JPS6039119A - Manufacture of hot-rolled high-tension steel plate - Google Patents

Abstract

PURPOSE:To manufacture a hot-rolled high-tension steel plate having high strength and cold workability comparable to those of a conventional high Si and high Mn steel plate by rolling a low Si and low Mn steel billet at a low temp. and coiling the resulting plate at a specified temp. CONSTITUTION:A cast steel billet contg. 0.02-0.25% C, <0.14% Si, <0.02- 0.28% Mn, <0.025% P, <0.005% S and 0.01-0.10% sol. Al is manufactured. The billet may further contain one or more among at least one among <0.05% each of Nb, V and Ti (i), <0.0100% Ca (ii) and <0.0050% B (iii). The billet is hot rolled to a plate, and the hot rolling is finished at Ar3+50 deg.C-650 deg.C. The plate is rapidly cooled at >=50 deg.C/sec cooling rate and coiled at <=450 deg.C. A hot- rolled high-tension steel plate having superior strength and cold workability is obtd. while using said small amounts of added elements such as Si and Mn.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention has excellent cold workability, and a weight of 45 kg/. A method of producing hot-rolled high-strength steel sheets having a strength higher than that of C-type steel at an extremely low cost. This article relates to a method for manufacturing hot-rolled high-strength steel sheets.

Conventionally, high-strength steel sheets include, for example, 0.09% C-0.
High Si-high Mn steels with a large amount of alloying elements added, such as ゜5%5i-1,50%Mn-0,03%Nbwi, were common (e.g., JP-A-54-65118). ).

However, such conventional hot-rolled high-strength steel sheets not only have high costs because they use large amounts of alloying elements such as Si and Mn, but also have continuous casting slabs (hereinafter referred to as CC slabs) obtained by continuous casting. ) cracks are also likely to occur. Therefore, it has been widely used in recent years due to its energy saving benefits.
Even if it is attempted to carry out the so-called direct rolling, which is currently carried out in which continuous casting is immediately followed by hot rolling, on this type of steel, it is quite difficult to apply it because of the necessity of slab care.

In addition, when the Si content is high, the descaling performance deteriorates, so the scale is not removed promptly during hot rolling, and the obtained hot rolled sheet has scale indentation defects, which reduces the value of the product. Not only this, but also defects such as deterioration of cold workability and fatigue properties occur due to such deterioration of surface properties.

Here, in order to solve the problems of the above-mentioned prior art, the present inventors first created a low-Si-low-Mn steel, and further, even if the content of Si, Mn was reduced in this way, the conventional steel As a result of intensive study to obtain a steel plate with the same high strength and workability as high Si-high Mn hot rolled high tensile strength steel plate,
For C-low Si-low Mn steel, high strength and cold workability comparable to conventional hot-rolled high-strength steel sheets are ensured by quenching after low-temperature rolling and then coiling at 450°C or less. The present invention was completed by discovering that a high tensile strength steel plate can be obtained.

Therefore, the gist of the present invention is, in weight %, C: 0.02 to 0.25%, Si: 0.14%
Below, Mn: 0.02-0.28%, P: 0
．． 0.025% or less, S: 0.005% or less, Sol
, A j! : 0. Ar3+5 is obtained by hot rolling a cast steel piece having a composition of iron ~0.10% and the remainder substantially iron.
After finishing hot rolling at 0℃~650℃, 50℃/sec
This is a method for producing a hot-rolled high-strength steel sheet, which is characterized by performing the above quenching and then winding at 450° C. or lower.

Furthermore, in the present invention, the cast steel piece may contain Ca: 0.0100% or less and/or N.
b: 0.05% or less, V: 0.05% or less and T
i: 0.05% or less of one or more kinds, and/or B: 0.0050% or less may be contained.

In addition, in the present invention, - the above-mentioned cast steel slab is a continuously cast slab;
In other words, it may be a CC slab, or it may be a steel slab obtained by blooming rolling, that is, a blooming slab.

Further, in any case, prior to hot rolling, the hot cast steel billet may be reheated or directly sent to the hot rolling process without being heated.

However, when considering the advantages of the so-called direct rolling described above, a preferred embodiment of the present invention is to use a CC slab as the cast steel slab, and directly send it to the hot rolling process without reheating. It is preferable to adopt the law.

Next, the reason why the &171 composition and rolling conditions are limited as described above in the present invention will be explained in detail below.

C: C, especially of the present invention! ! ! Under construction conditions, a hard second phase (
For example, it has the effect of increasing the volume fraction of fine pearlite, pseudo pearlite, bainite, etc.), increasing the strength of the steel plate, and easily imparting a tensile strength of 45 kg/mm or more. If the content is less than 0.02%, the desired strength cannot be obtained, while if the content exceeds 0.25%, weldability deteriorates. In addition, in order to economically obtain a high tensile strength steel plate with a tensile strength of 50 kg/mA or more, it is preferable to add C in an amount of 0.06% or more.

Si: Si has the effect of increasing the strength of steel sheets through solid solution hardening, but it is necessary to save this element in order to improve the surface properties of steel sheets and reduce costs. Therefore, in the present invention, Si is set to 0.14% or less. Preferably it is 0.10% or less.

Mn = Mn has the effect of improving the hardenability of steel and increasing the strength of the steel sheet, but in C-strengthened steel as in the present invention, it rather lowers the Mn content and improves the hardenability. It is more advantageous for cold workability to lower it. Furthermore, lowering Mn is advantageous for cold workability in terms of reducing MnS, which causes cold working cracks.

Therefore, in consideration of saving alloying elements and preventing cracks in CC slabs, the Mn content is reduced to 0.2 in the present invention.
Limit to 8% or less. Preferably it is 0.20% or less.

Note that the lower limit is set to 0.02% in order to stabilize S and prevent embrittlement during hot working.

P: P increases the hardness of the center segregation part in steel plates manufactured from CC slabs, and tends to cause cracks during cold working. Therefore, it is desirable to have as few as possible. In the present invention, from an economic standpoint, the content is set to 0.025% or less, preferably 0.010% or less.

S: S combines with Mn to become MnS, producing A-based inclusions and deteriorating cold workability. Moreover, when S is high, embrittlement tends to occur during hot working. Therefore, it is desirable that S is also as small as possible. In the present invention, from an economical point of view, 0.
0.005% or less, preferably 0.002% or less.

Sol, A1: Sol, An is effective as a deoxidizing agent and therefore
The lower limit is O1 old % at which the deoxidizing effect is expected, and the upper limit is 0.10% at which the deoxidizing effect is saturated.

Nh, V, Ti: All of these elements combine with C (or N) to form fine precipitates, resulting in large precipitation hardening. However, if too large a quantity is added, the above-mentioned hard second phase decreases and the precipitates become coarse.
Since it tends to become steel and the strength decreases, the amount of each element added is set to 0.05% or less.

B: B is a preferable element that can improve the hardenability of steel by adding only a small amount. However, addition of more than a certain content not only leads to increased cracking of the CC slab, but also saturates the hardenability effect of the steel, resulting in higher costs. Therefore, when adding B, its double is 0.0
050%.

Ca: In addition to the Bs effect, Ca has the effect of combining with A7!203 and MnS to change A-based and B-based inclusions into C-based inclusions. Therefore, Ca is a preferable element that greatly improves the cold workability of the hot-rolled high-strength steel sheet according to the present invention due to this effect. However, if it is added in a large amount, inclusions in the steel will increase and cold workability will actually deteriorate. Therefore, Ca
The upper limit of addition is 0.0100%.

Note that at least one of the above-mentioned desired additive elements such as Nb, (2), Ti, B, or Ca can be appropriately selected and added as an endogenous element.

Other incidental impurities include N and the like, but N is preferably limited to 0.02% or less.

Hot rolling end temperature: In the present invention, rolling is ended at a temperature of Ar3 + 50°C to 650°C. However, if rolling is ended at a temperature higher than ^r3 + 50°C, the effect of fine graining of the structure due to hot rolling will be reduced. This is not expected, and in particular, when attempting to obtain high strength using C, as in the method according to the present invention, the deterioration of cold workability is rather significant.

Furthermore, if rolling is completed at a temperature lower than 650° C., the ferrite after transformation will be significantly processed, resulting in large anisotropy and deterioration of cold workability. Moreover, if the rolling is completed at a temperature lower than 650° C., the hot deformation resistance becomes extremely high, making rolling actually quite difficult.

Note that the rolling start temperature is not particularly limited as long as it is 113 points or higher. Generally, the temperature is 1050°C or higher.

Cooling/coiling conditions after hot rolling: If the cooling rate after hot rolling is less than 50°C/sec,
A sufficient hardening effect does not occur, making it difficult to obtain a hardened structure, making it impossible to obtain the desired high strength. Note that it goes without saying that preferably the cooling rate is as fast as possible.

However, even if the cooling rate is faster than 50°C/sec, if the coiling is performed at a temperature higher than 450°C after cooling, the structure will still soften due to slow cooling after winding and the desired high strength will not be obtained. do not have.

Note that the winding temperature is preferably 450°C or lower, preferably 450 to 100°C. If the temperature is lower than 100℃, the hardness of ferrite underground due to slow cooling after coiling will increase.
This is because the 80 reduction effect is reduced and the cold workability is also deteriorated.

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

Each sample material having the chemical composition shown in Table 1 was melted and hot rolled under the conditions shown in Table 2. Table 1 shows three Ar points for each steel type for reference.

The mechanical properties of the obtained hot-rolled steel sheet were
They are summarized in the table.

Test numbers 1.2.15 to 17 are all the same steel type (A)
The results show the cases where the hot rolling conditions were changed. When the conditions of the present invention are not met, that is, when the hot rolling conditions are not met, the cold workability is poor (e.g. test number 15.16), and when the cooling and winding conditions are not met, the strength is low (e.g. test number 15.16). 17.18). On the other hand, all the cases of test number 1.2 show examples of the present invention, but in the case of test number 2, the winding temperature is as low as 100° C. or less, so the workability is slightly inferior.

In addition, in the case of test numbers 9 to 14, the chemical composition of the steel deviates from that of the present invention, and either they have extremely low strength (e.g. test number 9, L2) or high strength. However, the cold workability is not good (e.g. test number 1).
0.11.13.14).

0 Thus, as is clear from the results shown in Table 2,
In the cases according to the present invention (test numbers 1 to 8), the obtained hot rolled steel sheets all have high strength and excellent cold workability. In particular, in the case of test numbers 3.5 and 8 in which Ca was added, hot-rolled steel sheets with high strength and extremely good cold workability were obtained.

Claims (2)

[Claims] (1) Coo in % by weight. 02-0.25%, Si: 0.14% or less, Mn: 0.02-0.28%, P: 0.
0.025% or less, S: 0.005% or less, Sol,
Aβ: 0.01 to 0.10%, with the remainder essentially iron, hot-rolled to Ar35
A method for producing a hot-rolled high-strength steel sheet, which comprises hot rolling at 0°C to 650°C, followed by rapid cooling at 50°C/sec or higher, and then winding at 450°C or lower. (2) In weight%, C: 0.02-0.25%, St: 0.14%
Below, Mn: 0.02-0.28%, P: 0
．． 0.025% or less, S: 0.005% or less, Sol
, All: 0.01 to 0.10%, and one or more selected from the following groups (i) to (iii) or 2N or more; (i) Nb: 0.05% or less, V: 0.05% and Ti: 0.05% or less (both 1 type, (
ii) Ca: 0.0100% or less, and (i
ii) B: A cast steel piece having a composition of 0.0050% or less, the remainder substantially consisting of iron, is hot rolled to ^r3+5
A method for producing a hot-rolled high-strength steel sheet, which comprises hot rolling at 0°C to 650°C, followed by rapid cooling at 50°C/sec or higher, and then winding at 450°C or lower.