JPS5983722A - Preparation of low carbon equivalent unnormalized high tensile steel plate - Google Patents

Abstract

PURPOSE:To prepare a low carbon equivalent unnormalized high tensile steel plate excellent in low temp. toughness and weldability, by applying rolling, heat treatment and accelerated cooling to slab having a specific composition having reduced contents of C and N and contg. B and Ti therein under a limited condition. CONSTITUTION:A steel slab containing 0.01-0.12% C, 0.05-0.50% Si, 1.00- 2.50% Mn, 0.005-0.10% Nb, 0.0003-0.0030% B. 0.0080% or less N, 0.005-0.040% Ti, 0.005-0.080% Al and 0.008% or less S, further containing one or more of Ni, Mo, Cu according to necessity, or further containing one or more of V, Ca, REM and comprising the remainder Fe and inevitable impurities is heated to a temp. for forming a solid solution containing 0.01% or more of Nb and the heated slab is rolled respectively in a temp. range above (an Ar3-point + 150 deg.C), from (the Ar3-point + 150 deg.C) to the Ar3-point, and from the Ar3-point to (the Ar3-point - 80 deg.C) under cumulative drafts of >=50%, >=50%, and >=10% for each temp. range. Directly after rolling, the rolled slab is forcibly cooled to <=500 deg.F at a cooling speed of 2-30 deg.C/sec to obtain an unnormalized high tensile steel plate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a low carbon equivalent non-tempered high tensile strength steel plate, and in particular, the present invention relates to a method for producing a low carbon equivalent non-tempered high tensile strength steel plate. ~gO
The present invention relates to a method for manufacturing a kg/w class non-tempered high-strength steel plate for low temperature use. That is, the present invention is directed to steel plates for large-diameter line pipes of 60 to 70 kg/m, or 50 to 70 kg/m, mainly used for natural gas transportation in cold regions.
The present invention relates to a method for producing a steel plate that can be substituted for a RI' class tempered steel plate (hereinafter referred to as QT rust steel plate).

As the demand for energy has increased in recent years, there has been a desire to transport large amounts of natural gas, and the operating pressure of line pipes has been increasing from the conventional 75 atm to 700 atm to 7.20 atm. Along with this, the materials used are required to have higher tensile strength and thicker walls, and so
++ If it is grade 2, it must be thicker than 15un, and /g
If it is thick, high strength of qo-go kg/vr- class is required. At the same time, these Ll) line pipes are desired to have improved weldability from the standpoint of efficiency of circumferential welding on site, and are beginning to be required to have low carbon equivalents. For example, the strength is 6 o kg/2nd class of 0.30 or less,
In addition, there is a demand for a steel plate for line pipes that has a strength of 70 kg/v=n2 class, has a very low carbon equivalent of 0.35 coefficient or less, and has excellent low-temperature toughness. Q and T steel sheets containing 1% or more of N1 are known as conventional steel sheets that meet these requirements, but they have the drawback of being very economically disadvantageous in terms of component costs and productivity. Further, a steel sheet made of a fine ferrite + pearlite structure obtained by air cooling after conventional controlled rolling cannot fully satisfy the above requirements no matter how strong the precipitation strengthening is.

An object of the present invention is to provide a method for manufacturing a low carbon equivalent non-tempered high tensile strength m-plate that eliminates the drawbacks seen in the conventional manufacturing method, and by providing the method described in the claims, The above objective can be achieved.

Next, the present invention will be explained in detail.

As a result of repeated research by the present inventors on a manufacturing method that can achieve high tensile strength and high toughness with a low carbon equivalent,
The ferrite transformation of fine-grained austenite, which is obtained by processing extremely fine ferrite (3 μm at most), and the shear transformation (bainite transformation and martensitic transformation) instead of the pearlite transformation ), we came up with the idea that "low carbon fine bainite and island martensite" obtained by causing microfabricated ferrite and low carbon fine bainite and martensite can simultaneously increase strength and improve toughness. It has been newly discovered that island-shaped martensite can be simultaneously obtained by the production method of the present invention.

In other words, by rolling in a high-humidity austenite region using a Nb-containing steel to which the B element with high hardenability is added without increasing the carbon equivalent, some unrecrystallized austenite grains undergo ferrite transformation, and then Rolling is performed at an Ar point and within a temperature range of 3 to 0.degree. C. to produce "microprocessed ferrite" and at the same time cause the remaining 9 unrecrystallized austenite grains to undergo ferrite transformation, followed by bainitic transformation, and then martensitic transformation. , ゛Minute Be・
By generating r-night and island-like martensite, a rolled structure consisting of ferrite as a main component, fine bainite, island-like martensite, and micro-processed ferrite is obtained, and as a result, high strength can be obtained using component steel with a low carbon equivalent. and high toughness can be achieved at the same time.

