JP2018059143A - Method for manufacturing high carbon steel strip with excellent workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing efficiently a high carbon steel strip with excellent workability.SOLUTION: A method with excellent heat treatment efficiency for manufacturing the high carbon steel strip comprises: a first step of hot rolling a steel in austenite region and coiling subsequently at a temperature of 550°C or lower; and a second step of developing the coil, reheating to a temperature of 600°C or higher and 700°C or lower at a heating rate of 50°C/s or more, and coiling again. The steel contains, in mass%, C: 0.15 to 1.0%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.005 to 0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, N: 0.001 to 0.01%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a high carbon steel strip excellent in workability.

High carbon steel strips are increasingly used as automotive parts and the like that are required to have good formability such as light weight, high strength and hole expansibility.
This high-carbon steel strip is heat-treated after being molded by a processor, and is completed as various parts, but because it is high-carbon, it has high strength, is inferior in formability such as hole expandability, and has good workability. I can't say that.
Therefore, an iron manufacturer, who is a delivery company, usually heat-processes to ensure formability.
As an annealing heat treatment to soften the steel strip in order to impart workability, the method of box annealing of steel strip coils has been used in the past, but it is continuously heated while running the developed steel strip in a heating furnace. Since the development of annealing, this continuous annealing method is often used.

However, since the high carbon steel strip contains carbon in a high concentration, cementite precipitates and grows in the structure when it is wound at a high temperature after hot rolling is completed. It has a difficult hardness, and steel manufacturers are performing annealing for the purpose of spheroidizing cementite and the like in order to reduce the hardness to such an extent that a processor can perform processing such as cutting and forging.
Since this spheroidizing annealing requires a long-time heat treatment and it is difficult to cope with the continuous annealing method, coil box annealing is still employed.

  In addition, since the continuous annealing furnace has a high equipment cost and is premised on continuous operation, coil box annealing is also adopted for small-lot products.

Since this coil box annealing is indirectly heated from the outside of the coil, it takes a long time to heat the inside of the coil, and the heating cost is high, and the temperature inside the coil tends to be uneven. .
There are a number of techniques other than Patent Documents 1 and 2 for performing box annealing on high carbon steel, but all of them are limited to temperature-raising conditions and annealing atmospheres. There is no document that describes any temperature distribution to the steel strip.

JP 2011-012317 A JP 09-157758 A

  The present invention provides a technology capable of producing a high carbon steel strip excellent in workability with high productivity, and a secondary processing such as press molding or forging can be easily performed in a processor. In addition, it has excellent heat treatment properties such as quenching thereafter, and can provide a steel material suitable for automotive parts and the like that require formability and high strength.

The gist of the present invention is as follows.
(1) By mass%, C: 0.15-1.0%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03% , S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, N: 0.001 to 0.01% of steel, hot-rolled in the austenite region, 550 ° C or less After winding at a temperature of (first step), the coil is unfolded, reheated to 600 ° C. or higher and 700 ° C. or lower at a heating rate of 50 ° C./s or higher, and wound again (second step). A method for producing a high carbon steel strip with excellent heat treatment characteristics.

(2) By mass%, C: 0.15-1.0%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03% , S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, N: 0.001 to 0.01% of steel, hot-rolled in the austenite region, 550 ° C or less A method for producing a high-carbon steel strip excellent in heat-treating property, wherein the steel sheet is cooled again to 600 ° C. to 700 ° C. at a heating rate of 50 ° C./s or more, and wound again.

(3) The method for producing a high carbon steel strip according to (1) or (2), wherein the reheating is an electrical heating means.

(4) Reheating temperature distribution is provided in the longitudinal direction of the steel strip so that the outer periphery of the coil wound up after reheating has a high temperature distribution. The manufacturing method of the high carbon steel strip in any one.

(5) When the steel strip is coiled in the second step, heating in the first step is performed so that the temperature at both ends in the width direction of the steel strip is lower than the temperature at the center of the steel strip. The heat treatment method for a high carbon steel strip according to any one of (1) to (4), wherein the temperature is changed in the width direction of the steel strip.

(6) Any of the above (1) to (5), wherein the heating in the first step is Ac1 point or less, and the temperature of the steel strip wound in the second step is 400 ° C. or higher. A heat treatment method for the high-carbon steel strip described in 1.

(7) In mass%, Cr: 0.05 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.05 to 0.5%, and Mo: 0.01 to 1.0% of 1 type or 2 types or more are contained, The manufacturing method of the high carbon steel strip in any one of said (1)-(6) characterized by the above-mentioned.

