes to precipitation strengthening together with carbonitriding elements such as i, Nb and V, and also contributes to the formation of a martensite structure, which is the point of the present invention. To exert these effects, it is necessary to add at least 0.02%. However, if added excessively, the amount of martensite structure,
It affects the dispersion state and size, increases the degree of the Bauschinger effect, increases the amount of YS reduction after pipe formation, and further lowers low-temperature toughness and weldability. Therefore, the lower limit is 0.02%,
The upper limit is set to 0.12%.

Si: 0.1 to 1.5% Si facilitates the formation of a ferrite structure during cooling or winding immediately after hot rolling and promotes C enrichment in austenite. A martensite structure is likely to be generated during cold. If it is less than 0.1%, such an effect is too small. On the other hand, if it is added excessively, the surface defect increases and the toughness and weldability are adversely affected. Therefore, the lower limit is set to 0.1% and the upper limit is set to 1.5%. .

Mn: 2.0% or less Mn is effective as a solid solution strengthening element. However, when added excessively, sulfides such as MnS and segregation zones are generated, and remarkable deterioration of characteristics is caused. , And the upper limit is 2.0%.

P: 0.05% or less P is effective as a solid solution strengthening element like Mn, but when added excessively, a segregation zone is formed to deteriorate toughness or weldability. Therefore, the upper limit is set to 0.05%.

S: 0.01% or less S forms inclusions and causes deterioration of low-temperature toughness and hydrogen-induced cracking. For this reason, the smaller the better, the better, but the upper limit is made 0.01% in view of manufacturing costs.

Al: 0.01 to 0.10% Al is an element necessary as a deoxidizing material. For this reason, at least 0.01% or more is required. On the other hand, if the addition amount is excessive, casting defects become remarkable, so the upper limit is 0.10%.
And Preferably it is 0.02-0.08%.

Mo + Cr: 0.1 to 1.5% Mo and Cr can suppress the pearlite transformation and the bainite transformation during cooling or winding at a feasible cooling rate immediately after hot rolling. , And is effective for leaving an austenite phase. Such an effect can be achieved by Mo
The same effect can be obtained with r, and either one or a combination of these may be added. However, the total amount of Mo and Cr is 0.1
%, The effect is too small. On the other hand, if it is added excessively, the effect is saturated and the cost is increased.
% And the upper limit is 1.5%. 5 and 6 show Mo + C in low carbon steel (C: 0.07%, Mn: 1.2%).
The effect of the addition of r on various transformations was observed. It can be seen that the addition of Mo and Cr suppresses the pearlite transformation and the bainite transformation even during cooling or winding.

[0019] The steel according to the present invention contains the above components as essential components, and the balance consists of Fe and unavoidable impurities. By further containing one or more of the following components, it is possible to further improve the material properties. Can be.

Cu, Ni: 1.0% or less Cu is effective for improving strength by solid solution strengthening and also improves corrosion resistance. Ni contributes to prevention of surface defects during hot rolling caused by addition of Cu and improvement of corrosion resistance. Since these effects are saturated at about 1.0%, the upper limit is set to 1.0%. Note that Ni is preferably added together with Cu, and in this case, the addition amount is preferably set to be half to the same amount as the addition amount of Cu.

Ti, Nb, V: 0.08% or less Each of Ti, Nb, and V is an element effective for precipitating fine carbonitrides and improving strength, and is necessary for obtaining high strength. Will be added accordingly. However, if added excessively, the amount of solute C required to obtain martensite decreases, and not only a low yield ratio of the present invention cannot be sufficiently obtained, but also coarse precipitates are formed, which is a starting point of cracking. Become. For this reason,
The upper limit of each element is set to 0.08%.

Ca, REM: 0.005% or less Each of Ca and REM (rare earth element) is added for controlling the form of inclusions and improving weldability. The amount of addition is 0.005% or less, preferably 0.003% in consideration of economy.
The following is assumed.

Next, the structure of the hot-rolled steel sheet of the present invention will be described.

In order to obtain a low yield ratio, an area ratio of 1-2
It is important to have a two-phase structure mainly composed of 0% martensite and ferrite. 1% reduction effect of yield ratio
If it is less than 10%, it is too small.
If the amount of martensite exceeds 0%, YS after pipe forming
The amount of reduction increases. Therefore, the lower limit is 1% and the upper limit is 20%. However, since it is considered that the yield ratio of the normally required steel sheet material for line pipe is not particularly important up to 80% or less, the martensite amount of 10% or less is sufficient. The remainder of martensite is mainly composed of ferrite, where "mainly" means only ferrite or 15% to 1% in addition to ferrite.
It means that some pearlite or bainite of about 5% or less, preferably about 10% to 10% or less may be mixed.