The size of ferrite here is an average grain size of /θ μm or less, and the size of fine bainite is 7 μm or less.

The size of microfabricated ferrite is 3 μm or less, and the size of island martensite is 3 μm or less, and the ratio of these amounts is such that ferrite is approximately SO% or more, and the sum of microfabricated ferrite, fine bainite, and island martensite is approximately SO% or more. Approximately kOt4
The following is common, but the strength of the steel plate is 1.0, '1
0, gOkli'/y+J It is necessary to increase the amount ratio of the co-phase in order to reduce the ferrite ratio without increasing the ferrite ratio.

Note that the Nl) steel to which the above B element is added can obtain a fine bainite and island martensite structure even if it is left air-cooled after rolling, but the volume fraction of the above structure is low, and the steel is 40 kgf.
/my'' or higher strength is difficult to obtain.For this reason, V, N1.
It is necessary to add expensive components such as MO for the purpose of supplementing strength, but if large amounts of these elements are added, weldability tends to deteriorate.

In contrast to the above-mentioned conventional method, according to the present invention, by heating and cooling after completion of rolling, the volume fraction of fine bainite island martensite can be increased and high strength can be obtained. That is, according to the invention, Nl),! : Controlled rolling is performed using steel containing special components such as B, and rolling is performed in a limited (α + γ) phase region.
The volume fraction of fine bainite and island-like martensite can be increased by generating "finely modified efrite" and performing accelerated cooling immediately after rolling. An increase in these three second phase structures impairs toughness. Since it enables strengthening, high toughness and high tensile strength steel with low carbon equivalent can be produced.

One of the characteristics of the steel used in the present invention is that it contains 0.07 to 0.7.2% C. The purpose of adding a small amount of B is to promote bainite formation and island-like martensite formation, and to reduce the C component and other additive alloy components.

In the method of the present invention, the slabs used as starting materials are characterized by reduced O, N contents and B.

Effective use of T1. The primary purpose of reducing the C content is to improve the toughness of the bainite grains in the ferrite-bainite structure and improve the toughness of the base metal.The purpose of reducing the C content is to improve the toughness of the base metal by improving the toughness of the bainite grains in the ferrite-bainite structure. At the same time, the aim is to prevent Furthermore, in order to obtain a steel with high strength and toughness, it is important that B be solidly dissolved in the recrystallized austenite grains. whether to reduce the content (I
I) It is important to add Tt to fix N in the TiN precipitate.

Next, the reason for limiting the conditions for slab rolling and heat treatment among the constituent elements of the present invention will be explained.

As mentioned above, the purpose of the present invention is to manufacture steel having micromachined ferrite and microbainite structures, and in order to generate these micrograins, it is necessary to contain Nb, and Nl) is 0.0. The steel slab must first be heated to form a solid solution of N
When b is not in solid solution, the upper limit of the unrecrystallized austenite region is Ar3+! When 0.0/% or more of Nb is dissolved at rO°C, the unrecrystallized austenite region becomes Ar3+
/! ; Since the temperature rises to 0°C, in order to enable rolling of 50 mm or more in this unrecrystallized austenite region, it is necessary to expand the unrecrystallized austenite region, and furthermore, the solid solution Nb is It increases hardenability and makes it easier to generate bainite and island martensite.
The heating temperature of the slab needs to be such that it can be rolled at a temperature exceeding Ar3+ 130° C. so that the cumulative reduction ratio is at least go %.

The cumulative reduction rate of SOS in the high-temperature recrystallized austenite region exceeding Ar3 + 130° C. is the lower limit of the reduction rate necessary to make the grains finer than about 3 μ'm.

The reason why rolling is carried out so that the cumulative rolling reduction is at least sO% within the temperature range of the unrecrystallized austenite region of Ar3+ 130°C or higher and Ar3 point or higher is because ``unrecrystallized austenite grains are stretched into a pancake shape. The purpose of this is to introduce many deformation bands into the grains and generate ferrite grains.For this purpose, a cumulative reduction of at least 50% is required, so the lower limit of the reduction rate in this temperature range is 5. -(7%
It is necessary to

(γ+α) from Ar to Ar3”-g 0℃, 2
The reason for rolling in the phase region is to generate micro-processed ferrite grains from ungrown fine-grained ferrite produced by transformation from remaining unrecrystallized austenite grains, and to effectively accumulate strain in unrecrystallized austenite. This is because A is effective in refining ferrite grains and bainite grains.
Rolling in a temperature range lower than r310°C involves processing large ferrite grains and lowers the transition temperature (hereinafter referred to as vTrs), so rolling in the co-phase region is within the range of Ar3 to Ar -tO°C. It is necessary to Furthermore, if the rolling reduction in the co-phase region is less than 70%, there is no effect of increasing the tensile strength (hereinafter referred to as TS), so the rolling reduction in the co-phase region should be tO% or more. There is a need.