(8) By mass%, Nb: 0.01-0.5%, V: 0.01-0.5%, Ta: 0.01-0.5%, B: 0.001-0. One or more of 01%, Ti: 0.005 to 0.2%, and W: 0.01 to 0.5% are contained. The manufacturing method of the high carbon steel strip in any one.

(9) By mass%, Sn: 0.003-0.03%, Sb: 0.003-0.03%, and As: 0.003-0.03%, or one or more The manufacturing method of the high carbon steel strip in any one of said (1)-(8) characterized by containing.

  According to the present invention, compared with box annealing generally applied as a technique for annealing a high carbon steel strip, the time required for heat treatment is short and low in cost, and a high carbon steel material excellent in workability is high. Can be manufactured with productivity. Compared with box annealing, carbides mainly composed of cementite contained in steel do not coarsen, so it has excellent carbide dissolution characteristics during quenching in subsequent heat treatment, and ensures hardness even after quenching for a short time. It becomes possible.

It is a conceptual diagram which shows the aspect which transfers the steel strip wound by the coil shape currently stocked to a heat treatment process. It is a conceptual diagram which shows the embodiment which transfers to a heat treatment process directly from a hot rolling process or a cold rolling process.

  First, the reason for limitation relating to the component composition of the soft high carbon steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) will be described. Hereinafter, “%” means “mass%”.

C: 0.15-1.0%
C is an important element for securing the strength of the steel sheet, and is added by 0.15% or more to ensure the required strength. If it is less than 0.15%, the hardenability is lowered and the strength as a high-strength steel sheet for machine structures cannot be obtained, so the lower limit is made 0.15%. If it exceeds 1.0%, it takes a long time for heat treatment to ensure toughness and workability, so the upper limit is made 0.95%. Preferably, it is 0.25 to 0.85%.

Si: 0.05-0.4%
Si acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.05%, the effect of addition cannot be obtained, so the lower limit is made 0.05%. If it exceeds 0.4%, not only will the surface properties be deteriorated due to scale wrinkling during hot rolling, but also softening during reheating will be inhibited, so the upper limit is made 0.4%. Preferably, it is 0.10 to 0.3%.

Mn: 0.5 to 2.0%
Mn acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.5%, the effect of addition cannot be obtained, so the lower limit is made 0.5%. If it exceeds 2.0%, not only the impact properties after quenching and tempering are deteriorated, but also softening during reheating is inhibited, so the upper limit is made 2.0%. Preferably, it is 0.5 to 1.5%.

P: 0.005 to 0.03%
P is a solid solution strengthening element and is an element effective for the strength of the steel sheet. Since excessive inclusion will inhibit toughness, the upper limit is made 0.03%. Reduction to less than 0.005% causes an increase in refining cost, so the lower limit is made 0.005%. Preferably, it is 0.007 to 0.02%.

S: 0.0001 to 0.006%
S forms a non-metallic inclusion and causes a deterioration in workability and toughness after heat treatment, so the upper limit is made 0.006%. Reducing to less than 0.0001% causes a significant increase in refining costs, so the lower limit is made 0.0001%. Preferably, it is 0.001 to 0.004%.

Al: 0.005-0.10%
Al acts as a deoxidizing agent and is an element effective for fixing N. If it is less than 0.005%, the effect of addition cannot be sufficiently obtained, so the lower limit is made 0.005%. If it exceeds 0.10%, the effect of addition is saturated and surface flaws are likely to occur, so the upper limit is made 0.10%. Preferably, it is 0.01 to 0.05%.

N: 0.001 to 0.01%
N is an element that forms a nitride. If nitride precipitates during slab bending correction in curved continuous casting, the slab may crack, so the upper limit is made 0.01%. A smaller amount is preferable, but a reduction to less than 0.001% leads to an increase in refining costs, so the lower limit is made 0.001%. Preferably, it is 0.004 to 0.007%.

  In order to enhance the mechanical properties of the steel sheet of the present invention, a required amount of one or more of Cr, Ni, Cu, and Mo may be added.

Cr: 0.05-1.0%
Cr is an element effective for improving hardenability. If it is less than 0.05%, there is no effect of addition, so the lower limit is made 0.05%. If it exceeds 1.0%, softening during reheating is inhibited, so the upper limit is made 1.0%. Preferably, it is 0.07 to 0.7%.

Ni: 0.01 to 1.0%
Ni is an element effective for improving toughness and hardenability. If it is less than 0.01%, there is no effect of addition, so the lower limit is made 0.01%. If it exceeds 1.0%, the softening at the time of reheating is inhibited and the cost is increased, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.5%.

Cu: 0.05 to 0.5%
Cu is an element effective for ensuring hardenability. If it is less than 0.05%, the effect of addition is insufficient, so the lower limit is made 0.05%. If it exceeds 0.5%, it becomes too hard and the cold workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.08 to 0.2%.

