JPH03162521A - Manufacture of high tension steel sheet having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To form fine TiO even by the conventional continuous casting method and to obtain a high tension steel sheet having superior toughness of the weld heat-affected zone by specifying the amts. of Al, Ti, O and N in a steel having a specified compsn. and conditions in rolling and cooling. CONSTITUTION:A steel consisting of, by weight, 0.04-0.12% C, <=0.5% Si, 1.2-2.0% Mn, <=0.03% P, <=0.005% S, 0.04-0.09% Ti, <=0.004% Al, 0.002-0.005% N, 0.0015-0.0050% O and the balance Fe and satisfying the inequality is prepd., heated to 1,100-1,250 deg.C, rolled at 650-850 deg.C rolling finish temp. and >=40% total draft in the temp. range of <=950 deg.C and air-cooled or acceleratedly cooled to obtain a steel sheet. In the case of a small thickness of the sheet and relatively low welding heat input, the toughness of the weld heat-affected zone (HAZ) is especially considerably improved and a line pipe having superior toughness at low temp., especially toughness of HAZ and superior weldability can be manufactured.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is a high tensile strength steel plate for line vibes (tensile strength, TS of 55 kgf/mJ or more, with excellent low temperature toughness, especially weld heat affected zone (HAZ) toughness, The present invention relates to a method for producing a film having a thickness of 40 mm or less.

In the steel industry, it is most preferably applied to plate mills, but it can also be applied to hot coils. Moreover, steel sheets manufactured by this method can be used for large-diameter line pipes and other industrial machinery used in cold regions.

(Prior art) Line pipes used in cold regions are required to have high strength, excellent low-temperature toughness, and on-site weldability. In order to satisfy these characteristics at the same time, line pipe steel plates are made of N
B-added steel was generally manufactured by controlled rolling or controlled rolling-accelerated cooling method.

However, recently, based on Nb addition, Nb-1t4, Nb
- Steels such as Mo steel, Nb-B steel, and TiC steel (TiCl: precipitation hardened steel) have also been developed. Among these, TiC steel is a new steel that can replace Nb steel, and is characterized by high strength, excellent weldability, and low cost, but there have been some concerns about HAZ toughness.

Therefore, the present inventors conducted research on improving the HAZ toughness of TiC steel, and by increasing the solidification cooling rate during casting of steel with small amounts of JlTl and N, fine TI nitrides (T i N) were dispersed in the steel. He invented a method to refine the HAZ structure and improve its toughness (Japanese Patent Publication No. 58-009J115). However, in this invention, the minimum solidification cooling rate required to finely disperse TiN is high, and it is difficult to realize this using the current continuous casting method.

(Problems to be Solved by the Invention) The present invention provides HA without impairing the excellent properties of TiC steel.
This provides a method for improving Z toughness, and provides a manufacturing technology for inexpensive TI-added high-strength steel with excellent low-temperature toughness and weldability. The TIC steel produced based on the method of the present invention can obtain excellent HAZ toughness even in a normal continuous casting method.

(Means for Solving the Problems) The gist of the present invention is that C: 0.04-0.12%, S
i: 0.5% or less, Mn: l. 2-2.0%, P:
0.03% or less, S: 0.005% or less, T1: 0
．． 04-0.09%, AII: 0.004% or less, N
: 0.002-0.005%, O: 0.0015
~0.0050% and further Nb: 0.0 as necessary
05-0.05%, N I:0.05-0.3%, C
u: 0.05-0.3%, Cr: 0.05-0.5
% or two, and 0.03%≦Tl%
-2(O%)-3.4(N%)≦0.07%,
Steel, the balance of which is iron and unavoidable impurities, is heated to 1100°C.
Heating to a temperature range of ~1250°C, cumulative reduction of 40% or more below 950°C, rolling end temperature 650°C to 850°C
This is a method for manufacturing high-strength steel sheets with excellent low-temperature toughness, which is characterized by performing rolling at a temperature, followed by air cooling or accelerated cooling.

The present invention will be explained in detail below.

In order to improve the HAZ toughness of TiC steel, it is necessary to refine its structure. However, miniaturization using only TiN particles has the above-mentioned manufacturing problems, and there is also a limit to the level of toughness that can be achieved.

Therefore, the present inventors invented a miniaturization method using TI oxide (Tie). Regarding refinement of HAZ toughness by T1 oxide, it has already been proposed in Japanese Patent Application Laid-Open No. 63-210235, etc., but the Ti content in these inventions is at most 0.03% or less, and TiC due to the addition of a large amount of TI.
It was thought that this would significantly deteriorate the shape or HAZ toughness.