In order to reduce the amount of YS reduction after pipe formation, it is effective that the amount of martensite is small, its size is small, and its dispersion is sparse.
It preferably has a martensite amount of 10% or less, and the average size of martensite is 10 μm or less, preferably 5 μm or less.
It should be less than μm. In addition, the average size of martensite means the average value of the measured values of the length and width of each martensite in a tissue section observed with an electron microscope.

Next, a method for producing the steel sheet of the present invention having the above structure will be described. Needless to say, the production conditions in the production method of the present invention are not the only conditions for obtaining the steel sheet of the present invention.

First, the heating temperature of the steel slab to be subjected to hot rolling will be described. The heating temperature is Mo, Cr, Ti,
Heating is performed to a temperature necessary for dissolving additional elements such as Nb and V. Normally, the temperature is required to be 1000 ° C. or higher. If the temperature is lower than 1000 ° C., each additive element remains undissolved, so that the effect is reduced. On the other hand, when the temperature exceeds 1300 ° C., the amount of oxide scale increases, which leads to a decrease in yield and deterioration of the fire resistance of the heating furnace. Therefore, the upper limit is set to 1300 ° C.

The finishing temperature of the hot rolling is 750 ° C. or higher, preferably 800 ° C. or higher. If the temperature is lower than this, the hot deformation resistance increases, so that the load on the rolling mill is large and sufficient rolling cannot be performed. Further, the upper limit of the finishing temperature is set to 950 ° C. or lower because if it exceeds 950 ° C., oxidized scale increases and causes surface defects.

During cooling immediately after hot rolling, it is necessary to generate ferrite to make the structure a ferrite + austenite structure. In the steel component of the present invention, the isothermal transformation diagram is as shown in FIG. 5. When the temperature is lower than 10 ° C./s, the pearlite transformation is mainly performed, and when the speed is higher than 50 ° C./s, the ferrite transformation does not occur. Therefore, the lower limit of the cooling rate is 1
0 ° C./s, preferably 15 ° C./s, and the upper limit is 50
° C / s.

In the subsequent winding treatment, it is necessary to promote ferrite grain growth and to increase the C concentration in austenite. Therefore, it is necessary that the temperature range be such that bainite transformation and pearlite transformation do not occur. In the steel composition of the present invention, bainite transformation occurs below 480 ° C.
Since the pearlite transformation occurs at a temperature higher than ℃, the winding temperature has a lower limit of 480 ° C, preferably more than 500 ° C, and an upper limit of 600 ° C.

In the cooling after the winding process, since the C concentration in austenite is sufficiently performed during the winding process, martensite can be obtained even by ordinary cooling, and the cooling rate is not particularly defined.

[0032]

EXAMPLE A steel having the composition shown in Table 1 was melted to form a slab, heated at 1200 ° C. for 30 minutes, finish rolled at about 850 ° C., finished to a thickness of about 10 mm, and then heated to 10-60 ° C.
After cooling in the range of / s, winding treatment was performed at 450 to 630 ° C for 30 minutes.

[0033]

[Table 1]

The hot rolled steel sheet was examined for tensile properties and structure. The structure was observed with an electron microscope, and the amount of martensite was measured based on the area ratio. As for the size of martensite, the length and width of each martensite captured in the visual field were measured, and the average was taken.

Further, in order to evaluate the YS reduction amount after forming the pipe, this hot-rolled steel sheet was bent by three-roll bending to a thickness / radius of curvature = 2% (equivalent to a thickness / tube diameter of 1%). And the tensile properties after straightening were examined.

The results of the investigation are shown in Tables 2 and 3. In the table,
ΔYS is (YS of steel sheet after bending processing−Y of hot-rolled steel sheet)
S), which is an amount corresponding to the amount of YS reduction after pipe molding due to the Bauschinger effect. The remaining structure other than martensite was 15% as shown in the table.
Although pearlite and bainite structures of less than about grade were observed, the others were ferrite.

[0037]

[Table 2]

[0038]

[Table 3]

Samples Nos. 1 to 14 in Table 2 were obtained by examining the effect of the cooling rate (CR) after hot rolling.
FIG. 1 summarizes the effects on R. Nos. 2-4, 7- using the steel type added with Cr + Mo corresponding to the invention examples
In No. 9, the YR is 82% or less. Also, from FIG.
From 15 to 50 ° C./s, the YR becomes 85% or less.