Mo: 0.01 to 1.0%
Mo is an element effective for improving hardenability and improving resistance to temper softening. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 1.0%, softening during reheating is inhibited, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.5%.

  In order to further enhance the mechanical properties of the steel sheet of the present invention, one or more of Nb, V, Ta, B, and W may be added in a required amount.

Nb: 0.01 to 0.5%
Nb is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so the lower limit is made 0.01%. If it exceeds 0.5%, softening during reheating is inhibited, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

V: 0.01 to 0.5%
V, like Nb, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

Ta: 0.01 to 0.5%
Ta, like Nb and V, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the addition effect is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

B: 0.001 to 0.01%
B is an element effective for enhancing the hardenability by adding a small amount. If it is less than 0.001%, there is no effect of addition, so the lower limit is made 0.001%. If it exceeds 0.01%, the castability deteriorates, and a B-based compound is generated to reduce toughness. Therefore, the upper limit is made 0.01%. Preferably, it is 0.003 to 0.007%.

Ti: 0.005 to 0.2%
Ti acts as a deoxidizer and is an element effective for fixing N. From the relationship with the amount of N, addition of 0.005% or more is necessary. Even if Ti is added in excess of 0.2%, the effect of addition is saturated and the cost is not only increased, but also Ti-based precipitation due to promotion of nitrogen absorption during the production process, reduction of effective carbon amount due to carbide formation, etc. This increases the amount of material, inhibits the growth of austenite grains during quenching heat treatment, and deteriorates the hardenability, so the upper limit is made 0.2%. Preferably, it is 0.01 to 0.2%.

W: 0.01-0.5%
W is an element effective for strengthening a steel sheet. If it is less than 0.01%, the effect of addition does not appear, so the lower limit is made 0.01%. If it exceeds 0.5%, the workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.04 to 0.2%.

  When scrap is used as a raw material for the steel sheet of the present invention, one or more of Sn, Sb, and As are inevitably mixed in by 0.003% or more, and any of them may be 0.03% or less. For example, since the punchability and hardenability of the steel sheet of the present invention are not hindered, Sn: 0.003-0.03%, Sb: 0.003-0.03%, and As: 0.0. Inclusion of one or more of 003 to 0.03% is allowed.

  In the steel sheet of the present invention, the amount of O is not specified, but when the oxide aggregates and coarsens, the ductility decreases, so O is preferably 0.0025% or less. A smaller amount of O is preferable, but since it is technically difficult to reduce it to less than 0.0001%, a content of 0.0001% or more is allowed.

  When scrap is used as a melting raw material of the steel sheet of the present invention, elements such as Zn and Zr are mixed as unavoidable impurities. Allow mixing. In addition, elements other than Zn, Zr and the like are allowed to be mixed as long as the characteristics of the steel sheet of the present invention are not impaired.

FIG. 1 is a conceptual diagram showing a process of the present invention in a case where a steel strip is once wound after the hot rolling process is completed, and then the steel strip coil is newly developed, reheated under predetermined conditions, and wound again.
When the steel strip is wound into a coil after the hot rolling process, the steel strip temperature is cooled to 550 ° C. or lower.
Depending on the time required for cooling from the hot rolling end temperature to a coiling temperature of 550 ° C. or less, the difference in the thickness of the steel strip, etc., fine bainite and martensite are contained in the ferrite structure constituting the steel strip coil matrix Partially occurs.
In the present invention, these are called hard phases.

Since the ratio of the hard phase and the crystal grain size also change depending on the length of the standing time until reheating after winding, it is possible to select the required steel strip hardness by experiment.
If the standing time after winding is made longer, the crystal size of the hard phase and the matrix increases, and the steel strip tends to become soft.

  When reheating the steel strip coil, the coil is unwound and reheated to a predetermined temperature of 600 ° C. to 700 ° C. at a heating rate of 50 ° C./s or more by electric heating means such as energization heating or induction heating. It heats and passes through a predetermined cooling process, and is wound up into a coil shape again.

  By this reheating and cooling process, the fine hard phase deposited at the end of hot rolling is decomposed again, the steel strip is softened, and a steel material having a hardness suitable for press forming and forging is obtained.

FIG. 2 shows a conceptual diagram when the present invention is applied as it is without winding the steel strip coil after hot rolling.
The steel strip coil after hot rolling is cooled to a desired temperature on the run-out table at a predetermined cooling rate, and then the steel strip is heated at a heating rate of 50 ° C./s or more by direct current heating means or electromagnetic induction heating means. To 600 ° C. to 700 ° C. to a desired temperature.

  The effect | action by reheating and subsequent cooling is the same as that of the example shown in FIG.

  In the present invention, after reheating and cooling, the coil is wound and stored, so that the subsequent cooling is affected by the winding temperature, but the outer periphery of the coil is more quickly Because it will be cooled and there is a risk of uneven cooling in the longitudinal direction of the coil, if necessary, provide a reheating temperature distribution in the longitudinal direction of the steel strip, and cooling conditions for the steel strip in the finished state Can be made uniform on the inner and outer peripheries of the coil.

In test 1 (Table 1), steel types A to K having a thickness of 1.2 mm were hot-rolled, cooled to 420 ° C. on a ROT (runout table), and once wound on a coil, within 30 minutes. While re-expanding (coil unwinding), it was heated to 680 ° C. at a heating rate of 60 ° C./s with a high-frequency heating device, and then wound up.
On the other hand, Test 2 (Table 2) and steel types A to E having a plate thickness of 1.2 mm were manufactured under the conditions shown in the table.
In the present invention, the press workability was judged by whether or not contact bending was possible.
The hardenability was evaluated by heating the obtained steel sheet to 900 ° C. at 10 ° C./s using a heat treatment simulator, holding it for 0 s and 180 s, then cooling it at 50 ° C./s, and measuring the hardness. After securing a martensite structure of 70% or more, it was judged that the difference in hardness between holding for 180 s and holding for 0 s was Hv35 or less and that the hardenability was good.

  In any case, when the steel strip composition is within the scope of the present invention and the heating is performed at a predetermined heating (temperature increase) rate after the end of hot rolling to a range of 600 ° C. to 700 ° C., press workability As a result of its excellent hardenability after processing, it has become a high carbon steel strip useful as a material for various automotive parts.

  On the other hand, if the composition of the steel strip is outside the range specified in the present invention, or if the heat treatment specified by the present invention does not occur in the heat treatment after hot rolling, press formability and processing In at least one of the subsequent heat treatment properties, the evaluation is not satisfactory.

  According to the present invention, a high carbon steel strip excellent in workability can be produced with high productivity, secondary processing such as press molding and forging can be easily performed, and heat treatment such as subsequent quenching can be performed. It is excellent, and it is possible to provide a steel material suitable for automobile parts and the like that require formability and high strength.

Claims (9)

  In mass%, C: 0.15-1.0%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03%, S: A steel containing 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01% is hot-rolled in the austenite region and at a temperature of 550 ° C. or lower. After winding (first step), the coil is unfolded, reheated to 600 ° C. or more and 700 ° C. or less at a heating rate of 50 ° C./s or more, and wound again (second step). A method for producing high carbon steel strips with excellent heat treatment properties.   In mass%, C: 0.15-1.0%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03%, S: A steel containing 0.0001 to 0.006%, Al: 0.005 to 0.10%, N: 0.001 to 0.01% was hot-rolled in the austenite region and cooled to 550 ° C or lower. Then, it reheats to 600 degreeC or more and 700 degrees C or less with the heating rate of 50 degrees C / s or more, The manufacturing method of the high carbon steel strip excellent in the heat processing characteristic characterized by winding up again.   The method for producing a high carbon steel strip according to claim 1 or 2, wherein the reheating is an electrical heating means.   The distribution of the reheating temperature is provided in the longitudinal direction of the steel strip so that the outer periphery of the coil wound up after the reheating has a high temperature distribution. Manufacturing method of high carbon steel strip.   In the second step, when the steel strip is wound in a coil shape, the heating temperature in the first step is set so that the temperature at both ends in the width direction of the steel strip is lower than the temperature at the center of the steel strip. The heat treatment method for a high carbon steel strip according to any one of claims 1 to 4, wherein the heat treatment method is changed over the width direction of the strip.   Heating in a 1st process is Ac1 point or less, and the temperature of the steel strip wound up in a 2nd process is 400 degreeC or more, The high in any one of Claims 1-5 characterized by the above-mentioned. Heat treatment method for carbon steel strip.   Further, Cr: 0.05-1.0%, Ni: 0.01-1.0%, Cu: 0.05-0.5%, and Mo: 0.01-1.0 % 1 type or 2 types or more, The manufacturing method of the high carbon steel strip of any one of Claims 1-6 characterized by the above-mentioned.  (8) By mass%, Nb: 0.01-0.5%, V: 0.01-0.5%, Ta: 0.01-0.5%, B: 0.001-0. One or more of 01%, Ti: 0.005-0.2%, and W: 0.01-0.5% are contained, The any one of Claims 1-7 characterized by the above-mentioned. The manufacturing method of the high carbon steel strip as described in a term.   In addition, it contains one or more of Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003 to 0.03%. The method for producing a high carbon steel strip according to any one of claims 1 to 8, wherein:

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