However, according to the research conducted by the present inventors, fine TiO can be obtained and the HAZ toughness can be improved even in steel manufactured by a normal continuous casting process, if the composition and manufacturing method are limited to a specific range. It was revealed. We have also found that when the plate thickness is thin (less than 40 mm) and the welding heat is relatively small, such as in a line vibrator, a particularly large effect of improving HAZ toughness can be obtained.

In order to obtain the above-mentioned effects of increasing strength and improving HAZ toughness by adding TI, it is necessary to obtain an appropriate amount of TiC (precipitation hardening) and sufficient amounts of TiO and TiN. To do this, first, the Ap amount is set to 0.004% or less, and TI,0,
It is essential to limit Nffi and T l:o. 0
4-0.09%, N: 0.002-0.005%, O
: 0.0015 to 0.0050%, and the balance of these is 0603%≦T1%-2 (0%) -
It was limited to 3.4 (N%)≦0.07%.

Ag is an element normally included in steel as a deoxidizing agent, but in the present invention it is an unfavorable element, and the upper limit is set at 0.004%.
And so. The reason for this is that AI has a stronger binding force with O than TI, which prevents the formation of fine TiO (AJ
It is desirable to reduce ffi as much as possible).

TI is an essential element for making the HAZ structure finer by forming fine Tie and TiN and increasing the strength by fine TiC. The lower limit of the amount of Ti is the minimum amount to sufficiently obtain these precipitates. However, if there are too many Tlffi
Since precipitation of iC causes hardening of HAZ and deterioration of toughness, the upper limit was set at 0.08%.

Next is N, Offi, whose lower limit is Tie, Ti
This is the minimum required amount to generate N. Upper limit of N is 0.
The upper limit of 0.005% is to prevent deterioration of HAZ toughness due to solid solution N, and the upper limit of 0.0050% is to prevent deterioration of the cleanliness and toughness of steel due to the formation of nonmetallic inclusions.

Excellent properties cannot be obtained simply by limiting the amount of each element, so the balance should be further adjusted to 0.03%≦T1%.
-2(○%)-3.4(N%)≦0.07%. When the amounts of Ti, N, and O are within this range, the properties are dramatically improved. The above equation expresses the stoichiometric excess or deficiency of TI when considering that TiO (Ti203) and TiN are generated. The lower limit is Ti due to the lack of T1ffi.
This is to prevent insufficient production of N and TiO, and the upper limit is
This is to prevent excessive precipitation hardening due to C.

However, even if a sufficient amount of fine TiN, TiO, or TiC is obtained, an excellent balance of properties cannot be achieved unless the basic fraction is appropriate. This point will be explained below.

The lower limit of 0.04% of C is the minimum amount in order to ensure the strength of the base metal and the welded part and to exhibit the toughening effect of TI and Nb (precipitation hardening due to carbides, etc.). However, if the amount of C is too large, weldability will deteriorate significantly, so the upper limit was set at 0.12%.

Since adding a large amount of S1 deteriorates weldability and HAZ toughness, the upper limit was set to 0.5%. TI alone is sufficient for deoxidizing steel, and it is not always necessary to add S1.

Mn is an essential element for ensuring strength and toughness.
Its lower limit is 12%. However, too much MnE1 not only increases hardenability and deteriorates weldability and HAZ toughness, but also promotes center deflection of continuously cast slabs, so the upper limit was set at 2.0%.

In the steel of the present invention, the impurities P and S are each 0.0
The reason for setting it to 3% and 0.005% or less is to further improve the low-temperature toughness of the base metal and welded part. Reduction of P prevents intergranular fracture, and reduction of 3B prevents deterioration of toughness due to MnS. The preferable amounts of P and S are 0.01 and 0.01, respectively.
It is 0.003% or less.

Next, the reason for adding Nb, Ni, Cu, and Cr will be explained. The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is self-limited.

Nb is an element that has excellent effects such as improving low-temperature toughness and hardenability by refining the microstructure, and increasing strength by precipitation hardening. However, if the amount added is too large, it causes deterioration of weldability and HAZ toughness, so the upper limit was set at 0.05%.

N1 improves both strength and toughness without adversely affecting weldability and HAZ toughness, and is also effective in preventing hot cracking when Cu is added. However, if it exceeds 0.3%, it is unfavorable from an economic point of view, so the upper limit was set at 0.3%.

Cu is also effective in corrosion resistance and hydrogen-induced cracking resistance, but
．． If it exceeds 3%, hot rolling, especially Cu-cracks will occur,
Manufacturing becomes difficult. For this reason, the upper limit was set at 0.3%.

Cr is an element that increases the strength of the base metal and the welded part, but if it is present too much, it significantly deteriorates weldability and HAZ toughness. Therefore, the lower limit of the amount of Cr added is 0.05%, and the upper limit is 0.5%.

In order to sufficiently obtain the toughening effect of Tl addition as described above, the manufacturing method must be appropriate, and the reheating, rolling, and cooling conditions of the steel (slab) must be limited. First, the reheating temperature is limited to a range of 250°C to 1250°C.

The reheating temperature is TI. The temperature must be 1,100° C. or higher (preferably 1,150° C. or higher) in order to dissolve Nb precipitates and ensure the rolling end temperature. Below this temperature, TI is not sufficiently dissolved in solid solution and the desired strength cannot be obtained. However, if the reheating temperature is 1250° C. or higher, the austenite grains (γ grains) become extremely coarse and cannot be completely refined even by rolling, making it impossible to obtain excellent low-temperature toughness.

Therefore, the reheating temperature is set to 1250° C. or lower.

Further, the cumulative reduction amount below 950°C must be 40% or more, and the rolling end temperature must be 650 to 850°C.

This is because the γ grains refined by recrystallization zone rolling are stretched by low-temperature rolling to thoroughly refine the ferrite grain size and improve low-temperature toughness.

If the cumulative reduction amount is less than 40%, the γ structure is insufficiently stretched, and fine ferrite grains cannot be obtained. Further, if the rolling end temperature is 850° C. or higher, fine ferrite grains cannot be achieved even if the cumulative reduction amount is 40% or higher. However, if the rolling end temperature is too low, excessive rolling occurs in the (γ-α) two-phase region, leading to deterioration of low-temperature toughness, so the lower limit of the rolling end temperature was set at 850°C.

The cooling conditions after rolling are preferably air cooling or accelerated cooling.

The conditions for accelerated cooling are a cooling rate of 10 immediately after rolling.
It is desirable to cool down to an arbitrary temperature of 600°C or less at a rate of ~40°C/see, and then air-cool.

Note that even if this steel is reheated after manufacturing at a temperature below the A C point for the purpose of sufficient tempering, dehydrogenation, etc., the features of the present invention will not be impaired.

(Example) Steel plate (thickness lO~32m+
A) was manufactured and its strength, toughness, and HAZ toughness were investigated.

Examples are shown in Table 1.

All steel plates manufactured according to the method of the present invention (inventive steel) have good properties. On the other hand, comparative steels not according to the present invention are inferior in strength and HAZ toughness.

FIG. 1 shows the notch position of the Charby specimen.

Among comparative steels l2 to l5, steel 12 has a high AD amount and T
Because I oxide is not sufficiently produced and because steel I3 has too much oxygen, its HAZ toughness is poor.

Steel l4 has low Mnffi, so the base metal strength and HAZ
Both have poor toughness. #ll5 has a high f (TI) and therefore has poor HAZ toughness. Furthermore, comparative steel 16. In No. 17, steel 16 has a low f (Ti), and steel 17 has a low reheating temperature, so the strength of the base material is insufficient. Comparative steel
No. 8 has poor toughness of the base material because the rolling end temperature is too low.

(Effects of the Invention) The present invention has made it possible to manufacture a high-strength line pipe with excellent low-temperature toughness and weldability. As a result, on-site welding efficiency and pipeline safety have significantly improved.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the notch position of a Charpy test piece. Agent Patent Attorney Tatsuo Chanokiz77)77
1

Claims (1)

[Claims] 1. In weight ratio: C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.2 to 2.0%, P: 0.03% or less, S: 0.005% or less, Ti: 0.04-0.09%, Al: 0.004% or less, N: 0.002-0.005%, O: 0.0015-0.0050%, 0 .03%≦Ti%-2(O%)-3.4(N%)≦0
．． 0.7% steel with the balance consisting of iron and unavoidable impurities 1
Heating to a temperature range of 100℃~1250℃, 950℃
Cumulative rolling reduction of 40% or more, rolling end temperature 650℃~
A method for producing a high-strength steel sheet with excellent low-temperature toughness, which comprises rolling at 850°C and then air-cooling or accelerated cooling. 2. In weight ratio, Nb: 0.005-0.05%, Ni: 0.05-0.3%, Cu: 0.05-0.3%, Cr: 0.05-0.5%. 2. The method for manufacturing a high-strength steel sheet with excellent low-temperature toughness according to claim 1, wherein the steel contains one or two types of the above-mentioned.