Next, the reason for limiting the post-rolling accelerated cooling conditions of the present invention will be explained.

The purpose of the present invention is to mix fine bainite and island-like martensite into the ferrite structure.9 When the cooling rate is faster than 30°C/see, the original austenite grain boundaries are observed to form massive martensite. and bainite are generated, resulting in a so-called quenched structure, which requires tempering treatment, and the method for manufacturing non-tempered steel, which is the objective of the present invention, cannot be achieved.
On the other hand, if the cooling rate is slower than 2'C/θθC, the effect of accelerated cooling will not be seen, so the cooling rate should be 1.2~30℃78e
It must be within the range of C.

The temperature at which the accelerated cooling after rolling is stopped is 3000C or higher, and the volume fraction of bainite and martensite decreases, resulting in a small increase in strength.
If accelerated cooling is stopped below OO°C, high tensile strength can be obtained without deterioration of toughness, so the cooling stop temperature must be below SOO°C.

Next, the reason for limiting the component composition of the present invention will be explained.

If C is less than 0.07%, the strength of the steel plate will decrease and the weld heat affected zone (hereinafter referred to as HAZ) will be greatly softened, and the manufacturing cost will increase significantly. On the other hand, if C exceeds 0.12%, As the toughness of the material deteriorates, the welded part hardens,
Since the deterioration of cracking resistance is significant, C is 0.07 to 0. /
, 2%. 3.Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.005%, the toughness of the base metal will deteriorate, while the θ, 50 coefficient will decrease. If Si exceeds 0.0F, the promotion of bainite formation will be low.
It needs to be within the range of 0S to o,so%.

If Mn is less than 7.00%, the strength and toughness of the steel and plate will be low -Fl~, and the softening of the HAZ will be large, while λ,
If it exceeds 5θ, the toughness of HAZ deteriorates, so Mn needs to be within the range of 7.00 to 2.5θ%.

Deoxidation of H1 Steel - It is necessary to add Al with a minimum of 0°OOS so that it becomes a solid solution.On the other hand, if the solid solution Al exceeds o, og%, not only the toughness of the HAZ but also the toughness of the weld metal is also degraded, so AltOta/, is o,
It is necessary to keep it within the range of oos to 0.0g0%.

In the production method of the present invention, S must be less than o, og%.
Since the vTrs in the direction does not go below 110°C and the absorbed energy also becomes extremely low, S, among other impurities, must be kept at below 0.0θg%.

If the Nb content is less than 0.00jt4, a fine grain effect that improves the transition temperature cannot be obtained; /θ ratio, the weld metal toughness of the welded part will deteriorate, so Nb is o, o
It is necessary to keep it within the range of os to 0.70%.

If B is less than 3%, it is not effective in promoting bainite formation, and if it exceeds 0.0030%, hardening of the HAZ will be large, so B must be within the range of 0.0003 to 0.0030. be.

T1 improves toughness due to the fine effect of γ grains and improves toughness as mentioned above.
It is added to reduce the amount of solid solute N in unrecrystallized austenite grains due to the formation of Ti carbonitrides, thereby preventing the formation of B nitrides, but it has no effect below the o, oos ratio. If it exceeds 0.0g, the toughness deteriorates, so Ti needs to be within the range of O1θO~o, ollo%.

If N is o, oogo 56 or more, the amount of Al and Ti within the range of the present invention is insufficient to fix it as T1 nitride and Al nitride, and as a result, B will not form B nitride. Since N deteriorates the hardenability of
oogo Must be 96 or lower.

The above are the basic components of the steel slab used in the present invention.
, C! At least seven elements selected from a and RIM can be added, and by containing appropriate amounts of each element, specific effects are added as described below.

In other words, N1 improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ, but adding N1 in excess of 0096 will increase manufacturing costs and Since it is not necessary to achieve the purpose and effects of the invention, N1 should be less than /,00.

Ou not only has almost the same effect as N1, but also improves corrosion resistance, but if it exceeds o,so%, cracks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates, so aU is It is necessary to keep it below 0.03SO%.

MO makes the γ grains regular during rolling and also produces fine bainite, so it improves strength and toughness, but in order to achieve the purpose of this invention, it is not necessary to add more than o, s9A, Anything more than that will lead to an increase in manufacturing costs.
Mo should be o, 5 oqb or less.

■Improvement in base material strength and toughness of steel plates by this invention.

It is added to ensure the strength of the joint, but if too much is lost, the toughness of the base metal and HAZ will be significantly degraded, so it is necessary to keep the value of ■ below 10c4.