Sample Nos. 3, 8, 13 and 15-2
Reference numeral 3 indicates the effect of the winding temperature (CT). FIG. 2 shows the effect of the winding temperature on YR. As shown in the figure, when the winding temperature is 480 to 600 ° C. in the range of the present invention, Y
It can be seen that R is 85% or less.

Further, Sample Nos. 21 to 23 and 24 to 36
Indicates the effects of the components. Even if the cooling rate and winding temperature of the present invention are satisfied from Nos. 21 to 23, Cr,
In the case of not containing Mo (steel type No. 7), martensite was not generated, and the YR remained at a high value. Also,
Even if the amount of Cr + Mo is within the range of the present invention, in the case of Nos. 24 to 27 (steel types 1 and 4) where the amount of C is out of the range of the present invention, a predetermined amount of martensite and size cannot be obtained, and ΔYS is large. On the other hand, in Nos. 28 to 36 satisfying the components of the present invention and the production conditions, low values were obtained for both YR and ΔYS.

Regarding the effect of the amount of martensite on the YR, Nos. 1 to 5, 15 and 16
2), No. 6 to 10, 18 to 20 (Steel type No. 3),
No. 25 to 27 (the above steel type No. 4) are shown in FIG. In addition, regarding the influence of the average martensite size on the YS reduction (ΔYS), No. 2 to 4, 15, 1
6 (No. 2), Nos. 7 to 9, 18, 19 (No. 3) and No. 25 to 27 (No. 4) are shown in FIG. 3 and 4, it can be seen that both YR and ΔYS are low in the amount of martensite and average martensite size in the range of the present invention.

[0043]

According to the hot-rolled steel sheet of the present invention, C, Mo +
Contains a predetermined amount of Cr etc. and reduces the amount of martensite to 1 to 20%
Or further, the average martensite size is 10μ
m or less, a low yield ratio can be obtained, or further, a decrease in yield strength after pipe processing can be suppressed, and this is suitable as a high-strength steel sheet material for pipes. Further, the production method of the present invention can easily produce a hot-rolled steel sheet having an intended composition without quenching from an austenite temperature range, and is excellent in productivity.

[Brief description of the drawings]

FIG. 1 is a graph summarizing the effect of a cooling rate (CR) after hot rolling on a yield ratio (YR) in Examples.

FIG. 2 shows a yield ratio (Y) of a winding temperature (CT) in an embodiment.
It is a graph which arranged the influence on R).

FIG. 3 is a graph summarizing the effect of the amount of martensite (M) on the yield ratio (YR) in Examples.

FIG. 4 is a graph summarizing the influence of the average size of martensite (M) on YS reduction (ΔYS) in Examples.

FIG. 5 is a isothermal transformation diagram of the low carbon steel according to the present invention,
The drawing also shows the cooling pattern after hot rolling in the method of the present invention.

FIG. 6 is a isothermal transformation diagram of a low-carbon steel containing no Cr or Mo.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (4)

[Claims] C .: 0.02 to 0.12% by weight,
Si: 0.1 to 1.5%, Mn: 2.0% or less, P:
0.05% or less, S: 0.01% or less, Al :: 0.0
1 to 0.10%, further Mo + Cr: 0.1 to 1.5%
A hot-rolled steel sheet for high-strength low-yield-ratio pipes, comprising: a balance of Fe and unavoidable impurities, and having a structure mainly composed of martensite and ferrite in an area ratio of 1 to 20%. 2. The composition according to claim 1, further comprising:
And Cu: 1.0% or less, Ni: 1.0% or less, Ti:
0.08% or less, Nb: 0.08% or less, V: 0.08
%, Ca: 0.005% or less, REM: 0.005%
The hot-rolled steel sheet for a high-strength low-yield-ratio pipe according to claim 1, containing at least one of the following elements: 3. The average size of martensite is 10 μm.
The hot-rolled steel sheet for a high-strength low-yield-ratio pipe according to claim 1 or 2, which is: 4. After heating the steel having the composition described in claim 1 to 1000 to 1300 ° C.,
Hot rolling is completed in a temperature range of 0 ° C., and the cooling rate is 10
Cool at 50 ° C / s to winding temperature, 480-600 ° C
A method for producing a hot-rolled steel sheet for high-strength low-yield-ratio pulling, characterized by winding in a temperature range of: