JPS58120727A - Manufacture of high toughness nontempered high tensile steel sheet superior in weldability and causing less separation - Google Patents

Abstract

PURPOSE:To obtain the titled high tensile steel sheet by heating the steel containing specified amount of C, Si, Mn, Al to the specified temp. range, performing two stage rolling with the specified cumulative draft and cooling at the specified cooling rate. CONSTITUTION:The steel consists of, by weight %, 0.03-2 C, 0.03-0.6 Si, 0.5- 2 Mn, 0.005-0.06 Al if necessary, one or >=2 kinds among and the balance Fe with inevitable impurities (where <0.008 S in the inevitable impurities). This steel is heated to 900-1,150 deg.C, then rolled with the cumulative draft R satisfying the equation calculated for the heating temp. T when the heated temp. is above 900 deg.C until it is decreased to 900 deg.C; then rolling is continued with the cumulative draft above 50% during the temp. is reduced from 900 deg.C to Ar3 point and the rolling is finished at below Ar3 temp. This steel is then cooled at the rate of 2-25 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a non-temperature treated high tensile strength steel plate with less separation, excellent weldability, and high toughness.

The separation is observed on the fracture surface of the impact test specimen,
This refers to delamination that appears perpendicular to the fracture surface.

A so-called controlled rolling method is one of the conventionally practiced methods for producing high-strength, non-thermal-treated high-strength steel sheets with excellent weldability and high toughness. However, in the controlled positive arc, separation is increased because reduction is applied in the co-phase region of austenite and ferrite for the purpose of increasing toughness and increasing tensile strength. In order to avoid this, it becomes difficult to obtain the necessary strength if the rolling is finished in the austenite region.To overcome this problem, an increase in the alloying component of K is required, and the so-called carbon equivalent (Ceq) increases, making it difficult to weld. It is known that sex is impaired.

Note that the effect of separation on the safety of welded structures has not yet been clarified. However, there is a belief that separation is one of the causes of deterioration of toughness in the thickness direction, deterioration of stress corrosion cracking resistance, or unstable ductile fracture, so it is desirable to have no separation, and therefore, it is desirable to have no separation. There has been a demand for the establishment of a non-temperature manufacturing method for high-toughness, high-strength steel sheets with excellent weldability.

By the way, as mentioned above, if there is a method that can increase the tensile strength in controlled rolling without applying reduction gold in the co-phase region, it is thought that it is possible to manufacture steel sheets with less separation and excellent weldability. Conventionally, a method has been known in which rolling is finished in the r region and accelerated cooling is performed after piezoelectricity. However, in these accelerated cooling methods, the cooling stop temperature is S
Since the temperature is OO°C or higher, there is a drawback that the increase in strength is not so large.

Conventionally, it has been thought that if the accelerated cooling stop temperature is set to 500° C. or lower, martensite will be mixed into the ferrite ground, and although the strength will be increased, deterioration of toughness will be unavoidable.

However, the present inventors have previously discovered that even if the cooling stop temperature of accelerated cooling is set to 200°C or less in Yb-containing steel, if the introduced martensite is fine, high strength can be achieved without deterioration of toughness. After discovering the heating, forward arc and cooling conditions, a patent application was filed in Japanese Patent Application No. 1973/1/&'1.

However, according to the invention, it was necessary to incorporate Nb into the steel.

An object of the present invention is to provide a method for producing a high-toughness, non-heat-treated high-strength steel plate that eliminates and improves the drawbacks of the conventional method and has less separation and excellent weldability. The above objective can be achieved by providing the following.

Next, the present invention will be explained in detail.

As is well known, refinement is easily achieved in Wb-containing steel, but the present inventors have achieved a cooling stop temperature of 50% by optimizing heating and rolling conditions even without containing Nl.
The present invention was completed by demonstrating that it is possible to increase the strength without deteriorating toughness by incorporating fine martensite at temperatures below 0°C.

The progress of research into the present invention will be explained below along with the research results.

First, in research for this invention, the influence of the stopping temperature of accelerated cooling on strength and toughness was investigated.

Steel l shown in Table 1 is heated to 1ioo℃, /100℃~
After rolling at qoo°C with a cumulative reduction rate of 1,0*, q
Rolling is started again from oo℃ and finished at 110℃, the cumulative reduction rate during this period is 43%, and the cooling stop temperature when the subsequent cooling rate is io@c/s is the strength, toughness, and The effect on the total separation length was investigated. The results are shown in FIG. Note that the above strength and toughness are values in the direction perpendicular to rolling, that is, the C direction. Also, the Ar3 point of this steel is 100℃, and the above rolling is uneven.
Finished with 5 points or more.

Table 1: Composition of the sample steel According to the same figure, when the stop temperature of accelerated cooling is set to below SOO℃, the toughness increases by J = ak 70 days compared to the case where air cooling is performed after rolling, and the toughness decreases. It turns out that it does not deteriorate. Moreover, the strength is also increased by 3 to q kg compared to the case where the stop temperature of accelerated cooling is set to 200° C. or higher. Furthermore, the total separation length does not increase even if the cooling stop temperature is lowered and remains very small. This can also be seen by comparing the total length of separations that occur in a controlled rolled material of steel indicated by a black mark in the same figure, which is rolled by 30° in the co-phase region and finished at tlIo°C. That is, if accelerated cooling is performed under the above conditions, it is possible to increase the strength and toughness without increasing separation. Furthermore, by reducing the alloy components by the amount of 1+ increased strength, it is possible to lower Ceq.

Therefore, the first essential component of the present invention is its heating and rolling conditions.

According to this invention, the slab heating temperature is 900 ~ / /! r
Set to 0°C. If the slab heating temperature is Ac, the lower the better from the point of view of reducing the austenite grain size during heating, but from the point of view of reducing separation, the required cumulative rolling reduction rate should be In order to ensure this, a temperature of 900°C or higher is required. However, since current slab heating furnaces are not suitable for such low-temperature heating, it is often necessary to use even higher heating temperatures. In such cases, it is possible to refine the austenite grains before cooling, that is, at the end of the positive arc, by appropriately rolling at temperatures above 0°C, as described below, but heating above 1130°C This makes it impossible to reduce the grain size by over-rolling, where the austenite grain size during heating is large, and compensate for the drawbacks of the large grain size, so the upper limit was set at 1110°C.

Nine slabs heated under the above conditions have at least 100-100
Rolling is repeated in a temperature range of .degree. C. until the cumulative reduction ratio reaches 50 or more. Toughness after accelerated cooling is guaranteed by a reduction of 50 inches or more in this temperature range.

Figure 1 shows the steel shown in Table 7/ (Ars temperature 100℃)K
/ After heating to 100℃ / 60 at 100-900℃
After rolling at a cumulative reduction rate of -, the cumulative reduction rate is changed within the range of 00 to 110°C and rolling is finished at 110°C,
It is a figure which shows the change of toughness when cooling with woooCl at 10 degreeCh. From this figure, it is understood that a cumulative reduction rate of SOchi or higher is required. Strong pressure in a temperature range including this austenite non-recrystallization temperature range is one of the essential conditions for achieving the object of the present invention after subsequent cooling, and if this condition is not met, the structure after cooling will deteriorate. Coarse low-temperature transformed structures (bainite and martensite) are mixed in and the toughness is significantly impaired.

Note that the heating temperature is set to a high temperature within the scope of the claims of the present invention.
Specifically, 93O~//! 0° C.), coarse low-temperature transformation products will be mixed into the structure after cooling if the pressure is only reduced by more than θ as in the range of 90° C. to r3. In order to prevent this, the cumulative rolling reduction rate (-) R should be set within the temperature range of heating temperature T (℃) to 100℃.
h) It is necessary to roll it so that it becomes. Figure 3 shows the steel l in Table 1.
Heating using ff1 degree τ℃9! After varying the cumulative reduction rate in the temperature range from heating @ 1 m'I''C to 900 °C,
For steel that applied a cumulative reduction of 41% at 10°C, finished rolling at 110°C, and immediately accelerated cooling to 00°C at io°C/S, heated to 00°C → bst 4 reduction, finished at 0°C → 10 In comparison with the goo C cooled material, the case where the same toughness was obtained with °C oil is indicated by ○, and the case where toughness deteriorated is indicated by . From this figure, when heating is performed at 900°C or higher, the temperature range is between the heating temperature and qoo oci, and the cumulative rolling reduction ratio R>0. :1 da T-J/A (Sword.

It can be seen that it is necessary to perform rolling under the following conditions (T: heating temperature (C)).

Next, the main part of the constituent elements of this invention is cooling after the above-mentioned heating and pressure polishing. That is, accelerated cooling is started immediately following the above-mentioned rolling, but it is necessary to keep the cooling rate in the range of C to T and A. According to the present invention, as described above, it is necessary to mix fine martensite or bainite into the ferrite structure, and in order to create such a structure, the range of the cooling rate is limited. 2! If the speed is faster than r °C/s, a hardened structure will result, requiring tempering treatment, which is not a method for producing non-tempered steel.

On the other hand, if the cooling rate is slower than ℃/8, the effect of accelerated cooling will not be seen.For the above reasons, the range of the cooling rate was limited to around ℃ to ℃.

Furthermore, accelerated cooling at a cooling rate of -~x"Q/ls must be continued until below SOO℃. This is because, as mentioned above, the strength is even higher than the conventional method in which cooling is stopped at 500℃ or higher. As long as the above rolling conditions are adopted, high tensile strength can be achieved without deterioration of toughness.

Note that it is necessary to perform cooling immediately after the rolling is completed, and specifically, it is desirable to start cooling from the Ar point 3 or higher. However, even if it takes time to start cooling and the temperature of the steel plate falls below the Ars point, if it is between Ar5 and θ℃, it is true that the ferrite grains that precipitate after the Ar5 point is reached during air cooling will grow. Since the above can be ignored, the purpose of miniaturization can be achieved.

According to the present invention, it is necessary to limit the heating-rolling-cooling conditions as described above, but it is also necessary to limit the chemical composition of the steel, and the reason for this will be explained next.

If the C content is less than o, oy*, high strength cannot be obtained, and the weld heat affected zone (hereinafter abbreviated as HAZ) will be greatly softened, and if it is more than 0.20- The weldability is impaired, and the heating-rolling-cooling conditions of the present invention result in a quenched structure, which impairs the toughness and requires a tempering process, so it is necessary to set it to 0.03 to 0.20.

Sl promotes deoxidation of steel and increases strength, so it is added in an amount of at least O0θ3Is. However, if there is too much slack, toughness and weldability will be significantly impaired, so it should be kept at a maximum of 0.40.

Mn is O0! If the lower limit is less than 0, the strength and toughness of the steel plate will decrease, and the softening of the steel sheet will increase.
．． It was set as 3-. On the other hand, if too much Mn is present, the toughness of the HAZ will deteriorate, so the upper limit should be set at 2. It was set to O-.

For the deoxidation of steel, it is necessary to add IG to the minimum level of o, oos, and on the other hand, when the solid solution mulch exceeds o, obs, I
Not only the toughness of IIAZ but also the toughness of the weld metal deteriorates significantly. Therefore, soj Ju is o, oos〜o, o
bo Yutojiku.

8 is θ, 00g- If it is more than 00g, the impact absorption energy, especially in the C direction and the 2-direction (thickness direction), will decrease, which is not a disadvantage, but also from the standpoint of separation.
If more than s is present, inclusions will increase and occur frequently, making it impossible to achieve the object of the present invention.

Furthermore, in addition to the basic component system as described above, Ti may be added as necessary to achieve high tensile strength or other effects.
, Ni, Mo, (!u, V, Or, and Oa) can be added. Even if these elements are added, the features of the present invention are not lost. The addition of the above elements is effective because it can increase the tension appropriately and achieve the following effects.

Next, the purpose of adding the above components and the reason for determining the amount added will be explained.

T1 is added for the purpose of improving toughness by refining the r grains. However, if it exceeds o, o4I-, the toughness will deteriorate, so it should be less than o, oII-.

N1 is added because it improves the strength and toughness of the base metal without adversely affecting the hardenability and toughness of HAZ.
Since it is expensive, the upper limit was set at 7.0 inches.

Cu not only has almost the same effect as N1, but also improves corrosion resistance, but if it exceeds o, zots, it tends to cause hot embrittlement and the surface quality of the steel sheet deteriorates, so the upper limit is set at θ, jQ-.

MO makes the austenite grains fine and regular during rolling,
Furthermore, since it produces fine bainite and martensite, it improves strength and toughness, but it is expensive, so the upper limit was set at 0.50-.

■ is added to improve strength and toughness and to ensure weld joint strength, but if added in excess of 0.10-
Since it significantly deteriorates the toughness of AZ, the upper limit is set at 0 and IOIII.

Or produces fine bainite and martensite and improves strength and toughness, but addition of more than O and SO impairs weldability, so the upper limit was set to O and SO.

Ca and RIM are effective in controlling the morphology of MnS and improving the toughness in the C direction.Ap, i@, or a combination of both is added, but when Ca exceeds 0.0/* and RICM exceeds 0.104, respectively. Additions impair the cleanliness of the steel and cause internal defects, so the upper limit for each is set at 0. Ol- and 0,10-
And so.

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

Example Steel/-5 was melted to have the composition shown in Table 1.

Next, each of these test steels is made into slabs having the required thickness by ingot formation, blooming rolling, or continuous casting, and these slabs are heated, rolled, and heated in the manner shown in Table 2.
Processed under cooling conditions. The strength, toughness, and total length of the separation on the Charpy fracture surface at -9°C of the nine steel plates obtained were determined and were as shown in Table 1.

The final plate thickness was t5x and C2, and test pieces were taken in the direction perpendicular to the air pressure and subjected to a tensile test 211I V-notch impact test. The number /-j in each steel plate is the first
This means that the steel of fragrance/~S shown in the table was used, and the alphabetic characters in the suffix indicate the manufacturing conditions.

/JJ /F and /G are conventional steels, 11 is a steel with an accelerated cooling stop temperature of 300°C or higher, /G is (α+
γ) So-called strict control rolling is performed in which the amount of rolling in the co-phase region is increased.

Also, /1 is a book that has been subjected to so-called normal rolling.

/H, /E, and /J are comparative examples, and are manufactured by IJ Company.

The cumulative reduction amount in the temperature range of ~Ar5, /H is the heating temperature, and /X is the cumulative reduction amount from the heating temperature to too 'Ct, which are outside the scope of the present invention. , /B, /(1!.

/D, com, 3mu, dam, com are steel plates according to the present invention. In order to make the above relationship easier to understand, conditions outside the scope of the present invention are underlined in Table 2.

The steel l has Oeq = 0.30, and as seen in the conventional steel plate/R in Table 1, if it is normally rolled, τ
, 8 = Dakuyu 12. It only reaches vTra = - invasive °C. Here, using the steel/
The aim is to manufacture a steel plate that satisfies the separation length jW at ゜vTrs < -40°C and -J°C. In other words, it has an extremely low Oeq of 0.30% Ceq and excellent toughness! rOkQ f/sm2 steel is manufactured under conditions with very little separation.

7F using the conventional method has a stop temperature of 500°C or higher for accelerated cooling, so it has higher strength than /B! O' does not reach 9 f/, @''. The conventional method of /G involves strict control rolling and has sufficient strength, but separation occurs frequently. Steel plate/H that is out of the range of this invention in round beating temperature
Steel plates 1 and 1 and /J, whose rolling conditions are outside the scope of the present invention, both have insufficient toughness.

On the other hand, steel plates according to the present invention */mu, /B, /C.

/D all exhibit sufficient strength and toughness, and there is no increase in separation in particular. That is, as can be seen from the above examples, according to the method of the present invention, high tension with low Ceq and excellent weldability can be achieved.

High-toughness steel can be produced without separation and without heat refining.

The steel plates comb, 3mu, dam, and jA are examples in which the plate thickness is further increased, and all have low Oeq and less separation.

In addition, according to the present invention, one of the regular effects is that excellent productivity can be ensured without reducing rolling efficiency as in conventional controlled rolling.

Fig. 1 is a diagram showing the influence of the cooling stop temperature of accelerated cooling on the mechanical properties of the steel plate in an example of the steel of the present invention, and Fig. 2 is a diagram showing the cumulative reduction between 0°C and Ar3 in the example of the steel of the invention. Figure 3 shows the effect of heating temperature on the toughness of the steel plate, and the heating temperature in the example of the steel of the present invention and the heating temperature ~ 900 ° C.
FIG. 3 is a diagram showing the influence of the cumulative reduction amount during the time on the toughness of a steel plate.

Patent applicant: Kawasaki Steel Corporation Representative patent attorney Mura 1) Masaharu

Claims (1)

[Claims] l Cθ, 03-0. Koo whisper, BLO-03~0. A.O.
blood. Mn O,! ~ko, 0%, Aj O,00! r
~0.01a *Including T10.0 or less, N1/, 0 or less as necessary. cuo, ! S and below, Moa, 411 and below, vo
, l* or less, cro, s%* or less ca o, ot %
Hereinafter, a steel containing any one selected from the following RIMo, to JIG, or 11 or more, with the balance being Fe and unavoidable impurities, provided that 8 of the impurities is o, ooi- or less is referred to as a steel. 00~//jθ℃, and if the heating temperature exceeds 900℃, 900℃
Rolling is performed at the cumulative rolling reduction rate (chi) R that satisfies the following formula calculated from the heating temperature ("C) ↑, and then the temperature range from 00 °C to Ar. Rolling is performed at the above cumulative reduction rate, finishing the rolling at a temperature of at least Ar3, and immediately cooling at the cooling rate of Sekikai Tips B to reach zoo'C at the most. A method for manufacturing a high-toughness non-tempered high-strength steel plate with excellent weldability with little separation.