If Ca is less than 0.0011%, it is insufficient for controlling the morphology of MnS and has no effect on improving the toughness in the C direction.On the other hand, if it exceeds 0.070%, the cleanliness of the steel deteriorates and causes internal defects. must be within the range of O0θ02 to 0.0/θchi.

If REM is less than o, oo, or z, it is insufficient for controlling the morphology of MnS and is not effective in improving the toughness of the steel plate in the C direction.
It is also disadvantageous in terms of arc welding, so HEM is 0,
It is necessary to keep it within the range of θ0% to o,olo%.

Next, the present invention will be explained with reference to examples.

Example 1 Slabs of test steel types having the composition shown in Table 1/A to 3A,
/Bf: Rolling shown in the second coating.

The mechanical properties of steel sheets treated under different cooling conditions are shown in the same table.

Test Examples /16/ to /3 shown in Table 1 are slabs of /B steel having a composition that can be used in the present invention, and were manufactured under various rolling conditions. Second
According to the table, at /16/ and λ, the slab solid solution Nb is 0. O/
I63 is air-cooled, rot 6 has a cooling stop temperature of SOO°C or higher, and /16// has a reduction of 0 below the Ar3 point, that is, each of the configurations of the present invention. It can be seen that the strength of the manufactured steel plate does not reach '10 kgf/m- because the requirements are not completely met.

/I69 has a reduction amount of ro in the austenite recrystallization region.
Less than e4, /16/θ is Ar3 + 150℃
If the reduction amount in the unrecrystallized austenite region from to the A rB point is less than 5 degrees, and the finishing temperature is Ar
It can be seen that the 'vTrs of the manufactured steel sheets is -70°C or higher because the temperature is lower than 3-40°C, that is, the constituent requirements of the present invention are not completely satisfied.

Scrap 3. Da,? , g, 7.2 are rounded, manufactured within the scope of all the constituent requirements that can be used in the present invention, and T of 70 kg f /rsvt2 or more, respectively.
It can be seen that a very low carbon equivalent steel plate having vTrs of -70° C. or lower and superior high tensile strength and high toughness compared to the methods of the above-mentioned comparative examples can be obtained.

Example 2 Test steels having the chemical compositions shown in Table 3 were processed under the rolling conditions shown in Table 1, and the mechanical properties of /C steel plates are shown in the same table.

According to Table 1, /167 to B were manufactured using slabs of steel types that can be used in the present invention as starting materials and within the range of all the constituent requirements that can be used in the present invention, / I6/~3 is o, 3o
With a C equivalent of s or less, an intensity of θkg f/1n, 2 or more and a VTre of 0°C or less have been obtained, and AII, j
' is 0.3! ; Strength of qokgf/kari or more and vTrs of -70°C or less have been obtained with a C equivalent of less than 1.
It can be seen that 6A can obtain an intensity of gokgf/, - or more and a vTrs of -70°C or less with a C equivalent of o, qos or less.

As can be seen from the above examples, if the production method of the present invention is used, it is possible to easily produce low carbon equivalent non-tempered steel with high strength and toughness, and of stable quality, in large quantities and at low cost. can do.

125-

Claims (1)

[Claims] L 00.07-0.7! %, Bi0.03~o,
gochi. Mn/, 00~2. ! ;0%, NbO,0(
B; ~θ, IO'4°B O,0003~0. θ03
0chi, N O, 00g 0 section or less. T1 θ, 005-0. θg θ , A) 0.
oog ~ o, ogo chi. Contains s o, oog % or less, and if necessary N1/
, oo 嗟いTING, MOo, go 1 or less, au
Contains at least one of the following: o, so % or less, or 0.30% or more, and further contains p as necessary.
V O, 10% or less, Oa O, 002-0, θto
qA. I(EiMθ, θ(Nl) containing at least 7 of any selected from 7.2 to 0.070%, with the remainder consisting of unavoidable impurities and Fe) After heating to a temperature at which at least 0.07% solid solution is obtained, rolling is performed at a temperature exceeding 3 Ar points + 150°C so that the cumulative reduction ratio is at least 5<7%, and then rolling at a temperature exceeding 3 Ar points + 150°C, and then , and within the temperature range of the unrecrystallized austenite region with an Ar point of 3 or more, rolling is performed so that the cumulative rolling reduction rate is at least SO%, and then austenite and ferrite with an Ar of less than 3 points and an Ar of 3 points - go'C or more are rolled. Rolling is carried out within the temperature range of the phase region of , so that the cumulative reduction rate is at least 10 inches, and then immediately forced cooling is carried out at a cooling rate of 2 to 2 b/sec to a temperature below SOO ° C. A method for producing a low-carbon, heat-treated, non-heat treated high-strength steel sheet with excellent toughness and weldability, which is characterized by forming a structure mainly consisting of ferrite, fine bainite, island martensite, and microfabricated ferrite.