וַיַּ֥רְא יוֹסֵ֛ף אֶת־אֶחָ֖יו וַיַּכִּרֵ֑ם וַיִּתְנַכֵּ֨ר אֲלֵיהֶ֜ם וַיְדַבֵּ֧ר אִתָּ֣ם קָשׁ֗וֹת וַיֹּ֤אמֶר אֲלֵהֶם֙ מֵאַ֣יִן בָּאתֶ֔ם וַיֹּ֣אמְר֔וּ מֵאֶ֥רֶץ כְּנַ֖עַן לִשְׁבָּר־אֹֽכֶל׃
요셉이 보고 형들인줄 아나 모르는체 하고 엄한 소리로 그들에게 말하여 가로되 너희가 어디서 왔느냐 그들이 가로되 곡물을 사려고 가나안에서 왔나이다
Rashi
ויתנכר אליהם HE MADE HIMSELF STRANGE UNTO THEM — He made himself like a נכרי a stranger to them in his conversation, speaking harshly (Genesis Rabbah 91:7).
Bekhor Shor
And recognised them. Because he was searching for them, if they came because of the famine, since it's the way of the patriarchs to come to Egypt because of the famine. See for example "And there was famine in the land, and Avram descended Egyptwards" (Bereishit 12:1), and so too with Yitzchak until the Holy Blessed One said to him "Don't descend Egyptwards" (Bereishit 26:2). And they weren't looking for him and thinking he would be king! And my teachers have explained that the brothers had beards when they sold him, and now were unchanged, but Yosef didn't have a beard and had since grown one, so he looked different.
Chizkuni
ויתנכר אליהם, “he acted like a stranger to them.” His reasoning was that if he would reveal his identity, they would say to him: “if you expect us not to reveal your true identity, you are also sworn not to reveal our true identity.” Alternately, his reasoning was that he would frighten them into trying to flee. As a result, he would cause his father untold additional anguish. It would therefore be better that they would appear before him because they had no choice not to do so, as they needed the supplies that only he could provide. This would provide him with the chance to force them to bring Binyamin down to Egypt and he would detain him there. As a result, the fact that he was Joseph would become known.